Specifying Insulating Castable Refractories
Education is key for enabling decision-makers to select a product that is best-suited to each individual application.
Industrial furnaces and kilns rely on high-temperature insulation materials to optimize production yield and minimize energy costs, which can rise rapidly if excessive heat escapes from the point of operation. Insulating castable refractory materials are key to this energy-saving process due to their inherent low heat conductivity, as well as advantages derived from ease of placement and structural strength. However, with so many local, national and global manufacturers delivering to market myriad material technologies and products, accurate specification is a highly challenging task.
Customer requirements for higher performance products, and installer needs for easy-to-apply materials, drive the leading refractory manufacturers worldwide to continue to invest heavily in the research and development of next-generation industrial insulation materials. The aim is to bring to market castable products that combine optimum insulation performance with other important attributes, such as strength, operator safety and ease of installation.
Insulating castable refractory systems containing alternative, high-performance core monolithic ingredients, such as crushed insulating firebrick (IFB), are now an increasingly popular specification staple for complex high-temperature applications. That said, the use of conventional raw materials such as perlite (an amorphous volcanic glass) and vermiculite (a hydrous phyllosilicate mineral) remains prevalent in many sectors. Despite crushed IFB-containing insulating castable mixes outperforming conventional material choices in both application precision and product performance, habitual specification behavior is preventing customers in certain industries from moving in favor of better alternatives. As with any change in specification, education is key for enabling decision-makers to select a product that is best-suited to each individual application in accordance with environmental factors, application considerations, desired outcome and, of course, cost.
With advances in materials technology set to continue and product variety expected to increase even further, specification best practice will become continually more challenging to apply. With that in mind, it is vital for specifiers to obtain and uphold a detailed understanding of the essential products, their technical capabilities, application processes and how each one can facilitate or hinder primary drivers, including installation, lifetime cost, and energy efficiency.
Understanding the Ingredients of Each Castable Mix
On the face of it, all insulating castable refractories look the same, comprising a mixture of aggregates, cement and additives such as clay and fillers. When mixed with water, they form a slurry suitable for application via casting, gunning, ramming, pouring or plastering, and in some compositions, pumping and shot-creting. It is important to realize that all castable refractories can be different, and therefore should not be commoditized. By learning the difference between each castable type, specifiers, contractors and installers can select and install a product that is better-matched to their application, delivering improved energy and output performance, increased lifespan and associated cost efficiencies as a result.
The best way to facilitate an ongoing learning curve is by partnering with an established and knowledgeable manufacturer that can not only encourage best practice throughout the specification process, but will also assist specifiers and procurement teams in making the right purchasing decision on a site-by-site basis, in accordance with customer requirements. The difference between working closely with a manufacturer and seeking a commodity castable refractory solution is simple. A highly experienced and well-established manufacturer has refractory products to suit even the most complex insulation challenge, balancing properties such as density, strength, and thermal conductivity. This ability is particularly useful when specifying for an environment that is particularly harsh or requires a specific method of application.
It is also important to understand that while raw materials in insulating castables vary, three main “core” aggregate raw materials are used to form a variety of insulating castable refractory products: perlite, vermiculite and crushed IFB.
Perlite is a completely natural siliceous volcanic mineral that is formed by the sudden cooling and solidification of volcanic ash, which traps crystalline water into its masses. Used widely in construction, as well as agriculture for the aeration of soil, perlite is mined throughout the U.S., Greece, China and Italy.
World reserves of perlite are estimated at 700 million metric tons (Mt), with around 1.5 Mt being mined and processed each year. Characterized by its ability to expand to up to 20 times its original size when rapidly heated to 1,472°F and 1,742°F (800°C and 950°C), perlite is essentially a mass of minuscule glass bubbles, which give it the insulating properties for which it is known.
Vermiculite is a hydrous phyllosilicate mineral that occurs naturally as an alteration product when certain types of rocks form next to each other. When heated to around 572°F, exfoliation occurs and vermiculite expands to approximately 30 times its original size. Large commercial vermiculite mines are located in Russia, South Africa, China and Brazil, producing material for a variety of industries. For insulation purposes in certain mixes, vermiculite and perlite can withstand temperatures of up to 2,000°F and 2,100°F (1,093°C and 1,149°C), respectively, before excessive shrinkage occurs.
Used as an alternative core raw material for making insulating castable refractories, typical cast process crushed IFB offers superior heat-resistance capabilities of up to 2,800°F (1,538°C). Having already been fired to a high temperature during the brick manufacturing process, crushed IFB is a pre-shrunk aggregate which, when mixed to make a castable refractory, contracts very little during high-temperature use.
With the inherent structural strength capacity of an IFB and a density of 34 PCF (545 kg/m³) compared to perlite’s 8 PCF (128 kg/m³), monolithic castable mixes that use crushed IFB as the core material will not only perform extremely well in high temperatures, but can also be formulated specifically to offer increased strength and thermal insulation performance in harsh furnace and kiln environments. While a number of manufacturers worldwide promote IFB, very few crush special cast-produced IFB for use in monolithic castable refractories.
Key Specification Criteria
With a clearer understanding of the three main core raw materials in insulating castables, the next question is: Which base aggregate should be used? A number of criteria should be considered when specifying insulating castable refractories, including the method and complexity of application, the quality and cost of the product, and the environment in which the product is expected to perform. If we get these three elements right and the best-suited product is specified and correctly installed, the material should deliver optimum furnace or kiln performance and improved energy efficiency over a longer lifespan.
Taking an industrial or commercial furnace or kiln out of operation is inconvenient and incredibly expensive, so specifying an insulating castable refractory that is quick and efficient to apply while providing long and reliable service is of great benefit to the end user. Two main concerns should be taken into consideration when selecting a product that will facilitate a predictable and efficient application: ease of use (generally by casting or gunning) and product loss (usually via rebound or material compaction).
Insulating castable products that are deemed easy to install are consistent in production and can be applied under a variety of conditions. Cast process crushed IFB-based castables have a consistent density and particle size, enabling tight control on water addition and resulting in a smooth castable with good flow characteristics. These castables also lend themselves to installation by gunning and pumping, since a more porous aggregate tends to clog the hoses. It is this application downfall that has seen many specifiers and contractors move in favor of castable materials using raw material technologies such as crushed IFB, so that material costs can be more accurately controlled prior to application.
The other key consideration here is “rebound,” which is the term used during installation to describe the situation when gunned material falls off the walls or ceiling onto the floor. Waste caused by rebound is usually the aggregate, which is why leading manufacturers have engineered specific formulations to minimize rebound to as low as 10% while providing greater consistency of the installed product.
Finally, material compaction occurs when the gunned castable mixture compacts when being installed on the wall due to the force of application, requiring additional material in order to deliver the desired thickness. Despite its beneficial lightweight characteristics, perlite-based castable products are known to compact up to 20% when gunned, which can make what is at first glance a cost-effective material a more expensive overall installation. Meanwhile, IFB-based insulating castables suffer very little, if any, on the wall-gunned compaction since the hard-fired raw material does not easily break down during the application process.
Due to the uniform and reliable manufacturing methods used in creating crushed IFB insulating castable refractories, installers can also benefit from simplified and consistent application processes. Monolithic refractories with a core of crushed IFB mix into a smooth, homogenous “ball-in-hand” consistency, compared with other insulating castables that are typically grainy and less cohesive. The consistency of IFB mixes allows for more precise control during application, requiring less air or water adjustments and potential surging during the gunning process.
Quality vs. Cost
The quality vs. cost argument is an age-old specification problem, especially when working with large companies that have an in-house procurement team tasked with identifying cost savings. Tackling this issue in accordance with best practices means engaging with both the technical and purchasing teams to aid a process of understanding. Put simply, by encouraging an appreciation of the benefits that a better-quality product can offer in the long run, when compared with a lesser-quality material with a more attractive perceived initial cost, specifiers can guide other decision-makers in the purchasing chain to opt for a refractory that not only delivers enhanced performance and product reliability, but a more sustainable whole life cost as well.
It can even be said that an application that only requires a low to moderate level of thermal insulation could reap the benefits of “over-specifying” on quality in order to enjoy better whole life costs and minimize the risk of costly kiln failure. A good example of this would be the purchase of a $1,100/metric ton castable material rather than a $1,000/metric ton alternative, which might potentially deliver more reliable product service life, as well as added performance, insulation and speed of installation benefits that come with a better quality product. One has to look at the total cost: the price of the material, the installation production rate, the density on the wall, the installed material performance and service life.
Specifying on a Project-by-Project Basis
It is not uncommon for specifiers to have preferred manufacturers or suppliers for materials or building products that they use on a regular basis. For some materials, including insulating castable refractories, this approach is not always conducive to best practice.
Commercial and industrial furnaces and kilns can be subject to a variety of different application-specific factors, and a number of operational variables may be at play too, which will shape the specification requirement. The key here is to really get to know the environment you are specifying for, so that you can recommend a product that will provide adequate insulation, performance and lifespan.
The simplest example of having to specify on a project-by-project basis is that of operating temperature. While all furnaces rely on intense heat, a significant difference can still exist in temperature between one environment and the next. As not all monolithic refractories offer thermal resistance to the same level, a furnace or kiln that operates at 2,000°F, for example, could be insulated with a perlite, vermiculite or crushed IFB-based refractory; an alternative environment reaching much higher temperatures would rule out perlite and vermiculite mixes completely.
The formulation of the mix will change depending on the temperature requirements of each project, with more cement and a denser aggregate providing increased strength, and less cement but a better insulating aggregate being most suitable for higher temperature operations. This is true for a number of environments within the ceramics sector, such as the manufacture of small ceramic spheres for liquefied natural gas (LNG) fracking, which requires a high-strength castable capable of performing in extremely high temperatures. An established manufacturing partner will be able to assist in specifying the right mix for the job, providing guidance on best practice and how to accommodate the change in formation with appropriate application methods.
Additional important considerations include the presence of contaminants in the operator’s process, which will require a purer castable refractory, as well as the issue of “thermal cycling,” which describes the scenario where a furnace or kiln is heated then cooled frequently during operation. This constant change in temperature may cause cracking in a lower-strength castable, while an insulating castable mix formulated with a pre-shrunk core material, like IFB aggregate, would be more suitable.
Changing the Specification Habits of a Lifetime
Many areas of the supply chain can be resistant to change, especially in environments where planned downtime or furnace failure is extremely costly. It is this resistance, as well as a focus on simple material price, that is slowing the shift toward better materials technologies in some sectors, despite the obvious benefits. When considering best practices, the unfortunate truth is that the very nature of specification can bring about habitual behaviors, which can eventually lead to sub-optimal product choices if decision-makers do not keep up to speed with technological advances and market changes. However, it is crucial to remember that improved castable refractory materials offer enhanced performance, better insulation and ultimately, energy and costs savings over the life of the product, so they should be embraced as early as possible.
For additional information, visit www.morganthermalceramics.com/f&krefractories.