DE1285956B - Process for the desilification of water - Google Patents

Description

It is known that silica from aqueous solutions, e.g. B. of course occurring waters, removed with the help of strongly basic anion exchangers can be, provided the strongly basic anion exchanger by treatment beforehand with alkaline agents, such as. B. dilute sodium hydroxide solution, converted into the OH form have been. In this process, the anions contained in the solution are inclusive the silica is exchanged for OH ions, so that in the treated solution all cations are in the form of their hydroxides. The resulting alkalinity is in many cases, such as B. in the production of boiler feed water, undesirable, so that it is necessary to neutralize the hydroxides formed by adding acid or by subsequent treatment with an anion exchanger in the salt form, such as B. the chloride form, the OH ions against z. B. to exchange Cl ions. Included In this process, all anions present in the solution are exchanged for OH ions is an amount of alkali to regenerate the strongly basic anion exchanger required, which is determined by the amount of total ions in the solution. Since the most waters contain only a small amount of silica compared to the total anions, a very high excess of regenerant is therefore related to this process on the amount of silica removed.

Furthermore, in this process, if solutions with cations, which form sparingly soluble hydroxides are to be decilified, this is necessary Either remove cations or replace them with such cations prior to decilification, whose hydroxides or carbonates are soluble.

It has now been found that the disadvantages of such methods can be avoided if instead of the OH form the anion exchanger is the salt form, z. B. the chloride form is used and the solution to be desiccated before treatment is made alkaline with the anion exchanger.

When treating silicic acid solutions, their phenolphthalein alkalinity greater than half of the methyl organ alkalinity, i.e. p> m, with strongly basic ones Anion exchangers in the salt form surprisingly found an extensive one Silicification takes place, with simultaneous pH reduction due to the addition of silica binding taking place exchange of OH ions against z. B. Cl ions.

It has been suggested to use bicarbonate water to reduce their bicarbonate content with the chloride form of strongly basic anion exchangers to be treated, but this method does not remove silica from the water, since the p-value of natural bicarbonate-containing water for a desiccation with the Salt form of strongly basic anion exchanger is too low.

Example 1 Tap water decarbonized with lime (pH = 10.5; p = 1.6; m = 1.8; SiO2 = 18 mg / l) was transferred to a strongly basic anion exchanger in the chloride form (e.g. polystyrene with divinylbenzene crosslinked, chloromethylated and quaternized by introducing trimethylamine) filtered. When using 1 l exchanger and filtering 201 of the decarbonized water per hour, the following SiO2 values were measured during the filter run: Filtrate | SiO2 1 mg / l 5 2.0 10 1.3 15 1.0 20 0.9 25 0.8 30 1.5 35 2.0 40 2.5 If the non-decarbonised tap water containing bicarbonate (pH = 7.3; m = 3.8; SiO2 = 18 mg / l) is filtered through an anion exchanger of the type mentioned in the chloride form, the SiO2 content of the water remains unchanged.

Example 1 shows that when using the pure salt form of the exchanger the filtrate has a much higher SiO2 content at the beginning, which then increases over time the filtration gradually decreases.

It has been found that it decreases right at the beginning of the filter run SiO2 contents can be achieved if the salt form of the strongly basic anion exchanger is pretreated with small amounts of an alkaline agent.

EXAMPLE 2 First 200 ccm of saturated lime water and then 201 lime decarbonated tap water (p = 1.6; m = 1.8; SiO2 = 16 mg / l) were filtered over 11 of the chloride form of the anion exchanger used according to Example 1. The filtrate had the following composition: Filtrate p-value m-value Silo2 1 mgIl 4 0 0.1 0.35 8 0 0.1 0.35 12 0 0.1 0.35 16 0 0.1 0.3 20 0 0.1 0.25 24 0 0.1 0.25 28 0.05 0.20 0.35 32 0.2 0.35 0.75 36 0.45 0.55 1.0 40 0.50 0.65 1.5 44 0.70 0.80 2.1 The amount of lime water used in this example is very small compared to the total capacity of the anion exchanger used. Larger amounts of alkaline agents can also be used, but the upper limit is to be regarded as the amount which is equivalent to about 50% of the total capacity of the exchanger. Since the total capacity of the anion exchanger used is about 1.2 equivalents per liter, no more than 0.6 equivalents per liter of the anion exchanger of alkaline regenerants should be used. In most cases less than 10ovo of the amount equivalent to the total capacity is sufficient. If the amount of the alkaline regenerant is too high, the residual silica content is high. The most suitable amount for the exchanger and the water to be treated is determined experimentally.

For the actual regeneration of the exhausted anion exchanger can saline solutions such. B.

Saline solution can be used, provided that residual silicic acid contents of about 2 mg / l can be accepted.

If, however, the aim is to achieve a silica content below 1 mg / l, then it is preferable to first clean the exhausted anion exchanger with alkaline agents, z. B. dilute sodium hydroxide solution, and then with saline, z. B. saline solution, to treat.

Example 3 11 anion exchangers loaded with silica of the type used according to Example 1 were regenerated after backwashing with 1 liter of 50% sodium chloride solution and then with 20 liters of calcium carbonated tap water per hour (p = 2.3; m = 2.5; SiO2 = 16 mg / l) applied. The filtrate had the following composition: Filtrate SiO2 p-value m-value 1 mg / l 4 0 0.05 3.0 8 0 0.05 2.5 12 0 0.05 2.0 16 0 0.05 1.3 20 0.05 0.1 1.0 24 0.05 0.1 1.1 28 0.1 0.2 1.5 32 0.5 0.6 1.8 36 0.7 0.9 2.1 Example 4 11 anion exchangers loaded with silica of the type used according to Example 1 were regenerated after backwashing first with 11 1% sodium hydroxide solution and then with 1 1 5% NaCl solution.

Then 20 liters of tap water decarbonated with lime (p = 2.45; m = 2.65; SiO2 = 16 mg / l) were filtered per hour. The filtrate had the following composition: Filtrate p-value | m-value | SiO2 1 mg / l 4 0 0.1 0.8 8 0 0.1 0.8 12 0 0.1 0.8 16 0 0.1 0.6 20 0 0.1 0.6 24 0.15 0.25 0.8 28 0.55 0.65 0.95 32 0.8 0.9 1.1 36 1.05 1.15 1.2 40 1.15 1.25 1.4 To achieve the lowest possible SiO2 content, the amount of alkaline regeneration agent can be increased or the alkaline regeneration agent can be used in a hot solution.

Furthermore, a z. B. water then decarbonized with lime to be softened by ion exchange to the silicification according to the invention.

It is possible to have the silica removal and softening done in one Perform filter.

For this purpose, the water is used except with the strongly basic anion exchanger still charged with a cation exchanger.

When using a strongly basic anion exchanger of the aforementioned Type and a strongly acidic cation exchanger z. B. one based on polystyrene crosslinked with divinylbenzene and with sulfo groups as active groups, results the advantage of the salt solution used to regenerate the anion exchanger to be able to use simultaneously for the regeneration of the cation exchanger.

Example 5 A filter tube was filled with 1 l of anion exchanger according to Example 1 used type in the chloride form and 0.3 1 cation exchanger (z. B. polystyrene crosslinked with divinylbenzene and sulfonated) charged in the sodium form. After backwashing, the materials formed two separate layers. The specific The upper and the specifically heavier cation exchanger formed lighter anion exchangers the lower layer.

The following values were obtained in the filtrate when 20 liters of lime decarbonised tap water was applied per hour (p 1.75; m = 2.0; SiO2 = 18 mg / l; total hardness 7 ° d): Filtrate p-value m-value SiO2 residual hardness mg / l 4 0 0.1 0.6 0.02 8 0 0.1 0.6 0.02 12 0 0.1 0.55 0.02 16 0 0.1 0.5 0.02 20 0 0.1 0.5 0.02 24 0 0.1 0.5 0.02 28 0.05 0.15 0.6 0.02 32 0.2 0.3 0.6 0.02 36 0.4 0.5 0.6 0.02 40 0.6 0.7 0.7 0.02 44 0.8 0.9 1.0 0.02 48 1.0 1.1 1.1 0.02 52 1.2 1.3 1.3 0.02 In the subsequent regeneration of the exchanger with z. B. NaCl solutions often arise with such combined filters that the first fractions of the NaCl solution cause precipitations in the cation exchange layer due to their content of OH or carbonate ions. Such difficulties can be avoided if a collection system is arranged in the filter at the level of the dividing line between the two exchangers, as z. B. is common with the so-called mixed bed filters, and via this collection system the first portions of the regeneration solution containing the alkalinity are passed out of the filter and only the other portions of the regeneration solution are then passed through the cation exchanger.

Example 6 1 1 of the anion exchanger used according to Example 1 in the chloride form was treated with 400 cc of saturated lime water and then treated with 201 per hour of lime decarbonated tap water (p = 2.2; m = 2.4; SiO2 = 18 mg / l). The following values were obtained in the filtrate: Filtrate p-value | m-value | 510 1 mg / l 2 0.45 0.5 0.35 6 0.45 0.5 0.5 10 0.45 0.5 0.8 14 0.45 0.5 2.0 18 0.45 0.5 3.8 22 0.45 0.5 5.0 26 0.45 0.5 8.0 30 0.45 0.5 9.0 34 1 0.45 0.5 9.0 38 0.6 0.7 9.0 42 0.6 0.7 9.0 Methods are known which can be used to remove silica from solutions in the context of desalination, namely with an upstream debasement filter. The method according to the invention differs fundamentally from this in that it enables silica removal without simultaneous desalination. This is of particular advantage in all those cases in which no fully demineralized feed water is required, but, as in all other cases, the silica content of which must nevertheless be very low. According to the previously known processes, however, it is practically not possible to economically produce water that is largely free of silicic acid by means of ion exchange without at the same time bringing about complete desalination of the water. Fully demineralized water naturally also meets the requirements of boiler feed water, but in many cases the method according to the invention can be used to produce fully adequate boiler water in a much more economical way (lower regeneration costs and profitable partial desalination with lime).

Claims (2)

Claims: 1. A method for removing silica from water Pre-softening in a first stage and treatment with a strongly basic anion exchanger in a second stage with, if necessary, regeneration of the exhausted anion exchanger, characterized in that the pre-softening in a known manner by adding Lime is made in such an amount that the separated from the resulting precipitation Water contains excess free hydroxide, whereupon it in the second stage with the salt form, preferably the chloride form, of a strongly basic anion exchanger or with a mixture of the salt form with less than 50 ° l0 of the OH form of the exchanger is treated. 2. The method according to claim 1, characterized in that the exhausted Exchanger after, if necessary, prior treatment with an alkaline Means, preferably caustic soda, performed regeneration with a salt solution, preferably saline, which may require an aftertreatment with less than 50% of the amount of an alkaline equivalent to the total capacity of the exchanger By means of connects, is reused.