Case study modelling for an ettringite treatment process

Several technologies have been developed to treat acid mine drainage (AMD) and attention is shifting towards the removal of sulphate. The formation of ettringite, a hydrous calcium aluminium sulphate mineral, is an option to treat AMD and can reduce the sulphate concentration to well below the discharge specifications. The process modelled in this study includes the formation of ettringite and the recovery of gibbsite through the decomposition of recycled ettringite. The modelling of this process was done using PHREEQC and the results presented in this paper are based on the outcome of different case studies that investigated how the process is affected by a change in parameters. These include, changing the feed water pH, altering the split fractions in the hydrocyclone for the gibbsite recovery and varying the pH for ettringite formation.

Integrated treatment of acid mine drainage using cryptocrystalline magnesite and barium chloride

This study endeavours to report the efficiency of a hybrid approach in the treatment of acid mine drainage. Cryptocrystalline magnesite was used to pre-treat acid mine drainage and barium chloride was used to remove the residual sulphate through the formation of barium sulphate. Batch experimental approach was adopted and the documented optimum conditions for both cryptocrystalline magnesite and barium salts were used. This was 60 mins for pre-treatment and 60 mins for polishing the residual sulphate. Water quality was assessed using ICP-MS and IC. Mineralogical composition was done using XRD, elemental composition with XRF, morphology with SEM and transmission electron microscope. Magnesite removed >99% of heavy metal species and 40% sulphate except for base cations. Barium managed to remove 99% of residual sulphate from an aqueous system. This indicate that the water meet the industrial, discharge and irrigation standards as stipulated by the water quality guidelines. As such, it can be concluded that this integrated approach can be used to treat acid mine drainage to useable standards as defined in the water quality guidelines. This will be a pragmatic approach for mining houses to curtail the impact of acid mine drainage by using natural and locally available materials such as cryptocrystalline magnesite and polish the water with barium salts.

Destabilization dynamics of clay and acid-free polymers of ferric and magnesium salts in AMD without pH adjustment

The physicochemical treatment was employed to treat acid mine drainage (AMD) in the removal of turbid materials using clay only (exp A) and a combination of clay, FeCl3 and Mg(OH)2 (exp B) to form a polymer. A 5 g sample of clay (bentonite) was added to 1.2 L of AMD and treated in a jar test at 250 rpm for 2 min and reduced to 100 rpm for 10 min. A 200 mL sub-sample from the 1.2 L mother liquor was poured into five 500 mL glass beakers, and 20 mL dosages of a polymer of 0.1 M Fe3+ in (FeCl3) and 0.1 M Mg2+ in (Mg(OH)2) was added to the beakers. The samples were allowed to settle for 1 h, after which the supernatant was analyzed for pH, total suspended solids (TSS), dissolved oxygen (DO) and oxidation-reduction potential (ORP) (exp A). A similar set of experiments was conducted where 200 mL of the AMD sample was poured into 500 mL glass beakers and (20–60 mL) dosages of a combination of 5 g clay, 0.1 M Fe3+ (FeCl3) and Mg2+ (Mg(OH)2) polymer was added and similar mixing, settling time and measurements were conducted (exp B). The polymers used in exp A exhibited TSS removal efficiency (E%) which was slightly lower compared with the polymer used in exp B, above 90%. Clay has a high TSS removal efficiency in the treatment of the AMD, indicating that adsorption was a predominant process in exps A and B. The scanning electron microscope (SEM) micrographs of the AMD sludge of both exps A and B, with a rigid and compacted structure consisting of dense flocs surrounded by the smaller flocs bound together, corroborate the fact that adsorption is a predominant process.