From recovery boiler to integration of a textile fiber plantCombination of mass balance analysis and chemical engineering

Hans Magnusson

Hans M

agnusson | From recovery boiler to integration of a textile fiber plant | 2015:39

From recovery boiler to integration of a textile fiber plantModern chemical technology is an extremely efficient tool for solving problems particularly in a complicated environment such as the pulp and paper industry. Here, examples are studied during which chemical technology is of fundamental importance.

At normal conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank without hindrance. However, for certain kraft recovery boiler alternatives, knowledge of more precise data of the molten salts is required. The viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at relevant temperatures.

The main input of non-process elements (NPE) is down to the wood, and known problems include deposits in evaporators and decreasing efficiency in the causticization department. Green liquor clarification is an efficient kidney for many NPE. Magnesium added in the oxygen delignification does not form a closed loop.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be of significant interest, as the handling of both alkali and sulphuric compounds can be integrated. Problems will however arise as the capacity of the pulping line and the chemical recovery has to be adjusted.

LICENTIATE THESIS | Karlstad University Studies | 2015:39 LICENTIATE THESIS | Karlstad University Studies | 2015:39

ISSN 1403-8099

Faculty of Health, Science and TechnolgyISBN 978-91-7063-657-8

Chemical engineering

LICENTIATE THESIS | Karlstad University Studies | 2015:39

From recovery boiler to integration of a textile fiber plantCombination of mass balance analysis and chemical engineering

Hans Magnusson

Print: Universitetstryckeriet, Karlstad 2015

Distribution:Karlstad University Faculty of Health, Science and TechnolgyDepartment of Engineering and Chemical SciencesSE-651 88 Karlstad, Sweden+46 54 700 10 00

© The author

ISBN 978-91-7063-657-8

ISSN 1403-8099

URI: urn:nbn:se:kau:diva-37266

Karlstad University Studies | 2015:39

LICENTIATE THESIS

Hans Magnusson

From recovery boiler to integration of a textile fiber plant - Combination of mass balance analysis and chemical engineering

WWW.KAU.SE

1

Abstract Modern chemical technology is an efficient tool for solving problems, particularly within the complex environment of the pulp and paper industry, and the combination of experimental studies, mill data and mass balance calculations are of basic importance to the development of the industry. In this study various examples are presented, whereby chemical technology is of fundamental importance.

It is well documented that under normal conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank without problems. However, in the case of alternatives to the kraft recovery boiler, knowledge of more precise data of the molten salts is required for the design calculations. In this study the viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at temperatures relevant for a recovery boiler, i.e. similar to water at room temperature.

The presence of non-process elements (NPE) in a typical pulp mill has been investigated. The main input is with regards to the wood, and anticipated problems include: deposits in evaporators, high dead-load in liquor streams, plugging of the upper part of the recovery boiler and decreasing efficiency in the causticization department. Efficient green liquor clarification is of the greatest importance as an efficient kidney for many NPE. Mill data and calculations show that the magnesium added in the oxygen delignification does not form a closed loop.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be highly beneficial. The prehydrolysis liquor will contain both sugars and acetic acid. It is however not possible to fully replace the sulphuric acid of the viscose spinning bath with acetic acid of own production. The sulphuric chemicals from the viscose plant can be partly taken care of in the kraft recovery area as well as the viscose plant which can be supplied with alkali and sulphuric acid. Zinc-containing effluents from the viscose plant can be treated with green liquor to precipitate zinc sulphide.

Keywords: integrated production, kraft pulping, non-process elements, oxygen delignification, recovery boiler, regenerated cellulose, smelt properties, viscose manufacturing.

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Sammanfattning Modern kemiteknik är ett synnerligen effektivt verktyg för att lösa problem i en så komplicerad miljö som i massa- och/eller pappersbruk. I denna avhandling behandlas några olika exempel där kemiteknik är av fundamental betydelse för att nå framgång i problemlösandet.

I sulfatmassafabrikens kemikalieåtervinning återvinns processkemikalierna som en saltsmälta från sodapannan. Viskositeten för denna är av intresse för driften av sodapannan men i synnerhet en viktig parameter vid konstruktionen av några alternativ till den konventionella sodapannan. Viskositeten för saltsmältor av natriumkarbonat och 30 mol% natriumsulfid har mätts och är av storleksordningen 2 – 3 cP i relevant temperaturområde, dvs som vatten vid rumstemperatur.

Vid produktionen av pappersmassa finns vid sidan av processkemikalierna även så kallade processfrämmande grundämnen (PFG), exempelvis kalium, järn, mangan och magnesium. Förekomsten av PFG har undersökts i ett typiskt sulfatmassabruk. Det huvudsakliga tillflödet av PFG är med veden och med kalken till kalkcykeln. Vid ökad slutningsgrad kan problem väntas med beläggningar i indunstare, stor andel ballast i lutar, igensättningar i övre delen av sodapannan och minskad verkningsgrad i kausticeringen. Effektiv grönlutsklarning är av största betydelse som njure för många PFG. Andra ämnen anrikas i sodapannans rökgasstoft och kan åtgärdas genom utblödning av detta. Fabriksdata och beräkningar visar att magnesium som tillsätts till syrgasdelignifieringen inte ingår i ett slutet kretslopp.

Kemitekniska kunskaper och verktyg har använts i utredningen om integrering av ett sulfatmassabruk för produktion av förhydrolyserad dissolvingmassa med en anläggning som producerar viskosfiber. Avluten från förhydrolysen av veden innehåller bland annat ättiksyra som skulle kunna användas för koagulering av textilfibern i spinnbadet. Denna studie har dock visat att det inte fullt ut är möjligt att ersätta svavelsyran för viskosspinningen med egenproducerad ättiksyra eftersom syramängden är för liten. Däremot kan viskosanläggningen förses med alkali och eventuellt även svavelsyra från massatillverkningen. Avlopp från viskosanläggningen kan delvis tas om hand och nyttiggöras i sulfatmassabrukets kemikalieåtervinning. Zinkhaltiga avlopp från viskostillvekningen, som av miljöskäl måste tas om hand, kan fällas med grönlut för återvinning av zink som zinksulfid.

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List of papers

Appended papers The following publications, referred to as Papers I, II, III and IV, comprise the foundation of this licentiate thesis and can be found in the appendix of this thesis

Paper I Hans Magnusson & Björn Warnqvist Properties of sodium sulfide - sodium carbonate melts Svensk Papperstidning 78(1975):17, 614-616

Paper II Hans Magnusson, Karin Mörk & Björn Warnqvist Non-process elements in the kraft recovery system Tappi/ATCP Pulping Conference, Seattle, WA, USA. Sept. 1979. pp 77 - 83

Paper III Hans Magnusson & Karin Mörk Can magnesium be a problem in a kraft mill with oxygen bleaching ? Tappi Journal 63(1980):5 , 121 – 123.

Paper IV Hans Magnusson, Niklas Kvarnlöf, Gunnar Henriksson & Ulf Germgård Integrated chemical pulping and regenerated cellulose manufacturing. Manuscript

The author’s contribution to the papers Paper I HMN designed and planned the experiments, took part in the practical

experimental work and evaluated the results supported by BWq. The paper was written together with BWq.

Paper II This paper is a part of a larger project planned by HMN and evaluated with support from KM and BWq. Sampling was planned by HMN and carried out together with staff and mill personnel.

The calculations presented in the paper were made by HMN and KM. The paper was written by HMN with support of KM and BWq.

Paper IV The paper was based on data from the project reported in paper II. The calculations were made by HMN and KM and the paper was written by HMN.

Paper IV UG proposed the basis for this paper, HMN performed literature search and calculations. Experiments were carried out by NK and HMN. The paper was written by HMN with support from NK, GH and UG.

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Related non-appended papers Hans Magnusson & Björn Warnqvist A Survey of Possible Methods for Recovery of Kraft Pulping Chemicals Black liquor recovery symposium 1982. Helsinki, Finland 1982-08-31 – 09-01. p B7-1 – 13.

Hans Magnusson & Björn Warnqvist The NSP Project: Development of an Alternative to the Conventional Recovery Furnace. Chemical Engineering Progress 76(1980):2, 54-56.

Hans Magnusson Silica and the Recovery of Pulping Chemicals – Technology and Economy. 2nd Int. Non-Wood Fibre Pulping & Papermaking Conference, Shanghai, PR China, 1992-04-06--09. p. 914 – 922.

Hans Magnusson, Niklas Kvarnlöf & Ulf Germgård Integration of a dissolving pulp mill and a cellulose based textile fiber plant Poster. 13th European Workshop on Lignocellulosics and Pulp (EWLP 2014). Seville, Spain. 2014-06-24—27. p. 547 – 550.

Hans Magnusson & Ulf Germgård Pros and cons with integration of a dissolving pulp mill and a textile fiber plant. 4th Avancell Conference, Chalmers University of Technology, Gothenburg, Sweden. 2014-10-07—08, p. 23 - 25.

Hans Magnusson, Niklas Kvarnlöf & Ulf Germgård Some aspects on the integration of a dissolving pulp mill and a textile fiber plant Poster. The 6th Workshop on Cellulose, Regenerated Cellulose and Cellulose Derivatives. Karlstad University, Karlstad, Sweden. 2014-11-11—12, p. 63-65.

Ulf Germgård, Hans Magnusson, Niklas Kvarnlöf & Gunnar Henriksson Some aspects on the possibility to integrate a dissolving pulp mill and a cellulose based textile fiber plant. Poster. COST Action FP1205. Joint Working Groups & Management Committee meetings. “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania. 2015-03-10 – 11.

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Table of Contents Abstract .................................................................................................................................................. 1

Sammanfattning .................................................................................................................................... 2

List of papers ......................................................................................................................................... 3

Appended papers ............................................................................................................................. 3

The author’s contribution to the papers ......................................................................................... 3

Related non-appended papers ....................................................................................................... 4

Table of Contents ................................................................................................................................. 5

List of Figures and Tables ............................................................................................................... 6

Abbreviations ......................................................................................................................................... 7

1. Introduction .................................................................................................................................... 8

1.1 The evolution of the pulp mill and the recovery of pulping chemicals .......................... 9

1.2 Non-process elements in the kraft pulping process. ..................................................... 14

1.3 The manufacture of textile fibers from cellulose ............................................................ 16

1.3.1. Processes for the production of dissolving pulp..................................................... 18

1.3.2. The traditional viscose process and new alternatives. ......................................... 19

1.4 The bio-refinery concept .................................................................................................... 25

1.5 Aims of the work ................................................................................................................. 27

2. Materials and Methods. ............................................................................................................. 28

2.1. Measurements of density and viscosity of recovery boiler melts ................................... 28

2.2. Calculation of material and energy balances. ................................................................... 29

2.3. Measurements of non-process elements. ....................................................................... 30

2.4. Calculations on the integration of a pulp mill and a textile fiber plant......................... 30

3. Results and discussion. ............................................................................................................. 31

3.1 The pulp mill of today and the recovery of pulping chemicals. ................................................. 31

3.2 Non-process elements in the pulping process ................................................................... 34

3.3 Mill integration ......................................................................................................................... 40

3.3.1. Co-location of dissolving and viscose mills ............................................................... 40

3.3.2. Advantages of never-dried pulp ................................................................................... 41

3.3.3. Sodium hydroxide from the kraft pulp mill used in in a viscose plant. .................... 41

3.3.4. Acid for the cellulose coagulation ................................................................................ 42

3.3.5. Precipitation of ZnS with green liquor .......................................................................... 42

4. Conclusions. ................................................................................................................................ 44

5. Industrial significance. ................................................................................................................ 45

6. Future perspectives. ................................................................................................................... 46

7. Acknowledgements. ................................................................................................................... 47

8. References................................................................................................................................... 48

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List of Figures and Tables Figure 1. The internal connections of the different departments in typical

unbleached kraft pulp mill. 11 Figure 2. Statistics for recovery boiler explosions in North America 12 Figure 3. Inside of different recovery furnaces.

(With the permission of Diamond Superior,Sweden). 13 Figure 4. Emission of sulphur compounds to air and production of pulp. 14 Figure 5. The inerts in burnt lime increases when the lime cycle is

gradually closed. 15 Figure 6. The world market for textile fibers, million tons/year. 16 Figure 7. The world fiber market 2014. 16 Figure 8. The cellulose gap. 17 Figure 9. Principle flow sheet for the viscose process 19 Figure 10. CCA. Simplified flowsheet for production of

Cellulose Carbamate fibers. 22 Figure 11. CelluNova. Simplified flowsheet for production of

CelluNova fibers. 23 Figure 12. The phase diagram Na2CO3 – Na2S. 31 Figure 13. Green liquor sludge as kidney for Mg, Ca, Mn, Fe, Cu, Pb, Cd etc. 35 Figure 14. The build-up of elements such as potassium and chloride. 36 Figure 15. Outline of the magnesium flow in a kraft mill with oxygen

delignification 38 Figure 16. The solubility of zinc hydroxide and zinc sulfide 43 Table 1. Losses and make-up chemicals in Swedish pulp mills between

1919 and 2000. 10 Table 2. The evolution of steam data and size for recovery boilers. 12 Table 3. The main chemicals involved in the viscose process. 20 Table 4. Chemicals used in the Cellulose Carbamate process. 22 Table 5. Chemicals used in the CelluNova process. 23 Table 6. Dynamic viscosity of molten sodium carbonate – sulphide. 33 Table 7. Some of the main NPE in wood, unbleached and bleached pulp. 34 Table 8. Composition of clarified “normal” green liquor from Scandinavian

kraft pulp mills. 35 Table 9. Chemical composition of green liquor sludge from 3 different

kraft pulp mills. 36 Table 10. Typical composition of a white liquor. 41 Table 11. Spinning bath for some typical viscose products. 42

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Abbreviations

AOX Absorbable Organically bound halogens (according to ISO 9562:2004, EPA 1650C)

BAT Best Available Technique

BOD Biological Oxygen Demand (according to SS-EN 1899)

BREF Reference Documents on Best Available Technique

COD Chemical Oxygen Demand (according to ISO 6060:1989)

DTA Differential Thermal Analysis

GLS Green Liquor Sludge

HTM High Temperature Microscope

IED Industrial Emission Directive, EU Directive 2010/75/EU

LCA Life Cycle Analysis

NMMO N-Methylmorpholone N-oxide, ionic liquid, used as solvent for cellulose

NPE Non-Process Elements

PFG ProcessFrämmande Grundämnen. Swedish for NPE

PHK PreHydrolyzed Kraft pulp

PHL PreHydrolysis Liquor

SSVL The Forest Industry´s Water And Air Research Foundation ( Stiftelsen Skogsindustriernas Vatten- och Luftvårdsforskning)

α-cellulose Older name for pure cellulose, measured according to Tappi Classical Method T203cm-99

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1. Introduction

During the industrial revolution in the middle of the 19th century chemical engineering was beginning to develop as a discipline of its own. When chemical handicraft for producing useful products was converted into an industry, it was necessary to combine new knowledge from mechanical engineering and other areas to the traditional chemistry (Kim 2002; Sinnott 1989). However, much of the knowledge used in chemical engineering was of course not new, but instead collected from various other disciplines. Chemical engineering is by nature multidisciplinary, based on a solid scientific ground of chemistry, physics and mathematics. Three main areas can be recognized in chemical engineering: chemical reaction engineering, process design and plant design. The theoretical background includes thermodynamics, unit operations and transport phenomena.

The last five decades has been a revolution for chemical engineering. The engineers’ traditional tools, i.e. the pencil and paper, the slide rule and the drawing board are now replaced with personal computers. Powerful computer software and advanced post–treatment has rationalized time-consuming calculations. However, the basic knowledge of chemistry and other basic disciplines must not be forgotten. The pulp and paper industry is a good example of when applied chemical engineering is productive. In Sweden the pulp and paper industry was developed partly based on the mill traditions of the steel industry; long traditions that included both fundamental chemistry and applied chemistry (Berndtson, 1797). At the end of the 19th century many pulp mills were built, different process concepts were tested and new equipment and process solutions were developed. Inspiration was, in many cases, taken from other industries. The engineers responsible for designing, building and operating the mills often had sound knowledge in chemical engineering, earned not only from schools but also from trial and error in the mills (Jerkeman, 2015). The technical progress is a combination of evolution and revolution. Along with the development of new processes and equipment, there is also a continuous development of the presently used processes and equipment. New processes and new equipment will replace old ones only if substantial gains in product quality or process costs can be achieved. This is especially true for the pulp and paper industry which has a large amount of long-life, yet expensive equipment. Naturally, for an investment of this nature the returns must be reasonable and the reliability must exceed that of the existing equipment.

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1.1 The evolution of the pulp mill and the recovery of pulping chemicals

During the growth of the modern society in the middle of the 19th century, the increasing demand for paper and the replacement of hand-made paper with machine-made paper caused a severe shortage of old textile fibers, these fibers being the main source for papermaking. Mechanical pulping of wood gave a reasonably good fiber material for newsprint, but high quality pulp for archive grade paper required a superior pulp and the choice at that time was acidic sulfite pulp, which was relatively bright without bleaching. It was known that paper pulp could also be produced with the help of alkaline cooking liquor, but soda-pulping of wood was not competitive, as among other things, alkali in the form of sodium hydroxide or sodium carbonate was quite expensive. The losses of chemicals in the early alkaline pulp mills were often higher than 50 %. The breakthrough came when the loss of chemicals was replaced with cheap sodium sulfate. This gave the Swedish name of the process, the English name comes from the German word “Kraft”, meaning the pulp could produce a paper of high tensile strength. The evolution of the kraft process combined with the development of efficient bleaching processes and high quality equipment made the kraft pulp suitable for writing and printing paper as well as printable surface layers on packaging board. Great efforts have finally been made in the pulp mills to minimize the emissions to water and air by both closing the processes internal and adding equipment for efficient external effluent treatment. As previously stated above, it was necessary from the beginning to recover the chemicals used in order to minimize the costs of make-up chemicals, thus improving the economy of the kraft pulping process. In the first mills this was achieved by mixing the spent liquor from the cooking department with saw-dust and wood residues and burning it; the ash was leached with water to obtain green liquor (Öman 1944; Edling 1981). A notable milestone was the development of the first operating recovery furnaces Windsor no 1, in 1930 and Windsor no 2, 1934, both in Canada (Tomlinson II, 1976). This was closely followed by the first Swedish recovery boiler at Husum in 1936 (Götaverken 1989). The evolution of the processes and equipment increased the efficiency of the mills and the need for make-up chemicals decreased. These chemicals shifted over time; from sodium sulphate, to sodium carbonate, sodium hydroxide and sulphuric acid (SSVL, 1974). The emissions of sulphur-containing obnoxious gases and sulphur dioxide were minimized by new process solutions and new equipment. Today large amounts of chemicals are handled inside the pulp mill, but the losses are relatively small, see Table 1.

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Table 1. Losses and make-up chemicals in Swedish pulp mills between 1919 and 2000. Note that the data for the year 2000 is somewhat misleading as the make-up demand is usually not specified as kg Na2SO4 /t pulp, as the losses are more or less covered by recycling of internal streams of sodium and sulfur in the mill.

Year Losses /make-up Comments Ref. 1919 150 kg Na2SO4/t pulp Brahmer 1919 1968 82 kg Na2SO4/ t pulp Statistics for all 31 Swedish kraft mills SNV 1969:3 1973 30 kg Na2SO4/ t pulp

50 kg Na2SO4/ t pulp Typical examples of different process configurations.

SSVL 1974

2000 17 kg Na2SO4/ t pulp Losses are covered by alkaline effluents from bleach plant and sulphuric acid from chlorine dioxide generation.

KAM 2001

The pulp mill of today is a largely closed system with small emissions to water and air compared to the conditions of the earliest mills. Various parts of the evolution of the internal connections between different parts are shown in Figure 1.

The key piece of equipment in the recovery area is the kraft recovery boiler, also named the kraft recovery furnace, dependent upon whether the subject is the production of steam and power or the recovery of pulping chemicals. (Here both names are used without distinction).

One interesting aspect is the evolution of steam data for the recovery boilers, as shown in Table 2. The steam temperature 510 oC is close to the limit for use with alloyed carbon steel (at higher temperatures, the strength of the steel rapidly decreases). Today it is highly favorable with high steam temperatures and pressures, as this increases the production of electrical power in a mill with a steam turbine.

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Figure 1. The internal connections of the different departments in a typical unbleached kraft pulp mill are shown. The old mill is in principle open with high losses of sodium and sulphur. The new mill is partially closed, the main emissions of sodium and sulphur to air and with effluents are indicated. The figures are redrawn from (Wiklander 1974).

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Table 2. The evolution of steam data and size for recovery boilers. Typical data for the time periods collected from (Deeley & Kirkby 1967; McCarthy 1968; Vakkilainen 2009; Götaverken 1989; Tampella 1991; Ahlstrom 1989). Year Steam

pressure [bar]

Steam temp. [oC]

Black liquor Typical d.s. [%]

Typical capacity [ton d.s/day]

1937 10 350 -- 120 1950 40 425 -- 200 1960 62 450 64 600 1970 64 460 64 1000 1980 64 460 64 1500 1990 100 480 84 3000 2010 100 510 85 5000

The increasing steam pressure and more aggressive corrosion have from time to time caused explosions in recovery furnaces. In connection with support firing with oil, explosions of volatized fuel oil were identified. More severe are accidents where water entered the hot bed causing violent explosions, damaging the furnace and sometimes causing severe injuries. Figure 2 shows some statistics from North America. The trend in Scandinavia has been the same but the number and effects of the explosions have not been as severe.

Figure 2. Statistics for recovery boiler explosions in North America (BLRBAC, 2014).

0

1

2

3

4

5

6

7

8

1948

1951

1954

1957

1960

1963

1966

1969

1972

1975

1978

1981

1984

1987

1990

1993

1996

1999

2002

2005

2008

2011

2014

Number of explosions

Year

Kraft Recovery Boiler Explosions Not spec.

Aux. fuel

BL pyrolysis

Smelt-water

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The risk for explosions was reduced by a number of measures such as better materials, better supervision and maintenance of equipment. Today’s operators are better educated and trained. Alternatives to the conventional kraft recovery boiler have also been investigated (Whitty & Verrill, 2004). The theoretical basis for a new kraft recovery process has been studied by several research groups (Magnusson & Warnqvist, 1982; Grace, 1976; Voci & Iannazzi, 1965). The main problem is with the recovery of the sulphur as sodium sulphide at the same time as the remaining sodium is recovered as sodium carbonate thence the organics are burnt to obtain heat for the chemical reactions and for production of steam.

At the end of 1960’s, the knowledge about what was happening inside the kraft recovery boiler was limited (Lange et al, 1973), some was proprietary knowledge owned by the boiler manufacturers. The research in the area accelerated mainly due to the safety problems and as a result all aspects were covered: chemistry and physics of the process, design and operation. As mentioned above, alternatives to the kraft recovery boiler were presented, but the main efforts were in fact concentrated on improvement of the existing boiler concept. Efficient monitoring of the thickness of the tube material in the pipes exposed for corrosion minimized the risk for smelt-water explosions (Bauer & Sharp, 1991). The most important steps forward were the development of the bed camera and the use of computers for control and handling of large amounts of data. It was now possible to see what was happening in the furnace cavity and to connect this to the control measures instead of relying solely on guess work. See Figure 3.

Figur 3. The inside of different recovery furnaces. The shape of the bed is clearly shown as well as the air ports. The left picture shows the primary air ports at the right bottom and the secondary air register in the middle. The right picture includes spot measurement of the actual temperatures of the bed surface and the furnace wall. (With the permission of Diamond Superior, Sweden).

The kraft recovery boiler is a big and expensive piece of equipment with a long life span. With efficient maintenance and updating the boiler can be maintained in a serviceable condition for a long time. The average Scandinavian recovery boiler is around 30 years old, the technical life-time is said to be 30 – 40 years, however there are boilers over 50 years old still in operation (Ahlroth, 2011).

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1.2 Non-process elements in the kraft pulping process.

Non-process elements (NPE) are externally introduced via the wood used, the make-up chemicals and the process water; they do not take active part in the pulp mill processes. Examples include potassium entrained within the wood, aluminum and silica within the make-up lime, chloride within water and occasionally with the bleaching chemicals. In a pulp mill with relatively open systems, the NPE are not problematic as there is no accumulation in the mill system. When the kraft pulp mill changed into that of more closed system, compare Figure 1, the problems began to show up.

In Scandinavia great efforts have been concentrated on the issue to minimize the pollution of the environment by closing up the pulp mill chemical system. As a result the need for make-up chemicals in the kraft pulping industry have decreased and changed over time from sodium sulphate to sodium hydroxide or sodium carbonate. The main philosophy is to recover the effluents and emissions using internal measures in order to minimize the need for external treatment of air and water emissions. One result of these measures is that the temperature in the internal streams has increased and so has the concentration of dissolved material and different non-process elements. The resulting emissions from the Swedish pulp and paper industry have significantly reduced, an illustrative example is shown in Figure 4 for the years 1978 - 2013. It is in principle related to kraft pulping, as the sulphite pulping production is small compared to kraft pulp production. The emission of sulphur is mainly in the form of sulphur dioxide and obnoxious gases. Note the increase of the pulp production during the same period. The emission of COD, BOD and AOX shows the same pattern, in spite of the increased pulp production, the emissions have decreased to very low levels, both total and per ton of produced pulp.

Figure 4. Emission of sulphur compounds to air and production of pulp. The emission of sulphuric gases has been minimized by collection, treatment and recirculation of both concentrated and diluted gas streams (Skogsindustrierna 2014).

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With a few exceptions, the lime kiln and re-burning of lime mud did not come into more common use in Sweden until around 1950. Earlier the lime cycle of the pulp mill was open, lime stone or burnt lime was purchased and lime mud was used as a filling material for land and for soil improvement. With an open lime cycle, there was of course no build-up of NPE in the lime cycle: the green liquor sludge was disposed together with the lime mud. With a lime kiln and a more closed lime cycle it became necessary to have an efficient green liquor clarification plant. From time to time mills have tried to minimize the input of make-up lime to the lime cycle, which means to close it up as much as possible. The result was that the NPE not separated in the green liquor clarification is entrained into the lime cycle. Figure 5 shows the result of one such experiment over five years, during this time the make-up and expel of lime of the lime cycle was gradually decreased (Bristow & Kunz, 1961). The result was that some of the NPE formed low-melting compounds which sintered on the surface of the burnt lime particles and the amount of available burnt lime decreased. The savings in make-up lime did not cover the cost of extra fuel for the lime kiln and the circulation of the dead-load.

Figure 5. The inert matter in burnt lime increases when the lime cycle is gradually closed. Redrawn after (Bristow & Kunz, 1961).

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1.3 The manufacture of textile fibers from cellulose

With increasing population and increasing standards of living the global demand for textiles is estimated to increase by around 2.5 times from the year 2000 to 2050, as shown in Figure 6 (Johnson, 2001). Even higher numbers, above 200 million tons/year are predicted (Holmström 2011).

Figure 6. The world market for textile fibers, million tons/year (Johnson, 2001).

Textile fibers are used for apparel (clothes, fashion), for home textiles as curtains and furniture. Technical textiles are the fastest growing sector which includes many different things such as geotextiles, textiles in cars and medical textiles. Today the major part of textile fibers are cotton and synthetic fibers based on oil, while man-made fibers based on cellulose are of the magnitude 6 % of the total, see Figure 7.

Figure 7. The world fiber market 2014 (Carmichael, 2015).

Statistics in this area are often difficult to use, sometimes production capacity is shown, sometimes actual production or consumption. To increase the confusion, choice of data may be different from year to year, or the data is valid for only apparel or with/without fibers for non-woven.

syntetic fibers,petroleum based

man-made cellulosebased

cotton

other natural fibers

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The production of cotton requires large amounts of water for irrigation and high loads of pesticides, thus the carbon and water footprint is high compared to wood-based fibers (Hämmerle 2011). Synthetic fibers based on petroleum are produced from non-renewable raw material and thus it is not sustainable. They are neither biodegradable in most cases. There are several ways to compare the total impact of the production of different textile fibers; environmental, use of water or energy etc. It is also important to consider and define the boundary limits of the system. A combination of a modern wood based pulp mill and a modern textile fiber plant with highly closed chemical systems is the most favorable combination compared to production of cotton or petroleum based fibers (Shen et al, 2010).

Textile fibers based on cellulose are the only raw material which can fill the fiber gap that can be expected between the increasing demand and the possible expansion of the cotton production when compared to the more expensive petroleum based synthetic fiber production. Cellulose has the advantage that it does not affect the production of food. “The cellulose gap” is the established name for the future difference between the demand for textile fibers and the production of fibers, see Figure 8 (Hämmerle, 2011). As usual, when it comes to a breaking point, the forecasts cannot be used to predict what will happen. However, in an open market a too low supply vs demand will increase the price and thus reduce the demand until an equilibrium point is reached.

Figure 8. The cellulose gap, as the cotton production is predicted to decrease. Based on the pattern of required fiber properties in different applications, it can be projected that cellulosic fibers, natural and man-made, in the future will still make 33 – 37 % of the fiber demand (Hämmerle, 2011).

Today more than 60 % the market for dissolving pulp is covered by 6 producers and the main growing market is China and the Far East (PRNewswire 2013). In China a number of pulp mills have started production of dissolving pulp, which has resulted in overcapacity and fluctuating prices. The typical pulp mill has been converted to dissolving pulp to improve the economy, the raw material is local or long distance transported eucalyptus chips. The typical pulp mill is usually small and the equipment old.

As a result of the rapid economic evolution in “the Tiger Economies” many small and medium sized viscose textile fiber plants have been established, particularly in China. The

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market has not been developing at the same speed, thus the utilization of the capacity has sometimes been low. In fact, the price for ordinary viscose qualities is set by the cotton price; only special qualities without competition from cotton can set their own prices based on raw material and production costs. One of the real big challenges for the viscose industry is to develop products that don’t compete on price but rather on quality.

1.3.1. Processes for the production of dissolving pulp

A dissolving pulp must have high purity, excellent reactivity and process ability, and give a good process economy (Söderlund, 2012).The question of which process or raw material that gives the best dissolving pulp for viscose production has to be answered with an “it depends” answer. Local traditions and variations in equipment and processes, together with adaption to the known problems, will favor one pulp or the other. As shown by Eriksson (Eriksson 2014) there are differences between sulfite and PHK dissolving pulps, but it is not clear from laboratory tests which is the best in any particular case.

The kraft process is today the dominating pulping process for most pulp qualities. The demand for a dissolving pulp with low content of impurities has been met by the development of the prehydrolysis concept. As a first step in the production of kraft dissolving pulp, the wood chips are treated in a pre-hydrolysis step to remove hemicellulose followed by a kraft cook and delignification/bleaching to remove the lignin and remaining hemicellulose. The resultant product is a cellulose pulp with a high content of cellulose (94 – 97 %) and low content of impurities. The prehydrolysis of the wood chips prior to kraft pulping is an efficient process to remove the hemicellulose (Rydholm, 1965 a).

There are two different ways to carry out the pre-hydrolysis; with addition of diluted sulphuric acid, or with hot water/steam, in both cases at a temperature of 150-170oC. Common for both softwood and hardwood is that the hemicellulose in the PHL (prehydrolysis liquor) is more or less broken down and dissolved as sugar, monomers or oligomers. Acetyl groups are easily cleaved off, forming acetic acid and consequently lowering the pH to around 3.5. The pre- treatment PHL can then be drained and/or displaced by kraft black liquor and kraft cooking liquor to continue the dissolving pulp process.

The PHL contains a mixture of sugars, the composition depends upon the raw material used (Garotte et al, 1999; Amidon & Liu, 2009). Extraction of spruce wood (milled to wood meal) and chips showed release of galactoglukomannan as well as xylans, arabinogalactans, lignin and acetic acid (Song et al. 2008). The main focus in this area has concentrated on the use of hemicellulose sugars, as monomers and oligomers, for the fermentation to alcohol. (Hemicelluloses from hardwoods give mainly pentosanes, which are not yet suitable for fermentation to alcohols (Chandel et al 2011)). It is important to separate acetic acid and

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lignin residues from the sugar-rich PHL, as these components inhibit the fermentation process (Ahsan et al. 2014). Processes which have been tested include: adsorption of lignin and furfural on activated carbon, nanofiltration and reversed osmosis in cascade. For a softwood pulp mill, the acetic acid is 10 % of the organic material in the PHL (Song et al. 2008).

1.3.2. The traditional viscose process and new alternatives.

Figure 9. Principle flow sheet for the viscose process

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In the traditional viscose process, the cellulose (dissolving pulp) is treated with alkali, converted to cellulose xanthate with carbon disulfide in excess and coagulated to fiber in an acidic bath. In the coagulation bath large amounts of sodium sulfate are produced. Today an extensive process is necessary to recover the carbon disulfide and prevent emission of the obnoxious gases. In concentrated form the carbon disulphide is explosive, flammable and poisonous (AGA 2012). Today there are no problems to design and build a viscose fiber plant which meets very high environmental standards with very low emission of carbon disulfide. However, the necessary chemical recovery plant will substantially add to both the investment and operational cost.

The main chemicals involved in the viscose process are sodium hydroxide (NaOH), carbon disulphide (CS2), sulphuric acid and sodium sulphate. Other elements may also be of interest as they are added in the cellulose dissolution or coagulation process, and then must be taken care of in a closed mill system or an effluent treatment. Table 3 shows the main compounds and elements.

Table 3 The main chemicals involved in the viscose process. (Wilkes, 2001; Kvarnlöf et al, 2006)

Element Source Output as Treatment The viscose process NaOH for steeping and dissolution of the cellulose

Added at the steeping process and at viscose dissolution

Formation of Na2SO4 in spinning bath

Cooling and precipitation

Partly recovered as Na2SO4

H2SO4; Na2SO4 Viscose spinning bath

Overflow from spinning bath

Evaporation, cooling and crystallization of Na2SO4

Recovery of acid and Na2SO4

CS2 Dissolution of cellulose

Added at viscose dissolution

Formation of H2S and CS2 as gas during spinning

Cooling, condensation and absorption from gaseous effluent

Recovery of CS2

Zn as ZnSO4

Enhancement of dissolution and coagulation

Added at the dissolution and coagulation process

Effluent from coagulation bath

Precipitation with burnt lime

Precipitation as zinc hydroxide.

CoSO4 Enhancement of cellulose dissolution

Added at oxygen treatment of pulp before dissolution

Effluent from coagulation bath

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As a part of the efforts to harmonize the conditions for the industry within the EU, the European commission Joint Research Centre has published BREF documents covering many industries, among them pulp and paper (BREF 2014) and polymers including viscose process (BREF 2007). In these documents the best available technique (BAT) is described and the emissions to the environment using BAT are stated. According to IED (Industrial Emissions Directive) (EU, 2010) this data are normative for the EU; however individual countries can decide to implement stricter rules.

Production of textile fibers with the viscose process is not considered a major industry in Europe, and there is only one plant in full operation, Lenzing in Austria (Lenzing 2014). The information about the viscose process in the BREF document is mainly based on information from this plant (BREF 2007). There is also at least one European producer of cord and industrial products.

The search for solvents of cellulose has been going on since the middle of the 19th century (Woodings 2001). Some solvents have been utilized to dissolve cellulose in technically viable processes, and the products are often used for special purposes. Cellulose nitrate was earlier used for photographic film, but because it is highly flammable (explosive), it has been replaced by cellulose acetate, which is too used for packaging, textile fiber, filters and consumer goods. Cuproammonium rayon is used for membranes and filters. During recent years a more systematic search has been summarized (Heinze & Koschelia 2005; Wang 2008; Libert 2010). The theoretical background to the solubility of cellulose has also been investigated and revised (Le Moigne 2008; Medronho et al. 2013).

One interesting area is the development of cellulose carbamate (CCA), based on an invention of Hill & Jacobsen (1938), investigated and developed further by Kemira Oy Säteri (Huttunen et al. 1983) and at the University of Wuhan, Peoples Republic of China (Ruan et al. 2004). The cellulose carbamate can be produced either by heating the cellulose impregnated with chemicals to approx. 130°C; or cooling to a few degrees below zero. The product is stable and can be stored. The next step involves dissolution in sodium hydroxide to achieve a dope for conventional spinning. The recovery of chemicals and treatment of effluents has not yet been fully developed.

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Figure 10. CCA. Simplified flow-sheet for production of Cellulose Carbamate fibers.

Table 4 Chemicals used in the Cellulose Carbamate process. (Huttunen et al. 1983; Ruan et al. 2004).

Element Source Output as Treatment CCA - Cellulose Carbamate NH3 (liquid) Added for the

production of CCA

NH3 is evaporated from the CCA

Cooling and condensation

Recovery of liquid NH3

N species, such as CO(NH2)2 urea or CS(NH2)2 thiourea

Added together with NaOH for dissolution of cellulose

Overflow from spinning bath

External effluent treatment

Combustion

Zn as ZnSO4

Enhancement of dissolution and coagulation

Added at the dissolution and coagulation process

Effluent from coagulation bath

Precipitation with burnt lime

Precipitation as zinc hydroxide.

Spinning bath like viscose

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A more basic approach is based upon the phase diagram water – cellulose – sodium hydroxide (Sobue et al. 1939); with a small solubility window at around 6 -10 w% NaOH and temperature just above the melting point (-10 to +4 °C). The solubility has been shown to be improved by the addition of zinc ions and is the basis for the Swedish development project CelluNova (Kihlman, 2012).

Figure 11. CelluNova. Simplified flow-sheet for production of CelluNova fibers.

Table 5. Chemicals used in the CelluNova process (Kihlman, 2012).

Element Source Output as Treatment CelluNova NaOH for steeping and dissolution of the cellulose

Added at the steeping process and at viscose dissolution

Formation of Na2SO4 in spinning bath

Cooling and precipitation

Partly recovered as Na2SO4

Zn as ZnSO4

Enhancement of dissolution and coagulation

Added at the dissolution and coagulation process

Effluent from coagulation bath

Precipitation with burnt lime

Precipitation as zinc hydroxide.

Spinning bath like viscose

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Today, research in this area is concentrated on several aspects, such as the use of ionic liquids at room temperature, cellulose carbamate (CCA) and sodium hydroxide. The process Lyocell (Trade name Tencell) based on the solvent NMMO (N-Methylmorpholone N-oxide) has been developed and commercialized by Lenzing (White 2001).

At the same time as new processes for the production of textile fibers are developed, the development has not been terminated in other areas. The use of textiles is gradually enlarged into new areas, particularly in the field of technical textiles, with new demands placed upon quality, strength and composition. Some requirements can today only be met by non-cellulose fibers, but the nano-technology may include new interesting features.

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1.4 The bio-refinery concept

The bio-refinery concept is today a hot topic which may include several different aspects. One common aspect is the utilization of by-products from the pulp mill. Early wood based kraft pulp mills did engage in the recovery of sodium chemicals, although in the beginning these operated with a rather low degree of efficiency. With the introduction of the kraft recovery boiler (Öman 1944) both the chemical recovery and the recovery of heat improved, and the evolution has since continued achieving greater degrees of recovery. Today, a kraft pulp mill is in fact often also a producer of heat for district heating, and power for the grid (KAM 2001; Berglin et al. 2011). With the introduction of the LignoBoost process (Tomani 2013) it is also possible to produce a material, which can be stored for future production of energy or used as a raw material for production of further materials such as carbon fibers. Typical for the conditions in Scandinavia and other northern countries are the differences in energy balance between summer and winter; with a pronounced surplus of energy in the summer period. It would be a great advantage if this could be transferred to a storable, transportable fuel to be used in the cold winter period. The first attempt to create a bio-refinery was through fermentation of the neutralized spent sulfite liquor and production of ethanol, and this was for a long time been the standard for utilization of the spent liquor from calcium based sulfite pulp mills. As the kraft pulping has been the dominating process, numerous attempts have been made to utilize chemicals from the black liquor as raw material for profitable production of different chemicals or products. The tall oil from the kraft pulping is used as a raw material for the chemical industry. One problem is that the total production of spent liquor from one kraft pulp mill is so large, that only high-volume products can be of interest for a large-scale process replacing the whole conventional chemical recovery. However, there are possibilities to utilize a part of the black liquor stream for specialized purposes without disturbing the main chemical recovery loop.

Comprehensive research in the lignin area has shown not only different ways to produce lignin from spent pulping liquors but also indicated a great number of areas where lignin can be used as a raw material, or can be included in further processing (Ragauskas 2014). Promising uses include liquid fuel for vehicles, various types of glues and additives for plastics, carbon fibers, fertilizer for crops, and so on.

Both the sulphite and the kraft pulping process involve sulphuric compounds. An early process concept was “soda cooking” or cooking the cellulose material with sodium hydroxide, and this is still used as a pulping process for annual plants such as wheat and rice straw (Öman 1944). One early bio-refinery concept was proposed by Rinman (Rinman 1927), based on sulphur-free cooking and utilization of by-products from the lignin and

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hemicellulose together with chemical recovery of the cooking base. This concept has also been included in some of the modern bio-refinery concepts (St. Pierre, 2014). It is also important to remember that the concept of bio-refinery is not restricted to wood; other cellulose materials are also of interest (Keijsers et al. 2013).

The different aspects of closing up the systems of a bio-refinery are in principle the same as for a pulp and paper mill, however, as there are more products and production processes involved, the complexity will be greater. It can be expected that there must be buffers between the different production plants and that the demands for monitoring and taking care of effluents will be greater too. Sustainable production and the zero waste vision are key words in the development of the cellulose-based society (Beyond 2015).

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1.5 Aims of the work

This work summarizes some examples of how the tools of chemical engineering can be used in the development of processes and process equipment.

In the recovery of kraft pulping chemicals in a recovery boiler the chemicals are produced as a mixture of molten sodium carbonate – sulphide. What is the viscosity of the smelt from a kraft recovery boiler? This is of interest for the operation of the kraft recovery boiler but also of special importance for some of the alternatives to the conventional kraft recovery boiler. Paper I reports the laboratory measurements used to obtain the data.

The pulp and paper processes are continuously developed to higher degrees of closure and the use of chemicals and energy becomes more efficient. As part of this development the input (and subsequent output) of water has decreased. Important questions to be answered are: What is the input of elements that can give operational problems in the pulp mill? What are these elements? Are they accumulating in the system causing deposits and other problems? Are there ways to expel these elements? Paper II investigates these questions.

One element of interest is magnesium, which is added as magnesium(II)-salts in the oxygen delignification of pulp. Will this addition give rise to processing problems in the pulp mill? This is addressed in Paper III.

The benefits of co-location of a pulp mill and a paper mill are well documented. Are there also benefits associated with the co-location of a dissolving pulp mill with a textile fiber plant? Some aspects of this question are looked at in Paper IV.

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2. Materials and Methods.

Only limited remarks are given about special features here, more details are given in Paper I – IV.

2.1. Measurements of density and viscosity of recovery boiler melts

Investigations involving molten salts at high temperatures, in the order of 800 – 1200 °C are not only formidable but require the use of specialist equipment. In this case, with a molten mixture of sodium carbonate and sulphide the operating problems are especially difficult to overcome as the molten salt mixture is very corrosive to most metals, and the majority of ceramics are either porous, or brittle when exposed to temperature changes. Quartz glass is rapidly destroyed. Tests showed that one special quality of alumina (Al2O3) sintered to a dense and compact surface could be used as a crucible for the molten salt mixture. The methods for measuring density and viscosity were chosen according to published investigations referenced in Paper I. Sodium sulphide was prepared according to procedure described by (Rosén & Tegman, 1971); the colour was bright white indicating a high purity. All handling of sodium sulphide was carried out in a glove-box within an oxygen-free atmosphere. The atmosphere in the furnace was based on high purity nitrogen, with additions to prevent the decay of the molten salt mixture.

The density was measured using the “maximum bubble pressure method” as described by (White 1959). This was then used together with the data from the “oscillating crucible” to calculate the viscosity of the smelt (Janz & Saegusa 1963).

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2.2. Calculation of material and energy balances.

For presentations of the computer programs used by the industry, see summaries (Kappen, 2009; Dahlquist 2008). Several equipment suppliers have developed program packages of their own, specially adapted to study their own equipment and with the aim to support their business operation. The programs GEMS and EXCEL were used for the calculations presented in the Papers II – IV. (GEMS is an early version of WINGEMS). The programs WINGEMS and MASSBAL are largely based on process knowledge, Aspen+ has subprograms to include statistical data to define a “black box” content and an EXCEL program can, with help of other programs, be adopted to handle input from data collection programs. These programs were historically used for steady-state calculations, but possibilities to make dynamic calculations have been added.

WINGEMS is a modular computer program based on the GEMS software initially developed by Lou Edwards at the University of Idaho, Moscow, Idaho, USA (Lindberg & Warnqvist, 1978). The program is designed for steady state calculations of the mass and heat balances of pulp and paper mills. From the actual flow-sheet (P&ID) the program modules are connected with each other with streams and the data for the input data for the modules and streams is presented. The program is relatively user-friendly as there is a direct connection between the physical reality and the modules, and the results are shown in a way that is directly related to the flow-sheet. The experienced user can also obtain information about dynamic processes from the computer results. Recently, dynamic functions have been added to the main program.

Excel comes with the Microsoft Office program package and is the most widely-used spreadsheet program; easy to interface with and with certain features allowing for more complex calculations. It is easy to operate for straight-forward calculations and can also cope with iteration. In many cases this is a rapid and effective program to make limited calculations (Arne 2013). The difficulty with this approach is that it is easy to design a program which is difficult for more than the inventor to use.

Dynamic calculations using the program family PaperMac and PulpMac, based on the platform EXTEND, has been used by the industry mainly for special calculations (ExtendSim, 2015). The same platform is used by the program family developed by the company Frontway (Frontway 2015) for different purposes, not limited to the pulp and paper sector. The programs are built with modules and special features that are incorporated to show the dynamic behavior in a simple way.

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2.3. Measurements of non-process elements.

A kraft pulp mill was extensively studied during 24 hours of an ordinary production period, sampling of the interesting streams were made four times during this period. The samples were stored cold and dark, and before analysis the samples from each point were mixed to one general sample. Furthermore, one complete set of samples from another kraft pulp mill was provided as a support to this project. More details are given in Paper II. As a special case, the addition of magnesium to oxygen delignification was studied, see Paper III.

Some of the methods for analysis of different elements had to be developed in the project and this was done at the department of Chemical Analysis at STFI (now Innventia). This has been reported in Paper II, Paper III and (Bethge et al. 1976).

2.4. Calculations on the integration of a pulp mill and a textile fiber plant.

The investigation is based on literature data, calculations using EXCEL and by hand as reported in Paper IV. The main sources for data have been (BREF 2007; Söderlund, 2012; and personal experiences from one of the authors). Paper IV also reports the experiments with coagulation of cellulose dope in baths of sulphuric acid and acetic acid. Precipitation and separation of zinc as zinc sulphide from the used spinning bath using technical green liquor is also documented.

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3. Results and discussion.

3.1 The pulp mill of today and the recovery of pulping chemicals.

The pulp mill of today is a complicated process industry. The demands for operational safety are higher than ever, and downtime in excess of the planned maintenance stops is in principle not permitted. A new kraft recovery furnace is factored to have an operational time of more than 95 % calculated over the whole year (Brewster, 2007). Maintenance stops were at one time planned every year, now the intervals are often longer.

To replace the main key component such as the kraft recovery boiler, with a completely new piece of equipment with unknown properties is extremely insecure. The reliability of a new process or new process equipment has to be proven in practical operation; this can be realized as an additional new part parallel to a present unit. This can for instance, be a way to solve the problem of an overloaded kraft recovery boiler. One example is the installation of the atmospheric Chemrec unit (for black liquor gasification) at Weyerhaeuser New Bern, N.C. USA (Brown et al, 2007).

Research efforts both on the kraft recovery boiler and alternative recovery processes have together increased the understanding and knowledge of the chemistry and physics of the involved processes. One example of this is the knowledge concerning smelts of sodium carbonate – sodium sulphide from the high temperature recovery processes, like the combustion in the recovery boiler. This was studied (Paper I) as a part of the NSP –project (Björklund et al. 1985).

Figure 12. The phase diagram Na2CO3 – Na2S. ○ HTM data Δ DTA data (Tegman & Warnqvist, 1972).

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There has been previous ambiguity about the smelt temperature of sodium sulphide, but it has been shown that for pure sodium sulphide it is 1175 ± 10 °C and the phase diagram for sodium carbonate – sodium sulphide is a two-component diagram with the eutectic temperature 762 °C (Tegman & Warnqvist, 1972; Warnqvist, 1980; Råberg et al 2003). Other lower smelting-point data can be explained by the presence of impurities as sodium sulphate and polysulphides.

Of special interest is the viscosity of the molten salt mixture from the kraft recovery process. It has been observed that in high sulphidity mills (sodium based sulphite pulp mills operating a combustion furnace) there are sometimes problems to get the smelt out of the furnace, but this depends on the increased solidification temperature at high sulfidities, over the eutectic composition. Molten salts usually have low viscosity, of the magnitude a few cP (Janz 1980) compared to molten silicate slags with much higher viscosities (Seetharaman et al. 2004). As the molten mixture sodium carbonate – sodium sulphide is extremely corrosive, a method to measure the viscosity without incorporating metallic details had to be chosen, and the swinging crucible method was chosen as described by (Janz & Saegusa 1963). To calculate the viscosity from the results, it was necessary to know the density, and this was measured first. The details of the experiments are described and the results are reported in Paper I. The density of the molten mixture of sodium carbonate and sodium sulphide can be summarized with the formula :

ρ · 10-3 = (2.30 ± 0.04) – (0. 26 ± 0.02) · xNa2S – (0.000375 ± 0.000005) · t

where ρ = density [kg/m3] xNa2S = mol fraction Na2S (xNa2S < 0.6) t = temperature [°C]

The density of pure Na2CO3 is in accordance with the determinations by Janz & Lorenz (Janz & Lorenz 1961) and Spedding (Spedding 1970). The magnitude of the molten salt mixture sodium sulphide – sodium carbonate is 1980 kg/m3 at the melting point and decreases with higher temperature and content of sodium sulphide. For the determination of the viscosity, reference data for sodium carbonate was used for calibration of the system. The viscosity was then determined with the equation:

(δ’ – Δ) · ρt/ρtm = K·√ η · τ · ρt

Where δ’ = logarithmic decrement (corrected) ρt = density at temperature t

ρtm = density at the melting point η = dynamic viscosity τ = oscillation period Δ, K = apparatus constants

The dynamic viscosity was measured for a mixture of 30 mol% Na2S in Na2CO3 at three different temperatures. The results are shown in table 6.

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Table 6 Dynamic viscosity of molten sodium carbonate – sulphide.

Temperature [°C]

Dynamic viscosity [cP = mPa · s] Na2CO3

(Janz & Saegusa 1963) 30 mol% Na2S in Na2CO3

878 3.71 3.1 920 2.47 2.45 956 1.81 2.2

To compare with something common, the viscosity of water is 1.06 cP, olive oil 90 cP, both at 18 °C (Ingelstam & Sjöberg, 1960).

An attempt to produce data for the viscosity in the solid-liquid area of the phase diagram has been made by Tran et al.(2004). In this investigation a few samples of technical smelts were measured but no analysis of the composition was reported. Most measurements were done on salt smelt combinations without sodium sulphide, as these mixtures are easier to handle, not so sensitive to oxidation and in most cases the temperatures of interest are lower. The significance of carbonate smelts are connected to the development of high temperature fuel cells (Lee et al 2013) and the desulfurization of coal and flue gases in molten carbonate. Experience from the latter has also been incorporated in the black liquor gasification process (the Champion-Rockwell molten salt gasification process) developed by Atomics International - Rockwell International (Kohl et al. 1978) and reported by (Whitty & Verrill, 2004).

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3.2 Non-process elements in the pulping process

The studies reported in Paper II were aimed to establish knowledge about in- and output and identify the kidneys for some of the main non-process elements. It was also of interest to explain the presence of the deposits found in the mill equipment. Table 7 shows typical data of the content of NPE in wood, unbleached and bleached pulp. Different wood species and their origin may cause a much greater variation. It is important to remember that the equilibrium content to some extent is dependent on the degree of closure of the actual mill.

Table 7. Some of the main NPE in pine, spruce and birch wood, unbleached and bleached pulp (Paper III; Magnusson et al 1979).

Element Wood Unbleached pulp

Bleached pulp

Sodium Na mg/kg 6 - 10 9500 134 Potassium K 190 - 600 432 8 Calcium Ca 390 - 900 1305 162 Magnesium Mg 70 - 300 474 21 Iron Fe 51 - 135 6 6 Manganese Mn 51 - 180 16 0.4 Silicon Si 10 - 40 310 20 Aluminum Al 10 96 8 Chlorine Cl 50 - 100 Phosphorus P 20 - 200

The main input of many NPE is down to the wood. The stem wood of pulpwood has relatively low contents of NPE, the largest amounts are found in the growing parts, bark and smaller branches. Data indicate that the local conditions at the growing site, such as soil and minerals, will influence the content of NPE. In order to study the conditions at a mill it is thus of more interest to use a correct average value obtained from a large chip sample, rather than a value from one tree in the forest. Process water is also of interest, but most Scandinavian mills use surface water from rivers or lakes, where the content of NPE is relatively low. Mills located in areas with bedrock of limestone will likely have problems from entrained calcium and magnesium, causing deposits in digesters and liquor evaporators as well as water system of the paper machine. For mills located in dry areas with limited access to water, the problems can be severe and require extra water treatment of incoming water. Water from deep wells may contain high amounts of chlorides which can be compared to brackish water from for instance the Baltic Sea. Experience shows that in a pulp mill brackish water cannot be used to replace freshwater as this will result in process difficulties (Magnusson 1975).

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Sodium sulphate was once upon a time the make-up chemical for the kraft pulp mill, now it is usually a mixture involving sodium hydroxide, sodium carbonate, sulphuric acid and chemicals from the generation of chlorine dioxide. The content of NPE is usually low in these inputs.

For an unbleached pulp mill, the pulp is one of the main output streams, and for certain paper qualities as fluting it may be economical to put as much sludge and NPE as possible in the paper web, instead of trying to take care of it in an effluent treatment plant. The bleaching of pulp is in principle an acid wash and hence removes the main part of NPE from the pulp. This is illustrated in Table 7.

Figure 13. Green liquor sludge as kidney for Mg, Ca, Mn, Fe, Cu, Pb, Cd etc. (Paper II).

The main output streams for NPE are green liquor sludge and grits from the slaker. It is shown in Paper II that the green liquor clarification is an efficient kidney for some heavy metals, as the precipitation of metal sulfides with the help of green liquor is very efficient. Table 8 shows the composition of a typical green liquor.

Table 8. Composition of clarified “normal” green liquor from Scandinavian kraft pulp mills (Theliander 2006). The density of the green liquor will change with the composition; the magnitude 1140 – 1200 kg/m3 is typical for the given composition.

Salt/ion Na+ K+ Na2CO3 Na2S NaOH Na2SO4 Na2S2O3 NaCl Conc. [g/kg]

70 - 95 5 - 15 100 - 140 30 - 60

2 - 25 1 - 15 1 - 10 1 - 10

The content of sludge in unclarified green liquor is usually of the magnitude 1000 – 4000 mg/l. There are several different pieces of equipment used to clarify the green liquor and it is possible to get down to a sludge content of close to 0 g/l (Lindström et al 2001), however with the most common filters or clarifiers, the sludge content is typically 20 – 50 mg/l. (Gustafsson & Maripuu 1983).

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Table 9. Chemical composition of green liquor sludge from 3 different kraft pulp mills, the samples are green liquor sludge from clarification of green liquor. (Gustafsson & Maripuu 1983).

Mill I Mill II Mill III Ash, [% ds] 97.0 95.9 97.9 Dry solids [%] 26.8 40.8 20.7 Na [g/l] 48.1 92.2 101 Mg [g/l] 4.2 9.2 0.18 Al [g/l] 1.2 1.3 0.17 Si [g/l] 0.92 1.4 0.10 P [g/l] 3.5 0.21 0.32 Ca [g/l] 33 9.7 0.15 Mn [g/l] 0.12 1.8 0.20 Fe [g/l] 1.6 1.4 0.17 S [g/l] 12 13 16

Figure 14. The build-up of potassium and chloride (Paper II).

The build-up of elements such as potassium and chlorine are illustrated in fig. 14. There is no specific kidney for these elements like the green liquor clarification. Potassium is chemically very similar to sodium and enters the chemical loop as one of the main NPE ingredients of wood. Due to the higher volatility of potassium salts in the hot part of the kraft recovery furnace, the potassium will be enriched as potassium sulphate in the flue gas dust. The main part of the dust is circulated to the black liquor and a closed loop is formed. However, mixtures of potassium and sodium will cause problems on the heat transfer surfaces of the kraft recovery furnace, as the melting temperature of the dust in the flue gas will decrease and the dust will become sticky at lower temperatures than for pure sodium.

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The input of chlorine as ions and its effects in the mill and the kraft recovery boiler dust as a kidney has been extensively studied in connection with the Rapson “effluent free mill”. The main source is the internal circulation of bleaching effluents and in coastal areas the input of chlorides from sea- rafted wood (SSVL Kloridprojektet 1977).The chloride level in the liquor loop will build-up and cause severe corrosion on the equipment, unless high-alloyed and more expensive materials are used (Liljas 1995). The content of NPE in the green liquor is in principle the trace nutrition elements that the trees require to grow in a healthy way. It is quite obvious that these ought to be transferred back to the forest soil to prevent shortage. The main problem is that the nutrition elements must be released during a very long time to support the growth of the trees. (The life-time of trees in Sweden is 50 – 100 years). Pelletizing together with wood ash and fiber sludge has been tested. The use of GLS (Green Liquor Sludge) as reinforcement in forest roads or in dense layers covering landfills has also been tested. There has also been some interest from the steel industry to utilize the calcium carbonate content of the GLS as replacement for lime in the steel-making process, as the GLS has very low content of sodium. The washing of GLS is difficult to carry out, because the sludge is a mixture of small dense NPE particles and larger light unburnt carbon particles from the recovery furnace. Often a technique using lime mud as filtration media is used, giving a natural output of lime mud. One element that has attained special interest is magnesium, as this is added to the oxygen delignification to improve the pulp viscosity (SSVL Slutrapport, 1974). One of the advantages with oxygen delignification is that the spent liquor from the oxygen stage can be returned to the recovery cycle utilizing the heat of combustion from the dissolved lignin in the recovery boiler thus reclaiming the sodium ions and the residual hydroxide. In the beginning however, there were concerns that the added magnesium could cause problems with deposits, in the first case in the evaporators, and secondly within ancillary tanks and pipes. Other possible problem areas were the recovery furnace and the caustization or the lime kiln. The results obtained in Paper III and shown in Figure 15 demonstrate that the magnesium is forming two separate loops, one along the fiber line and the other formed by the lime loop. As the solubility of magnesium compounds in both green and white liquor is low, no closed loop will be formed in the liquor cycle. The magnesium added to the oxygen delignification will together with the magnesium of the wood end up in the bleach plant effluent and also in the green liquor sludge. Internal circulation in the oxygen delignification will form a minor loop as shown in the diagram. Depending on input, output and specific configuration of the equipment, the actual distribution will vary from mill to mill.

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Figure 15. Outline of the magnesium flow in a kraft mill with oxygen delignification. (Paper III).

Research within this area has to a great extent been concentrated on solving important practical problems. The flue gas dust from the recovery boiler is returned to the fired thick black liquor, and can cause an accumulation of potassium in the dust. This mixture of sodium and potassium sulphate has a lower melting-point than the pure sodium sulphate and can thus form sticky deposits on the heating surfaces in the upper part of the recovery boiler (Backman et al 1955). In this way the heat transfer in the boiler can be reduced and the level of corrosion may increase. An increased content of chlorides in the liquor cycle of the pulp mill will cause severe corrosion, both on carbon steel and 18/8-type stainless steels. A controlled expel of dust or the treatment of dust can remove both potassium and chlorides and act as the necessary kidney (Minday et al 1997). Aluminum and silica can cause the formation of deposits in the black liquor evaporators, deposits that are difficult to wash away (Ulmgren 1981).

The silica problems in connection with the pulping of annual plants like wheat, rice straw, bamboo and bagasse is a problem of its own (Panda 1989). These mills are often small, with a limited economy and cannot justify large investments. Chemically, the silicon is easy to separate with treatment of green liquor with carbon dioxide to form silica, but the particles are so small that they are almost impossible to separate. With knowledge about nucleation and growth it is possible to solve the problem and achieve larger separable particles (Magnusson, 1992).

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With both increasing closure and concentration of NPE in the closed loops, the associated complications have in turn increased. One problem experienced by mills addressed in Paper II was the formation of deposits of sodium-aluminum-silicates in the evaporators, as these are especially difficult to handle. The thermal conductivity of the deposits is low, they are hard and glossy and difficult to remove with routine washing procedures. Cracking of the hot glassy deposits with cold water or high pressure water jet cleaning are two possible solutions.

It has been observed in a soda-pulping mill that aluminum could be precipitated in the causticization as the double-salt hydrotalcite. A burnt lime with make-up containing high content of magnesium was used as make-up for the causticization (the lime loop). This observation was transferred to the kraft pulping process (Ulmgren 1987), aluminum was precipitated by the addition of magnesium sulphate in the dissolving tank and removed as hydrotalcite in the green liquor clarification. The formula of hydrotalcite is Mg6Al2(CO3)(OH)16 · 4 H2O, however as in many complicated minerals the composition is variable within certain limits.

It has also been observed that high contents of magnesium in the green liquor will decrease the filterability of the green liquor sludge. This is not surprising as the magnesium ions in alkaline solutions will form large hydrated ions (Hägg 1964). The addition of alumina ions will co-precipitate with magnesium and form hydrotalcite which is removed in the green liquor clarification (Wimby 1955).

It ought to be mentioned that the results obtained in Paper II and III were confirmed by the study presented by Kietaniemi and Virkola (Kietaniemi & Virkola 1978).

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3.3 Mill integration

3.3.1. Co-location of dissolving and viscose mills One promising solution for the co-localization would be to add a dissolving pulp line to a paper pulp mill, here the size effects could be fully taken advantage of in the paper pulp line and at the same time the more limited capacity of the dissolving pulp line is fully utilized for the textile fiber plant. This is the basis for Paper IV. The chemical recovery area takes care of the spent cooking liquors and some effluents from the textile plant. Several maintenance departments and also operational departments could be in common for the different production lines. At least one of the main pulp producers have combined production sites, and are successfully operating pulp mills together with viscose textile fiber plants (Aditya Birla, 2015).

The size of a typical textile fiber line is in the magnitude 50 – 60 000 tons/year (Reisinger1998). For higher production, several lines are built parallel. The typical pulp mill of today is usually built according to the philosophy “the bigger the better”. The largest one-line pulp mills today have a production capacity in the magnitude 1 500 000 ton pulp/year (Andritz 2013). To convert a pulp mill producing paper pulp to a dissolving pulp mill is mainly of interest to small and medium sized mills as a big mill would have difficulty to sell all the dissolving pulp. Other concerns include (Hamaguchi et al. 2013; Mateos-Espejel et al. 2013; Lundberg et al. 2012; Wiley 2011; Flickinger et al. 2011) :

• The yield for kraft paper pulp is typically around 50 %, and the hemicellulose content is high. Dissolving pulp yield is around 35 % and the content of hemicellulose is as low as possible. At constant pulp production, the wood supply and the cooking capacity has to be increased.

• Taking care of the PHL will require equipment and energy. The PHL can be evaporated and burnt in the recovery boiler, yet this requires capacity in these units. The alternative is processing to recover valuable components, and this will require a filtration – concentration plant of its own.

• The cooking process for dissolving pulp requires more chemicals, thus the recovery boiler and the causticizing department are likely to be the bottleneck. This may limit the production of the dissolving pulp.

• If the production is based on swing production of both paper and dissolving pulp, special efforts have to be made to avoid interfusion of the pulp qualities, as the dissolving pulp is not suitable for paper production.

• Dissolving pulp quality is sensitive to impurities of all kinds: sand from the woodyard, bark and other particles that not will be dissolved in the viscose process or cause clogging in the spinning of the threads, and this must be removed. Good house-keeping and efficient screening to remove particles is important to attain a high quality pulp. The final washing of the dissolving pulp requires fresh water with low contamination of metals.

• The liquor and water volumes per ton of pulp will in general be increased; this will influence the design of a new mill or the re-design of an old, for the co-location.

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It seems that for a large modern pulp mill, it would be natural to add a small dissolving pulp mill with the same size as the adjoining viscose fiber plant to utilize the total capacity as well as possible. Obviously the chemical recovery departments must have higher capacity than for a paper pulp mill at a constant wood consumption.

3.3.2. Advantages of never-dried pulp It is well known from the production of paper, that there is a major energy advantage to pump the paper pulp direct from the pulp mill to the paper production plant without drying. As pointed out in Paper IV the saving is around 15% calculated on the demand of energy for the dissolving pulp. Another factor which is more difficult to express in economic terms is the reduced risk for hornification. This can occur during drying, and it can give the pulp a lower reactivity.

3.3.3. Sodium hydroxide from the kraft pulp mill used in in a viscose plant. The white liquor from the caustization plant of the pulp mill contains sodium hydroxide and sodium sulphide, see Table 10.

Table 10. Typical composition of a white liquor. Data calculated from (Brännvall 2006) and (Rydholm 1965 b)

Main components

Composition [g component/l] (Brännvall 2006) (Rydholm 1965 b)

NaOH 100 98 Na2S 39 41 Na2SO4 6 5 Na2CO3 33 32

As shown in the table, the composition of the white liquor may vary to some extent depending on the raw material, the type of pulp produced, the degree of closure and the circulation of dead load in the liquor system. Within the limits of the capacity of the kraft recovery boiler and the causticizing department sodium hydroxide as white liquor can be delivered to the viscose plant. However, there are process problems to overcome. The white liquor contains sodium sulphide. This can be oxidized with air to sodium thiosulphate or with oxygen to sodium sulphate. The dead load of these compounds may influence the operation of the pulp pretreatment (mercerization and steeping) as well as the dissolution. If necessary the dead load can be separated from the sodium hydroxide solution with an ion-exchanger process (Jemaa et al 2001). The pretreatment, the steeping of the pulp requires a concentration of the sodium hydroxide solution up to 17 – 19%. (Wilkes, 2001). Thus, according to data in Table 10 the oxidized white liquor has to be evaporated to obtain a sufficiently high alkali concentration.

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3.3.4. Acid for the cellulose coagulation The viscose process first used neutral or alkaline spinning baths, however in 1904 a new type of bath based on sulphuric acid and sodium sulphate was patented; this is known as the Müller spinning bath (Woodings 2001). Together with stretching of the fibers, this made the production of high-strength fibers possible. The sodium hydroxide from the dope is neutralized by the sulphuric acid and substantial amounts of sodium sulphate are formed. For the production of 1 kg of viscose fibers 0.6 kg sodium hydroxide is needed for pretreatment of cellulose and the processing of the dope. For the spinning bath 0.8 kg sulphuric acid is consumed and approximately 1.2 kg sodium sulphate is formed (Söderlund, 2012).

As substantial amounts of acetic acid are formed at the prehydrolysis stage it is a priori interest to use this in the viscose production as coagulation agent. There are a number of patents covering the use of acetic acid (Ernst 1905;Richter 1952; Vermaas 1957). However, the experiments reported in Paper IV show that this is not a realistic way to operate, as the amount of acetic acid is not sufficient for the process. Sulphuric acid is stronger than acetic acid and much smaller amounts are needed for the coagulation. The cost for purchased sulphuric acid would be much lower than for the corresponding amount of acetic acid to the same degree of coagulation. The fact that acetic acid is not widely used in the viscose industry indicates that sulphuric acid is the better alternative for the spinning bath. However, the main content of PHL is sugar as monomers and oligomers, lignin residues and acetic acid. With modern separation methods involving an ion exchanger, ultrafiltration and reversed osmosis (Ahsan et al. 2014), it is possible to separate the different components. The sugars can be utilized for production of ethanol and the purified acetic acid can be sold. With a specially designed prehydrolysis process, this can give a substantial contribution to the economy (Mao et al 2010).

3.3.5. Precipitation of ZnS with green liquor The spent spinning bath from the viscose process contains substantial amounts of zinc. A typical spinning bath composition is shown in Table 11.

Table 11. Spinning bath for some typical viscose products. (Söderlund 2012).

Stable Fiber Modal Fiber Textile filament H2SO4 [g/l] 120 75 140 Na2SO4 [g/l] 320 150 150 ZnSO4 [g/l] 16 50 10

The presence of zinc ions is important for the initial formation of the skin formed on the filament by the initial formation of a zinc-cellulose xanthate. This is subsequently followed by disintegration to cellulose and the penetration of the acid into the filament and the solidification of the filament fibers.

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The main input of water and sodium is with the viscose dope which contains more than 80 % water. The overflow/output of the spinning bath contains both valuable chemicals such as zinc compounds, dissolved sulfuric gases and sulfuric acid but also low value sodium sulfate. Part of the output is often evaporated and handled to recover carbon disulphide and sodium sulphate. There are however effluent streams from the washing and treatment of the viscose filament which are so diluted, that recovery is not economically viable. To take care of the zinc content of these streams before external treatment, burnt lime is employed, which is added to get a precipitation of zinc hydroxide (American Enka, 1971). This process induces a number of problems as the precipitation leaves some zinc in solution and the precipitated zinc is sometimes left as a bottom layer in a water treatment pond.

Figure 16. The solubility of zinc hydroxide and zinc sulfide (EPA, 1980). The break in the solubility of zinc hydroxide depends on the formation of complex ions.

It is well known from the metal finishing industry that the solubility of zinc sulfide is much lower than zinc hydroxide (EPA, 1980). This fact is used for effluent treatment of spent pickling baths.

The green liquor of the kraft pulp mill is the natural source for the sulfide ions to be used for the precipitation of the zinc sulfide. This has been tested on effluent from a zinc plant but the conditions were not optimized (Wiklund 2007). Experiments reported in Paper IV indicate that the precipitation of zinc sulphide can be a very efficient way to deal with the zinc content. It is then possible to design a closed loop for the zinc compounds in the textile fiber plant with the help of chemical engineering.

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4. Conclusions.

It is generally considered that under normal operating conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank problem free. However, for some alternatives to the kraft recovery boiler, knowledge of more precise data of the molten salts is necessary for the design calculations. In this study the viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at temperatures relevant for a recovery boiler.

The presence of non-process elements (NPE) in a typical kraft pulp mill has been investigated. The major inputs of NPE are via the wood, make-up chemicals and in some cases with the process water. The closing up of a bleach-plant and recirculation of bleach-plant effluents may also give substantial addition of NPE. The problems that can be expected if the content of NPE are too high include deposits in evaporators, high dead-load in liquor streams, plugging of the upper part of the recovery boiler and decreasing efficiency in the causticization department and lime kiln. It is important to control the input of NPE by using good barking of the wood and sound removal of sand and soil. Efficient green liquor clarification is of the greatest importance as this is an efficient kidney for many NPE. It is also beneficial to keep the lime cycle sufficiently open to balance the input of NPE with the output of lime mud. Mill data and calculations show that the magnesium added in the oxygen delignification does not form a closed loop with accumulation of the magnesium compounds.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be highly beneficial. The dissolving pulp delivered from the pulp mill to the textile fiber plant is delivered as wet pulp (never dried) and as such has a higher reactivity. A considerable amount of energy is also saved as the pulp need not be dried. Alkali and sulphuric acid can be delivered from the chemical recovery area of the pulp mill and effluents can be managed to recover sodium and sulphur. Effluent from the textile plant containing zinc ions can be treated efficiently with green liquor from the pulp mill to precipitate and recover the zinc for reuse in the textile process.

The integrated pulp production (pre-hydrolysis) also produces sugars as by-products that can be used for fermentation and other products, as well as acetic acid. For economic reasons it may be more feasible to sell the acetic acid instead of using it in the regeneration process, as sulphuric acid is a better coagulant.

Modern chemical technology is an extremely efficient tool for solving problems particularly in the complicated environment of the pulp and paper industry. The combination of experimental studies, mill data and mass balance calculations are of fundamental importance for the development and improvement of the industry. It is often beneficial to use knowledge from one branch of the technology to solve problems in other branches.

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5. Industrial significance.

The development of the industry proceeds in both small everyday steps and giant leaps, and knowledge from different parts of the science is one of the things that will help on the road ahead.

The knowledge about the viscosity of the molten salts from the recovery boiler is usually not necessary for the everyday operation of the kraft recovery boiler, but may be of great importance for the design of a new piece of process equipment, not yet invented.

NPE are on the other hand will become increasingly problematic as the mill systems become more closed due to environmental restrictions (the polluter pays) and economic conditions. The knowledge about occurrence and function of the kidneys for different NPE in real mill situations is essential for solving problems in mill operation and maintenance. The operation of an efficient green liquor clarification is a good example. Related to this is the integration of other industries with the pulp mill, as the supply of raw materials, chemicals and effluents can be integrated between the plants to minimize the environmental impact and the costs for both investment and operation. In this work it has been demonstrated that the integration of a dissolving pulp mill and a viscose textile fiber plant can be advantageous and save resources. Knowledge in chemical technology is of fundamental importance for the engineers involved in the planning and operation.

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6. Future perspectives.

More studies of the integration of production and diversification of products are necessary for the industry to fulfill the visions about the road map for “the cellulose-based society” (in contrast to the crude-oil based society) and “the zero waste city” valorizing everything. As a small part of this, the integration of dissolving pulping and production of viscose textile fibers is supporting a sustainable production. Many other products are to be included in this bio-economy, one important tool will be the use of LCA, Life Cycle Analysis, using better criteria for the summary of different aspects. Another important tool is the calculation of material and energy balances, both static and dynamic for complicated configurations

In the pulp mill, the use of mill supervision systems and new types of sensors will supply better data for controlling and operation of the processes. The green liquor clarification and the kraft recovery furnace are key components that will continue to be improved, and possibly replaced by new kinds of equipment, such as new types of filters or a black liquor gasifier unit. Can the future of soda-pulping be altered thanks to the by-products?

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7. Acknowledgements.

During the projects presented here and my work as a consultant for the industry, I had the opportunity to meet and work together with many skilled and helpful people. Without the interest, good advice and support there would not have been any published papers.

To mention everybody is impossible, but all of you are remembered. However there are a number of colleagues I have to give a very special thanks to : Ants Teder, Björn Warnqvist and all of my colleagues at STFI (now Innventia)/KTH, especially the research group ÅSA/ÅKE. This was an excellent school for a young engineer.

Life as a consultant has its ups and downs. Thanks to many good friends and colleagues there have mainly been ups, I mention only a few: Lars Miliander, Eva Karlsson–Berg, Anders Gustafsson, Allan Nordström and Lennart Asplund, but all are remembered. Who knows in what project we will meet next time?

Ulf Germgård picked me up from the consultant work and asked me to write something. With his great patience and the support from Gunnar Henriksson and Thomas Nilsson this licentiate thesis slowly grew to a publication. I am also very thankful to all my colleagues here at Karlstad University, to be one of the gang is a great special favor. Finally, I would like to express my gratitude to my beloved wife Kerstin and to our family, for all your love and support throughout the years. I would also like to remember my father, who once introduced me to this exciting area of technology.

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From recovery boiler to integration of a textile fiber plantCombination of mass balance analysis and chemical engineering

Hans Magnusson

Hans M

agnusson | From recovery boiler to integration of a textile fiber plant | 2015:39

From recovery boiler to integration of a textile fiber plantModern chemical technology is an extremely efficient tool for solving problems particularly in a complicated environment such as the pulp and paper industry. Here, examples are studied during which chemical technology is of fundamental importance.

At normal conditions the molten salt mixture from the kraft recovery boiler flows down into the dissolving tank without hindrance. However, for certain kraft recovery boiler alternatives, knowledge of more precise data of the molten salts is required. The viscosity for the case of sodium carbonate and 30 mole% sulphide has been measured and is of the magnitude 2 – 3 cP at relevant temperatures.

The main input of non-process elements (NPE) is down to the wood, and known problems include deposits in evaporators and decreasing efficiency in the causticization department. Green liquor clarification is an efficient kidney for many NPE. Magnesium added in the oxygen delignification does not form a closed loop.

Integration of a prehydrolysis kraft pulp mill producing dissolving pulp with a plant producing viscose textile fiber could be of significant interest, as the handling of both alkali and sulphuric compounds can be integrated. Problems will however arise as the capacity of the pulping line and the chemical recovery has to be adjusted.

LICENTIATE THESIS | Karlstad University Studies | 2015:39 LICENTIATE THESIS | Karlstad University Studies | 2015:39

ISSN 1403-8099

Faculty of Health, Science and TechnolgyISBN 978-91-7063-657-8

Chemical engineering