Thermochemical reduction of pelletized gypsum mixed with carbonaceous reductants

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.

Heat-constrained modelling of calcium sulphate reduction

“…As reported by Yan et al (2014) and corroborated by Mbhele et al (2009) and Motaung et al (2015), temperature plays an important role in the conversion of the sulphate, with increasing temperature yielding an increased reaction ratewith. Mbhele et al (2009), who focused on the recovery of sulphur from waste gypsum using activated carbon, investigated several process conditions to determine their impact on the yield of CaS and subsequently sulphur.…”

“…Several authors have reported that the thermal reduction process occurs via a gaseous intermediate, and can mostly likely be described by (Gorkan et al, 2000;Oh and Wheelock, 1990): [6] To this end, it has also been found that the presence of O₂ and CO₂ is undesirable as they favour the formation of calcium oxide and calcium sulphate. Thus, as reported by Jagtap, Pande, and Gokarn (1990), to effectively favour the formation of CaS from CaSO4 an excess of carbon monoxide (if using a gaseous reducing agent) should be present in the kiln; alternatively, if a solid reducing agent is used, an environment that favours excess amounts of CO in the kiln is desirable (Motaung et al, 2015).…”

“…Although numerous authors have looked at catalytic effects, making use of catalysts such as ferric oxide (Strydom, Groenewald, and Potgieter, 1997b), potassium dichromate (Kale, Pande, and Gokam, 1992), semi-coke additives (Trikkel and Kuusik, 1994), stannous sulphate and vanadium pentoxide (Zadick, Zavaleta, and McCandless, 1972) to achieve increased yields and lower initiation temperatures, this is not applicable to the current study but is worth noting. Furthermore, as the typical choice of kiln to perform the reduction in is a rotary kiln, several authors have also reported the effects of pelletization and binder choice (Finney, Sharifi, and Swithenbank, 2009;Motaung et al, 2015;Nengovhela et al, 2007).…”

The Recent Progress China Has Made in Green Mine Construction, Part I: Mining Groundwater Pollution and Sustainable Mining

“…Outside China, water pollution caused by mineral processing has become a concern of experts in recent years. In 2015, Motaung, S. R. et al [42], researchers from South Africa, proposed that waste recovery and comprehensive utilization should be carried out in the process of beneficiation drainage and AMD so as to promote the recovery of treatment costs and prevent environmental pollution caused by gypsum waste dumping. In 2021, Malek, A.; Rao, G. R. and Thomas, T. [43], researchers from India, proposed that it is essential to remove heavy metals from wastewater before the discharge of beneficiation wastewater.…”