CA1210779A - Process for the removal of oxalate ions - Google Patents

Abstract

- ? -A B S T R A C T

A PROCESS FOR THE REMOVAL
OF OXALATE IONS

A process for the removal of oxalate ions from an aqueous solution containing iron chelate or chelates of nitrilotriacetic acid and decomposition products thereof, including oxalate ion, by contacting the solution with an amount of a composition capable of providing hydrogen ions in said solution sufficient to precipitate ferrous oxalate, under conditions to precipitate ferrous oxalate, and precipitating ferrous oxalate and separating precipitated solid from the solution.

Description

~l2~

A P~DCESS FO~ REM~V~L
OF OXAIAIE ICNS

The present m ~ention relates to a process for the remcval of oxalate ions from an aqueous solution containing polyvalent metal chelates of N trilotriacetic acid and dRccmposition products thereof, including oxalate ions.
The presence of significant quantities of H2S and 2 in various sour m dustrial gase ws streams poses a persistent problem. Althcugh various procedures have been develcped to remove and reccver these contaminants, m~st such processes are de~icie~t, for a variety o~ reaso~s.
In one cyclic pro¢ess currently attracting attention, the sour gas is contacted, preferably with a solve~t-reactant system which oomprlses a regenerable reactant, to produce solld ~ree sulphur whlch l~ recovered either prlor or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, such as iron, vanadlum, cepper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are ooordination complexes in which the polyvalent metals form chelates with specified organic ac'~. m e preferred polyvalent metal is iron.
Where ligands or chelates or polyvalent metals are employed, degradation or decomposition of the polyvalent metal chelates represent~ an important cost in the process, as well as requiring measures for deccmposition product bleed, remaval or treatment, and ~ Mition of fresh solution. ~ven in the case of preferred chelates such as those of nitrilotriace~ic acid, ligand dexxx~position, over a period of time, requires attention to prevent buill-up of decomposition.products and oonsequent loss of efficiency. As will be reoognized, th~ bleed from such ~Z~ '7~

processes contains, along with the decomposition products, a considerable amount of the valuable chelate or chelates.
Oxalate i~n is apparently a decomposition product of the ligands employed. While the presence of limited quantities of oxalate ion appears beneficial (see, e.g., U.S. patent specification No. 4,009,251), in practi oe , the concentration of oxalate ion is significant in deternunation of bleed rate of the solu~;on. Accordingly, selective rem~val of oxalate ion from the ligand solution wculd tend to minimize bleed rate and Improve the eccnomics o such processes. The invention provides for such r~l~val.
m erefore, the invention relates to a process for th2 removal of oxalate iors from an aquecus solution containing iron chelate or chelates of nitrilotriacekic acid dRccmposition products thereof, including cou~late ion, by contacting the solution with an a~LUnt of a composition capable of prcviding hydrogen ions in ~i~ solution sufficient to precipitate ferrous oxalate, under conditions to precipitate ferrous oxalate, and precipltating ferrous oxalate and separating precipitated solid from t~e solution. m e ferrous oxalate i5 precipitated, and then separated from the solution. As usPd herein, the term "under conditlons to precipitate" and variants thereof, merely implies the use of temperatures at which the ferrous oxalate exceeds it solubility in the solution treated. Such conditions ~ay be inherent in the steps performed, or may be accomplished simply by caoling. It is not necessary that all the oxala~e be precipitated; some ax21ate, as ncted, supra, is beneficial.
Preferably, the a4uecus solutian is a bleed stream from a process for removing ~ S from scur ~ S-contain m g gaseous streams in which the gasecus sLLeam is contacted with an axidizing re~ mt solutian cantaining the ferric chelate o~
nitrilokriacetlc acid as the oxidizing ~eactant The inventian - ~2~

is admirably suited even in those of such prccesses utillzing a selective absorbent.
In such case, the bleed stream aftPr having reduced the oxalate ion content, may simply be returned to a suitable point in the process. Because such a stream will be smQll in relation to the volume of solution in the system, mu$Linal pH adjustment will be required, and may be conduc~ed as p æ t of the overall pH
adjustment of the system. Separate pH ad~ustment may be made by Mition of any suitable basic materials, as will be recognized by tho æ skilled in the art. The oxidizing ferrlc chelates of nitrilokriacetic acid, will be used in the H2S removal according to the pro oe ss of the invention.
Accordingly, in this cc~teKt, the inventicn further relates to a process for the remcval o~ ~ S fram a scur gasecus stream compris m g:
a) oontacting the sour ga~x~ls stream in a contacting zone ~ith an aqueous rea~k~n solution at a temperature below the melting point of sulphur, the mixture comprising the ferric chelate of nitrilotriacetic acid as oxidizing reactant, to produce a sweet gas str~n and an aque w s admixture contai m ng sulphur and re ~ reactant;
b) remaving aqueous admixture from the c~ntac*ing zone, and removing solld sulphur fro~ said aqueous admixture;
c) re~enerating said aqueous admixture, prcducing a re-generated oxidizing reactant soluti~n, and returning regenerated axla~zing reactant soluticn to the c~nt zone;
d~ remav mg a bleed stream o~ntaining iron chelates of nitri-lotriacetic acid and deccmposition pro~ucts of said chelates Includi~g oxalate ion, from one or mDre loci in or b~tween steps a, b, or c;
e) oontacting said bleed stream with an amount of a compo-sition capable of praviding hydrogen ions in the bleed .

~l2~ '7~3 stream and under conditions to precipitate ferrous oxalate, but not remove the bulk o~ the polyvale~t chelate or chelates in the bleed stream, and precipitating said ferrcus oxalate and separating precipitated solid from the bleed stream.
As will be evident ~o thosP skilled in the art, the particular location or point of removal of the bleed stream in su~h a process is not critical, r~-~val of the bleed stx~#~n subsequent to the contact zone being preferred. ~ga m , the bleed l~ stream may be removed from a portion of the process stream if the stream is divided f~r any purpose, e.g., a portlan ~or sulphur remDval, and a portion sent directly to reg~nPration.
The speci~lcs of the ~ S removal p~ooess are not c,ritical; e.g., the sulphur and bleel stream may be removed prior to or subsequent to reg~n~ration. Mkreover, the sulphur may first be ooncentrated i~n a portian of the liquid ~n circulation in the proc~ss, and thi~ ~ay be done prlor to or subsequent to regeneraticn. I~ the sulphur-oontaim ng liquid is first ooncentrated into a slurxy be~ore final separation o~ the 20 sulphur, the liquid, or a po~n ~ereof, fr01 the slurry may be u;tilized as a bleed strean. ~he bleed stream may be ccntinu~us or intermitt~nt, alth~gh the overall H2~ rer~val pnx~dure is preferably oonti~a~. Fre 3h make-up chelate or chlelate-containir~ soluti~n may s~lularly be suppl;~
25 ~ti:~ly or ~nterrnittently. ~Che rate and vol~ne of the bleed stream will de~d ~n a variety of factor3, but, as indicated, the concentration of oocalate ian is the predaninan~
cansidRration~ A~cordingly, a precise volume of the bleed st~ n (and makerup) ~although obviously a quite minor portion of the total volume of liqyid in the system) cannot be given, but, preferably, 20 per oe nt to 0.5 per oe~t, by volume, of the tokal aqueous cdbL~chIre will suffi oe . The bleed stream may be returned to any suita'ble po~nt in the systen.

:

The Lnventlon also encampasses treating scur gaseous streams which contain, apart from H2S, significant quantities of C02, in the m2nner described. More particularly, the sour gas stream containin~ ~2S and 2 is contacted with a selective absorben~-aqueols reactant mixture preferably, at a temperature below ~he melting point of sulphur, the reactant m~xture and p m cedure being similar to that described, supra. Broadly, this acco~plished by the use of an absorbent mixture containlng a selective absorbent for 2 (and preferably for H2S, as well), and an effective amLunt o~ an oxidizing fexric chelate of nitrilotriacetic acld, as described supra. A purified or sweet gaseous stream is produced which meets general industrial and commercial H2S and CO2 specifications. The CD2 is absor~ed and the H2S is immedlat~ly conver~ed to sulphur by the ferric chelate. In the pro oe ss, the reactant is redNced, and the sulphur may be treated, as described supra. The ~ulphur m~y be removed prior or subsequent to regeneration of the admixture. A
bleed stream may suitably be removed, and treated, as described herein.
In a preferred ertxxL~nc~t of the process according to the present inYe~ion the H2S-conta m ing sour gaseous stream is divided into a major portion and a minor portion and the bleed stream is contacted with the ~inor portion of the sour gaseous stream to reduoe the H2S concentration in the minor portion and to produce hydrogen ions in the bleed stream.
Preferably, the minor por~i~n of the H2S-c~nt~n;ng saur gaseous stream co~prises 0.01 to 30 %vol of the total saur gaseous st~ m, de~3YdIng an the concentration o~ H2S in the saur gaseous stream. The pH-lowering in the bJeed stream, caus2d by t~e H2S from the m m or porti~n of the saur gase w s stream, is generally to a value of abcut ~ w~hich may be sufficient.
Preferably a pH-lowering substan oe is then added to lower the pH
o~ the bleed stream further. ~he pH of the bleed stream is ~2~'7~

preferably adjusted by a composition capable of providing hydrcgen ions to a ~alue of from 3 to 5.
If not all the ~ S m the minor portion lS remaved, e.g.
if the ooncentration of oxidiz mg reactant still present in the bleed stream, is insu~ficient to convert all of the ~ S in the m~nor portio~, the partially sweetened gas i~ then preferably not vented. After being oonta~ted wi~h the bleed stream the minor portion o$ the sour gaseous stream lS preferably passed to the sour gaseous stream to be conta~u#d with said aquÆous reaction soluticn.
In any event, the bleed strean containing the chelate is oontacted with a composltlon capable of providing hydrogen ions Ln solution. Any composition capable of prcvlding sufficient hydrogen lcns in solution to lower the pH the desired a~ount may be employed. A suitable composition lS selected from the grcup, ccmpris~ng H2S04, HCl, H3PO4, SO2, NaHS03, N~CH2COOH)3, N ( hydro~yethyl)ethylene dia~lne triacetic acid, ethylene diamine tetraacetic acid and mixtures thereof. The oompositions are supplied in an amount sufficient to precipitate the bulk of the oxalate without removing a subst~ntial qu~ntity o~ the iron oompleK or complexes o~ nitrilokriacetic acid in the bleed stream. Preferably, su~ficient pH lowering substance sh~uld be supplied to lower the pH of the blPo~ strea~m to a pH of from 3 to 5. This amcunt, o~ oourse, may be de~ermined routinely.
m e precipitated ferrous oxalate may be separated by any suitable means oh as by filtration, and the supernatant liquid is reoovered and returned to the process. Suit2~ble t~peratures will range from 10 C to 80 C, preferably fran 20 C: to 50 C.
Prior to retum of the r~uning solution to the process, the 30 r~nai~g solutian, now havin~ reduced a~alate content, may be t:reated or cantacted with a suitable baslc con4Osition. Or the r~r~g soluti~ may s~ly be returned to the system, the overall pH a~justment o$ the syst~m being ~lc~ed to arrive at ,~

~2~

the proper pH. Pre~erably, the pH of the rem~Lumng solution is ad~usted to a value of 4 to 9 and preferably at least the ~uIk of the remaining solutlon is returned to the aquecus admixt~re.
Sultable basic ccmposltlons for the pH-adjustment include, but are n~t limited to, ~H40H, NaOH, the Na salt of N-(2-hydroxy-ethyl)ethylene diamine triaacetic acid or the Na salt of ethylene diamine tetraacetlc acid, N~ ~ CCONa)3, and Na2C03. As will be reccgnized by those skilled in the art, not only m~st pH
be raised, but the iron metal precipit~ted must be replaced.
This may be d~ne in any suitable fashion. For example, the iron way be added or replaced as iron carbcnate. Alternatively, iron/ammonia mixtures may be used.
~ n the bleed stream the iron chelate or chelates way be pres~nt in m~re than one species; for example, the bleed stream might and probably wculd o~ntain both the ferrlc, Fe (III~, and ferrous, Fe (II), chelates o~ nitrilotriacetic acid. It is an adva~tage of the inventian that the kniLk of the chelate does not precipitate, but rema~ns in solution up~n the addition, and thus, an e~fective separaticn is achieved.
Ihe great val~3 of the ~nvention lies in the albility to reta~n the polyvalent metal nitrilotriacetic acid cc~plex so that it may be retu~ned to the aforementioned gas purificati~n processes. The polyvalent mekal chelates of nitrilotriacetic acld æ e readily formed in aq~ us solution by reaction of an appropriate salt, oxide, or hydroxide of the polyvalent metal and the chelating agent in the acid form or an aIkali o~
ammoniu~ salt of the chelating acid.
In the case of utilization of a bleed stream ~ram the afore-mesl:ka~el gas purifica~ion pro oe sses, the particular type of gaseous stream treated is not critical, as will ~2 evldent to thos2 skille~d in the art~ Streams particularly suited to remLval of ~ S by the practi oe of the inventlo~n a~2, as indicated, naturally-occurring gases, synthesls gases, process gases a~d , ~.Z~7'7~

effluents, and fuel gases produced by gasiflcation procedures, e.g., gases produced by the gasification of coal, petroleum, shale, tar sands and the liKe. Particularly preferred are sour gasecus streams selected from natural gas streams, streams derived f m m the gasification of coal and refinery feedst ~ s comFosed of gaseous hydrocarbon streams, especi~11y those streams of this type having a low ratio of ~ S to 2' and other gasecus hydlsx~libon streams. The term "hydrocarbon stream(s~ n as employed herein, is intended to include streams containing significant quantities of hydrocarbon (both paraffinic and aromatic~, it being recognized that such streams oontain si~nificant "lmpurities" not technlcally defin~d as a hydro-carb~n. Aga~n, streams containLng principally a ~i~gle hydro-carbon, e.g., ethane, are em m ently suited to the practioe of the ~nventi~n. Streams ~erlved from the gasification and/or Fa~ al oxidation o~ gaseous or liquid hydrocarbon may be tr~ated by the practice o~ the ~nvention. The H2S o~nt~ent of the type of streams c~ntemplated will vary extenslvely, but, in general, will range ~rom abcut 0.1 per clent to abcut 10 p~r ce~t by volume. CD2 content may also vary, but may range fro~ abcut 0-5 Fer c~nt to about 95 Fer cent or greater by volume. obvi the amcunt o~ ~ S and C02 present are not generally a l~miting factor in the practice of the m vention.
The temperatures employ~d in the contact m g zone are not generally critical, exoept that the reaction is pre~erably c æ ried out below the melting point of sulphur, and, if an absorbent is used, the temperatures emplcyed m~st permat acceptable absorption of C02. In many oommerc~ applications, such as ~he remLval of H2S and C02 from natural gas to meet pipeline specifications, absorptian at ambient temperatures is desired, since the oost of refrigeration w w ld exceed the benefits obtained due to mrn~ 3d abscrE~ n at the lower -temperature. In general, temperatures fram lO C to 80 C are ,~

~2~ 7~
g ~
suitable, and temperatures ~rom 20 C to 45 c are pre~erred.
Contact times wlll range from about 1 second to about 120 seoGnds, with contact times of 2 seconds to 60 seconds being preferred.
Simil~rly, Ln the regeneration zo~e or zones, temperatures may be varied widely. Preferably, the regeneration zone(s~
should be ~aintained at substantially the same temperature as the contacting æone. I~ heat is addel to assist regeneration, cooling of the aq~ us admixture is required be~ore return of the admlxture to the c~ntactlng zone. In general, temperatures of fron abcut 10 C to 80 C, preferably 20 C to 40 C, may be employed.
Pressure conditions in the contactiny zone may vary widely, dL~x3~lLng on the pressure of the gas to ke treated. For example, pressures in the contacting zone may vary from 1-200, and in particular ~ram 1-150 atmDspheres. In the regeneration zone or zones, pressures may also be varled considerably, and will preferably range from ab~ut 0.5 at m sphere to ab w t three or four atm~spheres. m e pressure-temperature relationships in-volved are well understood by those skilled in the art, and neednot be detailed herein. Preferably, if the ir~n chelate of nitrilotri.~cetlc acid is used, pH Ln the process of the in~ention will range from abo~t 6 to about 7.5, and the molar ratio of the nitrilotrlacetic acid to the iron lS from abcut 1.2 to 1.6. 'rhe procedNre is pre~erably csfYhx~bed continuously.
As indicated, the m ventian provides, in the H2S remcval 3mbDdimYnts, ~or the regeneration o~ the reactant. Preferably, the aqueous admlxture containing the reduced polyvalent me*al chelate and o~*~nally an absorbent, are regenerated k,y contacting the mixture in a regeneration zo~e or zones with oxygen. As used herein, the term "oxygen" mcludes oxygen-conta m ing gases such as air~ or air-enriched with a~ygen. If significant quantities of CO2 have ~een absorbed, the reactant-'7~

oontaining solution is pre~erably treated, such as by heating orpressure reduction, to remcve the bulk of the oO2 before regeneration of the reactant (either prior or subsequent to sulphur remov 1). Alternately, or if small quantlties of C02 are absorbed, the C02 may simply be stripped in the regeneration zone.
As noted, the regeneration of the reactant is preferably - acco~plished by the utilization o~ oxygen, preferably as aLr.
The oxygen Wlll accomplish two br~ ns, the oxidation o~ the reactant to its hlgher valence s ~te, and the stripping of any resldual C02 (if originally present) from the aqueous adnuxture.
The oxygen (in whatever form suppliF~) is supplied in a stoichiometrlc equivalent or ex oe ss wlth respect to the a~Dunt of reduoed nE~ual ion of the chelate or chelates present in the mi~ture. Preferably, the oxygen i5 suppli~d in an amount from about 1.2 to 3 times excess.
The H2S, when oontacted, is quickly co~verted ky the iron chelate of the nitrilotriacetic ac~d to elemental sulphur. The anx~nt of the iron chelate, or mixtures thereof, supplied is an effectlve amLunt, i.e., an amcunt sufficient to oonvert all or substant;~lly all of the H2S in the gas stream, and will generally be on the order of at least about one m~l per m~l of H2S. Ratios of from abcut 1 or 2 ~1 to about 15 mols of iron chelate per m~l of H2S nay be used, with ratios of fro~ abcut 2 mols per mDl to abcut 5 m~ls of iron chelate per mDl of H2S
being pr~ferre~d. m e manner of preparing the aqueous reaction solution is a mat~er of choice. The iron chelate solution will generally be supplied as ~n aqyeous solution h~ving a concen-tration of from about 0.1 mDlar to about 2 molar, and a concentration of about 0.5 molar is preferred.
Since the iron chelate of nitrilotriacetlc acid has limi~ed solubility in many solvents or absorbents, if an absorbent is used, the iron chelate is preferably suppli~ in a mixture of 7'7~

the liquid absorbent and water. ~he niu~ner of preparing ~he n~rbure containing an absorbent 1S a matter of choice. For example, the chelate may be added to the absorbent, and, if necessary, then water added. m e am~Dnt of water added wlll normally be just that an~nInt necessary to a~hieve solution o~
the iron chelate, and can ~e detern~u~3d by routine experl-ment~tion. Slnce the iron chelate may have a significant solubillty m the solvent, and since water is produced by the reaction of the H2S and the ions of the chelate, precise amaunts of water to be added cannot be glven. In the case of absorbents having a low solubll~ty for the iron chelate, approximately 5 percent to 10 per oe nt wat r ~y volume, based on botal volume of the absorbent mixture, will generally provlde solvency.
Preferably, however, the iron chelate is added as an aquecus solution to the liquid absorbent. Where the reactant lS supplied as an aqueous 301u~kan, the amcunt of solution Q lied may be abcut 20 perc~nt to akout 80 per oe nt ~y volume o~ the total mixture of absorbent and water. An iron chelate solution will g~nerally be supplied as an aqueous solution having a can-centration of fro~ about 0.1 molar t~ abcut 2 molar, and aconcentratl~n of abcut 0.5 mol~r is pre~erred.
If an absor~ent is employed, it is selected from those absorbents whlch have a high degree o~ selectivity in absorbing C2 (and pre~erably H2S as ~ell) fro~ the gaseous streams. ~ny of the known absorbents co~ventionally us~d (~r mix~lres there-of) which do not affect the activity of the iron chelate, or mixtures thereof, and which exh~bit su~ficient solubility ~or the react mt or reactants may be empJayed. As indicated, the absorbent preferably has good absorbency for H2S as well, in order to assist m the remcval of any ~2S present in the gaseaus streams~ The particular abscrbent chosen is a matter of cholce, given the~e qualifications, and selec~ion can be ~ade by rautine experimentation. For exa~ple, diethylene glycol ethyl mcnc-,~ , ~Z~ '7~

ether, prcpylene carbonate, tetraethylene glycol-dimethyl ether, N-methyl pyrrolidone, sulpholane, me~hyl isdbltyl ketone, 2,4-pentanedione, 2,~-hR~ 3dione, diacetone alcohol, hexyl aceta~e, cyclohRxanone, mesityl oxlde, and 4-methy1-4-methoxy-pentone-2 m~y be used. Suitable temperature and pressure relationshlps for different OOz-selective absorbents are known, or can be cal-culated by those skilled in the art.
As noted, if a bleed stream solution is the stream con-templabed, the bleed stream is preferably treated after sulphur removal, mDst preferably be~ore re~eneraticn. The advantage of pre-regeneratlon treatment is the hlgh~r ooncentration of ferrous lon. The manner of recovery of the sulphur is a ma~ter of choioe. For example, the sulphur may be reccvered by settling, filtration, or by suita`ble devices such as a hydrccyclone. MDreovsr, it may be advantageous to concentrate the sulphur first in a portion of the admixt~re, either before or after regeneration. For example, the sulphur-ocntaining admixture ~ro~ the ccntac*ing zone ~or fro~ the r~generation zone) may be separa~ed into two pDrtlOns, a portion or stream having reduced sulphur oonten~, and a portion or stream oontaining increased sulphur oantent, preferably a slurry. The manner of c~p ratiDn is a matter of ch~ice, and equiFment such as a hydrccyclc~e or a centrlfugal sep rator may be 2mployed. If a s~urry is Frxxh~a, the slurxy or concentrated stream will cQmprise 2 per oe nt to 30 percent, by volume, (on a continNous basls) of ~he tctal stream~ from the oontact or reg ation zone. It is not necessary that absolutely all sulphur be remcved on a continuous basis in the process; the process may suitably be op~rate~ with a very nLhlor invent~ry o~ significantly reduced content of sulphur in the syst~m.
In the case where a 51Urry iS produoed, the slursy may be filtered or sub~ected to further treatment to remave the sulphur, and the recovered admixture may be used as all or portian o~ the 7'~

bleed stream treated for oxalate remaval, or it may be retur~ed to the process cycle, either before or a~ter regeneration.
In order to descxibe the preferred er}xxliment o~ the mvention in greater det il, reference is made to the acoompanying schematic drawing. The values given herein relat m g to temperatures, pressures, compositicns, etc., are calculated or merely exemplary and should not be taken as delimiting the invention. Flgure 1 illustrates the enixxLhment of the invention wherein sulphur is removed prior to regeneratlon, while Figure 2 illustrates the remcval of the sulphur after regeneration.
In Figure 1, sour gas, e.g., natuxal gas conta m m g about 0.5 percent ky volume H2S, in line (1) enters contactor or unit (2) (tray type) into which also enters, from line (21), an aqueous reactlon solution comprising an aqweous 2.0 M solution of the Fe(III~ chelate of nitrilotria oe tic acid having a pH of 7. Prior to the entry of the ga~ in unit (2), a munor portion, 6 peroent by weight, ba~el on the tokal weight of the sour gas stream, is separat~d via line (3) and passed to seoordary contactor (4), which will be discussed more fully hereinafter.
The pressure o~ the feed gas in line (1~ lS abcut 12aO psig.
(about 84 atmDspheres), and the temperature of the aqueous reacti~n soluticn is about 35 C. A contact time of about 120 seconds is emplayed in unit (2) in order to react all th,e H2S in the scur gaseous stream. Purified or sweet gas leaves unit (2) thrcugh line (5). The sweet gas is of a ~urity suf~icient to me~t standard requirements.
In the adn~x~ure of unit (~), the H2S is converted to elemental sulphur by the Fe(III) chelate, the Fe(III) chelate in the process being ocn~erted to ~he Fe(II) chelate. A minor portion of the chelate ocnçx~L~d also degrades, giving rise to a minor concentration o~ oxalate ion in the admix~ure. The aqueous ~ acrixture containing elemen~l sulphur is remaved continNouslya~a sent thrcugh line (6) to a depressurization and degassing 7'~g unit (7), and then through line (8) to sulphur recovery unit (9). Sulphur reccvery unit (9) may be of any suitable type.
Preferably, unit (9) comprises a filtration unit. It is not necessary that all the sulphur ke remcved in the sulphur reocvery step, and some sulphur retention may be beneficial. m e type o~ unit chosen is thus flexible. Preferably, the amount of sulphur removed in the separation step is simply balanced with the rate o~ sulphur intake in line (8), which is of course, ~ependent on the amount of H2S in gas stream (1). Those skilled in the art may adjust the appropriate rates of withdrawal of the streams. From unit (9), ~he sulphur-free or substantially sulphur-free soluticn is sent via line (10) to regeneration zone (11). Prior to the entry of the sulphur-free or substantlally sulphur-free solution in regFn~ration zone (11~, a bleed streaml, representing perhaps 10 percent by weight of the streaml in line (10) is separated and forwarded via line (12) ~D ocntactor (4).
In contactor ~4) the bleed solution or admixture is contacted with the minor portisn of the sour gas~ous stream in line 3, thereby utilizing the H2S to reduce the pH of the solution.
~nGugh H2S is provided to reduce most of the ferric to ferrous ions, and to sNpply, on an active basis, sufficient hydro~en to lower the pH to a pH of about 5. Vent gas from (4), if con-taining H2S, is sent via line (18) to unit (2), or, if H2S-free, preferably to line ~5). Ccnsomit~ntly, a mixture of nitrilo-triacekic acid and NbHSO3 is a~ded via line ~13) to lower the pHto about 3. Temperature in oontactor l4) is about 45 C. Ferr~us cxalate precipitates, and a soluticn containing ferrous oKalate is removed via lm e (14) aNd sent tD ilter (or any other suitable separatlc~ devi oe) (15) where the ~errous oxalate is removed. Those skilled in the art will recogniæe that some sulphur and other solutisn components wlll also be removed as sollds. ~he remaining solution now containing reduced oxalate on content, i5 removed via l m e (16), and returned vla line (8) ~l2~ 7'7~

to the system. If desired, pH adjustment and iron concentration adjustment, e.g., addltion of FeOO3, may be made via line (17).
In regeneration zone or column (ll), the admlxt~re is contacted with excess alr ~rom line (19) to convert Fe(II) chelate in the admiKture to the Fe(III) chelate. The temperature of the regeneration column is about 35 C, and pressure in the column is maintained at about 2 atmospheres. Spent air is removed from col~mn (11) thrcugh line (20), while re~enerated aq~x~ls admixture is returned via line (21) to unit ~2)~
As indicated, Figure 1 illustrates the aspect of the m vention wherein the sulphur recovery is carried out prior to regeneration. Removal o~ the sulphur after regeneration may be preferred in some instances, and may be accompllshed by positioning of the sulphur recavexy unit af~er the regeneration zone. Thus, in a separate entxxl~me~t, regenerated liqu~d, still containing sulphur, may ~e passed to units analogcus or equivalent to unit (9~, sulphur reo~vered, and regenerated sulphur-fre~ solution returned to u m t (2).
Ascordingly, in Figure 2, the sulphur-contalning liquid is passed, after degassing in (7), via 1inP ~30) to regenerator (31) where it is regenerated, as previously describ~d. The regenerated sulphur oont2Lul~g admix*ure is rem3ved via line (~2), a~d passed to sulphur reccvery unit (33). Spent air is removed via line (34). Regenerated reactant solution is returned via line (35) to UNt (2).
As ~urther illu~trated in Figure 2, bleed st~ n (36) is removed from line (35), and is sent to contactor (4). Operation o$ contactor (4) and units ~13) thrcugh (15) is similar to that descr~bed previously, except that largPr amcunts o~ ~ S will be required to reduce the hi~her concentrations of ferric ion pres~nt. Supernatant liquid lg returned via l;n~ ~16) to reg~neration.

~lZ~'7~

In order to demanstrate the feasibility of the invention, the following experIment was run.
Approximately 200 ml of solutlon from a cyclic process fox the reaction of H2S to sulphur which had reached equilibxium composition trcugh make-up and bleed was obtained a~ter sulphur removal. The cyclic prooess solution contained about 3.14 - percent by welght ircn as Fe , ~ut 1.08 Fercent by weight iron as Fe , about 3.36 percent ~y weight thiosulphate lon, and contained principal portions of nitrilotriacetic acid and lmdncdiaoetic, as well as minor portions of N-cxalyl glycine diacetic acid, N-oxalyl glycine, glyoxalic acid, glycine, and oxalic acid (as determined ~y liquid chrcmatogxaphic analysis of the corresponding butyl esters).
A small volume (~ 20cc) of concentrated sodium oxalate soluticn was added to the cyclic process solution in an amcunt sufficient to raise the concentration of sxalate ion to abaut 0.18 M (about 1.9 percent ky weiyht). Aliquots of 96 percent by weight of aqueous H2~04 were then added to the cyclic process solution to gradually re & ce the pH. At about pH 5, ferrcus oxalate began to precipitate. At a pH of 3, cver 80 peroent by weight of the ferrcus oxalate had precipitated out of solution.
While the inventi~n has been illustrated wi~h part~ Ar ~pparatus, tho æ skilled in the art will appreciate that, except w~ere specified, c~her equivalent or anal~gclus units may be employed. The tenm "zones"~ as employed in t~e specification, includes, where suitable, the use of s~gmented equipment operated in series, or the division of one unit into multiple units be~ause of size constraints, etc. For ex2mple, a contacting column might comprise two ~eparate columns in which the solution from the lower portion of the first column wculd be introduced Lnto the upper portion of the second oolumn, the gaseous material ~rom the upper portion o~ the first column being ~ed into the lower portion of the second column. Parallel operation of units, is o~ course, well within the scope of the invention.

,3 ~

Again, as will be understood by those skilled in the art, the solutions or mixtures employed may contaln other materials or addltives for given purposes. For example, U.S. patent specification No. 3,933,993 discloses the use of bufferlng agents, such as phosphate and carbonate bu~rs.

Claims (9)

C L A I M S

1. A process for the removal of oxalate ions from an aqueous solution containing iron chelate or chelates of nitrilotriacetic acid and decomposition products thereof, including oxalate ion, by contact m g the solution with an amount of a composition capable of providing hydrogen ions in said solution sufficient to precipitate ferrous oxalate, under conditions to precipitate ferrous oxalate, and precipitating ferrous oxalate and separating precipitated solid from the solution.

2. The process of claim 1 wherein the aqueous solution is a bleed stream from a process for removing H2S from sour H2S-containing gaseous streams in which the gaseous stream is contacted with an oxidizing reactant solution containing the ferric chelate of nitrilotriacetic acid as oxidizing reactant.

3. A process for the removal of H2S from a sour gaseous stream comprising:
a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution at a temperature below the melting point of sulphur, the mixture comprising the ferric chelate of nitrilotriacetic acid as oxidizing reactant to produce a sweet gas stream and an aqueous admixture containing sulphur and reduced reactant;
b) removing aqueous admixture from the contacting zone, and removing solid sulphur from said aqueous admixture;
c) regenerating said aqueous admixture, producing a regenerated oxidizing reactant solution, and returning regenerated oxidizing reactant solution to the contacting zone;
d) removing a bleed stream containing iron chelates of nitri-lotriacetic acid and decomposition products of said chelates, including oxalate ion, from one or more loci in or between steps a, b, or c;
e) contacting said bleed stream with a composition capable of providing hydrogen ions in the bleed stream to precipitate ferrous oxalate, under conditions to precipitate ferrous oxalate, but not rewove the bulk of the iron chelate or chelates in the bleed stream, and precipitating ferrous oxalate and separating precipitated solid from the bleed stream.

4. A process as claimed in claim 3, in which the H2S-con-taining sour gaseous stream is divided into a major portion and a minor portion and the bleed stream is contacted with the minor portion of the sour gaseous stream to reduce the H2S concentra-tion in the minor portion and to produce hydrogen ions in the bleed stream.

5. A process according to claim 4, in which the minor portion of the H2S containing sour gaseous stream comprises 0.01 to 30 %vol of the total sour gaseous stream.

6. A process according to claim 3, 4 or 5, in which the pH of the bleed stream is adjusted by the contacting with the compound capable of providing hydrogen ions to a value of from 3 to 5.

7. A process according to claim 1, 3 or 4, in which the composition capable of providing hydrogen ions is selected from the group comprising H2SO4, HCl, H3PO4, SO2, NaHSO3, N(CH2COOH)3, N-(2-hydroxyethyl)ethylene diamine triacetic acid, ethylene diamine tetraacetic acid and mixtures thereof.

8. A process according to claim 3, 4 or 5, in which the pH of a remaining solution, obtained after separation of precipitated solid from the bleed stream, is adjusted to a value of 7 to 9 and at least the bulk of the remaining solution is returned to the aqueous admixture.

9. A process according to claim 3, 4 or 5, in which the sour gaseous stream is selected from natural gas streams, streams derived from the gasification of coal and refinery feedstocks composed of gaseous hydrocarbon streams.