Tungsten, designated by the chemical symbol W, is a metal known for its extreme physical properties. It possesses the highest melting point of all known elements, reaching approximately 3,422°C (6,192°F), which makes it invaluable for high-temperature applications. This metal also has an impressive density of about 19.25 grams per cubic centimeter, comparable to gold and uranium. Manufacturing usable, pure tungsten metal is a complex, multi-stage operation involving rigorous chemical refinement and specialized powder metallurgy techniques. Achieving ultra-high purity is necessary to preserve tungsten’s unique performance characteristics.
Ore Extraction and Concentration
The manufacturing process begins with sourcing tungsten from its naturally occurring mineral deposits. The two primary ores are wolframite (an iron-manganese tungstate) and scheelite (a calcium tungstate). These ores are typically found in veins or scattered throughout the rock, making their initial extraction a labor-intensive process.
Once the ore is mined, it contains a low concentration of tungsten and must undergo a series of physical separation steps known as beneficiation to increase the tungsten content. This process usually starts with crushing and grinding the raw ore into fine particles. The goal of this size reduction is to physically liberate the tungsten-bearing minerals from the surrounding waste rock, or gangue.
The high density of tungsten minerals allows for the effective use of gravity separation techniques, such as shaking tables and jigs, which separate the heavier tungsten particles from the lighter impurities. For scheelite, which is often finely dispersed, a flotation process is frequently used, where chemical agents and air bubbles are introduced to selectively attach to and float the scheelite particles to the surface. Magnetic separation is also employed to separate the magnetic wolframite from non-magnetic materials. These physical steps yield a concentrated ore that is ready for the intense chemical purification stage.
Chemical Purification and Intermediate Compound Formation
The concentrated ore still contains various unwanted elements, such as molybdenum, phosphorus, arsenic, and silicon, which must be removed to produce high-purity tungsten metal. The process requires the chemical decomposition of the tungsten minerals to dissolve the tungsten, leaving most impurities behind. One common method is alkaline digestion, where the concentrate is reacted with a sodium carbonate or caustic solution at high temperatures and pressures in an autoclave to form a soluble sodium tungstate solution.
Alternatively, an acid leaching process may be used, particularly for scheelite concentrates, where hydrochloric acid dissolves impurities and precipitates a crude tungstic acid. Regardless of the initial decomposition method, the resulting solution must be meticulously purified through a sequence of steps. This purification often involves adding chemical agents to precipitate specific contaminants or adjusting the solution’s acidity (pH) to isolate impurities like silica.
A crucial purification step is solvent extraction, which uses an organic solvent mixture to selectively pull the tungsten ions from the aqueous solution. The tungsten is then stripped from the solvent using an aqueous ammonia solution, yielding an ammonium tungstate solution. This solution is then evaporated and crystallized to produce the high-purity intermediate compound known as Ammonium Paratungstate (APT). APT is the form from which over 75% of all tungsten products are derived, serving as the benchmark for a pure starting material.
Reduction to Tungsten Powder
The next stage involves converting the purified APT into pure metallic tungsten powder, which is the foundational material for all subsequent shaping processes. APT is first heated to above 250°C to decompose it, often in a reducing atmosphere, to produce one of the tungsten oxides, such as tungsten trioxide. This oxide, typically yellow or blue in color, is the direct precursor to the metal.
The conversion of the tungsten oxide into metal is achieved through a controlled chemical reaction called hydrogen reduction. This process involves passing a stream of hydrogen gas over the tungsten oxide powder inside a furnace at high temperatures. The hydrogen acts as a reducing agent, reacting with the oxygen in the tungsten oxide to produce pure tungsten metal and water vapor.
The reduction typically occurs in two stages, often beginning at temperatures around 500°C to 700°C to form tungsten dioxide, followed by a second stage at 700°C to 900°C to complete the reduction to pure tungsten powder. Precise control over the temperature, the flow rate of the hydrogen gas, and the moisture level in the furnace atmosphere is necessary. These parameters directly influence the size, shape, and distribution of the resulting tungsten powder particles, with finer powders generally resulting from lower temperatures.
Consolidation and Shaping
The pure tungsten powder is not melted and cast due to its exceptionally high melting point, which makes conventional methods impractical. Instead, the powder is consolidated and shaped using specialized powder metallurgy techniques. The first step is to compact the powder into a desired preliminary shape, known as a “green compact.”
This pressing is accomplished using high-pressure mechanical or hydraulic presses, applying pressures that can reach up to 300 megapascals (MPa). For complex or large shapes, isostatic pressing is used, which applies uniform pressure from all sides to ensure even density throughout the compact. The resulting compact is fragile and porous, lacking the required density and strength for final use.
The green compact is then subjected to sintering, a high-temperature thermal treatment that fuses the powder particles together without reaching the melting point. This process involves heating the compact in a controlled atmosphere, typically hydrogen or vacuum, to temperatures between 2,200°C and 2,800°C. Sintering shrinks the component significantly, increasing its density to over 90% of its theoretical maximum and giving it the characteristic strength of metallic tungsten. The sintered product can then be further refined through mechanical processing, such as swaging or forging, to modify its internal grain structure and achieve the final dimensions and properties required for specific industrial applications.