Understanding the Effects of Heat Transfer on the Purification of Metallurgical Silicon by Directional Solidification in Cast Furnaces

Experiments on directional solidification were carried out to investigate how purification of metallurgical-grade silicon in cast furnaces is affected by changes in heat extraction from and heat supply to their melts. A reference condition analogous to that in the block-casting process was established using top/side heaters to supply heat and a water-cooled base to extract heat from the bottom of a graphite-clay crucible. This condition was modified by (a) changing the crucible bottom material to graphite, (b) increasing the length of the resulting ingot from 100 to 130 mm, and (c) turning off the heaters. Temperatures were measured within the melt and in the furnace environment. The grain macro-microstructures and the macrosegregation of impurities of the ingots were revealed. The cooling rates and solid–liquid interface velocity calculated with a mathematical model increase relative to the reference experiment when the graphite crucible bottom is used or when the top/side heaters are absent. The vertical temperature gradients also increase with the graphite bottom, but significantly decrease without the heaters. Most of the ingots exhibit a purified lower region of columnar grains with straight boundaries, free from intermetallic particles, and an upper region with mixed long and short columnar grains with serrated boundaries, precipitated particles, and higher impurity concentrations. Changing the crucible bottom material from graphite-clay to graphite increases the length of the purified region from 70 (reference condition) to 97 mm, whereas turning off the heaters completely eliminates this region. Although the graphite crucible bottom (with the top/side heaters) yields the longest purified region, the graphite-clay bottom (also with the heaters) gives the lowest impurity concentrations.