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The effectiveness of all classes of flotation agents depends to a large extent on the degree of alkalinity or acidity of the pulp solution. Consequently, a primary object of flotation testing is to discover the optimum pH value for a given reagent combination and ore. pH is one of the few factors which can be readily measured in a flotation pulp. Measurement may be by means of an occasional reading, or by use of a continuous recording Ph meter that is coupled to a reagent feeder to provide automatic control.
Almost all flotation mills treating sulfide mineral ores operate with an alkaline pulp. This produces optimum metallurgical results while also preventing corrosion of metal equipment. While a few plants still use acid pulps, the are usually located where the ore is very acid and lime is difficult to obtain, or where the minerals are floated from acid-leached pulp.
The alkalinity regulators most commonly used are lime and soda as. Equally good alkalinity control could be obtained with caustic soda and occasionally it is used, particularly in gold flotation. However, lime or soda ash is preferred.
Because of low cost, lime is used in substantially all flotation circuits – including most copper and zinc flotation circuits – where the calcium ion is not objectionable. Some minerals, notably pyrite and to a lesser extent galena, have a tendency to adsorb calcium ions in preference to the collector and, as a result, are less readily floatable in a pulp containing lime. This difficulty is eliminated by the use of soda ash, which will precipitat naturally occurring calcium ions and also provide the necessary alkalinity.
Acid pH control is usually obtained with sulfuric acid (H2SO4) because of its low cost.
Other reagents which are sometimes considered as pH modifiers include hydrofluoric acid (HF), sodium bicarbonate (NaHCO3), sodium phosphates, and alkali sulfides. Generally though, these compounds are used for form other purpose as well as for simple pH control.
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