Feasibility Study and Technical Report - Pretivm

Table 13.24 Head Sample Cyanidation Test Results (2009-2010)TestNoSampleIDGrind Size(P80 µm)Calculated Head(g/t)Extraction(%)ResidueGrade (g/t)Consumption(kg/t)Au Ag Au Ag Au Ag NaCN LimeC1 BZ 71 1.79 9.68 81.0 59.7 0.34 3.9 2.09 0.28C2 BZ 40 2.01 10.3 85.1 63.0 0.30 3.8 2.17 0.37C3 BZ 127 2.35 10.6 84.7 57.5 0.36 4.5 1.97 0.23C4 GH 72 1.41 40.2 77.9 67.6 0.31 13.0 1.91 0.24C5 GH 42 1.35 38.3 76.3 72.0 0.32 10.8 1.94 0.23C6 GH 119 1.49 36.6 72.4 68.6 0.41 11.5 1.77 0.23C7 R8 78 1.37 26.4 75.9 65.2 0.33 9.2 1.71 0.32C8 R8 44 1.24 24.5 75.0 68.2 0.31 7.8 2.02 0.32C9 R8 131 1.34 25.2 73.8 63.2 0.35 9.3 1.85 0.33At a leach retention time of 48 hours, gold extraction from the head samples ranged from72 to 85%; silver extraction from the head samples was lower, ranging from 58 to 72%.The influence of primary grind size on the gold and silver recoveries was relativelyinsignificant. The test results indicated that gold extraction from Composite BZ wasbetter than Composites GH and R8, possibly because Composite BZ had a higher goldhead grade than the other two samples. It appears that all the samples tested wouldneed a longer leach retention time because leaching was not complete when the testswere terminated. Sodium cyanide consumption varied from 1.7 to 2.2 kg/t.Further tests were conducted on the flotation concentrates that were reground to 90%passing 25 µm. The tests used a high sodium cyanide concentration of 5 g/L, and anincreased leach retention time of 96 hours. The test results are summarized in Table13.25.The test results indicate that between approximately 79 and 86% of the gold can beextracted from the reground concentrates. The tests also produced similar silverextraction results. The addition of potassium permanganate (KMnO4), lead nitrate(Pb(NO3)2), and oxygen did not improve gold extraction. The leach retention time requiredfor gold ranged from approximately 48 to 72 hours; however, silver required a longerleach retention time. Cyanide consumption was high, ranging from 13.7 to 16.0 kg/tsodium cyanide, which was possibly due to the high cyanide dosage (5 g/L sodiumcyanide) and fine grind size.Pretium Resources Inc. 13-35 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC