Percolation leaching

The diafiltration cyanide leaching process is generally suitable for the treatment of ore from -10 mm to +74 μm, coarser calcine and other loose porous materials. It is most suitable for processing raw materials containing clay , slime, excessively fine grinding and uneven size of ore particles. When the material with a large particle size is used for the percolation method, it must be sieved first, and the ore particles are classified and then separately leached. The raw material containing the slime or excessively finely ground is usually treated by a stirring leaching method, or sieved, and the fine mud and the fine ore are sent to be stirred and leached.

The percolation leaching method is a simple method for cyanide gold extraction. The method has low solvent consumption, power saving, simple equipment, and the cyanated pulp does not need to be concentrated or filtered, and is widely used in small mines at home and abroad. The large percolation leaching operations reported have been mainly applied to South Africa. However, due to the long working time of the percolation method, the equipment covers a large area, and the ore after the cyanidation is not sufficiently washed, so the recovery rate of gold is low.

I. Percolation leaching equipment

The main equipment for percolation leaching is the percolation leaching tank (Fig. 1). The leaching tank usually has a false bottom, and the bottom of the groove is slightly inclined (about 0.3%). The shape of the groove is cylindrical, rectangular or square. The structure can be wood, concrete or low carbon steel. The size of the tank used in small mines is about 5~12m, the height is 1.5~2.5m, and the ore is 75~150t per batch; the smaller scale percolation tank is 15~30t per batch. The large-scale percolation leaching tanks in foreign countries have a diameter of more than 17m and a height of 3m. Each batch can be processed for more than 1000t.

Figure 1 Percolation leaching tank

1-tank; 2-cement lining; 3-mineral layer; 4-false bottom; 5-outlet

The false bottom is about 100 to 200 mm from the bottom of the groove. The structure of the false bottom is usually composed of square wooden strips, and a filter layer such as canvas, sack or mat which can prevent the mineral sand from filtering and facilitating the infiltration of the gold-containing solution is laid on the lattice. The leachate flows out through the outlet tube. Some percolation leaching tanks are overhead and have working doors in the center of the bottom of the tank for unloading tailings.

In addition, there are sand pumps for conveying pulp, mining machinery and machinery for discharging tailings.

Second, the percolation leaching operation method

The diafiltration cyanide leaching operation includes several steps of charging, adding cyanide leaching, and discharging tailings.

(1) Loading

The pretreated, sieved ore is loaded into the leaching tank, and the dry charging method and the hydraulic charging method are separately performed. The dry charging method can be used manually or mechanically, and is used in many small mines after manual charging. Although this method is laborious, the ore layer is loose and uniform. Mechanical loading is usually carried out by a belt conveyor to the spreading disc installed in the center of the tank. The surface of the disc is provided with radial ribs, and the ore is sprinkled into the tank by the centrifugal force of the rapid rotation of the disc. The disadvantage of this method is that segregation is easy to occur in the ore sand, which makes the charging uneven. The use of dry charging, although air is present in the layer, is advantageous for increasing the leaching rate of gold. However, when used for wet grinding of ore, it is necessary to perform dewatering in advance to increase the complexity of the operation. When the dry feeding method is used, the lime is uniformly added along with the ore.

The hydraulic loading method is to dilute the slurry and then use the sand pump or the launder to feed the slurry into the tank. At this point, the ore is sunk, and excess water and part of the ore are discharged through the circulating overflow channel. After the loading is completed, the water in the ore is exuded from the false bottom and discharged through the liquid discharge pipe. The hydraulic loading is mostly applied to large-scale mines produced throughout the year, which increases the moisture in the tank, and the air present in the ore is less, and the dissolution rate of gold is slower.

(2) Percolation cyanide leaching

There are two ways to supply cyanide solution into the tank containing ore: one is that the cyanide solution permeates the ore layer from top to bottom by gravity; the other is that the cyanide solution is under pressure from the bottom. Overfiltrated the ore layer. Before use, the ore slurry is easily deposited on the filter cloth by the solution, thereby reducing the percolation rate; although the post-use method can overcome the above disadvantages, it is necessary to increase the mechanical equipment and power consumption. However, because the former method is simple and economical, the general method uses the former method.

The percolation operation mainly controls the percolation rate of the cyanide solution. Under normal circumstances, the falling or rising speed of the solution level should be 50-70 mm∕h. The factors affecting the percolation rate mainly include the size and shape of the ore particle size, as well as the thickness of the charge and the uniformity of the charge. When the percolation rate is too large, it may be caused by segregation of the ore particles or uneven thickness of the layer. When the percolation rate is too small, the pores of the filter cloth are blocked due to the precipitation of the slime and calcium carbonate. For this reason, the filter cloth should be sprayed with water regularly during production, and the calcium carbonate precipitated in the pores of the filter cloth should be washed away with dilute hydrochloric acid after the operation.

The diafiltration cyanide operation is different in the way of adding and releasing the cyanide solution. It is divided into two methods: batch method and continuous method. The continuous method is to continuously inject the cyanide solution into the tank, and keep the liquid level slightly higher than the ore material. Under the premise of the surface, the gold-containing solution is continuously discharged from the liquid discharge pipe. The batch method is to first inject a batch of concentrated cyanide solution (containing NaCN 0.1% to 0.2%) into the tank for 6~12h leaching, then release the first batch of gold-containing solution, and let the ore soak in the cyanide-free solution. Allow to stand for 6 to 12 hours under conditions to allow sufficient air inhalation between the ore particles. Then add a batch of medium concentration cyanide solution (containing NaCN 0.05% to 0.08%) and a third batch of dilute cyanide solution (containing NaCN 0.03% to 0.06%). The three batches of cyanide solution were leached for 6 to 12 hours, and the first and second batches of gold-containing solution were allowed to stand for 6 to 12 hours after being released. After the third batch of gold-containing solution is released, the tailings in the water washing tank are added. Since the ore that is intermittently worked can be saturated with air intermittently, that is, the solution contains more oxygen, it is about 2.5% higher than the continuous method.

In recent years, the percolation cyanide leaching method has been developed into a cyclic percolation method in some factories. The cyclic percolation operation was carried out in 5 percolation tanks (Fig. 2), the leaching period was over 5 days, and the amount of cyanide solution added was controlled by the first tank. The total operation time of each batch of raw materials is up to 9 days.

Figure 2 Continuous leaching operation and solution flow direction in a percolation leaching plant

(3) Unloading

The discharge of tailings is also divided into dry and hydraulic. Dry unloading has the use of artificial, but also the use of buckets. When there is a work door at the bottom of the tank, a hole can be made from the top with a rod, and the tailings are thrown into the hole to be released. The hydraulic discharge is to flush the tailings into the tailings ditch with high-pressure water (147.1～294.2 kPa, ie 1.5～3kg∕cm ² ), dilute with water, or pump it to the tailing mine with sand.

Third, percolation leaching technical and economic indicators

In the diafiltration cyanide operation, the total time for processing a batch of raw materials often takes 4-8 days due to the nature of the ore sand, the percolation rate, the loading and unloading efficiency, and the amount of cyanide solution used. When treating ore with poor particle size classification or containing slime, it sometimes takes 10 to 14 days.

The gold extraction rate of the percolation leaching method depends on the size of the gold particles, the size of the crushed ore, the content of the sulfide, the percolation rate, the leaching time, the concentration and amount of the cyanide solution, and the degree of washing of the tailings. When the gold-bearing quartz ore processing with filtration assay, the gold extraction rate of 85% to 90%; but crushing and grading is not good enough size, the gold extraction rate will be reduced to 70% or even 60%, if Coupled with other factors, the extraction rate of gold will also decrease.

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