Abstract
Classification of waste is an essential part of waste management to limit potential environmental pollution; however, global systems vary. The objective was to understand the waste classification of high density sludge (HDS) from acid mine drainage (AMD) treatment, according to selected global systems. Three sludges from two limestone treatment plants, and three others from a limestone and lime treatment plant from the Mpumalanga coalfields of the Republic of South Africa (RSA) were evaluated. Systems for the RSA, Australia, Canada, China, and the United States Environmental Protection Agency (USEPA) were considered. The USEPA system rated all six sludges non-hazardous, Canadian and Chinese systems allocated a hazardous status to one sludge from the limestone treatment plants based on Ni solubility. The RSA system considered two of the sludges from limestone treatment plants to be higher risk materials than did the other countries. This was due mainly to the RSA system’s inclusion of Mn and use of appreciably lower minimum soluble levels for As, Cd, Pb, Hg, and Se. None’s use of lime resulted in higher soluble Mn. Minimum leachable concentration thresholds for Cd, Hg, Pb, As, and Se in the RSA system were below method detection limits for Toxicity Characteristic Leaching Procedure (TCLP) extracts, making the guidelines impractical, and revision is advised. Considering all the systems, the probability that the HDS from the coalfields of Mpumalanga, South Africa will be classified as hazardous waste increases if the material is only subjected to limestone treatment because of Ni solubility.
Highlights
Neutralization of acid mine drainage (AMD) with lime (Ca(OH)2) or limestone (CaCO3) through conventional processes generates an inorganic solid, semi-solid or liquid waste referred to as sludge [1,2]
One consistency between the Canadian, Chinese, Australian, and Republic of South Africa (RSA) systems was the classification of HDSCaCO3 1 sludge from limestone–Site 2 as hazardous waste
For the RSA system it was for both Ni and Mn solubility because it is the only system that considers Mn
Summary
Neutralization of acid mine drainage (AMD) with lime (Ca(OH)2) or limestone (CaCO3) through conventional processes generates an inorganic solid, semi-solid or liquid waste referred to as sludge [1,2]. If the sludge is recycled and mixed with lime or limestone prior to AMD addition high density sludge (HDS) is generated [3,4,5,6,7]. The solution can be extremely acidic (pH 2.0 to 4.0) and contains transition metals and metalloids which can be toxic to the receiving environment. The treatment of AMD is needed to neutralize its acidity and remove/precipitate these metals and metalloids from solution. Flue gas desulfurization process facilitates dewatering of the sludge generated and encourages gypsum crystallization [12,13]. What makes HDS different is the appreciable iron oxide content and transition metals are often partitioned with the iron oxide fraction
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