A comparative study between the neutralizing capabilities of fly ash and green liquor dregs with acid mine drainage.

Abstract

In South Africa, acid mine drainage (AMD) is a huge problem and arises when sulphide-bearing materials become exposed to oxygen and water. AMD formation is catalysed by mining and mineral extraction activities. The AMD produced seeps into water bodies and this renders the water highly toxic and harmful to humans, animals and vegetation due to its high acidity, high concentration of toxic heavy metals and sulphates. The combustion of coal is the primary method of power generation in South Africa. A by-product of this process is fly ash (FA). Approximately 20Mt of FA is produced in South Africa a year and only 5% of this is used in other applications. Green liquor dregs (GLD) are a by-product waste produced in the pulp and paper industry. They are produced in the Kraft pulping process and primarily comprised of a mixture of sodium and calcium carbonates. Both these waste products pose massive environmental and disposal problems. GLD and FA are both highly alkaline; hence they can be used as neutralizing agents for the highly acidic AMD. The main aim of this project is to investigate the effectiveness of FA and GLDs in neutralizing AMD from coal mines. The effects of reaction time and neutralizing reagent (FA and GLD) concentration on the neutralization of AMD were studied. A 32 factorial design was employed for this research project. The reaction times used for the neutralization reaction were varied (1, 2 and 3 hours). The reagent concentrations used were also varied (0.4g/L, 1g/L and 2g/L). A total of 18 runs were conducted (9 for each reagent). The AMD was placed in beakers and electrical stirrers were used to ensure constant mixing. The reagent was added and the pH and electrical conductivity were measured after various reaction time intervals. From using the preliminary 18 runs, an optimum FA and GLD reagent concentration was obtained, and a run was conducted for each reagent, to achieve a theoretical goal pH of 7. The results indicated that the optimum FA and GLD concentrations were 0.728g/L and 0.422g/L, respectively. Hence, GLD would be a better neutralizing reagent as less of it would be required to neutralize AMD, when compared to FA. It was also proven that time plays a very small effect on the neutralization reaction. Overall, the results conclude that GLD would be better than FA for neutralization of AMD. The deposition of heavy metals caused issues and a future research study may be conducted to reduce this problem.