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This method is mostly used in South Africa. Many types of filters can be used for the filtration washing of cyanide pulp. Vacuum filters that can perform continuous operations are often used, and filter presses are less used.
In continuous vacuum filters, rotary cylinder or Feinc vacuum filters are most commonly used, because these two types of filters can effectively be used from concentrated slurry (containing 40% to 60% solids) or sometimes The fine particles are separated from the cyanide solution containing a small amount of solids, and it is convenient to wash the solid material. The disc type vacuum filter has the advantages of low investment, small floor space, etc., but the pulp is easy to form agglomerates thereon and affects the washing effect, and the filter cake is not easy to be discharged, so the use is less.
Another continuous vacuum filter worth mentioning is a belt filter. Belt filters have been used for at least 35 years in the paper and sugar industry sectors and have been successfully used for the filtration of cyanide pulp for nearly 20 years. Today, some cyanide plants in South Africa have adopted 60m 2 and 120m 2 belt filters, which consist of ordinary steel or stainless steel frames, drive wheels, tail wheels and reinforced rubber conveyor belts with horizontal discharge grooves and drain holes in the center of the groove. Composition (Figure 1). The conveyor belt is mounted between the drive wheel and the tail wheel and is supported by an air cushion driven by a variable speed motor. Below the belt are a vacuum box, a vacuum-sealed wear belt and a flexible filter drain hose. The conveyor belt is a grooved belt or pasted on both sides. When the belt passes the tail wheel, the rubber flange is flattened, and the filter cloth is in contact with the conveyor belt and is fixed by vacuum suction. The slurry is supplied to the filter cloth which is closely attached to the conveyor belt through the slurry distributor. The vacuum suction filter causes the solution to enter the vacuum box through the drain hole on the conveyor belt through the drain hole and then discharge into the storage tank. The separation crucible divides the belt filter into three zones, namely a (vacuum) suction filtration zone, a washing zone and a suction zone. The filter cloth belt (reverse) is separated from the conveyor belt by the drying zone and the driving wheel. After the filter cake is removed by the discharge roller, the filter cloth belt passes through the jet washer, the tensioner and the automatic ranger system, and again on the tail wheel. Continuous automation is combined with the conveyor belt. In order to grasp the condition of the filter cloth in time, South Africa developed a turbidity meter, and the turbidity of the filtrate was measured by the turbidity meter, so that the operator can know the working condition of the filter cloth at any time.
Figure 1 Schematic diagram of the belt filter
1-pulp and slurry distributor; 2-washing water dispenser;
3-drive wheel; 4-polar belt; 5-pole filter cloth; 6-tail wheel
The belt filter can be used for multi-stage filtration and washing without re-pulping, and the processing capacity is 1 to 3 times higher than that of the cylindrical vacuum filter. The filtered precious liquid can be returned to the washing to obtain a rich liquid, or it can be returned to the washing. The filter residue can be drained into a dry filter cake, and the wet tailings can also be drained. Although the belt filter investment in infrastructure, high maintenance costs, and requires careful operation and occasional loss of a large number of disadvantages such as expensive solution, but its low energy consumption, high efficiency, easy replacement of the filter cloth, which is expected in the production of gold A lot of adoption.
Among the filter presses used abroad, the best performance is the circular filter disc hydraulic press installed on the horizontal axis in the United States and the axial flow (candle) filter press in South Africa. Since they are all automated, they replace the plate and frame filter press.
In foreign countries, some old factories are still using outdated Moore or Butters type intermittent frame filters.
The filtration of the vacuum filter feeds the leached slurry into the filter. Under the suction of the vacuum pump, the gold-containing solution passes through the filter cloth, and the solid material is tightly precipitated on the filter cloth to form a filter cake. In the vacuum filtration of cyanide pulp, the suction of the filter can destroy the flocculation in the filter cake, and the undissolved gold is often dissolved (especially the mud slurry). Therefore, it is important to wash the filter cake. According to the production practice experience, when washing the filter cake with dilute NaCN liquid, washing with the washing liquid with the same amount of liquid in the filter cake, 80%-85% gold-containing solution can be washed out from the filter cake; if the filter cake is used instead A washing solution containing twice the amount of liquid washes out 98% of the gold-containing solution from the filter cake. Under normal circumstances, the filter cake should be washed several times. The first addition of dilute NaCN solution (or lean liquid), the filter cake is adjusted to 50% concentration of the slurry, washed and filtered, and then washed with water to discard the tailings.
Since the concentration of the slurry is usually low (about 30%) during cyanidation, in order to improve the processing capacity and filtration efficiency of the filter, a first-stage thickener is often added in front of the filter, and a certain amount of polypropylene is added to the thickener. The acyl gel flocculant makes the concentration of the filter to more than 55%. Figure 2 shows the concentration and two-stage filtration washing process of a cyanide slurry in a cyanide plant in China. The total wash recovery of the plant using this process gold is 98.27%.
Figure 2 Two-stage filtration and purification process of a cyanide slurry in a plant
Batch filters are commonly used to treat liquefied cyanide pulps that are difficult to filter because they can wash the filter cake for extended periods of time. However, due to its low production capacity and the use of many equipments, the plant covers a large area and is less used.
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