Metallurgy: Oxygen and Ore

Minerals are naturally occurring chemical substances in the earth’s crust obtainable by mining. Out of many minerals in which a metal may be found, only a few are viable to be used as sources of that metal. Such minerals are known as ores. Examples: Galena, PbS, etc. It is usually contaminated with earthly or undesired materials known as gangue. The extraction and isolation of metals from ores involve the following major steps: • Concentration of the ore,

• Isolation of the metal from its concentrated ore, and
• Purification of the metal. It is also called ‘Refining’.

Stages of extraction of metals:
After the concentration of the ore is done, there are two main steps or stages in extractive metallurgy. They are: Conversion of the ore into Metal oxide or metal compound.
Extraction of crude metal from metal from metal compound.

Concentration of ores:
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.

Principle: Concentration is done mainly to increase the percentage of pure metal. It is of 3 types.
1. Gravity separation (Hydraulic washing)
2. Froth floatation
3. Magnetic separation.

Froth floatation:
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 stabilizers are added. Collectors (e. g., pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilizers (e. g., cresols, aniline) stabilize the froth. The mineral particles become wet by oils while the gangue particles by water. 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. Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or 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 the froth.

Magnetic Separation:
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.

Extractive Metallurgy is broadly classified into:

1. Pyrometallurgy 2. Hydrometallurgy 3. Electrometallurgy.

1. Pyrometallurgy:
Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials. Solids containing valuable metals are reacted to form intermediate compounds for further processing or converted into their elemental or metallic state. Pyrometallurgical processes that involve gases and solids are typified by calcining and roasting operations. Processes that produce molten products are collectively referred to as smelting operations. The energy required to sustain the high temperature pyrometallurgical processes may come entirely from the exothermic nature of the chemical reactions taking place, usually oxidation reactions. Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.

2. Electrometallurgy:
Electrometallurgy involves metallurgical processes that take place in some form of electrolytic cell. The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. The metal of interest is plated onto the cathode, while the anode is an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode. Fused salt electrolysis is another electrometallurgical process whereby the valuable metal has been dissolved into a molten salt which acts as the electrolyte, and the valuable metal collects on the cathode of the cell. The fused salt electrolysis process is conducted at temperatures sufficient to keep both the electrolyte and the metal being produced in the molten state.

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 (for the reason see box). Thus isolation of metals from concentrated ore involves two major steps , they are: (a) conversion to oxide, and

(b) reduction of the oxide to metal

(a) Conversion to oxide
(i) Calcination: Calcinaton involves heating when the volatile matter escapes leaving behind the metal oxide: Fe2O3.xH2O → Fe2O3 (s) + xH2O(g)
ZnCO3 → ZnO(s) + CO2 (g)
CaCO3.MgCO3(s) → CaO(s) + MgO(s ) + 2CO2(g)

(ii) 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 + 3O2 → 2ZnO + 2SO2
2PbS + 3O2 → 2PbO + 2SO2
2Cu2S + 3O2 → 2Cu2O + 2SO2

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 Cu2S and FeS. FeO + SiO2 → FeSiO3.

(b) 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. MxOy + yC → xM + y CO.

Hydrometallurgy
Leaching: (agents; NaOH, Hcl, H2So4) Usually done for low grade ores. 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 SiO2, 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, Al2O3 is leached out as sodium aluminate (and SiO2 too as sodium silicate) leaving the impurities behind: Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH) 4](aq)

[Here, concentration is adding of metal with less reduction potential to a metal with lower reduction potential.] The aluminate in solution is neutralised by passing CO2gas and hydrated Al2O3 is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces the precipitation: 2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s) + 2NaHCO3 (aq)

The sodium silicate remains in the solution and hydrated aluminais filtered, dried and heated to give back pure Al2O3.

Al2O3.xH2O(s)(1470 K) Al2O3(s) + xH2O(g)

Smelting:
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes production of silver, iron, copper and other base metals from their ores. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gasses or slag and leaving just the metal behind. The reducing agent is commonly a source of carbon such as coke, or in earlier times charcoal. The carbon (or carbon monoxide derived from it) removes oxygen from the ore, leaving behind elemental metal. The carbon is thus oxidized in two stages, producing first carbon monoxide and then carbon dioxide. As most ores are impure, it is often necessary to use flux, such as limestone, to remove the accompanying rock gangue as slag. Process:

Smelting involves more than just melting the metal out of its ore. Most ores are a chemical compound of the metal with other elements, such as oxygen (as an oxide), sulphur (as asulfide) or carbon and oxygen together (as a carbonate). To produce the metal, these compounds have to undergo a chemical reaction. Smelting therefore consists of using suitable reducing substances that will combine with those oxidizing elements to free the metal.

Roasting
In the case of carbonates and sulfides, a process called “roasting” drives off the unwanted carbon or sulfur, leaving an oxide, which can be directly reduced. Roasting is usually carried out in an oxidizing environment. A few practical examples: * Malachite, a common ore of copper, is primarily copper carbonate (CuCO3). This mineral undergoes thermal decomposition to CuO and CO2 in several stages between 250°C and 350°C. The carbon dioxide is expelled into the atmosphere, leaving copper oxide which can be directly reduced to copper as shown below. * Galena, the most common mineral of lead, is primarily lead sulfide (PbS). The sulfide is oxidized to a sulfite (PbSO3) which thermally decomposes into lead oxide and sulfur dioxide gas. (PbO and SO2) The sulfur dioxide (like the carbon dioxide in the example above) is expelled, and the lead oxide is reduced as below.

Fluxes
Fluxes are used in smelting for several purposes, chief among them catalyzing the desired reactions and chemically binding to unwanted impurities or reaction products. Calcium oxide, in the form of lime, was often used for this purpose, since it could react with the carbon dioxide and sulfur dioxide produced during roasting and smelting to keep them out of the working environment. Flux and slag can provide a secondary service after the reduction step is complete: They provide a molten cover on the purified metal, preventing it from coming into contact with oxygen while it is still hot enough to oxidize readily.