TREATMENT OF IRON SULPHIDE DEPOSITS

1 . A method of treating a water system containing or in contact with a metal sulphide scale, the method comprising:

adding to said water system separately or together, a synergistic mixture of (i) a phosphorus compound selected from tris(hydroxyalkyl)phosphine (THP) and tetrakis(hydroxyalkyl)phoshonium (THP+) salts and (ii) a citrate salt, to provide a solution; then

contacting said solution with said metal sulphide scale,

thereby dissolving at least part of said metal sulphide scale in said solution;

and withdrawing said dissolved metal sulphide from the system.

2 . The method as claimed in claim 1 , wherein the synergistic mixture is in contact with the metal sulphide scale at a pH between 4 and 7.

3 . The method as claimed in claim 1 , wherein the THP+ salt is THP chloride, THP phosphate or THP sulphate.

4 . The method as claimed in claim 1 , wherein the metal sulphide scale is iron sulphide.

5 . The method as claimed in claim 1 , wherein the citrate salt is a salt of citric acid with a cation.

6 . The method as claimed in claim 1 , wherein the molar ratio of THP or THP+ salt to citrate salt is from 1:10 to 1:1.

7 . The method as claimed in claim 1 , wherein the molar ratio of THP or THP+ salt to citrate salt is from 1:8 to 1:1.5.

8 . The method as claimed in claim 1 , wherein the molar ratio of THP or THP+ salt to citrate salt is from 10:1 to 1:1.

9 . The method as claimed in claim 8 , wherein the molar ratio of THP or THP+ salt to citrate salt is from 8:1 to 1.5:1.

10 . The method as claimed in claim 1 , wherein the synergistic mixture comprises a polymer/copolymer of an unsaturated carboxylic acid or a copolymer of an unsaturated carboxylic acid with a sulphonic acid, said polymer or copolymer being either terminated by vinylphosphonic acid (VPA) or vinylidene-1, 1-diphosphonic acid (VDPA) or having such monomers incorporated into the polymer backbone.

11 . The method as claimed in claim 10 , wherein the polymer/copolymer is a polyacrylate terminated with vinylphosphonic acid, (VPA end-capped polymer) or with vinylidene-1, 1-diphosphonic acid (VDPA end-capped polymer), or may be a polyacrylate incorporating VPA and/or VDPA monomers.

12 . The method as claimed in claim 1 , wherein the polymer/copolymer is an acrylate/sulphonate copolymer terminated with vinylidene-1, 1-diphosphonic acid (VDPA end-capped copolymer) or with vinylphosphonic acid (VPA end-capped co-polymer), or may be an acrylate/sulphonate copolymer incorporating VPA and/or VDPA monomers.

13 . The method as claimed in claim 1 , wherein the mixture further contains a surfactant.

14 . The method as claimed in claim 1 , wherein the solution additionally contains biocides, water dispersants, demulsifiers, antifoams, solvents, scale inhibitors, corrosion inhibitors, oxygen scavengers, flocculants and/or non-surfactant biopenetrants.

15 . A synergistic composition, comprising:

(i) a tris(hydroxyalkyl)phosphine (THP) or tetrakis(hydroxyalkyl)phoshonium (THP+) salt; and;

(ii) a citrate salt; and optionally

(iii) a polymer/copolymer of an unsaturated carboxylic acid or a copolymer of an unsaturated carboxylic acid with a sulphonic acid, said polymer or copolymer being either terminated by vinylphosphonic acid (VPA) or vinylidene-1, 1-diphosphonic acid (VDPA) or having such monomers incorporated into the polymer backbone;

wherein the pH of the composition is of 4 7.

16 . A synergistic composition as claimed in claim 15 , wherein the molar ratio between component (i) and (ii) is 1:10 to 1:1.

17 . A synergistic composition as claimed in claim 16 , wherein the molar ratio between component (i) and (ii) is 1:8 to 1:1.5.

18 . A synergistic composition as claimed in claim 15 , wherein the molar ratio between component (i) and (ii) is from 10:1 to 1:1.

19 . A synergistic composition as claimed in claim 18 , wherein the molar ratio between component (i) and (ii) is from 8:1 to 1.5:1.