Bromination method

Summary Bromine is a strong leaching agent that dissolves gold quickly under the action of an aqueous solution. As early as 1881, Shaffer published a patent on the bromine gold extraction process (US Patent No. 267723), but until recently it was almost forgotten or neglected due to environmental protection and changes in the nature of the ore. The gold extraction process has been re-examined for many years. Certain bromine-containing leaching agents have also begun to take a place in the market.
Both bromine and chlorine are halogen elements and have similar chemical properties. In aqueous solution, they all react with most of the elements, and are both oxidants and complexing agents for gold, which can achieve a fast immersion speed, so it is an ideal gold leaching agent.
In recent years, countries such as Canada and Australia have published many articles, claiming that the D-method and K-method of bioleaching should compete with the cyanide leaching method, emphasizing that these new methods have a fast leaching speed and no The advantages of polluting the environment.
In the new process of bioleaching D-method, a leaching agent called Bi0-D (Bi0-D-Leachent) is used, which is prepared from sodium hydride and oxidant (halogen). A leaching agent that can be used to leach precious metals. The law was successfully researched in 1987 by the Bahamian Refinery Company of Arizona, USA, to replace cyanide leaching gold. It is said to be able to leaching at lower temperatures in addition to faster leaching, so some people think this is a new breakthrough in the gold extraction process.
The reagent is a halide, and has a higher affinity for a denser metal than a metal having a lower density, and can be used in a weakly acidic to neutral solution, the diluted solution is non-toxic, the reagent is easily regenerated, and has a bio-degradation decomposition effect. It is a good substitute for traditional cyanide leaching agents. Most ores use this leaching for 2.5 h to achieve a 90% leaching rate. However, since a considerable amount of bromine vapor escapes from the solution during the reaction, this not only increases the reagent consumption, but also causes serious corrosion and health problems, so it is still in the laboratory and semi-industrial test stage. Can be used in industrial production, will make a major change in the gold and silver extraction process.
The K-process is invented by Kalias, Australia, and is also known as the Kalias process (or K-process), which essentially utilizes a new process using bromide as the leaching agent. The reagent used in the process is a patent. It is estimated that it may include chlorine and bromine salts. It can be leached from ore under neutral conditions. However, it is still in the development and testing stage, and it is still difficult to promote it in industry.
According to a West German patent in 1985, a solvent consisting of sodium bromide (or sodium hydroxide, etc.) is about five times more soluble in gold than aqua regia, which is generally considered to be the strongest gold solvent. All of these indicate that certain bromine-containing reagents have a high ability to dissolve gold and can be economically and efficiently leached from refractory ores (or concentrates).
The basic principle of bromination gold extraction process 1) is similar to the chlorination method. The dissolution process of gold in bromine solution is also an electrochemical process, and can be simply expressed as follows:

Au+4Br- ==== AuBr ₄ ^- +3e ^- E ^Ó© =0.87V

The bromide concentration, gold concentration, solution pH, and redox potential (E _h ) are the main factors affecting the solubility of gold in bromine solution. The dissolution reaction of the sodium bromide leaching process can be written as:

Au+3Br0 ^- +6H ⁺ ==== AuBr ₃ +3H ₂ 0

AuBr ₃ +NaBr ==== Na(AuBr ₄ )

That is, first, Au is oxidized to AuBr ₃ , and then reacted with NaBr to form AuBr ₄ ^- complex ions into the solution. [next]
K. Osseo - Asare plotted the Au-Br-H ₂ 0 system potential-pH diagram, as shown in Figure 1. From this, as Br ^- increasing concentration, AuBr ₄ ^- stable area increases. At room temperature, the optimum gold dissolution region is between pH 4-6 and the potential is 0.7-0.9 V (based on the calomel electrode).

figure 1

An Au-Br-H ₂ 0 system potential-pH map was also drawn in the article by Brent and Hiskeg, as shown in Figure 2. This picture looks more complete. The presence of AuBr ₂ ^- is also indicated on this graph. The standard free energy of gold-containing components at 25 ° C is shown in Table 1.

figure 2
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3.5 g of bromine can be dissolved in 100 g of water at 20 °C. Liquid bromine is a reddish brown liquid with a relative density of 3.14 and a boiling point of 58.7 °C. If the pH of the solution is high, the following reaction will consume bromine:

20H ^- +Br ₂ —→ Br0 ^- +Br ^- +H ₂ 0

3Br0 ^- —→ 2Br ^- +Br0 ₃ ^-

Australia produces Br ₃ ^- in the olfactory solution. Therefore, the odor is more soluble in the hydrazine solution, and Br ₃ ^- has a strong oxidizing ability, which is favorable for the dissolution of gold.
2) Kinetics of leaching gold by bromination
The kinetics and Sergent Pesic Geobrom ^TM 3400 was dissolved gold rotating disk method. Gold dissolution rate. The change with the rotation speed is shown in Figure 3. The dissolution rate is linear with the speed. But the line does not pass through the origin. This indicates that the gold dissolution rate is partially controlled by the chemical reaction rate. The reaction is a first order relationship to the bromine concentration. The concentration of bromide ions is a 0.5 level relationship. Geobrom ^TM Geobrom ^TM 3400 series containing both bromine and bromide containing, 3400 experimentally determined so that 1.4 to 1.6. From the change of the dissolution rate v with pH (Fig. 4), it can be divided into three regions: the pH is 1 to 6, the dissolution rate v is independent of pH, the pH is 6 to 10, and the pH is increased. Reduce quickly. The pH is greater than 10 and the gold dissolution rate. Almost zero. The activation energy of the gold dissolution reaction was 24.85 kJ/mol. Copper, iron, manganese, and lead, zinc, sodium and potassium valency Ï… gold has no effect on dissolution rate. The dissolution rate decreases when [Mn ²⁺ ] is present in the solution.

Figure 3 Figure 4
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3) Process characteristics 1 Using Geobrom ^TM 3400 to leach gold from refractory ore. For the many bromide leaching agents tested, it is generally considered that the Geobrom series of reagents are the most detailed and technically economically studied. metrics are better, and all aspects of the analysis is the most promising be said to be 3114 (the bromo-chloro-dimethylhydantoin Thalidomide, i.e. an oxidizing agent, but a mixture of hypobromous acid and hypochlorous acid views) Geobrom ^TM , Geobrom ^Tm 5500 (the bromo-dimethylhydantoin Thalidomide) and Geobrom ^Tm 3400 and some organic bromine complexing agents, especially the effect of ^TM 3400 Geobrom best. In recent years, a large number of experimental studies have been conducted on such reagents in foreign countries, and some satisfactory results have been obtained.
Geobrom ^TM 3400 is a registered trademark of a bromine reagent produced by Great Lakes Chemical Company of Indiana, USA (the company is the world's largest producer of bromine and bromide products, and they also produce a wide variety of other codenamed Geobrom series reagents). It is a liquid bromine carrier with a lower vapor pressure and a patent. A large number of experimental results show that it can be used to obtain good technical and economic indicators when leaching gold from refractory gold ore.
With Geobrom when ^TM 3400 as a gold leaching agent for both refractory ore tested, due to the carbon-containing concentrate, with higher sulfur (10% ~ 13% C, 12% ~ 15% S), the former must first leaching The concentrate was dehydrated and dried at 110 ° C, and then calcined at 650 to 750 ° C. After cooling, the calcine is ground to -150 to 200 mesh. The gold content of concentrates I and II were 242g/t and 419g/t, respectively. The gold content in the calcined I and II obtained after pretreatment was 298g/t and 541g/t, respectively.
The results of the leaching test showed that the leaching rate of gold reached a maximum value (about 94%) when the concentration of Geobrom 3400 was 4 g/L and the concentration of NaBr was 6-8 g/L. It was also found during the leaching time (2 to 24 h) that 98% of the immersible gold was dissolved after 2 hours. Therefore, the time for all subsequent leaching tests was chosen to be 6 h. Optimal conditions are determined by the probe trial (Geobrom ^TM 3400 of 4 g / L, pH5.0 ~ 6.0 , leaching time 6 h) were further verification test. As a result, the calcined I sample contained 298-312 g/t of gold, the leaching residue contained 18.5-20.3 g/t of gold, the gold extraction rate was 94.2%-94.5%, and the corresponding index for the calcine II was 541-555 g/t. 22.3 ~ 24.0g / t, 96% ~ 96.3%.
It has also been reported that the gold leaching rate is 95.1% and 94.2%, respectively, when comparing the cyanide and bromination gold extraction of the above-mentioned calcine I, and the reagent cost per ton of ore is 11.7 and 11.6 US dollars, respectively. Almost very similar.
The recycling and recovery of the bromine carrier was also tested. Calculated for leaching gold from the concentrate Geobrom ^TM 3400 (price of $ 1.34 / kg) is an average consumption of 8.5 kg / t calcine. Therefore, the reagent cost of the bromination method is $11.4/t calcine. From the laboratory recovery test, the loading capacity of activated carbon to gold was calculated to be 25 kg/t. With gold Geobrom ^TM 3400 can rapidly desorbed from the supporting carbon, or zinc powder and then using hydrazine precipitated at room temperature. Therefore, the amount of carbon consumed in the recovery of gold by bromination is much lower than that of the cyanidation process. At the same time, the heat exchange, the electrolysis cell and the electrode required for the recovery of gold by the cyanidation method are omitted, and it is estimated that the cost can be greatly reduced.
Recently, A. Dadgar et al. studied in detail the leaching of gold from black sand concentrate with Geobrom ^Tm 3400 and the electrochemical regeneration of bromine. They used a very rich (6.2 kg/t) black sand concentrate to leach gold and then recover the gold by ion exchange and solvent extraction. The test results show that when immersing gold from black sand concentrate with Geobrom ^Tm 3400, the leaching rate of gold is extremely fast, about 90% of the gold is leached within the first 2 hours, and reaches the highest after 4 hours (94% to 96%). The leaching rate.
However, analysis of the leaching residue indicated that a significant portion of the gold remained in the residue after the first leaching. In order to achieve the highest gold leaching rate, must then leached twice with Geobrom ^TM 3400 with the new solution. When treated by ion exchange and solvent extraction, the loading rate and recovery of gold are almost 100%.
Preliminary economic accounting shows that consumption per ton of ore in about 130 kg of Geobrom ^TM 3400. Therefore, the cost of the leaching agent required to extract 31.1 g of gold from the black sand concentrate is only about $1.00. When the bromine is electrochemically regenerated, the cost can be greatly reduced. [next]
2 AuBr ₄ ^- Adsorbed on Dower 21 K resin. The adsorption kinetics experiments were all performed in a discontinuous manner on a mechanical oscillator. The temperature hardly affected the adsorption rate of the gold bromide ion on the anion exchange resin, so it was set at 25 °C. The adsorption rate of AuBr ₄ ^- on weak base and strong basic ion exchange resin is independent of the pH of the solution (in the range of 1 to 6), so the pH is adjusted to 3.0 to 3.5. It is important that bromine is converted to bromate in alkaline pH and gold is precipitated as gold hydroxide. Therefore, velocity studies in the alkaline range are meaningless. Therefore, the kinetic study was carried out at a temperature of 25 ° C, a pH of 3.0 to 3.5, and a 0.25 g wet resin was contacted with a 100 cm ³ gold bromide solution. Within 3 hours, one sample was taken every 15 minutes, and the gold content was analyzed by ICP.
The experimental results show that the adsorption rate constant of AuBr ₄ ^- on Dower 21 K is 0.029 mg/min, which is independent of Br ₂ concentration, and is the first-order velocity, strontium metal ions Fe ³⁺ , Zn ²⁺ , Cu ²⁺ and Ni ^{2 . +} In an acidic solution. Experiments have shown that the adsorption capacity and adsorption kinetics of the resin are not affected by these ruthenium metal ions, and are particularly effective for AuBr ₄ ^- adsorption.
3) Electrochemical regeneration of bromine To further improve and perfect the bromination process, a US patent was published in 1990. An electrolytic bromine leaching process is proposed, in which an electrode is inserted into a bromination leaching tank, and the active bromine produced by electrolysis can effectively carry out gold leaching. The gold-containing precious liquid that flows out through the percolation in the lower part of the electrolytic cell is pumped to the replacement tank to replace the gold with zinc powder, and a part is returned to the tank (or after the new liquid is added) to circulate. The lean liquid after the zinc powder replacement is also returned to the leaching tank, so that the bromide solution can be effectively recycled, thereby reducing the reagent dosage and cost.
Recently, Great Lakes Company further reduce costs Geobrom ^TM 3400 leaching process, has developed two electrochemical process for the recovery of gold from a leaching and exchanging ions after regeneration lean liquid Geobrom smell, when these methods are semi-industrial tests Has been successful. In the meantime, they electrolyzed the lean liquid with a mass fraction of 5% Br ₂ .
During the semi-industrial (250 kg/d) test, 20% to 35% of the slurry was stirred in the leaching tank for 6 hours to leach gold from the ore. After solid-liquid separation, the rich liquid is passed through an ion exchange column to recover gold. In addition to the adsorption of AuBr ₄ ^- , the ion exchange resin can also reduce residual olfactors into odorant ions. Therefore, the poor liquid will no longer contain gold and Australia. The albide ions in the lean liquid are anodized into olfactory and can be pumped back into the leaching tank for recycling, thereby reducing the consumption of olfactory reagents.
1 electrolytic cell device. The Lectranator system Lectranator tank is sold as a hypochlorite generator for electrolysis in swimming pools. The prototype used in the study consisted of six separate tanks, and the electrode area for the formation of chlorate was estimated to be 360 ​​cm ² . The Lectranator is a dipole cell that is polarized only when the two external electrodes are connected.
The cell was installed in a mobile unit consisting of a 0.2 m ² polyethylene storage bin with a lid and an Aquatron II centrifugal pump. A simulated gold leaching solution containing NaCl and NaBr was forced through the tank (102 dm ³ /min) and returned directly to the tank for circulation, and the return stream was inserted below the electrolyte level to accelerate mixing.
The Sorenson DCR 60-30B power supply was used to generate the electrode reaction, and the dial showed the cell voltage and current used, and electrolysis was performed at 6 A (corresponding to 100 mA/cm ² through 6 separate cells). The polarity of the electrode is inverted every 30 minutes to remove deposits such as calcium and external plating metals. These precipitates formed at the cathode 1/2 cycle and dissolved at the anode 1/2 cycle.
During the electrolysis process, the pH of the solution may naturally rise (note that the reverse reaction is the evolution of H _{2 to} form OH ^- with the cathode), and at the end of the reaction, a certain concentration of H ₂ S0 _{4 is added to} bring the pH to 5-6, at which time it is released. The leaching agent Br ₂ , the solution turned into a characteristic orange yellow, titrated with iodine to determine the Faraday current efficiency. [next]
2 mixed halide electrolysis. Initial studies related to the electrolysis regeneration of bromine leaching agents have shown that the bromine leaching process has high efficiency at current utilization rates of 80% to 90%. The intermediate scale electrolysis test uses a commercially available hypochlorous acid generation tank and a simulated leachate containing 0.5% to 5% Br ^- ion. Considering the reduction Br ^- ions to a very low concentration of the process will have a stronger attractive economically, so that changing the leaching agent component Br ^- maximum utilization. The basic idea of ​​the research is to use high C1 ^- ions and low Br ^- ion flow operations. During electrolysis regeneration, the current load is the oxidation of the anode C1 ^- ions to hypochlorite, and when the pH is lowered, the Br ^- ions are uniformly oxidized by the hypochlorite to release the Br ₂ leaching agent.
In this study, the gold leaching agent was processed using the Lectorator system of the swimming pool sterilization tank. The manufacturer considers this device to be a low current efficiency (40%) device, and in order to suppress side reactions that can cause low current efficiency, the operating conditions of 5% Cl ^- ion concentration are actually selected. As the concentration of C1 ^- ions increases, two other advantages arise: a) the conductivity of the solution increases, so the cell voltage is lower and the power cost is reduced; b) there may be an actual current density that becomes an industrial scale electrolysis feature, such as unit production. The ability has increased.
For efficient leaching, a typical oxidized ore requires about 0.2% Br ₂ . Since the aim is to make maximum use of Br ^- ion, the process stream is regenerated with a mass fraction of 0.5% Br ^- ion (introduced with NaBr). Electrolysis was carried out at 100 mA/cm ² to produce a solution having an active leaching agent mass fraction close to 0.2% Br ^- after acidification. Note that to secure oxidation Br ^- ions is not possible because: a) needs to be supplied free Br ^- ions, so that AuBr ₄ ^- anion is complexed oxidized material; b) free Br ^- ions to Br ₃ Br ₂ complex formation ^- So, to prevent an undesired high vapor pressure.
The data in Table 2 summarizes typical electrolysis conditions and results.

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