Kanowna Belle Pre-Oxidation Project

Transcription

1 Kanowna Belle Pre-Oxidation Project

2 Kanowna Belle Processing Kanowna Belle treats both free milling and refractory ores with the free milling material being sourced predominantly from the East Kundana Joint Venture a partnership between NSR and Rand & Tribune FY18 guidance for KB processing plant throughput is ~1.9 million tonnes, with gold production of ~ oz (including joint venture gold). Plant recoveries of ~95% are normally encountered when treating free milling ore. Page 2

3 Kanowna Belle Free Milling Flowsheet Crushing Gravity tailings Grinding (SABC) Gravity Separation (Knelson) Cyclone underflow Ground slurry Classification Cyclone underflow Conventional Leach/CIP Barren slurry Tailings Storage Gravity concentrate Cyclone overflow Loaded carbon Intensive Leaching (Acacia) Elution Pregnant solution Pregnant solution Goldroom Page 3

4 East Kundana Joint Venture (EKJV) Ore - Treatment East Kundana Joint Venture (EKJV) ores treated at Kanowna Belle can have high cyanide and dissolved oxygen consumption. Rubicon-Hornet and Pegasus ores normally have a cyanide consumption greater than 0.7 kg/t with one area of the Pegasus mine reaching 1.9 kg/t. Mineralogical analysis has shown that the ores contain significant quantities of reactive sulphides with pyrrhotite being the dominant species. XRD analysis of leach feed samples has shown that some leach feed Pegasus material has a pyrrhotite content of 6% by mass. It is the reactive sulphides in the ore that cause the high reagent consumption. High cyanide consumption is due to formation of ferrocyanide as per the reactions listed below (Chemistry of Gold Extraction Marsden and House, 2006) Page 4

5 Ferrocyanide Although ore sources are separated out, interactions are inevitable. Ferrocyanides act as a depressant in pyrite flotation reactive sulphides present in the process water impact processing of the refractory ore. Copper sulphate (flotation activator) addition is increased to counteract the ferrocyanide s action on flotation performance copper ferrocyanide complex is precipitated out of solution. Addition of copper sulphate requires close monitoring too much copper will start to impact adsorption kinetics of gold cyanide complex on activated carbon in adsorption train. Prussian blue indicating ferrocyanide Page 5 Tailings Storage Pit

6 Pre-Oxidation In 2015, KB metallurgy staff starting looking into pre-oxidative treatment of leach feed material to passivate the reactive sulphides in the ore. Drive this reaction forward Iron hydroxide protective layer forms over remaining pyrrhotite, preventing ferrocyanide production Chemistry of Gold Extraction Marsden and House, 2006 Laboratory scale test work was commenced, with the test work aiming to assess the cyanide savings achievable if the first leach tank was converted to a pre-oxidation tank. The test work indicated a cyanide consumption reduction of between 20 and 34% based off a pre-oxidation time similar to the residence time in our first leach tank. No noticeable decreases in total gold recovery were encountered during the test work for the planned pre-oxidation and leach/cip residence times. Page 6

7 Pre-Oxidation Plant trial setup involved redirecting all cyanide containing streams (including barren eluates) to second leach tank, and selecting second tank for cyanide process control. It was decided to move the lime addition and control to the second leach tank. Second leach tank all cyanide containing streams diverted here First leach tank converted to pre-ox tank The plant blend plan was adapted to allow for extended runs of the same feed material to the plant - this allowed for an ON/OFF configuration to be used during the trial. Page 7

8 Cyanide Consumption (kg/t) Plant Trial Results Predicted cyanide savings agreed with testwork with September exceeding expectations. Average cyanide consumption decreased from 0.71 to 0.50 kg/t over 4 month trial period, with the difference being statistically significant Effect of Pre-Oxidation on Leach Cyanide Consumption Based on FY17 figures, $ worth of cyanide could be saved per year No pre-oxidation Pre-oxidation Pre-oxidation showed potential to allow treatment of higher sulphide (and potentially higher grade) material with increased flexibility July 2016 August 2016 September 2016 October 2016 Copper sulphate savings would also be realised, due to decreased ferrocyanide production Page 8

9 Cyanide Recoverable Gold (g/t) Plant Trial Results Pre-oxidation had an overall negative impact to production tail grades increased, reduced gold recovery. The increased gold losses outweighed cyanide savings Cyanide Recoverable Gold in TK-10 Samples Pre-ox Test work on leach/cip discharge indicated that the quantity of cyanide extractable gold increased whilst on pre-oxidation configuration No pre-ox Recoverable gold Non recoverable gold It was concluded that decreased leach residence time had led to the increased tail grades during preoxidation plant trial Page 9

10 Solid Au (g/t) Solution Au (mg/l) Learnings from Plant Trial The second leach tank was short circuiting due to tank arrangement and damage to the tank agitator blades. Following the pre-oxidation trial, a downcomer was installed in this tank and repairs were made to the tank agitator Solid and Solution Profile 18th July Pre-Oxidation ON Au (solids) (ppm) Au (solution)(mg/l) Page 10

11 Benefits of Plant Trial Effect of pre-oxidation on Kundana Ore is well understood - an informed decision can be made on a permanent installation of a pre-oxidation tank. The project has not proceeded due to capital cost estimate (~$3M) Identified improvement opportunity use lead nitrate in the leach circuit. Lead nitrate can potentially improve recovery whilst also resulting in some reactive sulphide passivation cyanide savings. Laboratory test work has indicated a recovery increase of 1.39 to 1.81%, with cyanide savings of ~7.5%. The indicated profit on lead nitrate addition is $2.9M per year, based on FY 17 budgeted figures. The option of using pre-oxidation and lead nitrate together is currently being explored. Research into pre-oxidation also led to further test work examining the optimum cyanide/dissolved oxygen ratio in our conventional leach which could also lead to reagent savings. Page 11