Gold ores arsenical

Arsenic occurs primarily in sulphide minerals associated with copper ores, and to a lesser extent with zinc, lead and gold ores. Arsenic is produced as a by-product of the smelting of these metals. Primary arsenic production has now ceased in the USA and Europe, and most arsenic is now imported from China and Mexico. The volatility of arsenic represents a significant concern, and there is at present no known natural mechanism by which arsenic is immobilized in the environment. Anthropogenic activities account for an input of some 19000 tonnes into the atmosphere, compared with 12000 tonnes from natural processes, such as volcanism and forest fires (Ayres and Ayres, 1996). 

Arsenic is widely distributed about the earth and has a terrestrial abundance of approximately 5 g/t (4). Over 150 arsenic-bearing minerals are known (1). Table 2 fists the most common minerals. The most important commercial source of arsenic, however, is as a by-product from the treatment of copper, lead, cobalt, and gold ores. The quantity of arsenic usually associated with lead and copper ores may range from a trace to 2 —3%, whereas the gold ores found in Sweden contain 7—11% arsenic. Small quantities of elemental arsenic have been found in a number of localities. 

Gold ores can be concentrated by froth flotation, the resulting concentrate being roasted at 600-800°C to oxidize off sulphur and arsenic as their oxides. The product is extracted with cyanide under oxidizing conditions (using either peroxide or air itself) before displacement with powdered zinc. More reactive metals (silver etc.) can be removed by chlorination of molten gold. 

This hematite is not soluble in the cyanide solution. The oxidative pretreatment of gold ores thus reduces the cyanide consumption. Some impurity elements inhibit leaching reactions, examples include elements, carbon, sulfur and arsenic in gold ores are such impurities, but these can be removed by heating in air. 

Arseno A process for extracting gold from arsenic-containing ores, developed by Arseno Processing. Similar to the Cashman process. 

Cashman A high-pressure process for extracting gold from arsenic-bearing ores, concentrates, and flue dusts. 

Results and Discussion Arsenic in Refractory Gold Ores... 

Prior to gold extraction by cyanidation, refractory gold ores are either roasted or pressure oxidized to liberate the gold contained as submicroscopic particles or in solid solution in arsenopyrite and arsenic-rich pyrite. Gold extraction from such ores require roasting or pressure oxidation or bacterial oxidation prior to cyanidation to destroy the sulfide structure. 

Arsenical compounds that can form during pressure-oxidation of refractory gold ores in the autoclaves include Phase-3, Phase-4, Type 2 and Fe0HS04 (Fig. 1). In addition, scorodite can form in autoclave with sulfate substituting up to 20% arsenate. Jarosite is another phase with limited arsenate substituting for sulfate (Paktunc Dutrizac 2003). 

Fig. 1. Composition of ferric arsenate-sulfate compounds forming in the autoclaves treating refractory gold ores.

There are two major groups of arsenical gold ores of economical value. These are the massive base metal sulphides with arsenical gold (i.e. the lead-zinc Olympias deposit, Greece) and arsenical gold ores without the presence of base metals. Massive, base metal... 

Flotation of arsenical gold ores associated with base metals is accomplished using a sequential flotation technique, with flotation of base metals followed by flotation of gold-containing pyrite/arsenopyrite. The pyrite/arsenopyrite is floated at a weakly acid pH with a xanthate collector. 

Arsenical gold ores that do not contain significant base metals are treated using a bulk flotation method, where all the sulphides are first recovered into a bulk concentrate. In case the gold is contained either in pyrite or arsenopyrite, separation of pyrite and arsenopyrite is practiced. There are two commercial methods available. The first method utilizes arsenopyrite depression and pyrite flotation, and consists of the following steps ... 

Platinum is found in combination with arsenic as the rare mineral sperrylite, PtAs2, at Vermillion Mine in Ontario, Canada. It was first discovered by Sperry (whence its name) in 1887 along with copper and iron pyrites in contact with gold ore. It crystallises in cubes or, less frequently, in oetahedra hardness, 6-7 specific gravity, 10-602 at 20° C. It is tin-white in colour, possessed of metallic lustre, and contains traces of rhodium and antimony.4... 

Experience with bioleaching of arsenic-rich gold ores has shown that the ratio of pyrite to FeAsS is an important factor controlling the speciation of the arsenic released (Nyashanu et al., 1999). In the absence of pyrite, —72% of the arsenic released was As(III), whereas in the presence of pyrite and Fe(III), 99% of the arsenic was As(V). It appears that pyrite catalyzed the oxidation of As(III) by Fe(III), since Fe(III) alone did not oxidize the arsenic (Nyashanu et al., 1999). 

Nyashanu R., Monhemius A., and Buchanan D. (1999) The effect of ore mineralogy on the speciation of arsenic in bacterial oxidation of refractory arsenical gold ores. In International Biohydrometallurgy of Symposium IBS 99, Madrid (eds. R. Amils and A. Ballester). Elsevier, Oxford, pp. 431-441. 

Arsenic is widely distributed in the environment. It ranks twentieth in the abundance of elements in the Earth s crust. In the environment, arsenic is mainly associated with sulfide minerals. Soils contain arsenic usually in the range of 0.5 to 35 mg/kg [98]. Arsenic levels in gold ore deposits in Zimbabwe attain 20 g/kg in soils [99]. 

Arsenic is found in igneous and sedimentary rocks. The most common commercial source is as the by-product from the refining of copper, lead, cobalt, and gold ores. Although arsenic is actually a metalloid, it is grouped with metals for most environmental purposes. 

The first chemolithotrophic culture was isolated in the former Soviet Union by Ilyaletdinov and Abdrashitova (18). The organism, named Pseudomonas arse-nitoxidans by them, was associated with a gold-arsenic deposit (sulfidic gold ore). The culture consisted of aerobic, gram-negative, motile, straight rods (0.4 X 1.2-... 

Arsenopyrite FeAsS is the most important arsenic mineral. As a rule the arsenic-containing ores are mined mainly for their contents of the more attractive metals, gold, silver, copper, lead and zinc. On treatment of the complex ores, arsenic oxide As O is obtained. Reduction with charcoal gives metallic arsenic. The most important commodity is however the oxide. The world production of As Oj was 35000 tonnes in 2001, with China as the largest producer country (16000 tonnes), followed by Chile (8000 tonnes), Mexico and Peru (2500 tonnes each), Kazakhstan and Russia (1500 tonnes each), Belgium and France (1000 tonnes each). 

---