Summer Training at Hindustan Zinc Limited Udaipur

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

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)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) TABLE OF CONTENTS

Title Page No.

Acknowledgements.............................................................................................ITraining Certificate.............................................................................................IIPreface................................................................................................................III1. Company Profile ...................................................................................011. Vedanta ...................................................................................................012. Hindustan Zinc Limited ..........................................................................013. Zinc Smelter Debari ................................................................................022. Zinc ................................................................................................................041. Introduction .............................................................................................042. Properties of Zinc ....................................................................................053. Zinc Smelting ..........................................................................................053. Zinc Smelter Debari .....................................................................................071. General Process Overview ......................................................................072. Raw Material Handling Section ..............................................................094. Roaster Plant ................................................................................................111. Roasting of Zinc Concentrate .................................................................112. Fluidized Bed Roaster .............................................................................143. Waste heat boiler ....................................................................................144. Cyclone ...................................................................................................165. Hot Gas Precipitator ...............................................................................165. Heat and Mass Balance Over Roaster Plant .............................................191. Mass Balance ..........................................................................................192. Heat Balance ...........................................................................................266. Gas Cleaning Plant ......................................................................................321. Quench Tower ........................................................................................322. Packed Gas Cooling Tower ....................................................................32

Title Page No.

3. Wet Gas Precipitator ...............................................................................334. Mercury Removal ...................................................................................347. Acid Plant .....................................................................................................35 Drying and Absorption Section ...............................................................35 Converter System ....................................................................................36

Basic Operations In Plant1. Drying Tower .........................................................................................362. SO2 Blower .............................................................................................363. Converter Group .....................................................................................374. Preheater .................................................................................................385. Intermediate Absorber Section ...............................................................396. Final Absorber Section ...........................................................................39Important Process Criteria1. Gas drying and Water balance ................................................................402. Water Balance .........................................................................................413. Absorption of SO3 ...................................................................................414. Energy (Heat) Balance ............................................................................425. O2 /SO2 Ratio ...........................................................................................438. Leaching Plant ..............................................................................................441. Neutral Leaching .....................................................................................442. Acid Leaching .........................................................................................473. Neutralisation ..........................................................................................474. Residual Treatment Plant ........................................................................485. Magnesium Removal ...............................................................................496. Horizontal Belt Filter ...............................................................................507. Purification ..............................................................................................51

Title Page No.

8. Cadmium Plant ........................................................................................529. Gypsum Removal.................................................................................... 539. Electrolysis Plant ..........................................................................................5410. Melting and Casting ...................................................................................5511. References ...................................................................................................56

TABLE OF FIGURES

Fig.-1 Plants at Zinc Smelter Debari .......................................................07Fig.-2 General Processes In Plant ............................................................08Fig.-3 Raw Material Handling Flow Sheet ..............................................10Fig.-4 Process Flow Sheet In Roaster Plant ............................................12Fig.-5 Calcine Balance Over Roaster Plant .............................................19Fig.-6 Process Diagram For Acid Plant ...................................................37Fig.-7 Process Diagram for Neutral Leaching ..........................................45Fig.-8 Process Diagram for Acid Leaching and Neutralization ................47Fig.-9 Process Diagram for Residual Treatment Plant ............................48Fig.-10 Purification Plant .........................................................................52

Company ProfileVedantaVedanta is an LSE-listed diversified FTSE 100 metals and mining company, and Indias 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 LimitedHindustan 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) Hydrometallurgical (Debari, Vizag and Chanderiya Smelters) ISP pyrometallurgical (Chanderiya Lead Zinc Smelter) and Ausmelt (Chanderiya Lead Smelter) process routes.

Zinc Smelter, Debari-Udaipur

Location14 km from Udaipur, Rajasthan, IndiaHydrometallurgical Zinc SmelterCommissioned in 1968 Roast Leach Electrowining Technology with Conversion Process Gone through a series of debottlenecking 88,000 tonnes per annum of ZincCaptive Power Generation29 MW DG Captive Power Plant commissioned in 2003CertificationsBEST4 Certified Integrated Systems ISO 9001:2000, ISO 14001:2004, OHSAS 18001:1999, SA 8000:2001Covered Area (Ha)22.65 Total Plant Area (Ha)126 Products Range(a) High Grade Zinc (HG)(25 kgs) & Jumbo (600 kgs)(b) CadmiumPencils (150 gms)(c) Sulphuric Acid+ 98% concentrationAwards & Recognitions(a) International Safety Award: 2006 by British Safety Council, UK(b) ROSPA Gold Award for prevention of accidentsOperating Capacity (Per Year) Zn : 80,000MT Acid : 130,000MT Cd : 250MT Zinc dust : 360MTWork force876 Nos.Managerial & Engineering Staff 84 Nos.Supervisory & Technical Staff58 Nos.Labour729 Nos.(a) Skilled154Nos.(b) Semi-Skilled555Nos.(c) Unskilled250Nos.Raw Material Supplies:-(a) Zawar Mines(b) Agucha Mines(c) Rajpura Dariba MinesProduct Buyers:- (a) Tata(b) Bhel(c) Steel CompaniesProcess Collaborators:- (a) Krebs Penorrova, FranceLeaching, Purification, Electrolysis(b) Lurgi, GMBH, and GermanyRoaster and gas clearing(c) Auto Kumpu FinlandRTP, Wartsila Plant(d) I.S.C., ALLOY, U.K.Zinc dust plant, Allen Power PlantIntroductionZinc 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 earths 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. Its 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 zincs 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 worlds largest importer of refined zinc.

Properties of Zinc (metallic) at 293K1. Density7140Kg./m32. Melting Point693K3. Specific Latent Heat of Fusion10 J/ Kg4. Specific heat capacity385 J/Kg/K5. Linear expansivity31/K6. Thermal conductivity111 W/m/k7. Electric Sensitivity5.9 ohm meter8. Temp. Coefficient of resistance40/k9. Tensile Strength150 Mpa10. Elongation50%11. Young modulus110 Gpa12. Passions Ratio0.25Zinc SmeltingZinc 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 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 DebariGeneral Process OverviewThe 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;

Fig.-2 General Processes In PlantStep 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.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.

Fig.-3 Raw Material Handling Flow SheetBlended 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 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 SO3CuS + 1.5O2 CuO + SO2The 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 ConcentrateDebari roastingtechnology 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.

Fig.-4 Process Flow Sheet In Roaster PlantThe 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 800C to 975C 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 worlds 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 350C 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.

Debariis 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 PlantFluidized-Bed RoasterIn 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 BoilerThe 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 350C 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 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.

CycloneThe 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 precipitatorThe 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. 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

Heat and Mass Balance Over Roaster Plant III

Mass BalanceConcentrate feed= 39.75t/hr

feed in kg/hr= 39750kg/hr

Moisture content= 10 %

Relative humidity= 0.111

Dry feed=39749.89kg/hr

Concentrate composition

Component%Kg/hr

Zn5220669.94

Fe8.53378.74

Lead1.5596.25

Copper0.139.75

Suphur3011924.97

C0.9357.749

Cd0.1663.60

SiO22795.00

Insolubles1397.50

Fig.-5 Calcine Balance Over Roaster Plant1. Reactions of zinc:Zn+SZnS

65.432kg/hr97.4

20669.9410113.7330783.68

ZnS+1.5 O2 ZnO+SO2

97.44881.4kg/hr64

30783.6815170.6025726.8120227.47

2. Reactions of lead:Pb+S PbS

207.232kg/hr239.2

596.2592.08688.33

PbS+1.5 O2

PbO+SO2

239.248223.2kg/hr64

688.33138.13642.29184.17

3. Reactions of copper:Cu+SCuS

63.532kg/hr95.5

39.7520.0359.78

CuS+1.5 O2

CuO+SO2

95.504879.5kg/hr64

59.7830.0549.7740.06

4. Reactions of iron:Fe+SFeS

Fe+S2FeS2

Amount of Fe present in FeS75 % =2534.06Kg

Amount of Fe present in FeS225 % =844.69Kg

Fe+SFeS

563288

2534.063982.09

2 FeS+3.5 O2Fe2O3+2 SO2

176112160128

3982.092534.063620.082896.06

Fe+S2FeS2

5664120

844.69965.351810.04

2 FeS2+5.5 O2Fe2O3+4 SO2

240176160256

1810.041327.361206.691930.71

5. Reactions of cadmium:Cd+SCdS

11232144

63.6018.1781.77

CdS+O2CdO+SO2

144.003212864

81.7718.1772.6936.34

6. For Silica:SiO2SiO2

795.00795.00

7. For Carbon:C+O2CO2

123244

Feed and Product rate for different components:

ConcentrateFeed rateOxidesWt of oxidesOxygen req.SO2

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

ZnS30783.68ZnO25726.8088215170.599820227.4664

PbS688.33PbO642.2906757138.13184.17

FeS3982.09Fe2O33620.082534.062896.06

FeS21810.04Fe2O31206.691327.361930.71

CuS59.78CuO49.7730.0540.06

SiO2795.00SiO2795.0000

C357.75CO244.00320

CdS81.77CdO72.6918.1736.34

Insolubles397.50397.500.000.00

Total38955.9332510.8219250.3625314.81

Oxygen Required for 38.956 tonnes of concentrate = 19250.36 kg/hrOxygen Required for 1 tonnes of concentrate = 494.16 kg/hr =346.13 m3/hrOxygen Required for 39.75 tonnes of concentrate = 13758.49 m3/hrAir Required for 39.75 tonnes of concentrate = 65516.60 m3/hrExcess air =25 % =16379.15 m3/hrCO2 =82.1846667 m3/hrSO2= 8865.72238 m3/hrClean Air escaping from top =68137.26 m3/hr

Total Air at furnace inlet = 81895.75m3/hr

Gases escaping from furnace top =77085.17m3/hr

Calcine from top of the furnace =75 %

=24383.11kg/hr

Calcine from bottom of the furnace =25 %

=8127.70kg/hr

Dust removing capacity of boiler = 45 %

Dust removing capacity of Cyclone separator =40 %

Dust removig Capacity of HGP =15 %

Intletoutlet Underflow

Boiler24.38 t/hr13.41 t/hr10.97 t/hr

Cyclone separator13.41 t/hr3.66 t/hr9.75 t/hr

HGP3.66 t/hr0.01829 t/hr3.64 t/hr

\

Heat BalanceReactions involved :

Hf(KJ/mol)

ZnS+1.5 O2ZnO+SO2

Enthalpy-204.60-348-296.84KJ/mol-440.24

PbS+1.5 O2PbO+SO2

-98.120-218.08-296.84KJ/mol-416.8

CuS+1.5 O2CuO+SO2

-48.50-155.2-296.84KJ/mol-403.54

2 FeS+3.5 O2Fe2O3+2 SO2

-2010-825.5-593.68KJ/mol-1218.18

2 FeS2+5.5 O2Fe2O3+4 SO2

-3550-825.5-1187.36KJ/mol-1657.86

CdS+O2CdO+SO2

-208.40-235.6-296.84KJ/mol-324.04

C+O2CO2

00-393.5KJ/mol

Total Heat of Above reactions = - 4460.66 KJ/mol

Heat Balance for Furnace:Temp.of calcine at furnace inlet = 25 CTemp.of calcine at furnace outlet = 920 C t = 895 C

Calcine flyover

M = 24383.11 kg/hrQ = 21248538.45 kj/hr = 21248.53845 MJ/hr

Air = 68137.26 m3/hrM = 88158.32 kg/hrCp = 1 kj/kg-kQ = 102968915.5 kj/hr = 102968.9155 MJ/hr

SO2

M = 25314.81 kg/hr Cp = 0.645 kj/kg-kQ = 19071167.99 kj/hr = 19071.16799 MJ/hr

Calcine uderflow

M = 8127.70 kg/hrCp = 0.75 kJ/kg-kQ = 7082846.15 KJ/hr = 7082.84615 MJ/hr

CO2

M = 44.00 Kg/hrCp = 1.22 KJ/Kg-KQ = 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 heat output = 159015.486 MJ/hrHeat Input = 4460.66 KJ/mol

Total moles = 350.29 kmol/hrQ input = 1562537.85 MJ/hrQo/p = 159015.49 MJ/hrQ left = 1403522.37 MJ/hr

For cooling coils

Q =m*L

latent heat of vaporisation, L=2.42MJ/Kg

m =Q/L

M=17399.04Kg/hr

=17.40t/hr

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

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

h=4293.14kJ/kg

Q=mh

M=Q/h

=9.80766316t/hr

Heat Taken by cooling coil=3%

Q=h*A*tH=0.828MJ/hr-m2-C

A=54.98t=925C

Heat retained=97%

Heat Balance Over Waste Heat Re-Boiler:Temp.of calcine at boiler outlet = 350 CTemp.of calcine at boiler inlet = 920 Ct = 570 CCalcine underflow temp. in boiler = 900 Ct = 20 C

Calcine flyover

M=13410.71Kg/hr

Q=8434832.922Kj/hr

=8434.832922Mj/hr

Air=68137.26 m3/hr

M=88158.32kg/hr

Cp=1kj/kg-k

Q=74317462.16kj/hr

=74317.46216MJ/hr

SO2

M=25314.81 kg/hr

Cp=0.645 kj/kg-k

Q=13764550.18 kj/hr

=13764.55018MJ/hr

Calcine uderflow

M = 10972.40 kg/hrCp = 0.75 kJ/kg-kQ = 2398647.08 KJ/hr = 2398.64708MJ/hr

CO2

M = 44.00 Kg/hrCp = 1.22 KJ/Kg-KQ = 45252.24 KJ/hr = 45.25224 MJ/hr

Water evap.

Q = ml = 8581320 Kj/hr = 8581.32 MJ/hr

Radiation Loss=3%=0 Mj

Heat leaving Boiler

Q entering Boiler=1513349.34MJ/hr

Total Q leaving from top=105143.418MJ/hr

Heat leaving from boiler bottm=2398.64708MJ/hr

Q1 retaining inside boiler=1405807.27MJ/hr

Air = 68137.26 m3/hrM = 88158.32 kg/hrCp = 1 kj/kg-kQ = 28475136.75 kj/hr = 28475.13675 MJ/hr

Heat Balance Over Cyclone Separator:Temp.of calcine at Cyclone outlet = 300 CTemp.of calcine at Cyclone inlet = 350 Ct = 50 C

Calcine flyover

M = 3657.47 kg/hrQ = 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

Calcine uderflow

M = 9753.25 kg/hrCp = 0.75 kJ/kg-kQ = 2350437.64 KJ/hr = 2350.43764 MJ/hr

CO2

M = 44.00 Kg/hrCp = 1.22 KJ/Kg-KQ = 17338.64 KJ/hr = 17.33864 MJ/hr

water evap.

Q = ml = 8581320 Kj/hr = 8581.32 MJ/hr

Radiation Loss3%0 MJ/hr

Heat Entering at cyclone inlet =105143.418MJ/hrHeat leaving from Cyclone top =MJ/hrHeat leaving at cyclone bottm =2350.43764MJ/hrHeat Entering HGP =102792.98MJ/hr

Heat Balance Over Hot Gas Precipitator:Temp.of calcine at HGP outlet = 290CTemp.of calcine at HGP inlet = 300Ct = 10C

Calcine flyover

M = 18.29kg/hrQ = 3861.303327kj/hr = 3.861303327MJ/hr

Air = 68137.26 m3/hrM = 88158.32 kg/hrCp = 1 kj/kg-kQ = 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/hr

Calcine uderflow

M = 3639.18 kg/hrCp = 0.75 kJ/kg-kQ = 768399.362 KJ/hr = 768.399362 MJ/hr

Water evap.

Q = ml = 8581320 Kj/hr = 8581.32 MJ/hr

CO2

M = 44.00 Kg/hrCp = 1.22 KJ/Kg-KQ = 15191.44 KJ/hr = 15.19144 MJ/hr

Radiation Loss = 3% = 0 MJ/hr

Heat Entering HGP 102792.98 MJ/hrHeat Leaving at HGP bottom 768.399362 MJ/hrHeat 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: > 300C) 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 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 ESPs 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 ESPs, 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 ESPs 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-liningThe 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.

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. 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 PlantDrying towerColumn packed with ceramic packing to facilitate contact of SO2 Bearing gases with dilute sulphuric acidGases 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.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 mixerThe 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.

Fig.-6 Process Diagram For Acid PlantConverter 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 255C. 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 255C 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. The gas exits this heat exchanger at a temperature of approx. 400C 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. 425C 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. 433C 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. 400C before entering the fourth catalyst bed The gas leaving the fourth bed at a temperature of approx. 410C, 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. 180C 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 SO2- concentrations can be compensated. The separate preheater preheats air or, in start-up phases, SO2-gas to the required temperatures.

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. 400C. As the absorption of SO3 generates heat, the circulating acid at a temperature of 87C 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. 70C 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 180C 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 85C 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. 75C 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 clients 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 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. 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 180C 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. 38C 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 396C. 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 + O2 SO3The sulphur trioxide absorption and sulphuric acid formation: SO3+ H2O H2SO4 H2SO4 + SO3 H2S2O7H2S2O7 + 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 atmosphereO2 /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.

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.

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 LeachingCalcine 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 +H2OCuO+H2SO4 CuSO4 +H2OCdO+H2SO4 CdSO4 +H2OPbO+H2SO4 PbSO4 +H2OIn addition to the reactions above , MnO2 reacts in the first leaching pachukas2 FeSO4 + 2 H2SO4 + MnO2 Fe2 (SO4)3 +MnSO4 + 2 H2OWhere as in the last leaching pachukas oxygen is reacting and iron hydroxide is precipitated2 FeSO4 + 2 H2SO4 + 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

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+H2SO4ZnSO4 +H2OMeO+ H2SO4 MeSO4 +H2O (Me = Cu, Cd etc.)

Fig.-8 Process Diagram for Acid Leaching and NeutralizationNeutralization: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.

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 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 reactorsThe 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 beZnSO4 + Ca(OH)2 + 2 H2O Zn(OH)2 + CaSO4 .2H2OMgSO4 + Ca(OH)2 + 2 H2O Mg(OH)2 + CaSO4 .2 H2OH2SO4 + Ca(OH)2 2 H2O + CaSO4 .2H2OThe 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 flow2. Hot purification for removing of copper, cadmium , cobalt and nickel as major impurities3. 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.

Fig.-10 Purification PlantCadmium 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

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 86C to 34C. 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 157kg/h is removed from the solution.

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 35C (86 to 95F) 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 industrys largest and most efficient cell houses.

Melting and CastingCathode 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~24tonnes 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.

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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