Beneficiation of kraft pulp millwaste: using green liquor dregs in treatment of acid mine drainage as a new disposal solution in South Africa.

Abstract

Globally, water scarcity, depletion of non-renewable natural resources, handling and management of industrial wastes are not only significant environmental and economic burdens but also impact human and environmental health. In South Africa, the mining industry is essential to the country and a significant contributor to the GDP of the country. Two major ones are gold and coal mining. Unfortunately, the industries generate large amounts of acid mine drainage (AMD) that originate from the mining activities. The AMD is formed when the 3% pyrite mineral found in the mine effluent dams and voids of gold and coal mines is oxidised upon exposure to water and oxygen resulting in the formation of sulphuric acid which dissolves and leaches surrounding rock and soil matter thus introducing toxic metals into the aquatic waters and biota thus negatively impacting human, animal, and environmental health. Currently, this acidic discharge is neutralised by the use of alkaline reagents such as CaO, Ca(OH)2, NaOH, and CaCO3. However, these chemicals are expensive or are not sustainably resourced in the case of the widely used calcium carbonate. Possible landfilled, industrial, carbonic wastes such as the green liquor dregs (GLDs), from Kraft pulp mills, could be used as sustainable alternatives for the CaCO3 and its derivatives in pre-treating AMD. These wastes streams are produced at the rate of 7-15 kg/ton of dry pulp. In South Africa, this translates to ~100 000 tons of GLDs that are produced and landfilled annually. However, this is an environmentally challenging and not cost-effective practice. Thus, this study entailed characterising GLDs produced in South Africa, evaluating them for the potential of neutralisation of AMD, and optimising the neutralisation process variables. This is the first time that this type of study has been conducted in South Africa. Furthermore, the neutralisation of the coalfields AMD with GLDs has not yet reported in the literature and this is the first time the optimisation of AMD treatment by GLDs is being studied. The study entailed statistically designed experiments that employed a Box-Behnken Design and Response Surface Methodology to optimise the variables involved in the neutralisation process. The results indicate that although characteristics of GLDs from different mills differ they all are effective in neutralisation of AMD from gold or coal mines and their neutralisation potential is similar to that of calcium carbonate. Nevertheless, the SEM/EDX, ICP-AES/ICP-MS, XRF, and XRD analysis on the sludge that emanated from either of the neutralisation processes showed that the neutralisation of AMD using GLDs is effective and enables precipitation, co-precipitation, or adsorption of the different metals from the AMD. The pH of AMD could be raised to optimum pH process value and reduce the acidity at a low dosage, thus offering a competitive advantage over commercial CaCO3. Thus, using GLDs for neutralisation of AMD can be an effective symbiotic process that can benefit two industries in managing their waste discharges: the Kraft pulp industry and the mining industries. The results obtained from the optimisation of the variables (neutralising reagent dosage, process time, and stirring speed) involved in the neutralisation process showed that the reagent dosage was the most significant variable while the stirring speed was the least significant one. The models for all the GLDs and reference materials were proven to be significant because all of them had a p-value of <0, 000001 and their R2 and Adjusted R2 were close to 1.