SUCCESSFUL REPLACEMENT OF ALLOY 30, UNS N06030 WITH ALLOY 31, UNS N08031 AT A PHOSPHORIC ACID PLANT

摘要

In the fertilizer industry, phosphoric acid, ammonia and their derivatives along with potassium compounds are the major fertilizers providing the necessary soil nutrients for the agricultural industry. Ammonia, superphosphates, phosphoric acid and potassium chloride become the building blocks of the fertilizer industry and from these basic materials, hundreds of different formulations are produced to fit the individual soil and crop needs such as DAP (di-ammonium phosphate) and MAP (mono-ammonium phosphate) and others. More than 95% of the world's phosphoric acid production is by the wet acid process, balance being furnace acid grade for food and other additives. Almost 100% of this wet process acid is used in the manufacture of various fertilizers. Wet phosphoric acid manufacturing appears to be relatively simple involving the reaction of phosphate rock with concentrated sulfuric acid yielding phosphoric acid ( 26 to 28% P_2O_5 ) and calcium sulfate slurry, followed by filtration of the acid slurry to remove particulate matter, followed by concentration and purification of the phosphoric acid. Even though the process appears to be relatively simple and straightforward, severe erosion / corrosion problems have been encountered. Erosion is caused by particulate matter of phosphate rock and gypsum solids moving at high velocities whereas corrosion is due to the presence of sulfuric acid, phosphoric acid, hydrofluoric acid, hydrofluosilisic acid, chlorides, fluorides, organic compounds and oxidizing species such as ferric ions and other corrodents which can dramatically affect the active / passive behavior of metals and alloys used in this industry. The recent trend to increase the concentration of the final product involves use of higher operating temperatures, which increases the severity of the corrosive environments. This paper presents data, both lab and field, on some of the typical alloys and various components used in this industry to combat the corrosion problems with special emphasis on a newly developed super-austenitic high chromium 6 Mo alloy 31, UNS N08031 which has helped to solve/mitigate both the erosion and corrosion problems in this industry in a cost effective manner, both in USA and abroad (Morocco, Jordan, Israel). This alloy has successfully replaced alloy UNS N06030 in the author's plant in Rock Springs, Wyoming and for the last three years has been performing exceedingly well.