The Study On Interparticle Interaction And The Effect On Coal Flotation

A good hydrophobicity of coal surface is the prerequisite for a successful flotation.Slime coating,defined as fine and ultrafine hydrophilic particles coat coal surface,prevents the contact between coal and collectors/air bubbles,giving rise to a low flotation recovery.A search of open literature shows that slime coatings are ubiquitous in the flotation of various minerals,including sulfide minerals,oxide minerals,salt minerals,coal and bitumen.Particularly,when treating the complex multi-metal associated ores,slime coating is severer.Therefore,the adjustment and control of mineral interfacial property is a key factor for the improvement of flotation.In this work,aimed to mitigate the detrimental effect due to slime coating in coal flotation and improve the flotation recovery,the behaviors of aggregation and dispersion between coal and clays or original coal slimes were studied by using the clean lump coal,coal middlings,flotation clean coal,flotation feed and run of mine coal obtained separately from Huaibei coal preparation,Huoshaopu coal preparation,Linxi mine,Pangpangta mine and Greenhills mine.Meanwhile,the effect of interparticle interaction under different pulp conditions and the effect of agitation on interparticle interaction were explored.On the basis,the surface cleaning in high intensity conditioning was investigated and an approach to the control of slime coatings was proposed.Firstly,surface charge properties of mineral particles were studied.The effects of slurry pH,Ca²⁺and Mg²⁺on the surface charge of clays and coal particles with different densities were explored.It was shown that slurry pH determined the sign and magnitude of the zeta potential.Calcium and magnesium ions exhibited apparent compression on electronegativity of particles.The behaviors of aggregation and dispersion between particles were studied through zeta potential distribution and laser size analyses.The results showed that acidic slurry enhanced aggregation,but alkaline slurry mitigated particle aggregation,meanwhile,slight aggregation were observed under neutral condition.The interaction forces between particles were studied.The forces between clays and coal particles under different values of pH,Ca²⁺and Mg²⁺were calculated by using DLVO theory.It was shown that the slurry pH imposed great impact on interaction forces between clays and coal particles.Low slurry pH and high concentration of ions gave rise to small double layer repulsion and narrow interacting range,which aggravated slime coating.Meanwhile,the AFM measurement showed that the effect of pH on the interaction forces between coal and montmorillonite was in good agreement with DLVO theoretical calculation,indicating that the interaction between coal and montmorillonite obeyed classical DLVO colloid stability theory.The hydrophobic force between hydrophobic particles was analyzed.The agglomeration between pure coal and hydrophobized montmorillonite proved the existence of hydrophobic force.The conditions of occurrence and mechanism of slime coatings were explored from the prospective of particle size and surface property.The results showed that DLVO interaction forces determined the clays coatings on coal surface.The mechanisms of original slimes coatings on coal surface were different.High density slimes coated coal by weak Van der Waals force.Middle and low density slimes coat coal by both Van der Waals force and hydrophobic force.Therefore,the adhesion forces between coal and low/middle density slime were larger than that of high density slime.Theoretical analysis indicated that small particle was prone to coat coal surface due to lower energy barrier,higher ratio of adhesion/gravity force and weak shearing force by fluid.The effect of slime coatings on flotation of coal was investigated.Firstly,the effect of clays coatings was studied.It was showed that,in the presence of clays,the acidic condition exacerbated clays coatings,which greatly depressed coal flotation.However,in neutral slurry,clays slightly coated coal surface,which only impacts the concentrate ash content.In alkaline condition,clays did not depress coal flotation.Low density slime can greatly depress coal flotation by preferentially absorb reagents/air bubbles and depressed coal when under the addition of insufficient collectors.Nevertheless,hydrophilic high density slime did not absorb reagents but slightly coat coal,thus showed little effect on coal flotation.However,the middle density slime coated coal tightly due to hydrophobic force,leading to hydrophilic parts out-towards and hydrophobic parts in-towards,preventing the contact between coal and collector/air bubble.The depression on coal flotation by middle density slime was stronger than that of low/high density slime.The effect of agitation on the interaction between particles was studied.Based on focused beam reflectance measurement and other techniques,the intensity of agitation on behaviors of aggregation and dispersion was explored.The results showed that the mild agitation enhances kaolinite-coating,aggravating coal flotation by increasing collision probability between particles and providing kinetic energy for particles to overcome energy barrier.However,high intensity agitation mitigates the kaolinite-coating on coal by removing coated slimes.The mitigation of high intensity conditioning on slime coatings was explored.Meanwhile,the role of surface cleaning in high intensity conditioning was quantitatively analyzed by step by step conditioning test.It was showed that surface cleaning of high intensity conditioning removed most of coated slimes from coal surface and improved the hydrophobicity of coal particles,playing an important role in the mechanisms of high intensity conditioning.For Pangpangta flotation feed,4min surface cleaning at a linear velocity of 4.71 m/s increased combustible recovery from25.30%to 47.37%.In order to improve the surface cleaning,meanwhile avoid reagent detachment,a new approach to coal slime conditioning was proposed.