SMELTING | Unit of Pyrometallurgy

SMELTING

Definition : Smelting is defined as any metallurgical operation or comminution of operation there produces liquid metal.

Ore + Flux + Reducing agent + Slag + Gas

Smelting is a process of applying heat to ore in order to extract out a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silverironcopper, and other base metals. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slagand leaving the metal base behind. The reducing agent is commonly a source of carbon, such as coke—or, in earlier times, charcoal.
Smelting reduction
Smelting Reduction lay out

Characteristics of Smelting :

1. It is the heating process for the production of metal or matte.
2. The material to be smelted is charge in the solid state.
3. The product of a smelting furnace are as liquid form.
4. There are two type of smelting 
            * Reduction Smelting : It produces molten metal and molten slag
            * Matte Smelting : It may conducted in a B.F revervatory furnace.
● In generally reduction smelting occurs in B.F and matte smelting revervatory furnace. The necessary heat for supplied by fuel or electricity of the process may be autogenous.


Smelting Action :

● The iron blast furnace operates on a counter current principle. The charge moves slowly down in the furnace shaft and the current of gas that react with this charge moves up ward, this operation is reduction smelting.

● Smelting in Iron B.F. reduction in which coke serves both as a fuel as a reducing agent. This also an example of B.F smelting.
1. Non ferrous ore can be smelting in B.F.
2. Charge material is in lump state or lump material are used.
3. Coke is used as a fuel.

REDUCTION SMELTING

● Reduction and smelting are two independent unit processes employed in mental extraction through pyromettallurgical route. Reduction is carried mostly by carbon.
Blast furnace smelting process
Blast Furnace Redcution

 For example reduction of . Whereas smelting consists in heating the products to a temperature above the melting point to separate liquid metal from liquid slag. Both unit processes can be carried out simultaneously in one reactor or in two independent interconnected reactors.

● Typically lead, zinc and hot metal are produced in a single reactor called blast furnace. In Korex process two independent interconnected reactors are used to produce hot metal.  Shaft furnace is connected with a smelter–gasifies unit.

Here we know the reduction smelting becomes three types
1. Carbo-thermic reduction
2. Metallo thermic reduction
3. Calico thermic reduction

Carbo - Thermic Reduction :

A carbothermic process for producing an aluminium carbide containing mass by injecting carbon and alumina into molten aluminium superheated above 1400° C. The carbon reacts with molten aluminium to produce an aluminium carbide and alumina mass. The mass can be heated in the range of 1700° C. to 2000° C. to produce aluminium metal and carbon monoxide.

● A mass of solid aluminium carbide containing product is produced by injecting particulate alumina into a bath of molten aluminium metal; and injecting carbonaceous material, consisting of, containing or yielding carbon, into the bath.

● The bath of molten aluminium metal is maintained at a superheated temperature to heat and react carbon with molten aluminium to produce solid aluminium carbide which mixes with alumina to form a mass containing entrapped gas and entrapped molten aluminium metal and having a bulk or apparent density less than aluminium. 

● The mass is allowed to accumulate as a mass of solid aluminium carbide containing product on the upper surface of the bath. The carbonaceous material is a hydrocarbon material or is produced by pyrolysis, decomposition or cracking of a hydrocarbon material.


Carbo thermic reduction in blast furnace
1. Carbon is a reducing agent
2. Most common in manufacturing of pig iron in iron b/f.
3. Pre heated air is supplied to the coke.
4. Heat generation take place.
5. As reduction produced temperature rises.
6. Iron melts at the hearth.
7. Iron melt at the hearth at molten metal and slags moves at top.
8. Flux used as CaCO3 and dolomite.

Salient Future :

2Fe2O3 + CO = 2Fe3O4 + CO
Fe3O4 + CO = 3FeO + CO2
FeO + CO = Fe + CO2
FeO + CO = Fe + CO2
1. Metal phase is iron other reduced metal.
2. Gange material are aluminium and SiO2.
3. Slag will be flew if temperature is high.
4. Flux will reduced melting temperature of oxides
5. Basic slags are calcium oxides magnesium oxide.

Metallo Thermic Reduction :

● There are large tonnages of low-grade manganese ores which it is un-economic to produce ferroalloys by conventional submerged arc furnace smelting. A possible way to utilise these ores is to smelt them with carbon to produce pig iron and a manganese-rich slag, then to metallothermically reduce the molten slag to silicomanganese.

● Thermodynamic modelling of the metallothermic reduction stage using aluminium, silicon or aluminium-silicon alloy as the reductant at 1450°C showed that an acceptable grade of silicomanganese could be produced. Manganese recoveries were predicted to be about 80 per cent.

● Heat balances indicated that silicothermic reduction may not produce enough heat to maintain reaction temperature. In laboratory experiments showed that the manganese grade of the alloys, and the manganese recoveries, were close to those predicted, but those for silicon were much lower. Aluminothermic reduction produced the highest grade manganese alloy.

● The slag-metal equilibrium involving manganese, silicon and their oxides was investigated and it was found that equilibrium was not attained in any of the experiments, due probably to the very low rate of reactions involving silicon. This could be used to advantage in practice to achieve higher manganese levels in the alloys.

1. State if carbon as reducing agent metal is used.
2. Most commonly used reducing agent silicon and aluminum.
3. All reduction is practised where contamination is undersirable.
Example Thermit welding
Fe2O3 + 2Al = Al2O3 + 2Fe.

Calico - Thermic Reduction :

Calciothermic reactions are metallothermic reduction reactions, more generally thermic chemical reactions which use calcium metal as the reducing agent at high temperature.
● Calcium is one of the most potent reducing agents available, usually drawn as the strongest oxidic reductant in Ellingham diagrams, though the lanthanides best it in this respect in oxide processes. 
● On the other hand, this trend does not continue to other compounds that are non-oxides, and for instance lanthanum is produced by the calciothermic reduction of the chloride, calcium being a more potent reducing agent than lanthanum involving chlorides.
● Calciothermic processes are used in the extraction of metals such as uraniumzirconium, and thoriumfrom oxide ores.
1. Calicum produces most stable content CaO.
2. Metal alloy can be reduced by used some other medium.
3. Metal which is highly reactive
 
      TiCl (liq) + 2Mg (liq) = 2MgCl2 + Ti (solid).


MATTE SMELTING

Matte is the mixture of metal sulphides. This is commonly practised from Cu and nickel sulphide ore, which generally carried out in revervaratory furnace. Commonly slag and liquid metal obtained in this furnace is Cu2S + FeS.
Definition :
Matte is a molten mixture of sulphides. Of the greatest industrial importance is iron‐ copper matte which is an intermediate product in the extraction of copper from sulphide ores. 

● In production of nickel, matte of copper‐nickel is an intermediate product. Next is copper‐ nickel matte. One advantage of matte smelting is its low melting point which makes it possible to smelt sulphide ores at lower temperatures than required for metals. 

●Thus a matte with equal amounts of copper and iron sulphide melts below 1000 , whereas an alloy of Fe and Cu would melt at around 1400 . This leads to lower thermal energy requirements and gangue minerals can be separated easily as slag.

● If Cu present in majority then it will be present as metallic cupper in matte with some iron oxide.
● sometime small amount of dissolved elements of Nobel metal like Ag, Au and Pt etc.
● The solid charge material to the melting furnace consists of either a hot calcined from the rosters, be cold way concentrate directly from the mill heater.
● Flux, cement, Cupper precipitated are also charge in revervatory furnace.
● No, reductant is used.
● Depending on percentage of sulphur is charge some external flux may or mY not be used.
● Here sulphur produces a lot of heat.

Smelting Action :

1. It essentially a simple melting operation.
2. The fuel is burn with the current amount air so that maximum temperature is produced.
3. The charge consist at CuSO4, iron sulphide and sufficient flux form a suitable slag.
4.If the furnace is smelting calcined a good amount of iron and some of the cupper may be in the form of oxide.

5. If the cement Cu is charge there be some Cu in metallic form.
6. As the charge fuses the sulphide collects and molten metal.
7. As cupper has good affinity for Sulphur all oxide or metallic cupper will form stable Cu2S.
8. The grade at the matte are determine by Sulphur content of the charge, which in term is governed by the roasting operation.

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