Chemical identity and crystal structure
Ammonium paratungstate has the chemical formula (NH₄)₁₀[H₂W₁₂O₄₂]·4H₂O — or more commonly simplified as (NH₄)₁₀W₁₂O₄₁·5H₂O. It is a complex polyoxotungstate salt: twelve WO₄ tetrahedral units are linked into a large polyanion (the 'paratungstate B' anion), balanced by ten ammonium cations and crystallisation water.
In practice, APT is a white to slightly yellowish crystalline powder. The crystals have an orthorhombic structure and appear as irregular polygonal or tabular crystals under a microscope. Commercial APT is sold as a dry, free-flowing powder with a characteristic white appearance and mild ammonia odour (from surface ammonium groups). It is water-soluble (solubility approximately 100 g/L at 20°C in pure water) and decomposes on heating above approximately 150°C.
How APT is produced
APT is produced industrially by hydrometallurgical processing of tungsten-bearing raw materials — primarily scheelite ore (CaWO₄) or wolframite ore ((Fe,Mn)WO₄), or from secondary sources such as tungsten scrap and hard metal recycling concentrates.
The typical process route: (1) Ore is digested in NaOH (caustic soda) or hydrochloric acid to produce a sodium tungstate or tungstic acid solution. (2) The solution is purified by solvent extraction or ion exchange to remove impurities (Mo, As, P, Si). (3) The purified ammonium tungstate solution is concentrated and crystallised — APT precipitates as the product.
China produces approximately 80% of global APT from domestic scheelite and wolframite deposits in Hunan, Jiangxi, and other provinces. Other significant producers include Russia, Bolivia, Rwanda, and Vietnam. APT is the form in which tungsten is traded internationally as the primary refined intermediate — prices are quoted monthly in the Metal Bulletin/Fastmarkets APT European benchmark (USD/mtu, metric tonne unit of WO₃).
Key specifications and what they mean
WO₃ content (tungsten trioxide equivalent): APT is characterised by its WO₃ content, typically minimum 88.5% by weight. This is the primary assay — it reflects how much tungsten-as-oxide is present per unit weight, accounting for the ammonium and water of crystallisation. At exactly the theoretical formula, WO₃ content calculates to approximately 89.0%. Commercial specification min. 88.5% is standard.
F.S.S.S. grain size / crystal size: APT crystal size affects the downstream calcination and reduction behaviour. Larger crystals (F.S.S.S. 50–70 µm) produce coarser intermediate WO₃ and thus coarser W metal powder after reduction. Finer crystals (F.S.S.S. 20–40 µm) produce finer W powder. Specification is typically 30–70 µm.
Carbon content: Carbon in APT (from organic residues in the hydromet processing) is a critical impurity — max. 0.0010% (10 ppm) is the typical specification. Higher carbon causes carbon contamination of the W metal powder produced during reduction, making it unsuitable for electrical contact and other high-purity applications.
Impurities: Key impurities specified include Mo (molybdenum, max. 0.005–0.020%), As, Si, Fe, Na, K, and P. Molybdenum is the most critical — it is chemically similar to tungsten and very difficult to separate completely. Mo in APT carries through to W metal powder and WC powder, affecting electrical conductivity in W-Cu contacts and mechanical properties in WC-Co hardmetal.
Apparent density: Typically 30–55 g/in³ (approximately 1.8–3.4 g/cc) for commercial APT, measured by Hall flowmeter.
Uses of APT
Hydrogen reduction to tungsten metal powder: The largest use. APT is calcined (heated in air at 500–800°C) to tungsten trioxide (YTO, WO₃, or BTO, WO₂.₉), which is then reduced in hydrogen at 750–1,100°C to tungsten metal powder. This is the starting point for all WC powder production, tungsten heavy alloy, and tungsten electrical contacts.
Catalyst production: APT is used as a tungsten source in heterogeneous catalyst preparation — particularly HDS (hydrodesulphurisation) catalysts used in oil refinery operations, where W-Ni or W-Co active phases on alumina supports remove sulphur from petroleum fractions. APT is dissolved and impregnated onto the support before calcination and activation.
Electrochromic materials: WO₃ films produced from APT precursors are the active layer in electrochromic windows (smart glass), which change transparency when a small voltage is applied. Research into electrochromic devices uses APT-derived WO₃ nanoparticles and thin films.
Fireproofing and corrosion inhibition: APT and sodium tungstate (produced by dissolving APT in NaOH) are used as fire-retardant treatments for textiles and as corrosion inhibitors in water treatment and metalworking fluids.
Handling, storage, and safety
APT is classified as a nuisance particulate — inhalation of fine tungsten compound dust should be minimised using appropriate respiratory protection. It is not acutely toxic at typical industrial exposure levels but tungsten compounds have limited long-term toxicological data. APT is not classified as a hazardous material for shipping (non-DG) under IMDG for sea freight at commercial concentrations.
Storage: APT should be stored in sealed containers in a dry environment below 40°C, away from acidic vapours and heat sources. On heating above 150°C, it begins to lose ammonia and water of crystallisation, converting toward tungsten oxides — this decomposition is irreversible. Shelf life in sealed original packaging under correct storage conditions is typically 12–18 months with no change in specification.
Jayanth Tungsten supplies APT in 25 kg net fibre drums or bags with inner moisture-barrier liner, minimum purity WO₃ 88.5%, carbon max. 0.0010%, with full CoA and GHS/REACH SDS for international export.