Selective Flocculation-Flotation Separation Of Coking Coal’s Middllings

Re-separation of middlings can achieved deep desulfurization and ash reductionof coal. And it has important strategic significance to ensure sustainable supply ofscarce coal resources. Coal petrology characteristics and surface thermodynamiccharacterization of flotation middlings was researched firstly. Scale effect of particleswith its impact on flotation behavior and process characteristics of the fine coal wasstudied. Apparent size of fine particles was improved by dispersing and selectiveflocculation to enhance the fine coal flotation. In coal flotation system, selectiveadsorption behavior of flocculants PAM onto coal or kaolinite, surface modification ofparticles, the role of mineral particles surface-modified particles, action mechanismbetween PAM and particles and interfacial interaction between particles werediscussed by experimental study and theoretical calculations. Efficient separation offine coal by selective flocculation-flotation separation was achieved.Firstly, coal petrology characteristics, surface thermodynamic characterization offlotation middlings and scale effect of particles was studied. Particles will be fine ofthe re-separation which was determined by its petrology characteristics. After grindingof flotation middlings (-45μm about65.80%), macerals associated with mineralsdecreased from51.99%to20.75%. and it was well liberated. The flotation middlingswas strong hydrophobic with low surface free energy. The non-polar component andalkaline part based on hydrogen bonding of polar components of surface free energyof kaolinite is much larger than that of coal. Scale effect is reflected in geometriccharacteristics, interface chemistry and flotation behavior of particles. Therefore,when the surface hydrophobicity of coal was less affected, floatability difference canimprove by increase the apparent particle size of coal selectively, or mineral particleswere dispersed.Flotation behavior and process characteristics of fine coal was studied bymirco-flotation tests of pure mineral and artificial mixed samples, flocculation-flotation test, sedimentation test, zeta potential measurement and scanning electronmicroscopy test. It showed that heterogeneous agglomeration between coal andkaolinite was occurred in acidic environment or in Ca2+solution (>0.1mMconcentrations), which lead to reduce of coal recovery and increase of coal ashcontent. Collector aggravated the entrainment of kaolinite. Quartz particles weretransported into foam mainly by water entrainment. The finer the particle sizebecomes, the longer the flotation time. Thus, water recovery and entrainment degree increases. PAM can improve recovery of fine coal significantly, and water recoverywas increased together.It showed that selectivity of anionic polyacrylamide with molecular weight3×106(A401) is the best. The coal adsorbance of this polyacrylamide is2.15times largerthan that of kaolinite. Adsorption of A401onto coal or kaolinite was mainlyendothermic and spontaneous physical adsorption reaction. Hydrophobic bondingdominated the interaction between carbon chain of PAM and coal surface, whileelectrostatic and hydrogen bonding interactions was the main effect between PAM andkaolinite. When different polyacrylamide was absorbed onto different mineralsurfaces, the effect of compression or penetration with different intensity ofpolyacrylamide occurred.After adsorption of PAM A401, the hydrophobicity of coaldecreased. The hydrophobicity of Kaolinite increased slightly, and the polarcomponent was reduced from58.27mN m-2to55.05mN m-2.The laser particle size analyzer was used to measure the flocs size distribution ofcoal or kaolinite with different types of PAM, dispersing agent, stirring intensity andstirring time. Hexametaphosphate can inhibit the formation of flocs kaolinite strongly,which improved the adsorption selectivity of PAM. Quantum chemistry calculationproved that large π bonding effect was formatted between the hydrogen atom attachedto the nitrogen atom of PAM fragment and the phenyl ring of coal. The stability ofadsorption of PAM onto coal is higher than that of the constructed kaolinite siliconoxide layer and the aluminum oxide layer. Before and after PAM adsorption,interfacial interaction between particles of kaolinite was calculated by the extendedDLVO theory. Repulsion potential between coal particles after PAM adsorption and iskaolinite decreased, while the electrostatic repulsion increases, and the hydrophilicrepulsion potential reduced.Efficient separation of fine coal was achieved through the selectiveflocculation-flotation. Compared with the conventional flotation, collector dosage ofthe selective flocculation-flotation reduced, flotation rate of fine coal was improved.Clean coal recovery of selective flocculation-flotation was improved6.38percent,saving30%of collectors.