Milling:
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).
Quarry Process
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).
Magnetic Separation:
Magnetic separation is most commonly used to separate natural magnetic iron ore (magnetite) from a variety of less-magnetic or nonmagnetic material. Today, magnetic separation techniques are used to beneficiate over 90 percent of all domestic iron ore (See Table 1-4) (Ryan 1991). Between 20 and 35 percent of all the iron units being beneficiated in the United States today are lost to tailings because hematite is only weakly magnetic. According to the Bureau of Mines, techniques used-to-date to try to recover the hematite have proven uneconomic.
Typically, magnetic separation involves three stages of separation: cobbing, cleaning/roughing, and finishing. Each stage may employ several drums in a series to increase separation efficiency. Each successive stage works on finer particles as a result of the removal of oversized particles in earlier separations. Cobbers work on larger particles (3/8 inch) and reject about 40 percent of the feed as tails.
Quarry Process
Flotation:
Flotation is a technique where particles of one mineral or group of minerals are made to adhere preferentially to air bubbles in the presence of a chemical reagent. This is achieved by using chemical reagents that preferentially react with the desired mineral. Several factors are important to the success of flotation activities. These include uniformity of particle size, use of reagents compatible with the mineral, and water conditions that will not interfere with the attachment of the reagents to the mineral or air bubbles (U.S. EPA 1982).
Gravity Concentration:
Gravity concentration is used to suspend and transport lighter gangue (nonmetallic or nonvaluable rock) away from the heavier valuable mineral. This separation process is based primarily on differences in the specific gravities of the materials and the size of the particles being separated. Values may be removed along with the gangue material (tailings) despite differences in density if the particle sizes vary. Because of this potential problem, particle sizes must be kept uniform with the use of classifiers (such as screens and hydrocyclones). Three gravity separation methods have historically been used for iron ore: washers, jigs, and heavy-media separators (Weiss 1985).
Quarry Process
