Optimization Of Flotation Reagents Based On Surface Properties Of Hard-to-float Fine Coal

Flotation is an important method to reduce ash and improve quality of slime.China is rich in low-rank coal reserves,and its rational development and utilization is of great significance to China's energy economic supply,but the reagent consumption is quite large and the recovery and the selectivity are not ideal due to the fact that the coal surface has high oxygen content and large quatity of surface pores,Therefore,Low rank coal is considered as a kind of coal slime which is difficult to float,and its reasonable development and utilization is of great significance to China's energy economic supply.The low-rank coal slime samples were collected from No.2 Selian Coal Mine in Inner Mongolia,Donglutian Coal Washery in Shuozhou,Silaogou Coal Washery in Datong,Jinhuagong Coal Mine in Datong,and Xiegou Coal Washery in Lvliang,and proximate analysis,ultimate analysis,particle size analysis,XRD and XPS tests were carried out to determine their basic properties.The oxygen content of Seliann Coal Mine,Donglutian Coal Washery,Silaogou Coal Washery,Jinhuagong Coal Mine,and Xiegou Coal Washery are 11.25%,9.54%,9.43%,8.37% and 7.73%,respectively,Based on the laboratory flotation experiments,nonlinear fitting of the relationship between the clean coal recovery and elements content were conducted and it was found that with the increase of oxygen content in the sample,the flotation effect using diesel of the same dosage as collector becomes worse.Through nonlinear fitting,the function model between them was constructed,and a negative correlation between combustible recovery and oxygen content was obtained.The diesel oil was then mixed with three kinds of compounds to obtain mixed collectors and used in the flotation of the above five coal samples,which involve unsaturated carboxylic acid(oleic acid),saturated carboxylic acid(n-octanoic acid,n-decanoic acid,lauric acid),and soaps(Sodium Laurate,sodium oleate).Sodium hexametaphosphate was also used as regulator in the above flotation experiments as a comparison.The flotation results show that,under the same dosage of reagents,the flotation effect of the compound reagents of saturated carboxylic acid(octanoic acid,decanoic acid,lauric acid),unsaturated carboxylic acid(oleic acid)and diesel oil is better than that of diesel oil,indicating that they can promote the flotation of coal slime.Among them,oleic acid is the best,significantly improving the flotation of low-rank coal,especially dramatically increasing the combustible recovery of coal with low oxygen content.Take Jinhuagong coal containing 8.37%O as an example,the combustible recovery can be as high as 91.18% at the proportion of 10%.The consumption of the mixed collector based on oleic acid also decreases greatly compared to diesel,saving more than 50%.Among the three saturated carboxylic acids,lauric acid shows the best effect and its appropriate proportion is 20%.The mixed collectors based on the soap also show some improving flotation effect but the effect is not as significant as that of oleic acid,and show less selectivity as well.The addition of appropriate amount of sodium hexametaphosphate has a good effect on the flotation of both Selian coal and Donglutian coal.It can reduce the ash content of the clean coal and effectively increase the ash content of the tailing coal.The addition of 1000 g/t sodium hexametaphosphate can also increase the flotation rate of Donglutian coal.The clean coal difference between diesel and mixed collectors were analyzed in terms of the contact angle,infrared spectrum,surface oxygen unctional group,the dispersing size of collectors in water,and Zeta potential,and the reaction mechanism of the mixed collectors,sodium hexametaphosphate and the low-rank coal was discussed.The contact angle test shows that the contact angle is negatively correlated with the oxygen content in coal.The higher the oxygen content,the smaller the contact angle,the more hydrophilic the coal surface.The clean coal collcted by the mixed collector based on carboxylic acid has a bigger contact angle than that of diesel,which indicates an improved low-rank coal floatation.XPS measurements show that there are four forms of C in coal: C-C,C-H,C-O,C=O,and COO-.The mixed collector base on oleic acid increases the hydrophobic functional groups and decreases the hydrophilic oxygen-containing functional groups,which improves the hydrophobicity of low-rank coal.The infrared spectrum results show that the polar component COOH in the oleic acid hydrogen bond with the hydrophilic region of coal sample,and the non-polar ends adsorbe on the hydrophobic region,which akes the surface of coal sample more hydrophobic.In addition,Laser particle size test shows that among the three mixed collectors based on carboxylic acids,oleic acid is dispersed as the smallest particle size and presents the most concentrated size distribution,which may expain its best improvement effect on the low-rank coal floation.The Zeta potential test showed that sodium hexametaphosphate increases the absolute Zeta value of the coal sample,and thus increases the electrostatic repulsion force between coal and gangue mineral particles,which ensures a more stable and well dispersed slurry system,and helps to improve the flotation effect.