Research into industrial applications of thiourea has been increasing, says Guy Deschenes, a research scientist with the Canada Centre for Mineral and Energy Technology (CANMET) in Ottawa. Thiourea has been used commercially on cyanicidal products such as zinc and antimony residues. Heightened environmental concern, however, has placed a higher priority on its introduction in Canadian gold-processing.
During the past four years, CANMET has invested a good deal of effort in the development of a thiourea process. Research has also been under way at Laval University, the Centre de Recherche Minerale du Quebec (in Ste-Foy, Que., a suburb of Quebec City) and at Lakefield Research (in Lakefield, Ont.). A good number of other agencies have become interested in this field and have also contributed to its development.
Underground leaching with thiourea can be considered where an ore deposit is too difficult or too low-grade to mine by conventional means. A vein with appropriate porosity could be leached by a push-pull solution or in situ heap leaching. The use of cyanide, however, is ruled out by the difficulty of protecting personnel from the likely production of hydrogen cyanide (HCN). The deadly gas would not only affect the immediate area but could be spread throughout the mine by ventilation systems.
Thiourea, which does not present the gas problem, has many advantages over traditional cyanidation. It is less affected than cyanide by inhibitory agents such as zinc, lead, antimony and arsenic, as well as copper. These elements increase cyanide consumption and negatively affect, or even inhibit, the dissolution of gold. On the other hand, they have very little effect on thiourea. This allows a preferential dissolution where most of the gold is leached along with a certain amount of silver.
Gold-leaching by thiourea is also a solution for the development of an integrated circuit for hydrometallurgical copper-processing. “We have shown that thiourea is effective in extracting gold from a copper concentrate and even its residues,” Deschenes said. “We see this as a significant step in the use of thiourea for gold recovery.” The presence of copper in proportions of more than 1% to 2% causes serious problems when processing gold ore by cyanidation. Thus, designing a method to process a chalcopyrite gold-bearing concentrate by cyanidation is very problematic, so it seems thiourea offers the best alternative to recover gold from the concentrate. This has encouraged the evaluation of thiourea on a gold chalcopyrite concentrate from Chibougamau, Que. However, in order for this method to be used on a commercial scale, reagent costs must be optimized.
Reaction kinetics demonstrate the vitality of an acid thiourea solution. Some studies report a dissolution that is 12 times faster than with cyanidation. Other studies have achieved the extraction of at least 90% of the gold for a leaching time of 30 minutes to eight hours versus 24 hours for cyanide.
Thiourea leaching has to be performed under controlled ph and electromotive force (emf). Studies have shown that ph ranging from one to two is preferable to obtain good extraction of gold. Higher ph produces a decrease in recovery. The use of an oxidizing agent and sulphur dioxide (SO2) is helpful in obtaining optimum leaching conditions and in reducing thiourea consumption. It was found that the emf has to be higher than 185 mv (saturated calomel electrode) to recover more than 93% gold from the gold-bearing chalcopyrite concentrate. About 5 kg per tonne SO2 are required during the operation and 6 kg per tonne thiourea are consumed.
Research on the gold chalcopyrite shows that the initial leaching rate increases as a function of thiourea concentration when using up to 15 g per l of thiourea, and then decreases when the thiourea concentration reaches 25 g per l. By adding 0.25% hydrogen peroxide (H2O2 ), a maximum gold-extraction rate of 93.2% was obtained in one hour.
The second phase of the study on thiourea was related to an assessment of different options to recover gold from solution. The techniques examined were cementation, solvent extraction, electrowinning, hydrogen reduction and processes involving activated carbon and ion exchange resins. What follows is a description of the actual status of some of the methods.
. Cementation — The use of aluminum at 150 mg per l of solution made it possible to extract 97.2% of the gold. Bacon, Donaldson & Associates of Vancouver found that cementation must be preceded by pretreatment of the solution with SO2 to adjust the EMF.
. Activated Carbon — An industrial application of thiourea leaching has been developed in New South Wales. Leaching for a period of 15 minutes was carried out on an antimony concentrate and the gold recovered using activated carbon. Thiourea consumption was barely 2 kg per tonne and gold recovered was about 85%.
Thiourea has a tendency to be adsorbed on to the activated carbon, indicating that the use of carbon-in- pulp would be restricted to a system with a low thiourea concentration. Basic studies show that the adsorption of the gold-thiourea complex on to carbon is similar to the cyanide system, i.e., that it is formed by paired ions and that the initial adsorption kinetics is very fast. Using x-ray photoelectron spectroscopy, CANMET found that gold deposited on to carbon from a thiourea solution remains in a complex form.
. Electrowinning — Most of the research into gold electrowinning from a thiourea solution has been done in the U.S.S.R., where the process has been used commercially for more than 15 years. Studies at Laval University on the direct electrowinning of gold from a synthetic thiourea solution and a leach solution of a chalcopyrite concentrate gave a 99% gold recovery. The presence of sodium sulfite, which produces SO2, facilitated electrowinning, while an increase in the concentration of thiourea caused a decrease in the effectiveness of the process.
. Hydrogen Reduction — Reduction under hydrogen pressure to recover gold and silver represents an interesting choice because of its selectivity and the possibility of precipitating the precious metals. Temperature determines the reduction kinetics of the gold as well as the purity of the final precipitate. At 70 kpa hydrogen (H2) and with the use of nickel powder as catalyst, about 80% of the gold is precipitated after two hours at room temperature, while precipitation increases to 99% when temperature is increased to 95 degrees c.
The increase of the hydrogen pressure from 70 kpa to 6,000 kpa improves the gold recovery. About 75% gold is recovered at 70 kpa while the recovery reaches 98% at 6,000 kpa (two hours’ reduction). Another advantage of the process is the selective reduction of gold and silver by hydrogen. Impurities such as copper and iron remain in solution and lead to a highly pure powder. It has also been found that hydrogenation of a thiourea solution occurs without any decomposition of thiourea.
Basic studies in collaboratio
n with Laval University (Dr Ghali, et al.,) have made possible the construction of electrochemical Pourbaix diagrams. These diagrams help researchers understand the chemistry of the gold- thiourea system, an important element in the development of the thiourea process.
“There remains a lot of work to be done in order to improve and integrate thiourea gold-processing into Canadian industry,” Deschenes said. A good deal of progress has been made. Thiourea could be used on cyanicidal materials to recover precious metals and treat deposits in areas with stringent environmental restrictions. “We would welcome more interest on the part of industry in supporting our efforts. This would ensure the transfer of the technology,” Deschenes said. Norman Avery is a freelance writer based in Ottawa.
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