Note: Descriptions are shown in the official language in which they were submitted.
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K 7441 CAN
REMOVING H2S FROM A SOUR GASEOUS STREAM
The presence of significant quantities of H2S in various
"sour" industrial gaseous streams poses a persistent problem.
Although various procedures have been developed to remove and
recover this contaminant, most such processes are deficient, for a
variety of reasons.
In one cyclic method currently attracting attention, the sour
gas is contacted with an aqueous polyvalent metal chelate or
complex reactant system to produce solid sulphur which is recovered
either prior to or subsequent to regeneration of the reactant.
Preferred reactants are iron (III) complexes in which the iron
(III) forms complexes with specified organic acids and derivatives
thereof.
One of the disadvantages of such systems heretofore has been
the inability to maintain sufficiently high concentrations of the
chelate or complex to achieve efficient operation. Without suffi-
ciently high levels of the complex, these processes are limited in
their ability to handle streams containing significant quantities
of H2S. Again, the circulation of large volumes of dilute solutions
to handle even~moderate levels of H2S involves significant capital
~and energy costs, especially in high pressure applications.
Finally, degradation or decomposition of the polyvalent metal
complexes represents an important cost in the process, as well as
requirlng measures for decomposition bleed or removal and addition
of fresh solution. Even in the case of complexes such as those of
25~ nitrilotriacetic acid, decomposition of complexes, over a period of
; time~requires attention to prevent build-up of decomposition
products and consequent loss of efficiency. The invention addresses
these problems, and provides a novel composition and process for
the resolution thereof.
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To this end the process for the removal of ~2S from a sour
gaseous stream according to the invention comprising contacting the
sour gaseous stream in a contacting zone at a temperature below the
melting point of sulphur with an aqueous reactant solution, which
reactant solution contains an ammonium form of a coordination
complex of Fe(III) with nitrilotriacetic acid, an ammonium form of
a coordination complex of Fe(II) with nitrilotriacetic acid,
aqueous ammonia and thiosulphate ion, and which reactant solution
has a pH of between about 5 and about 8.5, under conditions to
convert H2S, and producing a gaseous stream having reduced ~2S
content and an aqueous mixture containing solid sulphur and
additional ammonium form of the coordination complex of Fe(II) with
nitrilotriacetic acid in solution, wherein the molar ratio of the
ammonium form of the coordination complex of Fe(III) with nitrilo-
triacetic acid to the ammonium form of the coordination complex of
Fe(II) with nitrilotriacetic acid in the reactant solution is
between about 0.2 and about 6, and wherein the thiosulphate ion is
present in an amount sufficient to inhibit degradation.
In a suitable embodiment the process according to the
invention further comprises regenerating the aqueous mixture by
contacting the aqueous mixture with oxygen under conditions to
convert the ammonium form of the coordination complex of Fe(II)
with nitrilotriacetic acid to the ammonium form of the coordination
complex of Fe(III) with nitrilotriacetic acid, and producing a
regenerated aqueous reactant solution having a molar ratio of the
: ammonium form of the coordination complex of Fe(III) with nitrilo-
triacetic acid to the ammonium form of ehe coordination complex of
Fe(II) with nitrilotriacetic acid of between about O.S and~about 6.
The regenerated aqueous reactant solution may be passed to the
~ 30 contacting zone for use as aqueous reactant solution therein, and
: prior to passing regenerated aqueous reactant solution to the
~ contacting zone sulphur may be removed from at least a portion of
: the regenerated aqueous reactant solution removed from the
:regeneration zone.
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3357
In a further suitable embodiment the process according to the
invention further comprises removing aqueous mixture from the
contacting zone and removing sulphur from at least a portion of the
mixture, regenerating aqueous mixture by contacting said aqueous
mixture with oxygen under conditions to convert the ammonium form
of the coordination complex of Fe(II) with nitrilotriacetic acid to
tbe ammonium form of the coordination complex of Fe(III) with
nitrilotriacetic acid, and producing regenerated aqueous reactant
solution having a molar ratio of the ammonium form of the
coordination complex of Fe(III) with nitrilotriacetic acid to the
ammonium form of the coordination complex of Fe(II) with nitrilo-
triacetic acid of between about 0.5 and about 6, and passing
regenerated aqueous reactant solution to the contacting zone for
use as aqueous reactant solution therein.
Ammonium hydroxide may be added to maintain pH in the con-
tacting zone in a range of from 5 to 8.5. Furthermore the total
iron content of the reactant solution in the coordination com-
plexes, may be between about 0.5 percent and about 7 percent by
weight, based on the weight of the reactant solution and the iron.
The amount of thiosulphate ion may be in the range of from
0.01 moI to 4 mol per mol iron in the aqueous reactant solution.
In addition the invention also relates to a reactant solution
suitable for converting H2S comprising an aqueous solution com-
prising an ammonium form of a coordination complex of FetIII) with
nitrilotriacetic acid, an ammonium form of a coordination complex
of Fe(II) with nitrilotriacetic acid, thiosulphate ion and aqueous
ammonia, wherein the molar ratio of the ammonium form of ths
~coordination complex of FetIII) with nitrilotriacetic acid to the
ammonium form of the coordination complex of FetII) with nitrilo-
triaceti acid is between about 0.2 and about 6, the pH of thesolution is between about 5 and about 8.5, and wherein the amount
of thiosulphate is between 0.01 mol to 4 mol per mol iron pressnt.
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The total iron content of the reactant solution may be between
about 0.5 percent and about 7 percent by weight, based on the
weight of the iron and the solution.
In a suitable embodiment of the invention the thiosulphate
ion is present in the form of ammonium thiosulphate, the thio-
sulphate ion may also be supplied from sodium, potassium or lithium
thiosulphate.
In the specification NTA will refer to nitrilotriacetic acid.
In a solution that has been used in the removal of H2S from a
gaseous stream, the total concentration of iron is not revealed by
the sum of the concentrations of the ammonium form of the
coordination complex of Fe(III) with NTA and the ammonium form of
the coordination complex of Fe(II) with NTA, since other iron
complex or complexes are present. It has been determined, for
IS example, that some degradation products of the complexes employed
(and degradation products thereof) are soluble complexea of iron.
Again, there is evidence, for example, that the ammonium form of
the coordination complex of Fe(III) with NTA may be present as a
dimer.
As used herein, the term "an ammonium form of a coordination
complex of Fe(II) with NTA" refers to those solubilized species,
which, upon precipitation, are believed to have the formula [NH43
[(NTA)Fe(II)(H20)23. Because of the complexity of the system,
however, and the difficulty of analysis of the components, the term
5~ may also be considered as simply defining the species in the
solution in which an ammonium ion and a coordination complex of
Fe(II) with NTA are chemically associated or related, whatever the
precise nature or character of the relationship or bonding.~The
term "an ammonium form of a coordination complex of Fe(IIIj with
NTA" refers, correspondingly, to the Fe(III) species ln the~
solution. The maximum solubility limit of an ammonium form of a
:
coordination complex of Fe(II) with~NTA is about O.S M in unused
solution.
As noted, the solutions employed will contain aqueous ammonia.
As used herein, the term "aqueous ammonia" is understood to include
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a3357
dissolved ammonia, ammonium hydroxide, and ammonium ion, as under-
stood from Advanced Inorganic Chemistry, 3rd edition; F.A. Cotton
and G. Wilkinson, especially page 349. The aqueous ammonia is
present in addition to the ammonium ion associated with or combined
with the coordination complex of Fe(III) with NTA and the
coordination complex of Fe(II) with NTA. The quantity will vary,
but will be at least about 0.1 percent, on a molar basis, with
respect to the total quantity of the ammonium forms of the
coordination complexes of Fe(III) and Fe(II) with ~TA used. ~re-
ferably, the aqueous ammonia will be present from, for example, 0.1percent, to 200 or 300 percent, or more, on a molar basis, with
respect to the quantity of the ammonium forms of the coordination
complexes of Fe(III) and Fe(II) with NTA present. In practice, this
quantity may be achieved by adjusting the pH of the solution with
ammonium hydroxide or carbonate to the levels described herein-
after. Ammonlum compounds having salt forming anions, with the
carbonate exception, are no~ desirable.
The presence of significant concentrations of sodium or
potassium ions is not desired in the solution of the invention. The
sodium and potassium salts of the coordination complex of Fe(II)
with MTA have been determined to have undesirably low solubilities,
so that they are unsuitable for hlgh concentration operations. By
avoiding these materials, the invention provides efficiencies not
; ~ attainable with prior art processes. For example, since con-
centrations of the complexes of the invention are higher, streams
; having greater concentrations of H2S may be treated, at similar
throughput. Pumping costs are reduced, and vessel slzes other
factors being equal, may be reduced. As noted, some sod;um or
potassium ions are tolerable, provided that they are not present in
sufficient amount to exceed the solubility of sodium or potassium
coordination complex of Fe(II) with NTA. In this regard, minor
quantities of sodium or potassium containing additives or buffers
may be present.
For this reason, pH ad~ustment~in the process of the invention
; 35 is accomplished by the use of ammonium hydroxide or carbonate.
~2933~7
Prior art practice of utilization of sodium hydroxide, potassium
hydroxide, or sodium or potassium carbonates, is generally unsuited
to the invention. The pH in the contacting zone will preferably be
maintained in a range of from about 5 to 8.5, preferably 6.5 to
8.5, and in the regenerator, from about 7 to 8.5.
It has been found that minor amounts of thiosulphate ion,
preferably supplied as a~monium thiosulphate, are useful in
inhibiting degradation of the ammonium form of coordination
complexes of iron with NTA. The combination of components,
parameters and steps disclosed herein provides a process in which
degradation rates of coordination complexes may be lowered to a
degree believed previously unattainable. The thiosulphate may be
supplied as the alkali metal salt, e.g., lithium, sodium or
potassium, provided the solubility considerations noted in regard
to the ammonium form of the coordination complex of Fe(II) with NTA
are taken into account. The thiosulphate ion is supplied in an
amount sufficient to inhibit degradation. Generally, the thio-
sulphate ion may be supplied in a concentration of about 0.2 to 2.5
mol/lJ preferably about 0.1 to 0.95 mol/l. The ion should be
supplied in 8 ratio of about 0.01 to 4 mol, preferably 0.1 to 0.95
; mol, of thiosulphate ion per mol of iron present, based on the
total concentration of the ammonlum forms of the coordination
complexes of iron and NTA.
As noted, the regeneration of the reactant is accomplished by
the utilization of oxygen, preferably as air. The oxygen will
accomplish two functions, the oxidation of Fe(II) of the reactant
to the Fe(III), and the stripping of any dissolved gas from the
mlxture. The oxygen (in whatever form supplied) is supplied in a
stoichlometric equivalent or excess wlth respect to the amount of
the ammonlum form of the coordinatlon complex of Fe(II) with NTA
present ln the mixture. Preferably, the oxygen is supplied in an
amount of from about 20 percent to about 300 percent excess.
As used hereln, the eerm "oxygen" ls not llmited to "pure"
oxygen, but includes air, air-enriched with oxygen, or other
oxygen-containin~ gases.
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The particular type of sour gaseous stream treated is not
critical, the only practical limitation being the reactivity of the
stream itself with the solution employed, as will be evident to
those skilled in the art. Streams particularly suited to removal of
H2S by the practice of the invention are, as indicated, naturally-
occurring gases, recycled C02 used in enhanced oil recovery,
synthesis gases, process gases, and fuel gases produced by gasifi-
cation procedures, e.g., gases produced by the gasification of
coal, petroleum, shale, tar sands, etc. Particularly preferred are
coal gasification streams, natural gas streams, produced and
recycled C02 streams, and refinery feedstocks composed of gaseous
hydrocarbon streams9 especially those streams of this type having a
low ratio of H2S to C02, and other gaseous hydrocarbon streams. The
term "hydrocarbon stream(s)", as employed herein, is intended to
include streams containing significant quantities of hydrocarbon
(both paraffinic and aromatic), it being recognized that such
streams contain significant "impurities" not technically defined as
a hydrocarbon. Again, streams containing principally a single
hydrocarbon, e.g., ethane, are eminently suited to the practice of
the invention. Streams derived from the gasification and/or partial
oxidation of gaseous or liquid hydrocarbon may be treated by the
invention. The H2S content of the type of streams contemplated will
vary extensively, but, in general, will range from about 0.005
percent to about 10 percent by volume. C02 content will also vary,
but may range from about 0.1 percent to about 99.0 percent (or
more) by volume. In this context, the invention may be used to
remove H2S from various C02 streams, e.g., supercritical C02
streams.
The temperatures employed in the contacting or absorption-
contact zone are not generally critical, except that the reaction
is carried out below the melting point of sulphur. In many commer-
cial appIications, such as removal of H2S from natural gas to meet
pipeline specifications, absorption at ambient temperatures is
desired, since the cost of refrigeration would exceed the benefits
obtained due to increased absorption at the lower temperature. In
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general, temperatures of from 10 C to 80 C are suitable, and
temperatures of from 20 C to 60 C are preferred. Total contact
times will range from about 1 second to about 120 seconds, with
contact times of 2 seconds to 60 seconds being preferred.
Similarlys in the regeneration or stripping zone or zones,
temperatures may be varied widely. Preferably, the regeneration
zone should be maintained at substantially the same temperature, or
somewhat lower, as the contacting zone. In general, temperatures of
from about 10 C to 80 C, preferably 20 C to 50 C may be
] employed.
Pressure conditions in the contacting zone may vary widely,
depending on the pressure of the gas to be treated. For example,
pressures in the contacting zone may vary from 0.1 MPa up to 15 MPa
or even 20 MPa. Pressures of from 0.1 MPa to about 10 MPa are
preferred. In the regeneration zone, pressures may also be varied
considerably, and will preferably range from about 0.1 MPa to about
0.3 or 0.4 MPa. Residence times for given volumes of admixture and
oxygen will range from 10 minutes to 60 minutes, preferably from 20
minutes to 40 minutes. The pressure, fluid flow, and temperature
relationships involved are well understood by those skilled in the
art, and need not be detailed herein. Other conditions of operation
for this type of reaction process are further described in US
Patent No. 3,068,065 to Hartley et al, dated December 11, 1962.
Preferably, the molar ratio of the nitrilotriacetic acid to total
iron is from about 1.0 to 1.5. The process is preferably conducted
continuously.
As indicated, the H2S, when contacted, is rapidly converted in
the process of the invention by the ammonium form of the coordina-
tion complex of Fe(III) with NTA to~solid elemental sulphur. The
amount of the ammonium form of the coordination complex of Fe(III)
with NTA supplied or employed in solution is an amount sufficient
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to reduce the H2S concentration or content of the stream to the
desired level. If total or substantially total removal is desired,
the total amount supplied will generally be on the order of at
least about two mol per mol o H2S. Ratios of from aboul 2 mol to
~L293357
about 15 mol of the ammonium form of the coordination complex of
Fe(III) with NTA per mol of H2S may be used, with ratios from about
2 mol per mol to about 5 mol of the ammonium form of the coordina-
tion complex of Fe(III) with NTA per mol of ~2S belng preferred. As
noted, the ratio of the ammonium form of the coordination complex
of Fe(III) with NTA to the ammonium form of the coordination
complex of Fe(II) with NTA present in solution wlll normally be
less than about 6, and will normally range from about 0.2 to about
6, preferably about 0.5 to about 6.
The manner of preparing the solutions of the invention is, to
some extent, a matter of choice. For example, the solutions
employed in the process of the invention may be prepared by
reaction of elemental iron with nitrilotriacetic acid, as described
in the US patent 3,115,511, followed by air oxidation, pH ad~ust-
ment with ammonium hydroxide, addition of the appropriate thio-
sulphate, e.g., ammonium thiosulphate, and appropriate water
dilution to achieve the desired concentration. Alternately,
nitrilotriacetic acid, Fe(II) carbonate, ammonium hydroxide and
oxygen (air) may be reacted to prepare the solution, with addition
~ of the thiosulphate. The novel compositions of the invention may be
produced, for example, by separate reduction of one of the above
mentioned solutions until the appropriate levels of the ammonium
form of the coordination complex of Fe(II) with NTA are formedJ or
the solutions mentioned above may simply be employed in the process
of the invention until the appropriate ratios of Fe(III) to Fe(II)
coordination complexes are reached.
The invention will now be described in more detail by way of
; example with reference to the accompanying drawings, wherein
Figure l shows schematically the embodiment of the invention
3Q wherein~sulphur removal is accomplished in a separate step prior to
regeneration; and
Figure 2 shows schematically the case where sulphur is removed
in a separate step after regeneration.
All values are calculated or merely exemplary, and all flows,
unless stated otherwisel are continuous.
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Sour gas, e.g. 9 natural gas containing about 0.5 percent H2S,
in line 1 enters a contactin~ zone in the form of contactor or
absorber 2 into which also enters an aqueous mixture comprising an
aqueous solution of 0.8 mol/l of an ammonium form of a coordination
complex of Fe(III) with NTA, which mixture also contains 0.2 mol of
an ammonium fonn of a coordination complex of Fe~II) with NTA and
0.3 moltl of ammonium thiosulphate through line 12. The solution is
produced by utilization of the reducing effect of the H2S in the
gaseous stream. That is, the initial solution in the contactor 2 is
an aqueous solution of 1 mol/l of the ammonium form of the
coordination complex of Fe(III) with NTA with a total cohcentration
of ammonium ion and aqueous ammonia of 3 mol/l. After startup, and
reaction with the H2S in the gaseous stream, regeneration,
described hereinafter, is controlled so that regeneration of the
ammonium form of the coordination complex of Fe(III) with NTA is
not complete. Absorber or contactor 2 may be of any suitable type,
such as a packed column or tray column, but is preferably a combi-
nation venturi-spray column system. Depending on the size of the
gas stream, the H2S content, etc., more than one contacting zone
may be employed, preferably in series. In any event, in the unit
illustrated, the pressure of the feed gas is about 8.5 MPa (gauge),
and the temperature of the aqueous mixture is about 45 C. A
; contact time of about 120 seconds i8 employed in order to react all
the H2S. Purified or "sweet" gas leaves absorber 2 through line 3.
~ The "sweet" gas is of a purity sufficient to meet standard require-
ments. In the mixture the H2S is converted to solid elemental
:
: ~ sulphur by the ammonium form of the coordination complex of Fe(III)
with NTA and the ammonium form of the coordination complex of
Fe(III) with NTA is converted to the ammonium form of the coordlna~
~ ~tion complex of Fe(II) with NTA. The aqueous mixture produced,
containing elemental sulphur and additional ammonium form of the
coordination complex of Fe(II) with NTA, is removed continuously,
and sent through line 4 to a depressurization and degassing unit 5,
which also serves~as a sulphur concentration or thickening zone.
35~ Gas desorbed~from the aqueoas mixture is removed from unit 5 via
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line 16. A minor portion, e.g., 5 to lO percent by volume of the
mixture in the depressurization and degassing unit 5, and con-
taining an increased sulphur concentration, is continuously with-
drawn from the lower portion of the depressurization and degassing
unit 5 and sent via line 6 to sulphur recovery unit 13, from which
sulphur is removed via line 14.
Sulphur recovery may be accomplished in any suitable fashion,
such as by filtration. Solution recovered during sulphur recovery
may be returned to any suitable point in the process, if proper
adjustment is made, but is preferably sent, as shown, via line 15
to a regeneration zone 8. The major portion of the aqueous mixture
in the depressurization and degassing unit 5, having a reduced
sulphur content, is removed via line 7 for regeneration. In
regeneration zone or column 8, the mixture is contacted with excess
air from line 9 to convert the ammonium form of the coordination
complex of Fe(II) with NTA to the ammonium form of the coordination
of Fe(III) with NTA.
Regeneration zone 8 comprises a sparged tower regenerator with
cocurrent upflow of oxygen (as air), supplied via line 9, and
aqueous mixture. Air velocity in the regenerator is in the range of
; from 0.03 to 0.1 m/s. The temperature in the column is about 45 C,
and overall pressure is about 0.2 MPa. Spent air is removed via
line 11, and regenerated mixture, having a ratio of the ammonium
form of the coordination complex of Fe(III) with NTA to the
;25 ~ ammonium form of the coordination complex of Fe(II) with NTA of
about 4 is returned via line 12 to contacting zone 2.
If required make-up reactant solution can be supplied via
line 17.
Reference is now made to Figure 2. Sour gas e.g., ~atural gas
~30 containing about 0.5 percent H2S~, and 32~percent by volume C02, in
line 31 enters~contacting~zone 32 (sparged column type) and
contacts an aqueous solution of 0.8 mol/l of an ammonium form of a
coordination complex of Fe(III) with NTA, also containing
0.2 mol/l of an ammonium form of an coordination complex of Fe(II)
35~ with NTA and 0.3 mol/l oE ammonium thiosulphate, the total
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concentration of ammonium ion and aqueous ammonia being 3 mol/l
supplied via line 42. The pressure of the feed gas is about 8.5 MPa
(gauge) and the temperature of the aqueous mixture is about 45 C.
A contact time of about 45 seconds is employed in order to react
all the H2S. Purified or "sweet" gas leaves the contacting zone 32
through line 33. The "sweet" gas is of a purity sufficient to meet
standard requirements. In the aqueous mixture, the H2S is converted
to elemental sulphur by the ammonium form of the coordination
complex of Fe(III) with NTA. The aqueous mixture, containing
elemental sulphur, a slight amount of absorbed C02, and about 0.5
mol/l of the ammonium form of the coordination complex of Fe(II)
with NTA, is removed continuously and sent through line 34 to
degassing unit 35. As shown, any dissolved gases are removed in
degassing unit 35 by reduction of pressure via line 47, and the
mixture forwarded via line 36 to regeneration zone 37.
In regeneration zone 37, mixture is treated in a similar
fashion to that described with reference to Figure l. The ammonium
form of the coordination complex of Fe(II) with NTA is converted by
oxygen supplied via line 38 to the ammonium form of the
coordination complex of Fe(III) with NTA, while maintaining
sufficient of the ammonium form of the coordination complex of
Fe(II) with NTA to inhibit degradation of the coordination
complexes of iron with NTA. The temperature of the regeneration
zone 37 is about 45 C, and the pressure in the column is
maintained at about 0.2 MPa. The regenerated mixture, which s~ill
contains elemental sulphur and spent excess air, is sent through
line 39 to degassing and thickening zone 40. Spent air is removed
from zone 40 through line 41. From zone 40, which corresponds to
depressurization and degassing unit 5 (see Figure l), major and
minor portions of the regenerated, solid sulphur-containing mixture
are separated, the ma;or portion being sent through line 42 to the
contacting zone 32. The minor portion, e.g., 5 percent by volume of
the mixture in zone 40, and having an increased sulphur content, is
sent through line 43 to sulphur recovery unit 44 in the manner
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described in relation to Figure 1. Mixture from sulphur recovery
unit 44 is returned to the system via line 45, and sulphur is
removed from the sulphur recovery unit via line 4O.
While the invention has been illustrated with particular
apparatus, those skilled in the art will appreciate that, except
where specified, other equivalent or analogous units may be
employed. The term "zones", as employed in the speclfication and
claims, includes, where suitable, the use of segmented equipment
operated in series, or the division of one unit into multiple units
because of size constraints, etc. For example, a contacting zone
may comprise two separate countercurrent columns in which the
solution from the lower portion of the first column would be
introduced into the upper portion of the second column, the
partially purified gaseous material produced from the upper portion
of the first column being fed into the lower portion of the second
column. Parallel operation of units, is, of course, well within the
scope of the invention. Mixture or solution withdrawal or intro-
duction may be made at any suitable sitets) or loci in the
particular zone. The return of solution to one or more multiple
contacting units in the contacting zone from a regenerator, or the
use of a common regenerator, is within the scope of the invention.
Again, as will be understood by those skilled in the art, the
solutions or mixtures employed may contain other materials or
additives for given purposes. For example, US Patent No. 3,9339993
discloses the use of buffering agents, such as phosphate and
carbonate buffers. Similarly, other additives, such as antifoaming
and/or wetting agents, may be employed.
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