Technology of using environmentally friendly gold extraction agents to replace NaCN for gold extraction production 

Abstract: Thiourea, thiosulfate and environmentally friendly gold extraction agents are gold leaching agent products. This type of product has the advantages of low environmental toxicity and recyclability in place of sodium cyanide. It is suitable for use in the leaching and gold extraction process of various gold-containing ores, thus changing the history of people using sodium cyanide as a single gold extraction agent in the past, which is toxic and harmful to humans, and solving the environmental pollution problems and safety concerns caused by the use of sodium cyanide. With the application practice in some gold mines at home and abroad and the market promotion in recent years, this type of product has entered a growth period, gradually been recognized by customers, and has a broad market prospect. Therefore, strengthening research, further improving and strengthening the performance and standards of this type of agent is of great significance for gold production to truly move towards "green, environmental protection and safety". 

Keywords: thiourea; thiosulfate; environmentally friendly gold extraction agent; sodium cyanide 

Introduction 

With the continuous advancement of mineral processing technology, low-toxic gold leaching agents aimed at replacing sodium cyanide are also constantly being introduced and innovated. The main components of this type of agent are thiourea, thiosulfate and ferrocyanide, which are added with urea and caustic soda chemical reagents to prepare environmentally friendly gold extraction agents. Compared with traditional cyanide, it is less toxic and safe to use. At present, the country has very strict use and management of cyanide. Thiosulfate and environmentally friendly gold extraction agents are used as an effective, convenient and low-toxic gold leaching agent to replace cyanide in the leaching and gold extraction production of certain types of gold ores, which is of great significance to gold production and environmental protection. 

1. Thiourea, thiosulfate and environmentally friendly gold extraction agents

The types of thiourea, thiosulfate and environmentally friendly gold extraction agents mainly include: thiourea, thiosulfate, and environmentally friendly gold extraction agents prepared by adding urea and caustic soda chemical reagents to ferrocyanide. 

1.1 Thiourea (abbreviated as Tu) 

Thiourea is also known as thiourea, and its structural formula is:

Molecular formula SCN2H4, with white and shiny rhombic crystals; easily soluble in water, its aqueous solution is neutral, unstable in alkaline solution, easily decomposed into thiocarbamide and aminocyanide, reducible in acidic solution, and can be oxidized to form dithiocarbamide and other products. The biggest feature of thiourea is that it has strong complexing ability and good selectivity, and is a good reagent for leaching precious metals. Under strong acidic conditions, thiourea has a strong complexing force on gold and silver, and can quickly form complexes with gold and silver and dissolve in the solution. The constant of the complex Au[SCN2H4]24+ formed by gold and thiourea is 21.96; its dissolution potential is: Au+2SC(NH2)2 = Au〔SC(NH2)2〕+2 + e;4°=0.38v

1.2 Thiosulfate 

Thiosulfate gold leaching agent is a sulfide containing S2O32-. Due to its strong negative charge, it forms polysulfides with alkali metals and alkaline earth metals. Precious metals such as gold and silver have a strong affinity with sulfur and are easy to form sulfides. At the same time, because sulfides are soluble, they provide conditions and theoretical basis for dissolving gold (silver). The reaction of gold, silver and polysulfide is as follows: In the absence of oxidant, S52- has both oxidation and coordination effects Au + 2S52- → AuS5- + S2- + e (Ag + 2S52- → AgS5- + S42- + S2- + e) In the presence of oxidant, S52- only plays a coordination role Au + S52- → AuS5- + e (Ag + S52- → AgS5- + e) S2O32- leaches Au (Ag) under the action of oxidation catalyst 2Au + 4S2O32- + H2O + 1/2O2 → 2Au(S2O3)23- + 2OH- + S2O32- (2Ag + 4S2O32- + H2O + 1/2O2 → 2Ag(S2O3)23- + 2OH- + S2O32- ) Further experimental studies revealed the anodic dissolution mechanism of gold: NH3 preferentially diffuses to the surface of gold particles and cooperates with gold ions to generate ammonia complex ions, which are replaced by S2O32- after entering the solution to form more stable gold thiosulfate complex ions. At the same time, the kinetic study of gold leaching by thiosulfate method revealed that copper and ammonia have catalytic effects in the leaching process of gold, and the mechanism of action of copper and ammonia ions in gold leaching, namely: ammonia can weaken the passivation effect of gold anodic dissolution and significantly increase the gold dissolution rate; copper ammonia complex ions catalyze the cathode reduction process of oxygen. The effect of thiosulfate leaching of gold depends on the type of ore. When thiosulfate is used to extract gold from oxidized ores, it is not as effective as cyanide, while the thiosulfate method obtains a relatively high gold extraction rate from carbonaceous ores, and the cyanide method extracts gold from similar difficult-to-treat sulfide ores, only a very low gold recovery rate. Comparing the effect of thiosulfate method in leaching gold from carbonaceous ores with the results of cyanide method, it can be seen that the thiosulfate method is technically superior and economically advantageous. 

1.3 Lime sulfur mixture method (LSSS) 

The lime sulfur mixture method is a new type of cyanide-free gold extraction technology pioneered in my country. The gold leaching reagent used is synthesized from lime or Ca(OH)2 and sulfur. The reagent has the advantages of being non-toxic, easy to synthesize, fast gold leaching rate, and can be used in alkaline media, so it has low requirements for equipment and materials. The main active ingredients of the lime sulfur mixture method for gold leaching are calcium polysulfide (CaSx) and thiosulfate. It has obvious advantages in treating difficult-to-leach gold ores because both polysulfide solvents and lime sulfur mixtures are alkaline gold leaching reagents, and the solubility of many metals in alkaline media is lower than that in acidic media. Therefore, when using alkaline media to leach gold ores, the content of other metal impurities in the precious solution is lower than that when leaching with acidic media, which is beneficial to the subsequent treatment of gold in the precious solution; since polysulfide and thiosulfate are both suitable for gold leaching, this method has good gold leaching performance. In the process of gold leaching, polysulfide ions Sx2- have the dual functions of oxidation and coordination, while S2O3- can be used as a ligand. Its main gold dissolution reaction is as follows: 6Au＋2HS-＋2OH-＋S42- → 6AuS-＋2H2O 8Au＋3HS-＋3OH-＋S52- → 8AuS-＋3H2O

2Au＋4S2O32-＋H2O＋1/2O2 →2Au(S2O3)23-＋2OH-

1.4 FKN environmentally friendly gold extraction agent 

FKN environmentally friendly gold extraction agent products are a new generation of environmentally friendly gold extraction agent products that replace NaCN and are developed by scientific researchers based on long-term scientific research and practice.

The principle of gold leaching with environmentally friendly gold extraction agents:

The environmentally friendly gold extraction agent is a mixture of various compounds such as ferrocyanide, urea or thiosulfate and alkalis. It is a gray-yellow solid particle, alkaline and easily soluble in water. 

The principle of gold leaching by environmentally friendly gold extraction agents is described differently depending on the synthetic components of the manufacturer's agents. Since the agent is a polymer, the molecular formula is unknown. The two main types that need to be verified are as follows: 

1.4.1 Thiosulfate is a compound containing S2O32- groups. Gold can react with S2O32- ions to form a stable complex ion Au(S2O3)23-. Under aerobic conditions: 2Au + 4S2O32- + H2O + 1/2O2 (gas) → 2 Au(S2O3)23- +2OH- 

1.4.2 The main active ingredient of environmentally friendly gold extraction agents is sodium polycyanamide or sodium carbonate cyanurate; sodium polycyanamide is a polymer formed by catalytic oxidation of general chemical raw materials such as ferricyanide, thiourea or urea, soda, etc. Under alkaline conditions, sodium polycyanamide solution decomposes and releases N(CN) group, which can also dissolve gold to form complex ions under aerobic conditions. Taking sodium dicyanamide as an example, its reaction process is: NaN(CN)2 → Na+ + N(CN)2- 

Au + N(CN)2- + H2O + 1/2O2 (gas) → Au N(CN)22- +2OH- 

It can be seen that the cyano group (CN -) in the carbonized sodium cyanurate is in a stable complex compound state under normal conditions due to the strong linking bond, rather than the free cyano group (CN -). This is the main reason why (Jinchan) gold ore dressing agent can detect a small amount of cyano group (CN-) but is non-toxic or low-toxic to humans and other organisms. 

As can be seen from the above, the environmentally friendly gold extracting agent may contain thiourea or thiosulfate and sodium polycyanamide or carbonized sodium cyanurate to leach gold. 

2. Overview and current status of the development of thiourea, thiosulfate and environmentally friendly gold extraction agents

2.1 Thiourea gold leaching

The main gold extraction processes using thiourea gold leaching include thiourea carbon slurry method, thiourea resin method, thiourea iron slurry method and thiourea electrolytic method. Each process has its own advantages and disadvantages. The replacement method is the earliest and most commonly used recovery method in the past. The method has a simple principle, low cost, simple process and easy operation, but it is easy to produce mechanical losses and cause metal imbalance. The first gold concentrate smelting plant built in the former Guangxi Longshui Gold Mine using thiourea gold leaching also failed mainly because of this. In modern times, ion exchange resin adsorption-electrolytic method is mostly used to recover gold from gold mine leaching liquid. This method does not require clarification, filtration and vacuum degassing of the leaching liquid. Although the process is relatively complex, it has become increasingly mature since its development, making its cost gradually reduced and becoming more and more widely applicable. It also laid the foundation for the later use of a mixture of thiourea, thiosulfate and ferrocyanide as a substitute for cyanide to leach gold.

The advantages of using thiourea to extract precious metals are mainly low toxicity, fast gold leaching rate, easy reagent regeneration, and not very sensitive to mineral compositions such as arsenic, antimony, copper, and sulfur that affect cyanide leaching, especially for the treatment of carbonaceous gold ores. Its disadvantages are that in addition to the limitations and requirements on the corrosion resistance of equipment for gold leaching in acidic media, the damage of thiourea to the soil is also the reason why people worry about this agent. 

2.2 Gold leaching with thiosulfate

Thermodynamic research and experiments have confirmed that the thiosulfate method of gold leaching needs to be carried out in an alkaline medium, so it is non-corrosive to the equipment. The process has a high gold leaching rate and the reagents used are not very toxic. Due to the poor thermal stability of the thiosulfate system, the large consumption of gold leaching agents, and the narrow allowable temperature fluctuation range, its initial application in production practice was limited. However, with the development of science and the advancement of technology, thiosulfate gold leaching has been transferred from past experimental research to practical application.

The use of ammonium thiosulfate to extract gold and silver has been proposed as an option. Previous studies have used synthetic relatively pure systems or pure gold. It can be determined that the main reaction and mechanism of extracting gold from raw ore or gold concentrate with thiosulfate are the same as those of dissolving pure gold with thiosulfate. Compared with the cyanide method, the thiosulfate method is worse than the cyanide method in extracting gold from carbonaceous ores, but the thiosulfate method is not as good as the cyanide method in extracting gold from oxidized ores. Practice shows that the thiosulfate method has the following advantages for gold leaching: 

(1) It is suitable for treating gold ores with low-grained carbon and arsenic; 

(2) A thiosulfate recovery and reuse process has been developed, which can be recycled multiple times, solving the problem of high reagent costs;

(3) It is a truly non-cyanide gold leaching process, which is safe and environmentally friendly and is recognized as the most environmentally friendly gold leaching process;

(4) The process is stable.

2.3 Gold leaching with lime sulfur mixture (LSSS)

As a new type of gold leaching process, the lime sulfur mixture has the characteristics of fast gold leaching rate, high leaching rate for difficult-to-treat gold ores, strong adaptability, non-toxic and pollution-free. It is reported that the use of solvent extraction to recover gold from precious liquid is more selective and operable than the metal replacement method, and the leaching of precious liquid does not require treatment; it is easy to recover the extractant after the organic phase reaction, and no high temperature and high pressure are required during the operation, and the energy consumption is lower than that of activated carbon elution and regeneration; at the same time, the organic solvent has high gold extraction efficiency after sufficient stirring and extraction, which overcomes the loss of precious metals carried away by the loss of activated carbon fine carbon, powdered carbon and resin. Polysulfide solvents and lime sulfur mixtures are both alkaline gold leaching reagents. The leached precious liquid has few solid impurities and low other metal content. It is very practical to use the extraction method to separate and enrich gold. However, due to the imperfect follow-up process, it is not widely used in industrial production and needs further research. 

2.4 FKN environmentally friendly gold extraction agent for gold leaching 

Through research, common chemical raw materials such as ferrocyanate, urea (thiourea), caustic soda and suitable catalysts are crushed and mixed, and then reacted in a high-temperature reaction pot to obtain a mixture of alkaline thiourea and sodium polycyanamide (or carbonized sodium cyanurate) to form a stabilizer. Whether this stable polymer contains sodium cyanide is avoided by manufacturers, and there is no clear official conclusion yet. Alkaline thiourea (SCN2H4) is easily soluble in water and is an organic complexing agent with reducing properties. It can form complexes with many metal ions. In the presence of oxidants, such as Fe, it can strongly form coordinated cations with gold and dissolve rapidly in the presence of oxidants (such as Fe3+). The reaction is: Au+2SC(NH2)2+Fe3+ →Au[SC (NH2)2]+2 +Fe2+

Since thiourea is unstable in alkaline solution and easily decomposes into sulfide and aminocyanide, through experimental research, stabilizers are added to thiourea, which can inhibit the irreversible decomposition of alkaline thiourea, realize the selective dissolution of gold from gold ore by alkaline thiourea solution, and enhance the stability of thiourea in alkaline medium; the gold leaching agent also contains sodium cyanurate. Under alkaline conditions, sodium cyanurate solution is converted into sodium cyanurate and produces OCN- ions to form complexes with gold to dissolve gold from the ore, thereby improving the extraction effect of gold and reducing the consumption of thiourea.

This type of agent includes both thiosulfate and thiourea, sodium polycyanamide or sodium cyanide. Leaching gold in a composite way has the advantages of selectivity, high leaching rate, and relatively low pollution. Production practice has also proved that the leaching solution can be recycled many times, and the method of use is simple, which is the same as the operation of using NaCN on site. The industrial test of Jigongshan gold ore of Shandong Gold Mining Co., Ltd. using environmentally friendly gold extraction agent to replace NaCN heap leaching is based on the past small-scale test to further verify the feasibility of using environmentally friendly gold extraction agent to replace NaCN in industrial heap leaching production. The results of the industrial test show that the gold grade of the upper ore is 1.16g/t, and the amount of environmentally friendly gold extraction agent is 73g/t; the leaching residue grade is 0.18g/t, the theoretical leaching rate according to the leaching residue is 84.48%, and the theoretical recovery rate according to the amount of gold-carrying carbon metal is 83.12%; the actual recovery of metal is 4042.61g, and the actual recovery rate is 78.74%; and there is a significant improvement in the consumption of reagents and the reduction of potential risks of environmental pollution, which greatly reduces the production cost and increases the gold leaching rate by more than 3%. According to Gaolong Company's 2 million tons/year processing scale, the annual revenue can be increased by 5.616 million yuan.

At present, there are many environmentally friendly gold extraction agent products that replace sodium cyanide in the environmentally friendly gold extraction agent market, which shows that this type of gold extraction agent has huge market potential.

3. Strengthen and improve the direction of thiosulfate and environmentally friendly gold extraction agent to replace NaCN gold extraction production technology

Thiourea, thiosulfate, lime sulfur and ferrocyanate are added with urea and caustic soda chemical reagents. Environmentally friendly gold extraction agents can be used to leaching gold. There is no doubt that these gold extraction agents have very similarities, but also have different characteristics and differences. The comparison of the properties and characteristics of each agent is shown in Table 1.