Heating – Shaft type – Having combustion products generated in or fed to shaft
Patent
1989-01-06
1991-05-07
Bennet, Henry A.
Heating
Shaft type
Having combustion products generated in or fed to shaft
432 96, 432 58, 34171, F27D 108, F26B 1712
Patent
active
050132377
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus for drying particulate materials such as sand, aggregates, powdered products and the like and which is particularly economical in energy consumption
The wet or dry calcining of gypsum crushed to fine and granular form is illustrative of drying processes for particulate materials, this example being of particular interest due to the poor heat conductivity of gypsum. Thus, when gypsum is dried at a pressure close to or less than 10.sup.5 Pascals (1 bar), water in the gypsum is eliminated as dry steam and a microporous solid product is obtained which consists of a loose assembly of microcrystallites, which is called .beta.-type plaster.
If gypsum is dried at a pressure exceeding 10.sup.5 Pascals in a liquid environment, a crystalline solid of hexagonal appearance is obtained which is .alpha.-type plaster. The latter has more desirable properties in subsequent use than the .beta. variety, because it requires a far smaller amount of mixing water. However, the manufacturing processes by which it is produced are quite costly due to the high level of energy consumption and the material facilities required.
Indeed, this variety of plaster is generally produced in high-temperature autoclaves wherein the internal pressure is raised from 5 to 7.times.10.sup.5 Pascals For this reason, the .alpha. variety is generally produced in only small quantities compared with the .beta. variety and is kept for just a few special applications where very high mechanical characteristics are required of the plaster.
In reality, .alpha. and .beta. varieties are only extreme terms in a series of intermediate varieties exhibiting more or less marked states of crystallisation. These varieties which include what are termed low-pressure varieties, exhibit mechanical strength properties which are vastly superior to those of the .beta. variety.
In industry, most plaster is used in mixture with .beta. plaster to enrich this latter so that it has intermediate characteristics identical to those of the low-pressure .alpha. varieties.
Conventionally, gypsum is dried in rotary ovens or in static cauldrons fitted with interior mechanical mixers or in any other system capable of producing heat and of transmitting it to the gypsum in order to evaporate the water of hydration therefrom; the increase in the temperature of the gypsum can occur in a dry or in a wet environment. The heat is produced by generators using industrial fuels and the temperature of the gypsum is raised either by direct contact with the combustion gases or by indirect contact through gaseous, liquid, refractory, metal or other heat exchangers. In ovens of this type, heat exchange between the products to be treated, the heated gases or the heated walls is carried out by agitation of the products and by their repeated passing through heated zones. In other processes, the product to be treated is substantially static, the heat exchange being carried out by multiple passages of recycled heated gases in such a way as to recover the maximum heat energy before the gases are discharged. All these prior art ovens for producing .beta. semi-hydrate or dead-burnt product work at atmospheric pressure in a dry atmosphere.
For example, in the case of the Renaudin oven which is the subject of French Patent No. 979,494, the inlets and outlets are open to atmospheric pressure and drying is performed in a dry atmosphere, the product, almost at the point of flow, is stationary in relation to non-recycled hot gas. However, such apparatus does not permit effective heat exchange, especially since special care is required to eliminate the water vapour produced in order to avoid the gypsum boiling in its organic water, the inventor finding this procedure preferable.
Another system described in German Patent No. 31 45 549 for calcining gypsum in the form of blocks likewise discloses a method in which the product to be treated is substantially immobile and virtually on the point of flow. This is likewise a system which is open to dry atmospheric pressure and which can onl
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Bennet Henry A.
Kilner C.
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