Pugmills are renowned for the blending and mixing of materials such as roadbase and concrete. However, there are signs pugmills are opening the door for quarries to diversify into cement and asphalt production. Damian Christie reports.
A pugmill is not part of a crushing and screening circuit, Jonny McMurtry told Quarry; it’s a circuit in its own right.
“A pugmill is a mobile system consisting of a chamber housing a dual shaft which rotates and blends material and water to optimally moisturise the system,” McMurtry, senior sales executive for national construction materials equipment supplier Precisionscreen, said.
“Predominantly it’s used for roadbase material, to optimally add water to aggregates which are compacted on-site. Other applications include soil stabilisation and creating a cement-based product, where cement or fly ash is added as a percentage to the product and then blended in the pughead to provide the optimum, efficient use of additives and the water to blend products.
“In rural areas, pugmills work where the optimum use of water is added to roadbase, so they can be carted to rural roads and laid in the most economic fashion. Certainly in rural areas, water is extremely precious, so producers are finding better ways of using it. Pugmills also blend sands, soils and waste products for an end use as a bedding soil.”
While not all quarries employ pugmills in their operations, McMurtry said some operations are eyeing them as a potential revenue stream.
“Pugmills extend quarries’ products and capabilities because they create cement-treated product,” he elaborated. “By and large, it’s expanded product they offer, if it’s a cement-treated application. In years gone by, quarries would add water and blend with a loader or an excavator, which was hard and labour-intensive on the capital equipment. Pugmills can blend materials at 200 to 250 tonnes per hour (tph), with the correct volume of water, in an efficient, economical way.”
OPTIMUM MOISTURE CONTENT
Over the years, Precisionscreen, an Australian family-owned business that specialises in the distribution and manufacture of mobile crushing and screening equipment solutions, has launched several iterations of the mobile Scorpion pugmill. The Scorpion can pug roadbase with optimum moisture content (OMC) at 300 tph.
The Scorpion can mix three products and cement powder simultaneously, thanks to a programmable logic controller (PLC) that uses weighscales on the main product belt to blend the mixture to achieve a homogenous product. The scales help to accurately determine the tonnage rate of the roadbase material. This rate is relayed to the PLC that regulates the percentage mix of each additional component, ie extra additives (eg clay, sand, etc), cement powder and water volume.
The Scorpion pugmill is available in two configurations: OMC and PLC. There are more than 80 Scorpion machines operating nationally; 20 are PLC versions.
“We have made additions to the Scorpion pugmill, where it’s now a PLC system, and more operator-friendly,” McMurtry explained. “A user can set parameters as per requirements, and return to it daily, understanding that the PLC system will anticipate what they’re trying to achieve. A single operator can run the machine.
Both versions of the Scorpion feature an elevated twin auger pug head with a 3.5m discharge height – it prevents segregation on the main conveyor belt and keeps the belt clean as material is carried to the pug head dry. They are each equipped with 50 reversible wear paddles and single on-board pumps.
“The discharge head on our pug head is about 3.5m so it can be loaded directly into the back of a truck off the pugmill itself,” McMurtry said. “All the wearparts within the pugmill are reversible, so you get optimum life out of them.
“There is the capability of running a pre-coating system off the pugmill. You can add on a pre-coat pump for diesel coating and blending of the material, so the diesel mix and pre-coat mix can be pumped into the pug head, and everything can be blended
in an economical fashion with the paddles and shafts.
“Two-thirds of the way up the conveyor, a flat door triggers the pre-coat pump as the material passes beneath it. Once it’s pumped to the pug head, the material is blended in a more efficient, effective manner.”
An on-board 2m3 weigh bin can be added to the Scorpion pugmill. The bin allows producers, McMurtry said, “to haul and add a certain percentage of cement, lime, fly ash or other additive to the product before it goes to the pug head”.
The weigh bin is complemented by the CTS-50 (vertical) or HTS-50 (horizontal) silo cement transfer system, each based on GP cement of 1.2 tonnes per cubic metre. “The HTS is a semi-mobile system that can store and feed up to 50 tonnes of additives and push the material through a screw conveyor into the additive bin from the pugmill,” McMurtry said. “It provides constant flow and production of cement and fly ash. It means the pugmill is self-sufficient, an economical way of producing the material.
“The CTS is a more permanent structure, it is built vertically. The HTS comes with jacking legs, and can be transferred from site to site in a cost-efficient manner.”
CLAM SHELL DESIGN
The pug head is available in an optional clam shell design for easier access, cleaning and maintenance. “The clam shell design allows the doors to open at the bottom of the machine, so you don’t have to lower the machine for cleaning and maintenance,” McMurtry said. “The operator can do the necessary checks and cleaning in a more time-effective manner. The reports we’ve had from customers who owned the older pugmills and now own the new ones is that on average it saves them between 30 and 40 minutes per day on checks and cleaning.”
According to Precisionscreen’s promotional literature, the clam shell design with additive bin can work as a mini-batch plant. “Most of the reports we’re getting is the pugmill, if it’s batching cement, can generate 180 to 200 tph,” McMurtry said. “It can do up to 300 tph if it’s moisture adding but when it’s cement or concrete, it’s restricted by the volume of the material.”
Another option is a blending hopper which, McMurtry said, is a similar size to the pugmill’s hopper and works alongside the machine, feeding to the main conveyor. “If producers have sands or another product to blend with aggregates or soils in the primary hopper, the blending hopper is a cost-efficient way of loading and blending materials on the pugmill’s main conveyor before it goes to the pug head and is blended as part of the pugmill process.”
McMurtry was asked what advice he had for quarry operators on the lookout for a pugmill. “The benefit of our pugmill is that it is mobile from site to site,” he said. “It’s Australian-designed and manufactured – built for what we call Australian conditions – right down to triple-sealed bearings and poly rollers to allow for the harsh conditions. The Scorpion has all the safety features that are necessary for the quarrying application, such as the automatic start-up feature, the grabwire switches, e-stops, all the necessary safety parameters. It has its own on-board water pump, so it is mobile, it can go from site to site and can integrate with water supplies on various sites.
“Now with the automation features that we’ve added to it, it’s more operator-friendly and easier to use. It’s the most mobile pugmill available for Australian quarrying operators.”