US20080053839A1

US20080053839A1 - Processes for the electrolysis of alkali metal chloride brines

Info

Publication number: US20080053839A1
Application number: US11/836,858
Authority: US
Inventors: Wolfgang Mentsches; Klaus Geisler; Alfred Soppe; Thorsten Leidig
Original assignee: Bayer MaterialScience AG
Current assignee: Covestro Deutschland AG
Priority date: 2006-08-10
Filing date: 2007-08-10
Publication date: 2008-03-06
Also published as: CN101187030B; DE102006037400A1; HU0700523D0; CN101187030A; HUP0700523A2

Abstract

Processes are described which include: (a) providing an initial crude alkali metal chloride brine; (b) mixing the initial crude brine with a recirculated brine to form a process brine steam; (c) subjecting the process brine stream to electrolysis to form chlorine gas and an electrolyzed weak brine; and (d) subjecting the electrolyzed weak brine to a post-electrolysis salt enrichment comprising (i) diverting a substream of the electrolyzed weak brine at a post-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) introducing the separated salt into the electrolyzed weak brine to form the recirculated brine.

Description

BACKGROUND OF THE INVENTION

In chlor-alkali electrolysis, the raw material salt is conventionally supplied to the electrolysis process via a brine circuit. The purified brine is usually introduced into the electrolysis process in virtually saturated form. In the conventional electrolysis processes, a portion of the salt content is converted into the products (i.e., chlorine and alkali metal hydroxide solution). Furthermore, the stream of brine in the circuit loses a portion of its water content by mass transfer through the electrolysis membrane, in the case of membrane electrolysis, and via moisture discharged with the gaseous product, such that the brine leaving the electrolytic cell is a smaller and more dilute brine stream than what is introduced into the electrolysis.
Water losses can be made up by the addition of water. The brine stream is usually re-fortified by the addition of fresh alkali chloride salt and, after purification, is reintroduced into the electrolysis. Impurities in the salt which cannot be separated during brine preparation are compensated by removing a sub-stream of the diluted brine (“purge”). The purge quantity to be removed is variable and can be determined based on the admissible concentrations of impurities in the purified brine in the electrolysis process and on the impurities which are introduced as a function of salt purity. This results in a costly and thus unwanted loss of the raw material salt.
Salt is generally introduced into the brine circuit, for example in NaCl electrolysis, not only as purified evaporated salt, but also as sea salt or rock salt. To this end, solid salt can be dissolved in water (for example, by solution mining a salt dome). The resultant brine stream can be pre-purified by a combination of precipitation and filtration. Further purification then proceeds by evaporating the brine and separating the salt as solid crystals (“evaporated salt”), wherein the predominant proportion of the impurities remains dissolved in the mother liquor and may be separated from the evaporated salt. This process demands an elevated energy input. For example; for each ton of evaporated salt transferred into the brine circuit, approx. 2.85 tons of water must be evaporated in the salt works.
The provision of an electrolysis process which facilitates using crude brine would therefore be advantageous. An electrolysis using crude brine has been described wherein the weak brine from electrolysis is concentrated in a vacuum installation, making use of waste heat from electrolysis which is exhausted with the weak brine. The vacuum-concentrated weak brine is then reintroduced into the input brine stream for electrolysis. The crude brine from the salt dome may be used directly.
However, in such a process, the weak brine is evaporated at most up to its saturation point, specifically avoiding crystallization of the salt. As a result, however, the impurities introduced with the crude brine remain in the electrolysis circuit. Moreover, an increase in temperature during electrolysis as described in such a process for use in the vacuum concentration does not occur in more energy-efficient electrolysis processes used today, such that the described crude brine process would not be economically sensible today.
Accordingly, there remains a need in the art for an electrolysis process which makes use of crude brine in an economic way and which avoids the concentration of impurities in the brine circuit.

SUMMARY OF THE INVENTION

It has been found by the present inventors that electrolysis of crude brine can be carried out economically and without detrimental over-concentration of impurities in the brine circuit by directly supplying the salt required for the electrolysis process to the brine circuit and electrolysis cell as a mixture of crude brine and a recirculated process brine stream, without further purification by brine evaporation and evaporated salt production.
More particularly, it has been found that crystallization of a portion of the salt in an electrolyzed weak (diluted) brine process stream, and re-introduction of the crystallized salt into the weak brine before the combined recirculated brine and crude brine enter the electrolysis cell can significantly improve the economics and impurity levels of the overall electrolysis. Advantageously, impurities are decreased by salt crystallization while energy requirements are lowered via the evaporation of a lower volume brine stream.
One embodiment of the present invention includes processes which comprise: (a) providing an initial crude alkali metal chloride brine; (b) mixing the initial crude brine with a recirculated brine to form a process brine steam; (c) subjecting the process brine stream to electrolysis to form chlorine gas and an electrolyzed weak brine; and (d) subjecting the electrolyzed weak brine to a post-electrolysis salt enrichment comprising (i) diverting a substream of the electrolyzed weak brine at a post-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) introducing the separated salt into the electrolyzed weak brine to form the recirculated brine.
Another embodiment of the present invention includes processes which comprise: (a) providing an initial crude sodium chloride brine having a sodium chloride concentration of at least 14 wt. %; (b) precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the initial brine and filtering out the precipitated ions to form a pro-purified crude brine; (c) mixing the pre-purified crude brine with a recirculated brine to form a process brine steam; (d) precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the process brine stream; filtering out the precipitated ions; and passing the filtered process brine stream over an ion exchange resin; (e) subjecting the filtered, ion-exchanged process brine stream to electrolysis to form chlorine gas and an electrolyzed weak brine; (f) removing at least a portion of any residual chlorine from the electrolyzed weak brine; and (g) subjecting the electrolyzed weak brine to a post-electrolysis salt enrichment comprising (i) diverting a substream of the electrolyzed weak brine at a post-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) introducing the separated salt into the electrolyzed weak brine to form the recirculated brine.
Processes in accordance with various embodiments of the present invention can also optionally include treatment of one or more brine streams, (e.g., an initial crude brine, a combined crude brine and recirculated brine, etc.) to remove at least some of various constituents which can be disruptive to electrolysis, such as, in particular, metal ions such as calcium, magnesium, iron, aluminium, strontium, and barium. In various preferred embodiments, one or more of the removal treatments can comprise precipitation of such ions and subsequent filtration, and/or subsequent ion exchange.
In various embodiments of the processes according to the invention, an initial crude alkali chloride brine can be preferably supplied as a 30 wt. % to saturated alkali metal chloride brine, more preferably as a saturated alkali metal chloride brine.
In processes according to the invention, a portion of the salt in the recirculating, electrolyzed weak brine is deliberately crystallized out of the weak brine so as to be able to remove the impurities which are concentrated in the mother liquor during crystallization. As noted above, in previously suggested crude brine electrolysis, all the impurities remain in the brine and have to be separated and removed during brine purification, while accepting economically damaging losses of salt. In comparison with conventional methods using purified evaporated salt, a further advantage of the invention resides in energy savings from the removal of a proportion of the process water.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings representative embodiments which are considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
FIG. 1 is a process flow diagram of a brine electrolysis circuit in accordance with one embodiment of the present invention; and
FIG. 2 is a process flow diagram of a brine electrolysis circuit in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular terms “a” and “the” are synonymous and used interchangeably with “one or more.” Accordingly, for example, reference to “a gas” herein or in the appended claims can refer to a single gas or more than one gas. Additionally, all numerical values, unless otherwise specifically noted, are understood to be modified by the word “about.”
Processes according to the invention include providing an initial crude alkali metal chloride brine and mixing the initial crude brine with a recirculated brine to form a process brine steam. In various preferred embodiments of the invention, the processes may further include treatment (e.g., purification) of one or more process streams. Thus, for example, preferred embodiments may include purifying the initial brine, purifying the process brine stream, and combinations thereof.
Purification of a process stream can comprise precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from one or both of the initial brine and the process brine stream; and filtering out the precipitated ions. In various preferred embodiments, additional purification of the filtered process brine can be carried out in order to remove divalent ions, in particular metal ions from the range: calcium, magnesium, strontium and barium. Thus, for example, purification can comprise subjecting the process brine stream to a purification comprising precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof; filtering out the precipitated ions; and passing the filtered process brine stream over an ion exchange resin.
In various particularly preferred embodiments, purification can comprise purifying the initial brine and purifying the process brine stream. Thus, for example, in such particularly preferred embodiments, purification can include purifying the initial brine via precipitation of one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the initial brine and filtration of the precipitated ions; and purifying the process brine stream via precipitation of one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof; filtration of the precipitated ions; and passing the filtered process brine stream over an ion exchange resin.
In various preferred embodiments of the processes according to the invention, the substream of the electrolyzed weak brine is diverted at a weight ratio of the substream to the remainder of the electrolyzed weak brine of 1:1 to 1:20. Preferably, the separated salt is reintroduced into the electrolyzed weak brine at a reintroduction point after the post-electrolysis diversion point. In various embodiments, the separated salt can be divided into two or more portions and one of the portions can be reintroduced into the electrolyzed weak brine at a first reintroduction point after the post-electrolysis diversion point, and another portion can be reintroduced at a second reintroduction point before the post-electrolysis diversion point.
In various preferred embodiments, processes according to the invention can further include subjecting the initial crude brine to a pre-electrolysis salt enrichment comprising (i) diverting a substream of the initial crude brine at a pre-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) reintroducing the separated salt into one or both of the initial crude brine after the pre-electrolysis diversion point and the mixing of the initial crude brine and the recirculated brine. Subjecting a substream of the crude brine to an evaporation before mixing with the recirculated brine can further reduce impurities by removing process water and separating crystallized salt from the remaining residual liquor containing the impurities, and then re-introducing the crystallized salt.
In such preferred embodiments of the invention, the substream of the initial crude brine is preferably diverted at a weight ratio of the substream to a remainder of the initial crude brine of 5:1 to 1:5.
In various preferred embodiments of the present invention, the processes can further include removing at least a portion of any residual chlorine present in the electrolyzed weak brine prior to salt enrichment. The removal of residual chlorine preferably proceeds by stripping the chlorine under reduced pressure.
Electrolysis preferably comprises membrane electrolysis, diaphragm electrolysis or amalgam electrolysis, more preferably membrane electrolysis.
The alkali metal chloride present in the brine of the present invention preferably comprises sodium chloride.
In various preferred embodiments, the alkali chloride concentration of the initial crude brine is at least 14 wt. %, more preferably 14 to 23 wt. %.
Chlorine obtained from the various embodiments of the processes according to the invention can be further used in various chemical production processes, such as for example, in the production of plastics precursors.
The following examples, in conjunction with the Figures, illustrate embodiments of processes according to the invention and are for reference, and do not limit the invention described herein.

EXAMPLES

In the Figures, the reference letters refer to the following:

- A: salt dome
- B: crude brine treatment (precipitation, filtration)
- C: extraction of evaporated salt (from crude brine)
- D: extraction of evaporated salt (from weak brine from electrolysis)
- E: restoration of the (electrolysis) brine circuit
- F: brine purification precipitation, filtration, ion exchanger)
- G: electrolysis
- H: dechlorination of weak brine
- I: cooler/condenser
- J: cooler/condenser

Example 1

Referring to FIG. 1, initial sodium chloride-containing crude brine is obtained from a salt dome A and subjected to a combination of precipitation and filtration B to remove Ca, Mg, Fe and Al ions. The initial crude brine 11 is combined with recirculated weak brine 9 to provide process brine stream 1. The process brine stream 1 is supplied to a brine purification stage F, in which contaminants are further removed by means of a combination of precipitation and filtration and subsequent passage of the brine through a membrane ion exchanger (Ca, Mg, Fe and Al, as well as Sr and Ba ions). The filtered, ion-exchanged process brine stream 2 is electrolysed in a membrane electrolysis stage G. The material flow is indicated by 15. The resulting chlorine gas is separated and water 12 is removed therefrom.
Any residual chlorine is removed from the resultant weak brine 3 in an evaporator H. The chlorine is separated and water 13 removed therefrom, the chlorine is combined with the chlorine stream from electrolysis and further used in another process.
In comparison with a purified evaporated salt brine, the initial crude brine 11 introduces a certain quantity of impurities into the brine circuit. In order to compensate this input and comply with the specifications for the membrane electrolysis stage G with regard to impurities, a substream 5 of the electrolyzed weak brine 4 is diverted and supplied to a weak brine evaporation stage D. Here, the weak brine substream 5 is concentrated to a solid crystal state. The salt 6 obtained in this manner is redissolved in the remaining weak brine 8 and recirculated as weak brine stream 9. The recirculated weak brine 9 is mixed into the crude brine stream 11 in the restoration stage E.
The impurities present in the weak brine substream 5 are likewise concentrated in the evaporation stage D and removed via a purge substream 7 to be discharged from the evaporator. An advantage is that, relative to the known process, the impurities may be retained and discharged from a considerably larger substream (approx. 10× larger), thus compensating for the increased input of impurities via the crude brine stream 11.
A substantial advantage obtained by the process is that, for each 1 ton of salt introduced into the brine circuit as crude brine, only approx. 1.33 tons of water 14 are evaporated, which amounts to a 53% reduction relative to prior process. Energy and operating costs and capital costs for the necessary evaporation plant and for steam generation or other energy sources are reduced commensurately.
Depending on the impurities present in the initial (optionally pre-purified, B) crude brine, it is possible with this process to supply at most the entire required quantity of salt to the electrolysis process; brine evaporation/evaporated salt production is then no longer necessary in the course of raw material preparation.

Example 2

Referring to FIG. 2, where individual impurities, which cannot be separated either via crude brine preparation or via brine purification of the brine circuit, such as for example bromine, do not permit sole use of crude brine as described in Example 1, a portion of the crude brine may also be pre-treated. Except as noted below, the process described in this example including the reference numerals and letters are the same as in FIG. 1 and example 1. A portion 11 a of the crude brine 11 is sent for evaporation and crystallization C and the resultant salt 10 is reintroduced into the crude brine. The size of the crude brine stream 11, and the size of the crude brine sub-stream 5 to be prepared in the weak brine evaporation stage is then determined in accordance with the impurities admissible in the process brine stream 2.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A process comprising:

(a) providing an initial crude alkali metal chloride brine;

(b) mixing the initial crude brine with a recirculated brine to form a process brine steam;

(c) subjecting the process brine stream to electrolysis to form chlorine gas and an electrolyzed weak brine; and

(d) subjecting the electrolyzed weak brine to a post-electrolysis salt enrichment comprising (i) diverting a substream of the electrolyzed weak brine at a post-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) introducing the separated salt into the electrolyzed weak brine to form the recirculated brine.

2. The process according to claim 1, further comprising a purification selected from the group consisting of purifying the initial brine, purifying the process brine stream, and combinations thereof.

3. The process according to claim 2, wherein the purification comprises precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from one or both of the initial brine and the process brine stream; and

filtering out the precipitated ions.

4. The process according to claim 1, further comprising subjecting the process brine stream to a purification comprising precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof; filtering out the precipitated ions; and passing the filtered process brine stream over an ion exchange resin.

5. The process according to claim 1, further comprising purifying the initial brine and purifying the process brine stream.

6. The process according to claim 5, wherein purifying the initial brine comprises precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the initial brine and filtering out the precipitated ions; and wherein purifying the process brine stream comprises precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof; filtering out the precipitated ions; and passing the filtered process brine stream over an ion exchange resin.

7. The process according to claim 1, further comprising removing at least a portion of any residual chlorine present in the electrolyzed weak brine prior to the salt enrichment.

8. The process according to claim 7, wherein removing at least a portion of any residual chlorine comprises stripping the portion of residual chlorine under a reduced pressure.

9. The process according to claim 2, further comprising removing at least a portion of any residual chlorine present in the electrolyzed weak brine prior to the salt enrichment.

10. The process according to claim 5, further comprising removing at least a portion of any residual chlorine present in the electrolyzed weak brine prior to the salt enrichment.

11. The process according to claim 1, wherein the substream of the electrolyzed weak brine is diverted at a weight ratio of the substream to a remainder of the electrolyzed weak brine of 1:1 to 1:20.

12. The process according to claim 1, further comprising subjecting the initial crude brine to a pre-electrolysis salt enrichment comprising (i) diverting a substream of the initial crude brine at a pre-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) reintroducing the separated salt into one or both of the initial crude brine after the pre-electrolysis diversion point and the mixing of the initial crude brine and the recirculated brine.

13. The process according to claim 12, wherein the substream of the initial crude brine is diverted at a weight ratio of the substream to a remainder of the initial crude brine of 5:1 to 1:5.

14. The process according to claim 1, wherein the alkali metal chloride comprises sodium chloride.

15. The process according to claim 1, wherein the electrolysis is selected from membrane electrolysis, diaphragm electrolysis and amalgam electrolysis.

16. The process according to claim 1, wherein the initial crude brine has an alkali metal chloride concentration of at least 14 wt. %.

17. The process according to claim 1, wherein the separated salt is reintroduced at a reintroduction point after the post-electrolysis diversion point.

18. The process according to claim 1, wherein the separated salt is divided into two or more portions and one of the two or more portions is reintroduced at a first reintroduction point after the post-electrolysis diversion point, and another of the two or more portions is reintroduced at a second reintroduction point before the post-electrolysis diversion point.

19. A process comprising:

(a) providing an initial crude sodium chloride brine having a sodium chloride concentration of at least 14 wt. %;

(b) precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the initial brine and filtering out the precipitated ions to form a pre-purified crude brine;

(c) mixing the pre-purified crude brine with a recirculated brine to form a process brine steam;

(d) precipitating one or more ions selected from the group consisting of Ca, Mg, Fe, Al and mixtures thereof from the process brine stream; filtering out the precipitated ions; and passing the filtered process brine stream over an ion exchange resin;

(e) subjecting the filtered, ion-exchanged process brine stream to electrolysis to form chlorine gas and an electrolyzed weak brine;

(f) removing at least a portion of any residual chlorine from the electrolyzed weak brine; and

(g) subjecting the electrolyzed weak brine to a post-electrolysis salt enrichment comprising (i) diverting a substream of the electrolyzed weak brine at a post-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) introducing the separated salt into the electrolyzed weak brine to form the recirculated brine.

20. The process according to claim 19, further comprising subjecting the pre-purified crude brine to a pre-electrolysis salt enrichment comprising (i) diverting a substream of the pre-purified crude brine at a pre-electrolysis diversion point; (ii) removing water from the substream; (iii) crystallizing salt present in the substream; (iv) separating the crystallized salt from the substream; and (v) reintroducing the separated salt into one or both of the pre-purified crude brine after the pre-electrolysis diversion point and the mixing of the pre-purified crude brine and the recirculated brine.