Whitening process and mechanism of coal gangue by sodium salt calcination

The accumulation of coal gangue results in serious environmental pollution, it is urgent and necessary to comprehensive, high value and efficient utilization of coal gangue. The resource utilization of coal gangue by calcination is important. In terms of the problems of high energy consumption and low efficiency, the use of sodium salt types and calcination process for coal gangue whitening has been investigated. Raw and calcined coal gangue were thoroughly characterized with respect to phases constituent, microstructures, pore structures and chemical bonds.Based on above investigations, the whitening mechanism and volatile products obtained from sodium chloride calcined coal gangue process were further analyzed by simultaneous thermogravimetry-Fourier transform infrared spectroscopy-Mass spectrometer (TG-FTIR-MS). Results show that after being calcined with sodium chloride at 860 ℃, the whiteness of raw coal gangue was upgraded from 38.5% to 86.0%. All the carbon and pyrite (FeS₂) contained in coal gangue was oxidized and correspondingly converted into hematite (Fe₂O₃) by desulfurization and the total amount of hematite and rutile (TiO₂) was reduced to 0.27%. The pore structural characteristics changed little at calcination temperature between 800 and 860 ℃. However, the calcination at 950 ℃ resulted in generation of spherical mullite, and a significant decrease in specific surface area and pore volume from 12.91 m²/g and 0.056 cm³/g for raw coal gangue to 6.58 m²/g and 0.031 cm³/g, respectively. The whiteness increased slightly at calcination time between 30 and 90 min (85.3% to 86.6%). Meanwhile, the whiteness changed little at calcination time between 90 and 150 min (86.6% to 86.7%). The addition of NaCl had little impact on the chemical bonds of calcined coal gangue. The evolution of mineral components in the calcination stage was relatively low, and the main volatile products were H₂O, HCl, CO₂ and H₂S. Moreover, NaCl reacted with SiO₂ and H₂O to decompose into HCl gas, which removes iron and titanium by the volatilization of the respective chlorides. The chlorinated products escaped from the surface of the material layer and discharged, resulting in a purified and whitened coal gangue.