Reducing the arsenic content in Copper

11 January 2016

Copper occurs in many forms throughout the world and has wide and varying applications; demand is still increasing and current forecast is some 17 million tonnes per annum. The main ore minerals of copper are in sulphide form dominated by Chalcopyrite (CuFeS2), with Bornite, Covellite, and Chalcocite as other important sources. In addition some copper occurs as Enargite a copper arsenic sulfo-salt mineral with formula (Cu3AsS4) Copper also occurs in carbonate form such as Malachite, Azurite and in oxide form as Cuprite, Tenorite, and also in metal form as Native copper.

The sulphides account for most of copper produced throughout the world, and generally occur in the lower regions lodes not exposed to weathering, which alters outcrops to carbonates, oxides and native metal by oxidation and other chemical reactions. Due to their characteristic green or blue colour, they are easily seen in the rocks in which they occur and some spectacular specimens continue to be found. 

Many large old mines are now depleted and more arsenic-rich copper deposits are now being processed. This presents some challenges as the concentrate needs further treatment before smelting to remove deleterious elements, in particular arsenic, as it is a health and safety risk.  

Methods to reduce arsenic content include:-  

  • Lowering copper grade in the concentrate by less cleaning in the flotation circuits
  • Blending the concentrate with cleaner material from another mines
  • Roasting 
  • Bacterial oxidation
  • Caustic leaching  

All of these results in sale of concentrate at a lower price to commodity traders who then sell to smelters   Copper is very important metal is one of the three which were the key to the development of man from the Stone Age to the Industrial Era. Prior to smelting ore the early metallurgists would have removed or sorted waste rock by hand picking using the weight and colour to identify the valuable mineral – the first form of pre-concentration. They also used the property of weight to sort ore by means of a primitive jig which was made from wood and mounted on a cradle to facilitate rocking – usually on the bank or in a small stream.  

Copper deposits containing native metal together with oxide and sulphide ore are still mined today, and whereas it was most useful to ancient man it can be problematic in modern process plants in that at coarse sizes it is difficult to crush as it is malleable. In grinding processes it can form into thin flat pieces and report to tailings in flotation circuit provided to float the copper ore.  

Modern metallurgists, much the same as their ancient counterparts, have considered the problem and devised a number of solutions, such as crushing with toothed rolls which shreds any coarse copper lumps rather than trying to break by impact, removal of  the coarse lumps by gravity or ore sorting ahead of grinding, removal of  fine native copper in the grinding circuits ahead of flotation by means of equipment such as spirals and shaking tables which use the specific gravity of copper to separate it from the rest of the waste and valuable minerals in the ore.  

In regard to flotation the sulphides are usually easy to float, however the oxides and carbonates require pre-treatment by a process known as sulphidation to promote flotation. In the case of arsenic bearing minerals such as enargite treatment of the concentrates is required to remove the arsenic and other deleterious elements.   STEINERT engineers study the process requirements for the total ore and offer practical solutions to mine developers.  

Ask us today to see how a hypothetical ore containing coarse and fine native copper, copper minerals in oxide and sulphide form, some arsenic bearing copper mineral and iron in the form of magnetite might be processed and is an example of how the properties of metals and minerals can be utilised in the beneficiation process.