Acid Mine Drainage Pilot Remediation System using Waste Products from the Steel Manufacturing and Sugar Industries
Naidu, Tamlyn Sasha (1,2,3,4); Sheridan, Craig Michael (1,2)
1: University of the Witwatersrand, South Africa; 2: Industrial Mine and Water Research Unit; 3: National Research Foundation; 4: Water Research Commission of South Africa
Acid mine drainage (AMD) has long been considered a global environmental hazard due to its long-term impairments to waterways and to the biodiversity of ecosystems. The amount of AMD produced and the high cost associated with treatment has led the United Nations to classify AMD as the second biggest problem that the Earth faces. As such, new cost-effective remediation methods are needed.
This research involves one such method; focusing on the operation of a waste beneficiation treatment scheme being tested at pilot-scale. It incorporates the use of two waste-products found in many AMD-affected countries: steel slag and sugarcane bagasse. The study has confirmed the potential of these products in AMD treatment through a two-step, chemical and biological process. Slag eluate contacted with raw AMD is used to buffer the AMD solution and precipitate heavy metals in a sedimentation tank. Overflow from this tank is then passed through a packed bed containing sugarcane bagasse inoculated with sulfate reducing bacteria (SRB) as a polishing step to remove sulfate, precipitate metal sulfides and elevate pH to near neutral conditions. A portion of the effluent is recycled through a packed bed of slag to create the eluate for pre-treatment of the raw AMD solution.
The AMD used in these experiments was characterized by: pH 2.4; 388 mg/L Al, 4256 mg/L Fe, 426 mg/L Mg, 96 mg/L Mn, 418 mg/L Ca and 15995 mg/L SO42-. Operation of the process at a laboratory scale treating 1 L/day 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 of <10 mg/L for Al, Fe, Mg, Mn and <200 mg/L for sulfate. Operation of the pilot scale system treating 1000 L/day has thus far shown maximum SO42-, Fe, Mn, Mg and Al removals of 75%, 50%, 100%, 5% and 96.4% respectively. The dissolution of the slag was found to follow a pseudo-first order model, and the alkalinity generation was found to occur mainly due to CaO and MgO dissolution. The SRB functioned in the slag eluate and a maximum sulfate reduction of 35% was found in the bioreactors.
If optimized, this treatment scheme could be implemented at mine sites throughout the world where these waste products are regionally available and could serve as either a low-cost pre-treatment program, or as a way for mines to recycle their acid waste for use as process or agricultural water.