COAL DRY BENEFICIATION TECHNOLOGY IN CHINA:THE STATE-OF-THE-ART

In China, coal is the major source of energy and its leading role in energy consumption would not change in the next 50 years. Coal preparation is the essential component of Clean Coal Technology. In China more than two-thirds of available coal reserves are in arid areas, which results in the unfeasibility with conventional wet processing for coal preparation. The uniqueness of dry coal beneficiation technology with air-dense medium fluidized bed is discussed in this paper and a detailed survey of the current status of theoretical study, commercial application and development of the new technology is given in this paper.

DEVELOPMENT OF COAL DRY BENEFICIATION WITH AIR-DENSE MEDIUM FLUIDIZED BED IN CHINA

In China more than two-thirds of available coal reserves are in arid areas, where, to beneficiate the run-of-mine coal, there is not enough water resource required by conventional processing. Developing efficient dry beneficiation technology is of great significance for efficient coal utilization in China, notably the clean coal technology (CCT). The dry coal beneficiation technology with air-dense medium fluidized bed utilizes air-solid suspension as beneficiating medium whose density is consistent for beneficiation, similar in principle to the wet dense medium beneficiation using liquid-solid suspension as separating medium. The heavy portion in feedstock whose density is higher than the density of the fluidized bed will sink, whereas the lighter portion will float, thus stratifying the feed materials according to their density. In order to obtain efficient dry separation in air-dense medium fluidized bed, stable fluidization with well dispersed micro-bubbles must be achieved to ensure low viscosity and high fluidity. The pure buoyancy of beneficiation materials plays a main role in fluidized bed, and the displaced distribution effect should be restrained. The displaced distribution effects include viscosity displaced distribution effect and movement displaced distribution effect. The former is caused by viscosity of the fluidized bed. It decreases with increasing air flow velocity. Movement displaced distribution effect will be large when air flow rate is too low or too high. If medium particle size distribution and air flow are well controlled, both displaced distribution effects could be controlled effectively. A beneficiation displaced distribution model may be used to optimize beneficiation of feedstock with a wide particle size distribution and multiple components in the fluidized bed. The rheological characteristics of fluidized beds were studied using the falling sphere method. Experimental results indicated that the fluidized bed behaves as a Bingham fluid. The plastic viscosity and yield stress can be obtained by measurement of the terminal settling velocity of spheres and linear regression of the experimental data. Both plastic viscosity and yield stress increase with increasing size of the fluidized particles. The drag coefficient can be calculated with favorable agreement with experimental data. The first dry coal beneficiation plant with air-dense medium fluidized bed was established by CUMT with an output of 320000 t·a-1 and a probable error Ep value up to 0.05 was achieved. The plant was accepted by the Chinese government in June, 1994. Since then, new applications have been found including a 700000 t·a-1 dry coal beneficiation plant put up for commercial testing. To realize coal dry beneficiation of full size range of 300-0 mm, further research on dry coal beneficiation of different size fractions has been under way at the Mineral Processing Research Center of CUMT, leading to the following results: Dry beneficiation technology with a vibrated air-dense medium fluidized bed for fine coal of size fraction 6-0.5 mm Ash content was reduced from 16.57% to 8.35%, with yield up to 80.20% and Ep value up to 0.065. Coal dry beneficiation technology with a deep air-dense medium fluidized bed for >50 mm coal An Ep value up to 0.02 was achieved. This technology is of great value for waste removal from 300-50 mm large feedstock, especially for big surface mines in China. Coal triboelectric cleaning technology for <1 mm pulverized Coal Coal is comminuted down to 320 mesh (0.043 mm) to fully liberate the embedded minerals, yielding an ultra-low ash coal (less than 2%). Currently a pilot system with triboelectric cleaning has successfully passed technical appraisal. Three-product beneficiation technology with dual-density fluidized bed This technology yields three products: clean coal, middling and tailing, with the following typical results: Ep value of 0.06-0.09 for the upper layer with a density of 1.5-1.54 g·cm-3 and Ep value of 0.09-0.11 for the lower layer with density of 1.84-1.9 g·cm-3.

Progress in developments of dry coal beneficiation

China’s energy supply heavily relies on coal and China’s coal resource and water resource has a reverse distribution.The problem of water shortages restricts the applications of wet coal beneficiation technologies in drought regions.The present situation highlights the significance and urgency of developing dry beneficiation technologies of coal.Besides,other countries that produce large amounts of coal also encounter serious problem of lack of water for coal beneficiation,such as American,Australia,Canada,South Africa,Turkey and India.Thus,dry coal beneficiation becomes the research hot-points in the field of coal cleaning worldwide in recent years.This paper systematically reviewed the promising research efforts on dry coal beneficiation reported in literature in last 5 years and discussed the progress in developments of dry coal beneficiation worldwide.Finally,we also elaborated the prospects and the challenges of the development of dry coal beneficiation.

Negative-pressure pneumatic separator:a new solution for hard-coal beneficiation

Hard coal is an important source of energy worldwide.Owing to the imperfections of excavation technology,most run-of-mine coals must be processed before they can be utilised as fuel.In this study,tests on the dry separation process were performed with numerous different raw hard coal and coal waste material samples,using a newly designed and constructed negative-pressure pneumatic separator(NPPS).The experiments revealed the effects of the feed-material properties and material processing conditions.These experiments were preceded by additional tests for evaluating the influence of each device operating parameter on the process,to determine the optimal method of supplying the feed material.The calorific value of the processed materials was improved by 5%–40%,while a minimal output yield of products(60%–70%)was maintained.The trials indicated that in coal material processing,the developed NPPS can be used for not only raw coal enrichment but also coal recovery from materials stored at coal-waste dumps.The main advantages of the device are its mobility,low material processing cost,and lack of water consumption during operation.

Evaluation of VSK separation in the classification of two mineralogically different iron ore fines

With gradually diminishing Fe grade in tandem with the ever-increasing demand for high-grade iron ores,iron ore industries are now focusing on the beneficiation of low-grade iron ore fines,mainly considered waste.Besides,the scarcity of water at many of the mines’sites and the new water conservation policies of the governments have necessitated research on suitable dry beneficiation routes.In this context,an effort has been made to evaluate the efficacy of a dry classification unit,such as the VSK separator,in upgrading the iron values of two low-grade Indian iron ore fines,named Sample 1 and Sample 2.The mineralogical studies,involving scanning electron microscopy and X-ray diffraction,suggest that Sample 1 is a low-grade blue dust sample(51.2wt%Fe)containing hematite and quartz as the major minerals,while Sample 2(53.3wt%Fe)shows the presence of goethite in addition to hematite and quartz.The experiments,carried out using Box-Benkhen statistical design,indicate that blower speed,followed by feed rate,is the most influencing operating parameter in obtaining a good product in the VSK separator.At optimum levels of the operating factors,a fines product with~55wt%Fe at a yield of~40%can be obtained from Sample 1,while Sample 2 can be upgraded to~56wt%Fe at a yield of~85%.The results suggest that the VSK separator can be employed as an efficient intermediate unit operation in a processing circuit to upgrade the iron contents of iron ore fines.

Clean Processing and Utilization of Coal Energy

The dominant status of coal on the energy production and consumption structure of China will not be changed in the middle period of this century. To realize highly efficient utilization of coal,low pollution and low cost are great and impendent tasks. These difficult problems can be almost resolved through establishing large-scale pithead power stations using two-stage highly efficient dry coal-cleaning system before coal burning,which is a highly efficient,clean and economical strategy considering the current energy and environmental status of China. All these will be discussed in detail in this paper.

Electrostatic concentration of phosphate flotation concentrate

The concentration of phosphate flotation concentrate with P2O5 grade lower than the commercially acceptable phosphate quality was upgraded in the use of tribo-electrostatic technique. The concentration of the flotation concentrate stream was conducted under both triboelectrification and inductive charging mechanism. Mineralogical analysis reveals that the phosphate ore utilized was dominated mainly by fluorapatite, crandallite, wavellite, and with quartz as the major gangue mineral. Flotations concentrate of about 28.87% P2O5 was obtained from the reverse flotation technique, after conditioning the phosphate ore at 80% passing 150 lm under Lilafloat. Constant parameters such as à6 kV charging voltage, 25 kV separating voltage, inlet air, different charger rotary speed and splitter distances were investigated. Commercially accepted grade of 35.50% P2O5 was attained after the second stage of separation with a recovery of 12.26%, another phosphate product of 34.02% P2O5 and 85.19% was also recovered under the same condition. Single stage of separation also shows a significant increase in the grade of the product obtained, with 33.41% P2O5 and 84.07% recovery achievable. The practicability of tribo-electrostatic separation technique for upgrading flotation concentrates resulted in a significant increase in grade for <150 lm product that do not meet the viable grade.

RHEOLOGY OF GAS-FLUIDIZED BED

Coal dry beneficiation with fluidized beds belongs to dry dense m ediu m concentration . Therheology is im portant for studying the behavior of an object settling in gas fluidized beds . Inthis paper , The viscosity of fluidized beds w as measured using falling spheres . The rheologi cal characteristics of fluidized beds can be described with Bingha m fluid m odel . The efficientviscosity of fluidized beds can be calculated and plastic viscosity and yield stress of fluidizedbed can be obtained by linearly regressing experi mental data . The calculated results are veryagreeable with the experim ental data .