Pugmills are instrumental in blending moisture, cement and other quarry materials to provide a homogenous roadbase material for the main roads and airport and airbase runways that transportation so relies on. Pugmills are used for optimum moisture content (OMC), which is generally around eight per cent moisture, or making cement treated base (CTB) for use in higher road specification for increased stabilisation and longevity.
Over the years, Precisionscreen, an Australian family-owned business that specialises in the distribution and manufacture of mobile crushing, screening, washing and recycling equipment, has launched several iterations of its enduring mobile Scorpion pugmill. The pugmill can pug roadbase with OMC at a rate of 300 to 335 tonnes per hour (the nominal capacity of a pugmill is normally around 250 tph). The Scorpion can mix three different 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 primary roadbase material. This is relayed back to the PLC that in turn regulates the percentage mix of each additional component, ie extra feed bin additives (eg clay/sand, etc), cement powder mix and water volume.
Traditionally, complementing the Scorpion pugmill has been a cement transfer system – the CTS-50 has a storage capacity of 50 tonnes (based on GP cement but Precisionscreen has the option to upgrade to 75-tonne and 100-tonne capacity silos). The CTS-50 has been offered to Precisionscreen customers in a vertical silo format, largely due to its portability. Precisionscreen have now gone one better – introducing a horizontal silo cement transfer system in the shape of the HTS-50. The Scorpion Pugmill PLC, the CTS-50 and the HTS-50 have been manufactured at Precisionscreen’s Wacol facility, in Brisbane.
MATCHING THE PUG
Like the vertical CTS-50, the HTS-50 also has a 50-tonne storage capacity, based on GP cement of 1.2 tonnes per cubic metre (1.2t/m3), and is primarily used as a filler silo to the pugmill’s on-board 2m3 weigh bin. The HTS-50 recurrently receives a signal to auto-fill the weigh bin when it reaches a certain low level and disengages when the weigh bin reaches the highest level. This allows for uninterrupted mixing throughout the day and there is no need for silo refilling. In conjunction with the pugmill, the HTS-50, like the CTS-50, is designed to pug roadbase materials at 300 tph.
The HTS-50 is equipped with four load cells that weigh the whole silo for verifying the discharge amount (with an on-board control cabinet for each load cell). Other features include four vibrators (two on each side of the hopper), a hydraulically driven 203mm fixed speed belly screw conveyor, a large on-board air filter and a battery box with lockable isolator as standard. Buyers also have the option of installing a ceiling screw, which is useful for lining up the silo with another screw conveyor. The silo’s external features include a heavy-duty structure and telescopic legs with wind-down feet for operation.
Paul Kerr, the general manager of Precisionscreen, recalled that the horizontal silo was a product that “we were going to release first but some clients wanted more of a standing silo. The HTS really fits in more with the mobility aspect of our business but the CTS was driven by customer requests”.
While horizontal silos are not new to the construction materials market, Kerr added, most are imported and Precisionscreen was keen to “build one that really matched up to the capabilities of the Scorpion pugmill”.
Kerr added that the HTS-50 was designed to be more mobile and compact than its vertical counterpart and to suit Australian, rather than European, transport rules.
“The HTS-50 transports by trailer but it’s designed so that it’s relatively easy to be folded up and for a client to drive a standard truck underneath,” Kerr explained. “It is portable but unlike the Scorpion pugmill, which can be moved around on-site fairly easily for repositioning, people are less likely to do that with a 50-tonne silo. Silos do bigger batches and there’s not really the requirement to do as much site manoeuvrability. The reason we decided to not go ahead and build it into a purpose-built trailer is just the availability and the low-cost of standard trailers in the marketplace.
It really means someone would be paying for an entire trailer, chassis for maybe 10 or 12 uses a year.
“As a standard product we think the HTS-50 fills the majority of categories where people want a mobile silo but, like everything, they need the costs back-up. It’s a very reasonable machine that’s easy for clients to get a good return on their capital.”
Logic might dictate that because of the similarities between the CTS-50 and HTS-50 that the latter will have aligned almost immediately to the Scorpion Pugmill PLC, which itself will have undergone few modifications. Kerr, however, said “nearly everything Precisionscreen does in its standard product range is built to order and that means nearly every machine gets a certain degree of customisation from the clients. Every time we put out a unit, we learn a new application. One of our advantages of being an Australian manufacturer is that as we learn something in the marketplace, we can literally get it onto the very next machine going through production rather than having to wait a year on a design cycle.
“The pugmill and every other machine are part of what we call a continuous improvement of the models. The type of things we’ve done with that Scorpion pugmill is easier connection to a PLC model that we’ve had for a number of years to make it easier to communicate with the HTS-50, so there’s been a bit of an upgrade on the base of the pugmill, to make it easier to collaborate with the HTS-50 and additive bin.”
The Scorpion pugmill features a large hopper capacity of 10.5m3 (2.5m3 without extension), a 650mm wide belt variable feed belt, a 650mm wide main belt with weighing system and a 63.5mm positive displacement water pump with variable feed drive and standard 100 psi wash down hose. The twin-shaft, two-speed action pughead is fitted inside a 3m long mixing chamber, complete with hydraulically variable angling. The pugmill is foldable for transport and ground level washout and servicing. It is also fitted with a drawbar and kingpin for site movement.
The main roadbase product is added to the pugmill’s hopper bin and this discharge is controlled via belt speed onto the main inclined conveyor to the pughead. The material travelling up the belt is layered like a cake (dry), with the cement powder being added to the roadbase via the 2m3 weigh bin before reaching the pug’s mixing chamber. Upon entry to the chamber, the correct dosage of regulated water flow is added. Once the material is in the chamber, the aggressive pugging nature blends the materials through the use of 50 paddles and is discharged to a stockpile or into the back of a haul truck.
The HTS-50’s (or CTS-50’s) weighscales are used as a manual check at the end of the day to ensure the correct dosage has been added.
Precisionscreen has recently installed a Scorpion Pugmill PLC with HTS-50 at a quarry in Orange, New South Wales. The machine was still being trialled by Boral at the time of writing but Kerr said that the initial feedback from the client had been promising.
“Boral was looking for a very mobile and modular pugmill that gave them flexibility,” he said. “I guess from the process point of view, what Boral wanted was a combination of machines that were flexible, modular, easy to move around, and that can do both OMC and CTB. The feedback from our fitter is that at the commissioning stage, Boral personnel found the machine easy to use, easy to understand and easy to calibrate set-up.”
Precisionscreen recently joined the Australian Made Campaign Limited, which means that most of the company’s local manufactured products will now carry the famous Kangaroo logo. Kerr said Precisionscreen joined the campaign to emphasise the company’s reliability and service rather than its outright patriotism.
“What I really want clients to understand is that being Australian Made means that everything is sourced locally which means that it’s easier for us to provide a more reliable machine with a better availability of spare parts,” he explained. “I think when we go through and explain that, we influence some clients’ decisions. Personally, I prefer to buy Australian Made but it has to be competitive and I believe that we are. The advantage of our product range being built local is that we are building with local know-how from local parts, with all the expertise of being local, so there is less chance in years from now of there being an issue with a machine. If someone needs a non-consumable stocking part, people won’t have to wait six or seven weeks for production in Europe or in Asia plus the transit times. Generally it will be about getting into our workshop and getting a replacement out the door.
“So those are considerations we’re telling our clients. Some people will see that naturally and some people will just want to see that Australian Made logo. I think it’s a recognisable logo and being part of the Australian Made campaign is something I’m actually very proud to be doing. The recent change in the Australian dollar versus the US dollar has obviously made it more attractive for us to be able to manufacture remotely and be highly competitive.”
While he was cautious about speaking on behalf of one of his customers, Kerr did not rule out the possibility that Boral was also pleased to be trialling an Australian product. “Boral has quite strict health and safety standards, so I think the comfort of working with an Australian manufacturer may have been a factor,” he said. “I know they were quite happy to be buying an Australian manufactured product.”