Aggregates Manager Digital Magazine
Issue link: https://read.dmtmag.com/i/103664
OPERAT 1 From this Fine Opportunities W hen life hands you lemons, as the saying goes… The creation of fines in aggregate processing is almost inevitable. The poor-quality, low-value byproducts of the crushing and washing processes have been the bane of aggregate producers for years — traditionally relegated to settling ponds until the land was reclaimed, stockpiled, or hauled offsite for dumping. With permitted reserves at a premium, and water an issue, it makes sense to try and find uses for fines. But what to do with those "lemons" that your processes hand you? As a byproduct of the limestone crushing process, limestone fines can — and do — become saleable fertilizer and soil improvement products. Producers have sold aglime products for years. The fines can often be difficult to handle and transport, however. One technology, which has been more commonly used in hard rock mining, has begun to gain interest from North American aggregate producers in recent years: pelletizing of fines. "The technology had been in use in iron ore mining in South America. But we saw that, by making improvements in the pelletizing disc, we could help other markets — such as mining of salt and aggregates," says Steffen Silge, product manager agglomeration of German-based Haver Niagara GmbH. "There, if a company produces a lot AGGREGATES MANAGER of minus-100 micron limestone fines through a slurry, and the fines go to a waste pond, there is a lot of pressure from the government to reduce the amounts of those fines because they consume land," he says. "There is (incentive) for these companies to find uses for the fines because their transport, dumping, and pond maintenance costs money." Silge says that dewatering the material to create a pellet form for fertilizer not only means the product has a use, but it also becomes easier and less costly to handle. "There is increasing demand in the industry for recycling. This is part of it. You are taking waste ponds full of fines and turning them into something valuable versus storing or dumping," he says. Aglime fertilizer is probably the most obvious use of pelletized limestone fines. Murray Wallinger, production manager for Western Mineral Fertilisers near Perth, in western Australia, says his company purchases raw fines from aggregate producers and mining companies to create fertilizer pellets using a combination of 65 different soil minerals and trace elements. Wallinger says that chemical fertilizers don't replace trace mineral elements in the soil, which become gradually depleted by crops as they are grown and harvested, whereas mineral fertilizers from the mining process are silicate-rich, containing minerals that are beneficial to soil biology. Producers have traditionally struggled with a means to use or dispose of the low-quality and low-value fines that are a byproduct of aggregate processing. Settling ponds and stockpiles of fines take up valuable land space. Settling pond maintenance and costs for transport and dumping of fines increase expenses. Environmental concerns with the ponds are a growing problem, as well. 4 Curing and screening When the pellets reach the desired size, they cascade from the disc onto a conveyor, which moves them to either a rotary style dryer or a vibrating fluid-bed dryer. The dried pellets are then screened. The undersize pellets can be recirculated for additional processing. Oversize pellets can be reduced and recycled. Optimally sized pellets are conveyed to storage for bagging or transport.