Five Common Issues with Refractory Castable Precast Components and Their Solutions
Refractory precast components offer advantages such as convenient and rapid on-site installation as well as an extended service life, leading to their increasingly widespread application in high-temperature industrial furnaces and kilns. The production process for these components is relatively straightforward and mainly includes the stages of batching, mixing, forming, and drying. However, certain issues frequently arise during manufacturing. This article primarily explores common production problems and their corresponding solutions.
PART.01 Pulverisation of Impurities in Bauxite Sintered Material
Bauxite sintered material is one of the commonly used refractory raw materials in the production of refractories, and its quality significantly affects the performance of refractory products. Aluminium bauxite sintered material, or simply bauxite sintered material, is produced through high-temperature calcination of bauxite ore, with an Al₂O₃ content exceeding 50%. The impurity content in the product must not exceed 2%, and no foreign inclusions such as limestone, loess, or high-calcium/high-iron materials should be present.
Due to the geological distribution characteristics of raw bauxite ore, it is often found in association with limestone, loess, and similar materials. If not adequately sorted after calcination, bauxite clinker may contain impurities such as limestone. When used in refractory precast components, during processes such as water mixing, forming, drying, firing, or service, the pulverisation of limestone can cause localised pitting defects. This not only compromises the product's appearance but also its intrinsic quality.
Therefore, before use, bauxite clinker must undergo pulverisation rate testing. The method involves taking a sample of bauxite clinker particles (grain size ≥3 mm) weighing M₁, immersing it in water for a specified period, then drying it at 110°C. After sieving through a 3 mm mesh, the weight of the particles retained on the sieve (M₂) is measured. The pulverisation rate is calculated as:
Pulverisation rate (%)=(M1-M2)/M1×100%
A pulverisation rate not exceeding 0.20% is desirable. If the measured pulverisation rate is excessively high, the batch of raw material requires pre-treatment to ensure product quality. This may involve initial water immersion, followed by drying and sieving before use.
PART.02 Pulverisation of Brown Corundum
In amorphous refractories, the use of corundum as a refractory aggregate and powder has become increasingly common, delivering significant improvements in performance. Corundum is typically produced by sintering or electrofusion of industrial alumina or bauxite, and includes white corundum, semi-white corundum, lamellar corundum, high-alumina corundum, and brown corundum. Among these, brown corundum is produced by electrofusion using lightly calcined high-alumina materials, coal, and iron filings as the main raw materials. The smelting process is carried out in either shell-removal furnaces or tipping furnaces.
Corundum produced in shell-removal furnaces shows considerable variation in crystallisation between different sections, along with a wider distribution of iron content. Brown corundum produced in tipping furnaces has more uniform quality and good bulk density; however, because of this uniformity, it undergoes less grading, which may result in slightly inferior overall performance indicators. Production experience indicates that brown corundum from shell-removal furnaces exhibits a much higher powdering rate than that from tipping furnaces.
When brown fused alumina with a high powdering rate is used in precast components, localised powdering and spalling may occur on the product surface after high-temperature firing. This not only reduces product quality but also drastically lowers firing pass rates, thereby increasing production costs. Given the severe quality risks associated with high-powdering-rate brown fused alumina, testing for powdering rate is essential.
At present, there are no established testing methods or standards for powdering rate. This paper adopts the following two approaches:
1. Qualitative Testing:
For each batch of brown fused alumina received, a single precast product is formed according to a specified formulation. After drying, it is fired at a low temperature of either 600°C or 1000°C to observe whether cracking occurs, thereby determining whether the batch exhibits powdering.
2. Quantitative Testing:
Take a sample weighing M₃ with a specific grain size (typically 3–1 mm), boil it in a pressure cooker for 60 minutes (or heat it in an electric furnace at 1000°C for 1 hour), then dry it and observe any changes in particle colour and size. Sieve the dried material through a 1 mm mesh, and record the weight of the material retained on the sieve as M₄. The pulverisation rate is then calculated as:
Pulverisation rate (%)=(M3-M4)/M3×100%
A pulverisation rate not exceeding 0.10% is considered acceptable. Control standards for pulverisation rates may vary depending on the specific refractory product.
PART.03 Delamination in Silica Microsphere-Containing Magnesium-Aluminium Precast Products
During the production of silica microsphere-containing magnesium-aluminium precast products, delamination often occurs due to surface swelling, significantly reducing the service life and yield of refractory products. Silica micropowder exists in two forms: one derived from high-purity quartz, and the other as a by-product of metallurgical silicon or ferrosilicon production. The silica micropowder typically used in refractories refers to the latter. It has a hollow spherical structure, exhibits high reactivity, resists agglomeration, and provides excellent filling properties.
At ambient temperatures, it displays pozzolanic activity, while at elevated temperatures it reacts with Al₂O₃ to form mullite. Both characteristics help enhance the strength of castables. However, it must have stable physicochemical properties; otherwise, it can negatively affect product performance.
During the production of precast refractories, fluctuations in moulding properties often occur due to variations between batches of silica powder. The most noticeable manifestation of this is surface swelling and delamination in the moulded products.
To address this swelling and delamination issue, the following measures are recommended: first, sieve the silica fume to ensure uniform composition. Second, during mixing, increase the amount of retarding agent, slightly raise the water content, and extend the wet mixing time before forming. Finally, appropriately reduce the curing temperature of the products. This combination of measures generally resolves the problem.
PART.04 Water Blistering in Alumina-Spinel Precast Components Containing Aluminium Micropowder
In the production of unshaped refractories, α-Al₂O₃ micropowder is a commonly used refractory powder. α-Al₂O₃ ultrafine powder is produced by calcining industrial alumina. Its characteristics include excellent dispersibility, fine particle size, ease of sintering at high temperatures, and minimal volume change.
During production, alumina-containing corundum spinel precast components often exhibit a milky-white liquid layer and honeycomb-like pits on the moulded surface during curing. Bubbles may also emerge from these pits. Upon removing the liquid, the moulded surface is found to consist almost entirely of powder. This phenomenon is known as "water blistering," with the thickness of the powder layer varying depending on the severity of the blistering.
This efflorescence is more pronounced during the winter months and poses significant quality risks for refractory precast products. It results in non-uniform microstructures, reduced strength, diminished thermal shock and erosion resistance, and a shortened service life. Extensive investigations have shown a correlation between efflorescence and the potassium oxide (K₂O) and sodium oxide (Na₂O) content in the aluminium micropowder used. When these contents exceed 0.2%, precast components formed from mixtures containing such aluminium powder show virtually no water seepage. Conversely, mixtures with contents below 0.1% inevitably produce water seepage during precast production, often to a considerable degree.
The following measures may mitigate or resolve water seepage issues:
1.Reduce the normal water addition by 0.1–0.3 percentage points.
2.Adjust the ratio of retarder to accelerator, increasing the accelerator proportion while reducing the retarder proportion.
3.Moderately increase the curing temperature after forming.
4.Incorporate a small amount of electrofused magnesia fine powder during mixing, with the addition not exceeding 0.5%.
PART.05 High-Temperature Treatment of Precast Embedded Hooks
The high-temperature treatment of precast embedded hooks is a common challenge in refractory precast production. Here, high-temperature treatment refers to temperatures exceeding 1100°C. Consequently, direct firing, as used for standard components, is inadvisable; protective measures must be implemented to prevent oxidation of the metal hooks during firing.
To investigate this, trials were conducted using steel bars of the same diameter as the hooks, testing three approaches:
1.Embedding in carbon.
2.Coating the steel bars externally with anti-oxidation paint.
3.Wrapping the steel bars in refractory cotton, then applying castable refractory as an external oxidation barrier.
The trial results, after firing in a high-temperature kiln, were as follows:
·The rebar buried in carbon remained entirely intact.
·The rebar coated with anti-oxidation paint suffered the most severe oxidation.
·The rebar with castable refractory as the external anti-oxidation layer experienced partial oxidation due to micro-cracks forming in the castable during firing, with an oxidation layer thickness of 1–2 mm.
It is evident that carbon burial is the optimal treatment. When implementing carbon burial, either localized or complete burial can be applied depending on the structural characteristics of the precast element.