Recovery of Silicon Powder from Kerf-Loss Slurry

The wire-saw process, in which the materials involved are silicon ingot, stainless wire, abrasive slurry containing silicon carbide particles and polyethylene glycol solution, is used to slice the silicon ingot into wafers for solar cell use. After the sawing stage, 30-40% silicon loss and metal fragments caused by attrition were carried by the used slurry, which usually was treated as the waste. At the present time, polyethylene glycol and silicon carbide have been recovered and recycled for abrasive slurry. If the loss silicon can be recovered either from the waste slurry, the shortage of feedstock and high cost of manufacturing wafers in the photovoltaic industry will be overcome.
The objective of this research is to recover the high-purity silicon powder from the kerf loss slurry using the centrifugation and particle phase-transfer method at ambient temperature. The starting material, which consisted of silicon and silicon carbide with a particle size distribution of 0.4-8 μm and 0.4-30 μm respectively, was obtained by washing the waste slurry to remove the polyethylene glycol solution and metal fragments using acetone and nitric acid respectively. In the washing step using nitric acid, the surface of silicon particle was oxidized to form a silicon dioxide layer.
In this work, the centrifugation was applied first by introducing a heavy-liquid with a density between that of silicon (2.33 g/cm3) and silicon carbide (3.23 g/cm3) into the starting material. After the mixing and centrifuging steps, the upper layer silicon cake was cleaned and dried to obtain the silicon-rich product. Several operating variables, including the solid volume concentration, heavy-liquid density, number of centrifugation, centrifugation time, mixing time of sonication, and the concentration of surfactant were investigated in this study. The results showed that the effects of heavy-liquid density, mixing time of sonication, and concentration of surfactant on the separation of particles were significant. However, the centrifugal force acted on the submicron SiC particles was insignificant so that the highest purity of Si-rich product was 93 wt% with a recovery lower than 50%.
Due to the limitation of centrifugation for separating submicron particles, the particle phase-transfer (PPT) method, which is based on the difference in the surface properties between particles, i.e., hydrophobicity versus hydrophilicity, together with the gravity and electrostatic force on particle surface, was investigated and applied to recover the silicon powder. At the beginning, the starting material was dispersed into water, and then an immiscible oil was added into it. After shaking or stirring, the hydrophobic particles would transfer into the oil phase and thus the particles with different surface properties were separated. In this work, the particle phase-transfer method was first used to remove the submicron SiC particles remaining in the Si-rich product from the centrifugation separation so that a high-purity silicon powder was obtained. Several variables were investigated, including type of oil phase, oil/water volume ratio, and concentration of sodium hexametaphosphate. Moreover, a two-stage of particle phase-transfer method was developed: in the first stage an oil with a density higher than 1 g/cm3 was used to separate the micron-size SiC particles, and in the second stage another oil with a density lower than 1 g/cm3 was applied to remove the particles of submicron and a few microns. The investigated operating variables included pH value of water phase, solid volume concentration, oil/water volume ratio, and number of separating stage. As far as the oil phase was concerned, the n-butanol showed the best results among the solvents tested. As to the pH value, which affects the zeta potential, its effect on the purity and recovery of Si product was significant near the isoelectric point. The best purity of Si product was 99.0 wt% accompanied with a recovery of 71.0%. In addition, the wire-saw slurries with different silicon content were investigated by the PPT method and the results showed that the PPT method can be proceeded successfully to recover high-purity silicon powder from kerf loss slurry.