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The NaF-KF-AlF3-based electrolyte is a promising low-temperature electrolyte for aluminum reduction. The solubility and dissolution rate of alumina in NaF-KF-AlF3-LiF-CaF2 melt with a cryolite ratio (CR) ranging from 1.5 to 1.8 were investigated by the rotating corundum disk method at 1073 K to 1153 K. The effect of the composition and temperature of the melt on the solubility and dissolution rate of alumina were discussed. The results show that the solubility and dissolution rate of alumina in the melt decrease with the increase of LiF and CaF2 content, and the effect of LiF on the alumina solubility is greater than that of CaF2. The decrease of the CR and temperature lead to a marked reduction of the alumina solubility and alumina dissolution rate. The dissolution rate of alumina depends on the dissolution rate in the first 5 minutes. The alumina dissolution is a zero-order reaction in the first 5 minutes, and a first-order reaction in the next 30 minutes.

This paper proposes a novel approach to sulfur removal by adding zinc during the digestion process. The effects of zinc dosage on the concentrations of different valence sulfur in sodium aluminate solution were investigated at length to find that high-valence sulfur (S 2 O 3 2− , SO 3 2− , SO 4 2− ) concentration in sodium aluminate solution decreases, but the concentration of the S 2− in the sodium aluminate solution increases as zinc dosage increases. This suggests that zinc can react with high-valence sulfur to generate S 2− at digestion temperature, which is consistent with our thermodynamic calculation results. In this study, as zinc dosage increases, sulfur digestion rate decreases while sulfur content in red mud markedly increases when zinc dosage was below 4%; the digestion rates of sulfur and sulfur content in red mud remains stable when zinc dosage was above 4%; the alumina digestion rate, conversely, increased slightly throughout the experiment. This suggests that high-valence sulfur in sodium aluminate solution can be converted to S 2− and then enter red mud to be removed completely by adding zinc during the digestion process.

Red mud is a hazardous waste generated from alumina refining industries. Unless managed properly, red mud poses significant risks to the local safety and environment. The Bayer Red Mud was considered as a low-grade iron ore with a grade of 5wt% to 20wt% iron. We adopted the reduction roasting-magnetic separation process to recover ferric from red mud by electromagnetic induction furnace. The effects of different parameters on the recovery rate of iron were studied in-depth. The optimum reduction reaction conditions were obtained that is 1wt% of carbon in red mud at 1450° roasting for 60 min and the magnetic field intensity is about 0.19T. The experimental results indicated that the grade of total iron and the iron recovery were 66.50% and 65.4%, respectively. The prove the electromagnetic induction furnace is more beneficial to iron recovery.

Bauxite residue (red mud) generated during alumina production is a highly alkaline solid waste. The red mud is mainly stored on land, but it can cause harm to the surrounding environment and human health. The transformation of red mud into soil is a feasible method for the large-scale disposal of red mud, but alkali removal is the key process that controls the transformation of red mud into soil. In this study, the calcification dealkalization of red mud with a small particle size was carried out below 100 °C. The results show that the sodium in red mud is predominately distributed in small particles, mainly because the lattice alkali and alkali present between the crystals are exposed to the surface of red mud particles by ball milling. The dealkalization process was controlled by the internal diffusion of the shrinking-core model (SCM), and the apparent activation energy was 23.55 kJ/mol. The dealkalization rate and the Na2O content of dealkalized red mud reached 92.44% and 0.61%, respectively. The dealkalization rate increased with increasing reaction time, reactant concentration, and leaching temperature, and this result was consistent with the results of the kinetic study. In addition, calcification enhances the flocculation of particles, so the filtration performance of red mud improved.

The cryolite ratio (CR) is an important parameter for the electrolyte in aluminum reduction cells. The measurement method for the CR of the KF-NaF-AlF3 system acid (CR < 3) electrolyte by means of electrical conductivity was initially developed, and the formula for calculating the CR was deduced. This method has the advantages of simple operation and high precision. In addition, the relative standard deviations (RSD) of the measurement are < 1.2 pct, and the analysis error of the NaF or KF content has little effect on the determination of the CR.

This paper proposes a novel approach to sulfur removal by adding iron during the digestion process. Iron can react with high-valence sulfur (S2O32−, SO32−, SO42−) to generate S2− at digestion temperature, and then S2− enter red mud in the form of Na3FeS3 to be removed. As iron dosage increases, high-valence sulfur concentration decreases, but the concentration of S2− increases; sulfur digestion rate decreases while sulfur content in red mud markedly increases; the alumina digestion rate, conversely, remains fairly stable. So sulfur can be removed completely by adding iron in digestion process, which provide a theoretical basis for the effective removal of sulfur in alumina production process.

The problem that sulfur causes in alumina production using the Bayer process can be eliminated by adding sodium nitrate, and, while the reaction mechanism is not perfect, this paper studies the reaction mechanism of sodium nitrate and different valence sulfurs in a sodium aluminate solution by combining thermodynamic calculations and experiments. Through thermodynamic analysis, it is deduced that sodium nitrate undergoes oxidation reactions with low valence sulfur (S 2− , S 2 O 3 2− , and SO 3 2− ) of different valence states in a sodium aluminate solution, while the oxidation effect of S 2− is the most obvious. By studying the influence of the sodium nitrate dosage, oxidation time, and oxidation temperature on the different valence sulfurs in a sodium aluminate solution, it is concluded that S 2− removal reaches 67.74% under the conditions of 3% sodium nitrate dosage, 260°Coxidation temperature, and 60-min oxidation time. However, the oxidation effect on S 2 O 3 2− and SO 3 2− is not obvious. The experimental results are consistent with the thermodynamic calculation results. Finally, the reaction mechanism of sodium nitrate with different valence sulfurs in a sodium aluminate solution is described in detail, which provides theoretical support for the desulfurization of high-sulfur bauxite in the production of alumina using the Bayer process.

In aluminum electrolysis, the iron-rich cover material is formed on the cover material and the steel rod connecting the carbon anode. Due to the high iron content in the iron-rich cover material, it differs from traditional cover material and thus requires harmless recycling and treatment. A process was proposed and used in this study to recovery F, Al, and Fe elements from the iron-rich cover material. This process involved aluminum sulfate solution leaching for fluorine recovery and alkali-acid synergistic leaching for α-Al 2 O 3 and Fe 2 O 3 recovery were obtained. The optimal leaching rates for F, Na, Ca, Fe, and Si were 93.92, 96.25, 94.53, 4.48, and 28.87%, respectively. The leaching solution and leaching residue were obtained. The leaching solution was neutralized to obtain the aluminum hydroxide fluoride hydrate (AHFH, AlF 1.5 (OH) 1.5 ·(H 2 O) 0.375 ). AHFH was calcined to form a mixture of AlF 3 and Al 2 O 3 with a purity of 96.14%. The overall recovery rate of F in the entire process was 92.36%. Additionally, the leaching residue was sequentially leached with alkali and acid to obtain the acid leach residue α-Al 2 O 3 . The pH of the acid-leached solution was adjusted to produce a black-brown precipitate, which was converted to Fe 2 O 3 under a high-temperature calcination, and the recovery rate of Fe in the whole process was 94.54%. Therefore, this study provides a new method for recovering F, Al, and Fe in iron-rich cover material, enabling the utilization of aluminum hazardous waste sources.

Herein, fluorine fixation, nitrogen removal, and the alkaline solution recovery of alumina were studied using calcium-based high-temperature soda roasting. In addition, the effects of roasting conditions, dissolution conditions, and additives (carbon alkali and calcium salt) on fluorine fixation and alumina recovery were studied. The results show that when the roasting temperature was 1100 ℃ and the roasting time was 2 h, the dissolution temperature was 80 °C, while the dissolution time was 20 min. The ratio of soda to dross was 1.0, and 30 wt% CaCO3 had been added as a fluorine fixing agent. The Al extraction rate from aluminum dross (AD) reached 86.21%, the denitrification rate reached 99.54%, the concentration of soluble fluoride ion was reduced to 0.111 g·L−1, and the fluoride fixation rate was 75.04 wt%. CaF2 is the main component of the leaching residue, which can be added as an insulation material for electrolysis. Alkaline oxidation roasting and calcium-based fluoride fixation are effective methods to separate aluminum, fluoride, and nitrogen. This study provides a new method for the safe and efficient utilization of AD.

Spent anodes (butts) are the remaining portion of prebaked anodes after consumption, which are reintroduced into the production of prebaked anodes after treatment. Among these residues, the main harmful element, sodium, tends to accumulate with the repeated recycling of the butts. This accumulation increases the air reactivity of prebaked anodes and consequently reduces their quality. In this study, aluminum sulfate hydrothermal acid leaching was used to remove sodium from the butts. The effects of leaching temperature, Al3+ concentration, leaching time, and liquid-to-solid ratio on the removal of sodium from the butts were investigated. The sodium removal rate reached 99.42% at a leaching temperature of 90°C, an Al3+ concentration of 1.5 mol/L, a leaching time of 2.5 h, and a liquid-to-solid ratio of 25 mL/g. This method significantly reduced the sodium content in the butts, offering a new approach for the high-quality recovery of the butts.