Effect of operating variables in Knelson concentrators: A pilot-scale study

Knelson concentrators are the most widely used semi-continuous centrifuge separators for the recovery of gold and platinum minerals by gravity methods. Bench scale characterization studies on these units provide information about the occurrence of gold in ore samples (e.g. gold particle size distribution, amount of gold recoverable by gravity) but not about the effect of operating variables for full-scale units such as top size of particle, feed rate, fluidization flow rate and rotation speed. Such work is not easily performed online on full-scale units owing to the inevitable variations in feed quality and to the impossibility of varying operating parameters systematically in the face of production requirements. To attack the problem, a pilot plant comprising a 12-in CD Knelson concentrator, a feed screen and tailing sump-pump arrangement was installed in the grinding-B circuit of Dome Mine, Porcupine Joint Venture (PJV), now Porcupine mine, Goldcorp Inc. Timmins, Ontario. The pilot plant received a bleed from the feed to the full-scale units. The pilot facility was extensively sampled in two campaigns. Fifteen tests were conducted in the first campaign and another sixteen in the second. In all 31 pilot tests, twenty six 30-minute recovery cycle tests, called "short tests" and five, 90-minute recovery cycle tests, dubbed "long tests", were conducted. Measuring recovery was the focus of the "short tests"; measuring the deterioration of recovery over time was the focus of the "long tests". The sampling protocols were designed accordingly.;Metallurgical results indicate that operating conditions including feed rate, rotation velocity, fluidization water flow rates and top feed particle size have little impact on the shape of the recovery compared to feed size distribution. A particle size hypothesis was tested using relevant industrial Knelson concentrator data. The analysis showed that a relatively coarse feed would impact negatively on the recovery between 106 and 425 microm. On the other hand, it would make it easier to recover particles between 25 and 106 microm. A finer feed would have a bigger impact on recovery around 25 to 106 microm and would yield a GRG recovery that decreases monotonically with the decreasing of particle size. This would be linked to the natural resistance offered by the gangue particles to the percolation of gold particles, which is significant at a particle size where the gangue is most abundant. The flowing slurry may be compared to a dynamic screen, with openings roughly the order of magnitude of the dominant particle size. This finding is useful for the simulation of the Knelson units, which uses the typical recovery curve "decreasing recovery with decreasing particle size" for estimating gravity recovery and it was thought that the shape of the curve had no impact on the estimation. Now, with this finding, either the fine or coarse recovery curve will be used depending on the size distribution of the gravity circuit feed. For example, for a coarse target grind, the coarse curve could be used and, for a fine target grind, the fine curve could be used.;Settling tests in the gravitational field were conducted in coarse and fine gangue beds. For the coarse gangue bed, dense particles recovery is high for coarser fractions (600 to 150 microm) when the bed is partially fluidized, indicating that these particles intruded the gangue bed due to their mass and the high momentum. Whereas, for the fine bed, most of the dense mineral percolated through the fine gangue bed easily, indicating that the resistance of the gangue bed to the percolation of dense particles is a function of bed voidage, particle size and density of the gangue bed.;Detailed metallurgical balances were made to analyze the effect of operating and design variables on the performance of 12-in pilot Knelson Concentrator as a step towards understanding full-size units and to study the mechanism of concentrate bed erosion. To gain some fundamental information about the recovery mechanism of the Knelson concentrator, percolation of dense particles in a gangue bed was investigated using a fluidized bed column in the gravitational field.