Boron and Phosphorus Removal from Silicon: A Hybrid Solvent–Slag Refining Approach

Investigating boron and phosphorus removal from silicon by Si-Ti and Si-Ti-Fe alloying systems

Al–Si solvent refining is a promising process for silicon purification, and the key impurity elements are boron and phosphorus. Thermodynamic calculation and experiments in particular temperatures show that boron and phosphorus segregation coefficients in Al–Si melt decrease with decreasing temperature, and it is not verified in traditional solidification. In order to character boron and phosphorus removal in Al–Si solidification over a temperature range, high purity hypereutectic Al–Si melts were solidified at relatively low cooling rate. It is found that boron and phosphorus removal rates increase with decreasing temperature. The tread that the boron and phosphorus segregation coefficients in the Al–Si melt decease with decreasing temperature is confirmed in solidification over a temperature range. In addition, aluminum content in purified silicon is close to the maximum solid solubility of aluminum in silicon. Removal rates of boron and phosphorus after solidification of high purity hypereutectic Al–Si melts

Al–Si solvent refining is a promising method to purify silicon. Boron and phosphorus are key impurity elements during the purifying process. The common raw materials for Al–Si solvent refining are metallurgical grade silicon and industrial aluminum. There are many impurity elements except boron and phosphorus in the metallurgical grade silicon and the industrial aluminum. The effect of these impurity elements on boron and phosphorus removal during Al–Si solvent refining is studied in this work. The hypereutectic Al–Si melts with these impurity elements were solidified, and the hypereutectic Al–Si melts without these impurity elements were solidified at the same cooling rate. Boron and phosphorus contents in purified silicon are determined by inductively coupled plasma optical emission spectrometer (ICP-OES). It is found that when silicon ratio increases from 20% to 70% in samples without other impurity elements, boron removal rates decrease from 0.4 to 0.3, and phosphorus removal rates decrease from 0.96 to 0.92. When silicon ratio increases from 20% to 40% in samples with other element impurities, boron removal rates decrease from 0.87 to 0.57, and phosphorus removal rates increase from 0.63 to 0.9. The different removal rates between samples with and without other impurity elements are attributed to the interaction between impurity elements. In addition, boron-containing intermediate compound forms and locates in the Al–Si matrix, while phosphorus-containing intermediate compound forms and locates in the primary silicon phase.

Separation of boron and phosphorus from Cu-alloyed metallurgical grade silicon by CaO–SiO2–CaCl2 slag treatment

Processes for Upgrading Metallurgical Grade Silicon to Solar Grade Silicon

Enhancement in extraction of boron and phosphorus from metallurgical grade silicon by copper alloying and aqua regia leaching

Effect of calcium addition on the silicon purification in the presence of low concentration of iron

Calcium and titanium as impurity getter metals in purification of silicon

Microwave plasma cleaning of ion implant ceramic insulators

A combination of solvent refining and flux treatment was employed to remove boron and phosphorus from crude silicon to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper, and the alloy was subjected to refining by liquid CaO-SiO2-Na2O-Al2O3 slags at 1773 K (1500 °C). The distribution of B and P between the slags and the alloy was examined under a range of slag compositions, varying in CaO:SiO2 and SiO2:Al2O3 ratios and the amount of Na2O. The results showed that both basicity and oxygen potential have a strong influence on the distributions of B and P. With silica affecting both parameters in these slags, a critical $$ P_{{{\text{O}}_{2} }} $$ could be identified that yields the highest impurity pick-up. The addition of Na2O to the slag system was found to increase the distributions of boron and phosphorus. A thermodynamic evaluation of the system showed that alloying copper with MG-Si leads to substantial increase of boron distribution coefficient. The highest boron and phosphorus distribution coefficients are 47 and 1.1, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.3 times and 17 times mass of alloy would be required, respectively.

Enhancing impurities removal from Si by controlling crystal growth in directional solidification refining with Al–Si alloy

Electrically-Enhanced Boron and Phosphorus Removal from Silicon by CaO–SiO2–Al2O3/–MgO Slag Treatment

The development of the solar market has been fast in the past decades, and the number of photovoltaic module installations is large. The photovoltaic modules have a lifetime of about 25 years and need recovery after that. The aluminium-back surface field (Al-BSF) module is the first kind of large-scale installed module and will come to its end of life in the next few years. The recycling of silicon material in the Al-BSF module is investigated in this work. The components of the module are separated, and the silicon material in the module is collected and then purified by (aluminium-silicon) Al-Si solvent refining for reuse. It is found that Al-Si solvent refining removed key impurity elements, namely boron and phosphorus, in the collected silicon. Kinetics has a great effect on boron and phosphorus removal, and boron and phosphorus contents in purified silicon decrease with decreasing cooling rate. The boron and phosphorus contents in silicon are lowered to 0.28 and 0.03 ppmw, respectively, after two times of Al-Si solvent refining with the cooling rate of 5.55 * 10-4 K second-1, and it meets the requirement of solar-grade silicon.

Solar radiation is a renewable and practically infinite source of energy that creates no greenhouse gas emissions such as CO2. Photovoltaic devices that turn solar energy directly into electricity are commonly made of high-purity solar-grade silicon, (SoG-Si). The SoG-Si is conventionally produced by a carbothermic reduction of quartz (SiO2), resulting in roughly 98 wt.% metallurgical grade silicon (MG-Si), which is then further purified into SoG-Si using the Siemens process. The carbothermic step releases a large quantity of CO2, and conventional purification methods require technically complicated equipment, consume intensive energy, and involve the use and production of highly toxic silane gases. This work has investigated a metallurgical purification method of MG-Si by solidification from aluminum-Si melt. Four purification rounds have been applied to ensure that boron and phosphorus are reduced to 314 ppba and 96 ppba respectively. A > 99.99 wt.% Si has been obtained, which can be further purified by the directional solidification method, and the purity of Si can be improved to 6N by effectively removing the other elements. The raw material of this process, the MG-Si, was produced by an aluminothermic process without direct CO2 emissions. Such a process, purification of MG-Si obtained from the aluminothermic reduction of a ferrosilicon slag, by a metallurgical route plus an additional step of directional solidification eliminates the use of highly hazardous silane gases associated with the conventional processes and requires much simpler equipment and lower operational energy, cost, and CO2 emissions.

Boron and Phosphorus Removal in Si Purification with Ca-Si Alloy under Air Atmosphere