Washing systems

Washing by successive dilution is used when the soHds are separated into a slurry, such as in filter thickeners. The soHds, thickened into a small amount of mother Hquor, are diluted into a wash Hquid and then separated again, diluted, separated, etc until clean of mother Hquor. The consumption of the wash Hquid can be reduced in countercurrent washing systems, sometimes referred to as countercurrent decantation. Cocurrent dilution washing, however, can be built into some dynamic filter-thickeners such as the Escher-Wyss filter. 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Sodium carboxymethylceUulose, NaCMC, greatly reduces redeposition in cotton-washing systems based on synthetic surfactants. It is effective at remarkably low concentrations of ca 1% of the standard washing compositions used at ca 0.1 to 0.2% in the bath. Thus, ca 0.001—0.002%, or 10—20 ppm NaCMC is sufficient to significantly inhibit redeposition. 

Specific gravity is the weight of the fuel in relation to water. This property is important in the design of centrifugal fuel washing systems. Sulfur content is important in connection with emission concerns and in connection with the alkali metals present in the ash. Sulfur reacting with alkali metals forms compounds that corrode by a process labeled sulfidation. 

Wet ESPs add to the complexity of a wash system, because of the fact that the resulting slurry must be handled more carefully than a dry product, and in many cases requires treatment, especially if the dust can be sold or recycled. Wet ESPs are limited to operating at stream temperatures under approximately 80 to 90°C (170 to 190°F), and generally must be constructed of noncorrosive materials (EPA, 1998 Flynn, 1999). 

This unit is good to pre-thicken feed to centrifugal filters and similar applications. One cyclone may satisfy a requirement, or the units can be arranged in parallel for large capacities or in series for removal of extreme fines. See Figure 4-53 for a counter-current wash system. Solids as small as 10 microns can be separated. 

Figure 4-53. Cyclones used for countercurrent washing system. By permission, Krebs Engineers.

Install a water-wash system for dilution and removal of cyanide from the unit. Cascading wash water from the high-pressure zone back to the main column overhead or to the first-stage wet gas compressor outlet is attractive, but it is better to use overhead water and pump it from low pressure to high pressure. 

Trend changes in the molecular weight of the gas at the first-stage suction. Verify that overhead cooling and wash systems are in order. 

After mixing for an appropriate time, clean soil particles and wash water containing the dissolved and suspended contaminants are separated. Separation techniques in soil-washing systems are similar to those applied in the mineral-processing industry.78 The most common separation techniques are as follows ... 

The big difference in application from the in situ flushing method is that this ex situ method can apply to soils with lower permeability, because soil is excavated and can be sufficiently washed. The following presents two ex situ soil washing processes for organic contaminants the BioGenesis soil cleaning process and the BioTrol soil washing system. 

FIGURE 18.16 BioTrol soil washing system process diagram. 

These difficulties have been thoroughly discussed in an exhaustive study [174]. Theoretical considerations were thoroughly explored and the economic and technical merits of four different washing systems were analysed ... 

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