REFRACTORY MATERIALS FOR INDUSTRIES
1.) INTRODUCTION TO REFRACTORIES & THEIR APPLICATION:-
Refractories may be defined as materials of construction with low thermal conductivity
which are capable of withstanding elevated temperature and are used to contain heat as
well as solid, liquid and gaseous substances in a structure. Stability at high temperature –
both physical and chemical – is the primary requirement for refractory materials.
2.) TYPES OF REFRACTORIES:-
Refractories are classified in many ways but the most important classification is on chemical
basis viz. acidic, basic and neutral.
(a) Acidic Refractories
These types of refractories are resistant to acidic conditions. Silica, fireclay, zircon and
zirconia come under acidic refractories.
(b) Basic Refractories
These types of refractories are resistant to basic conditions. Magnesite, dolomite,
chrome-magnesite and forsterite come under basic refractories.
(c) Neutral Refractories
High alumina, chrome, carbon, silicon carbide and mullite refractories come under neutral
refractories classification.
SOME OF THE REFRACTORIES USED IN INDUSTRIES:
Some of the important refractories used in chemical process industries are described
below in detail:
A. Bricks & Shapes:
i.) Dense type
– Moderate heat duty fire brick (general / critical uses, low iron type
– High heat duty fire brick
– 42-44% Al2O3 brick
– High alumina brick (45-99% Al2O3 )
– Phosphate bonded bricks
ii. Insulating type
– Mica brick
– Insulating fire brick ( hot face & back-up)
– Low B.D. high strength insulating firebrick
B. Monolithic:
i. Insulating castable (B.D =0.8 to 1.7)
– Normal strength
– High strength
– Low iron type
– Special type (low thermal conductivity, etc)
ii. Dense castable
– Conventional castable
– Low cement castable ( CaO more than 1%& less than 2.5%)
– Ultra low cement castable (CaO more than 0.2%& up to 1%)
– No cement castable (CaO maximum up to 0.2%)
– Abrasion resistant castable
– Ramming masses
iii. Ramming materials
iv. Plastics
C. Ceramic Fibre materials
– Blankets
– Module
– Vacuum formed board
– Bulk fibre
– Rope
– Paper, etc.
D. Minera Wool
– Blankets / Pads
E. Calcium silicate blocks
3.) GENERAL GUIDELINES ON INSTALLATION OF CASTABLES:-
Following is the general application procedure for proper application of castable
refractories, but in case of low cement castable special care for mixing, application , etc are
need to be taken as per the recommendation of the manufacturer.
1. Storage
a. The materials should be stored in clean, dry and enclosed area over wooden
pallets. Excessive stacking of pallets of insulating castable should be avoided
as it can cause increase in density of light-weight products and crushing of
larger insulating grains.
b. Bags should not be opened before use.
c. The normal shelf life of the castable material is about 9 to 10 months from
the date of manufacturing, unless otherwise specified. Therefore the
material should be used within the period of shelf life, as recommended by
the manufacturers.
d. Water used should be fresh, clean, cool and suitable for drinking purpose
with low chloride content. Water should be stored in a container free from
oil, grease and other impurities.
2. Preparation of Surface and Anchoring System
The surface to be lined should first be cleaned of rust, organic matter and other adhering
matters. If required, the surface should be sand-blasted.
Wherever anchors are to be welded, location should be marked properly. The proper
spacing of anchors perform an important part in a successful job. These can be diamond or
square shapes. Generally anchor pitch & quality of material are decided depending on the
lining thickness, type of castable being used, operang parameters, location of lining.
However, anchor pitch is around 2 or 3 times the lining thickness. Normally, the rows are
staggered; however, in special cases it can be square also. The top rows of anchors should
be located a minimum of 12″ from the top of wall and the bottom row should be no lower
than 18 to 24″ above the floor line.
Anchors should be secured firmly to the supporting frame or shell. Following two types of
anchors are mainly used:
(i) Metallic Anchors
(ii) Ceramic Anchors
Anchors mostly used are SS304or 310. Also SS316 quality of anchors depending on
requirement is used. For high temperature applications Inconel 601 is used. V and Y shapes
of these anchors are mostly used. The maximum length (height) of anchor is 80% of the
thickness of lining in most of the systems. Anchors must be coated with one to two mm
thick layer of beeswax, paper or bitumen in advance of placement in case of high
temperature service. For thinner lining (40-50mm) of castables, chain link anchors can also
be used. Hex steel mesh and S-bar anchoring system is used for abrasion resistant
castables.
For high temperature applications where metal anchors cannot be used, ceramic anchors
are used. One side of these anchors is tied with metals which are attached to the shell.
Ceramic anchors are useful for suspended arch lining.
3. Shuttering
The shuttering and the wooden form work should be rigid & water tight. It should be
sufficiently strong to support the hydraulic head of wet refractory and also retain and resist
any mechanical loads, such as vibration. Dimensional tolerances shall meet specified
requirements. For the ease of release of the form, grease or wax are used. For smooth
application of castables the inner portion of shuttering should be kept wet. It is also possible
to apply a separate layer of waterproof waxed paper or coating with wax or grease over the
inner surface of shuttering. Shuttering should be independent of working platform. For
special application metallic forms are designed and used.
4. Sectional wall support System
The high temperature alloy supporting shelf plates are available for use in extremely high
walls, stoker arches and burner walls, or where their use may be advisable due to a specific
condition. The use of support shelf plates permits replacement of any secon without
disturbing the lining above or below.
5. Castable Application
Different methods of applications are
A.) Gunning B.) ramming C.) troweling D.) Lining with hex Mesh E.) Injection F.) pouring and rodding G.) vibration casting,
Before mixing, material should be kept in cool place for a considerable period. On
application the castable structure would develop higher porosity in case the temperature
of the mix is high, hence lower crushing strength. In absence of manufacturers
recommendation, the mix temperature shall be kept between 15 to 25 deg. C.
Metal fiber reinforcement shall be used only when specified. Fiber addition shall be
uniformly dispersed in the castable without agglomeration.
It should be noted that castable should be applied within 15 minutes after mixing operation
is over. If panel size for application has not been specified in the drawings, the size and
sequence should be decided. If the size and shape of the vessel are suitable, square panels
of 1 m2 to 2 m2 are usually employed.
Prior to application , the contractor should take the materials for getting applicator
qualified for the job by using the equipment’s on the basis of specified procedure. This shall
be done by simulating the installation work, sampling and testing the applied material.
A. Gunning
The refractory material before charging into gun shall be pre-mixed with water for
better lining quality and reduction in dusting, segregation and rebound loss.
Gunning of castables can be done pneumatically using a shot gun, normally by dry
gunning. The moisture content in the gunning process should be adjusted at the
nozzle of the gun to ensure that the rebound losses are minimum. It should only be
done by trained gun operators. The rebound castables should not be reused. If
gunning is not proper, the refractoriness, density and conductivity of the castables
may change substantially. Gunning equipment shall provide a smooth and
continuous supply of water and material without causing lamination, void, or
rebound entrapment. If the gunning is stopped due to any reason for more than 30
minutes, only full thickness of lining shall be retained.
Gunning shall start at lowest elevation, building up the lining thinness gradually
over the area of about 1 m2 to full thickness and working in an upward direction to minimize the inclusion of rebound material.
Down hand gunning beyond 30 deg. below horizontal is not advisable. The
refractory material shall be placed by alternative method, such as, casting, hand
packing, etc to avoid the down hand orientation.
Depth gauge shall be used for thickness measurement. After gunning, once the job
is over the lining surface shall be trimmed with the help of serrated trowel and cut
back shall be performed before the initial setting.
For gunning material supply pressure and water supply pressure for the job at
equipment positioned at same level or higher level should be followed as per the
recommended practice.
Gunning application preferably should be carried out by rotating bowl machine. It
should be such the machine should be conveying small quantity of material on
continuous basis.
At the machine out let, the air pressure should not be less than 3.5 bar. But in case of
fibre in the mix, pressure should be higher.
In case of long distance between the machine and the application point within 25 m
without change of level addional pressure would not be required. But for each
additional 25 m o.5 bar increase in pressure would be needed. In case the material
have to be transported to higher level, other factors like bulk density, grain size, type
of gunning mix ,etc the air pressure shall have to be increased up to 5 – 7 bar.
The water used must be of right quality and served under pressure of 5 – 6 bar.
b. Ramming
In this method generally chemically bonded monolithic refractories are installed by
pneumatically operated rammers for compaction of the material in position.
c. Trowelling
This is suitable for thin lining and generally can be used for small area of repair also.
d. Lining with Hex mesh
The refractory mix is pressed into the hex mesh by hand to a thickness somewhat
exceeding the top level of hex mesh and tamped with a hammer, taking one mesh at a Time. After proper compaction the top surface is trimmed flush with hex mesh. Cut
back material can be used if workability is not diminished. But in no case dry
material shall be used. The trimming surface shall be tamped, as necessary, to
remove any imperfection.
e. Injection
The material is introduced into cracks, joints or gaps between the shell and the
existing lining under pressure with the help of injection machine.
6.) Curing:-
Forms should not be removed unl the cast has set hard. The installed castable lining
should be undisturbed till the initial seng of cement is over. The concrete surface should
be moistened and covered. Generally, the curing period is kept about 24 hours. This can be
done either by a fine spray of water or by applying a sealant compound on the exposed
surface.
7.) Drying and Firing:-
Improvements in refractory technology has resulted in products with higher bulk density,
lower porosity and lower permeability and has resulted in the need for more care to be
taken during dry out process in order to remove the small amount of water that are now
being used in many of the products. Initial heating of newly installed refractory linings shall
be performed by process heating devices or temporary equipment such as electric heating
elements or portable burners. The castable can be put into service 24 hours after casting
but 24 hours air drying is preferable. When heating up for the first me, a slow initial rate
and hold at 1000C/1500C is desirable to drive off the free water. A rate of 30-500C per hour
and hold at different temperatures is generally satisfactory for the removal of water of
crystallization from the various hydrated phases of calcium aluminate cement. Thereafter
the temperature can be increased more rapidly. Care must be taken to allow for the escape
of moisture, especially when the material is cast against an impervious surface. However,
the exact heating up schedule should be decided on the basis of quality and quantity of
material used, thickness of the lining, geometry of equipment, etc. When cooling down is
part of the schedule of dry out, cooling rates shall not exceed 80 C per hour.
Drying out of refractory brick work is also equally important . This helps in driving out the
water present in joinng mortar as well as in the brickwork. The length of drying schedule is
dependent on the mass of the brickwork and operang temperature. Heating should be
accomplished slowly enough to permit adjustments for the thermal expansion and mineral
transformation which may occur.
A proper dry out agency is helpful, because the permanent burners have a minimal heating
range, may be at best 5:1 turndown range , means lowest a burner can start at 20% of total
capacity. But a specialty dry out system has a range of 100:1 turndown. Therefore the
permanent system s lack the fine control necessary to increase temperature at a low rate of
a few deg. C per hour. At the same time it is possible to install the heating system in any area
of the unit, where a nozzle/manway exists to circulate heat.
8.) Inspection and Repairing of Lining:-
Before and after the heating up operaon, the lining should be visually inspected and
examined for the soundness of the lining and dimension. If repair is required accordingly to
the inspection, the defective lining should be repaired as per the recommended
procedures. This must be carried out by experienced persons.
After natural drying, visually checks whether crack 3 mm, or wider, have formed and, if so,
whether they are likely to effect operation. Irrespective of their width, if the cracks cross
each other forming separate blocks, which may tend to cause the section to fall off, remove
the portion and repair the area.
Also check expansion joints, separation joints, flange connections, dimensional accuracy
etc. as per the drawings.
After dry out, linings generally shrink more than naturally dried linings. Cracks 2 mm or so
in width and about 300 mm apart are normal and may be overlooked. However repairs are
necessary where a crack is 3 mm or more and extends to the shell, or if cracks cross each
other forming separate sections with a tendency to fall of.
A hammer test may also be done after dry out, with a test hammer. Normal and abnormal
linings give out clearly distinguishable sounds and faults are easier to locate. Note
however that sounds emitted by dried out linings tend to be dull.
