It is evident from the foregoing that pulp alkalinity plays a very important, though very complex, role in flotation, and, in practice, selectivity in complex separations is dependent on a delicate balance between reagent concentrations and pH.

Flotation where possible is carried out in an alkaline medium, as most collectors, including xanthates, are stable under these conditions, and corrosion of cells, pipework etc., is minimised. Alkalinity is controlled by the addition of lime, sodium carbonate (soda ash), and to a lesser extent sodium hydroxide or ammonia. Sulphuric or sulphurous acids are used where a decrease in pH is required.

These chemicals are often used in very significant amounts in almost all flotation operations. Although they are cheaper than collectors and frothers, the overall cost is generally higher with pH regulators per tonne of ore treated than with any other processing chemical. For example, the cost of lime in sulphide mineral flotation is roughly double that of the collector used, so significant operational cost savings can be achieved by the proper choice and use of pH regulators (Fee and Klimpel, 1986).

Lime, being cheap, is very widely used to regulate pulp alkalinity, and is used in the form of milk of lime, a suspension of calcium hydroxide particles in a saturated aqueous solution. Lime or soda ash is often added to the slurry prior to flotation to precipitate heavy metal ions from solution. In this sense, the alkali is acting as a "deactivator", as these heavy metal...