Determination of free cyanide in gold cyanidation process liquors by ion-interaction chromatography with post-column derivatization

Abstract

A chromatographic method is described for the determination of high levels of free cyanide in leach liquors encountered in the cyanidation process for the dissolution of gold from its ores. The method also offers simultaneous determination of metallo—cyanide complexes and thiocyante, which are also present in the sample. A 10-μl aliquot of the sample is first separated by ion-interaction chromatography using a C 18 column, 25% (v/v) aqueous acetonitrile containing 5 m M low-UV PIC A as mobile phase, and UV detection at 214 nm. Cyano complexes of Cu(I), Fe(II), Ag(I), Fe(III) and Au(I) are detected, together with thiocyanate. Linear calibrations were obtained over the concentration ranges in whic these species are found in leach liquors. The free cyanide in the sample is eluted at the column void volume and is detected by absorbance at 500 nm [together with thiocyanate and the copper(I) cyano complex] after post-column reaction with N-chlorosuccinimide, barbituric acid and isonicotinic acid to form a polymethine dye. The calibration plot for cyanide was linear over the range 0–300 ppm and no interferences from 1000 ppm levels of common anions were identified, except for sulfide which can be tolerated only up to 500 ppm. Alternative chromatographic hardware configuration offering heart-cut of the cyanide fraction and subsequent ion-exclusion separation of cyanide from other anions ae discussed. When applied to leach liquor samples taken from operating gold mines, the chromatographic method is shown to give clean chromatograms and to offer superior accuracy and precision to the conventional silver nitrate titrimetric procedure, especially in the presence of high levels of copper in the sample. On-line operation of the method at a throughput rate of 3 samples per hour is also described.