Electrofused vs. Sintered Ferro-Alumina Spinel: Key Effects on Refractory Product Properties
Due to the environmental pollution problems caused by the chromium valence transformation of magnesia-chrome bricks during high-temperature operation in the burning zone of cement rotary kilns (where the conversion of Cr³⁺ to Cr⁶⁺ increases sharply when the thermal medium contains high levels of calcium, sodium, and potassium), direct-bonded magnesia-chrome bricks used in the burning zone of cement rotary kilns have been widely replaced by magnesia-calcium bricks, magnesia-alumina spinel bricks, and magnesia-ferro-alumina spinel bricks.
Magnesia-dolomite bricks are prone to forming kiln coating, but their poor thermal shock stability and hydration resistance have hindered their development. Magnesia-alumina spinel bricks have good corrosion resistance and thermal shock stability; however, their poor kiln coating performance leads to an unstable service life, limiting their application to the upper and lower transition zones of rotary kilns.
Magnesia-ferro-alumina spinel bricks not only exhibit excellent kiln coating performance, but their room-temperature and high-temperature properties-especially thermal shock stability-are superior to those of direct-bonded magnesia-chrome bricks. They are well-suited for the demanding requirements of large and new rotary kilns, with a stable service life of over 10 months.
An important raw material for producing magnesia-ferro-alumina spinel bricks is ferro-alumina spinel. Ferro-alumina spinel is a natural mineral formed by the high-temperature metamorphism of iron-rich bauxite mudstone, appearing as black crystals with a hardness of 7.5 and a density of 3.95–4.39 g·cm⁻³. However, natural ferro-alumina spinel is rare, so it is usually artificially synthesized from steel scale and alumina through either the sintering method or the electrofusion method.
In the past, electrofused ferro-alumina spinel was the preferred raw material for producing magnesia-ferro-alumina spinel bricks due to its high density, low apparent porosity, and high spinelization rate, all of which contributed to higher bulk density and ensured product quality. However, electrofused raw materials are expensive. In recent years, with the continuous improvement and maturation of production processes, the sintering method for synthesizing ferro-alumina spinel has gained rapid popularity.
This paper analyzes the products synthesized from electrofused and sintered ferro-alumina spinel raw materials to identify the main differences between the two types of products and their practical application effects.
01. Performance Comparison of Products Manufactured with Electrofused and Sintered Ferro-Alumina Spinel Raw Materials
1.1. Performance Indicators of Raw Materials
The performance indicators of electrofused and sintered ferro-alumina spinel raw materials are shown in Table 1.
1.2 Basic Production Process
The bricks are classified into Grade A and Grade B. Grade A bricks are manufactured using electrofused ferro-alumina spinel (hereafter referred to as electrofused spinel), while Grade B bricks are produced by directly replacing electrofused ferro-alumina spinel with an equal weight of sintered ferro-alumina spinel (hereafter referred to as sintered spinel). For both Grade A and Grade B bricks, the amount and particle size ratio of other raw materials, such as high-purity magnesia, remain unchanged. Lignosulfonate pulp waste liquor is used as the binder. The bricks are formed on an LS automatic hydraulic press with a 2000-ton double-mold cavity, dried in a dryer at 110°C for more than 12 hours, and fired in a high-temperature tunnel kiln at 1650°C with a heat preservation period of 4.5 hours.
There is little difference between Grade A and Grade B bricks, and they are difficult to distinguish, both in terms of appearance and dimensional change after firing.
02. Test Results
A comparison of the performance indicators between Grade A and Grade B magnesia-ferro-alumina spinel bricks is shown in Table 2. From the comparison of conventional properties, it can be observed that, compared with Grade A products, Grade B ferro-alumina spinel products have slightly lower bulk density and cold crushing strength, and slightly higher apparent porosity. The refractoriness under load and thermal shock resistance are equivalent, and there is no significant difference in chemical composition. This is because the bulk density of sintered ferro-alumina spinel is lower, and the apparent porosity is higher than those of electrofused raw materials, which directly affects the bulk density, apparent porosity, and cold crushing strength of the final products. Due to the small differences, Grade B ferro-alumina spinel products can fully meet the construction and application requirements of the burning zone of large-scale cement rotary kilns.
Cut two samples each of Grade A and Grade B magnesia-ferro-alumina spinel bricks, with dimensions of 80 mm × 80 mm × 65 mm (one set of samples consists of these four pieces). Mix the newly obtained cement raw meal and kiln coating uniformly at a ratio of 7:3 (with a particle size ≤ 0.088 mm). Use the sandwich method to lay the mixture flat as the interlayer between each set of samples, with a dosage of 30 g per set. Place the samples, loaded with the mixture, into a re-firing furnace and hold them at 1500°C for 360 minutes. After cooling, remove the samples, separate them, and then observe the interlayer surfaces, as shown in Figures 1 and 2.
Observation of the samples separated after the kiln coating test shows that both Grade A and Grade B magnesia-ferro-alumina spinel bricks are prone to kiln coating adhesion. From the amount of kiln coating adhered to the upper part of the brick samples, it can be observed that Grade B magnesia-ferro-alumina spinel bricks have slightly more and firmer kiln coating adhesion compared to Grade A bricks. Since there is no significant difference in chemical composition between Grade A and Grade B magnesia-ferro-alumina spinel bricks, the higher apparent porosity of Grade B bricks allows more powder to penetrate into the brick's surface layer, making it more likely for the surface layer to react and form a firm kiln coating.
03. Application Effect
Batch-marked Grade A and Grade B magnesia-ferro-alumina spinel bricks were co-laid in the burning zone of a cement kiln with a daily output of 5,000 tons and operated continuously for 11.5 months. When the cement plant shut down the kiln for overhaul, the residual length of the 220-mm-long Grade A and Grade B bricks ranged from 140 mm to 160 mm. The strength within 100 mm of the brick ends remained high, and the wear and spalling areas were relatively uniform. A comparison of the residual Grade A and Grade B bricks revealed that their wear loss was very similar, with little difference, suggesting that their service lives are nearly identical with no significant gap.
04. Conclusion
Although the magnesia-ferro-alumina spinel bricks produced with sintered ferro-alumina spinel raw materials have slightly lower bulk density and cold crushing strength, and slightly higher apparent porosity compared to those made from electrofused raw materials, they still maintain good thermal shock stability and refractoriness under load, and are more prone to kiln coating adhesion. They can fully meet the construction and application requirements, and their service life is not significantly different from that of products synthesized with electrofused raw materials. Therefore, sintered ferro-alumina spinel is a raw material with high cost performance.