| Summary: | The pollution of South Africa’s water resources puts a strain on an already stressed natural
resource. One of the main pollution sources is industrial effluents such as acid mine drainage
(AMD) and other mining effluents. These effluents usually contain high levels of acidity,
heavy metals and sulphate. A popular method to treat these effluents before they are released
into the environment is lime neutralisation. Although this method is very effective to raise the
pH of the effluent as well as to precipitate the heavy metals, it can only partially remove the
sulphate. Further treatment is required to reduce the sulphate level further to render the water
suitable for discharge into the environment.
A number of sulphate removal methods are available and used in industry. These methods can
be divided into physical (membrane filtration, adsorption/ion exchange), chemical (chemical
precipitation) and biological sulphate reduction processes. A literature study was conducted in
order to compare these different methods.
The ABC (Alkali – Barium – Calcium) Desalination process uses barium carbonate to lower
the final sulphate concentration to an acceptable level. Not only can the sulphate removal be
controlled due to the low solubility of barium sulphate, but it can also produce potable water
and allows valuable by–products such as sulphur to be recovered from the sludge. The toxic
barium is recycled within the process and should therefore not cause additional problems. In
this study the sulphate removal process, using barium carbonate as reactant, was investigated.
Several parameters have been investigated and studied by other authors. These parameters
include different barium salts, different barium carbonate types, reaction kinetics,
co–precipitation of calcium carbonate, barium–to–sulphate molar ratios, the effect of
temperature and pH. The sulphate removal process was tested and verified on three different
industrial effluents.
The results and conclusions from these publications were used to guide the experimental
work. A number of parameters were examined under laboratory conditions in order to find the
optimum conditions for the precipitation reaction to take place. This included mixing
rotational speed, barium–to–sulphate molar ratio, initial sulphate concentration, the effect of temperature and the influence of different barium carbonate particle structures. It was found
that the reaction temperature and the particle structure of barium carbonate influenced the
process significantly. The mixing rotational speed, barium–to–sulphate dosing ratios and the
initial sulphate concentration influenced the removal process, but not to such a great extent as
the two previously mentioned parameters. The results of these experiments were then tested
and verified on AMD from a coal mine.
The results from the literature analysis were compared to the experiments conducted in the
laboratory. It was found that the results reported in the literature and the laboratory results
correlated well with each other.
Though, in order to optimise this sulphate removal process, one has to understand the
sulphate precipitation reaction. Therefore it is recommended that a detailed reaction kinetic
study should be conducted to establish the driving force of the kinetics of the precipitation
reactions. In order to upgrade this process to pilot–scale and then to a full–scale plant,
continuous reactor configurations should also be investigated.
The sulphate removal stage in the ABC Desalination Process is the final treatment step. The
effluent was measured against the SANS Class II potable water standard and was found that
the final water met all the criteria and could be safely discharged into the environment. === Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012.
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