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Scavenging flotation tailings using a continuous centrifugal gravity concentrator Ghaffari, Hassan


A study was conducted to evaluate the Knelson Continuous Variable Discharge (CVD) concentrator as a scavenger for coarse middling particles from flotation tailings. The goal was to recover a product of suitable grade for recycling to the grinding circuit to improve liberation and aid subsequent recovery in flotation. Such a hybrid flotationgravity circuit would result in improved metal recoveries, product grades and potentially lead to lower grinding costs. Froth flotation, a physico-chemical process, is the most commonly used process for treating base metal sulphides. Since separation is achieved on the basis of the surface hydrophobicity, mixed-phased particles or middlings are not efficiently treated by froth flotation. The coarser fractions in flotation feeds contain heavy and valuable liberated and non liberated minerals that cannot be floated efficiently. There is a top limit for particle size floatability which varies for different ores. The specific gravity of these particles plays an important role which causes decreasing the flotation performance. This study is based on the premise that the middling particles in tailings can be recovered efficiently by size enhanced density separation. The ability of gravity separators to treat fine particles has been limited by the lack of particle inertia relative to the surface drag forces. Particle inertia can be enhanced by the application of a centrifugal field. The Knelson C V D is a relatively new technology as a continuous centrifugal concentrator that recovers particles based primarily on density but also on size. Tailing samples from two mines were subjected to characterization, batch gravity and pilot scale C V D testing. Table 1 shows the specifications of the analyzed samples. The sample characterization of the both Eskay Creek and INCO Thompson samples indicated that there is potential to upgrade gold and nickel in the coarsest fractions. A preliminary assessment of gold and nickel recovery was obtained using a batch Knelson concentrator and the results were similar to those predicted from the characterization results. Based on the characterization and batch gravity tests, there appeared to be a greater opportunity to recover a recyclable product from the LGT and the SC-4, samples than the HGT and AS-4 samples. They were therefore selected for pilot scale tests using the CVD6 separator. The objective was to produce a concentrate with a low mass yield and high gold and nickel upgrade ratios. Products from these tests were subjected to size/assay analysis. The results for Eskay Creek sample show that the gold distributions are significantly higher than sulphur and arsenic distributions, which indicates that free gold or gold associated with poorly floated minerals is being recovered. The best result was obtained from Test 3 in which 28% of the gold was recovered in a concentrate grading 12.5 ppm Au representing 3.3% of the feed mass. This represents a gold upgrade ratio of 8.7. The results for INCO Thompson show that the gold distributions are significantly higher than nickel and sulphur distributions, which indicates that free gold is being recovered. The best result was obtained from Test 3 in which 12.9% of the nickel was recovered in a concentrate grading 0.85% Ni representing 6.7% of the feed mass. It is worth noting that the Mg grade is decreased dramatically in the concentrate products. The Mg distribution in concentrate was just 1.7%. This is a good indication that the centrifugal concentrator can also remove the magnesium bearing minerals such as talc. In the case of gold, remarkable results were produced and the metallurgical balances showed gold recoveries ranging from 12.6 to 67.3% and gold grades from 1.4 to 10.0 ppm. Size partition curves were obtained for all tests, which demonstrated that the CVD operates as a size classifier as well as a density separator (size enhanced density separation). For the Eskay sample (Tests 3 and 6) and the INCO sample (Test 7), the cut size was about 300 microns. In conclusion, the pilot scale testing indicated that the CVD was effective at recovering gold, gold bearing sulfides and nickel bearing sulfides from the coarse particles of the flotation tailings and also at de-sliming. The nickel and gold recovered were primarily in the coarse size fractions, which would likely contain middling particles (sulfides + silicates). The results showed that the CVD is capable of rejecting the Mg bearing minerals as well. These results support the application of continuous centrifugal concentrators into hybrid flotation-gravity circuits that could lead to improved metallurgical performance. Plant trials testing are recommended to confirm the results and potential benefits.

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