Ultrahigh Temperature Purification of Graphite for the Development of a Continuous Process.

This work presents a study of ultrahigh temperature purification of natural Canadian graphite flakes. The concentrated natural graphite flakes were purified using two test facilities, an ultrahigh temperature fixed bed furnace and an ultrahigh temperature fast-heating counterflow reactor. With the fixed bed furnace, the natural graphite flakes were purified at 2500 or 2800 °C for 15-120 min. With the counterflow reactor, the residence time was ∼20-25 min, with an average temperature of 2700 °C and higher local temperatures due to electric arcing. The heat-treated samples were characterized by using several different analysis techniques. The results showed that the samples treated with the fast-heating counterflow reactor reached a very high purity above 99.9 wt % carbon. The samples treated at 2800 °C in the fixed bed furnace reached a similar purity. At the lower temperature of 2500 °C, a similar purity could only be achieved with a duration of at least 60 min. Four elemental analysis techniques to quantify impurities in graphite were evaluated in this work, with a focus on elements that disrupt the performance of Li-ion batteries, such as magnesium, aluminum, iron, copper, and silicon. The analysis results with the original graphite flakes and the heat-treated graphite flakes showed that significant differences exist among the various analysis techniques. For some critical elements, such as iron and silicon, the detected concentrations could differ by more than 1 order of magnitude.