Bacterial oxidation is an option. Check the Biox Process on the attached link.

The gold mine I work for uses this. There are other variations on this technology from other companies such as Bacox.

For a non-biological process you can also look at the Albion Process. This uses ultra-fine grinding and oxygen injection.

http://www.biomin.co.za/

http://www.albionprocess.com/EN/Pages/default.aspx

Pressure oxidation is also an option but a little bit expensive as compared to bacteria leaching.

Dear Mr. Ndlovu,

I was a bit surprised to see you going straightforward to the processing and asking for the most environment-friendly method for without addressing what the bonding of gold is like. I think that this is one of the key questions to be answered before heading for the technique in processing gold.

I have listed in relation to your question some key elements. Maybe it will help you fine-tune your question.

• In mesothermal gold ores sulphide ore types can be subdivided broadly into pyritic (pyrite±pyrrhotite) and arsenical types (pyrite+arsenopyrite±pyrrhotite).

• Arsenopyrite in highly refractory ores is finer grained,

– contains high average concentrations of submicroscopic gold (60–270 ppm),

– but does not contain inclusions of particulate gold.

• Arsenopyrite in free-milling ores is coarser grained,

– contains low or negligible concentrations of submicroscopic gold,

– but contains inclusions and fracture fillings of particulate gold.

• Pyrite also contains moderately high levels of submicroscopic gold (20–40 ppm), the concentration of which is directly related to As content of the pyrite.

• Reasons for refractory behaviour in pyritic ores include very

– fine-grained native gold inclusions in pyrite, or the

– presence of gold-bearing tellurides.

With best regards

H.G.Dill

bioleaching

If you look at the overall picture as to the environmental effects of processing a sulphide gold concentrate, I would say the most environmentally friendly method is feeding the concentrate to a modern high sulfur capture copper smelter and refinery. Even more so if the concentrate has some copper, which will be then recovered. If it is however a gold only concentrate, gold will be collected in the blister copper and finally produced as metallic gold in a precious metals plant treating the anode slimes of the electrolytic refinery. Sulfur in the gold concentrate will appear as a useful product in the sulfuric acid plant of the smelter. Remember that now the best smelters in the wolrd have over 99.8% sulfur capture. From an overall process point of view, the only environmental effects of treating gold concentrate in a copper smelter would be the use of some additional oxygen (which requires power for its production) for smelting the concentrate, and the additional SO2 emissiions according to the sulfur capture associated to the specific smelter. Compare that to the assortment of chemicals and energy consumed in processes such as bioleaching or Albion process.

Even the change in oxygen consumption in the smelter caused by adding a gold concentrate to the feed can be minimimized or become negative if the gold concentrate in itself supplies enough sulfur to contribute to the autogeneity of the charge to the smelting furnace, which is the case when smelting low S copper concentrates.

What few people realize is that the process of copper smelting plus electrolytic refinery involves in itself a "free ride" for the precious metals which is a real gift from nature, where gold and silver will follow copper to the anode at high metallurgical recovery, and then in the electrolytic refinery the precious metals will be concentrated in a ratio of 1000 to 1 into the anode slime, without any expense other than collecting the slime from the bottom of the cells. Once you have the anode slime the PGMs have reached more than 95% of their final economic value, and the processing to metalli silver and gold by electrolytic refining or chemical process is straightforward and low cost.

Dear Herald 

Thank you ever so much for the detailed response. I found that to be very informative and academic. Of course I was aware of some of  other methods such as Biox. My question was basically meant to address problems faced by  small scale miners.

Kenneth,

You have your answer from Harald and Antonio.  Find out how much refractory gold concentrate you have and why it is refractory (send to a lab).  Then compare the costs of an autoclave against selling the con to a smelter (as Antonio suggests) - you can request a quote for terms from a smelter when you know what you would be giving them (composition and tonnage).  I think you will find that unless you have a significant amount of refractory concentrate, you should be selling to a smelter.  In any case, it is a route that allows you to have cash flow at the start without capital investment, then you can decide on capital for an autoclave or albion or bio or roasting if you have high tonnage.  You would need a lab that could do a trade off study on gold recovery for these different methods (we are one).  But the most straightforward route at the start is to sell to a copper smelter.  Good luck with it.

Bacteria leaching is one of the best methods for disintegration.  of sulphide mineral.The method is environmental friendly and energy saving.

I very much agree with Kenneth's recommendation, in the case of smal miners: to examine as the first option the sale of concentrate to a smelter or custom treatment for a fee. Its economic convenience to the miner will depend on sale terms which may be negociated, depending on the attractiveness of the concentrate as a supplement to the smelter present conc blend. Agree also that it may be a good first step before installing a physical gold production plant. In examining the convenience of installing such plant it will be important to know what other metal values are in the concentrate, specially copper, since in itself it may be interesting to recover or it may be a significant interference and source of additional costs in the processing of the concentrate. In case you have concentrate with high copper content besides gold, a small copper smelter may be the answer, as several of these have been installed lately in China, using their low cost SKS or BBR smelting process.

You might want to try Heap Leaching with thiourea solution. This will not produce gold concentrate but a rich gold solution that shoul be treated further with the gold refining techniques using oxalic acid

After crushing and washing the ore to get rid of unwanted by products such as clay you grind your sulfide ore down in accordance to the crystal intergrowth (hope that is the right technical term; in German: Verwachsungsgrad (more precise). The next step is flotation using terpenes for free gold; Xanthates forms with gold very stable complexes according to Pi-bonding which will be enriched in the effluent. Hope that will help you. - By the way oxalic acid is very toxic.

There are two issues, one is economics and the other is environmentally friendly.

Environmentally friendly is difficult because life cycle costs for the recovery of the gold, including plant and equipment, power, water, emissions etc should be considered.  In saying that if the economics don't stack up then the environmental aspects are a moot point. 

This statement may seem a little flippant, however the selection of the right processing route tends to provide a relatively good environmental solution as the efficiency of the process (lower operating costs) and lower capital costs (less raw materials converted to steel, less concrete for construction etc). 

The above is based on not selecting a particular technology based on environmental issues.  They all have plusses and minuses.  A heap leach operation may seem cheap, and for low grade ores, and low operating costs this may be true, but there is the legacy issues to deal with in terms of the heaps which need to be remediated for the long term safe storage or the material.

Typically the following processes are used for refractory sulphide ores as the most economic route.

If the gold in the ore can be upgraded through sulphide flotation, then a relatively simple crushing/grinding/flotation circuit can be used to remove the sulphides from the bulk of the ore.  If residual recoverable gold remains in the ore then it can be cyanide leached for recovery, with the sulphide material probably what is rendering the ore refractory.

A gravity recovery gold circuit can be used if there is native gold in significant quantities and the particles are of sufficient size.

The sulphide concentrate can then be oxidised to sulphates using pressure oxidation (in an autoclave) with the gold typically left in the solids and the iron and copper sulphides (other base metals as well) converted to sulphates in the acidic solution. The slurry is then neutralised and then can be leached with cyanide to recover the gold.

The sulphide concentrate can also be leached with strong oxidising acids in an intensive leaching step, but this would only be suitable on small amounts of concentrate due to the operating costs.

Tailings dams are then used to store the tailings from the process after neutralisation and cyanide destruction.

The sulphide concentrate can be sold (can be economically favourable) or roasted on site then leached to recover the gold. This is not necessarily the most environmentally friendly route, but cannot be discredited purely on emissions etc, as the SO2 is typically recovered at large smelters and can be used for fertiliser production, which could be considered environmentally friendly.

Heap leaching of sulphide ores is a possibility, but can be deceptively difficult to get right for sustainable long term predictable production, the risk is worthwhile for low grade ores, but high grade ores would be treated other ways. 

The above is all very general because it is quite difficult to put all sulphide refractory ores into the same basket, each ore body should be evaluated to find the best solution for that ore body.

Thanks so much for the information 

There are a small number of successful operations which use flotation of sulphides followed by biological oxidation of the concentrate to separate sub-micron gold that is dominantly locked inside sulphide minerals such as arsenopyrite,

Thanks Martin for the response

Bio-leaching is one of the eco-friendly method for leaching of sulphide minerals. we did some work in this field.

I visited such a mine last week - Fosterville gold mine in Victoria, Australia. Plant flow sheet is available - try the web. I imagine the Swaziland climate would be suitable for the bugs. I think the technique was developed in South Africa (when I was a student in Jo'burg long ago) - and it has also been used there.

Attached

Dear  All 

Thank ever so much for the information and the process flow chart from Martin is very illustrative 

Kenneth did start by saying that the gold was sulphide-locked and point out that the ore was of a type that would otherwise require roasting, so it is presumably not free-milling ore. Sulphide-locking and gold grain size in such ores is more relevant than bonding alone. Although one can divide sulphide ores into types, specific ore bodies often do not correspond to a simple pattern. A mine like Fosterville has gold in both pyrite and arsenopyrite, many hundreds of ppm in both, but no tellurides. Ores of this type commonly contain very fine particulate gold as well as Au (1) in the sulphide lattice, but the issue is releasing it. Mines like Fairview-Consort at Barberton contain both types of ore that Hartald mentions in a single ore body, but of course they can be separated in different circuits. 

Kenneth, do you mean artisinal miners or a hundred thousand ounces plus per year etc (I agree with Harald that sharper definition of the problem would be useful) - not sure what you mean by small-scale?

Dear Martin,

By small scale I mean artisinal miners