DE2448531C3 - Process for treating a residual product containing sodium chloride and sodium sulfate - Google Patents

Description

30th

The invention relates to a method for treating a containing sodium chloride and sodium sulfate Residual product, which in the form of a crystallized salt mixture on evaporation and cooling of a caustic soda obtained from a diaphragm electrolysis, and from which there is already residues of caustic soda removed.

Known methods for treating a residual product of the Ari described provide for the sodium sulfate to be dissolved by floating in water or in a low-sulfate sodium chloride brine, preferably at a suspension temperature at which a maximum of sodium sulfate and a minimum of sodium chloride is dissolved. The sodium chloride which remained undissolved in the process is then centrifuged off. The separated mother brine can be recovered if the sodium sulfate is precipitated as decahydrate (Glauber's salt) ^{by cooling to about 0 ° C.}

Such a method is e.g. B. in DE-PS 6 23 876 described. The centrifuged sodium chloride can prepared and dissolved, the brine can be added to the diaphragm electrolysis. Part of the won Glauber's salt can be used to obtain sodium hydroxide solution that is low in sodium chloride. The rest can deposited or deposited.

A relatively complex apparatus is necessary for these known methods and, moreover, the energetic effort considerable.

Furthermore, a dumping or a transport of the Glauber's salt, the sodium sulfate decahydrate, is included Difficulties are associated because its water of crystallization dissolves in the uncooled state and becomes watery. A conviction The Glauber's salt in anhydrous sodium sulfate requires, as is well known, an additional, apparatus-like energy-intensive process.

The inventor has set himself the task of rationalizing the treatment of the Reslprodukies in which one should get by with a simpler apparatus and lower energy expenditure, whereby but also a more rational use of the individual components obtained in this way can be made possible target.

According to the invention, this object is achieved in the above-mentioned process in that the sodium chloride-sodium sulfate mixture at a temperature of 60 ° to 100 ° C is so floated that a maximum of sodium chloride and optionally a first part of the sodium sulfate up to Saturation of the two components are dissolved and a second part of the sodium sulfate in thenardite form and optionally a remainder of the sodium chloride remain suspended, after which the solid is sodium chloride-sodium sulphate mother brine is separated.

Compared to the crystallization procedure previously used industrially here, this works according to the invention proposed the other way around: At the correspondingly increased temperature, a main component is dissolved of the chloride and allows a major portion of the sulfate to crystallize, which has the advantage of that the sodium sulphate is obtained in its anhydrous form, i.e. as thenardite.

Applied industrially, this means a saving in equipment and energy that used to be the Glauber's salt was obtained by cooling.

Further savings result if the sodium chloride-sodium sulphate mother brine obtained is used as an additive is used as the starting electrolyte.

Still another saving results from the anhydrous form of the sulfate obtained, so that if necessary, a complex conversion of a Glauber's salt to anhydrous sodium sulfate is omitted. Here, the thenardite obtained needs to be washed out or dried, if necessary so that they can be deposited or removed without hesitation or difficulty.

In the following the invention is based on a drawing, which schematically shows a system for Evaporation of sodium hydroxide solution with an example of a system for carrying out the invention Process represents, described and explained in more detail.

From a sodium chloride borehole 1, a brine is fed into a cleaning system 2, where the brine from a Calcium and a magnesium component is freed. Contains a pure brine obtained through the treatment another portion of sodium sulfate. The pure brine is mixed together with a brine, which consists of a Sodium chloride, which was obtained from a later stage of the plant - as will be explained later - is processed. This brine mixture is fed to a diaphragm electrolysis 3 as an electrolyte.

The sodium sulfate content in the electrolyte can usually be in the range up to 0.6%.

In the example shown, a solution obtained from the diaphragm electrolysis 3 contains 10% sodium hydroxide solution, 17% sodium chloride and 0.5% sodium sulfate, and the remainder is water. The solution is introduced in three (I, II, III) stages 4, 5 and 6 of an evaporation plant. Here the sodium chloride crystallizes, which is separated from the solution via separation devices 7 and 11 and fed back into the electrolysis 3 via a line 8 and a treatment tank 9.
In a following expander 12 and then-

the cooler 13 crystallizes the remainder (approx. 15%) of the precipitable sodium chloride and most of the Sodium sulfate. By means of a separating device 14, the now 50% sodium hydroxide solution is separated and used as Product set

Starting from this separating device 14 is a system according to the invention for carrying out the treatment the residual product, which is obtained after the separation of the caustic soda in the separating device 14, shown and described. ι ο

In addition to sodium chloride and sodium sulphate, the residual product also contains residues of up to 20% of caustic soda, which is undesirable for the reuse of the residual product in the diaphragm electrolysis are.

The mixture is therefore led to a device 15 for initiating the remains of the sodium hydroxide solution, where it was diluted by addition of water at a temperature between 60 ⁰ C and 100 ° C and is passed to the separator 16, in which the .erdünnte z. B. 25% sodium hydroxide solution is separated and returned to the third (III) stage 6 of the evaporation plant for evaporation.

The added water must be evaporated there. It is therefore more advantageous in the device 15 to use a dilute sodium hydroxide solution for dilution, namely part of that from the diaphragm electrolysis 3 derived 10% sodium hydroxide solution, as shown in the drawing with a dashed line This means that no additional water has to be evaporated.

It would also be possible to add the remainder of the sodium hydroxide solution in a preliminary stage by neutralization using hydrochloric acid remove. Such a measure, which is known per se, does not, however, need to be described in more detail.

A salt mixture separated after the separator 16 contains mainly sodium chloride and sodium sulfate. This salt mixture is fed into a suspension device 17 and mixed with water. A temperature is set which, for example, at 8O ⁰ C, a maximum solubility for sodium chloride and a limited or minimal solubility füt serving sodium sulfate. In this way, all of the sodium chloride and a first part of the sodium sulphate go into solution until saturation. A second part, namely the remainder of the sodium sulfate from the salt mixture downstream of the separating device 16, remains undissolved in suspension.

The solution and suspension are fed to a separator 18 -where the second is undissolved Part of the sodium sulfate is separated off and as a by-product in easily landfillable form as a solid, namely available as thenardite. The thenardite can further z. B. be refined by washing out. Even this measure, which is known per se, does not need to be described in more detail.

The delimited part of the sodium sulphate in the overall balance of the plant for extracting the caustic soda must correspond to the amount of sulphate which was freshly fed to the plant after cleaning the brine from a borehole with the pure brine of the electrolysis or the electrolyte.

The brine from the separating device 18, in which, in addition to the sodium chloride, also the one for the overall balance of the entire plant for extracting the caustic soda reliable proportion of unseparated sodium sulphate, d. H. the first part, which passed into the solution in the flotation device, is contained led back to the electrolyte via the preparation tank 9.

A part of sodium sulphate is therefore - because this means the above-mentioned permissible up to 0.6% sodium sulphate content in the electrolyte is not exceeded - left in the circulation.

If necessary, a larger amount of sodium sulphate could be separated from the circulation. with a small loss of sodium chloride, which is then separated off with the thenardite, in Would buy. The amount of water added for the suspension in the device 17 would have to be in should be increased accordingly in this case.

1 sheet of drawings

Claims (3)

1 claims: 1. A method for treating a residual product containing sodium chloride and sodium sulfate, which in the form of a crystallized salt mixture with evaporation and cooling of a diaphragm electrolysis Acquired caustic soda is obtained, and from which there is already residues of caustic soda were removed, characterized in that the sodium chloride-sodium sulfate mixture at at a temperature of 60 ° to 100 ° C is that there is a maximum of sodium chloride and optionally a first part of the sodium sulfate to be dissolved until the two components are saturated and a second part of sodium sulphate in thenardite form and optionally a remainder of the sodium chloride remains suspended, after which the solid is made up of sodium chloride-sodium sulfate mother-brine is separated. 2. Use of the sodium chloride-sodium obtained by the method according to claim 1 suIfat mother brine as an additive to the starting electrolyte. 3. Use of the thenardite obtained by the method according to claim 1, if appropriate after washing out the remaining sodium chloride, by processing in the form of a market-driven, anhydrous sodium sulfate.