seminar ppt

REPLACEMENT OF HALL- HEROUlT PROCESS

PRESENTED BY…… GAJENDRA GUPTA

TOPIC TO BE COVERED……INTRODUCTIONSTEPS FOR ALUMINIUM PRODUCTIONFLOW CHART OF PROCESSHALL HEROULT PROCESSWHY WE NEDDALTERNATIVES???ALTERNATIVES TO HALL HEROULT

PROCESS

INTRODUCTIONBAUXITE IS THE MAIN ORE OF

ALUMINIUM .

ALUMINA IS THE BASIC RAW MATERIAL FOR PRODUCTION OF ALUMINIUM.

THREE STEPS FOR ALUMINIUM PRODUCTION

1)PRODUCTION OF ALUMINA FROM BAUXITE

2)ELECTROLYTIC REDUCTION OF ALUMINA

3)REFINING OF ALUMINIUM

STEPS FOR ALUMINIUM PRODUCTION1)BAYER’S PROCESS-IT CONSIST OF

DIGESTING THE GROUND BAUXITE WITH SOLUTION OF CAUSTIC SODA AT HIGH TEMP. AND PRESSURE TO GET A Soln of SODIUM ALUMINATE .

THE IMPURITIES ARE DISCARDED AFTER SETTING AND FILTRATION TO GET PURE SODIUM ALUMINATE Soln FROM WHICH ALUMINIUM HYDROXIDE IS PRECIPITATED AND CALCINED TO PRODUCE PURE ALUMINA FOR ELECTROLYSIS

2)Reduction of Alumina: Reduction process consist of the electrolytic dissociation of alumina dissolved in bath of molten cryolite at a temp. 0f about 960°c

3)Refining of Aluminium : It is done by Three layer process. Electrolytic cell is made of iron, which is lined with carbon at the bottom. The lower layer consists of an alloy of impure  aluminum with copper. This

layer serves as anode. The middle layer consists of a solution of cryolite    (Na3AlF6) and barium fluoride. The upper layer consists of pure aluminum and serves as cathode.

These three layers are separated from each other due to difference in specific gravity.During electrolysis Al+3 ions from the middle layer migrate to the upper layer where they are reduced     to aluminum by gaining 3 electrons. Pure aluminum is tapped off from time to time. Hoope’s method gives  99.99% pure aluminum. 

HALL -HEROULT PROCESSAluminium cannot be produced by the

electrolysis of an aluminium salt dissolved in water because of the high reactivity of aluminium with the protons of water and the subsequent formation of hydrogen.

As in aqueous solution, protons (H+) are preferentially reduced before Al3+ ions, leading to hydrogen evolution, the reduction of Al3+ requires to perform the electrolysis in a molten aluminium salt in the absence of water

In the Hall–Héroult process alumina, Al2O3, is dissolved in molten cryolite, Na3AlF6 (sodium hexafluoroaluminate).

 Aluminium oxide has a melting point of over 2,000 °C (3,630 °F) while pure cryolite has a melting point of 1,012 °C (1,854 °F). With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1,000 °C (1,830 °F).

Some aluminium fluoride, AlF3 is also added into the process to reduce the melting point of the cryolite-alumina mixture.

The molten mixture of cryolite, alumina, aluminium fluoride is then electrolyzed by passing a direct electric current through it.

The electrochemical reaction causes liquid aluminium metal to be deposited at the cathode as a precipitate, while the oxygen from the alumina combines with carbon from the anode to produce carbon dioxide , CO2.

While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser than the molten cryolite at temperatures around 1,000 °C (1,830 °F).

so the aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. 

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The Hall Process for AluminumAt the anodes: solid carbon is oxidized  to carbon

dioxide gas. Carbon anodes disintegrate over time and need to be replaced periodically. Carbon dioxide gas is released into the atmosphere.

At the cathode: aluminium ions are reducedto elemental aluminium. 

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carbon-lined steel vesselacts as cathode

CO2 bubbles

Al (l)Al2O3 (l)

Drawoff Al (l)

-

+

Cathode: Al+3 + 3e- Al (l)

Anode: 2 O-2 + C (s) CO2 (g) + 4e-

frompowersource

Al+3

O-2O-2

Al+3

O-2

graphite anodes

e-

e- .

WHY WE NEED ALTERNATIVES?The electrolysis process produces exhaust

which escapes into the fume hood and is evacuated. The exhaust is primarily CO2 produced from the anode consumption and hydrogen fluoride (HF) from the cryolite and flux. HF is a highly corrosive and toxic gas.

The linings of aluminium reduction cell must be replaced periodically.

Handling of alumina and coke presents dusting problems.

The overall energy efficiency of this process is very low(36-48%) thereby it consumes more electricity for production of metal.

Suitable grade of bauxite ore has to be purified to produce alumina.this accounts for additional capital investment as also limits the choice to utilization of selected grade of the ore.

The process consumes large quantity of carbon electrodes.

Evolution of fluoride bearing fumes and other contaminants.

ALTERNATIVES TO HALL- HEROULT PROCESSSSub-Chloride processChloride electrolysisToth process

Sub-chloride processSmelting a mixture of bauxite and coke in

submerge arc furnace to produce a crude alloy containing the usual impurities such as silicon along with iron,titanium and carbon.

The impure aluminium alloy is reacted with preheated AlCl3 vapour at 1000-1400o c which gives aluminium monochloride vapour.

2Al + AlCl3 3AlClGaseous chloride are pssed to the decomposer

where they are cooled to 700-800o c in a stream of molten aluminium droplets.

aluminium monochloride is decomposed to AlCl3 and gives aluminium.

This AlCl3 is recycled.Most of technical problems such as

corrosion ,transfer of heat evolved as a result of higly exothermic decomposition reaction etc. were overcome.

Aluminium production by chloride electrolysisThe second process concerns the molten salt

electrolysis of aluminum chloride. Since aluminium chloride sublimes at 183° and is poor ionic conductor, it is usually dissolved in molten alkali chlorides.

In order to be able to separate the aluminium in liquid form, electrolysis temperatures of approx. 700° C. are chosen.

The material principally used for the anode and cathode is graphite.

Gaseous chlorine is released at the graphite anode.

Advantages of this processLower working temp. are required (700-790 c) as

compared to hall- heroult process(940-980°c).High current density can be used as the critical

density for anode effect is fairly high.Low specific electrical energy consumption of 9.9

kwh/kg as compared to 17.62 kwh/kg for hall- heroult process.

Aluminium metal of superior quality is obtained and the undesirable concentraition of sodium present in hall- heroult process is reduced.

Toth processThis was developed by charles toth.Based on exchange rxn between mangnese

metal and aluminium chloride at 300°c and 15 atmospheric pressure.

3Mn(l) + 4Al2Cl6 3(MnCl2.Al2Cl6) (l) + 2 Al(l)

ore with alumina content as low as 30-40 % can be used as raw material for this process

After mixing with coke,chlorination is done at 925°c using a mixture of chlorine and sicl4 to form AlCl3.

Then AlCl3 is purified and impurities comes out in form of Fecl3,TiCl4 etc.

Reduction of the aluminum trichloride by manganese metal to produce aluminum metal and a salt mixture of aluminum trichloride and manganese chloride.

Separation of the aluminum and salt mixture . Evaporation of the salt mixure to produce solid

manganese chloride and aluminum chloride vapor which is condensed and returned to the aluminum generator.

Oxidation of mangnese chloride is carried out at 600°c to produce Mn203. The manganese oxide carbothermally reduced to manganese metal in blast funace for recycling.

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