Abstract:
The ferromanganese industry is under pressure to deal with the slag
arising from the production of ferromanganese, which is discarded in
landfills or slag heaps. This material poses an environmental and health
risk to surrounding ecosystems and communities, and disposal costs are
increasing. Ferromanganese slag contains an appreciable amount of
residual manganese metal, which can be exploited. Previous work has
shown that the slag can be leached fully, while rejecting the silica to a
residue. The methods that were investigated to recover manganese from
the leach solution included hydroxide precipitation to upgrade the leach
solution followed by manganese carbonate precipitation to produce a
pure manganese carbonate product or a manganese carbonate furnace
feed material, which would be recycled to increase manganese recoveries
in the production of ferromanganese. In addition, electrowinning of
electrolytic manganese dioxide from the leach solution was studied. The
methods were compared in terms of selectivity, costs, and product
quality. Co-recovery of the leach residue, which is a potential cement
additive, is discussed.
Among the methods investigated to upgrade the pregnant leach
solution, hydroxide precipitation utilizing ammonia to adjust the pH
appears to be the most effective in removing major impurities such as
iron, aluminium, and silica to less than 1 ppm. The manganese
carbonate and impure manganese carbonate furnace feed products met
quality specifications. However, although the production of these
materials was technically viable, the large amounts of base reagent that
were required to raise the pH, and the associated high operating costs,
rendered the process uneconomic. An optimization study was therefore carried out with the primary
objective to determine the ideal acid amount to be utilized in the waterstarved
digestion stage, thereby decreasing acid and base consumption
while optimizing the quality of the pregnant leach solution, and
producing a leach residue that contained <1% Mn. The outcome was an
economically viable process. Additional benefits included an increase in
the manganese content of the impure manganese carbonate furnace feed
material, and a substantial reduction in the dilution of the pregnant
leach solution, thereby maintaining high manganese concentrations that
rendered the solution viable for electrowinning of electrolytic
manganese dioxide, the production of which yielded a current efficiency
of 74%.
