White coal crushing machine Nigeria
Screening process is defined: On the fenestrated screening surface, materials are classified according to its size. The product of screening process is fraction. Every fraction is identified by maximum and minimum size of the products, like 50~25 mm、25~13 mm, etc. The used machine in this process is screening machine (or sieve). There are many kinds of sieves, and according to the working principle, they can be divided into: gravity flow screen, revolving roll picker, shaking screen, trundle screen (drum sieve). The workpiece of screen is screening surface. In the screen, if the materials are bigger than sieve pore, this size of materials is called as oversize; smaller, belowsize.
Crushing process: By the action of external force, coal are crushed to ore particles of need size. According to the size of crushed products, we can divide the crushing process into rough process, secondary crushing, fine crushing, and crashing process. Generally, in rough crushing process ores are crushed to size above 50mm; secondary crushing, 25~6mm; fine crushing, 6~1mm; crashing process, below 1mm (which is also called grinding process). In rough coal crushing process, single toothed roller crusher, double roller crusher and jaw crusher are used. In secondary crushing and fine crushing process, hammer-type crusher, impact crusher, grinding mill, cage-type crusher are used. Ball mill is used in the last process. The ratio of raw materials’ biggest size before crushing to products’ size is called as crushing ratio. Commonly, when crushing ratio is below 6 (it can be bigger for fine crushing and crashing process), one-stage crushing process is adopted, and otherwise two or multiple stages of crushing process should be adopted.
According to diversity between various components’ densities, surface physiochemical properties and other properties, products containing different ash content are divided from coal. The main task of coal dressing process is to remove nonorganic minerals in coal, reduce ash and sulphur content of coal, improve coal’s quality. Many products can be produced, which can be applied to many fields, so as to improve using value and economy benefits of coal.
Ore Dressing Technology
Gravity concentration is still the main method for coal dressing draft. Gravity concentration makes use of the difference in density and consistency of coal. In gravity ore dressing plants, different coal has different acting situation, so they are separated from each other.
Generally water is made as the medium in coal jigging. In vertical-lift variable-speed stream coal is separated as density. Jigger is adopted to finish this process. Coal jigging is the most widely-used ore dressing draft so far, because of its features like simple plants, large output capacity, etc.
Dense Medium Coal Dressing Draft
(1) Basic Principle
The definition: Separate raw coal in gravity medium, of which the density is bigger than water. There are two kinds of mediums: heavy fluid and dense suspension. Heavy fluid is aqueous solution of various solvable high-density salts, like aqueous solution of zinc butter or some high-density organic solution (like chloroform, carbon tetrachloride, etc.). Adding high-density mineral fine powder (like magnetite, loess) into water, so dense suspension is made, like the mixed liquor in suspended state formed by magnetite.
(2) Dense Medium Coal Dressing Process
It is the most advanced gravity concentration of coal. The main advantages are:
- ① It can separate strictly on basis of density, and the separating efficiency is high;
- ② It can effectively separate coals which are hard and extremely hard to separate.
- ③ It has strong adaptability for raw coal, and in production it is slightly influenced by the fluctuation of coal quantity and raw coal’s quality.
- ④ The size range of feeding materials is wide, which can top 500～1000mm. Therefore manual separation can be replaced so as make mechanical working of the whole coal dressing process.
- ⑤ Dense medium dressing plants has simple structure and convenience adjustment in production operation and technological process.
According to the difference in surface physiochemical properties of materials, flotation separation can make coal separated from gangue. Flotation separation is widely used in concentration process of coal slurry and various useful minerals. Coal slurry is a kind of gunk produced in the washing process of coal powder, of which the size is smaller than 0.5mm. There are two main sources of coal slurry in coal dressing plant, one is raw coal slurry, which is generated in mining and transportation of the chosen raw coal, the other is some secondary coal slurry, which is newly produced in various crushing processes of coal dressing period. All these totally occupy about 15％～30％ of the feeding materials. For various gravity coal separations, their effective lower limit of separation can be about 0.5mm or smaller. However, flotation is the most effective method to select fine coal slurry.
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