Enhanced Coal Fly Ash Desilication Using Atmospheric NaOH Leaching with Simultaneous Magnetic Separation

Abstract

Coal fly ash (CFA) is a technogenic waste formed during coal combustion in thermal power plants (TPPs). The extraction of valuable components from CFA is complicated by the presence of a large amount of amorphous glassy mass and iron. Herein, a novel method of CFA desilication with complete extraction of the amorphous glassy mass without desilication product (DSP) precipitation and simultaneous magnetic fraction recovery in one stage is presented. The Fe recovery in the magnetic fraction using the proposed method was significantly improved from 52% to 68%. After conventional wet magnetic separation, followed by the proposed method for desilication and magnetic fraction separation, the Fe recovery was increased to 73.8%. Because of the absence of DSP precipitation, the Na2O content in the solid residue after desilication was lower than 1 wt.%. The simultaneous desilication and magnetic separation of magnetite was achieved by installing a belt of permanent magnets on the outer surface of the reactor, where the CFA was leached by the highly concentrated NaOH solution. The effects of different parameters on the extraction of Si, Al, and Fe from the raw CFA were elucidated by varying the liquid-to-solid ratio (L:S ratio) from 5 to 10, the temperature from 100 to 120 °C, the leaching time from 10 to 30 min, and the particle size from −50 µm to −73 µm. The optimal leaching parameters were determined to be temperature = 110 °C, L:S ratio = 7.5, and leaching time = 20 min. The extraction of Si and Fe under these conditions was higher than 66 and 73%, respectively. The Al extraction was lower than 10%. The solid residue of NaOH leaching and the magnetic fraction were examined by X-ray diffraction, X-ray fluorescence spectrometry, vibrating sample magnetometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller, and laser diffraction analyses.