Recovery of silicon powder from kerf loss slurry waste using superconducting high gradient magnetic separation technology

J Mater Cycles Waste Manag (2018) 20:937–945 https://doi.org/10.1007/s10163-017-0656-7 ORIGINAL ARTICLE Recovery of silicon powder from kerf loss slurry waste using superconducting high gradient magnetic separation technology Changqiao Yang1 · Suqin Li1 · Ruiming Yang1 · Jiaxing Bai1 · Zijie Guo1 Received: 11 August 2016 / Accepted: 2 August 2017 / Published online: 14 August 2017 © The Author(s) 2017. This article is an open access publication Abstract A major challenge in recycling of silicon powder from kerf loss slurry waste is the complete removal of metal particles. The traditional acid leaching method is costly and not green. In this paper, a novel approach to recover highpurity Si from the kerf loss slurry waste of solar grade silicon was investigated. The metal impurities were removed with superconducting high gradient magnetic separation technology. The effects of process parameters such as magnetic flux density, slurry density, and slurry flow velocity on the removal efficiency were investigated, and the parameters were optimized. In one lot of control experiments, the silicon content was increased from 90.91 to 95.83%, iron content reduced from 3.24 to 0.57%, and aluminum content from 2.44 to 1.51% under the optimum conditions of magnetic flux density of 4.0 T, slurry density of 20 g/L, and slurry flow velocity of 500 mL/min. The result indicates that the superconducting high gradient magnetic separation technology is a feasible purifying method, and the magnetic separation concentrate could be used as an intermediate product for high-purity Si powder. Keywords Silicon powder · Kerf loss slurry waste · Superconducting high gradient magnetic separation technology · Metal impurities removal · Purification * Suqin Li 1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 10083, China Introduction Conventional energy shortage and environmental concerns have made the solar energy industry popular globally. Solar energy has many unique advantages. It is inexhaustible, renewable, and pollution-free [1, 2]. The rapid development of photovoltaic industries leads to a shortage of polysilicon, which is the material of choice to fabricate photovoltaic converter, and its price has multiplied [3–5]. In the next few decades, there is no other material to replace polysilicon as the main material for photovoltaic industry. To fabricate the solar cell, polycrystalline silicon rods are sliced into 0.2–0.7 mm-thick silicon wafers with multi-wire cutting, in which the wire diameter is usually 0.2–0.5 mm, which is about the thickness of the silicon wafer [6]. While the theoretical calculation tells that 44% of the rod polysilicon material becomes powder and goes into the slurry waste in the process of wafer slicing, the actual loss is usually 50–52% in real process [7]. Each year, about 3800 t (according to the current global polysilicon production data) of solar grade polysilicon material has been lost in the slicing process, which amounts to $4.5 billion based on the current unit price of $120/kg. Effective recycling of the lost silicon material will alleviate the shortage of solar grade silicon with significant economic and environmental benefit. The conventional processes to reclaim silicon powder from kerf loss slurry waste are distillation-centrifugal separation, electrophoresis, gravitational settling [8], high-temperature treatment [9], etc. Jin et al. [10] patented a reclaim procedure: acid pickling and solid–liquid separation are carried out with the slurry waste, the obtained liquid then goes through a distillation–condensation–dehydration series, before the solidification product is treated with nitric acid and hydrofluoric acid to get the Si and SiC materials. Sousa et al. [11] used a thermal 13 Vol.:(0123456789) 938 plasma process to recycle silicon kerf loss for solar grade silicon feedstock. Their result shows that the deoxidation rate of the final silicon ingot was as high as 80% and the initial carbon concentration was reduced by 85%. Wang et al. [12] applied nitric acid to dissolve iron and a centrifuge separator to remove most of SiC before the Si was reclaimed and purified with high-temperature processing and directional solidification. To some extent, this method was suitable for industrial scale production, but there are some limitations. Acid corrosion of the surface of centrifuge may occur after long-term use of acid treatment. Lin et al. [13] reported about the use of centrifuge to separate Si and SiC from slurry waste. They used acetone at first to remove suspending agent and binder, and nitric acid to dissolve iron, before a centrifuge was used to separate Si and SiC. Silicon powder of 90.8% purity and 74.1% recovery rate were achieved with the following experiment conditions: solid volume concentration of 6.5%, medium liquid density of 2.35 g/cm 3, 60 min churning time, and 60 min centrifuge time. Further high-temperature process is needed as the purity of the reclaimed powder is not up to the solar grade standard, since Si and SiC particles have different density, surface charge, and particle size. Wu and Chen [14] used electrical field and gravity to separate Si and SiC particles. The obtained silicon powder still contains metal impurities elements such as Fe, Al, etc., from the wear-and-tear of the cutting wire, and the subsequent purification to achieve solar grade purity, which is still in stage of research at present, is a significant challenge. The superconducting high gradient magnetic separation (HGMS) technology, which was developed from the conventional ferromagnetic technique, is a new physical separation technology. It is a simple, energy-efficient, inexpensive, and non-destructive technique with high efficiency and no secondary pollution [15]. The prominent feature of superconducting HGMS technology is the high magnetic flux density which the maximum reach 5.5 T [16]. The high saturation magnetic matrix is filled in the uniform background magnetic field, so that the magnetic flux density gradient is greatly increased. A higher magnetic flux density gives higher separation efficiency [17], so the superconducting HGMS is more suitable for capturing fine weakly magnetic particles. The first fully developed superconducting HGMS process was implemented in the Kaolin Clay industry to help clean and brighten the china clay. Dwari et al. [18] applied a low-intensity wet magnetic separator to concentrate iron resources from low grade siliceous iron ore. It obtained a concentrate of 67% Fe by recovering 90% of iron particles below 200 μm size. Li et al. [19] analyzed the effect of a high gradient magnetic field on the distribution of the solute Si. They found that a high gradient magnetic field is capable of separating the solute Si and the primary Si phase from matrix. However, superconducting HGMS technology has 13 J Mater Cycles Waste Manag (2018) 20:937–945 yet to be used f (...truncated)