Morphology Of Gypsum Crystals Research Articles

Combined fouling of forward osmosis membranes: Synergistic foulant interaction and direct observation of fouling layer formation

This study investigated the combined fouling by organic and inorganic foulants in forward osmosis (FO) membrane processes. Alginate and gypsum were used as model foulants for organic and inorganic fouling, respectively. A synergistic effect between alginate fouling and gypsum scaling was observed in the combined fouling experiment: the coexistence of foulants caused faster flux decline than the algebraic sum of flux declines due to individual foulants. It was found that the synergistic effect was mainly a result of the aggravated gypsum scaling in the presence of alginate molecules: alginate molecules act as nuclei in gypsum crystal growth, thus significantly increasing the size of gypsum crystal and accelerating crystallization kinetics. Besides, in the presence of alginate, the dominating scaling mechanism switched from bulk crystallization to surface/heterogeneous crystallization. In order to better understand the effect of alginate on the kinetics of gypsum crystal growth, a membrane window cell coupled with a microscope was used to directly observe crystal formation on the FO membrane surface during the fouling experiments. The direct observation results showed that alginate shortens the nucleation time, increases the gypsum growth rate, and changes the morphology of gypsum crystals. Finally, cleaning experiments were performed by rinsing the fouled FO membranes with pure water and continuously introduced air bubbles. It was found that the cleaning efficiency for membranes fouled by combined foulants was lower than that for membranes fouled by individual foulants.

The Nature and Significance of Variations In Gypsum Crystal Morphology In Dry Lake Basins

Depressions in semiarid regions with a gypsiferous geological substrate commonly contain highly gypsiferous sediments, with gypsum that formed either as a synsedimentary precipitate during perennial or ephemeral lake stages or as a diagenetic precipitate during saline mudflat stages. The dry to ephemeral lake basins of the Monegros region in northern Spain are depressions of this type and provide a unique opportunity to examine whether and how conditions of gypsum formation are reflected by gypsum crystal morphology, yielding information that can contribute to a correct interpretation of the depositional environment of gypsum in ancient and modern formations. In most lake basins of the study area, the sediment fill consists of a similar sequence of three lithological units. The lower unit includes fragmented gypsum crystals in some basins, recording clastic reworking, and in other depressions it contains large lenticular crystals, formed as phreatic diagenetic precipitates. The middle unit typically contains synsedimentary gypsum with a non-lenticular morphology, modified by postdepositional processes, but it also comprises synsedimentary lenticular gypsum in some basins. In the upper unit, separated from the underlying interval by a marked hiatus, both synsedimentary and diagenetic gypsum are present, the former including layered gypsum deposits with features such as grading and variations in crystal morphology. The results show that a non-lenticular crystal habit of gypsum occurring within the groundmass (sediment matrix) indicates synsedimentary precipitation. In contrast, a lenticular crystal habit cannot be used in isolation as an indicator of the environment in which the gypsum initially precipitated. Unambiguously late-diagenetic gypsum includes fine-grained precipitates in voids created by soil fauna. These infillings consist of initially non-lenticular crystals, with a tabular-prismatic habit, recording that conditions promoting lenticular crystal growth do not always prevail outside the groundmass in soil-related settings and illustrating the importance of knowing the context of analyzed gypsum occurrences at the scale of a sample.

The growth morphology of gypsum crystals: an experimental study

The growth morphology of gypsum crystals: an experimental study

Hydration reactions and microstructural characteristics of hemihydrate with citric and malic acid

The hydration of calcium sulphate hemihydrate was investigated under several conditions of temperature, pH, water/solid ratio and the addition of citric or malic acid. Isothermal conduction calorimetry and scanning electron microscopy showed that introduction of the acids radically changed the mechanism of hemihydrate reactions and the morphology of gypsum crystals, as a result of adsorption processes divided in two stages. The setting times were reduced as the temperature increased from 20 to 40–60 °C and the time span of heat liberation was prolonged. The acids acted as setting retarders and as microstructure modifiers, producing compact crystals; such alterations induced higher strength.

Effect of phosphonate additive on crystallization of gypsum in phosphoric and sulfuric acid medium

AbstractUnderstanding the mechanisms of growth and inhibition during crystallization of calcium sulfate is of primary importance for many industrial applications. For instance, inhibition of the crystallization process may be required to prevent scale formation in pipes, boilers, heat exchangers, reactors, reverse osmosis membrane surfaces, cooling water systems, secondary oil recovery utilizing water flooding techniques and desalination evaporators, etc. On the other hand, control growth and morphology of gypsum crystals is desired in achieving higher filtration rate and higher productivity of phosphoric acid from phosphate rocks. In this regard, this basic study is carried out to understand effect of Aminotris (methylenephosphonic acid (ATMP) on calcium sulfate dihydrate (gypsum) crystallization. The time elapsed between the achievement of supersaturation and the appearance of a solid phase (termed as induction time) is measured under different supersaturation ratios ranging from 1.018 to 1.979. The data are used to calculate the surface energy, critical nucleus size, and crystal growth rates of gypsum under different conditions. The results show that, the induction time decreases exponentially with increasing the supersaturation ratio. In addition, the surface energy decreases with ATMP compared to the baseline (without ATMP). Interestingly, with addition of the ATMP, the crystals mean and median diameters are found to decrease. The inhibition efficiency ranges from 16% to 59% depending on supersaturation ratio.

The adsorption of additives at the gypsum crystal surface: A theoretical approach: I. Determination of the interfacial bond energies

By applying the concept of connected nets, all gypsum crystal faces are reduced to two-dimensional Ising lattices. For both the (011) and the ( 1 11) face, two possible connected nets are defined, one based on single layer growth and the other on growth in double layers of gypsum molecules. The relative morphological importance of the various gypsum crystal faces, as predicted from their dimensionless critical Ising temperatures, is compared with and concluded to be consistent with the actual morphology of gypsum crystals, except in pure water. From the dimensionless interfacial bond energies at the critical Ising temperature, the interfacial bond energies at room temperature are calculated, using the assumptions that the roughening temperature of the ( 1 11) face is about 100°C and that this roughening temperature is roughly equal to the transition temperature of the Ising model on the ( 1 11) face. The resulting bond energies are used to calculate the interfacial tension of each gypsum crystal face. The overall interfacial tension calculated for a gypsum nucleus is finally compared with the experimentally determined value. On the basis of this comparison it is not possible to discriminate between single and double layer growth of the (011) and ( 1 11) faces.