It has frequently been observed that if the iron concentration in an alkaline chelant boiler cleaning solution reaches a high value when dissolving magnetite, it sometimes continues to increase rapidly and not level, indicating corrosion of the boiler metal. A laboratory study of runaway iron increases investigated the cause and the best mitigation methods. Waste chelant solutions with high iron concentrations require extra treatment steps unless evaporation is used for waste disposal.
Over a period of several years, the authors have received a number of reports and been witness to instances of runaway iron increases occurring during chelant chemical cleaning of boilers. These reports have the following common parameters. Inhibited tetraammonium EDTA (TAEDTA) or diammonium EDTA (DAEDTA) was being used as the solvent for magnetite deposit removal at high temperatures from boilers. Iron concentrations in the solutions were always greater than 1% when the rapid dissolved iron increases initiated. Solvent concentrations as free EDTA chelant ranged between 1.0% and 4.0 %, and tended to rapidly spend to zero. Inhibitor concentrations were always reported to be 0.1%, suitable for the boiler?s S/V ratio according to laboratory data and long established successful field practice. When additional chelant was added, it too spent with iron continuing to increase beyond the point where all of the deposits should have been dissolved. Typically, pH ranged between 9.0 and 9.5 for TAEDTA and 5.0 to 6.0 for DAEDTA. Solvent temperature varied between 135 and 155°C (275 to 310°F) for TAEDTA and 88 to 100°C (190 to 210°F) for DAEDTA. For TAEDTA, temperature was often cycled in its range to achieve circulation of the solvent by convection. This occurred with different cleaning contractors, different boiler owner companies, and different corrosion inhibitors.
These problem-cleaning projects were generally terminated in one of two ways:
? A consensus decision between the contractor and the boiler operator was reached that the boiler must be clean in spite of the continuing increasing iron concentrations. Then the cleaning was terminated by cooling the boiler, adding sufficient TAEDTA to maintain a 0.5 wt.% minimum excess, proceeding with the copper removal and passivation stage, and then draining the solution, or
? A portion of the solution was drained, and fresh inhibited chelant solution was added to bring the iron concentration down to 0.8% maximum, with 0.1% inhibitor and 0.5 % minimum EDTA concentration. When this was done, the iron concentration would level in the normal way.
Although there was speculation, the reason for the runaway iron concentrations phenomenon remained a mystery. When it occurred, the cost to the boiler operating company was considerable due to:
? Additional inhibited chelant solution consumed
? Additional down time for the boiler
? Additional contractor time charges
? Additional waste generation, and
? The long-term cost of any possible corrosion damage to the boiler.
A boiler operating company requested that the authors determine the reason for the phenomenon so that it could be avoided in the future.
BACKGROUND
Theories
At the beginning of the project, the following ideas were considered:
1. The high concentration of iron in solution catalyzes the degradation of the free EDTA present in solution. If this is the case, the EDTA should disappear faster than the proportional iron concentration increases. This would result in an increasingly poor materials balance. This is not what was observed during the boiler cleaning projects.
2. The drain down allows sludge and sediment buil
