Iron sulfide inhibition and interaction with zinc and lead sulfides

Iron sulfide (FeS), zinc sulfide (ZnS), and lead sulfide (PbS) are considered to be among the most challenging scales in terms of inhibition and removal. They can form by the direct reaction of aqueous sulfide species with dissolved Fe, Zn, and/or Pb and by the exchange between aqueous sulfide species with preformed Fe compounds, such as Fe oxide hydroxide. These existing Fe compounds might have formed during production or intervention, such as an acid treatment. Similarly, PbS and ZnS can form by extracting sulfide from a more soluble sulfide scale (i.e., Zn exchanging with Fe in FeS). The objective of this work was to investigate FeS formation and inhibition under a range of conditions, including pH, temperature, and salinity, and to propose a mode of formation. In addition, the interaction between Fe, Zn, and Pb within solutions containing sulfide species was investigated.

The majority of this study was conducted under anaerobic conditions, with the scale-formation/inhibition experiments being monitored by inductively coupled plasma (ICP) analysis and pH and particle-size measurements. Among the tested scale inhibitors (SIs), two showed high efficiency against FeS, but high pH and salinity had detrimental effects on the performance of one of these products. Interestingly, these SIs prevented FeS deposition even under aerobic conditions (i.e., Fe hydroxide partially preformed). Moreover, at sufficiently high concentrations of SI, the deposition of ZnS and PbS was prevented even when these scales were formed by means of cation displacement (i.e., displaced ZnS and PbS ions from preformed FeS). The route of formation for FeS, ZnS, and PbS was seen to have a significant effect on the inhibition process.

The particle sizes of inhibited (suspended) FeS were significantly lower than those of the blank FeS samples, with this effect increasing with increased SI concentration. This difference in particle size might have an important influence on in-line filter-blocking tests and produced-water quality issues.