Ore Dressing Operations/ Mineral Dressing/Ore Beneficiation/
It is the process for separating the ore from gangue mechanically. It is also called mineral dressing, ore preparation or beneficiation. These physical operations are usually carried out in the sequence: crushing and grinding of the ores, concentration, sizing and classification, thickening, dewatering and filtering. Some important operations are discussed herewith:
1. GRAVITY SEPARATION
The separation is carried out on Wilfley tables or shaking machines called jigs. In Wilfley tables the crushed ore is carried by a stream of water over a surface provided with riffles or obstructions. The lighter gangue particles are washed away leaving the useful ore particles entangled in riffles. In jigs or shaking machines, the crushed ore is shaken in a box with a perforated bottom. In this way, the heavier ore particles carrying metals are separated from gangue.
2. FORTH FLOTATION PROCESS
This method is extensively used for concentrating low-grade ores such as sulphides of copper and zinc. The finely crushed ores are thoroughly agitated with the help of a blast of air in presence of water and a little pine or eucalyptus oil or camphor oil. The ore is preferentially wetted by oil and the gangue by water. The gangues (rocky material) are settled down slowly to the bottom. A forth appears at the top of liquids which carries ore particles and is skimmed off. It is worth noticing that froth flotation process is just the reverse of gravity concentration. In forth flotation process heavy ore particles rise to the top instead of sinking as in gravity process.
3. MAGNETIC SEPARATION
This method is usually applied for separating minerals of equal densities, especially when one of them shows magnetic behavior or can be easily converted into a magnetic product.
This tinstone (density 6.4-7.1) and wolfram (density7.1-7.9) which occur together in nature can be separated by this process. Wolfram is magnetic while tinstone is not. In order to separate them, the crushed ore mixture is allowed to run over a traveling belt passing over a magnetic roller, RM (Fig.6.2). The tinstone falls undefeated at I and wolfram is attracted by the magnet and falls at a separate heap. W.
Filtration is carried out to separate solids from the liquid. Filter medium is usually woven fabric such as cotton, woolen or synthetic fiber cloth or wire screen. When slurry is forced through the filter medium under pressure, the liquid passes through the medium while the solids are retained as “cake”. The filters are designed on the basis of high pressure.