Stage 1 — Concentrate to ammonium paratungstate (APT)
Tungsten arrives at a refinery as scheelite (CaWO₄) or wolframite ((Fe,Mn)WO₄) concentrate, or as recycled hard-metal scrap. The concentrate is digested — typically with sodium hydroxide under pressure — taking tungsten into solution as sodium tungstate. That liquor is then purified, usually by ion exchange or solvent extraction, to strip out molybdenum, phosphorus, arsenic, silicon and the metallic impurities that would otherwise travel with tungsten all the way to the finished powder.
The purified solution is then crystallised as ammonium paratungstate. This crystallisation is the single most important purification step in the entire chain. Impurities that remain in the mother liquor are rejected; only what co-crystallises carries forward. This is why APT — not tungsten metal — is the form the industry prices, contracts and ships against. A buyer purchasing APT is buying a verified purity baseline.
Jayanth Tungsten supplies APT at WO₃ min. 88.5% with carbon max. 0.0010%, average Fisher crystal size 30–70 µm, screened to −100 mesh.
Stage 2 — Calcination to tungsten oxide
APT is thermally decomposed, driving off ammonia and water and leaving tungsten oxide. What comes out depends entirely on the atmosphere in the calciner.
Calcined in air, APT gives yellow tungsten oxide (YTO) — stoichiometric WO₃, a stable yellow powder. Calcined under a reducing or oxygen-deficient atmosphere, it gives blue tungsten oxide (BTO), a sub-stoichiometric form of approximately WO₂.₇₂ to WO₂.₉ where some oxygen has already been stripped from the lattice.
That difference matters commercially. BTO is already partially reduced, so it needs less hydrogen and less time in the next stage. It also tends to yield a finer, more uniform metal powder, because the oxygen-deficient lattice nucleates metal differently during reduction. The trade-off is that BTO is more reactive and less forgiving in storage. Producers chasing fine grain sizes generally accept that trade; producers who value storage stability, or who are selling the oxide as a product rather than an intermediate, prefer YTO.
Stage 3 — Hydrogen reduction to metal
Oxide is reduced to metallic tungsten by passing it through a furnace under flowing hydrogen, typically in boats moving through a multi-zone pusher furnace. The reaction strips the remaining oxygen as water vapour, which the hydrogen stream carries away.
This is where grain size is decided, and the controlling variables are temperature, hydrogen dew point, hydrogen flow rate and residence time. A cooler, faster schedule with dry hydrogen produces fine powder: nucleation dominates and particles have little opportunity to grow. A hotter, slower schedule with a higher dew point produces coarse powder, because water vapour in the atmosphere enables chemical vapour transport, which moves tungsten from small particles to large ones and grows the grain.
That mechanism is the reason grain size and apparent density cannot be specified independently. Both are outputs of the same schedule. Jayanth Tungsten's four standard grades pair F.S.S.S. bands of 1.5–3, 4–6, 8–10 and 12–15 µm with apparent densities of 2.5, 3.4, 5.1 and 6.2 g/cc respectively. A buyer asking for 2 µm powder at 6 g/cc is asking for two settings of the same dial at once.
Why oxygen content is the specification to watch
Reduction never removes oxygen completely. What remains sits mostly as a thin oxide film on the particle surface, and because fine powders have far more surface area per kilogram, they inevitably carry more of it. Jayanth Tungsten holds oxygen to max. 0.06% across the grades.
Residual oxygen matters more than its small number suggests. In metal-matrix systems it interferes with wetting, so a diamond segment bond does not grip as it should. In sintering it reduces to water vapour inside a closing pore network, leaving porosity that will not close. In copper-tungsten infiltration it obstructs copper flow into the skeleton.
It is also the specification most likely to drift between the certificate and your loading dock, because tungsten powder picks up oxygen from humid air. This is why powder ships in sealed steel drums under nitrogen or vacuum: over a six to eight week sea-freight leg, an unsealed fine grade can pick up enough oxygen to make the certificate figure irrelevant.
What this means when you are reading a specification
Three practical conclusions follow from the process.
First, treat grain size and apparent density as a pair, not as two independent requirements. If your target combination is not one a supplier offers as standard, ask whether it is achievable before you order rather than assuming a premium will buy it.
Second, ask for the oxygen figure and the packing specification together. An excellent oxygen number in a drum that is not sealed under inert atmosphere tells you about the powder at the factory, not about the powder you will open.
Third, if you reduce in-house, buy APT or oxide rather than metal powder. You are then setting the reduction schedule yourself and controlling grain size directly, instead of inheriting another producer's process window.