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Concentration of ore:
Removal of the unwanted materials (for example, sand, clays, and so on.) from the ore is known as concentration, dressing or benefaction. It involves several steps and selection of these steps depends upon the differences in physical properties of the compound of the metal present and that of the impurity. Different non-metals are extracted by de-electronation using different processes. Metals are extracted by electronation using definite methods as mentioned below.
(b) Pine oil or eucalyptus oil to produce foam and work as collector / transforming agent.
(c) Cresols or aniline to stabilise foam.
(d) Xanthate (commonly employed xanthate is potassium ethyl xanthate collector to increase wetting of sulphide ore.
(e) Depressants and activators to collect the required sulphide part from the ore. For example, Galena contains a good quantity of ZnS with PbS into water soluble complex Na2[Zn(CN4)].
The powered ore is put in a tank containing a stirrer, a device of passing compressed air and all the above material. Foam is produced by wetting and lifting of sulphide ore to the surface. It is washed with acidulated water to collect the ore.
5. Leaching:- It is the method of collecting the required metal from an ore, in the form of aqueous solution of its selection compound, by treating the ore with acid or base or some other reagent. Leaching is often used if the ore is soluble in some suitable solvent.
IMPURITY + FLUX ---------> SLAG
- Pulverisation:- The lumps of the ore are converted to small pieces by using jaw crushers and to powder by employing stamp mill or ball mill.
- Magnetic Separation:- This method id employed to separate the magnetic and non-magnetic components present in the ore by carrying the powdered ore on a conveyer belt passing over an electromagnetic roller. This is based on differences in magnetic properties of the ore components. The non-magnetic part is thrown away while the magnetic part makes a heap below the magnet. For example, magnetic impurity of wolframite (FeWO4) is separated from tin stone ( the ore of tin). Other cases are chromite ore (FeO.Cr2O3) , magnetite (Fe3O4), pyrolusite(MnO2), etc are magnetic.
- Hydraulic Washing ( gravity separation method):- This is based on the differences in gravities of the ore and the gangue particles. This method is employed to purify heavier ore such as oxides ( example, haematite, tin stone etc.), carbonates ( example, calamine, malachite, extra), native gold, etc. The ore is washed with running water using big tubs or wilfley table that has sluices (grooves). The lighter impurities are washed away and the heavier particles of the ore settle down.
- Froth Floatation Method:- This method is employed to purify/concentrate sulphide ores. This method uses:
(b) Pine oil or eucalyptus oil to produce foam and work as collector / transforming agent.
(c) Cresols or aniline to stabilise foam.
(d) Xanthate (commonly employed xanthate is potassium ethyl xanthate collector to increase wetting of sulphide ore.
(e) Depressants and activators to collect the required sulphide part from the ore. For example, Galena contains a good quantity of ZnS with PbS into water soluble complex Na2[Zn(CN4)].
The powered ore is put in a tank containing a stirrer, a device of passing compressed air and all the above material. Foam is produced by wetting and lifting of sulphide ore to the surface. It is washed with acidulated water to collect the ore.
5. Leaching:- It is the method of collecting the required metal from an ore, in the form of aqueous solution of its selection compound, by treating the ore with acid or base or some other reagent. Leaching is often used if the ore is soluble in some suitable solvent.
Leaching of alumina from bauxite
In Bayer's process, for red bauxite, the ore is continuously stirred with 45% NaOH at 200-250°C to collect the solution of sodium meta aluminate NaAlO2 or Na[Al(OH)4]. It is then diluted and kept with Al(OH)3 (suds) for about 36 hours at 25-50°C to convert it to Al(OH)3(ppt). Ignition of Al(OH)3 at 1200°C produces Al2O3.
Al2O3.2H2O + 2NaOH -------> 2NaAlO2 + 3H2O ( 45% NaOH {200-250°C})
NaAlO2 + H2O ------> Al(OH)3 + NaOH ( AlOH3 {36 hours/25-30°C})
2Al(OH)3 ------> Al2O3 + 3H2O ( 1200°C)
In serpeck's methods white bauxite is generally heated with coke in the presense of N2 gas. AlN produced is reacted with water to produce Al(OH)3 which on ignition gives Al2O3.
Al2O3 + N2 + 3C ------> 3CO + 2AlN
heated
AlN + 3H2O ------> NH3 + Al(OH)3
6. Conversion of Ore to Oxide Form: Sulphide ores thermodynamically more stable than CS2 and hence cannot be converted by using coke (general and common reducing agent) as reducing agent. Moreover CO2 is more volatile and thermodynamically more stable than CS2. This all means that ΔfG' of a metal sulphide is not compensated by the energy of reaction of metal sulphide with coke.
Hence, it is better to convert the sulphide ore to oxide form and then apply smelting, i.e., carbon reduction method.
- Calcination:- Conversion of hydrated oxides, carbonates, basic carbonates and hydroxide ores to their oxides by heating in the absence of O2 below their melting point is known as calcination.
Calcination makes the oxides porous thus increasing the surface area for further reaction.
Fe2O3.xH2O(s) -----> Fe2O3(s) + xH2O(g)
limonite heated ferric oxide
ZnCO3(s) -----> ZnO(s) + CO2(g)
calamine heated zinc oxide
CaCO3 -----> CaO + CO2
lime stone heated calcium oxide
- Roasting:- In roasting, the ore is heated in a regular supply of air in a furnace at a temperature below the melting point of the metal.
(a) Oxidising Roasting:- It is done to remove Sulphur(S), Arsenic(As), Antimony(Sb), Phosphorous(P) in the form of their oxides.
S + O2 ------> SO2
sulphur sulphur dioxide
As + O2 ------> As2O3
arsenic arsenic oxide
P + O2 -----> P4O10
phosphorous phosphorous pentoxide
Sulphide ore of lead(Pb), Zinc(Zn), Nickel(Ni), Copper(Cu) etc are roasted by this method.
2PbS + 3O2 ------> 2PbO + SO2
ZnS + O2 ------> ZnO + SO2
FeS + O2 ------> FeO + SO2
Cu2S + O2 ----> FeO + SO2
(b) Sulphating:- when oxidising roasting is done at moderate temperature apart of sulphide ore is oxidised to oxide and another part to sulphate.
PbS + O2 ----> PbO +SO2
PbS + O2 ----> PbSO4 (at moderate temperature)
ZnS + O2 -----> ZnO + SO2
Zns + O2 ----> ZnSO4 (at moderate temperature)
(c) Chlorinating:- If extraction of metal is difficult from its oxide in such cases metal is obtained by reduction of its chloride.
Ag2S + 2NaCl ----> 2AgCl + Na2S
silver galance
7. Reduction of Oxide to metal:
- Smelting:- The process of extraction of metal in few states [Fe(iron), Cu(copper) etc] or in gaseous state [Zn(zinc)] is called smelting. For the extraction of less electro-positive metals powerful reducing agent may be used.
FeO + SiO2 ---------> FeSiO3
impurity acidic flux slag
SiO2 + CaO ---------> CaSiO3
impurity basic flux slag
- Reduction by C and CO (carbon reduction method):- Metals which are less electro-positive and do not form carbites with carbon are reduced by this method.
C + O2 ----> 2CO
900-1500K
FeO + CO ----> Fe + CO2
900-1500K
ZnO +C -----> Zn + CO
1120-1323K
NiO + C -----> Ni + CO
NOTE:- Below and at 710°C (CO) is best reducing agent while above 710°C (C) is best reducing agent.
- Reduction by metals:- Highly electro-positive metals (like Na, K, Li, Al etc) are used to reduce the less electro-positive metals (like Cr, Cu, Mn etc).
(a) Kroll's process for titanium
TiCl4 + 2Mg ----> 2MgCl2 + Ti
(b) Gold Schmidt alumino thermite process
Oxides of maganese, chromium, iron, etc. can be reduced by using aluminium powder as reducing agent. A mixture of BaO2 and Mg is used as ignition mixture to supply heat.
3Mn3O4 + 8Al ----> 4Al2O3 + 9Mn
- Reduction by Hydrogen or Water gas:- Though, the use of H2 is expensive and not very safe, Yet H2 is used as reducing agent when carbon becomes ineffective for reduction.
CuO + H2 ----> Cu + H2O
VO3 + H2 ----> V + H2O
NiO + CO + H2 ----> Ni + CO2 + H2O
Ag2O + H2 ----> Ag + H2O
It is used for less electro-positive metals.
- Self reduction or Auto reduction:- Sulphide ores of less electro-positive metals like Hg, Pb, Cu, etc are heated in air as to convert part of the ore into oxide or sulphate which then reacts with the remaining sulphide ore to give the metal and sulphur dioxide. No external reducing agent is used in this process.
2HgS + 3O2 ----> 2HgO + 2SO2
cinnabar
HgO + HgS ----> Hg + SO2
PbS + O2 ----> PbO + SO2
galena
PbO + PbS ----> Pb + SO2
- Amalgumetion:- This process is used for extraction of noble metals like Platinum, Gold, Silver etc.
METAL + Hg ----> AMALGAM ----> Hg(v) + METAL
dist. pure
