Page 147 - Chemical and process design handbook

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BORON COMPOUNDS

The important naturally occurring ores of boron are colemanite (Ca B O
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5H O), tincal (Na B O •10H O), and boracite (2Mg B O •MgCl ).
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Boron-containing brines and kernite, or rasorite (Na B O 4H O), are also
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sources of boron compounds.
Crude and refined hydrated sodium borates and hydrous boric acid are
produced from kernite and tincal. The ore is fed to the dissolving plant and
mixed with hot recycle liquor. Liquor and fine insolubles are fed to a pri-
mary thickener. The strong liquors are crystallized in a continuous vacuum
crystallizer.
Another process involves the use of an organic solvent to extract the
borax brines. Boric acid is extracted with kerosene, carrying a chelating
agent. In a second mixer-settler system, dilute sulfuric acid strips the
borates from the chelate. The aqueous phase with boric acid, potassium
sulfate, and sodium sulfate is purified by carbon treatment and evaporated
in two evaporator-crystallizers. From the first, pure boric acid is separated,
and from the other, a mixture of sodium and potassium sulfates.
Borax (tincal, Na B O •10H O), is the most important industrial com-
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pound of boron. The largest single use is in the manufacture of glass-fiber
insulation. Boric acid (H BO ) is a weak acid that is used in the manufac-
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ture of glazes and enamels for pottery. Its main uses are as a fire retardant
for cellulosic insulation and in the manufacture of borosilicate glasses and
textile-grade glass fibers.

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