Gypsum

Soft calcium sulfate mineral

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This article is about the mineral. For other uses, see Gypsum (disambiguation)

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[4] It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk.[5][6][7][8] Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.

Fine-grained white or lightly tinted forms of gypsum known as alabaster have been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England.

Etymology and history

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The word gypsum is derived from the Greek word γύψος (gypsos), "plaster".[9] Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon adding water, after a few dozen minutes, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or "set" in ways that are useful for casting and construction.[10]

Gypsum was known in Old English as spærstān, "spear stone", referring to its crystalline projections. Thus, the word spar in mineralogy, by comparison to gypsum, refers to any non-ore mineral or crystal that forms in spearlike projections. In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer.[11] Gypsum may act as a source of sulfur for plant growth, and in the early 19th century, it was regarded as an almost miraculous fertilizer. American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called "Plaster War" of .[12]

Physical properties

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Gypsum crystals are soft enough to bend under pressure of the hand. Sample on display at Musée cantonal de géologie de Lausanne.

Gypsum is moderately water-soluble (~2.02.5 g/L at 25 °C)[13] and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate (bassanite, often simply called "plaster") and, if heated further, to anhydrous calcium sulfate (anhydrite). As with anhydrite, the solubility of gypsum in saline solutions and in brines is also strongly dependent on sodium chloride (common table salt) concentration.[13]

The structure of gypsum consists of layers of calcium (Ca2+) and sulfate (SO24) ions tightly bound together. These layers are bonded by sheets of anion water molecules via weaker hydrogen bonding, which gives the crystal perfect cleavage along the sheets (in the {010} plane).[4][14]

Crystal varieties

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Gypsum occurs in nature as flattened and often twinned crystals, and transparent, cleavable masses called selenite. Selenite contains no significant selenium; rather, both substances were named for the ancient Greek word for the Moon.

Selenite may also occur in a silky, fibrous form, in which case it is commonly called "satin spar". Finally, it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly tinted variety of gypsum, called alabaster, is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called desert rose. It also forms some of the largest crystals found in nature, up to 12 m (39 ft) long, in the form of selenite.[15]

Occurrence

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Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Deposits are known to occur in strata from as far back as the Archaean eon.[16] Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface exposures. It is often associated with the minerals halite and sulfur. Gypsum is the most common sulfate mineral.[17] Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits.

Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Park in the US state of New Mexico have created a 710 km2 (270 sq mi) expanse of white gypsum sand, enough to supply the US construction industry with drywall for 1,000 years.[18] Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in when President Herbert Hoover declared the gypsum dunes a protected national monument.

Gypsum is also formed as a by-product of sulfide oxidation, amongst others by pyrite oxidation, when the sulfuric acid generated reacts with calcium carbonate. Its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by sulfate-reducing bacteria. This can lead to accumulation of elemental sulfur in oil-bearing formations,[19] such as salt domes,[20] where it can be mined using the Frasch process[21] Electric power stations burning coal with flue gas desulfurization produce large quantities of gypsum as a byproduct from the scrubbers.

Orbital pictures from the Mars Reconnaissance Orbiter (MRO) have indicated the existence of gypsum dunes in the northern polar region of Mars,[22] which were later confirmed at ground level by the Mars Exploration Rover (MER) Opportunity.[23]

Mining

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Commercial quantities of gypsum are found in the cities of Araripina and Grajaú in Brazil; in Pakistan, Jamaica, Iran (world's second largest producer), Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, Canada[25] and the United States. Large open pit quarries are located in many places including Fort Dodge, Iowa, which sits on one of the largest deposits of gypsum in the world,[26] and Plaster City, California, United States, and East Kutai, Kalimantan, Indonesia. Several small mines also exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities on soil for agricultural purposes.

Crystals of gypsum up to 11 m (36 ft) long have been found in the caves of the Naica Mine of Chihuahua, Mexico. The crystals thrived in the cave's extremely rare and stable natural environment. Temperatures stayed at 58 °C (136 °F), and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tonnes (61 short tons) and is around 500,000 years old.[27]

Synthesis

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Synthetic gypsum is produced as a waste product or by-product in a range of industrial processes.

Desulfurization

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Flue gas desulfurization gypsum (FGDG) is recovered at some coal-fired power plants. The main contaminants are Mg, K, Cl, F, B, Al, Fe, Si, and Se. They come both from the limestone used in desulfurization and from the coal burned. This product is pure enough to replace natural gypsum in a wide variety of fields including drywalls, water treatment, and cement set retarder. Improvements in flue gas desulfurization have greatly reduced the amount of toxic elements present.[28]

Desalination

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Gypsum precipitates onto brackish water membranes, a phenomenon known as mineral salt scaling, such as during brackish water desalination of water with high concentrations of calcium and sulfate. Scaling decreases membrane life and productivity.[29] This is one of the main obstacles in brackish water membrane desalination processes, such as reverse osmosis or nanofiltration. Other forms of scaling, such as calcite scaling, depending on the water source, can also be important considerations in distillation, as well as in heat exchangers, where either the salt solubility or concentration can change rapidly.

A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral called bassanite (2CaSO4·H2O).[30] This process occurs via a three-stage pathway:

1. homogeneous nucleation of nanocrystalline bassanite;
2. self-assembly of bassanite into aggregates, and
3. transformation of bassanite into gypsum.

Refinery waste

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The production of phosphate fertilizers requires breaking down calcium-containing phosphate rock with acid, producing calcium sulfate waste known as phosphogypsum (PG). This form of gypsum is contaminated by impurities found in the rock, namely fluoride, silica, radioactive elements such as radium, and heavy metal elements such as cadmium.[31] Similarly, production of titanium dioxide produces titanium gypsum (TG) due to neutralization of excess acid with lime. The product is contaminated with silica, fluorides, organic matters, and alkalis.[32]

Impurities in refinery gypsum waste have, in many cases, prevented them from being used as normal gypsum in fields such as construction. As a result, waste gypsum is stored in stacks indefinitely, with significant risk of leaching their contaminants into water and soil.[31] To reduce the accumulation and ultimately clear out these stacks, research is underway to find more applications for such waste products.[32]

Occupational safety

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People can be exposed to gypsum in the workplace by breathing it in, skin contact, and eye contact. Calcium sulfate per se is nontoxic and is even approved as a food additive,[34] but as powdered gypsum, it can irritate skin and mucous membranes.[35]

United States

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The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for gypsum exposure in the workplace as TWA 15 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an eight-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of TWA 10 mg/m3 for total exposure and TWA 5 mg/m3 for respiratory exposure over an eight-hour workday.[35]

Uses

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Old Alfarb kiln for making plaster as a construction material Map of gypsum deposits in northern Ohio, black squares indicate the location of deposits, from "Geography of Ohio",

Gypsum is used in a wide variety of applications:

Construction industry

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Agriculture

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Modeling, sculpture and art

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Food and drink

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Medicine and cosmetics

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Other

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Gallery

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See also

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References

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Introduction of raw materials of gypsum board product plant

The gypsum ores are crushed into small granules of the size less than 30mm, conveyed to mills to complete selecting and separating, and then the qualified powder are delivered into fluidized bed boiler for calcinations while the rest return to the mill. The qualified gypsum powders are carried into the storage bin or workshop for further use.

In the past, desulfurized gypsum slag was often treated by landfill, which seriously polluted the environment and occupied cultivated land. Now, the desulfurized gypsum slag can be made into gypsum powder for construction after being processed by DSJ series drying hammer mills, turning waste into treasure. Moreover, the performance of gypsum powder made by DSJ series drying hammer mills is exceeding than natural gypsum, and it is a high-quality raw material for manufacturing gypsum board.

Gypsum powder is one of the five major gel materials and occupies an important position in the national economy. The gypsum powder production line is to calcinate natural dihydrate gypsum ore (raw gypsum) or industrial by-product gypsum (desulfurized gypsum, phosphogypsum, etc.) at a certain temperature. It is ground to make the dihydrate gypsum dehydrated and decomposed. The product with β hemihydrate gypsum as the main component is the building gypsum (commonly known as plaster of gypsum).

The gypsum and FGD gypsum are fed from the inlet, effective contacting the surface of the heat exchange tube in the shell, and be burnt by the fume as a medium from the fluidized bed boiler and finally collected in the discharge outlet. Owing to the indirect heat exchange, the shell obtains higher regional vapor pressure which has positive effects on the quality of products. Moreover, there is no direct contact between the gypsum and heat carrier that keeps the maximum purity of the products. Indirect heat exchange model of rotating calcinations technique can effectively protect the gypsum against semi-hydrated gypsum by balancing the strength of the calcinations. This processing, adopting the advanced technology and taking the fluidized bed boiler as the heat source, which lowers the consumption of coal to its fullest extent and makes the full use of heat with output 100-t, is the ideal equipment for the calcinations of gypsum industry.

Based on Gypsum Plaster Manufacturing Process requirements, the construction gypsum product lines include crusher, mills, calcinations, storage and conveyor and control systems.

Crushing system

The gypsum ores are fed into crusher by the vibrating feeder, and crushed into small granules of the size less than 30mm for later use. Based on size of products and capacity requirements, the appropriate models can be adopted, like jaw crusher, hammer mills and impact crusher, etc. The dust collector is optional in order to keep the environment clean and satisfy the emission requirements.

Conveying system

The crusher gypsum is conveyed into the storage bin by the lifter. The design of the storage bin is based on requirements of the storage time to ensure stable supply.

Mill system

The materials are fed evenly and continuously into the mill by the vibrating feeder for grinding, and then the ground gypsum are blowing out by the blower for the analyzer to classify. The qualified powders go with the wind to the collector and are discharged by the tube as the final products which fall on the auger conveyors for the next stages of calcinations. It is close-recycled that the whole wind system is and adopts the bag filter between the wind collector and blowers, which filter the dust in the air and release into the atmosphere so as to prevent environment from pollution. The material sizes through mill system are changed from 0-30mm to 80-120 mesh and satisfy the gypsum requirements.

The mill system includes a lifter, storage bin, vibrating feeder, mills, auger conveyor and bag-type collector. The mill is adopting our latest patented Euro-typed miller (patented number is ZL 2 .8ZL 2 .8ZL 2 .9). There is an inside classifier, no necessity of outside one, which simplify the process.

Calcinations system

It includes the lifter, fluidized bed boiler, electro-static dust remover, roots blower, etc. The fluidized bed boiler is the most widely-used calcinations equipments in our country at present, which features smart shape, great capacity and simple structure, low failure rate and compact shape, low consumption, easy operation and self-control materialization, good quality of gypsum with ideal composure and stable physical performance, low operation cost, etc. It is widely used in the calcinations process of natural gypsum and chemical gypsum.

Control system

It takes the advanced centralized-control technology, DCS control and PLC control, adopting the well-known branded controls elements.

For more information, please visit gypsum powder manufacturing process.