Unmatched Durability and Power of Phosphate Castables in High-Temperature Environments

Phosphate Castables: High Impact Resistance and Excellent Performance in Industrial Furnaces

Introduction

In various industrial sectors, one material stands out for its exceptional refractory and wear-resistant properties: phosphate castables. Widely recognized for their outstanding performance, phosphate castables are indispensable in high-temperature furnace construction across many fields. Despite their prominence, many people are familiar with phosphate castables but lack a deep understanding of their characteristics. Let's delve into what makes phosphate refractory castables so unique and effective.

What is Phosphate Castable?

Phosphate castable is a high-performance material composed of clay clinker or bauxite clinker as aggregate, combined with industrial phosphate as a binder and high alumina cement as an accelerator. This blend results in a material that boasts high strength, high refractoriness, and excellent thermal shock stability. One of the key advantages of phosphate castables is the ability to adjust the AI₂O₃ content to meet various design requirements, making it suitable for a broad temperature range of 1450-1600℃. This adaptability positions phosphate castables as an ideal material for high-temperature applications.

Advantages of Phosphate Castables

The advantages of phosphate castables are significant and multifaceted. Firstly, they offer superior refractoriness and excellent insulation properties, maintaining stable physical and chemical characteristics in high-temperature environments. This makes them an ideal choice for protecting metals like steel under extreme conditions, ensuring they retain high quality and performance. Additionally, the impressive insulation capabilities of phosphate castables contribute to substantial energy savings and efficient energy use.

Moreover, phosphate refractory castables exhibit strong wear resistance and corrosion resistance. Even under high temperatures, they effectively withstand chemical erosion and reduce wear, significantly extending the service life of equipment. This durability not only enhances the longevity of the equipment but also reduces the frequency of maintenance and replacements, ultimately saving operational costs for businesses.

Another notable benefit is the ease of construction with phosphate castables. Unlike traditional refractory bricks, they do not require complex installation processes. Simply pouring the material into place allows for quick and straightforward construction, shortening project timelines and controlling costs effectively.

Performance Characteristics of Phosphate Castables

Phosphate castables, consisting of refractory aggregates, powders, and additives mixed in precise proportions, demonstrate exceptional performance characteristics. Their high strength, minimal strength degradation at intermediate temperatures, excellent high-temperature performance, and strong acid resistance make them suitable for various industrial applications.

During the preparation of phosphate castables, materials like phosphoric acid, magnesium phosphate, sodium hexametaphosphate, and sodium tripolyphosphate are used as binders. These materials form refractory castables that develop strength upon heating at ambient temperature, known as heat-setting refractory castables.

To promote material solidification, additives such as aluminate cement, aluminum hydroxide, ammonium fluoride, magnesium oxide, and talc are used as accelerators. A crucial step in production is the curing process, which requires at least 16 hours. This is because the binder and refractory raw materials' iron content may react, potentially causing expansion and cracking. However, using inhibitors to create a negative charge on the iron surface forms a thin film, preventing or slowing the reaction between anions and iron, thus allowing direct production and construction without the need for extended curing.

Phosphate castables are suitable for temperatures ranging from 1400-1600℃, outperforming aluminate cement castables in various industrial furnaces. Despite some disadvantages like higher costs and slightly complex construction processes, their outstanding performance ensures widespread use across numerous industrial sectors.

Construction Methods for Phosphate Castables

Mixing and Adding Water

Before construction, phosphate castables must undergo rigorous quality inspection to ensure they are within their shelf life and free from caking or hardening. Specialized castables should be used for high-temperature areas like kiln mouths and coal injection pipes, while manufacturer-specified materials can be used for other areas.

When switching between different types of castables, it is essential to thoroughly clean the mixer of debris and residues. To ensure construction quality, the mixer should be cleaned and soaked with adhesive. During construction, different types of castables or products should not be mixed.

Each bag of castables contains a small packet of powder. Before use, the large and small materials should be mixed for 1 to 2 minutes. Once evenly mixed, approximately 10% of adhesive should be added for mixing. It is crucial to use the entire bag of castables to ensure uniformity.

The amount of adhesive added to phosphate castables is critical. Different types of castables require varying amounts of adhesive. The adhesive amount significantly impacts the quality of the refractory castables and must be strictly controlled as specified. Typically, the adhesive amount for phosphate castables is around 10% to 12%.

Mixing should be done in a forced mixer, starting with 2 minutes of dry mixing, followed by adding adhesive and mixing for 3 to 6 minutes before pouring. To ensure uniform mixing, the weight of refractory castables for each batch should be controlled between 200 to 300 kilograms.

Additionally, castables mixed with adhesive must be used within 30 minutes, with time calculated from when the adhesive is added. Any castables not used within the specified time or that have hardened should not be mixed with adhesive again or reused to ensure construction quality and material performance.

Pouring

Before pouring, ensure the equipment is clean and dust-free for a conducive environment. The surface of refractory bricks or old castables in direct contact with the new castable should be pre-cleaned with adhesive to enhance high-temperature bonding.

Expansion joints must be set according to design requirements. If not specified, expansion joints are typically set at an area of 1.5~2.0M2. These joints can be precisely set using 3mm thick plywood or cardboard. It is crucial that each small area divided by expansion joints is poured continuously to ensure integrity. During pouring, no interruptions are allowed, and the entire area must be poured and vibrated before the castable begins to set.

After pouring into the mold frame, immediately use a vibrating rod for vibration. The process should be meticulous and moderate to ensure proper vibration without over-vibrating, which could cause segregation and affect quality. Generally, when surface slurry appears, gently remove the vibrating rod while vibrating.

Avoid sudden power outages during vibration to maintain continuity and uniformity. Once vibrated, do not reinsert the vibrating rod to prevent structural damage.

For dead corners caused by obstacles or areas where it is challenging to install molds and pour, manual layered tamping can be used. In such cases, increasing the adhesive amount appropriately ensures thorough pouring into every corner, leaving no unpoured areas.

Curing

After construction, curing phosphate castables is crucial. Generally, the poured body requires 24 hours of curing with the mold to ensure full solidification before demolding. In colder conditions or high-strength castable areas, extend the curing and demolding time to ensure material stability and strength.

After demolding, inspect the casting quality. If honeycombs, pitting, or voids are found, decide whether to repair or redo to ensure overall project quality. Also, repair non-essential protrusions or indentations promptly for a smooth surface.

During static curing, avoid water exposure to prevent performance degradation. Additionally, prevent heavy impacts to protect the curing castable from damage. These meticulous curing measures ensure phosphate castables achieve optimal performance post-construction.

Phosphate Refractory Castable Usage Considerations

When using phosphate refractory castables, several key considerations must be remembered. Firstly, if no accelerator is added during mixing and forming, natural setting for two hours allows heating and subsequent demolding. If an accelerator is added, curing can occur in an environment above 10℃.

At temperatures above 20℃, demolding is possible after 3 to 5 hours of static curing. If the temperature is not reached, extend static curing time by 5 hours before demolding. Throughout the curing process, ensure complete isolation from water to avoid adverse performance impacts.

The entire curing cycle lasts 3 days, requiring a suitable curing environment and strict adherence to demolding times and waterproofing. Following these guidelines ensures phosphate refractory castables maintain superior performance and stability during use.

Storage of Phosphate Refractory Castables

Storage conditions significantly affect the performance of phosphate refractory castables, especially between indoor and outdoor environments. When stored indoors, compressive strength increases with time, whereas outdoor storage may gradually reduce post-construction strength. Notably, indoor-stored phosphate castables exhibit significantly higher strength than those stored outdoors, with compressive strength differences of several MPa, and burn compressive strength at least 10MPa higher.

For optimal performance, store phosphate refractory castables indoors, particularly during winter, to avoid temperature fluctuations and the adverse effects of dry-wet cycles. Adjusting phosphate solvent amounts and adding suitable inhibitors can further enhance forming pressure.

Although storage environment impacts room temperature performance, it does not significantly affect high-temperature performance. However, for phosphate refractory castables without accelerators, heat treatment is necessary before storage to prevent deliquescence and maintain effectiveness.

In low-temperature storage environments, white crystalline deposits may form on the castable surface, primarily aluminum and calcium salts. These salts may dissolve at high temperatures or with prolonged heating, but excessive deposits can loosen the castable's internal structure, reducing strength. Therefore, maintaining an appropriate indoor environment and controlling temperature during storage is crucial to prevent negative impacts on the material.

Factors Affecting Phosphate Bonded Castables' Performance

1.Binder Concentration and Amount

The concentration and amount of binder play a crucial role in phosphate bonded castables' performance. Only with appropriate binder concentration and amount can the castable exhibit excellent performance. From a forming perspective, if the phosphate concentration is moderate but insufficient, the castable will be too dry and difficult to form. Conversely, excessive binder results in a slurry castable with slower hardening post-formation.

Binder concentration and amount also impact compressive strength at room temperature. With constant primary material composition and accelerator amount, compressive strength decreases with increasing phosphate concentration. However, adding high amounts of binder significantly enhances compressive strength.

Additionally, binder concentration and amount influence high-temperature compressive strength. When phosphate concentration and amount reach an optimal range, the castable exhibits excellent high-temperature performance and strength.

2.Amount of Primary Material

The amount of primary material significantly affects phosphate bonded castables' performance. In particular, the aluminum powder ratio must be reasonable. If the ratio is too high, aluminum powder will not fully react during the forming process, resulting in material wastage and decreased strength.

Conversely, an insufficient ratio will lead to incomplete phosphate reactions, reducing the castable's overall performance. Finding the optimal primary material ratio ensures phosphate bonded castables achieve excellent strength and stability.

3.Impact of Inhibitors

Inhibitors can effectively slow down the reaction rate between phosphates and the primary material, thereby improving the castable's setting time and performance. However, excessive inhibitors can adversely affect the castable's performance. Therefore, the type and amount of inhibitors must be carefully selected to ensure phosphate bonded castables maintain optimal performance.

Conclusion

Phosphate refractory castables are a versatile and high-performance material essential in high-temperature furnace construction across various industries. Their superior refractoriness, wear resistance, corrosion resistance, and ease of construction make them a valuable asset in industrial applications. Understanding the preparation, performance characteristics, and usage considerations of phosphate castables is crucial for maximizing their benefits and ensuring the success of high-temperature projects.