Precipitation processes description

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

The formation of the sponge-structured membranes can be easily rationalized utilizing the description of the precipitation process given above. With finger-structured membranes the formation process is more complex and cannot entirely be described by the thermodynamic and kinetic arguments of phase separation processes. 

In the description of the refining of aluminum, the molecular scale depiction of precipitation was not included. Use the web to determine which chemicals are involved in the precipitation process and draw a molecular scale drawing of this reaction. 

In this section the basic principles of membrane formation by phase inversion will be described in greater detail. All phase inversion processes are based on the same thermodynamic principles, since the starting point in all cases is a thermodynamically stable solution which is subjected to demixing. Special attention will be paid to the immersion precipitation process with the basic charaaeristic that at least three components are used a polymer, a solvent and a nonsolvent where the solvent and nonsolvent must be miscible with each other. In fact, most of the commercial phase inversion membranes are prepared from multi-component mixtures, but in order to understand the basic principles only three component systems will be considered. An introduction to the thermodynamics of. polymer solutions is first given, a qualitatively useful approach for describing polymer solubility or polymer-penetrant interaction is the solubility parameter theory. A more quantitative description is provided by the Flory-Huggins theory. Other more sophisticated theories have been developed but they will not be considered here. 

To avoid the cost and disposal problems of once-through processes employing alkali metal compounds, a considerable amount of research and development effort has been expended on techniques for regenerating this type of absorbent. Processes used employ precipitation of insoluble compounds (double alkali), and thermal decomposition (Wellman-Lord and Elsorb). Processes under development or which have been investigated include precipitation of insoluble compounds (zinc oxide), low-temperature reduction of sulfite (citrate and potassium formate processes), high-temperature reduction (aqueous carbonate process), electro-dialysis (SOXAL), and electrolytic (Stone Webster/Ionics Process). Descriptions of some of these processes are provided in subsequent sections. 

Two conceptual models were tested in this study and were implemented in the programs PRECIP (Noy 1990) and CHEQMATE (Haworth Smith 1994). The main difference between these models was whether the dissolu-tion/precipitation processes occurred fast enough for the aqueous phase to be considered to be in equilibrium with the primary and secondary minerals (as implemented in CHEQMATE), or whether a kinetic description of the system was necessary (as implemented in PRECIP). 

CRUD A term used in the nuclear reprocessing industry to describe unidentified materials it is an abbreviation of corrosive radioactive undetermined deposit. It is usually used in the description of precipitation processes involving fission products. 

Technology Description The function of reduction processes is to convert inorganics to a less toxic and/or more easily treated form. It also serves as a pretreatment step for inorganics in which chemical precipitation is used to remove the metal hydroxide from solution. 

Although both of these models provide a reasonable description of the precipitation polymerization process, they do not illustrate the relationship between the reactor variables and the polymer particle properties. 

In November 1997, Ferrari (France) invited Solvay to help them to solve a problem of recycling PVC coated textile for light mobile structures, especially ones used for shortterm applications (advertising). The Vinyloop is aphysical recycling process making use of an organic solvent to separate the PVC compound from the other materials in a PVC composite. It is a closed loop process the solvent is completely recycled. Aspects outlined include history and a description of the process, precipitated PVC compound evaluation and financial data... 

In cases of both the ionic and non-ionic homogeneous reduction processes, a reducing agent figures which oxidizes itself and, consequently, results in the precipitation of a metal. This description may wholesomely be conveyed by the following scheme ... 

The counterparts of dissolving particles are the processes of precipitation and crystallization the description and simulation of which involve several additional aspects however. First of all, the interest in commercial operations often relates to the average particle size and the particle size distribution at the completion of the (batch) operation. In precipitation reactors, particle sizes strongly depend on the (variations in the) local concentrations of the reactants, this dependence being quite complicated because of the nonlinear interactions of fluctuations in velocities, reactant concentrations, and temperature. 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

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