Crushed stone and sand & gravel are the main types of natural aggregate used in the United States. Despite the low value of its basic products, the crushed rock industry is a major contributor to and an indicator of the economic well being of the nation. Industry consolidation is on the rise as large producers buyout the smaller producers. Individual crushed-stone quarries range in size from 50,000 to 10 million tons of product annually.

In 2011, 1,260 companies operating 3,300 active quarries and distribution yards produced 1.5 billion tons of crushed stone valued at $8.6 billion and employing more than 70,000 people. Of the total crushed stone produced in 2011, about 71% was limestone and dolomite; 15%, granite; 7%, trap rock; and the remaining 7% was shared, in descending order of quantity, by sandstone and quartzite, miscellaneous stone, marble, calcareous marl, slate, volcanic cinder and scoria, and shell. Leading states, in order of production, were Texas, Florida, Pennsylvania, Missouri, Illinois, Georgia, Ohio, North Carolina, Virginia, and California, together accounting for 51.8% of the total output (USGS, 2011).

During the past 25 years, production of crushed stone has increased at an average annual rate of about 3.3%. These projections suggest that vast quantities of crushed stone and sand and gravel will be needed in the future and that much of it will have to come from resources yet to be delineated or defined. Further growth will be fueled by new legislation that is allocating significant monetary resources into the construction and repair of highways and bridges in the United States.

Asphalt Industry

Asphalt creation starts with a mixture of sand and small and large rocks (called aggregate), which are combined with heated asphalt cement - thus the name hot mix asphalt (HMA). There are two basic types of plant designs used to manufacture HMA: batch plants and drum plants. There are approximately 3,600 HMA plants in the United States, accounting for a total annual production of 450 to 500 million tons of asphalt paving material. About 2,300 of the HMA plants are batch plants, with the remaining 1,300 are drum mix plants. The asphalt pavement industry employs approximately 330,000 people in the U.S. About 94% of the nation's roads and highways are surfaced with asphalt.

Stone Crushing Process

Stone crushing involves the following processes: mining, processing, crushing, screening and transportation. These processes are discussed in detail below.

Mining

Geological investigations help determine the location, distribution, and nature of potential aggregate in an area. Open-pit mining and quarrying is the common means of extracting rock. Alternately, a small number of locations use underground mines. Mining methods involve removing the overburden to extract the underlying rock deposits. Tricone rotary drills, long-hole percussion drills, and churn drills are used to create the blast holes in the rocks. Blasting in smaller operations may be done with dynamite, but in most medium to large-size operations, ammonium nitrate fuel oil mixture (AN-FO) is used as a low cost explosive. The fractured rock or shot rock is then excavated and loaded onto trucks that either feed portable crushers or stationary crushers for further processing.

Processing

The shot-rock is typically transported to the crushing plants using trucks. The large end users in the crushing plant consist of feeders, crushers, conveyors and screens.

Crushing

Crushers mechanically break the stone into smaller pieces. Reduction in size is generally accomplished in several crushing stages, as there are practical limitations on the ratio of size reduction through a single stage. Crusher selection is based on rock type, required size reduction, output rock shape and production rate. Details on the different available crusher designs are presented below.

- jaw crusher

Jaw crushers are typically used as primary crushers. A jaw crusher consists of a set of vertical jaws, one jaw being fixed and the other moving. The progressive compression by the back-and-forth motion of the moving jaw breaks the stone. Jaw crushers operate best when fullyloaded but not flood-fed.

- impact crusher

Impact crushers are typically used in the primary and secondary crushing stages, are well suited for softer rocks and can achieve reduction ratios of up to 40:1. Impact crushers tend to wear faster than other crushers because of constant high-velocity collision with the stone.

- cone crusher

Cone crushers are a type of gyratory crushers that are well suited for applications that require a reduction ratio of 6:1 and provide a tight control over the rocks leaving the crusher. They are typically applied in the secondary and tertiary crushing stages. For proper reduction and consistent product gradation, cone crushers must be choke fed at all times.

- mobile crusher

A typical quarry face will recede approximately 45 feet every year. This extra distance increases the work of the haul trucks that feed the primary crushers with the shot rock. Mobile crushers offer the option of crushing near the blast site and using efficient conveyors to supply the secondary crushing operations. Mobile crushers are also popular for projects with a limited project life.

Crusher Machine Selection and Application

Table below shows a generic crusher selection guide, as well as unloaded power draw.

The primary crushing stage is the most economical segment of the aggregate plant in which to make material reduction, after the initial quarry blast. Primary crushing involves reducing shot rock from 40-inch minus to 9-inch minus rock. The secondary and tertiary crushers reduce the stone from 9-inch minus to less than 2-inch rock size. Different stages of crushing are required based on the intended final product size. Typically, a greater number of crushing stages are required to get smaller rock sizes.