The composition of coal macerals is directly related to the efficiency of coal liquefaction.It has been revealed that the oil recovery could be increased by 4%if vitrinite content is increased by 10%.However,due to the difficulty in liberation and the concentration based on maceral components,the development of enrichment technology for coal macerals is far from advance.Considering the size effect,heavy-medium separation has become one of the effective approaches for coal maceral beneficiation.However,it is difficult both theoretically and practically to achieve coal preparation at low density based on coal maceral concentration due to the poor suspension stability.The work comprehensively calculated the effect of the property and content of coal sludge on the volumetric concentration of solids in the suspension under low density circumstance.It was proposed to manipulate the property of the solids in the suspension to controlling the stability of low-density suspension.With the aim of concentrating coal macerals,according to studying the manipulation of the suspension stability,this thesis comprehensively investigated the liberation characteristics of the composition of macerals of the coals for direct coal liquefaction.This work also studied the mechanism for the stability of suspensions under different force fields,in combination with heavy medium separation for coal marcel concentration.Ultimately,this work developed the flowsheet for the concentration of coal macerals by heavy-medium separation tests at both laboratory scale and pilot scale.The main outcomes of this work are as below:This thesis studied the coal for direct liquefaction at Shendong Plant,with respect to the composition characteristics by focusing on the liberation properties and conditions.The distribution of particle size and the density of the coals were investigated.A relationship was observed between the content of vitrinite and ash.The power index increased and the vitrinite increased as the particle size decreases,while the content of vitrinite in coarse size fraction was higher than that in fine size fraction in the case of same ash content.The vitrinite content was found remaining a power-law relationship with the ash content in the different size fractions of coal crushed.The increment of vitrinite content decreased as decreasing the upper size for crushing.An appropriate upper size of crush process was 3mm.The content of vitrinite increased in the low-density fractions with particle size decreased.To reach the vitrinite content above 75%with recovery above 90%,the corresponding separation density was 1350Kg/m~3,and the content of?±0.1 was 67.28%,referring an extremely difficult separation process.The influence of particle size distribution of magnetic components,and the sludge content on the suspension stability in the static field and in the dynamic field were studied.It was found that the stability of suspension was affected by both the fineness and packing efficiency of magnetite powder,while the fineness was the dominant factor.Under the condition of isodensity,the addition of coal sludge could effectively improve the suspension stability in the gravity field,and the stability could be enhanced with the increase of the coal sludge content.This thesis focused on the correlation between the suspension stability and the sludge content.With the improvement of the suspension stability in the gravity field,the underflow concentration,the overflow concentration,the concentration efficiency and the density difference between the underflow and the overflow in the cyclone field all showed overall downward trends.Magnetite powder,montmorillonite and low-ash peat exhibited coupling impact on suspension stability.A coagulation phenomenon between magnetite powder and montmorillonite was observed,resulting in poor suspension stability of magnetite powder mixed with montmorillonite.However,the mixture of magnetite powder,montmorillonite and low ash peat showed explicit structure characteristics,resulting in the Z value greatly improved.In the cyclone field with high shear rate,the suspension made of magnetite powder,low ash coal sludge and montmorillonite were quite stable with marginal apparent viscosity increase,significantly facilitating the cyclone separation.The Euler multiphase flow model was introduced into the numerical simulation of the suspension system in the heavy medium cyclone.It was found that the density distribution was not even when only magnetite powder was present in the suspension.The content of coal sludge had an important influence on the stability of suspension.When the content of the sludge increased,the difference of average density between the underflow and the overflow of the cyclone was decreased,the stability of the flow field was enhanced,facilitating the stability of separation process and product quality.According to the shear rate distribution in the velocity field,estimated by numerical simulation,and the suspension rheological results,the apparent viscosity of the suspension in the cyclone field maintained stable under the low-density condition.Therefore,the movement obstruction of suspended medium particles to coal particles and the gradient of the density of the cyclone field were the main factors affecting the effect of separation process.According to studying the enhancement of cyclone separation process for coal macerals,based on magnetic and non-magnetic regulation,it was found that the separation effect was not positively related to the stability of the suspension in the gravity field and in the cyclone field.Excessive coal sludge content could inevitably lead to a significant increase in the number of solid particles in the suspension and in the volume fraction occupied,which could increase the migrating resistance of coal particles to an unacceptable level.However,over-low coal sludge content could lead to the increase of density gradient in the cyclone,and therefore would also lead to the decline of the separation effect.In general,the E value varied between 37-52kg/m~3.The E value increased firstly and reached a peak before declining with increasing the coal sludge.When the high ash coal sludge content was 30%,the E value reached the lowest 34kg/m~3,the vitrinite content was increased from 64.65%to 76.69%,and the recovery reached 95.52%.A pilot-scale low-density heavy medium cyclone study with capacity of 10t/h for coal macerals enrichment was carried out.The results showed that the separation effect was effectively improved by controlling the coal sludge content and size,achieving the separation density at 1350kg/m~3.The vitrinite content in concentrate was increased from 68.93%to 76.45%,the transformation rate and oil recovery was increased by 4.14%and 4.22%,respectively.It was proposed to the efficient concentration flowsheet for coal marcels by means of cyclone separation.In summary,this thesis proposed to manipulate the suspension stability by controlling fine non-magnetic solids and high-ash content.Based on this,a low-density coal macerals enrichment process system with coal crushing-fine sludge removal-suspension stability regulation-low density cyclone separation is constructed.The vitrinite content in the concentrate was above 75%,and the recovery exceeded90%.The outcomes of this work facilitates the industrialization of the concentration of coal macerals in future.This thesis includes 86 pictures,67 tables,and 156 references. |