Precipitation double-jet

David, R. and Marcant, B., 1994. Prediction of micromixing effects in precipitation Case of double-jet precipitators. American Institution of Chemical Engineers Journal, 40, 424M32. 

Another very useful technique that alleviates some of the aforementioned difficulties is the controlled double-jet precipitation (CDJP) process, which can be extended to a triple jet design. The technique was originally developed to produce uniform large particles (23-25), particularly of silver halides for photographic applications. Recently, it was demonstrated that the procedure may be used for the synthesis of a variety of uniform dispersions, including those of metal oxides of different modal sizes, ranging from several nanometers to several micrometers (26-29). 

Cu(I) oxide particles of different shapes were obtained by reductive hydrolysis of Cu(II) tartrate solutions in the presence of glucose (107,109) using a process originally described by Andreasen (106). More recently, an extensive study dealt with the precipitation of different copper (hydrous) oxide and oxide particles using controlled double-jet precipitation (CDJP). Thus, depending on experimental conditions, hexagonal platelets or rods of Cu2(0H)3N03 and ellipsoids or needles of CuO... 

Fig. 4.1.1 Scheme of a double-jet precipitation system of AgBr particles. 

The emulsions used in this work were composed of the suspension of AgBr grains in aqueous gelatin solution, and were prepared by the controlled double-jet precipitation method. The edge lengths... 

Figure 5. Schematic representation of double-jet precipitation of AgBr-...

Results of some double-jet precipitations (Figure 7) which led to the present investigation show that at a given pBr (—log Ubt- ) both the morphology and size of the AgBr grains are influenced by pH. From... 

Figure 6. Electron micrographs (preshadowed carbon replicas) showing the morphologies of AgBr crystals obtained from double-jet precipitation in bone gelatin at the indicated bromide concentration...

Figure 7. Electron micrographs of AgBr-gelatin dispersions prepared at 70° C in double-jet precipitations under the conditions indicated...

The course of precipitation of most sparingly soluble salts in double-jet precipitations can be illustrated by the schematic plot of the supersaturation ratio S vs. time (Figure 3). S is defined as the ratio of the bulk solute concentration C to the equilibrium solubility C. As soon as the reactants are introduced, S increases very rapidly and exceeds the critical value Sc for spontaneous nucleation at t = tj. S will now increase at a lower rate owing to solid-phase nucleation and subsequent growth until a maximum is reached. Thereafter, S will decrease and drop below Sc at t = observable nucleation will then... 

For the nucleation in double-jet precipitation of silver halides, a dynamic model was derived which relates the inal number of stable nuclei to various precipitation variables ... 

Its importance will become apparent during the discussion of experimental results. The validity of eq (1) was experimentally tested by double-jet precipitation of AgBr where reactant addition rate R, pAg and solubility, and temperature were closely controlled. In these experiments, initial reactor volume and gelatin concentration (2-8% grange) did not significantly affect the number of stable nuclei. 

The experimental results obtained from balanced double-jet precipitations of AgBr crystals can thus be qualitatively explained reasonably well by the dynamic model of nucleation which includes both diffusion and the Gibbs-Thomson effect. 

In any batch precipitation scheme, a selection must be made between a high-yield, high-supersaturation (without exceeding the critical supersaturation), short operation, and a longer batch time, low-supersaturation one. Such a decision must weigh both the product properties desired and the cost factor associated with the implementation of either scheme. Often, additional considerations may need to be taken into account. For example, as reported by Yokoyama and Toyokura (1993), different polymorphic forms of precipitate can be obtained by controlling the supersaturation level in a double jet-precipitation. 

Using the same technique, it is also possible to precipitate composite particulates. The latter can be homogeneous of exact stoichiometry, as exemplified by pure or doped barium titanates [9]. To achieve these conditions rapid mixing is required, such as by using the controlled double jet precipitation process. In contrast, slow precipitation results, as a rule, in internal inhomogeneity, that is, the composition changes from the center to the periphery, although the particles may still be perfectly spherical, as observed with mixed alumina/silica [10] or copper/ lanthanum and copper/yttrium oxides [11]. 

Y.-S. Her, E. Matijevic, and M.C. Chon Preparation of Well Defined Colloidal Barium Titanate Crystals by the Controlled Double Jet Precipitation. J. Mater. Res., 10, 3106-3114 (1995). Controlled Double-Jet Precipitation of Uniform Colloidal CiystalUne Sr-and Zr-Doped Barium Titanates. J. Mater. Res., 11, 3121-3127 (1996). 

Leubner, I.H., Jaganathan, R. and Wey, J.S. (1980) Formation of silver bromide in double-jet precipitation. Photographic Science and Engineering, 24, 268-272. 

Stavek, J., Sipek, M., Hirasawa, I. and Toyokura, K. (1992) Controlled double-jet precipitation of sparingly soluble salts. Chemical Materials, 4, 545 -555. 

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