Floatation and residency times are completely different issues. A concentrate derived from floatation usually needs to be further processed before there is a saleable concentrate. It is not amenable to cyanidation. Floatation is a surprisingly efficient process and the time to achieve floatation is not usually important.
However,the efficiency of cyanidation is strongly time dependent. If you have a long residency time then you need more tankage than for a short residency time. 48 hours is a long residence time and 24 hours is relatively quick. Some slurries will even leach efficiently in 12 hours. For low grade deposits it is usually better to accept a lower recovery and just increase the throughput. For high grade deposits it is usually worth putting more work (cost) into achieving higher recovery and thereby accepting a longer residence time. High recoveries may require fine grinding and higher consumption of reagents. All of these variables are established by bench scale testwork which enable a cost curve to be drawn up.
