People used bismuth in Greek and Roman times, but they didn't know it was a metal element. Bismuth gets its name from German white metal (Wismut). In about16th century, agricola latinized this name as bismntum. Bismuth has long been mistaken for lead, tin, silver and antimony. It was not until 1753 that C.Ggeoffroy and T Bergman determined that bismuth was an element. After 1860, bismuth began to have a scale in industry.

■ Characteristics of Bismuth

Bismuth is brittle and shiny. The volume of bismuth increases during solidification, and the expansion rate is 3.3%. Bismuth is the most diamagnetic metal. Under the action of magnetic field, the resistivity increases and the thermal conductivity decreases. Bismuth is the metal with the lowest thermal conductivity except mercury. Bismuth and its alloys have thermoelectric effect. Selenium and tellurium compounds of bismuth have semiconductor characteristics. At room temperature, bismuth is slightly oxidized in humid air, and when it is heated to the melting point, it burns to generate bismuth trioxide. Bismuth reacts slowly with hydrochloric acid, reacts with sulfuric acid to release sulfur dioxide, and reacts with nitric acid to generate nitrate.

■ Bismuth resources

There is a small amount of bismuth in nature, and the main minerals are: bismuth ore, biscite, biscite, natural bismuth, galena biscite, biscite and copper biscite. There are few individual bismuth deposits, which are often associated with minerals such as lead, zinc, copper, tungsten, molybdenum and tin, and the industrial grade of independent mining is 0.5%. The annual output of bismuth in the world is about 4400 tons. The amount of metallic bismuth in China is 500,000 tons, and the output of bismuth in 1993 is about 1052 tons.

■ Preparation of bismuth

Bismuth smelting is divided into two steps: rough smelting and refining. The method of rough refining varies with raw materials. When bismuth is smelted with bismuth sulfide concentrate, bismuth oxide and bismuth mixed ore, bismuth oxide slag and bismuth oxychloride as raw materials, a proper amount of iron filings, soda ash, fluorite powder and coal powder are added. Mix and smelt in reverberatory furnace to obtain crude bismuth, and send it to refining. The refining method of calcium, magnesium and bismuth scum produced in the pyrometallurgical refining of lead is as follows: firstly, heat the scum to make the lead contained in it sink and take it out. Continue to heat the slag, after melting, add lead chloride or introduce chlorine to remove calcium and magnesium to obtain lead-bismuth alloy rich in bismuth, and then send it to refining. Refining is generally divided into four steps: oxidation to remove arsenic, antimony, tellurium and so on; Adding zinc to remove silver; Chlorination to remove lead and zinc; High temperature dechlorination.

■ uses of bismuth

Bismuth is mainly used to prepare fusible alloy in metal form and drugs in compound form. The former has a melting point of 47-262℃, and the most commonly used alloys are binary, ternary, quaternary and pentanary alloys of bismuth with metals such as lead, tin, antimony and indium. By changing the percentage of these metals in the alloy, a series of alloys with different melting points and different physical properties can be obtained. These alloys are used in fire fighting equipment, heat sensitive parts of automatic sprinklers, safety plugs of boilers and compressed air bottles, solder, molten pool media for metal heat treatment, etc. Bismuth alloy does not shrink when it condenses, and is used for casting printing and high-precision molds. Bismuth and its alloys are usually used as additives for cast iron, steel and aluminum alloys to improve the machinability of the alloys. Bismuth alloy containing antimony 1 1% is used to manufacture infrared detectors. Bismuth tin and bismuth cadmium alloys are used as auxiliary electrodes of selenium rectifier. Based on the characteristic that the resistivity of bismuth drops sharply under the action of magnetic field, a magnetic measuring instrument is made. Bi-Mn alloy can be used to make permanent magnet alloy. Bismuth has small thermal neutron absorption cross section, low melting point and high boiling point, and can be used as heat transfer medium in nuclear reactors. Bismuth telluride is widely used to manufacture thermoelectric generator components of solar cells. Bismuth-silver-cesium alloy is used to manufacture photoelectric amplifiers. Silver bismuth sulfide is used to manufacture semiconductor instruments. Bismuth-cadmium temperature difference element is used in alarm device.