Chemistry Concentration and Extraction of Ores

Topics Covered :

● Concentration of Ores
● Hydraulic Washing
● Magnetic Separation
● Froth Floatation Method
● Leaching
● Extraction of Metal from Concentrated Ore

Concentration of Ores :

`text(Definition :)` Removal of the unwanted materials (e.g., sand, clays, etc.) 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 gangue.

● The type of the metal, the available facilities and the environmental factors are also taken into consideration.

● Some of the important procedures are described below.

Hydraulic Washing :

`text(Principle :)` This is based on the differences in gravities of the ore and the gangue particles. Therefore, it is a type of gravity separation.

`=>` In this process, an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind.

Magnetic Separation :

`text(Principle :)` This is based on differences in magnetic properties of the ore components. If either the ore or the gangue (one of these two) is capable of being attracted by a magnetic field, then such separations are carried out (e.g., in case of iron ores).

● The ground ore is carried on a conveyer belt which passes over a magnetic roller.

Froth Floatation Method :

`text(Principle :)` This method has been in use for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water. To it, collectors and froth stabilisers are added.

● Collectors (e. g., pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles.

● Froth stabilisers (e. g., cresols, aniline) stabilise the froth.

`=>` The mineral particles become wet by oils while the gangue particles by water.

`text(Procedure :)`

● A rotating paddle agitates the mixture and draws air in it.

● As a result, froth is formed which carries the mineral particles.

● The froth is light and is skimmed off. It is then dried for recovery of the ore particles.

`text(Note :)` Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using `text(depressants)`.

● Example : In case of an ore containing `ZnS` and `PbS`, the depressant used is `NaCN`. It selectively prevents `ZnS` from coming to the froth but allows `PbS` to come with the froth.

Leaching :

`text(Principle :)` Leaching is often used if the ore is soluble in some suitable solvent. The following examples illustrate the procedure :

(a) Leaching of Alumina from Bauxite :

● The principal ore of aluminium, bauxite, usually contains `SiO_2`, iron oxides and titanium oxide (TiO2) as impurities.

● Concentration is carried out by digesting the powdered ore with a concentrated solution of `NaOH` at `473 – 523 K` and 35 – `36` bar pressure. This way, `Al_2O_3` is leached out as sodium aluminate (and `SiO_2` too as sodium silicate) leaving the impurities behind :

`Al_2O_3 (s) +2NaOH (aq) +3H_2O(l) → 2Na [Al (OH)_4] (aq)` ..........(1)

● The aluminate in solution is neutralised by passing `CO_2` gas and hydrated `Al_2O_3` is precipitated.

● At this stage, the solution is seeded with freshly prepared samples of hydrated `Al_2O_3` which induces the precipitation :

`2Na [Al (OH)_4](aq) +CO_2 (g) → Al_2O_3 * x H_2O(s) +2NaHCO_3 (aq) ` ............(2)

● The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure `Al_2O_3` :

`Al_2O_3 * x H_2O (s) overset(1470 K)→ Al_2O_3 (s) + x H_2O (s)` ..............(3)

(b) Other Examples :

● In the metallurgy of silver and that of gold, the respective metal is leached with a dilute solution of `NaCN` or `KCN` in the presence of air (for `O_2`) from which the metal is obtained later by replacement :

`4M (s) +8CN^- (aq) +2H_2O (aq) O_2 (g) → 4 [M (CN)_2]^(-) (aq) +4OH^(-) (aq) \ \ \ \ ( M = Ag & Au )` .......(4)

`2[M(CN)_2]^(-) (aq) + Zn (s) → [Zn (CN)_4]^(2-) (aq) +2M (s) ` ...........(5)

Extraction of Crude Metal from Concentrated Ore

`=>` The concentrated ore must be converted into a form which is suitable for reduction.

`=>` Usually the sulphide ore is converted to oxide before reduction.

`=>` Oxides are easier to reduce.

`=>` Thus, isolation of metals from concentrated ore involves two major steps viz.,

(a) conversion to oxide, and

(b) reduction of the oxide to metal.

(a) `text(Conversion to Oxide :)`

(i) `text(Calcination :)` Calcinaton involves heating when the volatile matter escapes leaving behind the metal oxide.

`Fe_2O_3 * x H_2O (s) overset(Delta)→ Fe_2O_3 (s) +x H_2O (g)` ............(6)

`ZnCO_3(s) overset(Delta)→ ZnO (s) +CO_2 (g)` ...........(7)

`CaCO_3 . Mg CO_3 (s) overset(Delta ) → CaO(s) +2CO_2 (g)` ...........(8)

(ii) `text(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. Some of the reactions involving sulphide ores are :

`2ZnS +3O_2 → 2ZnO +2SO_2` ............(9)

`2PbS +3O_2 → 2PbO +2SO_2` ..........(10)

`2Cu_2S +3O_2 → 2Cu_2O +2SO_2` ........(11)

`=>` The sulphide ores of copper are heated in reverberatory furnace.

`=>` If the ore contains iron, it is mixed with silica before heating.

`=>` Iron oxide slags of as iron silicate and copper is produced in the form of copper matte which contains `Cu_2S` and `FeS`.

`=>` During metallurgy, ‘flux’ is added which combines with ‘gangue’ to form ‘slag’. Slag separates more easily from the ore than the gangue. This way, removal of gangue becomes easier.

`text(Flux) `: It is a substance that chemically combines with gangue (earthy impurities) which may still be present in the roasted or the caleined are to form an easily fusible material called the slag.

`text(Flux) +text(Gangue) → text(Slag)`

The slag thus formed melts at the temperature of the furnace . It is insoluble in the molten metal and also being lighter floats over the surface of the molten metal from where it can be skimmed off from time to time .

`text(Types of fluxes)` : Depending upon the nature of the impurities present in the ore, fluxes are classified the following two types :

`text(Acidic fluxes)`. For basic impurities like lime or metallic oxides `(CaO , FeO, MnO, etc)` present in the ore , acidic fluxes like silica `(SiO_2)` and borax `(Na_2B_4O_7. 10H_2O)` etc, are used .

`tt( (CaO , + , SiO_2 , → , CaSiO_3) , (FeO , + , SiO_2 , → , FeSiO_3) , (text{(basic impurities)} , , text{(Acidic flux)} , , text{(Fusible slag)}))`

`text(Basic fluxes)` . For acidic impurities like silica `(SiO_2)` , phosphorus pentoxide `(P_4O_(10))` etc. present in the ore , basic fluxes like limestone `(CaCO_3)` , magnesite `(MgCO_3)` , haematite `(Fe_3O_4)` etc. are used.

`tt((SiO_2 , + , CaCO_3 , → , CaSiO_3 , + , CO_2 ↑) , (SiO_2 , + , MgCO_3 , → , MgSiO_3 , + , CO_2 ↑) , (text{(Acidic impurities)} , , text{(Basic flux)} , , text{(Fusible slag)} , , ))`

`FeO+SiO_2 → underset(Slag)(FeSiO_3)` ..........(12)

The `SO_2` produced is utilised for manufacturing `H_2SO_4 .`

(b) `text(Reduction of Oxide to the Metal :)` Reduction of the metal oxide usually involves heating it with some other substance acting as a reducing agent (`C` or `CO` or even another metal). The reducing agent (e.g., carbon) combines with the oxygen of the metal oxide.

`M_x O_y + y C → x M +y CO` ...........(13)

`=>` Some metal oxides get reduced easily while others are very difficult to be reduced (reduction means electron gain or electronation). In any case, heating is required.

To understand the variation in the temperature requirement for thermal reductions (pyrometallurgy) and to predict which element will suit as the reducing agent for a given metal oxide (`M_xO_y)`, Gibbs energy interpretations are made.