Quarrying & Mining Magazine

Pugmill Rules

ALAN TITCHALL catches up with one of the busiest quarries in the Waikato region and finds out why the pugmill could become a more a common sight in the pit these days.

Over the past four years Huntly Quarry has produce massive volumes of metal for major roading contracts in the northern Waikato. Almost 700,000 tonnes was produced in one year alone.

The very capable three bin pugmill at Huntly.
The very capable three bin pugmill at Huntly.

As Stevenson Resources’ minerals executive Steve Ellis explains, when the company committed to these roading jobs, it had to also avoid interrupting aggregate supply from its static plants to existing customers, many of them concrete manufacturers.

“I suggested to the company CEO and the board that if we get the Te Rapa Fulton Hogan roading project we commit to installing a pugmill down at Huntly.

“To produce roading metal we could take our mobile to Huntly from Drury, rough the product out, crush a 10C40 and then put it through a pugmill and add what was missing in the grading, whether a bit of top-size or fines, or water.”

Stevenson won the contract to supply 300,000 tonnes of aggregate from Huntly over 100 weeks for the Te Rapa section of the SH1 Waikato Expressway. Delivery began in November 2010. And the company procured its second pugmill.

Initial lessons

The first pugmill was commissioned at Drury Quarry in 2006 and its performance convinced management that – through precise control and the addition of lime, cement, water, reclaimed aggregates, emulsions, polymers, recycled glass or crushed concrete to natural aggregates – these mixers make superior products, particularly better performing road bases, in terms of grading and grading curve shape control.

As Ellis explains, research for the first pugmill eight years ago took the company to Texas where he and colleagues saw Aaron Equipment pugmills in action. Short of aggregate, fast continuous mixers are commonly used in the state to produce a number of by-products for road base.

Shawn McLean, current manager (right), sits with Brien Golding (Chow), one of the original managers.
Shawn McLean, current manager (right), sits with Brien Golding (Chow), one of the original managers.

They liked features of the Aaron mixer, but considered the bins too small for their own use. Back in New Zealand they contacted subsidiary company Rocktec about modifying the original Aaron design. It happened that Fletchers was using an Aaron pugmill on an Auckland harbour project, and Roctec had been involved in modifying that machine for the specialist job.

The company designed and built a new pugmill, capable of producing 600 tonnes an hour at the Drury Quarry, where it has been used since 2006; receiving positive feedback from customers about the aggregates it produces.

Stevensons won the Te Rapa Alliance contract to supply 300,000 tonnes of roading aggregate over 100 weeks for a section of the SH1 Waikato Expressway. Delivery started in November 2010.

Once the Te Rapa Alliance contract was sealed, a smaller pugmill with three silos (with room for a fourth) and capable of producing 400 tonnes an hour, was designed and built at Huntly using five years of feedback from the Drury operation.

QM_P16_Feb_March_2014_3“The Huntly pugmill is semi-mobile, unlike the one at Drury, and we improved the gearbox sealing and other small things that all add up,” says Ellis.

A control hut was going to be built alongside the complex until a weighbridge operator suggested they run cable from the old plant office controlling the static plant some 100 metres away and operate it from there, which is what they did.

Stevenson’s success with the big roading contracts in the Waikato area has been ‘pudding proof’ of the pugmills meeting client specs and NZTA revisions of M4 specification. Suffice to add, quarry management are firm converts to using pugmills for grading control, rather than just relying on screening.

Both Huntly and Drury have produced modified aggregates with consistent performance within spec with every batch, iterates Ellis.

“And I believe they are the way of the future.

“Yes they are more costly to run, but they can’t be beaten for blending and correcting product.

“Screening will only take out what it is presented in the material from the face, and you end up with inconsistencies.”

For the record

He says that for a single roading job, the quarry was sending out the gate up to 100,000 with not one batch lot failing.

QM_P16_Feb_March_2014_4“Each lot was made up of 6000 tonnes. At times we had 40,000 tonnes stocked ready for jobs and every lot was traceable – we knew where the shot from the face came from and when it was tested. Nothing went out of the gate until the client had approved it.

“Each lot was put through four tests at the Stevenson’s lab at Drury Quarry.

“We spend around $250,000 on lab costs each year between the two quarries.”

And it is not all about roading material. The Drury pugmill has solved some interesting one-off challenges.

These include a speedway track in Auckland for a world-racing event. The finish involved fine clay and -6ml put through the pugmill.

“They reckon it was the best material finish they had seen. Sure, it wasn’t a big job – maybe 6000 tonnes – but they were paying $20 a tonne.”

Another small, but rewarding job, involved a temporary track for the 2013 UCI BMX World Championships held in Auckland’s Vector Arena.

“We played around with recycled concrete waste and then crusher fines to which we added small amounts of concrete. Different samples were laid down with a hand compactor in situ until they got the finish they wanted, which was superb they told us.”

A future with pugmills

Stevensons believes there are still many more products that can come out of a pugmill that are ideal for quarry clients.

Ellis says the idea of putting cement into the base roading aggregate during the Te Rapa section build was mooted as an alternative to using road stabilisers and mixing in situ.

“We did challenge them to do the numbers and it was very close [in cost] and they said after the job was finished they would do it our way if they were doing it over again.

“OK, the stabliser guys wouldn’t agree, but I think you have more control with a pugmill. We do it by weight – and you can’t get more accurate.”

Ellis sees pugmills becoming as common in New Zealand quarries as they are in Australia as the pressure builds to preserve the country’s good resources and we need to upgrade the poorer material with polymers and such like, as already done in blacktop production.

“We can’t keep burrowing into the good rock for every job. We have to start using lesser grades. Major roads come and go, but we have to keep developing our quarries.”

Production consents

Although they both produce aggregate from greywacke, Huntly and Drury are very different operations.

The greywacke resource is fine at Huntly and coarse at Drury and one operation is consented in the Auckland region and the other in the Waikato, although Ellis says the consenting is very similar. Local Government staff involved with consenting in both regions were probably trained by the old Auckland Regional Council, he jibes.

As with other quarries in the Waikato region, Huntly has a ‘five year’ production limit in its new consent, based on local authority road levies.

Its consented production average over five years is 550,000 tonnes a year. In one busy year recently, it produced around 700,000 tonnes, so is scaling back for the immediate future, while finishing its five-year consent cycle.

Reflections over cake and coffee

Just before Christmas Steve Ellis drove me down to Huntly for a site inspection and a meeting with its young manager, Shawn McLean, and one of the original managers, Brien Golding or Chow as he’s known amongst old workmates.

We were cooped up in the quarry’s front office for afternoon tea, eating cake and drinking mugs of coffee. Golding reminisced with infectious humour on the old quarrying days in Auckland, when Jack Stevenson astutely controlled his quarry managers by shuffling them around sites and pitching individual strengths and weakness among his workers.

Static plant built in 2000 and still going strong.
Static plant built in 2000 and still going strong.

That conversation, as you can appreciate, has to remain censored, but many respectful words were mentioned about the previous manager, and Shawn’s mentor, Pat Wallbank.

Pat first worked at Huntly Quarry in the 1970s when it was operated by the Roose/Stevenson alliance. He left in 1980 to become a mines inspector for Crown Minerals and was replaced by Brien Golding.

When Brien retired in 2003, Pat returned to the quarry as manager. In early 2011 he was tragically killed in an accident involving his ute and an out-of-control truck and trailer near Ngaruawahia. The northern Waikato roads are among the most dangerous in the country. Take it easy around there please readers. Not to say Pat stood much of a chance.

Perhaps this is the appropriate opportunity to recognise Pat Wallbank as a much-respected man within the quarry industry, who excelled at mentoring staff and was a founding member of the Institute of Quarrying of New Zealand when it was set up in 1969. He was made an Honorary Fellow in 2003.

Brien Golding, an ex manager at Stevenson’s short-lived Patumahoe Quarry in South Auckland during the 1960s, (covered in Hard quarry – tough quarrymen,Q&M, April-May 2012), had worked for W Stevenson & Sons for many years on numerous jobs, including drilling, before taking up his managing stint at Huntly in April 1980.

He is remembered for his hands-on approach to quarry operations and strong opinions on how it is done. He wasn’t keen on a new static plant for the site when it was proposed in the late 1990s. That was until his faithful cone crusher gave up the ghost and he called Steve Ellis and said, “How soon can you get me a new plant.” The original quote has a number of expletives.

The new plant was commissioned in 2000. Golding retired three years later, but the plant (photographed) is still faithfully grinding out stone. The only major change is coming up – a guarding refit for both Huntly and Drury to keep ahead of new specs.

The ‘principle’ of the plant follows the old plant, Golding tells me. Ellis adds that he used to take the first plant plans down to Huntly from Auckland to show Brien. “Leave them with me,” he would say. “I’ll look over them tonight.”

By the time Ellis returned, those plans had been almost completely redrawn.

Many of the old quarry managers got their first quarrying experience on a drilling rig, it was noted over coffee, and they had a panoramic view of what was going, including best practices and worse mistakes, that placed them at an advantage when they were finally in control. And there’s a large number of quarrymen out there today that will still tell you the best education is onsite, not just in a class room learning rules and regulations.

Maybe that is because quarrying has never been a precise science and every resource around the country is different and has to be approached differently.

“On top of everything else, a quarry manager has to have a ‘can do’ attitude, as operations are a percentage of science and a percentage ‘black art’, with no definitive rule book,” Ellis says with a smile.

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