Patent ID: 6926836

Claim:
A method of treating a water system containing or in contact with a metal sulphide scale selected from the group consisting of iron sulphide, lead sulphide, zinc sulphide and combinations of any two or more or said sulphides, said method comprising the following steps: (a) adding to said water system, separately or together: (i) sufficient of a phosphorus compound selected from the group consisting of tris(hydroxyorgano)phosphines (THP) and tetrakis (hydroxyorgano) phosphonium salts (THP+ salts) and (ii) sufficient of an amino-phosphonate chelant of the formula R 1 R 2 NCH 2 PO 3 X 2 , wherein at least one of said R 1 and R 2 is a —CH 2 PO 3 X 2 group and the other is selected from the group consisting of CH 2 PO 3 X 2 , (CH 2 ) n OH and [(CH 2 ) n NCH 2 PO 3 X 2 ] m CH 2 PO 3 X 2 , where each n is independently 2 or 3 and m is from 1 to 5, and X is hydrogen or a cation such that said amino-phosphonate is water soluble; to provide a solution containing from 0.1 to 50% by weight of said THP or said THP+ salt and from 0.1 to 50% by weight of said chelant; (b) contacting said metal sulphide scale with said solution (a), thereby dissolving at least part of said scale in said solution; and (c) withdrawing from said water system said solution containing said at least part of said scale.