An individual new to the refractories industry has one question on his or her mind: what materials are used in refractories? Refractory materials must be physically and chemically stable, as well as able to retain strength at high temperatures. They need to be chemically inert so they do not react with other materials or affect purity. They must also be thermal shock resistant and have a low coefficient of thermal expansion. Refractory materials typically include magnesite, bauxite, alumina and silica.
Types of refractories are chosen based on the environment in which they are used. Refractories are used in various product industries, such as cement and lime, energy, chemicals, non-ferrous metals, glass, iron, steel, and environmental technology. For example, tundish refractories are essential to the manufacture of cast steel. The tundish feeds molten steel into molds. Because steelmaking occurs under extreme temperature conditions (steel melts at 3,000°F), the tundish refractory must continue to perform under high heat while preventing thermal loss, oxidation and corrosion. This can be achieved with the addition of certain chemical additives.
In order to maintain a focus on performance and quality, the refractories industry looks to functional, high-performance additives in its formulations. One of the primary additive categories offering the highest value is phosphates. Phosphates improve refractories by enhancing chemical resistance, oxidation and durability at higher temperatures. They also provide abrasion resistance.
Sectors such as furnaces, kilns, incinerators, reactors and steelmaking rely on performance additives to create long-lasting, high-quality refractories. Binders allow control over hardening and setting when added to mixes. Deflocculants allow a better flow of materials to make a homogeneous mix. In ceramics, deflocculants create a significant increase in the viscosity of glazes.
Corrosion inhibitors are also important additives, preventing oxidation and maintaining the integrity of the refractory. Increasing rebuild intervals, coupled with decreased downtime, significantly improves the refractory output and profitability. For example, by reducing cement content and designing a phosphate-bonded castable vibratable, maximum strength can be reached at approximately 1,000°F after installation and maintain stable/intact bond strength through an elevated operating temperature of 2,000°F.
Aluminum phosphates are ideal for use in high-temperature, phosphate-bonded refractories based on alumina. Aluminum phosphates are also used in refractory mixes. When blended with dry raw materials, they become a single-component composition that can be stored on-site almost indefinitely and activated with water when needed. This allows the refractories industry to create ready-made dry products that can be activated with water on-site, which is especially suitable for spraying and gunning applications with monolithic refractories (specifically castable refractories). An example would be cement castable installation, where dry mixes can be transported to the worksite without the need to pre-mix the material. This also eliminates waste because material can be mixed as needed.
In materials containing clays, monoaluminum phosphates are widely used as a bonding agent, reacting at low temperatures with acidic raw materials by neutralization. At temperatures above 200°C, additional bonding occurs due to the formation of polymeric phosphates. The phosphate bond remains stable until the sintering starts, offering oxidation protection during start-up. Monoaluminum phosphate-bonded refractory masses and brick show excellent strength even at low temperatures, especially when clay or aluminum hydroxide is added. Monoaluminum phosphates are highly suitable for standard brick products, rotary kilns and refractory anchors. High-quality, high-purity aluminum metaphosphate and monoaluminum phosphate for refractories applications can reduce the possibility of out-gassing and spalling from tramp iron that can be found in other raw materials.
When seeking a binder that remains stable in high-temperature environments, boron phosphate anhydrous is an alkali-free, powder-type additive for use in unshaped refractories of raw materials containing alumina. It produces extremely strong bonds with the highest temperature resistance and is recommended to prevent spalling during repeated thermal cycling. It is much less susceptible to spalling than other cement-bonded materials. Because these bonds are so strong, this binder is often used to repair existing refractories. Another benefit of using boron phosphate is that it contributes to low thermal expansion, high thermal conductivity and high strength in the refractory. Like aluminum phosphates, boron phosphate anhydrous is available as a component of dry mixes. What’s more, it can extend the mix’s shelf life. Beyond the refractories industry, boron phosphate also has applications in flame retardants, ceramic bodies, frits and glazes.
An effective deflocculant in activated dry mixes is Fabutit 734, a modified sodium tripolyphosphate manufactured by Chemische Fabrik Budenheim. In dry mixes, it can be used as a dispersant to prevent clumping and reduce the amount of water needed. Compared to commodity sodium tripolyphosphate, Fabutit 734 allows for better deflocculation at a lower addition ratio, making it a cost-effective solution for refractories manufacturers. It also particularly excels in solubility and efficiency. It has been proven to be an effective additive in castables (e.g., floors surrounding furnaces, thermal cycling areas, incinerators). The difference between the Fabutit 734 vs. standard sodium tripolyphosphate is illustrated in Figure 1. High solids content can be obtained even with products considered difficult to liquefy.
As many producers are now focused on high-quality refractories, research and development work is undergoing a resurgence. Many producers are seeing that the combination of qualified raw materials and the appropriate binder systems yields refractories with the following properties:
- High strength, even at low temperatures
- High abrasion resistance
- Slag repellent
- Melt repellent
- Improved chemical resistance
- High refractory properties
- Improved durability
This is just a sampling of the properties that are achievable with the qualified ingredients, such as specialty phosphates. The modern refractories industry continues to be strong. According to research by Global Industry Analysts, the refractories industry will continue to be influenced by its main consumer, the steelmaking industry. Research and Markets, which published the Global Industry Analysts report, states, “Though end-user sectors other than the steel industry consume lesser amounts of refractories, their influence on the industry is still very important, and there are a considerable number of refractories manufacturers that offer specific high-value products for these sectors, which include cement, glass, ceramics and non-ferrous metals.” With the variety of high-quality additives available on the market today, manufacturers across these industries have the resources to create lasting refractories barriers that increase productivity, efficiency and profitability.
Source : https://www.ceramicindustry.com/articles/94619-choosing-the-right-raw-materials-for-refractories-production