Hindustan Zinc Limited Training Report

HZL Training Report

Summer Training ProjectSummer Training Project onon

STUDY OF ZINC PLANTSTUDY OF ZINC PLANT

Training PeriodTraining Period ::16-05-2011 to 15-07-201116-05-2011 to 15-07-2011

Submitted To: Submitted By: Dr. Madhu Agarwal Gajanand Pilaniya

2008UCH123

Department of Chemical Engineering

Malaviya National Institute of Technology, Jaipur

Department Of Chemical Engineering, MNIT Jaipur 1

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ACKNOWLEDGEMENT

I wish to acknowledge the encouragement received from Mr. S.K. JANA (HOD, Chemical engineering department, MNIT, JAIPUR) & Mr. Rohit Goyal (Training incharge) for initiating my interest in training.

I earnestly acknowledge my profound sense of gratitude to Mr. S.P. Sharma

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.

Gajanand Pilaniya


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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 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 a 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 technology.

Gajanand Pilaniya


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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

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


TABLE OF CONTENTS

Title Page

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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

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


TABLE OF FIGURES

Title Page

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Company Profile

Vedanta

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

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)


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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

(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


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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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Zinc (Zn)

Introduction

Zinc 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


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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.

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


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(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 similar to electrolytic refining. The

process most commonly used for low-grade deposits is heap leaching. Imperial

smelting is also used for zinc ores.


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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;


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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.

Roaster Plant

Roasting 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


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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.


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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.

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.


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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. 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


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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.

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


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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.

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


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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/hr

feed 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

39.75 20.03 59.78

CuS + 1.5 O2 CuO + SO2


kg/hr

kg/hr

kg/hr

kg/hr

kg/hr

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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 Kg

Amount 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

1810.04 1327.36 1206.69 1930.71


kg/hr

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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

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


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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 Balance

Reactions 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

-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


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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


Air = 68137.26 m3/hr

M = 88158.32 kg/hr

Cp = 1 kj/kg-k

Q = 102968915.5 kj/hr

= 102968.9155 MJ/hr

Calcine uderflow

M = 8127.70 kg/hr

Cp = 0.75 kJ/kg-k

Q = 7082846.15 KJ/hr

= 7082.84615 MJ/hr

CO2

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

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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.

40

t/hr

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

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


Total moles = 350.29 kmol/hr

Q input = 1562537.85 MJ/hr

Qo/p = 159015.49 MJ/hr

Q left = 1403522.37 MJ/hr

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∆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

= 8434.832922 Mj/hr

Air = 68137.26 m3/hr

M = 88158.32 kg/hr


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Cp = 1 kj/kg-k

Q = 74317462.16 kj/hr

= 74317.46216 MJ/hr

SO2

M = 25314.81 kg/hr

Cp = 0.645 kj/kg-k

Q = 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

= 8581.32 MJ/hr

Heat leaving Boiler


CO2

M = 44.00 Kg/hr

Cp = 1.22 KJ/Kg-K

Q = 45252.24 KJ/hr

= 45.25224 MJ/hr

Radiation Loss

=3%

=0 Mj

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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


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

HZL 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


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

HZL 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 Plant

Gases 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 Tower

In 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


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of the spray does evaporate, but the biggest portion will be collected in the

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 tower

The 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


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distribution system will be made of FRP. Plastic pumps (material: HP PE) will

be used. The pipes will be made of FRP.

Wet gas precipitator

Final 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, 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 Removal

For the production of sulphuric acid with a low mercury-content, so-called

Norzinc Mercury Removal Process (Calomel Process) is in foreseen. This


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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.

Acid Plant

Plant Description

The 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.

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


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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 system

The 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 tower

Column 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 removal of moisture from the gases heat is liberated which

increases the temperature of the circulating acid.


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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 Blower

Blower 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.


HZL Training Report

Fig.-6 Process Diagram For Acid Plant

Converter Group

After 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


HZL Training Report

entering the first bed of the converter the gas will pass first through the shell

side of that heat exchanger. 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 SO 3

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 SO2 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.

Preheater

The 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


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released within the process allows autothermal operation of the plant. In

addition lower or varying SO2- concentrations can be compensated. The

separate preheater preheats air or, in start-up phases, SO2-gas to the required

temperatures.

Intermediate Absorber Section

After 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 Section

The 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


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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.

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 balance

Gas 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.


HZL Training Report

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 Balance

The 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. 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


HZL Training Report

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.

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


HZL Training Report

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.


HZL Training Report

Leaching Plant

Leaching 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,

Calcine conveying

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

bucket elevator. This calcine is transferred to calcine hopper. When calcine


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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 leaching

The 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

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.


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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 Leaching

The 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

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

with an overflow launder. The solution flows by gravity from the first


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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 Removal

The 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.


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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

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 Filter

In 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)

Purification


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The 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

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 plant

Following 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

Gypsum Removal


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The 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.

Electrolysis Plant


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Zinc 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


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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.

References

“Plant Operating Manuals” Zinc Smelter Debari, Udaipur

McCabe, Smith, Harriott “Unit Operations of Chemical Engineering”

McGraw-Hill International Editions.

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

Limited [Viewed on 10-07-2011]

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

Vedanta Resources [Viewed on 10-07-2011]


Documents

Hindustan Zinc Limited Training Report