Abstract
Red mud is an iron-containing waste of alumina production with high alkalinity. A promising approach for its recycling is solid-phase carbothermic roasting in the presence of special additives followed by magnetic separation. The crucial factor of the separation of the obtained iron metallic particles from gangue is sufficiently large iron grains. This study focuses on the influence of Na2SO4 addition on iron grain growth during carbothermic roasting of two red mud samples with different (CaO + MgO)/(SiO2 + Al2O3) ratio of 0.46 and 1.21, respectively. Iron phase distribution in the red mud and roasted samples were investigated in detail by Mössbauer spectroscopy method. Based on thermodynamic calculations and results of multifactorial experiments, the optimal conditions for the roasting of the red mud samples with (CaO + MgO)/(SiO2 + Al2O3) ratio of 0.46 and 1.21 were duration of 180 min with the addition of 13.65% Na2SO4 at 1150 °C and 1350 °C followed by magnetic separation that led to 97% and 83.91% of iron recovery, as well as 51.6% and 83.7% of iron grade, respectively. The mechanism of sodium sulfate effect on iron grain growth was proposed. The results pointed out that Na2SO4 addition is unfavorable for the red mud carbothermic roasting compared with other alkaline sulfur-free additives.
Highlights
According to some estimates [1], the world industry produced over 132 million tons of alumina in 2019
We studied the effect of Na2 SO4 addition on carbothermic reduction and magnetic separation of two different samples of red mud with various contents of CaO and Fe by thermodynamic analysis and laboratory experiments
The roasting during 180 min with addition of 13.65% Na2 SO4 at 1350 ◦ C and 1150 ◦ C followed by magnetic separation resulted in obtaining of iron concentrates with 83.91% and 97% of iron recovery, 83.7% and 64.9% of iron grade for the red mud samples U and B, respectively
Summary
According to some estimates [1], the world industry produced over 132 million tons of alumina in 2019. It is well-known that alumina production is accompanied by considerable generation of waste: 1 ton of alumina produces about 1–1.5 tons of waste called the bauxite residue or red mud [2]. Total globally accumulated amount of red mud is about 4.6 billion ton [3]. Many studies investigated extraction of valuable components [7,8,9] with a particular focus on iron [10,11] as a major red mud Metals 2020, 10, 1571; doi:10.3390/met10121571 www.mdpi.com/journal/metals
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