Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group vib metal
Patent
1990-11-07
1992-05-12
Valentine, Donald R.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group vib metal
423 62, 423104, 423111, 423138, 204152, 204 89, C01G 3714, C25B 116, C02F 146
Patent
active
051125838
DESCRIPTION:
BRIEF SUMMARY
The subject of the present invention is a process for recovering chromium anhydride contained in soluble form (chromic acid) in exhausted chromium plating baths polluted by the presence of various metallic ions, particularly iron, copper, zinc, aluminium, nickel, cadmium, as well as by the appearance of trivalent chromium, which forms during chromium plating.
What is meant by exhausted solution is a chromium plating bath that can no longer be used for this purpose because it is polluted with metal ions (essentially iron, copper, zinc, aluminium, nickel, cadmium) and by the appearance of trivalent chronium. A typical example of a so-called exhausted chromium plating bath has the following composition:
______________________________________ CrO.sub.3 (total) =
35% (weight/weight)
CrO.sub.3 (hexavalent) =
30% (weight/weight)
CrO.sub.3 (trivalent) =
5% (weight/weight)
Fe = 30 parts per 1000 parts of total CrO.sub.3
Cu = 30 parts per 1000 parts of total CrO.sub.3
Zn = 5 parts per 1000 parts of total CrO.sub.3
Al = 1 part per 1000 parts of total CrO.sub.3
Ni = 0.1 parts per 1000 parts of total CrO.sub.3
V = 0.05 parts per 1000 parts of total CrO.sub.3
Cd = 0.2 parts per 1000 parts of total CrO.sub.3
______________________________________
Depending on the chromium plating processes from which the baths come, the following may also be present: Ca, Mg, Si, Na, SO.sub.4, Cl.sup.-. There may also be present chinonic organic substances whose disruptive action may easily be stopped by adding mercaptoacetic acid to the chromium sulphate obtained (as described in Italian patent application No. 21807 A/82).
A known method is that of precipitating metallic hydrates contained in aqueous solutions, at controlled pH values, and separating them on the basis of their relative separation factors, see Chemical Economy and Engineering Review; May 1975, vol. 7 No. 5 (No. 84) and references.
Another known method is that of extracting chromic anhydride from aqueous solutions where it is found together with polluting metallic ions, by means of ion exchange resins in solid or liquid form and with appropriate solvents (French patent application No. 2,179,649). Also known is the method (U.S. Pat. No. 4,045,340) consisting of precipitating the internal salt of hexavalent and trivalent chromium, known as chromium chromate, in the cycle of sodium bisulphate recovery from a chromium anhydride synthesis plant.
Now it has surprisingly been discovered that at certain pH values it is possible to precipate simultaneously metallic ions (which pollute an exhausted chromium plating bath) and chromium chromate, which is, as mentioned earlier, an internal salt of hexavalent chromium and trivalent chromium, which are present in the bath.
To be precise, it has been found in particular that the liquids separated by percolation from the chromium chromate precipitate, are suitable for use in a hexavalent chromium salt manufacturing process or to be transformed, by reduction of hexavalent and trivalent chroumium with sulphur dioxide, into a basic chromium sulphate suitable for use in tanning hides, as will be described in greater detail further on, on the basis of illustrative examples.
With this transformation, materials which, being toxic and harmful, would cause serious ecological problems if they were released into the environment, are recovered in the form of products of considerable economic and commerical value, with a much reduced effect on the environmental preservation.
In fact disposal problems are reduced to about 10% and the process claimed herein therefore proves generally more economical.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 graphically compares the relative curves of the titration at room temperature of a typical chromium plating bath with NaOH and the same titration carried out on a synthetically obtained solution, with only chromic anhydride dissolved in water, at the same concentration and temperature.
FIG. 2 graphically illustrates the separation factor pattern for iron, copper and trivalent chromium
REFERENCES:
patent: 4045340 (1977-08-01), Perrone
patent: 4108596 (1978-08-01), Hemming et al.
patent: 4318788 (1982-03-01), Duffy
Bruzzone Giuseppe
Ghelli Giovanni
Perrone Diego
Luigi Stoppani S.p.A.
Valentine Donald R.
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