The Bond ball mill work index test is thought to be additive because of its units of energy; nevertheless, experimental blending results show a non-additive behavior.  The SPI(R) value is known not to be an additive parameter, however errors introduced by block kriging are not thought to be significant .
A mill is a device that breaks solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an important unit operation in many processes. There are many different types of mills and many types of materials processed in them. Historically mills were powered by hand (e.g., via a hand