Thiourea extraction of gold and silver (3)

4 The test was carried out under the conditions of initial pulp pH of 1-1.5, H ₂ SO ₄ concentration of 8.96 g/L and thiourea concentration of 4.49 g/L. Due to the large amount of dissolution of the above various impurities, per ton of fine The mine consumes 100.5 kg of sulfuric acid and 6 kg of thiourea. Consumed by the job, the barren solution containing H ₂ SO ₄ concentration of 4.11g / L, SCN ₂ H ₄ concentration of 2.14g / L, drug consumption during operation than 50%.
5 The concentrate used in this time, the gold is very finely distributed in the concentrate, and the grinding grain size is fine in principle. In practice, the grinding size has reached 80%-85% -0.043mm (-325 mesh). If fine grinding is carried out, not only the grinding cost will be increased, but also the amount of impurities eluted will increase, which may cause deterioration of the gold leaching index.
6 Test proves: The thiourea iron slurry method is simple in process and convenient in operation. The equipment designed by Changchun Gold Research Institute for industrial testing has achieved mechanization and automatic program control, which can save labor and reduce labor intensity. Therefore, the process and equipment have been applied to industrial production after the completion of the test. Production practice has proven that it can meet production needs.
4) Treatment of gold mud The yield of sulphur urea ferrite gold mud is usually about 1% of concentrate, gold gold often contains 1%-5%, and the enrichment ratio is small. In view of the low gold content of gold mud, it is difficult to purify the next step, so fire smelting or hydrometallurgical treatment is often used.
The pyrometallurgical smelting of gold mud can be treated by furnace, small electric furnace, converter or ash furnace. Due to the low gold content and complex composition of gold mud, especially the presence of metals such as lead , copper and bismuth , long-term oxidative smelting is required to remove It brings difficulties to the melting of fire. For this reason, according to the actual composition of the gold mud, S, Cu, Fe, etc. may be removed in advance by conventional processes such as oxidation or sulphation roasting and acid or high-iron salt leaching, so that gold and silver are concentrated and then fired. The slag is added to the slag to produce the merging gold, which is then separated and purified by a conventional process.
The hydrometallurgy of gold mud may be acid leaching (cooking), cyanidation, thiourea, liquid chlorination, aqua regia, etc., but in most cases, it is preferred to carry out oxidation or sulphation roasting and leaching in advance. The impurities are removed, and the gold is concentrated and then treated. [next]
In the above hydrometallurgy, the gold obtained by the cyanidation method and the thiourea method is not high in purity and needs to be purified. The water-soluble gold of Wangshui is then reduced by a reducing agent such as SO ₂ , ferrous salt or sodium sulfite to increase the purity of the product gold. However, impurities in the gold mud must be removed in advance to increase the gold content in the gold mud to 50% or more. The liquid chlorination method is not limited by the gold grade and the amount of impurities. Because the water can be dissolved by the solution of aqua regia, the concentration of various ions in the leachate is quite high, but the reducing agent such as SO ₂ , ferrous salt or sodium sulfite can selectively reduce gold and produce high purity. Gold powder, which is then oxidized and cast, can produce gold ingots with a purity greater than 99%. However, using the aqua regia method or the liquid chlorination method, Ag in the gold mud will generate AgCl into the slag, and it is necessary to recover silver from the leaching residue by ammonia leaching method or sodium sulfite leaching method and other suitable processes, or to reduce and smelt the leaching slag containing AgCl. After slag formation, an alloy of copper, silver or the like is produced, and silver is separated therefrom by a conventional method and the valuable metal is comprehensively recovered. If sulfuric acid is used to extract and remove impurities, silver nitrate and aqua regia (or chlorinated leaching) are separated by a step-wise wet separation and purification process, and the effect is also good.
4. Carbon slurry or resin slurry method
The carbon slurry method or the resin slurry method for adsorbing gold or silver from the thiourea leaching slurry is the same as the carbon slurry method or the resin slurry method for cyanidation leaching, and the activated carbon used is also the same. If the resin slurry method is used, since the thiourea gold complex ion is a cation, a strong acid cation exchange resin or a thiol resin should be used. If the granular adsorbent is not used, the cation exchange resin fiber cloth or the activated carbon fiber cloth can be used, and the operation of sieving and recovering the gold-loaded granular adsorbent from the slurry can be eliminated, and the gold-loaded fiber cloth can be extracted from the slurry at regular intervals. The gold is sucked and another batch of spare fiber cloth is added to the tank to continue the adsorption.
Since thiourea gold is a positively charged complex cation, the adsorption performance of gold on the adsorbent is different from that of the negatively charged cyanogen cation anion, and the desorption method is also simpler. As long as it is washed by hot (about 50 ° C) acid or hot thiourea, the gold and silver on the adsorbent can be completely eluted.
When activated carbon adsorbs gold, it also adsorbs some thiourea. When these thioureas desorb gold under aerobic (air) conditions, the thiourea is rapidly oxidized and decomposed by the catalytic action of the carbon surface. For the same reason, if it is to be depleted from the poor liquid The thiourea can be recovered from the tail liquid, and then it can be adsorbed by activated carbon, and then washed with a small amount of warm water under anaerobic conditions to obtain a pure concentrated sulfur urea solution. Since this method has certain difficulty, it is usually to moderately neutralize the tail liquid or the lean liquid, and the excess Fe ^{3+ is} hydrolyzed and precipitated and returned to use. It saves both thiourea and non-sewage operations.
Guangxi Longshui Gold Mine is a sulfide deposit containing carbonaceous shale and graphite . Natural gold has a coarser particle size, most of which is 0.01-0.1 mm, a few are larger than 0.3 mm, and -0.01 mm only accounts for 15%. Gold particles are mainly found in the fissures of pyrite, and a small amount is encapsulated by pyrite. In view of the characteristics of the ore, after the test, the ore flotation, concentrate roasting and thiourea iron slurry gold extraction process have been used for many years. Since the pH of the pulp is about 1.5, the consumption of iron plate is about 10kg per ton of concentrate. The hole formed by acid etching on the iron plate is entrained with a lot of slime. The gold grade of gold mud is only 0.1%-0.2%, and some gold remains in iron. The recovery of the honeycomb honeycomb pores is not obtained, and the leaching rate of silver is low, resulting in waste of resources.
To this end, the Longshui Gold Mine has conducted a small exploratory experiment of the thiourea carbon slurry method. The scale of the experiment is 1kg, which is limited to the conditions of mining equipment. The test only uses the single factor comparison method, and the multi-factor optimal condition selection test is not carried out. In this test, a total of seven samples were taken in two batches, and the ore samples were all calcined by flotation concentrate. The first batch of three samples was high-grade calcine, which was not re-grinded and had a coarse particle size. It was used for the comparison test between the iron slurry method and the carbon slurry method. The test conditions are: at room temperature, 50 kg of sulfuric acid per ton of calcined sand, 4 kg of thiourea, leaching for 36 h by iron slurries, and immersed in an iron plate at ³ m ² · m ³ · trough ^-1 for immersion; carbon slurry method Leach for 24h, add 15-20kg of activated carbon per ton of calcined after leaching for 12h. The second batch of 4 samples were low-grade calcine with different gold content, and then re-grinded to 90%~95%-0.043mm (-325 mesh), all of which were leached by carbon slurry method, except for the addition of acid and thiourea. In addition to the above, another high-priced iron salt 2kg/t was added, the leaching time was 24h, and activated carbon 15-20kg/t was added after 12h leaching. The test methods and results are shown in Table 6. It can be seen from the table that with the thiourea carbon slurry method and the iron slurry method, the gold leaching and recovery indexes are similar, and the silver leaching rates are neither high. How to improve the leaching index of silver and other issues needs further exploration. [next]

5. Slurry electrolytic deposition method
The electrolytic deposition of gold in the thiourea leaching pulp is carried out by insoluble anolytic electrolysis using an external power source. This method was first tested by the Pinggui Mining Bureau in 1979. Later, the Guangdong Institute of Mining and Metallurgy conducted a systematic and small exploratory study using iron, lead and copper as plates. The methods and results are as follows:
The test was carried out in a slurry with a solid-liquid ratio of 1:2, sulfuric acid 10 kg/t, thiourea 3 g/t, room temperature 25-30 ° C, and a tank voltage of 7 V for leaching-electrolysis for 4 h. Due to the short time, the leaching rate of gold is less than 50%, but the recovery rate of dissolved gold is as high as 99%.
After the condition test, it is preliminarily considered that the slurry pH is 1.0-1.3 and the tank voltage is 3~5V. The cell voltage was monitored using a saturated calomel electrode to measure the cathodic redox potential. When the cathodic potential was -5 mV, gold was satisfactorily deposited on the cathode for recovery. If the cell voltage is too high, the potential and current density of the cathode electrode will increase significantly, especially the maximum increase in the anode potential, resulting in an increase in the acidity and temperature of the slurry, a thermal decomposition of thiourea, and an increase in the rate of anodic oxygen evolution. Since the gold concentrate used in the test contains 2.65%~3.12% of carbon, the increase of oxygen evolution rate of the anode will increase the oxidation of carbon, and a layer of viscous carbon foam will float on the surface of the slurry, which is not conducive to operation.
The test also proves that iron, lead and copper plates are used as cathodes respectively. Although the potentials of these metals are different, their electrode potentials are almost the same after the power is turned on. Since the anode continuously evolves oxygen during electrolysis, the oxygen concentration in the slurry can be maintained. Constant. There is enough oxygen in the slurry, and the mixing speed of the operation can be lower. These oxygens can also oxidize the Fe ²⁺ dissolved in the concentrate to form Fe ³⁺ without the need for an additional oxidant.

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