Abstract
A combination of smelting-reduction of bauxite and leaching treatment of the produced slag for alumina recovery is known as the Pedersen process. The process is considered to be more sustainable for producing metallurgical-grade alumina than the Bayer process as it does not produce bauxite residue (red mud), which is one of the most abundant industrial byproducts in the world. In this work, the leachability of a ternary CaO-Al2O3-SiO2 slag produced from smelting-reduction of low-grade bauxite has been studied. The obtained calcium aluminate-slag consists of Ca12Al14O33 and CaAl2O4 phases with minor amounts of complex oxide phases. A leaching series have been carried out at different temperatures, Na2O(carbonate):Na2O(caustic) concentration ratios. The composition of solids and leaching liquors were analyzed for measuring the recovery of the aluminum and silicon. The results show that the highest aluminum extraction extent in the current study is 46.7%, which is achieved at a temperature of 75 °C, 1 atm, in 60 g/L Na2O(carbonate) solution, in 30 min of leaching time. A passive calcium-containing layer at the slag's surface acts as a mass transfer barrier for the reactants and products of the leaching reactions, making their diffusion the rate-limiting step. A high concentration of Na2O(caustic) may decrease the aluminum extraction of the slag as a result of insufficient carbonate anions in the system, where the dissolved aluminum reacts with the calcium cations and produce 3CaO·Al2O3·6H2O phase.
Highlights
Excessive production of bauxite residue, which is estimated 150 million tonnes annually in 2017 (Tsesmelis, 2017) becomes one of the major concerns amongst alumina producers
The severe accidents on red mud dams occurred during the last decades (Norway's Norsk Hydro apologises for spills in Brazil river_2018.pdf WWW Document, 2018; Outrage as plant bosses acquitted over fatal toxic spill in Hungary WWW Document, 2016) boosted many research projects on red mud valorization worldwide
There has been limited information provided from the literature regarding the leaching characteristics of ternary CaO-Al2O3-SiO2 slag produced from the smelting-reduction of bauxite, as well as the physicochemical characteristics of residue generated from the leaching process
Summary
Excessive production of bauxite residue (red mud), which is estimated 150 million tonnes annually in 2017 (Tsesmelis, 2017) becomes one of the major concerns amongst alumina producers. Bauxite with relatively low Al2O3/SiO2 mass ratio is considered as low-grade bauxite. From a technical point of view bauxites with low Al2O3/Fe2O3 mass ratios can be treated with the Bayer process, from an economic point of view their treatment is unfavorably facing the challenge of disposal of vast amounts of red mud produced as mentioned earlier. There has been limited information provided from the literature regarding the leaching characteristics of ternary CaO-Al2O3-SiO2 slag produced from the smelting-reduction of bauxite, as well as the physicochemical characteristics of residue (grey mud) generated from the leaching process. The European Union under Horizon 2020 program has launched
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