Mining Mirror March 2018 | Page 40

Technology and innovation The rake mechanism of a gold pre-leach thickener. The underflow system of a tailings thickener. Bolted thickeners reduce risks The concept of bolted thickeners is growing in popularity. Although the concept has been around for at least a decade, it is becoming more commonly used. With a bolted tank, the entire thickener is constructed inside a purpose-designed fabrication facility, with all welding, sandblasting, and painting taking place under controlled conditions. This guarantees that everything fits as it should so there are no unexpected delays on site. Thickeners have traditionally been constructed on site; all rolled and bent plates are transported to the required location where they are welded onto the prefabricated support structure and radial beams. This process can take weeks to complete, as kilometres of welding runs are required. Moreover, the considerable weight of the material in [38] MINING MIRROR MARCH 2018 the finished tank — several thousand tonnes — means that the weld quality needs to be high and coded welders must conduct the work. These welders must be accommodated on site for extended periods, which adds to the cost; cross-border work often involves the added burden of obtaining work permits and special visas for these workers. Sandblasting of the welding runs is then necessary, as well as a final coat of paint. In addition to the extra costs, the work is highly weather dependent, and delays can be caused by rain or excessive dust. “Weather-related delays can cause time over-runs for the contractor, which can then lead to penalty costs,” says Ricus van Reenen, senior account manager at FLSmidth. “This sort of project risk is one of the main reasons why contractors are choosing bolted thickeners. As a contractor, your time on site is where your most significant risk will lie.” After the construction of a bolted thickener in a workshop, the entire tank can be transported to site where the bulk of the on-site work can be undertaken by a mechanical supervisor, who oversees the lifting and placement, with the support of a rigger and a crane driver. Local labour can be used to fit the necessary bolts into the structure, and a technical specialist can come in to torque the bolts. “Once in place, only touch-up painting is required, so poor weather won’t be a serious risk factor,” says Van Reenen. He adds that bolted thickeners are a fit-for-purpose, high-quality dewatering solution with reduced project construction risk and duration. A bolted gold pre-leach thickener.