The depressing action of cyanide on pyrite during froth flotation is well known and is widely exploited on many Flotation Equipment to selectively separate copper minerals from pyrite. Depression of pyrite by cyanide during froth flotation appears to take place in solutions containing free cyanide, ferro/ferri cyanide, thiocyanate, and cuprous cyanide mainly in the form of Cu . The depressing action in the presence of free cyanide seems to be caused by a decrease in the pyrite surface electrochemical activity, leading to lower collector adsorption. The depressing effect is reversible and is achieved by diluting the pulp with cyanide-free solution.
As an illustration of this washing and repulping effect, a cyanide residue from a gold mine with cyanide-free water was sufficient to negate the depressing effect of the cyanide on the pyrite. The most widely used method of reversing the depressing effect of cyanide in the past has been to condition the flotation pulp at a pH value in the range 3.5–4 with sulfuric acid, or bubbling sulfur dioxide into the slurry, and then add copper sulfate to complex the cyanide and subsequently float the pyrite. Amine collectors are also effective at floating cyanide-depressed pyrite in alkaline solutions, although the flotation rate is slow.
The ability of a mineral to float depends upon its surface properties. Chemical modification of these properties enables the mineral particles to attach to an air bubble in the flotation cell. The air bubble and mineral particle rise through the pulp to the surface of the froth or foam that is present on the flotation cell. Even though the air bubbles often break at this point, the mineral remains on the surface of the froth. The mineral is physically separated from the remaining pulp material and is removed for further processing.
Both mineral process techniques, in particular, the technique of froth flotation, as well as hydrometallurgical processes consume a wide variety of reagents. In many instances, especially in hydrometallurgical operations, the reagents are consumed as they combine with dissolved metal ions to form a metal compound. In such case, the reagents are recovered and recycled in the same step where metal is recovered from the compounds formed in the process.
In some operations, however, additional recycling steps are required to ensure maximum possible recycling. This specially applies to those areas where the reagent has to be used in excess and good portion of it remains unreacted and could be used for another cycle. Such recycling step is specially necessary where the reagent is expensive or toxic and, therefore, hazardous for health and environment. Some examples will be described.
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