Search

At present, the production of magnesium is mainly carried out semi-continuously with ferrosilicon as reducing agent under high temperature and high vacuum. In order to continuously produce magnesium, a new method of extracting magnesium from low-grade magnesite and calcium carbonate by silicothermal method in flowing inert gas was proposed. The effects of calcium fluoride (CaF2) on decomposition rate, decomposition kinetics, reduction rate of magnesia, and crystal type of dicalcium silicate in reduction slag were investigated in the paper. The experimental results showed that calcium fluoride could accelerate the decomposition of carbonate, and had no side effect on the calcined products. In addition, the analysis results of DTA curves showed that calcium fluoride could reduce the decomposition reaction activation energy and the reaction temperature of carbonate in the prefabricated pellets. The results of reduction experiments showed that proper calcium fluoride could promote the reduction rate of magnesia, and in the temperature range of 1250? ~ 1350?, with the same timeframe, the corresponding calcium fluoride contents were 5%, 3%, and 1% respectively when the reduction rate reached the maximum. Excessive calcium fluoride reduced the reduction rate of magnesia, but it promoted the transformation of dicalcium silicate to ? phase in the reduction slag.

With rapid economic development, a bauxite shortage has seriously restricted China’s aluminum industrial development. As a potential alumina secondary resource, high-alumina fly ash has alumina content comparable to China’s low-medium grade bauxite. Traditional alumina extraction processes have many problems, such as high-energy consumption, large amounts of waste acid and alkali, serious secondary pollution, and large amounts of slag discharge. In view of this situation, Northeastern University has proposed a new method for electrolyzing AlCl3 solutions to prepare alumina under ammonium chloride (NH4Cl) catalysis. This method has the advantages of catalyst recycling and no waste acid. In this study, the effects of initial NH4Cl concentration and stirring on the cathodic chamber pH and voltage were examined during electrolyzing AlCl3 solution under NH4Cl catalysis process. In addition, the phase, morphology, D(50) and D(90) of the products, the phase and morphology of subsequent roasted products, and Al recovery rate during electrolyzing AlCl3 solution under NH4Cl catalysis process were investigated. The results suggested that, with lower initial NH4Cl concentration, solution pH and voltage increased, while stirring can slow down the increasing trend of solution pH and voltage. The initial NH4Cl concentration and stirring were observed to have great influence on the product phase and the electrolytic products were all Al(OH)3. Stirring made products take on block and ball shapes. The initial concentration and stirring had no clear influence on product D(50) and D(90) nor the phase and morphology of the roasted products. The roasted products were all bulk amorphous alumina. Increasing initial NH4Cl concentrations and stirring can increase Al recovery rate. Without stirring and increased NH4Cl initial concentration, Al recovery rates increased from 10.2 to 21.2%, while with stirring and increased NH4Cl initial concentration, Al recovery rates increased from 19.5 to 21.8%.

With China’s bauxite resources depleting year by year, high-alumina fly ash has been regarded as a potential secondary aluminum resource, with alumina content as high as 40–60%. In view of recognized problems, such as large amounts of circulating acid and environmental pollution in the process of extracting alumina from high-alumina fly ash, Northeastern University has proposed a new electrotransformation method. This method has the main advantages of slag reduction, no waste acid, no waste alkali, and recycling. In this study, the effects of diffusion were examined during electrotransformation, in terms of electrode spacing and concentration difference on solution pH and temperature. In addition, the phase, FT-IR results, roasting weight loss rate of products, D(50), D(90), isothermal adsorption and desorption curves, BET surface area, pore volume, impurity chlorine content of roasted products, and Al recovery rate were investigated. The results showed that pH and temperature increased with prolonged electrotransformation time. And that these products were all amorphous containing-carbon aluminum salt precipitation. Roasted products were all alumina (Al2O3) with a steep broad X-ray diffraction peak. The effects of electrode spacing and concentration difference on isothermal adsorption and desorption curves, BET surface area, and pore volume were not clear. With decreased electrode spacing, the Al recovery rate increased and impurity chlorine content decreased. With increased concentration difference, the Al recovery rate tended to first decrease and then increase, while the impurity chlorine content increased. In other words, small electrode spacing and high concentration difference were beneficial for improving the Al recovery rate.

The condensation behavior of magnesium vapor in argon was studied. Thermodynamic analysis showed that magnesium vapor condensation was controlled by partial pressure and temperature and that temperature was the main factor affecting condensation. When the actual magnesium vapor partial pressure was 371.2 Pa, the magnesium vapor changed from gas to liquid and then to solid. Experimental results showed that droplet magnesium with particle sizes of 0.15 to 1.04 mm, transitional condensed magnesium with particle sizes of 2 to 25 μm and powdered magnesium with particle sizes of 0.5 to 8 μm were obtained at 1000 °C and 0.2 m3/h. This phenomenon occurred as the temperature in the constant temperature zone increased from 1000 °C to 1200 °C. The initial temperature of the droplet condensation zone increased from 618 °C to 776 °C; the initial temperature of transitional condensation zone was 492 °C to 552 °C and the initial temperature of the powder condensation zone 420 °C to 458 °C. When the argon flow rate increased from 0.1 to 0.4 m3/h, droplet magnesium decreased from 52.68 to 11.32 pct and the powder magnesium increased from 24.06 to 57.98 pct. Controlling the temperature and gas flow, different magnesium crystalline microstructures were obtained, which provided a means for magnesium vapor condensation in a continuous magnesium extraction process.

Metal magnesium is mainly produced from the calcined dolomite by the vacuum silicothermic process. However, it requires a high energy input. An improved process was proposed in which the magnesium was produced from pre-prepared pellets. In this work, diffusion and phase transformations in the pre-prepared pellets during the reduction process were investigated. The results suggested that the reduction ratio is remarkably increased as the reduction temperature increased or the dolomite/ferrosilicon molar ratio decreased. Only Si diffused into dolime, which caused the reduction reactions and Fe might not have been involved in the reactions. The liquid Ca-Si alloy was produced. CaSi was an important intermediate product to produce CaSi2 and Ca vapor. Both CaSi2 and Ca vapor were the effective reducing agents for MgO. This process for Mg production includes both solid–liquid and solid–gas reactions. A plausible reactions route was proposed based on experimental and thermodynamic analyses.

The Pidgeon process is the main extraction method of magnesium, but its continuous production cannot be achieved due to the switch between vacuum and atmospheric pressure. Therefore, it is vital to realize continuous extraction of magnesium under atmospheric pressure. In this paper, the process of extracting magnesium from prefabricated pellets in flowing argon was proposed. The isothermal kinetic analysis of the reduction process was carried out. The results showed that the reduction process was controlled by diffusion process in 1 h, and the apparent activation energy of extracting magnesium from prefabricated pellets in flowing argon was 218.75 kJ/mol. Then the influence of experimental factors on the reduction rate was explored, including briquetting pressure, carrier gas flow rate, ferrosilicon content, reaction temperature and time. Through analysis and calculation, it was concluded that the main control step of diffusion process was silicon diffusion.

High-alumina fly ash is an alumina-rich mineral resource that can potentially replace low-grade bauxite. While a wide range of different traditional processes are available for extracting alumina from high-alumina fly ash, these processes have many disadvantages. Northeastern University has recently proposed a new electrotransformation processing technique for transforming AlCl3 into Al2O3 in the process of extracting Al2O3 from high-alumina fly ash by the hydrochloric acid method. The feasibility of this new electrotransformation method was studied in this paper. The effects of the current intensity, electrotransformation time, initial electrotransformation temperature and initial HCl concentration on the electrotransformation process and roasting products were investigated. The results show that the pH of the electrolyte increases with increasing current strength, electrolytic time, initial electrotransformation temperature and HCl initial concentration, and its value tends to be 3.2. Under these conditions, XRD and SEM examinations confirm that the roasting products are Al2O3 sheets with a cubic crystal system. An examination of the grain size shows that the grain sizes are uniform. Elemental content examinations show that the Cl contents of the obtained products are

A new electrotransformation process based on bubbling CO2 into the electrolyte in an electrolytic process to cause precipitation of aluminum salt is proposed herein to solve problems such as the high energy consumption in the process of alumina extraction by acid leaching. The effects of different additives in this electrotransformation process applied to aluminum chloride solution and the resulting roasting products were studied. When using different additives (HCl, Na2CO3, and NaHCO3) at an initial concentration of 10 g/L, the solution pH value and solution temperature gradually increased with increasing electrotransformation time, and the final pH value approached 3.2. The roasting products were all sheet alumina with cubic crystal structure. Use of all the additives refined the granularity of the alumina product, improved the solution conductivity, and increased the chlorine content in the alumina product. Addition of HCl improved the recovery rate of Al, but use of the additives Na2CO3 and NaHCO3 greatly reduced the recovery rate of Al.

Carbide slag is a kind of industrial waste produced from the calcium carbide industry, which is difficult to treat and cannot be used reasonably. In this paper, carbide slag as a calcium source for extracting magnesium was proposed, i.e., carbide slag was directly mixed with low-grade magnesite instead of dolomite as raw material for extracting magnesium without pretreatment. The process is simple to operate and can obtain metal magnesium while reducing nearly 50% of CO2 emissions in the process of extracting magnesium. In the work, the strength of pellets, the effects of Ca/Mg ratio, and calcination temperature on the recovery rate of Mg were investigated. The results showed that low temperature calcination was beneficial to reduction and the recovery rate of Mg increased with the increase of Ca/Mg ratio, and the reduction rate was 87.95% when the Ca/Mg ratio was 1.2.

Contemporary processes for extracting alumina from fly ash involve various disadvantages, including high-energy consumption, heavy slag discharge, and severe equipment corrosion, so a new method for extracting alumina via electrotransformation was developed in this work. This novel method reduced slag discharge, enabled recycling, and produced no acid or alkali waste. The theoretical solution pH associated with Al(OH)3 precipitation and the actual solution pH during electrotransformation were also compared herein. Differential thermogravimetric analysis was performed for the direct electrotransformation products generated with and without NH4Cl as a catalyst; the activation energy and reaction order of these reactions revealed large fluctuations. The effects of the roasting temperature on the phase, morphology, D(50), and D(90) of the roasting products were also studied. The results indicated that the solution reached the theoretical pH (4.7) of complete Al(OH)3 precipitation in a short time when NH4Cl was implemented as a catalyst. Additionally, the D(50) and D(90) of the roasting products were nearly ten times higher when NH4Cl was used, relative to when it was not, reaching 227.1 and 519.6 μm, respectively. The activation energies of each reaction in the electrotransformation and roasting process were also reduced in the presence of the NH4Cl catalyst. Between 100 °C–250 °C, the physical adsorption of water occurred, and some of the structural water of the electrotransformation products generated with NH4Cl were removed in an endothermic reaction (reaction order, n1 = 0.43; activation energy, E1 = 2.13J mol−1). At temperatures between 250 °C–500 °C, the nitrogen-containing substances were removed in another endothermic reaction (reaction order, n2 = 0.4697; activation energy, E2 = 5.316J mol−1). Increasing the roasting temperature was beneficial for improving the crystal form of the roasting products, and the reduced electrotransformation products appeared more compact after roasting.

A novel process has been proposed for magnesium production, in which powder materials including dolomite, ferrosilicon, fluorite and binder are mixed to produce pellets. Kinetics of the silicothermic reduction using the pre-prepared dolomite pellets was investigated by a non-isothermal, flowing argon-protected gravimetric technique at a temperature range from 1323 K to 1673 K. The results suggest that the process is controlled by a chemical reaction, with the second-order chemical reaction model providing the best representation of the process. The apparent activation energy was determined to be 280 kJ/mol. Calcium silicide (was found in the pellet after calcinations. The silicothermic process in the pre-prepared dolomite pellets is possibly a solid–liquid reaction since calcium silicide transformed into liquid alloy, which reduced diffusion resistance of silicon during the reduction process. The gas–film mass transfer may have some effect on the reduction kinetics but is not the major rate-limiting step.

A novel silicothermic process was put forward in order to some the problems in Pidgeon process, including a loss of 5% fine powder produced during the calcination of dolomite and the easy deliquescence for calcined dolomite. For this method, pre-prepared pellets, which consist of dolomite, ferrosilicon, fluorite, and binder are prefabricated before calcination, and then the calcined pellets are used directly for magnesium production. The difference of decomposition behavior and thermal decomposition kinetics between pre-prepared pellets and pure dolomite were studied by thermogravimetry (TG) and differential thermal analysis (DTA) under non-isothermal conditions at different heating rates (4, 8, 16, and 30 K/min). The thermal decomposition data were analyzed using non-isothermal models and some isoconversional methods. The decomposition temperature of dolomite in pre-prepared pellets was lower than that in pure dolomite due to the higher thermal conductivity of ferrosilicon. The values of the activity energy for the two stages decomposition of dolomite are 270 and 280 kJ mol−1 respectively in pre-prepared pellets, which were lower than pure dolomite.