Difficult-to-float Mechanism And Floatability Improvement Of Taixi Oxidized Coal

Fine oxidized coal is difficult to float using common flotation processes since thelow floatability of oxidized coal surface, which is a serious waste of coal resource.The economical loss of Taixi Coal Preparation Plant is caused by its low flotationrecovery of oxidized coal. This dissertation based on Taixi oxidized coal, is to find outboth the formation and difficult-to-float mechanisms of Taixi oxidized coal. Theimprovement strategies for oxidized coal flotation are proposed as well as theirimprovement principles are also revealed. The effects of flotation processesoptimization are also discussed. The main conclusions are as follows:Formation mechanism of Taixi oxidized coals is revealed by simulating freshcoal oxidation processes under laboratory conditions. Oxidation processes containnatural weathering and high temperature oxidation processes. Natural weatheringprocesses contain inside and outside weathering processes. Inside weatheringprocesses are more moderate than outside. Coal surface after two years insideweathering processes is similar to fresh coal surface. However, Coal surface after3months outside is changed greatly and the floatability is decreased qucikly. Hightemperature oxidation processes increases the number of holes and cracks anddecreases the floatability of coal surface. High temperature oxidized coal surface canbe oxidized into a further state by water immersion. Both natural weathering and hightemperature oxidation processes can reduce coal hydrophobicity and change surfacemicroscopic morphology, and hence reduce the floatability of coal surface.Difficult-to-float mechanism of Taixi oxidized coal is investigated based onphysicochemical characteristics. Taixi oxidized coal contains both weathered coal andspontaneous combustion coal. It is representative that simulated oxidized coalproperties under laboratory conditions are similar to Taixi oxidized coal. FTIR, XPS,SEM and contact angle measurements are used to indicate oxidized coal properties.Floatability of oxidized coal is determined by surface groups and surface morphology.Hydrophilic oxygen containing groups can be bonded with water molecules, whichcauses oxidized coal surface to be covered by a thick hydrated film. This thickhydrated film always makes oxidized coal difficult to float. The holes, ravines andfissures on oxidized coal surface can be also filled with water, and hence the thicknessof hydrated film can be further enhanced. It is difficult for oily collectors, such asdiesel and kerosene to contact with oxidized coal surface. Floatability of oxidized coals is improved by grinding pretreatment. Grindingcan expose fresh, hydrophobic surfaces. Therefore, the floatability of weathered coalcan be improved. There are many holes, ravines and fissures on spontaneouscombustion coal surface, and hence the hydrated film on spontaneous combustioncoal surface is very thick. The inner surface of spontaneous combustion coal can bealso exposed by grinding. However, it is disappointed that the floatability ofspontaneous combustion coal is primarily affected by surface morphology. Grindingpretreatment may improve spontaneous combustion coal flotation by reducing the sizecomposition of feeding. The suitable size range should be between22and74μm.Floatability of spontaneous combustion coal can be enhanced by new effectivecollectors. Biodiesel and oxidized diesel can improve the floatability of groundspontaneous combustion coal. FTIR results show that biodesel and oxidized sieselhave many oxygen containing groups which can be bonded with the same oxygencontaining groups on spontaneous combustion coal surface, and hence the floatabilityof spontaneous combustion coal is enhanced. DCT collector is made under laboratoryconditions. DCT collector also has the similar performance of the improvement in thefloatability of spontaneous combustion coal surface. XPS results show that DCTcollector can increase the floatability by reducing the content of oxygen containinggroups but increasing the content of hydrophobic hydrocarbon groups.Improved flotation processes promote the flotation recovery of spontaneouscombustion coal. The flotation performance is affected greatly by prewetting time.The shorter prewetting time, the higher flotation recovery is. Flotation recovery is thehigheset while prewetting time is zero. There is one suitable collector conditioningtime related to one prewetting time. The suitable condition intensity and impellerspeeds are very important during flotation processes. The moisture content ofspontaneous combustion coal samples can be reduced by microwave pretreatment.The decreasing of moisture content can improve the flotation of spontaneouscombustion coal. The main ash minerals are silicate (SiO2and Al2O3). Dextrin andHTAB were used as depressant and collector, respectively. Reverse flotation issuitable for the recovery of coarse oxidized coals.