Water chlorination

The aqueous solution chlorination method had many patents in the late 1970s. Carlin used a secondary chlorination process to establish a continuous test unit for treating 500 t of ore per day, which greatly reduced chlorine consumption. The US patent reported that leaching with a chloride solution at 328 kPa (160 ° C), gold leaching The rate is higher than 98.5%.
The liquid chlorination method (also known as wet chlorination or solution chlorination) is carried out by adding chlorine or other chlorinating agent to an aqueous solution of a salt or an acid to chlorinate and extract the gold. This method uses the initial addition of sulfuric acid or chlorine bleach solution from the leaching of gold ore successfully, and with ferrous sulfate was precipitated gold from the leachate. Later, it developed into one of the main methods of leaching gold at the end of the 19th century. In general, any material in the raw material that is soluble in aqua regia can be dissolved by liquid chlorination. With the liquid chlorination method, the leaching rate of gold is higher than the cyanidation method, up to 90% -98%, the price of chlorine is lower than that of cyanide, and the consumption of chlorine is about 0.7 to 2. 5 kg / t concentrate. After the introduction of the liquid chlorination method, the cyanidation process also appeared at the end of the 19th century and began to be widely used for direct leaching of gold from ore. Therefore, at the same time, the liquid chlorination method was stopped in various factories. In recent years, Some hydrometallurgical methods pollute the environment, and the liquid chlorination method is used again to extract gold and silver . In the future, it may become one of the important metallurgical methods for gold and silver.
The process is characterized by low investment, high recovery rate and environmental protection. The liquid chlorination process is essentially an oxidative leaching. After the chlorine is dissolved in water, a hydrolysis reaction occurs to form a highly oxidizing hypochlorous acid to chlorinate gold into HAuCl ₄ or NaAuCl ₄ , and then the precipitate is reduced by using sulfur dioxide or ferrous sulfate. According to the different chlorinating agent and medium used, liquid chlorination is divided into three main processes: chlorination of aqueous solution of hydrochloric acid, chlorination of hypochlorite (sodium hypochlorite or calcium hypochlorite) and electrochlorination.
Basic principle The principle of water chlorination immersion gold is that gold is oxidized in an acidic chloride solution saturated with Cl ₂ to form a complex anion of trivalent gold.
Chlorine is a strong oxidant that reacts with most elements. For gold, it is both an oxidant and a complexing agent. In the potential-pH diagram of the Au-H ₂ 0-Cl ^- system, as shown in the figure below, gold is chlorinated to oxidize and complex with chloride ions, so the water is chlorinated to leaching gold, and the chemical reaction is:

2Au+3Cl ₂ +2HC1 ==== 2HAuC1 ₄

2Au+3Cl ₂ +2NaC1 ==== 2NaAuC1 ₄

This reaction is rapidly carried out under low pH conditions in which the chlorine concentration in the solution is significantly increased.
The potential of trivalent gold in the chloride solution is quite high:

Au+4C1 ^- ==== AuC1 ₄ ^- +3e ^-

E ^Ó© =1.00 V[next]

Therefore, the dissolved gold is easily reduced, so the chlorine must be saturated in the solution when the ore is leached. The biggest advantage of the water chlorination process is that it is cheap and the leaching speed is fast. The leaching agents used in the liquid chlorination process are mainly (wet) chlorine and chloride salts. Since the activity of chlorine is high, there is no problem that the surface of the gold particles is passivated. Therefore, under the given conditions, the leaching speed of gold is very fast, generally only 1-2 h is leached. This method is more suitable for treating carbonaceous gold, gold-bearing ore pickled off, the residue antimony, arsenic-containing ore concentrates, or the like, and recovering the gold from the solution is easy.
However, the water chlorination method also has serious limitations: when the sulphide ore is leached, some or most of the MeS is dissolved, which complicates the waste liquid treatment. Therefore, for the acid ore containing S < 0.5%, the water chlorine is used. The chemical method may be suitable. In addition, the water chlorination method also has the problem of the harm of Cl ₂ on the site and the complexity of the equipment, but with the application of the composite metal, the equipment problem may be solved.
South Africa has a large water chlorination pilot plant for the treatment of re-elected gold concentrates. The process used is: after the concentrate is desulfurized by oxidation roasting at 800 ° C, the calcined sand is leached in a hydrochloric acid solution of chlorine gas, and the leaching rate of gold is 99%. It is then reduced with SO ₂ to precipitate gold from the solution. The gold powder washed with an ammonia chloride solution has a purity of three nine.
Process characteristics
The actual process is that the ore mills to -200 mesh account for more than 65%, the slurry concentration is 45%, the temperature is 27-38 ° C, and the 500 t/d feed amount is added to 4 series stirred tanks, and the total stirring time is 20 h. The chlorination tank is lined with rubber and is coated with a urethane foam insulation layer. The chlorine gas is fed into the first three tanks through a distribution pipe, and the fourth tank is a storage tank to complete the chlorination reaction. The gas in the sealing tank is discharged to the washing tower. The tower is a packed tower, which is circulated through the soda ash solution. The chlorine reacts with the soda ash to form sodium hypochlorite, and then returns to the same slurry in the process, and the utilization rate of chlorine gas exceeds 99%. About 60 × 10 ⁴ t ore has been treated by chlorination. When the gold content of the ore is 8.71 g/t, the extraction rate is 83.5%, and the chlorine consumption per ton of ore is 18 kg.
This pre-oxidation treatment, which is suitable for cyanidation by means of chlorination, is suitable for use in at least two larger gold mines in the United States. Despite this, there are still different opinions. For example, in the argumentation about the selection of the Mok gold mine process, Marshall believes that if chlorine gas is used for pre-oxidation treatment, in the subsequent cyanidation operation, a higher gold extraction rate is required, and chlorine and other chemicals are consumed very high (chlorine gas 86.26). Kg/t ore, sodium carbonate 48.12 kg / t ore, gold cyanide leaching rate of up to 84%), it is considered that the pre-chlorination of the mine is not an economical and practical method.
The chlorination of aqueous solution by diffuse gold has received much attention as a pretreatment method and has been industrially applied in the treatment of stubborn ores or concentrates. One example is pulp chlorine oxidation process when using a carbon-containing hard mineral stone CARLIN GOLD processing plant. Activated carbon and long-chain organic carbohydrates are present in Carlin Oxide Ore, which is difficult to treat by conventional cyanidation process. However, it is found that the harmful effects of carbonaceous materials can be eliminated by adding oxidant to the slurry, that is, chlorine gas or in situ electrolysis of salt-containing pulp can be used. The resulting sodium hypochlorite oxidizes charcoal and organic carbohydrates to CO or CO ₂ . This chlorinated pretreated slurry can be fed directly into the cyanide circuit.
The aqueous solution chlorination process can also be used for subsurface leaching, which Nebela considers to be the most economical method for extracting gold from a poor ore containing 0.6-2.1 g/t of gold. U.S. patents have also described that for underground leaching, loose blasting of gold-bearing ore is followed by a solution of chlorine, oxidant and organic matter (sodium azide, hydroxyethylamine or ethylenediamine) flowing into the gold complex. Preliminary studies have shown that the extraction rate of gold is 80%-90% (three weeks leaching time), and it is confirmed that 80%-90% of gold can be recovered by adsorption, ion exchange or electrolysis. The industrial use of this underground leaching method depends mainly on geological conditions.
Nebela offers three formulations for chloride leaching in the subsurface: 1 HCl + 0.1 mol / L NaCl + Cl ₂ ; (2) Ca (OH) ₂ + C1 ₂ , 3 NaCl + 0.05 mol / L Na ₂ C0 ₃ + Cl ₂ , in which chlorine gas is saturated, and the leaching effect of the three is compared.
The liquid chlorination method has been put into practical use in industrial production. In the United States, carbonaceous gold ore was successfully treated by a hydrochloric acid aqueous solution chlorination process. In 1980, a carbonaceous ore processing plant was built in Nevada. Murchison United Minerals uses this process to treat slag, and the recovery rate of gold is over 98%. In addition, the treatment of gold-containing iron ore and arsenic pyrite by liquid chlorination has a higher leaching rate than the cyanidation method and the thiourea method. After oxidizing roasting or ore pulping at 650 ° C, the air is oxidized and pretreated at 75-100 ° C, and the ore is leached at a liquid-solid ratio of 2:1 for several hours, and the gold leaching rate is over 92%.
Because the chlorinating agent is easy to obtain, the price is cheap; the formed gold chloride is easy to separate, and the pure product is easily obtained; the harm of the chlorination operation to the human body is avoided, and the environmental protection is favorable. Therefore, the development prospect of the liquid chlorination process for gold extraction is very broad, and it will occupy an important position in the field of future gold and silver extraction.
In summary, the aqueous solution chlorination process is suitable for treating a relatively single gold-containing raw material or a carbon-containing gold ore, which has the advantages of a high gold leaching rate and a chlorine gas as an oxidant at a lower price than cyanide. The US Bureau of Mines has conducted intermediate industrial tests with chlorine. The main disadvantage of this method is that many impurities are easily dissolved at the same time to consume the agent, and it brings difficulties to the subsequent gold extraction process. The control potential leaching method can partially overcome the disadvantages in this respect.

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