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Summer Training Project MACHINE SHOP (ZINC PLANT) Training Period : 25-05-2015 to 07- 07-2015 Submitted By: Shubham Sen 1

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Page 1: HZL Shubham Sen Training Report

Summer Training Project

MACHINE SHOP (ZINC PLANT)

Training Period: 25-05-2015 to 07-07-2015

Submitted By: Shubham Sen

Submitted To:

Department of Mechanical Engineering

Sir Padampat Singhania University, Udaipur

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ACKNOWLEDGEMENT

I wish to acknowledge the encouragement received from Mr. Naveen Kumar (HOD, Mechanical engineering department, SPSU, Udaipur) & Mr. Satish Ajmera (Training Incharge) for initiating my interest in training.

I earnestly acknowledge my profound sense of gratitude to Mr. Naveen Kumar.

His mastery & work helped me in covering out this work smoothly. I am also grateful of all the workers of various departments who have helped me to improve my thinking as well as the practical knowledge.

Finally, I wish to add that I am indebted to god & My parents for everything good that has happened to me.

Shubham Sen

(12ME001672)

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PREFACE

Practical training is a way to implement theoretical knowledge to practical use to become a successful engineer. It is necessary to have a sound practical knowledge because it is only way by which one can acquire proficiency & skill to work successfully in different industries.

It is proven fact that bookish knowledge is not sufficient because things are not as ideal in practical field as they should be.

Hindustan Zinc Ltd. is one of the best examples to understand the production process & productivity in particular of Zinc.

This report is an attempt made to study the overall production system & related action of Zinc Smelter, Debari a unit of HZL. It is engaged in production of high grade zinc metal & other by products viz. Cd, sulphuric acid etc. since 1968 by adopting Hydro Metallurgical technolo

Shubham Sen

(12ME001672)

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TABLE OF CONTENTS

Acknowledgements.............................................................................................ITraining Certificate.............................................................................................IIPreface................................................................................................................III

1. Company Profile ...................................................................................01

1. Vedanta ...................................................................................................01

2. Hindustan Zinc Limited ..........................................................................01

3. Zinc Smelter Debari ................................................................................02

2. Zinc ................................................................................................................04

1. Introduction .............................................................................................04

2. Properties of Zinc ....................................................................................05

3. Zinc Smelting ..........................................................................................05

3. Zinc Smelter Debari .....................................................................................07

1. General Process Overview ......................................................................07

2. Raw Material Handling Section ..............................................................09

4. Roaster Plant ................................................................................................11

1. Roasting of Zinc Concentrate .................................................................11

2. Fluidized Bed Roaster .............................................................................14

3. Waste heat boiler ....................................................................................14

4. Cyclone ...................................................................................................16

5. Hot Gas Precipitator ...............................................................................16

5. Heat and Mass Balance Over Roaster Plant .............................................19

1. Mass Balance ..........................................................................................19

2. Heat Balance ...........................................................................................26

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6. Gas Cleaning Plant ......................................................................................32

1. Quench Tower ........................................................................................32

2. Packed Gas Cooling Tower ....................................................................32

3. Wet Gas Precipitator ...............................................................................33

4. Mercury Removal ...................................................................................34

7. Acid Plant .....................................................................................................35

Drying and Absorption Section ...............................................................35

Converter System ....................................................................................36

Basic Operations In Plant

1. Drying Tower .........................................................................................36

2. SO2 Blower .............................................................................................36

3. Converter Group .....................................................................................37

4. Preheater .................................................................................................38

5. Intermediate Absorber Section ...............................................................39

6. Final Absorber Section ...........................................................................39

Important Process Criteria

1. Gas drying and Water balance ................................................................40

2. Water Balance .........................................................................................41

3. Absorption of SO3 ...................................................................................41

4. Energy (Heat) Balance ............................................................................42

5. O2 /SO2 Ratio ...........................................................................................43

8. Leaching Plant ..............................................................................................44

1. Neutral Leaching .....................................................................................44

2. Acid Leaching .........................................................................................47

3. Neutralisation ..........................................................................................47

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4. Residual Treatment Plant ........................................................................48

5. Magnesium Removal ...............................................................................49

6. Horizontal Belt Filter ...............................................................................50

7. Purification ..............................................................................................51

8. Cadmium Plant ........................................................................................52

9. Gypsum Removal.................................................................................... 53

9. Electrolysis Plant ..........................................................................................54

10. Melting and Casting ...................................................................................55

11. References ...................................................................................................56

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TABLE OF FIGURES

Fig.-1 Plants at Zinc Smelter Debari .......................................................07

Fig.-2 General Processes In Plant ............................................................08

Fig.-3 Raw Material Handling Flow Sheet ..............................................10

Fig.-4 Process Flow Sheet In Roaster Plant ............................................12

Fig.-5 Calcine Balance Over Roaster Plant .............................................19

Fig.-6 Process Diagram For Acid Plant ...................................................37

Fig.-7 Process Diagram for Neutral Leaching ..........................................45

Fig.-8 Process Diagram for Acid Leaching and Neutralization ................47

Fig.-9 Process Diagram for Residual Treatment Plant ............................48

Fig.-10 Purification Plant .........................................................................52

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COMPANY PROFILEVedanta

Vedanta is an LSE-listed diversified FTSE 100 metals and mining company,

and India’s largest non-ferrous metals and mining company based on revenues.

Its business is principally located in India, one of the fastest growing large

economies in the world.

In addition, they have additional assets and operations in Zambia and Australia.

They are primarily engaged in copper, zinc, aluminium and iron businesses, and

are also developing a commercial power generation business.

Founder of this recognition is Mr. Anil Agarwal, who is chairman of this group,

a simple person without any special degree in management field but have a

great experience in this field and a sharp sight of the future conditions and

requirement.

Hindustan Zinc Limited

Hindustan Zinc Limited was incorporated from the erstwhile Metal Corporation

of India on 10 January 1966 as a Public Sector Undertaking.

In April 2002, Sterlite Opportunities and Ventures Limited (SOVL) made an

open offer for acquisition of shares of the company; consequent to the

disinvestment of Government of India's (GOI) stake of 26% including

management control to SOVL and acquired additional 20% of shares from

public, pursuant to the SEBI Regulations 1997. In August 2003, SOVL acquired

additional shares to the extent of 18.92% of the paid up capital from GOI in

exercise of "call option" clause in the share holder's agreement between GOI

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and SOVL. With the above additional acquisition, SOVL's stake in the company

has gone up to 64.92%. Thus GOI's stake in the company now stands at 29.54%.

Hindustan Zinc Ltd. operates smelters using

Roast Leach Electro-Winning (RLE)

Hydrometallurgical (Debari, Vizag and Chanderiya Smelters)

ISP™ pyrometallurgical (Chanderiya Lead Zinc Smelter) and

Ausmelt™ (Chanderiya Lead Smelter) process routes.

Zinc Smelter, Debari-Udaipur

Location 14 km from Udaipur, Rajasthan, India

Hydrometallurgical Zinc Smelter Commissioned in 1968 Roast Leach

Electrowining Technology with

Conversion Process Gone through a

series of debottlenecking 88,000 tonnes

per annum of Zinc

Captive Power Generation 29 MW DG Captive Power Plant

commissioned in 2003

Certifications BEST4 Certified Integrated Systems ISO

9001:2000, ISO 14001:2004, OHSAS

18001:1999, SA 8000:2001

Covered Area (Ha) 22.65

Total Plant Area (Ha) 126

Products Range

(a) High Grade Zinc (HG) (25 kgs) & Jumbo (600 kgs)

(b)Cadmium Pencils  (150 gms)

(c) Sulphuric Acid + 98% concentration

Awards & Recognitions

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(a) International Safety Award: 2006 by British Safety Council, UK

(b)ROSPA Gold Award for prevention of accidents

Operating Capacity (Per Year)

Zn : 80,000MT

Acid : 130,000MT

Cd : 250MT

Zinc dust : 360MT

Work force 876 Nos.

Managerial & Engineering Staff 84 Nos.

Supervisory & Technical Staff 58 Nos.

Labour 729 Nos.

(a) Skilled 154Nos.

(b) Semi-Skilled 555Nos.

(c) Unskilled 250Nos.

Raw Material Supplies:-

(a) Zawar Mines

(b) Agucha Mines

(c) Rajpura Dariba Mines

Product Buyers:-

(a) Tata

(b) Bhel

(c) Steel Companies

Process Collaborators:-

(a) Krebs Penorrova, France Leaching, Purification, Electrolysis

(b)Lurgi, GMBH, and Germany Roaster and gas clearing

(c) Auto Kumpu Finland RTP, Wartsila Plant

(d) I.S.C., ALLOY, U.K. Zinc dust plant, Allen Power Plant

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INTRODUCTIONZinc is a metallic chemical element with the symbol Zn and atomic number 30. In

nonscientific context it is sometimes called spelter. Commercially pure zinc is known as

Special High Grade, often abbreviated SHG, and is 99.995% pure.

Zinc is found in the earth’s crust primarily as zinc sulfide (ZnS). Zinc (Zn) is a metallic

element of hexagonal close-packed (hcp) crystal structure and a density of 7.13 grams per

cubic centimeter. It has only moderate hardness and can be made ductile and easily worked at

temperatures slightly above the ambient. In solid form it is grayish white, owing to the

formation of an oxide film on its surface, but when freshly cast or cut it has a bright, silvery

appearance. It’s most important use, as a protective coating for iron known as galvanizing,

derives from two of its outstanding characteristics: it is highly resistant to corrosion, and, in

contact with iron, it provides sacrificial protection by corroding in place of the iron.

Zinc ores typically may contain from 3 to 11 percent zinc, along with cadmium, copper, lead,

silver, and iron. Beneficiation, or the concentration of the zinc in the recovered ore, is

accomplished at or near the mine by crushing, grinding, and flotation process. Once

concentrated, the zinc ore is transferred to smelters for the production of zinc or zinc oxide.

The primary product of most zinc companies is slab zinc, which is produced in 5 grades:

special high grade, high grade, intermediate, brass special and prime western. The primary

smelters also produce sulfuric acid as a byproduct.

With its low melting point of 420° C (788° F), unalloyed zinc has poor engineering

properties, but in alloyed form the metal is used extensively. The addition of up to 45 percent

zinc to copper forms the series of brass alloys, while, with additions of aluminum, zinc forms

commercially significant pressure die-casting and gravity-casting alloys. Primary uses for

zinc include galvanizing of all forms of steel, as a constituent of brass, for electrical

conductors, vulcanization of rubber and in primers and paints. Most of these applications are

highly dependent upon zinc’s resistance to corrosion and its light weight characteristics. The

annual production volume has remained constant since the 1980s. India is a leading exporter

of zinc concentrates as well as the world’s largest importer of refined zinc.

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Properties of Zinc (metallic) at 293K

1. Density 7140Kg./m3

2. Melting Point 693K

3. Specific Latent Heat of Fusion 10 J/ Kg

4. Specific heat capacity 385 J/Kg/K

5. Linear expansivity 31/K

6. Thermal conductivity 111 W/m/k

7. Electric Sensitivity 5.9 ohm –meter

8. Temp. Coefficient of resistance 40/k

9. Tensile Strength 150 Mpa

10. Elongation 50%

11. Young’ modulus 110 Gpa

12. Passion’s Ratio 0.25

Zinc Smelting

Zinc smelting is the process of recovering and refining zinc metal out of zinc-containing feed

material such as zinc-containing concentrates or zinc oxides. This is the process of converting

zinc concentrates (ores that contain zinc) into pure zinc.

The most common zinc concentrate processed is zinc sulfide, which is obtained by

concentrating sphalerite using the froth flotation method. Secondary (recycled) zinc material,

such as zinc oxide, is also processed with the zinc sulfide. Approximately 30% of all zinc

produced is from recycled sources.

Globally, two main zinc-smelting processes are in use:

(a) Pyrometallurgical process run at high temperatures to produce liquid zinc.

(b) Hydrometallurgical or electrolytic process using aqueous solution in combination

with electrolysis to produce a solid zinc deposit.

The vast majority of zinc smelting plants in the western world use the electrolytic process,

also called the Roast-Leach-Electrowin (’RLE’) process, since it has various advantages over

the pyrometallurgical process (overall more energy-efficient, higher recovery rates, easier to

automate hence higher productivity, etc.).

In the most common hydrometallurgical process for zinc manufacturing, the ore is leached

with sulfuric acid to extract the Zinc. These processes can operate at atmospheric pressure or

as pressure leach circuits. Zinc is recovered from solution by electrowinning, a process

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similar to electrolytic refining. The process most commonly used for low-grade deposits is

heap leaching. Imperial smelting is also used for zinc ores.

Zinc Smelter Debari

Zinc Smelter Debari have following main plants

Fig.-1 Plants at Zinc Smelter Debari

General Process Overview

The electrolytic zinc smelting process can be divided into a number of generic sequential

process steps, as presented in the general flow sheet set out below.

In Summary, the Process Sequence is:

Step 1: Receipt of feed materials (concentrates and secondary feed materials such as zinc

oxides) and storage;

Step 2: Roasting: an oxidation stage removing sulphur from the sulphide feed materials,

resulting in so-called calcine;

Step 3: Leaching transforms the zinc contained in the calcine into a solution such as zinc

sulphate, using diluted sulphuric acid;

Step 4: Purification: removing impurities that could affect the quality of the electrolysis

process (such as cadmium, copper, cobalt or nickel) from the leach solution;

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Fig.-2 General Processes In Plant

Step 5: Electrolysis or electro-winning: zinc metal extraction from the purified solution by

means of electrolysis leaving a zinc metal deposit (zinc cathodes);

Step 6: Melting and casting: melting of the zinc cathodes typically using electrical induction

furnaces and casting the molten zinc into ingots.

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Additional steps can be added to the process transforming the pure zinc (typically 99.995%

pure zinc known as Special High Grade (’SHG’)) into various types of alloys or other

marketable products.

Raw Material Handling Section(RMH)

Smelters use a mix of zinc-containing concentrates or secondary zinc material such as zinc

oxides as feed to their roasting plant. Debari smelter is characterized by a relatively high

input of secondary materials. Smelters located inland receive their feed by road, rail or canal

depending on site-specific logistical factors and the type of feedstock (eg, secondary zinc

oxides come in smaller volumes and are typically transported by road). Concentrate

deliveries typically happen in large batches (eg, 5,000 to 10,000 tonnes).

Hindustan Zinc Smelter Debari is strategically located close to the Zawar mines that serves as

a global concentrate hub and provides for an extensive multi-modal logistical infrastructure.

It is 14 kms away from Udaipur well connected by rail, road and air. Most zinc smelters use

several sources of concentrates. These different materials are blended to obtain an optimal

mix of feedstock for the roasting process.

The zinc concentrate is delivered by trucks and is discharged into two underground

bins. Several belt conveyors transport the concentrate from the underground bins to the

concentrate storage hall. A Pay loader feeds the materials into two hoppers. By means of

discharging and transport belt conveyors including an over-belt magnetic separator, a vibro

screen and a hammer mill, the materials are transported to the concentrate feed bin. Dross

material from the cathode melting and casting process will be added to the feed

material before the vibro screen. For moistening of the concentrate several spraying nozzles

are foreseen in the concentrate storage hall, as well as on the conveying belt before

the concentrate feed bin.

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Fig.-3 Raw Material Handling Flow Sheet

Blended feed from the concentrate feed bin is discharged onto a discharge belt

conveyor, which in turn discharges onto a rotary table feeder. The roaster is fed then

by two slinger belts.

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ROASTER PLANTRoasting is a process of oxidizing zinc sulfide concentrates at high temperatures into an

impure zinc oxide, called "Zinc Calcine". This is a metallurgical process involving gas-solids

reactions at elevated temperatures. A common example is the process in which sulfide ores

are converted to oxides, prior to smelting. Roasting differs from calcination, which merely

involves decomposition at elevated temperatures. A typical sulfide roasting chemical reaction

takes the following form:

S + O2 → SO2.

2 ZnS + 3O2 → 2 ZnO

SO2 + O2 → SO3

CuS + 1.5O2 → CuO + SO2

The gaseous product of sulfide roasting, sulfur dioxide (SO2) is often used to produce sulfuric

acid.

Approximately 90% of zinc in concentrates are oxidized to zinc oxide, but at the roasting

temperatures around 10% of the zinc reacts with the iron impurities of the zinc sulfide

concentrates to form zinc ferrite. A byproduct of roasting is sulfur dioxide, which is further

processed into sulfuric acid.

Reduction of zinc sulfide concentrates to metallic zinc is accomplished through either

electrolytic deposition from a sulfate solution or by distillation in retorts or furnaces. Both of

these methods begin with the elimination of most of the sulfur in the concentrate through a

roasting process,

Roasting Of Zinc Concentrate

Debari roasting technology is characterized by lowest operating cost, minimum waste

material, safe and simple operation at high availability and the production of useful side

products as steam and sulfuric acid. Strongest environment regulations are met for solid,

liquid and gaseous products or emissions.

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Fig.-4 Process Flow Sheet In Roaster Plant

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The roaster has a cylindrical bed section, a conical intermediate section, a cylindrical

enlarged top section, and a grate area of 123 square meters. The enlarged cylindrical section

enables a complete roasting of even the finest calcine particles without the occurrence

of a secondary combustion phenomenon. For process optimisation 10 secondary air

nozzles are installed to be able to distribute additional roasting air above the bed. A slight

draught is maintained at the roaster gas outlet to ensure the safety of the roaster operation.

Depending on the raw material, the roaster operates with a capacity of 15 000 - 300 000 t/y

(zinc) and respectively 55 000 – 260 000 t/y (pyrite).

The combustion air serves both as a carrier medium for the fluid bed and as a source of

oxygen for the predominant reaction, which convert the metal sulfide to metal oxide and

sulfur dioxide. The combustion air is provided by a high pressure air fan, which is controlled

between the lower and a upper limit for a stable fluidization of the bed. The reaction in the

roaster is strongly exothermic, and the gas leaves the roaster with a temperature of

approximately 800°C to 975°C and an SO2 concentration of approximately 10 % by volume,

dry basis.

As combustion medium during the above described preheating diesel oil is used. The

maximum flow of diesel oil amounts to 3000 kg/h. The composition of offgas during furnace

heating is shown in below table:

The roasting process is fully automated, controlled and operated from a central control room.

Debari operates some of the world’s largest roasters, which are modelled after those used

throughout the industry.

The roasting step results in the production of calcine material (which is transported to the

subsequent leaching plant) and sulphur dioxide-rich waste gases. Waste heat boilers remove

the calcine contained in these gases as well as recovering the heat in the form of steam that is

used in the leaching plant and/or converted into electricity.

The hot dust-laden gas stream leaving the roaster is drawn into the waste heat boiler under

suction from the SO2 blower. In the boiler, the dust-laden gases are cooled down from the

roasting temperature to about 350°C before entering the dust precipitation system. Finally,

the sulphur dioxide is converted into sulphuric acid in a contact process, generating an

important smelter by-product.

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Debari is able to deliver the whole off-gas treatment and energy recovery system after the

roaster which includes following process steps:

Waste heat boiler

Hot Electrostatic Precipitator (ESP)

Wet Gas Cleaning

Sulfuric Acid Plant

Fluidized-Bed Roaster

In a fluidized-bed roaster, finely ground sulfide concentrates are suspended and oxidized in a

feedstock bed supported on an air column. As in the suspension roaster, the reaction rates for

desulfurization are more rapid than in the older multiple-hearth processes. Fluidized-bed

roasters operate under a pressure slightly lower than atmospheric and at temperatures

averaging 1000 °C (1800 °F). In the fluidized-bed process, no additional fuel is required after

ignition has been achieved. The major advantages of this roaster are greater throughput

capacities, greater sulfur removal capabilities, and lower maintenance.

Waste Heat Boiler

The hot dust laden gas stream leaving the roaster is drawn into the waste heat boiler under

suction from the SO2 blower. The waste heat boiler is a horizontal-pass boiler, gas-

tight welded, membrane wall-type, directly connected with the gas outlet flange of the roaster

by means of a flexible fabric expansion joint.

In the boiler, the dust-laden gases are cooled down from the roasting temperature to about

350°C before entering the dust precipitation system. The waste heat boiler is a forced-

circulation-type boiler for the production of superheated steam. The convection heating

surfaces of superheaters and evaporators are combined in bundles in a suspended

arrangement.

The waste heat boiler is equipped with a membrane tubed settling (drop-out) chamber ahead

of the front convection bundles. In the settling chamber, part of the dust carried along with

the gas is separated. Since the waste heat boiler handles roasting gases having a very

high dust content, a mechanical rapping device has to be provided. Pneumatic cylinders

drive these rappers. Depending on the degree of fouling, the rappers can be actuated by a

cylinder controller from a switching cabinet.

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The pneumatic cylinders are operated by compressed air, which can be taken from the plant

air system. The gas-flow velocity through the tube banks was designed to be very low to

avoid erosion. The tube banks can be easily removed for maintenance after the plant has been

taken out of operation. The rapping device is automatically actuated at certain time intervals.

The dust separating out in the boiler is collected in a chain conveyor and fed to the rotary

drum cooler. The combined system of cooling coils in the roaster, superheated tube bundles,

evaporator tube bundles, and membrane wall casing is designed for the maximum load

of the boiler. The boiler produces steam in a forced circulation system and is equipped with

two circulating pumps, one motor-driven and one turbine-driven. Each pump is capable of

handling the maximum rating of the boiler continuously. The stand-by steam-driven

circulating pump will start automatically when the electric power supply fails or when the

flow of circulating water falls below a preset quantity. The water-steam mixture, produced in

the forced circulation system, is separated in a steam drum by means of a demister. A

pressure relief system is included to exhaust steam directly to the atmosphere via a noise

damper in the event of curtailed steam usage in the leaching plant. The steam relief system is

designed for the full waste heat boiler production of 49 metric tons per hour. The level-

control valve installed in the incoming demineralized water line controls the feedwater

tank level. The demineralized water is deaerated in the deaerator on top of the

feedwater tank. Deaeration is accomplished by means of steam from the saturated

steam line. The feedwater tank pressure is maintained by the pressure control valve. The

deaerated feedwater is preheated and fed to the steam drum via the feedwater pumps, one

motor-driven and one steam-driven. The steam drum level is controlled using a three-

element control system. An additive preparation and dosing station for the boiler

feedwater is included in the system.

Cooled gases leaving the waste heat boiler flow into the hot electrostatic precipitator (ESP)

for final dust removal.

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Cyclone

The cooled and dust loaded gas enters the two parallel cyclones for pre-dedusting

with a temperature of approx. 350 °C. The gas leaves the cyclones at the top whereas the

dust is collected in the lower part of the cyclones and removed via rotary valves. Final

dedusting of the hot gas is achieved in the hot ESP.

Hot gas precipitator

The gas leaving the cyclones enters a three field hot gas ESP. The ESP consists of

the discharge electrodes, the collecting electrodes, gas distribution walls, casing, roof,

hoppers, horizontal inlet and outlet nozzles, pressure relief system, rapping systems, sealing

air system for the insulators with electric heater and transformer rectifiers with control

cabinet for the electrostatic fields. The precipitator is insulated. The collected dust is

removed from the ESP via the chain conveyor and the rotary valves. Rapping systems are

installed at the gas distribution plates in the inlet cone of the precipitator and at the

discharge and collecting electrodes. At their bottom end, the collecting electrodes run

through rapping bars, so that the rap is effected at the lower side end in the plate plane. A

hammer shaft rotates at the end of each electrostatic field, lifting the rapping hammers which

drop down in a free fall when the vertical position is exceeded. The acceleration amounts

to more than 200 g (200 x 9.81 m/s2) at the total collecting electrode area.

During start-up and shut-down the gas temperature in the precipitator can fall below the dew

point. To prevent condensation on the insulator a heating system is installed. This heating

system, consisting of a fan and a heater supplies hot air to all insulators. This hot air prevents

the penetration of gases to the insulators and also will keep the insulators at a temperature

above the dew point to prevent the formation of condensate, which could cause

electrical flash-overs.

Each discharge electrode system is supported by means of four high voltage insulators. The

discharge electrodes are tightened in tubular frames, which are vertically arranged between

the collecting electrodes. The high voltage insulators which support the discharge electrode

systems are located within box-type roof beams on top of the electrostatic precipitator

and a key-system is used to secure every door. By this way they are protected against

accidental contact to personnel.

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Separate transformer-rectifier sets per each electrostatic field (= 3 fields = 3 units) are

installed. The discharge electrode systems is supplied with high voltage DC by modern

transformer rectifier sets. Each transformer rectifier set will have its own cubicle for control

and regulation. Such a transformer rectifier set (power pack) contains a high tension

transformer and semi conductor rectifier pack. Both are installed in a steel vessel immersed in

oil. The vessels of the power packs are located in the high voltage room below the hot gas

electrostatic precipitator. The optimum values of the high voltage are controlled by a special

low voltage control system.

There is a high voltage switch on top of the T/R set for manual operation for disconnecting of

high voltage supply and earthing of the discharge electrode system

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Heat and Mass Balance Over Roaster Plant III

Mass Balance

Concentrate feed = 39.75 t/hrfeed in kg/hr = 39750 kg/hr

Moisture content = 10 %

Relative humidity = 0.111

Dry feed

=39749.8

9 kg/hr

Concentrate composition

Component % Kg/hr

Zn 52 20669.94

Fe 8.5 3378.74

Lead 1.5 596.25

Copper 0.1 39.75

Suphur 30 11924.97

C 0.9 357.749

Cd 0.16 63.60

SiO2 2 795.00

Insolubles 1 397.50

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Fig.-5 Calcine Balance Over Roaster Plant

1. Reactions of zinc:

Zn + S ZnS

65.4 32 97.4

20669.94 10113.73 30783.68

ZnS + 1.5 O2 ZnO + SO2

97.4 48 81.4 64

30783.6

8

15170.60 25726.81 20227.47

2. Reactions of lead:

Pb + S PbS

207.2 32 239.2

596.25 92.08 688.33

PbS + 1.5 O2 PbO + SO2

239.2 48 223.2 64

688.33 138.13 642.29 184.17

3. Reactions of copper:

Cu + S CuS

63.5 32 95.5

26

kg/hr

kg/hr

kg/hr

kg/hr

kg/hr

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39.75 20.03 59.78

CuS + 1.5 O2 CuO + SO2

95.50 48 79.5 64

59.78 30.05 49.77 40.06

4. Reactions of iron:

Fe + S FeS

Fe + S2 FeS2

Amount of Fe present in FeS 75 % = 2534.06 KgAmount of Fe present in FeS2 25 % = 844.69 Kg

Fe + S FeS

56 32 88

2534.06 3982.09

2 FeS + 3.5 O2 Fe2O3 + 2 SO2

176 112 160 128

3982.09 2534.06 3620.08 2896.06

Fe + S2 FeS2

56 64 120

844.69 965.35 1810.04

2 FeS2 + 5.5 O2 Fe2O3 + 4 SO2

240 176 160 256

27

kg/hr

Page 28: HZL Shubham Sen Training Report

1810.04 1327.36 1206.69 1930.71

5. Reactions of cadmium:

Cd + S CdS

112 32 144

63.60 18.17 81.77

CdS + O2 CdO + SO2

144.0

0

32 128 64

81.77 18.17 72.69 36.34

6. For Silica:

SiO2 SiO2

795.00 795.00

7. For Carbon:

C + O2 CO2

12 32 44

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Feed and Product rate for different components:

Concentrate Feed

rate

Oxides Wt of oxides Oxygen

req.

SO2

(kg/hr) (kg/hr) (kg/hr) (kg/hr)

ZnS 30783.68 ZnO 25726.80882 15170.5998 20227.4664

PbS 688.33 PbO 642.2906757 138.13 184.17

FeS 3982.09 Fe2O3 3620.08 2534.06 2896.06

FeS2 1810.04 Fe2O3 1206.69 1327.36 1930.71

CuS 59.78 CuO 49.77 30.05 40.06

SiO2 795.00 SiO2 795.00 0 0

C 357.75 CO2 44.00 32 0

CdS 81.77 CdO 72.69 18.17 36.34

Insolubles 397.50 397.50 0.00 0.00

Total 38955.93 32510.82 19250.36 25314.81

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Oxygen Required for 38.956 tonnes of concentrate = 19250.36 kg/hr

Oxygen Required for 1 tonnes of concentrate = 494.16 kg/hr

= 346.13 m3/hr

Oxygen Required for 39.75 tonnes of concentrate = 13758.49 m3/hr

Air Required for 39.75 tonnes of concentrate = 65516.60 m3/hr

Excess air = 25 %

= 16379.15 m3/hr

CO2 = 82.1846667 m3/hr

SO2 = 8865.72238 m3/hr

Clean Air escaping from top = 68137.26 m3/hr

Total Air at furnace inlet = 81895.75 m3/hr

Gases escaping from furnace top = 77085.17 m3/hr

Calcine from top of the furnace = 75 %

= 24383.11 kg/hr

Calcine from bottom of the furnace = 25 %

= 8127.70 kg/hr

Dust removing capacity of boiler = 45 %

Dust removing capacity of Cyclone separator = 40 %

Dust removig Capacity of HGP = 15 %

Intlet outlet Underflow

Boiler 24.38 t/hr 13.41 t/hr 10.97 t/hr

Cyclone separator 13.41 t/hr 3.66 t/hr 9.75 t/hr

HGP 3.66 t/hr 0.01829 t/hr 3.64 t/hr

\

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Heat BalanceReactions involved :

∆Hf(KJ/mol)

ZnS + 1.5 O2 ZnO + SO2

Enthalp

y

-204.6 0 -348 -296.84 KJ/mol -440.24

PbS + 1.5 O2 PbO + SO2

-98.12 0 -218.08 -296.84 KJ/mol -416.8

CuS + 1.5 O2 CuO + SO2

-48.5 0 -155.2 -296.84 KJ/mol -403.54

2 FeS + 3.5 O2 Fe2O3 + 2 SO2

-201 0 -825.5 -593.68 KJ/mol -1218.18

2

FeS2

+ 5.5 O2 Fe2O3 + 4 SO2

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-355 0 -825.5 -

1187.36

KJ/mol -1657.86

CdS + O2 CdO + SO2

-208.4 0 -235.6 -296.84 KJ/mol -324.04

C + O2 CO2

0 0 -393.5 KJ/mol Total Heat of Above reactions = - 4460.66 KJ/mol

Heat Balance for Furnace:Temp.of calcine at furnace inlet = 25 C

Temp.of calcine at furnace outlet = 920 C

∆t = 895 C

Calcine flyover

M = 24383.11 kg/hr

Q = 21248538.45 kj/hr

  = 21248.53845 MJ/hr

SO2

M = 25314.81 kg/hr

Cp = 0.645 kj/kg-k

Q = 19071167.99 kj/hr

   = 19071.16799 MJ/hr

32

Air = 68137.26 m3/hr

M = 88158.32 kg/hr

Cp = 1 kj/kg-k

Q = 102968915.5 kj/hr

= 102968.9155 MJ/hr

Page 33: HZL Shubham Sen Training Report

Total heat output = 159015.486 MJ/hr

Heat Input = 4460.66 KJ/mol

For

cooling

coils

Q = m*

L

latent heat of

vaporisation, L

= 2.4

2

MJ/Kg

m = Q/L

M = 173

99.

04

Kg/hr

= 17. t/hr

33

Calcine uderflow

M = 8127.70 kg/hr

Cp = 0.75 kJ/kg-k

Q = 7082846.15 KJ/hr

  = 7082.84615 MJ/hrCO2

M = 44.00 Kg/hr

Cp = 1.22 KJ/Kg-K

Q = 62698.24 KJ/hr

= 62.69824 MJ/hr

Radiation Loss

= 3%

= 40699.3869 MJ/hr

water evap.

Q=ml

= 8581320 Kj/hr

 = 8581.32 MJ/hr

Total moles = 350.29 kmol/hr

Q input = 1562537.85 MJ/hr

Qo/p = 159015.49 MJ/hr

Q left = 1403522.37 MJ/hr

Page 34: HZL Shubham Sen Training Report

40

enthalpy of liquid water, h1 = 104.86 kJ/kg at 25 C

enthalpy of superheated steam, h2 = 4398 kJ/kg at 920 C

 ∆h = 4293.14 kJ/kg

 Q = m∆h

 M = Q/∆h

= 9.80766316 t/hr

Heat Taken by cooling coil = 3%

Q = h*A*∆t H = 0.828 MJ/hr-m2-C

A = 54.98 ∆t = 925 C

Heat retained = 97%

Heat Balance Over Waste Heat Re-Boiler:Temp.of calcine at boiler outlet = 350 C

Temp.of calcine at boiler inlet = 920 C

∆t = 570 C

Calcine underflow temp. in boiler = 900 C

∆t = 20 C

Calcine flyover

M = 13410.71 Kg/hr

Q = 8434832.922 Kj/hr

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Page 35: HZL Shubham Sen Training Report

= 8434.832922 Mj/hr

Air = 68137.26 m3/hr

M = 88158.32 kg/hr

Cp = 1 kj/kg-k

Q = 74317462.16 kj/hr

= 74317.46216 MJ/hr

SO2

M = 25314.81 kg/hrCp = 0.645 kj/kg-kQ = 13764550.1

8

kj/hr

= 13764.5501

8

MJ/hr

Calcine uderflow

M = 10972.40 kg/hr

Cp = 0.75 kJ/kg-k

Q = 2398647.08 KJ/hr

= 2398.64708MJ/hr

Water evap.

Q = ml

= 8581320 Kj/hr

35

CO2

M = 44.00 Kg/hr

Cp = 1.22 KJ/Kg-K

Q = 45252.24 KJ/hr

  = 45.25224 MJ/hr

Page 36: HZL Shubham Sen Training Report

= 8581.32 MJ/hr

Heat leaving Boiler

Q entering Boiler = 1513349.34 MJ/hr

Total Q leaving from top = 105143.418 MJ/hr

Heat leaving from boiler bottm = 2398.64708 MJ/hr

Q1 retaining inside boiler = 1405807.27 MJ/hr

Heat Balance Over Cyclone Separator:

Temp.of calcine at Cyclone outlet = 300

C

Temp.of calcine at Cyclone inlet = 350 C

 ∆t = 50 C

Calcine flyover

M = 3657.47 kg/hr

Q = 881414.1164 kj/hr

=  881.4141164 MJ/hr

SO2

 M = 25314.81 kg/hr

 Cp = 0.645 kj/kg-k

 Q = 5273961.696 kj/hr

  = 5273.961696 MJ/hr

36

Radiation Loss

=3%

=0 Mj

Air = 68137.26 m3/hr

M = 88158.32 kg/hr

Cp = 1 kj/kg-k

Q = 28475136.75 kj/hr

= 28475.13675 MJ/hr

Calcine uderflow

M = 9753.25 kg/hr

Cp = 0.75 kJ/kg-k

Q = 2350437.64 KJ/hr

  = 2350.43764 MJ/hr

CO2

M = 44.00 Kg/hr

Cp = 1.22 KJ/Kg-K

Q = 17338.64 KJ/hr

  = 17.33864 MJ/hr

water evap.

 Q = ml

= 8581320 Kj/hr

  = 8581.32 MJ/hr

Radiation Loss

3%

 0 MJ/hr

Page 37: HZL Shubham Sen Training Report

Heat Entering at cyclone inlet =105143.418MJ/hr

Heat leaving from Cyclone top =MJ/hr

Heat leaving at cyclone bottm =2350.43764MJ/hr

Heat Entering HGP =102792.98MJ/hr

Heat Balance Over Hot Gas Precipitator:

Temp.of calcine at HGP outlet = 290C

Temp.of calcine at HGP inlet = 300C

∆t = 10C

37

Calcine flyover

M = 18.29kg/hr

Q = 3861.303327kj/hr

  = 3.861303327MJ/hrAir = 68137.26 m3/hr

M = 88158.32 kg/hr

Cp = 1 kj/kg-k

Q = 24948804.02 kj/hr

  = 24948.80402 MJ/hr

SO2

M = 25314.81 kg/hr

Cp = 0.645 kj/kg-k

Q = 4620839.505 kj/hr

= 4620.839505 MJ/hrCalcine uderflow

M = 3639.18 kg/hr

Cp = 0.75 kJ/kg-k

Q = 768399.362 KJ/hr

  = 768.399362 MJ/hr

Water evap.

Q = ml

= 8581320 Kj/hr

= 8581.32 MJ/hr

CO2

M = 44.00 Kg/hr

Cp = 1.22 KJ/Kg-K

Q = 15191.44 KJ/hr

= 15.19144 MJ/hr

Radiation Loss = 3%

= 0 MJ/hr

Page 38: HZL Shubham Sen Training Report

Heat Entering HGP 102792.98 MJ/hr

Heat Leaving at HGP bottom 768.399362 MJ/hr

Heat retaining inside HGP 102024.581 MJ/hr

GAS CLEANING PLANTGases leaving waste heat boiler are passed through cyclone to remove the calcine particles

and then passed through hot gas precipitator to remove the fine particles of calcine by the

application of electric field.

Quench TowerIn the quench tower, the hot gas is cooled by the evaporation of water. The heat of the

incoming gas (Temperature: > 300°C) is used for the evaporation of water that is sprayed into

the Quench Tower. The sensitive heat of the gas is converted into water vapor (latent heat).

This type of “cooling” can be considered as an adiabatic process. Adiabatic means, the

process step is operated without energy exchange with the environment. But besides this

quenching, also a part of the dust and condensable impurities in the gas will be scrubbed in

the quench tower. At the outlet of the tower, the gas contains water vapor. If the temperature

is lowered, water vapor will condense. The Quench Tower is designed as counter current

flow type quencher. The gas inlet is at the bottom part of the casing. The gas outlet is at the

top of the quench tower. The liquid is sprayed into the quench tower in counter-current flow

to the gas. A part of the spray does evaporate, but the biggest portion will be collected in the

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Page 39: HZL Shubham Sen Training Report

lower part of the tower which does serve as pump tank. A side stream of the spray circuits is

guided through a settling tank for removal of suspended solids.

Excess liquid from the washing and cooling system is discharged from the quench tower

circuit via strippers.

Packed gas cooling towerThe adiabatic cooling in the quench tower does result in water saturated gas and consequently

in a high content of gaseous water in the SO2-gas. If the water vapour would not be lowered

prior to the drying tower, the concentration of the sulphuric acid would be lower than

acceptable limits. Removal of water vapour is done in a packed gas cooling tower. The gas

enters the tower from the bottom and flows upwardly through the packing. In counter current

flow to the gas, cold cooling liquid (weak acid) is distributed over the packing. The

downward flowing cold liquid does cool the SO2-gas and water vapour is condensed. While

flowing downward, the cooling liquids heats up. The lower part of the tower serves as a pump

tank. From this pump tank, the cooling liquid is circulated via pumps and plate heat

exchangers back to the liquid distribution system. The plate heat exchangers are cooled with

cooling water which is circulated via evaporative type cooling towers.

The packed gas cooling tower is designed as cylindrical vessel made of FRP. Diameter and

packing height are calculated in a way, that filling bodies made of plastic (PP, PVC) with a

diameter of 2 inch and a specific surface of more than 90 m2/m3 are to be used. The gas side

pressure drop does not exceed 4 mbar under full gas load. The liquid distribution system shall

ensure an equal distribution of the liquid on the upper surface of the packing. The liquid

distribution system will have two feed points for cooling liquid. The liquid distribution

system will be made of FRP. Plastic pumps (material: HP PE) will be used. The pipes will be

made of FRP.

Wet gas precipitatorFinal removal of dust and aerosols will be carried out in the wet electrostatic precipitator

system which consists of two stages of precipitators. Downstream of the second stage of the

wet electrostatic precipitators, the SO2-gas will be optically clean. The wet ESP’s will be of

the tube type with gas flow in vertical direction. While flowing through the wet ESP tubes,

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Page 40: HZL Shubham Sen Training Report

the aerosols and particles are electrically charged due to high tension and migrate to the

collection tubes.

Particles and aerosols separated from the gas will be removed continuously together with the

condensate at the bottom of the ESP’s, respectively from the connected gas ducts. The liquids

will be directed to a tank, which does also serve for wet ESP flushing.

Normally wet ESPs do have a good self-cleaning. This self cleaning can be improved in the

second stage wet ESP’s by the installation of a continuous spraying system But from time to

time, depending on the operating conditions, each precipitator must be flushed. This is done

with liquid, that is withdrawn from a tank and that is pumped to the flushing nozzles installed

in the top part of the wet ESPs.

All parts of the wet electrostatic precipitators in contact with the gas stream will be made of

the following acid resistant materials: Homogeneously lead lined steel PVC, PP, FRP or FRP

with PVC-lining

The materials are selected in accordance with the operating conditions and mechanical loads

to which the equipment is subjected.

Mercury RemovalFor the production of sulphuric acid with a low mercury-content, so-called Norzinc Mercury

Removal Process (Calomel Process) is in foreseen. This process allows a cost efficient

removal of metallic mercury vapours from SO2-gases upstream of sulphuric acid plants. The

ingress of mercury into the sulphuric acid is prevented and a safe and reliable production of a

sulphuric acid with low mercury contents is achieved.

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Page 41: HZL Shubham Sen Training Report

ACID PLANTPlant DescriptionThe sulphuric acid plant mainly consists of 2 plant sections:

The drying and absorption section

The converter section with the gas to gas heat exchangers

Drying and Absorption Section The drying and absorption section mainly consists of the drying tower, the intermediate

absorber, the final absorber with individual acid pumps, the acid coolers and the acid piping.

These towers are of identical construction: each tower consists of a bricklined steel

shell with a filling of ceramic Intalox saddles. The layer of Intalox saddles is supported by a

supporting structure made of acid resistant stoneware.

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Page 42: HZL Shubham Sen Training Report

The irrigated acid is distributed uniformly over the packing by the irrigation system. At the

gas outlet of each tower gas filters are installed, for the drying tower a wire-mesh filter, for

the intermediate and final absorber candle type filters made of a casing plugged with

special glass wool. Most part of the acid mist and all acid droplets will be removed from the

gas by the filters.

The gas flow through the towers is countercurrent to the acid flow, i.e. the gas flows from the

bottom to the top of the tower. From the bottom of the tower(s) the acid flows to the pump

sump and is pumped from there by the acid pumps (via the acid coolers) back to the irrigation

system.

Acid transfer lines between the drying tower, the intermediate absorber and the final absorber

and injection lines for dilution water at the intermediate absorber and final absorber

enable to control the necessary acid concentration for each of the towers. The acid cooler for

the drying tower, intermediate and final absorption as well as for the product acid system are

plate-type acid coolers.

Converter systemThe converter system consists of a stainless steel 4-layer

converter. The arrangement of the catalyst beds is 3 + 1, i.e. the intermediate absorption is after the 3rd layer. The converter

itself is an insulated, vertical and cylindrical vessel divided in four sections: called layers or trays. The catalyst required for the

conversion of SO2 to SO3 is arranged on these layers.

Basic Operations in Plant

Drying towerColumn packed with ceramic packing to facilitate contact of SO2 Bearing gases with dilute

sulphuric acid

Gases leaving the Wet Gas Precipitator are passed through drying tower to remove the

moisture by spraying sulphuric acid from top and gases enter from bottom. During the

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Page 43: HZL Shubham Sen Training Report

removal of moisture from the gases heat is liberated which increases the temperature of the

circulating acid.

Cooling of circulating acid is done by passing the acid through PHE(Plate heat exchanger),

where filter water is used as coolant. The concentration of circulating acid is maintained by

crossing through FAT & IAT vessel acid. This is done automatically based on sensing of

concentration of circulating acid by concentration analyzer.

SO2 BlowerBlower to circulate the sulphur dioxide bearing gases in converter through heat exchangers

and mixer

The SO2 gas blower is arranged downstream the drying tower and transports the gas from the

roaster section via the gas cleaning plant through the sulphuric acid plant. The blower will be

provided with an electric motor.

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Fig.-6 Process Diagram For Acid Plant

Converter GroupAfter being discharged from the SO2 blower the gas will be then routed to the shell side of

heat exchanger IV. In this heat exchanger, the main part of the SO2 gas will be preheated to a

temperature of about 255°C. A small part of the cold SO2 gas will be withdrawn from the

lower part of the heat-exchanger IV and routed to the tube side of heat exchanger II where it

will be preheated to a temperature of about 255°C while cooling the gas between layer II and

III. Both gas streams will be collected at the entrance of heat exchanger I. Before entering the

first bed of the converter the gas will pass first through the shell side of that heat

exchanger.

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Page 45: HZL Shubham Sen Training Report

The gas exits this heat exchanger at a temperature of approx. 400°C and enters the

first catalyst bed. Bypass lines will be located around heat exchangers IV and I to the

inlet of the first bed.

These bypass-streams will be utilized for adjusting the gas inlet temperature to the first

and third bed of the converter. The gas leaving the first pass will be cooled in the tube side of

the heat exchanger I to approx. 425°C and enters the second bed. After passing through the

second bed the gas will be routed through the tube side of heat exchanger II thus

cooling the SO3 gas to approx. 433°C by preheating the SO2-gas coming from the

drying tower. The gas will be fed to the third pass of the converter. After the third layer the

temperature of the gas is approx. 443 °C.

The gas will be cooled will be cooled in the heat exchangers III A/B to temperature of

165 °C and leaving to the intermediate absorption. After the SO3 has been absorbed in the

intermediate absorber, the gas is returned to the converter via the shell side of heat exchanger

III A and III B where it will be preheated to a temperature of approx. 400°C before entering

the fourth catalyst bed The gas leaving the fourth bed at a temperature of approx. 410°C,

passes through the tube side of heat exchanger IV where it will preheat the cold SO 2 gas

before entering the first bed. After being cooled to 271 °C, the gas enters the economizer.

The gas leaving the economizer at approx. 180°C will then passed to the final absorber

where the remaining SO3 will be absorbed.

PreheaterThe preheating is needed to preheat the converter system (e.g. catalyst and heat

exchangers) from cold conditions to operating conditions and low SO2 strength,

whereas during normal operation of the plant the heat released within the process

allows autothermal operation of the plant. In addition lower or varying SO 2-

concentrations can be compensated. The separate preheater preheats air or, in start-up phases,

SO2-gas to the required temperatures.

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Intermediate Absorber SectionAfter the SO3 gas has passed through the tube side of heat exchangers III A/B this gas will

then pass through the packed tower section in countercurrent to the acid. After passing

through the candle filter, the gas enters the shell side of heat exchangers III A/B leaving them

preheated by the hot SO3 gas at approx. 400°C. As the absorption of SO3 generates heat, the

circulating acid at a temperature of 87°C will be cooled before returning to the absorber

distribution systems as described before. The acid collected in the sump of the intermediate

absorber will be pumped through plate type acid coolers by the vertical intermediate absorber

pump back to the absorber distribution system.

A bypass located around the cooler ensures a constant temperature of approx. 70°C to the

absorber. The concentration of the circulating acid, measured downstream of the cooler, will

be maintained by the addition of 96 % acid from the SO2 drying tower and by addition of

dilution water. The level in the absorber will be controlled by transferring 98.5 % acid to the

final absorption system

Final Absorber SectionThe SO3 gas routed from the economizer will enter the final absorption tower. The gas at a

temperature of 180°C will flow through the packed tower section. 98.5 % circulating acid

will be introduced counter-currently to the SO3 gas through an acid distribution system

located in the upper part of the absorber section. After passing through the packed bed

and candle filter assembly, the gas will be routed to the tail gas scrubbing system. As the

absorption of SO3 generates heat, the circulating acid at a temperature of 85°C will be cooled

before returning to the absorber. The acid collected in the pump tank will be pumped through

a plate type acid cooler by an vertical mounted final absorber pump. A temperature-

controlled bypass located around the cooler ensures a constant temperature of approx.

75°C to the absorber. The concentration of the circulating acid, measured downstream of the

cooler, will be maintained by the addition of process water. The level in the absorber pump

tank will be maintained by transferring product acid to the to the client’s storage facilities by

the product acid pump. In the product acid cooler the temperature will be reduced from

approx. 85 °C to 40 °C.

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Important Process Criteria

For the production of sulphuric acid SO2 containing gases from the zinc roasting are used.

There are four main process criteria in the production of sulphuric acid from these kind of

gases by the contact/converter process.

They are:

Gas drying and water balance

Energy/ heat balance

Conversion of SO2 to SO3

Required O2 /SO3 ratio

Gas drying and Water balanceGas drying is an important process step in this type of contact plant. Gas drying

protects cooler parts of the plant, such as heat exchangers, against corrosion by acid

condensation. It safeguards against formation of acid mist which can be very difficult to

absorb in a later stage of the process. It also protects the catalyst from acid

condensation during plant shut-downs. Therefore, the life of the plant and also the tail gas

purity depend in large measure on a sufficient gas drying.

A substantial amount of heat - not just the heat of dilution of the sulphuric acid but also the

heat of condensation of the water - is liberated in the gas drying stage. For that reason the

circulated acid is generally cooled by indirect heat exchange before being recycled to the

dryer.

Water BalanceThe water balance of contact sulphuric acid plants in general may simply be defined by the

specific amount of process water required for achieving the desired product acid strength

from the amount of SO2 converted to SO3.

In the case of cold gas acid plants (like roasting plants) the process water requirements are

usually balanced nearly completely by the water vapor content of the SO2-feed gases

entering the drying tower except for a small margin necessary for the automatic acid strength

control.

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Thus at a given SO3 gas concentration and SO2 conversion rate as well as at a fixed product

acid strength, the only variable that can and has to be controlled or limited is the water vapor

content of the feed gases entering the drying in order not to exceed the water balance of the

whole system.This is done in the wet gas purification system by cooling the SO2 gases down

to the dew point temperature corresponding to the maximum allowable water vapor content.

When evaluating the required dew point temperature, it is important not only to

consider the designed suction pressure of the gas purification system but also the

external barometric pressure which depends largely on the elevation of the plant above sea

level.

Absorption of SO3 Sulphur trioxide formed by the catalytic oxidation of sulphur dioxide is absorbed in

sulphuric acid of at least 98 % concentration, in which it reacts with existing or added water

to form more sulphuric acid. The process gas leaving the converter system is cooled, first

in a gas-gas heat exchanger to a temperature of about 180°C before entering the

absorber. The gas entering the absorber is therefore not completely cold and it passes heat to

the absorber acid as it passes through the absorber; by the time it reaches the outlet it is

virtually at the same temperature as the incoming absorber acid.

A substantial amount of heat is also generated in the absorber acid from the absorption of

sulphur trioxide and the formation of sulphuric acid, and the acid temperature rises in

consequence by a margin which depends on the acid circulation rate. The acid

concentration is maintained constant by adding process water to the acid leaving the absorber

and the acid crossflow from the dryer at a rate controlled by a concentration measuring

device. The circulated acid is cooled by indirect cooling.

Energy (Heat) Balance The SO2 gases leaving the wet gas cleaning system enter the acid plant at temperature of

max. 38°C in this case for reasons of the water balance. After removing the rest

water content in the dryer, the process gases must be heated up to the required

converter inlet temperature of min 396°C. This is achieved by indirect heat exchange with the

available sensible gas heat released from the SO2 oxidation in the converter.

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The main objective in designing such a cold gas plant is the attainment of autothermal

operation conditions which becomes primarily a question of the required gas heat

exchanger surface depending on the feed gas SO2 concentration.

The reaction heat in a double catalysis plant based on roaster gases is released from: The

catalytic oxidation:

SO2 + ½ O2 → SO3

The sulphur trioxide absorption and sulphuric acid formation:

SO3 + H2O → H2SO4

H2SO4 + SO3 ↔ H2S2O7

H2S2O7 + H2O ↔ 2 H2SO4

The dilution to acid production strength of 98.5 % H2SO4 and the condensation heat for

the water content of the gases entering the drying tower system. Sulphuric acid thus

produced is stored in acid storage tanks of capacity 6500 MT each. Finally gases are

discharged through chimney to atmosphere

O2 /SO2 Ratio For the conversion of SO2 to SO3 the proportion of O2 volume to SO2 volume in the feed gas

to the converter, called O2/SO2 ratio, is an important factor for the conversion rate.The design

of the contact plant is based on a gas composition which is calculated on the basis of analyses

of the roaster gases be processed. However, in practice other gas compositions may occur and

their different water content may lead to substantial fluctuations in the gas composition.

Considering the SO2 concentration determined for the design of the double catalysis plant,

this may often mean an essential shifting of the expected O2/SO2 ratio towards lower values.

On the other hand, the O2/SO2 ratio of the gases has a decisive influence on the final

conversion efficiency achievable in the contact plant.

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LEACHING PLANTLeaching is a widely used extractive metallurgy technique which converts metals into soluble

salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to

perform and much less harmful, because no gaseous pollution occurs. The only drawback of

leaching is its lower efficiency caused by the low temperatures of the operation, which

dramatically affect chemical reaction rates.

Neutral Leaching Of Zinc Calcine The first neutral leaching step is the most important section of the leaching plant, because

approx. 70% of the total dissolved Zn is dissolved in this leaching step. The main target is to

leach the zinc oxide from the calcine and oxidize the ferrous iron to the ferric state. In

addition to being an important zinc leaching step, the neutral leaching step is also an

important purification step. Impurities like Fe, As, Sb, and Ge are precipitated in the last

tanks of neutral leaching.

Neutral leaching consists of following main process equipment:

One 35 MT capacity calcine hopper

Seven 680 MT capacity calcine storage silos

Three screw conveyors, and five reddller conveyors

One classifier and ball mill

Nine 45 m3 leaching reactors and first reactor is called Ready for calcine

Two 16 m diameter thickners

Two 70 m3 thickner overflow tanks.

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Calcine conveyingCalcine from roaster plant is taken through bucket elevator. There are two bucket elevator.

This calcine is transferred to calcine hopper. When calcine hopper is full, calcine is diverted

to storage silos through reddler conveyors to any of the seven silos depending on the level of

calcine present in each of them.

Neutral leachingThe ready for calcine(RC) and remaining leaching pachukas (Reactor) are all covered and

equipped with agitators and stacks. Pachukas are equipped with injectors for oxygen gas.

Execpt RC and the eight leaching pachukas are arranged in a cascade and are interconnected

with an overflow launder, so that the solution fed to the first tank flows by gravity to all the

tanks and to the classifier without pumps. Also , the launder system enables the bypass of any

single pachuka.

Fig.-7 Process Diagram for Neutral Leaching

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Calcine from calcine hopper is fed through a screw conveyor and reddler conveyor. Ready

for calcine solution is prepared continuously in RC tank and in case of breakdown of RC tank

first pachuka is used for ready for calcine.

The main acid bearing solutions ,which is used to leach the calcine added into the neutral

leach step is the spent elecrolyte from cell house whith approx. 180 g/l free H2SO4 along with

ball mill slurry and occasionally conversion overflow acid overflow also can be added. In

addition MnO2 (To control the ferrous content) KMnO4 are to be added in RC tank. The

dosing of MnO2 is carried out by a magnetic vibrator.

The solution mixture from the RC tank, with a free acidity of about 100-120 g/l H2SO4

pumped to the pachuka. The acidity is lowered in the leaching tank series by calcine addition

in two steps.

The basic chemical reactions in the neutral leaching process are:

ZnO + H2SO4 → ZnSO4 + H2O

CuO + H2SO4 → CuSO4 + H2O

CdO + H2SO4 → CdSO4 + H2O

PbO + H2SO4 → PbSO4 + H2O

In addition to the reactions above , MnO2 reacts in the first leaching pachukas

2 FeSO4 + 2 H2SO4 + MnO2 → Fe2 (SO4)3 +MnSO4 + 2 H2O

Where as in the last leaching pachukas oxygen is reacting and iron hydroxide is precipitated

2 FeSO4 + 2 H2SO4 + ½ O2 + ZnO → Fe2 (SO4)3 + ZnSO4

+ 2 H2O

Fe2(SO4)3 + 3 ZnO + 3 H2O → 2 Fe(OH)3 + 3 ZnSO4

4 Fe(OH)3 + Sb/As/Al/Ge-complex → Fe-OH-Sb-As-Ge-Complex

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Acid LeachingThe main task of the acid leaching step is to dissolve the remaining zinc oxide, which was not

dissolved during Neutral leaching and to reach a zinc oxide leaching efficiency of more than

97%.

The main chemical reactions in weak acid leaching are:

ZnO + H2SO4 → ZnSO4 + H2O

MeO + H2SO4 → MeSO4 + H2O (Me = Cu, Cd etc.)

Fig.-8 Process Diagram for Acid Leaching and Neutralization

Neutralization:Neutralization is used to remove the impurities Sb, As , Al and Ge by neutralizing the over

flow from acid leaching thickner and jarosite precipitation thickners with calcine before

sending it to neutral leaching.

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Residual Treatment Plant(RTP):The conversion process main function is to simultaneously leach the zinc ferrite and

precipitate the iron as ammonium or sodium jarosite. The conversion process comprise the

following main equipment.

Five 300 m3 conversion reactors

Two 18 m diameter thickners

Two 70 m3 thickener overflow tank

One 20 m3 condensate tank

Two (NH4)2SO4 /Na2SO4 preparation tank

Fig.-9 Process Diagram for Residual Treatment Plant

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The five conversion reactors are arranged in a cascade and are interconnected with an

overflow launder. The solution flows by gravity from the first conversion reactor down the

cascade through all reactors into the thickner of ccd system without the need of pumping.

Each reactor is covered and equipped with an agitator, a vent stack and steam heating

elements.

The weak acid leaching underflow is pumped through a flow indicator and controller into the

launder before the first conversion process in order to compensate for the sulphate losses.

MnO slurry from ZE plant is pumped in to inlet and is added to oxidize ferrous ion.the

temperature of all reactors is kept at 95-100. Ammonium sulphate or sodium sulphate diluted

with condensate water is added at outlet for formation of jarosite.

Magnesium RemovalThe magnesium removal is required to maintain minimum magnesium in process solutions.

For the separation of Zn from the Mg, lime milk is used.

The magnesium plant consists of following main equipment

One continuous vaccum drum filter

Two magnesium reactors 20 m each

Two lime preparation reactors

The lime milk solution is prepared in the lime milk tank. The Ca(OH)2 is fed from lime bin

into the precipitation tank and the COF is fed using pump.Alternatively the feed can be

filtrate from the jarosite filtration step. The lime milk is circulated to the precipitation tanks

using pump.The neutralization process is continuous and dimensioned for about four hours

retention time.The precipitation tanks are run at pH 6.8-7.5. Zinc is precipated together with

which is not a desired situation.In pH 8 Zn is almost totally precipated and Mg has started to

precipitate. The zinc concentration will be below 10 mg/l after neutralization. Samples will

be taken and analyzed once/shift and correction in lime feed made if too much zinc is going

through.

The reactions will be

ZnSO4 + Ca(OH)2 + 2 H2O → Zn(OH)2 + CaSO4 .2H2O

MgSO4 + Ca(OH)2 + 2 H2O → Mg(OH)2 + CaSO4 .2 H2O

H2SO4 + Ca(OH)2 → 2 H2O + CaSO4 .2H2O

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The slurry from reactor is fed to drum under vaccum. The MgSO4 is filtered out and cake is

retained over the cloth which is re pulped with COF and charged in reactors through

pumpfrom where it is pumped to ETP.

Horizontal Belt FilterIn the section the jarosite and leach residue slurry is filtered and washed on horizontal

vacuum belt filters to maximize water soluble zinc recovery. This section comprises of the

following main equipment

Two vaccum belt filter units(one is stand by)

One cake slurry re pulping tank

The underflow slurry of door is pumped with pump to HBF over head tank. For proper

vaccum control each horizontal belt filter will be equipped with its own water ring type

vaccum pump system.

The jarosite cake is separated on polypropylene filter cloth repulped with ETP water and

pumped to ETP via HBF slurry tank. Speed of the belt input slurry flow to HBF and wash

water quantity is controlled by the concerned C/H in the HBF control room. Vacculm pipes

are connected beneath the mother belt and filter water is used for the vaccum sealing purpose.

Mother liquor (collected in the feed zone and drying zone)

PurificationThe neutral solution contains several impurities like copper, cadmium, cobalt and nickel.

Becide these major impurities also small amounts of arsenic and antimony can be analyzed in

the solution. Before the solution can be sent to the electrowinning these impurities habe to be

removed in the purification plant. The main reagent in the purification plant is the Zinc dust.

The basic reaction is that of cementation of those metals, whose positions in the

electromotive series for sulphates are below that of Zinc.

The purification of the neutral solution will be carried out in three steps.

1. Pre filtration (cold filtration) of neutral over flow

2. Hot purification for removing of copper, cadmium , cobalt and nickel as major

impurities

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3. Second step or polishing step to ensure top quality of purified solution.

In pre-filtration step suspended solids present in NOF tank are removed. Main impurities are

removed in hot purification. These process is based on antimony purification process. In this

process solution is passed through a spiral heat exchanger so that its temperature become 80-

82 C. This solution is passed through a reactor cascade and another reagent Zn dust is added.

To improve the reactivity of Zinc dust potassium antimony tartrate (PAT)is added. For

removing organic impurities charcoal solution is also fed. Reactor outlet is passed through a

cascade of filter press where impurities are removed as Cu-Cd Cake. Cu-Cd cake is sent to

the Cd plant for further purification. Filtrate is processed in polishing step. Here the solution

is again passed through Reactor and filter press cascade and remaining amount of Cd which

may be slipped during hot purification is also removed.

After polishing step solution is almost pure solution of zinc sulphate containing a small

amount of gypsum which is removed in gypsum removal plant.

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Fig.-10 Purification Plant

Cadmium plantFollowing are the main stages in the production of purified cadmium sulphate solution:

Enrichment of Cu-Cd cake

Enriched Cu-Cd cake leaching and Cu removal as Cu-cement

Cadmium cementation

Leaching and purification of Cd sponge

Iron and cobalt purification of solution before sending to zinc circuit

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Gypsum RemovalThe gypsum removal system will be necessary to remove gypsum from hot neutral solution

before it is taken to the cell house in order to reduce gypsum precipitation in electrolytic

cells, piping, launders and cooling towers. The neutral solution that is saturated with gypsum

will be cooled in three atmospheric cooling towers from 86°C to 34°C. This causes

crystallization of gypsum from the solution still remains saturated with gypsum. The gypsum

will be then removed in a thickener. Spent acid will be fed for pH adjustment and flocculent

(1 g/l aqueous solution) into the thickener. The thickener overflow will be taken to the

purified solution storage tanks. The underflow will be pumped to the jarosite step.

Prior to be directed to the cooling towers the pH of the solution will be adjusted to 4.9 to

prevent formation of basic zinc sulphates. The pH adjustment is made in the filtrate collecting

tank of the polishing purification step by adding a controlled amount of spent electrolyte.

The solution is fed to the top of the tank through nozzles. The falling solution meets the air

produced by the fan and solution will cool down when water is evaporated. The cooled

solution is collected on the bottom of the tower and leaves the tower trough an overflow box,

which is provided with temperature control and level detection. The overflow from the

thickener flows by gravity via the launder to the selected purified solution storage tank. In

this whole process, a part of gypsum, about 157 kg/h is removed from the solution.

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Electrolysis PlantZinc is extracted from the purified zinc sulfate solution by electrowinning, which is a

specialized form of electrolysis. The process works by passing an electric current through the

solution in a series of cells. This causes the zinc to deposits on the cathodes (aluminium

sheets) and oxygen to form at the anodes. Sulfuric acid is also formed in the process and

reused in the leaching process. Every 24 to 48 hours, each cell is shut down, the zinc-coated

cathodes are removed and rinsed, and the zinc is mechanically stripped from the aluminum

plates.

Electrolytic zinc smelters contain as many as several hundred cells. A portion of the electrical

energy is converted into heat, which increases the temperature of the electrolyte. Electrolytic

cells operate at temperature ranges from 30 to 35°C (86 to 95°F) and at atmospheric pressure.

A portion of the electrolyte is continuously circulated through the cooling towers both to cool

and concentrate the electrolyte through evaporation of water. The cooled and concentrated

electrolyte is then recycled to the cells. This process accounts for approximately 1/3 of all the

energy usage when smelting zinc

The electrolysis phase uses large amounts of electrical energy and is responsible for the high

proportion of the energy-cost in the overall smelting process (typically about one third of

total plant cash costs). Hence, cell house productivity (and electrical current and energy

efficiency in particular) is a crucial driver in overall plant efficiency. Debari runs some of the

industry’s largest and most efficient cell houses.

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MELTING AND CASTING

Cathode melting will be carried out in two identical electric induction furnaces. The furnaces

will have a guaranteed average melting rate of 22 tonnes per hour of zinc cathodes

(maximum ~ 24 tonnes per hour). The melting rate is infinitely variable between 0% and

100% of the maximum melting rate and is controlled by the automatic control system to

match the rate that molten metal is removed (pumped) from the furnace.

Each furnace is equipped with a still well equipped with one or more molten metal pumps.

The pump delivers molten zinc to a launder system feeding the casting machine. Each

furnace feeds a single casting line. In addition, provision is made to pump molten zinc from

one of the furnaces to the zinc dust production plant.

In addition to cathode bundles, the furnace chutes are designed to receive metallic zinc from

the dross separation plant and metallic zinc “skims” from the casting machines. This material

is fed to any chute (normally one dedicated chute) from forklift transported to hoppers that

have been raised to the charging floor by the freight elevator (lift). The required amount of

nh4cl to enhance the melting of this material is manually added to each hopper prior to

dumping in the charge chute.

When cathode zinc is melted, a layer of dross comprised mainly of zinc oxide entrained

molten zinc droplets is produced. This dross must be removed from the furnace once in every

24 hours by manually skimming the dross from the surface of the bath in a process called

drossing. This process consists of opening one of the doors on the side of the furnace,

manually spreading a few kg of NH4Cl onto the dross layer, manually agitating the dross

layer with a steel “rake” and finally using the “rake” to drag the dross through the open door

of the furnace into a forklift tote bin. During the drossing process, the furnace is operated

under conditions of increased ventilation to contain the fumes and dust that are generated by

the agitation and dross removal processes. The totes of furnace dross are transported by lift

truck to the dross cooling area, to await treatment in the dross separation plant.

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REFERENCES

“Plant Operating Manuals” Zinc Smelter Debari, Udaipur

McCabe, Smith, Harriott “Unit Operations of Mechanical Engineering” McGraw-Hill

International Editions.

http://www.hzlindia.com/index.aspx Official Website of Hindustan Zinc Limited

[Viewed on 10-06-2015]

http://www.vedantaresources.com/default.aspx Offical Website of Vedanta Resources

[Viewed on 10-06-2015]

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