Mechanism Of Reduction Roasting Of Coal Gangue-red Mud Under The Effect Of Co-melting Sodium Salts

Coal gangue and red mud are rich in valuable elements such as iron,aluminium and silicon,from which the recovery of valuable elements not only helps to solve the environmental pollution and ecological damage caused by their stockpiling,but also has important significance for alleviating the shortage of resources such as iron ore and bauxite in China.However,the existing recovery process requires high temperature conditions to achieve the same efficient recovery of iron-containing minerals and aluminium-containing minerals,with high energy consumption.How to achieve simultaneous and efficient enhancement of the magnetisation of iron-containing minerals and activation of aluminium-containing minerals in the low and medium temperature range has become a difficult task.In this paper,it is proposed that sodium salt eutectic can effectively regulate the activation reaction of the aluminium-containing minerals,which is expected to synchronize with the magnetization reaction of iron-containing minerals,thus helping to achieve the simultaneous improvement of the efficiency of iron-aluminium separation and aluminium activation in the medium and low temperature range of coal gangue and red mud.Based on thermodynamic calculations and experimental studies,the effect of sodium salt eutectic on the magnetisation of iron-containing minerals and activation of aluminium-containing minerals in coal gangue-red mud was investigated in terms of thermal effects,phase transformation and macroscopic properties during the sodium reduction roasting,so as to reveal the reaction mechanism of sodium salt eutectic in regulating the magnetisation of iron-containing minerals and activation of aluminium-containing minerals,and to propose a regulatory strategy for sodium salt eutectic in regulating the magnetisation of iron-containing minerals and activation of aluminium-containing minerals.The main findings of the study are as follows:（1）Thermodynamic methods such as Factsage and HSC were used to analyse the possible phases present at equilibrium in the sodium-aluminium-silicon-iron coexistence system and the ease of their generation.The results show that in the medium to low temperature range,Al₂O₃,Si O₂ and Na₂O may react within the Na₂O-Al₂O₃-Si O₂ system.Analysis of the magnitude of each reactionΔG reveals that the priority order of sodium salt production is:Na₆Si₈O₁₉>Na₆Si₂O₇>Na₄Si O₄>Na Al Si₃O₈>Na Al Si O₄>Na₁₀Si O₇>Na Al₉O₁₄>Na₂Si₂O₅>Na₂Si O₃>Na₂Al₁₂O₁₉>Na Al O₂>Mullite.Within the Fe₂O₃-C system there may be a reduction of solid phase carbon with Fe₂O₃ to produce Fe CO₃,Fe₃O₄,Fe O and Fe,with the priority order of Fe CO₃>Fe₃O₄>Fe>Fe₃C>Fe O for the production of the iron-containing phases.The TG-DSC method was used to study the effect of sodium salt ratio on the temperature rise of the mixed coal gangue-red mud-sodium carbonate-sodium hydroxide system.The results showed that the effect of sodium salt ratio on the reaction temperature of the mixed system of coal gangue-red mud-sodium carbonate-sodium hydroxide.The results showed that the sodium salt ratio had a significant effect on the decomposition temperature of calcium chalcocite,magnetite and other minerals in the range of 600°C～950°C.In other words,when the sodium salt ratio was 1:9,the activation temperature of aluminium-containing minerals and the magnetisation temperature of iron-containing minerals in the mixed system were lower than those of other sodium salt ratios.（2）XRD,FTIR and SEM-EDS methods were used to study the effects of roasting temperature,sodium salt ratio and total sodium salt on the transformation of the phases of coal gangue-red mud sodium reduction roasting.The results showed that with the increase of roasting temperature,the aluminium-containing minerals and iron-containing minerals showed regular changes,in which the aluminium-containing minerals（such as kaolinite,hydrated garnet,calcium chalcopyrite,etc.）could be transformed into chalcopyrite,sodium aluminosilicate;the iron-containing minerals（mainly hematite）could be transformed into magnetite→calcite→iron monomer→sodium ferrate and in turn.As the proportion of sodium hydroxide increases,the activation efficiency of aluminium-containing minerals and the reduction efficiency of hematite both show a trend of first increasing and then decreasing,and the conversion rate of both aluminium-containing minerals and the iron-containing minerals is higher when the sodium salt ratio is 1:9.With the increase of the total sodium salt,the activation efficiency of the aluminium-containing minerals and the reduction efficiency of hematite both gradually increased.The reduction temperature of the iron-containing minerals was significantly reduced when the sodium salt addition control Na:Al:Si was increased to 3.6:1:1.（3）The effect of roasting temperature,sodium salt ratio and total sodium salt on the recovery of aluminium,silicon and iron from the coal gangue-red mud sodium reduction roasting product was investigated by acid leaching and magnetic separation.The results showed that:with the increase of roasting from 500°C to 900°C,the dissolution rate of aluminium and silicon gradually increased,and the dissolution rate of aluminium could increase from 46%to 83%,and the dissolution rate of silicon could increase from 32%to81%;the total iron recovery showed a trend of first increasing,then decreasing and then increasing,and the dissolution rate could increase from 27%to 72%.As the proportion of sodium hydroxide in the added sodium salt increases,the dissolution rate of aluminium,silicon and total iron recovery all show a trend of first increasing and then decreasing,and reaches its maximum at a sodium salt ratio of 1:9,the dissolution rate of aluminium increasing from 55%to 78%,the dissolution rate of silicon increasing from 42%to 60%and total iron recovery increasing from 33%to 67%.As the total sodium salt is increased to Na:Al:Si=5.0:1:1,the dissolution rate of aluminium and silicon and the total iron recovery increase gradually from 40%to 82%for aluminium,from 27%to 72%for silicon and from26%to 67%for total iron recovery.In summary,when the sodium salt addition is controlled at Na:Al:Si=3.6:1:1,the sodium salt ratio is 1:9 and the roasting temperature is 500°C,the dissolution rate of aluminium and silicon and the total iron recovery rates can reach 78%,60% and 67% respectively.