CA1056521A - Process for slow softening water - Google Patents

Abstract

PROCESS FOR SLOW SOFTENING WATER
Abstract of the Disclosure Water having calcium compounds therein is softened by admixing an alkaline compound therewith to change the calcium compounds inside a reaction zone into calcium carbonate. A
finely divided crystalline material is mixed with the water being softened prior to the addition of a flocculating agent containing trivalent metal ions. The calcium carbonate produced in the reaction zone by sedimentation prior to addition of the flocculation agent is continuously withdrawn and recycled into the incoming water to be softened.

Description

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 ~95~

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 ~os~s~
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 l~)S6S2~
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 ` ~63565Z~
- 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 ` ~5652~

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 1~56~;iZ~

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.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for slow softening raw water having soluble calcium compounds present therein, wherein at least one alkaline precipitating agent is admixed with the water to be softened in a mixing zone whereby the calcium compounds are then changed inside a reaction zone through which the water passes after being admixed with the alkaline precipitating agent into insoluble calcium carbonate matter which is sub-sequently changed in a flocculation zone into settleable flocs by the addition of a flocculating agent containing trivalent metal ions, the improvement comprising:
(a) mixing the raw water in addition to said alkaline precipitating agent with finely divided crystalline calcium carbonate in said mixing zone to form seeds for said calcium carbonate matter which newly precipitates from the raw water inside said reaction zone, (b) immediately presettling and discharging at least a part of the insoluble calcium carbonate matter which has been precipitated inside said reaction zone, and (c) recycling said part of the presettled and dis-charged insoluble calcium carbonate matter to said mixing zone as said seeds for the precipitation in said reaction zone.

2. A process as defined in claim 1 in which the water is introduced into a sedimentation zone after said water has been mixed with the flocculating agent and has passed through the flocculating zone.