An Insight to Bismuth

An element that occurs native in small quantities, but is generally obtained as a by-product in the refining of lead, copper, tin, silver and gold ores (American Bismuth). It can also be extracted from the ores bismuthinite or bismuth glance (Bi₂S₃) and bismite (Bi₂O₃). Bismuth was originally confused with lead and tin, and it was not until Claude Geoffrey in 1753, who was able to distinguished it from lead.

Bismuth metal has a greyish white appearance with a reddish-pinkish tinge. Upon heating in air it burns with a blue flame, forming yellow fumes of the oxide. It is considered brittle and powders easily. Bismuth is highly crystalline with rhombohedral crystals. It is the most diamagnetic of all metals and exhibits the lowest thermal conductivity of any metal, except for mercury.

Bismuth is one of few metals that increase in volume upon solidification, where it expands 3.32% when changing from the liquid to the solid state. This property makes bismuth alloys particularly suited to the making of sharp casting of objects subject to damage by high temperatures. It also can be used in controlled amounts for casting alloys, so that they fill the mould without the effects of expansion or contraction upon cooling.

Applications

Elemental bismuth is used in applications including:

• When alloyed with tin cadmium and the like, bismuth forms low-melting alloys, which are extensively used for safety devices for fire detection and extinguishing systems.
• As a catalyst for making acrylic fibres.
• Bismuth wire (extruded) is used for thermocouple applications (has highest negativity known), which is drawn in glass tubes of diameters of 0.0062 to 0.0991cm (0.003 to 0.039in), and is ductile enough to be wound tightly.
• Is added in small amounts as a machining aid, especially for steel, as a substitute for lead. Bismuth has been incorporated both as an additive to resulphurised, rephosphorised, and leaded AISI 1214 steel and as a substitute for lead in AISI 1214 and 1215 steels.
• It has also been incorporated in aluminium alloy 6262 in equal parts with lead (0.6%) for improved machinability.
• It can be alloyed with lead providing more effective internal lubrication than lead alone. Its advantage over lead has been attributed to its lower density, allowing more uniform dispersion in steel matrixes.
• Bismuth is used also in amalgams.

A number of bismuth compounds also have commercial applications. These include:

• Bismuth salts are used in pigments, examples of which are bismuth oxychloride (BiOCl) a white crystalline powder, and bismuth chromate (Bi₂O₃.2CrO₃) an orange-red powder both of which are insoluble in water.
• In pharmaceuticals as it is used as an antacid. In medical applications bismuth must be completely free of trace amounts of arsenic. An example is bismuth phosphate (BiPO₄), a white powder insoluble in water.
• Bismuth telluride (Bi₂Te₃) is employed as a thermoelectric semiconductor.