The mineral processing industry usually evolves rather than revolutionizes but the horizontal shaft lmpactor (HSI) has revolutionized the crushing process in numerous industries. There are several varieties of HSI, and their simi larities are more numerous than their differences. All varieties feature externally fed horizontal rotors with breaker bars, which propel material into a series of apron-mounted breaker plates that crush or pulverize many different types of materials to specified degrees of fineness.