Industry Focus #mullite #alumina #silica #refractories #furnaces Understanding the Benefits of Electric Arc Furnace Technology

Minerals processed via electric arc furnace technology provide optimal purity, porosity and crystal size to meet the increasingly stringent needs of a range of industries.

hether a finishing process uses sandpaper or a grinding wheel, pres- sure blasting or thermal spraying, abrasive and ceramic grains and powders play an integral role in the appearance and performance of thousands of products that are used every day all over the world. WThese minerals, which are also used to produce ceramic parts and kiln furniture, through various size reduction and among other refractory products, undergo a meticulous production process in order powder processing steps to achieve the to meet the increasingly stringent requirements of various end-use applications. A material’s desired finished properties. vital aspect of that process is the electric arc furnace, which helps ensure the miner- Virtually any oxide material can be als’ optimal purity, porosity, and crystal size. processed in an electric arc furnace. Because the materials are melted and The Fusion Process actually reach the liquid state, the end Simply put, an electric arc furnace is a vessel that uses electricity to melt miner- result is a near-perfect fusion. Let’s take als or other materials. The process begins when the dry minerals are weighed and a look at mullite as an example. To pro- mixed together, then evenly distributed throughout the furnace via feed chutes. duce mullite, alumina and silica are put Power is supplied to the furnace by a transformer and graphite electrodes. The in the electric arc furnace and the elec- electrodes are lowered into the furnace, touching the materials, and an electric arc trodes produce the arc, heating the two is formed between the electrodes. The arc then melts the materials, resulting in a minerals and melting them together. liquid bath. After being poured from the furnace, the mixture cools within the mold as an aluminum silicate compound to form the mullite structure. The end product is Virtually any oxide material can a near 100% conversion of alumina and be processed in an electric arc silica into mullite. furnace. The Fusion Advantage Fused minerals see multiple benefits com- pared to those produced through other Temperatures in an electric arc furnace can be quite high, around 1,800-2,500°C, processes, such as sintering. Before a sin- depending on the melting point of the materials being fused. To avoid burn-through, tering process can even begin, the materi- the steel vessel is equipped with a water cooling system that chills the liquid material als need to be sized very carefully in order up against the shell and causes the formation of what is called a skull. The skull acts to ensure the desired reaction. The electric as a protective barrier between the molten liquid and the steel shell. arc furnace does not require such precisely Once the optimal chemistry is achieved in the melted material, it is poured into sized materials, therefore saving both time a mold and moved to a cooling area for 24 hours. The solidified ingot then goes and energy—and the resulting costs.

www.ceramicindustry.com • April 2015 33 ] Understanding the Benefits of Electric Arc Furnace Technology

A sintered material has more poros- prove to be quite aggressive, since the that was added during the size reduc- ity than a fused mineral, due to the cutting particles are larger. On the other tion process. Materials that don’t meet lower temperatures involved in the sin- hand, large crystals in a grinding mate- specifications can be sent through the tering process. Since materials in the rial with a fairly coarse grit (e.g., 24 or process again, eliminating scrap. fusion process are in their liquid state, 30 grit) would be difficult to achieve Quality control and assurance from porosity is reduced dramatically. In fact, via sintering, which typically results in start to finish is key to ensuring that the bulk specific gravity of a fused min- fairly small crystal sizes. the finished materials meet customers’ eral is very close to its theoretical spe- expectations. Raw materials are evalu- cific gravity, which is not the case with ated before they are sent to the electric a sintered material. Minerals can also arc furnace, as is the fused material Purity is another benefit of the elec- when it is poured out of the furnace and tric arc furnace technology. As men- be purified during again in the cooled state. Chemical and tioned previously, the fusion process physical tests are performed, depending results in a virtual 100% conversion the electric arc on the material(s), to determine factors of the minerals. If we look back at our such as purity, particle size and density. example of mullite, the same alumina fusion process. Visual inspections for color are per- and silica processed through sintering formed as well. would not be able to achieve the electric arc furnace’s 100% mullite end product. Custom Fusions In addition, a small percentage of unre- Some finished compositions might Companies can take advantage of acted alumina and silica could remain require the inclusion of 4-5 differ- opportunities to explore new, custom following sintering, creating waste and ent oxide minerals, which is difficult material formulations. A test furnace is process inefficiencies. to achieve through sintering. Blending used for initial tests to determine if the Minerals can also be purified dur- those oxides and then adding them to material in question is a feasible pros- ing the electric arc fusion process. the electric arc furnace to melt them pect for fusion. If the material melts and While the initial minerals are in their together results in a completely fused pours well, the process can be strategi- liquid state, other materials can be final material. cally scaled up and evaluated. added in order to increase purity or Other formulations might benefit During the scale-up process, the size change chemistry. Bauxite, for exam- from the low thermal conductivity pro- of the furnace increases from a smaller ple, is high in alumina but contains vided by alumina “bubbles,” which can test furnace to a larger experimental naturally occurring contaminants like serve as an ideal insulating material in furnace that melts the same material. silica, iron oxide and titania. Bauxite some applications. These bubbles can be The feasibility at that size is evaluated, is melted in an electric arc furnace in easily produced in an electric arc furnace including the required power input, the presence of carbon, which reduces by melting the alumina and pouring it chemistries, etc. If those results are the iron oxide down to an iron metal, through an air stream in such a way that satisfactory, a larger run (e.g., 50-200 and, since it has a higher specific grav- hollow spheres are formed. This process tons) will be performed in a production ity than the alumina, it settles to the is not possible with sintering. furnace. Again, all of the same tests are bottom of the furnace. Silica and some conducted to ensure that the finished of the titania in the bauxite are also After the Melt material meets the required specifica- reduced, increasing the alumina con- The cooled ingot of fused material tions—with no surprises. tent from 80-85% in the raw material can be 4-12 tons in mass, depending and up to 95-96% in the fused prod- on the size of the furnace and mold. Fusing a Winning Partnership uct. Sintering simply can’t facilitate From there, it has to be broken down Fused minerals produced in an electric these types of adjustments. Another and ground to the right size. The pro- arc furnace offer multiple benefits, includ- potential problem in sintering is con- cess begins when the ingot is dumped ing low porosity, high purity, and larger tamination from the furnace lining. onto a steel floor. An electromagnet is crystal sizes, compared to sintering and Electric arc furnace technology’s used to pick up a steel ball and drop it other processes. Partnering with a com- other main benefit lies in crystal size, repeatedly onto the ingot, breaking it pany experienced in electric arc furnace which can have a significant impact into smaller pieces. technology can provide not only a stable on the finished product—particularly When these pieces are small enough, supply of high-quality material for today’s in grinding materials. When the liquid they are sent through a series of crush- needs, but the opportunity to explore and material is poured into a mold, depend- ers and screens that eventually separate produce new material formulations for ing on the mold design, fairly large out coarse sizes, mid-sizes, and fine expanded success in the future. crystals can result. Large crystals of sizes. High-intensity rare earth magnets alumina in a grinding material would are used to remove any crushing iron For additional information, visit www.washingtonmills.com.

34 April 2015 • www.ceramicindustry.com