McLanahan ATE_7dec10 15/01/2010 16:15 Page 40
ASK THE EXPERT
Hybridising US sand washing John Best reveals how the sand washing industry has moved forward in the last two decades and why the hybridised approach has a proven background and strong future.
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mere 20 years ago the US market was dominated by a process technology which used classifying tanks (horizontal trajectory classification) and dewatering screws for the classifying and dewatering of construction sand products. Contrasting this to Australia, sand washing was based almost solely on pumps, sumps and cyclones; European producers (specifically in the Netherlands) were evolving a new design of plant using hindered-settling classifiers in ‘fractionation plants’ often known as ‘Recipe Plants’ – this design splits the sand products into three or four discrete fractions e.g. 4.25 mm x 2 mm, 2 mm x .5 mm, .5 mm x .3 mm and .3 mm x 75 μm. These fractions are stored and are re-blended to create multiple, accurately graded, final products. In 1998, this Recipe Plant technology was finally accepted in the US with the first installation of a four-fraction plant in central Florida; this was the most technically sophisticated plant of its type anywhere and was used in a fine, dune type sand deposit where glass sand was a primary product and multiple construction and industrial sands were needed. The plant was unique in that the operating software monitored dredging conditions, blended the fractions, monitored incoming feed mass and storage-levels of the fractions, and provided prompts to the plant operator when fractions were at high or low levels actually providing suggestions on possible alternative products based on feed and residual fractions. This reduced downtime and maximised production of high dollar value products. This plant produced excellent results, based on product quality, product consistency, final-product mix (14 different products), reduced cement usage (reports of >USD $1.10/M3 saving in cement), but most importantly deposit yield. Over subsequent years, several more plants of this design were installed, but more recently with the move to more and more manufactured (crushed) sand and fewer permits issued for natural sand, this technology has somewhat stagnated for the construction industry but remains active in the industrial/specialty sand (frac sand) market where the same principals of fractionation are used. For certain types of deposit and certain markets, the Recipe Plant technology is still very valid. In the late 1990s, plants based on dewatering screens and cyclones started to make inroads particularly for the manufactured sand market where the main requirement was
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to adjust the bottom end (<75μm) of the product gradation. These plants grew in popularity because the recovery of usable fractions in the ~150μm size was vastly improved over dewatering screws and the resultant dewatering screen product was much drier (around 10 percent lower moisture for any given sand fraction). In later designs, the combination of variable screen aperture, divided decks, cyclone geometry changes and other adjustments allowed for more flexibility than normally associated with such a simple concept. Today, for natural construction sand applications, classifying tanks are still being used where a middle fraction (e.g. 300μm x 600μm) requires adjustment. However, these plants have become more hybridised with the use of cyclones (for
“Producers can have confidence that the hybridised equipment and process designs have a proven background” John Best desliming/dewatering) and dewatering screens for final products; even in manufactured sand there has been a crossover in technologies. A good example is a 1000tph wash plant in Texas, US, where cyclones, classifying tanks and dewatering screens were combined to produce a manufactured sand for both specification concrete and mason sands. It should be remembered that the equipment once considered ‘new’ in the construction sand industry (particularly hindered setting classifiers) was used, modernised and proven for over 40 years in the minerals industry. The initial reticence in construction sand industry was often explained by ‘not wanting to be the ‘first’’. Producers can have confidence that the hybridised equipment and process designs have a proven background; however, the plants are, as with most things, only as good as the people providing the technology and the back-up service. n John Best is the General Manager of the Aggregates Division of McLanahan Corporation. Starting in 1974, he has been actively involved in the design, application, selection and operation of hydroclassification and dewatering equipment in the mineral and construction aggregate industries in Australia and North America.