Flotation of low-rank coal using sodium oleate and sodium hexametaphosphate

Abstract A mixture of low-ash coal and gangue particles was used to investigate the effects of sodium oleate (NaOL), kerosene, and sodium hexametaphosphate (SHMP) on low-rank coal flotation. X-ray photoelectron spectroscopy etching revealed that NaOL was adsorbed on the coal surface effectively. Furthermore, zeta potential results confirmed the adsorption of NaOL and SHMP on the coal surface and gangue surface. The yield of clean coal increased sharply with increasing NaOL dosage, while the clean coal ash content initially decreased with increasing NaOL dosage and then increased. Compared to the use of kerosene alone, the addition of 3000 g/t sodium oleate can increase the yield of clean coal by about 50%. Compared to treatment without SHMP, the ash content of clean coal can be reduced by about 8%, the volatile matter and fixed carbon content clean coal can be increased by about 3% and 5% than using 500 g/t SHMP. The fixed carbon content of clean coal decreased slightly with the SHMP dosage above 500 g/t. The contact angle results showed that NaOL enhanced the hydrophobicity of coal surface, while SHMP did not significantly affect coal surface hydrophobicity. SHMP increased the electronegativities of both coal and gangue, and enhanced the electrostatic repulsion between the coal and gangue particles. Scanning electron microscopy/energy-dispersive X-ray spectroscopy showed that the SHMP reduced the amount of gangue particles coated on the coal surface, which reducing the ash content of clean coal. The attachment time results showed that the coal particles adhered to the bubbles and agglomerated easily, which greatly increasing the carrying capacity of single bubble to the froth layer and enhancing low-rank coal flotation in the NaOL system. This study can recover combustibles from coal slime waste, which can not only reduce the environmental pollution of slime, but also increase economic benefits.