phrase originating from two words consist of the Greek numerical prefix nano
referring to a billionth and the technology word 1-3. As an outcome,
Nano-technology or Nano-scaled technology is commonly considered to be at a
size under 100 nm (a Nano-meter is 10-9 m) 1-2.
According to the ASTM C 71 , the refractories are a “non-metallic
materials having those physical and chemical properties that lead to them
applicable for structures or as components of systems that are exposed to
environments above 1000 °F (538°C) 11, 16. Also,
some references mentioned that refractories are in-organic non-metallic
material which can withstand high temperature without changing in their
chemical or physical properties while remaining in contact with molten slag,
metal and gases 11-13, 16-20. As well as, according to the operating situation,
they should to have high thermal shock resistant, be chemically inert, and have defined ranges of thermal conductivity
and thermal expansion coefficient 11- 21, 22. It is clear that refractories have an important role in
glassmaking, metallurgical, and ceramic industries, where they are generated
into a variety of shapes to line the interiors of furnaces or kilns or other
devices for processing the materials at high temperatures 23-25. Some of the
technological and scientific inventions and progresses would not have been
possible without refractory materials. Producing 1Kg of any metal without
utilize of refractory is almost quite impracticable 26-29. The history of using refractory materials dates back to since mankind start to develop metallurgical
process. The first refractor raw material was clay. Up to the nineteenth century, refractory
products were made of natural ores, such as magnesite, dolomite stones and clay.it
was at the end of the eighteenth century and beginning of nineteenth century that
the basis of modern metal beneficiation, the development of Portland cement and
of modern glass processes started to inflict higher requirements to the
refractory industry 30-33. The main
materials used in the producing of refractories are based to Fig.1 34-36. In recent years, with
the changing trends in steelmaking, the high performing shaped refractories are
on an increasing demand. The higher campaign lives and the mutability of the
newer steelmaking operations are decided by the accessibility and performance
of such shaped refractories with superior high-temperature mechanical strength,
erosion and corrosion resistance the selection of refractories to be utilized
is often according to the conditions dominating in the application zone
are divided based on chemical
composition, manufacturing method, and physical shape or based on their applications
(Fig.2) 11-20, 40-55.
2.2.1. Based on chemical
a) Acidic refractories:
These types of refractories are used in region that
slag and atmosphere are acidic. They have high resistance to acids but corroded
by alkalis. The main raw materials belong to the RO2 category, such as
SiO2, ZrO2 and etc.
b) Neutral refractories:
These categories of refractories are used in area
that atmosphere and slags are chemically resistant to both acids and bases. The
major raw materials related to, but not confined to, R2O3
category. The general examples of these materials are Al2O3,
Cr2O3 and carbon(C).
c) Basic refractories:
These categories of refractories are used in area
that atmosphere and slags are basic; these categories high resistance to
alkaline materials but corroded by acids.
The major raw materials related to the RO category
to which MgO is a very general example. Also, (Mg.Ca (CO3)2
and (MgO-Cr2O3) are in these categories.
2.2.2. according to
a) Dry press.
b) Fused cast.
c) Hand molded.
d) Formed (normal, fired or chemically bonded).
e) Un-formed (monolithic- plastic, ramming and
gunning mass, castables).
2.2.3. according to physical
These types have determined shapes and size.
These types divided into standard shapes and special shapes. The first type has
size that is confirmed by most refractory producer and is generally suitable to
furnaces or kilns of the same types. The second type specifically made for special
furnaces or kilns.
These categories are without clear format and
are only given shape upon application. Un-formed are known as
monolithic refractories. The common examples castables are, plastic masses, gunning
masses, ramming masses, fettling mix, mortars etc.
Monolithic refractory phrase is the
name usually given to all un-shaped refractory products, the word “monolithic”
extracted from the word monolith which means ‘big stone’56-58. Monolithic
refractories are specific batches or blends of dry granular or cohesive plastic
materials utilized to form nearly joint free linings. Monolithic refractory
are un-shaped products which are installed as some form of suspension that
finally harden to create a solid shape. Most monolithic formulations are
made of three constituent such as: large refractory particulates (an
aggregate), fine filler materials (which fill the inter particle voids) and a
binder phase (that gels the particulates together in the green state) Fig
359-65. Monolithic refractories show a great range of mineral
compositions and vary greatly in their physical and chemical properties. Some
of them have low melting point (low refractoriness) whiles others approach high
purity brick compositions in their ability to tolerate severe
environments. Monolithic refractories are replacing the conventional type
fired refractories at a much faster rate in many applications including those
of industrial furnaces 53-55, 66-68.
These refractories are used to advantage compare to brick construction in
different type of furnaces. Their use enhanced fast installation. Utilize
of monolithic refractories often delete difficult brick laying tasks, which may
be accompanied with looseness in construction. Protect of furnaces is very
importance because substantial repairs can be made with a minimum loss of time
69-74. Sometimes, monolithic refractories linings of the same
composition as firebrick provide better insulation, lower diffusion and
enhanced spalling resistance to the effects of repetitive thermal
shock. Other major benefits of monolithic refractory linings are as
Removing joints which is an inherent weakness.
Easier and faster application.
Better properties than pressed (sintered or tempered) bricks.
Simpler transportation and handling.
Better volume stability.
Possibility to install in hot standby state.
Higher mechanical resistance to vibration and impact.
confirming shrinkage and expansion to the application.
Different methods are used in the placement of monolithic refractories such as
ramming casting, spraying, gunning, sand slinging and etc. Heat setting monolithic
refractories have a very low cold strength values and rely on relatively high
temperatures to progress a ceramic bond 81-83. Furnaces wall having the usual
temperature drop across its thickness, the temperature in the cooler part is
generally not enough to progress a ceramic bond. However with the use of a
proper insulating material as backup, the temperature of the lining can be high
enough to progress a ceramic bond throughout its entire thickness. In
order to the installation and curing, monolithic refractories need an intently
controlled dry-out program. This led to the filler, binder and aggregate
to fire generating a high strength material 84-86.
3.1. Types of monolithic refractories
Usually the monolithic refractories are divided
according to Fig.4 56-60, 65-88
a) Castable refractories
Materials with hydraulic setting in nature are
name of Castables. These refractories are containing cement binder (commonly
aluminate cement), which creates hydraulic setting properties when blended with
water. By heat-up temperature, the material and binder either transforms or
volatilizes simplifying the generation of a ceramic bond. The most common
binder used in castables is high alumina cement. Other binders are consisting
of hydratable alumina and colloidal silica. These materials are installed by
casting and are also known as refractory concretes. Insulating castables are
specialized monolithic refractories that are used on the cold surfaces of applications.
These monolithic castables are composed of lightweight aggregate aggregates
such as vermiculite, bubble alumina, perlite and expanded clay. The main
function of castables is to create thermal insulation. Also, they are generally
had low density and low thermal conductivity. The castables are classified
according to following 48-58:
ü Low Cement
ü Ultra Low
Cement Castable (ULCC).
ü No Cement
ü Light Weight