I. Introduction

For the presence of gold ores containing copper and gold, direct cyanidation can achieve a higher leaching rate of gold, copper due to the presence of the cyanide consumption greatly increased, so that the process becomes uneconomical direct cyanide in the form of a non-wrapped . The high leaching rate of gold and the economics of gold recovery are the key issues to be solved in the study of such gold ore leaching. The selective immersion gold method is worthy of further study because its margin has a higher leaching rate and reduces the interference of copper on leaching.

For different purposes, there are many methods for selective immersion gold. In addition to the traditional step leaching, thiourea leaching, thiosulfate and ammonia cyanide have been deeply studied and applied. As a substitute for cyanide, thiourea and thiosulfate have been extensively studied in the past few decades from the perspective of “non-toxic gold extraction”. Thiourea in the acidic solution, with Fe 3 + as the oxidant, is favored for its faster leaching kinetics (5 times faster than cyanide) [1 , 2] . In addition to non-toxic thiosulfate method, the greatest success is carbonaceous gold ore leaching, successfully solved the "robbed gold" problem [3-5]. At the same time, many scholars have found that thiourea leaching gold and thiosulfate leaching gold method can also reduce the interference of metal impurities such as copper, arsenic , zinc and nickel in gold ore, but it is obviously not thiourea and sulfur. The key content of the research on the method of sulphate leaching gold. Since the ammonia cyanide method has been proposed since the 1990s, it has not yet formed a unified view on the mechanism of leaching, the action of Cu + and ammonia. This reflects the inherent complexity of the system. For copper-bearing gold ore, all research results of the ammonia cyanide system leaching gold show that the method can effectively reduce the cyanide consumption and increase or maintain a high gold leaching rate [6 , 7] . The author used four selective immersion gold methods, namely thiourea method, thiosulfate method, step leaching method and ammonia cyanide method, to carry out leaching test on copper-bearing gold concentrate in a certain area of ​​Shanxi, and the effect was better. The ammonia cyanide method was piloted and the leaching process of the gold concentrate was initially determined.

Second, the nature of the ore

The test concentrate sample was taken from a flotation plant in a gold mine in Shanxi. The deposit is a medium-temperature hydrothermal gold deposit, and the ore type is a primary ore. Metal ore minerals include pyrite, chalcopyrite, sphalerite and galena gangue minerals are quartz, feldspar, and calcite. The gold minerals in the ore are natural gold and silver gold. The gold minerals are sub-microscopic in size, with various shapes and forms, such as granular, flake, dendritic and fine veins. They are mainly produced in pyrite, chalcopyrite and quartz in the form of fissure gold and interstitial gold. . The spectral analysis results of the samples are shown in Table 1, and the main element analysis results are shown in Table 2. The natural grain size of the sample is: more than 200 mesh accounts for 20%, 200-320 mesh accounts for 26.5%, and less than 320 mesh accounts for 53.5%.

Spectral analysis results of the samples in Table 1

Table 2 Main element content in concentrate

The direct cyanidation leaching test of the sample showed that the gold leaching rate was higher, but the cyanide consumption was as high as 130 kg/t. The result is consistent with the ore properties, that is, the presence of gold in the form of fissure gold or interstitial gold makes the concentrate have good cyanide leachability, but the higher copper content causes high cyanide consumption. The test conditions and results of direct cyanide immersion gold are shown in Table 3.

Table 3 Direct cyanidation test conditions and results

Third, laboratory test

(1) Test method

In view of the characteristics of high cyanide leaching rate and high cyanide consumption of the gold concentrate, the process scheme of immersion gold was tested, including thiourea leaching, thiosulfate leaching, step leaching and cyanamide. Four kinds of schemes such as leaching.

Many scholars at home and abroad have conducted in-depth research on the factors affecting thiourea leaching, thiosulfate leaching and ammonia cyanide leaching.

In thiourea leaching, the main factors affecting the gold leaching rate are leaching time, thiourea concentration, Fe 3 + concentration and sulfuric acid concentration [1 , 2 , 8 ~ 11] : thiourea dosage 8 ~ 12kg / t (mass fraction 1%) When the mass concentration of sulfuric acid is 20g/L (0.15mol/L) and the mass fraction of Fe 3 + is 0.3%~0.4%, the leaching rate of gold is higher; after 4h leaching time, the leaching rate of gold no longer continues to rise. high.

The main influencing factors in thiosulfate leaching are temperature, pH, S 2 O 2

Concentration, Cu 2 + concentration, NH 3 concentration. The optimum range of leaching temperature is 35 to 50 °C. In fact, the temperature requirements, mainly consider the economics of the method and the feasibility of the process. Many researchers have conducted experimental studies on the temperature range achievable by the process. The pH range leached by this method is extremely narrow, only between pH 9-10. In the thiosulfate leaching system, Cu 2 + and NH 3 have a significant acceleration effect on the dissolution of gold. In addition, SO 2 - in the presence of 4, SO 2 - 3 an appropriate amount can be significantly reduced S 2 O 2 - consumption of 3, functions of S2O2-3 stable. Depending on the nature of the ore, the optimum conditions for leaching of thiosulfate are generally [3 ~ 5 , 12 ~ 15] : temperature 40 ~ 50 ° C, pH 9 ~ 10, S 2 O 2 - 3 concentration 0.4 ~ 018mol / L, SO The concentration of 2 - 3 is 0.25 to 0.5 mol / L, the concentration of SO 2 - 4 is 011 to 0.2 mol / L, the concentration of Cu 2 + is 0.01 to 0.04 mol / L, and the concentration of ammonia is 0.5 to 1.0 mol / L.

The most important factor in the leaching of ammonia cyanide is the cyanamide ratio. The different optimum cyanide ratios used by different scholars reflect the inherent complexity of the leaching mechanism of the cyanamide system. The cyanamide ratio ranges from 1:1 to 4:1. The concentration of ammonia should be determined by the dissolution of copper in the leaching system [6 ~ 7 , 16] : In general, in order to make ammonia and copper exist in the form of stable Cu(NH 3 ) 2 + 4 , NH 3 and Cu 2 The ideal ratio is 4:1; the pH range in which Cu(NH 3 ) 2 + 4 can be stabilized is 8 to 10.5, which is also the optimum pH range in the leaching system.

In summary, combined with the nature of the test sample, the conditions of thiourea leaching, thiosulfate leaching and ammonia cyanide leaching were first tested. For the thiourea leaching, four sets of conditions were tested for thiourea mass fraction (0.5% and 1.0%) and Fe 3 + mass fraction (0.3% and 0.5%). For thiosulfate leaching were temperature (23 ℃, 40 ℃ and 60 ℃), pH (pH8, pH8.5 and pH9), S 2 O 23 concentration (0.25mol / L, 0.5mol / L and 1.0mol / L), Cu 2 + concentration (0.02mol / L and 0.04mol / L), SO 2 - 4 concentration (0.05mol / L and 0.1mol / L) set of test conditions and the like 12. For the leaching of ammonia cyanide, the ratio of cyanamide (2:1 and 3:1), copper and no copper, ammonia addition type (NH 4 HCO 3 , NH 3 ·H 2 O, NH 4 Cl), leaching 14 sets of conditional tests such as time (12h, 24h and 48h), ore particle size (original particle size and 95% less than 320 mesh). The leaching rate of gold is optimized, and the optimum leaching conditions for the determined thiourea leaching, thiosulfate leaching and ammonia cyanide test are shown in Table 4, Table 5 and Table 6.

Table 4 thiourea leaching conditions

Table 5 thiosulfate leaching conditions

Table 6 Leaching conditions of ammonia cyanide system

The step-by-step leaching uses a test scheme of roasting-sulfuric acid immersion copper-cyanide immersion gold. After roasting at 600 ° C in a muffle furnace, copper immersion pretreatment was carried out with 20 g/L of sulfuric acid, and the copper slag was subjected to conventional stirring cyanide immersion gold.

All leaching tests were carried out using a variably powered electric mixer in a 1 L beaker with agitation. The mass of the stirring test sample was 100 g, and the liquid-solid ratio was 3:1. The thiosulfate immersion gold was carried out at room temperature except that it was carried out in a water bath at 40 °C. After the completion of the stirring leaching, the slurry was filtered, and the residue and the leachate were subjected to correlation analysis.

(two) analytical methods

The main analytical methods involved in the test are as follows: Au, Ag, and Cu were analyzed by atomic absorption. CS (CN 2 ) 2 analysis was performed by redox titration. CN - Analytical dithizone silver nitrate titration. The pH was measured with a pH test paper and a pH meter. The oxidation-reduction potential was measured by a potentiometer.

In the laboratory test, the reference reagent was purely standard, NaCN was industrial product (content 98%), and the remaining reagents were analytically pure. The leaching reagents in the expanded test were all industrial products.

Fourth, fruit and discussion

(1) Leaching effect of gold in the leaching system

The leaching rates of gold in thiourea, thiosulfate, stepwise leaching and ammonia cyanide leaching are shown in Table 7.

Table 7 Gold leaching rates in different selective leaching schemes

From the optimized leaching conditions and leaching results, it can be seen that in the thiourea immersion gold, the leaching time is short and the speed is fast, but the leaching index is low, the gold leaching rate is only about 77%, and the thiourea consumption is large, so that the process is not economically feasible, further, to the acidic leaching process difficult environment; thiosulfate gold leaching process requires Cu + 2, gold and copper, may be provided leaching Cu + 2 moiety, and the gold leaching rate Fast, only 5h can achieve higher leaching rate, but this method has higher temperature requirements, leaching conditions are harsh (pH range and reagent addition requirements are strict), leaching system is complex, reagent consumption is large; In the leaching, after the sample is calcined, copper sulphate leaching can remove about 48% of copper in advance, so that the cyanide consumption of cyanide immersion gold is greatly reduced, only 6kg/t, but the roasting is originally made.

The easy-dip natural gold package becomes difficult to be immersed, and the gold leaching rate is only 8154%; the ammonia cyanide system has good selectivity for gold leaching. When the cyanide consumption is 14.7kg/t, the gold leaching rate reaches 90.23%, and the reagent is cheap, and when the cyanide consumption is 14.7kg/t, the leaching rate of direct cyanide gold is 49.33%, and the leaching rate of ammonia cyanide method is about 40% higher than that of direct cyanide gold.

(The influence of two different leaching systems

In direct cyanidation, the dissolution rate of copper reaches 55.84%, so single cyanide is not selective for copper.

In the thiourea leaching, the dissolution rate of copper is 6.0%, which has no direct effect on the leaching of the thiourea system, which is theoretically the same. In addition, theoretically, copper leaching has a certain consumption of Fe 3 + in solution, but it does not affect the conversion of thiourea (the oxidation of thiourea to dithiocarbamidine), which is from copper, iron and thiourea. The standard electrode potential can be seen in the acidic solution (see Table 8). Therefore, the effect of copper on the leaching of the thiourea system is small.

Table 8 Standard electrode potentials for electrical pairs in acidic solutions (298.15K)

The effect of copper on leaching of thiosulfate is very complicated. Under the experimental conditions (the amount of added Cu 2 + 0.02mol / L), the sample of 4.4% of dissolved copper (corresponding supplemented 0.005mol / L of Cu 2 + in solution). If a Cu 2 + concentration of 0.04 mol/L is added, precipitation of copper occurs (the copper content in the sample precipitates 4.57%, that is, the concentration of Cu 2 + in the reduced solution is 0.017 mol/L). Therefore, under the optimal test conditions, the concentration of copper should be about 0.025 mol/L, and the ratio of copper to ammonia should be 1:32. Obviously, the ratio of copper to ammonia is not theoretically 1:4, but much smaller. Thiosulfate leaching, Cu 2 + involved in the gold leaching reaction, serves to S 2 O 2 - 3 stabilized. Since copper is a leaching agent in thiosulfate leaching, the selectivity of this method for copper is reflected in the use of copper in copper-bearing ores.

In the cyanamide system, gold leaching can be achieved after conditional testing without the addition of copper (results not listed). This is mainly because 8.8% of the copper in the sample is dissolved during the leaching process (corresponding to the addition of Cu 2 + 0.01 mol/L in the solution), which has met the needs of the leaching system. If Cu 2 + is additionally added, the leaching rate of gold will decrease. Many studies have shown that the ratio of copper to ammonia in the cyanamide system is preferably from 1:4 to 1:6 in order to stabilize the presence of Cu(NH 3 ) 2 + 4 . However, in this study, the ratio of copper to ammonia is 1:9. It may be that the presence of excess ammonia does not hinder the leaching performance of the cyanamide system, which deserves further study. Similar to thiosulfate leaching, copper is also utilized as a leaching agent in the cyanamide system.

In summary, among the four selective immersion gold methods, the ammonia cyanide immersion gold can not only achieve the ideal leaching index, but also realize selective immersion gold, and the process is simple and economical.

V. Expanding the experiment

According to the results of laboratory tests, the ammonia cyanide leaching was expanded. The expansion test was carried out in a mechanical leaching tank with a leaching tank volume of 1.5 m 3 and a sample mass of 365 kg. After the leaching slurry is separated by solid-liquid separation using a filter press, the gold in the leaching solution is adsorbed and recovered by activated carbon. The test conditions and results are shown in Table 9.

Table 9 Conditions and results of the expanded test

The results of the expanded test are basically close to those of the laboratory test, indicating the rationality of the leaching conditions. After that, continuous production of 5t concentrate was carried out, and the gold leaching rate was stable at around 90%.


For copper-bearing gold ore in the form of non-wrapped gold, the ammonia cyanide method has higher leaching rate and lower reagent than the selective leaching method such as thiourea method, thiosulfate method and step leaching method. The process is simple and obvious advantages. Compared with direct cyanidation, it reduces the consumption of cyanide and increases the leaching rate of gold. It is an effective method for recovering gold from copper ore.


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Author unit:

School of Environmental Science and Engineering, Chang'an University (Chengdonghui)

China University of Geosciences (Beijing) Energy College (Li Guobin)

Heap Leaching Technology Center of Shaanxi Provincial Bureau of Geology and Mineral Resources (Zhang Xiaoyan, Wang Liqun, Zhao Fangling)

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