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An Introduction to Boron: History, Sources, Uses, and Chemistry William G

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An Introduction to Boron: History, Sources, Uses, and Chemistry William G CORE Metadata, citation and similar papers at core.ac.uk Provided by PubMed Central An Introduction to Boron: History, Sources, Uses, and Chemistry William G. Woods Office of Environmental Health and Safety, University of California, Riverside, California Following a brief overview of the terrestrial distribution of boron in rocks, soil, and water, the history of the discovery, early utilization, and geologic origin of borate minerals is summmarized. Modern uses of borate-mineral concentrates, borax, boric acid, and other refined products include glass, fiberglass, washing products, alloys and metals, fertilizers, wood treatments, insecticides, and microbiocides. The chemistry of boron is reviewed from the point of view of its possible health effects. It is concluded that boron probably is complexed with hydroxylated species in biologic systems, and that inhibition and stimulation of enzyme and coenzymes are pivotal in its mode of action. - Environ Health Perspect 102(Suppl 7): 5-11 (1994) Key words: boron, borax, colemanite, glass, boric acid, health effects, enzyme, review Introduction retardants, as micronutrients in fertilizers were an important source of boric acid in Boron is an ubiquitous element in rocks, and for many other purposes, as well. Europe from about 1820 to the 1950s. soil, and water. Most of the earth's soils The varied chemistry and importance Borax also was made from Italian boric have <10 ppm boron, with high concen- of boron is dominated by the ability of acid in England, France, and Germany. trations found in parts of the western borates to form trigonal as well as tetrahe- The borate industry in Turkey com- United States and in other sites stretching dral bonding patterns and to create com- menced in 1865 with mining of the calci- from the Mediterranean to Kazakhstan. plexes with organic functional groups, um borate pandermite (priceite, The average soil boron concentration is many of biologic importance. 4CaO5B203 7H20). At about the same 10 to 20 ppm, with large areas of the The present review will give an time, several borate deposits were found world boron deficient. Boron concentra- overview of boron from its sources to its in California and Nevada, including ulex- tions in rocks range from 5 ppm in uses and chemistry to provide a back- ite (Na 202CaO 5B 20 3 16H 20) and basalts to 100 ppm in shales, and averages ground for the following papers dealing colemanite (2CaO3B203 5H20) in 10 ppm in the earth's crust overall. Soils with its health effects. Death Valley. These minerals could be have boron concentrations of 2 to 100 converted to borax by reaction with trona ppm. Seawater contains an average of 4.6 History and Sources (Na2CO 3-NaHCO 32H 20). ppm boron, but ranges from 0.5 to 9.6 The Babylonians have been credited with The Kramer deposit, at what is now ppm. Freshwaters normally range from importing borax from the Far East over Boron, California, in the Mojave Desert, <0.01 to 1.5 ppm, with higher concentra- 4000 years ago for use as a flux for work- was discovered in 1913, first as a cole- tions in regions of high boron soil levels ing gold. Mummifying, medicinal and manite ore source. In 1925, tincal ore was (1,2). metallurgic applications of boron are found and in 1926, the new mineral Highly concentrated, economically sometimes attributed to the ancient rasorite (kernite, Na202B20 34H 20) sized deposits of boron minerals, always Egyptians. None of this very old borax was encountered (4). It is the largest in the form of compounds with boron history has been verified, but solid evi- borate deposit outside of Turkey and has bonded to oxygen, are rare and generally dence exists that tinkar (i.e., supplied a sizable portion of world borate are found in arid areas with a history of Na 2B4 0 7- 10H 20, tincal, the mineral demand for over 50 years. volcanism or hydrothermal activity. Such borax) was first used in the eighth century Turkey has supplied colemanite for deposits are being exploited in Turkey, around Mecca and Medina, having been many years to boric acid producers in the United States, and several other coun- brought there (and to China) by Arab Europe. Sodium borates were discovered tries (3). Borate-mineral concentrates traders (4). The use of borax flux by at Kirka in 1960 and other deposits have and refined products are produced and European goldsmiths dates to about the since been found and developed in sold worldwide. They are used in a myri- 12th century. Anatolia. As a result, today Turkey is the ad of ways: in glass and related vitreous The earliest source of borax is believed largest producer of borate products in the applications, in laundry bleaches, in fire to have been Tibetan lakes. The borax world (5), exporting mineral concentrates was transported in bags tied to sheep, of tincal, colemanite, and ulexite, plus which were driven over the Himalayas to refined borax decahydrate, borax pen- This paper was presented at the International India. tahydrate (Na2O°2B203-5H20), anhy- Symposium on Health Effects of Boron and Its Compounds held 16-17 September 1992 at the Volatility of boric acid with steam is drous borax (Na2O-2B20 3 ), and boric University of California, Irvine, California. believed by geologists to be the primary acid (B[OH]3). Address correspondence to Dr. William G. Woods, mechanism for the formation of borate Borates in the United States are pro- Office of Environmental Health and Safety, University of California, Riverside, Riverside, CA 92521. deposits (3). A prime example of this are duced entirely in the Mojave Desert of Telephone (909) 687-1568. Fax (909)787-5122. the geysers (soffioni) in Tuscany, which southern California. United States Borax Environmental Health Perspectives 5 W G. WOODS Table 2. Estimated consumption of boron minerals and chemicals in the United States (6). Use Percent Glass and ceramics Insulation fiberglass 28 Textile fiberglass 12 Glass 9 Enamels and glazes 3 Detergents and bleaches 12 Alloys and metals 6 Fire retardants 5 Agriculture 4 Adhesives 2 Other chemicals 19 low sodium, so boric acid or colemanite are used. Glass batch melting is aided by B203 which acts as a flux to improve the melting rate of more refractory components (e.g., U..~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ... SiO2). As an added benefit, water given off Figure1 Aerial. view of the US Borax mine and refineryeat Boron CA (U Borax Inc photo) by the borates also accelerates the melting. The alkali/alkaline earth oxide content Figure 1. Aerial view of the US Borax mine and refinery at Boron, CA. (US Borax, Inc. photo) is quite important to the effect of B203 on the structure and properties of glass. Adding Na2O, for example, progressively Table 1. Important refined borate products and mineral concentrates. changes trigonal B03 groupings to tetrahe- Name Formula Percent B203 dral B04- groups, which initially causes an increase in the glass transition temperature Borax pentahydrate Na2[B405(OH)4] 3H20 47.8 (Tg). Further addition of Na)O converts Borax (tincal) Na2[B405(OH)4]F8H20 36.5 B04- bonding groups to nonbonding Colemanite Ca[B304(OH)3] H2O 50.8 BO3-n units, which decreases Tg as well as Ulexite NaCa[B506(OH)6]F5H20 43.0 the melt viscosity. The latter helps in elimi- Boric acid B(OH)3 56.3 nating bubbles from the finished glass and Anhydrous borax Na2B407(glass) 69.2 gives good homogeneity and flattens the viscosity-temperature curve. The latter allows more latitude during manufacture & Chemical Corporation, a subsidiary of and the former USSR also produce some (forming) of the finished glass product, and Rio Tinto Zinc (RTZ), is the largest borates. the lower viscosity allows higher pro- source, producing borax pentahydrate Table 1 gives a list of the commercially duction rates. and decahydrate, and anhydrous borax important refined borate products and Borosilicate glass has increased thermal from tincal ore, and boric acid from ker- mineral concentrates. The formulas are shock resistance because B203 lowers the nite ore (Equation 1). written in a form reflecting their structures. expansion coefficient. In fiberglass, boric Na2B407 4H20+H 2S04 + H20 oxide gives desirable drawing qualities and Uses increases mechanical strength and chemical 4B(OH)3 + Na2SO4 [1] Table 2 lists the uses ofboron minerals and durability. An aerial photo of the US Borax mine chemicals in the United States glass and Another 12% of United States B203 is and refinery is shown in Figure 1. Another related uses consume about half of the consumed in detergents and bleaches; the US Borax plant at Los Angeles harbor pro- borates as boric anhydride (B203) in the percentage is even higher in Europe. Borax duces ammonium and potassium borates, United States (6). Fiberglass accounts for has been used as a laundry additive since boric oxide, zinc borate, and spray dried the largest share, comprising about 77% of about 1900. It can contribute to the soft- polyborates, in addition to serving as an the total (54% for insulation fiberglass, ening of hard water by tying up calcium export terminal. 23% for textile grade). Heat-resistant pyrex ions, as well as acting as a buffer agent. A North American Chemical recovers and other low-thermal expansion glasses traditional use as a sweetening agent for borax and boric acid as byproducts from consume about 18%, while enamels, frits, diaper pails is thought to involve the borate potash, soda ash, and salt-cake extrac- and ceramic glazes consume 5%. Small inhibition of the urease enzyme, (prevent- tion operations on Searles Lake brine. amounts are used in sealing and optical ing ammonia formation). The same effect Ore concentrates are produced by glasses, Vycor, and vitrifying nuclear waste.
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