Page 203 - Industrial Ventilation Design Guidebook

4,4 WATER PROPERTIES AND TREATMENT i 6S

6. The value of the recovered solids or liquids
7. The volume of materials to be treated

The following list provides an indication of the various techniques on
which the separating methods are based.

• Relative vapor pressure of the solid and liquid
• The phase relationship between the solid and liquid
» The relative solubility of immiscible solvents
• Reduction of solubility
• Chemical precipitation
• Ion exchange
• Electrolytic deposition
• Adsorptive properties

The separation of solids in suspension in liquids can be achieved by either
of the following techniques
• Density difference
• The cross-section of the solid particulate matter
• The electrostatic properties of the solid

Next we briefly consider each of the above in turn.

Relative Vapor Pressure of Solid and Liquid. If the dissolved solids in a
liquid have a low vapor pressure relative to the liquid in which they are dis-
solved, provided the solid is not affected by the liquid boiling point, it is an
easy matter to vaporize the liquid, leaving a dry residue.

Phase Relationship between the Solid and Liquid. A phase relationship
may involve a number of crystalline forms from which materials can be sep-
arated. When a solid material is precipitated as a result of the solution be-
coming supersaturated, crystallization occurs. Crystallization may be
achieved by
• Cooling alone
• Concentration
• Concentration followed by cooling
• Simultaneous concentration and cooling

Relative Solubility of Immiscible Solvents. Many solid materials in so-
lution can be removed by transferring them to a second solvent; it is essential
that the solvents be mutually insoluble.
This approach will not produce a solid; it can, however, be used to remove
a solid from one solution or solvent and transfer it to another, from which it
can be readily removed.

Reduction of Solubility. It is possible to remove a solid from a solution
by changing the condition of a solvent. One method is the addition of a sec-
ond solvent miscible with the first, in which the solid in solution is relatively