Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1982-03-01
1983-11-29
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204149, C25B 100
Patent
active
044179634
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a process for removing cyanide ions from a cyanide-containing solution by anodic oxidation.
In several industries cyanide chemicals are used for different purposes resulting in liquid waste not allowable for disposal in available recipients. Therefore, the cyanide-containing liquid waste or solutions must be freed from their contents of cyanides before being discharged into recipients or municipal sewers. Examples of such industries are noble metal workshops, iron and steel industries, the photographic industries and surface finishing industries.
BACKGROUND ART
Up to now the cyanide-containing waste has been collected in destruction facilities within the plant, usually large basins, where destruction has taken place by oxidation with sodium hypochlorite. Small plants have to collect their wastes in closed vessels which have to be transferred to destruction plants with concomittant high costs. It is also known to destroy the cyanide contents of solutions by anodic oxidation, but the technology developed up to now has been complicated and requires long electrolyzing periods. Examples of the latter type of techniques are found in U.S. Pat. No. 3,764,499, Swiss patent specification No. 444,066 and published Swedish patent application No. 77-12278-6.
SUMMARY OF THE INVENTION
The present invention provides for a new process for removing cyanide ions from cyanide-containing solutions through which the disadvantages associated with the prior art are avoided or substantially reduced. In the process of the invention using anodic oxidation for destroying the cyanide ions the pH of the solution is adjusted to a value of at least about 11 and the halide content of the solution shall be at least equivalent to the cyanide concentration. The crux of the invention lies in the finding that when electrolysis is performed using intense agitation of the electrolyte the period of time required for full destruction of the cyanide ions will be substantially reduced. It has also been found that using the conditions outlined above results in deposition of solid carbon on the anode.
The technique of the invention enables treatment of any liquid waste containing cyanide ions. Thus, for example electroplating baths containing 10-20 grams of cyanide per liter of solution can be efficiently treated using the process of the invention. Of course, liquid waste containing smaller concentrations of cyanides may also be made harmless using the inventive technique.
It is preferred to provide the agitation of the electrolyte by stirring same. In providing for efficient stirring it is suitable to use a propeller to impart a rotary motion to the electrolyte. It is particularly preferred to provide for such intense agitation that air is sucked into the electrolyte. In addition to the fact that this will substantially shorten the period of time of electrolysis it also prevents uneven corrosion of the anode, particularly local corrosion at the interphase between surrounding air and electrolyte.
In a preferred embodiment of the invention the halide content of the solution is adjusted to about twice the equivalent of the cyanide concentration, and as a halide sodium chloride is preferred, but any alkali metal halide can be used, the chlorides being preferred.
The electrolysis is suitably performed using a current density of about 3-3.5 A per dm.sup.2, and to adjust the pH of the solution sodium hydroxide is the preferred choice of base.
When the process of the present invention is used for rendering harmless the cyanide-containing waste solutions resulting from the recovery of noble metals in accordance with patent application No. 355,570 filed 3-1-82, the cyanide ion removal can advantageously take place in the same electrolyzer. In this case new cathode plates are introduced into the electrolyzing vessel and the direction of current is reversed so that said plates become anodes onto which the deposition of carbon takes place. This will have for an effect that contamination of the walls of the e
REFERENCES:
patent: 2773025 (1956-12-01), Ricks
patent: 3764497 (1973-10-01), Tarjanyi
patent: 4029557 (1977-06-01), Christensen et al.
Andrews R. L.
Sodermark Nils-Erik
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