Clifford Y. Tai

Morphology and growth rate of calcium carbonate crystals in a gas-liquid-solid reactive crystallizer

The polymorphism of calcium carbonate crystal was studied by introducing CO2 gas through a gas sparger or a double-tube gas injection nozzle into a pH-stat crystallizer containing calcium chloride solution. Several operating variables were investigated and the pH of solution and calcium ion concentration were the important factors that affect the formation of polymorphs of calcium carbonate. At ambient temperature either calcite or vaterite was the major product, depending on the pH and calcium ion concentration. Then the same crystallization system was employed to study the growth kinetics of calcite crystals, using a double-tube gas injection nozzle instead of a gas sparger, which cannot be used to suppress nucleation, to introduce the CO2 gas into the crystallizer. The growth rates of calcite seeds increased with increasing supersaturation and crystal size. As a conclusion, the polymorphs and growth rate of calcium carbonate are controllable in a gas-liquid-solid reactive crystallizer, using a pH-stat operation mode.

Synthesis of spherical zirconia by precipitation between two water/oil emulsions

Abstract Microparticles of ZrO 2 are produced by using precipitation method between two emulsion solutions. First, two solutions of stable reverse emulsion (water-in-oil) are prepared and mixed to form gelled precipitates, using normal heptane as the continuous oil phase and aqueous solutions of zirconium oxyacetate and aqueous ammonia as the suspending droplets. Through a series of operations, including distillation, filtration and washing, the dried precursors are obtained. After calcining the precursors at 750°C, ZrO 2 powder with a tetragonal structure is obtained. Principle factors that influence the emulsion stability, which subsequently affects the morphology and particle size of ZrO 2 powder, are investigated, including the type and concentration of surfactant, volume ratio of water/oil, concentration of solute in water phases, and mixing intensity and time for emulsion formation. Four kinds of anionic surfactants are put to test for emulsion stability; among them Span 40 and Span 80 are considered as suitable surfactants for producing spherical microparticles of ZrO 2 , which has a size range from several hundred nanometers to micrometers depending on the synthesis conditions. ©

Interfacial supersaturation, secondary nucleation, and crystal growth

A theory describing the source of nuclei in secondary nucleation is presented and used to rationalize experimental data from the literature, some of which had appeared to be conflicting. The theory rests on a model in which an adsorption layer consisting of clusters of growth units of varying size is formed on the surface of growing crystals. The existence of the layer is related to the two-resistance model of crystal growth; by varying system conditions, the relative importance of the two resistances is altered and thereby changes the interfacial supersaturation even though overall supersaturation remains constant. Interracial supersaturation and contact energy determine kinetics in a system dominated by contact nucleation.

Kinetics study on supercritical fluid extraction of zinc(II) ion from aqueous solutions

Abstract This work studies the kinetics of metal ion extraction using an in situ chelation-SFE method with Cyanex 302 as the chelating agent, which is commercially available, and supercritical carbon dioxide as the solvent, which extracts the metal-chelate complex from aqueous solutions. Zinc(II) ion is studied as a model species. The experiments are conducted in a 1.3-dm3 batch stirred tank. The effects of pressure and stirring rate were investigated. The extraction efficiency at 313 K varies from 50 to 60% when the pressure varies from 8.3 to 13.8 MPa. A simplified model is used to estimate the effective mass-transfer coefficient. The results show that the extraction rate of zinc(II) ion increases with an increase in stirring rate, but decreases with pressure. The effective mass-transfer coefficient at 313 K and 8.3 MPa varies from 0.45 to 2.6×10−3 s−1 when the stirring rate increases from 7.2 to 17.7 s−1.

Crystal growth kinetics of two-step growth process in liquid fluidized-bed crystallizers

The fluidized-bed crystallizers have long been used in the chemical industry and recently for water treatment as the so-called pellet reactors. This report summarizes the experimental results regarding hydrodynamics and crystal growth rates in laboratory-scale fluidized-bed crystallizers. The two-step crystal growth model rather than the over-all model is adopted to explain the observed kinetic behaviors. In using the two-step growth model, the disadvantage is the uncertainty of the surface-reaction order. A reliable method is suggested to explore the surface-reaction order using a lean fluidized-bed crystallizer. The crystal growth process of sparingly soluble salts, which have been studied recently for a few systems, seems more complex than that of soluble salts. Then, a comparison of crystal growth kinetics of soluble salts is made between the dense and lean fluidized-bed crystallizers. As a conclusion, the two-step growth model is suitable for the estimation of crystal growth rates in the design of a liquid fluidized-bed crystallizer.

Effects of operating variables on the induction period of CaCl2–Na2CO3 system

The induction period is defined as the time elapsed between the creation of supersaturation and the formation of critical nuclei. A novel data acquisition system is developed in this experiment to determine the induction period on the desupersaturation curve. The effect of several operating conditions, including initial reagent concentration, temperature, pH, and presence of additive and seeds on the induction period of CaCO3 were studied experimentally. The results showed that the induction period decreases with an increase in initial reagent concentration, temperature, and pH of the solution. The presence of Mg2+ in solution prolongs the induction period. On the other hand, the presence of Na+ in solution has little influence on the induction period. Further, the presence of seed crystals in solution shortens the induction period.

The induction period of the CaCl2–Na2CO3 system: Theory and experiment

The induction period, tind, of CaCO3 precipitated from an aqueous CaCl2–Na2CO3 solution is investigated both experimentally and theoretically. The tind of primary nucleation is measured by applying a conductivity method and estimated by using the rapid coagulation theory. By using a novel data acquisition system in the experiment, tind can be easily estimated from the desupersaturation curve, which is a plot of solution conductivity against time. The desupersaturation curves obtained at various levels of supersaturation reveal the important features at different desupersaturation stages, including clusters coagulation, formation of critical nuclei, growth of critical nuclei, detection of visible crystals and equilibrium stage. These individual stages are affected greatly by the initial supersaturation of solution. An oscillation in conductivity, which arises from the fluctuation of solution concentration, is observed between tind and tv, tv being the instant that visible crystals are detected. A compariso...

Interpretation of crystal growth rate data using a modified two-step model

Abstract A two-step model with the surface integration step represented by the BCF equation was proposed to analyze the growth rate data of organic α-glutamic acid and inorganic magnesium sulfate crystals in pure and impure solutions. It was found that the parabolic law and the linear law which characterize the BCF equation were essentially obeyed by these two systems.

Crystal growth kinetics of the two-step model

Abstract The single crystal technique was used to measure the growth rate of the potassium alum (111) face and the magnesium sulfate (110) face. The two-step model was found appropriate to describe the growth kinetics with the surface integration order of two for potassium alum crystal and of one for magnesium sulfate crystal. The individual rate constants, K d and K r , were determined accordingly.

Crystal growth kinetics of calcite and its comparison with readily soluble salts

The crystal growth kinetics of calcite was investigated in batch crystallizers of stirred tank and fluidized-bed type, which were maintained at a constant pH by an autotitrator. Rhombohedron seed crystals were prepared using different techniques, then the growth experiments were conducted in the metastable region, which was explored as part of this research, to suppress nucleation. The crystal growth rates were evaluated from the consumption rates of sodium carbonate and calcium ion for the stirred-tank and fluidized-bed experiments, respectively. Several operating variables were investigated and the crystal growth rate data of constant pH and ionic strength were analyzed by the two-step crystal growth model. The mass-transfer and surface-reaction coefficients were thus obtained and used to explain the growth behaviors of calcite crystals. Then, the crystal growth kinetics of this sparingly soluble salt was compared with that of readily soluble salts. Although the effects of pH and ionic strength on crystal growth were not reported for the readily soluble systems, these effects were significant to calcite growth.