Screening of green inhibitors used during acid cleaning of heat exchanger tubes in the alumina industry

SUMMARY: Aluminosilicate scale fouling of heat exchanger tubes is a common, and serious, problem for alumina refineries. One of the most economical solutions to this issue is to pump dilute sulphuric acid mixed with an appropriate corrosion inhibitor through the pipes to remove the scale. Although some previous inhibitor formulations used in the acid clean have been extremely effective, a greater awareness of environmental issues has prompted inhibitor manufacturers to produce green inhibitors. Developing, validating and interpreting the results of a suitable test protocol that is acceptable for the target alumina industry is challenging. This paper discusses the testing of a variety of formulations in the quest for an inhibitor that is both green and as effective as its predecessor. A test protocol was developed to compare the inhibitors based on the service requirements within an alumina refinery. Linear polarization resistance measurement with a rotating cylinder electrode was found to be an appropriate technique for conducting the evaluations. The results and interpretations of the tests are presented in this paper. In addition an associated alkali inhibitor was evaluated using a laboratory based flow loop to simulate plant conditions. Some formulations, although performing poorly as a corrosion inhibitor, exhibited interesting electrochemical behaviour and this is also discussed in this paper.