Insulating Castable Refractories – Specification Best Practice for Furnace and Kiln Applications
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 a myriad of material technologies and products, accurate specification is a highly challenging task.
Lance Caspersen, from Morgan Advanced Materials, one of the world’s leading producers of Insulating Firebricks (IFBs) and lightweight castable refractories under the K® IFB and Kaolite® insulating castable brand names, examines the key considerations for specifying castable refractory insulation systems, offering advice on achieving thebest value solutions which minimise energy consumption, optimize outputs and meet the needs of both the installer and end user.
Customer requirements for ever higher performance products, and installer needs for easyto 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 which combine optimum insulation performance with other important attributes, such as strength, operator safety and ease of installation.
Specifying insulating castable refractory systems has become a real challenge, with those containing alternative, high-performance core monolithic ingredients, such as crushed IFBs, 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 behaviour is preventing customers in certain industries from moving in favour of better alternatives. As with any change in specification, education is key to enable decision-makers to select a product which 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 key products, their technical capabilities, application processes and how each one can facilitate or hinder key 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 will 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 realise that all castable refractories can be different, and therefore should not be commoditised. By learning the difference between each castable type, specifiers, contractors and installers can select and install a product which 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; something which is particularly useful when specifying for an environment which is particularly harsh or requires a specific method of application.
It is also important to understand that while raw materials in insulating castables vary, there are three main ‘core’ aggregate raw materials on the market, used to form a variety of insulating castable refractory products. Before we go any further, it is important to assess these key ingredients.
Perlite is a completely natural siliceous volcanic mineral, 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 US, Greece, China and Italy.
World reserves of perlite are estimated at 700 million tonnes, with around 1.5 million tonnes 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 which 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. There are large commercial vermiculite mines 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.
Crushed Insulating Firebricks
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 insulating fire brick and a density of 34PCF (545kg/m³) compared to perlite’s 8PCF (128kg/m³), monolithic castable mixes which 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 there are a number of manufacturers worldwide that promote Insulating Fire Bricks, there are very few which crush special cast produced IFBs for use in monolithic castable refractories, making Morgan Advanced Materials the leading innovator in this area of materials technology.
Key specification criteria
With a clearer understanding of the three main core raw materials in insulating castables, the next question is: which base aggregate to choose? There are a number of key criteria which would be considered best practice in specifying insulating castable refractories. These include the method and complexity of application, the quality and cost of the product and the environment in which the product is expected to perform. Get these three elements right and the product specified, assuming it has been correctly installed, should deliver optimum furnace or kiln performance and improved energy efficiency over a longer lifespan. Here we look at the three variables in more detail.
Taking an industrial or commercial furnace or kiln out of operation is inconvenient and incredibly expensive, so specifying an insulating castable refractory which is quick and efficient to apply, while providing long reliable service, is of great benefit to the end user.
There are two main concerns 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 which are deemed easy to install are consistent in production and can be applied under a wide variety of conditions. Cast process crushed IFB based castables have a consistent density and particle size enabling tight control on water addition resulting in a smooth castable with good flow characteristics. They also lend themselves to installation by gunning and by pumping since a more porous aggregate will tend to clog the hoses.
It is this application downfall which has seen many specifiers and contractors to move in favour 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 name 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 manufactures like Morgan have engineered specific formulations to minimize rebound to as low as 10% while providing greater consistency of the installed product.
Finally, material compaction is 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.
Thanks 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 which are typically grainy and less cohesive. The consistency of IFB mixes allow for more precise control during application, requiring less air or water adjustments and potential surging during the gunning process.
The quality vs cost argument
The quality vs cost argument is an age-old specification problem, especially when working with large companies with an in-house procurement team tasked with identifying cost savings.
Tackling this issue in accordance with best practice means engaging with both the technical and purchasing teams to aid a process of understanding. Put simply, by encouraging an appreciation of the benefits which 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 within the purchasing chain to opt for a refractory which not only delivers enhanced performance and product reliability, but a more sustainable whole life cost too.
It can even be said that an application which 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 which 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 whom they use on a regular basis. For some materials though, 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 there may be a number of operational variables 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, there can still be a significant difference in temperature between one environment and the next. As not all monolithic refractories offer thermal resistance to the same level, a furnace or kiln which operates at 2,000°F, for example, could be insulated with a perlite, vermiculite or crushed IFB based refractory, but 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 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.
Other important considerations here are 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 towards better materials technologies in some sectors, despite the obvious benefits.
When considering best practice, 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.