Recent Experience with Metallic Heaters for Phosphoric Acid Evaporation

The material used for phosphoric acid heaters is in many cases graphite, despite the many advantages of choosing a metallic material. The main reasons for selecting graphite is the material's good corrosion resistance and the tradition of using graphite. The corrosiveness of phosphoric acid during the wet-process concentration is complex and is dependent on several factors such as the presence of chlorine, fluorine, and diluted sulphuric acid. Metallic materials such as Sandvik Sanicro™ 28 can be used for this process with similar or even better results than graphite. This paper refers to the advantages of using metallic heaters in phosphoric acid production. It explains the important aspects of process design to take full advantage of the benefits available with a metallic material. It also includes the recent experience with process design, fabrication and performance of a metallic heat exchanger using Sandvik Sanicro 28 which was installed in a phosphoric acid plant in Tunis.