Causes and Solutions to Five Common Problems in Refractory Precast Shapes, Including Chalking, Delamination, and Efflorescence
Refractory precast shapes offer the advantages of convenient and efficient on-site construction as well as a long service life; therefore, they are increasingly used in high-temperature industrial furnaces and kilns.
The production process of refractory precast shapes is relatively simple and mainly consists of batching, mixing, forming, drying, and other procedures. However, a number of problems are often encountered during production. This paper primarily discusses common problems that occur during the production process and their corresponding solutions.
01 Chalking of Impurities in Bauxite Chamotte
Bauxite chamotte is one of the commonly used refractory raw materials, and its quality has a significant impact on the performance of refractory products. Bauxite chamotte, also called chamotte, is produced by calcining bauxite ore at high temperatures, with its Al₂O₃ content required to exceed 50%. The impurity content of the product should not exceed 2%, and external inclusions such as limestone, loess, and high-calcium or high-iron materials must not be present.
Due to the geological characteristics of raw bauxite ore, it is often associated with limestone, loess, and other minerals. If the sorting process after calcination is inadequate, impurities such as limestone may be incorporated into the bauxite chamotte. Once such chamotte is used in refractory precast shapes, chalking of the limestone during water mixing, forming, drying, firing, or service can cause local pit defects in the products. This not only affects the appearance of the products but also compromises their internal quality.
Therefore, prior to using bauxite chamotte, it is necessary to conduct a chalking rate test. The test method is as follows: take bauxite chamotte particles larger than 3 mm with a weight of M1, immerse them in water for a certain period, then dry them at 110 °C. Sieve the dried particles through a 3 mm screen and weigh the particles remaining on the screen as M2. The chalking rate can be expressed as:
Chalking Rate (%)=(M1-M2)/M1×100%
It is recommended that the chalking rate does not exceed 0.20%. If the measured chalking rate is relatively high, the batch of raw materials should be pretreated to ensure product quality. A feasible pretreatment method is to first immerse the raw materials in water, followed by drying and screening before use.
02 Chalking of Brown Fused Alumina
In unshaped refractories, the use of alumina as a refractory aggregate and powder has become increasingly prevalent, yielding significant benefits. Alumina is generally produced by sintering or electrofusion of industrial alumina or bauxite ore and includes types such as white fused alumina, sub-white fused alumina, tabular alumina, high-alumina fused alumina, and brown fused alumina. Among these, brown fused alumina is smelted via electrofusion using lightly calcined high-alumina material, coal, and iron filings as the primary raw materials.
The smelting process is divided into two types: shell-removing furnaces and tilting furnaces. Brown fused alumina produced in shell-removing furnaces exhibits significant variations in crystallization degree across different parts, along with a wide range of iron content. In contrast, brown fused alumina from tilting furnaces features uniform quality and high bulk density; however, due to its homogeneous properties, it has fewer grading categories and may have slightly lower comprehensive performance indicators.
According to production experience, brown fused alumina produced in shell-removing furnaces has a substantially higher probability of chalking than that from tilting furnaces. If brown fused alumina with a high chalking rate is used to produce precast shapes, localized chalking and cracking may occur on the product surface after high-temperature firing. This not only impairs product quality but also significantly reduces the firing qualification rate and increases production costs. Given the severe quality risks associated with brown fused alumina of high chalking rate, it is imperative to conduct chalking rate testing on such materials.
Currently, there are no established testing methods or standards for chalking rate. The two methods adopted in this paper are as follows:
Qualitative Testing: For each incoming batch of brown fused alumina, prepare a test sample according to a specific formula, dry it, and then fire it at a low temperature of 600 °C or 1000 °C. Observe for cracking to determine whether the batch is prone to chalking.
Quantitative Testing: Take a sample of a certain particle size with a weight of M3 (a particle size range of 1–3 mm is generally recommended). Subject the sample to autoclave boiling for 60 minutes (or heat treatment in an electric furnace at 1000 °C for 1 hour). After drying, observe changes in particle color and size. Sieve the sample through a 1 mm screen and record the weight of the retained particles as M4. The chalking rate can be expressed as:
Chalking Rate (%)=(M3-M4)/M3×100%
A batch of brown fused alumina is deemed qualified if its tested chalking rate does not exceed 0.10%. The chalking rate control standards may vary for different refractory products.
03 Swelling and Delamination of Magnesia-Alumina Precast Shapes Containing Silica Fume
During the production of magnesia–alumina precast shapes containing silica fume, surface swelling after forming is a common issue, which can lead to product delamination. This problem seriously affects both the service life and yield of refractory products.
There are two types of silica fume (SiO2 fume): one is made from high-purity silica stone, and the other is a by-product derived from the production of metallic silicon or ferrosilicon. The silica fume commonly used in refractories is of the latter type. It has a hollow spherical structure, high reactivity, good dispersibility without agglomeration, and excellent filling properties. It undergoes pozzolanic reactions at room temperature and forms mullite with Al2O3 at high temperatures, both of which help improve the strength of castables. However, it must possess stable physical and chemical properties; otherwise, the service performance of the final products will be compromised.
In the production of refractory precast shapes, fluctuations in forming performance often occur due to batch-to-batch variations in silica fume. The most prominent manifestation is swelling and delamination of products after forming.
The solutions to swelling and delamination are as follows:
Screen the silica fume to homogenize its composition.
Increase the dosage of retarder during mixing, moderately raise the water addition, and extend the wet mixing time before forming.
Properly reduce the curing temperature of the products.
These measures can generally resolve the problem.
04 Efflorescence of Corundum–Spinel Precast Shapes Containing Alumina Micropowder
In the production of unshaped refractories, α-Al2O3 micropowder is one of the commonly used refractory powders. α-Al2O3 ultrafine powder is produced by calcining industrial alumina. It is characterized by good dispersibility, small particle size, easy sintering at high temperatures, and low volumetric expansion.
During production, a common issue occurs with corundum–spinel precast shapes containing alumina micropowder: after forming and during curing, a layer of milky white liquid and honeycomb-shaped pits appears on the formed surface, accompanied by bubbles escaping from the pits. After removing the liquid, the surface is found to be composed almost entirely of powder. This phenomenon is called efflorescence. The thickness of the powder layer on the formed surface varies depending on the severity of efflorescence.
Efflorescence is more pronounced in winter. It poses severe quality risks to refractory precast shapes, leading to uneven product structure, low strength, reduced thermal shock resistance and corrosion resistance, and shortened service life. Extensive investigation and analysis have shown that efflorescence is related to the content of metal oxides (K2O and Na2O) in the alumina micropowder used.
When the total content of K2O and Na2O is above 0.2%, precast shapes formed with such alumina micropowder exhibit almost no efflorescence.
When the content is below 0.1%, efflorescence is inevitable-and may even be severe-in the corresponding precast shapes.
The efflorescence problem can be mitigated or resolved by the following methods:
Based on the normal water dosage, reduce the water addition by 0.1–0.3 percentage points.
Adjust the addition ratio of retarders and accelerators-appropriately increase the proportion of accelerators while reducing that of retarders.
Properly raise the curing temperature of the products after forming.
Add a small amount of fused magnesia fine powder during mixing, not exceeding 0.5%.
05 High-Temperature Treatment of Precast Shapes with Embedded Lifting Hooks
The high-temperature treatment of precast shapes with embedded lifting hooks is a common challenge in the production of refractory precast shapes. The high-temperature treatment referred to here involves temperatures above 1100 °C. Therefore, direct firing cannot be carried out as usual, and protective measures must be taken to prevent oxidation of the metal lifting hooks during firing.
To address this, steel bar segments of the same diameter as the lifting hooks were first used for testing. Three schemes were evaluated:
Embedding the steel bar segments in carbon.
Coating the steel bar segments with an anti-oxidation coating.
Wrapping the steel bar segments with refractory cotton and then applying castable as an external anti-oxidation layer.
Firing tests were conducted in a high-temperature furnace. The results showed that:
The steel bars embedded in carbon remained intact.
The steel bar segments coated with anti-oxidation coating suffered the most severe oxidation.
The steel bar segments with castable as the external anti-oxidation layer experienced partial oxidation due to microcracks in the castable formed during firing, with an oxide layer thickness ranging from 1 to 2 mm.
It can be concluded that embedding in carbon is the optimal method. During carbon embedding, either partial or full embedding can be applied depending on the structural characteristics of the precast shapes.
06 Other Production Specifications
In addition to the common problems described above, the forming process of refractory precast shapes must be carried out in strict accordance with the following specifications:
Pre-Vibration Equipment Check
Before vibration, the team leader of the precast shape group and quality management personnel shall check the following:
Ensure the vibration table operates vigorously.
Check that the screws of the vibration motor are tight.
Verify that the vibration table surface is level.
Confirm that all other electrical and mechanical equipment is in normal condition to ensure smooth casting and forming.
Mold Preparation
Clean all impurities from the molds.
Evenly apply mold release agent and assemble the molds according to the markings.
For mold sleeves, ensure templates fit into the grooves and screws are tightened, leaving no gaps for slurry leakage.
After tightening, place the molds steadily on the vibration table.
Verify that molds meet drawing requirements, then perform mold sleeve compression.
Conduct pre-vibration to check if molds are firmly pressed. If no abnormality is found, preparation for forming can proceed.
After aligning seat bricks or other molds with holes, quality management personnel shall check:
Whether shaft cores are eccentric.
Whether shaft cores, side plates, bottom plates, and top plates fit tightly without gaps.
Whether screws are tightened.
Forming production is prohibited if these requirements are not met.
Material Charging and Vibration
Ensure materials are poured evenly into the entire mold. Charging in a single location is strictly prohibited.
For flat-mold forming: when charging reaches half the thickness, level the refractory mixture and start vibration.
For vertical-mold forming: start vibration when the charging reaches approximately 200 mm; subsequent charging shall be gradual. Excessive one-time charging is prohibited as it affects gas discharge.
Vibrate until materials at edges and corners are fully compacted.
When using a vibrator, maintain a spacing of about 200 mm. Move and extract the vibrator slowly; do not pull it horizontally to avoid internal holes.
For upper layers, insert the vibrator into the lower layer to a depth of about 30 mm to ensure integral structure and prevent delamination.
Continue vibration until the mold is filled to the specified size and slurry bleeds on the surface with essentially no air bubbles.
For electric furnace covers, charge and vibrate materials from the four sides first; after the castable reaches the edges, charge the center and vibrate toward the four sides.
Lifting Ring Installation
Follow drawing requirements for installation.
If no requirements exist:
Precast shapes over 500 kg: lifting rings installed 100 mm above the surface (net dimensions).
Precast shapes under 500 kg: lifting rings installed 80 mm above the surface (net dimensions).
Follow drawing specifications for lifting ring thickness; if unspecified, arrange temporarily based on shape and size.
Embedded Parts Protection
Brush two coats of asphalt or varnish on embedded parts as required.
Wrap plastic film or adhesive tape (~1.5 mm thick) around parts prone to damage to prevent cracks from thermal stress differences during drying.
Post-Forming Surface and Mold Cleaning
Level the surface immediately after forming; do not wet the surface to prevent dry cracks.
Remove excess refractory mixture around mold edges to avoid flash on blanks, which affects appearance and dimensional accuracy.
Curing Environment
Move molds with precast shapes to a location with suitable temperature and humidity. Avoid excessively high or low temperatures.
In dry weather, cover blanks with wet sacks (without dripping water) to prevent excessive moisture evaporation.
In cold weather, use heating to maintain indoor temperature.
Production shall stop if ambient temperature is below 6 °C.