Growth inhibition and deposition prevention of sulfide scales using dispersants

Abstract

Although sulfide scales of FeS, PbS, and ZnS do not occur as frequently as carbonate and sulfate scales, it is difficult to remove them once they are formed and deposited. Effective prevention and remediation of sulfide scales have not been fully developed. For sulfide scales, the conventional threshold inhibition approach is not as effective as for carbonate and sulfate scales, presumably, due to strong ion pairs formation of FeS0 and fast precipitation kinetics of FeSs. Polyacrylamide (PAM), polyvinyl pyrrolidone (PVP), and carboxymethyl cellulose (CMC) were examined as dispersants for sulfide scales to prevent and mitigate sulfide scale deposition through controlling the particle sizes of FeS, PbS and ZnS. PAM, PVP, and CMC dispersed FeS particles more effectively at pH 6.7 than 5.0 and temperature 70 °C than 90 °C. PAM and CMC successfully dispersed FeS particles even in extremely high ionic strength (I) of 4.6 M, while, PVP, slightly less effective, up to I = 2.6 M. At pH 5.0 and 90 °C, CMC was able to disperse FeS particles up to I = 1.3 M and PVP up to I = 0.6 M. In contrast, PAM could not disperse FeS particles at all. When FeS particles were dispersed in the presence of polymeric dispersants, FeS particle sizes remained in nanometer ranges, from 100 to 600 nm. Our results demonstrated that CMC was the best dispersant for FeS particles among the three polymers we tested. For CMC, the larger molecular weight and the lower degree of substitution of carboxylic groups on cellulose, the more effective dispersant CMC became. PbS and ZnS were also well dispersed in the presence of CMC. Almost all fraction of PbS particles was smaller than 200 nm, while about 60% of ZnS particles were smaller than 200 nm. Results demonstrated that the promising treatment selection of CMC as a universal dispersant for sulfide scales.