No abstract

The effect of various benzoic acids on the precipitation of barite and calcite was investigated. The acids varied in the number of carboxylate groups, from di-benzoic acids (phthalic, isophthalic and terephthalic) through to the hexabenzoic acid (mellitic acid). It was found that the stereochemistry of the di-benzoic acids was important, as was the pH of the solution (trimesic acid was used as a test case and showed greatest inhibition was achieved with all carboxylate groups deprotonated). Interestingly, for both the calcite and barite systems, mellitic acid was found to be both a potent inhibitor and a significant crystal growth modifier. In the case of barite, the presence of mellitic acid produced nanoparticles that agglomerated. The nanoparticles were found to be ~20 nm in size from XRD linewidth analysis and 20-50nm from TEM. Humic acid was also tested and found to form bundled fibres of barium sulfate.

Calcium ions have been implicated in changing the precipitation of barium sulfate when organic additives are present, although whether it inhibits or promotes nucleation and/or growth has been disputed in the literature. We conducted a thorough investigation into the effect of calcium when additives are present and show that calcium cations do indeed promote nucleation of barium sulfate particles when compared to the appropriate control. This result is independent of the analytical method (conductivity or turbidity) used to assess precipitation. The nucleation promotion produces no change in the crystal morphology, thus morphology is not always a good indicator of nucleation or growth behavior. The extent of nucleation promotion depends on the functional group of the additive. r