NUCLEATION, GROWTH AND INHIBITION OF BARIUM SULFATE-CONTROLLED MODIFICATION WITH ORGANIC AND INORGANIC ADDITIVES

The crystal habit of barium sulfate formed by the rapid mixing of Ba2 and SO42- containing brines at 95 degrees C has been investigated and the effects of coprecipitated K+, Mg2+, Ca2+ and Sr2+ ions, supersatu ration ratio and barium-to-sulfate mixing ratio on the crystal morphol ogy studied. The most marked crystal morphological changes were induce d by varying the supersaturation and mixing ratios. The foreign ions p roduced relatively subtle effects. At high supersaturations (within th e homogeneous nucleation regime), eight-pointed star-like crystals wer e formed, whilst at low supersaturations, the equilibrium rhombohedral crystal form was recovered. All precipitates were single crystals. In the presence of crystal growth modifiers, marked morphological change s could be induced. For example, certain polymers induced the formatio n of millimetre long bundles of needles at pH approximate to 6 and fra ctal-like hollow cones at pH approximate to 5. In contrast, a range of phosphonate-based molecules, designed to act as barium sulfate scale inhibitors, produced oblate spheroids and very distorted star-like cry stals 15-20 times smaller than the unmodified crystals. These particle s were found to be porous on the nanometer scale. All the precipitates described were single crystals. We have found that the control and pr evention of barium sulfate precipitation by phosphonate-based material s at high Ba2+ concentration and low pH (ca. 4.5) are complicated. In the absence of Ca2+ ions in solution, little or no inhibition occurs a nd evidence suggests that calcium phosphonate complexes are the active inhibitors. In addition, although the inhibitors may act by the class ical mechanism of blocking crystal growth sites, they also act as nucl eation promoters. This provides an additional mechanism for scale inhi bition involving depletion of scaling ions from solution. As part of t his work, using dynamic light scattering, we have for the first time d emonstrated the presence of 1-10 nm microcrystallites in fully and par tially inhibited barium sulfate scaling systems.