Recycling silicon kerf waste: Use cryolite to digest the surface oxide layer and intensify the removal of impurity boron

A novel strategy for recovery of heavy metals and synthesis of Co-rich alloy from the alkali-treated tungsten residue using photovoltaic silicon kerf waste

The treatment of spent cemented carbides using the conventional alkali-acid leaching process results in the generation of hazardous solid waste tungsten leaching residue. This study proposed an alternative process using the alkali-treated tungsten leaching residue (AW-residue) without the acid leaching step, preserving Co in the residue. By using photovoltaic silicon kerf waste (SKW) as a reducing agent, heavy metals (Co, Ni, W, Nb, and Ta) were efficiently extracted from AW-residue and a Co-rich alloy was obtained. The silicothermic reduction process facilitated the recovery of iron group metals (Co, Ni, and Fe) and effectively captured trace refractory metals (W, Ta, and Nb). Phase separation occurred through reduction reaction and viscosity-driven processes between the Co-rich alloy and the slag. Optimal conditions were identified as 20% SKW addition, MgO crucible, and a holding time of 120 min, achieving a total recovery yield of 95.5%, with specific yields for Co (97.7%), Ni (97.0%), W (82.5%), Nb (76.3%), and Ta (70.5%). A 20 kg pilot-scale experiment confirmed the feasibility of the process, yielding 47.0% Co-rich alloy from AW-residue compared to 48.3% in lab-scale experiment, and producing a harmless slag phase. This environmentally friendly approach promotes sustainable recycling of valuable metals in the tungsten industry.

Recycling of Ti and Si from Ti-bearing blast furnace slag and diamond wire saw silicon waste by flux alloying technique

Two industrial solid wastes, Ti-bearing blast furnace slag (TBFS) and diamond wire saw silicon waste (DWSSW), contain large amounts of Ti and Si, and their accumulation wastes resources and intensifies environmental pollution. In the present study, DWSSW was used as the silicon source to reduce titanium oxide in TBFS by electromagnetic induction smelting, and meanwhile Na3AlF6 was added as a flux to improve the recycling of the wastes. Ti and Si of the two wastes were simultaneously recovered in the form of alloy. The effects of different addition amount of Na3AlF6 flux in the mixture of DWSSW and TBFS on chemical composition, viscosity, basicity and structure of slag were investigated. The dissolution behavior of SiO2 in Na3AlF6 flux was theoretically deduced and experimentally verification. The optimized recovery rate of Ti and Si were obtained, and the research realizes the efficient recycling of DWSSW and TBFS simultaneously.

Synergistic utilization of industrial solid wastes: Extraction of valuable metals from tungsten leaching residue by photovoltaic sawing waste

Solid waste challenges in both the tungsten and photovoltaic industries present significant barriers to achieving carbon neutrality. This study introduces an innovative strategy for the efficient extraction of valuable metals from hazardous tungsten leaching residue (W-residue) by leveraging photovoltaic silicon kerf waste (SKW) as a silicothermic reducing agent. W-residue contains 26.2% valuable metal oxides (WO3, CoO, Nb2O5, and Ta2O5) and other refractory oxides (SiO2, TiO2, etc.), while micron-sized SKW contains 91.9% Si with a surface oxide layer. The impact of SKW addition on the silicothermic reduction process for valuable metal oxides in W-residue was investigated. Incorporating SKW and Na2CO3 flux enables valuable metal oxides from W-residue to be effectively reduced and enriched as a valuable alloy phase, with unreduced refractory oxides forming a harmless slag phase during the Na2O-SiO2-TiO2 slag refining process. This process achieved an overall recovery yield of valuable metals of 91.7%, with individual recovery yields of W, Co, and Nb exceeding 90% with the addition of 8 wt.% SKW. This innovative approach not only achieves high-value recovery from W-residue and utilization of SKW but also minimizes environmental impact through an efficient and eco-friendly recycling pathway. The strategy contributes significantly to the establishment of a resource-efficient circular economy, wherein the recovered high-value alloy phase return to the tungsten supply chain, and the harmless slag phase become raw materials for microcrystalline glass production.

Three-Dimensional Porous Si@SiOx/Ag/CN Anode Derived from Deposition Silicon Waste toward High-Performance Li-Ion Batteries

The application of photovoltaic (PV) solid waste to the field of lithium-ion batteries is deemed to be an effective solution for waste disposal, which can not only solve the problem of environmental pollution but also avoid the loss of secondary resources. Herein, based on the volatile deposited waste produced by electron beam refining polysilicon, a simple and environmentally friendly method was designed to synthesize P-Si@SiOx/Ag/CN as an anode material for lithium-ion batteries. Remarkably, the presence of silver and the formation of a carbon-nitrogen network can enhance the electrical conductivity of the composite and boost the transport efficiency of lithium ions. Furthermore, the porous Si@SiOx structure is generated by silver-assisted chemical etching (Ag-ACE), and the carbon-nitrogen grid architecture is formed after lyophilization with NaCl as a template, which can jointly provide sufficient buffer space for the volume change of silicon during lithiation/delithiation. Benefitting from these advantages, the P-Si@SiOx/Ag/CN anode exhibits outstanding cycling performance with 759 mA h g-1 over 300 cycles at 0.5 A g-1. Meanwhile, the lithium-ion batteries employing the P-Si@SiOx/Ag/CN anodes present a superior rate capability of 950 mA h g-1 at 2 A g-1 and retain a high reversible specific capacity of 956 mA h g-1 at 1 A g-1 after 50 cycles. This work opens up a new economic strategy for the fabrication of high-performance silicon anodes and affords a promising avenue for the recycling of PV silicon waste.

Recycling of silicon from waste PV diamond wire sawing silicon powders: A strategy of Na2CO3-assisted pressure-less sintering and acid leaching

Recycling diamond wire sawing silicon powders (DWSSP) from photovoltaic (PV) silicon wafers production has become an urgent problem. The challenge of recovery is the surface oxidation and contamination of the ultra-fine powder with impurities during the sawing and collection process. In this study, a clean recovery strategy of Na2CO3-assisted sintering and acid leaching was proposed. Due to the Al contamination from the perlite filter aid, the introduced Na2CO3 sintering aid can react with the SiO2 shell of DWSSP to form a slag phase with accumulated impurity Al during the pressure-less sintering process. Meanwhile, the evaporation of CO2 contributed to the formation of ring-like pores surrounded by a slag phase, which can be easily removed by acid leaching. When 15 % Na2CO3 was added, the content of impurity Al in DWSSP could be reduced to 0.07 ppm with a removal rate of 99.9 % after acid leaching. The mechanism suggested that the addition of Na2CO3 can trigger the liquid phase sintering (LPS) process of the powders, and the cohesive force and liquid pressures difference generated during the process facilitated the transportation of impurity Al from the SiO2 shell of DWSSP to the formed liquid slag phase. The efficient silicon recovery and impurity removal of this strategy demonstrated its potential for solid waste resource utilization in the PV industry.

Oxygen removal and silicon recovery from polycrystalline silicon kerf loss by combining vacuum magnesium thermal reduction and hydrochloric acid leaching

With the strengthened awareness of environmental protection and the growing interests of wastes recycling, silicon recovery from polycrystalline silicon kerf loss (PSKL) has drawn extensive attentions all over the world. In this paper, an efficient and environmental friendly approach for oxygen removal and silicon recovery from PSKL by combining vacuum magnesium thermal reduction (VMTR) and hydrochloric acid leaching was proposed. The effects of temperature, duration and particle size on the reduction of PSKL were investigated thoroughly. It is proved that the amorphous SiO₂ in PSKL can be reduced by magnesium vapor at 923 K to generate MgO, and then the produced MgO can be dissolved by hydrochloric acid to eliminate the impurity oxygen. The oxygen removal fraction and the silicon recovery efficiency attained 98.43% and 94.46%, respectively, under the optimal conditions, indicating that a high efficiency recovery of silicon from PSKL was achieved. Compared to the existing PSKL deoxidation technologies, e.g., the high temperature process and the hydrofluoric acid leaching method, this method requires a relatively lower temperature and the waste acid can be easily recovered. Additionally, by taking into accounts the fact that the MgCl₂ in leaching liquor can be recycled for cyclic utilization with a molten salt electrolysis method, it should be suggested that an efficient and environmental friendly process for PSKL recycling was obtained, which shows good prospects for commercial application.

Preparation of Al-Si alloy from silicon cutting waste: Enabling oxide surface removing and silicon utilization improving via vacuum sintering

Environmentally harmful silicon cutting waste (SCW) generated during the production of silicon solar cells possesses a high reuse value. However, the presence of oxide surface and impurities restrict the Si-cores reuse. Herein, inspired by the structure and composition of SCW, designed a combined process consisting of vacuum sintering and alloying to reuse SCW into Al-Si alloy at a low cost. Vacuum sintering promotes the reduction of the oxide surface by Si-core. Oxygen content was decreased by 92.54 %, demonstrating the successful removal of the oxide surface. The discharge of reduction products contributes to the densification, and the Si-core has converged into dense Vac-ceramic (Si block), rendering a relative density of 96.17 %. More importantly, during the alloying process, the formation of Vac-ceramic dredges the mass transfer pathway from Si-core to Al melt. As a result, the Si utilization rate increased about seven times compared with the direct reuse of pristine SCW. Compared with commercial Al-Si alloys, the Al-Si alloys prepared by reusing silicon cutting waste in this work have satisfactory mechanical properties. The method has the prominent advantages of being protective-atmosphere-free, additive-free, and scalability, and may be a promising candidate for the silicon cutting waste purifying and reusing field.

Simultaneous recycling of Si and Ti from diamond wire saw silicon powder and Ti-bearing blast furnace slag via reduction smelting: An investigation of the effects of refractories on recycling

The industrial wastes diamond wire saw silicon powder (DWSSP) and Ti-bearing blast furnace slag (TBFS) are important Si and Ti secondary resources, respectively. During the industrial application of recycling DWSSP and TBFS via reduction smelting, the refractories can dissolve into the molten slag, which can change the composition of the slag and influence the extraction of Si and Ti. Unfortunately, few studies on the reduction smelting of DWSSP and TBFS related to refractories have been reported, making such studies urgently needed. Therefore, the main purpose of this work was to reveal the dissolution mechanism of refractories (alumina and magnesia bricks) and the effect of refractory dissolution on Si-Ti alloy preparation. The results show that during the reduction smelting, the dissolution of alumina and magnesia bricks changed from direct dissolution into the molten slag to indirect dissolution, and the amount of magnesia bricks dissolved was less than that of aluminum bricks. Al³⁺ (aluminum brick) entering the slag could replace Si⁴⁺ in [Si_nO_2n] to form [Al_xSi_n-xO_2n]^x-, increasing the viscosity of the slag. The O^2- (magnesia brick) entering the slag could dissociate [Al_xSi_n-xO_2n]^x-, decreasing the viscosity of the slag. Therefore, compared with alumina bricks, magnesia bricks can promote slag-alloy separation and improve the extraction ratios of Ti and Si. In the case of magnesia bricks, the maximum reduction ratio of TiO₂ was 98.4 %, and the maximum extraction ratio of Si was 95.8 %. This work provides essential experimental data for the Si-Ti alloys prepared via recycling DWSSP and TBFS.