1. Field of the Invention
The present invention relates to a method for the production of high purity calcium thiosulfate by a metathesis reaction of ammonium thiosulfate and calcium hydroxide or calcium oxide.
2. Description of the Prior Art
Although the prior art contains many references to the production of calcium thiosulfate, at present, there are no known commercial processes for the manufacture of high purity calcium thiosulfate.
Japanese Pat. No. 6,039, issued Feb. 22, 1973, discloses a process for the manufacture of calcium or magnesium thiosulfate by reacting calcium or magnesium sulfite with sulfur in alkaline solution. High yields are obtained only in the preparation of magnesium thiosulfate. This Japanese patent also discloses a metathesis reaction between calcium hydroxide and magnesium thiosulfate to produce calcium thiosulfate and insoluble magnesium hydroxide. However, the preparation of calcium thiosulfate solutions according to this Japanese patent requires a difficult filtration of a slurry containing magnesium hydroxide.
In Spanish Pat. 245,171, issued Nov. 15, 1958, a process is disclosed for the preparation of calcium thiosulfate by reacting a mixture of calcium chloride hexahydrate and calcium hydroxide with a concentrated solution of sodium thiosulfate at 30.degree. C. Not only are 0.77 tons of co-product sodium chloride produced for every ton of calcium thiosulfate produced but residual sodium chloride is an objectionable contaminant, e.g., when calcium thiosulfate is used as a non-corrosive hardening accelerator for concrete.
High purity calcium thiosulfate hexahydrate (CaS.sub.2 O.sub.3.6H.sub.2 O) is produced by a metathesis reaction between a concentrated solution of calcium chloride hexahydrate and sodium thiosulfate pentahydrate, described in Monatsh, 80, 220-31 (1949). A stream of nitrogen is bubbled into the reaction mixture to exclude air and CO.sub.2 and the pH of the metathesis reaction solution is adjusted to about 8 to 9 by addition thereto of calcium oxide. However, the solid calcium thiosulfate so produced requires a purification step at temperatures of 0.degree. C or lower.
It is accordingly an object of the present invention to provide a method for the production of high purity calcium thiosulfate by a metathesis reaction wherein difficult processing steps and objectionable impurities are avoided and conversion rates are high.
It is another objection of the present invention to provide a process for the production of aqueous solutions of high purity calcium thiosulfate up to the saturation concentration under conditions which minimize the decomposition of the thiosulfate ion.
It is still further an object of the present invention to provide a method for the production of calcium thiosulfate wherein integration of said method with a process for the manufacture of the ammonium thiosulfate feedstock is feasible.
Other objects and advantages of the present invention will become apparent from the description which follows.