Ores are found in natural abundance and contain our desired minerals along with impurities. It is important to remove these impurities from the ores to obtain the high-grade metals that we will use to cast different items. These impurities are called gangue and are usually removed by the various purification processes. The common flow of metal extraction from ores are as follows:
- The concentration of the ores
- Isolating the metal from its concentrated ore
- Purification of metal
We’ll discuss these steps in detail.
The concentration of ores:
Concentration is when we remove the unwanted materials like sand, dust, etc., from the ores, so we get concentrated ore full of our required metals. It is also called dressing or benefaction. First, the physical and chemical properties of gangue and the metals are accessed. A suitable process is then used. Before going for concentration, the ores go through crushing and grinding to make the small size ores.
Different methods of concentration:
In this type of separation, the fundamental property of their specific gravities is used. The difference in the gravity of the gangue and the metal is used as the separation mechanism. In this gravity separation method, an upward stream of running water is used to wash away the lighter particles. And the heavier particles are found at the bottom.
In this method, the magnetic properties of the ores or gangue are used to separate them. A roller made of magnetic wheels is used to attract the magnetic particles in a different container. In contrast, the non-magnetic compounds are gone in a different container.
This method is extensively used to remove gangue from sulfide ores. In this method, the crushed ore is mixed with water to form a suspension. Froth stabilizers and collectors are added to the suspension. Collectors increase the unwettable quality of the mineral particles, and froth stabilizers make the froth stable. Oils make the mineral particles wet, while water soaks the gangue. A rotating paddle andair stirs the mixture in the process. Consequently, froth carrying the mineral particles is formed. The light froth is skimmed off and then dried for the recovery of the mineral particles.
It is likely to separate two sulphide ores by adjusting the proportion of oil to water or using ‘depressants’. In case of an ore containing ZnS & PbS, the depressant used is sodium cyanide. It selectively prevents zinc sulfide from coming to the froth but allows lead sulfide to come with the froth.
- Leaching: This process is done when the ore is soluble in some suitable solvent.
Extraction of Metals from Ores:
For extraction, various methods are used. We will study the roasting and calcination methods.
- Roasting: In this process, the ore is warmed in a continuous air supply in the furnace at any temperature lower than their melting point.
Eg: 2ZnS + 3O2 → 2ZnO + 2SO2
- Calcination: Calcination involves heating when the volatile matter escapes leaving behind the metal oxide. E.g.:
Fe2O3.xH2O(s)-à Fe2O3 (s) + xH2O(g)
ZnCO3 (s) -à ZnO(s) + CO2 (g)