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 Ksp of 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.
Sulfide scales are encountered in high temperature as well as in low/medium temperature geothermal operations.1 Their formation is profoundly dependent on water chemistry, and, when deposited onto critical equipment surfaces, they are very difficult to manage. Zinc sulfide is a well-characterized mineral salt and has a solubility constant (Ksp) of about 2·10−25.2 The formation of zinc sulfide in aqueous solutions is not simple, because of zinc3 and sulfide speciation.4 Some crucial observations are warranted. An increase in pH reduces the actual concentration of Zn2+, due to zinc hydroxide precipitation, a competitive reaction to the formation of zinc sulfide. In addition, S2− is the dominant species in pH values greater than 11, while at pH values of about neutral, HS− plays the key role. The extremely low solubility constant of ZnS, allows the fast deprotonation of HS−, which shifts the equilibrium and reduces the pH.
