Preparation of Low-Boron Silicon from Diamond Wire Sawing Waste by Pressure-Less Sintering and CaO–SiO 2 Slag Treatment

With the shortage of high-grade silicon (Si) feedstock due to the rapid development of photovoltaic, electronic information, and organic Si industries, there is an increasing demand to recycle Si waste economically and efficiently. However, the deep removal of B and P from Si has always been a significant challenge for Si waste recycling. A novel application of electroslag remelting (ESR) refining has been developed to remove B and P for the purification and recovery of Si. In addition, industrial tests were performed to recover the diamond wire saw Si powder. The design principle is put forward for the Si alloy electroslag system. A new 35% CaO−30% CaF 2 −17.5% Al 2 O 3 −17.5% SiO 2 electroslag applicable for Si alloy ESR is designed to meet both physical and impurity removal requirements. Furthermore, the conductivity (2.67 Ω −1 •cm −1 ), density (2.97 g/cm 3 ), and viscosity (0.05 Pa•s) meet the requirements of ESR. The dynamic refining process of Si alloy passing through the slag layer in the droplet form is the core of ESR refining, and it is found that more than 80% of B and P removal is achieved in the dynamic refining process of the droplet penetration slag. In order to understand the dynamic refining process, the mathematical model of droplet movement and impurity removal model are proposed to study the droplet movement behavior in slag and the competitive influence mechanism of multiple parameters on B removal efficiency during the dynamic refining process. Mathematical models and experimental results show that there is an ultimate velocity of droplet entry into the slag, which increases linearly with the growth of droplet size under the same slag condition. In addition, the specific surface area of the droplet plays a dominant role in the removal of B. The successful industrial experiments of Si alloy ESR prove that the design principle of the Si alloy electroslag system and the size control mechanism of Si alloy are feasible in this work. The removal efficiency of B and P reached 80 and 65%, respectively, within 30 min. It is also believed that ESR will have an application in purification and recovery of Si waste.

“…(5) The influence of heat transfer of flow was ignored. (6) The interface between molten slag and crucible was set to be no slip wall.…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Novel Application of Electroslag Remelting Refining in the Removal of Boron and Phosphorus from Silicon Alloy for Silicon Recovery

Qian

Sun

Wang

et al. 2021

“…B and P are the most typical impurities, and their contents in Si are about dozens to hundreds of parts per million (ppm). Two kinds of impurities often come from the contamination of electroplating solution containing pyrophosphate on the diamond wire saw or B/P addition of P/N batteries, which limits the use of recycled Si powder as Si feedstock. − One of the biggest difficulties in the preparation of SoG-Si is how to deeply remove traces of B and P. The small separation coefficient is an important reason for the difficulty in deep separation of the two impurities. For example, the partition coefficient of B and P in the slag refining process is usually less than 10, , and the segregation coefficient of B and P in the solidification segregation process is close to 1 .…”

Section: Introductionmentioning

confidence: 99%

Enhanced In Situ Separation of Boron at the Silicon Alloy Solidification Interface through Innovating the Impurity Chemical Reconstruction Approach for SoG-Si

Qian

Zhou

et al. 2021

Removal of trace impurities such as boron (B) and phosphorus (P) has always been a challenge for the preparation of solar-grade silicon (SoG-Si). Chemical reconstruction has been thought to be an effective way to remove trace impurities by changing the phase structure and location of impurities. The traditional way of the chemical reconstruction of impurities at the grain boundary after solidification inevitably relies on the subsequent separation of crystalline Si and chemical reconstruction medium. Thus, we develop a new method for the chemical reconstruction of B at the front of the solidification interface of the Si alloy, which provides advantages for the in situ separation of the reconstructed impurity phase in the supergravity filtration separation process. The chemical reconstruction of B at the solidification front of the Si alloy has been realized by adding 25–75% of the liquation agent aluminum (Al) and about 10 000 ppm of the impurity modifier agent titanium (Ti) with the help of Thermo-Calc thermodynamic calculation and experimental verification. The influence mechanism of the supergravity field on the precipitation behavior of the chemically reconstructed phase TiB2 is revealed by the derived Maxwell–Stefan (MS) equation under the action of the supergravity field. A special filter-type combined crucible is skillfully designed and strictly controlled to cool the crucible containing the alloy melt at a constant temperature gradient under the supergravity field to achieve in situ separation of impurities at the solid–liquid interface of the Si alloy. The chemical reconstruction phase TiB2 is mainly retained on the surface of crystalline Si during the process of supergravity centrifugal filtration, which is more suitable for the deep removal of the B impurity and B content in Si drops below 1.5 ppm by simple pickling. In addition, the Si–Al–(Ti) alloy with a small amount of impurities obtained after centrifugal filtration separation can be further used for the chemical reconstruction of impurities, and the recycling of chemical reconstruction medium has been achieved. The present work proposes a new idea of chemical reconstruction of impurities at the front of alloy solidification and in situ separation, which expands the way of in situ separation of trace impurities at the solid–liquid interface of the alloy.

“…In the aspect of the preparation of high-purity silicon, DWSSP has more advantages than does silicon ore . Therefore, disposal and recycling of DWSSP have become the focus of the global photovoltaic industry. , …”

Section: Introductionmentioning

confidence: 99%

“…In the aspect of the preparation of high-purity silicon, DWSSP has more advantages than does silicon ore. 8 Therefore, disposal and recycling of DWSSP have become the focus of the global photovoltaic industry. 14,15 Various research had been carried out to find ways to recover silicon from DWSSP. The proposed disposal methods included acid leaching method to remove metallic impurities in DWSSP, 16−19 the two-stage thermal plasma process to recycle silicon from DWSSP, 20 the vacuum carbothermal reduction and hydrobromination reaction method to eliminate silicon oxide, 21,22 contamination, 23 and the slag treatment process for silicon recycling.…”

Section: ■ Introductionmentioning

confidence: 99%

Application of Pressure-less Sintering and Dynamic-Slag Treatment to Recover Diamond Wire Saw Silicon Powder

Chen

Wang

Huang

et al. 2020

Self Cite

Recycling silicon from environmentally hazardous diamond wire saw silicon powder waste (DWSSP) and effectively removing boron is still an elusive aim. Here, pressure-less sintering and dynamic-slag treatment were used to recover the powder waste. On the basis of thermodynamic simulation, the slag composition was designed. Dense structure of DWSSP ceramics could effectively reduce the oxidation loss. Dynamic-slag treatment can provide higher silicon recovery and better boron removal rate than can traditional one-step slag treatment. Slag-1 (Na 2 O-SiO 2 ) can remove boron while protecting molten silicon from oxidation. Interestingly, after Slag-2 (CaO−SiO 2 −Na 2 O) was added, relative displacement between the mixed slag and silicon can provide more favorable dynamic conditions. Using the dynamic-slag treatment, the boron removal efficiency reached 83.4%, and the silicon recovery rate reached 82.29%. Additionally, by introducing the solute penetration theory, the mechanism of boron removal during convective mass transfer process was discussed in detail. The simplicity of this combined approach provides a new possibility for recovery and purification of intractable fine powdery waste.