Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkaline earth metal
Patent
1990-01-26
1992-03-03
Straub, Gary P.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkaline earth metal
106778, 106784, 106785, 106786, 423555, C04B 1102, C04B 1126, C04B 4000
Patent
active
050930931
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for the preparation of calcium sulfate alpha hemihydrate.
THE RELATED ART
Various processes are known for the preparation of calcium sulfate alpha hemihydrate, namely so-called wet processes, wherein raw gypsum is continuously passed in a suspension through an autoclave and converted to calcium sulfate hemihydrate, as well as autoclave methods wherein the raw gypsum is introduced in pieces or as briquettes in an autoclave and converted. For instance, in the preparation of calcium sulfate alpha hemihydrate from raw gypsum originating from desulfurization installations of power plants and which has been pressed into briquettes, it is possible to obtain large and regularly shaped crystals (primary crystals) of calcium sulfate alpha hemihydrate, after the autoclave process. The briquettes consisting of primary crystals are broken in a crusher, and this way the primary crystal is broken down into a coarse-grained product with a specific surface of for instance 800 cm.sup.2 /g.
Such coarse-grained calcium sulfate alpha hemihydrate is in itself a very valuable product when used for the preparation of mortars or the like, due to its reduced water requirements. However, the coarse grain does not lead to a rapid strength development, since the reaction per time unit of the coarse grain with water is relatively reduced and incomplete. In order to accelerate this reaction, the usual accelerating agents can be used. However, they have the disadvantage of leading immediately to onset of strength development. Although this effect can be diminished by feeding additional water amounts, these additional water amounts are actually undesirable. Besides, usually a sufficient holding time is required, in order to prepare the mortar and to bring it to the place where it is needed.
It is therefore the object of the present invention to create a process of the aforementioned kind, which renders possible production of a calcium sulfate alpha hemihydrate with a sufficiently delayed onset of rigidity, but which still leads to a rapid strenth development and a high level of strength.
SUMMARY OF THE INVENTION
This problem is solved by bringing the calcium sulfate alpha hemihydrate, which basically is available in primary crystals, to a degree of fineness corresponding to the strength development required for the intended use, by removing fines through screening and by admixing at least one of the fruit acids and/or their salts as retardants or liquefiers.
By milling the initial coarse-grained product, specific surfaces in the range of approximately 1200 to 4000 cm.sup.2 /g, particularly 1500 to 3500 cm.sup.2 /g are selected. Due to the screening, a certain range of grain size can be selected, so that the finest grain, as well as the coarser grain can be eliminated. The fine grain is extremely reactive and has a high water requirement, so that thereby the desirable characteristics would be impaired. Coarser grain does not have a high water requirement, but is not sufficiently reactive.
The fruit acids, e.g. malic acid and tartaric acid, particularly citric acid, or their water-soluble salts, which can be used separately or in combination, delay the onset of rigidity. At the same time these compounds serve as liquefiers, whose effect starts immediately when the calcium sulfate alpha hemihydrate comes in contact with the mixing water. Besides, as a result, the compression strength is considerably increased. This is particularly surprising, since up to now citric acid has been considered as an additive with a delaying, strength-reducing effect, when used in small amounts under 0.1M. -%, while in larger amounts it even avoids strength development, compare with ULIMANNS ENZYKLOPADIE DER TECHNISCHEN CHEMIE, 4th Edition, Volume 12, Page 307, and R. A. Kuntze, THE CHEMISTRY AND TECHNOLOGY OF GYPSUM, ASTM Special Technical Publication 861, 1984, so that citric acid, as well as other related fruit acids were considered up to now as undesirable.
The fruit acid is added particularly i
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Dubno Herbert
ProMineral Gesellschaft Zur Verwendung von Mineralstoffen mbH
Sicowa Verfahrenstechnik fur Baustoffe GmbH & Co.
Straub Gary P.
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