Solid Barium Sulfate Formation from Aqueous Solution: Re-examination of Key Literature and Kinetics of This Classic System in Search of a Minimum Mechanism of Formation

The formation of solid barium sulfate, (BaSO4)n, known as barite when a crystalline solid in space group Pnma, is a classic system with a range of applicability in and consequences for the petroleum and other industries. Herein, we search for a minimum mechanism for (BaSO4)n formation via, first, a critical review of the literature since the 1950s, literature revealing that (BaSO4)n formation is an early, important model system with which to study nucleation, growth, and any accompanying aggregation. The literature also teaches that solid (BaSO4)n forms via non-classical, continuous nucleation, as first identified in 1997. Second, key literature kinetics data sets are reanalyzed in light of five classes of established minimum mechanisms for particle and solid formation. The resulting working hypothesis for (BaSO4)n formation is a 4-step mechanism consisting of nucleation (A → B), autocatalytic growth (A + B → 2B), bimolecular aggregation (B + B → C), and secondary autocatalytic aggregation (B + C → 1.5 C), with rate constants k1, k2, k3, and k4, respectively, and where A = precursor, B = smaller particles, and C = larger particles. Also provided are sections on caveats, engineering models, remaining questions, suggestions for moving forward, and a short summary.