Sugar and Steel By-Product Utilization in Acid Mine Drainage Remediation

Abstract

In this research, two industrial by-products produced close to South African mining regions were used to treat acid mine drainage (AMD): steel slag and sugarcane bagasse, i.e., the shredded cane stalk residual after sugar extraction. Basic oxygen furnace (BOF) slag has a high alkalinity that makes it ideal for neutralizing acids. Sugarcane bagasse has a high surface area, and its slow decomposition makes it an ideal host media and long-term carbohydrate source for sulfate reducing bacteria (SRB). Accordingly, this research explored the viability of remediating AMD in a two-step continuous process combining both materials. A design was proposed and tested, in which a mixture of BOF slag eluate (generated from a treated water recycle loop) and raw AMD was used to initially buffer the AMD solution and precipitate heavy metals into a sedimentation tank. This avoided toxic shocking the SRBs in a subsequent sugarcane bagasse bioreactor. Overflow from the sedimentation tank was then passed through a packed bed containing sugarcane bagasse inoculated with SRBs as a polishing step to remove sulfate, precipitate metal sulfides, and elevate pH to near neutral pH conditions. The effluent from this vessel represents the treated water, and a fraction was recycled through a packed bed of BOF slag to create an alkaline eluate for the pretreatment of the raw AMD solution. The effect of AMD composition and slag particle size on the treatment was studied. The operation of the designed process at the laboratory scale, between 1 and 10.4 L/day, has confirmed the buffering of the AMD solution to a pH of between 7 and 8 and the removal of heavy metals and sulfate to levels below 10 mg/L for Al, Fe, Mg, and Mn and <200 mg/L for sulfate. The amount of Ca increased in the system, which was attributed to the dissolution of slag. The SRB functioned well in the system with a maximum sulfate reduction of 35% (accounting for 2% of overall sulfate removal) occurring across the sugarcane bagasse bioreactor. Precipitation and sedimentation accounted for the bulk of sulfate removal with over 90% of sulfate being removed across the sedimentation tank. Residence times of between 44 and 4 days were achieved for different conditions—smaller particle sizes of slag exhibiting better performances in terms of recycle rate and residence times.