Precipitation from solution hydrothermal

The behavior of silica and barite precipitation from the hydrothermal solution which mixes with cold seawater above and below the seafloor based on the thermochemical equilibrium model and coupled fluid flow-precipitation kinetics model is described below. 

Deep-sea occurrences (see Velde (1985) for an overview) are varied, but the material is generally associated with basalts. Nontronite seems to be formed directly from basaltic glass weathering at a very low rate in deep-sea environments. It is, however, not directly associated with celadonite. XRD data are sparse, and hence there is always the possibility that there is a tendency to form a mixed layer nontronite/celadonite mineral. Nontronite can be found as a product of precipitation from solution around hydrothermal vents where there... 

Precipitation from solution under hydrothermal conditions has been widely used for the synthesis of fine crystalline particles of various oxides [178-180]. The process involves heating reactants, such as metal salts, oxide, hydroxide, or even metal powder, in the form of solution or suspension, at certain temperatures. Water is the most widely used solvent. In this case, the precipitation temperatures are set between the boiling and critical points of water, i.e., 100-374 °C, while the pressures are up to 22.1 MPa, which is the vapor pressure of water at the critical point. Due to the presence of high pressures, hardened steel autoclaves are usually used to carry out hydrothermal reactions. The autoclaves have inner surfaces of which are lined with a plastic, such as Teflon, to prevent corrosion of the vessels. Similar to chemical precipitation method, hydrothermal synthesis also offers almost unlimited flexibility in combination of types and concentrations of starting reactants, additives, pH levels, temperatures, time durations, and so on. 

Ultra-flne-grained highly reactive yttria powders, suitable especially for the preparation of transparent ceramics, are prepared by various methods including combustion synthesis [293], precipitation [294, 295], hydrothermal synthesis [296], electrospray pyrolysis [297], and sol-gel [298]. In order to improve the dispersion and sinterability of yttria powders, seed crystals are often added [296]. A significant refinement of yttria powders prepared by precipitation from solution may be achieved by the addition of sulfate ions to the reaction mixture [299] (Figure 1.25). 

Precipitation from solution under hydrothermal conditions has been known for decades as a method for synthesizing fine, crystalline oxide particles (68). Interest in the method has increased in recent years because of the need for fine, pure powders in the production of ceramics for electronic applications. The process involves heating reactants, often metal salts, oxide, hydroxide, or metal powder. 

Hydrothermal BaTiOs powders, particularly very fine powders (less than — 100 nm) prepared at lower temperatures, show some structural characteristics that are not observed for coarser powders prepared by solid-state reaction at higher temperatures. X-ray diffraction reveals a cubic structure that is normally observed only at temperatures above the ferroelectric Curie temperature of 125-130°C. The possible causes for the apparent cubic and nonferroelectric structure are not clear and have been discussed in detail elsewhere (74). They include the idea of a critical size for ferroelectricity and, particularly for powders prepared by precipitation from solution, the presence of a high concentration of point defects due to hydroyxl groups in the structure. 

Hydrothermal or solvothermal reactions are also an important method for the synthesis of unconventional zeolite-like compounds. The reagents are usually dissolved or dispersed in water and then heat-treated at elevated temperatures in an autoclave. For reactions at moderate temperatures up to about 250 °C the autoclaves are normally lined with Teflon. If higher reaction temperatures are desired, the reactions are, for instance, carried out in stainless steel or gold tubes. However, in favorable cases, the use of higher temperatures might not even be necessary. The products sometimes just precipitate from solution at room temperature. 

The catalyst is prepared by precipitation from solutions of ferric chloride and ammonium molybdate. The precipitate may not be homogeneous, with significant variations within a single batch. Hydrothermal aging of the precipitate may be necessary to provide a more uniform composition. Precipitation of the catalyst as a gel provides a more uniform ferric molybdate compositioa... 

Precipitation from solutions is also used in hydrothermal synthesis. It is also involved in geochemical phenomena and even some biological processes such as calcification and kidney stone formation. 

It is thought that the precipitation of amorphous silica is caused by conductive cooling from the hydrothermal solution which flows laterally in the chimney (Herzig et al., 1988). 

However, as already noted, the barite content in Kuroko ore inversely correlates to the quartz content and the occurrences of barite and quartz in the submarine hydrothermal ore deposits are different. The discrepancy between the results of thermochemical equilibrium calculations based on the mixing model and the mode of occurrences of barite and quartz in the submarine hydrothermal ore deposits clearly indicate that barite and quartz precipitated from supersaturated solutions under non-equilibrium conditions. Thus, it is considered that the flow rate and precipitation kinetics affect the precipitations of barite and quartz. 

Therefore, it is likely that Ag-rich electrum and large amounts of sulfide minerals including argentite could precipitate due to CO2 loss and pH increase under low /sj and /02 conditions. Therefore, this mechanism (boiling and gas loss from the hydrothermal solution with different /sj, /o2> CO2 concentration and pH) could explain why the Ag content of electrum correlates with Ag/Au total production ratio (Fig. 1.124). 

Farrell and Holland (1983) cited ba,sed on Sr isotope study on anhydrite and barite in Kuroko deposits that the most appealing model for the formation of Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately below above the vent exit in the overlying seawater (Eldridge et al., 1983). The study on the chimney ores from Kuroko deposits support this model which is discussed below. 

Barite and sphalerite tend to precipitate at lower temperature from the hydrothermal solution mixed with a large amount of cold seawater (but mixing ratio (seawater/hydrothermal solution) may be less than 0.2). These minerals precipitate on the seafloor and/or at very shallow subsurface environment. However, chalcopyrite tends to precipitate from high temperature solutions in ore bodies and/or at the sub-seafloor sediments. Usually shale which is relatively impermeable overlies the Besshi-type ore bodies. This suggests that hydrothermal solution could not issue from the seafloor and... 

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes clear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for cluster synthesis in the autoclave. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

The most representative example is seen in the comb-like texture of quartz veins formed by the precipitation of quartz crystals on the wall of a vein from a hydrothermal solution that has entered into small fractures in the rocks (Fig. 8.2). 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

Various fluorides may be precipitated from aqueous solution for use as constituent powders in solid state reactions. Co-precipitation offers very elegant access to intimate mixtures, but the actual products are strongly dependent on the fluoride ion activity within the solution but also on the stability constants of the respective metal complexes. Accordingly, not only anhydrous fluorides are obtained, but also hydrated fluorides or hydroxide fluorides, which may be very difficult to convert to pure fluorides. As noted already [3], reactive compounds, e.g. carbonates, acetates, oxalates, hydroxides etc., which quite easily dissolve in acidic HF solutions, are the preferred starting materials for fluoride syntheses. In contrast, many oxides which have been heated to rather high temperature are frequently unreactive and may not dissolve at all. To enhance reactivity but also improve crystallinity of the product, it has proved useful to perform reactions above the boiling point of water in adapting the hydrothermal method, which has already been shown to be useful in the recrystallisation of materials which are more or less insoluble at ambient temperatures and pressures. Up to about 240°C even PTFE vessels may be used. A number of selected examples with respective reaction conditions are listed in Table 3. 

In contrast to the dry methods, there are other conventional methods related to post heat treatment from precipitates from the aqueous solutions. The ambient aqueous routes and the hydrothermal methods at elevated temperature usually lead to rare earth precipitates like hydroxides, carbonates, instead of oxides. In fact, the hydrothermal treatment of rare earth oxide powders results in a hydration process to form hydroxides. Subsequently, the precursor could be used to produce rare earth oxide nanocrystals with post annealing at varied temperatures and in appropriate atmosphere. 

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