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PROCESSING OF ALUMINA-RICH INDIAN IRON ORE SLIMES

Pradip Tata Research Development and Design Centre, Pune, India

Email: pradip.p@tcs.com Introduction Indian iron ore industry is one of the world’s largest and growing at a rapid pace. Out of a total production of 142.7 million tones of iron ore produced in the year 2004-2005, a record 78.1 million tonnes (consisting of 13.5 tonnes of lumps and 64.6 tonnes of sinter fines) were exported. As per the national steel policy announced recently, India will be producing 110 tonnes million tonnes (more than double of the current level of around 42 million tonnes only) of steel per anum by 2020 requiring around 190 million tonnes of iron ore. In addition the iron ore exports are being projected to reach a level of 114 million tonnes and thus the total production of iron ore is expected to cross 300 million tonnes by 2020. It is an achievable target provided we employ the best in science and technology to harness this valuable but non-renewable resource for the benefit of our country (1). Powered by the steel boom in the country, multi-billion dollars (US$ 70 billion) worth of new investments by global steel producers like Mittals, Pohang Steel (POSCO), Tata Steel, Jindal and Essar groups, in green field projects in the states of Chattisgarh, Jharkhand and Orissa have been announced. The public sector giant, Steel Authority of India (SAIL) plans to invest close to USD 10 billion in next few years to achieve a production capacity of 22 million tons per year from the current level of 13 million tons only. There will thus be a proportionate quantum jump in the mining and processing of iron ore, chromite and coal to meet the demands of the steel industry in the country. POSCO alone for example will be developing a 30 million TPA iron ore mine. BHP and Orissa Mining Corporation have announced a joint venture project to beneficiate iron ores in Orissa. Optimum utilization of iron ore resources of the country has thus become a national priority. It has also opened up several national policy questions regarding mine ownership, export, waste utilization and disposal strategies. India is currently producing all the possible marketable products of iron ore, namely iron ore lumps, ore concentrates, pellets, iron oxide powder and iron ore sinter. One of the most immediate technological challenges facing the industry is to deal with the problem of processing alumina rich iron ore fines and slimes. For the sustainable growth of iron ore industry which is beset with serious problems of shortage of land and water, it is absolutely imperative that the state-of-the-art mineral processing technology is utilized to take the industry closer to a position of zero waste production. India is endowed with rich resources of iron ores in the form of hematite and magnetite ore deposits. Hematite reserves estimated to be 12.9 billion tonnes, located primarily in the states of Jharkhand, Chattisgarh, Orissa and

Reprinted from International Journal of Minerals, Metals and Materials Engineering, 59(5), 2006, pp 551-568

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Madhya Pradesh are high grade (> 62% Fe) but also high in alumina content. Total magnetite reserves, estimated at around 10.6 billion tonnes with an average grade of 35-40% Fe are found in the states of Karnataka, Goa, Andhra Pradesh and Kerala. According to Indian Bureau of Mines, Indian iron ore reserves are over 24 billion tonnes (1). Indian hematite ores are typically rich in iron but contain unusually high alumina (as high as seven percent) and in some cases, problem of high phosphorous content is also noted. The current practice of iron ore washing in India results in three products, namely coarse ore lumps, directly charged to blast furnace, the classifier fines, (3-5% alumina) which with or without beneficiation are fed to sintering plants and the slimes (6-10% alumina) which are currently discarded as waste. It is a well-recognized fact that in order to enhance the competitive edge of Indian iron and steel industry, an efficient alumina removal technology for Indian iron ores is absolutely essential. It is worth noting the following facts in this context: • The alumina content in iron ore fines used in sinter making all over the

world is less than 1%. In contrast, iron ore fines in India assay as high as 3.0-5.5%. The sinter quality produced from such alumina-rich ore fines, is thus much poorer. The adverse effect of alumina on sinter strength productivity and its reduction – degradation characteristics (RDI) are well documented and conclusively established (2-4).

• Whether in the form of alumina-rich lumps or sinter, the blast furnace

productivity is significantly affected due to the presence of alumina in the feed. High alumina slag which is highly viscous, requires larger quantity of flux (10% MgO) and relatively larger slag volumes resulting in an increase in coke consumption and a decrease in blast furnace productivity. According to one estimate, a decrease in alumina content in the sinter from 3.1 to 2.5% will improve the RDI by at least six points, lower blast furnace coke rate by 14 kg per tonne of hot metal and increase its productivity by about 30% under Indian operating conditions (4-7).

• The generation of iron ore slimes in India is estimated to be 10-25% by

weight of the total iron ore mined – the iron ore values are lost to the tune of 15-20 million tonnes every year. In addition, these slimes stored in massive water ponds pose enormous environmental hazard. SAIL alone has more than 50 million tonnes of slimes accumulated over the years. Considering the fact that iron ore production will more than double and rise to at least 300 million tonnes soon, finding suitable means of safe disposal/utilization of slimes is indeed urgent.

Motivation for the Beneficiation of Indian Iron Ores Iron ores are being beneficiated all around the world including at Kudremukh in India. Several techniques such as spirals, floatex density separators, jigs,

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multi-gravity separator, low and high intensity magnetic separator, conventional as well as column flotation, selective dispersion – flocculation are all part of current industrial practice (8 -19). Recent advances include Batac jigs (20), packed flotation column (21) packed column jigs (22) and centrifugal concentrators like Falcon Concentrator (23) Kelsey jigs (24) Knelson Concentrator (24) for the beneficiation of iron ore slimes. Processing of hematitic ores in India at present, however, does not involve any beneficiation except for whatever rejection of silica (and to some extent alumina) occurs during washing and classification of crushed ores. Several commendable efforts have been made by Tata Steel to come up with an economically viable beneficiation flow sheet for processing classifier fines (25-28). More recently at least two beneficiation plants have been set up to process sinter fines for value addition. Tata Steel has commissioned a Batac jig of 300 tonnes per hour capacity at Noamundi to beneficiate sinter fines to achieve alumina content less than 2% (28). Essar (29) has pioneered the pellet route of utilizing fines. A state-of-the-art iron ore fines processing plant including a 267 kilometers long slurry pipeline has been commissioned recently. Essar is operating an 8 million tonnes per anum pellet plant in Visakhapatnam based on the fines and slimes being pumped from NMDC’s Balladilla iron ore mine. The beneficiation flowsheet includes gravity separation (spirals) of the coarser fraction after desliming and high intensity magnetic separation of the finer fraction. The concentrates thus produced assay less than 2.5% by weight of combined silica and alumina content and are further ground to produce a feed acceptable for the pelletization circuit. The Essar group has also announced to set up another 8 million tonnes per anum pellet plant as a part of its integrated steel unit in Orissa. A comprehensive technology development programme needed to delineate the appropriate beneficiation/utilization strategy for Indian iron ore deposits must include (i) the nature of occurrence, association and liberation characteristics of the alumina containing minerals; (ii) a comparison of the separation efficiency of various unit operations for both hematite-goethite/kaolinite/gibbsite separation in terms of recovery-grade plots (separation characteristics) and as a function of particle size (iii) a preliminary techno-economic assessment of the various technology options for a typical iron ore mine in the country. A schematic diagram showing the basic technological elements of an integrated strategy to utilize alumina rich iron ore deposits in the country is presented in Fig. 1. The readers are referred to our earlier publications for more details (8-10). The discussion in this paper is confined to the possible beneficiation strategies for alumina- rich iron ore slimes and that also, primarily to separation by flotation and selective flocculation techniques followed by semi-dry disposal of residue produced during the beneficiation. It must however be stressed that with ever increasing premium on bringing down the alumina content of Indian iron ores to less than 2%, preferably less than 1%, the generation of slimes as a proportion of the total mined output is likely to increase further. The development of an appropriate strategy to deal with alumina-rich iron ore slimes is therefore of great relevance and it is an urgent necessity.

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Lumps

FinesSlimes

Blast furnace charge

Sinter plantIron ore (ROM)

Tailing pond

-40+10 mm1.9 % Al2O3

-10+0.15 mm2.7 % Al2O3

-0.15 mm~10 % Al2O3

High alumina fines & slimes

Lumps

FinesSlimes

Blast furnace charge

Sinter plantIron ore (ROM)

Tailing pond

-40+10 mm1.9 % Al2O3

-10+0.15 mm2.7 % Al2O3

-0.15 mm~10 % Al2O3

High alumina fines & slimes

Appropriate beneficiation strategy

Enriched low alumina product

Residual Waste

Size enlargement

Waste management

strategy

• Gravity separation• Magnetic separation• Froth flotation• Selective dispersion / flocculation• Bio - beneficiation

Blast furnace charge material

Value-added productsEco-friendly storage

• Semi-dry disposal • Iron rich cements • Thermal conversion

to iron • Glass ceramics

• Sintering• Pelletization• Briquetting• Cold bonded -pelletization

Appropriate beneficiation strategy

Enriched low alumina product

Residual Waste

Size enlargement

Waste management

strategy

• Gravity separation• Magnetic separation• Froth flotation• Selective dispersion / flocculation• Bio - beneficiation

Blast furnace charge material

Value-added productsEco-friendly storage

• Semi-dry disposal • Iron rich cements • Thermal conversion

to iron • Glass ceramics

• Sintering• Pelletization• Briquetting• Cold bonded -pelletization

Fig. 1: An integrated approach to high alumina iron ore beneficiation Beneficiation Strategies for Indian Iron Ore Slimes Considering the present magnitude of the iron ore slimes generation annually, the quantities of slimes accumulated over the years, the fact that these slimes are available in already ground form and assaying reasonably high %Fe, it is obvious that if properly beneficiated, these slimes can be considered a national resource rather than a waste of nuisance value. The alumina content of the slimes, if brought to less than 2% Al2O3 in the beneficiated product will (a) lead to better utilization of national resources (b)

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achieve higher mine output (enhanced production) with not much additional costs (c) reduce environmental hazards associated with storage and disposal of slimes and (d) result in higher blast furnace and sinter plant productivity A number of research groups in the country have explored the possibility of reducing alumina in iron ore slimes (8-10, 18, 19, 25-43) A critical review of the earlier R&D investigations, as presented and discussed in greater detail in our earlier publications (8 -10), clearly indicates the need to carry out a comprehensive study targeted to establish an integrated strategy of utilization of Indian iron ore slimes. It must address the following important issues: • A quantitative and definitive assessment of the extent of alumina reduction

possible with state-of-the-art beneficiation technology backed up by reliable scientific data

• A conclusive evaluation of the state-of-the-art agglomeration (sintering/

pelletization/ briquetting) technology which can successfully convert the beneficiated slimes into a product acceptable to blast furnace without adversely affecting its productivity

• A techno-economic assessment of the various options available to utilize

the residual “waste” product (it could be as high as 50%) containing high amount of alumina and iron oxide – for example, in the production of iron-rich cements, recycling of iron-rich wastes by thermal treatment including conversion into metallic iron and in the production of glass ceramic materials from iron-rich waste

• A comprehensive eco-friendly and safe technology (for example, semi-dry

disposal technology) to replace the conventional practice of storing iron ore slimes in tailings ponds

One of the more important findings of earlier investigations is that alumina in Indian iron ore slimes occurs in the form of two distinct mineral constituents namely, gibbsite (hydrated aluminium oxides) and kaolinite (and other clay minerals in minor quantities). Even though not adequately quantified, the liberation studies also indicate that a substantial portion of alumina is present in the liberated form and hence amenable to separation by physical means. The work conducted by Tata Steel on Noamundi iron ore slimes is by far the most comprehensive study available at present. Based on their data, Pradip (9) compared the efficiencies of different unit operations including wet high intensity magnetic separation (WHIMS) and multi-gravity separator (MGS). As illustrated in Fig.2, it was possible to produce concentrates, at least on a laboratory scale; assaying less than 2% alumina at an overall yield of around 50% from slimes feed analyzing 7-8 % alumina. Another noteworthy observation is that the separation achieved in MGS is remarkably close to the theoretical yield predicted based on the sink-float tests done on the same sample of slimes. The availability of high capacity MGS units is currently a serious limitation.

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Based on the published literature on the beneficiation of Indian iron ore slimes, it is not possible to conclude the reasons why the yield is limited to 50% only. Is it because of interlocking and/or because of lack of a suitable separation device? Any future work on this topic must address this question. More importantly, the efficiency of various unit operations must be compared on the basis of such plots rather than a single point result (recovery at a specified grade) which is often misleading. Another benefit of such plots is that one can also assess the pros and cons of specifying a particular grade of the concentrate. In case there is a sharp drop observed in recovery with grade, one has to be careful in targeting a particular concentrate grade since it may or may not be possible to achieve it through a robust industrial separation circuit. Sensitivity of these results with respect to different feed grades, for feeds from different mine areas and feeds of different size distributions must be studied since all physical separation processes are known to be sensitive to particle size, mineralogy, liberation and feed grade. Fig. 2: Beneficiation of Noamundi iron ore slimes – the separation efficiency

(yield as a function of alumina content of the concentrate) of various unit operations compared (After Ref. 9)

Separation processes based on the surface-chemical differences between iron and alumina containing minerals, for example, froth flotation and selective dispersion – flocculation are also promising but have not been investigated adequately for processing Indian iron ores. These two separation techniques are therefore discussed in greater detail in this communication. The availability of selective reagents capable of achieving the desired separation efficiencies is a serious limitation. It has been addressed by us in our work on iron ore slimes (8-10, 37-41).

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Froth Flotation of Iron Ore Slimes Flotation process for concentrating iron ores received a big impetus in USA immediately after the Second World War due to the dwindling resources of direct shipping iron ores in the Lake Superior District. Flotation of iron ores essentially for silica removal has been reviewed extensively in literature (8-10, 13, 16, 44-53). The iron ore industry in Minnesota and Michigan in US uses cationic flotation of silica from magnetic taconites at a rate of 40 million tons per annually (44). Column flotation technology for rejecting fine silica using a variety of cationic amines has also been commercialized in iron ore industry including at Kudremukh in India. Several iron ore producers in Brazil employ reverse flotation separation of silica from iron ore minerals for producing pellet quality concentrates. It has been reported that the presence of gibbsite and/or kaolinite as the major alumina containing minerals in iron ores does dictate the choice of beneficiation flow sheet (47) While kaolinite does not interfere with flotation selectivity, gibbsite tends to contaminate the flotation concentrate as it is depressed together with iron oxides and hydroxides during the reverse cationic flotation process (47). These observations are remarkably close to what Pradip and co-workers (8-10, 41) have reported based on their investigations on the beneficiation of Indian iron ore slimes by flotation and selective flocculation. Gibbsite tends to go with iron oxide because of its surface chemistry and chelating chemistry being very similar to iron oxide minerals. In fact the separation of iron oxide from gibbsite continues to remain a challenge before mineral engineers. In order to process finely disseminated large deposits of oxidized taconites containing predominantly hematite and goethite, US Bureau of Mines in the late sixties developed a process involving selective flocculation and desliming followed by cationic flotation of coarse silica. It was commercialized for the first time in 1974 at the Cleveland Cliffs Iron Co’s Tilden Concentrator in USA (17). The 4.1 million tons per year capacity plant was later expanded to produce 8.2 million tons per year of pellets assaying 64%Fe. The plant flow sheet involves dispersion of minerals using sodium hydroxide in combination with sodium silicate/ lignosulfonates / hexametaphosphate or tripolyphosphates during grinding followed by selective flocculation of iron minerals using starches (for example, tapioca flour starch). The settled (flocculated) concentrate is then subjected to reverse flotation with cationic amine reagents in order to remove coarse silicates. Starch thus works both as a selective flocculant and as a depressant for iron minerals. Samarco, a joint venture between BHP Billiton and CVRD operates a flotation concentrator producing an iron ore concentrate which is pumped via a 396 kilometer pipeline to a pellet plant in Brazil. In order to reduce the silica content from present level of 3.1 % to less than 1% (super concentrate), regrinding to 100 % passing 105 microns was found to be most appropriate for enhanced liberation before flotation (53).

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In summary, a critical review of the published literature on the flotation of iron ore minerals suggests that it is likely to be a cost-effective commercial proposition for the beneficiation of Indian iron ore slimes provided one is able to design a selective flotation collector for iron oxide – gibbsite separation. Flotation of Iron ores for Phosphate Removal So far as the separation of apatite from iron ore minerals is concerned, flotation has been observed to be the most appropriate process. Depending on the mineralogy of occurrence of phosphate in iron ores, appropriate selective reagents have to be selected and/or designed to achieve acceptable grades of flotation concentrates. The magnetite ore available at LKAB’s Kiruna deposit in Sweden contains about 1 wt % of phosphorous. The phosphate minerals are floated with the help of a proprietary reagent developed by AKZO Nobel for this separation (50-52). The flotation collector, ATRAC-1562, found to be most effective in this flotation separation is a modified fatty acid reagent. It is possible to achieve a flotation concentrate grade assaying 0.025 % P and 71% Fe (acceptable for pellet plant) from a feed assaying 61% Fe and 1% P at a throughput rate of 3.8 million tons per year. A process control system is installed to be able to maintain the concentrate grade. Besides reagent dosage, and pH, the pulp temperature (varies between 10 to 400C) is also observed to affect the flotation kinetics as well as the selectivity of separation (50). Cleveland Cliffs is the largest producer of iron ore pellets in North America with a combined production capacity of 38 million tons and operating six mines located in Michigan, Minnesota and Eastern Canada (54). While the majority of pellets assay in the range of 0.01 to 0.02% P, the pellets produced from martite ores (Tilden Mine) tend to have higher phosphorus and hence its reduction is an ongoing challenge. The Tilden plant flow sheet consists of very fine grinding to 80% passing 25 microns followed by selective flocculation – desliming process step whereby a significant portion of siliceous gangue, goethite slimes and phosphorous containing minerals are rejected. Reverse amine flotation is utilized to further bring down the silica content of the concentrate to meet the pellet quality grade. pH is maintained near 11. Previous research indicated two modes of phosphorus occurrence, namely in the form of discrete apatite grains as well as in the form of solid solution in goethite grains (54). Since apatite grains exhibits strong cathodoluminescence, the cathodoluminescence microscopy (CLM) technique has been successfully utilized to investigate the nature of phosphorus occurrence in iron ores (54). A CLM detector is added to the SEM for ore characterization.

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Selective Dispersion-Flocculation Studies on Iron Ore Slimes Pradip and co-workers (8-10, 37-41) have systematically investigated the possibility of achieving selective separation amongst hematite- alumina- kaolinite- montmorillonite minerals, the mineral constituent’s representative of Indian iron ore slimes. Two classes of commercially available flocculants namely starch based natural polymers and polyacrylamide (PAM) - polyacrylic acid (PAA) family of synthetic polymers were extensively tested. Statistically designed experiments were conducted in order to compare the efficiencies of the two polymers namely starch and PAA for selective separation of hematite from kaolinite. As illustrated in Fig. 3. Starch at pH 10 is found to be a much more selective reagent for this separation (41). Pradip et al. (8, 38, 41) have also established that as compared to the commonly used dispersant, sodium silicate, low molecular weight synthetic polymers such as polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) are more selective dispersants for hematite-kaolinite separation with PAA as a flocculant. It is interesting to note that this effect of various dispersants on selectivity is not observed in case starch is used as a flocculant. Fig. 3: A comparison of the efficiency of polyacrylic acid (PAA) and starch

flocculants in the separation of hematite from its mixtures with kaolinite in the pH range 6.5-9 for PAA and 9.5-11.5 for starch (After Ref. 41)

Pradip and co-workers have also modified starch as well as polyacrylamides by incorporating more selective functional groups. Modified polyacrylamides containing iron chelating groups such as hydroxamates, (PAMX) were, for example are found to be much more selective than PAA or PAM. The

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representative results on the separation efficiency of PAMX in hematite-kaolinite separation are presented in Table 1 (37). Table 1: Enhancement in selectivity in iron oxide/kaolin separation by the introduction of hydroxamate functional groups in polyacrylamide

Feed grade: 35% Fe (1:1 mixture) Pulp density: 1% Dispersant: 40 mg/l Na-Silicate

Test

Conditions Results

Flocculant pH Dosage

(mg/l) Grade Recovery

Name CONH2 COOH CONHOH (%) (%)

None 10 - 63.6 50.0

PAM 100 - - 9 1 55.5 57.5

PAA - 100 - 9 2 61.6 74.0

PAMX* 68.7 23 8.3 9.2 5 66.5 72.0

* 8.3% - CONHOH (hydroxamate) and 23% COOH (carboxylate) and rest CONH2 (acrylamide) functional groups

PAMX

CH2 CH

C O

NH

O-H+

[ ]x CH2 CH

C O

O-H+

CH2 CH

C O

NH2

[ ]z

[ ]y

The deleterious effect of the presence of montmorillonite in the system was also studied in detail (55). Flocculation experiments conducted on the synthetic mixtures of hematite-kaolinite-montmorillonite indicated that as little as 5% of montmorillonite when introduced in hematite-kaolinite mixtures could lead to a marked deterioration in the separation efficiency due to relatively less flocculation of hematite in presence of montmorillonite. Kaolinite flocculation remained largely unaffected. This deleterious effect of montmorillonite could be partially mitigated by the use of modified flocculants containing more selective functional groups. The selectivity could be partially restored when modified starch-mercaptan was used as a flocculant instead of conventional maize starch. The beneficial effect of a more selective flocculant like starch mercaptan was observed with synthetic dispersants as well (55). An interesting finding of our work is that starch, modified starches and modified polyacrylamides (PAMX) which were found to be excellent flocculants for selective separation of hematite from kaolinite and even montmorillonite, turned out to be disastrous in hematite-gibbsite separation. In fact, we established that starch flocculated both alumina (gibbsite) and

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hematite (iron oxide) equally well. It is well known that iron oxide and alumina have identical crystal structures. The results for hematite (iron oxide) – kaolinite and hematitie – alumina separation using starch are shown in Fig. 4 for illustration (8, 56). Fig.4.: Selective flocculation of hematite (Fe2O3) from its synthetic mixture with

alumina (Al2O3) and kaolinite using a natural maize starch flocculant at pH 10.5 + 0.1 in presence of sodium silicate dispersant (After Ref. 8)

We have also proposed a molecular recognition mechanism underlying starch interaction with iron oxide and alumina which explains this observation (41, 56). The adsorption mechanism for starch end group having O - O distance of 2.85 A° interacting with iron oxide substrate having Fe - Fe distance of 2.85 A° on its cleavage plane is schematically shown in Fig.5. Al-Al distance on the cleavage plane of corundum (Al2O3) is exactly same and hence strong interaction and flocculation of alumina (Al2O3) is observed with starch. Fig.5: A schematic diagram illustrating the mechanism of starch adsorption

through binuclear complexation with Fe sites on hematite surface or Al sites on Al2O3 surface (After Ref. 41)

0 10 20 30 40 50

WE

IGH

T P

ER

CE

NT

SE

TTLE

D

0

20

40

60

80

100Fe2O3 - KAOLINITE

Fe2O3

KAOLINITE

MAIZE STARCH 1% SOLIDS 1:1 MIXTURE pH 10.5

FLOCCULANT DOSAGE, mg/l

0 10 20 30 40 50

Fe2O3 - Al2O3

Fe2O3 Al2O3 SODIUM SILICATE (mg/l)

40 80

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Various research groups including ours have reported on the lack of any worthwhile success in the beneficiation of natural iron ore slimes using commonly used dispersant–flocculant combinations. Based on our systematic work on delineating the reasons for this observation, we attribute it to the remarkable similarities in the crystal structure as well as the surface chemical properties of the two constituent minerals of iron ore slimes namely hematite (iron oxide) and gibbsite (alumina). While kaolinite separation is possible with certain modified flocculants synthesized by us, even in presence of minor quantities of montmorillonite, it is not enough to reduce alumina to the desired levels in the concentrate because gibbsite remains with hematite. The key to solving the problem of iron ore slimes thus lies in developing selective reagents (flocculants, dispersants and flotation collectors) for iron oxide – gibbsite separation. This is by no means a trivial task but any breakthroughs on this front, in the author’s opinion will have far-reaching impact on the techno-economics of processing alumina-rich iron ore slimes. These reagents are not only essential for solving the problem of alumina rich iron ore slimes but those will also have a significant impact on the beneficiation of iron-rich bauxite deposits in India. Processing of Indian red muds, rich in alumina and iron oxide will also become economically more attractive with the availability of reagents capable of iron oxide– gibbsite separation. A sustained and systematic interdisciplinary effort by Indian researchers in this direction is therefore urgently needed. State-of-the-art molecular modeling techniques appear to be promising for the design of highly selective, tailor-made reagents for industrial applications (56-62). Pelletization and Sintering of Beneficiated Iron ore Slimes In India, Kudremukh Iron Ore Co. (KIOCL), Mandovi Pellets, Jindal Vijayanagar Steel Ltd (JVSL) and Essar are producing iron ore pellets of marketable grade. While a discussion on the overall techno-economics of pelletization vis-à-vis sintering of iron ore slimes is outside the scope of this paper, it is worth mentioning that major technological advances have taken place in both the processes leading to improvement in productivity and quality of pellets and sinter. One of the more recent innovations has been the demonstrated utility of High Pressure Grinding Rolls (HPGR) for the production of pellet feed. Modern iron ore pellet plants consist of HPGR for grinding the feed to acceptable levels of fineness with comparatively less consumption of energy (63). One of the very first installations of HPGR in the iron ore industry was at LKAB in Sweden in 1994. Currently there are 29 HPGR units operating worldwide in the iron ore industry including one at Kurdremukh in India (63). Besides low specific energy consumption and high throughput per machine, the pellet specific advantages of HPGR machine include increase in fineness, more appropriate size distribution and shape of HPGR product compared to grinding in conventional ball mills, increase ultrafines in the product, more uniform pellets

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resulting in reduction in circulating loads, better moisture control in filtration, higher density of pellets (average porosity decreased from 33 % to 25 %), lower binder requirement and increased green pellet strengths (upto 50% higher than conventional pellets), reduced fuel consumption, higher loading, better bed permeability and increased productivity during induration (63). Some of these benefits are illustrated with data reported by Ehrentraut and Ramachandra Rao (2001) on the quality of pellets obtained at Kudremukh Iron Ore Pellet plant in India (64).

Table 2: A comparison of pellet properties with and without roller press as obtained at Kudremukh Iron Ore Company (Ref 64)

Parameter Without

Roller Press

With Roller press

Blaine value of feed on pelletizing disc (cm2/g) 1450 1800 Green Pellets (wt%) of 9-16 mm 86 90 Green Pellets (wt%) of < 5 mm 2 1.5 No of drops – Green Pellets Strength 7 10 Tumbler Index after induration, wt% of > 6.3 mm fr. 94.5 95 Abrasion index, wt% of < 0.6 mm 4.5 4.0 Porosity (%) 29 26 Swelling Index (%) 17 15 Reducibility (%) 64 62 Bulk Density (t/m3) 2 2.24

Significant improvements in terms of quality and productivity in iron ore pellet plants have been achieved through the introduction of advanced process control systems including expert systems (65-67). It is well known that sintering plants can be appropriately modified to accept higher levels of fines/slimes, for example by micro-pelletizing the slimes before sintering. To the best of my knowledge I have not come across any investigation on Indian iron ore fines, carried out to establish the proportion of slimes one can accept in sinter plants without adversely affecting their performance. Such investigations must be conducted as a part of the R&D program on beneficiation of iron ore slimes. Next generation Technology for Safe Disposal of Iron Ore Slimes Residue after Beneficiation No discussion on processing of Indian iron ore slimes is complete without a mandatory section on the environmentally acceptable recycling and/or storage of residual waste after beneficiation. No matter which process of beneficiation is employed, it is clear that with existing technologies one is able to recover only 50% of the slimes as a valuable product. The iron rich alumino-silicate residue is thus available for further processing and/or safe disposal. Current practice of storing slimes in huge dams built for this purpose not only occupy precious land but also hazardous to the population. In response to this need to find an alternative to current tailings disposal schemes, an innovative

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thickened tailings disposal technique has been invented and commercialized recently. It is very much relevant to the present discussion on processing of iron ore slimes. The conventional technology of tailings disposal/storage involves building an expensive tailings dam or a tailings pond where million of tons of very dilute and fine particulate suspensions are allowed to settle under gravity for several years. The industry is well aware of the inherent problems/hazards associated with this strategy. The widely prevalent practice of disposal and management of tailings entails dilution with water, pipe line transportation of relatively dilute slurry and impoundment in a pond or tailings dam. This so-called third generation technology has associated with it immense hidden costs and potential for serious environmental damage (68). Apart from locking up vast quantities of water and valuable land for generations on end, it poses a man-made hazard to the soil and water sub-systems of the environment with unpredictable and far-reaching consequences for the ecosystem and the habitat. There is then a need for the development of an environmentally benign and cost effective technology for disposal and management of wet particulate wastes. The imperative for an alternate approach is even more urgent in the Indian context where water is a scarce resource, land is at premium, and environmental degradation has already reached alarming proportions. Thickened tailings disposal (TTD) is a concept introduced by Robinsky (1975) and commercialized only very recently in the mining industry. The success of a TTD system depends on the extent the tailing can be thickened to concentrated but pumpable slurry. Once thickened, the tailings can be pumped and discharged in the form of a self-supporting ridge/cone designed to attain a slope of 2% to 6% only. This new technology obviates the need of building dams/ponds and is therefore going to have a major impact in the industry (68 - 80). Excellent work of David Boger and coworkers (73-80) has provided the necessary scientific foundations to this new concept of tailings disposal. The basic science underlying the dewatering, pumping and stacking of mineral slurries (also called “particulate fluids”) is thus understood to a great extent and hence, this “fourth” generation technology of tailings disposal is ready for wider application in the industry. This concept has been adopted and perfected as a semi-dry disposal technology treating red-mud (a residue of bauxite processing) by Alcoa of Australia (75, 80). The results obtained after a comprehensive characterization of the Alcoa ‘red mud’, were utilized to optimize design and the semi-dry disposal system for the bauxite residue produced by Alcoa in Western Australia (75). Depending on the rheological characteristics of the given red-mud, it was shown that there exists an optimum solid concentration (wt% solids) in the slurry at which it should be pumped.

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The rate and extent of thixotropic break down of the red mud slurry as a function of mixing time are gainfully exploited in this process. The results clearly indicate that destruction of network structure by mechanical agitation leads to the reduction in yield stress (a measure of the slurry’s resistance to flow) by several orders of magnitude. It is even more interesting to note the rate of recovery of the structure (and hence rise in yield stress) of red mud as a function of time in the absence of shear. Recovery is slower by an order of magnitude than degradation. This property of red mud is exploited in pipeline transport at high percent solids followed by semi-dry discharge at the disposal site and subsequent consolidation of red mud slurries (75). Based on the work of Boger and coworkers, Alcoa commissioned a demonstration plant in Kwinana in 1984. Subsequently, a semi-dry disposal method was commissioned at Pinjarra refinery in 1985. Later three high density super thickeners to produce highly concentrated slurries were built and are in use. The largest of these operations consists of a thickener having a diameter of 90 metres with 10 metres deep compression zone, a capacity of 450 tons per hour, use 60 g flocculant per ton of red mud and produces a 50% solids underflow which is pumped to a 70 hectares drying area where by it is discharged by the slope thickened tailings disposal (TTD) method (75). Recent developments in multi-disciplinary researches at the intersection of colloid science, yield rheology and processing of particulate fluids suggest that it should now be possible to develop/optimize the fourth generation technology, which can be optimally tailored for a specific waste management problem, for example disposal/storage of iron ore slimes/residue. The sub-processes that need to be embedded in this novel technology for disposal/storage of iron ore slimes/residue include one or more of the following:

• Dewatering for efficient recovery and recycle of water to the plant by thickening and high pressure filtration operations, as dictated by the compressive yield stress of the iron ore slimes/residue.

• Pipeline transportation of highly concentrated suspensions, based on time-dependent and time-lag thixotropic flow of the particulate fluid for example, iron ore slimes/residue

• Above ground stacking, so called semi-dry disposal, which takes advantage of the shear yield stress of the residue/slimes

It will be appreciated that, unlike the conventional technology, the new approach is based primarily on exploiting the flow and rheological characteristics of the particulate fluid (for example, red-muds), which in turn depend on surface forces, particle interactions and the resulting structure of the particle network in suspensions. The immediate need is to first characterize iron ore slimes in India to ascertain whether it has thixotropic properties similar to red muds. If not, one can suitably design appropriate additives in order to manipulate the yield rheology as required.

Based on the recent advancements in our understanding of the rheology of suspensions, particularly its manipulation/control with the help of appropriate

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surfactant and polymeric additives (78-89) it should now be possible to design a semi-dry disposal strategy tailor-made for Indian iron ore slimes.

A mission mode R&D program aimed at customization and demonstration of this semi-dry disposal technology for iron ore slimes must be immediately undertaken in India so that one can replace the existing tailings dam technology with a more robust, more eco-friendly and more cost effective fourth generation technology of semi-dry disposal of iron ore slimes/residue.

Production of Value-added Items from Iron Ore Slimes/Residue : Eco-cements In order to accomplish the task of developing zero waste technology for Indian iron ores, it is important to find appropriate means of utilizing the ultra-fine iron-rich alumino-silicate residue obtained during the beneficiation of iron ore slimes. Amongst several industrially useful products being explored world-wide, made from waste materials, eco-cements are perhaps the most promising (90).

Considering the large volumes of cement and concrete products consumed and the rates of growth anticipated in the buildings/construction industry in India, it is only natural that efforts are being made to incorporate industrial and mining wastes as substitutes for raw materials, admixtures, fillers, binders etc. in the construction industry. For example, the use of granulated blast furnace slag, volcanic ash, certain kinds of fly ashes and other materials having adequate lime reactivity in cement and concrete applications is now a standard industrial practice. Standard specifications are for instance, available in almost all the countries for blended cements.

Since there are stringent specifications on the quality of raw materials permitted in the manufacture of Portland cements with respect of composition and the presence of certain impurities such as phosphate, chloride, sulfate, iron oxide, titania, magnesia, etc. the use of waste products is obviously limited. Recent work on special cements, in particular those based on novel alinite and sulpho-aluminate type solid solution cementitions phases, however indicates that good quality cement/concrete products could be manufactured almost exclusively from wastes such as the one produced during the beneficiation of iron ore slimes. These cements are thus called eco-cements (90-97).

In addition to converting wastes into value added products of commercial significance both these classes of cements are also energy efficient as compared to portland cement. As an illustration, the compressive strengths of alinite cements made from gold ore tailings sands are compared with those of Portland cements in Fig. 6 (96). The cement was used as a binder (a substitute for Portland cement) during the pelletization of the same tailings sands. The pellets were required for subsequent heap leaching in order to recover the residual gold from the tailings. All the properties specifications such as pellet strength and permeability for this particular application were met or exceeded by alinite cements. It was established during trials that alinite

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cements were indistinguishable from Portland cement in all respects and can replace it as an inexpensive substitute binder.

Fig.6 Low temperature alinite cements made from gold ore tailings sands – Compressive strengths are comparable to those of ordinary portland cement (After Ref 96)

The above-mentioned case study illustrates the challenges and opportunities inherent in finding appropriate sinks for industrial and mining wastes such as the iron ore slimes and residue. Based on our work on cements, this approach of producing eco-cements from iron rich alumino-silicate residue produced during the beneficiation of iron ore slimes is certainly worth exploring.

Need for an integrated approach

A critical review of the available data and prior work presented in the preceding sections thus suggests that the serious problem of safe disposal and/or finding a commercially viable utilization strategy of Indian iron ore slimes remains unsolved not due to lack of technical expertise but because of the lack of proper problem definition. A comprehensive and integrated approach for Indian iron ore slimes calls for a mission mode R&D program aimed at coming up with a commercially viable solution, backed up by hard pilot plant/plant scale data for the proposed strategy and an independently verifiable techno-economic assessment of alternatives, to be completed in definite predefined time-frame. No one institution/organization or expert can do all that is required but one organization/individual must be given total responsibility/freedom and adequate funds to drive the project through to the end. Then only we can ensure that the end result does not suffer from the

LIGHT TAILINGS SANDS PORTLAND CEMENT DARK TAILINGS SANDS

Low Temperature Alinite Cements

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inadequacies, narrow expertise, vision or interest of the particular institution and investigators involved. The final outcome of such an effort can form a basis for arriving at business decisions subsequently.

It is not only imperative but also a challenge to Indian professionals to demonstrate that they together as a team can solve the problem of iron ore slimes, a problem unique to India.

Concluding Remarks A critical review of the work done thus far on the processing of Indian iron ore slimes indicates the urgent need to undertake a comprehensive, mission oriented, multi-institutional R&D program, in partnership with the industry, and directed towards an integrated solution to the problem of alumina rich iron ore slimes, as proposed in this paper. Development of selective reagents capable of accomplishing hematite-alumina/gibbsite separation has also been identified as a key research challenge facing Indian mineral processing community. Acknowledgements The author greatly benefited from many helpful discussions with Prof. P.C. Kapur during the preparation of this manuscript. The encouragement and support from Prof. Mathai Joseph is gratefully acknowledged.

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88. Sasanka Raha, Kartic C. Khilar, Pradip and P.C. Kapur, Rapid Determination of Compressive Yield Stress of Dense Suspensions by a Mean Phi Model of High Pressure Filtration, Powder Technology, 155, 2005, pp 42-51

89. Sasanka Raha, Kartic C. Khilar, Pradip and P.C. Kapur, Pareto Profile Benchmark for

Kinetics of Filtration and Extent of Dewatering of Fine and Colloidal Suspensions, Industrial Engineering and Chemistry Research, 44, 2005, pp 9364-9368

90. Pradip and E. Forssberg, Stabilization and Utilization of Solid Mining Waste in

Effluent Treatment in the Mining Industry, (Ed.) S.H. Castro, F. Vergara, M.A. Sanchez, University of Concepcion, Chile, (1998), pp 1-55.

91. Pradip, E. C. Subbarao, P. C. Kapur, N. R. Jagannathan and C.N.R. Rao,

"Characterization of Alinite Cements through X-Ray Diffraction & MAS29 Si NMR Studies", Materials Research Bulletin, 22(8), (1987), pp 1055-62.

92. Pradip, D. Vaidyanathan, P. C. Kapur and B. N. Singh, "The Production and

Properties of Alinite Cements from Steel Plant Wastes", Cement and Concrete Research, 20, (1990), pp 15-24.

93. P.K. Mehta, “Investigations on energy-saving cements”, World Cement Technology,

21, (1980), 166-73 94. M. Singh, S.N. Upadhyaya and P.M. Prasad, “Preparations of iron-rich cements using

red mud”, Cement and Concrete Research, 27(7), (1997), pp 1037-46 95. M. Singh, S.N. Upadhyaya and P.M. Prasad, “Preparations of special cements from

red mud”, Waste Management, 16(8), (1996) 665-670 96. Pradip, D. Vaidyanathan, Maneesh Singh, Ajay Panjwani and P. C. Kapur

Manufacture of Energy Efficient Hydraulic Cements from Industrial and Mining Waste Materials Mineral Processing : Recent Advances and Future Trends, (Eds.) S.P. Mehrotra and R. Shekhar, Allied Pub., New Delhi, India (1995), pp 816-823

97. Pradip and P.C. Kapur, Manufacture of Eco-friendly and Energy-efficient Alinite

Cements from Flyashes and other Bulk Wastes, J. Resources Processing (Japan), 51 (1), 2004, pp 8-13