INNOVATIONS Catalyst for cyanide destruction

Degussa Corp. has developed a chemical process, using hydrogen peroxi de, that destroys toxic cyanide complexes found in waste streams by mills that process gold ores. A specially formulated catalyst, developed by Degussa, is used in the patented process. It enables mill operators to monitor the level of free cyanide in the waste stream throughout the course of the chemical reaction. This feature allows, for the first time, continuous-stream detoxification — a process that lends itself well to automation. Batch processing can also be used. But in either case, free-cyanide levels are reduced to less than 0.1 ppm, the company says. The process is being used at the Nickel Plate mine in British Columbia.

From an engineering point-of-view, this new technology appears to have some additional advantages over the classical method of alkaline chlorination and a more recent detoxification process which uses sulphur dioxide, air and copper sulfate. These engineering advantages include:

* no corrosion problems (polyethylene or stainless steel can be used);

* a minimal power requirement; and

* the fact that it can be used in remote areas such as the far north.

The process uses more or less the same equipment as hypochlorite treatment: a reaction vessel; dosing containers for hydrogen peroxide, caustic and acid; and storage facilities for these chemicals. Small systems often permit dosing directly from the plastic storage drums.

Existing hypochlorite detoxification units can, in most cases, be readily converted to the hydrogen peroxide process at minimal cost, Degussa reports. Only the potentiometric monitoring and control equipment may require modifications. In the potentiometrically controlled process, hydrogen peroxide is usually added gradually, in a series of small doses, until the silver-phthalamide electrode indicates complete detoxification. This allows for greater control when cyanide content and levels of other oxidizables vary significantly. More uniform effluents permit addition of the hydrogen peroxide in a single predetermined dose. The Degussa process calls for maintaining ph near the optimum value of 10.

Depending upon cyanide concentration, reaction times can range from minutes to hours, to assure that free- cyanide content falls below the permissible levels of less than 0.1%. Raising the temperature accelerates this reaction. At high cyanide levels the exothermic reaction between hydrogen peroxide and cyanide adds enough heat to speed up the reaction adequately and avoids the need for costly preheating.

Theoretically 1.3 lb of hydrogen peroxide will oxidize a pound of cyanide ion. Extra peroxide in the system will shorten reaction times considerably — especially important with low concentrations of cyanide.

Degussa is a major manufacturer of both sodium cyanide and hydrogen peroxide.