They come reluctantly and leave as converts.

Running a paste plant involves variety and skills that cross a raft of engineering disciplines, according to Dugald River backfill manager Allen Flemmer.

“It’s a very interesting job,” Mr Flemmer said.

“All the backfill engineers that I have trained in my time absolutely loved it.

“All dreaded coming, but by the time they left they loved it.”

Mr Flemmer estimates he has worked in backfill on and off for about 30 years and has been operating paste plants for more than 10 years.

“I’m a mechanical engineer and I started out designing fans for a big fan company, then moved into processing plants – mainly in the cement industry – and then ended up running a lime kiln for many years in New South Wales for Boral,” he said.

“It all came together to make paste relatively easy for me.

“The twists and turns of things that happen in your life just lead you down a certain path eventually.”

MMG’s Dugald River project came online recently, with the first load of zinc concentrate leaving the north-west Queensland site in November.

The paste fill plant at Dugald River was in its commissioning phase in November and was expected to come online in mid-December.

It takes wet tailings from the mill and filters them to a lower moisture content before they are mixed with binder to form the paste which is used to fill the voids left by underground mining.

Mr Flemmer said the Dugald River plant was the latest model Outotec design and incorporated some innovative features.

“The mixer sits on a platform that can pivot around a central column, so we can move it from sitting over the pump to a position directly over a borehole that goes into the mine,” he said.

“That allows us to use it either as a pump or a gravity fed system. It also allows us to perform maintenance on a bore hole while we are still operating.

“Dugald River mine has two bore holes, so if we need to work on one of the boreholes I can do that without interfering with the operation of the plant

“The belt filter is also a bit different to ones I’ve seen before. It has an additional belt towards the end of the filter belt which allows you to re-establish a good seal on the cake (filtered tailings).”

Mr Flemmer has been involved with the Dugald River project for about 18 months.

“When I came to site the paste plant was just a word on a piece of paper and we needed to select the successful tenderer and design the underground reticulation system that would take the paste from the surface down to the stopes,” he said.

It had been satisfying to see the project through, Mr Flemmer said.

“Paste is quite a challenging process as it is a non-Newtonian product – it doesn’t obey Newton’s Laws and can be a difficult process to start and operate,” he said.

“We tried to incorporate everything we’ve learned over many years when we built this plant so it’s as trouble-free and easy to operate as possible.

“Some minerals left (in tailings) after you extract the metal are not conducive to achieving good strength and we need to compensate for that with additional binder.

“You can also get a deterioration of strength over time just because you’ve got sulphides that produce an acid that may consume the cement in the paste. These are all factors you need to deal with when you are designing fill masses for the mine.”

Mr Flemmer’s previous paste plant experience has been in New South Wales, the Northern Territory and in Nevada USA.

“I used to work in Broken Hill many years ago and one of the tasks I had was to commission the backfill plant there – that was where I really started getting into backfill. That goes right back to the early ‘90s,” he said.

He also brought the paste plant online at Newmont Mining’s Tanami gold operations.

But Mr Flemmer owes his present position at Dugald River to the work he did at the Endeavour mine in Cobar, NSW, where he was able to get a problem-prone paste plant up and running.

“I worked with the GM (general manager project delivery Pierre Malan) at Endeavour mine and he wanted someone he knew could bring this project in,” Mr Flemmer said.

“It has been an interesting process and I’ve worked with a lot of great people. I think we have a very good plant and I’m looking forward to taking it into operation now and putting in infrastructure for the life of mine – setting up the systems and processes that will sustain it.”