General Principles & Processes of Isolation of Elements

Minerals and Ores

Most of the elements occur in combined states in the earth's crust. So, elements are needed to be extracted from their combined forms.

The process of extraction is different for different elements, but some process are common and are applied to the process of extraction in all of the elements.

Metallurgy

The whole scientific and technological process of extraction and purification of elements is commonly known as Metallurgy. Metallurgy is a branch of science which deals with the physical and chemical behaviours of metallic elements, their intermediate compounds and their alloys.

Minerals

A Mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes. This is the formal definition of mineral approved by IMA in 1995.

Or, Naturally occurring chemical substances in the earth's crust obtainable by mining is called MINERALS.

Ores

Minerals that are viable to be used as sources are called ores.

Gangue

Impurities present in ores is known as Gangue.

Common Process of Extraction and Isolation of Metals from Ores

(a) Concentration of Ores

(b) Isolation of the metal from its concentrated ore, and

(c) Purification of the metal

Occurrence of Metals

The abundance of metals is not uniform in the earth's crust. Aluminium is the most abundant metals and is the third most abundant element in the earth's crust.

In earth's crust the percentage of aluminium is approximately 8.3% by weight. Aluminium is the major component of many igneous minerals including mica and clays. Many gemstones are impoure forms of `Al_2O_3` and the impurities range from `Cr` (in 'ruby') to `Co` (in 'sapphire').

Iron is the second abundant metal in the earth's crust.

Concentration of the Ore

Removal of the unwanted materials, such as sand, clays, etc. from the ore is known as Concentration. Concentration of Ore is also known as dressing or benefaction.

Steps in the concentration of ores, depend upon the types of gangue present in the ores. Along with types impurities present in ores, the available facilities and the environmental factors are also taken into consideration in the concentration of ores.

Some of the important procedures of concentration of ores

Hydraulic Washing

Hydraulic washing based on the differences in gravities of the ore and the gangue particles. Hydraulic washing is a type of gravity separation.

In this process of Hydraulic washing, 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

The process of Magnetic separation is based on the differences in magnetic properties of the ore components. If either of the ore or the gangue is capable of being attracted by magnetic field, then magnetic separation is carried out, such as iron ores.

In the ground ore is carried out on a conveyer belt which passes over a magnetic roller. The magnetic substances is attracted by magnetic roller and separated out in separate container.

Froth Floatation Method

This process is used for selectively separating hydrophobic materials from hydrophilic.

In the process a suspension of 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-wettablility of the mineral particles. And froth stabilizers, e.g. cresols, aniline, etc. stabilize the froth.

The mineral particles become wet by oils while the gangue particles by water. The mixture is agitated and air is drawn in it using a rotating paddle. As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off, which is then dried for recovery of the ore particles.

This method has been used for removing gangue from sulphide ores.

Sometime, it is possible to separate two sulphide ores by adjustment of proportion of oil to water by using depressants. For 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 froth.

Leaching

Leeching is a process to use if the ore is soluble and impurities are insoluble in any of the suitable solvent.

(a) Leaching of alumina from bauxite

Bauxite `[AlO_x(OH)_(3*2x)]` is one of the primary ores of aluminium. Bauxite contains `SiO_2`, Iron oxide and titanium oxide `(TiO_2)` as impurities.

Concentration is carried out by digesting the powdered ore with a concentrated `NaOH` solution at a temperature of `473 - 523K` and a pressure of `35-36` bar. In this, `Al_2O_3` is leached out as sodium aluminate and `SiO_2` as sodium silicate leaving impurities behind.

The aluminate in the solution is neutralized by passing `CO_2` gas in which `Al_2O_3` is precipated. At this stage the solution is treated with freshly prepared samples of hydrated `Al_2O_3` which induces the precipitation.

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

Cyanide Process

In the metallurgy of silver and gold, the respective metal is leached with a dilute solution of `NaCN` or `KCN` in the presence of air from which the metal is obtained later by replacement.

Extraction of Crude Metal from Concentrated Ore

Concentrated ores are converted to a form suitable for reduction. Sulphide ores are usually converted into oxide which is reduced to respective metals.

Thus, extraction of crude metals from concentrated ores involves two major steps, these steps are conversion to oxide and reduction of oxide to metals.

(a) Conversion to oxide

Calcination

Calcination is a process of heating which is done in the absence or under the limited supply of air to separate volatile matters leaving behind the metal oxides. Calcination is applied to ores and other materials to bring about thermal decomposition.

Example:

Haematite ores (ores of iron) is put under calcinations after concentration, which escapes water as vapour leaving behind oxides.

`Fe_2O_3*xH_2O(s)` `stackrel{\Delta}->Fe_2O_3(s)+xH_2O`

Calamine ore (Ore of zinc) when put under calcinations, it decomposes into zinc oxide (`ZnO`) and carbon dioxide (`CO_2`).

`ZnCO_3(s)` ` stackrel{\Delta}->ZnO(s)+xCO_2\ (g)`

Similarly when `CaCO_3*MgCO_3` is put under calcinations, it decomposes into calcium oxide (`CaO`), Magnesium oxide (`MgO`) and carbon dixodie (`CO_2`).

`CaCO_3*MgCO_3(s) stackrel{\Delta}->` `CaO(s)+MgO(s)+CO_2\ (g)`

Roasting

Usually sulphide ores after concentration are converted into respective oxide by the process of Roasting. Roasting is a process in which ores is heated in a regular supply of air in a reverberatory furnace at a temperature below melting point of the metal. This (roasting) decomposes respective oxides from ores.

Example:

`2ZnS+3O_2 ->2ZnO+2SO_2`

`2PbS + 3O_2 -> 2PbO+2SO_2`

`2Cu_2S+3O_2->2Cu_2O+2SO_2`

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`.

`FeO + SiO_2 -> FeSiO_3`

The `SO_2` produced in the process of roasting is utilized for manufacturing of `H_2SO_4`

(b) Reduction of Oixde to the Metal

Metal oxide forms in the process of calcinations or roasting is further heated with reducing agent, such as `C` or `CO` or even another metal, which reduces metal oxides to respective metals.

`MxOy + yC ->xM + y CO`

[Where, `M` is metal]

Thermodynamic Principles of Metallurgy

Oxides of some metals reduce easily which others are very difficult to reduce, although heating is required in every case. In such cases the variation in the temperature is required to understand form thermal reduction (pyrometallurgy) and to know which element will suit as the reducing agent for a given metal oxide (`M_xO_`), Gibbs energy interpretation are made.

The change in Gibbs energy, `DeltaG` for any process at any specified temperature, is described by the equation:

`DeltaG = \Delta\ H-T\Delta\S` ----------(i)

Where `Delta\H` is the enthalpy change and `Delta\S` is the entropy change for the process.

For any reaction this enthalpy change could also be explained by the equation:

`Delta\G^(⊖) = -RT\ln\K` -----------(ii)

Where, `K` is the equilibrium constant of the 'reactant - product' system at the temperature, `T`. A negative `Delta\G` implies a +ve K in the above equation. And this can be happened only when reaction proceeds towards products. Thus,

(1) When the value of `Delta\G` is negative in equation (i), only then reaction will proceed. If `Delta\S` is positive on increasing the temperature `(T`), the value of `TDelta\S` would increase (`Delta\H < T\Delta\S`) and then `Delta\G` will become negative.

(2) If reactants and products of two reactions are put together in a system and the net `Delta\G` of the two possible reactions is negative, the overall reaction will occur. So the process of interpretation involves coupling of the two reactions, getting the sum of their `Delta\G` and looking for its magnitude and sign. Such coupling is easily understood through Gibbs energy (`Delta\G^(⊖)`) vs `T` plots for formation of the oxides.

Explanation:

The reducing agent forms its oxide when the metal oxide is reduced.

The role of reducing agent is to provide `Delta\G^(⊖)` of the two reactions, (oxidation of the reducing agent and reduction of the metal oxide) negative.

As we know, during reduction, the oxide of a metal decomposes:

`MxO(s)->xM(s\ or\ l) + 1/2O_2(g)` --------(iii)

The reducing agent takes away the oxygen. Eqation (iii) can be visualized as reverse of the oxidation of the metal. And then, the `Delta_fG^(⊖)` value is written in the usually way:

`xM(s\ or\ l) + 1/2O_2 (g) -> MxO(s) [DeltaG_(M*MxO)^(⊖)]` ------(iv)

If reduction is being carried out through equation (iii), the oxidation of the reducing agent (e.g. C or CO) will be there:

`C(s) + 1/2O_2(g) ->CO_2(g) [DeltaG_(C, CO)]` -------------(v)

`CO(g) + 1/2 O_2(g) -> CO_2(g)[DeltaG_(CO, CO_2)]` -----------(vi)

If carbon is taken, there may also be complete oxidation of the element to `CO_2`:

`1/2C (s) + 1/2O_2(g) -> 1/2 CO_2(g) [1/2\ Delta\ G_(C,CO_2)]` ----------(vii)

On subtracting equation (iv) from one of the three equations, we get

`MxO(s) + C(s) ->xM(s\ or\ l) + CO(g)` ------------(viii)

`MxO(s) + CO(g) -> xM(s\ or\ l) + CO_2(g)` -------------(ix)

`MxO(s) + 1/2C (s) ->xM(s\ or\ l) + 1/2CO_2(g)` -----------(x)

These reactions describe the actual reduction of the metal oxide, `MxO` that we want to accomplish. The `Delta,G^(⊖)` values for these reaction in general, can be obtained by similar subtraction of the corresponding `Delta_fG^(⊖)` values.

As it can be seen, heating favours a negative value of `DeltarG^(⊖)`. Therefore, the temperature is chosen such that the sum of `DeltarG^(⊖)` in the two combined redox process is negative. In `DeltarG^(⊖)` vs `T` plts, this is indicated by the point of intersection of the two curves.

After that point, the `DeltarG^(⊖)` value becomes more negative for the combined process including the reduction of `MxO`. The difference in the two `Delta\r\G^(⊖)` values after that point determines whether reductions of the oxide of the upper line is feasible by the element represented by the lower line. If the difference is large, the reduction is easier.

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