Dissolution-precipitation process

The starting material and shock-activated powder were mixed with 5-wt% MgO and heated for various periods. At the end of each period the phase content of the samples was determined with x-ray diffraction. In this environment it is thought that the phase is formed by a dissolution-precipitation process as shown in Fig. 7.8. As indicated in Fig. 7.9, the shock-activated silicon nitride displays substantially enhanced dissolution rates that are strongly dependent on shock pressure between 22 and 27 GPa. 

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

The initial compositions of both the infiltrating water and the solid materials may change due to their interaction, which in turn may affect the solubility and the pathway of dissolution-precipitation processes with time. When a particular component of the dissolved solution reaches a concentration greater than its solubility, a precipitation process occurs. Table 2.1 includes the solubility of selected sedimentary minerals in pure water at 25°C and total pressure of 1 bar, as well as their dissolution reactions. All of the minerals listed in Table 2.1 dissolve, so that the products of the mineral dissolution reactions are dissolved species. Figure 2.2 shows the example of gypsum precipitation with its increasing concentration in a NaCl aqueous solution. 

The kinetic mass transfer model developed to take into consideration the geochemical evolution of the Cigar Lake ore deposit was mainly done by simulating the evolution of the Al-Si system in the Cigar Lake ore deposit system. To this aim the system formed by kaoli-nite, gibbsite and illite as main aluminosilicate solid phases was considered and kinetics for the dissolution-precipitation processes were taken from the open scientific literature (Nagy et al. 

A 25-ml scintillation vial was charged with the step 1 product (7 g), tin(II)-2-ethyl-hexanoate, and ethylene glycol (7.507 mmol) and then thoroughly shaken using a KEM-Lab vortex mixer at 35 rpm. This mixture was then treated with 4,4 -methylene-bis(cyclohexylisocyanate) (11.262 mmol) and then further shaken by the vortex mixer for 1 minute. The vial was then placed into a heat shaker for 15 minutes and stirred to ensure its consistency and then returned to the heat shaker for 3.45 hours. Half of the hot mixture was removed from the vial and placed into a second vial, which was treated with 15 ml of /V, V-d i met h I ace tam i de and put onto the shaker until the biodegradable elastomer was dissolved. This solution was then precipitated in 1000 ml of water, the dissolution/precipitation process being repeated twice. Thereafter the precipitated polymer was isolated and purified by lyophilization. 

A number of attempts have been made to understand the mechanism of the adsorption of chelates on oxide minerals. For instance, IR spectroscopic studies10 have indicated the presence of a basic monosalicylaldoximate copper complex as well as the bis-salicylaldoximate complex on the surface of malachite (basic copper carbonate) treated with salicylaldoxime. However, other workers4 have shown that the copper chelate is partitioned between the surface and dispersed within the solution, and that a dissolution-precipitation process is responsible for the formation of the chelate. Research into the chemistry of the interaction of chelating collectors with mineral surfaces is still in its infancy, and it can be expected that future developments will depend on a better understanding of the surface coordination chemistry involved. 

Amorphous polyimide powders prepared by dissolution/precipitation processes, can be used to toughen thermosetting polymers. Polyethylene powders are frequently used in low-shrink unsaturated polyester formulations. 

The weathering of minerals can be understood as a continuous dissolution-precipitation process involving them. The process can be very complicated, leading to multiphase equilibria, in which more than one solid phase, solution, and gas phase may be present. For example, primary silicates transform to secondary silicate minerals via such weathering reactions (Stumm and Wollast, 1990), as in the formation of kaolinite (AljSijC OH) from anorthite (CaAl2Si208) ... 

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

Inorganic macrocomponents. Anions in leachate plumes are mainly important due to their ability to form complexes, take part in dissolution/ precipitation processes, and their role as electron acceptors. The formation of complexes may increase the mobility of cahons and heavy metals. In addition, many reactions are inhuenced by pH, which is governed, to a large extent, by the carbonic acid components, in parhcular HCOa". 

A material exhibiting high electrocatalytic activity in the HER was prepared by anodic oxidation of the amorphous alloy FegoCojoSiioBio in 30% aqueous KOH at 70°C (202). A dissolution-precipitation process involving the Fe is involved, giving an active surface oxide that is presumably reduced on subsequent evolution of Hj. A highly porous material results. 

Dissolution-precipitation models. Dubinina and Lakshtanov (1997) developed a kinetic model that describes isotopic fractionation between a mineral and fluid involved in one of three types of dissolution-precipitation processes (Fig. 11). Type I (mineral synthesis) considers successive dissolution of an unstable phase, A, of uniform isotopic composition and precipitation (crystallization) of phase B. Type II (Ostwald ripening) involves the partial dissolution of phase B which has a non-uniform isotopic composition... 

Type III dissolution-precipitation process involves the repeated transfer of mass M of mineral B in a series of steps resulting in a non-uniform isotopic composition. Unlike... 

Depending on the rate of formation and the rate of crystallization, the product may be amorphous or crystalline. Aging or slow growth of amorphous phases may result in a transition from the amorphous to the crystalline state. This process can occur through slow transformation in the solid state or through dissolution-precipitation processes. This is illustrated in the transition from amorphous to crystalline basic nickel sulfate, the former being less corrosion-resistant than the latter. 

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