Monolithic Refractories
Monolithic Refractories
Monolithic refractory is the name normally given to all unshaped refractory products, the word monolithic coming from the word monolith meaning ‘big stone’.
Monolithic refractories are special mixes or blends of dry granular or cohesive plastic materials used to form virtually joint free linings. They are unshaped refractory products which are installed as some form of suspension that ultimately harden to form a solid mass. Most monolithic formulations consist of 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).
Monolithic refractories represent a wide range of mineral compositions and vary greatly in their physical and chemical properties. Some are low in refractoriness while others approach high purity brick compositions in their ability to withstand severe environments. Monolithic refractories are replacing the conventional type fired refractories at a much faster rate in many applications including those of industrial furnaces.
Monolithic refractories are used to advantage over brick construction in different type of furnaces. Their use promotes quick installation, avoid delays for the manufacture of special brick shapes. Use of monolithics frequently eliminates difficult brick laying tasks, which may be accompanied with weakness in construction. 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. Under certain conditions, monolithic linings of the same composition as firebrick provide better insulation, lower permeability and improved resistance to the spalling effects of thermal shock. With little or no preparation, monolithic refractories can be applied to form monolithic or joint free furnace linings in new constructions or to repair existing refractory lining. Other major advantages of monolithic refractory linings are as follows.
- It eliminates joints which is an inherent weakness.
- Method of application is faster and skilled measures in large number are not required.
- Properties can be better than pressed bricks.
- Transportation and handling are simple.
- It offers considerable scope to reduce inventory and eliminate special shapes.
- It has better volume stability.
- It has ability to install in hot standby mode.
- Monolithic linings have improved mechanical resistance to vibration and impact.
- Another advantage is that shrinkage and expansion can be matched to the application.
Various means are employed in the placement of monolithic refractories like ramming casting, gunniting, spraying, sand slinging etc. Ramming masses are used mostly in cold applications where proper consolidation of the material is important. The same practice can be adopted with both air setting and heat setting materials. Proper ramming tools need to be selected.
When air setting or hydraulic activated monolithic refractories are used, the entire thickness of the lining becomes hard and strong at atmospheric temperatures. The strength can be somewhat lower through the intermediate temperature range, but increases at higher temperatures with the development of a ceramic bond.
Heat setting monolithic refractories have a very low cold strength and depend on relatively high temperatures to develop a ceramic bond. In the case of a furnace wall having the usual temperature drop across its thickness, the temperature in the cooler part is usually not enough to develop a ceramic bond. However with the use of a suitable insulating material as backup, the temperature of the lining can be high enough to develop a ceramic bond throughout its entire thickness.
When monolithic refractories are used as the primary furnace lining, they are usually held in place with either ceramic or high temperature steel anchors. Each method of anchoring has advantages, depending upon furnace conditions and installation techniques.
Subsequent to the installation and curing, monolithic refractories require a carefully controlled dry-out schedule. This causes the binder, filler and aggregate to sinter producing a strong material. One consequence of not controlling the dry-out schedule is explosive spalling. Explosive spalling of monolithic linings is very problematic to refractory installers and furnace operators, costing significant loss in revenue from down time and repair work. Explosive spalling is believed to be caused by water trapped within the pore structure of cementitious materials, which becomes heated rapidly, forming steam with very high vapour pressure. It is supposed that the steam in combination with thermal stresses developed during the heating causes catastrophic failure of the structure.
Types of monolithic refractories
Common usage divides the monolithic refractories in the following groups. Fig 1 shows the four major monolithic refractory groups.
Fig 1 Major monolithic refractory groups
Castable refractories
Castable by name implies a material of hydraulic setting in nature. These are the materials that contain cement binder usually aluminate cement, which imparts hydraulic setting properties when mixed with water. Following the heat-up of the material the binder either transforms or volatilizes facilitating the formation of a ceramic bond. The most common binder used in castables is high alumina cement. Other binders that are often used include 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 face of applications. These 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.
An article on refractory castables is at link http://www.ispatguru.com/refractory-castables/.
Plastic refractories
Plastic refractories are used to form refractory monolithic linings in various kinds of furnaces. They are especially adaptable for making quick, economical emergency repairs. These refractories are easily rammed to any shape or contour.
Plastic refractories are mixtures of refractory aggregates and cohesive clays which is prepared in stiff plastic condition at the proper consistency for use without further preparation. During application, the blocks are sliced into pieces and are rammed or poured into place with pneumatic rammer. Plastic refractories can also be pounded into place with a mallet.
The high refractoriness, the range of compositions, and the ease with which plastic refractories are rammed into place make them suitable for many important applications. Plastic refractories are often highly resistant to destructive spalling.
Plastic refractories can include all the fireclay, clay-graphite, high alumina, high alumina graphite and chrome types adapted for many different operating conditions. Special gunning versions are also available. These are in granulated form and are prepared at the proper consistency, ready to use.
Examples of some of the of plastic refractories are heat setting super duty fireclay plastic, super duty heat setting plastics with graphite, plastics in the 50 % alumina class, heat setting 60 % alumina class plastics, air setting high alumina plastics in 80 % alumina class, phosphate bonded high alumina plastics with alumina content ranging from 70 % to 90 %, phosphate bonded alumina chrome plastics, and silicon carbide based phosphate bonded plastics etc. Different plastic monolithic refractories have specified properties and are used for specified applications.
Ramming mixes
Ramming mixes consist essentially of ground refractory aggregates, with a semi plastic bonding matrix. Ramming refractory materials are very similar to plastic refractories but are much stiffer. They need some sort of form to restraint them when formed. The particle sizes are carefully graded and the final product is usually delivered dry and then mixed with a little amount of water just before application. Other ramming mixes are delivered in wet form and are ready for use immediately upon opening. Ramming mixes are placed with pneumatic rammer in layers of 25 mm to 40 mm.
High purity ramming mixes based on mullite grain are used in steel making, burner blocks, ports and similar applications. Ramming mixes of 80 % plus alumina content have excellent resistance to shrinkage and thermal spalling at high temperatures. Air setting high alumina mixes employ stabilized, chemically refined high purity alumina and hence have excellent resistance to thermal spalling at high temperatures and volume stability up to their temperature limit. Phosphate bonded alumina chrome ramming mixes typically feature very high strength at high temperatures and extremely good resistance to acid and neutral slags including coal ash slags. Alumina graphite ramming mixes have combination of high alumina aggregate and slag inhibitors which give them excellent slag resistance to acidic and slightly basic slags.
Dry ramming mixes based on high purity magnesite and a sintering aid are useful in steel making. magnesite ramming mixes of exceptional purity and stability are used primarily as lining materials for coreless type induction furnaces. Magnesite chrome fused grain ramming mixes provide exceptional density and strength.
Gunning mixes
More than a third of all monolithic refractories are installed by gunning. Gunning mixes consist of graded refractory aggregate and a bonding compound, and may contain plasticizing agent to increase their stickiness when pneumatically placed onto a furnace wall. They are granular refractory materials sprayed on application area using a variety of air placement guns. These are heat setting and are used for patching and maintenance works for kilns and furnaces. Typically gunning mixes are supplied dry. To use, they are pre-damped in a batch mixer, then continuously fed into a gun. Water is added to the mix at the nozzle to reach the proper consistency.
Dense, homogeneous monolithic linings can be gunned without the use of forms and with a marked saving 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 pre-damped and fed through a continuous dual chamber or rotary gun. Magnesite and hot gun mixes are not pre-damped 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.
Examples of gunning mixes include fireclay gunning mixes of multipurpose hard fired fireclay and standard calcium aluminate cement compositions, fire clay gunning mixes with high purity calcium aluminate bonding system, gunning mixes based on vitreous silica and a special combination of calcined fireclay and high purity calcium aluminate cement binder, high purity alumina mixes which combine high fired alumina aggregate and high purity calcium aluminate binder, basic refractory gunning mixes with magnesia content ranging from 60 % to 95 % with or without a phosphate bond etc.
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.
Mortars
Mortars are generally neither classified under refractory brick nor monolithic refractories. These are finely ground refractory materials, which become plastic when mixed with water. These are used to bond the brickwork into solid unit, to provide cushion between 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 must have good water retention properties and must not sediment. In this way, premature penetration of water in the refractory bricks after laying, causing the mortar to dry out, can be avoided. Refractory mortars can be with ceramic bonding (bonding starting at 800 deg C), chemical bonding or hydraulic bonding ( bonding starting at 20 deg C). The composition and characteristics of the mortar materials, grain size and consistency are the important properties of the mortars.
Fettling mixes
Fettling mixes are also granular refractory materials, with function similar to gunning mixes, but are applied by shoveling into the furnace needing patching.
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. Different criteria are essential for all tap hole mixes. These are correct consistency, setting, and carbonization at the right time, precisely controllable PLC, and above all drilling capability.
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