Improvements in bi-metal production at the Brunswick plant, Costerfield.

Transcription

1 1 Improvements in bi-metal production at the Brunswick plant, Costerfield. D Buchanan 1, V Cullinan 2 1. MAusIMM, Processing Plant Manager, Mandalay Resources Costerfield Operations, 1024 Heathcote Nagambie Rd Costerfield 3523, d.buchanan@mandalayresources.com.au 2. MAusIMM, Senior Metallurgist, Mandalay Resources Costerfield Operations, 1024 Heathcote Nagambie Rd Costerfield 3523, v.cullinan@mandalayresources.com.au ABSTRACT The Brunswick plant is located in Costerfield, approximately 40km east of Bendigo in central Victoria. Crushing, grinding, gravity gold concentration and flotation methods are used to produce a saleable gold concentrate and an antimony-gold flotation concentrate. This paper discusses the improvements made within the processing plant which have resulted in a 99% increase in throughput over nameplate capacity of average tonnes per month to average tonnes per month and increases of 12% gold (from 2007 average of 78% to 2014 average of 90%) and 16% antimony recovery (from 2007 average of 79% to 2015 average of 95%). This combined effort has resulted in an increase of equivalent gold ounces (AuEq oz) produced from of to a increase. AuEq oz = Au oz + Sb t Sb (US$/t) Au (US$/oz) INTRODUCTION History Mining in the Costerfield area dates back to mid In between that period and the current day mining operations have existed periodically. In 1995 a processing plant was constructed to retreat the old tailings and oxide material from the Brunswick pit. While the plant processed the material, exploration work discovered a new vein, the Augusta ore body. In 2006 development of the underground mine commenced. In 2007 the processing plant was redesigned to treat the new Augusta ore body. In 2014 mining of the new Cuffley ore body commenced via the existing Augusta decline. Processing of the Cuffley ore started in The mining lease is solely owned by Mandalay Resources. Geology The valuable metal/metalloids in the deposit are gold and antimony. Antimony is found in the most common sulphide form of stibnite (Sb 2S 3). The deposit formed in the Palaeozoic age approximately 340 to 450 million years ago in the Tasman orogenic system which lies approximately 1000 km in width down the eastern side of Australia. The deposits are located in quartz-sulphide veins and are low grade metamorphic. The veins averaging 30cm in width range up to 1.6m in width, with some traceable for up to 400m. Veins are said to generally run north-north-west (NNW) south-south-east (SSE) and there are two main types of veins, the older containing more coarse gold (high grade), while the younger veins contain less grade and more massive antimony. Process summary The Brunswick plant processes a sulphide gold antimony ore through a conventional comminution and flotation style concentrator. The concentrator operates 24 hours per day, seven days per week. Ore is delivered to the plant via road registered dump trucks due to location of the mine and the plant which are separated by a main road. The plant feed contains approximately 4% antimony and 10 grams per

2 2 tonne (g/t) gold (2014 operational data). Crushing is done during day shift and operates under noise restriction guidelines. The surface mobile crushing and screening system processes at 80 tonnes per hour (t/h) plant feed passing 16mm, suitable for milling at a rate of 18 to 19 t/h through a two stage, closed circuit ball milling circuit. A Knelson XD30 concentrator is used on the second stage of milling to recover a gold rich gravity concentrate which is upgraded using Gemeni GT1000 and GT250 tables in series to produce a gold concentrate which is sent to an independent refinery. Secondary milled products are classified on size to 80% passing (p80) of 54 microns (µm) and processed through a simple flotation circuit. The flotation circuit consists of two upfront tank cells installed in The rougher cells are four Denver DR100 cells in series with four DR100 scavenger cells. The combined rougher and scavenger concentrate reports to the cleaners, comprised of six DR15 cells, two of which were reinstalled in August The final antimony-gold concentrate reporting to the thickener prior to filtration is comprised of both the tank cells and cleaner concentrates. This is bagged for shipment to customers overseas, via the port of Melbourne. The tailings are thickened before being pumped to a tailings storage facility (TSF) adjacent to the Brunswick plant. A summary processing flow sheet is provided in Figure 1. Figure 1. Brunswick plant flow sheet THROUGHPUT INCREASE The original crushing plant consisted of a two stage jaw crushing circuit. The primary jaw crusher had a closed side setting of 45mm, with all the discharge fed over a scalping screen of 19mm with oversize being fed to the secondary jaw crusher set with intermeshing teeth to crush the material as fine as possible. This delivered a product of approximately 80% passing 16mm. Utilising this crushing circuit, the nameplate capacity was tonnes per month (t/m). Figure 2 below shows the dry tonnes per month processed from Data prior to this has not been used as ore supply was limited. Nameplate capacity of t/m was reached in March The work index of the Costerfield ore is 16 kwh/t with an abrasion index of 0.3 (Rayner S, Ore characterisation test on new Cuffley ore samples. ALS Ammtec).

3 3 In September 2012 a diesel powered mobile crushing plant was purchased (Finlay T-REX I-130RS). This resulted in a step change in the ability to mill at increased rates from a nameplate capacity of t/m milled to average t/m milled for all of Due to the old open circuit jaw crushing system, there was little control over the top size reporting to the primary mill circuit. The mobile crusher employs a closed circuit crushing circuit with a 16mm screen. The only exit from this crushing circuit is once the material is minus 16mm. This, combined with the impact design of the crusher resulted in a significantly finer product reporting to the existing milling circuit, 80% passing 10mm from the original circuit of 80% passing of 16mm. The percentage of fine material has significantly increased, from 30% passing 5mm to 52% passing 5mm. A diesel powered crusher was also chosen as the plant is at the limits of the existing power supply and taking the jaw crushers off this supply allowed additional equipment to be installed in the plant. The mobile crusher also prevented significant downtime that would have been a consequence of constructing a traditional fixed crushing circuit at a significant saving over the fixed plant in purchase price. A scope definition study report was completed in 2012 with the directive to increase throughput to tonnes per month. This report estimated $ for the project and a four month construction period. Instead, the mobile crusher was purchased and put into immediate use by utilising the existing conveyor circuit to fill the fine ore bins with the finer crushed material. Considering the cost of the fixed plant upgrade with the loss of revenue from the construction downtime, the initial purchase price of the crusher in 2012 and accounting for the running costs for mobile crushing at $5.83 per tonne, with purchasing a new mobile crusher every second year, it will be approximately 20 years before mobile crushing becomes a more expensive option. Figure 2. Historic monthly dry tonnes throughput from January 2012 to December 2014 The increase in throughput has continued to trend upward after the step change in September This is due to a number of smaller factors: redesigned ball mill feed chute to assist with flow of feed and reduced time to clear blockages in October 2012

4 4 installation of redundant pumps on all critical units by June 2013 which increased availability from an average of 85% in 2012 to 94% in 2013 and 2014 combined a reduction of the mobile crusher screen from 19mm to 16mm in May 2014 increase in crushing hours from August RECOVERY INCREASE Data used for recoveries dates back to April 2007, as this is from the time the existing plant was restarted in the basic layout it is today. There were only seven operational months in 2007 (April, then July to December). In April 2009 the plant was not operated. All data is reconciled recovery data. Gold recovery Gold recovery at the Brunswick plant is a combination of gravity gold and flotation gold. Figure 3 shows the trend from April 2007 to December Figure 3. Historic monthly gold recoveries from April 2007 to December 2014 Improvements in gold recovery at the Brunswick plant has been a culmination of numerous small changes. The significant notes and improvements are listed in chronological order: There is a marked change in recovery from 2010 to 2011with similar head grades of 7.41 g/t and 7.31 g/t respectively The entire flotation circuit is manually operated. For 2011 the operators were specifically targeted with on the job training to ensure operations were consistent between crews. This change made a bigger difference to overall recovery than any other change made to the plant In June 2011 the pulp depth of the rougher, scavengers and cleaners were increased using weir bar installation on the existing Denver cells

5 5 August 2011 a new Knelson XD30 was installed. This replaced an old, manually controlled Knelson that relied on operators to stop and clean for every cycle For December 2012 the old cyclones (Warman 10032C) were replaced with a bank of three 250CVX cyclones and new reagent pumps were also added at this time to give finer control Many small operational changes were made in 2011 and 2012, resulting in a year on year recovery increase from 86% in 2011 to 89% in 2012 Early 2013, reagent control was still quite varied from operator to operator so greater direction over reagent flow was given, specifically for lead nitrate addition in relation to the PAX addition, as test work had shown excessive lead nitrate addition had detrimental effects on recovery A purpose built gravity gold recovery circuit was built in November and December 2013 and commissioned in January This enabled constant collection of Knelson concentrates on one hour cycles. For 2014 the average gravity gold recovery was 43%. Prior to this installation, gravity gold recovery was 15% for This was due to limited capacity in the old gold room. This had surprisingly little impact on overall gold recovery In mid 2014 additional cleaner cells were added. These resulted in not only increased recovery, but additionally 2% increase in final concentrate grade. Antimony recovery Antimony recovery at the Brunswick plant is shown in figure 4 from April 2007 to December Figure 4. Historic monthly antimony recoveries from April 2007 to December 2014 Improvements in antimony recovery at the Brunswick plant has been a culmination of numerous small changes. The significant changes are listed in chronological order: In 2009 additional tank cells were added to the front end of the flotation circuit to take the antimony load off the rougher and scavenger circuits

6 6 There were significant differences in operational philosophy for each crew. Training consistent operational behaviour started in 2011 with specific on the job training. This is ongoing In June 2011 the pulp depths of the rougher, scavengers and cleaners were increased using weir bar installation on the existing Denver cells For December 2011 o the old cyclones (Warman model 10032C) were replaced with a bank of three 250CVX cyclones o The reagent pumping system was upgraded Many small operational changes were made in 2011 and Following the same trend as gold recovery antimony recovery had a year on year increase from 92% in 2011 to 96% in The operational changes combined with the cyclone installation smoothed out recovery significantly The reagent control and additional cleaner cell changes in 2013 and 2014 have helped maintain recoveries at increased throughput levels at similar head grades. CONCLUSIONS Critical equipment purchases have made increases to throughputs and recoveries, numerous and incremental improvements are also very important and have a huge impact over time. As the Brunswick plant is manually controlled, good training, operational discipline, good communication and supervision are critical for optimal plant performance. In addition to this, there must be a developed level of trust and cooperation between operators and the metallurgical team so changes requested are properly executed in the plant. All the small things add up to massive gains in throughput and recovery. ACKNOWLEDGEMENTS The authors wish to acknowledge Mandalay Resources for permission to publish and present this paper and contributions made by all present and past personnel in advancing the plant over the years. Special thanks to Sam Madin Costerfield Metallurgist and Toni Streczynski Mandalay Resources Vice President of Metallurgy and Processing for their input. Rayner S, Private Communication. REFERENCES