Cold Crucible Induction Melting: An Introduction

Cold Crucible Induction Melting (CCIM) is a combination of induction heating and water cooling to heat glass melts or metals to extremely high temperatures while the crucible remains cold.

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CCIM provides a number of advantages over traditional melting techniques, including higher process temperatures, the capacity to handle corrosive or reactive materials and the manufacture of astonishingly pure semiconductors, oxides and metals.

Basic Principles of Cold Crucible Induction Melting

First patented in 1931 by Siemens und Halske Company, Germany, cold crucible induction melting is predicated on the principles of inductively heating material inside a crucible while cooling the crucible itself with water.¹

A standard CCIM setup is comprised of a water-cooled steel crucible surrounded by one or more induction coils.^2,3 These induction coils transmit alternating high frequency currents, which induce electrical currents in the material being heated.

These currents, known as eddy currents, heat the material. Inductive heating of the material is enabled by segmenting the crucible into pieces divided by narrow slits to facilitate the permeation of magnetic fields.

The crucible is water-cooled — generally, the temperature remains below 200°C — causing any metal or glass that comes in to contact with the crucible wall to freeze solid. This means that the molten material is completely encased within a solid crust or ‘skull’ — leading to the alternative name of induction skull melting.

Configuration of CCIM systems is conducted so that maximum heating occurs in the center of the crucible.

With optimal power density also at the crucible’s center, the extremity of the temperature gradient between the hot center and the cold walls makes sure that the melt is stirred by convective means, as well as by electromagnetic effects.^4,5

Advantages of Cold Crucible Induction Melting

In CCIM, the solid crust inhibits contact between the molten material and the crucible. This safeguards the crucible by avoiding exposure to extreme temperatures and corrosion, giving CCIM a number of unique advantages.

High Operational Temperatures

Surprisingly, cooling of the crucible facilitates higher working temperatures within the melt in contrast to traditional electric melters. For instance, joule-heated melters (JHM) function by conducting an electric current between electrodes submerged in the molten material.

This design is naturally limited by the electrodes’ maximum operational temperature — joule-heat melting is usually limited to applications below 1200°C.⁶ However, there is no such limitation where current is induced rather than conducted in CCIM. Cooling of the crucible also protects CCIM systems from damage caused by heat exposure.

Compatibility with Corrosive Materials

The crucible is also protected from corrosion because of the layer of frozen material between the crucible and melt. CCIM is compatible with a broader range of materials than other electric melting techniques due to separation of the crucible from reactive substances in the melt, including reactive metals such as Zr and Ti.

High Purity Materials

In traditional melting techniques, high performance refractory materials line the crucible to protect them from corrosion and heat. Though these materials tend to be effective, extremely high working temperatures or reactive substances still lead to degradation, thereby contaminating the melt with small amounts of the crucible lining.

As CCIM inherently prevents crucible deterioration of the crucible, the material requirements of CCIM are lower than those of traditional electric melters, as the need for the refractory materials is eradicated.⁷ 

Crucially, because the melt is not contaminated by the crucible lining, the processing of very high purity materials such as refractory metals and their alloys is made possible.

Applications of Cold Crucible Melting

Nuclear Waste Processing

One of the principal applications of CCIM is the vitrification of nuclear waste. In this process, nuclear waste products are combined with glass-forming chemicals and heated to extreme temperatures, then formed and solidified into canisters to inactivate the waste in a non-leaching and durable form.

CCIM is appropriate for this process as it eradicates the stringent material requirements associated with joule-heating-based vitrification processes. Changing corroded refractories and electrode materials from failed joule heating melters carries a considerable risk of radiation exposure, which CCIM curbs.

Additionally, the cold crucible design is less expensive, smaller and in the long run, produces less waste for disposal.²

Production of High Purity Metals and Alloys

The inherent corrosion resistance of CCIM crucibles means that the process is relevant to the manufacture of high purity refractory metals and alloys, including Molybdenum, Niobium, Titanium and Tantalum. CCIM can also be used to manufacture exceptionally large pure silicon crystals and huge multi-crystalline silicon ingots as a base material for the production of solar cells.

Glass and Oxide Melting

CCIM is an outstanding method to melt materials like oxides and silica glass, which are distinguished by their high melting points and low electrical conductivity at low temperatures.

Materials that act as insulators at low temperatures (such as silica glass) must be pre-heated to a temperature where inductive coupling (i.e., transmission of energy from induction coils to material) can be accomplished. Resistance to corrosion means that CCIM techniques can be used to fabricate glass with high purity content.⁸

Phosphate glasses are extensively used in biomedical, nuclear, optical and many other industries. CCIM can also be another technique for the melting of phosphate glasses corrosive to contact refractories and metals at processing temperatures.

Mo-Sci possesses expertise in glass production and utilizes a wide variety of manufacturing processes to produce custom glass solutions at various scales, from commercialization to prototyping. To discover more about our custom melting and glass development services, get in touch today.

References

1. Mühlbauer, A. History of Induction Heating and Melting. (Vulkan-Verlag GmbH, 2008).
2. Gombert, D. & Richardson, J. G. Cold Crucible Induction Melter Technology: Results of Laboratory Directed Research and Development. INEEL/EXT-01-01213, 910987 http://www.osti.gov/servlets/purl/910987-kFgCJc/ (2001) doi:10.2172/910987.
3. Fluxtrol | Modeling and Optimization of Cold Crucible Furnaces for Melting Metals. https://fluxtrol.com/modeling-and-optimization-of-cold-crucible-furnaces-for-melting-metals.
4. Pericleous, K., Bojarevics, V., Djambazov, G., Harding, R. A. & Wickins, M. Experimental and numerical study of the cold crucible melting process. Applied Mathematical Modelling 30, 1262–1280 (2006).
5. Sombret, C. G. Cold Crucible Melting: A Multipurpose Technique. Proc. Vol. 1994–13, 705–712 (1994).
6. Cold Crucible Induction Melting :: Total Materia Article. https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=ktn&LN=ES&NM=389.
7. Goyal, P., Verma, V., Singh, R. K. & Vaze, K. K. Thermal Analysis of Joule Heated Ceramic Melter. 4 (2010).
8. Chen, R. et al. Glass melting inside electromagnetic cold crucible using induction skull melting technology. Applied Thermal Engineering 121, 146–152 (2017).

This information has been sourced, reviewed and adapted from materials provided by Mo-Sci Corp.

For more information on this source, please visit Mo-Sci Corp.