Abstract
The recovery of better quality waste gypsum during acid mine drainage (AMD) neutralization is one step closer to achieving downstream waste gypsum beneficiation for recovery of valuable materials. This can facilitate recovery of treatment costs and prevention of environmental pollution from gypsum waste-dumps. Thermal reduction using rotary kilns to recover valuable materials from waste gypsum remains a critical and controversial process because of waste gypsum handling problems, environmental pollution due to dust and gaseous emissions and poor conversion yields. In order to mitigate these problems and improve waste gypsum conversion yields, pelletization of waste gypsum in the presence of binders (starch and cellulose) was investigated. A laboratory-scale disc pelletizer was used to produce pellets from a mixture of coal and commercial gypsum or waste gypsum, generated during AMD neutralization, with starch and micro-crystalline cellulose used as binders. The pellets were subjected to high-temperature thermal treatment in a tube furnace to generate calcium sulphide (CaS), an important intermediate for waste gypsum beneficiation. The kinetics of thermal conversion of pelletized waste gypsum to CaS were found to be highly dependent on furnace temperature. Results also showed that pelletization affords improved handling of waste gypsum while use of binders as additives significantly improved the CaS yield, with starch giving the better yield compared to cellulose.
Highlights
CaSO4 formed during pre-treatment of acid mine drainage (AMD) with hydrated lime is a resource that can be converted to calcium sulphide (CaS), an important intermediate for synthesis of a number of calcium compounds including CaCO3, which is a feed stock for AMD neutralization
The current study investigated waste gypsum pelletization and the effects of using binders as well as binder content during thermal reduction in a tube furnace
This study has investigated pelletization of waste gypsum mixed with carbonaceous reductants as a means of improving handling of gypsum during thermal reduction to recover CaS, as well as promoting better conversion of the feed material
Summary
CaSO4 formed during pre-treatment of acid mine drainage (AMD) with hydrated lime is a resource that can be converted to CaS, an important intermediate for synthesis of a number of calcium compounds including CaCO3, which is a feed stock for AMD neutralization Other saleable products such as sulphur may be produced downstream and the recovery of these feed materials promotes the cost effectiveness of AMD treatment. A report prepared by a team of experts (Coetzee et al, 2012) appointed by the South African Government–appointed Inter-Ministerial Committee on AMD has recommended construction of a 20 Ml/d emergency neutralization plant for AMD treatment as one of the short-term interventions to deal with large quantities of uncontrolled AMD decanting from the Witwatersrand’s Western Basin This short-term intervention plan provides an opportunity for recovery of saleable materials from waste gypsum generated during AMD neutralization, thereby contributing towards providing economically sustainable AMD remediation. Zvimba et al (2011, 2012) reported some opportunities for the treatment and recycling of gypsum whereby CaCO3 is recovered
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