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". Refractory materials are used in linings for
furnaces, kilns, incinerators and reactors. They are also used to make crucibles.
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, be chemically inert, and/or have specific ranges of thermal
conductivity and of the coefficient of thermal expansion.
The oxides of aluminium, silicon and magnesium are the most important materials used in the
manufacturing of refractories. Another oxide usually found in refractories is the oxide
of calcium. 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 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.
Classification of refractory materials Refractories can be classified on the basis
of chemical composition, method of manufacture, physical form or according to their applications,
fusion temperature. Based on chemical composition
Acidic refractories They consist of mostly acidic materials like
alumina and silica. They are 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. 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. The common examples of these materials are alumina, chromia and carbon.
Basic refractories These are used on 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 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 as a lining material for the furnace.
Based on method of manufacture Dry press process
Fused cast Hand molded
Formed Un-formed
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, 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 shipmentProject 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
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 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. 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% weight cement. 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.
There 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: Normal refractory.
High refractory. Super refractory.
Normal refractory Fusion temperature = 1580 ~ 1780°C
High refractory Fusion temperature = 1780 ~ 2000°C
Super refractory Fusion temperature > 2000°C
Refractory anchorage All refractory require anchorage systems such
as wire formed anchors, formed metal or ceramic tiles to support the refractory linings. The
anchorage used for refractory on roofs and vertical walls are more critical as they must
remain able to support the weight of refractory even at the elevated temperatures and operating
conditions. The commonly used anchorages have circular
or rectangular cross-section. Circular cross-section 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 anchors material, shape, numbers and size has significant impact
on the useful life of the refractory. Refractory heat-up
Usually, refractories require special heat-up techniques to ensure that their performance
will be attained as designed, and to avoid thermal shock and drying stresses until the
operational status is achieved. See also
Fire brick Furnace
Masonry oven Refraction
References
External links The Refractories Institute
The European Refractories Producer Federation