Ferromolybdenum Production by the Outside-Furnace Method

Why the Outside-Furnace Method Dominates Ferromolybdenum Production

Ferromolybdenum is a master alloy used to introduce molybdenum into steel and cast iron. The outside-furnace method, also described as aluminothermic or thermite reduction, produces the alloy without any external power source, because the charge itself releases enough heat to melt both the metal and the slag. Molybdenum trioxide is reduced by aluminium, and iron is blended into the charge so that the reduced molybdenum is collected as an iron-molybdenum alloy rather than as pure metal. Once ignited, the reaction is self-sustaining, so the process depends on a properly prepared reaction vessel, a clean sand base and a tightly controlled charge mix rather than on fuel.

The smelting furnace is therefore the central piece of equipment. Its geometry, its lining and the condition of the sand nest decide how much metal is recovered and how cleanly the slag separates from the alloy.

Furnace Shell and Refractory Lining

The furnace shell is a plain circular cylinder welded from steel plate, typically about 10 mm thick. It works as a structural container and as the form that carries the refractory lining, not as a heat source. The inner face is lined with refractory clay bricks laid to a uniform thickness, so that the shell is shielded from the short but intense thermal pulse of the reaction. Bricks that are spalled, cracked or loose allow liquid slag to reach the steel plate and shorten furnace life.

Shell: welded circular steel cylinder, approximately 10 mm plate.

Lining: refractory clay bricks, laid tight and even.

Slag tapping hole: opens through the shell roughly 100 mm above the lower edge.

Bore diameter: about 120 mm on a typical production unit.

The tapping hole is deliberately placed low so that the slag layer, which floats above the denser alloy, can be drawn or guided away without disturbing the metal pool. A hole set too high leaves a heavy slag heel inside the furnace; a hole set too low puts the alloy at risk.

Preparing the Sand Base and the Sand Nest

The prepared furnace is set down on a sand base, and a hemispherical depression, usually called the sand pit or sand nest, is formed in that base. The nest is what actually holds the molten alloy at the bottom of the reaction vessel, so its shape and its compaction matter as much as the brick lining.

Dried river sand is used. After the furnace barrel is aligned, the nest is tamped firmly, with particular attention paid to the joint where the lower edge of the barrel meets the sand. A practical rule is to tamp the inner side to a finished height of 150-200 mm and the outer side to more than 400 mm. That gives the sand enough mass to resist the pressure of the melt and prevents slag from leaking through the joint while smelting is under way.

The nest must dry naturally. A wet sand nest is never acceptable: moisture trapped in the sand flashes to steam on contact with the melt, producing violent splashing that can injure the crew and disturb the reaction. Time spent on drying protects both safety and yield.

The Aluminothermic Charge

The charge is assembled from three functional groups:

Molybdenum source - molybdenum trioxide, the oxidic feed that supplies the molybdenum.

Reductant - aluminium, normally as granules or powder, weighed to match the oxygen balance of the oxide feed.

Iron carrier and flux - iron-bearing material plus a small amount of flux, which sets the density and melting behaviour of the alloy and keeps the slag fluid.

Charge preparation is a weighing and mixing operation rather than a melting operation. Aluminium that is too coarse reacts slowly and leaves molybdenum in the slag; aluminium that is too fine can drive the reaction too quickly and eject material from the vessel. Moisture in any component behaves much like a wet sand nest, so all feeds are kept dry and the mixed charge is used promptly.

Reaction, Cooling and Separation

Ignition starts from a single point and the reaction front then sweeps through the charge. The alloy forms as a dense liquid that sinks into the sand nest, while the alumina-rich slag floats on top. The vessel is left undisturbed so that gravity performs the separation: a proper settling period is worth more than any mechanical stirring afterwards.

Once the mass has solidified, the furnace is lifted clear and the sand nest is broken out. The alloy button is recovered, cleaned of adhering slag, then crushed and screened to the size range the customer ordered. The slag is inspected for entrapped metal before being discarded, because molybdenum locked in the slag is a direct loss of yield.

Typical Composition and Quality Control

The table below lists representative commercial ranges for ferromolybdenum. The exact limits for a given order are fixed by the applicable product specification and by the purchase contract.

Element	Typical range	Function in the alloy
Mo	55.0-75.0 %	Active alloying element delivered to the steel
Fe	Balance	Carrier that lowers the melting range of the alloy
Si	1.5 % max	Residual from reduction and raw feed
Al	1.0 % max	Residual reductant
C	0.10 % max	Controlled for low-carbon steel grades
S	0.10 % max	Controlled for cleanliness
P	0.05 % max	Controlled to limit embrittlement
Cu	0.50 % max	Residual element from feed materials

Trade designations such as FeMo60 and FeMo55 indicate the nominal molybdenum content of the alloy. Ferromolybdenum supplied against GB/T 3649 or ASTM A132 normally arrives with a certificate showing the analysed values for the elements listed above, and buyers should compare that certificate with the sand nest records, the charge weights and the screened size fraction, since together these three records explain most yield variations from heat to heat.

Frequently Asked Questions

Q: What is the outside-furnace method in ferromolybdenum production?
It is an aluminothermic reduction route in which molybdenum trioxide is reduced by aluminium inside a prepared vessel, with iron added so that the product is a ferromolybdenum alloy. The reaction supplies its own heat.

Q: Why is the furnace lined with refractory clay bricks?
The lining shields the steel shell from the intense heat of the reaction and from liquid slag. Worn or loose bricks let slag reach the shell and shorten furnace life.

Q: How large should the slag tapping hole be?
A bore of about 120 mm opened roughly 100 mm above the lower edge of the shell is typical for a production unit. That position keeps the hole near the slag layer without exposing the alloy pool.

Q: What happens if the sand nest is wet?
Trapped moisture turns to steam when the melt arrives, causing severe splashing and a real risk of injury. The nest is always dried naturally before use.

Q: Why must the joint between the barrel and the sand be tamped so carefully?
That joint is the weakest point of the vessel. Firm tamping on both sides, with an inner height of 150-200 mm and an outer height above 400 mm, stops slag from leaking during smelting.

Q: Is external fuel needed to start the reaction?
Only a small ignition charge is required. Once started, the reduction of molybdenum trioxide by aluminium is strongly exothermic and sustains itself through the rest of the charge.