Study On Iron And Aluminum Recovery From Red Mud By Alkali Roasting-two-stage Leaching

In recent years,with the shortage of local bauxite deposits,a large number of bauxite trihydrate laterite minerals from Guinea,Australia and other countries are imported in alumina enterprises in China.As a result,huge amounts of high-alkali and high-salt red mud(T_Fe≥30%)with high iron contents are generated,which not only occupy a large amount of land and endanger the surrounding environment,but also cause the waste of iron resources.The key problem for the ultilization of high-iron red mud is the separation of Al and Fe from the highly stable aluminum goethite mineral.To address this,an integrated process of alkali roasting-aqueous leaching-carbonation decomposition was proposed in this dissertation.Firstly,the phases of aluminum minerals in red mud were transformed to soluble aluminate by alkali roasting,and then the aluminum,sodium and calcium species were dissolved from the roasting slag by the subsequent aqueous leaching process.In this process,iron was retained in the slag as iron concentrate output,while the dissolved aluminum and sodium were enriched into high concentration sodium aluminate solution.Finally,through the carbonation decomposition process,aluminum was effectively exported as aluminum hydroxide product,and sodium can be reused for roasting process after evaporation and concentration.The main research contents and results are as follows:（1）Through roasting the mixture of red mud,Na₂CO₃ and CaCO₃,the phase decomposition and fusion of red mud were achieved,and the aluminum minerals transformed into soluble aluminate,while iron minerals were stably retained in the slag.The results showed that under the optimal condition of 25%Na₂CO₃,5%CaCO₃,roasting temperature of 1100 ^oC,roasting time of 3 h,and heating rate of 9 ^oC/min,the phases of hematite,perovskite,sodium aluminate and sodium silicate aluminate can be stably generated during the roasting process,and the leaching efficiency of Al and the grade of Fe in the roasting slag after water leaching can reach the optimal value.At this time,the leaching efficiency of Al was 72.01%,and the content of Fe in the leaching residue was55.45%.（2）The factors affecting the leaching efficiencies of impurity elements（Al,Na,Ca）during the static H₂O+FeCl₃ leaching process was explored.Specifically,the effects of liquid-solid ratio,temperature and FeCl₃ concentration were investigated.The results showed that the total leaching rates of Al,Na and Ca were 81.12%,95.11%and 33.10%,respectively,when the liquid-solid ratio was 4 m L/g,the leaching temperature was 55 ^oC and the concentration of FeCl₃ solution was 10g/L.To simulate the heap leaching process of red mud in the industrial production process and further optimize the quality of hematite and enrich the washed sodium aluminate solution,the dynamic H₂O+FeCl₃leaching experiment was carried out,and the influences of the leaching speed of water in the first stage and the concentration of FeCl₃ in the second stage on the leaching of impurity elements and the grade of iron-rich slag were further studied.The results showed that under the condition of water leaching rate of 0.64 L/min and FeCl₃ concentration of10 g/L,the iron grade of iron-rich slag was 55.56%,and the main phases were hematite,perovskite and sodium aluminasilicate.Al₂O₃ content and removal rate of slag were 2.24%and 84.60%,respectively.The content of Na₂O and Ca O in slag decreased to 0.56%and 3.79%,respectively.（3）The precipitation of Al（OH）₃ in the simulation solution after leaching was carried out through the carbonation decomposition experiment,aiming at the separation of aluminum and sodium.Al（OH）₃product with high decomposition rate and coarse particle size was obtained,while the sodium can be reused for the roasting stage after evaporation and concentration.The results showed that when the initial Al concentration was 75 g/L,the initial Na concentration was 120 g/L,the gas flow rate was 1.0 L/min,the seed crystal coefficient was 0.07,the reaction temperature was 95 ^oC,and the reaction was 150 min,the decomposition rate of Al and Na were 98.06%and 0%,respectively.The Al（OH）₃ product with high precipitation rate,high decomposition efficency and low alkali content was obtained,and its average volume particle size was 48.6μm.Through the alkali roasting-aqueous leaching-carbonation decomposition process,iron and aluminum in high-iron red mud can be effectively separated and recovered.The products were mainly iron-rich slag and aluminum hydroxide.Iron-rich slag basically met the grade condition of iron concentrate and can be used as the primary raw material for industrial iron smelting,and aluminum hydroxide can be used for the production of sandy alumina.This work provides a visual reference route for comprehensive utilization of high iron red mud.