Cyanide Recovery and Detoxification Study on Seabridge Gold’s KSM Project
*Mariam Melashvili1, Chris Fleming1, Inna Dymov1, Jim Smolik2, Dean Lindsay3 1
SGS Canada Inc. 185 Concession Street Lakefield, Ontario K0L 2H0 (*Corresponding author:
[email protected]) 2
Seabridge Gold Inc. 106 Front Street, Suite 400 Toronto, Ontario M5A1E1 3
Dean G. Lindsay Consulting Ltd. 378 – 55 Street Delta, BC. V4M3J5
ABSTRACT This paper presents results from the proposed cyanide detoxification circuit for Seabridge Gold’s KSM project. The proposed flowsheet uses a two-stage CCD (counter-current washing/decantation) circuit to transfer the flotation products CIL(carbon-in-leach) barren solution to a SART process (Sulphidization, Acidification, Recycling of precipitate, and Thickening of precipitate) to recover copper and free cyanide from the copper cyanide complex. The SART barren solution is then treated by an AVR process (Acidification, Volatilization of HCN gas, and Reneutralization) to concentrate the cyanide that was liberated in SART for recycling to leach. The washed CIL barren pulp (the CCD underflow) is treated with the SO 2 /Air process to oxidize residual cyanide. The goal of this complex flowsheet is to recover and recycle as much cyanide as possible and produce a final effluent containing less than 0.5 mg/L copper and CN WAD at the lowest cost. The SO 2 /air process is capable of achieving low residual CN WAD (200 mg/L each species) and the difficulties this might create for the SO 2 /air process was a key factor in the decision to consider including the washing, SART and AVR steps in the project flowsheet. The target concentrations of CN WAD and Cu were readily achieved after aging the SO 2 /air treated pulp, as would occur in a tailing or polishing pond. This suggests no additional treatment will be required to sustain low levels of these species in the final plant effluent.
KEYWORDS
cyanide destruction, cyanide recovery methods, stability of aqueous copper in CIL pulp
INTRODUCTION
The KSM Project is a proposed gold/copper mine located 65 kilometers northwest of the town of Stewart, B.C at the site of one of the largest undeveloped gold resources in the world. The KSM Project is made up of four large gold and copper deposits. The proposed mine has a 50-55 years life. Part of development work was conducted at SGS Minerals, Lakefield site. This work included a preliminary assessment of cyanide destruction and recovery methods designed to meet strict limits regarding cyanide and copper levels in the plant’s final effluent. The assessment was conducted in two stages. In the first stage, testing was conducted on a solution with a similar composition to the anticipated wash solution from two-stage counter-current washing of the KSM cyanide leach residue. A variety of cyanide destruction methods were evaluated, including SO 2 /air, peroxide, Caro’s acid and activated carbon adsorption to reduce the concentrations of aqueous cyanide and copper to the target levels. In addition, the SART process followed by AVR was evaluated to recover the cyanide for recycling purposes, and to lower the concentrations of copper and cyanide entering the cyanide destruction stage of the overall process. The second stage included preparation of actual feed material for testing, by conducting carbon-in-leach (CIL) experiments on flotation products – a cleaner scavenger tailing sample and a rougher concentrate sample. This test program consisted of feed sample preparation, gold recovery in batch cyanidation/CIL experiments and simulation of CIL residue washing by appropriate dilution and decantation. Cyanide destruction testing on the CIL washed residue was then conducted using the SO 2 /air process, followed by polishing treatment using carbon adsorption and peroxide oxidation. Recovery of cyanide and copper from the wash solution was also evaluated using the SART and AVR methods. The objective of the testwork was to identify and prove a conceptual flowsheet that would consistently produce a final effluent solution containing < 0.5 mg/L weak acid dissociable cyanide (CN WAD ) and 4 mg/L DO (dissolver oxygen) in solution. Sodium metabisulphite (as the source of SO 2 ) was added by pumping solution at the flow rate and concentration calculated to achieve the target ratio of SO 2 to CN WAD . The residual CN WAD was monitored by picric acid method. At first, a batch test was conducted to produce a reactor full of treated solution with low residual cyanide, which was then used as the
starting solution for a
continuous test. The treated product in the reactor from one test was used as the starting material for the next test. Four different methods of polishing dilute solutions, after most of the copper and cyanide had been removed, were tested. These methods involved treatment with Caro’s acid, hydrogen peroxide, ferrous sulphate and activated carbon. In each of these experiments except for the activated carbon test, a glass reactor with a magnetic stirrer was used. The Caro’s acid (25% peroxo-mono-sulphuric acid) reagent was prepared in advance using a procedure provided by mixing concentrated grades of H 2 SO 4 (minimum 95%) and H 2 O 2 (50-70%). In the peroxide tests, a ~3% H 2 O 2 solution was prepared and added in the amount required to achieve the target dosage. In the experiments with ferrous sulphate, a solution of analytical grade FeSO 4 *7H 2 O was prepared and added at the target ratio of iron to aqueous copper. Carbon adsorption polishing tests were conducted using fresh pre-attritioned activated carbon (Calgon GRC-22). The carbon was added to the solution or pulp and either placed on rolls or placed in a glass column through which the cyanide solution was pumped upflow. Samples were collected at the top of the column and analyzed to determine the kinetics of copper and cyanide adsorption.
Stage 1- Treatment of Plant Wash Water The objective of the test program was to investigate various treatment options to produce a final treated product containing less than 0.5 mg/L of residual Cu and CN WAD . Testwork was performed on the simulated CCD plant O/F (overflow) wash water sample and on several diluted CCD plant wash water solutions. These were prepared by adding gypsum saturated water to generate target copper concentrations of 90 mg/L (Solution A), 5 mg/L (Solution B), and 2 mg/L (Solution C), to simulate polishing treatment of the tailings pond effluent. The overall test program and the results achieved are summarized in Figure 1.
Feed: CCD O/F Wash Water Wash Water Cu 145 mg/L CN 176 mg/L
SART (1) Cu 0.4 mg/L CN 156 mg/L
Solution C Cu 2.1 mg/L CN 2.0 mg/L
Solution B Cu 4.9 mg/L CN 5.4 mg/L
Carbon (2) Cu 0.2 mg/L CN 0.5 mg/L
Cu CN
H2 O2 (2)
