Molybdenum Concentrate Smelting: Roasting, Leaching and Hydrogen Reduction
Feb 01, 2024
Feed and Product Forms
The primary feed is molybdenite, MoS2, which contains 59.94% molybdenum and 40.06% sulfur by stoichiometry. The smelter converts this to one of three commercial products: technical molybdenum oxide for steelmaking, ferromolybdenum for the same market where a master alloy is preferred, or pure molybdenum trioxide and ammonium molybdate for chemical and metal powder use. Ferromolybdenum is traded under ASTM A132 and, in international trade, under ISO 5451, with commercial grades carrying roughly 60% to 70% Mo.
Oxidative Roasting
In the conventional route molybdenite is roasted in air to molybdenum roasted sand, from which molybdenum trioxide is recovered by sublimation or by a wet route. Roasting the concentrate converts the sulfide to oxide and rejects the sulfur as SO2 in the off-gas, which must be captured and treated. Depending on the equipment and the additives used, the process is operated as rotary kiln roasting, reflector furnace roasting, multi-chamber furnace roasting, fluidised bed roasting or flash furnace roasting.
Roaster selection is a function of concentrate grade and gas handling. The older furnace types are simple but generate a large volume of dust and fume, give lower molybdenum recovery, and allow most of the rhenium in the concentrate to report to the flue dust rather than to a collected product. For low-grade and complex concentrates these routes are less suitable than fluidised bed or flash roasting, where gas volume per tonne of feed is lower and rhenium recovery from the scrubber circuit is practical.
Leaching, Purification and Reduction
Roasted calcine is leached with ammonia, which dissolves molybdenum as ammonium molybdate and leaves insoluble residue behind. The solution is purified, then concentrated and crystallised to produce ammonium molybdate crystals, or acidified to precipitate a molybdate that separates from soluble impurities. Either product is calcined to pure molybdenum trioxide.
Molybdenum metal is then produced by hydrogen reduction of the trioxide. Reduction is carried out in two stages in conventional practice, first to molybdenum dioxide and then to metal powder, because a single-stage reduction is difficult to control and produces coarse, poorly sinterable powder. Reduction temperature and the dew point of the hydrogen set the powder particle size, which in turn determines the sintering behaviour of the subsequent compact. Mill products made from that powder are ordered under ASTM B386 for plate, sheet, strip and foil and ASTM B387 for bar, rod and wire.
Hydrometallurgical Alternatives
| Route | Principle | Practical limitations |
|---|---|---|
| Oxidative roasting | Sulfide oxidised to oxide in air | Fume and dust, SO2 handling, rhenium loss to flue dust |
| Nitric acid pressure leach | MoS2 oxidised in autoclave with air or oxygen | High temperature and pressure, difficult process control |
| Sodium hypochlorite leach | Oxidative leaching of low-grade ore and tailings | High reagent consumption, cost penalty |
| Electro-oxidation | Anodic chlorine species oxidise the sulfide in slurry | Requires electrolysis plant and power |
In the nitric acid route the autoclave converts MoS2 to soluble molybdate using air or oxygen as the oxidant. The method consumes a cheap oxidant but demands high temperature and pressure, robust reaction equipment and tight control of the leaching conditions, and it carries safety hazards that have limited its use. Sodium hypochlorite leaching is applied to low-grade and tailings material; it dissolves molybdenum readily but the hypochlorite decomposes with oxygen evolution and also oxidises other metal sulfides, whose hydroxides can co-precipitate molybdate and return molybdenum to the residue. Electro-oxidation leaching is an improvement on that route, in which slurried concentrate is fed to an electrolytic cell containing sodium chloride solution and anodically generated hypochlorite performs the oxidation, giving high leaching rate, mild conditions and easier control of reaction direction and rate.
Emerging Routes and Recovery Economics
Two further approaches are in use. Molybdenite concentrate can be processed without an oxidising roast by oxygen pressure leaching directly to pure molybdenum trioxide. Low-grade oxidised ore can be leached with sulfuric acid, with molybdenum recovered from solution by ion exchange or solvent extraction. In both cases the economics depend on by-product recovery, particularly rhenium, and on whether the site can handle the reagent and effluent loads that the chosen route imposes.
Frequently Asked Questions
Q: What is the molybdenum content of molybdenite?
A: MoS2 contains 59.94% molybdenum and 40.06% sulfur by stoichiometry.
Q: Why is molybdenum reduced in two stages?
A: Reducing trioxide to dioxide and then to metal gives better control over powder particle size and sinterability than a single reduction step.
Q: Which roasting routes are least suitable for low-grade concentrate?
A: The older multi-chamber and reflector furnace routes, because of their high dust and fume load, lower recovery and poor rhenium capture.
Q: How is molybdenum recovered from the ammonia leach liquor?
A: The liquor is purified and then either crystallised as ammonium molybdate or acidified to precipitate a molybdate, and the product is calcined to molybdenum trioxide.
Q: Which specifications cover the finished mill products?
A: ASTM B386 for plate, sheet, strip and foil; ASTM B387 for bar, rod and wire; ferromolybdenum under ASTM A132.
