Minerals that are processed using electric arc furnace technology offer the right amount of porosity, optimal purity and crystal size to satisfy the highly stringent requirements of many industries.
Abrasives, ceramic grains and powders play a very important role in the way thousands of products appear and perform worldwide irrespective of whether the finishing process used is a grinding wheel, sand paper, thermal spraying or pressure blasting.
These minerals that are also used for producing kiln furniture and ceramic components go through a detailed production process to satisfy the strict requirements of a number of end-use applications. The electric arc furnace is a key part of the process helping to ensure the optimal porosity, purity and crystal size of minerals (Figure 1).
Figure 1. Minerals processed with electric arc furnace technology provide optimal purity, porosity, and crystal size to address the increasingly rigorous requirements of a number of industries.
The Fusion Process
An electric arc furnace is simply a large vessel in which minerals or other materials are melted using electricity. First, the dry minerals are weighed, then blended together and uniformly distributed all through the furnace with feed chutes. Using a transformer and graphite electrodes, power is supplied to the furnace. The electrodes are lowered into the furnace and come into contact with the materials resulting in an electric arc between them.
The materials are then melted by the arc causing a liquid bath. Electric arc furnace temperatures can be considerably high around 1800-2500°C, based on the melting point of the fused materials. In order to prevent burnthrough, the steel vessel is provided with a water cooling system, which cools the liquid material against the shell and helps create a skull. Hence, the skull behaves like a protective wall between the steel shell and the molten liquid.
On achieving the right chemistry in the melted material, it is transferred into a mold and then kept in a cooling area for 24h. In order to attain the desired finished properties of the material, the solidified ingot is subjected to several powder processing and size reduction steps. An electric furnace is capable of processing almost any oxide material.
Since the materials are melted and reach the liquid state, the final result is an almost-perfect fusion. An example of this is mullite. In order to obtain mullite, silica and alumina are added to an electric arc furnace, and when the arc is created by the electrodes, the minerals are heated and melted together. After transferring to the mold, the mixture is cooled and forms an aluminum silicate compound to create the mullite structure. The final product is a complete transformation of alumina and silica into mullite.
Benefits of Fusion
In comparison to products produced using other processes like sintering, fused minerals offer a number of advantages.
Before even commencing a sintering process, it is important to size the materials meticulously to obtain the required reaction. There is no need for this level of accuracy with fused materials, saving energy, time and the resulting costs.
A sintered material traditionally has much higher porosity than a fused mineral. Materials in the fusion process are in a liquid state, meaning they have a drastically lower porosity.
The bulk specific gravity of a fused mineral and its theoretical specific gravity are very close, which is not the case for sintered material.
Another advantage is that the fusion process causes 100% transformation of minerals. Considering the previous example of mullite, if alumina and silica were processed using sintering, it would have been impossible to achieve the same 100% mullite end product. In the case of sintering, there would be a small percentage of unreacted alumina and silica remaining, causing waste and process inefficiencies. In addition, while the minerals are in their liquid state, it is possible to add other minerals to improve the purity or change the chemistry. For instance, bauxite is high in alumina but includes contaminants such as silica, iron and titania. Melting of bauxite in an electric arc furnace in the presence of carbon results in reduction of iron oxide to an iron metal, and because it has a higher specific gravity when compared to alumina, it moves to the furnace bottom. A reduction of silica and certain amounts of titania in the bauxite also occur, improving the alumina content from 80-85% in the raw material and up to 95-96% in the fused product. These types of adjustments are simply impossible with sintering.
Crystal size can also have an effect on the finished product, especially in grinding materials. While pouring the liquid material into a mold it is possible to obtain considerably large crystals. Large alumina crystals in a grinding material may be aggressive considering that the cutting particles are bigger. However, large crystals in a grinding material having a coarse grit are tough to achieve with sintering, resulting in considerably smaller crystal sizes.
Certain finished products require the addition of four to five oxide minerals, which is difficult with sintering. Conversely, a completely fused final material is obtained by blending the oxides and adding them to the electric arc furnace.
Alumina bubbles, which offer low thermal conductivity, are a suitable insulating material in certain applications. These bubbles can be produced easily in an electric furnace by melting the alumina and pouring it through an air stream so that hollow spheres are created. With sintering this is not possible.
After the Melt
Based on the mold size, the cooled fused material ingot can be 4 to 12t in mass. It then needs to be ground to the optimum size. The ingot is first kept on a steel floor. A steel ball is repeatedly picked and dropped by an electromagnet onto the ingot. As a result, the ingot breaks into smaller pieces. When these pieces are very small, they are sent through several screens and crushers, eventually separating out coarse sizes, mid-sizes and fine sizes.
The removal of any crushing iron added at the time of the size reduction process is then performed with high-intensity rare earth magnets. Materials not complying with specifications are again sent through the process to eliminate scrap.
In order to ensure that the final product is compliant with customer specifications, quality control from the commencement of the process to the end is required. Raw materials are tested prior to sending them to the electric arc furnace, ands as is the fused material coming out of the furnace. Testing is again done after cooling. Physical and chemical tests are conducted based on the materials to determine particle size, purity and density. Visual inspections for color are also done.
The Product Development Service of Washington Mills enables customers to study new, custom material formulations. In order to determine if the material of interest can be fused, a test furnace is deployed for initial tests. In cases where the material melts easily and can be conveniently transferred, it can be strategically scaled up and studied.
It is possible to use larger-sized furnace shells in a larger furnace melting the same material. At that size, the feasibility is studied including the needed power input and chemistries. If satisfactory results are obtained, a larger run (e.g., 50-200 tons) will be conducted in a production furnace. All tests are performed to make sure the final product complies with required specifications.
In comparison to sintering and other processes, fused minerals in an electric furnace offer a number of benefits such as high purity, low porosity and large crystal sizes. Partnering with a company having experience in electric arc furnace technology can provide not just a stable supply of high-quality material for today’s needs, but also the chance to explore and create new material formulations for extended success in the future.
This information has been sourced, reviewed and adapted from materials provided by Washington Mills.
For more information on this source, please visit Washington Mills.