Overview of thiourea gold extraction technology and development

Huang Kaiguo Hu Tianjue

I. Introduction

Thiourea has a strong network together gold and silver, good selectivity, good noble metal is leached reagent. The study of leaching gold from ore with thiourea began in the 1940s and has not received much attention for some time. Until the 1970s, with the promotion and research of the non-cyanide gold leaching process, the importance of thiourea immersion gold was re-recognized, and an unprecedented research heat of thiourea immersion gold was formed worldwide. In the past ten years, the research reports on thiourea leaching gold at home and abroad have emerged in an endless stream, but they all focus on experimental research in a certain aspect. Therefore, this paper makes a general review on the basis of previous research.

Second, the principle of thiourea immersion gold

Thiourea (English name abbreviation Tu) is also known as thiourea, its structural formula is , molecular weight 76.12, rhomboid crystal with white luster, bitter taste, density 1.405, melting point 180-182, soluble in water, its aqueous solution is neutral, unstable in alkaline solution, easy to decompose into thio compound and cyanamide, It has reducibility in an acidic solution and can be oxidized to form various products such as dithiocarbamidine. The biggest feature of thiourea is its strong complexing ability. It can be quickly dissolved in solution with gold and silver under strong acidic conditions. Table 1 lists the complexing constants of thiourea with some metal ions.

Table 1 Several metal thiourea complex formation constants

It can be seen from Table 1 that in the presence of an oxidizing agent, gold and silver can form a stable thiourea complex dissolved in an acidic liquid. Its dissolution potential is:

Au+2SC(NH ₂ ) ₂ = Au[SC(NH ₂ ) ₂ ] ⁺ ₂ +e; 4°= 0.38v

Ag+2SC(NH ₂ ) ₂ = Ag[SC(NH ₂ ) ₂ ] ^- ₂ +e; 4°=0.025v

The standard potential of gold dissolution (0.38v) is very close to the standard potential of thiourea oxidation (0.42v). Controlling the acidity avoids the oxidation of thiourea and effectively dissolves gold. Figure 1 is an anodic polarization curve of gold in thiourea, showing that only gold dissolution occurs during this potential interval without rapid oxidation of thiourea. Figure 2 is a comparison of the dissolution rate of gold in thiourea and cyanide solutions. It can be seen from the figure that the dissolution rate of gold in thiourea is much faster than that in cyanide. Therefore, gold and silver in the ore can be leached according to these principles.

Figure 1 Current-potential curve of gold dissolved in thiourea

Figure 2 Comparison of acid thiourea method and cyanidation method

Third, the development of thiourea leaching gold

The dissolution of gold with thiourea was first proposed by Praxin et al. in the former Soviet Union in 1941, when it did not attract the attention of the world. Until the 1970s, due to the increasing environmental problems, thiourea immersion gold technology began to receive attention in the world. Countries such as the former Soviet Union, the United States, and South Africa have done a lot of work on thiourea gold extraction. In theoretical research, seventies Groenrwald gold dissolution rate in an acidic thiourea conducted intensive studies prove fast thiourea gold dissolution speed associated elements such as: a small copper, arsenic, antimony, lead and other radio interference. Reddy, Charley, and Bilston studied the reaction mechanism and leaching potential, and systematically explained the entire chemical reaction process of thiourea leaching. In the early 1980s, Gabra conducted in-depth research on the kinetics and leaching conditions of thiourea leaching gold, and determined the effects of Fe, thiourea, sulfuric acid concentration and temperature, which provided the determination of the optimal conditions for the acid thiourea gold extraction process. Theoretical basis. In the same period, domestic Zhang Jianmin, Sabenjia and Changchun Metallurgical Research Institute also carried out a lot of research work, which contributed to the promotion and application of domestic thiourea method. In the mid-eighties, Schulze proposed in the study of reducing the consumption of thiourea that sufficient SO _{2 was introduced} into the slurry during the immersion process, which could effectively inhibit the irreversible decomposition of thiourea and reduce the loss of thiourea. To a minimum, this research has greatly improved the application value of thiourea immersion gold. Since then, there have been many reports on the thiourea method, but they have only further improved the work of the predecessors, and there has been no major breakthrough in theoretical research.

In actual leaching process aspect of the study, the first to do Gabra of thiourea and cyanide comparative experiment, we were treated carbonaceous arsenic pyrite in two ways, the results shown in Figure 2. In the 1970s, Pyper and Eudrix and Groenwald also conducted experimental studies on thiourea immersion gold in some gold-bearing ore, but the results were not satisfactory. In the 1980s, Chinese-funded Chinese Chen Dengwen used a calcined and sulfuric acid pretreatment for refractory carbonaceous ore, which was leached with acid thiourea. The recovery rate of gold was 95%, and the consumption of thiourea and sulfuric acid was 1.5-. 2kg/t and 70kg/t ore. Research by the American Bilante Engineering Company pointed out that the thiourea process can be profitable in many cases by replacing the cyanide gold extraction process. The company has completed semi-industrial trials, designed preliminary processes and successfully conducted small pilot plant production. In addition, Belter Engineering has made great progress in extracting gold and silver from ores and concentrates using thiourea, with a focus on high-grade gold concentrates. The key to this work is the regeneration and return of the agent and the precipitation of precious metals. During the same period, the Real Estate Gold Co., Ltd. conducted an in situ thiourea immersion gold test in Victoria, Australia. In the test, a mixture solution of thiourea, thiosulfate and ferricyanide was first used in the "press-in-out" test to extract gold from a deep-coverage alluvial deposit. In a patent obtained in Montague, a method for recovering gold and silver from a sulfur-containing material using an acidic thiourea solution at 60 ° C to 80 ° C is described. In the Salseg patent, a method for recovering gold and silver from arsenopyrite is described. The process is that the raw material is calcined in two stages to produce slag, pre-soaked with sulfuric acid at 70 ° C, and then acid thiourea solution. Leach at this temperature. Both of these patents are widely used.

In order to reduce the amount of thiourea without affecting the leaching rate, Schulze invented a new process of thiourea immersion gold in the mid-to-late 1980s, as shown in Figure 3. It is reported that this process can solve the problem of high consumption of thiourea and unstable recovery of gold and silver. It is characterized in that SO ₂ is introduced into the leaching slurry and the leaching residue is washed with thiourea. As a kind of organic substance which is easy to be oxidized, in the case of feeding, the introduction of SO ₂ in the leaching slurry can solve the problem that the agent is not easily damaged and passivated. According to the experimental calculation, 1000 kg of dry material and 100 kg of wet material (including gold 35g) ), adding H ₂ SO ₄ 5kg, SO ₂ 0.5kg, H ₂ O ₂ (30%) 0.75kg, thiourea 1.5kg, the extraction rate of gold can reach 98%, after three-stage carbon adsorption, gold The recovery rate is 97.9%, and the total recovery rate of gold is above 95%.

Figure 3 thiourea leaching flow chart

In addition, the late eighties and early nineties, the bacterial oxidation invention Murthy - thiourea leaching of gold and silver extraction process from refractory sulfide ores of lead and zinc for practical production. The leaching material is a lead-zinc sulfide ore of the PeOS mine in New Mexico, and its ore contains gold and silver of 1.75 g/t and 22.5 g/t, respectively. Bacterial oxidation was carried out in a stirred reactor. All bacteria were Thiobacillus ferrooxidans grown in Fe ³⁺ medium, the leaching medium was 9K medium, the leaching temperature was 35 ° C, the pH was 2.3, and the leaching time was 1 day - 30 days. After the ore sample is oxidized and leached by bacteria, thiourea is directly added to extract gold. Thiourea leaching conditions: pulp concentration 25%, pH 2.3 and 1.3, temperature 35 ° C, thiourea concentration 0.5 mol / L, leaching time up to 4 hours. The leaching rate of gold is over 90%. If the material is not oxidized by bacteria and directly leached with thiourea, the gold intake rate is only 23%.

Domestically, the Changchun Gold Research Institute first proposed a new perspective on the leaching of gold by the thiourea method and the replacement of gold in the same equipment. It is pointed out that the iron plate replacement not only has a high replacement rate, but also accelerates the leaching reaction and increases the leaching rate, thus proposing a "dip-single-step method" for thiourea immersion gold. This is a major improvement in the thiourea gold extraction process. In the mid-1980s, a thiourea gold extraction industrial test workshop for processing 10 tons of gold concentrate was built in a gold mine in China and put into production.

According to reports, a gold mine in Guangxi has also established a thiourea immersion gold plant to improve the thiourea gold extraction process. Kunming Metallurgical Research Institute Pinggui Mining Bureau Experimental Institute and Guangdong Institute of Technology and other units have also carried out research in this area.

Fourth, the basic process and application

At present, the thiourea immersion gold process has been basically improved in terms of the results obtained by the research, which are briefly described as follows:

Figure 4 thiourea leaching gold process flow chart

Generally speaking, the ore pretreatment methods are: 1 oxidative roasting method; 2 hot pressing method; 3 chlorination method; 4 microbial oxidation. Which method is used depends on the type, structure and nature of the ore. The ore is pretreated to greatly improve the leaching rate.

The higher the concentration of thiourea in the solution, the better. As the concentration of thiourea increases, the leaching rate of precious metals increases, but the cost increases. Therefore, the concentration of thiourea is usually not higher than 3%.

The pH control of the leachate is very important, and the thiourea in the solution tends to be stable as the acidity of the medium increases. However, when the pH is less than 1.78, the high concentration of thiourea is easily oxidized. When the pH is >2, the consumption of thiourea is increased, and the dissolution rate of gold is also slowed down.

The oxidant also plays an important role in the leaching process. With a suitable oxidizing agent and a suitably high oxidizing agent, the concentration greatly affects the leaching rate. The commonly used oxidants are shown in Table 2. Usually, Fe ^{3 + is} used as an oxidizing agent.

Common oxidant standard oxidation reduction potential Table 2

The gold is recovered from the conventional leaching solution, and mainly includes an ion exchange method, a carbon adsorption-electrowinning method, a displacement, and a solvent extraction-electrowinning method. The ion exchange method has been studied very well and has its advantages, but the process is complicated and the cost is high, which limits its wide application. The solvent extraction-electrowinning method has been studied intensively, but it is still limited to small-scale production. The replacement method is the earliest and the most commonly used recycling method. The method is simple in principle, low in cost, uncomplicated in process and easy to operate. The most application prospects. When gold is recovered from the low-grade gold ore leaching solution, the activated carbon adsorption-electrowinning method is often used. This method does not require leaching liquid clarification filtration and vacuum degassing, and the concentration can be as low as 0.0016 g/l.

The application of the thiourea process for the extraction of precious metals can be said to have been studied for more than half a century. As mentioned above, it has been used in various mining factories around the world. It is especially suitable for the treatment of precious metal ore, acid leaching slag, anode slime and gold and silver waste which are difficult to handle by some cyanidation methods. Its low toxicity, fast leaching speed, simple purification process, no pollution to the environment and no interference from copper, lead, arsenic and antimony are incomparable by cyanidation. In the end, it is very likely to replace cyanide. A century of reagents for extracting precious metals.

Originally published in Gansu Nonferrous Metals, 1996, No. 4 ☺

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