p. 22-23 of cited doc -emphasis and formatting ad

p. 22-23 of cited doc -emphasis and formatting added
sounds like crushing to a pretty small size here is beneficial




Mining Association of Minnesota and the American Iron Ore
Association.
1.4.2.1
Milling
Beneficiation begins with the milling of extracted ore in preparation for further activities to recover
iron values. Milling operations are designed to produce uniform size particles by crushing, grinding,
and wet or dry classification. The capital investment and operation costs of milling equipment are high.
For this reason, economics plays a large part in determining the use of comminution equipment and the
degree of crushing and grinding performed to prepare ore for further beneficiation. Other factors
considered in determining the degree of milling include the value concentration of the ore, its
mineralogy, hardness, and moisture content. Milling procedures vary widely both between mills and
within individual mills depending on these variables.

Milling is a multistaged process and may use dry or wet ore feed. Typically, primary crushing and
screening take place at the mine site. Primary crushing is accomplished by using gyratory and cone
crushers (Weiss 1985). Primary crushing yields chunks of ore ranging in size from 6 to 10 inches.
Oversize material is passed through additional crushers and classifiers to achieve the desired particle
size. The ore is then crushed and sized at a secondary milling facility (Weiss 1985).

Secondary milling (comminution) further reduces particle size and prepares the ore for beneficiation
processes that require finely ground ore feed. The product resulting from this additional crushing is
usually less than 1 inch (1/2 to 3/4 inches). Secondary crushing, if necessary and economical, is
accomplished by using standard cone crushers followed by short head cone crushers. Gyratory crushers
may also be used (Weiss 1985).
Mining Industry Profile: Iron
1-17
Subsequent fine grinding further reduces the ore particles to the consistency of fine powder (325 mesh,
0.0017 inches, 0.44 microns).