| Lignite reserves are abundant in China,but such deposits currently have a low utilization rate mainly because of its special features.Since lignite has a low degree of metamorphism and a high tendency of argillization,it is difficult to carry out floatation of the generated coal slime owing to poor surface hydrophobicity and good pore development.An effective method to improve the floatation property of lignite is to modify the lignite structure.In recent years,mechanical hot pressing is an efficient approach to carry out such modification since heat treatment can reduce the number of oxygen-containing functional groups on the lignite surface,thereby enhancing the hydrophobicity.Lignite pores collapse under mechanical pressure and weaken the absorption of reagents in the pores.In order to further explore the mechanism of lignite processing through mechanical hot pressing,this study used the lignite samples from Holingol,Xilinhot for investigation.It used a mechanical hot pressing device to modify the lignite samples and analyzed the change in properties before and after modification.Further,a coal slime flotation test was carried out to determine the difference in the floatability of the samples before and after modification to further understand the mechanism of lignite processing through mechanical hot pressing.The main conclusions of this paper are described below.(1)The physical and chemical properties of the lignite samples from Holingol,Xilinhot were studied.The XRD and XRF techniques were used to analyze the mineral constituents of lignite,while an automatic industrial analyzer was used for the technical analysis.The Fourier infrared spectrometer and XPS techniques were used to analyze the oxygen-containing functional groups on lignite surface,while the mercury porosimetry,the specific surface area tester,and the scanning electron microscope were used to analyze the structural characteristics and surface morphology of lignite pores.The results showed that the lignite sample consists mainly of kaolinite and quartz,which accounted for 60% and 28%,respectively.There were many oxygen-containing functional groups on the lignite surface and the percentage of carbon-oxygen bonds was as high as 22%.Many pore structures were found in the lignite sample-large pores had a pore volume of 0.285 ml/g.The surface of the lignite samples was extremely rough.Therefore,it exhibited poor surface hydrophobicity,weak floatability,high reagent consumption,and less effective direct floatation.(2)A mechanical hot pressing device was used to treat the lignite sample from Holingol,Xilinhot by appropriately setting the pressure and temperature of the device in order to investigate how pressure and temperature can affect lignite floatation.Further,the principle and mechanism of mechanical hot pressing were also discussed.A laboratory floatation machine was used to carry out floatation test on the lignite sample before and after hot pressing and the results was compared.The results showed that for the same clean coal ash content,the clean coal productivity from the floatation of the hot-pressed lignite sample was higher than that of the direct floatation of raw coal.In the case of a hot-pressed setting at 9 MPa and 200 °C,the clean coal productivity from the floatation of the hot-pressed lignite was 21% higher than that of the direct floatation of the raw coal,which indicates that mechanical hot pressing can effectively improve the effect of lignite floatation.(3)Changes in the physical and chemical properties of the lignite surface before and after hot pressing were investigated,where some representative lignite samples were selected for qualitative analysis on changes in the surface properties.The testing tools used include automatic industrial analyzer,Fourier infrared spectrometer,XPS,scanning electron microscope,mercury porosimetry,and specific surface area tester.The results showed that after hot pressing,the moisture content in the samples decreased from 27.86% to 20.45%.The number of oxygen-containing functional groups on the surface decreased,and the percentage of carbon-oxygen bonds decreased from 22.06% to 10.23% after hot pressing.The total pore volume decreased from 0.56 ml/g to 0.13 ml/g.Furthermore,the surface changed to relatively smooth and angular according to the observation obtained from the scanning electron microscope.(4)To further understand the mechanism of the hot pressing device on the lignite floatation interface,this study investigated the change in free energy of the lignite surface after hot pressing,and the interactive free energy of the interface in water between collector and lignite sample.With the aid of a KRUSS100 surface tension meter and a DSA100 contact angle meter,the van Oss-Chaudhury-Good theory was used to detect and calculate the non-polar Lifshitz-van der Waals effect and the polar Lewis acid alkaline effect in order to calculate the change in free energy on the interfaces of the lignite,water,and collector.The results showed that the total free energy of the lignite sample after hot pressing decreased from 47.18mJ/m2 to 43.59mJ/m2.The free energy on the interfaces of the lignite,water,and collector decreased,where the surface energy of the lignite sample after hot pressing at 12 MPa and 200 °C was lower than that of the original sample by 2.59 mJ/m2.This study investigated how hot pressing affects the surface and pore of lignite samples and the mechanism of hot pressing on lignite floatation in order to provide theoretical guidance for the improvement of lignite floatation. |
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