Copper extraction techniques
Oxidised copper ore bodies may be treated via several processes, with hydrometallurgical processes used to treat oxide ores dominated by copper carbonate minerals such as azurite and malachite, and other soluble minerals such as phosphates like chrysocolla, or sulfates such as atacamite and so on.
In general froth flotation is not used to concentrate copper oxide ores, as oxide minerals are not responsive to the froth flotation chemicals or process (i.e.; they do not bind to the kerosene-based chemicals). Copper oxide ores have occasionally been treated via froth floatation via sulfidation of the oxide minerals with certain chemicals which react with the oxide mineral particles to produce a thin rime of sulfide (usually chalcocite), which can then be activated by the froth floatation plant.
Secondary sulfides - those formed by supergene secondary enrichment - are resistant (refractory) to sulfuric leaching. These ores are a mixture of copper carbonate, sulfate, phosphate, and oxide minerals and secondary sulfide minerals, dominantly chalcocite but other minerals such as digenite can be important in some deposits..
Supergene ores rich in sulfides may be concentrated using froth flotation. A typical concentrate of chalcocite can grade between 37% Cu to 40% Cu in sulfide, making them relatively cheap to smelt compared to chalcopyrite concentrates.
Supergene sulfide ores rich in native copper minerals are refractory to treatment with sulfuric acid leaching on all practicable time scales, and the dense metal particles do not react with froth flotation media. Typically, if native copper is a minor part of a supergene profile it will not be recovered and will report to the tailings. When rich enough, native copper ore bodies may be treated to recover the contained copper via a gravity separation circuit where the density of the metal is used to liberate it from the lighter silicate minerals. Often, the nature of the gangue is important, as clay-rich native copper ores prove difficult to liberate.
The modern froth flotation process was independently invented the early 1900s in Australia by C.V Potter and around the same time by G. D. Delprat.
The copper ore is crushed and ground to a size such that an acceptably high degree of liberation has occurred between the copper sulfide ore minerals and the gangue minerals. The ore is then wet, suspended in a slurry, and mixed with xanthate reagents (or other reagents of the thiol class), which react with the copper sulfide mineral particle to make it hydrophobic on its surface. (Besides xanthates, dithiophosphates and thionocarbamates are commonly used).
The treated ore is introduced to a water-filled aeration tank containing surfactant such as methylisobutyl carbinol (MIBC) which is an alcohol. Air is constantly forced through the slurry and the air bubbles attach to the hydrophobic copper sulfide particles, which are conducted to the surface, where they form a froth and are skimmed off. These skimmings are generally subjected to a cleaner-scavenger cell to remove excess silicates and to remove other sulfide minerals which can deleteriously impact the concentrate quality (typically, galena), and the final concentrate sent for smelting.