Stabilized ammonium polysulfide solutions and process utilizing same

A stabilized ammonium polysulfide solution comprising an aqueous solution of ammonium polysulfide and a stabilizing amount of a water soluble, stabilizing compound that can be an azacyclic compound, a compound having the formula R--O--(C.sub.n H.sub.2n)NH.sub.2 or a compound having the formula (CH.sub.3).sub.2 NR.sub.1, the pH of the composition being from 7 to 14, the stabilized composition being useful in controlling the cyanide concentration in aqueous streams.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the stabilization of ammonium polysulfide 
solutions and to a process for controlling cyanides in water streams using 
such stabilized ammonium polysulfide solutions. 
2. Description of the Prior Art 
Cyanides are found in numerous industrial streams such as, for example, 
aqueous waste streams in refineries, chemical plants and the like. The 
cyanides cause considerable corrosion, hydrogen blistering and waste water 
treatment problems. 
It is known, as a method of controlling cyanides in such waste water 
streams, to add polysulfides such as sodium and ammonium polysulfides into 
the waste water system. The polysulfides convert the cyanides into 
noncorrosive, biodegradable thiocyanates, which are water soluble and can 
be readily removed by stripping. 
Ammonium polysulfide is known only in solution and is generally obtained by 
passing hydrogen sulfide into a 28% ammonium hydroxide solution and 
dissolving an excess of sulfur in the resulting solution. Over time, and 
under typical storage conditions, the ammonium polysulfide decomposes, 
resulting in the precipitation of free sulfur. The resulting ammonium 
polysulfide solution containing the precipitated sulfur is largely unfit 
for use in the control of cyanides because of the fact that the active 
ingredient is no longer present. Further, the precipitated sulfur can clog 
valves, pumps and lines, leading to expensive downtime. Since it is 
typical. e.g., in a refinery operation, for the ammonium polysulfide 
solution that is used for the control of cyanides to be in storage for 
relatively long periods of time, a method is needed that will enhance the 
stability of the ammonium polysulfide without deleteriously affecting its 
cyanide control capabilities. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
stabilizing ammonium polysulfide solutions. 
Another object of the present invention is to provide a stabilized ammonium 
polysulfide solution. 
Still a further object of the present invention is to provide an ammonium 
polysulfide solution that is stable at sub-ambient temperature conditions. 
Yet a further object of the present invention is to provide a method of 
controlling the cyanide concentration in an aqueous stream using a 
stabilized ammonium polysulfide solution. 
The above and other objects of the present invention will become apparent 
from the description given herein and the appended claims. 
In one aspect, the present invention provides a method of stabilizing an 
ammonium polysulfide solution in which a stabilizing amount of a water 
soluble stabilizing compound selected from the group consisting of 
azacyclic compounds, compounds having the formula R--O--C.sub.n H.sub.2n 
NH.sub.2, wherein R is an alkyl group of from 1 to 3 carbon atoms and n is 
1 to 3, compounds having the formula (CH.sub.3).sub.2 NR.sub.1, wherein 
R.sub.1 is a hydrocarbon group having from 1 to 6 carbon atoms and 
mixtures thereof is added to an ammonium polysulfide solution, the pH of 
the stabilized solution being in the range of from about 7 to about 14. 
Another aspect of the present invention is a stabilized composition 
comprising an aqueous solution of ammonium polysulfide containing a 
stabilizing amount of a stabilizing compound as described above, the pH of 
the composition being from about 7 to about 14. 
The present invention also contemplates a method for controlling the 
concentration of cyanides in an aqueous stream comprising injecting into 
the stream an effective amount of a stabilized aqueous ammonium 
polysulfide solution containing a stabilizing amount of a stabilizing 
compound, as described above, the pH of said ammonium polysulfide solution 
being in the range of from 7 to 14. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The ammonium polysulfide solution (APS) used in the composition and methods 
of the present invention is an aqueous ammonium sulfide solution (usually 
available in concentrations of up to 70% by weight) containing net-free 
sulfur available for complexing weight simple cyanides. The net-free 
sulfur is the sulfur that dissociates from the ammonia complex and reacts 
with cyanides to form the desired thiocyanates. The ammonium polysulfide 
has the general formula: 
EQU (NH.sub.4).sub.2 S.sub.x 
wherein x is an integer from 2 to 8. 
According to the process of the present invention, APS can be stabilized by 
the addition of a stabilizing amount of certain amine-type compounds that 
are water soluble (soluble in the APS), do not deleteriously affect the 
reaction of the ammonium polysulfide with cyanides, and prevent 
decomposition of the ammonium polysulfide at ambient temperature 
(approximately 20.degree.-30.degree. C.), at sub-ambient temperature (less 
than about 15.degree. F.), or both. The useful stabilizing compounds are 
azacyclic compounds, compounds having the formula: 
EQU R--O--C.sub.n H.sub.2n NH.sub.2 
wherein R is an alkyl group of from 1 to 3 carbon atoms and n is 1 to 3, 
compounds having the formula (CH.sub.3).sub.2 NR.sub.1, wherein R.sub.1 is 
a hydrocarbon group having from 1 to 6 carbon atoms.and mixtures of such 
compounds. Thus, R.sub.1 can be methyl, ethyl, butyl, cyclohexyl, etc. 
Non-limiting examples of azacyclic compounds include morpholine, 
pyrrolidine, ethylenimine, quinuclidine, pyrroline, pyrrole, N-methyl 
pyrrolidone (NMP), 2-pyrrolidinone and the like. Particularly preferred 
are azacyclic compounds wherein the nitrogen is present as a secondary 
amine. Especially preferred as an azacyclic stabilizing compound is 
morpholine. Non-limiting examples of compounds having the formula 
R--O--C.sub.n H.sub.2n NH.sub.2 that are useful include 
3-methoxypropylamine, ethoxyethylamine, methoxyethylamine, etc. 
Non-limiting examples of compounds having the formula (CH.sub.3).sub.2 
NR.sub.1, that are useful include dimethylcyclohexylamine. 
The amount of the stabilizing agent added will depend upon the length of 
storage of the APS, the temperature conditions under which the storage 
occurs, the type of storage utilized for the APS, the type of stabilizing 
compound used, etc. For example, some stabilizing compositions are 
effective at ambient temperatures, but not at sub-ambient temperatures of 
less than about 15.degree. F. In general, the stabilizing compound is 
added in a stabilizing amount, i.e., an amount that significantly reduces 
the precipitation of free sulfur from the APS during storage or use. More 
specifically, a stabilizing amount of the stabilizing compound is that 
amount that prevents significant crystallization and precipitation of free 
sulfur either at ambient temperature, especially when ammonia is being or 
has been lost from the APS, or that prevents such crystallization and 
precipitation at sub-ambient temperatures of less than about 15.degree. 
F., especially less than about 10.degree. F. For example, it is desired 
that the APS remain stable for at least about 30 days without significant 
precipitation of sulfur. Most desirably, it is desired that the APS remain 
stable at ambient temperature for at least 45 days, especially at least 
about 75 days, without significant precipitation of sulfur. While an 
effective or stabilizing amount of the stabilizing compound can be readily 
determined by those skilled in the art, generally speaking the stabilizing 
compound will be present in an amount of from about 3 to about 15% by 
weight of the composition, i.e., the amount of stabilizing compound and 
APS. and APS. 
The stabilized APS can be prepared by mixing in any suitable fashion the 
APS and the water soluble stabilizing compound. 
The stabilized APS composition prepared according to the process of the 
present invention will have a pH of greater than 7, up to 14, more 
preferably from about 8 to about 12. It will be appreciated that acidic 
pHs are to be avoided since this results in rather rapid decomposition of 
the ammonium polysulfide. It is particularly desirable that the pH be 
maintained between about 8 and 12 since this results in optimum stability 
of the APS without introducing an excessively alkaline solution in waste 
(aqueous) streams being treated. 
The destabilization of APS solution, as noted above, is evidenced by the 
precipitation of sulfur. This can occur due to loss of ammonia from 
evaporation during manufacture, transportation and/or storage. 
Furthermore, low temperatures, e.g., about 15.degree. F. and lower, 
especially 10.degree. F. and lower, result in precipitation of the sulfur, 
thereby depleting the active cyanide-removing agent. Moreover, as noted 
above, the presence of precipitated sulfur can lead to plugging of lines, 
pumps, filters, etc., which is clearly undesirable. 
The stabilized APS composition of the present invention can be effectively 
used in methods of controlling cyanides in aqueous streams such as, for 
example, the water system of an FCC reactor such as taught in U.S. Pat. 
No. 4,508,683, which is incorporated herein by reference for all purposes. 
In such water streams or systems, the stabilized APS of the present 
invention is injected into the system at the desired location, which can 
be readily determined by those of skill in the art, the amount of APS 
solution being injected being sufficient to provide a weight ratio of 
net-free sulfur sufficient to complex cyanides, particularly weight-simple 
cyanides. In using the stabilized APS of the present invention, it is 
preferred to maintain the pH of the aqueous stream being treated at at 
least about 8, preferably in the range of from about 9 to about 11. As 
noted, the amount of stabilized APS added will be dependent upon the 
concentration of cyanides in the aqueous stream being treated, the 
concentration of the ammonium polysulfide in the APS solution and the 
degree to which it is desired to reduce the cyanides in the aqueous 
stream. Since the stabilized APS of the present invention substantially 
eliminates the problem of crystallization and precipitation of sulfur, it 
finds particular utility in treating aqueous streams or compositions that, 
for various reasons, albeit that they are present in or form a part of a 
process, exist in a substantially quiescent state, to a certain degree 
mimicking storage conditions to which the APS might be subjected. Since 
sulfur is not readily crystallized or precipitated from the stabilized APS 
compositions of the present invention, the likelihood that precipitated 
sulfur will foul pumps, lines and the like used in the process is greatly 
minimized. 
To more fully illustrate the present invention, the following non-limiting 
examples are presented. In all cases, the APS used had a pH of from about 
9 to about 13.

EXAMPLE 1 
This example demonstrates how the loss of ammonia at ambient temperature or 
subjecting the APS to sub-ambient temperatures results in crystallization 
and precipitation of sulfur from the APS. In a first set of runs, five 
separate, 40-gram samples of APS were subjected to a nitrogen purge at a 
purge rate of 0.5 cc/min to obtain a desired weight loss, i.e., loss of 
ammonia. The five runs were compared with an APS that had not been 
nitrogen purged. The time of crystallization at ambient temperature was 
noted. In a second set of runs, six separate 40-gram samples of APS in 
sealed glass bottles, duplicating the six 40-gram samples of the first set 
of runs, were placed in a freezer and the time of crystallization noted. 
The results for both sets of runs are shown in Table 1 below. 
TABLE 1 
______________________________________ 
% Weight Crystallization Time 
% Weight Loss 
Stabilizing 
Ambient Temperature 
of Ammonia Compound Temperature 
9.degree. F. 
______________________________________ 
0 0 &gt;24 hours 2-3 hours 
0.26 0 &lt;24 hours 5-10 minutes 
0.52 0 14 hours 5-10 minutes 
0.76 0 6 hours 5-10 minutes 
1.0 0 6 hours 5-10 minutes 
2.0 0 6 hours 5-10 minutes 
______________________________________ 
As can be seen from the data in Table 1, loss of ammonia at ambient 
temperature, a condition that occurs frequently in the manufacture, 
transportation and storage of APS, results in crystallization, i.e., 
destabilization of the APS, in a relatively short period of time. Note, 
for example, that when the ammonia loss amounts to 0.26 weight percent, 
crystallization time falls from greater than 24 to less than 24 hours, and 
when the weight loss is above about 0.5% by weight, crystallization occurs 
in about 6 hours. With respect to the six runs at sub-ambient temperature, 
it can be seen that even without any loss of ammonia, the APS begins to 
crystallize in approximately 2-3 hours. However, with only a minor loss of 
ammonia, crystallization takes place in a very short period of time. 
EXAMPLE 2 
In this example, four different charges of 40 grams of APS were purged with 
nitrogen to achieve a 1% weight loss of ammonia. Varying amounts of 
morpholine were then added to the four samples and the crystallization 
times determined at ambient temperature and at 9.degree. F. The results 
are shown in Table 2 below. 
TABLE 2 
______________________________________ 
% Weight 
Stabilizing 
Crystallization Time 
Compound Ambient Temperature 
% Weight Loss 
(Morpholine) 
Temperature 
9.degree. F. 
______________________________________ 
1.0 1.0 6 hours 5-10 minutes 
1.0 2.0 6 hours 5-10 minutes 
1.0 3.5 14 hours 60 minutes 
1.0 5.0 &gt;24 hours &gt;24 hours 
______________________________________ 
As can be seen from the results in Table 2, when the level of morpholine is 
above about 3.0%, the crystallization time, both at ambient temperature 
and at 9.degree. F., is greatly increased. Note particularly that at the 
5% level, crystallization times, both at ambient temperature and 9.degree. 
F., are greater than 24 hours. 
EXAMPLE 3 
In this example, samples of 80 grams of APS with and without stabilizing 
compounds at varying levels were placed into closed containers and then 
placed either into an ambient temperature bath or a freezer maintained at 
9.degree. F. Crystallization times of the various samples were determined. 
The results are shown in Table 3 below. 
TABLE 3 
______________________________________ 
Crystallization Time 
Stabilizing Ambient Temperature 
Compound % Weight Temperature 
9.degree. F. 
______________________________________ 
none 0 45 days 2-3 hours 
morpholine 1 45 days 2-3 hours 
morpholine 2 45 days 2-3 hours 
morpholine 3 45 days 2-3 hours 
morpholine 5 &gt;2.5 months 
&gt;45 days 
morpholine 10 &gt;2.5 months 
&gt;45 days 
N-methylpyrrolidone 
1 &gt;45 days 3 days 
N-methylpyrrolidone 
3 &gt;45 days 3 days 
N-methylpyrrolidone 
5 &gt;45 days 3 days 
N-methylpyrrolidone 
10 &gt;45 days &gt;7 days 
______________________________________ 
As can be seen from the data in Table 3, with no ammonia loss morpholine is 
effective both at ambient temperature and at 9.degree. F. at a level of 
above about 3% by weight. In the case of N-methylpyrrolidone (NMP) (up to 
a 5% by weight amount), satisfactory stabilization is exhibited at ambient 
temperature. However, at low temperatures (9.degree. F.), that level of 
NMP is a less effective stabilizing compound. Note, however, that at the 
10% by weight level, NMP is an effective stabilizer. 
EXAMPLE 4 
This example shows the results of testing the effectiveness of a wide 
variety of potential stabilizing compounds. In all cases, the procedure of 
Example 3 was followed with the exception that the stabilizing compounds 
were used at the 6% by weight level. The results are shown in Table 4 
below. 
TABLE 4 
______________________________________ 
Crystallization Time 
Stabilizing Ambient Temperature 
Compound Temperature 9.degree. F. 
______________________________________ 
None 45 days 2-3 hours 
dimethylcyclohexylamine 
&gt;90 days 3 days 
cyclohexylamine &lt;5 minutes 15 minutes 
triethanolamine 4 days 5 days 
diethylhydroxylamine 
5 days 3 days 
Primene 81R.sup.1 
2 phases 2 phases 
p-phenylenediamine 
3 days 2 phases 
diphenylamine insoluble insoluble 
methoxypropylamine 
&gt;90 days &gt;78 days 
______________________________________ 
.sup.1 A mixture of isomeric amines of the formula (C.sub.x H.sub.2x + 
2).sub.3 CNH.sub.2, wherein x is 12-14, sold by Rohm & Haas. 
As can be seen from the data in Table 4, with the exception of 
methoxypropylamine and dimethylcyclohexylamine, none of the compounds 
listed in Table 4 show any significant stabilizing effect either at 
ambient temperature or at a temperature of 9.degree. F. 
EXAMPLE 5 
Using the procedure of Example 3, pyrrolidine and 2-pyrrolidinone are used 
as stabilizing compounds. Both compounds are found to be effective at 
ambient temperatures and at 9.degree. F. 
The foregoing description and examples illustrate selected embodiments of 
the present invention. In light thereof, variations and modifications will 
be suggested to one skilled in the art, all of which are in the spirit and 
purview of this invention.