Solidification roasting-cyanide gold extraction test for a refractory gold concentrate in Guizhou

So far, the world of refractory gold ore oxidation pretreatment method, there are the traditional roasting oxidation, pressure oxidation, biological oxidation, nitric acid oxidation method. The first three methods have successful process practices. However, the conventional roasting oxidation method releases toxic fumes such as As ₂ O ₃ and SO _{2 to} pollute the environment; the pressure oxidation method requires strict technology, complicated equipment and high material requirements; the biological oxidation method is harsh and the leaching process is lengthy. The roasting oxidation method is a traditional industrial production method for pretreatment of refractory gold ore in China. Compared with the foreign roasting oxidation process, the domestic roasting process has the following two problems: 1 roasters are rotary kiln, soot and smoke are large. Dust and exhaust gas treatment is poor, and environmental pollution is serious. 2 The recovery rate of calcination and gold extraction is low. Therefore, research and development of a pretreatment process suitable for China's national conditions, cost-effective and environmentally-friendly is of great significance to the development of China's gold industry.

The innovations and features of this research are conducive to environmental protection. The innovation is that arsenic and sulfur in the gold concentrate are not polluted by the smoke or dust in the air during the roasting process, but are captured by the added curing agent and fixed in the form of salt. In the sand; the calcined sand produced at the same time is easy to be directly cyanidated and gold is extracted; the cyanide residue does not cause secondary pollution to the environment.

First, the nature of gold concentrate

(1) Mineral composition of gold concentrate

Gold concentrate samples required for testing from a gold processing plant in Guizhou. Through identification, 11 minerals were found (Table 1). There are 7 kinds of primary metal minerals and 4 kinds of gangue minerals. Semi-quantitative determination by visual estimation, to metallic minerals pyrite, arsenopyrite, realgar, accounting for about 60% of total mineral; gangue minerals in water mica, quartz, accounting for about 30% of total mineral ; no independent minerals of gold found.

Table 1 Gold concentrate ore identification results %

(2) Chemical analysis of gold concentrate

The results of multi-element chemical analysis of gold concentrate are shown in Table 2.

Table 2 Multi-element chemical analysis results of gold concentrates %

Note: The unit of Au content is g/t.

It can be seen from Table 2 that the main characteristic of the chemical composition of the sample is that the content of the target element to be recovered is high; the content of harmful components S and As is high, and other Pb, Cu, Zn, Sb, and C also have a certain content.

(3) Particle size composition of gold concentrate

The particle size composition of the gold concentrate is shown in Table 3.

Table 3 Gold concentrate particle size composition

5重量。 The gold concentrate - 200 mesh content accounted for 72.62%.

Second, test reagents

(2) Slaked lime [Ca(OH) ₂ ] curing agent. After the industrial type quicklime was aged in the room, it was sieved with a 0.5 mm sieve, and the sieved material (-0.5 mm) was taken for the test.

(2) Sodium cyanide leaching agent. The sodium cyanide leaching for gold cyanide is an industrial product produced in Japan, and is prepared to have a NaCN concentration of 10% for use.

(3) XG auxiliary additives. Industrial type, particle size is -0.5mm.

Third, solidification roasting - cyanide gold extraction test

(1) Ca(OH) ₂ dosage test

It is generally believed that the amount of curing agent added should be slightly larger than the theoretical amount, but the reasonable gold leaching rate and the solidification rate of arsenic and sulfur should be followed and the principle of depletion of the calcined gold grade due to the addition of the curing agent should be minimized. Fixation conditions of Ca(OH) ₂ dosage test: solidification roasting is 100g of ore sample, XG additive is 10g, calcination temperature is 550°C, calcination time is 1.0h, air flow rate is 1.2L/min; roasting cyanide leaching is finely ground for 7min , the leaching solution solid ratio = 2:1, NaCN dosage 4 kg / t, pH = 10, leaching time 20.0h. The Ca(OH) ₂ variable is 60%, 70%, 80%, and 90% of the mass of the ore sample. The test procedure and results are shown in Figure 1, Figure 2, respectively.

Figure 1 Ca(OH) ₂ dosage test procedure

Figure 2 Ca(OH) ₂ dosage test results

◆-arsenic solidification rate; ▲-sulfur solidification rate; ●-gold leaching rate

It can be seen from Fig. 2 that the curing rate of arsenic and sulfur increases gently as the amount of curing agent increases in the range of Ca(OH) ₂ selected. When the theoretical dosage (the theoretical dosage is 80% of the mass of the ore sample) is reached, the ideal arsenic and sulfur solidification rate can be obtained; on the contrary, the leaching rate of gold in the calcination increases with the amount of Ca(OH) ₂ to the ore. The best value is obtained at 80% of the weight, and then gradually decreases. This may be due to the excessive addition of the curing agent, which hinders the complete oxidation of the gold-bearing mineral. Therefore, it is determined that the amount of Ca(OH) ₂ is 80% of the mass of the ore sample. At this time, the solidification rates of arsenic and sulfur are 95.76% and 95.14%, respectively, and the leaching rate of gold is 82.93%.

(2) Calcination temperature test

Different gold concentrates have their own suitable calcination temperatures depending on their material composition and curing agent type. Temperature test fixed conditions: the amount of curing agent Ca(OH) ₂ is 80% of the mass of the ore sample, the heating time is 1.0h; the solid-liquid ratio of the calcined leaching is 2:1, the dosage of NaCN is 4kg/t, pH=10, The leaching time was 20.0 h; the other fixed conditions were the same as 3.1. The calcination temperature variables were 450, 500, 550, 600, and 650 ° C, respectively. The test procedure is shown in Figure 1, and the test results are shown in Figure 3.

Figure 3 Roasting temperature test results

◆-arsenic solidification rate; ▲-sulfur solidification rate; ●-gold leaching rate

It can be seen from Fig. 3 that the solidification rate of arsenic and sulfur increases slowly with increasing temperature in the selected temperature range, and the solidification rate remains substantially unchanged when the temperature reaches 550 °C or higher. The effect of the calcination temperature on the gold leaching rate is obvious. The temperature is too low, the oxidation rate is slow, the arsenopyrite and pyrite are not completely decomposed and oxidized, and the purpose of destroying the mineral structure and lattice wrapping is not achieved. When the temperature is too high, the calcined product Fe ₂ O _{3 is} sintered, and the gold is re-wrapped. In the calcined product, a secondary physical encapsulation is formed, resulting in a decrease in the gold leaching rate. The gold leaching rate reaches the optimum state when the temperature is 550 °C. Considering comprehensively, the selected calcination temperature is 550 ° C. At this time, the cyanidation leaching rate of gold is 82.29%, and the solidification rates of arsenic and sulfur are 95.09% and 94.43%, respectively.

(3) XG auxiliary additive dosage test

In the previous tests, the solidification rates of arsenic and sulfur were satisfactory, but the leaching rate of gold was low. Even if some cyanidation conditions were strengthened, such as prolonged leaching time and fine sanding, the intended purpose was not achieved. The gold concentrate sulfur, arsenic content higher, sulfur components in particular, it is the largest consumer of the curing agent, a large amount of calcium sulfate generated, is not conducive to leaching of gold; ore while still contain antimony, lead and other harmful ingredients It also has an effect on the leaching of gold. From the exploration experiments of many calcination conditions, it was found that a certain amount of XG auxiliary additive was added to the solidification roasting, which had a significant effect on the leaching effect of gold. Therefore, the amount of XG was tested. The amount of fixed Ca(OH) ₂ is 80010 for the quality of the ore sample, the calcination temperature is 550 ° C, the fine grinding time is 10 min, and the other conditions are the same as 3.2. The XG variables are 10%, 15%, 20%, and 25% of the mass of the ore sample, respectively. The test procedure is shown in Figure 1, and the test results are shown in Figure 4.

Figure 4 XG auxiliary additive dosage test results

It can be seen from Fig. 4 that the leaching rate of gold is gradually increased with the increase of the amount of XG auxiliary additive in the range of the selected XG auxiliary additive. When the amount of XG exceeds 20% of the mass of the ore sample, the leaching rate of gold slowly decreases. The reason is that the amount of XG is increased, the reaction speed is accelerated, and the reaction is more intense. The calcination phenomenon occurs in the calcine, and the looseness of the calcination is gradually decreased. Therefore, it is determined that the optimum amount of XG is 20% of the mass of the ore sample. The leaching rate of gold in the calcination reached 88.24%.

(4) Roasting time test

The amount of XG auxiliary additive is 20% of the mass of the ore sample, and the other test fixed conditions are tested with the XG auxiliary additive dosage condition. The calcination time variables were 0.5, 1.0, 1.5, 2.0 h. The test procedure is shown in Figure 1, and the test results are shown in Figure 5.

Figure 5 Roasting time test results

It can be seen from Fig. 5 that in the selected time range, when the calcination time is between 1.0 and 1.5 h, the main gold-bearing mineral arsenopyrite and pyrite in the gold concentrate are completely decomposed and oxidized, and the porosity of the calcine is increased. The contact degree of gold particles with cyanide solution is high, and the leaching rate of gold is above 88%. However, when the calcination time is increased, the leaching rate of gold is decreased, which may be caused by sintering during the firing process. Therefore, it is more suitable to select the calcination time of 1.0 h. At this time, the cyanidation leaching rate of gold is 88.51%.

(5) Air flow test

In some literatures, the conclusions about the influence of air flow on the solidification rate of arsenic and sulfur are different. This test mainly investigates whether the air flow has any influence on the leaching rate of gold. The test conditions except the air flow fluctuation and the best roasting time are 1.0 h. In addition, other fixed conditions are the same as in Section 3.4, and the test results are shown in Table 4.

Table 4 Relationship between air flow and gold leaching rate

It can be seen from Table 4 that when the air flow rate is increased from 0.5 L/min to 5.0 L/min, the quality of the calcine produced by solidification roasting remains basically the same, indicating that the air flow rate in this range has substantially no cure rate for sulfur and arsenic. Significant impact. At the same time, when the air flow changes, the leaching rate of gold in the calcine is ideal and remains basically unchanged. Considering comprehensively, the air flow rate is 1.2L/min.

After determining the best combination of curing calcination factors, finding the best calcination leaching conditions to increase the gold leaching rate becomes another important part of the whole process, because the effect of calcination cyanidation gold It is directly related to whether it is feasible to add the cooked lime roasting process. Analysis of the characteristics of the roasting process, fine sanding, cyanide leaching time, sodium cyanide dosage and other factors will have different effects on the leaching rate of gold. In the experimental study of the cyanide leaching stage, it is necessary to examine various factors in detail.

(6) Calcine fine grinding test

To improve the leaching rate of gold in the calcine, fine sanding is one of the more effective means. Fixation conditions for the fine grinding time test: the optimum conditions for solidification roasting Ca(OH) ₂ is 80% of the ore sample mass, the XG dosage is 20% of the ore sample mass, the calcination temperature is 550 ° C, and the air flow rate is 1.2 L/min. The calcination time was 1.0 h; the liquid-solid ratio was 2:1 when the cyanide was leached, the amount of NaCN was 4 kg/t, the pH was 10, and the leaching time was 20.0 h. The sanding fine grinding time variables were 7, 10, and 13 min, respectively. The test procedure is shown in Figure 1, and the test results are shown in Figure 6.

Figure 6 Calcination time test results

It can be seen from Fig. 6 that the effect of fine grinding on increasing the gold leaching rate is obvious. The unmilled sand was immersed in water for 20 min, and the content of -400 mesh reached 72.20%. However, it was found that the sieve was mostly brownish red and dense hematite (Fe ₂ O ₃ ). Grinding can expose the covered gold and increase the specific surface area of ​​the gold particles. With the increase of fine grinding time, the leaching rate of gold is slowly increased when it reaches 10 min or more. From the test requirements and production considerations, it is determined that the optimal fine grinding time is 10 min, that is, the fine grinding calcine - 400 mesh content 97.72%. At this time, the gold leaching rate was 88.88%.

The aerated mixing pretreatment of the finely ground calcined sand is designed to fully oxidize a very small amount of CaS, CaSO ₃ and As _x S _y generated during the roasting process, eliminating the possible reduction of gold cyanide leaching rate and increasing the amount of NaCN. Impact. The test showed that the aeration stirring time was 1 h.

(7) Cyanide leaching time test

The cyanide leaching time is closely related to the fine grinding time of the baking sand. The fine grinding time of the baking sand is determined to be 10 min. For other fixed conditions, see the sanding fine grinding time test in Section 3.6. The cyanidation leaching time variable is 8,14,20,26. , 32h. The test procedure is shown in Figure 1, and the test results are shown in Figure 7.

Figure 7 Test results of calcination cyanide leaching time

It can be seen from Fig. 7 that with the increase of cyanide leaching time, the leaching rate of gold in the calcining sand also increases. When the leaching time exceeds 20 h, the leaching rate of gold does not change much, so the cyanide leaching time is determined to be 20 h. The leaching rate of gold was 88.42%.

(8) Sodium cyanide dosage test

The NaCN tuning test was carried out by fixing the other conditions of the test to an optimum state. The fixed conditions of the test: the optimum conditions for solidification roasting are Ca(OH) ₂ dosage of 80% mineral sample mass, XG dosage is 20% mineral sample mass, calcination temperature is 550 ° C, air flow rate is 1.2 L/min, roasting time is 1.0h; calcination cyanide leaching solution solid ratio = 2:1, NaCN dosage 4 kg / t, pH = 10, leaching time 20.0 h. The amount of NaCN used was 2, 3, 4, 5 kg/t. The test results are shown in Figure 8.

Figure 8 NaCH dosage test results

It can be seen from Fig. 8 that as the amount of NaCN increases, the leaching rate of gold increases steadily. When the amount of NaCN is more than 3 kg/t, the gold leaching rate changes very little. This is because the roasting is relatively complete, and the fine grinding calcine is pretreated by aeration stirring. The harmful components of cyanidation in the gold concentrate have been eliminated. It can be confirmed from Fig. 8 that the NaCN dosage is 3 kg/t. At this time, the gold leaching rate was 88.33%.

(9) Cyanide leaching liquid-solid ratio test

The curve in Fig. 9 shows the leaching of gold in different liquid-solid ratios when the sand is finely ground for 10 min, the amount of sodium cyanide is 3 kg/t, and the cyanidation time is 20 h.

Figure 9 Leachate solid ratio test results

It can be seen from Fig. 9 that the liquid-solid ratio has little effect on the gold leaching rate in the normal range, and the optimum liquid-solid ratio is between 2:1 and 3:1. At this time, the gold leaching rate is between 88.0% and 89.0%. Such a liquid-solid ratio can maintain a relatively high concentration of sodium cyanide, accelerates gold leaching and reduces the consumption of sodium cyanide.

(10) Verification test for the best comprehensive conditions of roasting and leaching

After detailed experimental research and analysis on various factors affecting cyanidation, the relationship between various factors and gold leaching rate was found, and the optimal conditions of each factor were determined. The final comprehensive condition test of calcination cyanide leaching is now carried out in combination with the best conditions of various factors. In order to obtain a more reliable gold leaching index, two sets of parallel verification tests were carried out, and the average value was taken to calculate the final comprehensive condition test results. The leaching rate of gold is also expressed in terms of the theoretical leaching rate of the calcine and the actual leaching rate. The results of the comprehensive condition verification test are shown in Table 5.

Table 5 Test results of comprehensive conditions for calcination cyanide leaching

It can be seen from Table 5 that the gold grade of cyanide slag is relatively stable, the test results are reproducible and the indicators are reliable. Therefore, the optimum conditions for the calcination and gold extraction stage were determined: fine sanding for 10 min (ie -400 mesh accounted for 97.72%), sodium cyanide dosage 3 kg/t, cyanide liquid to solid ratio 2:1, cyanidation time 20 h.

(11) Experimental research final process

The condition test of the influencing factors in the two stages of solidification roasting and cyanide gold extraction of the gold concentrate, the screening of test data, the optimization of the test process and the verification of the best comprehensive condition test were confirmed. The final process flow of the gold concentrate concentrate roasting-cyanide gold extraction process is shown in Figure 10. The technical specifications of the process flow are shown in Tables 6 and 7.

Table 6 Hardening rate of arsenic and sulfur in solidification roasting

Note: F _As , F _s are the solidification rate of arsenic and sulfur in the calcine, %; F _Au is the distribution rate of gold in the calcine, %.

Table 7 Operational recovery rate and total recovery rate of gold in the process

The indicators in Table 6 are the average values ​​of three sets of parallel verification tests (calcin quality of 219.0, 219.5, 218.0 g, respectively) under the optimal comprehensive conditions. The test results are very stable, and the solidification rates of arsenic and sulfur are very satisfactory.

Figure 10 The final process flow of the solidification roasting-cyanide gold extraction test

In the solidification roasting process of gold concentrate, gold has a small amount (1.86%) of volatilization. Because it is a small laboratory test, it is impossible to carry out recovery test on volatilized gold. In the production process, the volatile gold can be comprehensively recovered while treating the exhaust gas and dust, thereby improving the total gold recovery rate of the entire process.

Fourth, the conclusion

(1) Process mineralogical studies have shown that gold is mainly present in the poisonous sand and pyrite crystal lattice, containing copper , antimony, lead minerals and carbon substances harmful to cyanidation, with "physical" and " The chemistry is difficult to handle in two ways.

(2) The results of curing with only slaked lime show that the solidification rates of arsenic and sulfur are ideal, but the cyanide leaching rate of gold is still low. When a proper amount of XG additive is added to the solidification roasting, the cyanidation leaching rate of gold is remarkably improved while maintaining a high arsenic and sulfur solidification rate. Appropriate amount of XG can shorten the roasting time and reduce the amount of slaked lime, which is conducive to the operation of subsequent cyanidation operations. The reason may be to speed up the reaction, increase the porosity of the calcine, form a soluble salt with the metal, and form a sulfate with SO _{2 in the} atmosphere. In a certain air flow range, the arsenic and sulfur solidification rate and the gold leaching rate do not change much. The best calcination conditions obtained by solidification roasting condition test: 80% of slaked lime sample quality, XG 20% mineral sample mass, calcination temperature 550 ° C, air flow rate 1.2 L/min, calcination time 1.0 h.

(3) It is necessary to finely grind the calcine before cyanidation. The purpose is to further expose the gold particles covered in the calcination and increase the specific surface area of ​​the gold particles, which is beneficial to the effective leaching of gold and shortening the cyanide leaching time. The best conditions for cyanidation leaching: fine sanding to -400 mesh accounted for 97.72%, sodium cyanide dosage 3 kg/t, liquid-solid ratio 2:1, leaching time 20 h.

(IV) Considering the progress of deep processing technology of refractory gold ore in China, the addition of slaked lime solidification roasting method is more suitable for the oxidation pretreatment of high arsenic and high sulfur carbonaceous gold concentrate.

(V) The research obtained a good index of arsenic solidification rate of 97.56%, sulfur solidification rate of 96.10% and gold leaching rate of 88.12%.

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