Silicon sawing waste treatment by electrophoresis and gravitational settling

Recycling of Silicon Carbide Sludge on the Preparation and Characterization of Lightweight Foamed Geopolymer Materials

This study used silicon carbide sludge (SCS) to prepare lightweight foaming geopolymer materials (FGPs) by the direct foaming method. Results showed that when the SCS replacement level was 10%, the bulk density of the lightweight FGPs with added foaming agent amounts of 0.5% and 2.0% was 0.59 and 0.49 g/cm³, respectively; at a curing time of 28 days, the lightweight FGPs with amounts of added foaming agent of 0.5% and 2.0% had bulk densities that were 0.65 and 0.58 g/cm³, respectively. When the SCS replacement level was 10%, and the amount of added foaming agent was 2.0%, the porosity ratio of the lightweight FGP increased from 31.88% to 40.03%. The mechanical strength of the lightweight FGPs with SCS replacement levels of 10% and 20% was 0.88 and 0.31 MPa, respectively. Additionally, when the amount of foaming agent increased to 2.0%, the thermal conductivity of the lightweight FGPs with SCS replacement levels of 10% and 20% were 0.370 and 0.456 W/m⋅K, respectively. When the curing time was 1 day, and the amount of added foaming agent was 0.5%, the reverse-side temperature of the lightweight FGPs with SCS replacement levels of 10% and 20% were 286 and 311 °C, respectively. The k value of the O₂ reaction decreased from 2.94 × 10⁻⁴ to 1.76 × 10⁻⁴ because the reaction system was affected by the presence of SiC sludge, which was caused the reaction to consume O₂ to form CO₂. The results have been proposed to explain that the manufactured lightweight FGPs had a low thermal conductivity (0.370–0.456 W/m⋅K). Therefore, recycling of silicon carbide sludge in lightweight foaming geopolymer materials has potential as fire resistance material for the construction industry.

High-Performance n-Type Ge-Free Silicon Thermoelectric Material from Silicon Waste

Silicon waste (SW), a byproduct from the photovoltaic industry, can be a prospective and environmentally friendly source for silicon in the field of thermoelectric (TE) materials. While thermoelectricity is not as sensitive toward impurities as other semiconductor applications, the impurities within the SW still impede the enhancement of the thermoelectric figure of merit, zT. Besides, the high thermal conductivity of silicon limits its applications as a TE material. In this work, we employ traditionally metallurgical methods in industry reducing the impurities in SW to an extremely low level in an environmentally friendly and economical way, and then the thermal conductivity of purified silicon is greatly reduced due to the implementation of multiscale phonon scattering without degrading the power factor seriously. Benefiting from these strategies, from 323 to 1123 K, for the sample made from purified silicon waste, the average zT, relevant for engineering application, is increased to 0.32, higher than that of the state-of-the-art n-type Ge-free bulk silicon materials made from commercially available silicon, but the total cost of our samples is negligible.

Recycling of silicon from silicon cutting waste by Al-Si alloying in cryolite media and its mechanism analysis

More than 40% of the crystalline silicon has been wasted as silicon cutting waste (SCW) during the wafer production process. This waste not only leads to resource wastage but also causes environmental burden. In this paper, SCW produced by the diamond-wire sawing process was recycled by Al-Si alloying process. Cryolite was introduced to the reaction system to dissolve the SiO₂ layer existed on the surface of the Si particles in SCW. Alloys with 12.02 wt% of Si were prepared and the mechanism of the alloying process was investigated in detail. The Si-Al-cryolite system and SiO₂-Al-cryolite system were studied individually to analyze the reaction process and transferring behavior of Si and SiO₂ in SCW. The SiO₂ shell was firstly transformed into Si-O-F ions. Then the Si-O-F ions diffused to the reaction interface by the effect of the concentration gradient and were reduced to Si by the aluminothermic reduction reaction: 4Al (l) + 3SiO₂ (dissolved in the melt) = 3Si (Al)+ 2Al₂O₃ (dissolved in the melt). Then the internal Si particles were released into cryolite after the dissolution of SiO₂ and transferred to the reaction interface by the effect of gravity. The influences of the mass ratio of Al/SCW and agitation modes on the Si content of the alloys and the Si recovery ratio in SCW were investigated. With the increase of the mass ratio of Al/SCW from 2.2 to 6.5, the Si recovery ratio in SCW increased from 44.08% to 69.05%, but the silicon content of the alloys decreased from 16.06 wt% to 8.83 wt%. Agitation can effectively improve the smelting effect during smelting by which the silicon content of the alloys and the Si recovery ratio in SCW increased from 12.02 wt% and 64.25% to 13.17 wt% and 69.46%, respectively.

Utilization of Silicon Carbide Sludge as Metakaolin-Based Geopolymer Materials

The recycling of SiC sludge material is crucial for resource reutilization and environmental protection. In the current study, the effect of the mass ratio between the Na2SiO3 and sodium hydroxide (NaOH) solutions (NS/SS ratio) and the effect of SiC sludge on metakaolin geopolymers was comprehensively investigated to determine the underlying performance of the geopolymerization system. During thermal evolution, the second exothermic peak of 1.6NS10SCS (NS/SS ratio: 1.6, 10% SiC sludge) showed a heat evolution value of 990.6 W/g, which was the highest among other geopolymers. Additionally, the 1.6NS10SCS sample after 28 days of curing showed the highest flexural strength (6.42 MPa), compared to that of the others, and the DTA/TG (differential thermal analysis/thermogravimetry) results showed that the weight loss percentage increased to 14.62% from 400 to 750 °C. For the 29Si nuclear magnetic resonance deconvolution, 1.6NS10SCS exhibited high fractions of Q4(3Al) (33.63%), Q4(2Al) (23.92%), and Q4(1Al) (9.70%). Thus, the geopolymer with the optimal SiC-sludge replacement level and NS/SS ratio contained more macropores and geopolymer gels, which benefit structural development. The experimental results indicated that SiC-sludge can potentially serve as a partial replacement for metakaolin and exhibited favorable mechanic characteristics.

Reutilization of silicon- and aluminum- containing wastes in the perspective of the preparation of SiO2-Al2O3 based porous materials for adsorbents and catalysts

The waste materials available as sources of silicon and aluminum for producing porous materials like amorphous silicas, aluminas, amorphous silica-aluminas, and zeolites, to be used as catalyst and adsorbents, are briefly summarized. The procedures for preparing these materials from wastes are also taken into account. The limits of this approach in terms of economy and environmental protection are also briefly considered. It is concluded that mesoporous materials can be prepared from wastes, but care to product quality and to overall process efficiency is needed.

Possibility of Recycling SiOx Particles Collected at Silicon Ingot Production Process as an Anode Material for Lithium Ion Batteries

Recently, some studies have utilized silicon (Si) as an anode material of lithium ion battery by recycling Si from the slurry of wafer slicing dust. The filtration of Si particles condensed from Si vapors that were exhausted from the ingot growing furnace could propose another method of Si recycling. In this study, we investigated the possibility of using such collected silicon oxides (SiO_x) particles as an anode material. After collecting SiO_x particles, FE-SEM, TEM, EDS, XRD, XPS analysis, and charge/discharge test were carried out to investigate characteristics and usability of these particles. FE-SEM and FE-TEM images showed that these particles mainly consisted of spherical primary particles with a diameter of 10 nm or less. Agglomerates of these primary particles were larger than 300 nm in diameter. In TEM image and EDS analysis, crystalline particles were observed along with amorphous particles. As a result of XRD analysis, amorphous silica (SiO₂) and crystalline Si were observed. Charge/discharge tests were carried out to determine the feasibility of using these particles as an anode material for lithium ion batteries. A cycle efficiency of 40.6% was obtained in the test in which the total number of charge/discharge cycle was 100 under the condition of C-rate 0.2 for the first three times and C-rate 1.0 for the remaining 97 times. Results showed that these collected particles could be used as an anode material for lithium ion batteries.

Recovery of cutting fluids and silicon carbide from slurry waste

The wafer slicing process generates large amounts of slurry waste. The recovery of cutting oil and abrasives from slurry waste can reduce both the cost and environmental damage. A process combining magnetic precipitation and flocculation was developed for the recovery of cutting oil. A magnetic precipitation tank was employed for storage of the slurry and acceleration of the settlement of suspended particles. The larger particles further aggregate upon adding a non-aqueous flocculant comprising polyacrylamide (PAM) and ethylene glycol (EG). The recycled oil product is obtained by centrifugation and bag filtration. The physical properties and wafer dicing tests indicate that the recycled oil is qualified. Wafer manufacturers can thus reduce costs by using this process to produce recycled oil. A magnetic reactor with alkaline aqueous successfully recycled the SiC powder from the slurry waste by converting all the silicon species into sodium silica for further use. The results demonstrate that the magnetic reactor is able to remove most metal species and that the alkaline aqueous medium can recover all the Si substances in a sodium silicate solution, also called water glass.

Recovering valuable metals from recycled photovoltaic modules

Recovering valuable metals such as Si, Ag, Cu, and Al has become a pressing issue as end-of-life photovoltaic modules need to be recycled in the near future to meet legislative requirements in most countries. Of major interest is the recovery and recycling of high-purity silicon (> 99.9%) for the production of wafers and semiconductors. The value of Si in crystalline-type photovoltaic modules is estimated to be -$95/kW at the 2012 metal price. At the current installed capacity of 30 GW/yr, the metal value in the PV modules represents valuable resources that should be recovered in the future. The recycling of end-of-life photovoltaic modules would supply > 88,000 and 207,000 tpa Si by 2040 and 2050, respectively. This represents more than 50% of the required Si for module fabrication. Experimental testwork on crystalline Si modules could recover a > 99.98%-grade Si product by HNO3/NaOH leaching to remove Al, Ag, and Ti and other metal ions from the doped Si. A further pyrometallurgical smelting at 1520 degrees C using CaO-CaF2-SiO2 slag mixture to scavenge the residual metals after acid leaching could finally produce > 99.998%-grade Si. A process based on HNO3/NaOH leaching and subsequent smelting is proposed for recycling Si from rejected or recycled photovoltaic modules. Implications: The photovoltaic industry is considering options of recycling PV modules to recover metals such as Si, Ag, Cu, Al, and others used in the manufacturing of the PV cells. This is to retain its "green" image and to comply with current legislations in several countries. An evaluation of potential resources made available from PV wastes and the technologies used for processing these materials is therefore of significant importance to the industry. Of interest are the costs of processing and the potential revenues gained from recycling, which should determine the viability of economic recycling of PV modules in the future.

Recycling silicon wire-saw slurries: separation of silicon and silicon carbide in a ramp settling tank under an applied electrical field

The growing demand for silicon solar cells in the global market has greatly increased the amount of silicon sawing waste produced each year. Recycling kerf Si and SiC from sawing waste is an economical method to reduce this waste. This study reports the separation of Si and SiC using a ramp settling tank. As they settle in an electrical field, small Si particles with higher negative charges have a longer horizontal displacement than SiC particles in a solution of pH 7, resulting in the separation of Si and SiC. The agreement between experimental results and predicted results shows that the particles traveled a short distance to reach the collection port in the ramp tank. Consequently, the time required for tiny particles to hit the tank bottom decreased, and the interference caused by the dispersion between particles and the fluid motion during settling decreased. In the ramp tank, the highest purities of the collected SiC and Si powders were 95.2 and 7.01 wt%, respectively. Using a ramp tank, the recycling fraction of Si-rich powders (SiC < 15 wt%) reached 22.67% (based on the whole waste). This fraction is greater than that achieved using rectangular tanks.

IMPLICATIONS

Recycling Si and SiC abrasives from the silicon sawing waste is regarded as an economical solution to reduce the sawing waste. However, the separation of Si and SiC is difficult. This study reports the separation of Si and SiC using a ramp settling tank under an applied electrical field. As they settle in an electrical field, small Si particles with higher negative charges have a longer horizontal displacement than SiC particles in a solution of pH 7, resulting in the separation of Si and SiC. Compared with the rectangular tanks, the recycling fraction of Si-rich powders using a ramp tank is greater, and the proposed ramp settling tank is more suitable for industrial applications.