Evaluation of Lime Precipitation for Treating Boiler Tube Cleaning Wastes

摘要

The report gives results of an evaluation of lime precipitation for treating boiler tube cleaning wastes. In this project, wastewater samples were collected from six boiler tubeside chemical cleanings, using complexing and chelating agents. The samples represented: (1) ammoniacal bromate/hydrochloric acid, (2) thiourea-hydrochloric acid, (3) hydroxyacetic-formic acid, (4) ammoniated citric acid, and (5) ammoniated EDTA cleaning systems. Wastewater samples were also collected from boiler fireside and air preheater washes. A treatment methodology was investigated that involved: dilution of the boiler tubeside cleaning wastewater with a mixture of the fireside and air preheater wash wastewaters, precipitation with lime, and addition of polymers for clarification. After settling of the solids, the supernatant was analyzed for total and dissolved iron, copper, nickel, zinc, and total suspended solids. Major variations in testing included adjustments in pH and dilution ratio. Results indicate that, on a bench scale, the treatment methodology effectively reduced the concentration of iron, copper, and zinc in the tubeside cleaning wastewater to < 1 mg/l. Attainable nickel residuals were also < 1 mg/l for wastes from all except the ammoniated EDTA system, for which nickel residuals were not < 5 mg/l.