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China is the world's largest producer and consumer of coal, coal accounted for 75% of primary energy consumption structure, while coal consumption structure, for combustion of coal accounts for more than 80%, and this situation is quite long in the future It will not change during the time. However, by the end of 1997, the proportion of raw coal in China was only 25.73%, which caused serious environmental pollution and waste of resources caused by coal burning. Therefore, we must accelerate the development of clean coal technology and protect the environment. Coal preparation technology, which is the source of clean coal technology, should be given priority.
However, more than 2/3 of China's coal recoverable reserves are distributed in the western part of severe drought and water shortage. It is impossible to adopt the current wet method of large-scale water separation, which seriously restricts the development of coal preparation technology. Therefore, research and development of new high-efficiency dry coal preparation methods is not only an urgent task in China, but also has international significance because water shortage is global.
The basic principle of gravity sorting is that the selected materials are sorted according to their different settling velocity in the gravity field or the centrifugal force field. The smaller the density of the sorting medium, the greater the sedimentation velocity of the different density materials is affected by the particle size, and the material must be divided into very narrow particle grades to obtain effective sorting. To improve the effectiveness of dry sorting, it is necessary to try to increase the density of the sorting medium. The dry sorting study of air heavy medium fluidized bed is based on this theoretical basis.
The fluidized bed sorting method proposed by JMBeeckmans of Canada, J. Laskovski of the Soviet Union, and E. Douglas of the United States has improved the density of the sorting medium, but only applied the material replacement principle of the chemical fluidized bed chemical reactor, and did not involve aggravation. The degree of plasmid gradation and the distribution of the second weighted particles were not studied.
In the distribution characteristics of coal sorting, the macroscopic backmixing of the aggravated mass is large, the sorting precision is low, and the dynamic stability of the fluidized bed density in the continuous sorting process is not involved, and it is still in the laboratory research stage.
Gas-solid fluidized bed genus air dense medium dense-phase fluidized high density, heavy medium generally using higher density of magnetic iron ore, in a fluidized bed expansion ratio of about 8%, the bed density is substantially equal to the density separation, And has fluid-like properties. In a fluidized bed having a certain density and being uniformly stable in a three-dimensional space, the selected materials are basically layered according to the Archimedes principle according to the Archimedes principle, thereby achieving effective sorting.
However, ordinary air heavy medium fluidized bed can not achieve effective sorting of full-grain coal, because the selected materials should be effectively sorted, and the minimum particle size must be sufficiently larger than the dominant particle size of the solid phase to make the fluidized bed as a whole. Density acts on the selected material. The lower limit of the separation particle size of the air heavy medium fluidized bed is 6 mm, and the effective separation of <6 mm fine coal cannot be performed. Therefore, we must not only study for different size fractions of coal sorting of Dense Phase High Density steady flow of separation theory, but also to study and solve moist fine coal dry-depth screening this worldwide problem, because as mining continuously improve the level of mechanization of coal, fine coal moisture content of raw coal increased year by year.
Therefore, the dry depth screening of coal (Φ6mm) can not only produce coal products of different grades such as high-quality power pulverized coal, but also provide conditions for dry sorting, and finally establish the theory of efficient dry sorting and screening. system.
2 Theoretical study on probability screening and fluidized bed sorting
2.1 Probability screening theory
Dry depth screening of moist fine coal is a difficult point in today's world screening technology. When dry coal is screened for wet coal (sieving size is equal to the size of the mesh), there will be a phenomenon that the slime clogs the mesh hole, resulting in a high rate of lowering, low screening efficiency, and no screening in severe cases. The issue is not resolved. In this regard, this study breaks through the traditional precision screening concept and the single particle permeable screening probability theory, and takes the lead in exploring the problem of wet fine-grain coal dry screening by the approximate probability screening theory.
(1) It is proposed to use large mesh holes (relative screening particle size) to solve the problem of coal plugging, using large dip angle to control the screening granularity, using the optimal screen surface length to improve the screening accuracy, and proposing the coal probability screening method. It provides an effective way for dry screening of moist fine coal.
(2) In order to reveal the mechanism of plugging of coal slime during the screening process, through the experimental and theoretical study on the permeability of wet fine coal in the screening process, it is found for the first time that the screening distribution curve caused by coal slime adhesion is The important phenomenon of abnormal uplift in the fine-grained interval establishes a mathematical model of probability screening described by a piecewise power function. To provide a theoretical basis for the successful interpretation of the characteristics of wet fine coal which is not easy to block holes during probabilistic screening.
(3) Based on the screening test of materials, the operation law of the particle group on the sieve surface was studied, and the distribution model of the probability of the particle group passing through the sieve surface length was proposed, namely P=1-exp(1-ALB). (In the formula, P is the permeability probability, A and B are parameters, and L is the screen length). The sieving process of the granules on the sieve surface is decomposed into two parts: the catenary and the sifting. The concept of the probability of the netting is proposed, the calculation of the sifting probability of the granules is solved, and the probability theory of granule permeable screening is established. The theory of single particle permeable screening probability proposed by Chinese and foreign scholars reveals the essence of the material screening process.
(4) For the different types of raw coal into the sieve, the process, structure and kinetic parameters of the coal using the probabilistic grading sieve were studied. The mathematical parameters of the sieved material properties, operating parameters and screening effects were established to carry out the parameters. Optimization, proposed by the general efficiency EC=α+[(β-α)/(β-θ)](100-2θ) (where α, β, θ are respectively the input, the sieve, the sieve The content of fine particles smaller than the specified particle size is used as an evaluation method for the approximate screening effect, and a calculation formula for separating the particle size and possible deviation is derived.
(5) The mechanical model and dynamics analysis of the string probabilistic sieve are proposed, and the design theory of the string probabilistic sieve is established, so that the stringer wire works in the resonance state, the screening machine can adopt smaller acceleration, and the string The screen can obtain a large acceleration, the screen is not easy to block the hole, and the power consumption is small, and the working life of the screening machine is long.
2.2 Fluidized bed sorting theory
The air heavy medium fluidized bed suitable for coal sorting is a dense phase high density fluidized bed, which is very different from the dilute phase low density fluidized bed widely used in the chemical industry, and the dilute phase low density bed applied in chemical industry. The bed is deep, the solid phase weighting plasmid is wide, and the back mixing is more serious. The fluidized bed suitable for coal sorting requires dense phase and high density microbubble fluidization with dense and narrow solid phase weighting. bed. Since the selected materials enter the fluidized bed and are stratified according to the Archimedes principle according to the bed density, the dense phase high-density fluidized bed not only requires the density to be uniform and stable in the three-dimensional space, but also requires aggravation of macroscopic back-mixing to avoid The sorted materials have been remixed. This is very difficult in theory and practice.
(1) To form a fluidized bed with uniform density, we must first solve the problem of wind distribution. Through careful theoretical analysis and experimental research, it is proposed to stabilize the air distribution with a two-stage composite large pressure drop gas distributor. Under the guidance of the theory, the gas distributor developed under the guidance of this theory achieves the purpose of large opening ratio, uniform distribution and easy control of gas flow rate, in order to form a dense phase suitable for coal sorting. The high density microbubble fluidized bed provides the basis.
(2) To achieve efficient separation of coal by dense phase high-density fluidized bed, to ensure high sorting accuracy, the bed density must be uniform and stable, and the heavy mass backmixing should be small, which requires low flow on one hand. Operating at a gas velocity, reducing the heavy mass backmixing, on the other hand, avoiding the grading and grading tendency of the weighting due to the low gas velocity, ensuring that the bed has sufficient activity and a small viscosity. For this reason, we put forward the idea that the weighting is dominant, the grain size is narrow (the screen ratio is only 2), the operation is performed at low gas flow rate, and the interaction effect of the macro-remixing and interference settlement of the agglomerated particles is rationally utilized. Aggravating the tendency of backmixing and grading, we established the theory of uniform density stability of air heavy medium fluidized bed. A dense phase high-density microbubble fluidized bed is formed, which is uniform and stable in three-dimensional space, and has a small mass-returning mixing, and is suitable for effective sorting of materials according to density.
(3) Starting from the analysis of the force of the selected materials in the fluidized bed, the separation mechanism of the feed in the fluidized bed is studied. It is considered that the net buoyancy of the feed is the dominant factor determining the stratification, and the complexity is studied. The dense phase high-density gas-solid fluidization system proposes the sorting theory of the mismatched effect of the selected materials in the fluidized bed, and divides the mismatch effect into two mismatch effects of viscosity and motion. The viscous mismatch effect is caused by the viscosity of the bed, generally decreases with the increase of the gas velocity, and the motion mismatch effect is larger when the gas velocity is too low or too high, so the sorting of the feed is subject to the above two types. The interaction interference of the mismatch effect gives the critical conditions for the selected materials not to be layered according to the Archimedes principle, but only the weighted plasmid is graded, the gas velocity control is appropriate, and the two mismatch effects can be obtained. Effective control. The larger the particle size of the selected material, the more significant the net buoyancy is, and the smaller the mismatch effect is. The operating parameters of the sorting machine should be optimized with reference to the fine particle sorting material. Under the condition of optimizing the parameters, a good sorting effect was obtained, and the possible deviation Ep value was 0.05-0.07.
(4) In the process of continuous sorting, the primary and secondary pulverized coal will be continuously produced. In this regard, the distribution law of different grades of coal in the fluidized bed is studied, and the coal of different grades in the fluidized bed is proposed. The third-order distribution theory quantitatively determines the lower limit of separation in theory, and successfully explains the distribution of different granular grade pulverized coal in fluidized bed. The core of the theory is through the main characteristics of different grades of pulverized coal and aggravated quality. The comparison calculation of parameters determines the distribution law of different size pulverized coals, that is, the pulverized coal is distributed in three sizes in the fluidized bed, the first granule is fine granules, distributed in the upper layer, and the second granule is equal granules. It will be mixed in the inherently weighted particles, which can be regarded as the second weighting. The third level is coarse, which will be affected by the buoyancy of the average density of the fluidized bed and stratified by the bed density. This theory provides a theoretical basis for the adjustment of bed density and stability control.
(5) In the continuous and stable sorting process, the mass balance of the sorting medium must be maintained to stabilize the bed density. To this end, a dynamic equilibrium model of the sorting medium is established, that is, QS=γ is entered into Q/[0.85+ln(1/Ï-0.08)] (where QS is the medium flow, and γ is the raw coal powder, Q is the feed amount, Ï is the bed density), and the correlation between bed density and coal powder content is Ï=1/[0.08+1.16exp(-1+γ)], which is the bed density online. Measurement and control and optimization of process parameters provide a mathematical model.
(6) On the basis of successful realization of >6mm coarse coal sorting in air heavy medium fluidized bed, in order to broaden its adaptability range, the characteristics of fluidized bed suitable for <6mm fine coal separation were studied. To achieve effective sorting of fine coal, one is to eliminate the generation of bubbles, improve the quality of fluidization, and achieve the vibration by introducing it. The second is to fully reduce the degree of weighting of the plasmid so that the fine grain material can receive the buoyancy of the overall density of the bed. effect. The vibrating air heavy medium fluidized bed formed by the interaction of airflow and vibration has the characteristics suitable for the separation of fine coal. Its superior characteristics use vibration waves to transfer energy and vibration to the weighted particles in the fluidized bed. The idea of ​​growing a big role to explain. On this basis, the flow model of vibrating fluidized bed with heavy mass in continuous sorting process is studied. Two parallel diffused flow models are proposed, namely 5-1.gif (79 bytes) Ï/5-1.gif (79 Bytes)t=DX5-1.gif (79 bytes)2Ï/5-1.gif (79 bytes)X2-u5-1.gif (79 bytes)Ï/5-1.gif (79 bytes)X( Ï is the bed density, DX is the axial effective diffusion coefficient, and u is the weighted flow velocity) to describe the flow state of the heavier mass, which provides a method for controlling the vibrating fluidized bed sorting process.
(7) To realize three different quality products in the same fluidized bed sorting tank, it is necessary to form a double-density fluidized bed in the same fluidized bed, which is not mentioned by the predecessors. Therefore, from the study of the two kinds of weighted particle size and density gradation and bed structure, the formation mechanism of double-density layer air heavy medium fluidized bed was studied, and magnetic beads (density of 3.6g/cm3) and magnetic were found. Iron ore fines (density 4.6g/cm3) of two-component mixed particles for weighting, can form two evenly stable upper and lower two sorting strips, each of which has its own density (1.52g respectively) /cm3 and 1.86g/cm3), the formation theory of two-element weighted particles to form a double-density fluidized bed was proposed, and the law of particle stratification, segregation and mixing during the formation of a double-density fluidized bed was revealed.
3 Conclusion
The probabilistic screening theory of high-end coal preparation technology and the fluidized bed dry sorting theory have opened a new beginning for mineral processing engineering, forming a series of high-end coal preparation technologies. Under the guidance of theoretical results, the coal probabilistic depth screening method for higher-end coal preparation technology is proposed, which produces huge social and economic benefits. These coal preparation technologies have opened up new avenues for the sorting and processing of water shortage in remote areas of China.
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