Vacuum Carbothermal Reduction Research Articles

Recovery of gallium from yellow phosphorus flue dust by vacuum carbothermal reduction

Yellow phosphorus flue dust (YPFD) is a kind of solid waste generated from yellow phosphorus production. Because it contains approximately 0.05% of gallium (Ga), the recovery of Ga from YPFD has attracted the interest of many researchers. To reduce environmental pollution and storage problems associated with YPFD, a new vacuum thermal reduction method to recover Ga from YPFD was proposed in this study. Thermodynamic calculations and thermogravimetric analysis were first used to determine the initial reaction temperature between Ga2O3 and C. High-purity Ga2O3 and activated carbon were evenly mixed and heated at 1273 K and 10 Pa for 2 h. Experimental results showed that metallic Ga could be obtained in the condensation zone by vacuum carbothermal reduction. Based on this result, we next attempted to recover Ga from YPFD by vacuum carbothermal reduction. YPFD containing 30 wt% carbon was heated at 1273 K and 10 Pa for 2 h, and the experimental results also yielded Ga enrichment in the condensation zone. Therefore, this study provides a new and potential useful technology for recovery of Ga from YPFD, as well as a certain practical approach to comprehensively utilizing such industrial waste.

Synthesis of high purity nano-sized transition-metal carbides

Nano-sized transition-metal carbides with high purities were synthesized by vacuum carbothermal reduction coupled with a novel method for the removal of free carbon by molten calcium treatment. Transition-metals oxides were firstly reduced by excessive carbon black under vacuum condition, and then the obtained product was treated in molten calcium to remove the free carbon. After acidic leaching the product, pure carbides were obtained. The excess carbon in the carbothermic reduction stage could not only increase the contact between reactants and make the reduction more thorough, but also act as barrier to restrict the grain growth of the products. The effects of carbothermic reduction temperature, calcium treatment temperature and particle sizes of oxides on the reaction products were discussed in detail. It was concluded that after carbothermic reduction of corresponding oxides at 1823K in vacuum and the following calcium treatment at 1173K, TiC, ZrC, HfC, V8C7, NbC and TaC with the particle sizes of 100nm, 70nm, 130nm, 200nm, 150nm and 120nm, could be prepared, respectively.

A new method of recycling gallium from yellow phosphorus flue dust by vacuum thermal reduction process

Gallium is widely used in the field of semiconductors, and is mainly recovered as a by-product from other production processes utilizing metals such as aluminum and zinc. As a waste generated during the production of yellow phosphorus, yellow phosphorus flue dust is a potential resource for recovery of gallium. In this study, vacuum carbothermic reduction technology was applied to extract gallium from yellow phosphorus flue dust. Under the optimal conditions of 1323 K, 40 min holding time, 50 wt% carbon content, and 1−10 Pa, the recovery rate of gallium was 92.5 %. The condensate was analyzed and the enrichment area of gallium in the condensation zone was determined. No hazardous gases and materials were generated during the process. Compared with other methods of recovering gallium from yellow phosphorus flue dust by hydrometallurgical process, this study has the advantages of short process, easy collection of products, low energy consumption. Overall, this work demonstrates a new and environmentally friendly method for the recovery of gallium from yellow phosphorus flue dust, and also provides a reference value for the comprehensive utilization of this material.

Kinetics of Magnesium Removal from Garnierite by Carbothermal Reduction in Vacuum

The removal of magnesium from garnierite in Yuanjiang area of Yunnan was performed by carbothermal reduction in vacuum. The effects of reduction temperature and reduction time on the removal rate of magnesium were investigated. The kinetics of the removal of magnesium by carbothermal reduction in vacuum was studied. The thermodynamic calculation results show that it is feasible to remove magnesium from garnierite by carbothermal reduction in vacuum. The experimental results show that the removal rate of magnesium in garnierite can reach 93.23% under the conditions of 1823K for 120min. The reduction process conforms to the chemical reaction kinetics model, which indicated that the reduction process is controlled by chemical reaction and whose expression is 1-(1-α)1/3=(-22850.1/T+2.6296) t, the apparent activation energy (Ea) and the pre-exponential factor (A) are 189.97 kJ/mol and 13.87 s-1, respectively. The results of XRD and SEM analysis show that the condensate obtained by carbothermal reduction in vacuum of the garnierite is magnesium, which is mainly obtained by the reduction reaction between magnesium silicate produced by the decomposition of serpentine in minerals and coal. At the same time, it is proved that it is feasible to directly extract magnesium metal from the garnierite.

Extraction of magnesium from garnierite by carbothermal reduction in vacuum

The aim of the traditional pyrometallurgical processes is to extract nickel and iron in garnierite, but it is difficult to extract magnesium of higher value in minerals. Aimed at solving this problem, the extraction of magnesium from garnierite is proposed and studied in this study. The effects of different reduction temperatures, different reduction times and dosage of CaO additions in experiments were investigated. The experimental results indicate that the extraction rate of magnesium can reach 93.23% when the mass ratio of mineral/reductant is 100:28.3, system pressure is 50 Pa, reduction temperature is 1823 K, and reduction time is 120 min, and, at the same time, the purity of metal magnesium in the condensate can be up to 91.88%. Under these conditions, the extraction rate of magnesium can increase to 99.45% by adding 25 wt% CaO, and the purity of the metal magnesium in the condensate exceeds 90%. CaO as the additive can effectively improve the extraction rate of magnesium and promote the recovery and utilization of magnesium. The mechanism is that CaO replaces –O–Mg– in the –O–Mg−O–Si−O– structure to form free MgO, which is reduced to Mg vapor by carbon and condensed into magnesium metal in the condensation zone to achieve the purpose of removing and collecting Mg from the garnierite. At the same time, the reduced slag is rich in Ni and Fe, which can be used for the extraction of Ni and Fe. Compared with the traditional pyrometallurgical process, the process can extract magnesium from garnierite, and the process is short and its operation is simple, which is a potential technology.

Mechanism of CaF2 under Vacuum Carbothermal Conditions for Recovering Nickel, Iron, and Magnesium from Garnierite

Nickel laterite ore is divided into three layers and the garnierite examined in this study belongs to the third layer. Garnierite is characterized by high magnesium and silicon contents. The main contents of garnierite are silicates, and nickel, iron, and magnesium exist in silicates in the form of lattice exchange. Silicate minerals are difficult to destroy so are suitable for smelting using high-temperature pyrometallurgy. To solve the problem of the large amounts of slag produced and the inability to recycle the magnesium in the traditional pyrometallurgical process, we propose a vacuum carbothermal reduction and magnetic separation process to recover nickel, iron, and magnesium from garnierite, and the behavior of the additive CaF2 in the reduction process was investigated. Experiments were conducted under pressures ranging from 10 to 50 Pa with different proportions of CaF2 at different temperatures. The experimental data were obtained by various methods, such as thermogravimetry, differential scanning calorimetry, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy. The analysis results indicate that CaF2 directly reacted with Mg2SiO4, MgSiO3, Ni2SiO4, and Fe2SiO4, which were isolated from the bearing minerals, to produce low-melting-point compounds (FeF2, MgF2, NiF2, etc.) at 1315 and 1400 K. This promoted the conversion of the raw materials from a solid–solid reaction to a liquid–liquid reaction, accelerating the mass transfer and the heat transfer of Fe–Ni particles, and formed Si–Ni–Fe alloy particles with diameters of approximately of 20 mm. The smelting materials appeared stratified, hindering the reduction of magnesium. The results of the experiments indicate that at 1723 K, the molar ratio of ore/C was 1:1.2, the addition of CaF2 was 3%, the recovery of Fe and Ni reached 82.97% and 98.21% in the vacuum carbothermal reduction–magnetic separation process, respectively, and the enrichment ratios of Fe and Ni were maximized, reaching 3.18 and 9.35, respectively.

Effect of particle size on vacuum carbothermal reduction of Panzhihua ilmenite concentrate

The effect of particle size on the vacuum carbothermal reduction of Panzhihua ilmenite concentrate was investigated by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction. The results showed that metallization of the reduced sample reached 96.84% and the mass loss was 30.02% when the particle size of ilmenite concentrate was less than 48 μm. The small particle size was beneficial to the further reduction of Fe and the transition of titanium oxide to low-valence states, and promoted the aggregation and growth of metal iron particles. Metallic iron, titanium oxides, and a small amount of ferrosilicon alloy were the main products.

Synthesis of β-SiC powders by the carbothermal reduction of porous SiO2–C hybrid precursors with controlled surface area

SiO2–C precursors with various surface areas were derived from tetraethyl orthosilicate and phenolic resin as Si and C sources, respectively, by a modified sol–gel process using the in situ precipitation of phenol resin in a prepared wet gel. The surface area of the SiO2–C precursors was varied from 20 to 175 m2/g by changing the C/Si molar ratio in the preform. β-SiC powders were synthesized using carbothermal reduction in vacuum at the temperature range of 1200–1600 °C. The effects of the temperature and heat treatment time as well as that of the surface area of the preform on the formation of β-SiC powders were studied. It was determined that the formation of β-SiC started at 1200 °C and was considerably promoted as the heat treatment temperature and time further increased during the carbothermal reduction of SiO2–C preforms with high surface area. When high surface area SiO2–C preforms were used, highly crystalline SiC powders were synthesized at 1600 °C in vacuum with a high yield of 85%.

Thermodynamic analysis and experimental verification for silicon recovery from the diamond wire saw silicon powder by vacuum carbothermal reduction

Recovering silicon from waste material is profoundly significant in terms of making profit and encouraging development within the photovoltaic manufacturing industry. Eliminating oxygen is essential for the preparation of high purity silicon. However, a substantial amount of oxygen exists in diamond wire saw silicon powder (DWSSP), creating a barrier for the effective silicon recovery caused by a nonnegligible silicon loss. In this paper, amorphous silicon dioxide and the Si core – SiO2 shell structure was systematically characterized and confirmed. A thermodynamic calculation was then carried out to analyze the reduction reaction probability of silicon dioxide by vacuum carbothermal reduction. The results indicated that the reduction reaction could occur under the vacuum condition with lower temperature, while it could not under that of the atmospheric condition. Further, experiments were conducted to verify the reliability of the thermodynamic analysis, which are keeping in good agreement with thermodynamic results. Finally, the results confirmed that the vacuum carbothermal reduction method is a feasible alternative for silicon recovery from the DWSSP. This research provides contributions to understanding the recovery of silicon resources from DWSSP with the vacuum process, making it competitive with existing silicon recovery methods.

Volatilisation behaviour of iron, silicon and magnesium during vacuum carbothermal reduction of ilmenite concentrate

ABSTRACTThe non-isothermal reduction kinetics and volatilisation behaviour of iron (Fe), silicon (Si), and magnesium (Mg) during the vacuum carbothermal reduction of ilmenite concentrate were investigated from 1350°C to 1550°C. Mg species was reduced to metallic Mg before volatilisation. Some Si elements were reduced to Si, which entered into the Fe phase to form ferrosilicon alloy, and others were reduced to SiO before volatilisation. The volatilisation of Fe came from the metallic Fe phase. For the comprehensive consideration of Šatava–Šesták and Coats–Redfern methods, the rate-limiting factors of Fe, Mg, and Si were diffusion control. The integral mechanism functions of Fe, Mg, and Si were G(α) = [1−(1−α)1/3]2, G(α) = 1−2α/3−(1−α)2/3, and G(α) = (1−α)ln(1−α) + α, respectively. The apparent activation energy for volatilisation processes of Fe, Mg, and Si were 1043.02 ± 12.29, 253.15 ± 7.63, and 314.46 ± 6.04 kJ mol−1, respectively.

Effect of temperature on the vacuum carbothermal reduction of low-grade phosphate ore

The effect of temperature on the vacuum carbothermal reduction of low-grade phosphate ore was investigated by FactSage 7.0 and experimental studies. The calculation indicated that the mass of PO and PO2 in the gas phase increased with the increase of temperature, while the volatilization ratio of phosphorus initially increased and then decreased under fixed carbon dosage and pressure. The maximum theoretical volatilization ratio of phosphorus was 96.70% at 1250 °C. Experimental results showed that the volatilization ratio of phosphorus obviously increased as the reduction temperature was increased from 1200 °C to 1250 °C, and the tendency slowed down subsequently. Theoretical calculation and experimental results were in fairly good agreement that the optimum reduction temperature was between 1250 °C and 1300 °C. The Ca5(PO4)3F and SiO2 in the ore were converted to Ca2SiO2F2, and the phosphorus in the ore was reduced to phosphorus vapor during the reduction.

Non-isothermal Reduction Kinetics of Iron During Vacuum Carbothermal Reduction of Ilmenite Concentrate

The non-isothermal reduction kinetics and mechanism of iron during vacuum carbothermal reduction of ilmenite concentrate were investigated by X-ray diffraction, backscattered electron imaging, and energy-dispersive X-ray spectroscopy at temperatures from 1273 K to 1673 K (1000 °C to 1400 °C). The pressure dropped and the increasing trend of mass loss and metallization ratios slowed down when the temperature was higher than 1573 K (1300 °C). The reaction products were iron, Ti2O3, and silicate. Iron was mainly obtained by carbothermal reduction of FeTiO3 and FeTi2O5. The irons continued to nucleate, aggregate, and grow during the reduction process. Comprehensive consideration of Satava–Sestak method and Coats–Redfern method, the apparent activation energy of the reduction process was 587.4 ± 2.98 kJ/mol, and the forms of both integral and differential for the mechanism function were G(a)= [(1−a)−1/3−1]2 and f(a) = 3/2(1−a)4/3[(1−a)−1/3−1]−1, respectively.

The effect of CaO on the recovery of Fe and Ni in a vacuum carbothermal reduction of garnierite

The effects of adding CaO during a vacuum carbothermal reduction on Fe and Ni recovery yields were examined. In addition, magnetic separation was investigated. Experiments were conducted under pressures ranging from 10 to 50 Pa with different proportions of CaO at different temperatures. The results indicated that at 1723 K, the mass ratio of ore/C was 100:65.5; when the amount of CaO was 22.64% (the mole ratio of CaO/Si was 1:1), the recovery of Fe and Ni reached 84.33% and 97.00% in the vacuum carbothermal reduction-magnetic separation process, and the enrichment ratios of Fe and Ni were maximized, reaching 6.32 and 6.72, respectively. In addition, the recovery of Mg in the reduction process reached 99.35%. An analysis of the experimental results also indicated that the addition of CaO could cause the Fe-Si alloy to transform into the Fe-Ni alloy in the nickel-rich residue, which increased the content of Fe-Ni in the magnetic separation material from 13.34% to 73.17%. In addition, the concentration in reduced ore of Si from 45.43% increased to 83.68%, which could reduce the evaporation of Si in the form of SiO at high temperatures. If Si volatilized in the form of SiO during the condensation process, SiO would disproportionate to form Si and SiO2 contaminated condensed magnesium. In summary, the valuable metals in the minerals were comprehensively utilized with the addition of CaO.

Fabrication of titanium carbide reinforced iron matrix cermet by vacuum carbothermal reduction of ilmenite

Iron matrix cermet reinforced with TiC has been produced by vacuum carbothermal reduction of ilmenite followed by sintering processes. The influences of reduction temperature and carbon mass ratio were discussed in detail. X-Ray diffraction (XRD), electron probe micro-analyzer (EPMA) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) were employed to characterize the phase composition and microstructures. After carbothermic reduction, most of Mg, Mn, Ca evaporated from the sample; Si and part of Al was dissolved in the iron matrix. The obtained powders were used as the raw materials to produce TiC-Fe cermet by vacuum sintering. Density, hardness and bending strength of the samples were examined. The optimal cermet products after heat treatment had a density of 5.38 g?cm-1, a hardness of 1125.5 HV and a bending strength of 667 MPa, which was obtained at the carbon/ilmenite mass ratio of 0.378:1 at 1773 K under the pressure of 10 Pa.

Non-isothermal reduction kinetics during vacuum carbothermal reduction of ilmenite concentrate

The non-isothermal reduction kinetics and mechanism during the vacuum carbothermal reduction of ilmenite concentrate were investigated by X-ray diffraction, backscattered electron imaging, energy-dispersive X-ray spectroscopy and FactSage 7.2 at temperatures ranging from 1000 °C to 1400 °C. The quick reduction reaction appeared at 1200 °C–1300 °C and slowed down after 1300 °C. At 1200 °C, magnesium was dispersed throughout the sample, and silicon, calcium, and aluminum were dispersed and distributed in the same region. At 1350 °C, magnesium began to gather and show the same distribution as those of silicon, calcium, and aluminum. The reduction process was mainly FeTiO3→FeTi2O5→Ti3O5→Ti2O3. During the reduction process, the iron continued to nucleation, aggregate and grow, and the high-valence titanium was reduced to low-valence titanium and gradually enriched. Comprehensive consideration of Šatava–Šesták method and Coats–Redfern method, the apparent activation energy of the reduction process was 233.16 ± 8.40 kJ/mol.

Preparation technology of Ti-rich material from ilmenite via method of vacuum carbothermal reduction

ABSTRACTA route of vacuum carbothermal reduction of Panzhihua ilmenite was introduced to prepare Ti-rich material. The effects of mass ratio of graphite to ilmenite, reduction temperature and vacuum condition on compositions of products and contents of impurities were investigated. Scanning electron microscope images showed that Mg mainly existed in MgSiO3, MgAl2O4 and MgnTi3−nO5 in ilmenite. Thermodynamic analyses showed that only after TiO2 was completely reduced to Ti2O3, the impurities Mg and Ca could be removed from the products in gaseous state. A Ti-rich material containing 0.39 wt-% Ca and 0.038 wt-% Mg was obtained when Panzhihua ilmenite was reduced with a graphite/ilmenite mass ratio of 0.130:1 at 1773 K for 1 h under the vacuum degree of 10 Pa. X-ray diffraction patterns showed that the final product was mainly composed of Ti2O3 and α-Fe.

Study on reduction of MoS2 powders with activated carbon to produce Mo2C under vacuum conditions

A method of preparing Mo2C via vacuum carbothermic reduction of MoS2 in the temperature range of 1350–1550°C was proposed. The effects of MoS2-to-C molar ratio (α, α = 1:1, 1:1.5, and 1:2.5) and reaction temperature (1350 to 1550°C) on the reaction were studied in detail. The phase transition, morphological evolution, and residual sulfur content of the products were analyzed by X-ray diffraction, field-emission scanning electron microscopy, and carbon–sulfur analysis, respectively. The results showed that the complete decomposition of MoS2 under vacuum is difficult, whereas activated carbon can react with MoS2 under vacuum to generate Mo2C. Meanwhile, higher temperatures and the addition of more carbon accelerated the rate of carbothermic reduction reaction and further decreased the residual sulfur content. From the experimental results, the optimum molar ratio α was concluded to be 1:1.5.

Preparation and purification of titanium carbide via vacuum carbothermic reduction of ilmenite

An economical route has been developed to prepare high purity titanium carbide from Panzhihua ilmenite, which has high contents of Mg and Si. In this paper, high purity (>99 wt%) TiC particle was prepared by vacuum carbothermal reduction followed with acid leaching and alkali leaching processes. The current process has a lower reaction temperature, and a shorter reaction time, meanwhile the used raw materials are much cheaper than the industrial method of preparing TiC which used TiO2 and C as the raw materials. Effects of atmosphere, reduction temperature and carbon ratio were studied and the purification mechanisms of elements Mg, Mn, Ca, Si, Al were put forward. Mg and Mn could be separated in gaseous state during vacuum carbothermal reduction. Al was reduced to Al4C3 after reduction and then was removed in acid leaching process. Meanwhile, CaO was also removed in the acid leaching process. SiO2 was reduced to the dissolved silicon in liquid iron firstly, then formed amorphous silicon after acid leaching, and finally was removed in alkali leaching process. The as-prepared TiC powder had a narrow particle size distribution within 1–20 μm and Dv50 was 5.29 μm.

Preparation of high purity zinc from zinc oxide ore by vacuum carbothermic reduction

Effects of the yield and quality of Zn were investigated by vacuum carbothermic reduction from zinc oxide ore. Results indicate that initial moles ratio of C/ZnTotal, pressure of system, reaction time affect significantly the Zn yield from zinc oxide ore, while sintering temperature and sintering time have important influences on the quality of Zn. The optimal operating conditions are obtained as follows: the sintering temperature 973 K, the sintering time 50 min, the molar ratio of C/ZnTotal 2.5, the reaction temperature 1173 K, the pressure system lower than 20 kPa, the reaction time 50 min. Under the optimal conditions, the yield and purity of Zn are more than 95% and 99.995% respectively, suggesting the vacuum carbothermic reduction is a promising method to extract high-purity zinc from zinc oxide ore.

Effects of Temperature on Vacuum Carbothermic Reduction of Panzhihua Ilmenite Concentrate

The vacuum carbothermic reduction of ilmenite concentrate was analyzed using SEM, EDS, and XRD, in combination with analysis of the Fe metallization rate. Results show that increasing the reduction temperature favors ilmenite reduction. At a reduction temperature of 1400°C, the Fe metallization rate reaches up to 97.15%. Increasing the reduction temperature is conducive to the reduction of Fe and Ti from a high to low state of matter. The microstructure of the reduced sample is divided into gray, graywhite, and white contrasts. The gray-white contrast comprises metallic Fe that contains small amounts of Si. An increase in reduction temperature enhances the generation, gathering, and growth of the graywhite contrast. Fe is the predominant component in the gray contrast, while Ti, O, and Mg are distributed throughout the gray contrast, which is a Ti-rich material phase. Fe is restored to the metallic Fe state. Small amounts of reduced Si and Fe combine to form a ferrosilicon alloy.