Pumping up gold production rates at Greenfields Mill

First class toll milling to the local gold mining industry

FMR Investments was formed in 2007 when specialist hard rock underground mining services provider Barminco - founded in 1987, divested its contracting business. Today, the company operates three mining projects in Australia: the Eloise Copper Mine in northwest Queensland, as well as the Greenfields Mill and Gordon Sidar mine, both of which are located in Western Australia.

Stockpile ore delivered by FMR’s customers initially passes through a three-stage crushing circuit consisting of a primary jaw crusher followed by secondary and tertiary cone crushers. The feed from the secondary and tertiary crushers is graded by a double deck screen that removes undersize material from the circuit. The crushing system produces a P80 product size between 6 and 8mm which is then transferred and stored in a 1000 tonne mill feed bin.

The mill circuit consists of 1300, 875 and 500 kW ball mills. Classification of leach feed product is handled by 15-inch cyclones. Free gold particles in the cyclone underflow are separated in a concentrator and sent to the gold room for direct smelting, while the remaining underflow goes back to the grinding circuit to be further reduced.

The overflow product from the cyclones is sent to a CIL (Carbon-in-Leach) circuit. In this process, gold is dissolved from the ore in a cyanide solution in the presence of oxygen, and the resultant gold cyanide complex molecule is then absorbed onto activated carbon. The gold is stripped from the loaded carbon, in a process known as elution. Gold is later recovered from the pregnant strip solution by electrowinning onto steel wool, and direct smelting before shipment to an external refinery.

Critical to the operation of the mill-to-classification process are the plant’s mill discharge pumps. They are responsible for transporting the milled slurry up a large vertical pipe to the classification cyclones. The pumps process a high volume of material that is highly abrasive in nature. This means that key wear components must be monitored regularly to ensure the pumps operate efficiently and prevent unexpected downtime.

Alternative Registered Manager at Greenfields Mill, Morgan Dombroski, says that the mill discharge pumps are a critical part of the site’s process. “The pumps feed the classification cyclones, which are the mother of the whole plant. If the pumps are down, we aren’t producing a product - which is a big deal for our site.”

Dave Scott, FMR’s Maintenance Supervisor describes how their pump setup has been designed to ensure there is always one pump available. “If the mill circuit is stopped, it basically costs the business about $40,000 per hour. That’s why we have a pair of pumps for this application. If the active ‘duty’ pump fails or is under maintenance, there is always a standby ready to take over.”

Innovative pump design slashes maintenance costs

In 2014, FMR invested in two Metso HH200 pumps to replace existing units that were nearing the end of their effective life.

During routine maintenance of their original pumps, operators had to disconnect the suction and discharge spools, front casings and case liners to access critical internal components. A mobile franna crane and a licensed operator were required to conduct the procedure. The labour-intensive nature of the job meant that it could take up to 8 hours to complete, which made it difficult to maintain the pumps on a regular basis.

The new mill discharge pumps are equipped with Metso’s unique slide-base technology. The slide-base allows operators to open the pump without removing the suction and discharge pipework, making it extremely easy to inspect wear components and conduct routine maintenance. FMR can now ensure that their pumps operate efficiently by regularly optimising wear components. The slide bases have also reducted the time required to get a blocked pump online again.

Dombroski says that the site’s mill discharge pumps are now checked on a weekly basis. “Our team here have a lot of experience at this site and are good at forecasting maintenance activities. We monitor the pumps through weekly inspections including a ‘shimming’ process. This optimises both front and rear impeller clearances which makes the pumps operate more efficiently and extends wear life.”

FMR’s Maintenance Fitter, Travis Dingle explains how shimming extends a mill discharge pump’s wear life. “The pump operates more efficiently when the impeller is closer to the volute liner. If you have a wider gap between these components, larger particles get in and wear the volute down at a faster rate. Shimming ensures the gap is reduced and the pump operates effectively.” The slide base technology helps the team to undertake this process quickly and with improved safety. “What makes it easy with the Metso pumps, is the hydraulic slide. We simply remove the volute bolts and pull the housing back to access everything we need - without the use of a crane,” he said.

By conducting weekly inspections and shimming both mill discharge pumps, the Greenfields Mill has been able to reduce the frequency of major overhauls. Scott says this has reduced operational costs for the site. “The total cost of a full rebuild can be significant. There was a time when we had to do this monthly, but now with regular maintenance we get a lot more life from our impellers and housings. This has reduced our costs by about half.”

According to Scott, the new pumps have also created a safer working environment on site. “The slide base allows the team to conduct routine maintenance without using a crane. This means there is no rigging gear for anyone to get struck by. The other factor is by not having to disassemble everything, most of the possible pinch points are removed from the process.”

The Pump Specialist at Metso believes that FMR’s pump arrangement and preventative maintenance program are an industry best practice. “Having two pumps in place ensures plant availability by providing a backup option that can be implemented without any impact to production. On top of this, the site takes full advantage of the slide-base technology and takes a very proactive approach to maintenance. This has benefits in terms of pump efficiency, but also means unscheduled servicing is much quicker and easier, as fasteners are exercised and anti-seized frequently”, he said.

Unblocking production when the going gets tough

A major issue for pump operators occurs if sediment or large foreign objects from the ball mill cause the pump to become ‘bogged’. At FMR, bogged pumps can occur if wear in the mill trommel allows a large rock or steel mill ball to pass into the pump, or if there is an unexpected electrical mains failure on site. In these instances, the pump must be dismantled to remove the blockage which can be a time-consuming process.

Dingle explains how this can be a significant job that must be carried out at short notice. “With most common pumps you have to take several components off, such as the front spool and cover. This requires a franna crane and a person available with a ticket to operate it. This can be a horrible job that can take over four hours to finish”.

Scott explains that the unique slide-base technology speeds the job up dramatically. “Un-bogging our Metso mill discharge pumps is a simple process. We don’t need to remove key components or require any cranage. Our guys can unblock the mill discharge pumps in forty-five minutes, which is significant if the blockage is causing downtime on site”, he said.

Dombroski adds that pump blockages can occur at any time, so there is peace of mind knowing the job can now be carried out quickly. “I’ve had a pump get bogged in the middle of the night and our operators need to drop everything and get it back online. The slide base design makes the job 100% easier. If you bog the pump you can have it unblocked again within the hour,” she said.

Benefiting from local support

Greenfields Mill runs a lean workforce on site and relies on OEMs to help support the range of equipment they have in operation. Scott highlights that it is an added convenience to have a local team ready to assist with the day-to-day operation of the site. “I’m in contact with Metso most days making sure we have all the parts we need. It’s good to have them close by and know that we can get any parts we require at short notice.”

For Dombroski, it is also important to have access to people who have experience with her company’s plant and equipment. “We have spent many days together with the Metso team working on rebuilds and other activities such as the installation of new products. Over this time, we have established a good rapport and they have proven to be easy to deal with in any situation. It is definitely good to have a local Metso presence in Kalgoorlie to check in on us from time-to-time and make sure all is well. They know the importance to the plant of the equipment we have purchased, and they make sure the right parts are available to avoid any downtime.”