Feasibility Study and Technical Report - Pretivm

Air/vacuum valves will be installed at critical points along the pipeline to prevent thepossibility of air entering the underwater section. A large volume of air entering theunderwater section could potentially float sections of the pipeline. The valves willfunction primarily during start up and shut down.Figure 18.9 shows the pipeline route in plan and elevation. A small quantity of coarsesand and gravel will be placed at the terminus of the outfall prior to tailings discharge.Tailings solids will further accumulate and cover the discharge point through operation.This feature will act as a filter to prevent suspended solids from entering the upper layersof the lake’s water column and from subsequently being discharged into BrucejackCreek, potentially in violation of strict receiving water quality regulations.There will be a constant flow through the pipeline to keep the deposit at the end of theoutfall fluidized. When the thickened tailings are used in the backfill plant, flow will bemaintained with water.Earlier Rescan designs for discharging tailings through a deposit (at Minahasa inIndonesia and at Compañía Minera del Pacífico in Chile) have been demonstrated to beeffective in reducing TSS. This configuration was also recently adopted for discharge in alake at a project in the US.During lake turn over, there may be some fine deposited tailings particles transportedupward toward the lake surface, potentially resulting in elevated suspended solidsconcentrations in the surface layer and at the outlet.Figure 18.10 shows the area on the lake bottom that the tailings would occupy at the endof the 22-year project life, assuming that 9.5 Mt of tailings are deposited over the LOM(Keogh 2013 pers. comm.) and that the density of the tailings deposit increases withaccumulation and consolidation. (Tailings dry unit weight estimates were taken from thelab report completed by Golder (2013)). The overall footprint of the tailings at the end of22 years occupies most of the lake bottom, to a depth of approximately 48 m at its edgeand a depth of 38 m at the apex of the deposition cone.A complete bathymetric survey of the lake, with detailed data collected along thesubaqueous pipeline alignment, will be accomplished in the detailed design stage. Inaddition, tailings slurry rheology, at a broad range of concentrations, will be definitivelymeasured in the laboratory.Pretium Resources Inc. 18-35 1291990200-REP-R0012-02.1Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC