Research On Strengthening Separation For The Flotation Of Coal-montmorillonite System

With the development of coal mining mechanization level,the content of gangue minerals in raw coal increased sharply,which had greatly hindered the coal separation.Clay minerals in water are easy to collapse into a large quantity of fine particles.These clay particles,with negative charge and good hydrophilicity,always agglomerate with coal slurry,which not only causes low flotation efficiency,high ash content of foam products,big reagent consumption and difficult dewatering of tails,but even threats to the order of the whole production system.Hetercoagulation had already became the bottleneck of efficient separation and clean utilization of coal resources.Many scholars had carried on the detailed research on methods to eliminate the adverse impacts of clay on coal.The main problems existing in the present study included that the underlying cause of coal slime rich in clay minerals difficult to separate efficiently was unclear,the internal mechanism of hetercoagulation impeding coal flotation was unknown,efficiency of clay dispersion was low,selectivity of clay minerals inhibitors was not adequate.How to separae clay minerals and coal effectively is still a problem.In this paper,strengthening dispersion and selective depression for montmorillonite in coal slime flotation were studied by sedimentation tests,flotation tests,optical microscope,Zeta potential,contact angle,atomicforce microscope analysis,absorption measurement,infrared spectrum analysis,X-ray photoelectron spectroscopy and the calculation of the extended DLVO theory.The research contents are as follows: Discussing the aggregation and dispersion behavior of the clay and coal particles in flotation system;Exploring the mechanism of hetercoagulation between montmorillonite and coal;Examining the the effect of hetercoagulation on coal slime flotation;Establishing flotation dynamics model for coal slime;Clarifying the mechanism of dispersion and selective inhibition of clay minerals;Proposing “strengthening dispersion-synchronous depression” flotation process.Efficient separation of montmorillonite and coal was achieved.Effects of slurry pH value,montmorillonite content,metal cations species and concentration on dispersion-flocculation behavior of mineral particles and the mechanism of hetercoagulation were studied.The results indicated that montmorillonite and coal particles coagulated together in whole test pH range,but the hetercoagulation between montmorillonite and coal weakened at alkaline pulp.The increase of montmorillonite content and calcium ion concentration would aggravate hetercoagulation.Sodium ions had weak effect on dispersion-flocculation behavior of mineral particles.The extended DLVO theory showed that interaction potentials were the key factors influencing particle gathering state.Hamaker constant,surface potential,polarization potential constant and the thickness of adsorption layer had direct impacts on dispersion-flocculation behavior of mineral particles.The decrease of pH value and increase of calcium ion concentration could increase surface potential of coal and montmorillonite,which reduced the scope of repulsion,weakened the energy barrier and promoted the hetercoagulation between coal and montmorillonite particles.The hetercoagulation between montmorillonite and coal was unfavorable to coal slime flotation: On one hand,hetercoagulation reduced the flotability of coal,resulting in flotation concentrate loss;on the other,hetercoagulation polluted foam products,leading to higher ash content;meanwhile,it reduced the coal flotation rate.A small amount of montmorillonite could not only reduce concentrate yield but increase concentrate ash.Montmorillonite with high content could even completely inhibit the flotation of coal.Calcium ions could promote the hetercoagulation between montmorillonite and coal,the higher concentration of the ions,the poorer performance of the flotation.Hydroxyl and hydroxide formed by ion hydrolysis could adsorb on minerals surface and form “ion electrostatic bridge” under the condition of alkaline,which was more likely to aggravate the adverse impacts of hetercoagulation on flotation.Regression models,which uncovered connection of flotation cumulative recovery,flotation rate constant with flotation time,pH value,metal cations and montmorillonite content were created by SPSS software.The correlation coefficient of flotation cumulative recovery and flotation time,pH value,metal cations,montmorillonite content was 0.850?0.103?-0.194 and-0.330.Flotation time was the key factor influencing cumulative recovery.The correlation coefficient of flotation rate constant and the four variables was -0.467?0.209?-0.384 and-0.624.Flotation rate constant decreased with the increase of the montmorillonite content and the extension of time.The regression models could predict the coal slime flotation index under different process conditions,but the accuracy was poor.BP neural network model established by Matlab software,which took flotation time,pH value,metal cations and montmorillonite content as input vector,flotation cumulative recovery and flotation rate constant as output vector,could predict the coal slime flotation index under different process conditions with high prediction accuracy and generalization capability.The effect of mechanical stirring on dispersion-flocculation behavior of coal and montmorillonite were studied.The mechanism of mechanical stirring improving coal slime flotation was also explored.The results showed that enhancing the stirring intensity and prolonging stirring time were helpful for mineral particles to disperse and improved the performance of coal slime flotation.Slime particle size determined the difficulty of desorption and dispersion.When the impeller speed was higher than 6.28 m/s,stirring time exceeded 120s,the flotation index was optimal.With similar flotation concentrate ash,flotation concentrate recovery with strong stirring was 10 points higher than that with weak stirring.It could be attributed to that high intensity stirring removed the fine montmorillonite from coal surface,which reduce slime coating,improved the hydrophobicity of coal surface,increased the surface properties difference of coal and gangue minerals.The paper represented the effect of depressants on coal slime flotation behavior.Experiments denoted that sodium silicate,SHMP and SCMG had powerful inhibition to montmorillonite.Compared with no depressants,the ash of concentrate with depressants was cut by 1 points,at proximately same concentrate recovery.Hydrolysis components of sodium silicate and SHMP varied with pulp pH value.Theoretical study showed that Si?OH?₄ was the main component to depress montmorillonite in sodium silicate and the sequence of depressibility was Si?OH?₄> SiO?OH?₃^-> SiO₂?OH?₂^2-;Catenulate polymers formed by SHMP hydrolysis was the main component to depress montmorillonite,other hydrolysis components,such as H₂PO_<sup>4-,H₃PO₄,HPO₂^4- and PO₄^3-,had effect on the separation of montmorillonite and coal.The different adsorption capacity of montmorillonite and coal made sodium silicate,SHMP and SCMG own selective inhibition.Poor adsorption ability made coal affected by depressants slightly,meanwhile,montmorillonite had priority to adsorb depressants in the pulp due to stronger adsorption ability.When the initial concentration of depressants was 50 mg/L,adsorption capacity of montmorillonite for the three depressants was 0.48,1.04,and 2.02 mg/g,the depressants quantity absorbed on coal surface was 0.26?0.36 and 1.02mg/g.There was a wide gap betweem montmorillonite and coal in adsorption quantity.At high dosage,a large number of depressants absorbed on the surface of montmorillonite and coal,which depressed montmorillonite in conjunction with coal,made depressants lose selectivity.The mechanism of sodium silicate,SHMP and SCMG adjusting the agglomeration and dispersion behavior between coal and montmorillonite was in accord with the extended DLVO theory.By adhering to the surface of coal and montmorillonite particles,depressants increased the absolute value of zeta potentials of the minerals,enhanced the electrostatic repulsion,improved the hydrophilic polarization effect,strengthened the hydrophilic repulsion.Moreover,long-chain molecules formed from SHMP and SCMG hydrolysis adsorbed on the particle surface,generating dimensional stabilization effect and increasing the steric hindrance when particles approach to each other.According to the adsorption capacity diffenence of coal and montmorillonite for kerosene,reagents addition order was changed to adjust the selectivity of the flotation reagents.Kerosene absorbed on coal surface in preferential order,which prevented adsorption of depressants on the surface of coal particles,enhanced the selectivity of flotation reagents for different minerals,improved the performance of flotation.Artificial mixed coal sample flotation tests showed that adjustment with mechanical stirring or depressants alone could not achieve the effective separation of coal and montmorillonite mineral,“strengthening dispersion-synchronous depression” was an effective regulation treatment of flotation separation.Finally,the results of flotation with Dongpang coal were used to illustrate the effect of “strengthening dispersion-synchronous depression” regulation in this study.The performance of Dongpang coal flotation was improved significantly with the regulation treatment.The recovery and ash content of the concentrate was 46.34% and 13.61% respectively.Compared with no regulation,the ash content of concertrate reduced obviously and the flotation perfect degree increased at proximately same concentrate recovery.