Analyzing Mining Products for the Ceramics Industry

Non-metallic industrial materials

A great many industrial materials begin as non-metallic minerals. Lime (calcium oxide), a widely available material, is used in cement, building construction, agriculture, paper making, chemical manufacturing, and even iron and steel. Silica, composed of silicon and oxygen of several varying crystalline forms or polymorphs, is a primary source for glass, filtration systems, foundry castings and metallurgical applications. Diatomite is a powdery mineral composed of the fossilized remains of microscopic single-celled aquatic plants called diatoms, and is used for additives to paints and plastics, filtering agents, and absorbents to mitigate spills in the automotive, industrial, janitorial and waste remediation industries. The list of industrial minerals includes talc, bentonite, saprolite, borate, gypsum, kaolin and feldspar, to name a few.

Ceramics

Ceramics are some of the most commonly found materials used for industrial applications and home use. Ceramic pottery is some of the oldest materials known, found in ancient earthenware, precious Chinese porcelain, and now in school art classrooms around the world. In the home, in addition to dishware, ceramics are found in sinks, countertops, tiles and flooring. Ceramics are widely used due to their initial plasticity, durability, hardness, poor conductivity and high melting points.

Because of the high strengths exhibited by their primary chemical bonds, many ceramics possess unusually good combinations of high melting point and chemical inertness. This makes them useful as refractories. Ceramics generally can withstand very high temperatures, ranging from 1,000 °C to 1,600 °C (1,800 °F to 3,000 °F).

Refractory materials

Refractory materials are used in furnaces, kilns, incinerators, and reactors. They must be chemically and physically stable at high temperatures. Depending on the operating environment, they must be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion. Refractory stems from the Latin word meaning unable to break (fract).

The most important materials used in the manufacturing of refractories are oxides of aluminum (alumina), silicon (silica) and magnesium (magnesia). Another oxide usually found in refractories is the oxide of calcium (lime). Fire clays are also widely used in the manufacture of refractories.

Kaolinite

Kaolinite is an abundant clay mineral used for pottery and ceramics and is also very important in the production of paper. Kaolinite is used in pharmaceuticals as an ingredient in some medications such as stomach soothers (Kaopectate) as the adsorbent. Kaolinite is also used as an ingredient in some cosmetics, soaps, paint gloss, and toothpastes. Kaolinite is named after the Kao-ling, a mountain in Jiangxi Province in China where this mineral was well-known from early times.

Kaolinite is part of the group of industrial minerals with the chemical composition Al₂Si₂O₅(OH)₄. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO₄) linked through oxygen atoms to one octahedral sheet of alumina (AlO₆) octahedra. Rocks that are rich in kaolinite are known as kaolin or china clay.

Under the electron microscope, kaolin consists of roughly hexagonal, platy crystals ranging in size from about 0.1 micrometer to 10 micrometers or even larger. Crystals may take vermicular and book like forms, and occasionally macroscopic forms approaching millimeter size. Found in nature, kaolin typically contains modest amounts of other minerals such as muscovite, quartz, feldspar, and anatase. In addition, crude kaolin is frequently stained yellow by iron hydroxide pigments. It is often necessary to bleach the clay chemically to remove the iron pigment and to wash it with water to remove the other minerals in order to prepare kaolin for commercial use.

When kaolin is mixed with water in the range of 20 to 35 percent, it becomes plastic (i.e., it can be molded under pressure), and the shape is retained after the pressure is removed. With larger percentages of water, the kaolin forms a slurry, or watery suspension. The amount of water required to achieve plasticity and viscosity varies with the size of the kaolinite particles and with certain chemicals that may be present in the kaolin. Kaolin has been mined in France, England, Saxony (Germany), Bohemia (Czech Republic), and in the United States, where the best-known deposits are in the southeastern states.

Magnesite

Magnesite is a mineral with the chemical formula MgCO₃ (magnesium carbonate) and is named after the presence of magnesium in its composition. Magnesite usually forms during the alteration of magnesium-rich rocks or carbonate rocks by metamorphism or chemical weathering. Iron, manganese, cobalt and nickel may occur as admixtures, but only in small amounts. Magnesium oxide is an important refractory material used as a lining in blast furnaces, kilns and incinerators.

Magnesite is used to produce magnesium oxide (MgO), which serves as a refractory material for the steel industry, ceramic processes and as a raw material for the chemical industry. Small amounts of magnesite are also used as a gem and lapidary material. When heated, magnesite dissociates into MgO and CO₂. MgO has an extremely high melting temperature. MgO is one of the most commonly used materials for making the bricks used to line kilns, industrial ovens, and blast furnaces. MgO is also used to make fertilizers, magnesium chemicals, and refined into magnesium metal.

Magnesite is commonly used to make tumbled stones, beads, and cabochons. White magnesite is porous. It can be cut and reliably absorb dye to produce almost any color. Magnesite dyed a turquoise color has been a disclosed and undisclosed substitute for turquoise for nearly 100 years. People have been fooled by magnesite dyed to look like turquoise or lapis lazuli.

Magnesite is mined around the world, particularly China, Turkey, Russia, Australia, North Korea, and several European countries. Magnesite was detected in meteorite ALH84001 and on planet Mars.

Feldspar

Feldspar describes a group of minerals distinguished by the presence of alumina and silica (SiO₂), including aluminum silicates of soda, potassium, or lime. They form the most abundant mineral group on Earth (an estimated 60% of exposed rocks), as well as soils, clays, and other unconsolidated sediments. Feldspar minerals are the principal components in rock classification schemes. The minerals included in this group are the orthoclase, microcline and plagioclase feldspars.

In making ceramics there are two basic categories of feldspars: potash feldspars (orthoclase), where the primary melting oxide is potassium, and soda feldspars (albite) where the primary melting oxide is sodium. Soda and potash have the highest thermal expansion and contraction rate of all the ceramic melter oxides. In making ceramics, they contribute color brilliance and luster at most firing temperatures, and they encourage specific color results.

Feldspars have both alkali and alumina content which makes them particularly valuable in industrial processes. Feldspars play an important role as fluxing agents in ceramics and glass production and are also used as functional fillers in the paint, plastic, rubber and adhesive industries.

Several feldspars are used as gemstones, including varieties that show opalescence sold as moonstone, labradorite with strong color flashes, sunstone with a yellow to orange to brown with a golden sheen, amazonite, a green variety of microcline used as an ornamental material.

Sanidine occurs with large noticeable crystals in extrusive felsic igneous rocks such as rhyolite and trachyte. Other feldspars include oligoclase, microcline, anorthite and bytownite.

In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.

Processing of raw materials into ceramics

Most minerals are not mined in pure form and commonly include fractions of mineral compounds. These materials undergo several processing steps: from crushing and milling to refinement techniques such as flotation, electrostatic and magnetic separation, and dewatering.

Companies that purchase raw materials, process ingredients and fabricate ceramic products need efficient laboratory procedures to determine mineral compositions and purities. X-ray diffraction (XRD) is a commonly used technique to characterize minerals in mining and processing industries. A non-destructive test method used to analyze the structure of crystalline materials, XRD identifies and quantifies the phases present in a material and thereby reveal chemical composition information. Identification of phases is achieved by comparison of the acquired data to that in reference databases.

XRD analysis for ceramic manufacturing

Recently, XRD specialists here analyzed samples of ball clay, kaolin, magnetite and feldspar with the ARL EQUINOX 100 X-ray Diffractometer to determine the phase composition in mineral samples used for the production of ceramic materials. A benchtop diffractometer, the ARL EQUINOX has been shown to be a convenient and cost-effective solution for QA/QC procedures in the ceramic industry and is also appropriate at mining sites due to its ease of use and robustness.

For details, read Quantitative Phase Analysis of Mining Products for Ceramic Industry

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