Barium sulfate nucleation

Klein, D.H. and Frontal, B. (1964) Heterogeneous and homogeneous nucleation of barium sulfate. Talanta, 11, 1231-1237. 

Nielsen, A.E. (1957) Nucleation in barium sulfate precipitation. Acta Chem. Scand., 11, 1512-1515. 

Takiyama (6) prepared highly monodispersed spindle-type particles of barium sulfate (BaS04) by decomposing the Ba-EDTA complex with hydrogen peroxide. In this reaction, the initial concentration of the Ba-EDTA complex is a decisive factor in separation of nucleation and growth stages. 

In studying barium sulfate precipitation, Nielsen (N4) found that the number of crystals formed depends upon the method of cleaning the glass vessel. Steam-washing the glass vessel reduced the number of barium sulfate nuclei from 2000 to 100 per mm . He concluded that for this system nucleation is primarily catalytic rather than homogeneous. 

Figure 8-4 schematically represents particle size as a function of initial concentration of solute prior to nucleation, with the number of particles assumed equal to the number of nucleation sites present. Generally, particle size can be expected to increase with concentration until homogeneous nucleation takes place, after which particle size will decrease sharply with concentration. In Figure 8-4 the difference between the concentrations S and S can be large or small. For barium sulfate it is larger than for silver chloride, with the result that barium sulfate tends to be nucleated much less extensively than silver chloride and consequently forms larger particles. 

On the assumption that heterogeneous nucleation is the important process, precipitation of barium sulfate from a supersaturated solution can be viewed as beginning by a series of steps on a nucleation site ... 

During the induction period this process continues on the surface of the nucleation site until the critical cluster has collected the next ion to be added triggers nucleation. Crystal growth then can follow. For barium sulfate, La Mer concluded that the slope of the line in Figure 8-1 is six and therefore the nucleation of barium sulfate is a seventh-order reaction overall. The critical cluster is then (Ba" " S04")3, and the addition of the seventh ion, either Ba " or 804 , constitutes the final step of the nucleation process. The question of the number of ions in the critical cluster, however, is by no means settled. Christiansen and Nielsen concluded that for barium sulfate the number is 8. Johnson and O Rourke also concluded that the nucleation rate of this salt is proportional to the fourth power of the concentration. The concept of a small critical nucleus is intuitively satisfying in that the nucleus then requires only a small number of steps for its formation. On the other hand, application of the... 

Nucleation Fischer found that, in PFHS, nucleation takes place early and only growth occurs thereafter. In the homogeneous precipitation of barium sulfate by sulfate generation from sulfamic acid in a solution containing 0.01 Af barium ion, he found (Section 8-2) that the number of particles was sensibly constant from the beginning to the end of precipitation. Evidently nucleation was complete within the first small fraction of the total precipitation time. From the rate of sulfate generation it... 

Barium sulfate precipitation has been reported to be controlled initially by nucleation reaction and, finally, by growth reaction. The growth kinetics was represented by the equation ... 

The vigorous mixing regime in the thin films of the SDR has also been exploited for crystallization processes.Homogeneous nucleation in the thin films results in a narrow crystal size distribution for crystals formed in the SDR. Uniformly shaped barium sulfate crystals of 0.5-1 pm have been produced with a reasonably low power dissipation of 115W/kg.f ... 

Figure 4-8 Measurement of nucleation kinetics for barium sulfate. Schubert and Mersmann (1996), by permission.

Marchisio, D. L., Barresi, A. A. Garbero, M. 2002 Nucleation, growth, and agglomeration in barium sulfate turbulent precipitation. AIChE Journal 48, 2039-2050. 

Sulfate salts are sometimes added to batteries to depress the formation of large lead sulfate crystals that are difficult to recharge. Failure by sulfation is common when the battery is deeply discharged or left on open-circuit in the discharged state. Because crystals of barium sulfate and strontium sulfate are isomorphous with lead sulfate and are relatively insoluble, they can nucleate the lead sulfate crystals and thus reduce their size. 

The influence of barium sulfate additions on the performance of negative plates has been studied extensively [21,25]. Barium sulfate is isomorphous with lead sulfate and therefore functions as a nucleation centre ( seed ) for the precipitation of the discharge product and favourably restricts its crystal size (see Section 4.5.1, Chapter 4). Strontium sulfate behaves similarly. It has been suggested that the effectiveness of barium sulfate is directly attributable to the number of nucleii present rather than to the amount added. The optimum amount of barium sulfate when combined with other additives for use in automotive batteries has been found to be between 0.3 and 0.5wt.%. 

The probability for nucleation to occur depends strongly on the degree of supersaturation of the solution. In the case of a very slight supersaturation, precipitation can take weeks or month to initiate, and at a certain degree of supersaturation nucleation and precipitation will happen immediately. This is called the critical supersaturation ratio. In the case of barium sulfate, a relatively high critical supersaturation ratio of about 32 has been determined. The supersaturation ratio is defined by the formula ... 

One should remember that the formula is used only to describe how fast after supersaturation is achieved one can expect precipitation, and not whether it will happen. The solubility product describes the supersaturated system thermod5mamically and predicts that precipitation will take place. The supersaturation ratio describes the supersaturated system kinetically by prediction of when it will take place. Also one should remember the role of impurities in nucleation. In the original determination of the critical supersaturation ratio of barium sulfate, solvent and reagents of the lower purity were used, giving a value of 21 instead of 32. 

At lower supersaturations, where heterogeneous nucleation takes place, the concentration of the heteronuclei formed is dependent on the concentration of the impurity particles and is not greatly influenced by the ionic concentration, so that particle size increases as the concentration of precipitate ions increases. However, when the supersaturation ratio exceeds that required for homogeneous nucleation to take place, the concentration of nuclei increases rapidly with increasing ionic concentration. At very high supersaturation ratios the increase is so rapid that the ultimate particle size decreases with increasing ionic concentration. Table 1 shows the variation of the form of barium sulfate precipitate as a function of supersaturation of the solution. 

Nucleating agent is selected from talc, kaolirt, mica, calcitrm sulfate, and barium sulfate for PBT article production by injection molding. ... 

See, e.g., the formulation and the references in Wattis, J. A. D. Coveney, P. V. Mesoscopic models of nucleation and growth processes a challenge to experiment, Phys. Chem. Chem. Phys. 1999, 1, 2163-2176. This paper draws conclusions from comparisons with experiments for barium sulfate. For an example with organic materials see Burnham, A. K. Weese, R. K. Weeks, B. L. A distributed activation energy model of thermodynamically inhibited nucleation and growth reactions and its application to the P-8 phase transition of HMX, J. Phys. Chem. B 2004,108,19432-19441. 

Cheng J, Yang C, Mao Z-S, Zhao C (2009) CFD modeling of nucleation, growth, aggregation, and breakage in continuous precipitation of barium sulfate in a stirred tank. Ind Eng Chem... 

---