Comparison of hold-up measurements between clarified solution and solvent in pulp operations.

Abstract

Lowering the processing cost of liquid-liquid extraction by eliminating the clarification process, requires an understanding of the hydrodynamics and the operating regions of pulsed column when treating unclarified solutions. In order to achieve such an objective, an investigation has been carried out to measure and compare the total hold-up of clarified solutions and solvent in pulp operation for two potential commercial operations, uranium and gold tailings. Preliminary bench scale separation tests were carried out in order to evaluate phase separation characteristics and assist in the selection of the extractant phase with a minimum of emulsion formation. The results of these tests indicated good phase separation for the uranium aqueous tailings system. However, for the gold tailings system slight crud formation was noticed for the system chosen. Based on these bench scale tests, total hold-up measurements were made for eight systems using a pulsed sieve plate extraction column. The systems included both clarified solution (five) and solvent in pulp processing (three). Results of the clarified solutions test showed that, the hold-up increased with an increase in the amplitude of pulsing, dispersed phase flow rate and pH of the aqueous phase; while, hold-up is unaffected by the variation in the continuous phase flow rate. An increase in the pulse frequency decreased the hold-up in the mixer-settler region till it reached a minimum at the dispersion region. Then, in the emulsion region the hold-up increased with a further increase in pulse frequency. These results were in agreement with results obtained by Koren, Sehmel and Babb. Maintaining the operating conditions the same as in the clarified solution test; when spent uranium tailings were added to the aqueous phase, the column operated in the transition region between the mixer-settler and the emulsion region. The column remained in this region of operation till the pulse frequency or the pulse amplitude or both were increased. Stable emulsions obtained in gold tailings system prevented reliable hold-up measurements and suggested that this system can not be studied unless the crud formation is eliminated. A fractional factorial design was carried out during the experiments in order to minimize the number of runs and avoid erosion to the tailings pump. The experimental data of each system were fitted to a second order model as proposed by Bell and Babb in order to correlate the hold-up data. No common form of these equation models for predicting hold-up was noticed, each model had its unique identity. Different equations obtained from literature were used to predict the hold-up. In the case of uranium tailings system, The models suggested by Sato et al was found to be the best when uranium concentration was 10% by weight, while, Arthayukti's model gave better agreement for 25% uranium tailings.