Tungsten (W) is an elemental metal with a unique combination of properties. It has the highest melting point of all metals, approximately 3,422 degrees Celsius (6,192 degrees Fahrenheit). This high thermal resistance, combined with its hardness and density, makes it valuable in demanding applications. Tungsten does not occur naturally in pure form, requiring complex industrial processes to extract and refine it from mineral compounds.
From Ore to Concentrates
Tungsten extraction begins with mineral ores like wolframite (iron, manganese, tungsten oxide) and scheelite (calcium tungstate). These ores undergo beneficiation to separate tungsten-bearing minerals from waste rock, or gangue. This involves crushing the raw ore into smaller pieces, then grinding it into a fine powder.
After grinding, physical separation methods concentrate the tungsten minerals. Gravity separation, using techniques like shaking tables or spirals, utilizes density differences. Flotation can also be used, where chemicals attach to tungsten particles, causing them to float while waste sinks. Magnetic separation may be applied, especially for wolframite, which is magnetic, further enriching the tungsten content before chemical processing.
Purifying Tungsten Compounds
After physical concentration, tungsten concentrates undergo chemical processing for higher purity. This involves dissolving tungsten from the concentrate using strong chemical solutions. Alkaline methods (e.g., sodium hydroxide) or acidic methods (e.g., hydrochloric acid) are common. The choice of reagent depends on the ore type and desired purity.
Once dissolved, tungsten is selectively precipitated as an intermediate compound, such as ammonium paratungstate (APT) or tungstic acid (H2WO4). This precipitation step is important for removing impurities not eliminated during initial physical separation. High-purity APT is a primary intermediate product in the tungsten industry, enabling further refinement into various materials.
Creating Tungsten Powder
The purified tungsten compound, typically ammonium paratungstate (APT), is converted into tungsten metal powder. This process begins by calcining the APT, heating it in a controlled environment to drive off ammonia and water, yielding tungsten trioxide (WO3). This yellow powder is the direct precursor to metallic tungsten.
Tungsten trioxide then undergoes reduction in a hydrogen atmosphere at elevated temperatures. Temperatures range from 500 to 1000 degrees Celsius, depending on the desired particle size and properties of the final powder. During this reaction, hydrogen acts as a reducing agent, reacting with oxygen in tungsten trioxide to form water vapor, leaving pure, elemental tungsten in a fine powder form. Control over temperature and hydrogen flow influences the particle size, shape, and distribution of the resulting tungsten powder, impacting the final solid tungsten product.
Forming Solid Tungsten
The fine tungsten powder is not yet in a usable solid form for most applications. Two primary methods are employed to consolidate this powder into solid shapes: powder metallurgy through sintering, and high-temperature melting techniques. Each method is suited for different applications based on the desired properties and scale of production.
Powder metallurgy involves pressing the tungsten powder into a specific shape, creating what is known as a “green” compact. This compact is then heated in a process called sintering, in a hydrogen atmosphere at temperatures below tungsten’s melting point, around 2,200 to 2,800 degrees Celsius. During sintering, the individual powder particles bond together and densify through atomic diffusion, forming a solid, yet often porous, body with increased strength and integrity. This method allows for the creation of near-net-shape components directly from the powder.
For applications demanding high purity or full density, tungsten can be melted and cast. Due to tungsten’s high melting point, specialized techniques are necessary, such as vacuum arc melting or electron beam melting. These methods create an intense heat source capable of liquefying the metal in a controlled, inert environment. The molten tungsten is then allowed to solidify, forming dense, solid ingots that can be further processed through conventional metalworking techniques like forging or drawing.