Thiourea gold extraction technology and development overview

Huang Kaiguo Hu Tianjue

Central South University of Technology

I. Introduction

Thiourea has a strong network together gold and silver, with 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 are all focused 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), 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 1-2 lists the complexation constants for thiourea with some metal ions.

Table 1 Formation constants of several metal thiourea complexes (1)

Table 2 Formation constants of several metal thiourea complexes (2)

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 (0.38 V) is very close to the standard potential of thiourea oxidation (0.42 V). 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, according to these principles, gold and silver in the ore can be obtained.

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, thiourea leaching technology began to receive attention throughout the world due to the growing emphasis on environmental issues. The former Soviet Union, the United States and South Africa have done a lot of work in the thiourea gold extraction. In terms of theoretical research, Groenrwald in the 1970s conducted intensive research on the dissolution rate of gold in acid thiourea, proving that thiourea dissolves. gold speed and associated elements such as: copper, arsenic, antimony, lead, etc. with small interference thereof: and Reddy, charley, Bilston the reaction mechanism and the potential of leaching were studied systematically explained the whole of thiourea Chemical reaction process. 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, were treated carbonaceous arsenic pyrite in two ways, the result is as in FIG. 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 used for the first time in the "press-in-extraction" test to extract gold from a deep-covered 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 materials are calcined in two stages to produce slag, which is top-dipped with sulfuric acid at 70 ° C, and then leached at the temperature with an acidic thiourea solution. 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 solves the problem of high consumption of thiourea and unstable recovery of gold and silver. It is characterized by the introduction of SO ₂ in the leaching slurry and the washing of the leaching slag with thiourea. As an easily oxidized organic matter, thiourea will produce a series of products during the leaching process. In terms of feedstock, the introduction of SO ₂ into the leaching slurry can solve the problem that the agent is not easily destroyed and passivated. , 1000Kg dry material and 100Kg wet material (35g gold), H ₂ SO ₄ 5Kg, SO ₂ 0.5Kg, H ₂ 0 ₂ (30%) 0.75Kg, thiourea 1.5Kg, gold extraction can reach 98%, after three-stage carbon adsorption, the recovery rate of gold is 97.9%, and the total recovery of gold is above 95%.

In addition, the late eighties and early nineties, Murthy invention the bacterial oxidation of a thiourea leach gold and silver extraction from refractory sulfide ores lead-zinc and used in the actual production process. 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 agronomy 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 leaching 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 higher replacement rate, but also accelerates the leaching reaction and increases the leaching rate, thus proposing a "one-step one-step method" for thiourea gold extraction. 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. The Kunming Institute of Metallurgy, the Pinggui Mining Bureau Experimental Institute and the Guangdong Institute of Technology also carried out research on this aspect of the process.

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:

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. 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. The use of a suitable oxidizing agent and a suitably high concentration of oxidant greatly affects the leaching rate. The commonly used oxidants are shown in Table 2. Fe ³⁺ is usually used as the oxidizing agent.

Table 2 Standard Oxidation Reduction Potentials of Common Oxidants

The gold is recovered from the conventional leaching solution, mainly by ion exchange method, carbon adsorption-electrowinning method, displacement and 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. Solvent extraction - The electrowinning method has been studied in depth, 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, complicated in process, simple in operation, and most promising. 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.

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