Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Background of the Invention
This invention relates to a proeess for slow softening
wa~er as distinguished from water softening which takes place
under pressure inside a reactor. As is well ~nown in the art,
slow`softenlng of water is a method for removing the temporary
hardness of water by admixing the water with an alkaline com-
pound, usually Ca(OH)2 or possibly NaOH or Na2CO3, passing
the water through a reaction zone wherein the various calcium
earbonates present in the water, in particular soluble ca~HCO3),
axe ehanged into insoluble CaCO3, contacting such compounds
with a floeculating or preeipitating agent containing trivalent
metal ions, such as FeC13, settling the flocs thus produced
in a sedimentation zone or separating such flocs by means of
a sand filter.
A very difficult problem encountered with the known slow
softening process lies in the fact that the reaction must be
completed before the liquid`leaves the reaction zone. That
is, the total amount of calcium which can be precipitated
3~ must be changed into calcium carbonate so as to be completely
flocclllated and preeipitated in the following stages. Other-
wise, precipitation and sealing will occur inside pipes, duets,
heaters or other plant equipment through whieh the liquid
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subsequently passes. In known slow softening systems satis-
factory reaction will occur only if the temperature of the
liquid to be softened is greatly increased or if, for pre-
cipitation of calcium carbonate from supersaturated solutions,
the aforementioned introduction of salts containing trivalent
metal ions is combined with a retention time of several hours
inside the flocculation and sedimentàtion zones. But even
under these co~ditions and even if a series of successive
flow-through, completely mixed reaction stages are used, it
may occur, especially at temperatures below 5C, that reaction
is incomplete and the calcium carbonate remaining in solution
will later precipitate and scale in subsequent systems, pipes
and ducts.
Summary of the Invention
An object of the present invention is to provide, by
simple means, an improved method of slow softening water which
considerably reduces total treatment time and achieves complete
reaction, precipitation and removal of calcium carbonate. In
accordance with our invention, at least from time to time and
before being contacted with the flocculation or precipitating
agent, the water being softened is admixed with a finel~
divided crystalline material, preferably finely divided
crystalline calcium carbonate. The admixture preferably takes
place before the water enters the reaction zone and preferably
simultaneously with the addition of an alkaline compound.
The admixed finely divided crystalline material acts as
an inoculant or seeding agent which accelerates restoration of
the equilibrium in the reaction zone, whereby a fast and
satisfactory reaction takes place e~en before the salts con-
taining trivalent metal ions are added to the liquid. Thefinely divided crystalline material, in particular calcium
carbonate, may be supplied to the system from an outside source
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and may consist of fine particulate marble or a similar
mineral. On the other hand, our improved softening process
lends itself as a particularly advantageous and inexpensive
source of calcium carbonate. That is, a portion of the calcium
carbonate which is formed in the reaction zone is caused to
settle within or downstream of the reaction zone hut prior to
the addition of the salts containing trivalent metal ions.
This portion of calcium carbonate is then recycled to the inlet
end of the reaction zone or into the mixing stage. Additonal
1~ material such as particulate marble or similar additives
supplied by an outside source will only be needed to start up
the process. The fine particulate crystalline material and/or
the alkaline compound are preferably added to a deflected
portion of the incoming stream of raw water, and after this
operation the deflected portion of the liquid stream is fed
back into the main stream. In this improved manner it is
possible to feed stoichiometrically exact dosages corresponding
to relatively low concentrations. The alkaline compound added
may be ~a(OH)2 in the form of lime-water (solution) or lime-milk
(suspension). As another advantageous alternative CaO (unslaked
lime) may be added directly to the incoming raw liquid, such as
by adding CaO to a 2eflected portion of a stream thereof,
whereby Ca(OH)2 is thus obtained, slaking o lime is avoided,
the volume of the material added is reduced and the temperature
increase resulting from the exothermic reaction produces
desirable results.
The process according to the invention provides for
essentially complete reaction within the reaction zone and
thus prevents calcium carbonate scaling in subsequent treatment
stages of the plant, therefore the flocs obtained by the addition
of a flocculating or precipitating agent containing trivalent
metal ions may be caused to settle beyond the flocculation stage
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in a high-rate sedimentation apparatus, particularly conventional,
inclined sedimentation plates or tubes.
The invention is discussed more in detail in the following,
reerence being made to the accompanying drawings in which:
FIG. 1 is a flow diagram showing one embodiment of the
process according to the invention; and,
FIG. 2 is a 10w diagram showing another embodiment of the
p~ocess.
Detailed Description
Referring now to FIG. 1, the raw water entering at 1 is
ed into a cyclone mixer 2 in which it is admixed with lime-
milk from a container 3 and with finely divided crystalling
calcium càrbonate, such as particulate marble, from a container
4. The water being softened is then caused to enter a reaction
zone 5 comprising a series of successive reaction stages,
wherein the liquid is thoroughly mixed. While we show four
such reaction stages, it will be apparent that the number of
stages may be varied. Water being softened passes into a
flocculation ~one 6 where it is contacted with-a flocculation
2Q agent in the form o one or more salts containing trivalent
metal ions, such as FeC13, with the flocculant being supplied
from a contalner 10. If desired, the flocculant may be dosed
into the last of the successive reaction stages in the
reaction zone 5. After a retention time sufficiently long for
the formation of 10cs, the water being soPtened is admitted
to a sedimentation or filtration zone 7, where the flocs settle
out to form a sludge consisting mainly of CaC03, Fe(OH)3 and
possibly Al(OH)3 and other compounds. The sludge is withdrawn
at 8 while the softened water is discharged at 9 and may then
be fed to further treatment systems, such as a conventional
sand filter. The flocculation zone 6 and the sedimentation or
filtration zone 7 may be in the form of tanks having conventional
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- internal structures therein to improve flow conditions. Also,
the sedimentation or filtration zone 7 may consist of a high-
rate sedimentation unit, having a conventional plate or tube
separator, with the water flowing through according to the
concurrent or countercurrent principle.
It has been found that the addition of finely divided
crystalline CaCO3 even in quantities as small as 5 g/m3, but
preferably in excess thereof, combined with the addition of
calcium hydroxide in the feed concentrations ranging from 0.5
lQ to 2 g/l (grams per liter) will result in considerably reduced
reaction and ~locculation times. Accordingly, total reaction,
flocculation and precipitation throughout the system may be
obtained during retention times totalling far less than one hour.
These results compare very favorably with those obtained in
known processes, in which, especially at low temperatures,
retention times of several hours or even days are customary.
In the embodiment shown in FIG. 2 only a portion of the
incoming liquid stream is admitted to the cyclone mixer 2,
lime-water or lime-milk being added thereto from the container
3. That is, a portion o the main liquid stream 11 is deflected
to the mixer 2 with the main liquid stream 11 bypassing the
cyclone mixer 2 to be mixed or blended again with the deflected
stream in a mixer 12. The mixed liquid then passes successively
through the multistage reaction zone 5, the flocculation
zone 6 where the flocculant is added rom container 10, and
then through the sedimentation or filtration zone 7. Even
before flocculant is added from container 10, calcium carbonate
having formed in the last stage of the reaction zone 5, and
possibly in previous stages, is withdrawn as a sediment and
returned via conduit 13 to the cyclone mixer 2. The stream of
liquid deflected from the main stream is thus continuously
admixed with crystalline calcium carbonate produced in the
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process itsel. An initial addition of calcium carbonate
such as particulate marble, is required only for starting up
the process.
Obviously, dosing of additives into a deflect~d portion
of the main stream of raw water can also be realized in the
embodiment shown in FIG. 1.
As a variation to both the embodiments described herein-
above, the raw water is not mixed with lime-water or lime-
milk but with solid Ca(OH)2 or solid CaO (unslaked lime) in
1~ the form of either finely or coarsely granulated particles
directly dosed into the cyclone mixer 2. If solid CaO
particles are added, the alkaline calcium hydroxide required
in the reaction zone is ormed within the water itself, thus
resulting in the aforementioned favorable conditions.
In all of the embodiments the zone 7 may well be a
sedimentation zone or a filtering zone, for instance in the
form o a conventional sand filter.
The reaction zone 5 which usually comprises a series of
successively arranged mixing tanks or containers may be equi~ped
with conventional devices for the withdrawal of the previously
precipitated calcium carbonate which is then recycled i~to the
process through conduit 13. Also in the embodiment of FIG. 2
the reaction zone 5 may be a conventional high-rate sedimentation
unit. Recycling the calcium carbonate produced in the process
through conduit 13 enables dosage of finely crystalline calcium
carbonate into the raw liquid with concentrations ranging from
200 to 500 g/m3, whereby conditions in the reaction zone are
effectively and rapidly equilibrated and reaction is fully
completed inside the reaction zone. Retention ti~es range well
below one hour even at low temperatures. While the invention
provides for the addition of calcium carbonate as a preferred
fine, crystalline compound, the use of other fine crystalline
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additives such as silver sand (argentiferous sand), calcium
magnesium carbonate, and the like are possible.
While we llave shown our invention in two forms, it will
be obvious to those skilled in the art that it is not so
limited, but is susceptible o various changes and modifications
without departing from the spirit thereof.
