After being transported to the processing plant, the wet sand and gravel raw feed is stockpiled or 

emptied directly into a hopper, which typically is covered with a "grizzly" of parallel bars to 

screen out large cobbles and boulders. From the hopper, the material is transported to fixed or 

vibrating scalping screens by gravity, belt conveyors, hydraulic pump, or bucket elevators. The 

scalping screens separate the oversize material from the smaller, marketable sizes. Oversize 

material may be directed to a crusher for size reduction, to produce crushed aggregate. Crushing 

generally is carried out in one or two stages, although three-stage crushing may also be 

performed. 

Following crushing, the material is returned to the screening operation for sizing. The material 

that passes through the scalping screen is fed into a battery of sizing screens, which generally 

consists of either horizontal or sloped, and either single or multideck, vibrating screens. Rotating 

trommel screens with water sprays are also used to process and wash wet sand and gravel. 

Screening separates the sand and gravel into different size ranges. Water is sprayed onto the 

material throughout the screening process. Afier screening, the sized gravel is transported to 

stockpiles, storage bins, or, in some cases, to crushers by belt conveyors, bucket elevators, or 

screw conveyors. The sand is hd from clay and organic impurities by log washers or rotary 

scrubbers. After scrubbing, the sand typically is sized by water classification. Wet and dry 

screening is rarely used to size the sand. After classification, the sand is dewatered using screws, 

separatory cones, or hydroseparators. After processing, the sand is transported to storage bins or 

stockpiles by belt conveyors, bucket elevators, or screw conveyors. 

1.2 Concrete atc chin^^,^ 

Concrete is composed essentially of water, cement, sand (fine aggregate) and coarse aggregate. 

Coarse aggregate may consist of gravel, crushed stone or iron blast furnace slag. Some specialty 

aggregate products could be either heavyweight aggregate (of barite, magnetite, limonite, 

ilmenite, iron or steel) or lightweight aggregate (with sintered clay, shale, slate, diatomaceous 

shale, perlite, vermiculite, slag, pumice, cinders, or sintered fly ash). Concrete batching plants 

store, convey, measure and discharge these constituents into trucks for transport to a job site. In 

some cases, concrete is prepared at a building construction site or for the manufacture of 

concrete products such as pipes and prefabricated construction parts. A generalized process 

diagram for concrete batching is included in Appendix A of this document. 

The raw materials can be delivered to a plant by rail or truck, but in the case of existing sources 

on GRIC land, they are collocated with nonmetallic sand/gravel mining and processing facilities. 

The cement is transferred to elevated storage silos pneumatically or by bucket elevator. The 

sand and coarse aggregate are transfmed to elevated bins by front-end loader, clamshell crane, 

belt conveyor, or bucket elevator. From these elevated bins, the constituents are fed by gravity 

or screw conveyor to weigh hoppers, which combine the proper amounts of each material. 

Truck mixed (transit mixed) concrete is the process practiced at one of the existing GRIC 

concrete batch plants. At these plants, sand, aggregate, cement and water are all gravity fed from 

the weigh hopper into the mixer trucks. The concrete is mixed on the way to the site where the 

concrete is to be poured. Central mix facilities (including shrink mixed) constitute another 

concrete production process in use on GRIC lands. With these, concrete is mixed and then 

NMMProc TSD 6-14-02 Created on 6/18/2002 9:39:00 AM 2