How is Tungsten Extracted?
How is Tungsten Extracted?
Tungsten is a unique metal valued for its exceptional physical and mechanical properties. With the highest melting point of all metals along with high density, strength, and hardness, tungsten is essential for numerous industrial applications ranging from light bulb filaments to radiation shielding.
Tungsten rarely occurs naturally in its pure form. Rather, it is found in chemical compounds like tungsten oxides and tungstates embedded in mineral ores. Extracting tungsten metal from these ores is a complex, multi-step process. Two primary tungsten ores are wolframite and scheelite, which contain tungsten bound with iron/manganese and calcium, respectively.
To produce pure tungsten metal, these ores must go through an array of physical and chemical extraction processes. Crushing, grinding, gravity separation, magnetic separation, froth flotation, roasting, and high temperature hydrogen reduction are among the key techniques used to first concentrate the tungsten minerals and then separate out impurities from the matrix.
Continued advancements in tungsten extraction aim to improve efficiency and reduce environmental impacts as global demand for this unique metal keeps rising.
Role of Scheelite and Wolframite in Tungsten Extraction
Tungsten is a critical metal that is widely used in various industries, including aerospace, automotive, electronics, and defense. It has exceptional properties, such as high melting point, hardness, and density, making it an essential component in many applications. However, the extraction process of tungsten is complex and requires several steps to obtain pure tungsten metal. Scheelite and wolframite are two primary minerals from which tungsten is extracted, playing a crucial role in this process.
Scheelite is calcium tungstate (CaWO4), while wolframite is an iron-manganese tungstate [(Fe,Mn)WO4]. Both minerals are important sources of tungsten, but they differ in their composition and properties. Scheelite is usually found in low-temperature hydrothermal veins, while wolframite is commonly associated with granitic intrusions.
The extraction of tungsten from scheelite and wolframite involves several stages, starting with mining and ore concentration. In the mining process, the ore is first drilled and blasted to loosen it from the surrounding rock. It is then loaded onto trucks and transported to the surface for further processing. The ore undergoes crushing and grinding to reduce its size, facilitating the subsequent concentration process.
The concentration process is carried out to separate the valuable minerals from the gangue (unwanted material). Froth flotation is the most commonly used method for tungsten ore concentration. In this process, the crushed ore is mixed with water and various chemicals, including collectors and frothers. Air bubbles are then injected into the mixture, which attach to the desired mineral particles and float them to the surface, forming a froth. The froth, containing scheelite or wolframite, is collected and further processed.
Once the froth is collected, it goes through additional steps to separate the tungsten-bearing minerals from other impurities. These steps include drying, magnetic separation, and gravity separation. Magnetic separation utilizes magnetic properties to separate magnetic minerals from non-magnetic ones, while gravity separation exploits differences in density to separate heavy minerals from lighter ones.
After the separation process, the tungsten-bearing minerals are further processed to obtain pure tungsten metal. One common method is roasting, where the minerals are heated in the presence of air or oxygen. This step removes impurities and converts tungsten minerals into tungsten oxides. The oxides are then reduced using hydrogen or carbon, resulting in the formation of tungsten metal powder.
Scheelite and wolframite play a critical role in tungsten extraction due to their high tungsten content. Scheelite typically contains 60-70% tungsten, while wolframite can have a tungsten content ranging from 20% to 30%. Therefore, the presence of these minerals significantly contributes to the overall tungsten yield.
Moreover, scheelite and wolframite are also valuable sources of other metals, such as calcium, iron, and manganese. These by-products can be extracted and utilized in various industries, further enhancing the economic value of tungsten extraction.
Scheelite and wolframite are essential minerals in the extraction of tungsten. They are the primary sources of tungsten metal and contribute significantly to the overall tungsten yield. The extraction process involves several stages, including mining, ore concentration, and purification. Froth flotation, magnetic separation, and gravity separation are commonly used methods to separate the tungsten-bearing minerals from impurities. The extracted minerals are further processed to obtain pure tungsten metal, which is highly sought after for its exceptional properties.
What was tungsten originally called?
Tungsten was originally known as wolfram. The name "wolfram" originated from the mineral wolframite, which is one of the main ores of tungsten. The word ""wolfram"" itself has an interesting history. It comes from the Swedish words ""wolf"" (meaning wolf) and ""ram"" (meaning foam), referring to the difficulties miners faced when extracting tungsten from the ore.
How is Tungsten Extracted from the Earth?
The Tungsten Mining Process
Tungsten is extracted from the earth through both surface and underground mining techniques. Surface mining methods like open-pit mining allow miners to access shallow tungsten deposits by digging large open pits in the earth. Underground mining involves tunneling deep below the surface to reach buried tungsten ore bodies. The method used depends on factors like the location and depth of the ore deposit.
Heavy machinery like bulldozers, excavators, and trucks are utilized in tungsten mining to remove and transport large volumes of rock and ore. Techniques like drilling and blasting are used to break up ore deposits. Modern mining equipment allows for more efficient and higher-volume extraction. Strict safety protocols protect miners from hazards like rockfalls, explosions, and toxic dust.
Tungsten mining presents environmental challenges due to the waste rock and tailings it generates. Proper waste disposal systems must be in place to prevent contamination of surrounding areas. Careful monitoring of mining operations is necessary to minimize ecosystem disruption. Some mines aim to restore previously mined sites to their original ecological state through remediation efforts.
How is Tungsten Mined?
Open-Pit Mining
Open-pit mining involves stripping surface layers of soil and rock with heavy machinery to expose the tungsten ore deposit underneath. This method is advantageous for ore deposits located close to the surface, generally 0-200m deep. It allows for high productivity and low operating costs compared to underground mining.
Underground Mining
Underground mining is used to access deeper tungsten deposits, usually 200-1200m below the earth's surface. Tunnels, shafts, and ramps are constructed to allow workers, equipment, and ore to be transported to and from the underground mine. This method poses greater safety risks and is more expensive than open-pit mining.
Environmental Considerations
Proper environmental management systems are necessary during tungsten mining to mitigate ecosystem damage. Tailings dams safely contain mining waste rock and prevent sediment runoff. Air and water quality monitoring ensures contamination is minimized. Some mines aim to restore biodiversity through re-vegetation, wetland construction, and soil rehabilitation post-mining.
Tungsten Extraction Methods
Crushing and Grinding of Tungsten Ores
The first step in the processing of tungsten ore is crushing. This involves breaking the ore into smaller pieces to facilitate the extraction of the desired mineral. In the case of tungsten ores, the ore minerals often appear in veins where the valuable tungsten-bearing minerals are dispersed within a host rock. Crushing the ore helps to expose the valuable minerals, making it easier to separate them from the gangue, or waste materials.
There are several methods of crushing tungsten ores, depending on the desired outcome and the characteristics of the ore. One common method is jaw crushing, where large chunks of ore are fed into a stationary jaw crusher. The jaws compress the ore against a fixed plate, breaking it into smaller pieces. Another method is cone crushing, which uses a cone-shaped crushing head that gyrates inside an inverted, cone-shaped crushing chamber. This method is particularly useful for finer crushing and for ores with a higher degree of hardness.
After the ore has been crushed, the next step is grinding. Grinding involves reducing the crushed ore to a fine powder, which is then subjected to further processing. The purpose of grinding is to liberate the valuable minerals from the gangue, as well as to increase the surface area of the ore for chemical reactions to take place.
Various types of grinding mills are used for the grinding of tungsten ores. Ball mills are commonly used, where the ore is rotated along with steel balls, causing the balls to fall back into the mill and onto the ore, thereby crushing and grinding it. Rod mills are similar to ball mills but use long rods instead of steel balls for grinding. These rods grind the ore by tumbling within the mill, causing impact and attrition between the ore particles.
Once the ore has been ground to a fine powder, it can be subjected to further processing steps, such as flotation or leaching, to separate the valuable minerals from the gangue. Flotation involves adding chemicals to the ground ore, which selectively attach to the desired mineral particles and float them to the surface, where they can be collected. Leaching involves using chemicals to dissolve the valuable minerals from the ground ore, leaving behind the gangue.
Gravity and Magnetic Separation Techniques
Two primary methods used for tungsten extraction are gravity and magnetic separation. These techniques rely on the differences in specific gravity and magnetic properties of minerals to separate them efficiently.
Gravity separation is a method that utilizes the force of gravity to separate minerals based on their specific gravity. In tungsten ore processing, this technique is used to separate wolframite from impurities such as quartz and cassiterite. Wolframite, being heavier than the impurities, settles at the bottom when mixed with water in a jig machine or spiral chute. The lighter impurities, on the other hand, float to the top and are washed away. This process can be repeated multiple times to ensure a high purity of tungsten concentrate.
Magnetic separation, on the other hand, uses the magnetic properties of minerals to separate them from a mixture. In the case of tungsten extraction, this method is used to separate ferromagnetic minerals such as magnetite and ilmenite from non-magnetic minerals such as scheelite and cassiterite. A magnetic separator is used to generate a magnetic field that attracts the magnetic minerals while repelling the non-magnetic ones. The magnetic minerals are then collected and separated from the non-magnetic minerals.
Both gravity and magnetic separation techniques have their advantages and limitations in tungsten extraction. Gravity separation is relatively simple and cost-effective, requiring minimal equipment and energy consumption. It is also environmentally friendly as it does not involve the use of chemicals. However, it may not be effective for separating fine particles or when the specific gravity differences between minerals are small.
Magnetic separation, on the other hand, is highly efficient for separating minerals with strong magnetic properties. It can achieve a high recovery rate and produce a clean concentrate. However, it requires sophisticated equipment such as high-intensity magnetic separators, which can be costly and consume significant energy. Additionally, magnetic separation may not be suitable for separating minerals with weak magnetic properties or when the magnetic field strength is not sufficient.
To optimize tungsten extraction, a combination of gravity and magnetic separation techniques is often employed. This approach takes advantage of the strengths of both methods to achieve a higher recovery rate and better concentrate quality. The ore is first subjected to gravity separation to remove the majority of impurities. The resulting concentrate is then further processed using magnetic separation to remove any remaining magnetic minerals. This two-step process ensures a high purity of tungsten concentrate suitable for further processing.
Flotation Extraction
One of the common methods used for tungsten extraction is flotation. Flotation is a process that utilizes the differences in surface properties of particles to separate valuable minerals from gangue minerals. In the case of tungsten ores, the main objective of flotation is to recover the tungsten minerals while suppressing the gangue minerals.
The flotation process involves several stages, starting with the crushing and grinding of the ore to obtain a fine particle size suitable for flotation. The finely ground ore is then mixed with water and various reagents, including collectors, frothers, and modifiers. These reagents help to selectively attach to the desired tungsten minerals, making them hydrophobic and enabling their separation from the hydrophilic gangue minerals.
Collectors are chemical compounds that selectively bind to the tungsten minerals, promoting their attachment to air bubbles in the flotation cell. Commonly used collectors for tungsten flotation include fatty acids, phosphonic acids, and alkyl sulfates. The choice of collector depends on the specific ore type and its associated gangue minerals.
Frothers are another crucial component in the flotation process. They help to generate and stabilize a stable foam or froth layer on the surface of the flotation cell, which facilitates the transportation of the hydrophobic tungsten minerals to the concentrate launder. Common frothers used in tungsten flotation include alcohols, glycols, and polyglycols.
Modifiers are chemicals added to the flotation process to modify the surface properties of the minerals, enhancing their selectivity and recovery. Modifiers can change the pH of the flotation pulp, increase the dispersion of reagents, or modify the surface charge of minerals. Sodium silicate, sodium carbonate, and lime are commonly used modifiers in tungsten flotation.
After the addition of reagents, the pulp is subjected to flotation in a flotation cell. Air bubbles are introduced into the cell, and the hydrophobic tungsten minerals attach to these bubbles and rise to the froth layer. The froth containing the tungsten concentrate is collected from the top of the cell, while the gangue minerals settle at the bottom as tailings.
Once the tungsten concentrate is obtained, it undergoes further processing to remove impurities and obtain a high-grade tungsten product. This may involve additional flotation stages or other beneficiation techniques such as gravity separation or magnetic separation.
The tungsten flotation extraction method has several advantages over other extraction methods. Firstly, it is a relatively simple and cost-effective process compared to other methods, such as gravity separation or magnetic separation. Additionally, it allows for the recovery of valuable tungsten minerals from low-grade ores that would otherwise be uneconomical to process using other techniques.
However, there are also challenges associated with the tungsten flotation extraction method. The presence of complex mineralogy in tungsten ores, especially the association of tungsten minerals with sulfide minerals, can affect the efficiency of the flotation process. The presence of impurities, such as iron and calcium, can also interfere with the flotation process and reduce its effectiveness.
To overcome these challenges, researchers and engineers are continually developing new and improved flotation reagents and techniques. These advancements aim to enhance the selectivity and efficiency of the tungsten flotation extraction method, ultimately improving the overall extraction process and reducing the environmental impact.
Extraction of Tungsten from Scheelite: The Sodium Method
Scheelite (CaWO4) is an important tungsten ore mineral that contains both tungsten and calcium. Extracting the tungsten from scheelite can be done using a sodium hydroxide or sodium carbonate solution in a process called alkaline digestion.
Decomposing Scheelite with Sodium Solutions
In the alkaline digestion process, the scheelite ore is first crushed and ground into a fine powder. This powder is then mixed with a hot, concentrated solution of sodium hydroxide or sodium carbonate. The sodium hydroxide or carbonate breaks down the crystal structure of the scheelite, converting the tungsten and calcium into water-soluble sodium tungstate and calcium compounds.
Key factors that affect the decomposition of scheelite using sodium solutions include:
- Temperature - higher temperatures improve dissolution
- Alkali concentration - more concentrated NaOH or Na2CO3 solutions work better
- Solid/liquid ratio - lower ratios give higher tungsten extraction
- Agitation - stirring or agitation accelerates the reaction
Producing Sodium Tungstate
After the alkaline digestion process, the resulting slurry contains sodium tungstate and other sodium compounds in solution. This solution is then separated from the insoluble ore residues and sent for further processing
To obtain pure sodium tungstate crystals, the solution is heated to evaporate excess water. As the volume decreases, sodium tungstate precipitates out due to its low solubility. This precipitation process requires precise temperature and pressure control to maximize yields.
The purified sodium tungstate can then be processed further to produce tungsten metal or tungsten compounds. Meanwhile, the leftover alkaline solution can be recycled back into the extraction process to decompose more scheelite ore.
Tungsten Extraction from Tungsten Oxide
Tungsten oxide, also known as tungsten trioxide (WO3), is the main intermediate compound in extracting pure tungsten metal from tungsten ores. Tungsten oxide is obtained from the ores through a series of crushing, grinding, flotation, and chemical processing steps.
The key process in extracting tungsten from tungsten oxide is reduction - removing the oxygen atoms from the compound. This can be done using hydrogen gas in a high temperature reaction to produce tungsten metal powder and water vapor as byproducts.
More specifically, the tungsten oxide powder is mixed with hydrogen and heated to temperatures between 600°C and 1,100°C in a furnace or retort. The extreme heat provides the energy needed to break the tungsten-oxygen bonds and release the oxygen as water vapor.
Other reducing agents like carbon and carbon monoxide can also be used instead of hydrogen gas. However, hydrogen reduction is preferred because it produces higher purity tungsten metal.
After the reduction process, the tungsten metal powder goes through further heat treatment and consolidation processes like sintering and hot isostatic pressing to produce tungsten metal ingots, sheets, wires or other end products.
Extracting tungsten from tungsten oxide provides very pure tungsten suitable for applications like filaments, electrodes, heating elements, and radiation shielding where high purity levels are critical.
The hydrogen reduction method also allows for easier purification compared to extracting tungsten directly from its ores. Overall, this extraction process generates tungsten metal powder in an efficient, cost-effective and environmentally-friendly manner.