Enzymatic degradation of carbonaceous matter in contrasting South African refractory gold ores using cell-free spent medium from Phanerochaete chrysosporium

Abstract

Two carbonaceous gold ores (ZAFB and ZAFV) underwent sequential biotreatment to expose gold and minimize preg-robbing before cyanidation. These included comparable organic carbon contents (6.0%wt in ZAFB and 4.4%wt in ZAFV) but were contrasting in the graphitic degree of the carbonaceous matter. Based on Raman spectra, the carbonaceous matter in ZAFB was more physically and chemically disordered than the ZAFV. To reduce extraneous factors, ≥ 90% of sulfides in both samples were oxidatively dissolved by the thermophilic mixed culture. Afterwards, the secondarily-formed minerals (mainly jarosite) were dissolved by oxalic acid washing. The carbonaceous matter was then treated for 15 days with a cell-free spent medium (CFSM) with lignin-degrading enzymes produced from a Phanerochaete chrysosporium culture. The enzymatic treatment reduced the carbonaceous matter content in the ZAFB by 0.73%wt from an initial value of 7.56%wt, while the ZAFV was unchanged. Raman and 3D-fluorescence analysis of the carbonaceous matter showed that the CFSM treatment was much more effective on the ZAFB sample. This indicates that the graphitic degree of the carbonaceous matter is a significant factor when applying crude enzymes derived from P. chrysosporium. The lignin-degrading enzyme treatment improved the gold extraction of ZAFB from 88% to 93%. It was found that Raman spectroscopy could serve as a handy tool for determining carbonaceous gold ores that are suitable for enzymatic treatment.