Tailings pulp carrying traces of cyanide and the discard of barren solutions in some cases constitutes a hazard to both humans and animals, and methods have been devised for destroying the contained cyanide.
A recent paper “The Treatment of Cyanide Wastes”by Chlorination by J. G. Dobson published in the Sewage Works J., November, 1947, discusses the subject as follows;
Free cyanide is one of the most toxic components of industrial wastes that are often discharged into sewers and streams. Because its toxicity to fish varies with pH, concentrations of other ions present, temperatures, and oxygen content of the receiving water—as well as activity of development and species of fauna in the steam—it is difficult to set up exact limits of concentration which can be safely discharged. Under some conditions, as little as 0.1 p.p.m. has proved fatal to fish. From 0.1 to 0.3 gram of CN is fatal to humans.
For 35 years or more various investigators have sought a satisfactory method of treating cyanide wastes. Among the methods that have been used are acidification and removal of the resulting HCN gas by air blowing, reaction with “lime sulphur,” aeration, treatment with ferrous sulfate, and oxidation with potassium permanganate.
The first of these methods is, of course, the standard scheme used for cyanide regeneration already described; the second involves the addition of commercial lime-sulphur solution, whereby the CN is converted to the cyanate. A definite yellow precipitate indicates complete reaction. The third (aeration) has been discussed by C. H. Clevenger and H. Morgan in “Atmospheric Decomposition of Cyanide Solutions,” p. 413, Alining and Scientific Press, 1916; the fourth, or ferrous sulphate method, is described by J. Moir and J. Gray in “The Destruction of Cyanide” J. CM.S. S. A., Vol. 10, p. 433, 1909; Vol. 11, p. 152, 1910; and the fifth, oxidation with permanganate, is discussed by E. F. Eldridge in “Reducing the Toxicity of Cyanide Waste” Eng. News-Record, Vol. 111, No. 23, p. 677, Dec. 7, 1933.
All the above methods have been used commercially for the reduction of the cyanide content of concentrated solutions, but they all leave a substantial cyanide residual. This residual may be serious unless extremely large dilutions are available. Another objection to all these methods is that the pollution load is increased by the addition of objectionable chemicals. None of these treatment methods are applicable to the treatment of dilute wash solutions such as are obtained from plating operations. Chlorination of solutions at pH above 8.5 has now proved to be an economical and satisfactory method for treatment.
Referring to the chemical reactions involved, Dobson says:
When chlorine is added to a free cyanide solution with sufficient free alkali present to maintain a pH above 8.5, the cyanide is oxidized to cyanate:
NaCN + 2NaOH + Cl2 → NaCNO + 2NaCl + H2O
This reaction is practically instantaneous. Under all conditions which we have tested, it is complete in less than 1 min.
Theoretically, it requires 2.73 parts of chlorine per part of CN and 3.08 parts of caustic per part of CN. However, most trade wastes contain a substantial portion of the required free alkali.
If an excess of chlorine is added, free chlorine residual, as measured by the O-T-A test, will be found in the solution after 1 or 2 min.
In all trade wastes and laboratory samples that we have tested, no cyanide within the limits of titration accuracy has been found in the presence of free available chlorine at pH 8.5.