Study On Surface Properties And Flotation Mechanisem Of Liangdu Oxidized Coal

Flotation is one of the most widely used and the best separation method of fine coal.Due to containing a greater amount of oxygen functional groups on its surface,the oxidized coal has high hydrophilicity which makes the coal difficult to float with conventional collectors.Therefore,seeking efficient flotation reagents and establishing reasonable system of flotation reagents which enchance the adaptability of coal and collectors and reduce the dosage of collector is one of the ways to reduce the cost of oxidized coal flotation and realize the reasonable utilization of resources.Sample of raw coal was obtained from Shanxi Liangdu coal mine,and it was oxidized under the laboratory condition of low-temperature oxidation.Through the analysis of surface property of raw coal and oxidized coal,the increase of oxygen functional groups,the decrease of surface negative charge,the reduce of hydrophobicity were found after oxidized,which leaded to lower flotation.The blend of kerosene and oleic acid was selected as collectors,and the flotation tests were carried out by controlling slurry pH value,composite rate,collector dosage,pulping time and addition of inorganic salts to determine the best flotation condition.The effect mechanism of collectors and CaCl₂ on oxidized coal surface was measured and analyzed by FTIR,zeta potential and immersion heat under different conditions.It is shown that the optimum condition of this oxidized coal flotation was slurry p H value of 5,1:1 blend of oleic acid and kerosene,1.80L/t collector dosage,1min pulping time,and 0.05mol/L calcium chloride concentration.Under this condition,the concentrate yield was 72.23%and the combustible recovery was 82.79%with 11.15%product ash.In addition,the effect of collectors on the surface of oxidized coal was studied FTIR,Zeta potential and immersion heat.The results showed that the ether groups increased and methyl groups decreased after adding collectors and inorganic salts comparing to no collector.It is shown that the studied collectors were beneficial to the flotation of oxidized coal via selectively masking the hydrophilic sites on oxidized coal surface and enhancing the attachment of bubbles.Kerosene,oleic acid and their blends can reduce the Zeta potential and immersion heat of oxidized coal.Among kerosene,oleic acid and 1:1 oleic acid/kerosene mixture,kerosene is the least effective reagents to reduce the Zeta potential of coal,while oleic acid is the most effective one.The immersion heat of coal decreased by 0.344J/g,0.299J/g,and 0.548J/g after adding kerosene,oleic acid,and the 1:1 blend of oleic acid and kerosene,respectively.Because of attraction between hydrocarbon chains of kerosene and hydrocarbon on coal surface,kerosene will adhere to oxidized coal surface and spread out,so as to cover on oxygen functional groups on the coal surface,which reduces the coal surface electronegativity and immersion heat to some extent.When oleic acid is added,the carboxyl group of oleic acid will combine with the oxygen functional groups on the coal surface by hydrogen bonding,which leads to the covering of oleic acid on the oxygen functional groups on the coal surface.In the meantime,the nonpolar functional group of oleic acid will be exposed on the coal surface.Thus the influence of the oxygen functional groups on the coal surface will be reduced,which results in the decrease of Zeta potential and immersion heat of oxidized coal and the improvement of hydrophobicity and flotability of oxidized coal.When the blend of oleic acid and kerosene was used,the Zeta potential of coal is between that using oleic acid and kerosene.However,the immersion heat was lower than using kerosene and oleic acid alone because of oleic acid anchoring and adsorbing on the hydrophilic site which is benefical to the adhesion and spreading of kerosene on the coal surface.At last,the influence of inorganic salt CaCl₂ and collector on FTIR,Zeta potential and immersion heat of oxidized coal was studied.The results showed that the ether and methyl groups increased and the Zeta potential and immersion heat will be significantly reduced using Ca²⁺and collector simultaneously,which was consistent with the flotation results.As a strong polarity cation,Ca²⁺could associate with the dissociated carboxyl anion of oxidized coal surface and formed–COOCa⁺.When oleic acid was in the form of anion in the pulp,it would associate with–COOCa⁺further to form–COOCaCOO–R and deposit on the oxidized coal surface.At the same time,the nonpolar chain of oleic acid is exposed to the coal surface which will act as a wedge to improve the hydrophobicity of coal surface.As a result,it will promote the attachment and spreading of kerosene droplet on the oxidized coal surface,leading to the formation of hydrophobic oil film.Thus the surface electronegativity and immersion heat of oxidized coal were decreased.