Unseeded Precipitation Research Articles

Production of Rutile TiO2 Pigment from Titanium Slag Obtained by Hydrochloric Acid Leaching of Vanadium-Bearing Titanomagnetite

An alternative route was proposed for preparing rutile titanium dioxide (TiO2) white pigment. The process involves the digestion of titanium slag with sulfuric acid, subsequent hydrolysis of unenriched titanyl sulfate solution with ammonia, low-temperature calcination, and unseeded crystallization, producing a high-quality rutile TiO2 white pigment. The experimental data indicated that the conversion ratio of dissolved Ti in the unenriched titanyl sulfate solution was highly dependent on the NH3/TiO2 molar ratio. Under the optimal conditions, approximately 92.0% of dissolved Ti in the solution could be converted to metatitanic acid (H2TiO3) that was then calcined at 850 °C to form rutile TiO2 pigment with purity of 98.41% and mean particle diameter of 0.1–0.3 μm. The presence of NH4+ during the hydrolysis improved the stability of the charged H2TiO3 particles and blocked further coagulation. The fast phase transformation at relatively low temperature could be attributed to the presence of self-generated rutile nuclei. In addition, the experimental validation for the unseeded precipitation of ferrous ammonia sulfate ((NH4)2Fe(SO4)2·6H2O) in the crystallization step was demonstrated.

Model reduction for prediction of silver halide precipitation

A fully mechanistic model was developed to simulate the turbulent mixing and precipitation of silver bromide in a semi-batch reactor. Instantaneous primary nucleation, multiple species thermodynamic equilibria, size-dependent growth kinetics, crystal population balances and accurate modeling of turbulent flow in a stirred tank are fully coupled in a multi-scale, Eulerian–Lagrangian approach. Several significant advances are achieved in the course of the model development. First, a novel symmetric geometric length scale is developed to allow resolution of the scales of nucleation, growth, and dissolution in a single model. This significantly improves the model's ability to predict the crystal size distribution, and eliminates a severe numerical instability in the population balance equations. Second, the multi-species equilibria are accurately represented by a linear equation over most of the range of interest. Third, nucleation occurs primarily in the feed plume, while growth occurs primarily in the bulk, although the two processes remain coupled through the mixing model and the population balances. Seeded and unseeded precipitation experiments were conducted to allow model validation. A comparison of the model predictions with the experimental crystal size distributions shows excellent agreement. The model is also able to isolate a weakness in the growth model in the Gibbs–Thompson controlled range, between the lower limit of the experimentally available data and the size of the nuclei. The validated model provides several insights into the precipitation process, and is suitable for further examining mixing effects on the precipitation process.

Kinetics of Precipitation of Calcium Carbonate in Alkaline pH at Constant Supersaturation. Spontaneous and Seeded Growth

The seeded and unseeded precipitation of calcium carbonate from supersaturated aqueous solutions was investigated in a closed system under conditions of constant supersaturation (supersaturation 1.88−3.39) at 25 °C and at pH 9.0 and 10.0. The analysis of our results based on expressions for the supersaturation dependence of the induction time in seeded and unseeded precipitation according to the classical nucleation theory provides additional information on the mechanism of crystal growth and nucleation of calcium carbonate from supersaturated solutions. High pH values and relatively high supersaturations favor the precipitation of vaterite. The induction times were shorter in the seeded precipitation compared to those in the unseeded precipitation and displayed a marked inverse dependence on the solution supersaturation. Crystal growth proceeds via the spiral growth mechanism. The relatively low surface energy calculated for vaterite from the kinetics data, and the very low nucleation rate constant, indi...

Precipitation of calcium and magnesium carbonates from homogeneous supersaturated solutions

Homogeneous (unseeded) precipitation of calcium and magnesium carbonates in the CaO–MgO–CO 2–H 2O system in supersaturated solutions was investigated. The induction period ( τ) of spontaneous nucleation and the composition of precipitated solid phases were determined. The influence of saturation degree (chemical affinity) and Mg 2+/Ca 2+ activity ratio in solution on the kinetics of CaCO 3 nucleation was studied. The order of CaCO 3 homogeneous nucleation equals to 4. The surface energy of CaCO 3 nucleus was measured to be 63 mJ/m 2. The main factor which controls the Mg content in the solid phase being formed during unseeded precipitation of carbonates is the saturation index of MgCO 3 in solution.

Water treatment in a Membrane-Assisted Crystallizer (MAC)

In many cases it is necessary to remove ions from aqueous streams because of environmental reasons or process demands. Crystallization is one of the available techniques for the removal of many unwanted ionic components from aqueous streams. The removal of ions by means of crystallization on foreign seeds offers technical, economical and environmental benefits compared to the unseeded precipitation. Both the Pellet Reactor (PR) and the Membrane-Assisted Crystallizer (MAC) make use of a seeded crystallization. The main difference between these techniques is the relatively small size of the seeds in a MAC, which offers a number of advantages.

Effect of additives on nucleation rate, crystal growth rate and induction time in precipitation

A theory is proposed and general expressions are derived for the nucleation rate, the rate of crystal growth by two-dimensional nucleation and the introduction time in unseeded precipitation in the presence of additives. According to the theory, the additives behave as active centres for nucleation and do not alter the surface free energy and edge free energy of the nuclei by adsorption on them, because of the short lifetime of the nuclei and/or their small surface area. The surface free energy and edge free energy of the nuclei formed on the additives, however, can be different from the surface free energy and edge free energy of the nuclei on the original active centres in heterogeneous nucleation. Induction times in unseeded barium sulphate precipitation are measured as a function of the concentration of an additive, a co-polymer of maleic acid and vinyl sulphonic acid, and interpreted by the proposed theory. It is concluded that this theory describes fairly well the obtained experimental data, which show a stimulation of the precipitation process at very low additive concentrations and a change-over into retardation at an additive concentration of ≈ 0.005 ppm.

Precipitation of barium sulfate: Induction time and the effect of an additive on nucleation and growth

On the basis of recently derived theoretical expressions for the induction time in seeded and unseeded precipitation, nucleation and growth rates of BaSO 4 are separately determined in the absence and presence of a copolymer of maleic acid and vinyl sulfonic acid (PMA-PVS). From the seeded precipitation experiments at different supersaturations, BaSO 4 is found to grow by the mechanism of two-dimensional nucleation and its growth rate is determined. Complete determination of the BaSO 4 nucleation rate is also done via a combined analysis of the induction times in both seeded and unseeded precipitation. The presence of PMA-PVS in the solution appears to retard the growth and to stimulate the nucleation of BaSO 4.

Determination of nucleation and growth rates from induction times in seeded and unseeded precipitation of calcium carbonate

Theoretical expressions for the supersaturation dependence of the induction time in seeded and unseeded precipitation are derived for different mechanisms of crystallite growth. On the basis of the theory a combined analysis of the induction time in both seeded and unseeded precipitation is presented, which allows separate determination of the nucleation and growth rates. Induction times in both seeded and unseeded precipitation of calcium carbonate in the absence and presence of additives are measured and interpreted in the light of the proposed theory. The analysis provides full information about the growth and nucleation rates in the investigated systems.

The Disposition of Silver Released from Soviet Oblako Rockets in Precipitation during the Hall Suppression Experiment Grossversuch IV. Part I: Measurements of Background and a Preliminary Seeding Test

Abstract In association with Grossversuch IV, a program designed to test the Soviet hail suppression method by seeding clouds with AgI from Oblako rockets, a complementary program was conducted by l'Observatoire du Puy-de-Dome and the Desert Research Institute to study the diffusion of the seeding material (AgI) in the clouds, based on the analysis of silver in precipitation. This program covered the summers of 1977 and 1978, and this paper describes the results of measurements of natural background silver concentrations in unseeded precipitation. It also describes a new automatic precipitation collector, five of which were first tested in the field in 1977. A more extensive network of 15 collectors was deployed during two months of the 1978 summer. Based on the analysis of 118 unseeded precipitation samples collected in 1977, the natural background concentration of silver was estimated as 0.9 × 10−11 g mL−1(σ = 0.6 × 10−11 g mL−1). Although the standard deviations overlap, the 1978 season results appear ...

Background Silver Concentrations in Illinois Precipitation and River Water

Abstract This work was undertaken to measure background silver in Illinois precipitation in the absence of any known seeding operations in on near Illinois. The rainfall-weighted mean concentration in samples from several stations was 73 ng l−1. This is somewhat higher than has been found in unseeded precipitation in other parts of North America, but not high enough to preclude wind-blown soil dust as a primary source.