Electrochemical conversion of sulfur-containing anions to sulfur

An aqueous solution comprising one or more sulfur-containing anions is introduced into a diaphragmless, mercury, electrolytic cell wherein, in the presence of an impressed direct electric current, the sulfur-containing anions are electro-chemically converted to elemental sulfur. The invention will be found most advantageous in treating spent aqueous absorbents recovered from processes for removing SO.sub.2 and SO.sub.3 from stack gases.

BACKGROUND OF THE INVENTION 
This invention relates to a process for electrolytically converting sulfur 
compounds dissolved in aqueous liquids to elemental sulfur. The invention 
further relates to a process for removing SO.sub.x compounds from stack 
gases by absorption in an aqueous absorbent and electrochemically 
regenerating the resulting spent absorbent. 
It is known in the art of sulfur chemistry that SO.sub.x compounds (i.e., 
SO.sub.2 plus SO.sub.3) can be removed from stack gas streams by 
absorption in alkaline, aqueous liquids, e.g., an aqueous solution of 
sodium hydroxide. It is further known that spent absorbent solutions 
obtained from such absorption processes, containing, for example, large 
concentrations of sodium cations and assorted sulfur-containing anions, 
may be electrochemically regenerated so as to produce a fresh absorbent 
solution of sodium hydroxide. Processes typifying this approach are shown 
in U.S. Pat. Nos. 3,607,001 and 3,515,513. In these processes, the spent 
absorbent solution is first treated, as by stripping or heating, to desorb 
as much SO.sub.2 as possible, thereby producing a concentrated stream of 
SO.sub.2 suitable as one component of a Claus plant feed. The remainder of 
the spent absorbent, largely comprising an aqueous solution of sodium 
sulfate, is directed to an electrolytic cell containing two diaphragms (or 
ion-permeable membranes) separating a feed compartment from anode and 
cathode compartments. Under the influence of an impressed direct electric 
current, the sulfate anions migrate from the feed compartment through one 
diaphragm to the anode compartment, producing therein an anolyte solution 
comprising sulfuric acid. Simultaneously, cations pass through another 
diaphragm to the cathode compartment, producing a catholyte solution of 
sodium hydroxide. Hence, in the usual case, the products removed from the 
electrolytic cell comprise a sulfuric acid solution and a caustic or other 
alkali metal hydroxide solution useful as fresh absorbent for removing 
SO.sub.2 from the stack gas. 
Several problems are involved in using the electrochemical processes as 
above described. First, considerable power loss occurs across the 
diaphragms, thereby reducing the efficiency of the cell. Further, the 
production of sulfuric acid from such processes is usually undesirable 
because sulfuric acid is not an economic product to store or transport 
when produced in large quantities. Moreover, because that portion of the 
spent absorbent fed to the electrolytic cell contains sulfite, bisulfite, 
and bisulfate ions as well as sulfate ions, the sulfuric acid produced 
from the electrolytic cell is impure, and thus of much less economic value 
than more purified forms of sulfuric acid. 
In view of the foregoing, it would be far more desirable to 
electrochemically convert the sulfate and other oxysulfur anions in such 
spent absorbent solutions to a single product, preferably a solid product 
such as elemental sulfur, which is more easily stored and more marketable 
than sulfuric acid. However, producing sulfur in a typical diaphragm cell 
raises the obvious problem that the elemental sulfur will easily plug the 
ion membrane pores and thus render the cell inoperative. Additionally, 
sulfur could collect around the cathode of the cell and thus interfere 
with the efficiency of the cell for the intended conversion. 
Accordingly, it is an object of the invention to electrochemically produce 
elemental sulfur in a diaphragmless cell in which one or more 
sulfur-containing anions are converted to the single, homogeneous product 
of elemental sulfur. It is a further object to provide a process wherein, 
by electrochemical conversion in a diaphragmless, mercury, electrolytic 
cell, the sulfur-containing anions in spent aqueous absorbents recovered 
from processes for removing SO.sub.2 and SO.sub.3 from stack gases are 
converted to elemental sulfur. It is yet another object to provide a 
diaphragmless, mercury, electrolytic cell in which elemental sulfur is 
produced from an aqueous electrolyte without the elemental sulfur 
collecting around the electrodes, especially the mercury electrode, and 
thus interfering with the cell efficiency. Other objects and advantages 
inhering in the invention will become apparent to those skilled in the art 
from the following detailed description. 
SUMMARY OF THE INVENTION 
In its broadest aspect, this invention comprises subjecting a feed aqueous 
solution comprising sulfur-containing anions to electrolysis in a 
diaphragmless, electrolytic cell having elemental mercury as one electrode 
and a material such as graphite or platinum as the other electrode. In the 
presence of an applied voltage with the mercury electrode acting as 
cathode, the sulfur-containing anions are converted to elemental sulfur, 
which elemental sulfur may be recovered from the cell as a froth floating 
upon an aqueous liquid product. The aqueous liquid product has a reduced 
concentration of sulfur-containing anions in comparison to the 
concentration of sulfur-containing anions in the feed aqueous solution. 
One especially beneficial feature of the invention is that elemental sulfur 
is produced within the cell regardless of the valence state of the sulfur 
atoms contained in the sulfur-containing anions. If the sulfur is present 
in a positive valence state, as is the case with sulfate ion, sulfur is 
formed at the mercury cathode. If in the negative valence state, as in the 
case of sulfide ion, sulfur is produced at the anode. And if sulfur is 
contained in both a positive and a negative valence state, as is true for 
thiosulfate ion, sulfur is produced at both electrodes. Thus, the 
invention may be utilized to treat an aqueous solution containing an 
assortment of sulfur-containing anions, and since the sulfur-containing 
anions are converted in a diaphragmless cell, the elemental sulfur product 
is advantageously collected in a single electrolyte chamber. 
The process of the invention is particularly useful for regenerating spent 
absorbent solutions obtained from the removal of SO.sub.2 and SO.sub.3 
compounds from stack gases and the like in a SO.sub.x absorber. Spent 
absorbent solutions recovered from such processes usually contain one or 
more oxysulfur anions and one or more alkali metal cations. Electrolysis 
is a diaphragmless, mercury, electrolytic cell yields a mercury-alkali 
metal amalgam and a slurry comprising an aqueous product and dispersed 
particles of elemental sulfur. After the slurry is separated into 
elemental sulfur and the aqueous product, the amalgam is contacted with 
the separated, now sulfur-free, aqueous product to produce elemental 
mercury, which elemental mercury is recycled to the cell to replenish the 
mercury cathode therein. Also produced by the contact of amalgam and 
aqueous product is an alkali metal hydroxide solution useful as fresh 
absorbent for removing SO.sub.x compounds in the SO.sub.x absorber. Hence, 
in the overall process for removing SO.sub.x compounds from stack gas by 
absorption in an alkali metal hydroxide absorbent solution, regeneration 
of spent absorbent containing alkali metal cations and oxysulfur anions is 
accomplished by electrochemically producing elemental sulfur and 
converting the amalgam by-product into a fresh alkali metal hydroxide 
absorbent solution. 
As used herein, the term "oxysulfur anions" includes all anions containing 
oxygen and sulfur, Illustrative of such "oxysulfur anions" are 
SO.sub.4.sup.-2, SO.sub.3.sup.-2, S.sub.2 O.sub.3.sup.-2, S.sub.2 
O.sub.6.sup.-2, S.sub.2 O.sub.5.sup.-2, S.sub.2 O.sub.7.sup.-2, SO.sub.3 
F.sup.-1, HSO.sub.3.sup.-1, and HSO.sub.4.sup.-1. 
A diaphragmless electrolytic cell is defined herein as an electrolytic cell 
containing at least one electrolyte chamber in which both the anode and 
cathode are in intimate contact with the same electrolyte liquid.

DETAILED DESCRIPTION OF THE INVENTION 
In the process of this invention, an aqueous solution containing oxysulfur 
anions or other sulfur-containing anions is introduced into a 
diaphragmless, electrolytic cell having a suitable anode and a mercury 
pool cathode. When a direct electric current of sufficient voltage is 
applied across the electrodes with the aqueous solution acting as 
electrolyte, the sulfur-containing anions are converted to elemental 
sulfur, which elemental sulfur, under ambient conditions, is dispersed in 
the electrolyte in solid particulate form. If, as will be the usual case, 
the aqueous solution contains an alkali metal cation, such as sodium or 
potassium, in additon to sulfur-containing anions, the high hydrogen 
overvoltage of elemental mercury will reduce such cations and produce an 
alkali metal-mercury amalgam. Thus, as applied to an aqueous solution 
containing a dissolved compound of sulfur and an alkali metal, the process 
of the invention results in the simultaneous formation in the cell of two 
products: an alkali metal-mercury amalgam and an aqueous liquid containing 
dispersed particles of elemental sulfur. 
As is conventional with mercury cells, the amalgam produced at the cathode 
is preferably withdrawn from the cell and decomposed by contact with an 
aqueous liquid into elemental mercury and an aqueous solution of a metal 
hydroxide. After separation of the mercury from the aqueous metal 
hydroxide solution, which is usually accomplished by taking advantage of 
the great difference in density between mercury and aqueous solutions, the 
product mercury is recycled to the cell to replenish the mercury pool 
cathode while the aqueous metal hydroxide solution is recovered as a 
by-product. 
An alternative but non-preferred method by which the amalgam may be 
converted to mercury involves periodically reversing the voltage across 
the electrodes, i.e., after operating for a certain period of time with 
the mercury as cathode, thereby forming the amalgam, the voltage is 
reversed so that the mercury becomes the anode. The amalgam is thereby 
reconverted to mercury without the necessity of withdrawing the amalgam 
from the cell. In carrying out this embodiment of the invention, it is 
desirable before the mercury is made the anode that the cell be purged of 
electrolyte containing dispersed sulfur so that no elemental sulfur is 
present when the mercury becomes anodic. Otherwise even as the amalgam is 
being decomposed, the mercury so produced would be converted to mercurous 
or mercuric sulfide and purpose of reversing the voltage, i.e., to 
replenish the mercury pool with elemental mercury, would be lost. 
The design of the mercury cell is not critical and may be similar to 
diaphragmless, mercury cells utilized in the chlorine industry. Usually, 
the cell design will be such that the cathode comprises a pool of mercury 
lying at the bottom of the cell while the anode, composed of any material 
having good electrical conductivity and corrosion resistance, such as 
platinum, palladium, gold, silver, copper, carbon, or graphite, is 
suspended or supported from the upper portion of the cell chamber. The 
cell should be designed for high efficiency, as by providing for the anode 
and cathode to have high surface area exposure to the electrolyte, thereby 
lowering the current density as low as possible. Additionally, the 
distance separating the anode and cathode is preferably made small so as 
to provide a short conductive path through the electrolyte. And if means 
are employed to keep both the electrolyte and the mercury cathode stirred, 
efficiency losses due to non-uniform conductance through the electrolyte, 
or to reduced conductance through the mercury amalgam, are kept to a 
minimum. Lastly, for maximum efficiency, it is a critical feature of the 
invention to eliminate energy losses across ion-permeable membranes by 
utilizing a diaphragmless cell. Preferably, the diaphragmless cell 
contains a single electrolyte chamber, and the chamber is so designed that 
the anode and mercury pool cathode therein will both be in contact with 
the flowing, aqueous electrolyte. 
Since in the preferred embodiment sulfur is removed from the cell as 
dispersed particles carried within an aqueous liquid, the cell is 
preferably constructed so as to operate near or at ambient conditions and 
thus produce solid sulfur. However, in alternative embodiments of the 
invention, the cell may be designed to operate at elevated temperatures 
and pressures so that advantage may be taken, if desired, of recovering 
sulfur as a molten liquid. 
The most advantageous use of the invention is in treating spent aqueous 
solutions employed in the removal of SO.sub.x compounds from stack gases 
and the like. The gas components found in stack gases largely comprise 
nitrogen, carbon dioxide, and water vapor, with the balance consisting 
essentially of some combination of oxygen, carbon monoxide, argon, 
SO.sub.x, and NO.sub.x present in individual proportions no greater than 
about 10 mole percent. Suitable stack gas streams are those containing at 
least 50 ppmv SO.sub.x, preferably at least 500 ppmv SO.sub.x, with 95% or 
more of the SO.sub.x being present as SO.sub.2. The typical concentrations 
of gaseous compounds in stack gas obtained from the combustion of a 
sulfur-containing fuel is shown in Table I: 
TABLE I 
______________________________________ 
Component Mol % Component ppmv 
______________________________________ 
O.sub.2 1-5 CO 0-500 
CO.sub.2 10-20 NO.sub.x 0-2000 
H.sub.2 O 5-25 SO.sub.2 50-50,000 
N.sub.2 70-75 SO.sub.3 0-200 
______________________________________ 
Referring now to the drawing, a preferred embodiment of the invention will 
be described with relation to removing SO.sub.x compounds from a stack gas 
containing SO.sub.2 and SO.sub.3. A stack gas, having a gaseous 
composition falling in the typical ranges listed in Table I, is fed 
through inlet 1 at a convenient temperature, usually less than about 
200.degree. F., and at a rate between about 1000 SCF/hr and about 100,000 
SCF/hr and at a pressure above atmospheric but preferably about 15 psig 
into SO.sub.x absorber 2. The absorber may comprise suitable gas-liquid 
absorption equipment such as a packed tower, a multi-plate column, a 
splash-deck column, a disk and donut column, or a venturi scrubber, but 
the design should be such that sufficient contact time is provided for the 
SO.sub.x components to react as fully as possible with the abosrbent 
recirculating through the absorber. Preferably, absorber 2 is of a packed 
tower design, and the stack gases pass countercurrently to the flow of the 
absorbent. An essentially SO.sub.x -free (and thus desulfurized) product 
gas stream is discharged to the atmosphere by line 4 while recovered 
absorbent containing dissolved SO.sub.x compounds is withdrawn via line 5 
and recycled via line 6, pump 7, and lines 8 and 9. 
In the event the stack gas in line 1 contains fly ash or other particulate 
matter, as would be typical for a stack gas obtained from the combustion 
of coal, such particulate matter or fly ash is collected in the lower 
portion of the SO.sub.x absorber. It is removed therefrom as a slurry with 
some of the absorbent utilized in the SO.sub.x absorber and directed by 
line 10 to a fly ash pond or other waste facility. 
The absorbent as introduced into the SO.sub.x absorber via line 3 may 
comprise any alkaline, aqueous solution useful for the removal of SO.sub.2 
and SO.sub.3 from stack gas. The most typical absorbent comprises sodium 
hydroxide, although potassium hydroxide and even solutions of potassium 
thiosulfate and potassium formate, as used in the well-known Consol 
process, may also be utilized. Suitable concentrations of dissolved alkali 
compounds in the absorbent solution range between about 5 and 50 percent 
by weight, usually between about 10 and 20 percent by weight. These 
absorbents, when recycled at an appropriate rate and pH through SO.sub.x 
absorber 2, remove essentially all SO.sub.x compounds from the stack gas, 
producing an absorbent solution containing such oxysulfur anions as 
sulfite and sulfate ions, the latter being formed not only by the 
dissolution of SO.sub.3 but also by the absorption of SO.sub.2 in the 
presence of oxygen. 
As the absorbent solution becomes increasingly more concentrated in 
sulfur-containing anions, it must be replenished with fresh absorbent. 
Thus, at steady state, fresh absorbent must be introduced into absorber 2 
while spent absorbent is removed partly by line 10 but mostly by line 11. 
To reduce the rate at which make-up absorbent must be introduced via line 
3, it is usually required that the spent absorbent removed via line 11 be 
regenerated. In most cases, this is accomplished, as in the Wellman-Lord 
process, by heating the spent absorbent solution in a suitable heating or 
distillation vessel 12 to drive off absorbed SO.sub.2, thereby obtaining 
in line 13 a product gas rich in SO.sub.2 and useful as a feed to a Claus 
plant for the manufacture of sulfur. Also obtained is a solution once 
again active for the removal of SO.sub.x in absorber 2, and accordingly 
this solution is recycled to absorber 2 via pump 14 and lines 15, 16 and 
17. 
Since some SO.sub.x compounds dissolve in forms not readily decomposed to 
SO.sub.2, a bleed stream comprising sulfate ions and, in the usual 
instance, one or more oxysulfur anions selected from the group consisting 
of sulfite, bisulfite, bisulfate, and thiosulfate ions, is withdrawn from 
heating means 12 via line 18. According to the process of this invention, 
the bleed stream in line 18 is introduced to one or more diaphragmless, 
electrolytic, mercury cells, represented in the drawing as the single cell 
20. 
Electrolytic cell 20 comprises a cell housing 21 constructed to provide an 
inlet 22 for the bleed stream, an outlet 19 for the product slurry 
consisting essentially of an aqueous liquid product containing dispersed 
particles of elemental sulfur, another outlet 23 for gases produced by the 
chemical reactions occurring within cell 20, and an outlet 43 and inlet 44 
for the withdrawal of amalgam and return of mercury. Cell housing 21 may 
be composed of any suitable material customarily used in the construction 
of diaphragmless, mercury, electrolytic cells, including such materials as 
concrete, ebonite-lined steel, reinforced plastic or ceramic, and rubber 
convered steel. A suitable a.c.-d.c. converter 26 impresses a voltage or 
potential difference across a mercury pool 41 acting as cathode and 
several graphite rods 42 acting as anode, with the magnitude of the 
potential difference being adjusted to produce elemental sulfur without 
also producing a significant amount of hydrogen sulfide from the 
particular aqueous stream entering inlet 22. 
The reduction of positive valence sulfur atoms to elemental sulfur occurs 
at the mercury pool cathode 41 while the consumption of water and the 
oxidation of negative valence sulfur atoms occurs at the graphite anode 
42. Assuming that the electrolye solution in the mercury cell contains 
sulfite, sulfate, and thiosulfate anions in equal molar ratios, the 
chemical half-reactions occurring within the cell may be formulated as 
follows: 
Cathodic Reactions 
EQU SO.sub.3.sup.-- +4e.sup.- +3H.sub.2 O.fwdarw.S+6OH.sup.- (I) 
EQU SO.sub.4.sup.-- +6e.sup.- +4H.sub.2 O.fwdarw.S+8OH.sup.- (II) 
EQU S.sub.2 O.sub.3.sup.-- +6e.sup.- +6H.sup.+ .fwdarw.S+3H.sub.2 
O+S.sup.--(III) 
Anodic Reactions 
EQU 7H.sub.2 O.fwdarw.14H.sup.+ +31/2O.sub.2 +14e.sup.- (IV) 
EQU S.sup.-- .fwdarw.S+2e.sup.- (V) 
with the overall anion reduction being formulated as: 
EQU S.sub.2 O.sub.3.sup.-- +SO.sub.3.sup.-- +SO.sub.4.sup.-- +3H.sub.2 
O.fwdarw.4S+6OH.sup.- +31/2O.sub.2 (VI) 
Simultaneously with the above theorized reactions is the side reaction 
involving the reduction of alkali metal ions to alkali metal in amalgam 
form, which side reaction may be formulated (if the alkali metal is 
sodium) as: 
EQU 6Na.sup.+ +3H.sub.2 O .sup.H.sbsp.g 6Na(Hg)+6H.sup.+ +11/2O.sub.2 (VII) 
and the net overall reaction occurring within the cell is the sum of 
reactions (VI) and (VII): 
EQU Na.sub.2 S.sub.2 O.sub.3 +Na.sub.2 SO.sub.3 +Na.sub.2 SO.sub.4.sup. 
H.sbsp.g 4S+5O.sub.2 +6Na(Hg) (VIII) 
Withdrawn from diaphragmless, electrolytic, mercury cell 20 are an aqueous 
liquid in line 27 containing dispersed particles of sulfur, elemental 
oxygen from outlet 23, and a sodium amalgam in line 28. The amalgam is 
introduced into decomposer 29 wherein, by countercurrent contact in a bed 
of graphite packing with an aqueous liquid interconnected from line 30, 
the amalgam is decomposed according to the following chemical reaction: 
EQU 2Na(Hg)+2H.sub.2 O.fwdarw.2Hg+2NaOH+H.sub.2 (IX) 
The products obtained from Reaction IX separate into a hydrogen gas phase 
that collects in the upper portion of decomposer 29 and a two-phase liquid 
of caustic solution and mercury, the latter of which collects in the 
lowest portion of decomposer 29 due to its high density. A stream of 
elemental hydrogen suitable as a fuel is removed from the upper portion of 
decomposer 29 by line 31 while a stream of elemental mercury is removed 
from decomposer 29 via line 32 and then recycled to the electrolytic cell 
20 by mercury pump 33 and recycle line 34. Also obtained from decomposer 
29 is a relatively pure, aqueous solution of caustic, which may either be 
recovered as a by-product or, if the circumstances permit, be directed by 
recycle line 35 as fresh SO.sub.x absorbent for absorber 2. 
The sulfur-aqueous liquid slurry recovered from cell 20 via line 27 may be 
treated in any convenient manner to separate elemental sulfur from the 
aqueous carrier. One method involves heating the aqueous stream obtained 
in line 27 under suitable pressure such that the sulfur is liquefied and 
then separated from the aqueous carrier by density difference in a 
decanter. Another is to pass the stream to a suitable liquid-solid 
separation zone 36 wherein elemental sulfur is separated, as by 
centrifugation or filtration, from the aqueous carrier. The particulate 
sulfur is thus ejected as a solid through conduit 37 while the aqueous 
filtrate is collected in line 30 and preferably used as the aqueous media 
required in decomposer 29. 
In the preferred embodiment of the invention, the liquid mercury and the 
aqueous electrolyte in cell 20 are agitated to maintain cell efficiency 
and prevent polarization. Accordingly, means are included, such as a 
mechanical stirrer not shown, for agitating the liquids in cell 20. As an 
alternative to a mechanical stirrer, recycle pumps may be used to keep 
both the electrolyte and mercury liquids circulating through the cell. 
Keeping the liquids agitated by either of the foregoing or equivalent 
methods aids in keeping the dispersed elemental sulfur separate from the 
mercury. It would obviously be undesirable to allow a layer of solid 
sulfur to accumulate on the mercury surface, and accordingly, it is 
necessary in the preferred embodiment to separate the dispersed elemental 
sulfur from the mercury. This is preferably achieved by collecting the 
sulfur as a froth floating on the surface of the electrolyte. Elemental 
sulfur produced in cell 20 tends to float to the surface of the 
electrolyte because gas streams bubble from the anode, and some of the gas 
becomes entrapped in the dispersed sulfur particles, thereby sufficiently 
reducing the density thereof to cause flotation. Agitation enhances this 
flotation effect and renders it more efficient because sulfur particles 
are constantly being stirred into, and thus being more intimately 
contacted with, the gas bubbles emanating from the anode. 
One method by which the electrolyte may be agitated and the flotation 
effect enhanced still further is by introducing one or more gas streams 
into the lower portion of the cell. In one embodiment, air or other gas is 
bubbled from the bottom of the cell through both the mercury and the 
electrolyte liquids to be collected with the product oxygen in outlet pipe 
23. A more preferred embodiment, in which energy losses involved in 
pressurizing a gas through mercury are minimized, the injection gas, which 
is preferably a portion of product oxygen obtained in line 23, is 
introduced into cell 20 at a location therein just above the 
mercury-electrolyte boundary. This may be accomplished by injecting the 
gas through pipe 38 into header 39 to bubble the gas into the electrolyte 
from a number of distribution points 40 lying near the mercury-electrolyte 
boundary. 
Yet another means for introducing a multitude of gas streams into the cell 
from the mercury-electrolyte boundary is by periodically increasing the 
voltage across the electrodes of the cell to the point that hydrogen gas 
is produced from the mercury cathode. After bubbling up through the 
electrolyte and aiding in producing the sulfur froth, the hydrogen is 
collected with the oxygen gas leaving the cell by outlet 23. 
The following illustrative example is provided to demonstrate the 
feasibility of producing elemental sulfur in a diaphragmless, mercury, 
electrolytic cell from an aqueous solution containing oxysulfur anions. 
EXAMPLE 
Into a 3-liter flask was introduced a quantity of mercury, which lay in the 
bottom portion of the flask in contact with a platinum wire sealed through 
the glass. Two liters of an aqueous solution comprising 30-35 wt.% 
Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O were then introduced into the flask. 
Subsequently, carbon rods of dimensions 3/4 inch diameter by 6 inch length 
were placed through the four openings at the top of the flask and 
contacted with the aqueous solution. When a voltage of between about 5 and 
8 volts was impressed across the carbon rods acting as anode and the 
mercury pool acting as cathode, elemental sulfur was found to form in the 
electrolyte while a gas stream bubbled from each of the four carbon rods. 
Although the invention has been described in conjunction with an example, 
many variations, modifications and alternatives of the invention as 
described will be apparent to those skilled in the art. As an 
illustration, although much attention has been devoted in the description 
of the invention to treating a solution containing cations of the alkali 
metals, it is clear that aqueous solutions containing other cations, 
particularly ammonium cations if the pH within the cell is properly 
maintained to prevent the evolution of ammonia gas, may be 
electrochemically treated to convert the oxysulfur anions in such 
solutions to sulfur. In so doing, advantages not realized with solutions 
containing alkali metal cations may be obtained. For example, when 
ammonium ion-containing solutions are treated, no amalgam will form in the 
cell, thereby obviating an amalgam decomposer and mercury pump with their 
attendant costs. Accordingly, it is intended to embrace within the 
invention all such variations, modifications, and alternatives that fall 
within the spirit and scope of the appended claims.