Metals and Non-Metals - Part IX

Concentration of ore
Unwanted rocks, sand and grit from the mineral ore are called gangue or matrix. These have to be removed so that the mineral ore is concentrated with higher percentage of metal. Ores are mined from deep within the earth’s crust in the form of rocks. The minerals are embedded in these rocks. The rocks are first crushed into smaller pieces by crushers. Then they are ground to powder by ball mill and other processes so that powdered ore is obtained. Depending on the type of ore, hydraulic washing, froth floatation process, magnetic separation and chemical separation techniques are applied for concentrating an ore.

i) Hydraulic washing : As the name suggests, hydraulic washing process is done by washing the ores with streams of water. If an ore is heavier or denser than the gangue, then the gangue particles are washed way with the stream. The heavier or denser ore particles remain behind and can be collected. Hydraulic washing is done for ores that have tin or lead, as they are found to be heavier than the gangue.  

ii) Froth-floating process : This process is used for sulphide ores. Oils can wet sulphides. Oil floats on water. Sulphide ores are first ground to powder and water is added. Then pine oil is added and the emulsion is agitated by passing compressed air. Oil and froth float on the surface along with the sulphide ore. The gangue particles being insoluble in oil remain at the bottom of the water tank. The froth is removed and allowed to settle down. This is called the froth-floating process. This process is used for sulphide ores of Cu, Pb and Zn.  

iii) Magnetic separation : This method of concentration can be applied when the gangue and the ore particles have different magnetic properties. For example if the ore particles are magnetic in nature and if the gangue particles are non-magnetic, then a strong magnet can be brought and the magnetic ore particles can be sucked out from the powdered ore. The figure below shows how magnetic separation is done. The powdered ore is poured over a conveyer belt. One of the rollers of the belt is made out of magnet. The magnetic roller makes the magnetic ore particles stick on the belt and these are moved at a distance before they are collected. The gangue particles, being non-magnetic in nature, do not get attracted to the roller and fall in a heap below the roller itself. Iron ores like magnetite, chromite, and manganese ore like pyrolusite are concentrated by this process. 

Sometimes, a reverse situation may occur : the ore is non magnetic and the gangue is magnetic. In this case also magnetic separation may be used for concentration of the non-magnetic ore.

iv) Chemical separation : Different chemical affinity between the ore and the gangue is utilized for  concentrating the ore. One example shown below will make this process clear.

1. Separation of Al from bauxite ore : This process is known as Bayer’s process. Bauxite ore is reacted with hot NaOH. A water soluble sodium aluminate is formed. The gangue is insoluble in NaOH as well as water. The sodium aluminate formed is removed by adding water and filtering the solution.  

2. The solution containing sodium aluminate is treated with HCl. A precipitate of aluminium hydroxide is obtained.  

3. The precipitate is separated and dried, then heated. Pure aluminium oxide is obtained. The oxide can be reduced to obtain pure Al.  

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