Block Ferrotungsten: Smelting Routes, Size Rules and Cost Drivers

Ferrotungsten as a Melting-Shop Addition: What the Buyer Actually Controls

Ferrotungsten belongs to the iron-alloy family: an alloy of tungsten and iron used as an alloying additive in steelmaking and in the production of tool steels, hot-work steels and certain wear-resistant grades. Commercial material is normally supplied at tungsten contents of the order of 70% and 80%, written FeW70 and FeW80, and it reaches the melting shop as cast blocks that have been crushed and sized to an agreed band.

From the buyer's point of view, the chemical result in the finished heat depends on three things that are decided before the material ever arrives: which grade and carbon level is ordered, which size fraction is specified, and how the addition is actually made in the furnace. Purchase price matters, but it is the delivered cost per kilogram of tungsten recovered that determines the real cost of alloying. That distinction - nominal price against recovered units - runs through the whole purchasing decision.

This article deals with the properties that distinguish ferrotungsten from other ferroalloys in day-to-day procurement: the production routes and what they imply for carbon, the physical delivery form and size specification, the surface and internal soundness rules, the cost drivers, and the inspection points where a cargo is either settled cleanly or becomes a claim.

Two Production Routes and What They Mean for Analysis

Ferrotungsten cannot be tapped and cast like ferrosilicon or ferromanganese. The alloy's melting range is very high - well above ordinary steelmaking temperatures - and the reduced metal is viscous, so it will not flow out of a conventional taphole. Two industrial routes developed around this limitation.

Agglomeration, or block-fusion, smelting. The charge consists of refined tungsten ore concentrate, a carbon reductant such as pitch coke or petroleum coke, and a slagging agent such as bauxite. The mix is charged in batches to an electric furnace, where carbon reduces the tungsten oxide and the reduced tungsten alloys with iron. The metal pool thickens as melting proceeds and solidifies as a large mass; the furnace is stopped, the body drawn out and the upper section removed so that the mass can condense and cool. The block is then extracted, crushed and finished, and the off-grade portions, edges and slag-rich material are returned to the furnace for remelting. The classic agglomeration product contains about 80% tungsten with carbon not exceeding about 1%.

Iron extraction, or thermite-type reduction. Here a reactive metal rather than carbon performs the reduction, producing a more fluid metal that separates from slag more cleanly and lowering the carbon content of the alloy. This route is the practical answer when a steelmaker needs a ferrotungsten with a tighter carbon ceiling, because there is no large carbon charge to be carried into the metal.

The practical consequences for procurement are straightforward. Two offers of nominally the same tungsten grade may carry different carbon guarantees depending on the route and the refining practice applied, and carbon is exactly the element that matters most when tungsten is added to a low-carbon or medium-carbon tool steel. The correct approach is to state the required carbon maximum in the enquiry and to confirm which production route the supplier intends to use, rather than to assume that a grade designation alone settles the question.

Historically, agglomeration smelting was carried out in relatively small single-phase electric arc furnaces of roughly 100–500 kVA, and the later adoption of three-phase furnaces together with the extraction method expanded both furnace capacity and the range of analyses available. That history explains why the same grade designation may represent different impurity profiles between producers today.

Delivery Form and Size Specification: How to Write the Size Clause

Ferrotungsten is delivered as cast blocks in a range of sizes, and the size clause is one of the most frequently under-specified parts of a purchase order. A specification that simply says "crushed" leaves both parties exposed, because an agglomerated and then crushed product naturally contains a distribution of sizes, some fine and some coarse.

A well-written size clause does three things. It states the nominal size band, it sets a maximum permitted undersize and oversize fraction, and it says how the fractions are to be determined. A typical commercial specification covers blocks from about 10 mm to 130 mm, with the material smaller than a 10 × 10 mm screen limited to no more than about 5% of the batch weight, and with the maximum dimension of an individual block in any one direction not exceeding about 150 mm. Those figures are the shape of a workable clause: a usable coarse fraction for charging, a defined cap on the fines that are prone to oxidation and loss, and a defined cap on oversize that will not fit a charging bucket.

Physical parameter	Typical commercial specification	Why it is specified	How to verify
Nominal block size	About 10–130 mm	Chargeable by bucket or chute without bridging	Sieving a representative sub-sample
Undersize fraction	Material below 10 × 10 mm limited to about 5% of batch weight	Controls fines, which oxidise and blow out of the bath	Screen analysis of the sub-sample
Maximum single dimension	About 150 mm in any one direction	Fits charging equipment and dissolves uniformly	Manual measurement of oversized pieces
Surface condition	Smooth and clean, free of obvious non-metallic inclusions and heavy rust	Surface contamination adds slag and unusable mass	Visual inspection of the pile, cross-section check on broken pieces
Packing and marking	Drums, bags or lined containers marked with grade, heat number and net weight	Traceability and tare settlement; packing provisions per GB/T 3650	Weighing against a calibrated scale; check of marks

Sieve testing should be carried out under a defined particle-size test method so that both parties apply the same procedure; the ferroalloy granularity provisions of GB/T 13247 exist for that purpose. Without an agreed method, results from a supplier's plant and a buyer's yard can differ simply because of screen aperture, sample mass or shaking time.

Fines deserve particular attention. Fine ferrotungsten has a high surface-to-mass ratio, oxidises more readily in storage and in transit, and is more easily carried into the slag or lost to the dust extraction system when charged. A limited undersize allowance is therefore a quality requirement, not a formality, and it should be paired with a price adjustment mechanism for material that exceeds the allowance rather than a simple accept-or-reject rule.

Surface Condition, Slag and Internal Soundness

Because ferrotungsten is produced as an agglomerated mass rather than as a cleanly cast metal, the surface of the finished blocks carries information about the quality of the heat. The specification normally requires that the surface be smooth and clean, without obvious non-metallic inclusions, and that neither the surface nor a broken cross-section show significant rust or slag-rich areas.

Three defects should be looked for specifically. Slag or furnace-lining material adhering to block surfaces adds mass that contains little or no tungsten, so a cargo that appears to weigh a lot may yield fewer tungsten units per tonne than expected; this is a direct money issue on a per-contained-unit basis. Heavy scaling or rust staining indicates that surface tungsten has already been oxidised, which is again a loss of units and a source of oxygen in the bath. Large internal cavities or porous zones, visible on broken faces, indicate incomplete fusion and can lead to erratic dissolution when the material is charged.

The practical response is to combine visual inspection with a physical check on a small number of pieces: break representative blocks and examine the fracture face for cavity and inclusion content, and compare that impression against the certificate of analysis. A supplier whose material is consistently sound at the break will also be consistent in its tungsten recovery.

Cost Drivers and Quotation Practice for Block Ferrotungsten

Ferrotungsten quotations respond to the same underlying cost blocks as other ferroalloys, but the weighting is unusual because the tungsten charge is so dominant. The main variables are:

Contained tungsten feed. Tungsten ore concentrate, priced on contained WO₃ with penalties for impurities, accounts for the majority of the cost. The concentrate market also responds to policies affecting mining, beneficiation and export, so availability can tighten independently of steel demand.

Reductant and flux. Pitch coke or petroleum coke in the agglomeration route, a reactive metal in the extraction route, and slagging agents such as bauxite. A change in reductant quality affects both the furnace performance and the residual carbon in the alloy, so the cheapest reductant is not always the cheapest route to a given grade.

Electric power. Agglomeration smelting is energy-intensive and is normally scheduled around available power, particularly where tariff structures or allocation agreements make off-peak operation cheaper. Two plants with identical equipment can therefore have materially different conversion costs.

Yield and returns handling. Tungsten that reports to slag, dust or off-grade blocks is lost unless it is recycled. A plant that routinely remelts its returns and recovers dust carries a lower specific consumption of concentrate, which appears directly as a more competitive offer.

Size, packing and analysis deviation. A narrow crushed band, steel-drum packing, a very low carbon ceiling or a restricted impurity profile all add cost over the basic bulk block product.

Freight, insurance and currency. Block ferrotungsten is dense and high in value, so freight per tonne can be significant relative to the product value, and the choice of basis - ex-works, FOB, CFR or delivered - changes the delivered number without changing the plant economics. Currency of contract and the payment mechanism complete the picture.

Quotation practice follows from this. Compare offers per kilogram of contained tungsten, confirm whether the analysis is guaranteed or merely typical, confirm the size clause and packing, and establish the validity period. A firm offer that is valid for a short window and a soft indication valid for a long one are not comparable instruments, and treating them as if they were is a common source of disappointment when the market moves.

Inspection, Sampling and Weight Settlement

Settlement of a ferrotungsten cargo rests on three things: a representative sample, an agreed analytical method, and a reliable weight. Each deserves attention at the contracting stage rather than at the quayside.

Sampling. Sampling for chemical analysis should follow the ferroalloy provisions of GB/T 4010, with the iron-alloy sampling framework of ISO 3713 as the reference where an overseas laboratory is used. Increments must be drawn from enough blocks and enough positions within the lot to be representative, then reduced by a documented procedure into a laboratory sample, a reserve sample and, where the contract provides for it, a referee sample held sealed by a third party.

Analysis and dispute. Agree in advance which tungsten determination method is the referee method and what tolerance applies between the supplier's certificate and the buyer's result. A well-drafted clause states that a difference within the tolerance is settled on the supplier's certificate, and that beyond it a referee analysis on the sealed sample decides.

Weight. Weighing must be against a calibrated scale, with the tare of the packing determined rather than assumed. Where material is supplied in drums or bags, gross-for-net conversion errors accumulate quickly across a whole consignment, and because ferrotungsten is dense and valuable, even a small percentage error is worth pursuing.

Documentation. Packing, marking and transport provisions consistent with GB/T 3650 ensure that grade, heat number and net weight travel with the material, which is what allows a lot to be traced back to the furnace heat if a downstream problem appears.

Common Buying Mistakes and How to Avoid Them

Buying on gross weight instead of contained tungsten. Two grades with different tungsten contents cannot be compared on price per tonne of blocks. Convert every offer to contained units first.

Leaving the carbon ceiling unstated. Carbon is the analysis item most likely to create a problem in low-carbon tool and die steels, and it is also the item that responds most to the production route. State it in the enquiry and require it in the guarantee.

Accepting an undefined size clause. Without a stated undersize allowance and a defined test method, fines can only be argued about, not settled. Put the numbers and the method in the order.

Ignoring the returns and recovery question at the shop floor. Even a correctly specified alloy can deliver poor economics if it is added into an oxidising bath or held for too short a time. Addition late in the heat, into a well-deoxidised bath, with adequate holding time after the addition, is the standard practice for protecting tungsten recovery.

Treating a quotation as a number rather than a basis. Grade guarantee, size, packing, freight term, currency, payment and validity are all part of the offer. A cheap offer on a different basis is not a cheap offer.

FAQ

Q: Why is ferrotungsten produced by agglomeration instead of being cast like other ferroalloys?
Because its melting range is very high and the reduced metal is too viscous to be tapped, the alloy is melted in an electric furnace and allowed to solidify in place as a large mass, which is then crushed into graded blocks. A thermite-type extraction route is used where a lower carbon content is required.

Q: What is a reasonable size specification for block ferrotungsten?
A workable clause covers blocks of roughly 10–130 mm, limits material below a 10 × 10 mm screen to about 5% of batch weight, caps the maximum single dimension at about 150 mm, and states the sieve test method, for example under the ferroalloy granularity provisions of GB/T 13247.

Q: How much carbon does ferrotungsten contain?
The classic agglomeration product contains no more than about 1% carbon, while grades intended for low-carbon steels are supplied with substantially tighter carbon guarantees. The maximum should be stated in the purchase specification because it depends on the production route and the refining practice.

Q: How should ferrotungsten be sampled for chemical analysis?
Follow the iron-alloy sampling provisions of GB/T 4010, cross-referenced to ISO 3713 where offshore laboratories are involved, and retain sealed reserve and referee samples so that a disputed tungsten determination can be resolved without re-sampling the cargo.

Q: What physical defects should be checked when accepting a delivery?
Adhering slag or lining material, heavy rust or scaling, and internal cavities visible on broken faces. Slag adds mass with little tungsten, scaling means surface units have already oxidised, and porosity indicates incomplete fusion that affects dissolution behaviour.

Q: Which cost variables explain a gap between two ferrotungsten quotations?
The contained tungsten feed cost, the reductant and flux, electric power arrangements, process yield and returns handling, and then the commercial terms: grade guarantee and carbon ceiling, size band, packing, freight basis, currency and validity. Compare on contained tungsten before drawing any conclusion.

Q: How should ferrotungsten be stored before charging?
Under cover and dry, with drums or bags stacked on dunnage off the floor and older stock consumed first. Damp material should be dried before charging, both because moisture introduces hydrogen into the melt and because wet fines oxidise and skew sampling.