US3645867A - Method of treating cyanide-containing liquors in surface treatment installations - Google Patents

Abstract

A METHOD OF REMOVING CYANIDE WASTES FROM CYANIDECONTAINING SURFACE TREATING BATHS IN A METAL-TREATING PROCESS WHICH INCLUDES AN ELECTROLYTIC ALKALINE CLEANING BATH AND A CYANIDE-CONTAINING SURFACE TREATING BATH COMPRISING PASSING THE WASTE CYANIDE-CONTAINING WATER FROM THE CYANIDE TREATMENT BATH TO AN ALKALINE ELECTRLYTIC CLEANING BATH, WHEREBY THE CYANIDE IS ELECTROLYTICALLY OXIDIZED.

Description

Feb. 29, 1972 mcso EI'AL 3,645,857

METHOD OF TREATING GYANIDE-CONTAINING LIQUORS IN SURFACE TREATMENT INSTALLATIONS Filed June 19, 1969 United States Patent US. Cl. 204130 Claim ABSTRACT OF THE DISCLOSURE A method of removing cyanide wastes from cyanidecontaining surface treating baths in a metal-treating process which includes an electrolytic alkaline cleaning bath and a cyanide-containing surface treating bath comprising passing the waste cyanide-containing water from the cyanide treatment bath to an alkaline electrolytic cleaning bath, whereby the cyanide is electrolytically oxidized.

The present invention is concerned with a method of preventing the discharge of cyanide-containing liquors from chemical metal surface treating processes wherein aqueous cyanide solutions are used. Such cyanide-containing aqueous baths are used extensively in electrolytic plating processes, such as zinc-plating, copper-plating, silver-plating etc. processes.

Since for sanitary reasons increasingly higher demands are put upon the purification of industrial waste water, this has resulted in great technical and economical problems. This is illustrated by the fact that for depositing one kilogram of metal, normally about 5 cubic meters of water are used.

The destruction of the cyanide in the waste water from the electrolytic plant is normally carried out by decomposing it by oxidation, using chlorine as the oxidizing agent, the chlorine being supplied either as gaseous chlorine or as a hypochlorite. To avoid the formation of the toxic chlorocyanogen the oxidation must be carried out at a rather high pH value. The cost will therefore be relatively high and normally amounts to three or four times the cost of the cyanide used.

The object of the present invention is to provide a more economical solution of the problem of disposing cyanidecontaining liquids obtained in chemical processes for surface-treatment of metals. The method of the invention also provides several other advantages, inter alia a drastic reduction of the consumption of water for the process. The invention is based on the principle of electrolytically decomposing the cyanide in electrolytic pretreatment baths.

Electrolytic oxidation has been used to some extent in practice to destroy cyanides. However, due to several practical problems the method has found only limited use. One of the great disadvantages has been high installation costs.

In the anodic oxidation of cyanide, carbonates and ammonia are formed. The process is accelerated by elevated temperatures and a high pH. Also the over-voltage of the anode material for oxygen is important. The best material is platinum which has the highest oxidation potential. Provided that the pH is about 11 and the temperature about 70 0., iron and carbon are also satisfactory and a high current yield is obtained in the decomposition process.

Each electrolytic plating process is normally preceded by an electrolytic cleaning of the articles in a hot alkaline bath. Such a bath provides ideal conditions for an anodic decomposition of the cyanides.

According to the present invention an electrolytic decomposition of cyanides from surface treatment baths is carried out in one of the electrolytic process steps, in particular in the electrolytic cleaning baths.

The invention is illustrated but not limited by the following example which is diagrammatically shown on the accompanying drawing. The example relates to electrolytic zinc-plating in cyanide-containing electrolytic baths.

As shown on the drawing, the installation comprises a sequence of baths 1-12 for treating metal articles for zinc-plating. These baths have the following functions:

The metal articles to be treated pass through the baths mainly in the order mentioned. The thin arrow lines to the right in the figure illustrate the flow of liquid to and from and between the baths as explained in detail below.

More particularly, the baths may operate in the following manner:

Bath 1 is a degreasing bath containing sodium hydroxide, sodium carbonate and metasilicate, operating at a temperature of about C.

Bath 2 is an electrolytic degreasing bath of the same composition as bath 1. The article is connected as a cathode. The anode consists of graphite. The bath operates at a temperature of about 80 C. and at a current density (anodic and cathodic) of about 5 to 10 amperes per sq. dm.

Bath 3 is also an electrolytic degreasing bath of the same composition as bath 2 and operated under similar conditions, except that the article is coupled as the anode and the cathode consists of steel. Suitably, a complexing agent of gluconate type is added to this bath.

The evaporation of water from baths 1, 2 and 3 (in dicated by the outwardly directed arrow at bath 1) is about 8 to 10 liters per sq. m. of bath surface an hour. To compensate this liquid loss, and to achieve the desired destruction of cyanides, liquid is passed from bath 4 to baths 1, 2 and 3 as indicated by the arrows.

Bath 4 is a cold water bath for rinsing the articles after they have been treated in bath 3.

Bath 5 is also a cold water bath for further rinsing the articles before they are treated in the plating bath. The articles may also be rinsed in this bath after they have been treated in the plating bath.

Bath 6 is a cyanide-containing electrolytic zinc-plating bath, containing about 35 g./l. Zn, 30 g./l. NaOH and g./l. NaCN. The bath operates at a temperature of not above 30 C. and at a current density of about 3 to 6 amperes per sq. dm.

Bath 7 is a cold water bath for rinsing the electroplated articles, which may previously have been rinsed in bath 5. Fresh water is introduced into bath 7 at a rate corresponding to the evaporation in baths 1, 2, 3, the water flowing through baths 7, 5, 4 and thence to baths 1,2,3 as indicated by the arrows in the diagram on the drawing.

Thus, zinc and cyanide accompany the water to the degreasing baths 3, 2, 1. The zinc is precipitated in bath 3 3 on the cathodes. The cyanide is decomposed by electrolytic oxidation in baths 3 and 2. To avoid introducing cyanide into bath 1 it may be suitable in some cases to introduce water from bath 4 only into baths 3 and 2 and take water for bath 1 from bath 2 as indicated by the valve and the broken line arrow.

Bath 8 is a cold water bath for further rinsing the article. From this bath, water is circulated through a basic anion exchange unit 13, where any residual zinc cyanide complexes are taken up. The ion exchange composition is regenerated, when necessary, with NaOH and the regenerate may be passed to the zinc bath 6.

Bath 9 is an acid pickling bath containing about 1% HNO Bath 10 is a so-called one-step ehromating bath containing about 2 g./l. alkali bichromate and fluoride and sulfate ions. Total salt content about 4 g./l.

Bath 11 is a cold water rinsing bath, wherein the water is circulated through a two bed (anionic and cationic) ion exchange filter unit 14, 15 so thatthe water will be completely deionized. Fresh water is introduced into this bath (through the filter unit) to compensate evaporation in bath 12.

Bath 12 is a final hot water rinsing bath operating at about 80 C. Evaporation is about 8 liters per sq. m. an hour, which is replaced by water taken from bath 11.

We claim:

1. In a method of surface treatment of metal articles in cyanide-containing surface treatment baths, wherein the surface-treatment is preceded by cleaning the articles in electrolytic alkaline cleaning baths and the articles after surface-treatment are rinsed in water, the improve- 1 ment which comprises passing cyanide-containing water from the rinsing baths to said electrolytic cleaning baths,

whereby the cyanide will be submitted to electrolytic oxidation in said baths.

References Cited UNITED STATES PATENTS OTHER REFERENCES Disposal of Waste Cyanides by Electrolytic Oxidation by R. W. Oyler, Plating, April 1949, pp. 341-342.

Destruction of Cyanide Copper Solutions by Hot Electrolysis by L. B. Sperry and M. R. Caldwell, Plating, April 1949, pp. 343-347.

JOHN H. MACK, Primary Examiner T. T UFARIELLO, Assistant Examiner US. Cl. X.R. 204-149

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