What is the difference between roasting and calcination on the basis of removal of impurities of an ore?i mean...which process removes volatile impurities and which one removes moisture impurities.?
Extraction of metals from ore is different, depending by metal chemical combination. When ore is composed of carbon, it is subjected to an ignition in advance, for removing carbon dioxide, metals remaining so in the form of oxides, which are treated moreover as oxide ores.
Sulfide ores generally are subjected to a prior roasting, to remove sulfur (as sulfur dioxide) and processing sulphide to oxide.
In principle, the procedures for obtaining metals are divided into three methods:
1) by chemical reduction;
3) thermal dissociation of combinations.
Electrolysis of aqueous solutions and melts.
Some metals are obtained easily from their combinations by means of electrolytic reduction. Electrolysis will be done in solutions or melts.
Metals which cannot decompose water can be separated from aqueous solutions by electrolytic way. These metals are arranged after hydrogen in the series of oxidation potentials. Among metals placed before hydrogen, only those with high voltage (Pb, Ni, Cd, Zn) may be obtained by electrolysis in a solution.
Mercury, with a high voltage is used as cathode in electrolysis of solutions of metal salts which decompose in water, such as alkali metals,with which forms amalgams.
Separation of metals by electrolysis of aqueous solutions is used mainly in metal purification technique.
Van Arkel and de Boer procedure allows obtaining of metals in very pure and compact state. It is based on the fact that volatile iodine of some metals thermal dissociates,in vacuum, at temperatures below the melting point of the metal.
Thus, if in formed iodine vapor is introduced a tungsten wire, heated to a temperature of dissociation of iodide and the melting point of the metal, the metal deposited on the heated wire is forming crystals, while iodine diffuses into the surrounding space. Very thin tungsten filament inside the metal rod is not practically an formed impurity.
The method is applied to obtain the metals Ti, Zr, Hf, Th.