Insulating Castable Refractories: Best Practice for Kiln and Furnace Application

Industrial kilns and furnaces rely on high temperature insulation materials to optimize production yield and reduce energy costs, which can increase rapidly if excessive heat escapes from the operation point.

Due to their inherent low heat conductivity and other benefits derived from structural strength and ease of placement, insulating castable refractory materials are key for this energy-saving process. However, accurate specification is an extremely challenging task for many local, national, and global manufacturers who supply a wide range of material technologies and products.

Lance Caspersen, from Morgan Advanced Materials, one of the world’s leading manufacturers of Insulating Firebricks (IFBs) and lightweight castable refractories under the K® IFB and Kaolite® insulating castable brand names, examines the main factors for specifying castable refractory insulation systems and offers suggestion for achieving the best value solutions that optimize outputs, reduce energy consumption, and meet the requirements of both the end user and installer.

The installers’ requirements for easy–to-apply materials and the customers’ needs for higher performance products drive the world’s leading refractory manufacturers to continuously and significantly invest in the research and development of advanced industrial insulation materials.

The goal is to bring insulating castable products to the market. These castable products combine optimal insulation performance with other important attributes, like strength, ease of installation, and operator safety.

Insulating castables found as primary and back-up linings in many Industrial and Energy applications.

Figure 1. Insulating castables found as primary and back-up linings in many Industrial and Energy applications.

Insulating castable refractory systems that contain alternative, high-performance core monolithic ingredients, like crushed IFBs, are now an increasingly popular specification staple for complex high-temperature applications, and specifying these systems has become a real challenge.

That said, the application of coventional raw materials such as vermiculite, a hydrous phyllosilicate mineral, and perlite, an amorphous volcanic glass, remains prevalent in many sectors. Although crushed IFBs, which contain insulating castable mixes, outperform traditional material choices in terms of product performance and application precision, habitual specification behavior continues to prevent customers in certain industries from selecting better alternatives.

As with any modification in specification, education is key to allow decision-makers to choose the best suitable product for each individual application according to environmental factors, desired outcome and cost, and application considerations.

As advancements in materials technology are set to continue, and product variety is likely to increase even further, applying the best specification practice will become more and more challenging. Keeping this factor in mind, it is important for specifiers to obtain and uphold a thorough knowledge of the key products, their technical capabilities and application methods, and how each product can hinder or facilitate main drivers, including installation, energy efficiency, and lifetime cost.

Understanding the Ingredients of Each Castable Mix

All insulating castable refractories look the same, containing a mixture of cement, aggregates and additives, like fillers and clay. When mixed with water, these castable refractories will form a slurry that is suitable for application via pouring, casting, plastering, ramming or gunning, and in some compositions, shot-creting and pumping.

It must be realized that all castable refractories can be different, and hence they should not be commoditized. By understanding the difference between each type of castable, contractors, specifiers and installers can select and install a product that suits their application, and as a result, deliver improved energy and output performance, increasing lifespan and associated cost efficiencies as a result.

The best way to enable an ongoing learning curve is by partnering with a knowledgeable and established manufacturer that can encourage best practice throughout the specification process and also help specifiers and procurement teams to make the correct purchasing decision on a site-by-site basis, according to customer needs.

The difference between working in close cooperation with a producer and looking for a commodity castable refractory solution is simple. A well-established and highly experienced manufacturer can offer refractory products to meet the requirements of even the most complicated insulation challenge, balancing properties such as strength, density, and thermal conductivity. This capability is especially useful when specifying for harsh environments or for environments where a specific method of application is required.

While raw materials in insulating castables can differ, there are three key ‘core’ aggregate raw materials available on the market that can be used to create a range of insulating castable refractory products. It is important to evaluate these key ingredients.

Perlite

Perlite, a completely natural siliceous volcanic mineral, is formed by the rapid cooling and solidification of volcanic ash, which traps crystalline water into its masses. It is widely used in construction and in agriculture for soil aeration. It is mined across the United States, China, Greece, and Italy.

Perlite, 0.5 lbs

Figure 2. Perlite, 0.5 lbs

The estimated world reserves of perlite are 700 million tons, with around 1.5 million tons being mined and processed every year. When rapidly heated to 1,472°F and 1,742°F (800°C and 950°C), perlite expands up to 20 times its original size. It is essentially a mass of tiny glass bubbles, which give it the insulating properties for which it is known.

Vermiculite

A hydrous phyllosilicate mineral, vermiculite occurs naturally as an alteration product when certain types of rocks form next to each other. Exfoliation occurs and vermiculite expands up to 30 times its original size when heated to around 572°F.

Large commercial vermiculite mines are located in South Africa, Russia, Brazil, and China, producing material for a wide range of industries. In certain mixes for insulation purposes, 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.

Vermiculite, 0.5 lbs

Figure 3. Vermiculite, 0.5 lbs

Crushed Insulating Firebricks

Standard cast process crushed IFB is used as an alternative core raw material for making insulating castable refractories, and it also provides better heat-resistance capabilities of up to 2,800°F (1,538°C). The crushed IFB, which is already fired to a high temperature during the brick manufacturing process, is a pre-shrunk aggregate that contracts slightly during high temperature use when mixed to make a castable refractory.

Insulating Firebrick, crushed, 0.5 lbs.

Figure 4. Insulating Firebrick, crushed, 0.5 lbs.

When compared to perlite’s 8 PCF (128 kg/m³), monolithic castable mixes have a density of 34 PCF (545 kg/m³) and an inherent structural strength capacity of an insulating fire brick. As monolithic castable mixes employ crushed IFB as the core material, they perform exceptionally well in high temperatures and can also be formulated specifically to provide improved strength and thermal insulation performance in harsh kiln and furnace environments.

While Insulating Fire Bricks are promoted by several global manufacturers, there are very few that crush special cast produced IFBs for use in monolithic castable refractories. This makes Morgan Advanced Materials a top innovator in the field of materials technology.

Key Specification Criteria

With a better understanding of the three core raw materials in insulating castables, the next question is: which base aggregate to select? There are several key criteria that would be considered best practice when specifying insulating castable refractories, including the cost and quality of the product, the environment in which the product is anticipated to perform, and the method and complexity of application.

If these three main elements are right, the product specified, assuming it has been properly installed, will be able to provide optimum kiln or furnace performance as well as improved energy efficiency over a longer lifespan. The following sections review the three variables in more detail.

Application Consistency

Taking a commercial or industrial kiln or furnace out of operation is inconvenient and very expensive. Therefore, specifying an insulating castable refractory that is quick and efficient to apply and provides long reliable service is highly advantageous to the end user.

The selection of a product that facilitates a predictable and efficient application involves two major concerns: Ease of use generally by gunning or casting and product loss usually via rebound or material compaction.

Insulating castable products are seen as easy to install, consistent in production, and can be applied under various conditions. Cast process crushed IFB based castables have a uniform particle size and density, enabling tight control on water addition. This results in a smooth castable with good flow characteristics.

Cast process crushed IFB-based castables lend themselves to installation by pumping and by gunning since a more porous aggregate will tend to clog the hoses.

It is this application drawback which has seen many contractors and specifiers to move in favor of castable materials using raw material technologies such as crushed IFB in order to more accurately control material costs before application.

‘Rebound’ is another key consideration and is the name used during installation to explain the situation when gunned material falls off the ceiling or walls onto the floor. Waste caused by rebound is typically the aggregate, which is why leading manufacturers like Morgan have developed specific formulations to reduce rebound to as low as 10%, while delivering greater consistency of the installed product.

Gunning application for a furnace roof

Figure 5. Gunning application for a furnace roof

Finally, material compaction is when the gunned castable mixture compacts due to the force of application when being installed on the wall, requiring additional material to provide the desired thickness.

Despite their beneficial lightweight characteristics, perlite-based castable products tend to compact up to 20% when gunned, which can make what is at first sight a cost-effective material, a more expensive overall installation. Conversely, IFB-based insulating castables suffer very little, if any, on the wall gunned compaction because the hard fired raw material does not easily break down during the application process.

The uniform and reliable manufacturing techniques used in producing crushed IFB insulating castable refractories provide the benefit of simplified and consistent application processes to installers.

Compared with other insulating castables that are normally grainy and less cohesive, monolithic refractories with a core of crushed IFB mix into a smooth, homogenous ‘ball in hand’ consistency. Thanks to the consistency of IFB mixes, more precise control can be achieved during application with less water/air adjustments and potential surging during the gunning process.

The Quality vs Cost Argument

The quality vs cost argument is a longstanding specification problem, particularly for large companies with an in-house procurement team tasked with identifying cost savings.

Addressing this issue according to best practice means engaging with both the technical and procuring teams to aid a process of understanding.

In simple terms, by encouraging an appreciation of the advantages that can be offered by a better quality product in the long term compared to a lesser quality material with a more attractive perceived initial cost, specifiers can direct other decision-makers in the purchasing chain to select a refractory that not only delivers better product reliability and performance, but also a more sustainable whole life cost.

It can even be said that applications that only requires a low to moderate level of thermal insulation could reap the benefits of ‘over-specifying’ on quality to reduce the risk of costly kiln failure and enjoy better whole life costs.

One good example is purchasing a $1,100/metric ton castable material instead of a $1,000/metric ton alternative, which might potentially offer more reliable product service life in addition to better insulation, performance, and speed of installation benefits that are expected from a better quality product.

The total cost should be considered: the price of the material, the density on the wall, the installation production rate, 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 who they use on a regular basis. However, this approach is not always conducive to best practice for some materials including insulating castable refractories.

Industrial and commercial kilns and furnaces can be subject to various different application-specific factors, and there may be several operational variables at play too, which will determine the specification requirement. The key here is to really understand the environment that is to be specified, so that a product that will offer adequate performance, insulation, and lifespan can be recommended.

Operating temperature is the simplest example of having to specify on a project-by-project basis. While all furnaces rely on intense heat, there can still be a considerable difference in temperature between one environment and another.

As not all monolithic refractories provide thermal resistance to the same level, a kiln or furnace which operates at 2,000°F, for instance, could be insulated with a vermiculite, perlite, or crushed IFB-based refractory. However, perlite and vermiculite mixes would not be considered in an alternative environment that reaches much higher temperatures.

Depending on the temperature requirements of each project, the formulation of the mix will differ, with more cement and a denser aggregate providing increased strength, and less cement but a better insulating aggregate being ideal for higher temperature operations.

This applies for several environments in the ceramic industry, including the manufacture of small ceramic spheres for LNG fracking, which needs a high-strength castable that has the ability to perform in extremely high temperatures.

An established manufacturing partner will be able to help in specifying the right mix for the project, offering guidance on best practice and how to accommodate the change in formation with appropriate application methods.

The presence of contaminants in the operator’s process, which will need a purer castable refractory, and the problem of ‘thermal cycling', which describes the situation where a furnace or kiln is heated and cooled frequently during operation, are other significant considerations.

This constant change in temperature may lead to cracking in a lower strength castable, while an insulating castable mix formulated with a pre-shrunk core material, like IFB aggregate, would be ideal.

Changing the Specification Habits of a Lifetime

Many areas of the supply chain can be resistant to change, particularly in environments where furnace failure or planned downtime is highly expensive. It is this resistance and a focus on simple material price that is slowing the shift towards more efficient materials technologies in some industries, despite the obvious benefits.

The unfortunate truth when considering best practice is that the very nature of specification can lead to the development of habitual behaviors, which can lead to sub-optimal product choices over time if decision-makers do not keep up with market changes and technological advances.

However, it is important to remember that improved castable refractory materials offer better performance and insulation, leading to cost and energy savings over the product’s lifespan – so they should be adopted as early as possible.

This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.

For more information on this source please visit Morgan Advanced Materials.

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