Diatomite
Diatomite, also known as diatomaceous earth, is a soft, chalklike sedimentary rock primarily composed of the fossilized silica shells of microscopic algae called diatoms. Its unique finely porous structure makes it an effective filtration medium, widely used for filtering water, beer, and various chemicals. Diatomite is mined from deposits found near ancient and present-day bodies of water, with significant production occurring in countries like the United States, China, Denmark, and Japan. The U.S. is the leading producer, accounting for a substantial portion of the world's diatomite exports.
In addition to filtration, diatomite serves multiple purposes, including insulation, as a filler in products like paints and plastics, and as a mild abrasive in toothpaste and polishes. Its historical uses date back to ancient Greece, while industrial applications gained prominence in the late 19th century, particularly for its role in manufacturing dynamite. The processing of diatomite involves various steps to preserve its structure, which is crucial for its filtration capabilities. Its affordability and versatility make it a widely utilized resource across diverse industries, despite the availability of alternative materials.
Diatomite
Where Found
Diatomite is found in deposits near present-day or ancient bodies of water, because it is composed of the silica shells of water-dwelling diatoms. Diatomite deposits are found throughout the world; major producers include the United States, China, Denmark, and Japan. The United States is the main producer of diatomite, accounting for at least 50 percent of the world’s diatom exports every year.
Primary Uses
Uses for diatomite fall into four main categories: for filtering, for insulating and building, as a filler material, and as a mild abrasive. Diatomite is commonly used to filter a wide variety of substances, ranging from oils to drinking water. As an abrasive, it is used in toothpastes and metal polishes. Many products, ranging from ceramics to paints, use diatomite as a filler to add volume.
Technical Definition
Chemically, diatomite consists primarily of silica with trace amounts of magnesium, sodium, iron, and other elements, although exact proportions vary. Purified diatomite is essentially silica (SiO2), with an average molecular mass of 60.8. Diatomite has a melting point of 1,710° Celsius and a density of 2.3 grams per cubic centimeter. Heating it to high temperatures forms crystalline silica.
Diatomite is usually white (if pure), buff, gray, and rarely black. In situ, it is generally found as a soft sedimentaryrock or as powder. Raw diatomite is typically processed by a series of crushing, drying, size-reduction, and calcining procedures to produce different grades of diatomite for different specialized applications.
Description, Distribution, and Forms
Diatomite is a soft, chalklike, fine-grained sedimentary rock composed primarily of the fossilized silica shells of microscopic algae called diatoms. It is finely porous, is low in density, and has low thermal conductivity. Diatom frustules are composed of two symmetrical silica valves, which can be elaborately ornamented with tiny holes and protrusions. These tiny holes are what make diatoms an ideal material for filtration. The word “diatom” comes from Greek diatomos, meaning “cut in half,” because of the two valves.
Diatoms live in a wide range of moist environments, although most abundantly in marine (oceanic) and lacustrine (freshwater) environments. Three main types of diatomite deposits are recognized in the United States: marine rocks near continental margins, lacustrine rocks formed in ancient lakes or marshes, and sedimentary rocks in modern lakes, marshes, and bogs. Another commonly used term for diatomite, diatomaceous earth, more properly refers to unconsolidated or less lithified forms of diatomite.
One of the most important marine diatomite deposits is near Lompoc, California, reported to be the world’s largest producing district by volume. Economically important lacustrine deposits in the United States are found in Nevada, Oregon, Washington, and eastern California. In 2007, the United States produced 33 percent of the world’s diatomite. Other leading producers were China (20 percent), Denmark (11 percent; all moler diatomite, containing 30 weight percent clay), Japan (6 percent), and France (4 percent).
History
Some of the earliest references to diatomite are to the ancient Greeks’ probable use of it to form lightweight bricks for building; they also used diatomite as an abrasive. In 535 c.e., the Roman Emperor Justinian I used diatomite bricks in building the church of St. Sofia in Constantinople (now Istanbul).
Diatomite use became industrially important to Western Europe after 1867, when Alfred Nobel invented dynamite. Pulverized diatomite was commonly used to absorb and stabilize the nitroglycerine used to make dynamite. By the late 1800’s, the United States had become the primary producer of diatomite. By 1900, diatomite’s uses had expanded to include many of its present-day uses, including beer filtration and building materials.
During the 1920’s, techniques for calcining (thermally treating) and grading diatomite enabled a wider variety of uses for this resource. By World War II, the U.S. Army and Navy made wide use of diatomite to purify drinking water, to remove oil from boiler and engine water, and to create low-light-reflectance paints for ships.
Obtaining Diatomite
Because of its abundance and usual occurrence near the surface in the United States, most diatomite produced is obtained from open-pit mines. The diatomite is excavated by machine after the overburden is removed. Outside the United States—particularly in China, Chile, and France—underground diatomite mining is fairly common. These mines are usually pit-and-pillar mines excavated by machine, although some small mines are excavated using hand tools. In Iceland, diatomaceous mud is dredged from Lake Myvatn. Diatomite is often dried in the open air near the mine before processing.
Diatomite processing is often carried out near the mine from which it is extracted. Raw diatomite may contain up to 65 percent water and is expensive to transport. Primary crushing of ore is usually done with spiked rolls and hammer mills, reducing the ore to 1.25-centimeter pieces while limiting damage to the diatom structure.
Passage through heated air, milling fans, and air cyclones further dries the diatomite and begins to classify for size as well as remove impurities of different density. Processing aims to separate individual diatom valves without destroying their structure, which is key to filtration uses.
Calcining, which increases filtration rate, specific gravity, and particle hardness, as well as oxidizing iron, is usually done with rotary kilns. Calcining is particularly important for filter grades.
Uses of Diatomite
Diatomite is primarily used as a filtration medium but also is used for insulation, as a filler and absorbent, and as a mild abrasive, in addition to some specialized medical uses. The most common use of diatomite is in filtration, because of its finely porous nature. These uses include water purification, beer and wine filtering, and the removal of oils from water. As a water filtration element, diatomaceous earth usually is used as a layer on a filter element or septum (a permeable cover over interior collection channels), called pre-coat filtration. Diatomite water filtration systems are lightweight, cheap, and simple and can remove bacteria and protozoans as well as cysts, algae, and asbestos. This usage of diatomite first became important during World War II, when the U.S. Army needed a water filter suitable for mobile military operations. The first municipal diatomaceous-earth water filtration system was set up in 1948, and more than two hundred operate presently in the United States. Diatomite is also used to filter nonpotable water, such as that which is used in swimming pools.
Diatomite began to be used after Prohibition to filter beer and wine in the United States, replacing wood pulp in filters. It is also used to filter liquid sweeteners, oils and fats, petroleum and other chemicals, and pharmaceuticals.
Another major use of diatomite is in building, where it is used for lightweight blocks and bricks and for thermal insulation (high clay-content Danish moler in particular). Diatomite is also a frequent cement additive; diatomite for cement requires less processing.
As a filler, diatomite has many uses. In addition to providing bulk, diatomite can reduce reflectivity in paints, reduce caking in granular mixtures, and provide a variety of effects in plastics, including preventing film sticking. Diatomite is absorbent and often used for cleaning industrial spills and in cat litter.
As an insecticide, diatomite is less toxic than chemical pesticides, as it works by absorbing lipids from insects’ exoskeletons, causing dehydration. However, it harms beneficial insects as well as pests. Diatomite also is used as a growing medium for hydroponics and an additive in various types of potting soil, because it retains water and nutrients while draining quickly, similar to vermiculite. Medical-grade diatomite is sometimes used for deworming, as the sharp edges of the frustules are thought to kill parasites, but the efficacy of this is questionable.
Diatomite is also used in cosmetics—for example, in facial masks to absorb oil—and as a minor abrasive in jewelry polishes and toothpastes. Some processes for extracting and purifying DNA use diatomite, which will remove DNA but not RNA or proteins. Diatomite and a highly concentrated denaturing agent are used to remove DNA, and then a slightly alkaline, low ionic strength buffer (such as water) can be used to extract DNA from the diatomite.
While diatomite can be replaced by other materials—such as silica sand, perlite, talc, ground lime, ground mica, and clay—for most of its applications, its abundance, availability, and low cost make it a popular and heavily used resource.
Bibliography
Fulton, George P. Diatomaceous Earth Filtration for Safe Drinking Water. Reston, Va.: American Society of Civil Engineers, 2000.
Stoermer, Eugene F., and John P. Smol, eds. The Diatoms: Applications for the Environmental and Earth Sciences. New York: Cambridge University Press, 1999.
U.S. Geological Survey. Minerals Yearbook. Washington, D.C.: Author, 2008.
U.S. Geological Survey.