Recovery of gold from pyrite cinder

Since the air-permeable material, in particular due to the gold and other minerals symbiotic conductive dissolution of gold from concentrates and various pyrite cinder, found gold leaching rate increases as the leaching material decreases in sulfur content, which is , the anodic dissolution of gold is caused by the passivation effect. This can be explained by the current potential curve of gold oxidation and dissolved redox.

Generally, gold co-existing with pyrite or arsenopyrite is not suitable for extraction by cyanidation. To this end, some gold recovery plants use flotation sulphide concentrates and calcine them at 700-850 °C. In this way, gold is freed to make it more suitable for leaching, and sulfur dioxide is formed, and finally sulfur dioxide is made into sulfuric acid.

A complex reaction of the calcination of sulfides, during the calcination process, produces several intermediates, since not all of the particles are in equilibrium during the calcination process, and a mixture of these products is obtained. In order to maximize sulfuric acid production, the sulfur content of the residue should be minimized, but the furnace must be calcined with a slight deficiency of oxygen to prevent the formation of sulfur trioxide. It has been found that gold is leached from the produced cinder and its leaching rate increases as the degree of oxidation increases. In the practice of the factory, taking into account the production of sulfuric acid and gold, the generated slag contains a large amount of hematite and several magnetite. The reason why the leaching rate of gold is leached from the partially calcined slag has not been thoroughly investigated, and thus it is necessary to study.

1. Calcination temperature, residual sulfur and degree of sulfation

It is believed that at high calcination temperatures, due to agglomeration of the particles, physical encapsulation of gold is formed, resulting in a lower extraction rate of gold. The effect of residual sulfur on the gold leaching rate during leaching has many factors, including the consumption of oxygen in the leachate (FeS+6CN ^- +2O ₂ →Fe(CN) ₆ ^4- +SO ₄ ^{2 -} , S ₂ O ₃ ² +CN ^- +1/ 2O ₂ →SCN ^- +SO ₄ ^{2 -} ), that is, cyanide is consumed by the same reaction. During the calcination, a porous oxide structure is locally formed (which means that a small amount of gold is exposed), and a film of a reaction product is formed on the surface of the partially leached gold, thus causing passivation.

The effect of vulcanization (sulfation) roasting on the next leaching of gold is not well understood. However, lower calcination temperatures appear to be a major factor affecting the next step in gold dissolution.

It is well known that the dissolution of gold in a cyanidation medium is an electrochemical reaction. The anodic reaction is the dissolution of gold:

Au+2CN ^- →Au(CN) ^- +e

The cathodic reaction is the reduction of oxygen:

O ₂ +2H ₂ O+4e→4OH ^-

The anodic reaction has been studied in depth and will not be discussed in detail for this purpose, but the influencing factors are discussed in combination with the two reaction processes.

Second, the results of experimental research affecting the factors of leaching gold

The materials and test results used in the test are shown in Table 1.

Table 1 Effect of sample sulfur content on leaching rate

From Table 1, it was found that the leaching rate of gold in various raw materials decreased as the sulfur content increased. The reason can be explained by physical capsule action and electrochemical action.

(1) Consumption of cyanide

Leaching conditions: material 50g, NaCN2.5g, CaO2.5g, H ₂ O 100mL, time 24h.

The leaching of the partially calcined slag is higher than the leaching of the completely calcined material, as shown in Table 2. In factory practice, it is assumed that the cyanide used is slightly excessive, which is responsible for the low leaching rate of gold. In fact, the cyanide used in these tests greatly exceeded the amount, and the extraction rate did not change much, so the consumption of cyanide was not the main reason for the low gold leaching rate.

Table 2 Effect of cyanide consumption on gold leaching rate

(2) Oxygen consumption

Leaching conditions: material 50g, CaO2.5g, H ₂ O 100mL, time 24h.

In some cases, the dissolution rate of gold in the slag is controlled by the rate of oxygen reduction. Therefore, a decrease in the oxygen content causes a decrease in the dissolution rate of gold, and a dissolution reaction such as iron sulfide consumes oxygen. Therefore, when there is a large amount of iron sulfide in the partially calcined slag, oxygen deficiency may occur in the solution, but oxygen is used instead of air as the oxidant, and the degree of reaction is not affected after 24 hours. If the oxygen deficiency is the cause of the low gold leaching rate in the partially calcined slag, the gold can be more dissolved when the oxygen is sufficient.

In the second set of tests, 25 g of partially calcined slag A and 25 g of slag were mixed and leached, and the gold leaching rate was the same as that of the separately treated materials. If oxygen deficiency occurs during partial leaching of the leaching, the dissolution of gold in the slag is also affected when the mixed slag is leached. Therefore, the consumption of oxygen is not a major factor in the low leaching rate of the local calcined slag.

(3) Physical capsule (parcel) function

When the pyrite is calcined, the volume and structure change, and the particles become porous. It can be seen from Table 3 that the increase in surface area indicates that the gold leaching rate increases with the increase of porosity. Therefore, the leaching rate of gold from pyrite and partially calcined slag is very low, probably due to The physical wrap of gold is caused. This possibility can be confirmed by the reaction rate. In the experiment, the leaching of gold started quickly for all samples, but after 1 h, leaching stopped, indicating that some of the easily leached gold had dissolved, and the residual gold was not easily dissolved in cyanide.

Table 3 Effect of calcination and grinding on surface area

When the sample is finely ground, the gold leaching rate is high. However, despite the large surface area, the leaching rate of gold from the finely ground pyrite and the partially calcined slag is still much lower than the leaching rate of gold in the unpolished slag. This indicates that the pyrite and the local calcined slag have been exposed, or have been completely free, thereby increasing the solubility, but there are also some factors that cause the gold leaching rate to be low. However, it is very important to rupture the particles and expose the gold during the roasting process because it is beneficial to increase the leaching rate of gold. Of course, this is not the only factor to be considered.

(4) Passivation of gold in the slag

Leaching conditions: material 50g, NaCN2.5g, CaO2.5g, time 24h.

The oxidation of gold in cyanide is a complex reaction that has not been satisfactorily explained. All the literature in this respect indicates that the initial stage of oxidation follows the normal Tafel standard characteristics, but the passivation occurs as a result of the change in the anode potential. The anode current achieved prior to passivation depends on the concentration of cyanide in the solution and the amount of impurities. Curve B in Fig. 1 is a current potential curve which is obtained by immersing in a pure solution by a gold electrode just polished and then scanning. When the electrode is immersed in the solution after a few minutes, or lead salt or a thallium salt was added, the resulting current-potential curve A. The gold surface potential was scanned from A to -0.4 V, and then another scan was performed immediately from -0.9 V to obtain a curve similar to B. The reason for this nature is not fully understood. However, the two obvious forms of surface appearance, namely activation and passivation, are described in detail. When the cathode potential is maintained at -0.6 V, typically, gold is converted from passivation to activation. Conversely, when the anode potential reaches -0.6V, gold will be converted from passivation to passivation. The comparison of the current potential curve of redox on the gold surface with the passivation and activation oxidation curve of the gold surface indicates that in this case When the current is about 200 μA/cm ² , gold dissolves. Since the shape and size of the gold particles in the slag are unclear, this image cannot be directly converted to the leaching rate. However, it is clear that this rate appears to be quite high.

Fig.1 Current potential curve of gold oxidation and redox

Anode conditions: Au, 0.2 mol/L NaCN, pH = 12.4,

Argon, 500r/min, 22°C, 10mV/s;

Cathode conditions: Au, pH=12.4, air, 500r/min, 22°C, 10mV/s

Mineral studies of slag particles have shown that gold is tightly wrapped around the surrounding minerals. If the contacted mineral is electrically conductive, oxygen reduction occurs on the entire surface of the mineral. The value of the current for oxygen reduction can exceed the value of the current for gold oxidation when the potential for gold is passivated in the cathode range. From this point, it can be seen from Fig. 2 that under these conditions, the gold is dissolved under the mixed potential (gold, oxygen potential), and the mixed electric displacement is toward the anode region, and the surface of the gold is passivated. In this case, the dissolution rate of gold is slow. The approximate conductivity of minerals that are symbiotic with gold in concentrates and slag, such as pyrrhotite, pyrite and magnetite, has a high electrical conductivity, while hematite is an insulator. It is foreseen that the gold coexisting with magnetite, pyrite and pyrrhotite will be passivated due to the increase in cathode current.

Figure 2 Current potential curve with increasing cathode area

Among the completely oxidized cinders, only hematite is present, and therefore, oxygen reduction cannot be performed except for the surface of gold, and dissolution of gold is not hindered. In the presence of other conductive minerals (i.e., in the local calcined slag and pyrite), it is expected that the dissolution current of gold will be reduced to less than 10 μA/cm ² . In addition, the size of the gold particles in the slag is unclear, and this value cannot be directly converted to the leaching rate. However, the dissolution rate is much lower than the dissolution rate of unpassivated gold. The dissolution of the activated gold may take only a few hours, while the dissolution of the passivated gold takes several days.

The effect of soluble components in a single ore on the dissolution rate of gold may be small. It has been found that the dissolution current of gold before passivation is related to impurities in the solution (such as lead, antimony, mercury and antimony ), which causes gold to become state. The tendency to change.

According to the passivation effect of gold, it can be explained that the fine grinding after calcination can accelerate the dissolution of gold in the pyrite and the partially calcined slag. When the material is ground, the amount of gold separated from a group of conductive minerals increases, and at the same time, the passivation time during the leaching period is not too long, due to the occurrence of oxygen reduction on the smaller gold surface. To. Therefore, gold will dissolve well within 24 hours of leaching. However, the decrease in the dissolution rate of gold from pyrite and partially calcined slag is not entirely due to these reasons, which will be explained below.

In both tests, the passivation mechanism of Table 4 for preventing gold dissolution has been confirmed. In the first test, the first three materials were leached for 21 days (one cycle). There is no more dissolved gold from the fully oxidized cinder, but the gold is more effectively dissolved from the partially calcined cinder and the pyrite, indicating that the reaction proceeds very slowly. If the dissolution of gold from these materials is very slow within the first 24 hours, purely due to physical encapsulation, there is no need to increase the leaching time. Since the leaching rate of gold from the slag did not change after the leaching period was extended, this indicates that the residual gold is not easily leached by the leaching agent. As expected after 21d, all of the gold was exposed according to the amount of relative dissolution, but the passivated gold was dissolved. Gold is leached from pyrite and partially calcined slag, and the leaching rate is much lower than that of leaching gold from the slag. This indicates that a portion of the gold is not exposed to the leachate. This proves that the physical encapsulation is one of the reasons why the gold leaching rate in the partially calcined slag is low.

Table 4 Dissolution of gold in a long time

Leaching conditions: material 50 g, NaCN 2.5 g, CaO 2.5 g, temperature 22 ° C, H ₂ O 100 mL.

After prolonged leaching, the concentration of bismuth metal in the solution was significantly different from that after leaching for 24 h. Therefore, the additional dissolution of gold from pyrite and partially calcined slag is not considered to be due to the dissolution of the minerals encapsulating the gold.

In the second test, the slag sample was reduced to a material containing a portion of magnetite at 900 ° C in hydrogen. Although the surface area of ​​the material was not changed by the pretreatment, the gold was very little dissolved from this material within 24 hours, which was due to the slow dissolution of the gold embedded in the magnetite, so it was in a passivated state.

The passivation of gold in cyanide media is well known. Cathro used it to explain why the rate of gold dissolution decreased when the atmosphere changed from air to oxygen. Mrkusic proposed several possible reasons for the low gold leaching rate.

It has been shown in the above experiments that local calcination affects further dissolution of gold. This partial firing also prevents gold from being exposed to the leachate. In fact, the only way to choose the best conditions for the subsequent dissolution of gold from the slag is to produce fully oxidized slag or to dissolve the partially oxidized material prior to cyanidation.

From the partially calcined slag, the leaching rate of gold leaching is low, in part due to incomplete development of the pore structure. However, there is another reason for the low leaching rate of gold for the characteristics seen, which is caused by the occurrence of oxygen reduction on some gold surfaces, causing the passivation of gold. Oxygen deficiency in the solution and excessive consumption of cyanide do not contribute to the dissolution of gold in the tests carried out. However, in production practice, anoxic and excessive consumption of cyanide may cause gold. The dissolution rate is reduced.