An Evaluation of Nickel‐rich Alloys In Wet Process Phosphoric Acid

Abstract

IntroductionPhosphoric acid is an important chemical intermediate and is used primarily in fertilizer production. It is;lso used extensively as an intermediate in the production of detergents, water treatment, chemicals and leavening agents.Phosphoric acid is produced by two basic methods: the wet process involving reaction of phosphate rock with an acid (sulfuric, nitric, hydrochloric) and the furnace process in which phosphorous is burned with air to produce PzO5 gas which is then hydrated to form phosphoric acid. This report is concerned only with phosphoric acid that is produced by the wet process using sulfuric acid digestion.Extensive corrosion data have been published on the resistance of metals to reagent grade phosphoric acid. In contrast however, only limited test data are available on the resistance of metals in the wet phosphoric acid process. It should be noted that all the published corrosion data were obtained from immersion tests and are not useful in predicting the resistance of metals under heat transfer conditions.Operating experience with chemical process equipment has indicated that the corrosion rates of metals are accelerated when heat is transferred through the metal. For this reason, it is often necessary to fabricate the heating coils of a process reactor from a more corrosion resistant alloy than the walls of the reactor vessel. This accelerated corrosion attack of the coils is attributed to the extreme heating of a thin layer of fluid adjacent to the metal surface. In the case of very high solution temperatures, localized boiling is also responsible for increased corrosion attack. (1-14) describe the use of laboratory equipment to reproduce corrosive plant conditions involving heat transfer and elevated temperatures. The studies indicated that a hot wall test apparatus could satisfactorily simulate the heat flow through metal walls and its effects on corrosion. The test method was originally developed by N. D. Gyoves (1) and was used to predict the life of heat transfer surfaces in service in atomic energy installations. The investigations performed by Fisher and Whitney (2) concluded that the skin temperatures of the metal is the controlling factor in the corrosion Several publications JahrgangHeft 711971 attack and not the amount of heat flux flowing through the metal.Producers of phosphoric acid indicated interest in the potential use of corrosion-resistant metals for process equipment. Of specific interest were materials for evaporators for processing superphosphoric acid at higher temperatures. Several companies had indicated that they were evaluating processes for production of superphosphoric acid that required materials exceeding the temperature and pressure capabilities of impervious graphite which is commonly used in heat transfer equipment. Thus they were interested in metals and alloys suitable for these conditions and displaying acceptable corrosion resistance. Acid producers from Virginia and Florida cooperated in the study by providing samples of their crude phosphoric ...

Comparative corrosion behaviour of stainless steels and their welds in wet-process phosphoric acid

“…The concentration of each species in this solution is almost similar to that observed in WPA. The major impurities responsible for higher corrosivity of WPA are fluoride, chloride and sulphate [1][2][3][4]14,18,[26][27][28][29][30][31] and therefore these were added to 37% phosphoric acid to prepare reagent grade solution in which corrosion resistance of these three alloys was examined. Both the base material and the weld of SSs exhibit a similar trend (Figure 11 and Table 6).…”

“…Phosphoric acid is used in various industries such as chemical, fertilisers, mineral leaching, water purification, petroleum refining, food and metal production [1,2]. One of its major use is in fertiliser industries for manufacturing phosphate fertilisers.…”

“…Two main processes are used to produce phosphoric acid: (a) the wet-process phosphoric acid (WPA) by leaching phosphate rocks rich in calcium phosphate minerals such as apatite, Ca 10 (PO 4 ) 6 (F,OH) 2 with H 2 SO 4 and (b) a relatively novel process by treatment with HCl followed by separation of phosphoric acid applying solvent extraction technique [2,3]. But more than 95% of phosphoric acid is obtained by the WPA route [4].…”