Friday 29th May, 2020

Pumping up gold production rates at Greenfields Mill

By implementing two new Metso HH200 pumps, the Greenfields Mill has been able to almost halve operational costs through simplified labour and equipment.

By implementing two new Metso HH200 pumps, the Greenfields Mill has been able to almost halve operational costs through simplified labour and equipment.

The Greenfields Mill, located in Western Australia’s goldfields three kilometres east of Coolgardie, has supported the local gold mining industry for more than 20 years.

It provides crucial toll milling services to local miners with a unique setup of three ball mills. The circuit can be optimised for gold recovery depending on the needs of different clients and in its current configuration, the plant can process up to one million tonnes each year. Investment organisation FMR Investments operates the mill.

Stockpile ore is delivered to the site before passing through a three-stage crushing circuit consisting of a primary jaw crusher then through secondary and tertiary cone crushers.

A double deck screen grades feed from the secondary and tertiary crushers that remove undersized material from the circuit. The crushing system produces a P80 product size – 80 per cent passing size of the circuit product – between six and eight millimetres, which is then transferred and stored in a 1000-tonne mill feed bin.

The mill circuit consists of 1300-, 875- and 500-kilowatt ball mills, with classification of leach feed product 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.

Overflow product is sent to a carbon-in-leach circuit, where gold is dissolved from the ore in a cyanide solution in the presence of oxygen. The gold cyanide complex molecule is then absorbed into activated carbon and stripped in a process known as elution. Gold is later recovered from the pregnant strip solution by electrowinning onto steel wool and direct smelting before it is shipped to an external refinery.

Regular inspections and maintenance helps FMR to ensure mill discharge pumps are ready for action.
Regular inspections and maintenance helps FMR to ensure mill discharge pumps are ready for action.

Mill discharge pumps a critical component of the plant

Critical to the operation of the mill-to-classification process are the plant’s mill discharge pumps.

The pumps transport the milled slurry up a large vertical pipe to the classification cyclones, processing high volumes of highly-abrasive materials. This means key wear components need to be monitored regularly to ensure the pumps can continue operating efficiently and to prevent any 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,” she says.

Dave Scott, FMR’s Maintenance Supervisor, says the 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,” Scott says.

“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 purchased two Metso HH200 pumps to replace existing units nearing the end of their effective life.

To maintain the previous pumps, the operators had to disconnect the suction and discharge spools, front casings and case liners to access critical internal components, which required a mobile franna crane. Because of the labour-intensive nature of the job, it can take up to eight hours to fully complete, making it difficult to maintain the pumps regularly.

The new mill discharge pumps are equipped with Metso’s unique slide-base technology. The slide-bases allow operators to open the pump without moving the suction and discharge pipework, making it easier to inspect wear components and conduct routine maintenance. The slide-base also has reduced the time required to get a blocked pump online again.

Dombroski says 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,”
she says.

“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.”

Ball mill, pumps, tec-taylor valve and pipe arrangement.
Ball mill, pumps, tec-taylor valve and pipe arrangement.

Travis Dingle, FMR’s Maintenance Fitter, 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,” he says.

“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.”

Dingle adds that the slide-base technology helps the team 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 says.

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 by about half while also creating a safer work environment.

“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,” he says.

As the slide-base allows the team to conduct maintenance without a crane, there is no rigging gear that could potentially strike someone. Because the team doesn’t have to disassemble everything, most of the pinch points have also been removed.

Roger Doyle, Pump Specialist at Metso, says 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,” he says.

“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.”

By implementing two new Metso HH200 pumps, the Greenfields Mill has been able to almost halve operational costs through simplified labour and equipment.
Roger Doyle and Dave Scott discuss pump maintenance.

Unblocking production when the going gets tough

Sediment or large foreign objects from the ball mill can cause the pump to become bogged, which can dramatically affect productivity. Bogged pumps most often 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.

If this occurs, the pump must be dismantled to remove the blockage, which can be a time-consuming process.

Dingle says 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,” he says.

“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.”

Metso’s slide-base technology helps to speed up the job significantly, making the process simple. No key components need to be removed and no cranes are required. The team at FMR are able to unblock the mill discharge pumps in around 45 minutes, which can slash unplanned downtime.

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,” she says.

“The slide-base design makes the job 100 per cent easier. If you bog the pump you can have it unblocked again within the hour.”

Local support

Greenfield Mill has a lean workforce on site and relies on original equipment manufacturers to help support the equipment in operation.

Metso’s local team are ready to assist with the day-to-day operation of the site and are able to provide any spare parts necessary 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,” she says.

“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.”