Ceramics & Refractories/Insulation: Micro-Porous Castable Refractory for Molten Aluminum - Part 1
The aluminum industry is the second largest producer of metal in the United States, behind the iron and steel industry, and the largest consumer of energy on a per-weight basis. Smelting, which utilizes the electrolytic reduction Hall-Héroult process, is the largest energy consuming operation.1 Process heating is required to melt, hold, purify, alloy, and heat treat the metal, and accounts for 25 percent of the energy consumed to manufacture aluminum.
Refractories are used to contain thermal processes and provide protection for personnel and furnace structures. The refractory must possess several beneficial properties including resistance to various wear mechanisms inherent to the process, and thermal conductivity that suits the energy needs of the process. Historically, dense, high thermal conductivity refractories have been used as working linings to provide wear resistance and process containment. Secondary layers of less dense, more thermally insulating materials have been used to reduce heat flux through the lining.
Industry ProblemsThe energy efficiency of secondary aluminum production is affected greatly by the properties of refractories used. This is true not only at installation but also throughout a refractory's working life. This efficiency, or lack of it, translates directly to process cost for aluminum producers. Using annual production tonnages, the cost per BTU of various energy sources, and estimates of energy consumption by the industry, the aluminum industry spent over $500 million on fuel for production operations in 2000.2 Every 1 percent improvement in energy efficiency would result in more than $5,000,000 in annual savings.
It is also important to consider the decrease in greenhouse emissions that result from burning less fuel. Greater than 3.75 million metric tons of CO2 were emitted by aluminum production processes in 2000.3 The same 1 percent improvement in efficiency would result in a reduction of more than 96,500 MT of CO2.
In February 2003 the aluminum industry released its technology roadmap.4 Among the things this document highlights are the process areas where the need for technological improvement is greatest. One of the biggest recognized short falls listed is, "Low fuel efficiency in melting and holding furnaces; furnaces are not optimized for scrap heating and waste heat recovery."
In the roadmap, six R&D needs were identified. One of these is energy-efficient technology. Within this category, the top priority is design and development of a furnace that improves cost effectiveness and improves fuel/energy efficiency.
Production IssuesUndesirable reactions sometimes occur within a furnace, either within the refractory lining or adjacent to it within the vessel. These can reduce the service life of the refractory lining, impede production, or cause unscheduled maintenance outages.
Corundum Formation - Corundum (alpha alumina or -Al2O3) is a naturally occurring oxide of aluminum that forms as a result of the oxidation of liquid aluminum in a vessel. Once corundum crystals reach a certain size and volume, they interfere with the furnace's physical operation. They can impede the charging process, cause doors and ports to close improperly or incompletely, deflect burner flame, and insulate the metal from the desired heat transfer. Manufacturers remove deposited corundum by physically scraping the vessel walls with a mechanical arm or other device.
This scraping can damage the refractory in several ways. First, the corundum is strongly bonded to the refractory surface. Removing the corundum typically removes the surface of the refractory, thereby damaging the lining. Second, corundum penetration into and formation within the lining will cause refractory stress fractures, making it susceptible to thermal cycling or thermal shock, and increasing the likelihood that mechanical cleaning will damage it.
Air is the primary source of oxygen for the formation of corundum. Since the vessels are frequently opened for charging, and because of poor sealing around doors and ports, it is difficult to completely eliminate available oxygen. A second oxygen source is the reduction of SiO2 (silica), which is often a refractory lining component. Aluminum that penetrates the porous refractory can reduce the silica, making oxygen available for the formation of corundum within refractory pores. Physical damage to the refractory and the continued reduction of its thickness allows aluminum to penetrate further, continuing the destructive process. This increases the thermal conductivity of the refractory, which wastes heat. The shear stress of the differential expansion causes the material to fracture. These fractures allow further penetration, and repetition of this cycle will cause the refractory lining to wear to the point of having to be replaced.
Spinel Formation - A spinel mineral phase can be formed by the reaction of aluminum and magnesium in the presence of oxygen (O2) or of alumina (Al2O3) with periclase (MgO). It occurs naturally as MgAl2O4, (MgO-Al2O3). Certain alloys of aluminum, such as 7075, contain magnesium. When these are exposed to free oxygen, spinel can form. This crystal expands on formation and if this occurs within the pores of a refractory, the ceramic can fracture. It is more commonly present near door seals and ports, where free oxygen is available due to atmospheric leakage. Formation of spinel can cause the same operational problems as corundum.
Development of Castable RefractoryThe development of a new castable refractory was undertaken. It was focused on thermal efficiency and suitability for molten aluminum contact. Ease of installation was a secondary objective. In addition to laboratory work, the development process also involved service trials of the product at cooperating manufacturing facilities.
The aggregate was chosen based on its specific form and mineralogy. Once it was characterized to determine chemistry, mineralogy, and porosity, it was used to create a prototype castable product. The aggregate was screened into specific size distributions and additional fractions were utilized to form bond phases and enhance its resistance to aluminum penetration. Several iterations of the formulation were evaluated, and one best met the necessary criteria. That formulation was designated "D-1" and the results for that mix are listed below:
- 60 percent porosity, with an average effective pore diameter of less than 5 microns, creating a surface tension differential with aluminum that prohibits penetration and provides excellent insulating capacity.
- Less than 0.30 percent silica (SiO2), eliminating one source of oxygen that can contribute to corundum formation.
- A coefficient of thermal conductivity of less than 5 BTU-in/hr-ft2-°F at normal operating temperatures, allowing for significant reductions in energy consumption.
- Homogeneous mineralogy resulting in uniform resistance to aluminum penetration and thermal shock resistance comparable to amorphous silica compositions.
- Strength approaching that of low cement castables, and exceeding the strengths of most ceramic fiber boards, limiting degradation of the lining because of mechanical cleaning operations.
- Conventional castable installation requiring only blending with water, vibration casting, and a linear temperature increase for dry out.
Physical Properties Compared to Industry StandardsTable 1 compares key properties of D-1 to industry standard products.
Coefficient of Thermal Conductivity - An independent testing organization used the Hot Wire method for testing the Coefficient of Thermal Conductivity (K Factor) of the formulation. Those data were compared to typical data for a low-cement castable composed of 65 percent alumina with aluminum penetration inhibitors added and to a low-cement amorphous silica- (SiO2) based castable. The K Factor relates directly to the predicted heat flux calculated for any lining configuration relative to operating conditions. A lower K Factor indicates better insulating capacity, and a reduction in heat flux through the refractory lining. Thermal conductivity measurements for D-1 will be given in Part 2 of this article.
Silica (SiO2) Content - Since silica is a source of oxygen and can contribute to the formation of corundum, lower silica content will reduce the likelihood of corundum formation. Although corundum can form on the refractory surface from other oxygen sources, a working lining refractory with virtually no silica will not develop internal corundum, and surface corundum will be easily removed during the cleaning process.
Density - Comparison of the quantity of refractory needed to line a molten aluminum vessel can be calculated if the volume of the lining and the density of the refractory are known. Although strength and thermal conductivity properties are often correlated to density for standard refractory materials, the same correlation cannot be made for refractories based on micro-porous aggregate. D-1 is approximately 60 percent porous, resulting in a very low K Factor compared to products of similar density, but has a high strength bond phase making it much stronger than refractories of similar porosities.
Maximum Use Temperature - Additives typically blended with alumina-silica refractories to enhance their resistance to aluminum penetration and adhesion limit the recommended service temperature to around 2,600°F. Amorphous silica products will undergo crystallization if they cycle to temperatures above 2,000°F. D-1 is comparable to the alumina-silica products in terms of service temperature, but compares favorably to silica-based systems in terms of thermal cycling resistance and density.
Laboratory TestingA preliminary iteration of D-1 was subjected to an aluminum immersion test. In this test a sample bar of material is submersed for 72 hours in 7075 alloy at 850°C. The sample is then removed from the molten aluminum and cross-sectioned to allow visual examination of penetration into the sample. Figure 1 shows the cross section of D-1 on the bottom and the cross section of a typical 60% alumina composition containing no penetration inhibiting additives on the top.
The standard sample shown in the top image shows discoloration as a result of the material's exposure to a reducing environment. This gives a black coloration but does not affect the refractory properties of the material. However, penetration of aluminum into the sample is evident around the edges. In addition, penetration into forming cracks was observed. The bottom image shows that D-1 is not penetrated, nor is there a reduction reaction. The aluminum that is on the surface of the material does not adhere and is easily removed. This favorable result allowed us to progress to field testing service applications.
Field Trial TestingBased on the lab testing results, several customers agreed to field trials of the material. Because D-1 offered the best combination of properties as tested, the field trials were run with this mix.
Trials were run in three different application areas:
- Aluminum holders for die cast operations (two trials at different customers).
- Over-the-road crucibles at a recycling operation providing molten metal to another production facility.
- High temperature reaction vessel used to create specialized aluminum alloys.
Two separate customers agreed to test the refractory in aluminum holding furnaces. Both used electrically heated furnaces of slightly varying geometry.
Case Study - Efficient Holding FurnaceCustomer 1 agreed to design and cast a single furnace. Both in the choice of lining materials and in the electrical heating system used, the design focused on energy efficiency. Its design and fabrication were contracted out to a refractory installer, who utilized proprietary heating elements in combination with the new refractory formulation as the ‘working' or ‘hot face' lining for the vessel. The lining was approximately 4 inches thick. Thermocouples were placed at several different depths within the cast refractory lining in order to monitor in-service temperature performance.
This furnace was designated DOE3. Because its design was atypical, there were no means of comparing the furnace's temperature performance to a ‘typical' one on an apples-to-apples basis. Furnace shell temperatures were taken to evaluate the overall performance of the lining, and to confirm steady state heat flux calculations made during the engineering process. Figure 2 is a picture of the data collection device mounted on the furnace. Figure 3 shows the temperature profile of the lining during temperature ramp up to service conditions and subsequent performance with aluminum in the vessel. The observed spikes are a result of varying metal levels as metal was removed from the furnace to feed the die casting machine, and subsequently replenished from a transfer ladle.
The shell temperature of the test furnace (DOE3) was measured at an average of 38.5°C while in operation, as compared with this customer's typical furnace shell temperatures that range from 93.3 - 204.4oC. This represents a 58.7 - 81.1 percent improvement. However, not all this improvement was attributed to the lining, since the design of this furnace used a different back-up lining configuration compared to a standard furnace. Unfortunately, the furnace was taken off line only after a few weeks in service due to a malfunction in the heating elements. This did provide a unique opportunity to examine the inside of the furnace at the contractor's shop. Figures 4a and 4b show the interior of the furnace with aluminum on the refractory wall and how easily the aluminum is removed. For standard refractory linings, removal of the aluminum often damages the refractory.
After the repair was made to the heating element system, this furnace was placed back into service. In the short term, observed shell temperatures show an improvement in efficiency but no quantitative conclusions can be drawn on the specific contribution of D-1.
In Part 2 of this article we will review the industry problems that prompted us to develop the castable D-1 refractory lining. Additional field trials will also be presented.
References:1. U.S. Energy Requirements for Aluminum Production, (Washington, D.C.:U.S. Depart-ment of Energy, February 2003 v 1.1). The report was prepared for the Energy Efficiency and Renewable Energy Industrial Technologies Program.
2. U.S. Department of Energy Grant Number DE-FG02-04ER84118, Westmoreland Advanced Materials, LLC pages 3,4, April, 2005.
3. Ibid, page 4.
4. Aluminum Industry Technology Roadmap Washington, D.C., The Aluminum Association, February 2003). Listed priority research areas for primary and secondary aluminum.
Additional related material may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: aluminum, refractory, alumina, corundum, spinel, silica.