Drying and gas or vapor contact with solids – Process – With fluid current conveying or suspension of treated material
Patent
1996-05-23
1997-11-11
Sollecito, John M.
Drying and gas or vapor contact with solids
Process
With fluid current conveying or suspension of treated material
34 89, 34 66, F26B 308
Patent
active
056850891
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for producing powdered ceramic products by spray drying an aqueous suspension, including dispersants end colorants.
Examples of products produced according to this process are oxides and silicates and similar of Aluminium, Magnesium as well as other metals, such as clay, ferrites, kaolin and stearite. The powdered products are used within various fields such as filler and coating of paper, ceramics, catalyst carriers etc.
Dispersants and colorants are organic components which will decompose if they are exposed to high temperatures for a relatively long period of time. It is therefore important, when drying suspensions including dispersants and colorants, to use a drying process that expose the suspensions to a minimum of heat treatment.
A well-known industrial process to dry suspensions of ceramic products is one-stage spray drying, cf. Masters, K., Spray Drying Handbook, 1991 p. 569 ff. By this process a kaolin powder with 0.5-3.0% residual free moisture is obtained by using drying air inlet temperatures of e.g. 400.degree.-600.degree. C. and outlet temperature of 95.degree.-120.degree. C. (p. 504).
The emphasis An prior art has been to reduce total energy consumption by scrubbing and/or heat exchanging the suspension with the spent drying gas, which has a relatively high temperature.
This forms a part of a number of prior art patents.
U.S. Pat. No. 4,642,904 describes a system in which a clay slurry is dewatered by scrubbing and then preheated by heat exchangers before it is spray dried in one stage to a powder with about 1% residual free moisture.
U.S. Pat. No. 4,717,559 describes a method in which calciner waste heat is used for supplying heat to the suspension before a one-stage spray drying process.
WO 92/14530 describes a process in which a feed of kaolin slurry passes a heat exchanger and a flash concentrator before being spray dried in a one-stage spray drying process.
U.S. Pat. No. 3,867,234 describes a process in which a ceramic product is spray dried to a dry powder in a one-stage spray drying process. The dry powder is guided to the discharge opening by ambient air in a double helix-shaped, sloped fluidized bed. The ambient air assumes free flow of the dry powder to the product outlet.
Known processes for spray drying of ceramic suspensions are one-stage drying processes. The produced powders are suitable for storage, however, they are often very dusty due to a fine particle size end low free moisture content and they have poor flow properties.
With a view of avoiding these drawbacks it has been suggested to produce dust free and free flowing ceramic powders with large particle size and increased free moisture content in a one-stage spray drying process. This is possible by operating at decreased outlet temperatures, a process well known to a person skilled in the art. By doing so, however, powders will inevitably form unacceptable lumps during storage.
It has now turned out that ceramic suspensions, as defined above, can be spray dried into a powder with the above mentioned good properties using a process characterized in that: moisture content in the powder that is higher than the free moisture content in the final powder, with ambient air to reach its final free moisture content.
The powder produced according to the invention is dust free, stable during storage, it has good flow properties, and has a uniform and high bulk density as well as good dispersing properties. Suspensions dried as per this process are exposed to a minimum of heat treatment, which is important for suspensions containing organic components such as dispersants and brighteners.
The process is specially appropriate for spray drying of products which, due to a small primary particle size, are not easily spray dried into a dust free, free-flowing, high quality powder suitable for storage. Furthermore, the process opens possibilities of savings in operation as well as investment costs.
By using this procedure the powder is cooled as a consequence of evaporation of moisture in the p
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patent: 5223088 (1993-06-01), Hansen
Fredsted S.o slashed.ren
Getler Jens
Gibson Stuart
Nielsen Per
APV Anhydro AS
Gravini Steve
Sollecito John M.
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