Green Conversion of Coal Fly Ash into Soil Conditioner: Technological Principle and Process Development

Abstract

White mud is residue discharged during the acid method in the aluminum extraction process from coal fly ash, and this material is harmful to the environment. The implementation of an environmentally friendly and valuable way to use white mud is a key factor restricting the commercial application of the acid method in the fly ash alumina extraction technology. An analysis of white mud revealed the following: (1) it was highly enriched in SiO2 (70–80%) while concentrations of acid-soluble elements, such as Na, Al, and Fe, and some hazardous heavy metals, including Pb and Cr, were significantly lower than raw fly ash; (2) approximately 80% of SiO2 had relatively high reaction activity because of the foregoing Al-extraction treatment. Through an ingenious green chemical process, the complete conversion of white mud into silicon–calcium fertilizer (SCF) was achieved under very mild reaction conditions (approximately 100 °C and atmospheric pressure). Waste liquor was totally recycled, and no secondary solid waste was generated. The SCF had an available silicon content (ASC) of 35%, significantly higher than the commercial standard (20%). Converting them into soil conditioners or ecological remediation materials with the lowest possible energy consumption and secondary pollution may be the most promising approach for the future disposal of aluminosilicate industrial solid wastes.

Conversion of Harmful Fly Ash Residue to Zeolites: Innovative Processes Focusing on Maximum Activation, Extraction, and Utilization of Aluminosilicate

“…Then, the extraction, separation, and reuse of Si and Al would be quite easy. However, actually, the alkali leaching conditions of Si and Al partially overlap with those of the hydrothermal syntheses of zeolites; Si and Al in the alkali hydrothermal environment tend to combine with Na to form sodalite (Na 8 Al 6 Si 6 O 24 (OH) 2 ) precipitation. − Zeng et al (2007) adopted a two-step dissolution strategy (water-leaching for Na 2 SiO 3 and NaOH-leaching for NaAlSiO 4 ) to overcome this common issue. Its LR value after two-step dissolution was estimated to be less than 50%, nevertheless.…”

Spectroscopic characterizations of silicate fertilizers prepared by chemical deashing of coals

“…In recent years, alumina extractions have been realized by acid leaching of fly ashes in China, and the silica rich residues (white mud) with highly depleted heavy metals could be ideally used for the preparation of quality silicate fertilizers. Hydrothermal treatment or high temperature calcination were used for the preparation of silicate fertilizers by the white mud [ 13 , 14 ]. In particular, high temperature calcination should necessitate the addition of large amounts of alkalis as the fluxing agent [ 13 ], hydrothermal treatment could alleviate sodium in silicate fertilizers by using alkalis as the catalytic agent [ 14 ].…”

“…Hydrothermal treatment or high temperature calcination were used for the preparation of silicate fertilizers by the white mud [ 13 , 14 ]. In particular, high temperature calcination should necessitate the addition of large amounts of alkalis as the fluxing agent [ 13 ], hydrothermal treatment could alleviate sodium in silicate fertilizers by using alkalis as the catalytic agent [ 14 ]. As a result, considerable sodium would have to be added or left in the silicate fertilizers prepared by the white mud, which could lead to the saline alkali deterioration of soils by long term applications [ 15 ].…”