Editorial Feature

Refractories - An Introduction to the Types of Monolithic Refractories

Monolithic refractory is the name generally given to all unshaped refractory products, the word monolithic coming from the word monolith meaning 'big stone'. These are materials which are installed as some form of suspension that ultimately harden to form a solid mass.

Market Share

Monolithic refractories have increased in market share of the total refractories industry over the last two decades and will continue to do so. The main drivers for this have been economic considerations  (rapid installation time reducing the down time of a given application, cost of raw materials for their manufacture, less manpower required in their installation), and the availability of skilled people within the industry - as the refractories industry has decayed there are less skilled people available such as refractory brick layers to install conventional products.

Versatility

Monolithic refractories have a myriad of industrial applications throughout the steel, cement, non-ferrous metallurgical, waste disposal and petrochemical industries. They are available in many forms and different formulations. The main properties of these materials are their respective chemical inertness, mechanical integrity, abrasion resistance and thermal shock resistance at high temperatures.

Composition

Most monolithic formulations consist of large refractory particulates (an aggregate), fine filler materials (which fill the interparticle voids) and a binder phase (that gels the particulates together in the green state).

Installation and Curing

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

Castable Refractories

These are materials which consist of precision graded coarse and fine refractory grains. They are gelled by means of a binder system in the materials green state. Following the heat-up of the material the binder either transforms or volatilises facilitating the formation of a ceramic bond. The most common binder used in castables is HAC (high alumina cement). Other binders that are often used include hydratable aluminas and colloidal silica. Castables are mixed with water and then installed by either pouring or pumping. Placement of the material then requires vibration.

The cement-containing castables are often classified by the amount of cement they contain. Conventional castables can contain around 15-30% cement binder. As refractory technology evolved chemical additives were included in the package to reduce the amount of cement and water the product required - the impact of this was material with improved strengths and durability. Low cement castables contain between about 3-10% weight cement. Ultra low cement castables contain less than 3% cement component.

A specialised type of refractory castable is the free flow castable which is able to be installed without vibration. They require a much lower water addition than traditional castables. This is due to the fact that they have particle packing and dispersing agents which modify the surface chemistry of the fine particles to improve the flow of the material.

Certain castable formulations may be installed via gunning techniques which involves spraying the material through a nozzle at a high speed. At the nozzle, cement accelerators are often added to promote rapid hardening of the material. This technique allows applications to be lined very quickly.

Plastic Refractories

These are monolithic refractory materials which are tempered with water and/or added with a binder. They have sufficient plasticity to be pounded or rammed into place.

Ramming Refractories 

These materials are very similar to plastic refractories though are much stiffer mixes.

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.

Refractory Mortars

Mortars consist of finely ground refractory materials which are then mixed with water to form a paste. They are used for laying and bonding shaped refractory products such as bricks. They are normally applied by trowelling.

Insulating Castables

Insulating castables are specialised monolithic refractories that are used on the cold face of applications. There 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.

Source: CERAM Research Ltd

For more information on this source please visit CERAM Research Ltd.

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