Natural oxidation of coal tailings from Middelburg area (South Africa), and impact on acid-generation potential

N Van Wyk,E Fosso-Kankeu,M Le Roux,F.B Waanders,D Moyakhe,Q.P Campbell

doi:10.17159/2411-9717/1205/2020

N Van Wyk, E Fosso-Kankeu + Show 4 more

Open Access

https://doi.org/10.17159/2411-9717/1205/2020

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Abstract

SYNOPSIS The aim of this study was to determine the acid-generating potential of coal mine tailings located in the Middelburg area, South Africa, using conventional assessment techniques. Four coal-tailing samples were collected from different layers of a coal-tailing pile. The acid-base accounting (ABA), acid-buffering characteristic curve (ABCC), and net acid generation (NAG) methods were used in conjunction with a mineralogical investigation to assess the potential of acid mine drainage (AMD) formation from the tailings. The results showed that the top layer of the pile, which was exposed to the atmosphere, was most likely to form AMD, exhibiting the lowest paste pH (2.5) and ABA of zero kg H2SO4 per ton. The ABA results were compared to ABCC results and mineralogical calculations to confirm the findings. The results from the ABCC test and calculations based on mineralogy indicated that the ABA method overestimates the effective or readily available acid neutralization capacity (ANC) and the true MPA of each sample; only three of the four samples could therefore be classified as acid-forming, but with low acid-generating capacity. Although the findings show that the top layer of the coal tailings in the Middelburg area is most likely to form AMD, it is important for future studies to further investigate the kinetics of acid formation. Keywords: acid mine drainage, acid-base accounting, mineralogy, net acid generation, coal mine tailings.

Highlights

South Africa has mined coal commercially since 1857 and is one of the biggest coal producers in the world
The results showed that the top layer of the pile, which was exposed to the atmosphere, was most likely to form acid mine drainage (AMD), exhibiting the lowest paste pH (2.5) and acid-base accounting (ABA) of zero kg H2SO4 per ton
The findings show that the top layer of the coal tailings in the Middelburg area is most likely to form AMD, it is important for future studies to further investigate the kinetics of acid formation

Summary

Introduction

South Africa has mined coal commercially since 1857 and is one of the biggest coal producers in the world. Coal is currently the main source of energy for power generation in South Africa and will continue to be so for the foreseeable future (Hancox and Götz, 2014; Fosso-Kankeu, 2019). The mined coal is subjected to a beneficiation process whereby the less valuable fraction, which cannot be processed economically, is separated. This leftover material is commonly deposited in large piles (tailings). Tailing piles can reach up to a hundred metres high and stretch for kilometres These tailings generally contain materials with low coal grade but still contain a high concentration of sulphide minerals (Dolt, 2010; Fosso-Kankeu et al, 2011; Koch et al, 2017)

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