I. Introduction

It has long been recognized, containing high concentrations of dissolved oxygen in cyanide leaching solution will speed up the dissolution rate of silver and gold, and sometimes increase the overall extraction recovery, leaching oxygen plus oxygen system technology is not yet used widely in industrial practice The reason is that a practical and economical oxygenation method has not been found, and the ATEC oxygen conversion technology has broken through this constraint.

The patented ATEC ZX Oxygenated ^Tb1 technology has been used commercially in wastewater treatment and aquaculture to increase the dissolved oxygen content of aqueous solutions. This novel ATEC technology utilizes an oxygen source and an absorption unit in the leaching process. The liquid phase produces dissolved oxygen above its saturated content. The oxygen utilization efficiency is close to 100%. The dissolved oxygen has good stability and high content, up to 170% of saturated oxygen content, making this technology ideal for use. Accelerate the cyanidation of gold.

In the opinion of most mine personnel and operators, if the final leaching rate of gold in the refractory ore can be increased by a certain amount, for example, 10%, then the potential benefits can be obtained, which is obviously reasonable, and the increase in gold leaching will be The recovery rate of gold can be increased to obtain greater benefits, and the cost-benefit increase is easy to understand.

However, the benefits from the increase in gold leaching speed may not be fully justified. It is achievable to increase the leaching rate of gold by oxygenation by five times or to reduce the time required for leaching by 80%. For new ore bodies or new In terms of process facilities, this can save investment and production costs, and the benefits associated with reduced electronics and cyanide consumption and increased production scale are possible.

The increase of leaching speed may have more significant significance. For example, simple modification of existing equipment with a certain volume may increase the total recovery rate, increase the processing capacity of the equipment, and reduce the production cost, which will be detailed later in this paper.

Second, the role of oxygen in gold leaching

The role of oxygen in gold and silver cyanide leaching has been recognized for almost half a century, and the Elsder equation was well known as early as 1846.

4Au+8NaCN+O ₂ +2H ₂ O=4NaAu(CN) ₂ +4NaOH

This equation shows that oxygen plays an important role in the cyanidation leaching of gold. However, due to the explanation of the electrochemical leaching of gold dissolution and the reasons for the chemical calculation method, some confusion has arisen. The Janin equation proposed in 1892 does not require oxygen to participate in the reaction.

2Au+4NaCN+2H ₂ O=2NaAu(CN) ₂ +2NaOH+H ₂

During this period some producers believed that the leaching of gold did not require oxygen. Later, the equation proposed by Bodlaender showed that oxygen is needed, but it participates in the reaction with a peroxide as a vehicle.

2Au+4NaCN+2H ₂ O=2NaAu(CN) ₂ +2NaOH+H ₂

During this period, some producers believed that the leaching of gold did not require oxygen. Later, the equation proposed by Bodlaender showed that oxygen was needed, but it was involved in the reaction with a peroxide as a vehicle.

2Au+4NaCN+2H ₂ O+O ₂ =2NaAu(CN) ₂ +2NaOH+H ₂ O ₂

2Au+4NaCN+H ₂ O=2NaAu(CN) ₂ +2NaOH

The overall response is the same as the Elsler equation. Based on this, MaClaurin pointed out in 1895: "Oxygen is necessary for the dissolution of gold in cyanide solution. Without the participation of oxygen, there is no dissolution of gold. According to Elsler's equation, the dissolution of gold The ratio to the required amount of oxygen is 197:8.

The US Mines Bureau proposed some silver mineral leaching data in 1931, indicating that the use of oxygen or peroxide is significantly better than immersion only. These data are shown in Table 1.

Table 1 Effect of Oxygen on Silver Cyanide Leaching (US Mines Bureau 3064, January 1931)

NOTE: argentite; 2, sulfur, antimony, copper, silver; 3, brittle silver; 4 sulfur magnetic silver (Condition: 3 pounds of NaCN / t 23 ℃, solids concentration 25%)

The cyanidation data of these refractory silver minerals showed that the leaching effect achieved by only filling the air with the addition of about 5 lbs/t sodium peroxide (leaching time 24 hours) was comparable, the amount of peroxide added was increased, and the leaching rate was also Increasingly (increased by 15% to 25%), the oxygen is charged at atmospheric pressure, and the leaching rate of leaching for 24 hours is equivalent to the result of leaching for 48 hours when air is charged. The result of adding pure oxygen is very close to the result of adding 10 lbs/t NaO ₂ . One layer of meaning is clear, that is, a longer oxygenation leaching time may result in higher economic efficiency and leaching rate.

Third, the impact on the speed of gold leaching

Barsky et al. of the American Cyanamide Company conducted a basic study to determine the relationship between the leaching rate of pure gold in cyanide solution and the ratio of oxygen in the gas phase, and their conditions at atmospheric pressure and 23 ° C. These tests were carried out with a 10 cm ² gold foil. Figure 1 is a plot of the gold leaching speed (mg/cm ² /h) plotted against the percentage of oxygen in the air. The relationship between velocity and oxygen content in the leaching environment is linear. A linear regression equation: speed = 0.1258 × percentage of oxygen makes this curve have a high coefficient of relationship (R ² = 0.9989).

Figure 1 Gold leaching speed Barsky, Swuinson phase Hedley, 1954

Many studies have reported the ratio of the same ore using the same leaching equipment and operating conditions to leaching gold with oxygen and air. Many of Kamyr's papers suggest oxygen-enhanced cyanidation, vertical cyanidation, Oxygen pipeline reactors and other types of leaching processes are the subject of the latest papers. Improving the initial leaching speed is an important factor. Even if the final gold leaching effect can only be limitedly improved, some of this information is to illustrate this point. Published.

The data obtained for the enhanced cyanidation test of the belt re-election concentrate of the Welkom gold smelting plant in South Africa is shown in Table 2.

Table 2 Gold and silver leaching speed

If the 90% leaching rate is used as a basis for comparison, these data indicate that for gold or silver, the time required to use air is three to four times that of oxon. For gold, the time required to reach the final leaching with oxygen (4.5 h) is about 22% of the time required to use the air (20.0 hours). Similarly, for silver, the time required to achieve the final leaching with oxygen, 7.5 h is about 37% of the time required to use only air (20.0 h).

For gold and silver, the use of oxygen is 37% and 74% higher than the initial leaching rate of air alone. The significance of this is that for a given size of the leaching vessel, the daily treatment of the equipment when using oxygen as the oxidant The capacity will be 3 to 4 times that of air as an oxidant.

Fourth, the significance of speed increase

(1) Stirring cyanide

The advantage of rapid leaching is that for new installations, leaching with smaller vessels or shorter residence times does not have much of a problem with conventional leaching. The above conclusions will greatly reduce investment and production costs, but for factories in normal production, if the time taken to achieve optimal recovery when using air is not long enough, then the use of oxygen produces faster Leaching kinetics result in higher recovery. This can also be understood as achieving greater production capacity under current production conditions and, as a result, higher gold productivity.

The reduction in production costs can be achieved by increased production capacity, reduced energy costs, and reduced cyanide consumption. The savings in cyanide are not only a result of reduced leaching time, but also a result of the reaction of oxygen with potentially depleted cyanide.

Excessive residence time is a kind of “cushion” that makes the leaching rate more safe, and also causes recovery of the cyanide system due to process disturbances such as pulp concentration, pH, cyanide concentration or ore type change. The sensitivity of the rate loss is reduced. This excess or “cushion” provides the excess time required to replenish the process turbulence or abnormal production problems, and the potential for this improvement is important for the current stirred cyanide leaching circuit.

(2) Charcoal load

Some literature suggests that maintaining and controlling dissolved oxygen concentration and potential (Eh) in a carbon adsorption system will help increase the loading of gold on charcoal. Increases in adsorption rate, adsorption selectivity, and final loading have been reported in oxygenation systems.

In theory, this is partly related to maintaining the stability of the gold cyanide complex by controlling the potential and adjusting the ratio of the mixed metal complex system. These potential benefits, including the paper, are confirmed in terms of oxygen to improve the carbon loading. There is no hard data in it, but this possibility is interesting.

This is a promising area of ​​research for research institutions or suppliers of charcoal.

(3) heap leaching

Until now, various discussions have focused on conventional agitation cyanidation. It is also of great significance to improve the recovery rate of heap leaching, which is more significant than stirring.

In the various heap leaching of metal and uranium , the aeration or oxygenation of various mines is common and often used. In this case, where the oxidation of the sulfide begins to occur, the amount of oxygen required is equivalent to and often a limiting factor in the leaching process. The filling of the gas is carried out by a wind well or a wind gun extending from the top surface of the heap to the bottom of the pile. The management of the drilled hole is buried at the bottom of the pile, and the oxidized leachate is injected into the surface of the pile or injected into the pile. This is the method used in some technical trials or commercial applications. The ATEC ZX Oxygenation System can utilize any of these methods.

Bacterial activity plays an important role. For example, in steel heap leaching, the air blown into the heap is suitable to provide a temperature control method and participate in the metabolism of the bacteria. For heap leaching of gold, unless a large amount of sulphide is present, the amount of oxygen required is not very high, but maintaining a higher amount of oxygen through the heap will make cyanide leaching faster and more efficient, for energy consumption. The effect of doing so is very significant, as the leaching period is shortened, and the total recovery per unit time is increased, as the leaching period is shortened and the total recovery rate per unit time is increased, as the leaching of the oxygen in the liquid and thus the leaching of the oxygen and the leaching process of the gold and silver leaching process is carried out. Reduced consumption, which reduces production costs.

According to the latest report by Camille et al., oxygen has been used to improve gold heap leaching conditions and column leaching simulation industrial tests.

V. ATEC ZX Technology

The ATEC ZX Oxygen System is basically a columnar system. The oxygen and liquid streams (leachate or fresh water preparation) are introduced into the system at a time when the gas phase is effectively dissolved in the liquid phase. This process is carried out under pressure. The discharged liquid phase is a highly supersaturated solution of oxygen.

Oxygen enters the liquid phase and then converts to dissolved oxygen as a process solution. The conversion efficiency is very high (almost 100%). Compared with other conversion methods, the gasification tower, the blowing column, other types of microbubble dispersing devices, and agitation and aeration are used. Usually less than 50% oxygen utilization efficiency per segment. These systems sometimes require multiple stages or countercurrent configurations to achieve satisfactory oxygen and efficiency. Compared to these methods, the simple and highly efficient single stage ATEC system benefits significantly.

Figure 2 shows that the equilibrium dissolved oxygen concentration of pure water in contact with oxygen and a 500 ppm NaCN solution is a function of oxygen pressure. It has also been shown that the ATEC system can be used to increase the dissolved oxygen concentration (dissolved oxygen plus effective supersaturated oxygen). The Henry's law model can be used for calculations. Please note that the oxygen concentration of the leachate is 10% lower than that of pure water. This is due to the presence of dissolved components. The field measurements have been shown to be very consistent with the model's foresight.

Depending on the temperature, pressure and discharge environment, the available oxygen available through the ATEC system into the liquid phase may exceed 70% of the saturation concentration. The data is shown in Table 3.

Figure 2 Oxygen in solution

(oxygen, water, 20 ° C, NaCN 0.5 g / L)

Table 3 Available oxygen in water (ppm.20 ° C, 1 atm)

Sixth, ATEC system application

The aerated solution is ideal for heap leaching. (Because of supersaturation) some of the residual oxygen that is not absorbed by the process solution exists in the form of microbubbles. These microbubbles are relatively stable and do not aggregate into large bubbles. These remaining microbubbles are suspended in the liquid phase, along with the leaching solution. The oxygen is consumed and the microbubbles are easily absorbed by the leachate, thus allowing the leach solution to maintain a high oxygen content.

The potential for improvement in heap leaching operations is partially related to the oxidation of sulfides. For low permeability ores and/or heap leaching, high oxygen transmission forces are maintained in the leachate to improve dispersion control or dispersion. Limited leaching facilities are also important.

Stirring leaching is rarely restricted by dispersion limits, because the ore particle size is very small, the leachate is thoroughly stirred and mixed, and the resistance of the diffusion membrane is greatly reduced. However, sufficient reactant concentration (cyanide and oxygen) in the leachate is maintained. In order to increase the gold leachate rate) is still important, the ATEC system can provide dissolved oxygen to the stirred leaching system (using the supersaturation method). In the process solution, some of the oxygen will precipitate into microbubbles which are suspended in the slurry and added as a reserve in the stirred leach tank. No adverse effects on the slurry mixing system were observed or affected by the slurry mixing system.

Seven, ATEC process

The conceptual flow of the ATEC oxygen conversion technique for a stirred cyanide leaching system is shown in Figures 3a and 3b. The difference between the two is that the former provides a separate oxygenated leaching solution to each of the leaching tanks, and the latter uniformly adds an oxygenated liquid to each tank. The recycled leachate (de-sludged) or the formulated new leach solution can be used in the second process. In both processes, the highly oxygenated liquid prepared by the ATEC ZX unit is fed to a stirred leaching tank to be thoroughly mixed with the slurry.

Figure 3a Corresponding relationship between the ATEC system and the agitated leaching tank (each leaching tank leaching solution is circulated separately)

Figure 3b Single ATEC system with agitation leaching (single ATEC system connected to all leaching tanks)

In order to maintain the oxygen level in the leaching tank at 20-25 ppm, the circulating flow volume is required to be less than 10% of the tank volume, so that for a leaching tank with a residence time of 4 hours and a volume of 100,000 gallons, The circulating flow rate of the ATEC system is only 40 to 50 plus/min. If a formulated solution is used, the slurry concentration gradient throughout the system is strictly planned in order to accommodate the new solution.

A similar conceptual flow for the ATEC system for heap leaching is shown in Figure 4. The circulating leachate passes through the ATEC system in whole or in part. When all the leachate passes, the heap leach system can be designed to operate at a lower pressure. In this case, a distribution device and a drip device are required. The high pressure ATEC system can be used to treat part of the leachate. The high concentration oxygen leaching solution is mixed with the remaining leaching solution before being poured into the heap. Partial leachate at high oxygen concentration can also be mixed with the remaining leach solution. Partial leachate at high oxygen concentration can also be separated from the remaining slope and added to the heap separately to provide a positioning oxidant on demand. This is also possible.

Figure 4 ATEC ZX system with Ding heap dipping (new water prepared into a circulating leaching solution)

Existing solutions for desliming, dispensing, mixing, and oxygenation of existing solutions can be applied without the need for process research or production expansion studies.

Brief Introduction