Processing of coal gasification fine slag by different physical separation methods: Fate of typical heavy metals and comparison analysis on products

Abstract

The separation of carbon-mineral components in coal gasification fine slag (CGFS) is an important prerequisite for achieving large-scale utilization of the CGFS resources. This study aims to reveal the influence of three physical separation methods on the distribution features, occurrence modes, and environmental risk of typical heavy metals (i.e., V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) in the CGFS. The results show that all three physical separation methods can separate carbon-rich or mineral-rich fractions from the CGFS at different enrichment levels. Thereinto, froth flotation has a high balance between high yields and excellent separation performance. Furtherly, Ni, Cu, Zn, and Pb are more concentrated in the two carbon-rich fractions obtained by froth flotation while other heavy metals are mostly enriched in the mineral-rich fractions. Density separation is possible to obtain carbon-rich fractions with a fairly high fixed carbon content (68.29 %, dry basis). In addition, a rough separation of the carbon-mineral fractions in CGFS can be performed by particle size classification. The various concentrations and distribution features of typical heavy metals in different separated products are associated with their specific structures. The carbon particles derived through the froth flotation have a higher specific surface area than those produced from the density separation and size classification. It is found a high correlation between mineral content and typical heavy metal concentrations, while porous carbon particles with a high specific surface area and particle size effect could reduce the dependence of some heavy metals (e.g., semi-volatile or volatile heavy metals Ni, Cu, Zn, and Pb) concentrations on mineral content. In addition, the typical heavy metals in separated fractions with a high content carbon particles as well as small-sized particles tend to have a high proportion of mobile fractions. Moreover, the eco-risk assessment code (RAC) analysis results indicate that priority attention should be given to the potential environmental impacts of Mn, Ni, Cu, Ba, and especially Zn in high-carbon content products and small-sized fractions during their resource utilization process.