Monolithic Refractories:
Monolithic or monolith-forming refractories are special mixes or blends
of dry granular or cohesive plastic materials used to form virtually jointfree linings. They represent a wide range of mineral compositions and
vary greatly in their physical and chemical properties.
Some are relatively low in refractoriness, while others approach high
purity brick compositions in their ability to withstand severe
environments.
Advantage of Monolithic Refractories:
Monolithic refractories are used to advantage over brick construction in
various types of furnaces:
1. Their use promotes quick installation, avoiding delays for the
manufacture of special brick shapes.
2. Using monolithics frequently eliminates difficult bricklaying tasks,
which may be accompanied by weakness in construction.
3. They are of major importance in the maintenance of furnaces because
substantial repairs can be made with a minimum loss of time and, in some
cases, even during operations.
4. Under certain conditions, monolithic linings of the same chemical
composition as firebrick provide better insulation, lower permeability and
improved resistance to the spalling effects of thermal shock.
5. With little or no preparation, they can be applied to form monolithic or
joint-free furnace linings in new construction, or to repair existing
refractory masonry.
Types of Monolithic Refractories:
Common usage divides monolithic refractories into the following groups:
• Plastic Refractories
Plastic refractories are mixtures of refractory aggregates and cohesive
clays, prepared in stiff plastic condition at the proper consistency for use
without further preparation. They are generally rammed into place with
pneumatic hammers, but may also be pounded into place with a mallet.
• Ramming Mixes
Ramming mixes consist essentially of ground refractory aggregates, with
a semi-plastic bonding matrix which can be purchased ready-to-use or
prepared by adding water in the mixer at the construction site. Ramming
mixes are placed with pneumatic hammers in 1-1.5 inch layers. They
supply a denser, stronger refractory body than plastics, but need some
sort of form to restrain them when rammed.
• Gunning Mixes
Gunning mixes consist of graded refractory aggregate and a bonding
compound, and may contain plasticizing agents to increase their
stickiness when pneumatically placed onto a furnace wall. Typically,
gunning mixes are supplied dry. To use, they are predamped in a batch
mixer, then continuously fed into a gun. Water is added to the mix at the
nozzle to reach the proper consistency.
• Castables
Castables consist of graded dry refractory aggregates combined with a
suitable hydraulic-activated bonding agent. Castables are furnished dry
and form a strong cold set upon mixing with water. They are usually
poured or cast in much the same manner as ordinary concrete, but are
sometimes vibrated, trowelled, rammed or tamped into place, or applied
with air placement guns. They form strong monolithic linings, possessing
combinations of properties that make them ideal for many applications.
1. Plastic Refractories:
Plastic refractories are used to form refractory monolithic linings in
various kinds of furnaces, and are especially adaptable for making quick,
economical emergency repairs. They are easily rammed to any shape or
contour. The high refractoriness, the range of compositions, and the ease
with which they can be rammed into place make plastics suitable for
many important applications. Plastic refractories are often highly resistant
to destructive spalling. Plastics can include all the fireclay, clay-graphite,
high-alumina, high- alumina graphite and chrome types adapted for many
different operating conditions.
Products: (Heat-setting superduty fireclay plastics, Cold setting
superduty fireclay plastics, Superduty heat-setting plastics with graphite,
Plastics in the 50% alumina class, Heat-setting 60% alumina class
plastics, Air-setting high-alumina plastics, Phosphate-bonded highalumina plastics, phosphate-bonded alumina- chrome plastics).
2. Castable Refractories:
Castables are generally referred to as refractory concretes. They are
available in a wide variety of base materials and typically consist of a
refractory aggregate, special purpose additives and a cement binder. The
bonding systems used are often used to classify the types of castables into
four categories: (conventional, low cement, ultra-low cement and limefree castables).
a) Conventional castables have a cement-bonded matrix where,
typically, a calcium-aluminate type of cement fills in the spaces between
aggregates. This kind of castable is the most versatile for placement
purposes in that normally it can be poured, vibrated, rammed or gunned
into place while maintaining its designed properties.
b) Low cement castables are materials with lime contents of roughly
between 1% to 3%. High densities and strength are achieved by careful
particle packing and the use of additives to reduce the water needed to
cast.
c) Ultra-low cement castables contain from 0.2% to 0.8% lime. Like
low cement castables, they consist of sized particles to achieve maximum
particle packing. Because of the low cement content, these mixes are not
usually as strong in the low to intermediate temperature range as other
types of castables; but they tend to have higher hot strength and
refractoriness compared to chemically similar mixes with conventional or
low cement bonds.
d) Lime-free castables have been developed with bonding systems
containing no cement. These castables have desirable properties for use in
certain chemical applications and where the highest possible hot strength
and high temperature load resistance is required, such as metallurgical
operations and other high temperature furnace applications. Some of
these materials can approach the properties of pressed and fired brick.
Products: (Fireclay Castables, High-Alumina Castables, Silica Castables,
Basic Refractory Castables).
3. Refractory Gunning Mixes:
In some industrial furnaces, turnaround time and installation costs are the
major factors when choosing a refractory lining. In other cases, repairs
need to be made with little or no downtime. In both circumstances,
pneumatic convey-ing of material, or gunning, is often the method of
choice. Dense, homogeneous monolithic linings can be gunned without
the use of forms and with a marked savings in time.
Gun mixes include siliceous, fireclay, high-alumina, dead-burned
magnesite and chrome types. Many castables, ramming mixes and
specially designed plastics can also be applied successfully with
pneumatic guns.
Acid gun mixes are normally predamped and fed through a continuous
dual chamber or rotary gun. Magnesite and hot gun mixes are not
predamped and are placed in a batch pressure gun. Gun mixes should wet
up well, have as wide a water range as possible, and provide excellent
coverage in a variety of applications.
Products: (Fireclay Gunning Mixes, High-Alumina Gunning Mixes,
Silica and Silicon Carbide Gunning Mixes, Basic Refractory Gunning
Mixes).
4. Refractory Ramming Mixes:
Refractory ramming mixes consist of refractory aggregates and a semiplastic bonding phase. When properly installed, ramming mixes offer a
way of placing a cementless monolithic lining at high density and
relatively low porosity.
A well balanced selection of ramming materials includes compositions
with base materials of silica, high-alumina, corundum, mullite, deadburned magnesite, chrome ore, zircon and others. These materials are
particularly suited for forming dense monolithic, lining construction and
numerous other monolithic constructions. Ramming mixes are typically
supplied in both wet and dry forms, depending on the binder system.
Products: (High-Alumina Ramming Mixes, Basic Refractory Ramming
Mixes).