Refractory materials

A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000 °F (811 K; 538 °C)".

Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles and moulds for casting glass and metals and for surfacing flame deflector systems for rocket launch structures.

Refractory materials

Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, bechemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion.

The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fire clays are also widely used in the manufacture of refractories.

Refractories must be chosen according to the conditions they will face. Some applications require special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon (graphite) are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize and burn.

Binary compounds such as tungsten carbide or boron nitride can be very refractory. Hafnium carbide is the most refractory binary compound known, with a melting point of 3890 °C. The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds (4215 °C).

Based on chemical composition

Acidic refractories

Acidic refractories consist of mostly acidic materials like alumina (Al₂O₃) and silica (SiO₂). They are generally not attacked or affected by acidic materials, but easily affected by basic materials. They include substances such as silica, alumina, and fire clay brick refractories. Notable reagents that can attack both alumina and silica are hydrofluoric acid, phosphoric acid, and fluorinated gases (e.g. HF, F2).^[7] At high temperatures, acidic refractories may also react with limes and basic oxides.

Neutral refractories

These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The main raw materials belong to, but are not confined to, the R₂O₃ group. Common examples of these materials are alumina (Al₂O₃), chromia (Cr₂O₃) and carbon.

Basic refractories

These are used in areas where slags and atmosphere are basic; they are stable to alkaline materials but could react with acids. The main raw materials belong to the RO group to which magnesia (MgO) is a very common example. Other examples include dolomite and chrome-magnesia. For the first half of the twentieth century, the steel making process used artificial periclase (roasted magnesite) as a lining material for the furnace.

Based on method of manufacture

1. Dry press process
2. Fused cast
3. Hand molded
4. Formed (normal, fired or chemically bonded)
5. Un-formed (monolithic-plastic, ramming and gunning mass, castables, mortars, dry vibrating cements.)
6. Un-formed dry refractories.

Shaped

These have standard size and shapes. These may be further divided into standard shapes and special shapes. Standard shapes have dimension that are conformed by most refractory manufacturers and are generally applicable to kilns or furnaces of the same types. Standard shapes are usually firebrick that have a standard dimension of 9 x 4-1/2 x 2-1/2 inches and this dimension is called a "one brick equivalent". "Brick equivalents" are used in estimating how many firebrick it takes to make an installation into an industrial furnace. Special shapes are specifically made for particular kilns or furnaces. Precast refractory shape technology has become a specialized field within the refractory industry in recent years. As demands increase for greater refractory lining performance and lower maintenance costs, refractory users are finding that one effective way to achieve those goals is to incorporate a broader use of precast refractory shapes into their lining systems. Across virtually all industries – petrochemical, steel, power generation, metal casting and treatment, wood products, minerals processing and others - the applications for precast shapes are limited only by the imagination, and almost invariably their use will result in better performance and true cost savings. This article will discuss the design and manufacture of precast refractory shapes, and the benefits to be gained from the standpoint of both material properties and installation logistics.

Precast shape design and manufacturing

In order to realize the true benefits to be gained from the use of precast shapes, a thorough knowledge of how the shape system will be used and installed in the field is an absolute requirement during the design phase. The successful design and manufacture of a high-performance refractory shape system requires a unique understanding of refractory materials, manufacturing, anchoring systems, and construction practice. Dimensional tolerances, construction sequencing, lifting and handling capabilities at the site, anchoring facilities, and the actual service demands within the refractory lining environment are all factors that must be well known before the shape is designed.

Precast shape manufacturing inherently requires the use of a mold or pattern to form the shape. There are several methods for mold-making which are routinely employed, and the type of mold construction and materials used depends on the size, complexity, and dimensional tolerances required in the shape, and sometimes the quantity of shapes required. For simplistic shapes with loose dimensional tolerances (+/- 1/16”), plywood forms or metal fabricated forms can be used. Toward the other extreme, some shapes may require extremely tight tolerances, which require the use of a more sophisticated mold made from wood, plastic or metal. These molds may be of the type made by a foundry pattern maker or machine shop.

Another factor in the design of a precast shape has to do with the schedule and sequencing of the actual field installation. The shape design must take into account job accessibility, what other lining components will already be in place when the shapes are to be installed, and how the shape can be handled physically on the job site. Weight and lifting limitations must be considered and planned for, as well as the type of access available into the furnace or vessel. If necessary, lifting lugs or other fixtures can sometimes be incorporated into the shape design.

The design of the anchoring system to be used in the shape is of tremendous importance. In addition to the normal considerations of alloy type and anchor size, the precast shape design must also consider all alternatives for attaching the shape to the structure. Numerous methods can be used, including threaded stud attachments through the wall, welded fixtures, or bolted assemblies.

Perhaps most importantly, the proper refractory material must be selected to suit the demands of the application. Factors such as the desired temperature profile through the lining, expected mechanical stresses, potential chemical attack on the lining, erosion mechanisms, and expansion allowance must all be understood prior to selecting a material to be used in the precast shape.

A well-equipped precast manufacturing facility should include high-energy, large capacity mixers, automated mixing stations with conveyors for material delivery, vibration tables, digitally-controlled water addition, mixing time controllers, and adequate lifting capabilities for large shapes. Firing of shapes is accomplished with a digitally-controlled furnace with burners capable of firing to at least 1300 deg. F. In-house mold/pattern fabrication capabilities and CAD-generated drawings for design assistance should also be expected.

Benefits from material property enhancement

Regardless of how complex or sophisticated the refractory castable is that is selected for an application, the physical properties of the material can be drastically reduced if care is not taken during the mixing, pouring, and curing processes. Particularly with the use of more complex refractory castables to solve specific wear issues, installation variables become even more critical to the performance of a lining. Unfortunately, lining quality is often compromised by field conditions during material placement. Precast roof panels ready for shipment. Project schedules, crew skill levels, equipment availability, job cost pressures, or other demands can sometimes have an impact on proper refractory installation. Improper water addition, mix time variations, over- or under-vibration, and improper curing can drastically affect material quality. With precast shapes, cast in a controlled shop environment, the physical properties of a castable can be more fully optimized.

Initial drying and firing of a refractory castable is a critical installation variable that can influence lining performance. Precast shapes are typically fired in a digitally-controlled furnace prior to shipment, ensuring that the refractory manufacturer’s recommended bake-out schedule is closely followed. Since the shapes are fired slowly from all sides, the moisture is removed through the entire thickness of the shape in a controlled manner. Depending on the temperature to which the shape is fired, this can optimize the physical properties of the material through the entire thickness of the shape, not just the hot face surface. This results in a truly homogeneous lining. Micro-cracking within the shape, which are often introduced during field bake-out but may go unnoticed, may also be reduced since the initial firing is more controlled.

In service, linings composed of precast shapes often see less stresses and cracking, due to the independent, “floating” nature of the lining. The performance of the lining can also be more predictable, resulting in better opportunities to plan for maintenance and repairs.

Benefits from installation logistics

Other major benefits to be gained from the use of precast refractory shapes are related to simplified installation and repair logistics, which can lead directly to reduced costs and shorter down times. With the use of precast shapes, forming labor, materials, equipment costs, actual placement time and expense, and associated costs during form removal, curing, and cleanup are all eliminated. These costs are shifted back to the manufacturer of the shape, who can absorb them much more efficiently when spread over his overall production capacity.

Refractory installation contractors have begun to consider precast refractory shapes much like they do any other pre-manufactured item such as block insulation, ceramic fiber blanket, anchors, etc. These items can be bought and then re-sold as a component of their installation projects. Whenever any portion of refractory repair work can be completed prior to crews being on site, costs are automatically reduced. Installation contractors have also found that the use of precast shapes can often give them a substantial advantage in competitive bid situations.

With the use of precast shapes, crew sizing can be minimized. Speed of installation is another obvious benefit to both the installer and the owner, resulting in reduced costs due to shorter job duration. Material usage is also reduced, when compared to other installation methods such as guniting, where as much as 45% of extra material is required to compensate for rebound and other job losses. Environmental hazards such as dusting and tripping hazards associated with equipment and hoses are also reduced substantially, if not eliminated. Future repairs also become much more economical and quicker to accomplish. Repair areas can be isolated to just the immediate wear area within the boundaries of a shape. Anchor attachment points can typically be reused. Replacement shapes, purchased early and kept as spare parts on site, can be easily installed in a fraction of the time required for conventional repair methods. The initial bake-out of a new refractory lining on site can be a very expensive and time-consuming component of a refractory repair project. The use of precast and prefired refractory shapes can sometimes reduce or even eliminate the need for an extensive initial bake-out. If an entire repair is made with a prefired system, then normal furnace start up schedules can be used, without the fear of steam spalls or other damage during the initial heating. Bake-out of multi-component linings, which may include a combination of precast shapes and other materials placed in the field, can often be reduced by the pre-firing of the castable shapes, particularly if that material would have been the critical item determining the bake out schedule. This can have a positive impact on not only job costs, but in reducing down time as well.

Typical precast shape applications

• Burner Blocks
• Checker Wall Blocks
• Burner Pipe Covers
• Hearth Shapes
• Delta Sections
• Impact Pads
• Burner Ports
• Flare Tips
• Skimmer Blocks
• Troughs
• Pier Blocks
• Wall Panels
• Divider Walls
• Jambs, Lintel & Sills
• Well Blocks
• Flue Caps
• End Caps
• Roof Sections
• Safety Linings
• Runners
• Manway Plugs
• Air Grid Tiles
• Curb Blocks
• Peep Sites
• Nozzles
• Flue Walls
• Exhaust Ports
• Spouts
• Tap Blocks
• Sleeves

Precast refractory shapes will continue to be a growing specialty in the refractory industry in coming years. With the improved quality that can be achieved through controlled manufacturing processes, their expanding use will play a major role in improving refractory lining performance and reducing maintenance costs across all industries.

Unshaped (monolithic refractories)

These are without definite form and are only given shape upon application. These types are better known as monolithic refractories. The common examples are plastic masses, Ramming masses, castables, gunning masses, fettling mix, mortars etc.

Dry vibration linings often used in Induction furnace linings are also monolithic, and sold and transported as a dry powder, usually with a magnesia/alumina composition with additions of other chemicals for altering specific properties. They are also finding more applications in blast furnace linings, although this use is still rare.

Types of monolithic refractories

Castable refractories

These are materials which consist of precision graded coarse and fine refractory grains. They are gelled by means of a binder system in the materials green state. Following the heat-up of the material the binder either transforms or volatilises facilitating the formation of a ceramic bond. The most common binder used in castables is HAC (high alumina cement). Other binders that are often used include hydratable aluminas and colloidal silica. Castables are mixed with water and then installed by either pouring or pumping. Placement of the material then requires vibration.

The cement-containing castables are often classified by the amount of cement they contain. Conventional castables can contain around 15-30% cement binder. As refractory technology evolved chemical additives were included in the package to reduce the amount of cement and water the product required - the impact of this was material with improved strengths and durability. Low cement castables contain between about 3-10% cement by weight. Ultra low cement castables contain less than 3% cement component.

A specialised type of refractory castable is the free flow castable which is able to be installed without vibration. They require a much lower water addition than traditional castables. This is due to the fact that they have particle packing and dispersing agents which modify the surface chemistry of the fine particles to improve the flow of the material.

Certain castable formulations may be installed via gunning techniques which involves spraying the material through a nozzle at a high speed. At the nozzle, cement accelerators are often added to promote rapid hardening of the material. This technique allows applications to be lined very quickly.

Plastic refractories

These are monolithic refractory materials which are tempered with water and/or added with a binder. They have sufficient plasticity to be pounded or rammed into place.

Ramming refractories

These materials are very similar to plastic refractories though are much stiffer mixes.

Patching refractories

These materials are similar to plastic refractories though have a very soft plasticity allowing them to be pounded into place.

Coating refractories

This type of product is used to protect refractory linings usually against chemical attack. Coating refractories are normally intended to cover just the working surface of a lining. They tend to be fairly thin layers.

Refractory Mortars

Mortars consist of finely ground refractory materials which are then mixed with water to form a paste. They are used for laying and bonding shaped refractory products such as bricks. They are normally applied by trowelling.

Insulating castables

Insulating castables are specialised monolithic refractories that are used on the cold face of applications. They are made from lightweight aggregate materials such as vermiculite, perlite, extend-o-spheres, bubble alumina and expanded clay. Their main function is to provide thermal insulation. They are typically of low density and low thermal conductivity. Insulating refractories have inferior mechanical strength to that of conventional castables.

Based on fusion temperature

Based on fusion temperature, they are classified into three types:

1. Normal refractory.
2. High refractory.
3. Super refractory.

Normal refractory

Fusion temperature = 1580 ~ 1780 °C (e.g. Fire clay)

High refractory

Fusion temperature = 1780 ~ 2000 °C (e.g. Chromite)

Super refractory

Fusion temperature > 2000 °C (e.g. Zirconia)

Refractory anchorage

All refractories require anchorage systems such as wire formed anchors, formed metal (for example, hexmetal) or ceramic tiles to support the refractory linings. The anchorage used for refractories on roofs and vertical walls are more critical as they must remain able to support the weight of refractories even at the elevated temperatures and operating conditions.

The commonly used anchorages have circular or rectangular cross-sections. Circular cross-sections are used for low thickness refractory and they support less weight per unit area; whereas the rectangular cross-section is used for high thickness refractory and can support higher weight of refractory per unit area. The number of anchors to be used depend on the operating conditions and the refractory materials. The choice of an anchor's material, shape, quantity, and size has significant impact on the useful life of the refractory.

Refractory dry-out / heat-up

Refractories require special heat-up techniques to ensure that their performance will be attained as designed, and to avoid damage due to drying stresses (spalling) and thermal shock until the operational status is achieved.