Froth flotation depressants
In mineral processing term, depression is define to increase the selectivity of froth flotation by rendering certain minerals hydrophilic (water-avid), thus preventing their froth flotation. They are key to the economic froth flotation of certain ores such as platinum and nickel sulphides. There are many types of froth flotation depressants and their actions are complex and varied. In most cases not fully understood, making depression more difficult to control than the application of other types of reagent. It is in particular when the froth phase is also affect by their action.
Sodium silicate is also used as a froth flotation depressants in some systems, being one of the most widely used regulating agents in the froth flotation of non-sulphide minerals, such as scheelite, calcite, and fluorite. Sodium oleate is the major collector in the froth flotation of these minerals, but the selectivity in the separation of scheelite from calcite and fluorite is often inadequate. Therefore, Sodium silicate can be use to improve selectivity.
Cyanides as inorganic froth flotation depressants
In order to desorb the surface copper, Cyanide need to add to the pulp. The cyanide also intended to react with copper in solution forming soluble cyanide complexes. Sodium cyanide is most commonly use as froth flotation depressants in some conditions. Example which hydrolyses in aqueous solution to form free alkali and relatively insoluble hydrogen cyanide.
The depressive effect of cyanide depends on 3 factors. The concentration of cyanide itself, and on the concentration of the collector and the length of the hydrocarbon chain. The longer the chain, the greater is the stability of the metal xanthate in cyanide solutions. Thus, the higher the concentration of cyanide required to depress the mineral. Relatively low concentrations of xanthates with short hydrocarbon chains are therefore used for selective froth flotation where cyanides are used as froth flotation depressants.
Cyanides are, of course, extremely toxic and must handle with great care. They also have the disadvantage of being expensive. Cyanide also depress and dissolve gold and silver, reducing the extraction of these metals into the froth product. Despite these disadvantages, they are widely use due to their high degree of selectivity in froth flotation. They also have the advantage of leaving the mineral surface relatively unaffected, so that subsequent activation is relatively simple, although residual cyanide ions in solution may interfere with the activator.