Tungsten (W) is a metal prized for its extreme physical properties. It has the highest melting point of all known metals, reaching approximately 3,422°C (6,192°F), and a density comparable to gold. This combination of heat resistance and high density makes tungsten indispensable in manufacturing. Tungsten is primarily used in cemented carbides, which are second only to diamond in hardness and are used extensively in cutting tools. It is also used in heavy alloys for applications like radiation shielding and counterweights. Transforming raw tungsten ore into a dense, usable metal is a complex, multi-stage chemical and metallurgical undertaking that requires careful control over chemical purity and powder characteristics.
Sourcing and Initial Concentration of Ores
Tungsten is not found in pure metallic form but is bound within mineral ores, primarily scheelite (\(CaWO_4\)) and wolframite (\((Fe,Mn)WO_4\)). Wolframite, an iron and manganese tungstate, is often associated with tin ore, while scheelite, a calcium tungstate, typically occurs in high-temperature veins. These raw ores contain a very low percentage of tungsten, often less than 1% of the total rock mass, necessitating initial physical concentration after mining.
The first step after crushing the mined rock is beneficiation, which separates the heavy tungsten-bearing minerals from the lighter waste rock. Gravity separation techniques are highly effective due to tungsten’s density. Equipment such as shaking tables and jigs wash the crushed material, allowing the denser tungsten minerals to settle out.
For fine-grained scheelite, flotation separation may be employed, using chemical reagents to make the mineral particles adhere to air bubbles. These physical processes significantly increase the tungsten content, resulting in a concentrate containing 60% to 75% tungsten oxide (\(WO_3\)). This concentrate is the starting point for subsequent chemical purification.
Chemical Refinement: Producing High-Purity Tungsten Compounds
The concentrated ore still contains impurities, such as phosphorus, arsenic, molybdenum, and tin, which must be removed before the metal can be produced. The standard intermediate product for tungsten manufacturing is Ammonium Paratungstate (APT), a white crystalline salt. The purity of the final tungsten metal is directly related to the purity of the APT used.
One common method for producing APT is alkaline digestion, where the concentrated ore is treated with a basic solution, such as sodium carbonate or sodium hydroxide, under high heat and pressure. This process dissolves the tungsten, forming a soluble sodium tungstate solution while leaving many impurities behind in a solid residue. The resulting solution then undergoes purification stages, including precipitation, filtration, and solvent extraction, to remove contaminants.
Alternatively, an acid-leaching process uses strong hydrochloric acid to break down scheelite concentrate, precipitating the tungsten as solid tungstic acid. This tungstic acid is then washed and dissolved in an ammonia solution to form ammonium tungstate. In both processes, the final step involves crystallizing the purified ammonium tungstate solution to yield APT crystals, often reaching purities of 99.9% or higher.
Hydrogen Reduction: Creating Tungsten Powder
The refined APT must first be converted into an oxide form, typically yellow tungsten trioxide (\(WO_3\)) or a blue tungsten oxide, by heating the APT in a furnace above 250°C. This oxide powder is the precursor to the final metal powder.
The tungsten oxide is then chemically transformed into metallic tungsten powder (W) using hydrogen reduction. The reaction occurs in specialized furnaces, often using a flow of hydrogen gas (\(H_2\)) at high temperatures. The chemical reaction for the trioxide is \(WO_3 + 3H_2 \rightarrow W + 3H_2O\).
The reduction typically takes place in multiple temperature zones within the furnace, ranging from 550°C to 850°C. Controlling the temperature and hydrogen flow rate is essential, as these parameters directly influence the characteristics of the resulting tungsten powder, such as particle size. This control determines the quality and final application of the consolidated metal product.
Consolidation and Forming Usable Metal
Tungsten powder cannot be formed into a useful shape by traditional melting and casting methods because its high melting point makes that process impractical and inefficient. Instead, the powder is converted into a dense, solid form using powder metallurgy. This process begins by loading the fine tungsten powder into a mold and applying immense pressure to create a pre-formed object called a “green compact.”
The green compacts are then subjected to sintering, a high-temperature treatment. Sintering is performed in a reducing atmosphere, typically flowing hydrogen, at temperatures up to 3,050°C, which is just below the metal’s melting point. During this heating, the individual powder particles fuse together, and the compact shrinks significantly, achieving high density and strength.
This densification transforms the porous compact into a solid, bulk metal product, often in the form of rods, bars, or billets. The resulting tungsten metal can then be further processed by forging, rolling, or drawing into its final shape, ready for use in applications like filaments, heavy-duty machinery, or specialized electronic components.