Sand

Background

Sand is a loose, fragmented, naturally-occurring material consisting of very small particles of decomposed rocks, corals, or shells. Sand is used to provide bulk, strength, and other properties to construction materials like asphalt and concrete. It is also used as a decorative material in landscaping. Specific types of sand are used in the manufacture of glass and as a molding material for metal casting. Other sand is used as an abrasive in sandblasting and to make sandpaper.

Sand was used as early as 6000 B.C. to grind and polish stones to make sharpened tools and other objects. The stones were rubbed on a piece of wetted sandstone to hone the cutting edge. In some cases, loose sand was scattered on a flat rock, and objects were rubbed against the sandy surface to smooth them. The first beads with a glass glaze appeared in Egypt in about 3,500-3,000 B.C. The glass was made by melting sand, although naturally-occurring glass formed by volcanic activity was probably known long before that time.

In the United States, sand was used to produce glass as early as 1607 with the founding of the short-lived Jamestown colony in Virginia. The first sustained glass-making venture was formed in 1739 in Wistarburgh, New Jersey, by Caspar Wistar. The production of sand for construction purposes grew significantly with the push for paved roads during World War I and through the 1920s. The housing boom of the late 1940s and early 1950s, coupled with the increased use of concrete for building construction, provided another boost in production.

Today, the processing of sand is a multi-billion dollar business with operations ranging from very small plants supplying sand and gravel to a few local building contractors to very large, highly automated plants supplying hundreds of truckloads of sand per day to a wide variety of customers over a large area.

Raw Materials

The most common sand is composed of particles of quartz and feldspar. Quartz sand particles are colorless or slightly pink, while feldspar sand has a pink or amber color. Black sands, such as those found in Hawaii, are composed of particles of obsidian formed by volcanic activity. Other black sands include materials such as magnetite and homblende. Coral sands are white or gray, and sands composed of broken shell fragments are usually light brown. The white sands on the Gulf of Mexico are made of smooth particles of limestone known as oolite, derived from the Greek word meaning egg stone. The white sands of White Sands, New Mexico, are made of gypsum crystals. Ordinarily, gypsum is dissolved by rain water, but the area around White Sands is so arid that the crystals survive to form undulating dunes.

Quartz sands, which are high in silica content, are used to make glass. When quartz sands are crushed they produce particles with sharp, angular edges that are sometimes used to make sandpaper for smoothing wood. Some quartz sand is found in the form of sandstone. Sandstone is a sedimentary, rock-like material formed under pressure and composed of sand particles held together by a cementing material such as calcium carbonate. A few sandstones are composed of almost pure quartz particles and are the source of the silicon used to make semiconductor silicon chips for microprocessors.

Molding sands, or foundry sands, are used for metal casting. They are composed of about 80%-92% silica, up to 15% alumina, and 2% iron oxide. The alumina content gives the molding sand the proper binding properties required to hold the shape of the mold cavity.

Sand that is scooped up from the bank of a river and is not washed or sorted in any way is known as bank-run sand. It is used in general construction and landscaping.

The definition of the size of sand particles varies, but in general sand contains particles measuring about 0.0025-0.08 in (0.063-2.0 mm) in diameter. Particles smaller than this are classified as silt. Larger particles are either granules or gravel, depending on their size. In the construction business, all aggregate materials with particles smaller than 0.25 in (6.4 mm) are classified as fine aggregates. This includes sand. Materials with particles from 0.25 in (6.4 mm) up to about 6.0 in (15.2 cm) are classified as coarse aggregates.

Sand has a density of 2,600-3,100 lb per cubic yard (1,538-1,842 kg per cubic meter). The trapped water content between the sand particles can cause the density to vary substantially.

The Manufacturing
Process

The preparation of sand consists of five basic processes: natural decomposition, extraction, sorting, washing, and in some cases crushing. The first process, natural decomposition, usually takes millions of years. The other processes take considerably less time.

The processing plant is located in the immediate vicinity of the natural deposit of material to minimize the costs of transportation. If the plant is located next to a sand dune or beach, the plant may process only sand. If it is located next to a riverbed, it will usually process both sand and gravel because the two materials are often intermixed. Most plants are stationary and may operate in the same location for decades. Some plants are mobile and can be broken into separate components to be towed to the quarry site. Mobile plants are used for remote construction projects, where there are not any stationary plants nearby.

The capacity of the processing plant is measured in tons per hour output of finished product. Stationary plants can produce several thousand tons per hour. Mobile plants are smaller and their output is usually in the range of 50-500 tons (50.8-508 metric tons) per hour.

In many locations, an asphalt production plant or a ready mixed concrete plant operates on the same site as the sand and gravel plant. In those cases, much of the sand and gravel output is conveyed directly into stockpiles for the asphalt and concrete plants.

The following steps are commonly used to process sand and gravel for construction purposes.

Natural decomposition

• 1 Solid rock is broken down into chunks by natural mechanical forces such as the movement of glaciers, the expansion of water in cracks during freezing, and the impacts of rocks falling on each other.
• 2 The chunks of rock are further broken down into grains by the chemical action of vegetation and rain combined with mechanical impacts as the progressively smaller particles are carried and worn by wind and water.
• 3 As the grains of rock are carried into waterways, some are deposited along the bank, while others eventually reach the sea, where they may join with fragments of coral or shells to form beaches. Wind-borne sand may form dunes.

Extraction

• 4 Extraction of sand can be as simple as scooping it up from the riverbank with a rubber-tired vehicle called a front loader. Some sand is excavated from under water using floating dredges. These dredges have a long boom with a rotating cutter head to loosen the sand deposits and a suction pipe to suck up the sand.
• 5 If the sand is extracted with a front loader, it is then dumped into a truck or train, or placed onto a conveyor belt for transportation to the nearby processing plant. If the sand is extracted from underwater with a dredge, the slurry of sand and water is pumped through a pipeline to the plant.

Sorting

• 6 In the processing plant, the incoming material is first mixed with water, if it is not already mixed as part of a slurry, and is discharged through a large perforated screen in the feeder to separate out rocks, lumps of clay, sticks, and other foreign material. If the material is heavily bound together with clay or soil, it may then pass through a blade mill which breaks it up into smaller chunks.
• 7 The material then pass through several / perforated screens or plates with different hole diameters or openings to separate the particles according to size. The screens or plates measure up to 10 ft (3.1 m) wide by up to 28 ft (8.5 m) long and are tilted at an angle of about 20-45 degrees from the horizontal. They are vibrated to allow the trapped material on each level to work its way off the end of the screen and onto separate conveyor belts. The coarsest screen, with the largest holes, is on top, and the screens underneath have progressively smaller holes.

Washing

• 8 The material that comes off the coarsest screen is washed in a log washer before it is further screened. The name for this piece of equipment comes from the early practice of putting short lengths of wood logs inside a rotating drum filled with sand and gravel to add to the scrubbing action. A modern log washer consists of a slightly inclined horizontal trough with slowly rotating blades attached to a shaft that runs down the axis of the trough. The blades churn through the material as it passes through the trough to strip away any remaining clay or soft soil. The larger gravel particles are separated out and screened into different sizes, while any smaller sand particles that had been attached to the gravel may be carried back and added to the flow of incoming material.
• 9 The material that comes off the intermediate screen(s) may be stored and blended with either the coarser gravel or the finer sand to make various aggregate mixes.
• 10 The water and material that pass through the finest screen is pumped into a horizontal sand classifying tank. As the mixture flows from one end of the tank to the other, the sand sinks to the bottom where it is trapped in a series of bins. The larger, heavier sand particles drop out first, followed by the progressively smaller sand particles, while the lighter silt particles are carried off in the flow of water. The water and silt are then pumped out of the classifying tank and through a clarifier where the silt settles to the bottom and is removed. The clear water is recirculated to the feeder to be used again.
• 11 The sand is removed from the bins in the bottom of the classifying tank with rotating dewatering screws that slowly move the sand up the inside of an inclined cylinder. The differently sized sands are then washed again to remove any remaining silt and are transported by conveyor belts to stockpiles for storage.

Crushing

• 12 Some sand is crushed to produce a specific size or shape that is not available naturally. The crusher may be a rotating cone type in which the sand falls between an upper rotating cone and a lower fixed cone that are separated by a very small distance. Any particles larger than this separation distance are crushed between the heavy metal cones, and the resulting particles fall out the bottom.

Quality Control

Most large aggregate processing plants use a computer to control the flow of materials. The feed rate of incoming material, the vibration rate of the sorting screens, and the flow rate of the water through the sand classifying tank all determine the proportions of the finished products and must be monitored and controlled. Many specifications for asphalt and concrete mixes require a certain distribution of aggregate sizes and shapes, and the aggregate producer must

The preparation of sand consists of five basic processes: natural decomposition, extraction, sorting, washing, and in some cases crushing. Sand is extracted from the location at which it occurs by either a floating dredge or front loader. The dredge delivers a slurry of sand and water to the processing plant via pipeline, while the front loader simply scoops the sand up and into trucks or onto conveyor belts for transporation to the plant. The sand is sorted through a series of screens that separate differently sized particles. The sand is washed, and the smaller particles are sent to the sand classifying tank, where the particles are further separated. Some particles may be crushed if smaller particles are needed.

The Future

The production of sand and gravel in many areas has come under increasingly stringent restrictions. The United States Army Corps of Engineers, operating under the Federal Clean Water Act, has required permits for sand extraction from rivers, streams, and other waterways. The cost of the special studies required to obtain these permits is often too expensive to allow smaller companies to continue operation. In other cases, residential development in the vicinity of existing aggregate processing plants has led to restrictions regarding noise, dust, and truck traffic. The overall result of these restrictions in certain areas is that sand and gravel used for construction will have to be transported from outside the area at a significantly increased cost in the future.