Dewatering of flotation clean coal

      1、 Main factors affecting filtration effect

Filtration and dehydration of floatation foam products is an extremely complicated process, which is affected by many factors. These factors include the following aspects:

Feed size composition

When the particle size composition of the feed is uniform and the average particle size is coarse, the filter cake with lower moisture content can be obtained. This is because the feed particle size composition is uniform, and the gap composed of coarse particles can be filled with appropriate fine coal. The filter cake formed in this way is beneficial to the water penetration, and does not cause the air permeability of the filter cake to be too good and the vacuum degree inside the filter fan to drop. Coal particles adsorb water on its surface. The total surface area per unit weight of slime with coarser average particle size is smaller, so less moisture is carried on the surface of unit weight slime after filtration, so the products with lower moisture content can be obtained.

If there is a large amount of fine particles in the feed (less than 120 mesh, accounting for more than 80%), properly adding some coarse slime with low ash content (such as coarse slime after being graded by cyclone at the bottom of pyramid sedimentation tank) can improve the filtering effect and reduce the moisture content of filter cake. However, the amount of coarse coal slime mixed should not be too much, and the particle size should not be greater than 2 mm, otherwise the material will be easy to layer and precipitate in the filter tank, which will make the filter not work normally. When the content of very fine particles (less than 200 mesh) in the feed is large, especially when the content of impurities in the fine mud is large, the filtration effect will be significantly deteriorated. It is shown in the following aspects: the filter cake becomes thinner and the moisture is larger, the filter cake is difficult to unload, and the solid content in the filtrate is more.

2. Feed concentration

Under the condition that other conditions remain unchanged, thicker coal cake can be deposited on the filter cloth by increasing the feed concentration of the filter, which is very intentional to improve the processing capacity of the filter. Due to the increase of feed concentration, the amount of filtrate separated from the coal cake with the same thickness is less, and the suction filtration time required to form the necessary thickness filter cake is short. The solid particles in the filtrate are mainly brought in by the infiltration of water in the slurry when the filter fan is in the filtration zone, so the solid loss in the filtrate decreases with the increase of the feed concentration. In addition, increasing the feed concentration of the filter is also conducive to increase the stability of the slurry in the trough, and weaken the stratification effect of coarse particles in the slurry, which is particularly important for Pulp Filtration with high content of coarse particles. However, when the feed concentration is too high, the filter cake formed is too thick and the filtration resistance is too large, resulting in poor filtration effect and increase of cake moisture; therefore, the feed concentration should not be too high. When the feed concentration is 350-400g / L.

3. Pulp viscosity

The effect of pulp viscosity on filtration effect is obvious. If the pulp viscosity is large, the thickening speed of filter cake will be greatly reduced, and the filter cake will become thinner and the dehydration effect will be worse. The factors affecting the viscosity of flotation foam products are mainly the content of fine mud in foam products and the performance of flotation reagents. When the content of fine mud in the filter feed is high, the slurry viscosity increases. At this time, not only the filter cake is thin, but also the filtration resistance is increased due to the presence of a large amount of fine mud in the filter cake, resulting in a large amount of moisture in the filter cake. The frother used in the process of coal slime flotation, especially the foaming agent with high viscosity (such as coal tar), also has adverse effects on filtration. Because they often generate stable and firm foam, which not only improves the viscosity of the filter feed, but also increases the difficulty of dehydration when the foam is adsorbed on the filter fan. The use of agents which can produce brittleness and easy defoaming has little effect on the filtration effect. When selecting and using flotation reagents, we should choose frothing agents which have no adverse effects on the dehydration of foam products.

The content of fine mud in flotation clean coal can be affected by the concentration of slurry and the amount of foam scraping, which should be paid attention to in production. In the process of operation, the content of fine mud in flotation clean coal should be reduced as much as possible to create good conditions for its filtration and dehydration.

4. Vacuum degree

The higher the vacuum degree, the larger the filter capacity and the lower the moisture content of filter cake. Generally, the vacuum filter should work under the condition of 400-500mmhg. At present, the vacuum degree of most vacuum filters can only be maintained at about 200mmhg, resulting in low processing capacity per unit area and high moisture content in filter cake. There are many factors that affect the vacuum degree of the filter. In addition to the working condition of the filter system, it is also related to the operating factors such as the level of slurry in the tank. The following discussion is about the measures to improve the vacuum degree of the filter.

1) The maintenance and overhaul of vacuum pump should be strengthened to ensure that the vacuum pump has enough vacuum degree and air pumping capacity, and the filter can reach 0.6-0.8 m3 / m2 · min air extraction volume and more than 400 mmHg vacuum degree. The main reason for the low vacuum degree is the insufficient air pumping capacity of the vacuum pump, and the wear of the relevant parts of the vacuum pump is the main reason for the insufficient air extraction. Therefore, the vacuum pump parts must be kept under the technical conditions. Experience shows that it is better to use softened water to cool the vacuum pump; if hard water is used, the dirt accumulated in the pump should be removed regularly. As the cooling water is the working medium of water ring vacuum pump, the cooling water added must be appropriate. Too much or too little will reduce the pumping capacity of vacuum pump. The temperature of cooling water should be about 30 ℃.

2) Keep the vacuum system in good sealing condition. There should be no air leakage at the joint of the pipeline and the connection between the hollow shaft of the filter and the filter fan. The gas-water separator with automatic water discharge device should be cleaned regularly and maintained in time to keep the valve open and close flexibly.

3) Adjust the friction plate of the distribution head frequently, and replace the seriously worn parts in time.

4) Clean the filter cloth and filter fan frequently, and repair the damaged filter in time.

5) The pulp liquid level in the filter tank should be kept at a certain height, so as to avoid that the pulp can not be complete when the filter fan is in the filtering area

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