Work was undertaken to systematically investigate the factors that affect the formation of zinc sulfide in an aqueous system. Experiments were performed at a series of temperatures from room temperature up to 90 °C, at a range of initial pH values and in two brines systems. The effect of pH was further examined by changing the salt from Na2S.9H2O to NaSH.xH2O, therefore changing the initial sulfide source from S2- to HS-, as part of an ongoing method development strategy.
A variation of the standard barium sulfate static bottle test was used, in which the two bottles to be mixed contained aqueous "H2S" and zinc, respectively. Having pH adjusted the zinc brine to values calculated by the FAST sulfide model, the brines were pre-heated to the required temperature and mixed. Aliquots were removed at 2, 4 and 24 hours to perform elemental analysis by ICP and pH measurements were performed on all samples once they had returned to room temperature. In addition, particle size analysis and ESEM examination of the resulting precipitate were also performed for a subset of the samples prepared.
The reaction between zinc and aqueous "H2S" was quantitative at all temperatures up to 90 °C and in both brines. The final pH values of the supernatant were independent of the zinc brine pH and instead were dependent on the molar ratio of zinc and sulfide ions. A high pH, sulfide dominated, and a low pH, zinc dominated, plateau region were seen with a sharp inflection between the two. As a consequence, reaching field representative pH values was seen to be extremely difficult while retaining the ability to alter the relative concentrations of the reacting ions. Altering the sulfide source yielded the same trend, albeit with different absolute values. These observations have been rationalised with reference to the thermodynamic constants governing the reaction through scale prediction modelling.
The work presented here provides a greater understanding of the factors governing the formation of zinc sulfide scale and the considerations required for more industrially relevant formation and inhibition experiments in the future.