Effect Of High-temperature Heating Treatment On Hydrophobicity Of Coal Surface

Hydrophobicity is a key factor affecting coal flotation.The high-temperature environment caused by spontaneous combustion of coal,power plants and blast furnace injection would change the hydrophobicity of the coal surface.In this paper,two ranks of low-ash coal,the non-caking coal and anthracite,are selected as experimental samples.The high-temperature tube furnace is used to simulate the actual high-temperature environment under the N2 and oxygen-deficient atmosphere?3%O2+97%N₂?.Then,Modern analytical instruments are employed to characterize the physical and chemical properties of coal surface.The influence of high-temperature on the hydrophobicity of coal surface and its mechanism are studied.The main conclusions are as follows:The decomposition reaction of non-caking coal during the pyrolysis is violent,and a large amount of gaseous/liquidoid products are generated,mainly including H₂,CH₄,CO,phenol and unsaturated hydrocarbons.The pyrolysis process of anthracite is relatively mild and only the gaseous products can be detected,which consists of dominated by CH4,CO2 and CO.After pyrolysis,the induction time of non-caking coal and anthracite is reduced,and the flotation yield increase significantly.When the final temperature of pyrolysis reach to 800°C and 900°C with the constant temperature time exceeded 15 min,the flotation yield of coal samples is more than 90%.The-CH₂-CH₂-structure on the coal surface and the C on the aromatic nucleus are oxidized through the attack of the O atom,which caused the sharp weight loss of coal samples with the large generation of CO₂.The induction time of non-caking coal and anthracite increase rapidly when the oxygen deficient oxidation temperature exceeds500°C and 600°C,respectively.The floation yield of the two coal samples decrease sharply after the high-temperature oxygen deficient oxidation,and when the temperature exceeds 600°C,the floation yield is less than 10%.The content of C-C/C-H hydrophobic group on the coal surface decreases and the content of oxygen-containing functional groups increases after high-temperature hypoxia.When the temperature is 800°C,the most hydrophilic-COOH group content on the non-caking coal surface increases to 9.16%,while the content of-COOH groups in anthracite has been increased to three times that of raw coal.The surface energy analysis results show that the polar probe molecules can not effectively adhere to coal surface after pyrolysis,and the dispersion component is also reduced by pyrolysis,which leads to the lower‘coal-water'adhesion work.As a result,the wettability of the coal surface is reduced after pyrolysis.On the contrary,after the high-temperature oxygen deficient oxidation,the intereaction between both of the non-polar and polar solvent probe molecule and the coal surface become extremely strong,and the elution time of the solvent increases sharply,which results in the larger net retention volume V_N.The coal surface energy and the‘coal-water'adhesion work becomes larger,resulting in a decrease in the hydrophobicity of the coal samples surface.The pores on the coal surface increase significantly after pyrolysis During the flotation process,the pores on the coal surface are filled with gas,forming microbubbles,and the microbubbles attract each other to become a‘coal-gas-coal'hydrophobic flocculation,which hence the flotation yeild.Although high-temperature oxygen deficient oxidation can increase the pores on the coal surface,it also significantly increases the polar oxygen-containing functional group content and enlarge the surface energy of the coal sample,making the pores easily filled with water and causing thicker around the coal surface.As a result,the coal sample is not easy to adhere to the bubbles,and the flotation yeild of the coal sample is reduced.Taking kerosene as the collector cannot improve the surface hydrophobicity of coal samples after high-temperature oxygen-deficient oxidation.Even when the dosage of the agent reaches 50 kg/t,the increase rate of floatation yield is still less than 4%.It indicates that the linear hydrocarbon oil collector should not be employed as the flotation agent for oxidation coal samples,but more efficient agents should be explored to improve the flotation behaviour of such oxidated coal samples.