Variation in apparent recovery in CIL/CIP plants Part 1 – the effect of delay and mixing in the tanks

Abstract

Sampling of mineral processing circuits for metallurgical control is typically done by compositing cuts of feed, tails and (if applicable) concentrate over eight or twelve hour periods. An exceptional problem exists in CIL/CIP circuits since the residence time in the leaching/adsorption tankage may be ∼20 h and the tails from any period are derived from material entering the circuit an average of 20 h or so earlier and thus do not relate to the head from the same sampling period. In addition, mixing in the series of tanks in a circuit also modifies the correspondence between input and output of the plant. A special purpose simulator has been therefore developed for a simple CIP/CIL plant in order to allow for the mixing and delay in the tankage and permit estimation of a head assay corresponding more truly to any tailings sample. In this way more accurate assessment of the day to day recoveries of gold and silver would be obtained. The model, programmed in Excel, assumes that the tanks are perfect mixers. By inputting the sequential head grades and considering that no leaching has occurred, the head grade which corresponded to the tailings for each period can be estimated. Allowance is made for variations in feedrate over the sampling period by computing the average flowrate of slurry and thus the residence time of each increment of the tailings. For a typical circuit even this simple model was able to greatly improve consistency of recovery data with variations being approximately halved by eliminating much of the anomalous variation arising from the lack of correspondence between head and tail particularly when significant changes in head grade are occurring. This has permitted genuine variations in metallurgy to be identified sooner and with greater confidence and thus operational shortcomings to be addressed more quickly. From the work reported here it has become clear that for meaningful recoveries to be obtained in a CIL/CIP plant on a daily or shorter basis then correction for delay and mixing in the tanks is essential.