Zn Extraction

8/4/2019 Zn Extraction

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Zinc Ores

ZnS - Sphalerite or zinc blende(Zn,Fe)S - Marmatite

ZnCO3 - Smithsonite

ZnSiO4 H2O - Hemimorphite or Calamine


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nc res

Zinc ores range from 2.0 to 7.5 % Zn metal.Zinc concentrates assay 47 to 60 % Zn, 1.5 to 2.5 %Pb up to 3.5 % Cu, 3 to 10% Fe about 0.2 % Cd and

29 to 33 % S as well as minor quantities of thallium,Indium, gallium, germanium, selenium and tellurium.A zinc concentrate is a powdered material with 50-95% particles less than 0.07 mm on side, the grains

measuring more than 0.6 m accounting for not over0.1 to 0.3 %.Oxide concentrates usually consist of smithsonite andcalmine.

Metallurgy of Zn is divided into two fields:pyrometallurgy and hydrometallurgy.Former is presented by the distillation processes andthe latter by the electrolytic processes. The distillation

processes appeared well before the electrolytemethods.


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Flow Sheet for Pyrometallurgical Extraction of Zn


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A mixture of roasted Zn concentrate and anthracitescreenings or coke breeze is charged in to a grog retortwhich is then placed in a furnace heated to1400 C.In the retort zinc is reduced according to the reaction:

ZnO + CO =Znvapour + CO2The nose of the retort opens into a condenser of fire clay.In the condenser, the retort gas cools in contact with itswalls, the droplets run down the sides and fall from the top,

collecting in the pool at the bottom.The liquid zinc is drawn from the pool as it accumulatesthere.Zinc vapour that escapes from the condenser is precipitated

in the prolong, a sheet-iron extension to the condenser asdust.The other oxides present in the charge, such as those of

Cd, Pb and Cu may also be reduced in the retort.Only Cd and Pb vaporize to an appreciable extent.

Material left in the retort is called the retort residue.


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Horizontal retort process is simple but of low capacityproducing metallic zinc contaminated with Cd and Pb.These retorts are made from 300 to 370 mm in diameter,with walls 30 to 50 mm thick; thicker walls would hamperheat transfer inside the charge and slow down the rate odistillation.

Length should not exceed 1700 to 1900 mm as at atemperature of 1400 C the retort will fail under the

bending load.Condensers and especially retorts must be fire resistant ,

strong and impervous to Zn vapours.Retorts and condensers are extruded from a damp andwell pugged mixture of fire clay grog and coke.Green retorts and condensers are dried and fired.


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A single charge of such a retort weighs 80 to 90 kgcontaining up to 30 kg metallic zinc.

With a distillation cycle of 24 hrs and a liquid zinc yield of80 to 83 % of the charge weight, a single retort canproduce 25 kg of zinc a day at the most.An average smeltery have several thousand retorts inoperation simultaneously.Retort process involves arduous manual labour. Thesize of the retort furnace varies with its construction.

Three row furnaces for 240 retorts measure about 20 min length, 5.3 to 5.5 m in width and .8 m in height.The bottom of the furnace is 2.75 m above the floor ofthe shop.


8/73Horizontal retorts arrangement


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Arrangement of retorts in furnace


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As the art of zinc smelting stands at present, the production

of 100 tons of zinc a day requires over 3000 retorts.The retorts are stacked in vertical rows.

Their closed ends rest on shelves inside the furnace , while

their open ends or noises stick out of the furnace through

openings.The retorts are inclined towards their noses.

High temperature required for operation has to be distributed

uniformly throughout the furnace space.

Gaseous fuel is used for heating up the furnace.The basic reaction of zinc distillation is the reduction of solid

zinc oxide with gaseous carbon monoxide.


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Higher iron oxides begin to be reduced to FeO at 640C.Impurities copper, cadmium and lead are easily reduced

at the beginning of the cycle due to the weak affinity offor oxygen.Liquid zinc separates out of the retort gas as a slowlycoagulating metallic fog at 830 to 870 C.

The equilibrium of vapour pressures above the dropletsis attained at a lower temperature than over the flatsurface of the molten pool.The retort gas remains in the condenser for 13 to

15seconds and the zinc vapour that escapes from thecondenser into the prolong is precipitated there aspowder due to rapid cooling.


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Incomplete condensation of zinc vapours in thecondenser is also due to the changes in thecomposition of the retort gas with cooling.

The ratio of CO to CO2, which is at equilibrium at thetemperature of the retort is shifted towards a greater

partial pressure of CO2 due to dissociation of CO:

2CO = CO2 + C

The carbon dioxide partly oxidises the zinc vapoursin the condenser:

CO2 + Zn = CO + ZnO


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When droplets of molten metal have experiencedsuperficial oxidation they fail to coalesce and blue

powder is formed in the condenser.The zn vapours, condenser blue powder, Zn oxide andsoot that escape from the condenser precipitate in theprolong to form prolong blue powder which contains60 to 70% metallic zinc and 13 to 15% zn as oxide andcarbonate.Cd is reduced in the retort before Zn and is more

volatile and very little Cd vapour reaches the prolong.Retort residue carries 10 to 13% of all zinc, gold, silverand copper as well as excess reduction coal and isusually retreated for the recovery of its values.


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Distilled zn contains

98-99 % Zn,

up to 1.8 % Pb,0.05 -0.1 % Fe,

0.004-0.02% Cu

and 0.2-0.4% Cd.

Prolong dust consists of finely divided particles measuring a few

microns in diameter.

It carries from 0.7 to 2.3 % Cd and is treated for Cd recovery.

Condenser dust is produced in limited quantities and carriesabout 0.1 % of all Zn and the impurities mechanically carried off

with the gases and returned to the retort.


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Broken retorts are inevitable for the service life of the

retort is 15-25 days at the most.Approximate % of distillation products:Distilled Zn 82.8;Prolong dust- 4.8;

Retort residue-6.3;Broken condensers: 1.1;Broken retorts:0.7;loam-3.2;

Losses with gases-1.1;

V ti l R t t F


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Vertical Retort Furnace

The basic disadvantages of the horizontal retort

process are its intermittent nature, cumbersome

furnaces an difficulty of mechanisation.

Vertical retort furnace operates continuously and

use s carborundum retorts.

Powdered carborundum is mixed with 6-12% of

fireclay.


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The batch obtained is wettened, pressed in moulds,dried and fired at 1400 to 1600 C.

The refractories retain mechanical strength up to2000C are chemically neutral and are 3-4 times asheat conductive as grog.

A vertical retort is a shaft rectangular in cross section,made of SiC blocks or bricks.


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Vertical Retort Process 1-Retort 2- gas combustion chamber 3-charging device 4-discharging device 5- condenser


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Retort is fired with producer gas which is burned in

chambers on either side of the retort.

The heated portion is about 7.5 m high while the internal

dimensions may be 0.3 m by 2.2 m.

The charge is introduced in the form of briquettes from

above through a firebrick chamber called the charge column

which also serves to provide an escape for zinc vapour.


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The bottom portion of the retorts terminates in aresidue extraction mechanism which is an iron box fittedwith a water seal.The spent residue is discharged continuously andretains the original shape of briquettes.The briquettes are prepared from a mixture of roastedzinc concentrate, anthracite, coking coal and suitable

binders such as coal tar.After mixing, the batch is passed through a briquettingpress and heated to 750-900C.


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The coal and tar are coked and the briquettes attain astrong and porous structure.

Chemistry of the process does not differ from that of thehorizontal retort process.

The rate of heat transfer through the SiC walls of thevertical retort and the briquetted charge is higher than inordinary distillation.

Zn recovery is more complete and its content in theretort residue is usually under 3 to 5 %.


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The normal life of vertical retort is 3 to 5 years and itsproductive capacity is 4 to 5 tons of zinc per calendarday or upto 90 kg per square metre of heated area (asagainst 15 kg per square metre of heated area in the

horizontal retort).Zinc vapour leaving the retort is condensed to liquid zincin condensers which are made of fire brick and haveinside partitions to extend the path covered by the zinc

vapours.


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85% of zinc is condensed to liquid metal.

The remaining uncondensed zinc vapour andentrained blue powder passing out of the condenser withthe zinc is recovered as a fine dust.

The scrubbed carbon monoxide gas is pumped backinto the retort setting which saves about 20% fuel.


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Vertical Retort Zinc Condenser


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Comparative Performance and Economics of he Horizontal andVertical Retort Processes (per ton of zinc)

Variable Horizontal retort Vertical retort

Heating andreduction fuel

(coal), tons

2.2-2.39 1.54-1.71

Labour, man-hours

29-43 7.7-12.5

Zinc recovery, % 86.4-87.8 90-94


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Electrothermic Furnace

Though more efficient than the HR process, the VR

process is disadvantageous in that heat has to betransmitted through the retort walls.The heating of the charge with in would raisedistillation temperature and consequently zinc

recovery while the walls would remain relatively cold.


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It is also a continuous process with a zinc productionof close to 100 tons per day at an efficiency of 92%

recovery.

Heat is generated internally by resistance to currentflow by the charge in the furnace between sets ofgraphite electrodes inserted through the top andbottom furnace walls.

The furnace is circular 45 feet high and 7.5 feet indiameter and constructed of a series of sections of highand superduty firebrick.Steel water jackets enclose the lower half of the mainsmelting zone of the furnace below the vapour ring.


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This annular vapour ring is about in the middle of the

furnace and through it the distilled zinc vapor and furnacegases pass to the condenser.

The vapor ring and condenser are both lined with SiC bricks.

Charge is primarily ZnO sinter and coke but other Zn bearing

materials such as granules, dross, screenings etc. are also

used.

Furnace is kept full of charge and the time required for the

charge to pass down through and out of the furnace is about

22 hrs.


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Electrothermic Smelting Furnace


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Heating is accomplished by the resistance of thecharge to the current flow between eight equispaced

pairs of graphite electrodes, one of the sets of pairs issituated near the top of the shaft and other set of pairs

is close to the bottom.


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Total power to the furnace will be 10,000 kW at 200-

230 volts and resistance heating gives a temperature inthe center of the furnace 1200-1400C andtemperature of 1300C at the lower electrode elevation.

The gases escape from the furnace at 850C :45% Zn vapour and 45% CO and the remainder N2, H2and CO2.

The condenser is situated below the vapour ring is U-shaped with a vertical inlet and outlet.

The bottom part of the U- is the liquid zinc pool and


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The bottom part of the U is the liquid zinc pool andfurnace gases are drawn in to this to increase the poolvolume operating under a vacuum of 25-30 cm Hg.

This zinc is tapped periodically and cast into slabs orgoes to the refinery.

The gases pass through the condenser and in to ahigh velocity water scrubber for the removal ofentrained solids and blue powder.


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This slurry is treated to recover the solids which arebriquetted and returned to the furnace.The clean gas (80% CO) is piped as a fuel.The spent residue from the bottom of the furnaceconsists of coke, sinter residue and liquid slag.

A rotating table is provided at the bottom of the furnace

for the removal of spent residue.

Zinc Blast Furnace


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Zinc Blast Furnace

Introduced in 1950.Largest production unit 300 tons

HR 0.06 tonsVR 10.0 tons

elec.-thermic furnace 100 tons

Charge hot lump sinter, preheated coke & smalladdition of lime


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Sinter analyses 42% Zn & 20% Pb

Coke 5 & 7.5 cm coke preheater 800C.

Accurate proportioning (coke, sinter andhard burned lime) important.


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rectangular in shape rounded ends Pb blast

furnace.Upper part of the furnace fire brick-lined steel shaft,

lower part hollow water cooled steel jackets.Two stoves are used air to 950C operates on a 40min on blast cycle & 30 min firing cycle.Dome shaped roof supports two double bell & hopper

charging units


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Cold air is introduced between the two bell valves toprevent gas leakage.4-equispaced top air inlets located in the furnace sidesabove charge level.

Preheated air is bled from tuyere bustle pipe throughthese inlets.


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Mixed sinter of PbO & zinc oxide is used in the BF in theproportion of about two zinc to one lead.

The reduction of ZnO to metal is endothermic but thereduction of PbO is slightly exothermic:

PbO = CO =Pb + CO2


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CO gas diluted with CO2 gas from lead reduction, butstill high enough in CO to preheat coke and tuyere airblast.

Furnace gases :20% CO and 12% CO2.

Liquid along with precious metals, Cu, other reducedtrace metals & slag flows to bottom of furnaceTapped every one and half hours through a water cooled

copper tapping block into a fore hearth.Slag runs over top into a granulation system & fromthese goes to the dump.Metal values in slag : 5% Zn and 0.5% Pb.


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Two lead splash condensers connected to gas offtakes on either side

Curtain of Pb droplets by rotor blades absorbs heat fromvapour & reduces 550C.Zn vapor is quickly condensed to liquid metal &absorbed in liquid lead.Common type of condenser three stages, with fourin first stage, two rotors in second stage & two in third

stage.Rotors are rotating vertical shafts with blades dip intopool of Pb bottom of condenser & throw up thecondensing & collecting spray of metal droplets.


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Droplets small size maximum liquid tovapour contact

Droplet size shape of the rotor bladesimmersion depth of bladesspeed of rotation.

Hot Pb with Zn in solution is run at 550C fromcondenser through an under flow baffle forming a gas

seal in to a cooling launder.

Hot Pb as it leaves condenser contains 2.5% Zn and


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is unsaturated;cooled down to 440C in the launder, Pb has lesssolubility for Zn and is saturated at 2.25% Zn.Excess Zn containing 1.2% Pb rises as liquid layerabove Pb in a mild steel quiescent separation bath,overflows continuously from here & into a heated Zn

holding bath.

Pb from separation bath runs by gravity under a weir holds


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Pb from separation bath runs by gravity under a weir, holds

back separated Zn layer & returns to metal pool in condenser

Cooling duty in condenser is heavy & this cooled Pb is quicklyreheated by great amount of heat it absorbs from entering

hot furnace gases.

For heat extraction requirements 400 tons of Pb must be

circulated through closed system- condenser, cooling launder& separation bath/ for a furnace output rate of 300 tons of Zn

per day, 120,000 tons of Pb must be circulated.


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Furnaces gases leaving condenser at 450C & lowin temperature as to carry minimum amounts of

uncondensed Zn.

Gases pass through two stages of cooling:washing in spray towers

mechanical scrubbers,producing a clean gas below 40 C.Solids removed are returned to the sinter plantGas containing 20% CO used as fuel in the coke

preheater and BF stoves.

Zinc Vertical Fractional Distillation


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Zinc Vertical Fractional Distillation

To remove Pb & Cd impurities 98%-99.995%+Priciple: Divergent boiling points

Pb (1620C), Zn (907C) & Cd (778C) Zn & Cd vaporised in 1st column at 1220C Pb

remains liquid in two-column vertical unit In second column at 850C, Zn remains as liquid

Cd vaporises.Liquid Zn runs from vertical retort /electrothermicfurnace condenser through a launder to a holding

furnace & from there to 1st

distillation column.


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Vertical Fractional Distillation


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Section Through Fractional Distillation Column


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25 tons Zn is fed into circuit & 15 tons recycled Znfrom 1st column total of 40 tons per day being treated.

Recycled 15 tons: Pb, Fe & other high boiling pointimpurities that flow out in to a two compartment holdingpot is separated by liquation.

First compartment is cooled to settle out a high densityPb layer & Zn-Fe dross with liquated Znnc nowreasonably free of Pb and Fe overflowing into thesecond compartment and back to the initial holding

furnace for feeding into the distillation column.Each of the two distillation columns is made of 50rectangular silicon carbide trays, 2 feet by 4 feet and 6inches deep.

These trays are super imposed to fit tightly together and


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These trays are super imposed to fit tightly together andprevent vapour leakage.These are adopted to hold a pool of metal, the depth of

which is set by an overflow dam around a 3 inchdiameter opening near the end of the tray.The trays are stacked with the openings on successivetrays alternatively from side to side so that the metal

overflow from one tray is caught on the tray below andthe descending flow of metal follows a zigzag pathdownward from tray to tray.

The design facilitates heat transfer and provides anextremely large surface from which the vapour canescape as the whole tray is covered with molten metal,but only to a shallow depth as each tray is an

independent unit.

Th di t l f ll th


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The ascending metal vapours follow the samepath upward as the liquid takes downward, so

that here is good liquid-vapour contact and liquidscrubbing of the vapour as it passed up throughthe column.Liquid zinc from the holding furnace runs into thefirst so called lead column in a steady stream at

a height of two or three trays above the top of theheating furnace, and this flow is regulated to

ensure that all the trays are full of metal and overflowing steadily into the trays below, in order thatno tray may run dry or become overheated.

Th d i f th t diff h t i id th


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The design of the trays differs somewhat inside thefurnace in that these distilling trays have a trough

extending around the outer periphery and a raisedcenter, to present the maximum amount of heated traywall to the liquid zinc and at the same time to have agreater holding capacity for the liquid feed.

The refluxing trays, positioned above the furnace areflat with no peripheral trough and hold a much smallerquantity of liquid metal.Controlled cooling (refluxing) takes place in these upper

trays with some condensed liquid metal from the risingvapour phase produced in the furnace zone collectingand overflowing to run down from tray to tray.


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The vapour phase passes off at the top of the lead column

consists of two thirds of the zinc fed in and all of the Cd and

this travels into a second so called Cd column.Two Pb columns can operate in series with one Cd

column, as the greater amount of impurities are removed in

the first processing stage and a much smaller amount of

material is treated in the second stage.

The design and operation of the Cd column is essentially

the same as that of the Pb column except for the lower

furnace temperature and the fact that all the trays are flat type

similar to those used in the upper refluxing section of the Pb

column.


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Liquation


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qo Used to separate Pb and Fe from retort Zn but has no

effect on Cd.

o The principle of this refining operation is to reheat ormelt the zinc in a reverberatory furnace and then allowthe temperature to drop to near the freezing point.

o Pb in excess of that soluble in the Zn at the lower

temperature will now settle out as a liquid layer on thebottom of the and can be withdrawn.

o Zn purity can be increased from 98% to 99% with Pb inthe Zn being decreased from 2% to 1% and iron

decreased from 0.1% to 0.025%.o A two compartmented reverberatory furnace is used

fired with coal and kept short of air to have a reducing

atmosphere above the liquid metal bath to prevent Zn

Slab zinc is added to the large compartment of he


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Slab zinc is added to the large compartment of hefurnace and melted at temperature above 800C atwhich temperature the Pb and Zn are completely

miscible.

The furnace is then cooled to slightly above 418C atwhich point the eutectic temperature is reached and Znwill dissolve only 0.85% Pb.

The excess Pb over this eutectic quantity comes out of

solution and settles as a bottom layer in the furnace.

The Pb is still quite fluid at this temperature because ofits low melting point 327C and is easily tapped from the

furnace.


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Flow Sheet for Hydrometallurgical Extraction of Zn

Leaching of Zinc Concentrates


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The primary purpose of leaching a zinc concentrate isto dissolve zinc oxide:

ZnO + H2SO

4= ZnSO

4+ H

2O

Process is complicated due to the dissolution ofunwanted elements and compounds which have anadverse effect on the electrolysis and recovery ofzinc.

A two prong approach is adopted for leaching operation: Firstly to minimise the amount of impurities passing into

solution; Secondly to purify the solution. Many of the impurities can be reduced in amount or

eliminated by neutralising the zinc sulphate solution withZnO, with the formation and precipitation of ferrichydroxide.


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Iron purification is usually carried out simultaneously with

leaching.Any ferrous iron present is first oxidised to the ferric state withmanganese dioxide and by hydrolysing the ferric sulphate:

2FeSO4 + MnO2 + 2H2 SO4 = Fe2 (SO4)3 + MnSO4 + 2H2 O

Fe2 (SO4)3 + 2H2O = 2Fe(OH)SO4 + H2 SO4

First reaction is possible in an acid medium; therefore the iron has to beoxidised at the beginning of the leaching operation.The second reaction is feasible only at the end of the leaching when theacid has been depleted and the solution is almost neutral.As and Sb are removed from the solution simultaneously with iron.

Good elimination of the two impurities is obtained if thef i i i h f A d


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amount of iron present is 10 to 20 times that of As and20 of 40 times that of Sb.Complete recovery of Zn is requires an excess ofsulphuric acid.

On the other hand the solution should be neutral

towards the end of the leaching operation if the iron is tobe withdrawn successfully.

To meet these conflicting requirements, the leaching

operation is carried out in two stages (double leaching).First roasted concentrate is treated with acid solution ofZnSO4 containing about 100 to 130 gm of Zn per litre ofand 1 to 5 gm of free sulphuric acid per litre, and it is

used for neutral leaching.


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Rate of leaching depends on the concentration of acid.Other factors affecting leaching are temperature, the

grain size of the roasted concentrate, agitation etc.

Equipment for leaching may be either air-operated ormechanical.

Air operated Pachuka tank is a tall cylinder with aconical bottom made of wood or steel plate.

The tank measures 3 or 4 m in diameter and 6 to 8 m inheight and has a capacity of up to 100 cu m.

A wooden pipe opened at both ends is placed centrally.C d i d f b 2 i


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Compressed air under a pressure of about 2 atm issupplied to its bottom end.

Pulp is fed into the tank from above and overflowscontinuously as it fills the tank at a level somewhatbelow the top end of the central pipe.

Bubbling through the pulp, the air raises the level of thepulp causing it to flow out of the pipe and in to the tank.

Bubbling through the pulp, the air raises the level of thepulp causing it to flow out of the pipe and in to the tank.Pulp is continuously drawn in to the pipe from below.As a result pulp is made to circulate in the tank.


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Pachuka Tank


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Purification of Neutral Zinc Sulphate Solution


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p

Impurities such as Cu, Co, Fe, As and Sb may

appreciably reduce current efficiency in subsequentelectrolysis.

In the presence of some ions such as As, Sb and Cothe cathode deposit will be rough and of low density.

Neutral solution be purified of its copper and cadmiumprior to elctrolysis.

The principal reaction of Cu and Cd elimination is that

of electrolytic reduction.

Electrolytic Precipitation


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Zinc sulphate leach solution after purification ispumped to storage house and flows from there to theelectrolytic cells for precipitation:

ZnSO4 + H2O + e = Zn + H2SO4 +0.5 O2As electrolysis proceeds and the acid strength rises to

150 to 200 gpl some of this high acid solution iswithdrawn from the cells and sent to the acid storagetanks from where it will be recycled to the leach circuitfor reuse.

This withdrawn solution is replaced in the cells byfresh neutral leach solution, which reduces the cellacid strength to about 115 gpl, the normal range for

electrolysis.


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The fresh leach solution in addition to being low in H2SO4 carries

from 100 to 160 gpl zinc as zinc sulphate.

Electrolysis removes from 50 to 70% of this by deposition of Zn

metal on the cathodes, depleting the solution to about 50 gpl and

the remaining Zn sulphate content is withdrawn in the high acids

solution recycled to the leaching circuit and forms a consistent

circulating load.

The cells are constructed of Pb- lined concrete and have typical

dimensions of 15 feet long by 2.8 feet wide by 5.5 ft deep.

These cells can be arranged in cascades of 6 to 12 cells each sosituated that solution will flow by gravity from the highest head

cell of each cascade to the second from the second to the third and

so on down the series with the last cell discharging in to a launder.

A cell room has 300-400 cells divided in to 2 or 4 units, with eachunit having its own electrical circuit for flexibility.


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unit having its own electrical circuit for flexibility.

Cell temperature is important and is held at 35 to 45C by

circulating water through lead coils placed in each cell .

High temperatures intensifies the bad effect of impurities in thecell; and as heat is generated during electrolysis, the cooling coilsare needed to keep this heat with in reasonable limits.

Anodes are fabricated form 99 % Pb -1% Ag sheet (1.25 cm thick)are inert to the electrolyte solution.

A cell typically has 46 anodes and 45 cathodes and smaller sizedcell has 28 anodes and 27 cathodes.

Most plants operate with a current density in the range of 20-40asf of cathode area, with variations at different plants ranging from20 to over 100 am s.

The voltage required for the electrolysis of a zinc


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The voltage required for the electrolysis of a zincsulphate solution is theoretically 2.35 volts, but in actualpractice 3.25 to 3.5 volts is required due to current lossand leakage throughout the electrical circuitry.The decomposition voltage of zinc sulphate is above thatof hydrogen, and normally it would be expected that

hydrogen would evolve instead of Zn.However, hydrogen over voltage w.r.t. Zn in an acidsolution is high enough to let zinc plate out of the zincsulphate solution without the evolution of a great amount

of hydrogen at the cathode.Ampere efficiency : 87- 94%; Cathodes are pulled fromthe cells every 24 48 hours to strip off the layer ofdeposited zinc and 99 995 % pure

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