Molybdenum

MOLYBDENUM (symbol Mo, atomic number 42, atomic weight 96). Pure compact molybdenum is a silvery white metal, softer than steel and quite malleable. Its melting point is very high, probably 2,500°C., whilst its tensile strength when drawn into wire, is about half that of tungsten or steel wire of the same diameter. Crude grey molybdenum, an electric-furnace product made direct from molybdenite, is impure owing to ab sorption of carbon. It is very brittle, with a hardness greater than that of quartz. Varying figures, e.g., 9.01, 8.95, 10.28, are given for the density of the pure metal. The electrical resistance of ductile molybdenum is 5.6 microhms per c.c. for hard-drawn wire and 4.8 for annealed wire. Molybdenum is not appreciably affected by air at ordinary temperatures but at a dull-red heat the oxide is slowly formed. It is somewhat resistant to the action of acids but less so than tungsten. Concentrated hydrochloric and sulphuric acids attack it only very slowly, whilst moderately dilute nitric acid, like aqua regia, rapidly reacts with the metal. Concentrated nitric acid induces passiv ity. Molybdenum is attacked by fused but not by aqueous caustic alkalis. The most important alloy of molybdenum is ferro-molybde num which is made in the electric furnace by fusing molybdenite con centrates with varying proportions of coke, lime, scrap iron, pyrites, etc. This is the usual form in which molybdenum is added to steels. There are also many types of non-ferrous molybdenum alloys. The non-ferrous stellite alloys, which are silver-white, insoluble in nitric acid and only slowly attacked by hydrochloric acid, are said to possess exceptional qualities when used for high-speed cutting tools. A typical molybdenum stellite gave the following analysis: Mo, 22.50; Co, 59.so Cr, 10•71 ; Fe, 3." ; Mn, 2.04 ; C, 0-87 ; Si, S, and P, balance. Other alloys are chrome-molybdenum, nickel-molybdenum, ferro-molybde num-tungsten and copper-nickel-molybdenum. Metallic molybdenum has come into use for a variety of electrical purposes, such as a support for lamp filaments, winding for electrical resistance furnaces, X-ray apparatus, plates used in wireless telegraphy, etc. High mo lybdenum steels, containing over 1% of the metal, are used for per manent magnets, rustless steels, and high-speed tools ; whilst low mo lybdenum steels, containing less than I% of the metal, find use for automobile parts, agricultural implements, railway forgings and track bolts.

The name molybdena which occurs in the writings of Pliny was employed by him to denote various substances containing lead. Later, this name was used to designate galena—the naturally occurring com pound of lead and sulphur—or substances of similar appearance, whilst by the middle of the 18th century it was applied solely to graphite and the mineral sulphide of molybdenum which now bears the name of molybdenite. In view of their similar appearances it is not sur prising that these two substances were classified together at this period. K. W. Scheele first pointed out, in his "Treatise on Molyb dena" (1778), the essential difference between them. He showed, that unlike graphite, this other mineral, on treatment with nitric acid produced a "peculiar white earth" with acidic properties to which he assigned the name, molybdic acid. Further, since this same mineral, on heating gave rise to sulphurous fumes, he concluded correctly that molybdenite was a sulphide of molybdenum. Finally, in 179o, ap peared an account, by P. J. Hjelm of the isolation of the new element molybdenum, as a metallic powder, by heating the oxide with carbon. Molybdenum is never found free in nature but always chemically combined with other elements. Although a large number of minerals

containing molybdenum have been described, only two of them are so far of any practical importance, namely, molybdenite and wulfenite (q.v.).

Concentration of Molybdenum

the percentage of pure molybdenite present in the molybdenum ore is generally only 1/4%, the importance of a successful method for separating and concentrating the molybdenite can be readily appreciated and at one time represented a real difficulty to the industry. The application of oil flotation, however, has gone far to solve this difficulty, and mo lybdenite, when fairly clean, is now one of the easiest minerals to con centrate. The concentration of wulfenite ores is simplified by the fact that the mineral is heavy and easily wetted, so that any of the gravity methods may be applied. The presence of heavy minerals such as lead ores, including vanadinite, however, complicates the separation. Preparation of the Metal.—There are several methods by which molybdenum may be obtained from its ores. Using the molybdenite concentrates, two methods are generally employed. In the alumino thermic process—the one mainly in use—the metal is obtained by igniting a mixture of finely divided aluminium and the concentrate. The metal may contain as much as 2% of iron and small quantities of silicon. In the electric fusion process, the molybdenite concentrates are heated in a carbon tube by a current of 35o amperes at 6o volts. Sulphur dioxide is expelled, but to expel completely the sulphur the current is afterwards increased to 90o amperes at so volts. The metal so produced may contain considerable carbon but this can be removed by heating with molybdic oxide. The special treatment of wulfenite, from which about io% of the world's supply of molybdenum is de rived, has been studied by J. P. Bonardi, who has made a detailed report on this subject in Chem. and Met. Eng., p. 364, 1919.

Chemical

compounds exhibit valen cies of 6, 5, 4 and 3 while halogen derivatives exist corresponding with the empirical formula but having the molecular formula The most important and most stable compounds of molyb denum are those in which the metal is hexavalent. The trioxide, which may be made by roasting molybdenite in a current of air, is a white powder which can be completely volatilized at approximately 700°C., although appreciable volatilization occurs at Heated to in a current of hydrogen chloride the trioxide sublimes as Mo032HC1. The oxide has both acidic and basic properties, for it dissolves in hot concentrated sulphuric or hydrochloric acid while with bases it forms molybdates containing varying ratios of to metallic oxide. Molybdenum forms many complex salts. In Aga the molybdenum has a co-ordination number of four, and X-ray analysis shows that the oxygen atoms are tetrahedrally distributed round the central molybdenum atom. The complex salts exhibit molybdenum with a co-ordination number of six, and there is evidence that in such substances the associating units are arranged octahedrally round the molybdenum. In other derivatives, such as a co-ordination number of eight is indicated.

Field tests for molybdenite: (a) Rubbed on porcelain or a white enamelled cup, molybdenite leaves a grey-green streak. (b) Heated on charcoal in the oxidizing flame, the powdered mineral emits a strong, sulphurous odour and deposits molybdic oxide, which is yellow when hot and white when cold. Nearer to the heated mineral, the coating is copper red. If the white coating is touched intermittently with the reducing flame, it becomes deep blue.