ROASTING | Unit Process of Pyrometallurgy


The general method of extraction method is three type
1. Pyrometallurgy
2. Hydrometallurgy
3. Electrometallurgy

PYROMETALLURGY

● It is deals with extraction of metal form their refining based on physical and chemical change occurring at high temperature that is 500 - 200℃.

● This is the process which is carried out at high temperature. Pyrometallurgy involves heating the ore in air to formed an oxide. It is then heated with a reducing agent such coal, charcoal which freeze the metal.

Classification of Pyrometallurgy :

The process can be classified in to
1. Solid state processing.
2. Liquid state processing.

Solid State : In solid state processing it doesn't involve any melting. It is typically carried at a temperature range of 500 - 1200℃.
Example : Rosting of sulphides, calcination and solid state reduction of metal oxide by CO, H2 etc.

Liquid State : This involves melting of at least metal containing phase and this is carried out at a higher temperature above 1200℃.
Examples : Blast furnace smelting, steel melting, refining of zinc from impure lead.

Advantage of Pyrometallurgy :

1. This method of metal production are usually cheaper and suitable for larger scale production.
2. Reaction reates are greatly accelarated at high temperature, small unit can therefore achieved at high productions rate.
3. Some reaction which are not possible at low temperature becomes so at high temperature.
4. There is greater lose of separation of the product metal from gangue, if the product can be melted and vopourised at high temp.
Example : All relevant an expensive metals iron, lead, zinc, aluminum, cupper etc are usually extracted by this process.

Unit Process of Pyrometallurgy 

The various unit process are used in pyrometallurgy are
1. Calcination.
2. Roasting.
3. Smelting.

CALCINATION

● The temperature required 1000 - 1200℃ and this is the thermal treatment of the ore to bring,  about its decomposition and determined the volatile product usually carbon dioxide and water. The furnace use for calcination are
Lime calcination through shaft kiln
Lime calcination Shaft klin 

1. Shaft furnace.
2. Rotary furnace.
3. Fluidised bed furnace.

● The calcination resembles drying in that the material must be heated enough, so that the associate pressure of the decomposition compound is greater that pressure of the gas being evolved. The temperature required are generally much higher than these needed for drying.

Example :
                 * The burning limestone to make quick llme
                  CaCO3 =  CaO + CO2  ∆H  =  39900 cal.
                 * Periodically MgO is formed by calcination of MgCO3 at about 1000℃ to drying of carbon dioxide and convert MgO to a stable crystalline structure.
                 * In the refining of bauxite a precipitated of Al tri-hydrate is filled and then calcined at about 1000℃ to drying of common water.
                  2Al(OH)3 = Al2O3 + 3H2O.

Mechanism of Calcination :

● Calcinations is the process of subjecting a substance to the action of heat, but without fusion, for the purpose of causing some change in its physical or chemical constitution. The objects of calcination are usually:

1. To drive off water, present as absorbed moisture, as "water of crystallization," or as "water of constitution";

2. To drive off carbon dioxide, sulphlur' dioxide, or other volatile constituent;

3. To oxidize a part or the whole of the substance. 'there are a few other purposes for which calcination is employed in special cases, and these will be mentioned in their propel' places. The process is often called "roasting," "firing," or "burning," by the workmen. It is carried on in furnaces, retorts, or kilns, and very often the material is raked over or stirred, during the process, to secure uniformity in the product.

● The furnaces used for calcining substances vary much in their construction, but there are three general classes: muffle, reverberatory, and shaft furnaces or Rotray kilns.

ROASTING

● It is defined as heating the ore below the fission point in presence of air to obtain the ore amenable to subsequent treatment. The temperature at which this operation is carried out is 500 - 1000℃. This is mostly done for sulphide ore.

● Roasting consists of thermal gas–liquid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis. In roasting, the ore or ore concentrate is treated with very hot air. This process is generally applied to sulfide minerals. During roasting, the sulfide is converted to an oxide, and sulfur is released as sulfur dioxide, a gas. For the ores Cu2S (chalcocite) and ZnS (sphalerite), balanced equations for the roasting are:

2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2
2 ZnS + 3 O2 → 2 ZnO + 2 SO2
The gaseous product of sulfide roasting, sulfur dioxide (SO2) is often used to produce sulfuric acid. Many sulfide minerals contain other components such as arsenic that are released into the environment.
Gold roasting overview
Gold Roasting

Objective of Roasting :

1. To oxidized the sulphide particles to oxides or sulphate.
2. To volatilized centrain impurities.
3. The removal of impurities by volatisation of certain oxides that is N2O, Sb2O3.

Mechanism of Roasting :

● Air is the universal oxidiser in all the cases sulphide particle are exposed to a current of air and ignited . As the air sticks the heated particles they burn to give some of these reaction.

● And the current of air carries sulphen dioxide away. The resting reaction being at the surface of the particles and are the reaction continue the oxide remain at a porous coating in order for the oxygen to continue to react with the sulphides for the SO2 to scare the hoses must pass through the particles.

● This implies that as the layer of oxides became thicker, the rosting  reaction tents to vevers because the concentration O2 decrease and concentration of SO2 in creases as a result the removal of sulphur is very difficult, by regulating the speed at which the roaster, it is possible to produce a roasted product having any desired sulphur content. A dead roast regards to a roasting operation in which all the sulpher is eliminated.

TYPES OF ROASTING 

1- Oxidising roasting.
2- Chloridising.
3- Sulphatizing.
4- Blast or Sinter.
5- Flash or suspension.
6- Autogenous.
7- Hearth.
8- Fluidised.
9- Fixed bed roasting.

1. Oxidising Roasting

Air is inversely used as anoxidising agent in roasting process is called Oxidising roasting. It is used to oxidise sulpher.
Example:  MS + 3/2 O2 = MO + SO2
                    PbS + 3/2O2 = PBO + SO2

2. Chloridising Roasting

This is carried out can convert certain metal compound to chlorides under Oxidising an reducing condition.
Example: 2NACL + Ms + 2O2 = Na2SO4 + MCL2
This reaction is particularly suitable for sulphide ore.

3. Sulphatizing Roasting

It is a modification of Oxidising roasting carried out under oxidizing condition and at lower temperature 
Example:  2PbS + 4O2 = 2PbSO4  low temperature
                   2PbO + PbS = 3Pb +SO2 high temperature

4. Blast or Sinter Rosting

Here charge to be roasted is held stationary and a current of air is drawn though it. A certain amount of limestone with the roaster charge is mixed and heat is absorbed in heating and decommissioning the calcium carbonate, lower the temperature to a point where function is avoided. In Italy the off draft roasting put with grated bottom method was employed. This method was intermittent at today, it has largely replaced by the continuously down draft roasting.

5. Flash Roasting

When the rosting operation is designed that all the roasting is done while the particularly are following through heating air, this process is known as flash roasting. This method roast rapidly and a flash roasted has a much greater capacity then the breath of same size.

6. Autogenerous Roasting

When the Oxidising reaction transplant during roasting is highly exothermic, then there is a minimum temperature about which the reaction is sustained by heat liberated by itself. Thus a roasting reaction when once installed continued to process even in the absence of any external heat is called Autogenous roasting process.

7. Hearth Roasting

The multiple Hearth roaster is in the shape of a vertical cylinder containing a number of super horizontal hearth, steel shell to support the Hearth and a rotating central shaft which carries the rabble hearth, air enters from the bottom and is heated by contact with roasting material. The ore concentration enters at the top and drops from Hearth of reach is succession until it is section at the bottom. The temp at this furnace must hat high enough to cause any fussion 

8. Fluidise bed Roasting

If we construct an apparatus where are finely divided solid material are kept in suspension by roasting a current of air. We produced a fluidised bed. This providing an ideal condition for gas solid reaction because each particle is in constant motion and is continuously in contact with the moving gas stream on all sides. The temperature must be kept low enough so that there is no fussion of the particle. In fluidization the temperature in particles partially constant through out all parts of the bed.

Uses - It is used as proposed such as cracking limestone produce quick lime.

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