Durable Draglines Double Efficiency at IMC Gypsum Storage Area

International Minerals & Chemical Corp.'s New Wales operation near Mulberry, FL, is reportedly the world's largest concentrated phosphate chemical fertilizer and phosphate-based animal feed ingredients complex. Its latest expansion represents a $191-million investment. Two Northwest draglines were put to work at the operation early last year and, according to a company spokesman, total production from the units have more than doubled that from older rigs. This article describes how the draglines are used and provides details on the New Wales operation. The principle product at New Wales is phosphoric acid, produced by reacting ground phosphate rock with sulfuric acid in large concrete attack tanks. For each ton of phosphoric acid produced, New Wales must dispose of about 4.5 t (5 st) of waste gypsum. New Wales stores the gypsum on-site, and uses durable Northwest draglines to help handle the material. Two Units Maintain the Gypsum Stack After the gypsum is separated from the phosphoric acid through 23-m-diam (75-ft-diam) vacuum filters, it is mixed with water from a 121-hm2 (300-acre) recirculating cooling pond and, in slurry form, is pumped to the top of a 121-hm2 (300-acre) gypsum stack located adjacent to the cooling pond. The slurry contains mostly gypsum (CaSO4), with small traces of silica and other elements. The water at this point is corrosive, containing from 1-2% phosphoric acid. Its pH usually ranges from 1.5-2.0. Water drains from the gypsum stack into the cooling pond. Only about 15% of the pond's water, however, comes from the gypsum stack; the balance is from various sources throughout the complex and is held in the pond to cool before being returned for reuse in the plant. About 97% of New Wales' water needs come from this recycling program. The gypsum remains on the stack, inert, ready for future processing if it becomes economically feasible. Throughout the handling process, Northwest draglines work around the stacks' rim, continually directing the waste flow and digging out dried solids to heighten the perimeter dike to provide more storage volume. When wet, gypsum is extremely soft. When dry, it compacts well and provides footing with enough bearing capacity to support the machines. Processing at New Wales started in 1975 and the gypsum storage area has already been built up 12 m (40 ft) above grade. A total of 14.8 km3 (12,000 acre-ft) of solids are now stored. The draglines dredge material from near the perimeter dikes and place it to build parallel berms. This