Firing and Drying / Raw and Processed Materials / Refractories

SPECIAL REPORT/REFRACTORIES: Assessing Monolithic Refractories

October 1, 2011
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Aluminum producers have developed a practical set of laboratory tests for furnace linings.

Figure 1. Furnace zones in an aluminum melt-hold reverb furnace.

Monolithic refractories are well established as linings for a range of holding and melting applications during aluminum processing, as they provide optimum productivity and cost effectiveness. A wide range of products is available and, since aluminum furnaces have their own unique set of operating conditions compared to other refractory applications, suppliers offer specifically tailored material solutions.

Figure 2. Mold for sample preparation, Method 1.

Background

Monolithics are used to line the metal and non-metal contact regions in typical melt-hold gas-fired reverberatory furnaces. Each region is divided into sub-regions (see Figure 1), all of which experience a different set of operating conditions-and hence environment-for the furnace lining. A variety of refractories is therefore required for a complete furnace lining.

End users melt and hold a variety of fluxing materials, so the monolithic products need to cope with the specific chemistry present in the furnace. In addition, different operating practices with respect to furnace management (e.g., methods and frequency of cleaning) mean that diverse physical conditions can influence different parts of the furnace.

The diverse nature of the furnace environment means aluminum producers need to maintain a complex and lengthy testing scheme for furnace linings. The purpose is to subject potential materials to the full range of conditions that they are likely to experience in service. Since a broad range of conditions exists, it is not practical or cost effective to test materials for all types of conditions. Instead, aluminum producers have developed a practical set of laboratory tests.

Figure 3. Mold for sample preparation, Method 2.

The two main failure mechanisms that limit service life are chemical attack (e.g., corundum growth or corrosion from flux addition) and mechanical damage (e.g., ingot loading, cleaning practices or thermal shock). Producers have developed tests to simulate these effects as part of their approval program. The investigation of tests in this article focuses on the metal contact region, as this produces the most aggressive set of conditions and represents the most demanding part of the furnace in terms of lining performance.

Corundum growth is the most significant threat in this area and therefore receives the most attention when designing and testing furnace lining materials. Corundum forms when liquid aluminum reacts with free silica in refractories, as shown in the formula:

4Al (l) + 3SiO2 (s) --> 2Al2O3 (s) + 3Si

This transformation leads to a very large expansion in volume, causing severe distortion and cracking of the lining. The most prevalent laboratory test for corundum growth resistance is the aluminum "cup" test. The objective of this investigation is to understand how different test conditions affect the behavior of the lining materials by evaluating how existing furnace lining materials behave when subjected to aluminum producers' contact "cup" test methods.

Figure 4. Materials A, B and C tested by Method 1.

Test Details

The standard metal contact "cup" tests of three large aluminum producers are outlined below. These procedures are routinely used to assess the suitability of monolithic refractories for use in melt-hold furnace linings.

Method 1
To prepare the sample, a series of 100 mm cubes are cast from compositions mixed at standard water addition. The mold is shown in Figure 2 (p. 29). Each cube has a 50 mm deep, slightly tapered hole (55 mm diameter at top, 53 mm at base). Samples are set overnight, then de-molded, cured and dried at 230°F for 18 hours. Half of the dried sample cups are then pre-fired to 2192°F for five hours. Lids of the same material (25 mm thick) are also made to minimize loss of volatiles.

Typically, 7075 alloy is used for testing, supplied as 52 mm bar and cut to 50 mm lengths. The cut alloy sample is inserted into the hole in the sample cup, and the lid is placed on top (unsealed). Both as-dried and pre-fired samples are tested at the same time for comparison.

The assembled cups are placed in a kiln, heated to 1832°F at a rate of 302°F/hour and held at temperature for 100 hours. The kiln is then allowed to cool naturally. After cooling, the samples are sectioned vertically; dried; visually assessed for the degree of metal penetration, corundum growth, or ease of removal of the aluminum; and photographed.

Figure 5. Materials A, B and C tested by Method 3.

Method 2
Following the supplier's mixing recommendation, a standard brick size (e.g., 230 mm high x 114 mm wide x 76 mm deep) of the test material is cast into a mold that incorporates a curved face to form a cup shape with a maximum depth of 32 mm for holding the alloy. The mold is shown in Figure 3. After the recommended curing time, the sample is fired according to the supplier's recommendation to 1499°F, with a 10-hour hold, and left to cool naturally in the kiln. The curved cup section is then roughened using a diamond saw to expose the refractory grain.

The cup sample is raised to 1499°F in a furnace at a rate not exceeding 302°F/hour. Meanwhile, 7075 alloy is melted in a silicon carbide crucible, heated to 1499°F and sampled for analysis. The molten alloy is then ladled into the brick cavity at 1499°F to about 3 mm below the top of the brick, and held at temperature for 72 hours.

The alloy is raked every half-hour for the first three hours to remove the oxide film barrier at the metal/refractory interface. After 72 hours, the oxide formed on the top of the molten alloy is cleaned and a sample of alloy from the cup is taken for analysis.

Any remaining metal is poured off and the cup surface is cleaned with a Superwool blanket pad. The cup is air cooled and sectioned through the center (along the short axis) to assess the degree of metal attack. The initial and final chemical analyses of the alloy are compared to determine silicon and iron pickup.

Method 3
Samples are prepared according to the supplier's recommendations and cast into the same molds as those used in the first method. Following the same setting, curing and drying process, half of the dried sample cups are pre-fired to 1472°F for five hours and half to 248°F.

Four test pieces are heated simultaneously in an electric furnace alongside a quantity of the test alloy in a crucible at 50°F/min to 1472°F +/- 41°F. Pure aluminum (160 g, > 99.8%) is ladled into the sample hole and the cups are held at 1472°F for 72 hours. The melt is stirred daily to break the oxide film. After the melt is left to cool naturally in the furnace, it is cut diagonally and the cut face inspected for penetration and reaction with metal. It is also photographed.

Results

As characterized in Table 1, three monolithic materials were tested using the three "cup" test methods to assess how the different test conditions used by the aluminum producers affect the outcome of the test results. As shown in Figure 4, none of the materials tested using Method 1 show any significant corundum growth, as would be expected since all three materials are routinely used in aluminum furnaces. Material C, which has been pre-fired to 2192°F, does show a thin layer of corundum formed at the interface with the metal; this suggests that corundum resistance begins to degrade as firing temperature increases. This behavior would have performance implications in service when furnaces are operated more aggressively.

The results of Method 2 show no corundum growth on any sample at lower temperatures of 1499°F, despite roughening of the contact surface to try to promote reaction (see Figure 5, p. 30). However, the alloy analysis reveals that silicon pickup increases going from material A to B to C. "Cup" test failures are normally accompanied by increased concentration of silicon and iron in the alloy after testing (see Figure 6).

Figure 6. Sub-hearth gunning material tested by Method 2.

Table 2 reveals that the trend in increasing silicon pickup matches the reduction in alumina/silica ratio in the material. In addition, as silica content increases, more Si is detected in the alloy. Despite the low testing temperature of Method 2, material C is close to the failure threshold for the maximum allowable silicon pickup of 0.5%.

As with Method 2, the results of Method 3 show no visible signs of corundum growth on any sample. The results indicate that testing at 1832°F accelerates the corundum reaction and that pre-firing the sample at higher temperatures can cause the non-wetting additive to react with other material constituents and to lose its effectiveness.

As all the materials studied are already in use in many furnaces, it was expected that all the materials tested would pass these cup tests; for most of the studied test conditions, that has been observed. However, as the severity of the test conditions increases, more metal/refractory interaction has been observed, specifically in material C.

This matches general operational observations, where it has been noted that material C starts to suffer from corundum growth in more aggressively run furnaces. According to these laboratory tests, metal contact performance appears to start deteriorating as the temperature increases to 1832°F.

In the past, such high test temperatures were considered unrealistic, since holding temperatures tended to be well below this level. However, in more recent times, as aluminum furnaces continue to be pushed harder, chamber temperatures have risen and conditions have become more aggressive for the refractory lining. Therefore, test conditions that accelerate the reactions involved by increasing temperature above traditional aluminum holding temperatures are now more valid.

In particular, corundum growth is often seen to start at hot spots in the furnace, where temperatures can be measured in excess of 1832°F. This situation is exacerbated by exothermic reactions from salt and dross build-up on the lining. As industry needs have changed, so too has the furnace environment. Material test methods must therefore evolve to reflect this.

In light of modern aluminum test practices, the testing temperatures of Methods 2 and 3 appear too low, as they do not accelerate corundum growth reactions adequately. In addition, the high melt surface area in Method 2 promotes excessive dross formation and volatilization. Methods 1 and 3 use relatively small alloy samples, which also suffer from volatilization, but this can be controlled to improve test repeatability by covering the sample "cup" with a refractory lid of test material.

"Cup" test results are further complicated when salts are introduced into the metal contact "cup" tests. These studies have shown that resistance to corundum growth can alter considerably in the presence of salts. Further investigation on this subject is under way.

Updates Needed

The metal contact "cup" test methods used by three aluminum producers for furnace lining selection have been investigated using monolithic materials currently in use in several melt-hold furnaces around the world. Aluminum producers have worked to increase productivity to remain competitive. This is normally achieved by increasing heat input to the furnace with more powerful burners to melt the metal faster. However, this practice leads to increased metal losses as a result of surface oxidation, as well as larger heat gradients across the metal that result in the segregation of alloying elements and a reduction in metal quality.

These effects are countered by increased use of fluxes to suppress surface oxidization and increased stirring of the metal to achieve homogenization. Given the increasingly challenging environment that the refractory lining has to work in, aluminum producers must ensure that their material assessment tests also reflect these changes in conditions. Otherwise, the tests will produce unrealistic results and material selection may be compromised.

The results of this investigation suggest that those "cup" tests using lower temperatures are not aggressive enough for assessing lining materials in today's furnace environment. In the past, such test conditions were adequate, but the test methods have not evolved in line with the furnace conditions that they are trying to simulate.

For more information, contact Morgan Thermal Ceramics at (706) 796-4200, email northamerica.tc@morganplc.com or visit www.morganthermalceramics.com.

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