|Fig. 1. Depleted uranium casting|
Microwave metal-melting is a potentially disruptive technology due to its efficient use of electrical energy. The concept has been developed by MS Technology, Inc., an engineering company located in Oak Ridge, Tenn. As a result of the company’s research and development, an alternative to vacuum induction melting (VIM) equipment is being developed to significantly cut energy costs and operations time.
Microwave casters do not require the expensive vacuum equipment and infrastructure needed to support operations, as do other comparable industrial melters. A small microwave caster can easily reach temperatures exceeding 2000°C in less than 30 minutes and be operated using only 110 volt line current.
|Fig. 2. Depleted uranium alloy plate after rolling|
Microwave Metal-Melting History
Microwave metal-melting represents a technology that allows efficient, volumetric heating without a conventional radiant heat source. An object to be heated is placed inside an oven, and microwave energy is introduced. This energy is absorbed by the object, and heat is created as a result of this absorption. The object itself becomes the only source of radiant heat. The more absorbent an object is to microwave energy, the hotter it will become for a given amount of power applied. The technology represents a real potential benefit for metal-casting operations. It is the same benefit that slowly caused a revolution in home cooking when it was released to the public in 1967.
For the past five years, MS Technology has operated a full-scale microwave melter in a commercial foundry. The melter is used to test microwave casting technology under actual manufacturing conditions. To date, the melter has been used extensively for casting depleted uranium (Fig. 1) for federal and commercial clients in the U.S. and abroad. Depleted uranium is used for development purposes because it is an ideal surrogate for enriched uranium.
For safety reasons, factories processing both enriched and depleted uranium together treat each as though it is enriched. Thus, being able to process depleted uranium in a foundry having no enriched material represents a unique development opportunity. Knowledge gained while working with depleted uranium is applicable if one were working with enriched materials. Depleted uranium can be melted, cast, machined, alloyed and rolled (Fig. 2). Foundry experiments are performed using depleted uranium so valuable knowledge can be gained without encountering the risks associated with handling enriched materials.
|Fig. 3. An 800-watt microwave oven shown melting a small metal charge at a temperature of 2236°C (4057°F)|
Melting Metal with Microwave Energy
Metal objects can be specifically designed and shaped to absorb microwave energy. However, placing metal objects in a microwave field will not result in a puddle of molten metal. Normally, when metal is exposed to microwave energy, the majority of the energy will reflect off its surface. However, some non-metallic objects are capable of absorbing microwave energy. Objects that absorb microwave energy can convert this energy into heat. An empty ceramic cup, for example, will get hot when placed inside a microwave oven.
If a properly designed ceramic susceptor is covered with insulation and placed inside a microwave oven, it can get hot enough to melt metal objects. If a metal object is placed inside the cup, the metal itself would be exposed to minimal microwave energy. Thermal energy radiating from the cup would then be transferred to the metal, eventually causing it to melt.
Melting metal with microwave energy requires a properly designed ceramic cup (e.g., a crucible). The metal itself will never absorb sufficient microwave energy to create usable heat, even in its molten phase. The metal must be placed in a crucible made from a special blend of ceramic materials. These materials are designed to optimize the formation of heat when exposed to microwave energy. Microwave metal-melting thus represents an indirect heating process.
There will always be a performance penalty associated with using any form of indirect heating. However, one advantage to microwave metal-melting is the heating efficiency of the ceramics used to hold the metal. If properly engineered materials are used, the heating rates and temperatures that can be achieved are quite impressive. In fact, unmodified home microwave ovens in the 800-1,000 watt range are capable of heating small, insulated ceramic objects to more than 2000°C in minutes (Fig. 3). Microwave metal-melting is exciting because it consumes minimal electrical energy and quickly creates very high temperatures.
|Fig. 4. Metal drum containing depleted uranium machine turnings|
Melting Uranium with a Microwave Melter
For the past 60 years, uranium metal has been cast and machined. A result of this work is radioactive machine turnings, saw fines and chips. These materials are stored in metal drums (Fig. 4). Since the materials are pyrophoric, they must be kept submerged in de-mineralized water or oil. At some point, the materials must be dispositioned. They are normally reduced to oxide, and chemical methods are used to recover the base metal. These types of chemical processes require a large infrastructure to support the activity.
|Fig. 5. Uranium-oxide waste remaining in a crucible after a melt|
A direct-recovery effort through melting would be preferred. Success in recasting these materials using existing vacuum induction melters, however, has been mixed. The casting quality is typically poor and yields are low. After melting, a large amount of waste in the form of oxide remains in the crucible (Fig. 5). If the materials could be successfully melted into new castings, the environmental and fire safety hazards would be mitigated. Additionally, large chemical processes could be avoided. After melting, the material could be more easily inventoried. The material would also be reduced to a casting shape appropriate for storage or future use.
|Fig. 6. Uranium machine turnings melted into a usable casting|
Microwave vs. VIM
Microwave metal-casting has a distinct advantage over existing VIM technology when melting machine turnings, saw fines and chips. The shape and physical size of these materials make them capable of direct microwave absorption. When exposed to microwave energy, the resulting plasmas and other electrical discharges cause the loose materials to fuse together. This consolidation occurs within a very short time. Once the loose materials have fused together, they can no longer absorb microwave energy. The result is a single, consolidated mass of material. The process of melting the material then continues using heat created by the crucible. The net result is the same casting quality with a yield similar to what is obtained when melting solid chunks of metal.
Microwave energy is already being used to recover thousands of kilograms of uranium machine turnings. During these melts, typical casting yields are greater than 85%. This means only about 15% of the original feed material – in the form of oxides and other waste products – remain for disposal. The resulting castings only require about 3% of their original storage footprint (Fig. 6).
MS Technology provides microwave production melters to test your own materials and processes for suitability. The company can provide metallography, chemical and other laboratory analytical support to evaluate new products. IH
For more information: Contact Paul Steneck, PE manager, at MS Technology, Inc., 137 Union Valley Road, Oak Ridge, TN 37830; tel: 865-483-0895 x216; fax: 865-482-5396; e-mail: firstname.lastname@example.org; web: www.mstechnology.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: microwave metal melting, vacuum induction melting, depleted uranium, pyrophoric, VIM