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Jul 2006 - Bioxidation

Is bioxidation the only acceptable way to treat refractory gold ores ?

Refractory gold from a western viewpoint usually means where the gold is associated with arsenic, whereas its more general definition (at least in China) is any ore that is not free-milling. This hence includes by-product gold being produced with other base or precious metals, or when gold is associated with pyrite and floated to produce a concentrate that is leached as shown in Figure 1 of the concentrates waiting to be processed at Michelago’s (becoming Golden China’s) BioGold plant in China’s Shandong Province.

Now that most of the easy non-refractory gold orebodies appear to have been mined, the focus appears to be switching towards refractory orebodies and how to acceptably (environmentally) treat them without producing drums of arsenic powder (for burial somewhere) and other fumes in a roasting process.

Michelago (MIC) has recently signed an agreement with the Skouries mine near Thessaloniki in Greece to treat their refractory tailings and dumps, shipping them to their plant in China and supplying the main feed for MIC’s BioGold plant expansion for the next 4 years. Shipping ore from Greece to China does appear to be extreme but highlights the difficulty in treating such ores in restrictive environments.

I can recall when I visited the then TVX owned Skouries mine in mid-1997 that it reputedly had a higher grade core and they were hoping to re-open the mine once they received approval to build a new plant – which never happened because it occurred at the same time as NDY’s Ovacik mine in Turkey encountered major opposition to opening their gold mine for fear of potential cyanide pollution.

Bioxidation (by Gold Fields’ BIOX® or bacoxidation by Bactech’s Bacox©) are both well-known methods that resulted from the bioxidation process in which bugs eat sulphides and produce gold. Originally yellow coloured moss was found on rocks underground in a defunct South African gold mine and when sampled was discovered to have a gold content and consist of millions of minute bugs or organisms.

The bioxidation process consequently evolved at Fairview in the Barberton area of South Africa with its extreme temperatures, endeavouring to create an environment that simulates old hot underground workings in which the bugs multiply and do their work efficiently.

The bugs or organisms require a narrow operating temperature range about 42ºC. Initially heat is required to get them working (as if it is too cold then they sleep on the job), however, cooling is then soon required as they generate heat when they are working and if they are not cooled down then they frizzle and die very quickly as Wiluna learnt in the early days of its biox plant.

At Wiluna, a cooling pipe burst and there was then no backup system so they all died within a matter of minutes (I think you have up to 1.5hours before they are all gone). Once dead a new batch can be built up to full strength over a period of about 3 months, the difficulty is that they evolve as they go, so the experienced bunch are very different from the starting ones and can handle the odd change in the mix such as oil etc, akin to learning to fight off viruses by vaccinations. Most systems now incorporate an ability to isolate each tank if a problem arises.

Up until now the biox process has been restricted to that relatively high operating temperature environment, but that may be able to change, since Marengo (MGO) has green malachite coloured moss (that contains relatively high grades of copper) next to a waterfall in a rainforest environment at its Yandera Project in PNG.

After biox, the next consideration has become LionOre’s hydro-metallurgical Activox process which once fully operational may spawn smaller plants that can treat refractory ores and produce gold (gold has been produced from its pilot plant in Botswana). China are also reputedly using their own hydro-met based process in to treat refractory Carlin-type ores in Guangxi in China.

Newmont experimented with a biox heap leach approach for some of their extremely refractory ores from the Carlin Trend in Nevada, but apparently it was then not commercially viable. Although at current prices, maybe it can be reconsidered, since BioNic and BioCu heap leach operations have reputedly had some success.

Conventional heap leaching achieving 65% to 70% recoveries remains standard for most gold ores, agglomerating firstly with cement if the ore is too clayey, although Minara’s new heap leach for laterite nickel is reputedly also being similarly successful (applying agglomeration) and may become a consideration as an add-on to existing nickel laterite plants or for small nickel laterite resources.

MIC’s Biogold plant also treats concentrates from the flotation of gold-in-pyrite ores (which are common in China’s Shandong Province), taking them through a cyanide-leach process. Interestingly Gallery Gold (now Iamgold’s) plant at Mupane in Botswana started a flotation circuit in early 2006 that does not produce a concentrate. Instead the overflow was being thickened and sent to tank No 2 of the existing CIL circuit, while the underflow went to tank No 1 and about a 45-hour resolution time.

So, out of biox, heap-leaching and hydro-met, it appears that currently the only commercially viable proven process on almost any scale to treat refractory gold ores is still bioxidation, and that is why it is feasible to ship ore from Greece to China for treatment. However, other treatment processes are steadily evolving and could develop as future alternatives.

Disclosure and Disclaimer : This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, (ERA, an independent research company) who is an Authorised Representative with Taylor Collison Ltd, and with his associates, may hold interests in some of the stocks mentioned in this article. The opinions expressed in this article should not be taken as investment advice, but are based on observations by the author. The author does not warrant the accuracy or completeness of any information and is not liable for any loss or damage suffered through any reliance on its contents.

Figure 1. The Michelago (becoming Golden China) BioGold Plant in Shandong Province, ChinaGDNjul06-A

  • Written by: Keith Goode
  • Saturday, 01 July 2006

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