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Technical Paper


remainder of the lining, except of course with the final sections and the furnace atmosphere as such will not differ much from ambient. Generally sufficient ventilation is provided already for the working crew, but should be controlled in order not to be in excess of what the 'curing' lining would require.


Under tropical conditions external shading from direct sun radiation or even cooling with water shall be applied if temperatures will still become too high. Ideal for lining and crew would be a temperature of 20 - 25 °C.


When refractory linings would be installed, cured and dried, e.g. under open covers or outside at all, the installed parts shall after completion immediately covered with wet tarpaulin, gunny bags, or similar, to prevent too fast evaporation.


Practice has shown that under wet gunny bags temperatures can be 10 °C or more lower than ambient !!


At the other hand, in very cold climates external protection may be required in order to keep the casing above 5 °C. Especially cold casings slow down the hydraulic (and chemical reactions) considerably, even bring them to a halt. Cold brought in by natural ventilation will hardly affect the lining, except perhaps of the surface layer. In such cases heated air could be circulated, but it will also increase evaporation rates.


After the appropriate period of curing of cement containing refractories, as described before, both in hot or cold conditions furnace linings could be left idle for a period of time. Contrary, in such events chemically bond refractories must then be heat treated, because they may react/deteriorate under influence of the moist from the atmosphere.


Adversely, complete fired linings will pick up moist and loose some strength (15 - 20%), but upon start firing all strength is regained again, hence the influence of moisture on the strength is largely reversible.


3.2: Curing of plastic and ramming refractory mixes


For plastic as well as chemically bonded refractories applies: Water spraying shall never be allowed


These mixes are supplied as moist and generally as air-setting products, necessary for placement. They contain little free water. The moist texture is achieved by a certain content of refractory clay, which behaviour resembles that of common mud; with sufficient water it is paste-like, when dried it shows a 'crackled' conduct because of drying shrinkage.


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Mainly because these mixtures are normally dense materials, evaporation of the clay-bonded water occurs gradually. Hence, no special curing period or special measures are required, except for some extra venting holes (slightly pointing upward) and maintaining sufficient air ventilation to avoid condensation. Moistening of plastic and rammed lining makes the refractory material weak again and may even affect the quality of the lining.


Though, fresh placed materials can be left unattended for a short period (night-breaks, lunch breaks, etc.) if they are adequately protected by plastic foil (e.g. in which they are supplied) or tarpaulin to avoid drying out of at least the working surface on which application will continue.


Anticipating on par. 3.3, certain heat-setting plastic refractory materials are hygroscopic and thus weaken rapidly again. They should therefore be fired directly after installation. If not possible these materials must be protected by plastic foil or tarpaulin.


Drying of rammed refractory materials has to take place in a controlled matter, avoiding excessive (surface-) shrinkage. Although the total water content is rather low, very dense refractory materials require considerable drying time in order to loose the free water, because of the inherent low permeability. Careful heat-treatment is then required in order avoid "steam-explosion".


3.3: Curing of chemical bonded refractory mixes


Chemically setting refractory materials are mainly based on phosphate- bond or colloidal silica-bond and although principally heat-setting, they are mostly supplied as air-setting products.


Except for appropriate air-drying they do not require a curing period. Here as well applies: 'No water spraying shall be allowed', because they are water soluble until firing.


After installation and air-setting, these materials form also hydrates with alumina-phosphates and silica-gels respectively. The longer the air-setting period at 'room' temperatures, the stronger the ultimate properties will become.


4: Drying and Firing of monolithic refractory materials


Due to their relatively closed bonding matrix of the cement, hydraulically setting refractories have a low open porosity (± 12 %v) and a low permeability in fresh, unfired state. Even concretes with highly porous


Figure 4: Thermal Expansion of refractory castables


Figure 5: Example of the thermal conductivity of an insulating refractory castable with an apparent density of ~1,5 kg/dm3


12 ENGINEER THE REFRACTORIES May 2018 Issue


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