The Selective Aggregation Mechanism Of The Clay Mineral In Coal Slime

The contamination of the flotation concentrate by ultra-fine gangue minerals is currently the bottleneck problem that limits the flotation performance and the concentrate grade in mineral flotation industry.Diverse research has shown that the gangue minerals can report to the flotation froth and contaminate the concentrate by slime coating,water entrainment,mechanical entrapment,etc.The smaller the size of the gangue minerals,the more severe the contamination is of the flotaiton concentrate.In the flotation of oxidized and sulphide minerals,researchers succeeded in diminshing the concentrate contamination through the selective aggregation of the gangue minerals by adding depresant or flocculant.In coal flotation,however,the selective aggregation of the gangue minerals is difficult to achieve and can not be found in current publications,mainly owing to the fact that coal is hydrophobic in nature so coal is more easily to be aggregated via hydrophobic interaction compared to the gangue minerals.Clay mineral usually refers to the phyllosilicate,which is characterized of the ultra-fine particle size and the sheet-like structure.Clay mineral is the main contaminating mineral to the flotation clean coal.Kaolinite is the clay mineral that extensively exists in flotation coal slime.After experimental exploration and theoretical analysis,it is for the first time discovered that polyaluminum chloride(PAC)could induce the selective aggregation of kaolinite in coal slime flotation and could also reduce the clean coal ash content.Afterwards,research in the effects of PAC on kaolinite water entrainment,flotaiton froth stability,and coal floatability was conducted.The aggregation rule of kaolinite and coal with varied pH,electrolyte concentration,and PAC concentration was also investigated.Alumina and quartz were innatively used to respectively represent the alumina octahedral and silica tetrahedral basal surfaces of kaolinite.Combined with zeta potential measurement,the DLVO theory,and the direct force measurement between particle surfaces,the selective aggregation mechanism of inhomogeneous sheet-like kaolinite was proposed.The main conclusions are as follows:In the selective aggregation exploring experiments,the effects of organic flocculant polyethylene oxide(PEO)and the inorganic coagulant PAC on coal flotaiton performance and on the selective aggregation of coal and kaolinite were respectively studied.The results show that with the combined use of sodium hexametaphosphate and PEO,the aggregation of kaolinite would be restrained and the aggregation of coal would keep profound.As a result,the selective aggregation of coal was achieved and the combustible matter recovery and the ash concent of the flotation clean coal both increased.The sole use of PAC aggregated kaolinite and dispersed coal simultaneously,so PAC diminished the contamination of kaolinite and reduced the clean coal ash content.PAC could work as the selective coagulant for kaolinite in coal slime.Through the flotation expreiments of the artificial mixture of low-ash coal and kaolinite and the low-ash coal,it was found that PAC decreased the water entrainment of kaolinite and the effect was more profound with low electrolyte concentration.PAC also decreased the flotation froth stability.This is mainly because PAC changed the surface property of coal and decreased the hydrophobic particles in the froth.PAC increased the zeta potential,decreased the hydrophobicity,and decreased the bubble attachment probability of coal.In the research of the aggregation rule of kaolinite and coal,the settling velocity,the particle size distribution,and the fractal dimension of flocs of kaolinite and coal were measured with varied pH,electrolyte concentration,and PAC concentration.The results show that the aggregation of coal was very weak in all conditions while the aggregation of kaolinite was more prodound with acidic pH,low electrolyte concentration,and low PAC concentration.In the research of the selective aggregation mechanism,kaolinite was first regarded as the homogeneous spherical particle according to the traditional view.The van der Waals and the electrostatic double layer forces between kaolinite surfaces were calculated in the framework of the DLVO theory.The results show that the calculated DLVO force was consistent with the kaolinite aggregation rule when the electrolyte concentration kept constant and the pH varied.But when the electrolyte concentration varied,the DLVO force calculation became contradictory with the aggregation rule of kaolinite.Afterwards,kaolinite was regarded as the inhomogenous sheet-like particle.Alumina and quartz were respectively used to represent the alumina octahedral and the silica tetrahedral basal surfaces of kaolinite.The DLVO forces between the same and different kaolinite basal surfaces were calculated and they were merged to be the resultant DLVO force between the sheet-like kaolinite.It was found that the DLVO force between the sheet-like kaolinite was consistent with the aggregation rule of kaolinite with vaired pH and electrolyte concentration.The attractive electrostatic double layer force between the different basal surfaces was the main reason why kaolinite aggregated more significantly with low electrolyte concentration.PAC decreased the electrostatic double layer repulsion between the silica tetrahedral basal surfaces so enhanced the aggregation of kaolinite.Afterwards,the direct force measurement between the silica and alumina surfaces was conducted using atomic force microscope.It was found that without PAC,the surface force between silica and alumina complied well with the DLVO theory.But a non-DLVO attractive force would appear when PAC was present.This attractive force was not obvious when the two surfaces were approaching each other but it was significant when the two surfaces were retracting from each other.This attractive non-DLVO force was assumed to be the charge patch attraction due to the inhomogeneous adsorption of the cations that dissociated from PAC on particle surface.