Hydrothermal chemical reactions

Low-temperature treatment of low-level mixed wastes has also been accomplished by solidification/stabilization with chemically bonded phosphate ceramics (CBPC). These are made by hydrothermal chemical reaction rather than by sintering. Chemical bonding develops when acid phosphates react with oxides to form crystalline orthophosphate (Singh et al. 1997). The ceramic matrix stabilizes the wastes by microencapsulation. The low temperature of the reaction allows volatile radionuclides to be treated (Singh et al. 1997). 

Alteration mineral assemblages characterize the many varied hydrothermal-chemical reactions that occur in active geothermal systems. The effect of chemical and temperature conditions on the occurrence of secondary minerals is well known, based on field observation and mineral synthesis laboratory experiments. It is less clear, however, how specific chemical species become enriched in one part of an active hydrothermal system. 

Because the chemical reaction is faster at higher temperature, aging can be accelerated by hydrothermal treatment, which increases the rate of the condensation reaction (8). 

The flow of hydrothermal solutions iato the oceans from hydrothermal vents, ie, springs coming from the sea floor ia areas of active volcanism, and the chemical reactions occurring there by high temperature alteration of basalts ate of significance ia the mass balance of and. Eurthermore,... 

For the formation of hydrothermal deposits the following are essential (i) the availability of mineralising solutions capable of dissolving and transporting mineral matter, (ii) the availability of openings in rocks through which the solutions may be channelled, (iii) the availability of suitable sites for deposition and localisation of ore minerals, (iv) chemical reactions that result in deposition, and (v) sufficient concentration of mineral matter to constitute economic deposits. 

Lichtner, P. C., 1985, Continuum model for simultaneous chemical reactions and mass transport in hydrothermal systems. Geochimica et Cosmochimica Acta 49, 779-800. 

Liquefaction of wet biomass streams is done by hydrothermal processes at elevated pressures. The feed stocks for these high pressure liquefaction processes are slurries of biomass particles and water. Feeding these slurries into the high pressure equipment, at reasonable costs, is an important hurdle in the process development. For example, the reported operating conditions for high pressure liquefaction are in the range 280-360 °C and 90-250 bar [25, 26]. Under these conditions, biomass is converted, in a complex sequence of chemical reactions, into various compounds. Upon cooling, the reactor effluent consists of three... 

The chemical reactions that occur in hydrothermal systems are largely the result of interactions between seawater and relatively yoimg ocean crust. During these reactions, some elements are solubilized and released to seawater as ions or gases. Others are precipitated, forming minerals that end up as a component of new oceanic crust or the metalliferous sediments. For some elements, the resulting elemental fluxes rival those associated with river input, making hydrothermal activity a very important process in the crustal-ocean-atmosphere factory. 

CHEMICAL REACTIONS THAT OCCUR IN HYDROTHERMAL SYSTEMS... 

Chemical Reactions That Occur in Hydrothermal Systems 481... 

In addition to the hydrothermal controls, the elemental cycles of Ca and Mg are also linked to those of CO2 and Si02 because the mobilization of Ca and Mg during terrestrial weathering from silicates, such as plagioclase, involves consumption of CO2. In the case of calcium, the net chemical reaction is... 

Another potential source of energy supporting primary production at the ridge crests vents is radiation. Two types of radiation have been detected around hydrothermal systems (1) visible light probably generated by chemical reactions, bubble formation, and... 

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

In solid phase reaction s3mthesis, there are three types of chemical reactions oxidation or reduction of a solid, thermal decomposition of a solid, and solid state reaction between two t3 s of solid. With liquid phase ssmthesis of ceramic powders, there are five different methods drying of a liquid, precipitation, sol-gel sjmthesis, hydrothermal S5m-thesis, and reactions of a liquid metal melt with a gas to give a solid ceramic. There are basically three operational principles for precipitation temperature change, evaporation, and chemical reaction. Sol—gel... 

Solid state reactant Thermal decompositions of a solid Oxidation or reduction of a solid Precipitation Solution heating or cooling Evaporative salting-out Chemical reaction with insoluble product Hydrothermal synthesis Forced insolubility Dissolution reprecipitation Evaporative Condensation Gas phase reaction with solid product Thermal decompositions Oxidation or reduction reactions Combination reactions with a solid product Solvent removal Spray drying Freeze drying Spray roasting Sol-gel synthesis Melt solidification... 

Solvothermal process is now becoming a powerful technique for preparing nanomaterials. It is analogous to hydrothermal synthesis, except that non-aqueous solvents replace water as reaction medium. From the chemical reaction point of view, solvents in supercritical conditions play a significant role in reaction and crystallization. New materials, especially those having metastable phases and special nanostructures, can be obtained under mild conditions. By sealing the reaction system in an autoclave, the reactants and products prevent effectively from oxidation, hydrolysis and volatilization, and the reaction and crystallization can be realized synchronously. 

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