Study on improving gold leaching rate in two-stage roasting roasting of arsenic-bearing gold concentrate

The pre-oxidation treatment of gold concentrate before gold extraction mainly includes three methods of roasting oxidation, pressure oxidation and bacterial oxidation. Practice shows that roasting, as a traditional process, is still the most attractive treatment method in gold concentrate pretreatment process because of its mature technology, strong adaptability, simple operation, reliable technology and relatively low investment cost. At present, mainly two-stage roasting refractory gold concentrate containing arsenic treatment process of sulfur, in the first stage furnace a reducing atmosphere produced by roasting arsenic removal of arsenic trioxide as a white product, in the second stage roasting furnace arising from desulfurization Sulfur dioxide flue gas is used to produce sulfuric acid, and cinder is used for cyanidation gold leaching. This process can realize the comprehensive utilization of gold, arsenic and sulfur resources.

Since 2000, the two-stage roasting technology developed for the treatment of arsenic-containing refractory gold concentrates developed on the basis of boiling oxidizing roasting of gold concentrates has been rapidly developed, and six professional gold smelting enterprises have been built and put into operation. The processing scale of the five manufacturers is between 100 and 200 tons, and four of the six professional gold smelting enterprises are designed and provided technical services by the Beijing Research Institute of Mining and Metallurgy. China's largest two-stage roasting unit for arsenic-containing refractory gold concentrate is a two-stage roasting furnace of Qinghai Dachaidan Mining Co., Ltd., which is supplied by Beijing Research Institute of Mining and Metallurgy and completes the project EPCM. The treatment scale has reached the treatment of arsenic-containing gold concentrate. Mine 480t∕d.

We believe that in the inability to incorporate arsenic-containing gold concentrate into the heavy metal smelting process to recover the valuable metals in the process of comprehensive recovery, it is necessary to realize the real estate gold for the arsenic-containing refractory gold concentrate and comprehensively recycle it. arsenic sulfur, gold and silver and other resources, two-stage firing process is still the preferred treatment choice.

Although China's domestic two-stage roasting has developed rapidly in the difficult process of arsenic-containing gold concentrate roasting and gold extraction, the gold content of the cyanidation tailings in the two-stage roasting gold smelting process is still high, and the average slag gold is 4g. Above t, for some particularly difficult to treat arsenic-containing gold concentrate, its cyanide tailings contain a higher gold grade. Therefore, how to improve the leaching rate of gold in the two-stage roasting slag, reduce the grade of gold in the tail slag, avoid resource waste and improve the economic benefits of the smelting enterprise are urgent problems to be solved.

In this paper, the leaching and gold extraction test of the roasting in the two-stage roasting industrial production process of a refractory gold concentrate in western China is carried out, and the optimal leaching process conditions and the best gold leaching rate are obtained, which provides practical guidance for industrial production.

The basic composition of one or two stages of roasting and baking

Multi-element analysis of the calcine and soot was carried out and the results are shown in Table 1.

Table 1 Main chemical components of calcined sand and soot

Mineralogy and scanning electron microscopic observation showed that the calcine microscope to see the gold particles, but the size is very small, typically <2μm, part of the gold fine particles as monomers, partially wrapped porous hematite, a small amount of Vein wrapped. The energy spectrum analysis of the calcine and soot shows that the basic composition except for Fe and Si, there are still a significant number of Al, Ca, K and Mg, S, also see a small amount of Cu, As, and a higher S in the soot. It can be seen that the basic mineral of gold concentrate, which is the raw material for roasting, that is, the pyrite is mostly desulfurized after being calcined in two stages, but the calcined sand still contains a significant amount of sulfur.

The sulfur content in the soot is significantly higher than the sulfur content in the calcine. The form of sulfur in the soot is mainly sulfate, but there is still a small amount of uncalcined pyrrhotite. The As in the soot is significantly higher than that of the calcined sand, because the residence time of the soot during roasting is relatively short, resulting in incomplete arsenic removal.

Second, cyanide leaching verification test

(1) Effect of cyanide leaching particle size on gold leaching

The particle size analysis of the two-stage calcination, the -0.055mm particle size accounted for 71.16%, the -0.039 mm particle size accounted for 63.09%, indicating that the calcine did not reach the required particle size for the refractory gold concentrate bake cyanide leaching. Therefore, it is necessary to further finely grind the calcine before the cyanide leaching to improve the cyanide leaching rate of gold.

Using conventional cyanidation leaching, 80 g of calcined sand with different particle sizes after grinding were respectively placed in a cyanide roller bottle and cyanated and leached at normal temperature. The leaching slurry concentration is 40%. Firstly, the CaO (6kg/t) is used to adjust the pH of the slurry to ~10.5, and the rolling pretreatment is carried out for 4h, then the NaCN solution is added to leaching. The NaCN dosage is 6kg/t, and the leaching is carried out for 32h at pH=11.5. The results of the cyanidation leaching test are shown in Table 2.

Table 2 Effect of grain size on gold leaching

It can be seen from Table 2 that as the grain size of the blasting leaching becomes finer, the leaching rate of gold is remarkably improved. When the cyanide leaching particle size reaches -0.039mm and accounts for 87.82%, the leaching rate of gold reaches 88.86%, and the fineness of the blasting of the blasting continues to increase, and the leaching effect of gold increases slowly. On the other hand, the grinding fineness is increased, power consumption, steel ball loss, and grinding operation time are also increased; in addition, too fine grinding will bring a lot of trouble to subsequent operations. It is recommended that the particle size of the calcination cyanide leaching is about -9039 mm, which is about 90%.

(2) Effect of sodium cyanide dosage on gold leaching

The calcined granules have a particle size of -0.039 mm and 90%, and the amount of cyanide is changed. Other conditions are the same as above, and cyanide leaching is directly carried out. The test results are shown in Table 3.

Table 3 Effect of sodium cyanide dosage on gold leaching rate

It can be seen from Table 3 that when the amount of NaCN is 4.0 kg/t, that is, the leaching cyanide concentration is 0.267%, the gold leaching rate is 88.86%. Continue to increase the amount of NaCN, the gold leaching rate did not increase significantly. The recommended dosage of NaCN is 4.0 kg/t.

(III) Selection test of baking cyanide leaching protection alkali

In the cyanidation leaching process, it is necessary to add a protective base to adjust the cyanide leaching slurry solution to a pH of 10 to 11.5. The presence of a protective base has three effects: (1) The oxygen required for the leaching process is provided by introducing air into the leaching slurry solution. Acid gases in the air, such as CO _{2 ,} will lower the pH of the leaching slurry solution, allowing the cyanide to hydrolyze, which can be eliminated when the protective base is present; (2) the side reactions of many associated minerals during the leaching process become acidic. The compound must be neutralized with a protective base; (3) ions formed by dissolution of some associated ore during the cyanidation leaching process inhibit the gold leachate or consume cyanide, and the protective base can react with it to eliminate this effect.

Calcium oxide, calcium hydroxide and sodium hydroxide are used as the protective alkali, and the pH of the slurry is about 9, after pre-stirring treatment, cyanide is added to cyanide leaching, and the leaching particle size is -0.039 mm, accounting for 90%. The test results of the protective base selection are shown in Table 4.

Table 4 Effect of different protective alkalis and dosages on gold leaching rate

It can be seen from Table 4 that when the above three protective bases are used for cyanidation leaching, there is no substantial difference in the leaching rate of gold. At present, which kind of protective alkali is selected in production mainly considers the production cost and is convenient to use.

This test did not use sodium carbonate (or ammonium carbonate) as a protective base for testing. The main reason is that sodium carbonate (Na ₂ CO ₃ ) or ammonium carbonate is easily hydrolyzed to H ₂ CO ₃ , and H ₂ CO ₃ can decompose NaCN. Therefore, when sodium carbonate (or ammonium carbonate) is used as a protective base, the possibility of reducing the amount of sodium cyanide is small.

Third, to improve the gold cyanide leaching rate in the calcine

(1) Cyanide leaching of roasting soot

Process mineralogy studies indicate that the presence of a certain number of pyrrhotite calcine dust and calcined while, since the gold concentrate roasting the presence of a small amount of copper ore yellow, blue calcined see a small amount of chalcocite is present, so baking These two indicators of sand seriously affect the leaching of gold and the consumption of cyanide. The cyanide leaching results of the two-stage roasting soot (leaching conditions: 0.039mm, 87.8%, NaCN dosage: 4.0kg/t, CaO dosage: 6.0kg/t) indicate that the leaching rate of cyanide gold after calcining is 89.15%. The slag gold grade is 4.28g/t; the cyanide leaching rate of gold in the soot is 88.42% on average, and the slag gold grade is 6.82g/t.

(2) Calcination and soot re-baking-cyanide leaching To illustrate the influence of the quality of calcined and calcined soot on cyanide leaching, the calcination and calcination of soot and roasting soot were studied. The chemical analysis results of the main elements of the roasting and roasting soot muffle furnace re-baking are shown in Table 5.

Table 5 Chemical analysis results of calcined and soot muffle furnace re-baking elements

Table 5 shows that the calcined re-baking still has a 28% desulfurization rate, indicating that some of the undecomposed sulfide minerals are still oxidatively decomposed during the re-baking process. Since As and Sb in the calcined sand are present in the form of a calcium compound in the calcination, they are fixed in the calcined sand and are difficult to be decomposed and removed at the calcination temperature. When the soot in the production process is re-fired, the desulfurization rate is 55% to 60%. Arsenic and Sb are reacted to form arsenic (or bismuth ) acid salt due to excessive oxidizing atmosphere during calcination, and arsenic (or bismuth) acid salt is a very stable compound, which can be decomposed only at very high temperatures, so arsenic (or bismuth) ) Continue to remain in the baking sand. Calcination and soot 650 ° C re-baking - cyanide leaching results are shown in Table 6 (leaching conditions: -0.039mm accounted for 90%, NaCN dosage 6.0kg / t).

Table 6 Calcination and soot re-baking - cyanide leaching results

It can be seen from Table 6 that when the calcined sand is subjected to re-baking-cyanide leaching, the leaching rate of gold reaches about 92.69%, and the leaching rate of gold is increased by about 4% compared with the direct grinding-cyanide leaching of the calcined sand. It can be seen that after the re-baking of the soot produced by the two-stage roasting at the production site, the gold content in the cyanide slag is significantly reduced.

Fourth, the phase composition of the leaching slag and the state of the gold in it

The main phase of the leaching slag consists of hematite phase obtained by oxidative desulfurization of pyrite. In addition, FeS remains in different degrees, especially in soot. The main mineralogical factor affecting the effect of Au cyanide is that the natural gold has a very fine particle size and is partially encapsulated by hematite or gangue (such as quartz ). The chemical phase analysis of gold in the leaching residue shows that gold exists mainly in the form of iron oxide wrapped gold, the distribution rate is 69.50%, followed by the form of sulfide-coated gold, the distribution rate is 17.20%, only a small part is in the form of bare gold. Existence, the distribution rate is only 5.80%, and a small part is encapsulated in the silicate phase, the distribution rate is 7.50%.

The porous iron oxide fraction in the cyanide leaching slag has formed densified hematite. If the leaching slag is further ultra-finely ground, it will continue to leach a small amount of the originally wrapped natural state Au, but most of the slag is Au. Still not leaching. This part of gold is invisible gold in the presence of concentrate. Although these invisible gold are leached with the oxidation of sulfur in the sulfide or the formed sulfate in the oxidative roasting process, most of them have become immersible gold, but Some are still wrapped in iron oxide and become non-immersible. This kind of gold, which is closely related to iron oxide, can only be continuously exposed with the continuous dissolution of iron oxide to become leachable gold.

V. Conclusion

(1) Calcine (after the belt filter) belongs to the typical refractory multi-metal containing gold concentrate calcine; there are undecomposed pyrite particles in the calcined sand and incompletely decomposed FeS and undecomposed complete magnetic yellow iron Minerals; there are magnetite and maghemite phases in the roasting soot, and more pyrrhotite (FeS) is found in the soot;

(2) Cyanide leaching residue under the conditions of calcining grinding fineness -0.039 mm, 90%, CaO dosage 6kg/t (calcining), NaCN dosage 4kg/t (calcining), cyanidation time 32h cyanidation The gold grade is 4.28g/t, and the gold leaching rate reaches 89.15%.

When the calcined re-baking-fine grinding-cyanide leaching, the cyanidation leaching of the calcined calcined gold reached 92.61%, and the gold grade in the slag was 2.92 g/t.

(3) The key problem in the two-stage roasting and gold extraction of the arsenic-containing refractory gold concentrate is that the quality of the gold concentrate roasting product is not good, which directly affects the cyanide leaching rate and cyanide consumption of gold. It is the key to improve the gold leaching rate and reduce the consumption of cyanide in the production process to improve the two-stage roasting process conditions of arsenic-containing gold concentrate and to strengthen the pretreatment before calcination.

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