PH measurement and control in solvent extraction using column contractors

Abstract

pH measurement and control in a solvent extraction process using column extractors is a considerable challenge. This paper summarises the progress of WMT's understanding of the emulsion characteristics, hydraulic and kinetic regimes existing in the column extractors and their effects on pH measurement and control. Three sets of pilot scale test results are provided for a case study of nickel solvent extraction using Versatic 10 in a column extractor, with ammonia solution as the pH modifier. Initially the pH in extraction columns was controlled by directly dosing ammonia into the column to achieve the pH set point. The pH was measured in-situ by a pH meter inserted directly into the column. It was found that the ammonia tended to flow upwards with the organic rather than downwards with the aqueous flow, as would be expected, in an organic continuous operation. In the second test, the barren organic was pre-equilibrated with ammonia via an in-line mixer prior to entering the column. The pre-equilibrating ratio was proportional to the nickel pregnant liquor solution (PLS) tenor and the flowrate of the various phases. The final aqueous pH was used to tune the ratio. This resulted in a significant improvement in control of the pH in the column. In the third test, a combination of pre-equilibration and direct ammonia injection was used. The pH control in the column was very stable and a desirable pH profile in the column was achieved to maximise nickel recovery and reduce impurity co-extraction.