A cut-off grade for gold and gallium in coal

Abstract

The contents of Au, Ga, As, Sb and other elements in two hundred and thirty-four samples of coal, partings, roof and floor collected from Guizhou Province, China were analyzed by Instrumental Neutron-Activation Analysis (INAA). In order to discuss the relationship between Au, Ga in coal and in coal ash, the contents of Au and Ga in 815°C coal ash were also determined by INAA and partial Ga by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analysis results showed that: (1) the average contents of Au and Ga in upper Permian coal from Guizhou are 6.92ng/g and 10.65μg/g, respectively, far higher than that in Chinese coal and world coal; the average contents of Au (59.73ng/g) and Ga (19.16μg/g) in partings, roof and floor are higher than that in the coal and the Clarke value of sedimentary rock (especially for Au); (2) during the coal ashing process, Ga is enriched in the residues more easily than Au, which is mainly because of the difference in the mode of occurrence of the two elements. Gallium mainly occurs as the substitute of Al in the form of isomorph, which makes it hard to volatize; while Au shows a high association with organic matter, sulfide, and carbonates, and some may be nano-gold, which is easily volatile during coal combustion. Only Au occurred in the aluminosilicate is easily left in the residues; (3) we collected data on the Au and Ga contents in coal and coal ash from China and then conducted correlation analysis by combining data from coals and coal ashes worldwide; the cut-off grades of Au and Ga in coal are considered to be 200ng/g and 60μg/g, respectively. This method could also be used to determine the cut-off grade of other valuable elements in coal.