DE2530447C2 - Process for the production of crystals from sodium carbonate heptahydrate or decahydrate - Google Patents

Description

The invention relates to a method for producing crystals from sodium carbonate heptahydrate or -decahydrate by cooling an aqueous solution of sodium carbonate close to saturation in contact

In such a procedure, crystallization devices with heat exchangers are used.
It is already known to crystallize the decahydrate or the heptahydrate of sodium carbonate by cooling an aqueous solution of sodium carbonate in a device which, based on the crystallization container, comprises internal or external cooling surfaces. The cooling is carried out by bringing the cooling element into contact with a cold liquid, the calories being transferred from the coolant to the sodium carbonate solution through the metal wall.

For economic reasons, the exchangers consist of tubes or other metallic ones Elements are formed, generally from steel or from stainless steel or from a cheap, any metal that is coated with plastic.

It was found, however, that the cooling surfaces on the side of the solution formed an encrustation as a result are exposed to the deposition of crystals to such an extent that one is forced to use exchangers to be used in the majority or to sew on the fabric periodically to clean the encrusted surface. Various means have already been proposed to reduce such encrustation, in particular

the maintenance of a high flow rate of the solution, the observance of the maintenance of a small temperature difference between the solution and the cooling surface (/ It), working with the highest possible density of the suspension and the use of well-polished metal surfaces (see Industrial and Engineering Chemistry [1961], August, page 624). However, these funds have not been found to be sufficient.

The appearance of the encrustation of heat exchanger walls has recently been described again in NL-PA 70 14 888, this publication relating to the production of dense, anhydrous soda by a complex process that uses carbon dioxide with carbon dioxide from a salty, caustic lye, which obtained by electrolysis of a sodium chloride brine in diaphragm cells, the crystallization of sodium carbonate decahydrate from the carbonated, saline, caustic liquor thus formed, the melting of the decahydrate to form the monohydrate and finally the calcination of the latter compound to form dense, anhydrous soda. For the part of the process which _{comprises the preparation of the sodium carbonate decahydrate from the solution of Na 2} CO 1 and NaCl, it has also been proposed to carry out the crystallization by evaporation under reduced pressure, which is inherently associated with cooling in order to prevent encrustation to avoid illegal occurrences when working by cooling the solution using heat exchangers.

The specified procedure 1st powerful, but it requires the formation of large vacuums,

which are obtained by using steam jets; However, this is a technically difficult one and aufv.eruilges means.

The applicant has now set itself the task of finding a material that the incrustation

less favored and thus the use of the cooling system through heat exchange and not the Allows application of low pressures. These investigations led to one due to the state result that was not foreseeable by technology.

The inventive method for solving this problem, namely the production of crystals from Sodium carbonate heptahydrate or decahydrate by cooling an aqueous sodium carbonate solution close at saturation In contact with a heat exchanger is distinguished by the fact that one has an exchanger uses the surface of which come into contact with the solution containing the sodium carbonate must, made of copper or an alloy with a predominant content of copper.

The method according to the invention is particularly efficient in the long term if the temperature difference (Δt) between the cooling surface and the solution subjected to crystallization is kept below 10 ° C. It is particularly advantageous to keep this temperature difference below 9 ° C.

Of course, it is not necessary that the heat exchanger elements are made entirely of copper or are made of a copper alloy; they can also consist of a different, metallic Constructed and simply made with copper or an alloy predominantly copper on the material

or the surfaces which are intended to be plated with the one containing the sodium carbonate Solution to get in touch.

Surfaces made of non-alloyed copper are preferred.

The method can be used to treat aqueous solutions which only contain sodium carbonate as a dissolved salt, or on solutions of sodium carbonate, which also contain an or contain several other salts. For example, one can apply it to sodium carbonate decahydrate To precipitate cooling an aqueous solution containing sodium carbonate and sodium chloride, as described in the aforementioned NL-PS 70 14 888 was described.

The crystallization according to the invention can take place under atmospheric pressure or under positive or negative pressure which differ little from atmospheric pressure.

The method according to the invention is explained in more detail using the following example. example

This example gives a comparison between the encrustation of stainless steel and copper exchanger _{tubes in the course of the crystallization of sodium carbonate decahydrate from a solution fed in which contains 126 g Na 2} COi, 153 g NaCl and 6 g Na ₂ SO ₄ per kg of solution. The crystallization temperature is 8 ° C., the circulation speed of the suspension in the exchanger tube is 0.5 m / sec and the density of the suspension is 0.25 kg / kg.

The device: comprises a crystallization device and an exchanger which, in an external circuit, in relation to the direction of the Crystal Island is included. The crystallization device consists of a thermally insulated, cylindrical vessel with a simple wall and a flat bottom; she is using a screw stirrer equipped. The sludge is drawn off by means of an overflow. The circulation loop begins at the bottom of the crystallization device and leads back perpendicular to the rare wall. Of the Exchanger consists of two concentric, vertical tubes; the inner tube has a diameter of about 50 mm and a useful length of 4.6 m, while the outer tube has a diameter of 75 mm owns. The circulation of the coolant - which is taken from a secondary circuit - takes place from bottom to top In the double jacket of the exchanger by means of a pump. The circulation of the mud In the exchanger, from bottom to top by means of a centrifugal pump, and the fed solution is introduced into the circulation loop by means of suction by the pump. Dk. Temperature in the double jacket the exchanger is controlled by automatic admission of the cooling fluid, the admission through a solenoid valve is regulated, the? τη of the temperature is controlled; the throughput of cooling fluid is by means of of a counter.

To start the experiments, the solution to be fed in is cooled down to the temperature corresponding to the saturation, then it is inoculated _{with solid Na 2} COj · 10H _{2 O.}

Since the crystallization temperature and the composition of the fed solution are predetermined, one can maintain a given heat flux by maintaining a constant throughput of the Maintain solution and the temperature of the double jacket is controlled so that a provided Heat transfer ensured and in this way the crystallization temperature is kept constant. One other working conditions, consists in setting a temperature deviation equal to the exchanger and the To regulate the throughput of the fed solution in such a way that this deviation is at the same time as the crystallization temperature is kept constant.

Experiments 1 to 8, the results of which are summarized in the following table, were carried out By keeping the temperature deviation constant at the level of the exchanger, this deviation Was 6, 9 or 12 ° C and using exchanger tubes made of stainless steel or copper.

Tabel Experiment Type of pipe Duration of experiment

1 deoxidized drawn copper 12

(containing phosphorus)

2 stainless steel 12

3 deoxidized drawn copper 13

(containing phosphorus)

4 stainless steel 13

5 deoxidized drawn copper 9

(containing phosphorus)

6 stainless steel 9

7 deoxidized drawn copper 12

(containing phosphorus)

8 stainless steel, drawn 12

FIuB (kcal / h ■ m ² )
Mean, mean,
first hour last hour 2400 2500 1600 3700 1900 3500 Δι medium thickness
of the crusts 1050 ( ⁰ C) (mm) 4300 12th 3.6 2600 12th 5.2 6th 0 2000 6th 1.1 9 0 2600 9 1.9 9 0

800

4.7

These tests show that copper allows a flow of 3700 kcal / h · m ^{2 to} pass through for 9 hours and of 3500 kcal / h · m ² for 12 hours without any incrustation being observed, whereas with the stainless steel wall it is it would not be possible to avoid the formation of crusts, although the flow was limited to 1600 kcal / h · m ² . From this it can be seen that stainless steel, despite its polished surface, does not prevent the formation of crusts, even with low zir values, while with copper it is possible to achieve this result for much higher values for At and longer test periods.

This specific effect of copper is pronounced even for a / St value of 12 ° C, but in this case the encrustation is only reduced without being completely suppressed (see experiment 1).
Experiment 5 shows that under operating conditions at a passage speed of the suspension = 0.5 m / sec, density of the suspension = 0.25 kg / kg, for a pipe made of copper with a diameter of approximately 50 mm, the / St value is up to 9 ° C can be adjusted without incrustation, with a constant heat flow of 3700 kcal / h Im ^{2 being} achieved. When using the method with a / St value of 9 to 10 ° C, it is possible to avoid encrustation and ^{to achieve heat flow values of the order of about 4000 kcal / h · m 2} , which are very satisfactory values for industrial practice are.

The use of metallic materials other than copper or its alloys makes it possible not to avoid the incrustation and in this way to ensure an effective and constant flow of heat; in such a case it would be necessary to work with such low / 7 values that the operation would be incompatible with the demands of industrial practice.

Claims (4)

Patent address: 1. Process for the production of crystals from sodium carbonate heptahydrate or decahydrate by Cooling an aqueous solution of sodium carbonate close to saturation in contact with a heat exchanger, characterized in that an exchanger is used whose surface, which must come into contact with the solution containing the sodium carbonate, is made of copper or of a Alloy with a predominant content of copper. 2. The method according to claim 1, characterized in that the temperature difference between the Surface and the solution is kept below 10 ° C. 3. The method according to any one of claims 1 or 2, characterized in that the heat exchanger element Constructed entirely of copper or of an alloy predominantly copper. 4. The method according to any one of claims 1 to 3, characterized in that the heat exchanger element Constructed of any metal, but predominantly copper or an alloy Copper content is plated on the surface exposed to the solution.