Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1988-07-29
1990-12-04
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 57A, 34164, F26B 308
Patent
active
049743341
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a procedure and a device for drying and/or calcining powdery materials.
BACKGROUND OF THE INVENTION
In various fields of technology, powdery materials have to be dried and/or burned (calcined) in order to remove surface humidity and/or chemically bound water, and in order to prepare material for further processing.
For example, in the gypsum industry, it is necessary to dry the natural gypsum, which was excavated above or below ground, and which has a quarry or inherent water content of about one to three per cent by weight, in order first to remove the adsorbed surface humidity and in order to subsequently calcine, with different calcium sulfate hydrate qualities being formed as a function of the temperature and time.
Numerous procedures and devices are known for drying and calcining natural gypsum.
In contrast, problems arise when treating particularly fine powdery materials, especially calcium sulfate products, during the treatment of synthetic calcium sulfate materials, such as the so-called chemical and desulfurization gypsums. The latter are the end product obtained during desulfurization, for example during a washing procedure with limestone (CaCO.sub.3) or with calcium hydroxide (Ca(OH).sub.2) or with calcium oxide (CaO). Such desulfurization gypsum is called calcium sulfate dihydrate (CaSO.sub.4. 2H.sub.2 O).
Such desulfurization gypsum differs in many ways from natural gypsum. Already in the raw or crude state, the desulfurization gypsum is much more finely crystallized and has due to the method of production, grain sizes of sometimes much less than 50 .mu.m. The crystalline appearance also differs from that of natural calcium sulfate dihydrate. Furthermore, the adsorbed surface humidity of desulfurization gypsum is approximately ten per cent by weight greater than that of natural gypsum (about one to three per cent by weight).
Obviously, such products cannot be treated by conventional drying and/or calcining procedures and devices. Even the transport of the finely grained and moist desulfurization gypsum causes difficulties due to the undesirable "formation of lumps", which quickly results in the clogging of conveying units and transport mechanisms. The moist desulfurization gypsums are therefore dried prior to delivery to customers (discharge systems), because available conveying and calcining units cannot operate in a disturbance-free manner with moist material (Zement Kalk Gips 5 (1983) 271, 273).
However, the drying itself causes difficulties due to the described properties of the synthetic calcium sulfate products, and relatively expensive dryers, such as centrifugal hydroextractors, are used to aid in the process, though they permit only a partial removal of water. Thereafter, transport is in fact somewhat easier, but the described technical difficulties have not yet been eliminated. Moreover, the material, which is still very finely grained, cannot be calcined in conventional calcining equipment without further preparation.
Projects known up to now (Zement Kalk Gips (see above); Umwelt 6 (1983), 435) propose a nodulizing or agglomeration and a subsequent comminution or pulverizing to improve the unfavorable grain structure and the particle structure, as well as to influence the rheological characteristics. The agglomeration is to be carried out with briquetting presses or with pelletizing machines, which lead to coarsening of the grain.
Thus, the material, which is still moist, has to be additionally treated. Supplementary equipment is necessary. Such equipment is not only expensive, but considerably increases the cost of preparation of the aforementioned products, and in addition is susceptible to breakdown.
Another proposal (Umwelt, see above) proposes the production of material in pieces or lumps by pan mills: however this results in the same drawbacks.
The known procedures for preparing such finely grained and powdery materials have, for the aforementioned range of application, have the further drawback that already for reasons of cost, the compacti
REFERENCES:
patent: 2832584 (1958-04-01), Plante
patent: 3063848 (1962-11-01), Van Gelder
patent: 3161483 (1964-12-01), Morris
patent: 4305210 (1981-12-01), Christensen et al.
patent: 4371375 (1983-02-01), Dennis, Jr. et al.
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