Sintered Vs Electrofused Hercynite: Production Process, Performance & Application in Refractory Bricks

Due to a series of environmental pollution problems caused by the chromium valence transformation in magnesia-chrome bricks during high-temperature operation in the burning zone of cement rotary kilns (the content of Cr³⁺ converts to Cr⁶⁺ 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-hercynite spinel bricks.

Magnesia-dolomite bricks are easy to form kiln coatings, but their poor thermal shock stability and hydration resistance have long hindered their development. Magnesia-alumina spinel bricks boast excellent 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.

In contrast, magnesia-hercynite spinel bricks not only have good kiln coating performance, but also exhibit superior room-temperature and high-temperature properties-especially thermal shock stability-compared with direct-bonded magnesia-chrome bricks. They are suitable for the harsh operating requirements of large-scale and new rotary kilns, with a stable service life of over 10 months.

An important raw material for producing magnesia-hercynite spinel bricks is hercynite. As a natural mineral, hercynite forms from 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 hercynite is scarce, so it is usually artificially synthesized from steel mill scale and alumina via sintering or electrofusion methods.

In the early stages, electrofused hercynite became the preferred raw material for hercynite brick production due to its high density, low apparent porosity, and high spinelization rate, which helped improve the bulk density of products and ensured product quality. Nevertheless, electrofused materials are relatively costly. In recent years, with the improvement and maturity of production processes, the synthesis of hercynite by sintering has been rapidly promoted.

This paper analyzes products synthesized from electrofused and sintered hercynite raw materials, aiming to identify the main differences between products made from these two types of raw materials, as well as their actual application effects.

01.Performance Comparison of Products Manufactured with Electrofused and Sintered Hercynite Raw Materials

1.1 Raw Material Performance Indicators

The performance indicators of electrofused and sintered hercynite raw materials are shown in Table 1.

1.2 Basic Production Process

The manufactured bricks are classified into Grade A and Grade B. Grade A bricks are produced using electrofused hercynite (hereafter referred to as "electrofused hercynite"). For Grade B bricks, sintered hercynite is used to directly replace electrofused hercynite in an equal weight ratio (hereafter referred to as "sintered hercynite"). The amounts and particle size ratios of other raw materials, including high-purity magnesia, are kept consistent for both Grade A and Grade B bricks. Lignosulfonate pulp waste liquor is used as the binder. The mixture is formed using a 2000-ton LS automatic hydraulic press with dual mold cavities, then dried in a dryer at 110°C for more than 12 hours, and finally fired in a high-temperature tunnel kiln at 1650°C with a holding time of 4.5 hours.

02. Test Results

The performance indicators of Grade A and Grade B magnesia-hercynite spinel bricks are compared in Table 2.

A comparison of the conventional properties shows that, compared to Grade A products, Grade B hercynite products exhibit a slight decrease in bulk density and cold crushing strength, along with a marginal increase in apparent porosity. Their refractoriness under load and thermal shock resistance are comparable, and there are no significant differences in chemical composition.

This is because sintered hercynite has a lower bulk density and higher apparent porosity than electrofused raw materials, which directly affect the bulk density, apparent porosity, and cold crushing strength of the final products. Given that the magnitude of these differences is minimal, Grade B hercynite products are fully capable of meeting the construction and operational requirements of the burning zone in large-scale cement rotary kilns.

Cut two samples measuring 80 mm × 80 mm × 65 mm from both Grade A and Grade B magnesia-hercynite spinel bricks (two samples per grade, forming one group). Mix the newly collected cement raw meal and kiln coating in a ratio of 7:3 thoroughly (particle size ≤ 0.088 mm). Using the sandwich method, evenly spread the mixture as an interlayer between the two samples in each group, with a dosage of 30 g per group. Place the samples, loaded with the mixture, into a reheating furnace and hold them at 1500°C for 360 minutes. After cooling, remove the samples, separate them, and observe the interlayer surface (see Figure 1 and Figure 2).

Judging by the separated samples from the kiln coating test, both Grade A and Grade B magnesia-hercynite spinel bricks can easily form kiln coatings. As observed from the amount of kiln coating adhered to the upper part of the brick samples, Grade B magnesia-hercynite spinel bricks have slightly more kiln coating adhered than Grade A bricks, and the adhesion is also stronger. Since there is no significant difference in the chemical composition between Grade A and Grade B magnesia-hercynite spinel bricks, the higher apparent porosity of Grade B products allows more powder to penetrate the brick's surface layer. This makes the surface layer more prone to reaction, thereby forming a firm kiln coating.

03. Service Performance

Batch-labeled Grade A and Grade B magnesia-hercynite spinel bricks were co-laid in the burning zone of a 5,000 t/d cement rotary kiln. After 11.5 months of cumulative operation, the cement plant shut down the kiln for overhaul. The residual length of both Grade A and Grade B bricks (originally 220 mm) ranged from 140 mm to 160 mm, and the section within 100 mm from the brick end still maintained high structural strength. The wear and spalling areas were relatively uniformly distributed.

A comparison of the residual bricks revealed that the wear loss of Grade A and Grade B products was nearly identical, with negligible differences. It can be inferred that their service lives are comparable, with no significant gaps.

04. Conclusions

Although magnesia-hercynite spinel bricks made from sintered hercynite raw materials exhibit slightly lower bulk density and cold crushing strength, along with marginally higher apparent porosity compared to those made from electrofused raw materials, they still retain excellent thermal shock stability and refractoriness under load.

Moreover, they demonstrate better kiln coating adhesion and can fully meet the requirements for both construction and operation. Their service life is comparable to that of products made from electrofused raw materials, making sintered hercynite a cost-effective raw material option.