roasting plant

8/12/2019 Roasting Plant

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Company Profile

Vedanta

Vedanta 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 inthe 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 greatexperience in this field and a sharp sight of the future conditions and requirement.


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Zinc Smelter Debari

Zinc Smelter Debari was commissioned in the year 1968 with an initial production

capacity of 18,000 tonnes per annum of zinc. In the past several years the capacity of

the smelter has been expanded five folds to its current production capacity of

88,000 tonnes per annum of zinc.Zinc Smelter Debari is a Hydrometallurgical zinc

smelter situated at Debari, about 13 kms from Udaipur, in Rajasthan, India. The

primary product of Debari is High Grade (HG) zinc and it also recovers cadmium and

Sulphuric Acid as by-product.

The plant has three roasting section, one leaching & purification section, one

electrolysis & one melting & casting sections.

Zinc Smelter Debari have following main plants:


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General Process Overview

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

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

In Summary, the Process Sequence is:

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

zinc oxides) and storage;

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

materials, resulting in so-called calcine;

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

zinc sulphate, using diluted sulphuric acid.Step 4: Purification: removing impurities that could electrolysis process (such as

cadmium, copper, cobalt 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


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Roasting 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 calcination, which merely involves decomposition at elevated temperatures. A typical sulfide

roasting chemical reaction takes the following form:

S + O2 SO2.

2 ZnS + 3O2 2 ZnO

SO2 + O2 SO3

CuS + 1.5O2 CuO + SO2

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

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

around 10% of the zinc reacts with the iron impurities of the zinc sulfide concentrates to form zinc ferrite. A

byproduct of roasting is sulfur dioxide, which is further processed into sulfuric acid. Reduction of zinc

sulfide concentrates to metallic zinc is accomplished through either electrolytic deposition from a sulfate

solution or by distillation in retorts or furnaces. Both of these methods begin with the elimination of most of

the sulfur in the concentrate through a roasting process.

Roasting Of Zinc Concentrate

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

simple operation at high availability and the production of useful side products as steam and sulfuric acid.Strongest environment regulations are met for solid, liquid and gaseous products or emissions. The roaster

has a cylindrical bed section, a conical intermediate section, a cylindrical enlarged top section, and a

grate area of 123 square meters. The enlarged cylindrical section enables a complete roasting of even

the finest calcine particles without the occurrence of a secondary combustion phenomenon. For

process optimisation 10 secondary air nozzles are installed to be able to distribute additional roasting air

above the bed. A slight draught is maintained at the roaster gas outlet to ensure the safety of the roaster

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

(zinc) and respectively 55 000260 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 asrecovering the heat in the form of steam that is used in the leaching plant and/or converted into electricity.


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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. Debari is able to deliver

the whole off-gas treatment and energy recovery system after the roaster which includes following process

steps:

Waste heat boiler

Hot Electrostatic Precipitator (ESP)

Wet Gas Cleaning Sulfuric Acid Plant

Fluidized-Bed Roaster

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

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

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

operate under a pressure slightly lower than atmospheric and at temperatures averaging 1000 C

(1800 F). In the fluidized-bed process, no additional fuel is required after ignition has been

achieved. The major advantages of this roaster are greater throughput capacities, greater sulfur

removal capabilities, and lower maintenance.

Waste Heat Boiler

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

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

membrane wall-type, directly connected with the gas outlet flange of the roaster by means of a

flexible fabric expansion joint. The hot dust laden gas stream leaving the roaster is drawn into the

waste heat boiler under suction from the SO2 blower. The waste heat boiler is a horizontal-pass boiler, gas-tight welded, membrane wall-type, directly connected with the gas outlet flange

of the roaster by means of a flexible fabric expansion joint.

Cyclone

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

temperature of approx. 350 C. The gas leaves the cyclones at the top whereas the dust is collected

in the lower part of the cyclones and removed via rotary valves. Final dedusting of the hot gas is

achieved in the hot ESP.


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Heat and Mass Balance Over Roaster Plant III

Mass Balance

Concentrate feed = 39.75 t/hr

feed in kg/hr = 39750 kg/hr

Moisture content = 10 %

Relative humidity = 0.111

Dry feed =39749.89 kg/hr

Concentrate composition

Component % Kg/hrZn 52 20669.94

Fe 8.5 3378.74

Lead 1.5 596.25

Copper 0.1 39.75

Suphur 30 11924.97

C 0.9 357.749

Cd 0.16 63.60

SiO2 2 795.00

Insolubles 1 397.50


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HZL Training Report

4. Reactions of iron:

Fe + S FeS

Fe + S2 FeS2

Amount of Fe present in FeS 75 % = 2534.06 Kg

Amount of Fe present in FeS2 25 % = 844.69 Kg

Fe + S FeS

56 32 88

2534.06 3982.09

2 FeS + 3.5 O2 Fe2O3 + 2 SO2

176 112 160 128

3982.09 2534.06 3620.08 2896.06

Fe + S2 FeS2

56 64 120

844.69 965.35 1810.04

2 FeS2 + 5.5 O2 Fe2O3 + 4 SO2

240 176 160 256

1810.04 1327.36 1206.69 1930.71


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Gas Cleaning Plant

Gases leaving waste heat boiler are passed through cyclone to remove the calcine particles and

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

application of electric field.

Quench Tower

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

incoming gas (Temperature: > 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 tower

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

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

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

acceptable limits. Removal of water vapour s 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 backto the liquid distribution system. The plate heat exchangers are cooled with cooling water

which is circulated via evaporative type cooling towers.


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Acid Plant

Plant Description

The sulphuric acid plant mainly consists of 2 plant sections:

The drying and absorption section.

The converter section with the gas to gas heat exchangers .

Drying and Absorption Section

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

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

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

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

supporting structure made of acid resistant stoneware. The irrigated acid is distributeduniformly 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 controlthe necessary acid concentration for each of the towers. The acid coolers for the drying tower,

intermediate and final absorption as well as for the product acid system are plate-type acid

coolers.

Converter system

The converter system consists of a stainless steel 4-layer converter. The arrangement of the

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

itself is an insulated, vertical and cylindrical vessel divided in four sections: called layers or

trays. The catalyst required for the conversion of SO2 to SO3 is arranged on these layers.


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Leaching Plant

Leaching is a widely used extractive metallurgy technique which converts metals

into soluble salts in aqueous media. Compared to pyrometallurgical operations,

leaching is easier to perform and much less harmful, because no gaseous pollution

occurs. The only drawback of leaching is its lower efficiency caused by the low

temperatures of the operation, which dramatically affect chemical reaction rates.

Neutral Leaching Of Zinc Calcine

The first neutral leaching step is the most important section of the leaching plant,

because approx. 70% of the total dissolved Zn is dissolved in this leaching step.

The main target is to leach the zinc oxide from the calcine and oxidize the ferrous

iron to the ferric state. In addition to being an important zinc leaching step, the

neutral leaching step is also an important purification step. Impurities like Fe, As,

Sb, and Ge are precipitated in the last tanks of neutral leaching.

Neutral leaching consists of following main process equipment:

One 35 MT capacity calcine hopper

Seven 680 MT capacity calcine storage silos

Three screw conveyors, and five reddller conveyors

One classifier and ball mill

Nine 45 m3leaching reactors and first reactor is called Ready for calcine

Two 16 m diameter thickners

Two 70 m3thickner overflow tanks,
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Calcine conveying

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

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

hopper is full, calcine is diverted to storage silos through reddler conveyors to any

of the seven silos depending on the level of calcine present in each of them.

Neutral leaching

The ready for calcine(RC) and remaining leaching pachukas (Reactor) are all

covered and equipped with agitators and stacks. Pachukas are equipped with

injectors for oxygen gas. Execpt RC and the eight leaching pachukas are arranged

in a cascade and are interconnected with an overflow launder, so that the solution

fed to the first tank flows by gravity to all the tanks and to the classifier without

pumps. Also, the launder system enables the bypass of any single pachuka.

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

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

breakdown of RC tank first pachuka is used for ready for calcine.

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

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

H2SO4 along with ball mill slurry and occasionally conversion overflow acid

overflow also can be added. In addition MnO2 (To control the ferrous content)

KMnO4 are to be added in RC tank. The dosing of MnO2 is carried out by a

magnetic vibrator.

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


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H2SO4pumped to the pachuka. The acidity is lowered in the leaching tank series

by calcine addition in two steps.

The basic chemical reactions in the neutral leaching process are:

ZnO + H2SO4 ZnSO4 + H2O

CuO + H2SO4 CuSO4 + H2O

CdO + H2SO4 CdSO4 + H2O

PbO + H2SO4 PbSO4 + H2O

In addition to the reactions above , MnO2reacts in the first leaching pachukas 2

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

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

2 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


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Acid Leaching

The main task of the acid leaching step is to dissolve the remaining zinc oxide,

which was not dissolved during Neutral leaching and to reach a zinc oxide

leaching efficiency of more than 97%.

The main chemical reactions in weak acid leaching are:

ZnO + H2SO4 ZnSO4 + H2O

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

Fig.-8 Process Diagram for Acid Leaching and Neutralization


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Neutralization:

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

the over flow from acid leaching thickner and jarosite precipitation thickners with

calcine before sending it to neutral leaching.


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Residual Treatment Plant(RTP):

The conversion process main function is to simultaneously leach the zinc ferrite

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

comprise the following main equipment.

Five 300 m3conversion reactors

Two 18 m diameter thickners

Two 70 m3thickener overflow tank

One 20 m3condensate 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


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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.


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Horizontal Belt Filter

In the section the jarosite and leach residue slurry is filtered and washed on

horizontal vacuum belt filters to maximize water soluble zinc recovery. Thissection 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)


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Purification

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

nickel. Becide these major impurities also small amounts of arsenic and antimony

can be analyzed in the solution. Before the solution can be sent to the

electrowinning these impurities habe to be removed in the purification plant. The

main reagent in the purification plant is the Zinc dust. The basic reaction is that of

cementation of those metals, whose positions in the electromotive series for

sulphates are below that of Zinc.

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

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

2.

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

major impurities

3.

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

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

impurities are removed in hot purification. These process is based on antimony

purification process. In this process solution is passed through a spiral heat

exchanger so that its temperature becomes 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. Herethe solution is again passed through Reactor and filter press cascade and remaining

amount of Cd which may be slipped during hot purification is also removed.

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

small amount of gypsum which is removed in gypsum removal plant.


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roasting plant

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