Chemical Weathering in a Hypersaline Effluent Irrigated Dry Ash Dump: An Insight from Physicochemical and Mineralogical Analysis of Drilled Cores

Accumulation of high ionic strength effluents (brines) that require disposal in inland industries where water recycling is necessary due to scarcity is a major challenge. A coal combustion power utility in South Africa utilizing a dry ash disposal system produces 1.765 Mt of fly ash per annum and also employs the zero liquid effluent discharge policy (ZLED) to manage its liquid effluents. Fly ash is conditioned for dust suppression before being conveyed to the ash dumps with the high saline effluent. The saline effluents results from various processes employed for maximum utilization, upgrading and re-use of various mine water and industrial effluents such as RO, EDR, softening and ion exchange in an effort to adhere to ZLED policy. In the ash dumps it is further conditioned by irrigation with the high saline effluents, therefore the ash acts as a repository for the salts. This study is an attempt to understand the chemical weathering of the effluent conditioned fly ash and species mobility in a dry disposal scenario. A combination of leaching tests was performed for fresh ash and drilled cores to estimate the highly leachable species. Results from DIN-S4 tests of the fresh and weathered ash reveal that Ca, K, Na, Mg, Ba, SO42-, Se, Mo and Cr are highly leached. Leaching tests also revealed that major soluble components in the solution at equilibrium are Ca, Na, SO42- and K. Weathering profiles of the ash dump cores were observed to follow a similar trend. The greatest weathering was observed to take place at the top layer (0.55-3 m depth) in the weathered ash cores (15 years and older), showing that infiltration of rain water over time has a profound effect on the decrease of the pore water pH.  Analysis of the extracted pore water in each of the different weathered ash cores by depth indicated the mobility of several elements through the ash. Increased cation exchange capacity at 4-5 m depth suggests a transient mineralization zone.Key words:  Weathered fly ash; Pore water; Ash dumps; Hypersaline effluents; X-ray diffraction analysis; DIN-S4 test; Cation exchange capacit

Passive remediation of acid mine drainage using cryptocrystalline magnesite: A batch experimental and geochemical modelling approach

Acid mine drainage is generated when mining activities expose sulphidic rock to water and oxygen leading to generation of sulphuric acid effluents rich in Fe, Al, SO4 and Mn with minor concentrations of Zn, Cu, Mg, Ca, Pb depending on the geology of the rock hosting the minerals. These effluents must be collected and treated before release into surface water bodies. Mining companies are in constant search for cheaper, effective and efficient mine water treatment technologies. This study assessed the potential of applying magnesite as an initial remediation step in an integrated acid mine drainage (AMD) management system. Neutralization and metal attenuation was evaluated using batch laboratory experiments and simulations using geochemical modelling. Contact of AMD with cryptocrystalline magnesite for 60 min at 1 g: 100 mℓ S/L ratio led to an increase in pH, and a significant increase in metals attenuation. Sulphate concentration was reduced to ≈1 910 mg/ℓ. PH redox equilibrium (in C language) (PHREEQC) geochemical modelling results showed that metals precipitated out of solution to form complex mineral phases of oxy-hydroxysulphates, hydroxides, gypsum and dolomite. The results of this study showed that magnesite has potential to neutralize AMD, leading to the reduction of sulphate and precipitation of metals.SP201