Civil Engineering Reference
In-Depth Information
step in the recalcination process is the burning of softening sludges at a temperature
of 1,850
F (1,010
C). The reactions during recalcination are:
CaCO
→
CaO
CO
(23-14)
3
2
Mg(OH)
→
MgO
H O
(23-15)
2
2
Since the lime-soda process produces approximately two parts of lime for every part
of calcium carbonate applied, it is theoretically possible to recover up to twice as
much lime as originally used. In practice, the yield is somewhat reduced by side
reactions involving impurities and inefficiencies associated with preliminary dewater-
ing, as well as capture of the recalcined lime.
12
Among the factors that lead to decreased lime recovery, the effects of magnesium,
silica, and typical surface water suspended solids must be considered. Of these ma-
terials, the magnesium content of the raw water is the most important. Softening
sludges can be recarbonated to redissolve magnesium selectively. Magnesium oxide
recovered from recalcination will not slake, but will pass through the softening process
as the oxide. It has been noted that one part of silica will combine with six parts of
calcium oxide to form an inert complex that will not slake.
27
A significant loss of lime
can occur if high concentrations of silica are evident.
Calcium carbonate is a high-density solid, whereas the hydroxides of metals such
as magnesium, iron, and aluminum are more light and flocculent by nature. These
three metal hydroxides are undesirable contaminants in a lime recalcination process.
Lime recalcining plants generally consist of the following components:
12, 15
Sludge thickening from an initial 3 to 10 percent solids, to 18 to 30 percent
•
Recarbonation using stack gases, 15 to 27 percent CO
2
, to redissolve magnesium
hydroxide selectively
•
Dewatering, usually by centrifuges, to 45 to 65 percent dry solids by weight
•
Flash dryers and cyclone separators using hot off-gases from the recalciner
•
Recalcining furnace
•
Available furnace types include the rotary kiln, the flash calciner, the fluidized-bed
calciner, and the multiple-hearth calciner. Features of some existing lime recalcining
plants are summarized in Table 23-20.
15
Recalcining yields a calcium oxide product of approximately 90 to 93 percent purity
at a fuel rate of about 8.5 to 12 bil Btu / ton (8,967 to 12,660 bil J / 907 kg).
24
High
energy use apparently has limited use of the recalcination process.
ULTIMATE SOLIDS DISPOSAL
Surveys of Disposal Methods
Water treatment plant sludges historically have been discharged either directly or in-
directly into a surface water. In 1953, 92 percent of 1,600 coagulation and softening
plants surveyed disposed of their sludges in streams or lakes.
71
A 1969 survey of 80
primarily large plants showed that the disposal of water treatment plant wastes in
surface waters had decreased to 39 percent for softening plants and 49 percent for
