Leaching behaviors of impurities in metallurgical-grade silicon with hafnium addition

Hf was employed as an impurity getter to enhance the removal of impurities from metallurgical-grade Si (MG-Si) via the solidification of Si or a Si-33 wt% Al solvent. The leaching behaviors of the impurities (B, Fe, Al, Ca, P, Zr, Ti, V, Mn, Hf, and Ni) within MG-Si, in the presence of 5 wt% Hf, were investigated using various leaching approaches. Compared with aqua regia and HF, HCl + HF was determined to be the optimal lixiviant for the elimination of impurities from Hf-containing MG-Si. The use of a combination of HCl + HF and aqua regia reduced the quantity of impurities from 6126 ppmw to 94 ppmw. Eh-pH diagrams were calculated to discuss the leaching of HfSi2 in aqua regia and HF solutions. The presence of Hf in the MG-Si enhanced the removal of impurities, especially P, which cannot be efficiently removed via solidification refining and hydrometallurgical treatments. Hf-containing Si-Al solvent refining is considered the most efficient approach for the elimination of impurities (except Al). The removal fractions of B and P were 94.2% and 86.2%, respectively, achieved via the solidification of the Si-33 wt% Al solvent. Moreover, 99.94% and 99.9996% of the Hf, used as an impurity getter, could be eliminated through the solidification of the Si and Si-33wt% Al solvent, respectively, decreasing from 50,000 ppmw, to 28 ppmw and 0.2 ppmw, respectively

Investigation of the moisture classification and moisture removal behavior of diamond wire saw silicon powder waste cake

When processing silicon wafers for photovoltaic power generation, approximately 35 % of high-purity silicon is lost as a diamond wire saw silicon powder (DWSSP) waste cake with a high moisture content. The high moisture content of DWSSP results in the oxidation of high-purity silicon, which hinders the recovery and utilization of silicon resources. Disappointingly, a method for the removal of moisture from the DWSSP waste cake, which is crucial for silicon recovery, has not yet been revealed. In the present study, a novel investigation was conducted to determine the moisture classification of a DWSSP waste cake and to reveal the moisture removal behavior. The results indicate that the moisture content in the DWSSP waste cake can be divided into two distinct categories: surface adsorbed water and capillary water. Moreover, the kinetics analysis and simulation demonstrated that the elimination of moisture predominantly takes place during the constant-rate period and the falling-rate period. Increasing the drying temperature and reducing the equivalent diameter of the DWSSP waste cake will contribute to improving the efficiency of moisture removal. The application of the findings of this study can help to reduce the harmful environmental impacts of DWSSP and achieve the efficient recovery of silicon resources

Silicon recovery from diamond wire saw silicon powder waste with hydrochloric acid pretreatment: An investigation of Al dissolution behavior

Silicon recovery from diamond wire saw silicon powder (DWSSP) waste is of great significance for increasing production profits and alleviating hazardous effects on the ecological environment. The purity of recovered silicon powder is determined by the purification efficiency during acid leaching pretreatment. Because the metallic impurities present in DWSSP are mostly physically mixed rather than chemically bound, the reaction rate is very fast in the initial stage of acid leaching, whereas it is difficult to dissolve the retained impurities in the later stage with the depletion of metal fragments adhered on the surface of the silicon matrix. Many prior studies have failed to consider the retained metallic impurities that reside in the inner silicon particle surfaces. Therefore, this study investigates the dissolution behavior of retained impurities via the dissolution of Al in HCl solution as an example. Thermodynamic results indicate that the Al dissolution process is dominated by entropic changes (Delta S-0), rather than enthalpic changes (Delta H-0). Furthermore, the dissolution behavior of Al is in accordance with the diffusion-controlled step in the Avrami mode, and the kinetic parameters were found to be A = 5.85 x 10(7), E-a = 49.27kJ . mol(-1), and m < 1. The determined dissolution behavior provides a clear understanding of the removal of retained metallic impurities from DWSSP via an acid leaching pretreatment. This study provides enlightenment for the further purification of silicon recovered from DWSSP waste. (C) 2020 Elsevier Ltd. All rights reserved

Silicon recycling and iron, nickel removal from diamond wire saw silicon powder waste: Synergistic chlorination with CaO smelting treatment

During the processing of silicon wafers for photovoltaic power generation, nearly 30-40% of silicon ingot will be lost as diamond wire saw silicon powder (DWSSP) waste; this can result in reduced profits, potential danger, and environmental pollution. Driven by the rapid development of the photovoltaic industry, the demand for highpurity silicon and the production of silicon waste generated during silicon processing will continue to increase. Against this trend, the recycling of silicon from DWSSP waste is a desirable alternative to meet the growing demand for high-purity silicon. In this study, a novel process of chlorination with CaO smelting treatment is first proposed for the recycling of silicon and the removal of Fe and Ni from DWSSP waste. The results of this research indicate that the SiO2 shell can be absorbed by the added CaO, while eutectic NaCl-MgCl2 has a significant effect on the maximum recovery of silicon by reducing the viscosity of the molten slag and the removal of metals through the micro concentration cells. CaO and eutectic NaCl-MgCl2 play a synergistic role in the promotion of silicon separation and the removal of Fe and Ni. Silicon with a purity of 99.83% and a recovery rate of nearly 98.96% was obtained under the smelting conditions of DWSSP:CaO:NaCl:MgCl2 = 150.0:23.42:7.50:8.85 with a holding time of 2 h at 1823 K. This process overcomes the technical bottleneck of the removal of Fe and Ni via smelting treatment, which is favorable for the recycling of silicon from DWSSP waste at the industrial scale

Role of Oxygen Potential and Oxygen Ions on Phosphorus Removal from Silicon via Addition of FeO into Slag

In present work, the role of the oxygen potential (P-O2) and oxygen ion (O2-) concentration for removing phosphorus (P) during CaO-SiO2-Al2O3-FexO slag refining was studied by on-line measurement of oxygen activity in molten silicon (Si), FactSage calculation, Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The results show that the addition of FeO from 0 to 9.25 wt% in slag can increase the activity of dissolved oxygen (a([O])) in Si and the mole fraction of O2- in slag. Moreover, the increase of O2- concentration leads to the increase of non-bridge oxygen (NBO). The value of L-P (the partition ratio of phosphorous between slag and Si shows a first increase and then decrease trend and reaches a maximum value of 1.95 at 5 +/- 0.1 wt% FeO. It is believed that the increase of a([O]) and NBO can promote the removal of P as FeO content is less than 5 +/- 0.1 wt%. the chain structure unit (Q(2)) of silicate network as the main intermediate structure to capture PO43- from the charge compensation of P2O5 by O2- to form the sheet structure unit Q(3)(Si and P). When FeO content is increased to more than 5 +/- 0.1 wt%, L-P value gradually decreases although the values of NBO and a([O]) are increasing. NBO plays a leading role in this process, it can be speculated that more NBO can depolymerize the Q(3) (Si and P) to destroy the stability of P in silicate network. As a result, a mount of PO43- is present at the interface to prevent the oxidation of phosphorous, which leads to the decrease of L-P value

Investigation of Na2CO3-CaO-NaCl (or Na3AlF6) additives for the remanufacturing of silicon from diamond wire saw silicon powder waste

The demand for high-quality silicon for solar cell preparation will continue to grow due to shortages of silicon feedstock driven by the photovoltaic industry. Consequently, the silicon lost in diamond wire saw silicon powder waste during silicon wafer production must be recovered for silicon production. Depending on the current yield of silicon and level of residual impurities, an optimization treatment for the increase of the recovery rate and purity of silicon collected from diamond wire saw silicon powder should be further developed. In this study, NaCl and Na3AlF6 are designed as additives to optimize Na2CO3-CaO to increase the silicon recovery rate and purity. The action mechanisms of NaCl and Na3AlF6 for silicon recovery and the removal of metal during the direct smelting process are investigated. It is found that NaCI and Na3AlF6 improve the silicon migration channels and stabilize the impurities in slag by reducing the slag viscosity and tuning the slag composition. After induction smelting at 1823 K for 2 h with the furnace feed mass ratios of Na2CO3 :CaO:NaCl = 12.8:1.7:8.6 and Na2CO3:CaO:Na3AlF6 = 12.8:1.7:8.6, the obtained silicon recovery rates were respectively 76.39% and 79.25%, and the silicon purities were respectively 99.985% and 99.986%. Therefore, NaCl and Na3AlF6 could be used as potential additives to promote the efficient remanufacturing of silicon collected from diamond wire saw silicon powder waste, and are technically and economically feasible for large-scale application. (C) 2020 Elsevier Ltd. All rights reserved

Dissolution and mineralization behavior of metallic impurity content in diamond wire saw silicon powder during acid leaching

Metallic impurity removal is essential for the recovery and purification of silicon from diamond wire saw silicon powder during acid leaching process. However, the related dissolution and mineralization behavior are unclear, which are the two main factors governing the capacity of metallic impurity removal during the acid leaching process for silicon recovery from diamond wire saw silicon powder. Metallic impurities have different characteristics in diamond wire saw silicon powder, especially the mineralization hindering the deep removal of impurities. In this study, the dissolution behavior in out-field and mixed lixiviant assisted intensification acid leaching were investigated, respectively. Further, it is of interest in this study to search the root cause of the dissolution behavior from impurity sources with a unique insight, where related theory was used to explain both the mineralization behavior and dissolution behavior. Results revealed that the acid leaching dissolution occurred from the surface inwards, as well as the root cause, which is caused by mineralization behavior. Owing to the fact that the bare silicon surface developed on the amorphous SiO2 layer, the retention of metallic impurity was the result of the SiO2 layer obstacle. The dissolution behavior could be explained as the disintegrating corrosion of SiO2. For this reason, the 4 M HCl + 0.5 M HF mixture with a total removal efficiency of 99.28% outperformed out-field assisted leaching. This study can provide a suitable foundation for understanding the dissolution behavior and mineralization behavior for impurity removal, as well as contribute a roadmap for further silicon purification from diamond wire saw silicon powder. (C) 2019 Elsevier Ltd. All rights reserved

Occurrence State and Dissolution Mechanism of Metallic Impurities in Diamond Wire Saw Silicon Powder

The facilitation of metallic impurities removal via acid leaching pretreatment plays a key role in silicon recovery from diamond wire saw silicon powder (DWSSP) waste. Disappointingly, the occurrence state and dissolution mechanism of the metallic impurities present in DWSSP have not yet been revealed. For this reason, in this study, three different kinds of raw DWSSP were used for acid leaching tests and leaching behavior verification to investigate the dissolution mechanism. After two-stage acid leaching with 4 M HCl and 2 M HCl + 2.5 M HF, the results indicate that the removal efficiency was superior to that of other previously reported leaching processes. Furthermore, two occurrence states of metallic impurities present in DWSSP were defined based on their different dissolution behaviors, and the intrinsic relationship between dissolution behavior and the thinning process of the amorphous SiO2 shell was derived. The findings indicate that the first type of impurity was more easily dissolved in HCl solution; however, the second type of retained metallic impurity was restricted by the SiO2 shell barrier and was removed by the added HF solution. For this reason, the disintegration dissolution of the amorphous SiO2 shell was found to have a significant impact on the facilitation of the removal of impurities retained in DWSSP. The findings of this study are significant for the recovery of silicon from DWSSP with an effective acid leaching flow design

Novel Reaction Media of Na2CO3-CaO for Silicon Extraction and Aluminum Removal from Diamond Wire Saw Silicon Powder by Roasting-Smelting Process

Exploring a sustainable process to dispose diamond wire saw silicon powder (DWSSP) waste produced in the solar crystal silicon wafering process is a research field with significant theoretical research value and industrial engineering application. The current consensus on silicon extraction from DWSSP is that SiO2 oxide layer digestion and high-content Al removal are two main factors governing the yield and purity of silicon. To solve these problems, a novelty reaction media of Na2CO3-CaO in the induction furnace roasting-smelting process for silicon extraction and aluminum removal from DWSSP was proposed. The mechanisms of SiO2 digestion and Al removal during the roasting-smelting process were investigated by thermodynamic analysis, mineral transformation, and roasting-smelting experimental investigation. The obtained silicon with a Al removal efficiency of 48.44% and silicon yield of 55.5% was improved from 89.125% to 99.31% under the furnace feed mass composition of 38.4 g Na2CO3, 5.1 g CaO, and 300 g DWSSP, and holding time for 2 h in 1823 K. The novelty reaction media of Na2CO3-CaO is expected to achieve efficient silicon extraction and aluminum removal from DWSSP. This process provides an alternative choice for the commercial production of silicon alloy, silicone, and high purity silicon preparation with a process that is low in cost, is easy to operate, exhibits short flow, and can be sustainably manufactured