METHOD FOR CHEMICALLY CLEANING SLIGHTLY SOLUBLE SCALE

摘要

PURPOSE:To perfectly dissolve and remove slightly soluble scale formed and stuck on the surface of an iron-based metal by applying a mixed soln. prepd. by adding an amine type alkali agent and hydrazine hydrate to polyaminocarboxylic acid and adjusted to a prescribed basicity. CONSTITUTION:An amine type alkali agent is added to an aq. soln. of polyaminocarboxylic acid (e.g., EDTA) having 10-30wt.% concn. to adjust the pH to 6.5-9.0. The polyaminocarboxylic acid is converted into salt and a chelating agent which easily forms chelate with a scale forming metal is produced. Hydrazine hydrate is then added to the chelating agent to adjust the pH to 7.0-9.5 and a corrosion inhibitor such as an org. amine type inhibitor is further added. By applying the resulting mixed soln., slightly soluble scale formed and stuck on the surface of an iron-based metal is perfectly dissolved and removed at 120-160 deg.C. One or more among an aq. ammonia soln., gaseous ammonia, morpholine, cyclohexylamine, mono-, di- and triethanolamine are used as the amine type alkali agent.