Challenges in copper flotation

How flotation continues to, and needs to, evolve is perhaps no better illustrated than by the case of copper, the commodity where it finds its most important use, and produces the greatest annual tonnage. Total world tonnage is around 15 million tonnes per year, and with developing countries demanding more each year, this is bound to increase.

The development of froth flotation had an enormous impact on copper mining, enabling chalcopyrite and other sulphides to be efficiently separated from ores of relatively low grade and fine grain size. Until 1907 practically all the copper mined in the USA was from underground vein deposits, averaging 2.5% Cu, whereas flotation allowed the mining of porphyry deposits of only 0.6% grade.

Nevertheless, the flotation of copper ores still presents challenges to the minerals engineer. Copper is characterised by having a number of economic ore minerals, many of which may occur in the same deposit, and in various proportions according to depth. Chalcopyrite is the dominant primary mineral, and floats readily with anionic collectors such as xanthates, as do other economic sulphides such as chalcocite, covellite and bornite. However many other minerals respond poorly to flotation, and may even impede the flotation of readily floatable minerals, as at Palabora in South Africa, where the ore contains small amounts of valleriite, a copper-iron sulphide containing Mg and Al groups in the crystal lattice, and occurring intergrown with other sulphide minerals. Valleriite has a low flotation response, and it is a very soft mineral. During comminution breakage occurs along the soft and friable valleriite, leaving grains of other copper sulphides with a valleriite coating, preventing these grains from floating.

But perhaps the biggest challenge in copper flotation is that of copper oxides which have very poor floatabilities and are usually treated by hydrometallurgical methods, such as vat and heap leaching, sometimes in association with solvent extraction. Oxidised minerals such as malachite and azurite can to some extent be upgraded by flotation after sulphidisation, but minerals such as cuprite and chrysocolla, respond very poorly.

There are a number of papers on copper flotation scheduled for Flotation ’11 but two of particular interest to those involved with oxidised and oxide copper minerals will be given by workers from Australia, who have found encouraging flotation response of malachite and chrysocolla using hydroxamate reagents.

Also of interest will be papers on the depression of the arsenic-containing copper sulphides, tennantite and enargite. Arsenic is a toxic, volatile element that has little commercial use. This is causing some concern to copper smelters as they are obliged to dispose of arsenic materials produced as a by-product to the smelting process in accordance with ever tightening environmental guidelines. The onus is moving back to concentrate producers to remove toxic elements, such as arsenic, earlier in the concentrate supply chain.

To complicate matters, the common copper-arsenic bearing minerals in copper ores, enargite (Cu3AsS4) and tennantite (Cu12As4S13), also contain significant amounts of copper; 48.4% and 51.6% respectively. Removal of these minerals from the concentrate removes valuable metal, hence income. There is a dearth of literature concerning the selective removal of enargite and tennantite from sulphide ores, but what there is reports some success using either chemical oxidation or potential control. This has applications in mines such as Rosebery in Australia, where arsenic levels in concentrate are becoming prohibitive.

A paper by workers from MMG Rosebery, the JKMRC and the University of Queensland, will present a review of the literature with regard to the selective removal of the arsenic bearing minerals from copper ores during flotation, summarizing the pros and cons of the various options, and will recommend a strategy for evaluation, while a paper from Japan will also consider the selective flotation of an arsenic rich sulfide copper ore.