Method for removing chlorate from caustic solutions with electrolytic iron

A process for removing chlorate from a caustic solution by the reduction of the chlorate with metallic iron. The process is particularly distinguished by the steps of contacting the caustic solution with iron to reduce the chlorate to chloride, contacting the caustic solution containing dissolved iron therein with a conductive substrate to electrolytically precipitate the iron on the substrate, thus providing a caustic solution which is substantially free from iron and chlorate ions, separating the purified portion of the caustic solution, and periodically using the iron coated substrate for treating an untreated portion of the caustic solution to reduce the chlorate to chloride, and periodically reusing the iron depleted substrate, for recovering dissolved iron from the caustic solution. The process of this invention decreases the chlorate content of caustic solutions to an acceptable level of a few ppm and also recovers dissolved iron on an iron depleted substrate for reuse and further reduction of chlorate of an untreated portion of the caustic solution.

BACKGROUND OF THE INVENTION 
The present invention resides in the removal of chlorate from caustic 
solutions with iron and the consecutive recovery of said iron in metal 
form. 
The removal of unwanted excess chlorate in sodium hydroxide solutions 
obtained from the diaphragm electrolysis of brine is carried out by using 
reducing agents, a number of which have been described, including iron. 
Thus, U.S. Pat. No. 2,404,453 discloses the reduction of chlorate in 
sodium hydroxide solutions containing about 50 percent NaOH using iron in 
comminuted form (chips or turnings), said iron particles being coupled 
with a more noble metal such as copper. Also, U.S. Pat. No. 2,403,789 
discloses the purifying of 30-60 percent NaOH solution from excess 
chlorate by means of iron powder or filings. In such processes, the iron 
dissolves in the caustic forming Fe.sup.+2 and Fe.sup.3+ ions and 
simultaneously reducing the chlorate to chloride. 
The dissolved iron which is then present as an impurity in the caustic 
solution as the result of the removal of chlorate should thereafter be 
eliminated by means involving, for example, oxidation, precipitation or 
electrolytic reduction. Thus, the cathodic removal of iron and other metal 
ions has been disclosed in the following publications: U.S. Pat. No. 
3,244,605 and French Pat. No. 1,505,466. According to the prior art, the 
caustic solution containing the iron to be removed (and possibly other 
metal impurities) is subjected to electrolysis whereby the iron deposits 
as a metallic coating on the cathode of the electrolytic cell. However, it 
has not been reported that such electrolytically deposited iron can be 
re-used for again reducing the chlorate content of a yet untreated caustic 
solution. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to use electrolytically reduced 
iron for removing chlorate from caustic solutions obtained by the 
diaphragm process. 
A further object of the invention is to decrease the chlorate content of 
caustic solutions containing from 300 to 1000 ppm ClO.sub.3.sup.- down to 
acceptable levels of a few ppm. 
Another object of the invention is to provide economical and practical 
means for achieving the above task using iron as the reducing material. 
Another object of the invention is to remove the iron impurity from caustic 
solutions in which it is present as the result of the removal of 
ClO.sub.3.sup.-. 
Still another object of the invention is to recover the dissolved iron 
having been used for the reduction of the chlorate by cathodic reduction 
into its elemental form and to re-use said recovered iron for further 
reducing the chlorate of a still untreated portion of the caustic 
solution, this operation being very economical since overall consumption 
of iron is then strongly minimized. 
Said objects which will become apparent hereinbelow are achieved by the 
process for removing unwanted chlorate from caustic solutions by the 
reduction of said chlorate with metallic iron which comprises using 
electrodeposited iron as the reducing agent. 
The invention also resides in a process for removing unwanted chlorate from 
a caustic solution by dissolution therein of metallic iron and 
simultaneous reduction of said chlorate by said iron and consecutively 
electrolytically removing said iron from the chlorate free solution which 
comprises 
(A) contacting said solution of caustic to be purified with the iron metal 
in a form appropriate for its easy dissolving in the solution and 
consequent reducing of the ClO.sub.3.sup.- to chloride, then 
(B) contacting the resulting solution containing dissolved iron with at 
least one conductive substrate, the latter being suitably adapted and 
polarized for electrolytically precipitating the iron in a form suitable 
for subsequently reducing chlorate as under (A) above, thus causing said 
iron to deposit thereon in pulverulent metallic form, thus providing a 
solution substantially free from iron and ClO.sub.3.sup.- ions, 
(C) separating said purified portion of the solution and 
(D) periodically using said substrates when sufficiently coated with said 
metallic iron to treat a yet untreated portion of the caustic solution as 
under (A) above and periodically, after consumption of said available 
deposited iron, re-using said substrates for recovering said dissolved 
iron as under (B) above. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The conditions required for carrying out the process of the invention are 
not critical. However, preferably step (A) is carried out at temperature 
above room temperature, preferably at about 100.degree. C., with or 
without agitation. The contacting time depends on the temperature and can 
be conducted over a period of time from 1 to 20 hours, this time also 
depending on the ratio of available iron to the caustic to be treated. 
Such ratio is also not critical provided, naturally, that the available 
iron be at least stoichiometrically sufficient for reducing the chlorate 
to be removed. In practice, it has been found that when nearly 100 percent 
of the original chlorate has disappeared, that is when there remains from 
5 to 15 ppm of ClO.sub.3.sup.-, the amount of dissolved iron is from 400 
to 600 ppm. Therefore, the amount of iron to be used is somewhat dependent 
on the total amount of chlorate originally present in the caustic solution 
to be purified. However, in practice it is always advantageous to use a 
large excess of metal iron since its physical bulk relative to the 
solution is not a problem and, actually, no more than the quantity 
required for effecting the required reduction needs to be dissolved. 
Therefore, the amount of iron to be used, and its physical arrangement 
within the solution will be easily determined by the skilled practitioner, 
this being also dependent on whether the practical implementation relates 
to a batch or a continuous process. 
The parameters for carrying out step (B) of the above described process are 
also not critical. It is indeed very fortunate that the electrodeposition 
of iron under that strongly alkaline conditions prevailing within 
concentrated caustic solutions will provide that form of metallic iron 
which is suitable for reducing the chlorate. Thus, not depending on the 
nature of the conductive substrate used as the cathode, the iron will 
deposit in porous spongy form particularly suited for being easily 
re-dissolved because its "sintered" or agglomerated structure allows a 
very large surface of metal to be contacted by the solution. Practically, 
electrodes of porous graphite or metal mesh, e.g. iron or nickel are 
suitable. Current densitites of from 0.1 to 10 A/dm.sup.2 (amperes per 
decimeter.sup.2) and temperatures of from room temperature to 80.degree. 
C. are suitable for recovering the iron according to step (B) above in a 
form adequate for being re-used according to step (A). Preferably, the 
solution is agitated or circulated during said recovery. The counter 
electrodes (anodes) must naturally not dissolve in the caustic solution by 
oxidation and are preferably made of noble metals or anodes having a 
substrate coated with a noble metal or a noble metal oxide such as, for 
example, a titanium core coated with a noble metal oxide. In practice, 
platinized expanded titanium is suitable. It should be remarked that, even 
in cases where electrolytic recovery conditions of the iron are such that 
they provide homogeneous, inherently non-porous deposits, the porous 
nature of the cathodically polarized substrates will ensure that a 
sufficient amount of internal discontinuities exist within the coated 
layer for still having the iron in highly divided form. 
There are, of course, many possible embodiments for physically implementing 
the invention. For instance, in a first embodiment, the solution to be 
treated is introduced in a conventional glass or polytetrafluoroethylene 
(PTFE) laboratory container and contacted at a moderately elevated 
temperature of from 80.degree. to 100.degree. C. with a plate of nickel 
mesh covered by electrolytically plated iron in pulverulent form. The 
solution is periodically analyzed by conventional means for chlorate and 
iron and when the initial chlorate of from 400-500 ppm has been decreased 
by 90 to 95 percent, the temperature is lowered to about 50.degree. C. and 
an anode plate is immersed into the liquid and electrolysis is carried out 
against the iron clad plate until practically all iron has been plated out 
and redeposited thereon. Conditions for achieving this are those described 
in technical literature, namely for instance the references mentioned 
hereintofore. Then, when the iron content has been reduced to nearly zero 
(practically a few ppm) the electrolysis is stopped and the purified 
solution is poured out and replaced by a new portion of untreated caustic. 
The temperature is raised again and the whole process is repeated thus 
avoiding undue losses of iron. It has been found that during the iron 
recovery step, other heavy metals which may be present in the solution, 
e.g. Ni, Co, Mn, Pb, are also plated out which constitutes a further 
advantage of the invention. During the dissolution stage, some of these 
metals will stay inert and, as a consequence, the substrate will get 
progressively enriched in such plated metals other than iron in the course 
of repetition of the successive dissolution and plating cycles. Therefore, 
there will come a time when the substrate is over-enriched and the plate 
will have to be taken out and replaced. However, this is not a dead-loss 
since such metals other than iron can then be recovered from the plate.

The annexed FIGURE is a schematic representation of an exemplary system for 
continuously purifying caustic solution from unwanted chlorate using iron 
in a closed cycle. 
The system comprises an inlet conduit 1 for the unpurified caustic and a 
general outlet conduit 2 for the purified solution. The impure caustic 
solution can be directed at will from conduit 1 through a two-way valve 3 
to a first cell A or to a second cell B, the inlet conduits to these cells 
being controlled by two-way valves 4 and 5. Therefore, the solution can 
also flow at will from cell A to cell B, or vice-versa, through conduit 6. 
Whichever way, the solution, after flowing through cells A and B will 
leave through one of the valves 4 or 5, the latter having been properly 
adjusted for the desired purpose, and will finally flow out of the general 
outlet conduit 2 after having passed through a two-way control valve 7. It 
is easily seen that, in order to properly provide the desired flow 
direction throughout, the various valves must be linked together by 
conventional means not represented here for the sake of simplicity but 
which will operate so as to simultaneously actuate the various valves in 
order to either direct the caustic solution from the inlet conduit to cell 
A, to cell B, and to the outlet conduit or from the inlet conduit to cell 
B, to cell A and to the outlet conduit. 
Each of the cells contains one plate covered with electrolytically reduced 
iron, respectively 8a and 8b, and a plate, respectively 9a and 9b, made of 
an electrochemically inert material, e.g. platinized titanium or titanium 
covered with a noble metal oxide, these plates being connectable at will 
to a suitable DC generator not represented here. 
Under operating conditions, the present system will function as follows: In 
a first mode, the valves will be adjusted so that the path of the caustic 
solution is directed from cell A to cell B; the plates of cell A will stay 
unpolarized whereas plate 8b of cell B will be made negative to plate 9b 
by means of the above-mentioned generator. Thus, the caustic solution 
flowing along plate 8a of cell A will become purified from excess chlorate 
by contacting the iron deposited on said plate but will simultaneously be 
loaded with iron. Then the iron containing caustic solution will flow 
through cell B whereby the iron will deposit electrolytically on plate 8b. 
After some time, the iron supply on plate 8a will be depleted and plate 8b 
will be fully plated; then, in a second mode, the valves will be 
re-adjusted to provide the solution flow from cell B to cell A, the b 
plates will be disconnected and the a plates will be polarized as 
described above in the case of the first mode for the b plates. The whole 
process is then allowed to continue, the only operation being to 
periodically switch back from the second to the first mode and vice-versa. 
Of course, the process can be operated semi-continuously, the valves being 
fully closed and operation being carried out on a finite portion of the 
caustic, then opened to allow the content of one cell to flow to the other 
cell while re-filling the one cell with an untreated solution and emptying 
the other cell from the purified portion, such sequence being repeated 
until exhaustion of the iron on the reducing plate of the one cell. Then, 
the series of sequences will be continued after reversing the order of the 
cells as described above for the fully continuous embodiment. 
In the present embodiment, the materials used for making the conduits, the 
valves and the cells can be any resins inert to concentrated caustic 
solutions and resisting moderately elevated temperatures. As such, 
polyolefins, melamine-formaldehyde and polytetrafluoroethylene (PTFE) can 
be contemplated. Otherwise, metals such as monel, stainless steel or 
bronze can also be used. 
The invention is further illustrated in more detail by the following 
examples. 
EXAMPLE 1 
First phase 
A 20 cm.sup.2 sintered iron electrode plate, prepared by manually 
depositing a 4 mm layer of iron powder (50-80 .mu.m particles) on a 0.5 mm 
ironplate, then sintering 1 hour at 900.degree. C. under nitrogen plus 5 
percent H.sub.2, was introduced into a 400 ml PTFE cylindrical container 
provided with agitation and reflux condenser. 250ml of a 35 percent sodium 
hydroxide containing 445 ppm chlorate was introduced into the container 
and heated to 110.degree. C. The reaction was allowed to proceed under 
moderate agitation, samples of the solution being removed at intervals for 
analysis. Table I, below, shows the results of said analysis performed by 
known methods described hereinafter. Such results indicate that most of 
the ClO.sub.3.sup.- was removed after about 6 hours reaction time. 
TABLE I 
______________________________________ 
Time, hrs 
ClO.sub.3.sup.-, ppm 
Fe, ppm ClO.sub.3.sup.-, removal, 
______________________________________ 
% 
0 445 0 0 
1/2 445 3.5 0 
1 410 32 8 
11/2 374 57 16 
21/4 240 139 46 
3 160 238 64 
4 72 480 85 
6 18 530 96 
______________________________________ 
Second phase 
Thereafter, a 20 cm.sup.2 platinized anode was introduced into the 
container and adjusted parallel to the iron plate at about 5 cm thereof. 
Then the solution was electrolyzed at 80.degree. C. under 4 A/cm.sup.2 for 
a day after which the iron level was found to be 12 ppm (about 98 percent 
removal). 
The container was refilled with chlorate contaminated caustic and the 
process was repeated with essentially the same results. The whole cycle 
including phase one followed by phase two was repeated several times with 
no significant change in behavior of the reducing aand recovery 
conditions. 
The analytical methods used were the following: 
Chlorates 
An amount of solution corresponding to approximately 60 mg of NaClO.sub.3 
was measured exactly and introduced into a 10 ml volumetric flask, then 3 
ml of concentrated H.sub.2 SO.sub.4 (98%) were carefully introduced from a 
3 ml safety pipette by making the tip of the pipette touch the inside wall 
of the upper stem of the flask and releasing the acid slowly. The solution 
was cooled to room temperature, 0.020 grams of FeSO.sub.4 (NH.sub.4).sub.2 
SO.sub.4.6H.sub.2 O (MOHR's salt), were added, the solution was levelled 
to the mark with distilled water and mixed thoroughly. A reagent blank was 
prepared in the same manner. Then, the absorbances (vs H.sub.2 O) of the 
reagent blank and the sample solution were measured at 301 m.mu. in a 1 cm 
silica cell. The reagent blank was stable for at lest one hour and its 
absorption only accounted to about 0.060 of an absorbance unit. The 
chlorate content was calculated from: 
##EQU1## 
Iron 
10 ml of the sample, 10 ml of concentrated hydrochloric acid, 1 ml of 
hydrogen peroxide and 10 ml of ammonium thiocyanate were introduced into a 
100 ml volumetric flask. After dilution to volume, a spectrophometric 
measurement was performed at 470 m.mu.. As this method is very sensitive 
(works in the range of 1 to 10 mg/l), it was necessary to choose an 
adequate volume of sample in the measurable concentration range. 
EXAMPLE 2 
A 25 cm.sup.2 nickel grid plate, 40 mesh, (pore openings =.apprxeq.500 
.mu.m) was immersed for several minutes in 2 N HCl, then rinsed in 
distilled water. Then it was used to de-ironize at 50.degree. C., 600 ml 
of 50 percent sodium hydroxide under 4 A/dm.sup.2. The electrolysis was 
carried out for 21/2 days, after which the iron removal was 99 percent. 
Then the iron plated nickel grid was used to reduce the chlorate content of 
200 ml of a 50 percent untreated caustic solution containing 500 ppm of 
ClO.sub.3.sup.- for 5 hours at 100.degree. C. exactly as described in 
phase 1 of Example 1. After that period, the chlorate was 99 percent 
removed and the iron content was 650 ppm. Then the iron was removed as 
described in phase 2 of Example 1 and the whole cycle was repeated several 
times with no significant change in the behavior of the plates and the 
reagents. 
EXAMPLE 3 
A sintered iron plate similar to that used in Example 1 was used as 
described in said Example for treating chlorated (550 ppm) 50 percent 
sodium hydroxide solution. Every twelve hours, the cell was emtied, the 
de-chlorated solution was put aside and a fresh portion of caustic was put 
to work for a new 12 hours period without changing the iron plate. After a 
total of 5 lit. of caustic had been treated, the iron was about fully 
consumed and the plate became inoperative. 
Then, the iron contaminated de-chlorated solution was poured into a 10 lit. 
PTFE tank and subjected to electrolysis at 50.degree. C. using a 
platinized titanium anode and the above iron stripped plate as the 
cathode. During electrolysis, the solution was kept under moderate 
agitation. After about 80 percent of the iron was plated out, the iron 
clad cathode was removed and re-used to de-chlorate untreated portions of 
caustic with subsequent removal of iron as described in Example 1. This 
plate provided the same service as the plate of Example 1. 
EXAMPLE 4 
A stainless steel tank was filled with 2.5 lit. of a 45 percent NaOH 
solution containing 375 ppm of chlorate and reduction was carried out at 
100.degree. C. under slight agitation with a 125 cm.sup.2 sintered iron 
plate. The results are shown in Table II below. 
TABLE II 
______________________________________ 
Time, hrs ClO.sub.3.sup.-, % removal 
Fe, ppm 
______________________________________ 
0 0 0 
2 16 74 
3 33 172 
4 49 298 
5 62 314 
15 98 450 
______________________________________ 
Then the solution was put into a 5 liter cylindrical PTFE cell and 
electrolyzed under agitation with a 58.6 cm.sup.2 vitreous carbon cathode 
and a 50 cm.sup.2 platinized expanded titanium anode. The current density 
was about 2 A/dm.sup.2 and temperature 110.degree. C. The results are 
shown in Table III. 
TABLE III 
______________________________________ 
Time, hrs Fe, ppm Iron recovered, % 
______________________________________ 
0 450 0 
11/4 154 66 
23/4 23 95 
42/3 18 96 
6 5 99 
______________________________________ 
The process of iron recovery was repeated four times using fresh iron 
containing solutions but still using the same plate as the cathode. 
Thereafter such iron clad plate was used in a series of chlorate 
purification and iron removal cycles as described in the previous Examples 
with equally good results. 
EXAMPLE 5 
The set up of Example 1 was used but with the difference that during phase 
1 (ClO.sub.3.sup.- removal) an iron sheet electrode was immersed in the 
cell, and a DC generator was connected to that electrode (cathode) and to 
the sintered iron plate (anode). 
Then, the reduction of chlorate was carried out in the same conditions as 
for Example 1, but with the addition of a small current between the 
sintered iron anode (0.1 A/dm.sup.2) and the other electrode in order to 
speed up iron dissolution. The results are shown in Table IV and indicate 
that the removal of chlorate is somewhat accelerated compared to operating 
without current. 
TABLE IV 
______________________________________ 
Time, hrs 
ClO.sub.3.sup.- , ppm 
ClO.sub.3.sup.- , % removal 
Fe, ppm 
______________________________________ 
0 400 11 86 
1/2 325 27 161 
1 277 38 234 
11/2 210 53 189 
2 165 63 167 
21/2 108 76 234 
4 not measurable 
.about.100 500 
______________________________________ 
Thereafter the sintered iron electrode was used to perform iron recovery as 
described in Example 1. The above two-phase cycle could be repeated 
several time with no significant changes to the results obtained.