IMST2017 Alternative Evaluation of the grindability of ... IOPscience
Then, HGI (Hardgrove grinding index) is calculated and expressed in °H. The method was designed for grinding coal, but it later expanded to other materials as well [6]. Another method is the Zeisel method, which was invented in 1953 and is based on Hardgrove. It uses a similar apparatus but operates at 100 to 200 mill revolution instead of 60.