| The preparation and application of ultra clean coal acts as an important part in clean energy technology.The ash content of ultra clean coal is generally less than 1 wt%,which can be used as raw materials for preparing high value-added carbon materials and liquid fuels.The preparation and application of ultra clean coal can effectively reduce the environmental pollution caused by coal combustion,increase the utilization value of coal,and improve the economic and social benefits of coal.In this paper,Taixi anthracite and Shendong long flame coal are used as raw materials to study the mineral composition of raw coal,the physical coexistence relationship between organic matter and mineral matter,the dissociation of coal matrix and mineral matter of different particle size samples,the change rule of physical and chemical properties of coal particles in the process of ultrafine comminution,the characteristics of flotation deep deashing of ultrafine coal particles and the mechanism of further deep demineralization and purification of ultra clean coal samples by using the deep demineralization method of flotation combined with chemical treatment.The main conclusions of this study are as follows:1.Mineral composition in the samples and physical occurrence relationship between organic matter and mineral.1)The occurrence state of carbonate minerals in Taixi anthracite are distributed in a large size in a lamellar form,coexisting with mainly dolomite and partly calcite.The silicate minerals are widely distributed in small size.Silicate minerals are disseminated in coal matrix.2)Calcites are disseminated in organic matter with the form of clastic particles,and the maximum particle size is about 20?m.Quartzs are disseminated in organic matter with the form of monomer particles,and the particle size is about 10?m.There are many kaolinite minerals in Shendong long flame coal,most of them are disseminated in the form of lamellae,and some of them are disseminated in the coal matrix in the form of impregnation.2.Study on the properties of ultrafine comminuted particles1)When the grinding time of Taixi anthracite is 60 min,its volume average particle size D[4,3] has been reduced to 8.01?m,less than 10?m.When the grinding time reaches 180 min,the particle size distribution of the sample appears in the form of bimodal distribution.At this time,coal particles are easier to agglomerate.When grinding for 30 min,the volume average particle size D[4,3] of Shendong long flame coal sample is 10.48?m.When grinding time continues to increase,the change of particle size is relatively slow.2)With the increase of grinding time,the particle size is decreased.The macropores in the sample are destroyed to form smaller pore structure.The micropore content in the particles will be increase,and then new surfaces will be generated.Therefore,the specific surface area of Taixi anthracite and Shendong long flame coal is gradually increased,the average pore diameter is decreased and the pore volume is increased.3)With the increase of grinding time,the decrease of coal particle size in Taixi anthracite and Shendong long flame coal makes the contact angle decrease gradually.The surface hydrophobicity of coal particle is weakened,and the surface tension of coal particle is increased.The surface tension between coal and water interface is decreased.The adhesion work between coal particles and water is gradually increased,and the surface of coal particles is easy to be wetted.4)The specific surface area and surface energy of Taixi anthracite and Shendong long flame coal are increased with the decrease of coal particle size.The reaction activity and oxidation rate of the particle surface will be increased.This will increase the functional groups containing oxygen on the surface of coal particles and further weaken the hydrophobicity.The electronegativity is enhanced,and the hydration film on the particle surface is thickened.It is adverse for the adsorption of flotation reagents on the surface of coal particles.Thus it is not easy to adhere between particles and bubbles,which reduces the floatability of ultrafine particles.5)In this paper,the dissociation effect of coal matrix and minerals in the process of micro dissociation is analyzed by the methods of floating and sinking experiment and acid-alkali deashing.It is found that when the particle size of anthracite sample is less than 10 ?m,the coal matrix and minerals have been well dissociated.It shows that the ultrafine particles of Taixi anthracite sample(D[4,3] is about 8 ?m)grinded for 60 min are more suitable for subsequent flotation experiments.When the particle size of Shendong long flame coal is about 10 ?m,the coal matrix and minerals have been well dissociated.The ultrafine particles(D[4,3] is 10.48 ?m)obtained by grinding for 30 minutes are suitable for subsequent flotation experiments.6)A model of describing the micro dissociation between organic matter and mineral matter in coal is established.3.Flotation separation characteristics of ultrafine coal particles1)Through the optimization experiment of micro bubble flotation of ultra-fine anthracite particles,it is found that when the amount of collector is 3.36 kg/t,the amount of foaming agent is 1.81 kg/t and agitation rate is 4560 r/min,the flotation effect is ideal with the lowest ash content of ultra clean coal.The ash content is 0.52%,and the concentrate yield is 66.25%.By he microstructural characterization studies of flotation concentrate,the factors limiting the degree of deashing in micro bubble flotation of ultrafine anthracite particles are as follows: The concentrates contain composite particles with the size of 2 ?m,where the organic matter accounts for more than 95% and the inorganic mineral(aluminosilicate)only accounts for a small amount.Besides,clay minerals(kaolinite and illite)are easy to be covered with fine mud during flotation.By adhering to larger organic matter particles,they enter the flotation concentrate.2)For ultra fine long flame coal particles,it is found that through optimization experiment of micro bubble flotation,the flotation effect is ideal when the amount of collector is 2.31kg/t,the amount of foaming agent is 9.24kg/t and the mechanical stirring speed is 2400r/min.Ultra clean coal with ash content of 1.26% and yield of84.63% is obtained.The poor deashing effect of Shendong long flame coal by microbubble flotation is attributed to the fact that the kaolinite in the sample is a kind of clay mineral,which is prone to argillization in water.Therefore,in the flotation process,the fine mud entrainment and the cover are easy to pollute the concentrate.Besides,the surface of long flame coal has strong hydrophilicity due to many functional groups containing oxygen.The following ultrafine grinding will further lead to worse hydrophobicity.This reduces the selectivity of flotation reagents and leads to poor flotation deashing effect.3)According to the agglomeration flotation experiment of ultra fine Taixi anthracite particles,when the mechanical stirring speed is 5500r/min,the mechanical stirring time is 10 min,the addition amount of sec-Octyl alcohol is 0.33kg/t,the addition amount of kerosene is 8kg/t,and the p H value of pulp is 2.And the ash content of flotation concentrate reaches at the lowest value of 0.77%.The corresponding yield is42.06%.The flotation concentrate contains quartz and aluminosilicate minerals in the form of composites,whose particle size is less than 2 ?m.In the process of forming flocs by stiring pulp,the conjoined particles are easy to enter the flocs and be sorted out.4)Through the agglomeration flotation of ultra-fine Shendong long flame coal particles,it is concluded that the mechanical stirring speed is 4000r/min,the mechanical stirring time is 15 min,the pulp p H value is 2,the amount of sec-Octyl alcohol is4.91kg/t,and the amount of collector is 24kg/t.Under this flotation condition,the ash content and the yield of ultra clean coal are 1.71% and 61.48%,respectively.At the same time,the research shows that the effect of emulsifying with spar-80 or tralaton X-100 is better than that of using collector alone.When the dosage of spar-80 is 1.5%,the ash content of ultra clean coal is 1.57%,and the yield is 78.86%.When the dosage of tralaton X-100 is 1%,the ash content of ultra clean coal is 1.59%,and the yield is65.26%.Although Tween-80 surfactant can also significantly improve the yield of concentrate,it can reduce the selectivity of emulsified kerosene,resulting in the increase of ash content of concentrate.4.Establishment of the deep demineralization method of flotation combined with chemical treatment1)The sample size has a significant effect on the demineralization of Taixi anthracite.The results show that the ash content of sample is decreased significantly with the decrease of sample size.When the volume average size D[4,3] of sample is8.01?m,the demineralization effect is optimum.The products with demineralization of86.81% and ash content of 0.36% can be obtained.The low reactive quartz,illite of Taixi anthracite and the fine silicate minerals coated by coal matrix mainly affect deep chemical demineralization.2)Because the pores of Shendong long flame coal are relatively developed and the mineral types are relatively single,it is suitable to prepare ultra clean coal directly by using mild acid-alkali deashing method.Ultra clean coal with ash content of 0.13% and calorific value of 30.55 m J/kg can be obtained.3)Based on the limitation of deep demineralization of Taixi anthracite in both microbubble flotation methods and Na OH-HCl method,the deep demineralization method of flotation combined with chemical treatment is proposed for further deep demineralization of Taixi anthracite.Firstly,the total amount of minerals with low reactivity such as quartz and illite are reduced by flotation pretreatment.Then the minerals in flotation concentrate react with Na OH to form water-soluble sodium silicate and acid-soluble sodium aluminate silicate,then can be further removed.Based on this mechanism,the model of deep demineralization process is further built up.The ultra clean coal with ultra low ash(0.12%),high carbon and high calorific value is finally obtained,which can be used to prepare high value-added carbon materials. |
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