As the largest producer of coal and electricity in southern Africa, South Africa finds herself in desperate need of efficiently alleviating the impacts of mining and power plants on her environment. Acid mine drainage (AMD) refers to the creation and movement of acidic water saturated with metals which originates from operational and closed coal and metal mines. Fly ash (FA) is an alkaline residue of coal combustion obtained from power plants which contains high concentrations of various metals. As a result of the negative effects of both fly ash and acid mine drainage, Vadapalli et al. (2012) created and tested an efficient zero-waste system in an attempt to recycle the by-products. The researchers intended to neutralize the acid mine drainage with fly ash and test the viability of the solid residue from the previous experiment as a possible filler for the excavated mining sites. They also attempted to synthesize zeolite-P from the AMD-FA mixture. Zeolite-P has a considerable industrial and commercial value as well as the ability to further clean the neutralized water. Vadapalli et al. found that fly ash was a significant neutralizer of acid mine drainage as it removed high levels of major contaminants. Though the solid residue did not have the required strength to be used as a filler, the researchers hypothesized that increased concentrations of cement and/or a longer setting time would increase strength. Even though the zeolite-P synthesis was faulty in terms of yield, they found that zeolite-P was able to ‘polish’ or further clean the water obtained from the AMD-FA treatment of selective elements. These results could allow South Africa access to a cost-effective waste management system —Monkgogi Otlhogile
Vadapalli, V R K., Gitari, M W., Petrik L F., Etchebers O., Ellendt A., 2012. Integrated acid mine drainage management using fly ash. Journal of Environmental Science and Health: Toxic/Hazardous Substances and Environmental Engineering 47, 60–69.
Vadapalli et al. used acid mine drainage samples from a coal mine and fly ash samples from a power plant in the Mpumalanga Province in South Africa. Both the fly ash and the acid mine drainage samples were analysed for the concentrations of various elements. Various ratios of AMD and FA (3:1, 2.5: 1, 2:1, and 1.5:1) were mixed and stirred for six hours. The pH and the Electrical Conductivity (EC) of the solutions were measured at different time intervals during the six hours. The solid residue tested as a filler was obtained by a 4:1 ratio of the AMD and FA. After decantation, the residue had water and 3% ordinary Portland cement (OPC) added to it. Much like cement, the solid residue mixture was allowed to settle or ‘cure’ in tubes for a period of 410 days during which strength and elasticity were tested. The zeolite-P was synthesized by obtaining 3:1 solid residue and then drying and crushing it. The crushed solid residue was added to sodium hydroxide, aged, and added to pure water. The product then underwent a thermal treatment and was rinsed until the rinse water had a pH of 9. The solid product was dried and then evaluated using x-ray analysis. This product was then used to clean the previously neutralized water by adding the solid product to the water at a ratio of 1:100 for an hour. The water was filtered and analysed for various elements.
Vadapalli et al. found that the introduction of FA almost immediately raised the pH of all the AMD-FA mixtures and continued to increase the pH after a buffer region during which the pH remained constant for a significant amount of time. During this buffer region, the reaction of major components of AMD and water such as iron took place which then increased the pH. However, because these reactions can only take place at a pH of 4–7, the pH remained constant as the reactions were taking place and then increased as the products of the reactions were released into the system. They also found that the final pH was directly proportional to the amount of FA added which leads to the conclusion that it was the FA that was responsible for the pH change. The electrical conductivity of the almost all the mixtures almost immediately decreased and after the same buffer region as the pH values, there was a gradual decline in EC. However, the researchers observed an EC increase in the 1.5:1 AMD-FA mixture at the end of the experiment as a result of transient sulphate. The researchers found that the contaminant removal rate except for aluminium and sulphate was directly proportional to the amount of FA added to the AMD. Though not all of the ratios were proportional, the FA particles played a major role in the significant removal of iron, sulphate, aluminium, manganese, magnesium, zinc, nickel, copper, and lead. Magnesium had an initial concentration of 2661 ± 35 and was decreased in the 1.5:1 mixture to 1.5 ± 0.02 while iron had an initial concentration of 5600 ± 81 and was decreased in the 1.5: 1 mixture to 5 ± 0.7. This showed that FA had a significant contaminant removal rate when added to AMD.
After measuring the strength and elasticity of the solid residue mixture over a 410 day period, the scientists found that the mixture’s elasticity and strength increased steadily. However, the elasticity had a temporary decrease after 180 days. At the end of the 410 days, the solid residue mixture displayed strength of approximately 0.30 MPa which fell short for the mixture to be used a filler for the excavation sites. However, the scientist suggest that using more ordinary Portland cement, increasing curing time, and a lower ratio of AMD to FA could increase the strength of the solid residue mixture. In the final stages of the system, the ratio of aluminium and silicon used in the synthesis process determined that the scientists had created zeolite-P. However, during the x-ray analysis, the scientists found that there was a significant amount of mullite which represented the unconverted fly ash. This suggests that the authors had not picked the optimum conditions for the complete synthesis of zeolite-P. As a secondary treatment agent for the neutralized water, zeolite-P was able to selectively remove elements that the FA was unable to effectively remove. Though, zeolite-P added sodium and silicone to the water, it was able to slightly decrease pH as well as significantly remove calcium, strontium, iron, manganese, vanadium, beryllium, and barium. The percentage removal ranged from the 34% removal of calcium to the 99% removal of manganese. The cumulative results of the authors’ study points to a zero-waste system which would effectively manage the by-products of mining and power plants in South Africa and other countries with the same environmental problems.