It is evident from the foregoing that pulp alkalinity plays a very important, though very complex, role in froth flotation. In practice, selectivity in complex mineral processing separations is dependent on a delicate balance between reagent concentrations and pH. Froth 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. In order to modifiers the pH as what we desire, then we can use froth flotation pH modifiers.
How pH affect the froth flotation?
pH is a major variable for controlling froth flotation because it directly influences the adsorption of collector by the electron transfer mechanism. One of the products of this reaction mechanism are hydroxyl ions (OH-). Raising the pH increases the concentration of OH- ions. This slows or even stops the reaction of collector with mineral due to the shift in chemical equilibrium. When this occurs, the collector no longer adsorbs onto mineral surface.
pH has other effects on froth flotation in addition to affecting the adsorption reaction. For example, at too high a pH, metal hydroxides can precipitate. Thus form coatings on the surface of the mineral thus blocking the surface to collector. This makes it important to establish the correct target pH.
Types of froth flotation pH Modifiers
The types of froth flotation pH modifiers used are also important due to economic considerations and the particular side affects that can occur in some cases. For example, soda ash contains carbonate ions and can form metal carbonate precipitates (scale). Lime introduces calcium ions which can combine with sulphate ions in the water and precipitate as gypsum (calcium sulphate). Soda ash and lime are both able to raise pH.
There are at least two type of froth flotation pH modifiers. Acids and bases pH modifiers. As expected, the acids froth flotation pH modifiers use to lower the pH while the bases froth flotation pH modifiers use to raise the pH.
Lime as froth flotation pH modifiers
Alkalinity is able to control 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 use 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 than with any other processing chemical. For example, the cost of lime in sulphide mineral flotation is roughly double that of the collector used. Significant operational cost savings can achieve easily by the proper choice and use of pH regulators.
The froth flotation pH modifiers, Lime, being cheap, is very widely use to regulate pulp alkalinity. It is being use in the form of milk of lime. The milk lime made from a suspension of calcium hydroxide particles in a saturated aqueous solution.
Lime as froth flotation deactivator
Lime or soda ash is often add to the slurry . The purpose is to precipitate heavy metal ions from solution. In this sense, the alkali is acting as a “deactivator”, as these heavy metal ions can activate sphalerite and pyrite. It is also in fact, can prevent their selective flotation from lead or copper minerals. Since the heavy metal salts precipitated by the alkali can dissociate to a limited extent and thus allow ions into solution, cyanide is often used with the alkali to complex them.
Hydroxyl and hydrogen ions modify the electrical double layer and zeta potential surrounding the mineral particles. Hence the hydration of the surfaces and their floatability is affected. With xanthates as collectors, sufficient alkali will depress almost any sulphide mineral, and for any concentration of xanthate there is a pH value below which any given mineral will float, and above which it will not float. This critical pH value depends on the nature of the mineral, the particular collector and its concentration, and the temperature
Lime as froth flotation depressant
Not only as froth flotation pH modifiers, Lime can also act as a strong depressant for pyrite and arsenopyrite when using xanthate collectors. Both the hydroxyl and calcium ions participate in the depressive effect on pyrite by the formation of mixed films of Fe(OH), FeO(OH), CaSO4, and CaCO 3 on the surface, so reducing the adsorption of xanthate. Lime has no such effect with copper minerals, but does depress galena to some extent. In the flotation of galena, therefore, pH control is often affect by the use of soda ash. In the meanwhile pyrite and sphalerite being depress by cyanide.