CH621753A5 - Process for the slow decarbonation of water - Google Patents

Description

The object of the invention is to improve a method of the type mentioned with simple means in such a way that the most complete possible reaction and removal of the calcium carbonate is achieved with a sharp reduction in the total treatment time.

The process according to the invention is characterized in that a finely divided crystalline material is added to the water at least temporarily before passing through the reaction zone.

The finely divided crystalline material added acts as a seed material and is able to accelerate the equilibrium in the subsequent reaction zone, so that a sufficiently rapid and complete reaction takes place before the addition of the salts of trivalent metal ions. The finely divided crystalline material, especially calcium carbonate, can be external to the system, e.g. in the form of finely powdered marble or similar minerals. However, it is particularly advantageous and cost-saving if the finely divided crystalline calcium carbonate is obtained from the decarbonization process itself. For this purpose, a portion of the calcium carbonate formed is allowed to settle in or behind the reaction zone, but before the addition of trivalent metal ions, and is returned in the reflux to the beginning of the reaction zone or to the mixing device. Foreign material in the form of powdered marble or the like then only needs to be added at the beginning of the start-up process.

The finely divided crystalline material and / or the alkaline compound are preferably mixed into a partial stream of the raw water flowing in and this is then combined with the main stream. In this way, the addition of stoichiometrically precisely dosed and relatively low concentrations corresponding amounts is possible in a particularly advantageous manner. Ca (OH) 2 in the form of lime water (solution) or lime milk (suspension) can in particular be added as an alkaline compound. Another advantageous possibility is to produce Ca (OH) 2 by directly adding CaO (undeleted lime) to the incoming raw water or a partial stream thereof. This saves the previous extinguishing of the burnt lime, the volume of the material to be added is less and the increase in temperature caused by the heat of reaction is quite desirable.

Since the method according to the invention ensures a very complete reaction sequence in the reaction zone and thus calcium carbonate deposits in downstream system parts are avoided, it is possible in a further embodiment of the invention that the sedimentation of the s

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The flocculant containing trivalent metal ions is added after passing through a flocculation zone in a high-performance sedimentation device, in particular an inclined plate or tube separator.

The invention is explained in more detail below with the aid of a few exemplary embodiments.

1 shows a flow diagram of an embodiment of the method according to the invention.

Fig. 2 shows the flow diagram of a modified embodiment of the method.

According to FIG. 1, the raw water flowing in at 1 reaches a mixing cyclone 2, in which it is mixed with lime milk from a container 3 and with finely divided crystalline calcium carbonate, e.g. Marble dust, from a container 4, is mixed. The water then reaches a reaction zone 5 with several, e.g. four reaction stages connected in series, in which it is thoroughly mixed each time. The water then passes into a flocculation zone 6, where a flocculant in the form of one or more salts of trivalent metal ions, in particular FeCb, is added to it from a container 10. If necessary, the flocculant can also be added to the last stage of reaction zone 5. After a sufficient time for flocculation in the flocculation zone 6, the water reaches a sedimentation zone 7, where the flakes settle as sludge, which consists in particular of CaCC> 3, Fe (OH) 3 and optionally Al (OH) 3 and other compounds can. The sludge is drawn off at 8, the softened water runs off at 9 and can optionally be subjected to further purification and treatment stages, e.g. a sand or gravel filter. Flocculation zone 6 and sedimentation zone 7 can be made from pools with special fittings to improve the flow conditions, sedimentation zone 7 also from a high-performance sedimentation unit, e.g. Plate or tube separators, which are operated in cocurrent or countercurrent.

It has been shown that the addition of finely divided crystalline CaCCb in amounts of only 5 g / m3, but preferably more, in conjunction with the calcium hydroxide added in concentrations of about 0.5 to 2 g / 1 to a very sharp reduction in Reaction and flocculation times result, so that a complete reaction and flocculation in the system can be achieved even with water in the system for far less than an hour. This is in marked contrast to the previous processes, in which dwell times of many hours and even days can be expected, especially at low temperatures.

621753

In the embodiment according to FIG. 2, only a partial stream 1 of the raw water reaches the mixing cyclone 2, where it is mixed with lime water or milk of lime from the container 3. The main stream 11 of the water is bypassed the mixed cyclone and reunited with the partial stream in the mixed cyclone 12 before it flows through the multi-stage reaction zone 5, the flocculation zone 6 with the addition of the flocculant from the container 10, and the sedimentation zone 7. Even before the addition of the flocculant of 10, calcium carbonate formed in the last stage of the reaction zone 5 (if appropriate also in earlier stages) is drawn off in a sedimented manner and likewise led via line 13 to the mixing cyclone 2. Crystalline calcium carbonate, which originates from the process itself, is thus continuously fed to the partial flow of raw sewage. Only at the beginning can the addition of foreign calcium carbonate, e.g. Marble dust, may be required.

Of course, the metering into a branched partial stream of the raw sewage can also be carried out in the embodiment according to FIG. 1.

In a further variant which is possible in both embodiments, solid Ca (OH) 2 or solid CaO (undeleted lime) in finely ground or coarse-grained form is added to the raw cyclone 2 in the mixed cyclone 2 instead of finished lime water or milk of lime. When using solid CaO, the alkaline calcium hydroxide required for the reaction zone only forms in the water. This results in the advantages mentioned above.

In both embodiments, a filtration zone, for example a gravel bed filter, can advantageously be provided instead of the sedimentation zone 7.

The reaction line, which in the usual way consists of successively mixed tanks or containers connected in series, can be provided with special devices for removing the calcium carbonate which has precipitated beforehand and is to be returned in the return flow. The reaction zone 5 can also be designed as a high-performance sedimentation unit in the embodiment according to FIG. 2. The return of the calcium carbonate obtained in the process via line 13 enables the fine crystalline calcium carbonate in concentrations of e.g. Add 200 to 500 g / m3 to the raw water, which gives a particularly good and quick equilibrium and complete reaction in reaction zone 5 with residence times of well under an hour, even at low temperatures. While calcium carbonate is the fine crystalline compound preferably added according to the invention, the use of other fine crystalline materials, such as e.g. Silver sand, calcium magnesium carbonate or the like, conceivable.

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1 sheet of drawings

Claims (11)

621753 1. A process for the long-term carbonation of water, in which the calcium compounds contained in the water are converted into CaC03 by adding one or more alkaline compounds as precipitants and then passing through a reaction zone, and this is then sedimented after addition of a flocculant containing trivalent metal ions characterized in that a finely divided crystalline material is added to the water at least temporarily before passing through the reaction zone. 2. The method according to claim 1, characterized in that the finely divided crystalline material is CaC03. 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that the finely divided crystalline material is added to the water simultaneously with the alkaline compound. 4. The method according to claim 2, characterized in that the CaCCb in powdery form, for. B. powdered marble is added. 5. The method according to claim 2, characterized in that CaCCb obtained from the decarbonization process itself by sedimentation before addition of the flocculant is continuously drawn off and admixed to the incoming water in the reflux. 6. The method according to claim 4, characterized in that CaC03 not originating from the decarbonization process is only added at the beginning to start up. 7. The method according to any one of claims 1 to 6, characterized in that the finely divided crystalline compound and / or the alkaline compound is mixed into a partial stream of the incoming raw water and then combined with the main stream. 8. The method according to any one of claims 1 to 7, characterized in that Ca (OH) 2 in the form of lime water or milk of lime or in solid form is added as an alkaline compound. 9. The method according to claim 7, characterized in that the Ca (OH) 2 is mixed in concentrations of 0.5 to 2 g / 1. 10. The method according to any one of claims 1 to 7, characterized in that serves as an alkaline compound Ca (OH) 2, which is generated by adding CaO (undeleted lime) in the inflowing raw water or a partial stream thereof. 11. The method according to any one of claims 1 to 10, characterized in that the sedimentation takes place after adding the flocculant and passing through a flocculation zone in a high-performance sedimentation device, in particular inclined plate or tube separators or in a filter, in particular gravel filter. Long-term carbonation (in contrast to the rapid decarbonization carried out in a reactor under pressure) is a softening process in which the water contains an alkaline compound to remove its carbonate hardness, usually Ca (OH) 2, optionally NaOH or Na2C03, is admixed, the water then passes through a reaction zone in which the calcium compounds present, in particular the soluble Ca (HCOs) 2, are converted into insoluble CaC03 and this is then added by adding a flocculant containing trivalent metal ions, for example FeCb, is flocculated and sedimented as sludge in a sedimentation stage or can be separated, for example, in a gravel filter. A major problem here is to allow the reaction to proceed to completion within the reaction zone in such a way that all calcium has been converted into calcium carbonate and is then caught by the subsequent flocculation and precipitation. Otherwise, calcium carbonate can be deposited in downstream system parts, pipelines, filters, heaters, etc. In plants for long-term carbonation known to date, a sufficient reaction has only been achieved either by significantly increasing the temperature of the raw water to be decarbonized or by adding the salts of trivalent metal ions in combination with a residence time of several hours in the flocculation to precipitate the calcium carbonate from supersaturated solution - and used sedimentation system. But even here, especially at low temperatures around or below 5 ° C, it can happen that the reaction remains incomplete even when several reaction stages are connected in series and mixed well and calcium carbonate, which has not been recorded, separates out in downstream system parts or pipes.