Bayer Process Heat Exchangers Cleaning Efficiency: Optimizing the Acid Dissolution of Sodalite Scale

Aluminum metal is produced by the electrolytic reduction of aluminum oxide (Al2O3), mainly extracted from bauxite ore digested in a hot caustic soda solution. This solution is supersaturated with respect to sodium aluminate and silicate, which react together to form sodalite (3[Na2O⋅Al2O3⋅2SiO2⋅2H2O]⋅Na2CO3) at high temperatures. Heat exchanger tubes are scaled with this compound, which seriously reduces heat transfer coefficient, thus increasing energy costs. Heat exchanger tubes are cleaned periodically with a sulfuric acid solution that contains a corrosion inhibitor. An investigation was undertaken in order to study the parameters affecting sodalite scale dissolution by sulfuric acid: temperature, solution concentration, flow rate, etc. A custom-made set-up was used, simulating a heater cleaning operation, using scaled tube sections as test specimens. It was found that by increasing the temperature, the flow rate and the acid concentration, the scale dissolution rate increases. However, it was also determined that the presence of a corrosion inhibitor reduces the efficiency of the cleaning operation at high temperatures. This result was further investigated by adding organic compounds to the sulfuric acid solution.