Figure 50 – Feed grade (Pb %) and recovery during a flash flotation cell trial. The alternation of periods where the flash flotation cell was either ON or OFF is shown with the background grey line (high value = ON; low value = OFF). The objective was to assess whether the flash flotation unit was providing any benefit to Pb recovery, or whether it should be switched off permanently to simplify the process and reduce costs. Due to the variability in feed, throughput, mineralogy, circuit conditions etc., it was extremely difficult to visually isolate the impact of the unit upon overall recovery. However, this was easily overcome with analysis using a paired t-test, which can be performed in Microsoft Excel (Table 1). Table 3 – Microsoft Excel paired t-test output. Rec (%) - ON Rec (%) - OFF Mean 88.6 87.3 Variance 6.4 4.8 Observations (no. of pairs) 30 30 df 29 t Stat 4.0 P 0.00018 The mean recovery difference between the ON and OFF conditions was 88.6% - 87.3% = 1.3% recovery. Whether this 1.3% recovery benefit was indeed ‘real’ is answered by the P-value in the final row of Table 3. The level of confidence that the result was statistically significant is given by 100(1-P) = 99.98%. In other words, there was over 99% confidence that the flash flotation unit provided a real metallurgical recovery improvement when it was switched on, and that the observed mean 1.3% recovery change was not simply an artefact of experimental error. This was 0 1 2 3 4 5 6 7 8 9 80 82 84 86 88 90 92 94 96 0 10 20 30 40 50 60 Pb feed grade (%) Pb recovery (%) Day Recovery (%) Feed grade
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