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Investigating Precipitation Kinetics of Sparingly Soluble Salts using Shock-Freeze TEM 

Umraan Hendricks ,  Marcos Rodriguez-Pascual ,  Jillian F. Banfield ,  Alison E. Lewis 

UNIVERSITY OF CAPE TOWN (UCT), RODENBOSCH, CAPE TOWN, CAPE TOWN 7701, South Africa

Abstract
Reactive precipitation is defined as the process whereby a solid is formed after two reactant solutions are mixed together. The kinetic processes in precipitation include, nucleation, crystal growth, agglomeration and breakage of the particles. These processes are used extensively in the pharmaceutical industry, the fine chemical industry as well as the mining industry. When precipitation is used in industry, optimal process conditions are required in order to manufacture a product with specific physical properties. These physical properties include; particle structure, morphology and particle size distribution and are a function of several variables. Supersaturation, temperature and mixing rate are variables that can be controlled in order to achieve specific physical properties of the precipitate. Optimal conditions can be obtained by understanding the competing kinetic processes that occur during precipitation. Due to the short nucleation time in the precipitation of sparingly soluble salts, a fast mixing time is required in order to obtain experimentally measurable nucleation rates. Fast flow rates in the order of 100-250 m.s-1 and accurate particle sizing techniques in the size range of 10-100 nm are required in order to achieve an experimental setup capable of measuring nucleation rates for these salts. Sparingly soluble salts produce nuclei that consist of 5-20 monomer units, which fall in the nanometer range and smaller. Therefore, in this study, cryofixation-TEM was used to measure the nucleation rate and kinetics during the precipitation of the reacting solutions. Liquid nitrogen (-196°) was used to quick freeze the nuclei in the solution and prevent further nucleation and growth. Copper nano tubes, with an outer diameter of 0.4 mm and an inner diameter of 0.3 mm were used to form the T-mixer reactor because of their high thermal conductivity. The use of TEM allowed for the determination of the number and size of the particles in micrometric slices taken along the reaction tube length. In this manner, the nucleation rate and kinetics, versus residence time of the precipitation reaction were determined. Residence times were calculated based on the flow rate and position of the micrometric slice along the copper tube. Three reactions were investigated.

BaCl2 + Na2SO4 -> BaSO4 + NaCl                    (1)

ZnSO4 + Na2S -> ZnS + NaSO4                              (2)

CuSO4 + Na2S -> CuS + NaSO4                        (3)

The precipitation of barium sulfate has been widely studied, and therefore in this study it was used as a calibration/verification system. Barium sulfate (ksp=1X10-10) for a supersaturation range of 1.0E+2 - 1.0E+7 and zinc sulfide (ksp=2X10-25) and copper sulfide (ksp=8X10-37) for a supersaturation range of 1,0E+12 - 1,0E+16 were investigated. The number of particles for specific residence times along the reaction channel was measured, and the nucleation rates were calculated. The morphology and size of the nuclei were also determined using Cryo-TEM. Experimental data was compared to the corresponding theoretical values and correlations between the data were made. The mass and energy balance for each system was carried out in order to determine the role of each kinetic process.

Cryo_tem.png 

Figure 1. Cryofixation Experimental Procedure

 

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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 4, by Umraan Hendricks
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-22 10:45
Revised:   2013-04-02 13:48