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Monolithic refractory is the term normally used to refer all unshaped refractory products. The word “monolithic” is derived from the word monolith meaning “big stone.” These are materials that are incorporated into some form of suspension that eventually harden to form a solid mass.
The market share of total monolithic refractories in the refractories industry has increased over the last 20 years and will continue to do so. The key drivers for this have been economic considerations and the availability of trained people within the industry.
The fiscal concerns comprise cost of raw materials involved in the manufacture of these refractories, quick installation time that decreases the downtime of a particular application, and less manpower needed in their installation. Since the refractories industry has dwindled, there are less trained people available, such as refractory bricklayers, to install traditional products.
Monolithic refractories have many industrial applications across the non-ferrous metallurgical, steel, cement, petrochemical, and waste disposal industries. They are available in many different forms and formulations. The key properties of these materials are their individual chemical inertness, thermal shock resistance at high temperatures, abrasion resistance, and mechanical integrity.
The majority of monolithic formulations contain large refractory particulates (an aggregate), a binder phase (that gels the particulates together in the green state), and fine filler materials (that fill the interparticle gaps).
Installation and Curing
Following the installation and curing, monolithic refractories need a properly regulated dry-out schedule. This causes the filler, binder, and aggregate to sinter, creating a strong material. If the dry-out schedule is not controlled, then explosive spalling will occur. For furnace operators and refractory installers, explosive spalling of monolithic linings poses a major challenge and the associated repair work and downtime can lead to a substantial loss in revenue.
It is believed that explosive spalling is caused by water trapped inside the pore structure of cementitious materials. These materials get heated quickly, forming steam with extreme vapor pressure. It is also assumed that the steam along with thermal stresses formed during the heating, leads to disastrous failure of the structure.
Types of Monolithic Refractories
Castable refractories are materials that contain precision-graded fine and coarse refractory grains. They are gelled with the help of a binder system in the material’s green state. After the material is heated, the binder either changes or volatilizes allowing the formation of a ceramic bond. High alumina cement (HAC) is the most typical binder used in castables. Other binders that are commonly used include colloidal silica and hydratable alumina.
Castables are blended with water and then set up by either pumping or pouring. Vibration is needed to place the material.
The cement-containing castables are usually categorized by the amount of cement they contain. About 15%–30% of cement binder can be present in conventional castables. With the advent of refractory technology, chemical additives were added to the package to decrease the amount of water and cement needed by the product. This resulted in a material with better strength and durability. Low cement castables have approximately 3%–10% weight of cement. Ultra-low cement castables have less than 3% cement component.
Free-flow castable is a type of refractory castable that can be set up without vibration. They need relatively less amounts of water than traditional castables. This is because they contain particle packing and dispersing agents that alter the surface chemistry of the fine particles to enhance the flow of the material.
Some castable formulations can be installed through gunning methods, in which the material is sprayed via a nozzle at a high speed. At the nozzle, cement accelerators are usually incorporated to harden the material quickly. Through this method, applications can be lined rapidly.
Plastic refractories are monolithic refractory materials that are tempered with water and/or integrated with a binder. They have adequate plasticity to be rammed or pounded into place.
Ramming refractories are just like plastic refractories but are much stiffer mixes.
Patching refractories are akin to plastic refractories and have very soft plasticity. This allows them to be pounded into place.
Coating refractories are used to safeguard refractory linings typically against chemical attack. These products are typically used to cover only the working surface of a lining. Coating refractories have fairly thin layers.
Mortars contain finely ground refractory materials that are blended with water to create a paste. They are utilized for laying and bonding shaped refractory products like bricks. They are usually applied by trowelling.
Insulating castables are essentially specialized monolithic refractories that are used on the cold face of applications. They are produced from lightweight aggregate materials such as bubble alumina, perlite, vermiculite, extendospheres, and expanded clay. Thermal insulation is the primary function of these castables. In addition, insulating castables usually have low thermal conductivity and low density. They have low-grade mechanical strength when compared to conventional castables.