The use of solar heat for the production of Glauber’s salt has been described by
HOLLAND(22,23)
. Brine is pumped in hot weather to reservoirs of 100,000 m2 in area to a
depth of 3–5 m, and salt is deposited. Later in the year, the mother liquor is drained off
and the salt is stacked mechanically, and conveyed to special evaporators in which hot
gases enter at 1150–1250 K through a suitable refractory duct and leave at about 330 K.
The salt crystals melt in their water of crystallisation and are then dried in the stream of
hot gas. BLOCH et al
(24)
, who examined the mechanism of evaporation of salt brines by
direct solar energy, found that the rate of evaporation increased with the depth of brine.
The addition of dyes, such as 2-naphthol green, enables the solar energy to be absorbed in
a much shallower depth of brine, and this technique has been used to obtain a significant
increase in the rate of production in the Dead Sea area.
The submerged combustion of a gas, such as natural gas, has been used for the
concentration of very corrosive liquors, including spent pickle liquors, weak phosphoric
and sulphuric acids. A suitable burner for direct immersion in the liquor, as developed by
SWINDIN(25)
, is shown in Figure 14.16. The depth of immersion of the burner is determined
by the time of heat absorption and, for example, a 50 mm burner may be immersed by
250 mm and a 175 mm burner by about 450 mm. The efficiency of heat absorption is
measured by the difference between the temperature of the liquid and that of the gases
leaving the surface, values of 2–5 deg K being obtained in practice. The great attraction
of this technique, apart from the ability to handle corrosive liquors, is the very great
heat release obtained and the almost instantaneous transmission of the heat to the liquid,
typically 70 MW/m3.