As aluminum producers strive to increase productivity, the environment within furnaces for holding and melting aluminum is becoming more aggressive. Chamber temperatures are increasing and more aggressive fluxes are being used, necessitating more frequent and severe cleaning operations of the refractory wall.

A key requirement for maintaining high levels of productivity is the need to minimize the frequency and duration of furnace downtime. The more aggressive conditions within which the refractory lining has to work today means that the aluminum-resistant lining materials developed in the past to cope with these applications are now being used beyond their originally intended design boundaries. As a result, their service performance is under threat, leading to more frequent lining repairs. In order to minimize the frequency of furnace downtime, a new breed of aluminum-resistant product is needed, specifically designed to perform within today’s more aggressive operating environment.

Over the last 30 years, a group of monolithic technologies has emerged that have been designed specifically to perform within the unique environment of aluminum melt-hold furnaces. These aluminum-resistant grades often contain “nonwetting” additives, particularly in the metal-contact areas, to minimize interaction between the refractory and the melt, which suppresses damage to the lining from “corundum growth.”[1]

Morgan Thermal Ceramics (MTC) has recently developed a monolithic material specifically to improve performance in the superstructure zone – roof, upper walls and flue areas – which copes with excessively high levels of alkali vapor and thermal shock. This article reviews the operating conditions found in the superstructure areas (Fig. 1) of a typical melt-hold furnace and the implications these have on monolithic lining material design and performance. The improved behavior of the newly developed monolithic material against the critical performance criteria in these furnace regions is demonstrated in the laboratory by comparing to existing industry-leading materials using industry-standard test methods.

Key Performance Parameters

To understand the operating conditions in the superstructure region of an aluminum melt-hold furnace in more detail, MTC worked with a number of leading aluminum producers. By studying working practices and furnace operating conditions and through post-mortem analysis of exhausted furnace linings, the company identified that the two primary factors that limit service life of linings are excessive alkali vapor and extreme thermal shock.      

An increase in the use of more powerful fluxes to deliver higher productivity and more exotic alloy compositions, coupled with higher chamber temperatures (particularly in melt furnaces) is leading to higher concentrations of alkali vapor in the upper chamber region. In the vapor state, these alkalis can easily enter refractory linings through the pore structure to chemically interact with the base materials. Such reactions can often lead to expansion effects, creating extreme changes in volume, which causes cracking and ultimately the catastrophic failure of a lining. The rate of chemical attack is affected by temperature, furnace atmosphere, furnace housekeeping, and composition and structure of the refractory lining.

The superstructure region is also subject to considerable thermal stress as chamber temperatures rise and fall rapidly during the opening and closing of the furnace door and in the areas around the gas burners. This leads to rapid contraction/expansion of the surface of the furnace lining, microcracking the structure and ultimately resulting in lining failure.

Other environmental factors can also have a secondary contribution in limiting service life of the refractory lining in the superstructure, particularly in the upper-wall region. The upper walls are subject to mechanical abrasion during cleaning operations, so some degree of abrasion resistance is necessary in the refractory lining to cope with the mechanical stress endured during cleaning. Also, although not normally in contact with molten metal, the upper walls can be subject to intermittent contact from metal splashing produced during stirring and cleaning operations. It is therefore important that the refractory in this region also has some degree of “nonwetting” capability to ensure corundum does not nucleate and grow at these splash points.

Testing the Materials

Three existing monolithic materials used by several aluminum producers in the superstructure region of melt-hold furnaces were selected as baseline materials for the study. The performance of these materials in service is well established, so any test results can serve as useful benchmarks against which to compare new developments.

A detailed analytical investigation of the baseline materials was undertaken in order to identify those aspects of the materials technology that were considered to be either promoting or constraining performance in terms of alkali and thermal-shock resistance and were thus controlling failure mechanisms. This knowledge of the strengths and weaknesses of the existing material technologies was then used as the basis for a series of reformulations.

The goal was to find the optimum balance of bond and aggregate chemistry and product granulometry that produced the maximum improvement in alkali and thermal-shock performance without negatively affecting other important properties. The results of performance and property measurements of the final, optimized development composition compared to the baseline standards are presented below. All materials in the study were tested against the four key performance parameters using industry-standard test methods.

Primary Performance Parameters
1. Thermal-shock resistance test (ASTM C1100-88 {1998} – ribbon test):  Pre-fired samples (230 x 115 x 64 mm) are subjected to five alternating heating and cooling cycles on one face using a ribbon burner. The Modulus of Elasticity (E-modulus) of samples is measured nondestructively by ultrasonics before and after testing. The percentage of retained E-modulus is used as a measure of retained strength.
2. Alkali resistance “cup” test:  Nine “cup” samples (5-cm cubes, each with a 2.2 cm diameter, 2.5-cm-deep hole) are prepared for each test composition (Fig. 2) along with a lid (5 cm square, 0.6 cm thick) for each cube. Samples are allowed to set overnight, then demolded, cured and dried at 110°C for 18 hours. Three of the dried sample cups produced are each filled with 8 grams of potassium carbonate, three with 8 grams of sodium carbonate and three with a 50:50 mixture of 8 grams of both potassium carbonate and sodium carbonate. Samples of each of the alkali mixtures are fired to 900°C, 1000°C and 1100°C for five hours. After sectioning vertically, samples are visually inspected for cracks, bulges, depth of penetration and color change.

Secondary Performance Parameters
1. Abrasion resistance test (ASTM C704):  Samples pre-fired to 815°C (1500°F) are blasted with a stream of silicon-carbide grit of specified grain size for a set time. Tested samples are then cross-sectioned, and the amount of material abraded across the section is measured and reported in cubic centimeters.
2. Aluminum resistance “cup” test:  Sample preparation is similar to the alkali resistance cup test except no lid is used during testing. Instead of alkali, the samples are filled with 7075 alloy. Samples are heated at 1000°C (1832°F) for 100 hours. After cooling, the samples are sectioned vertically and visually assessed for the degree of metal penetration and corundum growth. Full details of the test method are described in the literature.[2]

Results and Discussion

The physical characteristics and chemical composition of the optimized new material compared to the standard baseline materials are displayed in Tables 1 and 2.

Primary Performance Parameters
Thermal-shock resistance test results of several of the materials studied are presented in Fig. 3. After five test cycles, Standard 1 lost 50% of its E-modulus and Standard 2 lost 95% compared to only 40% loss for the new optimized material. These results suggest that the new material is capable of delivering a 20% improvement on thermal-shock resistance compared to Standard 1 and a twelvefold improvement compared to Standard 2.

Since the baseline materials are used routinely in aluminum melt-hold furnaces, it was expected that all would possess some degree of resistance to alkali attack. However, the alkali resistance testing displayed some extremes of behavior. Standards 1 and 3 displayed very poor resistance to alkali attack, with samples being severely disrupted by major cracking and erosion all over (Figs. 4-6).

This behavior contrasted with the results of Standard 2, which demonstrated excellent resistance to the alkali tests (Figs. 7-9). The final optimized new composition, which builds on the performance mechanisms in Standard 2, passed all alkali contact testing with potassium carbonate and sodium carbonate and delivered an excellent rating (Figs. 10-12). 

Secondary Performance Parameters
The abrasion resistance test results of the materials – an important performance parameter for materials in the upper walls to resist the abrasive action of cleaning operations – are presented in Fig. 13. Standards 1 and 3 display relatively poor resistance to abrasion compared to Standard 2. The target for abrasion resistance in this region was greater than 10 cm³, and Standards 1 and 3 do not achieve this. The newly developed material incorporates technologies utilized in Standard 2 to deliver excellent abrasion resistance.

To ensure the new material was safe from metal splashing in the upper-wall region, such that corundum growth would not be an issue, it was pre-fired at 1200°C (2192°F) and tested against metal contact with 7075 alloy at 1000°C (1832°F) for 100 hours. This is a more aggressive test than most aluminum producers use in their material approval procedures.[2] Standard 1 produced a poor result. All other materials achieved a good rating in the test, with only minimal interaction with the test alloy (Fig. 14).

Summary

The results of the tests, which can be seen in Table 3, show that some baseline materials demonstrated superior behavior for one of the primary performance parameters but poor performance for the other. The summary shows that the new material combines all the best performance features of the baseline materials but without any of the weaknesses. Therefore, it displays the optimum blend of performance features required for service in the superstructure of aluminum melt-hold furnaces. For improved ease of installation, the new monolithic material has been designed as a vibrocast grade, requiring only minimal water addition to achieve good flow, and is now on trial in the superstructure area of melt-hold furnaces at several aluminum producers around the world.  

Conclusion

Aluminum producers continue to increase productivity through their melt-hold furnaces to maintain competitiveness, and the use of more powerful burners to increase heat input to the furnace is therefore becoming an increasingly common practice. But faster melting leads to increased metal losses from surface oxidation and segregation from large heat gradients. These effects are countered by increased use of fluxes and more stirring. Given the increasingly challenging environment within which the refractory lining has to work, traditional lining solutions can no longer be relied upon to provide the service lives that were previously achieved. Therefore, a new generation of lining materials is required to cope with today’s aluminum furnace.

The results from Morgan Thermal Ceramics’ tests suggest that the new material should be capable of surviving the unique set of service conditions in the superstructure region of aluminum melt-hold furnaces better than the existing materials used in the industry and thus deliver longer service life. Extended service life in the superstructure area is expected to reduce the frequency of furnace downtime, and allowing aluminum producers to run longer production campaigns, increase productivity and minimize the need for expensive repairs. IH

For more information:  Contact Alicia Puputti, account executive, McNeil, Gray & Rice Strategic Communications, 1 Washington Mall, Boston, MA 02108; tel: 617-367-0100 ext. 155; fax: 617-367-0160; e-mail: alicia.puputti@mgr1.com; web: www.mgr1.com