So nearly all rocks on the planet have some metal or mineral content. However, the concentration of this metal is too less for it to be valuable. Ores usually have a very high content of the desired mineral, but it still must be separated from other mixed impurities, this method is known as the concentration of ores. Let us discuss this topic.
Concentration of Ores
An ore of a metal is an impure source, this means other than the metal it consists of many other impurities such as sand, grit, clay, rocks etc. These impurities are collectively known as gangue. Gangue is generally commercial valueless and we must separate it from the ore. This entire process is the concentration of ores or even dressing or benefaction.
Now there are numerous ways to carry out the concentration of ores. The correct method is chosen based on the physical and chemical properties of the metal. Let us take a look at some of the most common methods.
Learn the process of Refining the Metal.
This method is based on the differences in weight between the gangue and the metal. Generally, the metal particles of the ore are heavier than the impurities. So we use the principle of gravity separation to separate the two.
Here crushed ore is mixed with an upward stream of running water. The lighter particles of the gangue wash away in the running water. The heavier metal particles settle down and can be separated easily. We use this method for ores that contain lead and tin since these are relatively heavier.
Browse more Topics under General Principles And Processes Of Isolation Of Elements
- Occurrence of Metals
- Thermodynamic Principles of Metallurgy
- Extractions of Crude Metal from Concentrated Ore
- Uses of Aluminium, Copper, Zin, and Iron
Just like we saw earlier, the physical properties of metals can allow us to separate them from the gangue. One such physical property is magnetic properties of metals. In this method of Magnetic Separation, we use the magnetic properties of certain metals to divide them from the non-magnetic gangue.
Here we place the powdered ore on a roller belt. One belt of this roller is a magnet. So the magnetic material attracts to the metal belt and stays on the belt. The non-magnetic gangue falls in a heap from the belt. Also, the reverse may be the case. The gangue may be the magnetic material in certain cases.
Iron ores and manganese ores are the main ones for which we use magnetic separation. Magnetite and Chromitite ores (both iron ores) use this method on a large scale to remove their impurities.
Froth Flotation Process
Another method of concentration of ores is the Froth Flotation Method. This is the process for concentration of primarily sulphide ores. It has the advantage over gravity separation because it can collect even the extremely fine particles of minerals.
The process uses the difference in the wetting characteristics of minerals and gangue. Minerals are generally wetted by oil and gangue is wetted by water. So they prepare a mixture of the crushed ore with water, oil and other substances.
And rotating paddles will then agitate such mixture and introduce air. The air will cause bubbles to form, which attaches itself to the solid particles of minerals. And then a froth will form and it will rise to the top. This froth will contain the mineral particles which we can now separate. There are three main types of additives in this process, namely
- Collectors: These increase the non-wettability of mineral particles. Basically, they form a film on the particles which makes them water repellent. Some common collectors are carboxylic acids and xanthate salts.
- Froth Stabilizers: As the name suggests, they stabilize the foam that rises to the top, to make the separation easier. Pine oil, soaps, cresols etc are froth stabilizers.
- Depressants: Some time two similar minerals are both found in the ore and both react to collectors similarly. This will mean that both will rise to the top of the foam. To separate such similar compounds we use depressants. For example in an ore containing both ZnS and PbS (both sulphides) we use NaCN as a depressant. It reacts with ZnS not allowing it to froth and only PbS comes up in the foam.
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Leaching is a chemical process of concentration of ore. Here the ore dissolves in a chemical solution. The minerals will react and dissolve leaving behind the impurities only which filter out. And then the metal will be obtained from the chemical solution. Let us now take a look at an example of how we obtain Aluminium by leaching.
The leaching of aluminium from its core bauxite on an industrial scale is known as the Bayer process. This process will give us the end product of Alumina (aluminium oxide) from bauxite. The other impurities of silica, titanium oxide and varies other oxides will separate.
The first step is to heat the bauxite ore to temperatures of 200°c along with a sodium hydroxide solution. This will convert the aluminium oxide to a solution of sodium aluminate. The silica will dissolve itself in the process. The chemical equation is
Al2O3 + 2 NaOH → 2 NaAlO2 + H2O
From the above solution, we remove solid impurities by filtration. Then we bubble carbon dioxide through the aqueous solution of sodium aluminate to neutralize it. And aluminium hydroxide precipitates as a result.
2 NaAlO2 + CO2 → 2 Al(OH)3 + Na2CO3 + H2O
This hydrated alumina is then heated to 1470 K. This gives us a pure form of aluminium hydroxide without the impurities.
2 Al(OH)3 → Al2O3 + 3 H2O
Solved Question for You
Q: Which one of the following ores do we concentrate by chemical leaching method?
- Copper Pyrite
Sol: Gold and silver ores are concentrated by leaching. In this method, we treat the ore with a suitable reagent in which ore is soluble but impurities do not. Thus, impurities are removed by filtration.
Among galena, copper pyrites, cinnabar and Argentine, Argentite being silver ore is concentrated by chemical leaching method.