Advantages of oxygen enriched leaching
2020-09-29 10:19Introduction：Advantages of oxygen enriched leaching The traditional leaching process mostly uses the method of filling compressed air to provide oxygen for the leaching process. The main defect of this method is that the dissolution efficiency of gaseo
Advantages of oxygen enriched leaching
The traditional leaching process mostly uses the method of filling compressed air to provide oxygen for the leaching process. The main defect of this method is that the dissolution efficiency of gaseous oxygen passing through the liquid phase is too low. Even violent inflation can only produce very low dissolved oxygen concentrations in the slurry. Therefore, cyanide operation had to be carried out at low oxygen concentration, which greatly restricted leaching kinetics and reduced gold and silver production. In recent years, the results show that the strong oxidant can be used to increase the leaching rate of gold and silver under suitable conditions without adverse effect on cyanide. So that this technology in gold production has been rapidly spread and applied.
At present, most cyanide leaching processes provide oxygen for leaching by filling compressed air. However, a large number of investigations on cyanide plants show that the oxygen required in the leaching process is often lower than the saturation limit (about 8 × 10 -6). There are many reasons for this: the presence of active minerals in ore oxygen consumption has significantly reduced the actual content of dissolved oxygen in slurry, such as organic matter, divalent iron, pyrrhotite, arsenopyrite, stibnite and chalcopyrite, and other active sulfides; In some areas, the high salt content in the water for production also reduces the solubility of oxygen. However, the main reason for the low dissolved oxygen content in the solution is the defects of the air-filled process itself, because the efficiency of gaseous oxygen entering the liquid phase through the phase interface is very low. It was also found that most cyanide plants were leached at high cyanide concentrations. We know that the ratio of cyanide concentration [CN-] to oxygen concentration [O2] must be kept at about 6 in order to reach the limit of gold dissolution rate. Because the dissolved oxygen concentration is too low, the ratio is much higher than 6, which leads to the slow dissolution rate of gold.
Experience shows that when gold is dissolved in saturated air-saturated solution of pure water (the dissolved oxygen concentration is about 9.0 × 10-6), the concentration of NaCN in excess of 125 × 10 -6 is of no benefit to leaching. If the concentration of dissolved oxygen is very low, even with a small amount of cyanide, the dissolution rate of gold will not be greatly affected. In 1987, Nicol (Nicol), Fleming (Fleming) and Paul (Paul) calculated the maximum effective oxygen concentration corresponding to the given cyanide content based on cathodic and anodic reactions of gold dissolved in a clean solution, as shown in Table 8.1-1. Thus it is possible to increase the dissolved oxygen concentration according to the actual cyanide concentration in the cyanide plant to increase the dissolution rate of gold.
Table 8.1-1 maximum effective oxygen concentration corresponding to different cyanide concentrations
| NaCN concentration /mg·L-1 | oxygen concentration /mg·L-1 |
|
50 100 200 300 400 |
3.2 6.5 13.1 19.6 26.1 |
2. Advantages of oxygen enrichment leaching
⑴the main advantage of oxygen rich leaching is to accelerate leaching kinetics, reduce leaching time, and sometimes increase gold and silver leaching recovery. This means that the oxygen-enriched leaching process has the potential to save investment for the newly built cyanide plant, and the conversion of the cyanide plant under production to the oxygen-enriched leaching process can multiply the scale of production without adding leaching equipment. Both measures will bring considerable economic benefits.
Not all cyanide plants can improve gold and silver leaching rate by oxygen-enriched leaching process. The results of production and test show that the increase of leaching rate and the extent of increase are related to the properties of ore. Generally speaking, the leaching rate of refractory ores (such as high sulfur, arsenic, copper, etc.) can be improved by oxygen rich leaching process, but the leaching rate can only be increased by easy leaching ore.
⑵It can reduce the consumption of cyanide. There are two main components of cyanide consumption which are not related to the dissolution of gold and silver in the leaching process. One part is the loss of HCN gas in the form of volatile air from the solution. This part of the loss increases as much as possible. The other part is consumed by the cyanide-consuming substance in the ore. This part of the loss is related to the nature of the ore. After oxygen-enriched leaching the amount of HCN volatilized from the solution is greatly reduced because the amount of gas in contact with the solution is very small. In addition, under oxygen-enriched conditions, the dissolved oxygen content in the solution is very high and the oxidation ability is very strong. The cyanide-consuming substance in the ore is oxidized, and the free sulphide and other cyanide consuming ions in the solution are greatly reduced. The active sulphide minerals were inhibited and the consumption of cyanide decreased significantly. According to the report, cyanide consumption was reduced by 25% at a cyanide plant in Canada when pure oxygen was used instead of air.
⑶The results show that the gold leaching kinetics of fine grinding ores is slower than that of coarse grinding ores when air-filled agitation is used for the same ores, because fine grinding ores have larger surface area and the consumption of cyanide, oxygen and lime will increase. The results show that the dissolved oxygen concentration can reach 15 × 10 -6 after the fine grinding ore is filled with air for 24 hours, however, the oxygen filling can reach 14 × 10-6 in 4 hours. In a short time, enough oxygen can be used to make use of the large surface area of fine grinding ore, thus increasing the leaching rate and reducing the consumption of cyanide and lime.
⑷Some literatures suggest that keeping and controlling the dissolved oxygen concentration and potential (Eb) in the carbon adsorption system will help to improve the adsorption rate, adsorption selectivity and final gold load of the carbon. Theoretically, this is partly related to the stability of the gold-cyanide complex by controlling the potential and adjusting the ratio of the mixed metal complex system. However, there is no reliable data to confirm the improvement of oxygen to carbon load gold, but this possibility is interesting.
