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Continuous through mix for a consistent product

They 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.
Som Sharma is Precisionscreen?s technical expert for pugmills and has over a decade?s experience in the manufacture of pugmills. He explained that the pugmill mixing chamber has two horizontal shafts with 25 paddles mounted to each one that aggressively mix and churn the various materials together with the aid of moisture additive.
?Many products can be blended together to get the desired finished roadbase product and this may include binder agents like cement,? Sharma said. ?All aggregate materials are loaded into the respective feed bins and metered out accurately at the correct percentages. If cement is being added to mixture then a fully enclosed cement transfer system is used to bring the powder to the pughead for mixing. Water is injected at the pughead entry point at an accurate rate through the on-board positive displacement water pump and is mixed with the aggregate product, along with the cement powder or additive. All materials then travel through the 10 foot long mixing chamber to create a homogenous product.?
Sharma said that quarries may have traditionally used cement readymixers to blend various products together to meet roadbase specifications but reasoned that the use of a pugmill helps to control the product mix more effectively.
Sharma believes a continuous through mix is far more efficient and cost-effective than a cement readymixer. ?In a continuous through mix, mixers blend in suspension by collision and division,? Sharma explained. 
?A high speed collision between mix materials produces high quality product. The pugmill is required to mix all particles uniformly throughout the end product, to intimately mix binder particles so that they coat all the stones. The major emphasis on the continuous mixing approach is on proportioning of materials. At any instant, all of the ingredients in the feed stream are in correct proportion one to another. The twin shaft pugmill takes that stream and through mix to achieve homogeneity. The pugmill can produce CTB up to 300 tonnes per hour.?
The problem with a batch mix, by comparison, Sharma said, is that you need to determine ?the required quantity of each ingredient, weigh individually, dump these materials into the mixer in their segregated state, wait until the mixer can make the whole homogeneous and discharge the batch. The whole process takes lots of time which reduces the throughput. Every batch will be different. The quality of the final product is based on mixing interaction and speed?.
Precisionscreen has launched a new iteration of its longstanding mobile Scorpion pugmill. Whereas the nominal capacity of a pugmill is around 250 tph, this new model can pug roadbase with OMC at a rate of between 300 tph and 335 tph. It is capable of mixing three different products and cement powder simultaneously.
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 2.5? 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.
Sharma said that the point of difference between the Scorpion and other pugmills on the market is that the Scorpion has a unique, elevated design of pughead that discharges material directly to a stockpile or into the back of a haul truck at the end without segregation. The Scorpion also has a large throughput (starting at 300 tph an hour) and is built in Australia to suit Australian conditions. ?The Scorpion pugmill is a design that is built with simplicity and without compromise,? Sharma summed up.
He added that the Scorpion is also a modular plant that can be used in static, semi-static and mobile applications. The pugmill can also be adapted as a washplant, subject to additional costs.
Precisionscreen?s Scorpion pugmills have been engaged in a range of projects in recent years, including the Lismore City Council?s local quarry, where a pugmill is being used to mix CTB for future road upgrade projects. Scorpion pugmills have also worked on projects for Brisbane Airport and the RAF airbase, the Inpex project in Darwin and the Bundaberg Airport runway upgrade. ?
Source: Precisionscreen

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