Chemistry of inorganic compounds – Halogen or compound thereof – Sulfur containing
Reexamination Certificate
1993-08-12
2001-06-05
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Sulfur containing
Reexamination Certificate
active
06241958
ABSTRACT:
DESCRIPTION
This invention relates to a process of preparing sulfate-containing basic solutions of polyaluminumchloride. Such solutions are used as floc- culating agents to the neutral aluminum salts, which have been used for that purpose for a long time. Basic aluminum chlorides effect a flocculation and precipitation of the pollutants contained in the water within a relatively wide pH range and the resulting flocs are so large that they can easily be removed. The flocculation can be improved further by the incorporation of polyvalent ions, preferably sulfate ions, in the basic aluminum chloride because sulfate-containing basic aluminum chlorides will effect a faster hydrolysis than basic aluminum chlorides which are free of sulfates.
German Patent Publication 1,907,359 and German Patent Specification 2,547,695 disclose processes of preparing sulfate-containing basic solutions of aluminum chlorides. In said processes, sulfate ions are added to basic solutions of aluminum chloride. In accordance with German Patent Publication 1,907,359 the basicity of the solutions is adjusted by an addition of calcium compounds and barium compounds. A disadvantage of the known processes resides in that the sulfate ions are precipitated in part as CaSO
4
or BAO
4
and that said difficultly soluble compounds must be removed as by-products, which cannot be re-used. Besides, the solutions prepared by the known processes tend to be unstable so that difficultly aluminum compounds are undesirably precipitated or salted out and the flocculation activity is thus strongly decreased.
It is an object of the invention to provide for preparing sulfate-containing basic solutions of polyaluminumchloride a process by which products are produced which have a high flocculating activity and high stability whereas a formation of waste products is avoided. Besides, the basicity of the solutions should be adjustable within wide limits without an occurrence of instability.
The object underlying the invention is accomplished in that aluminum-containing substances are digested by a treatment with hydrochloric acid, the insoluble residue is filtered from the resulting solution, the solids-free solution is subsequently concentrated by evaporation, AlCl
3
.6H
2
O is recovered from the concentrated solution by crystallization, the AlCl
3
.6H
2
O is converted to a solid basic aluminum chloride by a thermal decomposition at 150 to 200° C., the solid basic aluminum chloride is charged into aqueous sulfuric acid and is dissolved therein with formation of Al(OH)
x
Cl
y
(SO
4
)
z
and the basic solution of aluminum chloride is subsequently heat-treated at 40 to 70° C. for 1 to 3 hours.
The aluminum-containing substance which is used may consist, e.g., of alumina hydrate and the hydrochloric acid used as a digestant contains 15 to 35% HCl. The AlCl
3
.6H
2
O can be recovered by a crystallization in one or more stages and it is necessary to produce a crystallizate which has an optimum particle size for the subsequent thermal decomposition and a low residual moisture content. A centrifuge is desirably employed to separate the crystallizate from the mother liquor, which is recycled to the crystallizing process. The basic aluminum chloride which has been produced by the thermal decomposition is required to have a high solubility and an optimum basicity (mole ratio of OH to Al). The HCl-containing gases which have been liberated by the thermal decomposition are reacted to form aqueous hydrochloric acid, which is re-used to digest the aluminum-containing substance. The Al(OH)
x
Cl
y
(SO
4
)
z
formed as the basic aluminum chloride dissolved in aqueous sulfuric acid is present not only as monomers but also as oligomers but the desired degree of polymerization is not achieved until the heat treatment.
The invention affords the advantage that raw materials are used which consist of aluminum-containing substances which have previously been discarded as waste material or had to be utilized with only a low technological and economic benefit. Besides, the process in accordance with the invention is an almost self-contained process, in which substantially no by-products and/or waste products are formed. Specifically, the hydrochloric acid used to digest the raw materials is recovered in part during the thermal decomposition of the AlCl
3
.6H
2
O and the hydrochloric acid is desirably recycled. Besides the required basicity is adjusted without a need for chemicals which would have to be disposed of as waste products. Finally, the solutions prepared by the process in accordance with the invention have extremely good properties in use. This is particularly due to the production of a solid basic intermediate product, the charging of the basic intermediate product into aqueous sulfuric acid, and the heat treatment of the solution of aluminum chloride sulfate.
According to the invention, aluminum-containing waste products are used as an aluminum-containing substance. For instance, waste solutions may be used, which are formed in the chemical industry, e.g., by the etching of aluminum or in organic syntheses catalyzed by AlCl
3
. It is also contemplated according to the invention that the solids-free solution produced by the digestion is concentrated by evaporation to an AlCl
3
content of about 30 weight percent and an optimum crystallization of AlCl
3
.6H
2
O is subsequently effected. In accordance with the invention it is particularly desirable to form an AlCl
3
.6H
2
O having a median particle diameter d
50
between 250 and 450 micrometers and a residual moisture content of 3 to 10 weight percent. It will also be particularly desirable in accordance with the invention to form a solid basic aluminum chloride which has a basicity—i.e., a mole ratio of OH to Al— of 1.35:1 to 2.25:1. In that case the basic aluminum chloride will have a high solubility because 92 to 98 weight percent of that product are soluble in water or in aqueous sulfuric acid. It is contemplated according to the invention that the basic aluminum chloride is dissolved in water before it is added to the aqueous sulfuric acid. A particularly good dissolving behavior will be achieved if, in accordance with the invention, the solid basic aluminum chloride is dissolved in aqueous sulfuric acid or in water at 40 to 70° C. It is also contemplated according to the invention that the basic solution of aluminum chloride sulfate has an aluminum concentration of 3 to 10 weight percent, preferably 5 to 7.5 weight percent. The solutions prepared by the process in accordance with the invention will have particularly desirable properties if the basic solution of aluminum chloride sulfate has after the heat treatment a basicity (OH:Al mole ratio) of 1.05:1 to 1.95:1 and a sulfate content of 1 to 6 weight percent, preferably 2 to 4 weight percent. The properties of the solid basic aluminum chloride will be particularly constant and good if the AlCl
3
.6H
2
O is thermally decomposed in a fluidized bed and it is desirable to form AlCl
3
.6H
2
O crystals having a particle size of 250 to 450 micrometers. Finally, it will be necessary in some cases to filter the basic solution of aluminum chloride sulfate after the heat treatment in order to remove any solids which may be present and which can be recycled in the process as raw materials.
The subject matter of the invention will now be explained in detail with reference to examples.
REFERENCES:
patent: 2369691 (1945-02-01), Schmerling
patent: 4264569 (1981-04-01), Sinha
patent: 4560541 (1985-12-01), Davis
patent: 1907359 (1970-08-01), None
patent: 211543 (1984-07-01), None
patent: 2547695 (1984-12-01), None
patent: 2289447 (1976-05-01), None
patent: 1347413 (1974-02-01), None
patent: 53506 (1968-03-01), None
Geiler Gunter
Haake Gerhard
Haupt Frank
Griffin Steven P.
Hendrickson Stuart L.
Metallgesellschaft Aktiengesellschaft
Striker Michael J.
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