Coal washing belongs to the physical processing method of coal. Its basic principle is to separate the components (clean coal, medium coal, gangue, pyrite) in the raw coal according to their different physical and chemical properties. These physical and chemical properties are mainly density, hydrophilicity, hydrophobicity, color, hardness, shape, electromagnetic properties, friction coefficient, elasticity, etc. When the separation medium used in coal preparation is a fluid, it is called wet coal preparation, or simply coal washing. Coal preparation methods include jigging coal preparation, heavy medium coal preparation, trough selection, cyclone (centrifugal field) coal preparation, shaking table coal preparation, flotation coal preparation, and air selection. The determination of the coal preparation method depends on the grade of coal, selectivity, and user requirements for product quality, and also on technical and economic rationality and the capacity and supply of equipment manufacturing in my country.
1.Coal selection of lump coal
Before coal washing, large pieces of gangue are manually picked out by a flat belt conveyor at a speed of 0.2~0.3 m/s according to the differences in color, gloss and appearance between coal lumps and gangue. Gangue can also be picked out by electric gangue picking, friction sorting and other methods.
2.Wind coal separation
Wind coal separation does not require coal slime treatment and water supply equipment. Therefore, areas with drought and water shortage or long cold winters can consider using wind jigs or wind elimination discs for wind separation. Its separation efficiency is poor and it is only suitable for easy-to-separate coal.
3.Wet coal preparation
Since the density of bituminous coal is 1.5 and the density of gangue is generally greater than 1.8, the density difference can be used to separate them in a fluid medium. This coal preparation method is the most widely used and has many types, including: jigging coal preparation, chute coal preparation, heavy medium coal preparation, cyclone coal preparation and shaking table coal preparation.
4.Jigging coal preparation
Jigging coal preparation is the most widely used method in coal preparation production practice at home and abroad. Although heavy medium coal preparation has emerged since the 1950s and its application scope has been continuously expanded, jigging coal preparation still occupies a dominant position.
Jigging coal preparation is suitable for processing easy-to-select and medium-selectable coal. Its separation particle size range is relatively wide, with an upper limit of 50~100 mm, or even larger, and a lower limit of 0.5~0.3 mm. Its advantages are simple and easy process flow, large production capacity, convenient maintenance and management, high quantitative efficiency and clean coal quality index for washing easy-to-select and medium-selectable coal, and low coal preparation cost. When processing difficult-to-select coal, its process index is second only to heavy medium selection. Jigging coal preparation has strong adaptability to the properties of raw coal, and the quantitative efficiency of easy-to-select coal can reach more than 90%, and the quantitative efficiency of difficult-to-select coal can reach 70~80%. Therefore, areas with conditions should try to use jigging coal preparation.
5.Coal separation by chute
The coal separation chute is used to separate coal and gangue by using the difference in speed of movement in the inclined water flow. In order to reduce the influence of particle size on separation, raw coal is usually washed in different chute according to different particle sizes. The medium block size of 100~10 mm is separated by lump coal chute, the small block size of 10~0.5 mm is separated by fine coal chute, and the coal dust of 0.5~0 mm is separated by coal slime chute.
The production capacity of the chute is determined by the width of the chute and the selectivity of the coal. If the chute is too narrow, it is easy to get clogged, and if it is too wide, it is difficult to evenly distribute the material and water along the chute. Therefore, the width of the lump coal chute is limited to between 350 and 800 mm. Calculated per centimeter of chute width, the production capacity of the lump coal chute is: 1.6 to 2.0 tons/hour for easy-to-select coal, 1.3 to 1.6 tons/hour for difficult-to-select coal, and 1.0 to 1.3 tons/hour for difficult-to-select coal.
Although the efficiency of chute coal preparation is not as high as that of jigging coal preparation, it has the advantages of simple equipment, low power consumption, large production capacity, and low infrastructure investment. It is suitable for processing anthracite, power coal, and easy-to-select and medium-selectable coking coal. The amount of raw coal washed by this method in my country ranks second.
6.Heavy medium coal separation
A solution or heavy suspension with a density between clean coal and washed slag is used as a separation medium to separate clean coal from medium coal and gangue. In the process of heavy medium coal separation, the suspension is required to have a stable density and low viscosity.
Heavy medium separation is the most advanced coal separation method at present, with the advantages of accurate separation accuracy and high quantitative efficiency. Therefore, high-quality clean coal and better separation indicators can be obtained. The particle size range of separation is wide, and the upper limit can reach 300~500 mm, or even larger. The lower limit can also reach 0.3~0.5 mm when separated in a centrifugal field, which can effectively separate difficult-to-separate coal and extremely difficult-to-separate coal. Most of my country’s coal belongs to difficult-to-separate coal and extremely difficult-to-separate coal, and heavy medium separation can achieve good separation results. At the same time, heavy medium separation still has the advantages of simple production operation and process adjustment, and it is easy to realize the automation of coal preparation plants. Among them, heavy medium separation of lump coal is more common, and heavy medium separation of fine coal mainly uses heavy medium cyclone. Production practice shows that the efficiency of the inclined wheel separator in separating lump coal can reach about 95%, while the efficiency of the fine coal cyclone is about 90%.
7.Coal separation by cyclone
Fine-grained and low-density materials can be separated more effectively in the centrifugal field caused by the rotating flow than in the density field. The heavy medium cyclone uses this principle for separation.
The heavy medium cyclone is a device for separation in the centrifugal field, so it greatly enhances the separation of ore particles and breaks through the limit of 13 (or 6) mm for the separation of heavy medium coal, thus becoming a limited device for separation of fine coal of 13 (or 6) ~ 0.5 (or 0.3) mm.
The water medium cyclone is easy to separate fine coal and can also obtain clean coal products with low ash content. It can effectively separate oxidized coal with a particle size lower limit of 100 mesh and remove pyrite sulfur below 28 mesh. However, as an independent separation equipment, the water medium cyclone cannot obtain pure gangue and high-quality clean coal products at the same time. Therefore, the water medium cyclone can be used in combination with a shaking table. The water medium cyclone is used as a roughing equipment to achieve a high-density separation in one step. The water medium cyclone equipment is compact, the system is simple, the infrastructure investment is low, the operation is convenient, and no chemicals or media are required, thus greatly simplifying the medium recovery system.
