adhesives because of its low oil-absorption, whitening effect, chemical inertness and ability to disperse uniformly within the matrix.
Chapter 3
BORAX AND RELATED MINERALS
Borax (so named after Persian “burah” or”burak”) is a sodium borate with 10 water molecules of crystallization. But, commercially, the term borax is often used to denote a group of closely related sodium borates differing in the contents of crystal water and of a natural derivative of borax, namely, boric acid. The compounds are:
1. Anhydrous borax (Na2B4O7, 69.2% B2O3)
2. Tincalconite or borax pentahydrate (Na2B4O7·5H2O, 47.8% B2O3)
3. Tincal or borax decahydrate (Na2B4O7·10H2O, 36.5% B2O3)
4. Kernite (Na2B4O7.4H2O, 50.9% B2O3)
5. Natural boric acid (H3BO3, 56.3% B2O3)
Of these, the decahydrate or tincal is the most common form and (unless otherwise specified) borax is understood by this form (“suhaga’ in Indian language). But, on exposure, it tends to dehydrate to the pentahydrate (tincalconite). Besides, there are some other boron compounds, some of which can substitute borax in some of its uses. These are:
1. Ulexite (Na2O.2CaO.5B2O3.16H2O, 42.9% B2O3)
2. Colemanite (2CaO.3B2O3.5H2O, 50.8% B2O3)
3. Szaibelite (2MgO.B2O3.H2O, 41.4% B2O3)
4. Hydroboracite (CaO.MgO.3B2O3.6H2O, 50.2% B2O3)
Of these, ulexite and colemanite are important from the point of view of industrial usage. Commercially sold borax is usually partially dehydrated. In nature, borax occurs naturally in evaporite deposits (resembling cotton balls) produced by the repeated evaporation of seasonal lakes. Borax can also be produced synthetically from other boron compounds and from some lake brines. It is believed to be known over 4,000 years ago.
In India, borax deposits occur in Puga Valley in eastern Ladakh, Jammu and Kashmir which is an extension of the borax-rich belt of south-western Tibet. The earliest record of these occurrences dates back to 1563. Historically, crude borax used to be imported to India across the Indo-Tibetan border which reached a peak at 1,625 tons in 1885 and refining used to take place in India. Of the total import, the major part used to be exported from India to UK. After this year, exports dwindled due to discovery of deposits in the USA. Subsequently, in 1925, the transborder trade was stopped. Some insignificant amounts continued to be
produced from Ladakh, but since the 1940s, there is no production of borax in India and the entire requirement is met by imports.
In USA, borax has been mined near the surface in Death Valley, California since the late 1800s. Besides, other commercially important deposits are found in Turkey and near Boron, California and other locations in the South western United States, Russia, Argentina, Tibet and the Atacama desert in Chile. Of the total world production of 4.64 million tons during 2000, Turkey, USA, Russia and Argentina accounted for over 95 percent.
C
RITERIA OFU
SEBorax is usually a white powder consisting of soft colourless crystals. If left exposed to dry air, borax slowly loses part of its water of hydration and becomes a white and chalky mineral tincalconite (Na2B4O7·5H2O). The important criteria determining the practical use of
borax including those of the related minerals and its derivatives are as follows.
1. Low cost: Borax is plentiful in the world, easy to mine due to softness and near- surface occurrence and hence relatively inexpensive.
2. Specific gravity: It is light having specific gravity 1.73.
3. Water solubility:- It dissolves easily in water, its water-solubility being 5.1 gm/100 ml at 20°C. Boiling water can dissolve two times its weight of borax.
4. Chemical nature: The solution of borax in water is mildly alkaline having pH of about 9.5. Borax can be easily converted to boric acid (H3BO3) by reacting a solution
of borax with sulphuric acid. The reaction is:
Na2B4O7·10H2O + H2SO4 → 4H3BO3 + Na2SO4 + 5H2O
Boric acid in its turn can be converted to boric oxide (B2O3) and other borates, which
form the basis for many of its applications. When melted, borax has the ability to melt many chemical elements like iron, copper, nickel, etc.
4. Thermal properties: Its melting and boiling temperatures are low, being 75°C and 320°C, respectively. When borax is burned, it produces a bright orange-colored flame, but it is not inflammable.
5. Electrical conductivity The low electrical conductivity of sodium has been carried into its compound borax.
6. Colour: Borax is a white powder. When fused, it becomes a colourless glass-like substance — an attribute of elemental boron carried into it.
7. Refractive index: Refractive index of borax is low, being only in the range of 1.447 to 1.472 (c.f., ordinary glass 1.54).
8. Elemental boron: Boron is a metalloid belonging to the borderline between metal and nonmetal. Its characteristic properties are as follows.
(a) It is hard and it retains its strength at high temperatures.
(b) It is extremely reactive at high temperature with oxygen and nitrogen. (c) Amorphous boron burns with a distinct green flare.
Borax and Related Minerals 33 (d) It behaves like a nonmetal inasmuch as it is a very weak electrical conductor
(semiconductor), but it conducts heat (like a metal).
(e) The heat conductivity gives it a low coefficient of thermal expansion.
(f) One of its isotopes B10 is capable of absorbing neutrons without emitting gamma rays, while another, B11 has low neutron absorption ability.
(g) It is light compared to many metals, having specific gravity varying from 2.34 to 2.37.
(h) Its melting point is high, 20790C, and it boils and sublimates at 25500C. But its melting rate is slow.
(i) Of all the elements of the Periodic Table, boron is unique inasmuch as it has the ability to form glass.
9. Ulexite and colemanite: Ulexite crystals possess natural strength and flexibility while colemanite has a high B2O3-content combines with calcium instead of sodium. These
attributes are advantageous in some uses. Besides, these minerals can be processed to yield boric acid by reacting them with sulphuric acid and the boric acid is converted to borax by treating it with soda ash (Na2CO3). Borax can also be prepared directly
from colemanite by boiling its powder with soda ash solution, filtering and then allowing borax to crystallize from the filtrate.
USES
Borax is generally processed by dissolving in hot water under pressure whereby the impurities are made to remain in suspension and are be filtered out. Then the filtrate is cooled and crystals of commercially usable borax are obtained. Borax has a wide variety of uses. Its important uses are:
1. Flux
2. Insecticide and fungicide 3. Medicinal applications
4. Cleaning material, soaps and detergents 5. Glass
6. Ceramics 7. Enamel 8. Silly-Putty
9. Buffer solution in biochemistry 10. Ultra-hard materials
11. Recovery of elemental boron 12. Ferro-boron
13. Metallic glass
1. Flux: The low melting point (75°C) and low boiling point (320°C) of borax enable it