CA2180034A1

CA2180034A1 - Method of producing an alkali metal hydroxide by electrolysis of sodium sulphate

Info

Publication number: CA2180034A1
Application number: CA002180034A
Authority: CA
Inventors: Gary Derdall
Original assignee: Individual
Current assignee: Dirdal Rand Technologies
Priority date: 1996-06-27
Filing date: 1996-06-27
Publication date: 1997-12-28

Abstract

A process is disclosed whereby an anion or base ion exchange resin in the hydroxy form is used to absorb sulphuric acid produced in the electrolysis of an alkali metal such as sodium sulphate and thereby enable production of an alkali metal hydroxide, e.g., caustic soda, without production of chlorine. Base resin in the sulphate form from contact with sulphuric acid is then treated with an alkali metal chloride, e.g., potassium chloride, to produce alkali metal sulphate.
Regeneration of the chloride laden base resin with calcium hydroxide, magnesium hydroxide or aqueous ammonia leads to the hydroxy form and a chloride salt which can be recovered or recycled, as with ammonium chloride.

Description

BACRGROUND OF THE lNVh~. ~lON
The present invention relates to a process for producing alkali metal hydroxides such as caustic soda (sodium hydroxide) or potash (potassium hydroxide) by electrolysis of alkali metal salts. More particularly, the present invention relates to the production of sodium or potassium hydroxide by electrolyzing non chloride salts such as sodium sulphate so that the co-product of the process is not chlorine but other valuable material such as potassium sulphate which can be used in industrial chemical or fertilizer markets.
In the classical electrolytic process for producing caustic soda, sodium chloride is used almost exclusively. In some instances, there is sustained demand for caustic soda while chlorine markets are steadily declining for a variety of market and environmental factors, such as found with the pulp and paper industry.
There is therefore a need to provide caustic soda production without the usual chlorine production in a manner that is economical and is sufficiently effective and efficient that investment can be justified.
It is well known in the art that treating soda ash with lime provides caustic soda and there are commercial plants in the United States that do this. However, the economics are such that these operations are only attractive to operate when caustic soda prices are high. The primary reason for this ~AW or~lcEs FINNEGAN,HENDERSON, cyclical operation is that soda ash based caustic soda plants FARABOW, GARREl~
~ DWNER,L.L.P.
1300 I STQEET, N. W
'NAS~INGTON, DC 20005 202 408 '-000 ~ 18003~

produce only one valuable product whereas chlor-alkali plants produce two valuable products.
A number of processes have sought to produce caustic soda by electrolysis of sodium sulphate such as those disclosed in United States Patent Nos. 2,829,095; 3,135,673; 3,222,267;
3,398,069; 3,907,654; and 4,561,945, all incorporated herein by reference. This art, in one manner or another, relates to the production of caustic soda along with sulphuric acids of varying strengths.
From an economic perspective, more dilute sulphuric acid is less desirable. Attempts t-o produce sulphuric acid of higher concentrations can result in significantly lower cell efficiencies. These techno-economic difficulties have resulted in very limited application of the art.
U.S. Patent No. 5,098,532 (incorporated herein by reference) relates to the electrolysis of sodium sulphate to produce caustic soda. This involves the use of a so-called three compartment cell and the co-production of ammonium sulphate. In this patent, back migration of protons from the anolyte compartment, which causes reduced current efficiency, is reduced by introducing ammonia to the cell compartment and results in the production of ammonium sulphate.
While this approach reduces a techno-economic limitation inherent in two or three compartment cells in the electrolysis of sodium sulphate, the costs for utilization of ammonia may L AW 0~1 C ES
FINNEGAN,HENDE~ON, well not be covered by the revenues due to ammonium sulphate. FARABOW, GARRETT
6 DWNER,L.L.P. This can then place the system at an economic disadvantage to 1~00 I STF~EET, N. W
W~ShlNGTON, DC 20005 Z02 - 40e - 4000 - 2-- ~18003~

the production of caustic soda by conventional chlor-alkali technology.

SUMMARY OF INVENTION
The features and advantages of the present invention include providing a process for the production of an alkali metal hydroxide such as sodium or potassium hydroxide which will provide improved methods for utilizing non-chloride alkali metal salts in electrolysis to produce caustic soda or potash and other valuable products which do not contain chlorine.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the present invention relates to a process for producing an alkali metal hydroxide, e.g., caustic soda or caustic potash, by electrolysis using non-chloride salts such as sodium sulphate or potassium sulphate. The sulphuric acid produced in an anolyte compartment by electrolysis of the non-chloride ~w orr~crg FINNEGAN, HENDE~ON, salt, such as sodium sulphate, is removed and brought into FMABOW, G~RRE~
~3~NNERLL.P contact with a base ion exchange resin in the hydroxy form to 1300 I ST~EET, N. W.
W~S~I~NGTON, OC 20005 202 .~0~ 000 218003~

neutralize the sulphuric acid and thereby load the resin with sulphate. The de-acidified or acid stripped anolyte liquor is then returned to the anolyte compartment separately or with additional non-chloride salt as needed. This enables the anolyte compartment to be maintained at pH levels sufficiently high enough to minimize back migration of protons and achieve high current efficiencies. The alkali metal hydroxide is produced as a catholyte, e.g., in a catholyte compartment.
After the base ion exchange resin is loaded with sulphate ion, the resin is given successive displacement washes to remove anolyte entrained in the resin. The resin then is eluted with alkali metal chloride, as with a hot concentrated alkali metal chloride brine, such as potassium chloride and optionally containing some alkali metal sulphate, such as potassium sulphate. The resultant brine, now rich in alkali metal sulphate, such as potassium sulphate, is cooled in a crystallizer and treated with alkali metal chloride such as potassium chloride, to yield alkali metal sulphate crystals, such as potassium sulphate crystals, that are separated from the brine and dried. The crystallizer liquor (primarily alkali metal chloride) is heated and recycled to elute fresh base ion exchange resin in the sulphate form. The base ion exchange resin, now in the chloride form, is then subjected to displacement washes to remove entrained alkali metal chloride and then treated directly with an aqueous alkaline earth metal LAW orrlcr~
FINNEGAN, HENDE~ON, hydroxide, such as aqueous calcium hydroxide, e.g., in a FARABOW, GARREl-r ~D~NNE~,LLP slurry, to regenerate the base resin to the hydroxy form. The 1300 ~ STF~EET, N. W.
WASHINGTON, OC 20005 202-40~-4000 ~180034 resin is subjected to a wash to remove entrained alkaline earth metal hydroxide solids, such as calcium hydroxide, and then a displacement wash to remove alkaline earth metal chloride brine, such as calcium chloride. The displacement wash may include use of sodium sulphate brine. The regenerated resin is returned to absorption of sulphuric acid from the anolyte.
The present invention also relates to a process wherein the base resin in the hydroxy form is used to absorb sulphuric acid from the anolyte as described above to produce alkali metal hydroxide with subsequent treatment of the base resin by ammonia, preferably in solution, to regenerate the base resin to the hydroxy form and produce a solution of ammonium sulphate. As in the preceding process, the base resin in the hydroxy form is given a displacement wash to remove liquors (primarily ammonium sulphate and ammonia solution) entrained in the resin. The displacement wash may include the use of solution of dilute caustic soda. The ammonium sulphate solution can be brought up to higher concentrations by additions of ammonia, e.g., in solution, and contacted with a fresh bed of resin in the sulphate form. The solution can be evaporated, the solids (e.g., ammonium sulfate) separated, and dried. Ammonia can also be used to regenerate the resin from the chloride form to the hydroxy form as well.
Additionally, the concentrated ammonium sulphate solution L~W orrlcE~
FINNEGANHENDERsoN is passed over a potassium laden cation ion exchange resin to FMAsow, GARRETT
~D~ERLLP produce potassium sulphate brine preferably of high 1300 ~ 5TREET, N. W
W~S~II~IGT0~, DC ZOOO~S
20Z 40e-4000 " 2180034 concentration which can be processed in a manner similar to the salt-out crystallizer cited above to yield potassium sulphate. The resin now in the ammonium form is regenerated in an alkali metal chloride, e.g., potassium chloride, which then subsequently provides a resin in the potassium form and forms ammonium chloride. The ammonium chloride brine can be treated with an alkaline earth metal hydroxide such as calcium hydroxide or magnesium hydroxide to regenerate the ammonia.
Strong or weak base resin in the chloride form can be regenerated in two ways. In one approach, the resin is regenerated with lime which will yield a resin in the hydroxy form. The second approach is regeneration with ammonia. This will form ammonium chloride solution and yield a resin in the hydroxy form. The ammonium chloride solution can then be treated with lime to produce ammonia and calcium chloride brine. Lime based hydroxide ion for regeneration is likely to be the most economic method to carry out the overall process.
Magnesia or partially calcined dolomite can also be employed if warranted by economics. Other regenerants such as caustic soda could be employed but normally this would not be economic within the context of electrolytic production of caustic soda.
The process can be placed on recycle of sodium sulphate by use of sodium rather than potassium chloride. The above process conditions have application in two compartment cells because the base resin in contact with the anolyte acts only to strip ~w o~r~c~s FINNECAN,HENDE~ON, acidity (reduce acidity) and leaves sodium sulphate in FARABOW, GARRErE
8 DUNNER, L. L.P.
1300 I STF?EET, N. W
WA5 1~1 ~JGT01~, 0 C 200 OS
20Z 40e - 4000 - 6-- ~18003~

solution which is desirable for operation of lower cost two compartment cells.
The present invention may also be applied to electrolysis of sodium sulphate using three compartment cells wherein sulphuric acid in the anolyte compartment is processed as above with an ion exchange resin to yield potassium sulphate and calcium chloride. These methods can be preferred when low levels of sulphuric acid in the anolyte are desired for reasons of minimal back migration of protons, high current efficiencies, high quality caustic, and valuable co-products.
In an alternative approach, a cation resin may be used to process acidic anolyte by passing the anolyte over a bed of cation resin in the potassium form. In this manner, protons from the acid liquor exchange with potassium ions, reducing acidity. This results in the formation of a potassium sulphate rich anolyte. This can be taken up to 120-180 g/l of potassium sulphate at about 50~ to about 80~C. This liquor can be bled from the system to an evaporative circuit with recycle of water or the potassium sulphate brine can be cooled and potassium chloride added to salt out the potassium sulphate. The potassium sulfate can be separated out, dried, and formulated into product by various means. The cation resin in the hydrogen form can be treated with a base such as aqueous ammonia, calcium hydroxide, magnesium hydroxide, or partially calcined dolomite. The resin is then treated with ~AW or~lc~, FINNEGAN,HENDERSON, potassium chloride to regenerate the resin. If ammonia is FARABOW, GARRETT
~ ~UNNER,L.L.P. used with the anolyte to produce ammonium sulphate liquor, 1300 1 5TREET, N. W.
WASI~INGTON, OC Z0005 202 408-'~000 - ~180034 this can be contacted with the cation resin in the potassium form to produce potassium sulphate liquor which can be processed as above to form solid potassium sulphate. The cation resin now loaded with ammonium ion is treated with potassium chloride to regenerate the resin to the potassium form and the ammonium chloride is treated with calcium hydroxide to release ammonia for recycling and to produce calcium chloride. It is evident that ammonium sulphate and potassium chloride can be reacted directly without the use of ion exchange.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates one embodiment of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING
The Figure is a schematic diagram of an embodiment of the process of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The Figure is a schematic diagram of an embodiment of the L~W orrlcr~
FINNEGAN, HENDER50N, present invention. Unit A is an electrolytic cell. Anolyte FARAEOW, GARRE~
~ ~NNER,L.L.P. is withdrawn from the cell and passed through an anion ion 1300 ~ STF~EET, N. W
WAS~lNGTON~ ~C 20005 202-40~-~ooo --8--exchange resin bed in the hydroxy form, Unit B, to reduce acidity. The exhausted resin is given a displacement wash at Unit C. The resin is then treated at Unit D with potash brine to produce potassium sulphate for crystallization at Unit E.
The resin is given a displacement wash at Unit F and regenerated to the hydroxy form with an alkaline earth metal hydroxide, e.g., calcium hydroxide at Unit G which also forms an alkaline earth metal chloride brine, e.g., calcium chloride brine. The resin is given a displacement wash at Unit H
whereupon the resin is returned to reduction of acidity if the anolyte.
The processes of the present invention provide improved methods for utilizing non-chloride alkali metal salts, such as sodium sulphate and potassium sulphate and the like, in electrolysis to produce an alkali metal hydroxide such as caustic soda and other valuable products, which do not contain chlorine. For example, with sodium sulphate or potassium sulphate as a feedstock to the electrolysis cell, a base resin in the hydroxy form can be used to remove the required amount of sulphuric acid from the anolyte liquor such that improved cell current efficiencies are achieved. The anolyte is preferably replenished with additional alkali metal sulphate and returned to the anolyte compartment where additional electrolysis occurs to form or produce alkali metal hydroxide in the cathode compartment. Control of the acid level is ~AW orrlc~s FINNEGAN, HENDE~ON, achieved through the volume of anolyte contacted with the FAR~BOW, GARRE1T
~D~NNER,LLP hydroxy form of the base resin per unit time. The base resin, ~300 I STREET, N. W
WASNINGTON, DC 20005 202-40~3-4000 - ~180034 now in the sulphate form, can be treated directly with alkali metal chloride (e.g., potassium chloride) to yield an alkali metal sulphate (e.g., potassium sulphate), a valuable product.
The base resin, now in the chloride form, can then be directly regenerated to the hydroxy form with an alkali earth metal hydroxide (e.g., calcium hydroxide) to yield an alkali earth metal chloride (e.g., calcium chloride). Alternatively, the base resin in the chloride form can be treated with ammonia to yield ammonium chloride. The ammonium chloride can then be treated with a base such as calcium hydroxide and the like to yield ammonia for recycle and calcium chloride. The regeneration system produces base resin in the hydroxy form for absorption of additional sulphuric acid.
U.S. Patent Nos. 4,504,458 and 4,707,34?, and European Patent Application No. 86103871.9 (EP 0 199 104 A2), all incorporated herein by reference, mention that sulphate may be loaded on a base resin in the chloride form by means of calcium, magnesium, and sodium sulphate brines, in solutions of varying acidity. However, these processes have limitations because sulphate only effectively displaces chloride ion on a base resin within a dilute concentration range. Outside this range, mixed brines are formed requiring expensive recovery and separation methods to achieve effective operation.
Furthermore, it is not taught that under acid conditions, acidity can build up in the production zone and steadily L~W orr~c~
FINNECAN, HENDERSON, reduce productivity if not controlled.
F~RAsow, GARRErr ~ Dl.'N~ER, L. L. P.
1300 ~ STREET, N. W
W~SIIINGTON, ~C 20005 Z02 40~-4000 '-- 2180034 In the present invention, a base resin in the chloride form is first placed in the hydroxy form, by contact with an alkaline earth metal hydroxide base such as calcium hydroxide or aqueous forms thereof. The base resin in the hydroxy form can then be readily and effectively loaded with sulphate ions for the eventual production of potassium sulphate by contacting the hydroxy form of the base resin with sulphuric acid solutions, such as those found in anolytes of electrolytic cells used for the production of caustic soda from sodium sulphate.
According to a preferred process of the present invention, anolyte typically containing about 10~ to about 40 sodium sulphate is electrolyzed at temperatures in excess of 50~C to produce caustic soda in the cathode compartment and sulphuric acid in the anode compartment. The acidic anolyte is contacted with the base resin in the hydroxy form to reduce acid levels and thereby load the base resin in the sulphate form. Before contact, steps must be taken to ensure that any residual chlorine or hypochlorous acid is destroyed to prevent damage to the resin. An agent suitable for this purpose is sodium sulphite although those skilled in the art will recognize that other agents can be used. The produced chloride ion is absorbed by the base resin in the sulphate form and is taken out of the system. Similar means may be used to polish sodium sulphate feedstocks.
L~W orr-c~
FtNNEGAN,HENDE~ON, This contact may be carried out in several different FARABOW, GARREl~' ~D~NNERLLP means including a stirred batch type reactor, a stirred 1300 I STFIEET, N. W.
WAS~INGTON, OC 20005 202-~0~-4000 - 11 -!, _ ~co-current reactor, a conventional column type absorption reactor or a moving bed of resin contacted with sprays of anolyte. The type of contact process selected depends upon the preferred steady state acid concentration of the anolyte leaving the cell required for the effective and efficient operation of the electrolytic cell, the required contact time, and the volume per unit time of anolyte required to be acid stripped by a given volume or equivalent of base resin.
In general, stirred contact reactors can be employed when volumes of anolyte and resin are similar. Contact time is a factor since neutralization of- acid with a base resin can be significant in this mode. When anolytes need to be maintained at less acidic conditions, due to operational requirements of the cell, moving anolyte through a bed of resin in some manner is preferred since this minimizes contact time needed between resin and anolyte.
With a stirred batch reactor, acidic anolyte is added to a base resin in the hydroxy form such that the pH of the resin/anolyte slurry does not go below a pH of about 4 or 5.
The resin is now loaded in the sulphate form. At this point the anolyte is drained and moved to an anolyte holding tank for recycling and use in the cell. Additional purified sodium sulphate is added as needed to the anolyte tank for proper cell electrolysis conditions and this liquor is then returned to the anolyte compartment in the cell.
LAW orrlcr~
FINNEC~, HENDERSON, The resin in the batch reactor is slurried to a FMASOW, GARRETT
~ D~NNER,L.L.P. displacement washing device such as a pan filter, horizontal 1300 ~ 5'rREET, N. W.
WASIIINGTON, DC 20005 ZOZ- 408 - ~000 -12-21 ~003~

vacuum filter or a fixed bed column wherein the resin is washed, further processed as needed, and removed for further processing, such as with a Himsley device noted in Perry's Chemical Engineers' Handbook; the description of this device is incorporated herein by reference.
With these devices, a displacement wash is carried out wherein an amount of wash water, typically equal to 0.2 to 0.3 bed volumes of resin, is added to hydraulically displace or force out anolyte entrained in the resin. This minimizes carry over of sodium sulphate and residual acidity into the alkali metal sulphate (e.g., potassium sulphate) production circuit. In the case were there is significant residual acidity in the resin, the displacement wash can first be carried out with concentrated neutral sodium sulphate brines and then followed with a water displacement wash.
A conventional fixed bed absorption column may be used to reduce acidity in anolytes. This works quite effectively in reducing acidity and produces anolyte effluent that is non acidic until the capacity of the bed is exhausted. However, a consideration with column absorption is that the column is entrained with acidic anolyte when the resin bed acid absorption capacity has been exhausted. As well, the bed contains sodium sulphate in the entrained liquor.
Regarding entrained sodium sulphate, significant carry over to the potassium sulphate production zone may not be LAW OFI'IC~g FINNEcANHENDER50N desirable under most circumstances. Carry over of sodium FAR~sow, GARRErr ~ D~NER,L L.P. sulphate entrained in the base resin to the potassium sulphate 13O0 ~ 5~EET, N. W.
WASHlNGToN~ DC Z0005 Z02 - 40~- 4000 -13-production zone is not in itself a problem. The processes of the alkali metal sulphate (e.g., potassium sulphate) production zone can accommodate sodium sulphate without affecting the ion exchange production process provided that a sodium bleed is provided for in an evaporator used for volume control in the alkali metal sulphate (e.g., potassium sulphate) production zone. However, carry over of large amounts of sodium sulphate is not desirable since it is an expensive feedstock for the electrolytic process. The sodium sulphate is preferably removed by a displacement wash.
With respect to acidity, the pH of the residual anolyte entrained in the resin can be quite acidic and in the range of a pH of about 0.6 when the acid content of the anolyte exiting the cell is in the range of 50 g/l of sulphuric acid. It can be difficult to wash this acidity out with water alone, particularly when using a weak base resin.
Acidity entrained with the resin can be transferred into the salt out crystallizer or crystallizer from the resin when the resin is treated with an alkali metal chloride (e.g., potassium chloride). A displacement wash with concentrated sodium sulphate brine can be used to reduce acid transport to the potassium sulphate crystallizer.
Acid entrainment by the resin in fixed bed columns is more severe with the use of a weak base resin rather than a strong base resin due to absorption or complexing of sulphuric ~AW orrlcr~
FINNECAN, HENDE~ON, acid with the weak base resin. With the weak base resin, and F~sow, G~RRE~
~D~NNE~LLP depending upon the extent of water washing, as much as 20~ to 1300 I ST~EET, N. W
V~lASlllNGTON~ DC 2000~i 20Z - 4O~- ~000 ~oo~

40% of the sulphate ion in the crystallizer can be due to entrained acidity.
If acidity builds up in the salt out crystallizer, this would gradually reduce the efficiency of the alkali metal sulphate (e.g., potassium sulphate) production step since undesirable and highly soluble potassium bisulphate and/or HCl can be produced. Knudsen, U.S. Patent No. 4,504,458 (incorporated herein by reference), does not teach this nor does this patent teach that acidic double salts of potassium sulphate can be formed. Therefore, the process of Knudsen would be unsuitable for this type of application.
Control of acidity levels in the alkali metal sulphate (e.g., potassium sulphate) production zone can be carried out by including a buffering system connected to the salt out crystallizer whereby clarified liquor from the crystallizer is passed through a reactor containing a cation exchange resin in the potassium form. The cation resin absorbs acidity and releases potassium into solution to form potassium sulphate.
This avoids the need to add expensive bases like potassium hydroxide.
The cation resin in the hydrogen form can then be converted to a calcium form with a hydrated lime slurry and the resin can then be regenerated with potash to yield the starting resin. In this manner, acid values originating from the anolyte brine can be managed, particularly if upsets in ~w orrlc~s FIN~EGAN,HENDERSON, the system occur. Base resin in the hydroxy form could also FARA~OW, GARRE~
~D~NER,LLP be used to adjust the acidity of the crystallizer circuit.
1300 I STREET, N. W
WASIIINGTON, OC Z0005 20Z - 40~ - ~000 -15-2i 80U3~ ;

A preferred base resin desirably absorbs the full acid equivalency above pH 4. The exchange of hydroxy groups on strong base resin for sulphate is generally near completion above pH 6 and this resin is preferred when used in conjunction with aqueous ammonia. With a weak base resin, an acceptable operating endpoint is usually lower. With weak base resin such as Rohm and Haas IR-93, endpoints are in the range of pH 4-5 leading to suitable working capacities.
When a high ratio of anolyte to resin, such as 10 to 1, 20 to 1, or higher, is used to maintain low levels of acidity in the anolyte compartment, the natural flow rate, in terms of USGPM, through batch reactors or fixed bed columns becomes difficult.
With fixed bed columns, pressurization is needed and even then, the geometry requires such a high ratio of diameter to height that it is less than desirable for the plug flow conditions needed for the remainder of the process wherein potassium sulphate is produced. While removing the resin to a more appropriate ion exchange device for further processing can be done, there is a more convenient option.
An alternative approach to stripping acid from anolyte is to employ a high surface area liquid/solid contacting device such as a horizontal belt filter or pan filter. For continuous operation, a horizontal belt filter equipped with multiple spray bars may be used whereby base resin in the ~w Ol'~lC~g FINNEC~,HENDERSON, hydroxy form is slurried on to the belt and the resin cake is FARABOW, GARRETT
~ ~NNER,L.L.P. sprayed or eluted with anolyte to load the resin in the 1300 I STQEET, N. W
WAs~llNGToN~ l~c ZOOOS

218~039 sulphate form. The anolyte may be passed over the bed one or more times in a counter current or co-current mode to ensure effective acid stripping and thereafter washed. For high anolyte to resin volumes, resin bed depths may be on the shallow side for best stripping efficiencies when the belt is operated in a single pass mode for high anolyte to resin volumes.
In a multiple pass or recycle mode for resin employed to achieve more suitable anolyte to resin volumes, one or more belt filters may be run in a continuous batch mode wherein resin is passed over the belt several times in a thicker layer from at least about 2 inches, more preferably from about 2 to about 18 inches under successive anolyte sprays, preferably in the countercurrent mode, such that the pH of stripped anolyte rises to an appropriate level of a pH 4-6 or more. When the acid stripping capacity of the bed is exhausted, it is sent to the potassium sulphate production zone and fresh base resin in the hydroxy form is placed on the horizontal belt device.
Operations in this manner increase the contact time between the resin and anolyte, which is desirable, and yield increased resin bed depths which is known to be desirable by practitioners of the arts in conducting effective ion exchange processes such as stripping.
Those skilled in the art will recognize from the present invention that use of a horizontal belt filter to carry out ~w orrlca3 FINNECAN, HENDE~ON, ion exchange in the foregoing manner can be extended beyond an F~RABOW, GARRETT
1300 ~ ST~ET, N W. acid stripping operation to production, washing and WA5~INGTON, DC 20005 z02-.Oe-4ooo -17-- 2180Q~4 regeneration steps as used in other types of devices listed in the Encylodpedia of Chemical Technology or Perrys' Chemical Engineers' Handbook.
Use of two horizontal belt filters, one for production and one for regeneration, with appropriate sprays for production liquor, regeneration liquors, and wash sprays, offers an alternative approach that separates out individual unit operations in ion exchange rather than combining them in a single device. This separation allows processing flexibility and close control that permits optimization of individual steps in a simple, open and continuous manner.
The shallower than normal bed depth can be compensated for by multiple passes of the resin over the belt as needed along with recycle of liquors. This could be termed a moving bed, horizontal moving bed, or a thin layer bed.
In a preferred method of the present invention, once the anolyte has been stripped by contact with base resin in the hydroxy form, the sulphate loaded base resin, which has had a displacement wash with sodium sulphate brine and then water to remove entrained anolyte, passes into the potassium sulphate production zone. The sulphate loaded resin and brine of potassium chloride and potassium sulphate, saturated with respect to each other at about 25~C to minimize potassium sulphate content, are contacted at about 25~C to about 95~C
but preferably at about 50~C to about 75~C in an ion exchange ~AW OrFlC~
FINNECAN, HENDE~ON, reactor such as a fixed bed column or a moving bed device.
FAR~BOW, GARRErr ~ DUNNER,L.L.P.
1300 I STREET, N. W.
WA51-IINGTON, DC 20005 202-40~--1000 - 21800~4 The volume of anolyte to resin is preferably in the range of two to one. In one arrangement of the process, the exit anolyte, rich in potassium sulphate, is passed through a flash cooler and then on to one or more crystallizer tanks wherein solid potassium chloride is added to saturate the brine at or near room temperature. Potassium sulphate, also known as sulfate of potash (SOP), is precipitated out during the addition of potassium chloride to the process. SOP is separated from the liquor, sent to the dryer, and then to a granulation circuit for formulation into granular product.
High grade SOP product can be made by repulping centrifuged potassium sulphate crystals in potassium sulphate brine, centrifuging, then drying the product. Alternatively, a potassium sulphate wash on the centrifuge may be employed.
The chloride ion content of the product that has been treated in this manner is 0.05%.
The salt out crystallizer mother liquor which is rich in potassium chloride is made ready for the production step by heat exchanging the brine at about 50~C to about 70~C prior to contact with the resin. The design contains a provision for evaporative capacity to control circuit volume. The evaporative load can range between 1-3 tons of water per ton of product although lower evaporative loads can be achieved by using a displacement wash of potassium chloride amounting to about 0.2 to 0.3 bed volumes of base resin in the sulphate L~W orrlcrg FINNEC~N,HENDE~ON, form to displace wash water entrained in the resin as it FARABOW, GARRErP
1300 I ST~EÉT', N. W enters the potassium sulphate production zone. In an W~SHINGTON, DC 20005 202 40~-4000 - 21~0034 alternate process configuration, a conventional crystallizer can be used to recover potassium sulphate from the production liquor.
When the base resin in the sulphate form is treated with hot liquor containing primarily potassium chloride, the resin is converted to the chloride form. A displacement wash is used to remove potassium liquors entrained in the resin. This is sent to the production brine circuit.
The base resin in the chloride form can be converted to the hydroxy form by slurrying the resin into a stirred reactor at about 50~ solids and adding typically 5-7~ excess hydrated lime to the slurry in a controlled manner so that the rate of addition does not significantly exceed the rate of consumption. Partially or fully calcined dolomite may be used in place of lime. U.S. Patent No. 4,708,804 (incorporated herein by reference) does not teach that hydrated lime can be used in this manner nor does it teach that chloride ion, which is difficult to displace from base resin, can be removed in this manner and the resin placed in the hydroxy form.
After treatment with a hydrated lime slurry, the resin and the residual hydrated lime slurry are separated. The base resin in the hydroxy form is thoroughly washed in a conventional manner to clean the resin surfaces and by a displacement method to ensure no calcium carryover. The displacement wash can include the use of 0.2 to 0.3 bed LAW orrlc~s FINNECAN, HENDERSON, volumes of hot brine rich in sodium sulphate. Resin is then FARA80W, GARRE~
6 DUNNER, L. L P.
1300 ~ STREET, N. W
WASH~NGTON, OC 2000~5 Z02 40e-'-000 - 2:18003~

returned to duty in the absorption of sulphuric acid from the anolyte liquor.
The hydrated lime slurry is sent to a settling tank and some material is sent for recycle and some is discharged. The settling circuit contains a bleed for calcium chloride brine.
This brine can be concentrated up for sales by conventional evaporation as outlined in the Encyclopedia of Chemical Technology (lst Ed.). Alternatively, initial concentration steps can be achieved by cation exchange using sodium chloride or reverse osmosis.
In an alternative method of regeneration, ammonia is used as the working base and lime is used to regenerate ammonia.
In this method, base resin in the chloride form leaving the production zone for regeneration is contacted with aqueous ammonia. This can be done using a fixed bed column that is capable of resin removal such as the Himsley device or a stirred reactor can be used wherein ammonia gas is sparged into a higher density solids/liquids slurry of resin and water.
In contacting resin with ammonia solution in a column, there is a better opportunity to achieve higher ammonium chloride solution concentrations than with a stirred reactor.
With a fixed bed, a displacement production volume of aqueous ammonia in an amount equal to 0.2 to 0.3 bed volumes of resin can be introduced and taken out of the column by displacement ~AW O~IC'S
FINNEGAN,HENDERSON, washes of water. In either case, ammonium chloride liquor is FARABOW, GARRErr 6 DU~N~R,L.L.P.
~300 I ST~EET, N. W
WAS~INGTON, DC Z0005 202 40~ 4000 ~1 8003~

produced and the base resin is transformed from the chloride form to the hydroxy form.
Care may need to be taken to remove residual ammonia values from the resin before it is returned to acid absorption from anolyte so ammonia values do not end up in the cell or lost to other parts of the circuit. This can be accomplished by a displacement wash of water, followed by one of sodium sulphate and/or dilute caustic soda. The resultant ammonium chloride brines can be treated with hydrated lime to release ammonia for recycle and generate calcium chloride brines for concentration to saleable products.
The individual method chosen for regeneration of base resin in the chloride form to the hydroxy form depends upon several factors. Weak base resin in the chloride form regenerates better than strong base resin and hydrated lime or ammonia may be used to regenerate weak base resin. Ammonia solutions can be used to generate higher initial concentrations of calcium chloride than direct use of lime and this is beneficial in economically concentrating calcium chloride brines to saleable products.
Lime is not very effective in regenerating strong base resin from the chloride form to the hydroxy form; however the higher levels of hydroxide with ammonia solution over hydrated lime enable strong base resin to be regenerated with ammonia solution, preferably in a column type operation where a ~AW O~IC~S
FINNE~AN,HENDE~ON, displacement volume of aqueous ammonia equal to about 0.2 to FAR~OW, GARRE~
~ DUNNER,L.L.P. O . 3 bed volumes is used to remove chloride ion from the resin.
1300 I STREET, N. W.
WA5~INGTON, DC Z0005 202 - '~0_- ~000 --22--~ 2'1gO03~ ~

Strong base resin in the hydroxy form can be usefully employed to absorb sulphuric acid from anolyte since strong base resin generally is neutralized by acid at higher pHs than weak base resin.

EXAMPLES
A number of experiments were carried out whereby solutions of 50 g/l of sulphuric acid at 50-70~C, with and without sodium sulphate, and simulating anolyte liquor, were contacted with base ion exchange resin in the hydroxy form in a fixed bed column to strip the sulphuric acid from the solution and load sulphate ion on the base resin. Elution of the resin was stopped when the pH of the effluent liquor declined from basic values and reached a pH of 4-6. The base resin was then treated with a hot potash brine at about 50-70~C which had been prepared from a solution saturated with respect to potassium chloride and potassium sulphate at room temperature. The eluant was cooled to crystallize out potassium sulphate and further potassium sulphate was precipitated by the addition of potassium chloride to saturate the brine with respect to each potassium salt. The amount of potash added varied with dilution due to water entrained in the resin, wash water and pH of the production brine. This liquor was then used as a production brine for another trial.
The resin in the chloride form was then regenerated to the L~W OrFlC~9 FINNECAN,HENDE~ON, hydroxy form by making up a 50~ slurry of resin with water.
FARAsow, CARRE~
~ DUNNER,L.L.P. Calcium hydroxide was then added in portions, with stirring, 1300 I STQEET, N. W.
W~SH~NGl'ON, DC 20005 ' ~ 2180034 ' until the slurry became permanently cloudy. The results are contained in Table 1 using about 0.65 L of IR-93 weak base resin.

Volume (L) Pota~sium of Anolyte Sulphate 50 g/l Thermal Calcium Sulphuric Step Hydroxide Acid (q) (~) A. 1.3 34 30 B. 1.05 39 30 C. 1.10 45 26 D. 1.1 44 26 E. 1.1 45 24 With Run E, a full material balance was taken. The amount of potassium sulphate recovered through salt out as potassium chloride was added to the cooled production brine amounted to 10.8 g for a total yield of 55.6 g of potassium sulphate. A density gauge was used to indicate the saturation point of potassium chloride addition. The amount of potassium chloride added was 57 g.
On average about 0.55 moles of acid was absorbed and entrained in the resin producing about 0.32 moles of potassium sulphate through the addition of 0.77 moles of potassium chloride while about 0.37 moles of calcium hydroxide was needed to regenerate the resin. The results are consistent indicating that the regeneration step works effectively and LAwOr~,c~5 consistent yields of potassium sulphate are obtained.

FINNECAN, HENDERSON, FAR~OW,GA~ Entrained acid, as measured by titration with KOH solution, ~i DIJ'NNE~, L, L. P.
WA5~1NGTON, DC Z0005 can be significant amounting to about 40~ of total acid Z02- 40~3-4000 - 2~8~34 absorbed when this is carried out in a fixed bed column.
Entrained acid can be partly washed out or neutralized in the crystallizer mother liquor or this can be minimized by a displacement wash with sodium sulphate solution. With sodium sulphate added to the acidic anolyte to simulate a cell exit solution, the initial sodium level was 12,200 ppm while it was 12,400 ppm after being passed over the weak base resin. This indicates little or no interference due to sodium sulphate with the acid stripping process. In a measurement to examine potential calcium carryover from the lime regeneration step and thereafter into the cell anode compartment, the initial calcium level for the anolyte was 25 ppm while the effluent level was 22 ppm, indicating little change as anolyte passed over the resin.
In another type of contact between anolyte and resin whereby anolyte is added to base resin in the hydroxy form with agitation, the addition of 0.5 equivalents of acid to 0.6 equivalents of resin dropped the pH of a resin/water slurry from about 10.8 to 4.6 using IR-93 resin and the like. Ion exchange resins are also described in Encyclopedia of Polymer Science and Eng'r., Vol. 8, pp. 341-393 (1985), which is incorporated herein by reference. Selection of a base resin that exhausts most of its acid absorption capacity by pHs above 4-5 is desirable. Resin could be regenerated with ammonia to a pH of about 9.0 before a type buffering set in.
LAW OrCICf5 It will be recognized that these examples are indicative FARABOW, GARRETr ~ 3WNER,L.L.P. of how base ion exchange resin can be usefully employed to 1300 I STI~cEr, N. W
W~SHINGTON, OC 20005 20Z - ~-0~ 000 - ~1 80034 remove sulphuric acid from the anode compartments of two and three compartment cells used in the production of caustic soda from sodium sulphate.
In view of the present invention, it will be evident to those skilled in the art that while acid stripping from anolyte is operationally simpler with higher acid levels in the anolyte leaving the cell, the nature of cell membranes, with two compartment cells in particular, is such that lower levels of acidity may be preferred for high current efficiencies. The process and equipment specified will allow sufficient flexibility for overall economic operation.
In another manner of acid removal, cation resin may be usefully employed with three compartment cells wherein a potassium laden cation resin is contacted with anolyte. The sulphuric acid is effectively converted to potassium sulphate which can be built up to as high as 150-180 g/l, at temperatures up to 95~C, and removed from the anolyte system and processed to recover potassium sulphate by evaporation.
Use of strong acid resin typically yields a more acidic pH for anolyte liquor than weak acid resin.
The choice of cation ion resin depends upon the most desired operating and pH conditions for the anolyte compartment. Weak acid cation resin is preferred for cells requiring less acidic anolyte liquor for efficient functioning. A two staged absorption may be employed whereby ~AW or~l c ~
FINNEG~,HENDE~ON, anolyte first contacts strong acid resin in the potassium form FAR~sow, G~RRErr ~ ER,L.L.P. and then weak acid cation resin in the potassium form.
1300 I ST~EET, N. W.
W~S~INGTON, DC Z0005 202 40~1-4000 --26--21800~4 Contact of this resin with a base such as calcium hydroxide neutralizes the acidity of the resin and loads the resin with calcium. The resin can then be more effectively treated with potassium chloride to regenerate the potassium form of the resin.
U.S. Patent No. 3,096,153 (incorporated herein by reference) uses cation ion exchange to produce potassium sulphate from sulphuric acid; however, this method uses - potassium chloride directly to regenerate the resin while the present invention uses first a base and then potassium chloride, which is more efficient in terms of minimizing losses of costly potassium chloride. Potassium ion is not very effective in displacing hydrogen ions from cation resin.
Processes have been presented whereby two or three compartment cells may be employed to electrolyze non chloride alkali metal salts such as sodium sulphate or potassium sulphate. The choice of cell type depends upon the comparative capital and operating cost of each and consideration of factors related to cell operations such as membranes, appropriate electrolyte concentrations, and suitable circulation rates for anolyte.
Generally speaking, two compartment cells are less costly in terms of capital and operating expense, and are therefore preferred. However, with two compartment cells, it is more difficult to select membranes and efficient cell operating FINNEGAN,HENDERSON, conditions suitable for the higher acid anolyte conditions FARABOW, GARRETT
~ D~ E~, L. L. P.
~300 ~ STi~EET, N. W
WASI~NGTON, DC 20005 202- ~01~ 000 which are more desirable for balanced volume contact between anolyte liquor and resin.

L~W o~rlcr~
FINNEGAN, HENDERSON, .
FARAEOW, GARRErT
li D UNN ER, L. L. P .
1300 I STREET, N. W.
WAS~INGTON, OC 20005 20Z - ~0~ - "000

Claims

1. A process for producing alkali metal hydroxide by electrolysis of an alkali metal sulphate, comprising removing anolyte having acidity due to production of sulphuric acid during the electrolysis from an anolyte compartment and contacting the anolyte with a base ion exchange resin in the hydroxy form to reduce said acidity and load the base ion exchange resin to the sulphate form; replenishing said anolyte with alkali metal sulphate; and returning said anolyte to said anolyte compartment where additional electrolysis occurs producing said alkali metal hydroxide in a cathode compartment.

2. The process of claim 1, further comprising contacting the resultant base ion exchange resin in the sulphate form with a solution comprising alkali metal chloride optionally containing an alkali metal sulphate to form an alkali metal sulphate and a resin in the chloride form and then recovering the alkali metal sulphate and regenerating the resin in the chloride form with an alkaline earth metal hydroxide or partially calcined dolomite.

3. The process of claim 1, further comprising contacting the resultant base ion exchange resin in the sulphate form with a solution comprising alkali metal chloride optionally containing an alkali metal sulphate to form an alkali metal sulphate and a resin in the chloride form and then recovering the alkali metal sulphate and regenerating the resin in the chloride form with ammonia to produce ammonium chloride solution.

4. The process of claim 3, further comprising treating said ammonium chloride solution with a base to produce ammonia.

5. The process of claim 4, wherein said base is lime.

6. The process of claim 2, further comprising subjecting the base resin in the chloride form to a displacement wash to remove entrained alkali metal chloride prior to said regenerating with said alkaline earth metal hydroxide.

7. The process of claim 3, further comprising subjecting the base resin in the chloride form to a displacement wash to remove entrained alkali metal chloride prior to said regenerating with ammonia.

8. The process of claim 2, further comprising subjecting said resin to a displacement wash to remove entrained alkaline earth metal chloride after treating said resin therewith.

9. The process of claim 3, further comprising subjecting said resin to a displacement wash to remove entrained ammonium chloride and ammonia solution.

10. The process of claim 1, further comprising treating said base resin in the sulphate form with ammonia to regenerate the base resin to the hydroxy form and produce ammonium sulphate solution or solid.

11. The process of claim 10, further comprising contacting said ammonium sulphate solution with a potassium laden cation ion exchange resin to produce potassium sulphate and an ammonium laden cation ion exchange resin and regenerating said ammonium laden cation ion exchange resin with potassium chloride to produce a solution comprising ammonium chloride and said potassium laden cation ion exchange resin.

12. The process of claim 1, further comprising treating the resultant base ion exchange resin in the sulphate form with ammonia to regenerate the base resin to the hydroxy form and produce ammonium sulphate solution.

13. The process of claim 1, further comprising subjecting the base ion exchange resin in the sulphate form to a displacement wash to remove entrained anolyte from the resin.

14. The process of claim 11, further comprising treating said ammonium chloride with an alkaline earth metal hydroxide to form alkaline earth metal chloride and ammonia.

15. A process for producing potassium sulphate by electrolysis of an alkali metal sulphate, comprising removing from an anolyte chamber anolyte having acidity due to production of sulphuric acid during the electrolysis and contacting the anolyte with a cation exchange resin in the potassium form to reduce said acidity and load the cation exchange resin to the hydrogen form and produce a solution comprising potassium sulphate.

16. The process of claim 15, further comprising separating the potassium sulphate from said solution comprising potassium sulphate.

17. The process of claim 15, further comprising treating said cation exchange resin in the hydrogen form with ammonia, an alkaline earth metal hydroxide, or partially calcined dolomite.

18. The process of claim 17, further comprising treating said cation exchange resin with potassium chloride to regenerate said resin to the potassium form.

19. The process of claim 15, wherein said anolyte has a pH of about 6 or less.

20. The process of claim 15, wherein said anolyte has a temperature of from about 20°C to about 75°C.

21. The process of claim 1, wherein said anolyte has a pH of about 6 or less.

22. The process of claim 1, wherein said anolyte has a temperature of from about 20°C to about 25°C.

23. The process of claim 1, wherein a horizontal belt vacuum filter or pan filter is used as a means of contacting said anolyte with said base ion exchange resin in the hydroxy form.

24. The process of claim 2, wherein a horizontal belt vacuum filter or pan filter is used as a means of contacting said alkali metal chloride with said base ion exchange resin in the sulphate form.

25. The process of claim 3, wherein a horizontal belt vacuum filter or pan filter is used as a means of contacting said alkali metal chloride with said base ion exchange resin in the sulphate form.

26. A process for producing caustic soda wherein sodium sulphate is passed through the anolyte compartment of a two compartment cell or the central compartment of a three compartment cell and caustic soda is produced in the cathode compartment and sulphuric acid is produced in the anolyte compartment by passing a direct electric current between the anode in anode compartment and the cathode in the cathode department producing caustic soda in the cathode compartment and sulphuric acid in the anode compartment, said process comprising reducing the sulphuric acid by removing the anolyte and contacting the anolyte with a base or an anion exchange resin in the hydroxy form to load the resin with sulphate ion.