The removal of arsenic bearing minerals from concentrates is becoming more important as environmental laws become ever stricter with regard to smelter emissions. The onus is shifting to concentrate producers to remove these minerals from their product, with penalties applying to materials containing greater than background amounts.
The Rosebery mine, located on the West Coast of Tasmania, is a poly-metallic mine producing a gold dore and concentrates of copper, lead and zinc sulphide through sequential flotation. The zinc concentrate is the major product in terms of both concentrate volume and cash income. Lead concentrate is the secondary earner; copper concentrate and gold dore are only minor products. Copper concentrates from this orebody have reported arsenic concentrations as high as 8000 ppm, well above the 5000 ppm limit set by some importers. The only method currently available to control the concentration of arsenic in copper concentrates is to control the amount of arsenic in the feed through an ore blending regime prior to crushing. This has however proved to be an unreliable method of controlling arsenic concentrations below the target, so this work seeks to find a metallurgical solution to controlling arsenic levels.
A mineralogical examination undertaken to identify the mineralogical form of the arsenic bearing minerals indicated that the majority of arsenic entered the mill as arsenopyrite (FeAsS), but tennantite (Cu12As4S13) carried most of the arsenic through to the copper concentrate. A plant survey was undertaken to determine the baseline flotation conditions within the copper circuit. Results indicated that the majority of arsenopyrite was depressed in the rougher circuit, with depression of tennantite in the cleaners, mainly through depression of middling particles. Tennantite contains 51.6% copper and the mineralogical study detected the presence of freibergite (Ag12As4S13). Arsenic reductions from this copper concentrate will also results in the loss of copper and silver.
From a literature survey, several means of reducing arsenic concentrations were identified. These include magnetic separation, roasting, regrinding to improve the liberation of arsenic bearing minerals from non-arsenic bearing minerals, chemical depression using lime, cyanide or sulphides or pulp potential controlled flotation. The Rosebery site is located adjacent to environmentally sensitive temperate rainforest, so arsenic removal options had to focus on environmentally neutral methods that will not harm local flora and fauna. Roasting has proven to cause damage to the environment, in particular the Queenstown, Tasmania, area, so was not considered. In addition, with copper concentrates being only a minor product, a flotation solution was sought to minimise capital expenditure and flowsheet changes.
Potential flotation tests to reduce the arsenic content was initially investigated, along a series of tests to investigate the effects of regrind, pH and froth depth as well as the additions of cyanide and peroxide. It was found that regrinding had the potential to improve the liberation of the arsenic bearing minerals. Samples of fresh rougher concentrate were ground to produce P80’s of 10, 20 and 30 µms. Flotation tests determined that a P80 of 10 µms produced the greatest selectivity between copper and arsenic, but at a reduced copper recovery.
An economic evaluation undertaken on the products of the regrind tests indicated that arsenic penalties were quite low relative to income, and that income from silver exceeds that from copper. Regrinding to 10 µm improved copper/arsenic selectivity, but the reduction in silver recovery dramatically reduced total cash returns.
A review of the flotation testwork results has determined that there is a relationship between the copper/arsenic ratio of the copper rougher concentrate and the arsenic content of the final copper concentrate. This will provide an indication of when steps to reduce final arsenic concentration need to be used, while reducing silver losses from overgrinding.