Wj. Benton et al., NUCLEATION, GROWTH AND INHIBITION OF BARIUM SULFATE-CONTROLLED MODIFICATION WITH ORGANIC AND INORGANIC ADDITIVES, Faraday discussions, (95), 1993, pp. 281-297
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.