The Economic, Environmental, and Quality Advantages of the Kiln Phosphoric Acid Process over the Wet Acid Process for the Manufacture of Phosphoric Acid
Dr. Joseph A. Megy March 2008
Introduction The wet acid process has been used to produce phosphoric acid from phosphate ore and sulfuric acid commercially since approximately 1842. The process yields an impure, black phosphoric acid solutionwith about 26% P2O5 that is concentrated to make solid fertilizers (diammonium phosphate [DAP] and monoammonium phosphate [MAP]) or purified to make liquid fertilizers or super phosphoric acid (SPA). The solution can also be further purified to make a technical-grade acid for various industrial markets. Except for a period during the last century when the electric furnace acid process producednearly as much phosphoric acid as the wet acid process, it has been the dominant manufacturing method for producing phosphoric acid. However, it does require highly beneficiated ores as a raw material, makes a rather weak and impure grade of phosphoric acid, and has a negative environmental footprint. In 1981, Dr. Robert Hard, who was working for Occidental Research Corporation (ORC), made animportant discovery that opened the door for a long-sought, efficient, high-temperature, kiln-based process to convert agglomerated mixtures of petroleum coke, low-grade phosphate ore, and silica into high-purity phosphoric acid for the $20 billion dollar world market (1). ORC tested the process in two continuous pilot kiln tests in December 1981 and May 1982 (2, 3). High-purity phosphoric acid was madeduring these tests with yields of over 70% in a commercial operating mode and over 80% in a test mode. However, the operating window was small, throughput was less than commercially desired, and operating parameters were not optimized. The ORC laboratory closed down in 1982 for reasons unrelated to the development of the kinetic phosphorescence analysis (KPA) process. Although the biggest hurdlefor the development of the process (e.g., melting of the kiln burden) had been overcome, it was still not ready for commercialization. Partially funded by two grants from the U.S. Department of Agriculture, Jamegy Development Corporation (JDC) reactivated a KPA research program conducted principally at Pacific Northwest National Laboratory, and a development program conducted at JDC facilities andvendor laboratories. Over the last 4 years, several important discoveries have been made that result in a modification of the Hard process that delivers high yields, high throughputs, optimized operating parameters, and a broad, stable operating range (4, 5). This work included engineering
studies that evaluated the 2004 operating and capital cost of a commercial Improved Hard Processfacility. The new discoveries have resulted in process modifications that are included in a recently allowed patent (6). The Improved Hard Process will permit the use of leaner phosphate ores because it has greater tolerance of some common impurities such as silica, organic content, and magnesium. It will also reduce the environmental impact of gyp stacks because an inert solid aggregate is the byproductof the process, not gypsum. This process produces super phosphoric acid (SPA) grade at a significantly reduced cost compared to the wet acid process. Higher grade acid can be more efficiently utilized by the fertilizer industry and other end users. This paper quantifies these advantages when the process is used with ore from the Southern Extension of the Central Florida phosphate deposit, theNorth Carolina phosphate deposit, and Idaho deposits of the Meade Formation, three diverse ores that are important to the U.S. phosphate fertilizer industry. Together, they supply the majority of the current U.S. phosphoric acid production plants. The following three analyses each consider a facility containing one large kiln producing 200,000 tons per year of phosphoric acid of SPA grade (76% P2O5)...
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