Metal Sulfide Scale Inhibitors

Abstract Metal sulfide mineral scaling, fouling and deposition are frequently encountered problems in geothermal systems. Their formation, crystallization and deposition occur principally because of their extremely low solubility, based on the low solubility product (Ksp). Among the metal sulfides that cause problematic issues, the most common ones are iron sulfide (FeS), zinc sulfide (ZnS), lead sulfide (PbS), and, less frequently, antimony sulfide(s) (Sb2S3 and Sb2S5). Zinc sulfide, for example, has a K of -sp 2·10 25 mol²/L², whereas for PbS, it is 1·10-28 mol²/L² (~three orders of magnitude less soluble). ZnS can precipitate either as Sphalerite or Zinc Blende, and PbS commonly crystallizes as Galena. Mitigation of such ZnS and PbS precipitates and deposits can be achieved by chemical interventions, by the addition of organic chemical additives to the water. Herein, we report the inhibitory effects of phosphonate-based chemical additives for ZnS and PbS scales. These additives can inhibit formation of sulfide scale, and, significantly, prevent its deposition on metal surfaces. The efficiency of these additives is dosage-dependent, and relatively high inhibitor concentrations are needed for their inhibitory activity to take place. Possible mechanisms will be discussed focusing on inhibition and dispersion.