Ferrosolar UMG Process

Ferrosolar process begins with the crushing of MG Si, to adapt its grain size to subsequent treatments. Crushers have been carefully designed to minimize contamination. The final grain size is 100% less than 1 cm.

Next, this silicon is charged in induction furnaces with a capacity up to one ton of silicon. Ferrosolar uses reactive slags in graphite crucibles with the objective of a selective removal of certain impurities, including boron. Once the treatment is finished, the molten silicon is transferred to a suitable vessel and directionally solidified. The degree of purification in this last step depends on its segregation coefficient of a given impurity. The overall result of this first stage of slagging and controlled solidification is the reduction of boron and most of the metallic impurities (Al, Ca, Fe, Ti, etc), as well as the partial reduction of other impurities like phosphorus (50% reduction) among others.

In the next step, the directionally solidified silicon is grinded bellow 1.5 cm and leached in acid, washed in deionized water and dried to prepare it for next stage which is a thermal treatment at temperatures in the range 1550°C – 1700°C under high vacuum (Souto et al. 2014). The main objective of this process is the removal of phosphorus, given its high volatility under vacuum at high temperatures. Other impurities such as Al, Ca, K, and Na, are also partially evaporated. In this step, the silicon is firstly introduced into a sealed chamber equipped with internal atmosphere control. This chamber, acts as the silicon charging system and is connected to the high vacuum furnace through a valve. Once the silicon has been loaded, the chamber is purged to reduce its pressure before opening the connection valve with the vacuum furnace. When the purge has been completed, the internal pressure in the charge chamber is adjusted to 3000 Pa to match that of the vacuum furnace. When internal pressures on both sides are equivalent the valve opens and the silicon is transferred into the vacuum furnace, where is placed in a high-density graphite crucible with a capacity up to 1000 kg of silicon.

When the transfer of silicon has been completed the valve closes, and the vacuum furnace is ready to start with the thermal treatment. The furnace is equipped with two independent systems to control temperature; a W-Re thermocouple placed close to the crucible, and an optical pyrometer that measures the temperature through a quartz crystal window. Internal pressure is initially 3000 Pa, and this pressure is kept constant until the melting of the silicon has been completed and the temperature stabilized at the chosen setpoint. At this point, to increase the evaporation kinetics it is necessary to lower the internal pressure in the furnace from 3000 Pa to below 5 Pa . In fact, evaporation of phosphorus reaches its maximum rate at pressures from 1 to 5 Pa at the range of temperatures of these treatments. Evaporation lasts between 4 and 8 hours, depending on the initial phosphorus concentration of the silicon.

Figure 1. Overview of the procedure followed in this study. HPM stands for High Performance Multicrystalline. The starting point is the silicon purification explained in section 2.1. A feedstock: silicon from Ferrosolar´s standard process, B feedstock: same silicon but intentionally mixed with contaminated silicon in the charge of multi growth.

At the end of the evaporation, the furnace is refilled with Ar to an internal pressure of 3000 Pa. Then, it is connected to a solidification furnace and the purified silicon is casted and directionally solidified. The result of this evaporation/controlled solidification process is a silicon block of 500-1000 kg depending on the furnace capacity. Finally, this block is placed in a diamond wire saw machine to remove the laterals, upper and lower parts where the residual impurities are concentrated. These contaminated parts will be recycled and re-melted in previous stages of purification. The silicon thus obtained after the cutting step has very low boron, phosphorus and metals content and is very suitable for photovoltaics.