Sand and gravel are created when eroded rocks are transported downstream by glaciers, streams, and rivers. The river bed is formed by the materials that settle out of the water. Consequently, sand and gravel deposits are left behind as the river channel travels through the area. These deposits can be extensive, such as the ones in the Platte River valley which can be more than 150 feet deep, and frequently include remains such as mammoth and bison bones along with petrified wood.
Sand and gravel deposits, which are the unconsolidated granular materials left over after the natural breakdown of rock or stone, are typically found in near-surface alluvial deposits as well as subterranean and subaqueous beds. Rocks and unconsolidated, or poorly consolidated, materials weather into siliceous and calcareous products. These deposits are widespread across the US.
The mining of aggregate, also known as sand and gravel, is significant to Nebraska’s economy. There are several uses for sand and gravel including:
- Concrete production
- Road building
- Glass manufacturing
- Petroleum extraction
- Steel foundries and sand casting in iron
Construction of Gravel
Open pit excavation or dredging methods are often used in sand and gravel mining in damp or wet conditions. Power shovels, draglines, front-end loaders, and bucket wheel excavators are used for open pit excavation. Light charge blasting is occasionally used to remove deposits. Dredging is a method of extracting sand and gravel from the bottom of water bodies by using suction or bucket-type dredges which are mounted on boats or barges. Following extraction, the materials are moved to the processing facility using a suction pump, an earth mover, a barge, belt conveyors, a truck, or another vehicle.
The majority of residential sand and gravel is treated before use, even though large amounts are used for fill, bedding, subbase, and base course without processing. Sand and gravel mining processing for a particular market comprises a variety of washer, screen, and classifier configurations to separate particle sizes. They also include crushers to reduce large material apart from storing and loading facilities.
The wet sand and gravel raw feed are stockpiled or dumped straight into a hopper at the processing facility. The hopper is often coated with a “grizzly” of parallel bars to screen out larger cobbles and boulders. From the hopper, the material is moved by belt conveyors, gravity, hydraulic pumps, or bucket elevators to fixed or vibrating scalping screens. These scalping screens distinguish between larger, unmarketable sizes and oversized stuff.
Sand and gravel Processing
The material that makes it past the scalping screen is fed onto a battery of sizing screens which typically consist of vibrating screens that are either horizontal or slanted, single or multi-deck. Wet sand and gravel are processed and washed using rotating trommel screens and water sprays. Sand and gravel are divided into several sizes and ranges through screening. The material is continuously sprayed with water during the screening process. After screening, belt conveyors, bucket elevators, or screw conveyors are used to move the sized gravel to stockpiles, storage bins, or, in some situations, crushers.
Rotary scrubbers or log washers are used to remove clay and organic contaminants from the sand. The sand is normally sized by water classification after cleaning. Rarely is wet and dry screening utilized to size the sand. The sand is dewatered using screws, separatory cones, or hydro separators after classification. The material can also be rod milled to create smaller-sized fractions. However, this is not a usual technique in the sector. Following processing, the sand is moved by belt conveyors, bucket elevators, or screw conveyors to storage bins or stockpiles.
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Industrial Sand and Gravel
Open pits of naturally occurring quartz-rich sand and sandstone are often where industrial sand and gravel are mined. The degree of cementation of the rock determines mining techniques in large part. Blasting may be necessary for some deposits to release the material before processing. Before being delivered to the processing facility, the material may go through primary crushing at the mining site.
The mined rock is transported and heaped up at the processing facility. After that, the material is crushed. Depending on the level of cementation, several stages of crushing may be necessary to accomplish the desired size reduction. Primary and secondary crushing is accomplished using gyratory crushers, jaw crushers, roll crushers, and impact mills.
Following crushing, the material is further ground using smooth rollers, media mills, autogenous mills, hammer mills, or jet mills to reduce its size to 50 micrometers (m) or less. The ground material is then separated into several categories via wet screening, dry screening, or air classification. After initial crushing and screening, some factories may divert a portion of the sand for use as construction sand.
Industrial sand and gravel are first crushed and screened before being cleaned to eliminate undesired dust and debris. They are then rescreened and categorized. The sand or gravel is then sent to an attrition scrubbing system, where it is rubbed in an agitated, high-density pulp to remove surface stains.
The cleaned sand or gravel is diluted with water to a solid content of 25 to 30 percent, then pumped to a set of cyclones for additional desliming. The deslimed sand or gravel enters a froth flotation process with sodium silicate. Sulfuric acid is added if the deslimed sand or gravel includes mica, feldspar, or iron-bearing minerals.
The contaminants are then suspended in a froth and directed to waste as the mixture enters a series of spiral classifiers. When the purified sand is transported to drainage bins, the moisture level is reduced to around 6%. The material is subsequently dried to a moisture level of less than 0.5 percent in rotary or fluidized bed dryers.
Natural gas or oil are typically used to fire the dryers, while alternative fuels like propane or diesel are also an option. The material is cooled after drying, followed by a final round of screening and classification before being stored and packaged for export.
The residual sand is converted into vast, sandy beaches when extinct mines are prepped for real estate development. In addition to possible residents, terns and plovers – who may have nested at the location when it was still a mine – are attracted to these areas.