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Breaking new ground in phosphogypsum dewatering

As one of the largest private food and agricultural companies in the USA, the J.R. Simplot Group based in Boise, Idaho, operates worldwide. The firm produces phosphate products at two facilities – Don Plant, Pocatello, Idaho, and Rock Springs, Wyoming. At the Don Plant facility established in 1944, 350 employees produce more than one million tons of phosphate fertilizer, feed phosphates, and pure phosphoric acid per year. The two million tons of phosphogypsum this generates are dewatered on four horizontal vacuum belt filters from FLSmidth. With the type 1003 mesh belt, the Don Plant is testing a belt type from GKD – GEBR. KUFFERATH AG that has never before been used for dewatering phosphogypsum.

As a key component for the development of roots and stems, phosphate is irreplaceable in fertilizer production and is therefore extremely important for agricultural yield. The main raw material for fertilizer production is phosphoric acid. It is obtained from phosphate rock. Each year, the Don Plant processes between 1.6 and 1.8 million tons of phosphate rock from the Smoky Canyon Mine near Afton, Wyoming, which belongs to the group. The phosphate rock mined here is washed, ground, and slurried. An underground pipeline then carries it to the Don Plant for further processing. There, the phosphate is dissolved from the rock flour by the addition of sulphuric acid and separated into phosphoric acid and calcium sulfate. The Don Plant produces 400,000 tons of phosphoric acid (P2O5) with a concentration of 25 to 28 percent in this way each year. It is concentrated into different qualities including super phosphoric acid, or is processed into granulated fertilizer (MAP) with the addition of ammonia. Overall, seventeen different qualities of liquid or solid fertilizer and related products are made here. The phosphogypsum generated in the production of phosphoric acid has been dewatered on vacuum belt filters from FLSmidth since 1985. Prior to that, the Don Plant used pan filters, as the Simplot plant in Rock Springs still does today. Significantly greater plant efficiency prompted the switch to horizontal vacuum belt filters, which have been continuously increased in number since then. The stability and throughput of the filter belts used are major contributors to the productivity of these plants. At the Don Plant, the phosphogypsum, which reaches a maximum temperature of 82 degrees Celsius, is dewatered with the addition of flocculents in a semi-hydrate process. The filter cake, which is around one centimeter thick, is washed with process water three times in reverse flow in order to remove the residual moisture from the gypsum by altering the capillary effect between the gypsum crystals. This lowers the moisture content of the gypsum suspension from 60 percent to 30 percent residual moisture.

Individual solution for highly efficient dewatering
At the start of 2017, GKD USA reached out to Lyman Asay from the process laboratory at the Don Plant. One year later, GKD delivered the first mesh belt for the vacuum belt filter. Unlike in flue gas desulphurization, the consistency of phosphogypsum differs in each system. Depending on the nature of the phosphate rock, the particle size of the gypsum crystals, their distribution in the gypsum suspension, and process temperatures vary. For years, GKD VACUBELT 3356 mesh filter belts have proven their worth in various plants for dewatering phosphogypsum in Russia, Australia or the USA. They have a pore size of 220 µm and air permeability of 290 cfm. However, the Don Plant from Simplot requires a much more open belt with a pore size of approximately 350 µm and air permeability of 350 to 400 cfm for its very coarse gypsum crystals. The Don Plant also required the filter belt to handle a throughput of 3,785 liters per minute and provide temperature resistance up to 82 degrees Celsius, mechanical robustness and resistance to aggressive chemical loads. GKD recommended a belt type for this plant that is well established worldwide on belt presses in the fruit juice industry and sewage sludge dewatering: a type 1003 mesh belt made from polyester. As vacuum belt filter systems do not have any pulling forces, unlike belt presses, but work with far higher temperatures, significant shrinkage would have been unavoidable without corresponding treatment of the belt. That’s why GKD fixed the mesh at a high temperature and without stretching. The type 1003 filter belt has a pore size of 340 µm and air permeability of 360 cfm. Its production on ultra-modern looms guarantees high transverse stability and thereby robustness against creasing, even in the dimensions of the Don Plant systems – belt length of over 66 meters with a width of 2.9 meters. With a maximum throughput of almost 48,000 liters per minute, the GKD belt exceeds the requirements of the Don Plant more than ten fold.

Mesh impresses, seam has room for improvement
Lyman Asay rated the mesh positively: “We like the mesh a lot. It holds well and displays significantly less shrinkage than the belts we have used previously.” However, the service life of the seams still requires some work. Because of the type of weave, GKD does not recommend a spiral seam for this belt. That’s why the Don Plant and GKD tested alternative seam solutions in various trials. Neither a clipper seam with Hastelloy clamps and flexible seam wire made from PES nor a variant with triple seam wire made from PEEK were able to stand up to the enormous loads of the system. A variant with Hastelloy seam and seam wire is currently undergoing practical testing. Lyman Asay is confident that by working together, a perfect solution can be found. “The support from GKD is amazing: We receive quick responses and well-founded suggestions for solutions on every topic.” She found the involvement of the GKD engineers from Germany on site at the Don Plant especially positive. They are actively working on solutions to problems with the development department at the German headquarters. So she is certain: “The belt runs better than our previous belts and has proved itself in terms of the mesh. That’s why we are doing all we can to find a permanent solution to the seam problem.”

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