Tungsten (W) is a remarkable element known for its exceptional physical characteristics. It possesses the highest melting point of any pure metal, reaching approximately 3,410 °C, and exhibits an extreme density comparable to gold. This combination of properties makes it indispensable in modern industry, particularly for applications requiring high heat resistance and strength. Tungsten is a fundamental material in the production of high-speed tool steels, lighting filaments, and durable tungsten carbide for cutting and drilling tools.
Locating and Extracting the Raw Ore
Tungsten is not found in metallic form but is bound within mineral compounds in geological deposits. The two primary tungsten-bearing minerals that are economically viable to mine are Wolframite (an iron and manganese tungstate) and Scheelite (a calcium tungstate). These deposits typically appear as veins, stockworks, or skarn deposits. The extraction method depends heavily on the deposit’s geological structure.
Underground mining is employed for vein deposits, where the mineralization is concentrated in narrow seams deep beneath the surface, involving drilling, blasting, and hauling rock out through tunnels. Conversely, open-pit mining is used when the tungsten mineralization is disseminated over a large, shallower area. This method involves excavating large volumes of rock from the surface downward in a series of benches.
Physical Concentration of Tungsten Ore
Once extracted, the raw ore undergoes physical concentration, or beneficiation, at the mine site to separate valuable tungsten minerals from the non-valuable rock (gangue). This process is necessary because raw ore contains a very low percentage of tungsten. The first step is comminution, where large rocks are reduced in size through crushing and grinding using equipment like jaw crushers and ball mills.
Tungsten minerals, especially Wolframite, have a high specific gravity, or density, which is utilized in the most common technique: gravity separation. Equipment such as shaking tables, jigs, and spiral concentrators exploit this density difference, separating the heavier tungsten minerals from the lighter gangue. For finer particles and Scheelite, which do not separate effectively by gravity, flotation separation is employed. Flotation involves grinding the ore finely and introducing chemical reagents that selectively attach to the tungsten minerals, making them hydrophobic. Air is bubbled through the mixture, causing the mineral-coated particles to float to the surface in a froth that is skimmed off. Further cleaning steps, such as magnetic separation, remove impurities like iron oxides, resulting in a concentrate enriched to about 65% tungsten trioxide (WO₃) content.
Chemical Refining and Powder Production
The physical concentrate is not pure metal and must undergo chemical refining to remove remaining impurities and produce a high-purity tungsten compound. This stage often begins with pretreatment, such as roasting, to eliminate sulfide and arsenide contaminants. The concentrate is then chemically treated, typically through one of two main methods: alkaline leaching or acid digestion. In the alkaline process, finely ground ore reacts with a hot solution of sodium carbonate or sodium hydroxide, dissolving the tungsten to form sodium tungstate. Acid digestion uses hydrochloric acid to break down the scheelite concentrate, yielding solid tungstic acid.
Both methods separate the tungsten into a liquid solution, leaving behind insoluble impurities. The resulting solution is subjected to multiple purification steps, including solvent extraction or crystallization, to achieve a high-purity product. The final intermediate product is almost always Ammonium Paratungstate (APT), a white crystalline salt. APT is the primary precursor for most tungsten products and is converted to pure tungsten oxide (WO₃) by heating it to about 600 °C. The final step to produce pure metal powder involves reducing the tungsten oxide in a furnace by passing hydrogen gas over it at temperatures ranging from 550° to 850 °C, which yields elemental tungsten powder.
Environmental and Worker Safety Considerations
Tungsten mining and processing generate significant volumes of waste that require careful management to mitigate environmental harm. Mine tailings, the finely ground waste rock from concentration, can contain residual tungsten and heavy metals, posing a risk of soil and water contamination. The use of chemical reagents, such as acids and bases during refining, necessitates stringent protocols to prevent accidental release and water pollution. Acid mine drainage is a concern, occurring when exposed sulfide minerals react with water and air to produce acidic runoff. Companies must implement water recycling systems and efficient tailings management. Worker safety focuses on exposure to dust and chemicals, especially during crushing, grinding, and powder production. Inhalation of fine tungsten dust or fumes can cause respiratory issues, highlighting the need for proper ventilation and personal protective equipment.