The Abstract: In recent years, the use of

The
impact of Nano-technology on recent advances in monolithic refractories: A
review

 

Heidar
Ali*

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1Department of Materials Science, UNSW University,
Australia

*Corresponding
Author, Tel: +98 9133330848, E-mail address: [email protected]

 

 

 

Abstract:

In recent
years, the use of Nano-technology (Nano-particles, Nano-material and Nano-additives)
has attracted attention of scholars, engineers, and scientists in all
scientific fields such as chemistry, medicine, material, agriculture, electric,
and etc. The use of Nano-technology has also become widespread in the
refractories products (which mainly used by various industries such as steel,
casting, cement, glass, and etc.). Therefore, many researchers have evaluated
the effect of using different types and contents of Nano-materials (oxides and
non-oxides) on the properties of shaped (bricks) and un-shaped (monolithic)
refractories products and they have achieved very interesting results.  One of the most consumable refractory products
in various industries is monolithic refractories, which has been widely used
because of their great benefits to the other refractories products (bricks).
Hence, in this paper, recent advances in monolithic refractories by Nano-technology are presented. This article can be considered as a complete
reference and guidance for researchers, students and artisans in order to easy
access to experimental research results of the impact of Nano-technology on monolithic
refractories.

Keywords: Nano-technology, Nano-particles, Refractory, Monolithic

                                                        

                                              

 

                                                         

 

1.   
Introduction:

1.2.       
Nanotechnology(Introduction
)

 

The Nano-technology
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.

 

2.     
Refractories:

2.1. Introduction:

 

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
36-40.

 

2.2.       
Classification
:

Generally, refractories
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
composition:

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
producing method:

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
shape:

a) Formed:

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.

b) Un-formed:

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 Refractories:

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
follows 75-80:

ü
Removing joints which is an inherent weakness.

ü
Easier and faster application. 

ü
Better properties than pressed (sintered or tempered) bricks.

ü
Simpler transportation and handling.

ü
Better volume stability.

ü
Possibility to install in hot standby state.

ü
Higher mechanical resistance to vibration and impact.

ü
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 Conventional
Castable.

ü Low Cement
Castables (LCC).

ü Ultra Low
Cement Castable (ULCC).

ü No Cement
Castable (NCC).

ü Light Weight
Castables.

ü Self-Flow
Castables (SFC).

ü Insulating
Castable.

 

b) Plastic refractories

Plastic refractories are used to form
refractory monolithic linings in different types of furnaces. These
refractories are suitable for making quick, economical emergency repairs and
they are easily rammed to any shape or contour. Plastic refractories are consisting
of refractory aggregates and adhesive clays which are prepared in stiff plastic
condition at the proper consistency for use without more preparation. During utilization,
the blocks are tasked into pieces and are rammed or casted into place with
pneumatic rammer.  These refractories can also be casted into place with a
mallet. These refractories suitable for many important applications due to the
high melting point (high refractoriness), the range of compositions, and the
ease with which plastic refractories are rammed into place make them. Also,
they have often highly spalling resistant. Plastic refractories can consist of
all the, clay-graphite, fireclay, high alumina, high alumina graphite and
chrome types adapted for many various operating situations. Specific gunning types
are also accessible. These are in granulated shape and are produced at the
proper consistency, ready to use.

Some examples of plastic refractories are
65-69, 76-80:

ü Heat setting
super duty fireclay plastic,

ü Super duty heat
setting plastics with graphite,

ü Plastics in the
50 % alumina class,

ü Heat setting 60
% alumina class plastics,

ü Air setting
high alumina plastics in 80 % alumina class,

ü Phosphate
bonded high alumina plastics with alumina content ranging from 70 % to 90 %,

ü Phosphate
bonded alumina chrome plastics,

ü And silicon
carbide based phosphate bonded plastics.

 

c) Ramming mixes

Ramming mixes composed essentially of ground
refractory aggregates, with a semi-plastic bonding matrix.  These refractory materials are like to plastic
refractories but are much harder. They need some sort of form to maintenance them
when formed. The grain sizes are carefully classified and the final product is
usually rendered dry and then mixed with a little content of water just before utilization.
Other ramming products are rendered in wet state and are ready for use
immediately upon opening. Ramming mixes are placed with pneumatic rammer in
layers of 25 mm to 40 mm. Steel making, burner blocks, ports and similar
applications used of High purity ramming mixes based on mullite grain. Ramming
mixes consist of 80wt. % alumina content have good shrinkage resistance and
thermal spalling at high temperatures. Some ramming mixed such as,
stabilized high alumina air setting, have good thermal spalling resistance at
high temperatures and volume stability up to their temperature limit. Also, phosphate-bonded
alumina-chrome ramming mixes typically have very high strength at high
temperatures and very good resistance to acid and neutral slags consist of coal
ash slags. Alumina-graphite ramming mixes have mixture of high alumina grain
and slag inhibitors which give them well slag resistance to acidic and slightly
basic slags. In steel making industry, the dry ramming mixes based on high
purity MgO and a sintering aid are useful. Magnesite ramming mixes of exceptional
purity and stability are used firstly as lining materials for coreless type
induction kilns. Magnesia-Chrome fused grain ramming mixes can create special
strength and density 52-60, 64-73.

d) Gunning mixes

The install method of more monolithic
refractories is gunning. The constitution material of gunning mixes are different
particles sized of refractory aggregate, a bonding compound, and may contain
plasticizing agent to enhance their stickiness when pneumatically placed onto a
kiln surface.  These refractory materials are sprayed on application surfaces
using a gun device. Usually gunning refractory mixes are supplied dry. In order
to application, they are pre-damped in a batch mixer, and then continuously poured
into a gun device. Water is added to the mix at the nozzle to achieve the
proper consistency. Typically, Gun mixes are including high alumina, siliceous,
fireclay, dead burned magnesite and chrome types. Magnesite and hot gun mixes
are not pre-damped and are placed in a batch pressure gun. Gun mixes should provide
good coverage in a variety of applications 40-52, 61-68. Some types of
gunning mixes are:

ü Fireclay
gunning mixes of multipurpose hard fired fireclay and standard calcium-aluminate
cement compositions.

ü Fire clay
gunning mixes with high purity calcium-aluminate bonding system.

ü Gunning mixes
based on vitreous silica.

ü High purity
alumina mixes which combine high fired alumina aggregate.

ü High purity
calcium aluminate binder.

ü Basic refractory
gunning mixes with magnesia content ranging from 60 % to 95 % with or without a
phosphate bond.

e) Patching refractories

 These type refractories materials
are like to plastic refractories though have a very soft plasticity let them to
be casted into place 35-43, 71-76.

F) Coating refractories

 These types of refractories materials are
used to maintain refractory linings usually against chemical attack. Coating
refractories are usually intended to coat just the working surface of a lining.
They tend to be justly thin layers 60-76.

g) Mortars

 Generally, mortars are neither classified as
refractory brick nor monolithic refractories. They are very fine refractory
materials, which become plastic when mixed with water. These are used to bond
the brickwork into solid unit, to provide cushion among the slightly irregular
surfaces of the brick, to fill up spaces created by a deformed shell, and to
make a wall gas-tight to prevent penetration of slag into the joints. Mortars should
have good water keeping properties and must not foul. In this way, premature
penetration of water in the refractory bricks after laying, causing the mortar
to dry out, can be avoided. Different types of refractory mortars are consisting
of 50-58:

Ø Mortars with
ceramic bonding (bonding starting at 800 C)

Ø Mortars With
chemical bonding

Ø Mortars with
hydraulic bonding (bonding starting at 20 C)

Also, the important properties of the mortars
are consisting of:

ü Composition and
characteristics of the mortar materials,

ü Grain size

ü Consistency

 

e) Fettling mixes

 Fettling mixes are also granular refractory
materials, with function like to gunning mixes, but are applied by shoveling
into the kilns needing patching 43-51.

f) Tap-hole mixes

Tap-hole mixes are resin bonded. In these mixes
the higher strength which is normally desired for monolithic refractory
products, is not that important. Some criteria are necessary for all tap-hole
mixes. These criteria are consisting of: correct consistency, setting, and
carbonization at the right time, precisely controllable PLC, and above all
drilling capability 72-78.

 

3.      Application of
Nano-technology in refractory industrial:

 Nano-technology is usually introduced by size
and consist of the visualization, properties, production and manipulation of
structures which are lower than 100 nm 89- 90. Specific mechanical, optical,
electrical, and magnetic properties which can differ substantially from the
properties of the same materials at larger dimensions can show for the
structures that the dimensions of which range from 100 nm down to approx. 0.1
nm. Hence, nano-technology is a very active research field and has applications
in a number of areas. Today, considerable attention has been paid to the use of
nano-technology in the progress of refractories products 91-93.
Nano-technology has been entered to refractories. It has been expressed that
the efficiency of
the refractories was extremely improved for the well dispersion of nano-sized grain
in the microstructure and reaction activity. Some efforts have been done by different
researchers to enhance the properties of refractories (bricks and monolithic)
by using Nano-particles. The application of nano-technology is aimed at achieving
the following specific properties of brick and monolithic refractories 90-95:

ü Ultra-high compressive strength,

ü Relatively high tensile strength and
ductility,

ü More efficient cement hydration,

ü Increased aggregate-paste bond strength,

ü High corrosion resistance

ü Control of cracks and self-healing

ü High thermal shock resistance and  

ü High Abrasion resistance

ü High chemical corrosion résistance

 

4.     
Use of Nano-technology in monolithic
refractories:

According to
the above, in this section, the results of carried out activities by various researchers
using nano-technology in monolithic refractories have been expressed (Table 1). It is observed that the use of
Nano-technology has been heavily used by researchers in recent years.

Conclusion:

Recently, Nano-technology is used for
production to refractory products and it is a necessary tool included in many activities.
A lot of research has been working on the adding of different types of
additives in ceramic goods, and some of them have concentrated their
investigations on the use of Nano-additives, because of the mentioned
advantageous of adding nano-particles to the ceramic goods.  In this review article, all researches which done to enhance
the performance of monolithic refractories is reported and it was concluded that the
application of nano-additives has the best results. Results show that recently, researchers
have been using nano-technology and have reached interesting results. 

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