Introduction to Metallurgical Processes

Introduction

The biggest source of metals is earth's crust. Metals are also found in rocks, sea water, some impurities, etc depending upon their chemical nature. Less electropositive metals have less affinity for oxygen, moisture and hence occur in free or native or uncombined state, e.g. Au, Pt, Ag, etc. While the metals with higher electropositive character occurs in combined state as compounds.

The naturally occurring chemical substances in the earth's crust which are obtained by mining are known as minerals. Minerals which can be used as a source for commercial recovery of a desired substance are termed as ores. Earthy substance associated with ores are called gangue.

Sixteen most abundant elements in the earth's crust are given as:

(Total percentage weight of these elements = 99.77)

Ores can be divided into the various categories according to table.

Some important compounds

Some important alloys and their uses are as follows:

Illustration 1. (i) List three metals that are found in nature as oxide ores.

(ii) If earth had the atmosphere that contained only $O_{2}andS{O}_2$and no $C{O}_2$then what would be the composition of the minerals you would expect to find.

Solution: (i) Fe as$F{e}_3{O}_4,TiasTi{O}_2,SnasSn{O}_2$

(ii) $MS{O}_4,MS{O}_3$, Oxide

Illustration 2. An ore which does not contain oxygen is

(A) bauxite (B) haematite

(C) chalcopyrite (D) calamine

Solution: (C)

Chalcopyrite is a sulphide ore.

Metallurgy

The whole process of obtaining a pure metal and its alloy of desired property from one of its ores is known as metallurgy. In order to extract the metal from ores several physical and chemical methods are employed depending upon the nature, availability and properties of metals.

The following flow sheet shows general metallurgical operations. Function of processes are given on the right side whereas methods employed are indicated on the left of the flow sheet.

Metallurgical Processes

The various metallurgical operations in the extraction of pure metals from their respective ores are given below:

(i) Concentration of the ore: Removal of gangue or matrix from the powdered ore is known as concentration or dressing of the ore. It can be carried out in the following ways:

(a) Levigation: Also called gravity separation or hydraulic washing where the lighter gangue particles are removed from heavier ore particles by washing in a current of water. It is generally used for oxide ores and carbonate ores. It is based on difference in densities of the ore particles and impurities.

(b) Froth – floatation: It is used for concentration of sulphide ores. Foaming agent like pine oil is added which raises the lighter ore particles to the top in the form of froth while the heavier gangue particles settle down.

Adsorption phenomenon is involved in this method.

(c) Electromagnetic separation: This method is used when either the ore or the impurities are magnetic in nature e.g. chromite $(FeC{r}_2{O}_4),$magnetite $(F{e}_3{O}_4)$and pyrolusite $(Mn{O}_2)$ are all magnetic ores and are separated from their non – magnetic gangue by this process.

(d) Chemical method-leaching: This is a chemical method of concentration of ore. A suitable reagent is added which dissolves the ore but not the impurities.

e.g. Noble metals (Ag and Au) are leached with dilute aqueous solution of NaCN or KCN in presence of air, while other impurities remain undissolved.

$A{g}_{2}S+4NaCN\to 2Na[Ag{(CN)}_2]+N{a}_{2}S$

Argentite Sodium argentocyanide

(ii) Roasting: The ore is heated strongly below its melting point in presence of excess of air which removes impurities of non – metals at their volatile oxides. This process is used for the conversion of sulphide ores to their respective oxides (de – electronation of ores)

$2ZnS+3O_2\to 2ZnO+2S{O}_2$

$ZnS+2O_2\to ZnS{O}_4$

(iii) Calcination: Ore is heated strongly in absence or limited supply of air (air is used here as carrier of heat and moisture). Carbonates and hydrated oxide ores are converted to their respective oxides.

$CaC{O}_3\to CaO+C{O}_2$

$A{l}_2{O}_3.2H_{2}O\to A{l}_2{O}_3+2H_{2}O$

(iv) Reduction of the metal oxide to free metal: The roasted ore is reduced to metallic state by the following methods:

(a) Reduction by carbon – smelting: Metal is extracted by heating calcined or roasted ore with powdered coke in presence of a flux.

$PbO+C\to Pb+CO$

$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$

(b) Reduction by aluminium – Goldschmidt aluminothermic process:

$F{e}_2{O}_3+2Al\to A{l}_2{O}_3+2Fe$

$C{r}_2{O}_3+2Al\to A{l}_2{O}_3+2Cr$

$3M{n}_3{O}_4+8Al\to 4A{l}_2{O}_3+9Mn$

Mixture of metallic oxide such as $F{e}_2{O}_3$and aluminium powder in 3:1 ratio by mass is known as thermite.

(c) Reduction by precipitation (hydrometallurgy):

By this technique at normal temperature the metal can be extracted.

For example : Ag, Au are precipitated by this preparation.

$2Na[Ag{(CN)}_2](aq)+Zn(s)\to N{a}_2[Zn{(CN)}_4]+2Ag\downarrow$

$2Na[Au{(CN)}_2](aq)+Zn(s)\to N{a}_2[Zn{(CN)}_4]+2Au\downarrow$

(d) Electrolytic Reduction:

This is applied for the active metal chloride or $A{l}_2{O}_3$

$MX\to M^{+}+X^{-}$

At cathode: $M^{+}+e\to M$

At anode: $X^{-}-e\to X$

$2X\to X_2$

(e) Reduction by magnesium (Kroll's process) :

This method is applicable for the reduction of $TiC{l}_4$

$TiC{l}_4+2Mg\to 2MgC{l}_2+Ti$

(f) Auto reduction:

$2HgS+3O_2\to 2HgO+2S{O}_2$

$2HgO+HgS\to 3Hg+S{O}_2$

In this sulphides of Hg, Cu, Pb etc. are oxidized to their respective oxides which further react with sulphides to yield the metal.

(v) Refining or purification of crude metal:

(a) Liquation: This method is used for those metals which are readily fusible (i.e. having low melting points) such as Pb, Sn, Bi and Hg as compared to impurities. The metal melts and flows down the sloping hearth leaving behind infusible material called dross on the hearth.

(b) Distillation: This method is used for the refining of such metals which have low boiling points such as Hg, Zn, Cd, etc.

(c) Poling: This method is used when impure metal has impurities of its own oxide. The molten impure metal is stirred with green wood poles. Gases such as $C{H}_4$are liberated which reduce any oxide present in the metal.

(d) Cupellation: The impure metal is heated in a cupel or oval shaped crucible made of bone ash or cement and a blast of air is passed over the molten mass. The impurities get oxidised and removed with the blast of air. For example the impurity of lead present in silver is removed by cupellation process.

(e) Bessemerisation: The impure metal is heated in a furnace and a blast of compressed air is blown through the molten mass. The impurities get oxidised. For example, the molten pig iron is taken in a Bessemer converter and compressed air is passed which oxidises the impurities.

$2Mn+O_2\to 2MnO$

$Si+O_2\to Si{O}_2$

$2C+O_2\to 2CO$

$MnO+Si{O}_2\to MnSi{O}_3$

Slag

(f) Van Arkel process: This method is generally applied for obtaining ultrapure metals. The impure metal is converted into a volatile compound while the impurities are not affected. The volatile compound is then decomposed electrically to get the pure metal.

Ti, Zr, Hf etc., have been refined by this method.

$Ti(s)+2I_2(g)\stackrel{523K}{\to }Ti{I}_4(g)$

Impure

$Ti{I}_4(g)\stackrel{1700K}{\to }Ti+2I_2(g)$

(g) Zone refining or Fractional crystallisation: Elements such as Si, Ge, Ga, etc., which are used as semiconductors are refined by this method. Highly pure metals are obtained. The method is based on the difference in solubility of impurities in molten and solid state of the metal.

Summary of the Extraction of Metals

Illustration 3. The roasting of an ore of a metal usually results in conversion of the metal to the oxide. Why does the roasting of cinnabar, HgS, produce metallic mercury rather than an oxide of mercury?

Solution: $HgS\to HgO\to Hg$

Since Hg lies much below in the electrochemical series hence HgO is further reduced to Hg.