Roasting molybdenum

In roasting molybdenum sulfide concentrates, it has been found that the efficiency of dust collection in electrofilters decreases with increasing temperature. This can be explained on the basis that there is no material in this dust that is capable of reacting with SO3. 

On roasting molybdenum sulfide concentrates there is a reduction in the trapping of dust by the electric filter as temperature rises. This may be explained by the fact that there are no substances capable of interacting with SO3 in the dust formed. 

Molybdenite concentrate contains about 90% M0S2. The remainder is primarily siUca, with lesser amounts of Fe, Al, and Cu. The concentrate is roasted to convert the sulfide to technical molybdic oxide. Molybdenum is added to steel in the form of this oxide. In modem molybdenum conversion plants, the oxidized sulfur formed by roasting M0S2 is converted to sulfuric acid. 

The principal direct raw materials used to make sulfuric acid are elemental sulfur, spent (contaminated and diluted) sulfuric acid, and hydrogen sulfide. Elemental sulfur is by far the most widely used. In the past, iron pyrites or related compounds were often used but as of the mid-1990s this type of raw material is not common except in southern Africa, China, Ka2akhstan, Spain, Russia, and Ukraine (96). A large amount of sulfuric acid is also produced as a by-product of nonferrous metal smelting, ie, roasting sulfide ores of copper, lead, molybdenum, nickel, 2inc, or others. 

On molybdenum sulfate roasting, downstream of a scrubber, to remove ammonium sulfite and sulfate aerosols which form in the ammonia scrubbing stage, and SO2,... 

Workers at a molybdenum-roasting plant with time-weighted average (TWA) exposures of approximately 9.5mgMo/m to soluble dusts had increased plasma and urine levels of molybdenum the only adverse biochemical findings were large elevations in serum ceruloplasmin levels and some increase in serum uric acid levels. ... 

The concentrated molybdenite ore is then roasted in air, converting molybdenum sulfide to molybdenum trioxide M0O3. This is harvested in high purity by sublimation. An alternative is to leach molybdenite concentrate with dilute ammonia solution, which converts the metal to ammonium molybdate, (NH4)2Mo04. Molybdenum trioxide or ammonium molybdate product is then heated with hydrogen at elevated temperatures from 500 to 1,150°C in a furnace to produce molybdenum powder. 

Amine salts have been used to recover molybdenum from solutions arising from a variety of sources. Most of the western world s supply of this metal is derived from molybdenite (MoS2) concentrates obtained as a byproduct of copper production in the USA and Chile. Such concentrates are roasted to molybdenum(VI) oxide (volatile Re207 can often be recovered as a valuable byproduct from the roaster gases) and leached with dilute sulfuric acid to remove the copper from the crude M0O3 product. Some molybdenum also dissolves and can be recovered, for example, by the same technique as that practised at Kennecott s Utah Copper Division smelter,213 i.e. by extraction into a solution of a tertiary amine in kerosene at an aqueous pH value of about 1. 

Molybdenum occurs naturally as the mineral molybdenite, which, by roasting, produces molybdenum trioxide (up to 90% purity). 

By-product processing is also largely by crushing and flotation, but the flotation processes are more specialised because of the variety of ores involved and the need to separate the small molybdenum content from the major proportion of copper or other primary metals. Because the product is mainly used in steelmaking, oxidation to molybdic oxide is acceptable, and an intermediate roasting may also be used. 

The bulk of the concentrate separated from molybdenite ore by flotation is further processed to produce molybdenum. A typical extraction and purification procedure is outlined in Figure 2.1. The concentrate is roasted to convert the moiybdenum disulphide to molybdic oxide. The product is called roasted concentrate, and about 30% is marketed as Technical Oxide, mainly for alloy manufacture. A typical range of compositions is shown in Table 2.6. 

Between 40% and 50% of the roasted concentrate is converted to ferromolybdenum, either by means of an electric furnace or by a thermite process. The thermite process involves ignition of a mixture of the roasted concentrate with aluminium and an iron source (iron ore and ferrosilicon) together with a flux. The resulting ferromolybdenum contains between 55% and 70% of molybdenum, and is used in alloy steel and cast iron manufacture. Some of the roasted concentrate is converted to briquettes by pressing with a pitch binder. The briquettes, weighing about 5 kg., are also used in manufacture of alloy steels and cast irons. 

Preparation.—Although the purest molybdenum is obtained from wulfenite, the chief commercial source is molybdenite, which is converted into the trioxide by roasting in air either with or without the addition of sand, and, on dissolving the residue in ammonia, a solution of ammonium molybdate is obtained. This salt, freed from copper by treatment in ammoniacal solution with ammonium sulphide, and from aluminium by the addition of potassium carbonate, on ignition yields molybdenum dioxide alternatively, heating with excess of sulphur yields pure molybdenum disulphide, MoS, which on roasting, or by treatment with nitric acid, is converted into the trioxide MoOj. ... 

The oxygen-enriched blast has been successfully tried in the zinc sulfide concentrates roasting in fluidized-bed, multiple-hearth, and suspension roasters. As early as 1956, Electrolytic Zinc Company of Australia improved calcine grade, increased oxidation rates, and facilitated use of cold blast to the hearth roasters for better temperature controls. Other nonferrous metals, such as molybdenum, copper, nickel, and lead, that use roasters to oxidize sulfide concentrates are technically and economically amenable to oxygen enrichment. 

I. From molybdenite, the extraction may be made in several ways, (a) The mineral is roasted as long as sulfur dioxide is given off. The residue which contains MoOs is leached with dilute ammonia and the solution evaporated until the ammonium molybdate crystallizes. Japanese patent 37420 (1920) extracts the roasted ore with Na2COj solution, then precipitates calcium molybdate by adding CaCb. (6) The finely ground ore is heated with nitric acid and the MoOj dissolved in ammonia, (c) A current of chlorine is passed over the dry pulverized ore at a temperature of 208°. The molybdenum chloride distills over and may be separated from sulfur and other chlorides by fractional condensation.2 (d) A British patent describes the extraction with an alkaline sulfide or polysulfide solution which removes the molybdenum from the ore as the soluble thio-molybdates. These may be converted to the molybdates by acidification or by contact with more ore. 

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