Principles of Pretreatment of Ores for Metal


Defination of Metallurgical Terms 

Ore : Ore may be defined as naturally occurring agents or combination of minerals from which one or more economically metal extracted.

Minerals : Minerals is defined as a naturally inorganic compound of one or more metals is associated with non metal such as O.S halogen. A mineral has fixed composition and well define physical and chemical properties.

Gangue : When ore deposit are exploted in extractive metallurgy there is generally waste product having there, thus waste product is composed of some minerals in the ore which are not used from the point of view of the metal extraction. These are known as gangue.

Flux :
This is a substance when used in smelting it used to lower both liquid temperature and viscosities of slag.
Types of flux's are :
1. Oxidizing - KNO3, potassium nitrate, NaNO3.
2. Natural flux - CaF2, calcium floride.
3. Reducing flux - NaCN, sodium cyanide.
4 many other are like SiO2, MgCO3, NaSO4, CaCO3 etc.

Slag : 

The two main function of slag are to collect the unreduced gangue mineral so as to from a separate layer in a metal extraction process and provides a medium in which impurities in a metal can collect during the refining process.
Properties of Slag :
1. The difference between the specific gravity of the slag and the metal should be sufficiently different, so one can easily separated.
2. The slag most be fluid enough to permit it's easily separation from metal and also to bring about a mass transfer faster than that the metal.
3. The slag have a chemical composition which answer that the activity of the impurities and of the dissolve gangue mineral.

Smelting : It is defined as any metallurgical operation or comminution of operation produces liquid metal
Ore + flux + Reducing agent = Metal + Slag + gas

Matte : A matte is a metallic sulphide sloutios that contains miner amount of oxygen and sometimes some metal too. In the metal smelting the sulfide ore is fuse with a flux to produce a mountain mixture of sulphide known as matte.

Speiss : When arcenical ore are smelted arcenied of two or more metals are form the heterogeneous mixture so obtained is known as speiss. The important components of speiss arcenic cobault, cupper, iron nickel etc.

Metals : An elemental chemical substance that is a good conducted of heat and electricity and when polished a good reflected of light. Mist elemental metal are malleable and ductile and ore in general than any other elemental substance.

Alloys : An alloys may may be defined as the combination of two or more elements mixture in different propertion. One of the element should be metal in major quantity will other should be metal or non metal in minor quantities.

Principle of Pretreatment of Ore for Metal

Drying :

● The temperature of 200℃ it usually means the removal of water or moisture from concentrate ore or other solid material. It is accomplished by converting the water to vapour which is carried away by a gas current. Drying is commonly done in drying furnace or kiln which is designed so that the wet material is brought in to contact with hot gas or air. The dryer must be constructed so that all the solid material comes in to contact with the hot gases.

● It is necessary to supply heat to drying furnace for reason of raise the latent heat of vaporisation absorbed when the vapour forms

H2O (L) + H2O (g) + H = 43.9 KJ.

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

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

● 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.

● 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.

Different Between the Calcination and Drying

1. Calcination resembles drying in that the material most be heated enough so that the dissociation pressure of the gas being evolved.

2. The temperature for calcination are much higher then those for drying.
3. Drying refers to removal of mechanically held water, but calcination refers to removal of chemically combination of water.

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