Concentration of Ores
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Ores are solid material from which a pure metal can be obtained. The process of removal of unwanted material from the ore is known as concentration or dressing or benefaction of ores. It involves several steps. The separation of required material from the ore is dependent on the differences in physical properties of the compound of the metal present and that of the gangue.
Gangue is an unwanted material which is a part of a mineral. The only useful material is ore. Gangue is removed from the mineral for the extraction of the ore.
It is based on the difference in gravity of the particles of the gangue and the ore. So, it is considered as a type of gravity separation. During this process, an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed leaving the heavier ore particles. Hydraulic washing is used for ores that have tin or lead, as they are heavier than the gangue.
Fig. 1. Hydraulic washing
It is based on the principle of magnetic properties of the ore components. If either the ore particles or the gangue are capable of being attracted in a magnetic field, magnetic separation can be used. Ore is kept in a conveyer which passes through the magnetic roller.
Fig. 2. Magnetic separation
This method is used to separate gangue from the sulphide ores. This process is used for sulfide ores of Cu, Pb and Zn. Suspension of powdered ore is prepared using water. To this suspension, collectors and froth stabilizers are added. Pine oils, fatty acids are used as collectors to enhance the non-wettability of the mineral particles. Whereas froth stabilizers such as cresols, aniline, are added to stabilize the froth. A rotating paddle agitates the mixture and draws air in it. This results in the formation of froth which carries the mineral particles. The froth is light and is scanned off. It is then dried for recovery of the ore particles. Sometimes depressants are used to separate the sulphide ores by adjusting the proportion of oil to water. Such as, in case of an ore containing Zinc sulphide and lead sulphide. The depressant used is sodium cyanide.
Fig. 3. Froth floatation process
Floatation stages are as follows:
- Roughing is the first stage of floatation. During this process, maximum amount of valuable material is removed. Sometimes pre-floatation step is followed for before roughing. This is done to remove contamination such as organic carbon.
- Cleaning is the next step for floatation. During this step, undesired material is removed. The product of this step is known as cleaner concentrate.
- Scavenging is the last step of the floatation process. This is to recover those important materials which are not recovered during roughing.
- Collectors are anionic Sulphur ligands. For example, xanthate salts. Other collectors used are fatty acids, fatty amines.
- Frothers are used to stabilize the froth. For example, pine oil, alcohols etc.
- Modifiers are used to optimize the separation process. pH modifiers include lime (CaO), Caustic soda (NaOH), etc. Anionic modifiers include phosphates, silicates, and carbonates. Organic modifiers include the thickeners dextrin, starch, glue, etc.
It is used when ore is soluble in some solvent. It is explained based on following example-
- Leaching of alumina from bauxite. Bauxite is an ore of aluminum. It contains silicon dioxide, iron oxides and titanium oxide. Concentrated solution of sodium hydroxide at a pressure of 473 – 523 K and 35 – 36 bar is used for concentrating the ore. During this process, bauxite is leached out as sodium aluminate, leaving other impurities:
Al2O3(s) + 2NaOH(aq) + 3H2O(I) → 2Na[AI(OH)4](aq)
The carbon-dioxide is passed through it and hydrated bauxite is obtained.
2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s) + 2NaHCO3
Hydrated alumina is then heated and dried to obtain pure bauxite or Al2O3.
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