Effect Of Ultra Fine Particle Properties On The Separation Of Ultra Clean Coal

Based on China’s energy situation, consumption structure and environmental problems caused by coal consumption, how to improve the clean utilization efficient of coal and reduce the pollution to the environment at the same time, has become the dominant content of relevant coal energy policy. As a kind of little impurity, high value-added products, the ash of ultra-clean coal is lower than 2%, even lower than 1%, thus it can be used as raw materials of the low pollution clean coal fuel – ultra-fine coal water slurry as well as a variety of carbon materials. It can effectively reduce the pollution to the environment and increase the utilization value of coal. The ultra clean coal preparation methods include chemical method and physical method. In the preparation process of chemical method, strong acid and alkali are used, which lead to the high cost and the environmental pollution. So its application is restricted. Ultra-fine grinding of coal is the prerequisite for the preparation of ultra clean coal by physical method, so that the organic components and minerals in coal are fully separated, and then the super clean coal is sorted out by fine particle separation technology. But the particle size after ultrafine grinding is below 10μm, also below the lower limit of effective separation flotation. Therefore, flocculation flotation is used for the separation of ultra clean coal. First of all, floccules with a certain scale are generated, in order to make the particle size to achieve effective separation range of flotation. And then the conventional flotation is used for the separation of ultra clean coal. In addition, coal superfine grinding process is not only the process of size reduction and mineral dissociation, mechanical and chemical effects of physical and chemical properties are also generated due to the ultrafine ginding process. The effect can affect the adsorption of super clean coal particles to the reagent and the formation of the flocculation, and finally affect the separation results of ultra clean coal.Shenmu non-caking coal(SM), Huainan gas coal(HN), Taixi anthracite(TX) were selected as coal samples, variation of energy consumption in the ultra-fine grinding processes using stirring mill and jet mill was discussed. And the changes of coal particle surface properties(size, degree of dissociation, pore structure, functional group, potential, wettability, etc.) were also explored. Moreover, in combination with floc separation mechanism, the effects of these physical and chemical changes on floc separation were also studied.Firstly, the energy consumption in the process of ultrafine grinding was studied. Based on the fractal theory, fractal dimension was used to represent different particle size distribution of different kinds of ultra-fine grinding. Combined with fractal research, crushing mechanism of the superfine grinding process was studied. Compared to the single fractal analysis, three stage fractal analysis is more suitable to describe the particle size distribution of ultrafine particles, and the fractal dimension of each stage is between 2.30~2.95, 1.04~1.98 and 0.006~0.750, on this basis, combined with surface fractal results of superfine coal particles, the energy consumption calculation method in the process of superfine grinding was proposed. The calculation results show that the finer the particle size is, the more energy it consumed, and the energy consumed grows exponentially. And the higher degree of metamorphism of coal particle leads to greater energy consumption in the ultrafine grinding process. Energy consumption of Huainan gas coal is higher due to the agglomeration in the process of crushing.Malvern laser particle size analyzer was applied to test the change of particle size, and the effect of particle size on the separation of ultra clean coal was discussed. The smaller size of ultrafine coal particles, the lower the collision efficiency between the particles in the flotation cell, the weaker Van der Waals force between particles, which leads to the requirment of greater stirring intensity to produce more large scale flocs. When the stirring speed is 2500r/min, SM coal particles with the size of more than 10μm can generate more floccules in the floccules formation process. But when the size is less than 10μm, there are more flocs with large scale formation until the stirring speed turns into 4500r/min. TX coal particles with the size of more than 10μm can generate more floccules in the floccules formation process when the stirring speed is 800r/min. But when the size is less than 10μm, there are more flocs with large scale formation until the stirring speed turns into 1500r/min. When there are more large scale flocs, the higher yield of super clean coal is obtained.Scanning electron microscopy(SEM) and Atomic force microscopy(AFM) were used to measure the changes of particle shape and surface roughness. SEM results show that there are more plastic fracture in the processes of ultra-fine grinding of SM and HN coal. However, the fracture characteristic of TX coal is typically brittle. In addition, agglomeration phenomenon appears when the particle size of HN coal is 8.47μm. Compared with the particles grinding with jet mill, the surface of particles ground with stirring mill is smoother and less small particles adhered. Combined with fractal theory, it can be concluded that the shape and surface roughness of coal particle after ultra-fine grinding have fractal characteristics. The shape fractal dimension and surface roughness fractal dimension of particles after grinding with jet mill were significantly higher than that with stirred mill. SM coal and TX coal were respectively grinded to the similar particle size with stirring mill and jet mill, and sequentially selected to study the effect of particles surface morphology on floccules separation. The results show that the greater the fractal dimension of particles, the more content of large scale floccules in the process of floccules formation, moreover, the more compact and stable of froth in flotation process, and the higher yield of ultra-clean coal.Physical adsorption analyzer, UV visible spectrophotometer were applied to measure the changes of porosity and agent adsorption. Based on low temperature nitrogen adsorption results, it can be concluded that in the grinding process, large open holes in SM coal particles gradually reduce, and the pore shape is mainly holes closed at one end. The pore morphology of HN coal is mainly the holes closed at one end. The pore of Taixi coal particles is the holes closed at one end and open type. With the prolongation of superfine grinding time, coal pore shape becomes open type air holes. The damage to pore morphology is less in the gringding process with jet mill. BET results suggest that with the decrease of particle size, particle specific surface area increases. Through the reagent adsorption capacity measurement, it can be obtained that with the decrease of particle size, the equilibrium adsorption increases, and the reagent dosage is increased in the flocculation flotation stage. The equilibrium adsorption amount of coal particles grinded with jet mill is lower than that of the coal particles grinded by stirring mill.Through image analysis and acid-base deliming rate measurement, the dissociation degree of particles in superfine grinding process were studied, and the corresponding flocculation flotation tests were carried out for coal particles with different dissociation degree. The consequences indicate that the smaller the particle size, the greater the degree of dissociation, and the yield of ultra-clean coal separated is higher, the ash content is lower. The dissociation degree of inorganic minerals in the ultrafine grinding of jet mill is larger than stirring mill. Inorganic minerals have been fully dissociated when the particle size of SM coal is 10.77μm and TX coal is 15μm. The clay mineral of HN coal is still not fully dissociated when it is crushed to 5μm.Fourier infrared spectrometer was applied to study the the variation of functional groups. The results show that ultrafine grinding changes the absorption peak area ratio of hydrophilic groups and hydrophobic groups. With the decrease of particle size, the surface of the particles is oxidized after grinded by stirred mill. The content of oxygen functional groups on the surface of the particles grinded by jet mill is decreased.Zeta potential analyzer was used to analyze the variation of zeta potential. Analysis results indicate that with the increase of metamorphic grade, the isoelectric point of coal sample increases. After crushed by stirring mill, zeta potential of SM coal and TX coal increases first and then decreases with the decrease of particle size. Due to the dissociation of clay minerals and the coverage on the particle surface in the ultrafine grinding process, the electronegativity increases gradually with the particle size decreasing. The zeta potential of the sample after grinding with jet mill is higher than with the stirred mill. The larger potential absolute value of the particles leads to the less formation of large scale floccules under the same agitation speed, and then as the result, the yield of super clean coal is lower. The increase of electrostatic repulsion between the floccules and the bubbles can lead to the increase of electrostatic potential energy, which leads to the difficulty of the adhesion between the floccules and the bubbles.Contact Angle tester was employed to measure the changes of wettability of coal particles in the process of super fine grinding. Through the contact angle measurement, it can be drawn that after grinded by jet mill, the coal particle surface hydrophobicity enhances. After grinded with stirring mill, the hydrophobicity of SM coal particles is the strongest when the size is 10.77μm. The contact angle of HN and TX coal particles is reduced with the decrease of particle size The enhancement of hydrophobicity is helpful to the formation of large scale floccules. The increase of contact angle makes the hydrophobic action potential energy increase between floccules and bubbles. Then it makes the contact between floccules and bubbles much easier, so as to promote the process of bubbles mineralization. From the point of dynamics, the enhancement of hydrophobicity can significantly improve the flotation probability of the floccules in the pulp. The surface tension of coal particles and the adhesion work of coal- water system can be calculated by contact angle. The smaller the surface tension of pulverized coal, the greater the surface tension of coal water interface. When the particle is in contact with bubble, it is more easily to exclude the water molecules on particle surface and contact with bubble. At the same time, the adhesion work of coal water interface is smaller, the critical thickness of hydration shell rupture is larger, and the hydration film can be broken when the distance between the particles is relatively long.Micro calorimeter was used to measure the wetting heat of coal particles with diesel. The results show that after ultra-fine grinding by stirring mill, with the decrease of the particle size, the wetting heat of unit surface area of SM coal increases first and then decreases. The wetting heat of unit surface area of HN coal is decreasing with the diminution of the particle size. The wetting heat of unit surface area of TX coal is larger at the granularity of 11.15μm. After ultrafine crushing with jet mill, the wetting heat between three kinds of coal samples and diesel are significantly higher than the samples grinded with mixing mill.The floccules size of SM coal particles is around 20 ~ 70 μm. The floccules size of HN coal particles is about 20 to 100 μm. The floccules size of TX coal particles is up to 100 μm.By the method of stepwise regression analysis of Matlab, significant analysis and regression analysis of seven kinds of physical and chemical properties were completed, and these surface properties would affect the ultra clean coal separation. The results show that the yield is significantly affected by oxygen functional groups and contact angle. Whereas, the ash is significantly affected by the metamorphic grade, dissociation degree and surface potential of particles.