Hindustan zinc limited leaching Project

DR.B.R.AMBEDKAR NATIONAL INSTITUTE OF TECHNOLOGY, JALANDHAR

Summer Training Report on:

Total Zinc Loss in Jarosite Cake

At

HINDUSTAN ZINC LIMITED, DEBARI, UDAIPUR

SUBMITTED BY:

ANKUR KULDEEP(12112012)

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ACKNOWLEDGEMENT

I take great pleasure in thanking Hindustan Zinc Limited, Debari Zinc Smelter for providing me with an opportunity to do my industrial training. In this esteemed organization, I express my gratefulness to Mr. Mahesh Todkar, Unit Head of Debari Zinc Smelter. The immediate gratitude goes to Mr. Nikhil Sharma, Head of HR Department, Officer HR for providing the necessary arrangements required during the training. I express my heartfelt thanks to department heads of Safety Department, Roasting Department, Leaching & Purification Department, Electrolysis Department, Mechanical Department and Electrical Department for providing the introduction of each and every department.

I express my heartfelt thanks to Mr. A. K. Singh, The Plant Manager of Leaching Department, Mr. Vijay Rana, Associate Manager –Leaching Department for assigning me the right project and all the necessary information and guidance to work with it. I am also grateful to Mr. DebashishSahuand Mr. Shashi Prakash Singh, Engineer (Leaching Department) for providing me with valuable information during the project completion. I take honor and pride in thanking each and every staff member for being friendly and co-operating with me in my project activities.

Finally, I wish to add that I am indebted to god &my parents foreverything good that has happened to me.

ANKUR KULDEEP

12112012

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PREFACE

Practical training is a way to implement theoretical knowledge topractical use to become a successful engineer. It is necessary to havea sound practical knowledge because it is only way by which one canPractical training is a way to implement theoretical knowledge topractical use to become a successful engineer. It is necessary to havea 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 theproduction process & productivity in particular of Zinc. This report is an attempt made to study the overall productionsystem & related action of Zinc Smelter, Debari a unit a HZL. It isengaged in production of high grade zinc metal & other by productsviz. Cd, sulphuric acid etc. since 1968 by adopting HydroMetallurgical technology.

ANKUR KULDEEP

12112012

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ABSTRACT

In this report firstly, the introduction of Hindustan Zinc Limited and Debari Zinc Smelter is given. The second section of this report includes the introduction of every department of this plant. Detailed information about the leaching department of this unit is given in this report. In the last section of this report project details are given. This project includes the continuously monitoring of some parameter of RTP (Residual Treatment Plant) and HBF (Horizontal Belt Filter) sections of leaching. To reduce the total zinc losses in jarosite cake some graphs are drawn between these noted parameters and zinc losses. Conclusions are drawn from these graphs about the ranges of running parameters in which the zinc losses in jarosite cake will be minimum. These parameter ranges of the running conditions can be used as reference in the operation of these sections of the leaching.

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TABLE OF CONTENTS

TOPIC PAGE NO.

Company profile 6

Introduction to Zinc 10

Plant induction program 13

Safety department 13

Roaster and Acid plant 15

Leaching and Purification department 17 1. Neutral leaching 21 2. Acid leaching 23 3. RTP & HBF 25 4. Purification 29

Electrolysis department 31

Melting & Casting 33

Project details 35 o Introduction to the project 35 o Noted down parameters of RTP section 37 o Noted down parameters of HBF section 40 o Conclusions and recommendation

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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, aluminum 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. Vision: Be the world’s largest and most admired Zn-Pb and Ag company. Mission:

Enhance stakeholder value through exploration, innovation, operational excellence and sustainability.

Be a globally lowest cost Zn producer.

Maintain market leadership and customer delights. 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

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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.

Operations:

Mining: HZL operates the world’s 3rd largest open-pit mines. World’s largest Zinc mines in RampuraAgucha, Rajasthan. World’s lowest cost Zn producer.

Smelting: operates Zn &Pb smelters and refineries at Chanderiya(Chittorgarh), Debari(Udaipur) and Dariba(Rajsamand).

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`ZINC SMELTER, DEBARI-UDAIPUR

Location ==14 km from Udaipur, Rajasthan, India Hydrometallurgical Zinc Smelter== Commissioned in 1968 Roast Leach Electro wining Technology with conversion process gone through aeries of debottlenecking 88,000 tons per annum of Zinc. Captive Power Generation==29 MW DG Captive Power Plant Commissioned in 2003. Certifications==BEST4 Certified Integrated Systems ISO9001:2000, ISO 14001:2004, OHSAS18001:1999, 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% concentrated

India Technology: Roast Leach Electro-winning (RLE) Technology

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: 88,000 MT Acid: 130,000 MT

Cd: 250 MT

Zinc dust: 360 MT

Raw Material Supplies:

(a) Zawar Mines (b) Agucha Mines (c) RajpuraDariba Mines

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(d) 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 cleaning (c) Auto Kumpa Finland – RTP ,Wartsila plant (d) I.S.C.,ALLOY,U.K.—Zinc dust plant, Allen power plant

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ZINC Introduction:

Properties of Zinc (metallic) at 293K:

Density ==7140 Kg/m3

Melting point == 693K

Specific latent heat of fusion == 10 J/Kg

Specific heat capacity == 385 J/Kg/K

Linear expansivity == 31/K

Thermal conductivity == 111W/m/K

Electric sensitivity == 5.9 ohm-meter

Temp. coefficient of resistance == 40/K

Tensile strength == 150MPa

Elongation == 50%

Young’s modulus == 110 GPa

Poisson’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 obtainedby 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. The vast majority of zinc smelting plants in the western world use the electrolytic process also called the Roast-Leach-Electro winning (RLE) process. Application of Zinc: About 13 million tons of zinc is produced annually worldwide. Around 58% of the amount is used for galvanizing to protect steel from corrosion. Approximately 14% is used the production of zinc die casting alloys. Nearly 9% of the zinc is also utilized for oxides and chemicals and about 10% is used in Brass semis and castings.

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Galvanizing:

Zinc is one of the best forms of protection against corrosion and is used extensively in building, construction, infrastructure, household appliances, automobiles, steel furniture, and more. Galvanizing accounts for around 58% of global zinc usage.

Zinc Oxide:

The most widely used zinc compound, zinc oxide is used in the vulcanization of rubber, as well as in ceramics, paints, animal feed, pharmaceuticals, and several other products and processes. A special grade of zinc oxide has long been used in photocopiers. 9% of global zinc usage is in this segment.

Die Castings:

Zinc is an ideal material for die casting and is extensively used in hardware, electrical equipments, automotive and electronic components. 14% of zinc used in the word is through Die Castings.

Alloys:

Zinc is extensively used in making alloys, especially brass, which is an alloy of copper and zinc. Alloy accounts for around 10% of global zinc usage.

Rolled Zinc:

Zinc sheets are used extensively in the building industry for roofing, flashing and weathering applications. These are also used in graphic art to make plates and blocks, as well as battery callouts and coinage.

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ZINC SMELTER DEBARI HAS FOLLOWING MAIN PLANTS:

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.

Figure 1: Zinc Smelter, Debari

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.

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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.

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.

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.

PLANT INDUCTION PROGRAME:

Plant induction program includes the introduction of the whole plant includes the visit of the safety department, roaster and acid department, leaching and purification department, zinc electrolysis department, mechanical department, electrical department, instrumentation department, maintenance department.

SAFETY DEPARTMENT:

Various safety instructions were given to us on the first day of training, which was supported by video of the plant.

The following instructions are given:

Personal safety is must.

Shoes and helmet are must in the plant.

No person less than 18 years of age is allowed in the plant.

Distance must be maintained from the conveyor belt and loose clothes should not wear.

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In the roaster and acid plant H2S gas is converted to sulphuric acid. In case of leakage of gas move against the wind and cover mouth with wet cloth.

While going near to these acid storage tanks, acid proof suits and goggles must be used.

Eye and body washers are provided at different places of the plant.

Mobile phones and watches are prohibited in electrolysis unit area.

Zinc ingots are formed which are around 400oC, cooled by air cooling for around 8 hours, so it has not to be touched.

Safety score card and physical condition tour audit have been implemented to assess the safety performance. Safety score card is a point based system which mainly assess lead indicators like safety training, near misses reporting, personal protective equipments(PPE) compliance, analysis mechanism etc., to monitor the safety status of the unit on monthly basis.

Physical condition tour is a field audit giving thrust to substandard condition in areas of general work place condition, facilities, material handling, equipment health, hazard control, emergency system and PPE compliance.

Foe continuous improvement in our safety performance, we conduct internal and external safety audits that assess the functionality, adequacy and adherence of safety management system across all our operations, to further improve and make a benchmark performance year after year. The journey of continuous improvement continues.

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ROASTER AND ACID 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 the process in which sulfide ores are converted to oxides, prior to smelting. Roasting differs from calcinations, which merely involves decomposition at elevated temperatures.

At this unit, three roasters are used for roasting of zinc sulphide to calcine. Roasters are mainly furnaces which are maintained at temperature of around 950oC. These furnaces are fluidized type in which fluidized bed is made and air is passed from bottom for oxidation. These furnaces are autogenously, so only to start the reaction or to obtain required temperature, fuel is required. A typical sulfide roasting chemical reaction takes thefollowing form:

S + O2 SO2

2ZnS + 3O2 2ZnO

SO2 + O2 SO3

CuS + 1.5 O2 CuO + SO2

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PROCESS FLOW DIAGRAM FOR ROASTER PLANT:

Figure 2:Roaster Plant

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ACID PLANT

Gases coming out of the hot gas precipitator in roaster unit have 7%-8% SO2 and at 330oC. In this section SO2 gas at temperature of around 330oC is passed through scrubbing tower, which has sedimentation tank and SO2 stripper and wet gases precipitator, the gas obtained at temperature of 55oC. SO2 gas has the moisture. The SO2 gas is first dried in a drying tower with H2SO4.

LEACHING AND PURIFICATION SECTION

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. All process activities related with Neutral leaching, Weak acid Leaching, Jarosite precipitation, Horizontal Belt Filter, Magnesium removal and Jarosite Operations (Effluent Treatment Plant).

Products Major Parameters to be Controlled Range

NOF Zinc (gpl) 130-150

pH 4.8-5.1

Fe+2 (ppm) <= 5

Ni (ppm) <10

Under flow of 07 thickener T/Zn in jarosite < 3.0 %

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Definitions:

Calcine Bin: Intermediate Storage Bin for storing Calcine

NL: Neutral Leaching

Flocculent: Chemical used to enhance solid liquid separation

MnO2 Slurry: Slurry obtained after anode cleaning in Cell House

Fe+2: Ferrous ion

Fe+3: Ferric ion

AL: Acid leaching

RTP: Residue Treatment Plant

HBF: Horizontal Belt Filter

U/F: Underflow of thickener

O/F: Overflow of Thickener

PPE: Personal Protection Equipment

RC: Ready for calcine

Pachuka: The tank/reactor in which solution and slurries are taken to carry out the reaction and it is equipped with motor and agitator system.

pH Analyzer: This is the online pH measuring instrument which measures the reduction / oxidation potential in terms of redox value.

Classifier: It’s a rubber-lined screw, mounted inside a semi cylindrical horizontal vessel & main screw having adjustable shaft angle.

Thickener: It’s a cylindrical tank with conical bottom provided with four centrally

Mounted rake arms (2 big & 2 small) to push the solid settled, to the center towards the bottom cone, provided with rake lifting arrangement of 600 / 900 cubic meter capacity.

Diaphragm Pumps: These are the motor driven pumps to draw underflow of door thickeners.

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ODS Pumps: These are the compressed air driven diaphragm pumps to draw underflow from door thickeners.

AOF Tank: Acid leaching Overflow tank

COF Tank: Conversion Overflow tank

Figure 3: Types of Leaching

LEACHING

NEUTRAL

LEACHING

ACID

LEACHING

RTP

PURIFICATION

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CALCINE

SPENT

O/F

U/F

O/F

U/F O/F

CELL HOUSE U/F

O/F

RECYCLE U/F

ETP (EFFLUENT TREATMENT PLANT)

Figure 4: General Process Flow Diagram-Leaching

NL1: NEUTRAL LEACHING NL2: ACID LEACHING PN: NEUTRALIZATION SECTION

PUR: PURIFICATION SECTION HBF: HORIZONTAL BELT FILTER

NL1

NL2

RTP

HBF

PUR

PN

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NEUTRAL LEACHING:

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 the following main process equipment:

• One 35 MT capacity calcine hopper

• Seven 680 MT calcine storage silos

• Three screw conveyors, and five reddler conveyors

• One Classifier and ball mill

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

• Two 16 m diameter thickeners

• Two 70 m³ thickener overflow tanks.

Calcine conveying: Calcine from roaster plant 1 & 2 is taken through bucket elevator no. 10, and No. 15/16 (max capacity 20 ton each), respectively. In case of breakdown of any of the two bucket elevators, calcine from both the roasters can be taken through single bucket elevator. When calcine hopper is full which is shown by high-level indication in DCS control room, calcine is diverted to storage silos(0 to 6) through reddler conveyors(11 & 15/21) to any of the seven silos depending on level of calcine present in each of them. In case of shortage calcine is withdrawn through reddler / retake conveyors (9 & 15/23) from any of the seven silos depending upon level of calcine.

NEUTRAL LEACHING:

The ready for calcine (RC) and remaining leaching pachukas are all covered and equipped with agitators, and stacks. Pachukas (N-3 to N-7) are equipped with injectors for oxygen gas. Except RC tank 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

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classifier without pumps. Also, the launder system enables the bypass of any single Pachuka.

Figure 5: Neutral Leaching

Neutral leaching input consists of calcine, spent electrolyte from cell house with approx 180 gpl free H2SO4, ball mill slurry, conversion and acid overflows, oxidizing agent MnO2.The pH in the first leaching pachuka is 1.8 to 2.0 which goes to pH 4.7 to 4.8 in the last leaching pachuka so as to take formation of iron hydroxide from ferric ions and to form complex compounds.

The basic chemical reactions in the neutral leaching process are:

ZnO + H2SO4 ZnSO4+ H2O

CuO + H2SO4 CuSO4 +H2O

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CdO + H2SO4 CdSO4 + H2O

PbO + H2SO4 PbSO4 + H2O

In addition to the reactions above, MnO2 reacts in the first leaching Pachuca's

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 precipitating

2 FeSO4 +2 H2SO4 + ½ O2 +ZnO Fe2(SO4)3 +Zn SO4 +2 H2O

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

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

Analysis of Mn, Mg, F-, Zn, Co, Ni, Cu, Cd, As, Ge, Sb & Fe++ are done in the control lab.

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 %. All leaching tanks are covered and

equipped with agitators, indirect steam heating elements and stacks. The steam

heating elements are to maintain a temperature of about 80°C in the leaching

tanks. The four leaching pachukas are arranged in a cascade and are

interconnected by an overflow launder, which enables the bypass of any single

tank. The solution, fed into the first tank, flows by gravity through all the leaching

tanks, the thickener and the overflow tank.

Underflow slurry from neutral thickeners (01 & 02) and neutralization thickener

(04) is withdrawn with help of diaphragm pumps and ODS pumps. Underflow is

discharged to acid leaching launder, mixed with spent electrolyte and it flows to

A4 pachuka. A4 pachuka outlet pH is maintained at 1.5 to 2.5.

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After solid liquid separation the solid is separated by rake movement and is

withdrawn by diaphragm pumps. 03 thickener underflow goes to RTP and

overflow collected in a tank called AOF tank and sent to Neutralization.

ZnO + H2SO4 ZnSO4 +H2O

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

NEUTRALIZATION:

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

the over flows from Acid leaching thickener and Jarosite precipitation thickeners

with calcine before sending it to Neutral leaching.

A1 pachuka outlet pH is so maintained such that A3 pachuka outlet pH of 3.7 to

4.0 is achieved. The slurry then flows to 200 m3 rector and is then pumped to

screw classifier. The pH of 200 m3 outlet is measured at regular interval and is

kept 4.7 to 4.9. The coarse particles are separated out and go to the ball mill and

the solution along with fine solids goes to 06 thickener where flocculent diluted in

water is added for solid liquid separation. Clear neutralization overflow is

collected in 223 tank and is pumped to neutral leaching.

The main reactions in Neutralization step are:

ZnO + H2SO4 ZnSO4 + H2O MeO + H2SO4 MeSO4 + H2O (Me = Cu, Cd, Pb...) Fe2(SO4)3 + 3 ZnO + 3 H2O 2 Fe(OH)3+3 ZnSO4 4 Fe(OH)3 + H-O-Me-complex Fe-O-H-Me-complex (Me = Sb/As/Al/Ge)

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Figure 6: Acid and Neutralization Section

RESIDUAL TREATEMENT PLANT: Objective of Residue Treatment section is to break ferrite Zinc and precipitation of Jarosite

using Ammonium or Sodium Sulphate, Sulphuric acid and high temperature to remove

impurities and recovering Zinc.

Products Major Parameters to be Controlled

Range & Alarm levels

COF Acidity 15-25 gpl

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(Conversion over flow)

Fe+2 0.5-2.5gpl

Fe+3 1.0-2.5 gpl

Thickener Underflow T/Zn in jarosite < 2.7 %

The conversion process main function is to simultaneously leach the zinc ferrite and precipitate the iron as Ammonium or sodium jarosite. The conversion process comprises the following main equipment:

Five 300 m³ conversion reactors (R-26, R-27, R-28, R-29 and R-30)

Two 18 m diameter thickeners

Two 70 m³ thickener overflow tank

One 20 m³ condensate tank

Two (NH4)2SO4 / Na2SO4 preparation tank

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

with condensate water is added at R-26 outlet for formation of Jarosite. The separation

thickener 08 overflow is collected in the over flow tank COF (Tailing as storage), from where it is

then pumped to the beginning of the neutralization. Further, the thickener 07 underflow is

pumped directly from the thickener underflow cone is pumped with thickener underflow

pumps directly to the feeding boxes of the horizontal vacuum belt filter in operation. The

addition of concentrated acid is done from the conc. acid tank. The condensates from the

conversion reactors and the heat exchangers of the hot purification plant are collected into the

condensate tank. The hot condensate collected in this tank will be used mainly as wash water

on the jarosite vacuum belt filters to reduce the zinc content in the final jarosite cake moisture.

The formation of jarosite will occur as shown in the following reaction-

3 Fe2(SO4)3 + 12 H2O + Na2SO42 Na[Fe3(SO4)2(OH)6] + 6 H2SO4

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Figure 7: RTP and HBF section

Parameters to be maintained are:

Temperature of reactors 90-100oC

Acidity in R-26 reactor =30-35gpl

Acidity in conversion overflow=15-25gpl

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HORIZONTAL BELT FILTER:

Figure 8: Horizontal Belt Filter

In this 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 vacuum belt filter units (one is stand by)

• One cake slurry re pulping tank.

Product Major parameter to be controlled

Range

HBF cake

T/Zn % (Total Zinc) W.S. Zn%(Water Soluble Zinc)

<2.7 <0.2

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Process:

The jarosite cake 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 & wash water

quantity is controlled by the concerned C/H in the HBF control room. Vacuum pipes are

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

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

washing zone and dry zone) are collected via vacuum pipes and pumped to 07 thickener

launder. To minimize the soluble zinc content in the final jarosite, the jarosite cake is washed on

HBF by counter current arranged washing step. The cake sample & slurry sample are collected

at the beginning of the shift and sent to Control Lab. For analyzing water soluble zinc &

moisture. Filter water is used for belt and cloth washing.

Parameters to be maintained:

Vacuum at HBF==220-260 mm Hg

pH of HBF Cake> 4.5(otherwise the complex formed will dissociate back into impurities)

HBF feed inlet density =1.40-1.42 g/cm3

Sample checking of water soluble zinc, feed slurry, mother liquor and wash liquor solid.

Purification:

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

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

solution. Before the solution can be sent to the electro winning these impurities have 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.

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

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

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

82o C. This solution is passedthrough a reactor cascade and another reagent Zn dust is added.

To improve thereactivity 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-CdCake. 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 small amount of gypsum which is removed in gypsum removal plant.

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Figure 9: Purification Section

The impurities levels are reduced to Cu traces, Cd= 1ppm, Co= 0.3ppm, Ni= 0.1ppm and Sb=

0.01ppm.the purified solution at a temperature of 60-65oC and at flow rate of 80-85 m3/hr is

supplied to cell house through a door thickener where gypsum and suspended basic zinc salts

are separated. Cake from purification section conveyed through screw conveyors to Cadmium

plant.

ZINC ELECTROLYSIS:

Figure 10: Zinc Electrodes Zinc is extracted from the purified zinc sulfate solution by electro winning, 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 (aluminum sheets) and oxygen

to form at the anodes. Sulfuric acid is also formed in the process and reused in the leaching

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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.

At cathode:

Zn2+ + 2e- Zn

At anode:

O2 ½ O2 + 2e-

Overall cell reaction is :

ZnSO4 + H2O Zn + H2SO4 + ½ O2

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 houseproductivity (and electrical

current and energy efficiency in particular) is acrucial 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 tons per hour of zinc cathodes (maximum ~ 24

tones per hour). The melting rate is infinitely variable between 0% and 100% of the maximum

melting rate and is controlledby 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,

provisionis 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 thecasting machines. This material is

fed to any chute (normally one dedicated chute) from fork lift 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 inthe

charge chute.

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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 kgs 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 fork lift

bin. During the drossing process, the furnace is operated under conditions of increased

ventilation to contain the fumes and dust operated under conditions of increased ventilation to

contain the fumes and dust furnace dross are transported by lift truck to the dross cooling area,

to awaittreatment in the dross separation plant.

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PROJECT DETAILED

# To reduce the total zinc losses in jarosite cake.

Introduction:

In this project we have to reduce the total zinc losses in jarosite cake. The total zinc content in

jarosite is the combination of acid soluble and water soluble zinc. This jarosite is the complex

compound of iron {2Na[Fe3(SO4)2(OH)6]). Dried jarosite cake formation takes place in HBF

section, which takes input of jarosite slurry from the RTP section thickener underflow. This

jarosite slurry from RTP thickener fed to the HBF made of polypropylene. This slurry is washed

with the condensate flow and water soluble zinc is removed is by use of vacuum beneath the

belt filter. The resultant dried jarosite cake on the belt filter is send to the ETP(effluent

treatment plant) section.

Fig: jarosite cake thickener.

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To reduce the Zn content in jarosite we have to reduce the zinc content in thickener underflow

i.e. acid soluble zinc and also to decrease the water soluble zinc content in jarosite cake at HBF.

To achieve this some parameters like acidity, temperature, free NH4 and total Zn in thickener

underflow of RTP are studied continuously for 19 days, similarly the parameters water soluble

zinc, vacuum, feed rate, condensate flow of HBF are studied.

By use of these parameters and plotting the graph between total zinc loss in thickener

underflow and RTP parameter, we have selected optimum parameter range foe which the total

zinc in underflow will be less. And again in the same way, by plotting graph between water

soluble zinc and HBF parameters, we have selected the optimum parameter range for which

the water soluble zinc in jarosite cake will be less. By use of these graphs some inferences are

drawn for the total zinc losses and its affecting parameters that are detailed in conclusion

section of this report.

RTP SECTION:

It mainly deals with activities related with conversion reactors, door thickener, sodium sulphate

addition, steam lines and flocculent preparation operations. To break zinc ferrite and

precipitation of jarosite using sodium sulphate, sulphuric acid and high temperature to remove

impurities and recovering zinc. We have to reduce the total zinc content in jarosite, for this we

noted down the zinc content in jarosite with other running parameter like acidity of R-26 ,

temperature, free Na and total inlet flow. We plot down the of these parameters with zinc

content in jarosite and see how these parameters affects the content of zinc in jarosite.

Noted down parameters of RTP Section:

Date

12-Jun

13-Jun

14-Jun

15-Jun

16-Jun

17-Jun

18-Jun

19-Jun

20-Jun

21-Jun

22-Jun

23-Jun

24-Jun

25-Jun

Total Zn loss %

3.05

3.18

3.32

3.88

3.46

3.02

2.86

2.7

3.02

3.3

2.66

3.02

3.2

3.08

Temperature(oc)

92.47

91.38

94.78

95.81

98.05

97.88

97.79

97.72

94.46

95.36

94.07

94.4

85.56

95.24

Acidity

31

37.3

35.5

36.5

37.5

38.6

37.7

36

34

38.3

35.1

35.8

34.6

35.5

Total Flow Rate

15.85

13.33

15.88

15.15

11.81

14.47

15.52

14.37

14.13

14.84

15.29

14.09

15.65

15.04

Free Na

0.35

0.76

0.62

2

1.36

1

0.71

1.35

1.13

1.05

0.95

1.08

1.15

0.69

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26-Jun

27-Jun

28-Jun

29-Jun

30-Jun

3.02

2.88

2.6

2.86

2.9

96.5

94.52

98.08

94.3

97.72

36.3

35.8

34.9

34.3

35.3

15.64

14.43

14.59

13.59

13.28

0.72

0.39

1.13

1.84

1.44

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Tota

l Zn

Lo

ss %

Total Flow Rate

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0.35 0.76 0.62 2 1.36 1 0.71 1.35 1.13 1.05 0.95 1.08 1.15 0.69 0.72 0.39 1.13 1.84 1.44

Tota

l Zn

Lo

ss %

Free Na

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0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Tota

l Zn

loss

%

Temperature(oC)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

31 37.3 35.5 36.5 37.5 38.6 37.7 36 34 38.3 35.1 35.8 34.6 35.5 36.3 35.8 34.9 34.3 35.3

Tota

l Zn

Lo

ss %

Acidity

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HBF SECTION:

It mainly deals with activities related with jarosite slurry, vacuum pump, pH analyzer and water

spray nozzle operations. The jarosite and leach residual slurry is filtered and washed on

horizontal vacuum belt filter to maximize water soluble zinc recovery. We have to reduce the

water soluble zinc content in jarosite with other running parameters like feed rate of jarosite

slurry, vacuum and condensate flow. We plot down the graph of these parameters with water

soluble zinc content in jarosite and see how these parameters affects:

Date

12-Jun

13-Jun

14-Jun

15-Jun

16-Jun

17-Jun

18-Jun

19-Jun

20-Jun

21-Jun

22-Jun

23-Jun

24-Jun

25-Jun

26-Jun

27-Jun

28-Jun

29-Jun

30-Jun

Water Soluble Zinc

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.11

0.27

0.1

0.1

0.11

0.27

0.4

0.15

0.1

0.1

0.33

0.1

Feed Rate(m3/hr)

25.5

24

26

26

26

25.42

26

26

24

24

24.08

25.87

23.33

25.56

24.33

27

26

27

25.5

Vacuum(mm of Hg)

253.75

236.25

242.5

241.81

220

211.42

235.83

240

247.5

253.12

266.08

256.66

253.33

258.69

259.16

241.25

240

240

250

Condensate Flow(m2/hr)

12

12

12.06

13

13

13

13

12

12.5

12.68

12.13

12.79

12

12.3

12

12

12

12

12

B.L.T.(Before Lime Treatment)

1.87

1.13

0.84

0.65

0.78

0.48

2.26

1.19

1.21

1.54

1.89

2.08

1.64

1.01

2.13

0.95

0.4

0.53

1.92

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

1.87 1.13 0.84 0.65 0.78 0.48 2.26 1.19 1.21 1.54 1.89 2.08 1.64 1.01 2.13 0.95 0.4 0.53 1.92

Wat

er

Solu

ble

Zin

c

B.L.T.(Before Lime Treatment)

Water Soluble Zinc

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Wat

er

Solu

ble

Zin

c

Feed Rate(m3/hr)

Water Soluble Zinc

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Wat

er

Solu

ble

Zin

c

Condensate Flow(m3/hr)

Water Soluble Zinc

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Wat

er

Solu

ble

Zin

c

Vacuum(mm of Hg)

Water Soluble Zinc

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CONCLUSIONS AND RECOMMENDATION:

IN RTP SECTION WE OBSERVED-

1. As the temperature in reactors increases, the acid soluble zinc in the jarosite slurry

decreases.

2. As the acidity of reactors increases, the acid soluble zinc in the jarosite slurry decreases.

3. As the free Na increases, the acid soluble zinc in the jarosite slurry decreases.

4. As the total flow rate decreases, the acid soluble zinc in the jarosite slurry decreases.

IN HBF SECTION WE OBSERVED-

1. As the total slurry input decreases, the water soluble zinc in the jarosite cake decreases.

2. As the condensate flow increases the water soluble zinc in the jarosite cake decreases.

3. As the cake thickness decreases the water soluble zinc in the jarosite cake decreases.

Hindustan zinc limited leaching Project

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