Tungsten Refining & Hardmetal Carbide Lab

Tungsten ores occur predominantly as scheelite (CaWO4) or wolframite ((Fe,Mn)WO4). Scheelite concentrates (typically 65% WO3) are digested in high-pressure autoclaves at 200°C to 225°C with sodium carbonate (soda ash) solutions under 2 to 3 MPa steam pressure: CaWO4 + Na2CO3 -> Na2WO4 + CaCO3. Wolframite concentrates can alternatively be digested with hot concentrated sodium hydroxide (NaOH) at 110°C to 150°C. Insoluble calcium carbonate and iron-manganese oxides are filtered out, while soluble sodium tungstate (Na2WO4) pregnant liquor advances to chemical purification to remove silica, phosphorus, arsenic, and molybdenum via sulfide precipitation.

Ammonium Paratungstate (APT, chemical formula 5(NH4)2O · 12WO3 · 5H2O) is the universal intermediate chemical commodity in global tungsten trade, quoted in metric ton units (mtu) of contained WO3 (1 mtu = 10 kg WO3). It is produced from purified sodium tungstate solutions via solvent extraction using high-molecular-weight quaternary ammonium salts or tertiary amines in kerosene, stripping with aqueous ammonia, and evaporative crystallization. APT crystallizes as exceptionally pure white monoclinic crystals containing 88.82% WO3 (70.43% pure W). APT's high purity makes it the universal feedstock for thermal decomposition into tungsten oxides and metal powder.

Calcining APT yields Tungsten Blue Oxide (WO2.9) or Yellow Oxide (WO3). This oxide is reduced to pure metallic tungsten powder in multi-tube pusher furnaces at 750°C to 1,000°C under counter-current dry hydrogen gas: WO3 + 3H2 -> W + 3H2O. Crucially, the tungsten grain size (measured from 0.5 to 10 microns via Fisher Sub-Sieve Sizer) is governed by the volatile chemical transport of tungsten through gaseous hydrated species (WO2(OH)2). Higher temperatures, high bed depths, and higher hydrogen moisture dew points accelerate vapor-phase transport, growing coarse grains; conversely, rapid dry hydrogen flow and lower temperatures produce ultrafine sub-micron powders required for high-hardness micro-drills.

Cemented tungsten carbide (hardmetal) consists of hexagonal tungsten monocarbide (WC) grains embedded in a ductile cobalt metallic binder (typically 6% to 12% Co). Synthesized by reacting tungsten powder with carbon black at 1,400°C–1,600°C (W + C -> WC) followed by liquid-phase vacuum sintering at 1,450°C, WC-Co achieves extreme hardness (1,200 to 1,900 HV), exceptional compressive strength, and high thermal conductivity. It is the indispensable material for CNC cutting tool inserts, rock drilling bits, metal stamping dies, and wear parts that machine the entirety of global aerospace, automotive, energy, and electronics hardware.

Spent cemented carbide cutting inserts and mining drill tips are recycled via the pyrometallurgical zinc process. Scrap hardmetal is heated with molten zinc at 850°C to 900°C in an inert atmosphere. Molten zinc diffuses into the cemented structure and alloys with the cobalt binder, causing massive volumetric expansion that disintegrates the rigid WC skeleton into loose powder. The retort is then evacuated to high vacuum at 950°C, boiling off and recovering 100% of the metallic zinc for reuse. The resulting spongy WC-Co cake is milled into ready-to-press carbide powder with identical metallurgy, consuming 75% less energy than primary ore refining.