Separation of dissolved solids

On an industrial scale, evaporation and crystallization are the main processes used for the recovery of dissolved solids from solutions. 

Membrane filtration processes, such as reverse osmosis and micro and ultra filtration, are used to filter out dissolved solids in certain applications see Table 10.9. These specialized processes will not be discussed in this book. A comprehensive description of the techniques used and their applications is given in Richardson et al. (2002), Scott and Hughes (1995), Cheryan (1986), McGregor (1986), and Porter (1997). 

Evaporation is the removal of a solvent by vaporization, from solids that are not volatile. It is normally used to produce a concentrated liquid, often prior to crystallization, but a dry, solid product can be obtained with some specialized designs. The selection of the appropriate type of evaporator is discussed by Cole (1984). Evaporation is the subject of a book by Billet (1989). 

Various evaporator designs have been developed for specialized applications in particular industries. The designs can be grouped into the basic types described in this section. 

Direct-heated evaporators include solar pans and submerged combustion units. Submerged combustion evaporators can be used for applications where contamination of the solution by the products of combustion is acceptable. 

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Ion exchange, the separation of dissolved solids, is covered in Chapter 18 of Volume 2. Through ion exchange is usually associated with water purification the technique has applications in other industries. 

Reverse osmosis is a demineralization process that relies on a semi-permeable membrane to effect the separation of dissolved solids from a liquid. The semipermeable membrane allows liquid and some ions to pass, but retains the bulk of the dissolved solids. Although many liquids (solvents) may be used, the primary application of RO is water-based systems. Hence, all subsequent discussion and examples will be based on the use of water as the liquid solvent. 

When the analyte is already present in a particulate form that is easily separated from its matrix, then a particulate gravimetric analysis may be feasible. Examples include the determination of dissolved solids and the determination of fat in foods. 

The injected fluids usually have a chemical composition that is different from that of the in situ reservoir fluids, particularly when steam has been separated from the liquid phase and only the separated water is re-injected. The steam phase will be enriched in non-condensable gases, whereas the separated water has a higher concentration of dissolved solids, but a lower gas content. Changes in the chemistry of the produced fluids are to be expected in all fields where re-injection of separated water has been applied. When the same water is recycled, successive steam loss will increase its salinity, which may in turn lead to scaling problems (see Amorsson, 2004). 

Reverse osmosis processes for desalination were first applied to brackish water, which has a lower TDS concentration than seawater. Brackish water has less than 10,000 mg/L TDS seawater contains greater than 30,000 mg/L TDS. This difference in TDS translates into a substantial difference in osmotic pressure and thus the RO operating pressure required to achieve separation. The need to process feed streams containing laiger amounts of dissolved solids led to the development of RO membranes capable of operating at pressures approaching 10.3 MPa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

Extraction or separation of dissolved chemical component X from liquid phase A is accomplished by bringing the liquid solution of X into contact with a second phase, B, given that phases A and B are immiscible. Phase B may be a solid, liquid, gas, or supercritical fluid. A distribution of the com-... 

It is apparent from these results that the onset of passivity is associated with the saturation of the solution in the immediate vicinity of the anode and the separation of the solid. In several instances deposits of this kind have been removed from the anode and identified. Immediately before the metal becomes passive a visible deposit forms on its surface, but it is thrown off, and dissolves in the bulk of the electrolyte, when the electrode is actually passive and oxygen is evolved the electrode then appears quite clean. 

In Exp. 34 an excess of undissolved solid was present in all cases when the solution was saturated. If, however, solutions can be brought to the point of saturation, either by cooling or by evaporation, frequently no solid will separate from the liquid. That is, solutions can exist which contain a larger amount of dissolved solid than is required for saturation at a given temperature. These solutions are called Supersaturated. 

The density of the leachate is a function of the temperature and the concentration of dissolved solids. Leachate with a total dissolved solid concentration of 2 X lO mgL is not uncommon (see Table 1) the density of such a leachate is >1% higher than the groundwater density. Density differences may significantly affect the vertical positioning of the plume just below the landfill. Field observations on the downward movement of the plume are often difficult to separate from the effect of local water table mounds (Christensen et al., 2001). A better understanding of the effects of higher leachate densities in field situations is needed. Density effects could be the major cause of vertical leachate spreading in aquifers since normal vertical dispersion is usually very small. 

The term leaching as used hereafter is taken to mean the dissolving of a constituent of a comminuted material and the separation of the resultant solution from the undissolved portion or residue. Two general methods are in use first, percolation, in which dissolution and the separation of the dissolved material are done coincidently, and second, dissolution by agitation of a finely divided pulp held in suspension and separation of the solids by decantation or filtration or both. 

In the case in which a water-miscible solvent is used, such as ethanol, methanol, or acetone, fractional distillation equipment will be required. This type of equipment also separates any dissolved solids in the wastes, such as those generated when water washing the marc. One potential problem with significant dissolved solids in the distillation feed is that these solids can precipitate out in the distillation equipment as the organic solvent is removed, potentially plugging the distillation column. The... 

There is also a slow filtration variant of filtration used by some water treatment plants, which combines both a physical separation of suspended solids plus some biological consumption or adsorption of undesirable dissolved substances in the water supply in a single-, two-, or three-stage unit [6]. 

FIGURE 7.4 Cross section through the drum and slurry trough of a drum filter, used for the continuous separation of a solid from a liquid suspension. This version includes a water wash for scavenging the last traces of a solution of value. Sodium hydroxide washings are retained in the process by collecting them separately in the filter and using this to dissolve fresh sodium carbonate, and to slake burned lime. (Modified from Codd et al. with permission [35]). 

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