Gold Leaching using Thiourea
Thiourea, CSN(NH2)2 is an organic compound which dissolves easily I acid solution in a stable molecular form. Gold dissolves in acidic thiourea solution to form a stable complex,
2Au + 4CS(NH2)2 + 2Fe3+ = 2Au(CS(NH2)2)2+ + 2Fe2+
In the thiourea reaction, ferric iron is used as an oxidizing agent, whereas the cyanide process uses oxygen from the air, dissolved in the leach solution. Part of the ferric iron needed is most the time present in the ore. In the case of highly oxidized ores, enough ferric ions will be set free, and then the addition of an oxidant can be reduced. But these ores could content higher amounts of carbonates, which will increase the acid consumption. One could use hydrogen peroxide, sodium peroxide, ozone, potassium permanganate or formamidine disulphide as oxidant; however ferric ion is the most effective.
Formamidine is interesting because it can be formed in acid solution by the thiourea oxidation by the presence of an oxidant agent such as ferric iron. Successfully leaching of thiourea depends on an understanding of the role of formamidine which is a compound very active during gold leaching.
In the first step thiourea is oxidized to formamidine disulphide,
2CS(NH2)2 + 2Fe3+ = C2S2(NH)2(NH2)2 + 2Fe2+ + 2H+
Oxidation thiourea is reversible. Thus, with a specific reducing agent, formamidine can be converted back into thiourea. In the next step, gold is oxidized by formamidine and forms a cationic gold thiourea complex,
2Au + C2S2(NH)2(NH2)2 + 2CS(NH2)2 + 2H+ = 2(Au(CS(NH2)2)2)+
Formamidine acts as an oxidizer as well as a complexing agent, supplying about 50% of the ligands to the complexation. This explains the higher leaching rates observed with thiourea compared to cyanidation. The general reaction is as follow,
2Au + 4CS(NH2)2 + 2Fe3+ = 2Au(CS(NH2)2)2+ + 2Fe2+
Thiourea must be present in a stoichiometric excess. The ratio of complexing and oxidizing agents must be carefully adjusted. An uncontrolled oxidation of the thiourea solution will lead to unwanted reagent consumption.
In a final and irreversible step, formamidine breaks down to cyanamide and elemental sulphur. This forms two effects. First, the elemental sulphur will come in a fine, sticky form, which might passivate the feed ore. Second, loss of silver could occur because of a reaction which leads to precipitation of silver salts. The breakdown can be unpredictable, but it seems that the event is accelerated by high concentrations of formamidine. The corrective measure is to keep the concentration of thiourea itself low and prevent uncontrolled oxidation. High amounts of free thiourea result in rapid leaching, but they are vulnerable for a fast breakdown.
The use of acidic solutions at potentials well below those required for the formation of oxide film on the gold surface obviates any possible passivation of the gold by oxide films and extractions in thiourea solutions are high. Of the several oxidants, only ferric ions offer any promise of practical application. The initial rate of dissolution of gold in freshly prepared solutions of thiourea containing ferric ions is high, and is controlled only by the rate of diffusion of the oxidant to the gold surface. However, due to the slow oxidation of thiourea and the partial passivation of the gold surface by the products of oxidation, the rate of dissolution decreases with the age of the lixiviant, and this can lead to excessively high consumption of the reagent. The excess can be more than 5 kg/t.
One of the main advantages of thiourea is the high rate of gold dissolution. The leaching rate can be four to five times faster than cyanidation. This could be an important aspect for plants which process ore containing coarse gold particles. The much higher leaching time will reduce the investment for a new processing plant. The thiourea leaching can be adapted to given plant and ore conditions, whereas the leaching conditions of the cyanide process can only be modified within a narrow range.
The economics of gold leaching with thiourea are principally determined by thiourea consumption, which is related to thiourea and oxidant concentrations, pH and the solution potential. Thiourea concentrations between 5 to 50 g/l have been used in lab and pilot test. Sufficient oxidant (i.e. ferric ion) is required to oxidize thiourea to formamidine disulphide for optimal leaching conditions. The presence of excess oxidant increases thiourea consumption significantly. For this reason, close control of solution potential would be required through all stages of leaching in any commercial process. Thiourea consumptions of 1-4 kg/t have been projected for optimized thiourea leaching systems based on currently available technology, although estimates as high as 10 kg/t could be obtained. Such high consumptions, coupled with the requirements of sulphuric acid for pH control, and agents for potential control, trends to be a part very important of the cost, perhaps between 1.5 to 2.1 times the cost of a cyanidation process.
The advantage of thiourea is that sulphuric acid has little effect on sulphidic copper minerals which dissolve almost completely in cyanide solutions. Obviously, copper carbonates will be dissolved easily. In general, the observed level of copper in thiourea leach solutions in lower than those in cyanide leaches.
Arsenic and antimony present in the form of sulphidic minerals can result in tremendous difficulties for cyanidation. This is true not so much for arsenopyrite, but more for realgar, orpiment and stibnite. These minerals dissolve in alkaline solutions at the high ph level used in cyanidation, resulting in the formation of thioarsenites and thioantimonites. These compounds react with oxygen and then less oxygen is available. Refractory ores of this type are more suitable for thiourea. Arsenic and antimony sulphides will not dissolve at pH range of 1 to 2. This is demonstrated at the New England Antimony Mine.
The New England Antimony Mine is located in New South Wales (Australia). The plant leached a refractory Aurostibnite flotation concentrate using thiourea. About 50-60% of the gold content is extracted in 10-15 minutes from the flotation concentrate after as much free gold as possible had recovered by gravity separation. Total recovery is about 80%. Critical parameters for the thiourea leaching were:
- pH, 1.4 adjusted with sulphuric acid.
- Redox potential, 150 to 250 mV.
- Thiourea concentration, 1%.
- Thiourea consumption, 2 kg/t.
- Leach time, 10-15 minutes.
Over 250 mV, thiourea consumptions increase to excessive levels. Below 150 mV, gold is not leached in thiourea. The potential was initially controlled by adding peroxide, but later was used MnO2. About 6000 gm/m3 ferric iron is needed for the process to work. Thiourea consumption is reduced by keeping the time of thiourea contact with the sulphide mineral to an absolute minimum. For this reason, leaching time is kept to 10-15 minutes, and the pulp is filtered as rapidly as possible. After leaching, gold is recovered by adding carbon to the pregnant solution, and allowing loaded carbon to settle before decanting the barren solution to recover thiourea values for reuse.
Gold thiourea can be recovered from solutions or slurries by adsorption on activated carbon or ion exchange resins, or it can be precipitated on steel wool or lead metal. Approximately, gold thiourea can be loaded in carbon to 10-20 kg/t gold.