Drying and gas or vapor contact with solids – Process – With treated material recirculation
Patent
1996-09-23
1999-09-07
Bennett, Henry
Drying and gas or vapor contact with solids
Process
With treated material recirculation
34169, 423215, 423625, F26B 700
Patent
active
059468181
DESCRIPTION:
BRIEF SUMMARY
This application is the national stage of International Application No. PCT/GB94/02594, filed Nov. 11, 1994.
The present invention relates to a process and apparatus for drying solid material borne in a liquid, and particularly to a process and to an apparatus for continuously drying, preferably with agglomerating and sizing, and separating a solid product from a potentially sticky liquid feed material, optionally with a heat treatment, especially without encrustation of the equipment used.
One particularly preferred application of the present invention is in the treatment of slurries of bauxite in Bayer process liquor, and slurries of Bayer process salt cake, which are obtained by evaporating Bayer process liquors to a high concentration of caustic, thereby causing the precipitation of the sodium salts of the organic impurities and of sodium carbonate present in these liquors. The present invention is capable of converting such viscous liquids or slurries into dry, free-flowing, non-sticky, abrasion and attrition resistant particles of mainly carbonates or oxides of the metallic elements originally present in the starting material.
When solutions, slurries or moist solids are dried the material frequently passes through a sticky phase as the moisture content is reduced. This is particularly true if the liquid phase contains dissolved solids.
The standard approach to the problems this creates is to back-mix some of the dried product with the fresh feed so that the mixture passes through the sticky phase before the moisture content of the fresh feed is reduced. This technique may suffer from the disadvantages of high solids recirculation rates, high mixer power consumption, encrustation and wear of the equipment, etc.
We have now discovered a relatively simple apparatus and process for drying liquid-borne solid materials, particularly sticky moist solids, which largely avoids the difficulties mentioned above. The described apparatus and process are not limited to aqueous solutions, suspensions and slurries, but may be applied to any system in which a carrier liquid, such as a solvent, is at least partly removed from a liquid-borne solid material, such as a solution, and particularly those which pass through a transient sticky phase as the carrier liquid is progressively removed. However, in order to simplify the following description of the present invention the terms "drying", "moisture", etc. will be used, taking an aqueous slurry containing solid particles as an illustrative, but non-limiting example. In a particularly preferred embodiment the dried solid material is also agglomerated, classified and heat treated during the drying process.
The present invention is based on the principle of feeding upwardly into a rising gas stream a liquid which carries solid material, and incorporates a solids reactor design derived from a known "Gas Suspension Dryer", in which the reacted material is allowed to fall counter-currently past the feed inlet point.
Debayeux et al in U.S. Pat. No. 4,335,676 disclose the basic principles of spouted bed drying. Importantly it is disclosed that the dried product is withdrawn from the top of the bed, which is different from the present invention, where the product is collected after falling counter-current through the stream of rising heat carrier.
In DK-A-5888/83 there is disclosed a so-called "Gas Suspension Dryer" for removing pollutants such as SO.sub.2 and other acid gases from flue or combustion gases in which the gases are absorbed on, and reacted with, the absorption agent in the presence of water to make a dry powder and a cleaned gas. It comprises a tubular reaction chamber with an annular bottom wall, and inlet ducts for the gas, the absorption agent, and an outlet at the top for the scrubbed gas. It is taught that the disclosed method is characterised by subjecting an axially-introduced rising stream of hot flue gas to a rapid reduction in velocity so as to cause a boundary layer separation in the lower part of the reaction zone. The method is also characterised by d
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patent: 4761893 (1988-08-01), Glorioso
Baxter Hamish
Carruthers Andrew Nicolson
Elkjaer Hans-Peter
Fenger Jens
Hiscox Bryan
Alcan International Limited
Bennett Henry
Wilson Pamela A.
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