Note: Descriptions are shown in the official language in which they were submitted.
FMC 1685
lO90.~
This invention relates to the purification of aqueous
waste streams containing triazines such as cyanuric acid
and ammelide.
Chlorinated isocyanuric acids and their alkali metal
salts are familiar chemical entities which are useful as a
source of active chlorine. Especially important members
are sodium dichloroisocyanurate and trichloroisocyanuric
acid. These are high-purity, white crystalline solids,
available in a variety of mesh sizes. Although active
oxidizers, they can be handled and transported with rela-
tive ease and safety. One of the important commercial
applications of these products is in the area of water
treatment where they have proved effective and convenient
for controlling algae and pathogenic bacteria. The water
in swimming pools, for example, is readily maintained in a
clean and sanitary condition by the addition of chlori-
nated cyanuric acid derivatives. Other volume uses are as
a dry bleach in cleansing, laundering and sanitizing com-
positions and the like.
Alkali metal dichloroisocyanurates and trichloroiso-
cyanuric acid are produced commercially by the chlori-
nation in aqueous media of alkali metal cyanurates. The
process is well known and documented exclusively in the
technical and patent literature and in this connection
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reference is made to U.S. Patent Nos. 3,299,060
(British Patent No. 1,083,404), 2,969,`360 (British Patent
No. 902,539), and 3,035,056. The principal reactions
involved, omitting intermediate stages and species, can be
depicted by the following simplified chemical equation in
which the alkali metal is sodium:
ICl
O=IC ~ C-ONa + 2Cl2 ~ o=f/ ~f=o + 2NaCl
\C~ ~ C~N Cl
Na O
Dichloroisocyanuric Acid
NaOH
fl
NaO-C~ \C=O
I ~ bl / -Cl
2C1 2 o
N~Sodium Dichloroisocyanurate
C~ N Cl
ONa
3Cl2O=IC C=O + 3NaCl
~C~
o
Trichloroisocyanuric Acid
The chlorination can be carried out by passing
chlorine through an aqueous slurry of the di- or trisodium
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cyanurate thereby forming the corresponding di- or
trichloroisocyanuric acid. These are filtered from the
reaction mixture and the resulting filtrate, which con-
tains up to about 2.0 percent dissolved chlorinated iso-
cyanuric acids, constitutes an acid waste stream having a
pH range of about 0.5 to 5Ø The dichloroisocyanuric
acid is neutralized with base, for example aqueous sodium
hydroxide and the so-formed sodium dichloroisocyanurate
filtered off. The filtrate from the neutralization con-
tains up to about 25 percent dissolved chlorinated sodium
isocyanurate and constitutes a near neutral waste stream
having a pH range of about 6.5 to 7Ø
Alternatively, the sodium dichloroisocyanurate can be
realized directly by selective chlorination of trisodium
cyanurate wherein two of the sodiums are replaced with
chlorine while the third sodium remains attached to the
cyanurate ring. The waste stream from this operation
exhibits a pH in the vicinity of 5.0 to 7.5. In general,
the pH of chlorinated isocyanurate waste streams will run
from about 0.5 to 7.5. Where a neutral and an acid waste
stream are produced these may be combined to yield a
single waste stream.
Of all the chlorinated isocyanurate derivatives,
sodium dichloroisocyanurate en~oys the greatest commercial
usage since it possesses such desirable attributes as
stability, water solubility and high available chlorine
content. Although essentially water-insolublè, trichloro-
isocyanuric acid has the highest available chlorine and
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because of this may be preferred for certain applications.
Generally speaking, however, both derivatives are important
industrial chemicals.
While entirely satisfactory from a purely technical aspect,
the manufacture of sodium dichloroisocyanurate and trichloroiso-
cyanuric acid is accompanied by the production of aqueous waste
streams containing dissolved cyanurates, the pollution-free dis-
posal of which presents a special problem. A practical solution
to this difficulty is a requirement for a commercially success-
ful operation.
The increasing public concern over the pollution of our
lakes and streams, coupled with the adoption of Federal, State,
and local regulations governing the discharge of waste materials
has occasioned increased effort in developing new and improved
processes and systems for treating waste streams prior to dis-
charge.
One approach to the problem is to heat the aqueous waste
stream in a pressure vessel at about 225C to 275C to hydrolyze
and decompose the cyanurate compounds, as described in Belgian
Patent No. 840,817 issued October 15, 1976, in the name of
Sidney Berkowitz and Charles V. Juelke and assigned to the assignee
of the instant application. Although this process is highly
effective in eliminating from the waste stream all traces of
cyanurate compounds, it is costly in terms of energy input. In
addition, the ammonia that is formed by decomposition of the
cyanurate compounds is in itself a pollutant.
--4--
1~ ~ O ~ 2ti
Another approach to the problem is to treat the
aqueous waste stream with active carbon powder. The
active carbon was found to exhibit a strong affinity for
dissolved cyanurate compounds so that they were removed
from the waste streams upon contact. Although this pro-
cess effectively cleans up the waste stream, the need to
purchase and handle large quantities of active carbon
powder increases operating costs. The disadvantage is
partially off-set by recycling cyanurate values recovered
from the exhausted carbon. Even so, the process is not as
economically attractive as might be desired and further
improvements in the treatment of chlorinated isocyanurate
waste streams are being actively pursued.
Yet another method for removing dissolved chlori-
nated cyanurate compounds from aqueous waste streams is
described in British Patent No. 1,450,003. That patent
discloses a method of dechlorinating the waste stream by
treatment with hydrogen peroxide. It is an advantage of
this process that the cyanuric acid (or its sodium salt)
precipitates from the waste stream and may be recycled
back to the chlorination zone. Approximately 65 to 98%
of the cyanurate compounds dissolved in the waste streams
are recovered in this manner. However the waste stream,
after the solid cyanurate values have been precipitated,
retains in solution a low level of organic matter - of
the order of 200 to 3,000 parts per million (ppm).
The present invention provides a means for
reducing the levels of cyanurate compounds present in
lU~O~
the chlorinator waste stream to less than 15 ppm by
reaction of the waste stream with sodium hypochlorite in
either a batch or continuous process, and has particular
application to the treatment of such waste streams that
have been dechlorinated with hydrogen peroxide to reduce
soluble cyanuric acid levels from 2 percent to 200-3,000
ppm. Further oxidization of such waste streams, con-
taining as much as 3,000 ppm of cyanurate compounds with
sodium hypochlorite can result in an effluent that meets
zero nitrogen discharge requirements. The reaction rate
is related to such variables as pH, temperature, initial
concentration of cyanurate compounds and initial sodium
hypochlorite concentration. Excess sodium hypochlorite
can be subsequently destroyed by reaction with hydrogen
peroxide.
In accordance with the present invention ammelide and
cyanurate compounds such as cyanuric acid can be removed
from aqueous waste streams by treatment with sodium hypo-
chlorite solution. The amount of sodium hypochlorite
used, the temperature and the pH at which the waste stream
is maintained during contact with the sodium hypochlorite
solution are critical for maximum process performance.
In carrying out the invention, sufficient sodium
hypochlorite is added to the waste stream liquor to
chemically oxidize the cyanuric acid or its sodium salt
to carbon dioxide. The chemistry of the reaction of
sodium hypochlorite with cyanuric acid may be represented
by the following equation indicating a 4.5 to 1 molar
30.~
stoichiometry of sodium hypochlorite to cyanuric acid.
H
O ~ N~C O
2 f I + 9NaOCl + 3N2 + 6CO2+9NaCl+3H20
HN~ ~NH
li
On a weight basis, this reaction requires a ratio of
259.5 parts by weight of sodium hypochlorite to 100 parts
by weight of cyanuric acid. At least 90 percent of the
cyanuric acid nitrogen is oxidized to nitrogen gas, the
balance being oxidized mostly to nitrates. The reaction
of sodium hypochlorite with cyanuric acid proceeds most
rapidly at pH 9.0 to 10 and increases in rate 2-3 times
for every 10C increase in temperature. The effect of the
initial concentration of cyanuric acid and sodium hypo-
chlorite on the reaction rate will be discussed below.
The residence time for destruction of 95 percent of the
cyanuric acid present in the waste stream can range from
more than 200 hours to less than 5 minutes depending upon
the reaction conditions.
Cyanurate compounds that may be removed from aqueous
waste streams by the process of this invention include
cyanuric acid, alkali metal salts of cyanuric acid, and
amide derivatives of cyanuric acid such as ammelide.
The waste streams that may be advantageously treated
by the process of this invention may contain up to about 2
percent of dissolved cyanurate compounds. Typical
solutions will contain from 200 to 3,000 ppm of cyanurate
compounds. Other soluble compounds may also be present
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lS)~O~Ztj
in the waste stream such as sodium chloride (up to 10
percent by weight), sodium bicarbonate, ammonium dihydrogen
orthophosphate, ammonium bisulfate, sodium nitrate, hydro-
chloric acid, sulfuric acid and orthophosphoric acid.
The process of the present invention has application
to solutions ranging in pH from essentially zero to 12.
However, the waste stream should be adjusted to a pH
of from 8 to 12 prior to contact with the sodium hypo-
chlorite solution. The preferred operating range of the
process of the present invention is between about 9 and 11.
Particularly preferred is an operating pH in the range of
9.0 to 10.
The amount of sodium hypochlorite reacted with the
waste stream may range from a low of about 2 moles of
sodium hypochlorite per mole of cyanurate compound present
in the waste stream to as much as 9 moles of sodium hypo-
chlorite per mole of cyanurate compound present in the
waste stream. Preferred operating conditions require a
molar stoichiometry of sodium hypochlorite to cyanurate
between 6 to 1 and 8 to 1. The source of the sodium hypo-
chlorite reactant may be either sodium hypochlorite added
as such, gaseous or liquid chlorine, hypochlorous acid, or
nitrogen chlorinated isocyanuric acid derivatives.
In addition to cyanuric acid, other triazines such as
ammelide may be destroyed by the process of this invention.
The chemistry of the reaction of sodium hypochlorite with
ammelide may be represented by the following equation
indicating a 6 to 1 molar stoichiometry of sodium
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hypochlorite to ammelide.
H
H 2 N-Ç ç=o
2 ¦ ¦ + 12NaOC1 ~ 4N2 + 12NaCl + 6CO2 + 4H2O
~NH
o
The benefits and advantages of the present invention will
become apparent upon a reading of the description of the
preferred embodiments taken in conjunction with the
accompanying drawings, wherein:
Figure 1 is a graph that illustrates the effect of pH
on the rate of cyanuric acid decomposition in the presence
of sodium hypochlorite.
Figure 2 illustrates a continuous process for treating
a waste stream containing cyanurate compounds in accordance
with the process of the present invention.
The invention is illustrated in greater detail by the
following examples in which all percentages are by weight
unless specified otherwise.
EXAMPLE I
One hundred parts by weight of a simulated waste
effluent composition containing 0.3 percent cyanuric acid,
6 percent sodium chloride and 0.5 percent hydrochloric
acid is mixed with 12.4 parts by weight of 5.25 percent
sodium hypochlorite solution and the pH is adjusted to
10.5. The weight ratio of the sodium hypochlorite to the
cyanuric acid present in solution is 217:100. This mix-
ture is stirred at room temperature for a period of two
hours, during which time extensive gassing is observed
due to the elimination of nitrogen. During this reaction
l~90~Z~
the pH of the mixture decreases to 10.0-10.1 due to decom-
position of the cyanuric acid to form carbon dioxide and
conversion of the carbon dioxide to sodium carbonate
and sodium bicarbonate. The pH of the effluent is
periodically readjusted during the reaction to pH 10.5 by
addition of 10 percent sodium hydroxide solution. After
the simulated waste effluent composition had been stirred
for two hours, it is treated with hydrogen peroxide to
destroy residual sodium hypochlorite and subsequently
acidified and heated at pH 2 to dispel carbon dioxide.
The solution following this treatment is analyzed for
cyanuric acid and found to contain 0.113 percent indi-
cating that about 63 percent of the cyanuric acid present
in the simulated waste solution was degraded. No ammonia
or chloramine odors are detected in the solution during
the two hour reaction.
EXAMPLE II
A number of batch experiments are conducted with a
simulated waste effluent containing 480 ppm cyanuric acid
and 6 percent sodium chloride. One hundred parts by
weight of this simulated waste effluent is reacted with
1.19 parts by weight of 10.8 percent sodium hypochlorite
solution at room temperature under varying conditions of
pH and reactor residence time. The pH of the simulated
waste effluent is maintained at the desired level by the
addition of 10 percent sodium hydroxide solution as
described above in Example I. The data from this Example
appear in Table I below and show a good correlation
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~go~
between the decomposition of cyanuric acid and sodium
hypochlorite, thus substantiating the 4.5 to 1 sodium
hypochlorite to cyanuric acid molar stoichiometry.
--11--
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.
O Q
c a
W o
~ J
æ o
a) o
C
~J
H '~1 _
1~ t~ .~
~: C C ~ O O O O O O O
O O
~0 ~ t~
o o&
.,, a ~
~ OoD C ~
'.L~ ~ rl
U~ Il~ 21 J~
a 5~
a ~ ~ ~ ~ ~ ~ ~ O O O
~ O ~: h ~ ~ ~ ~ ~ ~ t~
,~ ~ ~1 ~ a~
o ~ Q,
_1 ~ z ~
~C~ u~- C E~
O ~ H ~ ~ t~
R u~ u~ o u~ o o o o
C ~ ~ O ~
Zl ~
o C ~
~C
--12--
~U5~0~32~i
The data in the abo~Te table are plotted in Figure 1
and suggest that under stoichiometric operating conditions
at about 20C the reaction rate reaches a maximum at a pH
value between 9 and 10.
EXAMPLE I I I
A simulated waste effluent containing 1,714 milli-
grams per liter of cyanuric acid and about 6 percent by
weight of sodium chloride is treated with a solution con-
taining 4,804 milligrams of sodium hypochlorite. The molar
ratio of sodium hypochlorite to cyanuric acid is 4.86:1 (an
8 percent excess of sodium hypochlorite). The decom-
position rate (25C) is measured at pH 8.5, 9.5 and 10.5.
The residual undecomposed cyanuric acid present in the
simulated waste solution is measured by total Kjeldahl
nitrogen analysis. These data are listed in Table II
below which shows the advantage of close pH control at
about pH 9.5 for maximum rate at a temperature of 25C.
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TABLE II
Effect of pH on Rate of Decomposition
of Cyanuric Acld Wlt Sodlum Hypochlorlte
Conditions: 25C
NaOCl/CA mole ratio 4.86/1
Initial CA 1,714 mg/l
Elapsed Time % CA Decomposition 10
(Hours) pH .5 pH 9.5 pH .5
1 22 53 14
2 27 71 15
4 36 85 21.5
6 44 92 26
8 51 95
55.5 96.5
12 59 97
EXAMPLE IV
A simulated waste effluent containing 1,714 milli-
grams per liter of cyanuric acid and about 6 percent by
weight of sodium chloride is treated with a solution con-
taining 4,804 milligrams of sodium hypochlorite ~an 8 per-
cent excess). The decomposition rate is measured at
temperatures of 12, 25 and 35C while maintaining the
operating pH of the solution at 9.5. The data listed in
Table III below show that the reaction rate increases
markedly between 12 and 35C.
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z~;
TABLE III
Effect of Temperature on Rate of Decomposition
o Cyanuric Acl Wlth Sodium Hypochlori~te
Conditions: pH 9.5
NaOCl/CA mole ratio 4.86/1
Initial CA 1,714 mg/l
Elapsed Time % CA Decomposition
(Hours) 12C 25C 35C
1 18 53 74
2 32 72 88
4 46.5 85
6 56 92
8 64.5 95
72 96.5
12 78 97
EXAMPLE V
To evaluate the effect of changing the initial molar
ratio of sodium hypochlorite added to the cyanuric acid
waste stream, a simulated waste effluent containing 1,714
milligrams per liter of cyanuric acid and about 6 percent
by weight sodium chloride is treated with a solution con-
taining 2,175 milligrams of sodium hypochlorite. The
molar ratio of sodium hypochlorite to cyanuric acid is
2.2:1. The mixture is agitated for 4 hours while main-
taining the temperature at 25C and the pH at 9.5. Fifty-
one percent of the cyanuric acid is decomposed.
This experiment is repeated increasing the amount of
sodium hypochlorite from 2,175 milligrams to 9,390 milli-
grams. The molar ratio of sodium hypochlorite to cyanuric
acid is 9.5:1. The mixture is agitated for 4 hours while
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maintaining the temperature at 25C and the pH at 9.5.
Ninety-one percent of the cyanuric acid is decomposed.
EXAMPLE VI
This Example and Figure 2 describe a continuous process
for treating a waste effluent containing cyanuric acid
compounds with sodium hypochlorite. Referring now to
Figure 2 containers 11, 12 and 13 are 2-1/2 gallon
(9.46 liters) battery iars, each of which contains 8
liters of 6 percent sodium chloride splution. Vessel 11
contains 100 ppm of cyanuric acid and 0.02 moles per
liter of sodium hypochlorite. Vessel 12 contains 40 ppm
of cyanuric acid, and 0.017 moles per liter of sodium
hypochlorite. Vessel 13 contains 15 ppm cyanuric acid
and 0.014 moles per liter of sodium hypochlorite.
The levels of cyanuric acid and sodium hypochlorite
present in containers 11, 12 and 13 represent typical
concentrations obtained under continuous steady-state
operating conditions.
The contents 14, 15 and 16 of each of the three
vessels 11, 12 and 13 are maintained at 45C with gentle
stirring and the pumps (P-l through P-5) are started.
The pump P-l is adjusted to deliver 63 ml per minute of a
6 percent sodium chloride solution containing 660 ppm of
cyanuric acid to the container 11. The pump P-2 is adjusted
to deliver 4 ml per minute of an aqueous 0.64 molar sodium
hypochlorite solution to the container 11. The pumps
P-3, P-4 and P-5 are adjusted to maintain 8 liters of
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11)9~
solution in each of the vessels 11, 12 and 13. The pH in
each of the reactor vessels 14, 15 and 16 is maintained at
9.5 + 0.2 for 12 hours. After six hours of operation the
total Kjeldahl nitrogen in the three vessels is determined.
The residual cyanuric acid and the amount of cyanuric acid
decomposed in each of the vessels is described in Table IV.
TABLE IV
VesselCyanuric Acid
Present Decomposed (%)
11147 ppm 92
1231 ppm 98
13 6 ppm 99
Operation of the three-vessel continuous system
described above reduces the organic nitrogen content (as
cyanuric acid) from 200 ppm to less than 5 ppm in 6 hours
at 45C, in 3 hours at 55C, or in 1/2 hour at 85C.
The pH is an important variation and should be control-
led to 9.5 + 0.5 when operating in the temperature range
of 45 to 85DC. The amount of sodium hypochlorite most
effective in the continuous process described by this
Example is between 6.8 and 9.0 moles of sodium hypo-
chlorite per mole of cyanuric acid (a 50 percent to
100 percent excess).
EXAMPLE VII
To 4,240 parts of 6 percent sodium chloride solution
is added 2.4 parts of ammelide and the pH is adjusted to
9.5. Seventy parts of a 14.3 percent sodium hypochlorite
solution is added and the temperature is increased to
~090~
45C with gentle agitation while maintaining the pH at
9.5. After 15 minutes a sample of solution is removed
and analyzed for ammelide content by U.V. spectroscopy.
The analysis indicated 106 ppm of residual ammelide
corresponding to 81 percent decomposition during the
15 minute interval.
Pursuant to the requirements of the patent statutes,
the principle of this invention has been explained
and exemplified in a manner so that it can be readily
practiced by those skilled in the art, such exemplifi-
cation including what is considered to represent the best
embodiment of the invention. However, it should be
clearly understood that, within the scope of the appended
claims, the invention may be practiced by those skilled
in the art, and having the benefit of this disclosure
otherwise than as specifically described and exemplified
herein. In the following claims, the term "cyanurate
compounds" shall include but not be limited to ammelide
and water soluble salts thereof.
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