Alloys of bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead.
It is a brittle metal with a silvery white color when newly made, but often seen in air with a pink tinge owing to the surface oxide. Bismuth metal has been known from ancient times, although until the 18th century it was often confused with lead and tin, which each have some of the metal’s bulk physical properties. The etymology is uncertain but possibly comes from Arabic “bi ismid” meaning having the properties of antimony or German words weisse masse or wismuth meaning white mass.
Bismuth is the most naturally diamagnetic of all metals, and only mercury has a lower thermal conductivity. Bismuth compounds (accounting for about half the production of bismuth) are used in cosmetics, pigments, and a few pharmaceuticals. Bismuth has unusually low toxicity for a heavy metal. As the toxicity of lead has become more apparent in recent years, alloy uses for bismuth metal (presently about a third of bismuth production), as a replacement for lead, have become an increasing part of bismuth’s commercial importance.
The only primordial isotope of bismuth, bismuth-209, was traditionally regarded as the heaviest stable isotope, but it had long been suspected to be unstable on theoretical grounds. Bismuth has the longest known alpha decay half-life, although tellurium-128 has a double beta decay half-life of over 2.2×1024 yr. Bismuth-213 is also found on the decay chain of Uranium-233.
The most important ores of bismuth are bismuthinite and bismite. Native bismuth is known from Australia, Bolivia, and China. World 2008 bismuth refinery production was 15,000 tonnes, of which China produced 78%, Mexico 8% and Belgium 5%. The difference between world bismuth mine production and refinery production reflects bismuth’s status as a byproduct metal. Bismuth will behave similarly with another of its major metals, copper. Thus world bismuth production from refineries is a more complete and reliable statistic.
Next in recycling feasibility would be sizeable catalysts with a fair bismuth content, perhaps as bismuth phosphomolybdate, and then bismuth used in galvanizing and as a free-machining metallurgical additive.
Bismuth in uses where it is dispersed most widely include stomach medicines (bismuth subsalicylate), paints (bismuth vanadate) on a dry surface, pearlescent cosmetics (bismuth oxychloride), and bismuth-containing bullets that have been fired. The bismuth scattered in these uses is unrecoverable with present technology.
The most important sustainability fact about bismuth is its byproduct status, which can either improve sustainability (i.e., vanadium or manganese nodules) or, for bismuth from lead ore, constrain it; bismuth is constrained.
The life-cycle assessment of bismuth will focus on solders, one of the major uses of bismuth, and the one with the most complete information. The average primary energy use for solders is around 200 MJ per kg, with the high-bismuth solder (58% Bi) only 20% of that value, and three low-bismuth solders (2% to 5% Bi) running very close to the average.
The global warming potential averaged 10 to 14 kg carbon dioxide, with the high-bismuth solder about two-thirds of that and the low-bismuth solders about average. The acidification potential for the solders is around 0.9 to 1.1 kg sulfur dioxide equivalent, with the high-bismuth solder and one low-bismuth solder only one-tenth of the average and the other low-bismuth solders about average. There is very little life-cycle information on other bismuth alloys or compounds.
Bibrocathol is an organic bismuth-containing compound used to treat eye infections. Bismuth compounds were formerly used to treat syphilis, and today bismuth subsalicylate and bismuth subcitrate are used to treat peptic ulcers. Bismuth subnitrate and bismuth subcarbonate are also used in medicines.
Wood’s metal, also known as Lipowitz’s alloy or by the commercial names cerrobend, bendalloy, pewtalloy or MCP 158, is a eutectic, fusible alloy with a melting point of approximately 70 °C (158 °F). It is a eutectic alloy of 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight. It is named after American metallurgist B. Wood.
Wood’s metal is useful as a low-melting solder, low-temperature casting metal, high temperature coupling fluid in heat baths, and as a fire-melted valve element in fire sprinkler systems in buildings. Medical gas cylinders in the United Kingdom have a Wood’s metal seal which melts in fire, allowing the gas to escape and reducing the risk of gas explosion.
A common use is as a filler when bending thin-walled metal tubes. For this use the tubing is filled with molten Wood’s metal. After the filler solidifies the tubing is bent, and the filler prevents collapse of the tubing. The Wood’s metal is then removed by heating, often by boiling in water. Other uses include making custom-shaped apertures and blocks (for example, electron-beam cutouts and lung blocks) for medical radiation treatment, and making metal inlays in wood. Wood’s metal is also useful for repairing antiques. For example, a bent piece of sheet metal may be repaired by casting a Wood’s metal die from a good example. The low melting temperature of Wood’s metal makes it unlikely this will harm the original. The damaged piece can then be clamped in the die and slowly tightened to form it back into shape. Wood’s metal has long been used by model railroad enthusiasts to add weight to locomotives, increasing traction, and the number of cars that can be pulled. Wood’s metal is also used in the making of extracellular electrode for the electro-physiological recording of neural activity. Like other fusible alloys, e.g. Rose’s metal, Wood’s metal can be used as a heat transfer medium in hot baths. Hot baths with Rose’s and Wood’s metals are not in routine use but are employed for temperatures above 220 °C.
Cerrosafe Metal is non-eutectic alloy melts at 165 °F (74 °C). It is mainly used by gunsmiths for making a reference casting of the chamber of a firearm. When it solidifies it first shrinks, allowing easy removal from the chamber.
When it cools it expands back to the exact size of the chamber. The casting can then be directly measured with calipers or a micrometer to determine the dimension of the weapon’s chamber, which is important for safety.
CERROSAFE alloy was originally made for toy soldier castings. One of its most popular uses today is casting to check gun chambers. Cerrosafe shrinks during the first 30 minutes and at the end of one hour is EXACTLY the chamber size.
Some other uses are casting cavities such as threads, dies, molds, blind holes, duplicate patterns in foundry matchplate making, support work pieces while machining; spray coating wooden patterns, dental lab techniques, masks for electroplating.
Rose’s metal, Rose metal or Rose’s alloy is a fusible alloy with a low melting point. Rose’s metal consists of 50% bismuth, 25–28% lead and 22–25% tin; its melting point is 100 °C (212 °F). The alloy does not contract on cooling. Rose’s metal is typically used as a solder.
It was used to secure cast iron railings and balusters in pockets in stone bases and steps. Its other uses are similar to Wood’s metal. Rose’s metal can also be used for bending of tubes (when the tube is filled with molten metal, solidified, bent, and then the metal is remelted and poured out), and as a heat transfer medium in constant temperature baths. The hot baths with Rose’s and Wood’s metals are usually not in routine use and are employed for temperatures above 220 °C.
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