Flotation is the common method for fine low rank coal upgrading while it is always inefficient due to the rough surface morphology and rich oxygen-containing functional groups. Hydrothermal pretreatment is widely used in dewatering for low rank coal. The physicochemical properties of coal surface considerably vary following the hydrothermal pretreatment. In this study, hydrothermal pretreatments at different temperatures for different times were applied to enhance lignite flotation. The hydrothermal pretreatment at 200 °C can significantly improve the flotation efficiency, while the temperature raised to 300 °C, the flotation recovery did not increase or even decrease. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Zeta potential, Brunauer–Emmett–Teller, and particle size distribution analysis were conducted to explain the underlying mechanism. The contents of pore and crack on coal surface considerably decreased and more fresh and smooth surfaces exposed following hydrothermal pretreatment, according to SEM analysis. The FTIR spectroscopy and XPS showed that the contents of oxygen-containing functional groups were decreased, while the hydrophobicity of lignite was increased following pretreatment. The decrease of Zeta potential also indicated the decrease of oxygen-containing functional groups and increase of hydrophobicity on coal surface. However, excessive temperature of hydrothermal pretreatment could increase the pore diameter and fine particle fraction, leading to a higher collector consumption and lower bubble–particle collision efficiency. Consequently, the flotation recovery was not improved or even decreased at the hydrothermal pretreatment temperature of 300 °C.

浮选是低阶煤精制的常用方法，但由于表面形态粗糙和含氧官能团丰富而总是效率低下。水热预处理广泛用于低等级煤的脱水。水热预处理后，煤表面的物理化学性质发生很大变化。在这项研究中，采用了在不同温度，不同时间进行水热预处理来增强褐煤浮选的方法。在200°C的水热预处理可以显着提高浮选效率，而温度升至300°C时，浮选回收率没有增加甚至没有下降。扫描电子显微镜（SEM），傅立叶变换红外光谱（FTIR），X射线光电子能谱（XPS），Zeta电位，Brunauer–Emmett–Teller，进行粒度分布分析以解释其潜在机理。根据SEM分析，经过水热预处理后，煤表面的孔隙和裂纹含量显着减少，并且暴露出更新鲜，更光滑的表面。FTIR光谱和XPS分析表明，预处理后，含氧官能团的含量降低，褐煤的疏水性提高。Zeta电位的降低也表明含氧官能团的减少和煤表面疏水性的增加。但是，水热预处理的温度过高可能会增加孔径和细颗粒分数，从而导致收集器消耗增加，气泡-颗粒碰撞效率降低。所以，