| China has a completely various but uneven distributions of coal types,high-quality coking coal resources are relatively scarce. Because of backward process,coking middlings with high ash content, sulfur content and low calorific value isnormally added to the power station, which will cause waste of scarce coking resourceand environmental pollution. The mineral occurrence of coking coal determines theliberation of minerals from coal is the premise to achieve reconcentration of cokingcoal. Machinery mechanical properties of coal samples can be changed by microwavepretreatment to improve the grinding efficiency and strengthen the subsequentflotation to recover the low ash coal, which have important economic andenvironmental benefits.The mineralogy characteristics of a typical middlings samples that supported byHuaibei coal preparation plant from China were studied. The ash content of coalsamples is28.41%, which indicates the middlings have high recovery value. The mainmineral compositions of the coal samples are kaolinite, quartz, illite, smectite, calciteand with small amounts of dolomite and pyrite. In those minerals, silicates, carbonatesand sulphates are transparent to microwave radiation, while, pyrite with low content isconsidered to be the best microwave absorber. Minerals linked with coal closely anddisseminated fine granularity.The results of crushing tests showed that crushed products of coal samples withdifferent microwave pretreatment time have approximate size distribution and densitycomposition, which indicated that crushing and liberation characteristic were notimproved by microwave pretreatment under the extrusion and rupture conditions.Density analysis showed that the coal sample that was crushed to-0.5mm cannotachieve the desired liberation.Factors that have influence on grinding fineness were studied. Grinding productsof coal samples with "Crushing-Pretreatment-Grinding" pretreatment process hasbetter grinding fineness than grinding products with "Pretreatment-Crushing-Grinding" pretreatment process. Grinding tests studied the influence of pretreatmenttime, feeding quantity and grinding time on the grinding fineness throughwholly-factor test.The result showed that microwave pretreatment can significantlyimprove the grinding fineness of grinding products, grinding velocity has been greatlyimproved; Interaction of various influencing factors have obviously influence on thegrinding fineness, the relationship model: Characteristics of microwave selective heating can produce micron cracks at theinterface of different minerals after microwave pretreatment. Mineral matters achievedesired liberation degree by pretreatment; Mechanism of improvement of grindabilityof coal sample by microwave pretreatment was preliminarily analyzed. Generation oflarge number of cracks with micron-sized on the surface of coal is the most importantreason to improve the grindability of coal. Effect of microwave heating cansignificantly reduces the moisture content of coal and improves grindability of coal toa certain extent.Effect of microwave pretreatment on flotation of coking coal was investged.Surface properties can be changed by Microwave pretreatment to some extent.Flotation test results show that microwave pretreatment enhance flotation of cokingcoal, concentrate yield increased with the growth of pretreatment time. Microwavepretreatment promote grinding efficiency and surface modification have improved theefficiency of the reconcentration of coking middlings, concentrate yield of coalsamples with three minutes pretreatment increased12.36percentage points comparedwith the untreated coal sample.In summary, microwave pretreatment can significantly improve the grindingefficiency and strengthen the subsequent flotation process of coking middlings. As aresult, reducing the grinding energy consumption and improving the yield of theflotation concentrate. The findings provide an effective pretreatment method for there-election of coking coal. |
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