Primary Crushing

Abstract

Crushing, grinding and other words or phrases associated with the size- reduction of ore and other rock are all comprehended in the word “comminution”. This (Truscott) is “the whole operation of reducing the crude ore to the fineness necessary for mechanical separation, or for metallurgical treatment…”¹ It is usual to make an arbitrary division of comminution into convenient stages. Primary crushing brings run-of-mine ore down to a maximum size of the order 4″ to 6″ in average diameter; secondary crushing receives feed at −6″ and reduces it to below “Dry” crushing includes work on ore as mined, which may be somewhat moist when delivered. It is succeeded by comminution in water, arbitrarily called “grinding”. Although a considerable amount of fine grinding is done by dry methods, this book follows usage by reserving the word “crushing” for an operation predominantly dry and “grinding” for work on a suspension of ore particles in water. One important difference between dry and wet comminution lies in the mode of seizure of the particle. In the former case the particle is large enough to be gripped between two solid steel members as they are pressed together by mechanical forces. One or both of these members moves to and from in a fixed-path cycle. The rock gravitating through the rapidly expanding and contracting gap thus produced is nipped and crushed. In wet grinding the bulk of the ore is already too finely divided for a particle to be seized in this manner. It is therefore exposed while more or less free to move, to random blows. There are exceptions to this generalisation.