Super-gravity Separation Research Articles

Sustainable recovery and reutilization of molten salt from magnesium salt slag via supergravity separation

Magnesium salt slag is generated during the refining of crude magnesium and usually contains a large amount of soluble chloride salts, which will cause serious environmental problems. In this study, a new method was proposed for sustainable recovery and reutilization of molten salt from magnesium salt slag via supergravity separation. The phase transformation of molten salt with temperature was studied. The molten salt was in a fully molten state at 800–750 °C, while the NaCl precipitated at 750–700 °C, meantime combined with MgCl2 and tightly encapsulated the fine MgO particles. Therefore, recovery of molten salt was conducted at 800 °C enhanced by supergravity separation, where the molten salt and MgO-rich particles were efficiently recovered within 5 min with high recovery rates of 98.9 % and 99.5 %, respectively. Moreover, the recovered molten salt removed 98.6 % of oxide inclusions in the crude magnesium refining process. Meanwhile, the fine MgO-rich particles with w(MgO)=76.3 % and 1–10 µm was recovered from the magnesium salt slag, where the leaching amount of Cl- was decreased to 0.586 g/L, which was below the Chinese standard for chlorine ion content in solid wastes. Therefore, this manuscript provided a novel process for recovering molten salt from magnesium salt slag without additives and secondary pollution, meanwhile reducing the generation of solid waste and hazards.

Crystallisation behaviour and enhanced separation of RE-phase in RE-bearing blast furnace slag

With the wide application and consumption of rare earths (REs), the secondary resources of RE-bearing blast furnace slag are being investigated as viable resources for the recovery of these critical elements. In this study, the crystallisation behaviour including nucleation and growth kinetics of the RE-phase in RE-bearing blast furnace slag was studied and the optimal precipitation conditions for the RE-phase during the cooling process was obtained, providing an important reference and guidance for the efficient extraction and subsequent functionalised reuse of REs. Subsequently, the supergravity enhanced separation of RE-phase from the slag system was conducted in the optimal precipitation conditions of cooling rate (2 °C/min), experiment temperature (1200 °C), and holding time (100 min). At G = 1000, the RE-phase was successfully separated from molten slag, and the mass fraction of CeO2 in the separated sample increased from the original 20.51% to 74.85%, and the recovery ratio reached 92.06%. This indicated that the supergravity separation technology is considered to be an effective technology for the enrichment and recovery of REs from RE-bearing blast furnace slag.

A green method to clean copper slag and rapidly recover copper resources via reduction-sulfurizing smelting and super-gravity separation at low temperature

Massive copper slag containing heavy metals is produced in copper making and 0.5 - 8.0 wt% Cu is lost into it, deserving to be recovered. In this study, the waste coke and gypsum were employed to clean the copper slag, the lost copper was reduction-sulfurized and enriched to the matte droplets. However, the free-settling of matte droplets under normal gravity needed a higher temperature of 1350 ℃. On this basis, the matte droplets were efficiently separated from the cleaned slag via super-gravity at a low temperature of 1200 ℃ within 3 min, the recovery ratio of Cu was up to 99.56%, and the grade of Cu in the matte phase and cleaned slag was 85.84 wt% and 0.08 wt%, respectively. Moreover, the migration, distribution and leaching behavior of heavy metal elements (Pb, Zn, Ni, etc.,) were performed and analyzed, and the treatment and utilization of volatilized vapors and tailings were also discussed. This study proposed a green method to clean the copper slag and simultaneously recover copper resources via reduction-sulfurizing smelting and super-gravity separation at a low temperature, providing scientific guidance and application prospects for the synergistic treatment of hot copper slag with waste coke and gypsum.

Sustainable recovery of metallic Al and reuse of molten salt in Al dross: Salt flux erosion and super-gravity separation

Al dross is a hazardous solid waste produced in the smelting of electrolytic Al and contains amounts of metallic Al droplets encapsulated by dense oxidized films. The conventional Al recycling processes from Al dross are either inefficient or prone to producing harmful gases and salt cake. In this study, equimolar amounts of KCl and NaCl were employed to attack and erode the solid oxidized films and the metallic Al and molten salt were recovered simultaneously via super-gravity separation. The erosion behavior of salt flux on Al dross indicated that the Al–Al2O3 interface structure was changed, and the cracks and fractures were formed on the oxidized films, which provided the prerequisite conditions for the movement of Al liquid. On this basis, the metallic Al and molten salt were efficiently recovered from Al dross by the enhanced separation of super-gravity with recovery ratios of 98.51 % and 99.03 %, respectively, preventing the production of hazardous gases and unmanageable salt cake. The recovered molten salt was reused in the process of erosion on oxidized films and Al separation from Al dross, where the recovery ratios of metallic Al and molten salt were also up to 94.87 % and 98.58 % after five cycles of recycling. The final residue can also be utilized in appropriate ways and further provides a sustainable method for Al recovery and resource recycling from Al dross.

Efficient purification of scrap 1060 aluminum alloys contaminated with Fe and Si by super-gravity separation

1060 aluminum alloy has been used in high-volume commercial applications and correspondingly the mass of its downgraded recycling after contamination is huge, which is primarily due to the gradual accumulation of impurity elements such as Fe and Si. The separation of excess Fe is considered to be a key step towards the same-level recycling of aluminum alloy scrap with extremely economic and environmental benefits. A supergravity-induced technology was employed to purify the contaminating elements from molten aluminum scraps through Al2O3 CFF (Al2O3 ceramic foam filter). During the filtration process, the premature precipitation of Fe-rich phases and Si particles in the molten aluminum scraps were intercepted near the inner wall of Al2O3 CFF. Resultantly, the removal rate of Fe and Si reached 83.98 % and 49.67 %, respectively, at T = 690 ℃ and G= 30 via Al2O3 CFF of P = 80 ppi with H= 40 mm. A continuously supergravity-induced separation on industrial scale was further performed and the removal rate of Fe and Si were 83.5 % and 48.3 %, respectively.

Fundamental research on recovering metals from hot-dip Zn–Al–Mg dross by supergravity separation

Fundamental research on recovering metals from hot-dip Zn–Al–Mg dross by supergravity separation

An environmental-friendly method for recovery of aluminum droplets from aluminum dross: Mechanical activation and super-gravity separation

Aluminum dross produced from the smelting process of aluminum is a toxic solid waste that easily hydrolyzes and releases poisonous gases in contact with water, which contains large numbers of fine oxidized aluminum droplets. In this study, an environmental-friendly method for efficient recovery of aluminum droplets from aluminum dross was proposed through mechanical activation and super-gravity separation. Firstly, the aluminum dross with different particle sizes of 8.0–50.0 mm, 4.0–8.0 mm, and < 4.0 mm was mechanically activated, and the oxidized films of metallic aluminum droplets were greatly destructed via milling. However, the driving force of gravity was not sufficient to drive the molten aluminum droplets to detach from the oxidized films at high temperatures. With the enhancement of the super-gravity field, almost all of the aluminum droplets efficiently escaped from the oxidized films and fully recovered from the aluminum dross within 5 min with a high recovery ratio of 97.14% and a high purity of 99.17 wt%. Compared to the conventional process of separating aluminum, this study provides an environmental-friendly method to efficiently recover aluminum droplets from aluminum dross without the problems of secondary aluminum dross production, dust pollution, and gas pollution.

A new process for efficient purification of 6061 aluminum alloy scrap under semi-solid and super-gravity conditions

A new method for efficient and sustainable recovery of pure Al from 6061 aluminum alloy scrap based on semi-solid processing and supergravity separation was proposed in this study. Firstly, the microstructural variations of the alloy scraps after semi-solid isothermal treatment (SSIT) were investigated, and the liquid phase accumulated with a large amount of impurity elements would gradually aggregate at the grain boundaries and eventually form a dendritic network, while the spherical Al particles within the grain had high purity. Subsequently, the inter-dendritic liquid was successfully extruded from the intergranular by super-gravity separation technology and the pure Al was efficiently recovered. In this process, the effects of separation time (i.e., isothermal treatment time, t), separation temperature (i.e., isothermal treatment temperature, T) and gravity coefficient (G) on the purification effect were evaluated. The recovery rate of pure aluminum (Al ≈ 99.3 wt%) was 81.98% at the optimum conditions of t = 15 min, T = 640 °C, G = 500. In addition, it was found that increasing the separation time and temperature was beneficial to the purification effect of the alloy scrap but had a negative impact on the recovery rate. Therefore, this study has designed a new process for super-gravity purification and recovery, which realized cyclic recycling and significantly improves the recovery efficiency of aluminum scrap.

Solidification and recovery of Cr from hazardous Cr-bearing steel slag: Selective solidification, super-gravity separation and crystal characterization

Massive hazardous Cr-bearing steel slag is produced from the electric furnace smelting process of stainless steel, while the soluble poisonous hexavalent chromium (Cr(VI)) seriously limits the sustainable utilization of the slag. In this study, a novel method of selective solidification and super-gravity separation was proposed to recover Cr resources from Cr-bearing steel slag safely and efficiently. Firstly, Cr elements were selectively solidified into Cr-spinel in Cr-bearing steel slag with wt.%(MgO) = 6.00, and the high-purity Cr-spinel crystals ((Mg0.43Fe0.28Mn0.29)(Cr1.67Al0.33)O4) were effectively recovered from the molten slag via super-gravity separation with a significant Cr2O3 content of 58.08 wt% and a high Cr recovery rate of 88.58 %. On this basis, the chemical formulas and crystal structures of various high-purity Cr-spinel crystals were characterized and the solidification mechanism of Cr in Cr-spinel was determined, to verify that Cr3+ was accordingly substituted by Al3+ when Fe2+ and Mn2+ were substituted by Mg2+, and Cr elements were efficiently solidified and recovered into the Cr-spinel in stable trivalent chromium (Cr(III)) based on the XPS results. Thus the effective leaching amount of Cr in Cr-spinel was as low as 0.011 mg/L, which provided a basis for sustainable utilization of Cr-bearing steel slag.

Effective separation of fusing agent from refined magnesium slag by supergravity technology

Supergravity separation by high-temperature centrifugation is an effective and environmentally friendly method for separating and recycling waste refined magnesium slag. Based on the close density and greatly different wettability characteristics of fusing agent and inclusion in the refined magnesium slag, present work focused on the recovery of fusing agent from refined magnesium slag by supergravity separation. In a supergravity field, fusing agent and inclusions appeared obvious stratification phenomenon in the enrichment. The fusing agent with the purity of 97.9 wt% was obtained and filtered from waste refined magnesium slag by supergravity separation. The gravity coefficient (G) and reaction temperature (T) were favorable to the enrichment and separation of fusing agent. Under the optimal conditions of G = 400 and T = 500 °C, the fusing agent yield and recovery rate were 50.34% and 70.81%, respectively. The content of inclusion in the recovered fusing agent was less than 2.01 wt%, which meets the recycling demand of refined magnesium. The results indicated that supergravity technology provided a clean and efficient method for reclaiming valuable components from waste refined magnesium slag and creating significant economic and environmental benefits.

An environmentally-friendly method to recover silver, copper and lead from copper anode slime by carbothermal reduction and super-gravity

In the present study, an effective and green method to recover silver, copper and lead from copper anode slime by carbothermal reduction and super-gravity separation was investigated. During the pretreatment process, 99.9% of selenium was removed from copper anode slime after sulphation roasting, and the valuable metals of lead, copper and silver were concentrated as the lead-silver-copper multiphase complex by carbothermal reduction. The influence of temperature on the reduction efficiency was studied and the results obtained were supported by thermodynamic analysis. Under the optimal super-gravity separation conditions of G = 600, T = 1423 K and t = 5 min, the filtrated lead-silver-copper multiphase complex and residual phases were obtained. Results showed that the yield of the filtrated Pb-Ag-Cu phase was about 83% and the recovery efficiencies of lead, silver and copper exceeded 98%, 96% and 89%, respectively. These residues could be recycled to the pretreatment processes. Finally, based on the phase diagram analysis and equilibrium calculations, the lead-silver and copper-silver phases can be obtained and this was demonstrated after super-gravity enrichment. This work confirmed that super-gravity technology was a feasible approach in the recovery of valuable metals from copper anode slime due to its significant advantages of high efficiency, simple operation, and being more environmentally-friendly.

Removal of iron for aluminum recovery from scrap aluminum alloy by supergravity separation with manganese addition

In this study, a novel method using supergravity separation with manganese addition to remove iron for aluminum recovery from scrap aluminum alloy was explored. The effects of separating temperature (T), Mn/Fe mass ratio, and gravity coefficient (G) on iron separation efficiency were evaluated. Increasing the separating temperature improved the Al yield but decreased Fe removal. The higher Mn/Fe mass ratio resulted in a rise in Fe removal but a drop in Al yield, since Mn addition transformed the needle-like β-AlFeSi to compact α-AlSiFeMn phase in aluminum alloy. The gravity coefficient showed no effect on the Fe removal but greatly influenced Al yield. At the optimal conditions of T = 670 °C, Mn/Fe=1.0, and G = 500, the Fe removal and Al yield were well balanced at 60.4% and 82.5%, respectively, and the purity of recovered Al was 94.6 wt%. In addition, the Scheil-Gulliver solidification model of the Thermo-Calc software was used to calculate the solidification diagram and solidified phase fraction of scrap Al alloy, and it was found that the addition of Mn altered the solidification path, increased alloy solidification temperature, and promoted the formation of α-AlSiFeMn phase in the melt, which was able to reduce the balanced content of iron in the liquid phase.

Recovery of aluminum-zinc alloy from 55%Al-Zn dross by supergravity separation.

As a by-product of hot-dip 55%Al-Zn coating processing, the dross contains high levels of Al and Zn. However, no cost-effective recycling methods have been reported to date. Therefore, this work aims at developing a novel method for Al-Zn alloy recovery from an industrial 55%Al-Zn dross using supergravity separation. The separation efficiency was analyzed as a function of separation time and temperature as well as the gravity coefficient (G). Al-Zn alloy was recovered at G values above 70. Separation at higher temperatures benefited the recovery of Al-Zn alloy but decreased the removal rate of Fe impurities due to the increased Fe solubility. With the optimal conditions at 590 °C and G > 500, over 86 wt. % of Al-Zn alloy, containing only 0.4 wt. % of Fe, was recovered. According to the FactSage software calculation, the solubility of Fe increases from 0.06 to 1.11 wt. % when the temperature rises from 550 to 650 °C. This is consistent with the experimental results. The purified alloy could then be further used in the hot-dip 55%Al-Zn bath for production. This study demonstrates the feasibility of supergravity separation as a promising process efficient for recovering Al-Zn alloy from 55%Al-Zn dross.

Preparation of TiC ceramics from hot Ti-bearing blast furnace slag: Carbothermal reduction, supergravity separation and spark plasma sintering

TiC ceramics, a kind of ultra-high temperature ceramics, are primarily prepared by the synthesized TiC powders from high purity metallic titanium or titania. In this work, a novel method for sustainable utilization of hot Ti-bearing blast furnace slag to prepare TiC ceramics was developed. Firstly, the Ti was efficiently transformed into the TiC in hot Ti-bearing blast furnace slag through carbothermal reduction. The high-purity TiC powders were fully recovered from molten carbonized Ti-bearing slag through supergravity separation, where the mass fraction and recovery ratio of Ti in TiC powders were up to 77.89 wt.% and 95.58 %. The TiC ceramics with a relative density of 98.23 % were prepared from the recovered TiC powders via spark plasma sintering method, which possess the favourable mechanical properties including Vickers hardness (Hv) of 23.3 ± 0.4 GPa, fracture toughness (KIC) of 3.96 ± 0.24 MPa m1/2 and flexural strength of 371.9 ± 15.4 MPa.

Rapid removal of copper impurity from bismuth-copper alloy melts via super-gravity separation

Rapid removal of copper impurity from bismuth-copper alloy melts via super-gravity separation

Direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag: Super-gravity separation and microwave dielectric properties

In this study, a novel method was developed for direct preparation of magnesium borate (Mg2B2O5) ceramic from boron-bearing slag. The amorphous boron was efficiently enriched into Mg2B2O5 via crystalline phase transformation in the condition of B/S (B2O3/SiO2) = 1.20 and wt.% (MgO) = 36.00. High-purity Mg2B2O5 crystals were fully recovered from molten slag by super-gravity separation at G ≥ 500 and t ≥ 240 s. The Mg2B2O5 ceramic was directly prepared at various temperature by using the recovered Mg2B2O5, and the Mg2B2O5 ceramic displayed a favourable microwave dielectric properties including the dielectric constant (εr) of 6.05, the quality factor (Q × f) of 38,147 GHz and the temperature coefficient of resonant frequency (TCF) of -20.98 ppm/K. Compared with synthetic methods that use high-purity oxides, this study provides a green and economical method for direct utilisation of industrial slag to prepare microwave dielectric materials.

Recovery of rutile from Ti-Bearing blast furnace slag through phase transformation and super-gravity separation for dielectric material

High-purity rutile (TiO2) possessing favorable dielectric properties was sustainably recovered from Ti-bearing blast furnace slag through phase transformation and super-gravity separation in this study. Firstly, the phase transformation behavior of Ti was studied, the favorable conditions for transformation from perovskite to rutile were determined, and the Ti elements were efficiently enriched into rutile in the Ti-bearing blast furnace slag. Subsequently, the condition for solid and liquid phases of rutile and slag in coexistence was acquired by high-temperature CSLM, the rutile was effectively recovered from the Ti-bearing blast furnace slag through super-gravity separation, and its high purity was verified by the results of XRD, SEM-EDS, XRF, EPMA and Raman. Moreover, the rutile ceramic was prepared, and its dielectric properties were investigated, the dielectric constant was up to around 200, and the dielectric loss was as low as 0.0047 at about 900 Hz. The excellent frequency stability, high dielectric constant and low dielectric loss of the rutile recovered from Ti-bearing blast furnace slag reflect its favorable energy storage capability for dielectric material.

Recovery of crown zinc and metallic copper from copper smelter dust by evaporation, condensation and super-gravity separation

Copper smelter dust is an ultrafine hazardous waste containing various heavy metallic elements. This manuscript proposed a novel method for efficient recovery of crown zinc and metallic copper from copper smelter dust, including evaporation and condensation of zinc vapor, and super-gravity separation of copper droplets. Firstly, the fine zinc oxide powders mixed in the dust were efficiently transformed into zinc vapor and escaped from the dust through carbothermal reduction at 1373–1573 K, which was effectively condensed into the metallic zinc named as “crown zinc” above its melting point. Subsequently, the dispersed fine copper droplets were evidently separated from the residue above the melting point of Cu as driven by super-gravity. Accordingly, crown zinc and metallic copper, with high purity of 98.57 wt% and 99.99 wt%, were efficiently recovered from the copper smelter dust with the high recovery ratios of 99.94% and 98.86%, respectively.

Concentration of precious metals from waste printed circuit boards using supergravity separation

Printed circuit boards (PCBs) comprise valuable metals, precious metals, and hazardous materials. Thus, they are considered both attractive secondary sources of metals and environmental pollutants. This study is based on the selective separation of Pb-Sn, Sn-Cu, and Cu-Zn alloys, where supergravity separation was used to concentrate precious metals (i.e., Ag, Au, and Pd) from PCBs in Cu-Zn alloy and final residue. The temperature and gravity coefficient were found to have great influence on the concentration of precious metals in said alloy and residue. At the optimized temperature of 1300 °C, gravity coefficient of 1000, and separation time of 5 min, the Ag, Au, and Pd contents in the Cu-Zn alloy increased by 1.65, 2.05, and 1.54 times, respectively, compared to their concentrations in the original PCBs, while those in the final residue increased by 0.63, 1.02, and 2.62 times, respectively. By combining an appropriate hydrometallurgical process with the present supergravity separation and concentration of precious metals, this clean and efficient process provides a new pathway to recycle valuable metals and prevent environmental pollution by PCBs.

High-temperature centrifugal separation of Cu from waste printed circuit boards

Supergravity separation by high-temperature centrifugation is an effective and environmentally friendly method for separating and recycling comminuted waste printed circuit boards (WPCBs). Present work is focused on the recovery of Cu from WPCBs by supergravity separation at high temperature. A two-step separation process was adopted to selectively recover Cu alloys at different temperatures. The results indicated that temperature has a significant influence on the composition of the resultant SnCu alloy, while the gravity coefficient, which may be adjusted by varying the centrifugation speed, has little effect on alloy composition and only affects the Cu recovery rate in the separation of the SnCu alloy. Following separation, the Cu contents of the alloys were more than 85 wt%. Under optimized conditions, i.e., a temperature of 1300 °C, a gravity coefficient of 1000, and a separation time of 5 min, the total recovery values achieved for Cu, Zn, Pb, and Sn were 93.56%, 65.07%, 92.92%, and 95.52%, respectively, for the whole separation process. This process provides a clean and efficient method for reclaiming valuable metals from WPCBs and preventing environmental pollution.