FUNDAMENTAL INVESTIGATIONS OF THE SO_2/AIR, PEROXIDE AND CARO'S ACID CYANIDE DESTRUCTION PROCESSES

摘要

Fundamental investigations have been conducted on the SO_2/air, peroxide and Caro's acid cyanide destruction processes to establish a more detailed understanding of the reaction mechanisms and kinetics. The major findings are summaries below.For the SO_2/air process it was found that: 1. Upsets to the process (for example the loss of sulfite or oxygen addition) which result in the presence of free cyanide in the reactor will stop the oxidation of cyanide.2. In a CSTR or series of CSTR's, the DO concentration provides an indicator to the residual oxygen capacity available in the process; zero DO in the last reactor indicates insufficient oxygen addition for the rate of cyanide and sulfite addition.3. The addition of hydrogen peroxide to the SO_2/air process to potentially increase the cyanide oxidation is not beneficial and is not recommended as sulfite is preferential oxidised over cyanide.The copper catalysed peroxide destruction of cyanide investigations found that the solution composition (especially metal ions) and pH have a significant impact on the reaction chemistry, particularly the inception of precipitation and the subsequent stoichiometry of peroxide to cyanide oxidation.Investigations of the Caros' acid process found that: 1. Free cyanide and thiosulfate are preferentially oxidised prior to the oxidation of copper cyanide and thiocyanate which occur in parallel.2. The control of pH is important since at low pH, HCN forms which is not readily oxidised and the rate of cyanate oxidation increases. This can occur if pH control is subsequent to Caro's acid addition or within the localised zone where Caro's acid is added, and can significantly reduce the cyanide oxidation efficiency.Thiosulfate is detrimental to all the cyanide destruction processes due to the oxidation reaction having a high oxidant demand. Thiocyanate has an impact only on the Caro's acid process (high oxidant demand), particularly with high copper and thiocyanate concentrations. Most metal cyanides precipitate from solution once the copper cyanide has been destroyed, however zinc precipitates before copper whilst mercury is not precipitated by any of the destruction processes.