Process and a device for the decomposition of free and complex c

Details Process and a device for the decomposition of free and complex c Process and a device for the decomposition of free and complex c

1995-02-24

1996-11-12

McCarthy, Neil

Liquid purification or separation

Processes

Chemical treatment

210748, 210757, 210759, 210912, 210913, 210205, C02F 148, C02F 162, C02F 170, C02F 172

Patent

active

055736769

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process and a device for the decomposition of contaminants in waste waters being loaded, for example, with cyanides, complexing agents, mineral oil, AOX, COD, chromate, nitrite, and further contaminants as metals.
A decontamination process widely used for the decontamination of waste waters loaded this way essentially consists of cyanide decontamination with chlorine bleaching leach and a subsequent reduction of chromate with sodium hydrogensulphite (Hartinger, Handbuch der Abwasser- und Recyclingtechnik, Hanser Verlag Munchen, 2nd edition, pp 292 ff.).
The decontamination of cyanide with chlorine bleaching leach takes place at a pH value of 10 and is combined with a considerable salting. In addition, reacting of chlorine bleaching leach with organic ingredients such as brighteners and tensides leads to the formation of so-called absorbable organic substances (AOX).
The so-called easily releasable cyanides can be decontaminated quantitatively by this process. Complex cyanides, as those of copper and nickel, require a very long reaction time, whereas extremely strong complex cyanides as those of iron are not or not quantitatively decomposed.
Incompletely decomposed complex cyanides render the precipitation of the central atom as a hydroxide impossible and lead, by back dissociation, to an increase of free cyanide content (for this see Hartinger, Handbuch der Abwasser- und Recyclingtechnik, 2nd edition 1991, p 44). Additional complexing agents such as EDTA or NTA are not decomposed by chlorine bleaching leach.
Subsequent reduction of the chromate with sodium hydrogensulfite takes place after adjustment of the pH value to .ltoreq.2.5 and is combined with another salting of the waste water. As cyanide decontamination by chlorine bleaching leach, chromate decontamination may be followed and also controlled by measuring the redox potential.
Using various devices, a waste water liberated from cyanides and chromates this way may be cleaned from dissolved metals by precipitating the corresponding hydroxides. For this as well unless this is not rendered impossible by the presence of additional complexing agents as EDTA or NTA being not decomposed by chlorine bleaching leach (Hartinger p 206 ff.), and provided that chromates were to be reduced--further addition of an auxiliary chemical--in this case a base--is required leading to further salting of the waste water.
Altogether easily releasable cyanides and chromates can be decomposed for certain using chlorine bleaching leach and sodium hydrogensulfite as decontamination process. However, for complex cyanides a quantitative decomposition is not achievable, such that the metals cannot be precipitated completely during hydroxide precipitation but are discarded in form of easily soluble complex cyanides into the waste water and contributing therefore to an increased metal loading.
Due to the chlorine bleaching leach and sodium hydrogensulfite treatment decontamination process the waste water becomes strongly saline due to the chlorine bleaching leach and the acids and bases required to adjust the pH value and particularly becomes loaded with AOX due to hydrochloric acid usage. An additional considerable AOX source consists of the unspecific reaction of chlorine bleaching leach with organic ingredients, so that the AOX limits of 1 mg/l and 0.5 mg/l, respectively, cannot be lowered as a rule. Consequently, up to the present these limits are postponed in galvanic industry.
Therefore attempts to modify the chlorine bleaching leach and sodium hydrogensulfite decontamination process or to replace it by other processes not suffering from the drawbacks mentioned above have not been spared. In this connection particularly it has been tried very often to replace chlorine bleaching leach, e.g., by hydrogen peroxide.
In this way DE 40 14 023 describes a process, during which the waste water is acidified, the hydrogen cyanide released this way swept off by means of an air stream and collected by an absorption liquid, e.g., sodium hydroxide solution. Then t

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