Abstrict A process for fixing elemental mercury (Hg.degree.) in a spent
molecular sieve desiccant is disclosed. The process includes isolating
gspent molecular sieve desiccant containing elemental mercury, and
contacting the spent desiccant in a dilute aqueous solution of an
alkaline metal salt. The reaction traps the mercury within the spent
desiccant as an insoluble heavy metal salt. The alkaline metal salt
can include, for example, sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.3),
sodium polysulfide (Na.sub.2 S.sub.x), or potassium peroxomonosulfate
(KHSO.sub.5). The reaction with any of these aqueous solutions acts
to fix the elemental mercury (Hg.degree.) as a mercury compound
within the solid desiccant. If sodium thiosulfate or sodium polysulfide
is utilized, then the process also includes introducing hydrochloric
acid (HCl) in a suitable quantity to liberate sufficient elemental
sulfur to react with the elemental mercury (Hg.degree.) to form
an HgS salt. The formation of HgS fixes the elemental mercury (Hg.degree.)
in the form of a heavy metal salt in the solid desiccant. If, however,
an aqueous potassium peroxomonosulfate (KHSO.sub.5) solution is
utilized, then HCl is not needed. The aqueous potassium peroxomonosulfate
solution is used in a suitable concentration to convert the elemental
mercury (Hg.degree.) in the spent desiccant to HgO, so that the
elemental mercury (Hg.degree.) is fixed as an insoluble heavy metal
oxide in the solid desiccant.
Claims I claim:
1. A process for fixing elemental mercury in a spent desiccant,
comprising:
isolating a solid desiccant containing elemental mercury;
determining the quantity of elemental mercury present in the desiccant;
contacting the desiccant in an aqueous mixture with molar quantities
of an alkaline metal salt sufficient for converting the elemental
mercury in the solid desiccant to a water insoluble oxide or salt,
resulting in a wetted desiccant impregnated with the alkaline metal
salt, said alkaline metal salt is selected from the group consisting
of sodium thiosulfate, sodium polysulfide, potassium peroxomonosulfate,
and combinations thereof,
whereby said mercury is fixed as a water insoluble mercury oxide
or salt compound in the solid desiccant.
2. A process as recited in claim 1 wherein said alkaline metal
salt is selected from the group consisting of sodium thiosulfate
and sodium polysulfide; and
said process further comprising introducing hydrochloric acid to
the wetted desiccant in a suitable quantity to liberate elemental
sulfur, thereby releasing sufficient elemental sulfur to react with
said elemental mercury to form HgS,
whereby the elemental mercury in the solid desiccant is fixed within
the desiccant as a water insoluble HgS.
3. A process as recited in claim 1 wherein said alkaline metal
salt is potassium peroxomonosulfate in a suitable concentration
to convert the elemental mercury in the solid desiccant to HgO,
whereby said elemental mercury in the solid molecular sieve desiccant
is fixed as water insoluble HgO.
4. A process as recited in claim 1 wherein the solid desiccant
is a molecular sieve.
5. A process for fixing elemental mercury in a spent molecular
sieve desiccant, comprising:
isolating a molecular sieve desiccant containing elemental mercury;
determining the quantity of elemental mercury present in the desiccant;
contacting the molecular sieve desiccant in an aqueous mixture
with molar quantity of an alkaline metal salt sufficient for converting
the elemental mercury in the desiccant to a water insoluble oxide
or salt, resulting in a wetted molecular sieve desiccant impregnated
with the alkaline metal salt, said alkaline metal salt is selected
from the group consisting of sodium thiosulfate, sodium polysulfide,
potassium peroxomonosulfate, and combinations thereof,
whereby said mercury is fixed as a water insoluble mercury oxide
or salt compound in the molecular sieve desiccant.
6. A process as recited in claim 5 wherein said alkaline metal
salt is selected from the group consisting of sodium thiosulfate
and sodium polysulfide; and
said process further comprising introducing hydrochloric acid to
the wetted desiccant in a suitable quantity to liberate elemental
sulfur, thereby releasing sufficient elemental sulfur to react with
said elemental mercury to form HgS,
whereby the elemental mercury in the spent molecular sieve desiccant
is fixed within the desiccant as water insoluble HgS.
7. A process as recited in claim 5 wherein said alkaline metal
salt is potassium peroxomonosulfate in a suitable concentration
to convert the elemental mercury in the molecular sieve desiccant
to HgO,
whereby said elemental mercury in the spent molecular sieve desiccant
is fixed as water insoluble HgO.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the fixation of elemental mercury (Hg.degree.)
present in spent molecular sieve desiccants, which are used in the
drying of hydrocarbon gases containing trace amounts of elemental
mercury (Hg.degree.), prior to disposal.
2. Background of the Related Art
Drying, or dehydration, is a chemical engineering operation that
has extensive applications in plants in which gases, and specifically
hydrocarbon gases are processed. For example, in a cryogenic gas
liquefaction plant, in order to minimize the risk of plugging by
the formation of ice, it is essential to reduce the moisture content
of the gas feed to very low levels, about 1-5 ppm. This reduction
in the moisture content is achieved by the use of desiccant beds.
These beds usually contain molecular sieves which operate continuously
and are regenerated in a cyclic manner. Desiccants or molecular
sieves have a finite life and have to be replaced after their efficiency
becomes reduced. When the desiccant becomes spent it is a waste
product for which an avenue of disposal is required. Spent desiccants
have been used in landfills, in road construction, and as aggregates
in cement related construction projects. In the case of spent molecular
sieve desiccants that contain heavy metal ions, however, fixing
procedures that prevent the metal from leaching are necessary and
must be applied before the spent molecular sieve desiccant is eliminated.
In the case of gases, such as hydrocarbon gases that usually contain
elemental mercury ("Hg.degree."), the concentration of
Hg.degree. in the gas, which is carried into the desiccant bed is
usually low. However, Hg.degree. is a hazardous substance and even
when low concentration of Hg.degree. are present in the feed, sufficient
Hg.degree. is deposited on the spent desiccant to make normal landfill
disposal environmentally unacceptable. In addition, the usual fixing
procedures which have been devised for fixing metal ions are not
generally applicable to Hg.degree..
A process for in situ stabilization of soluble mercury in deposits
of mercury-containing materials is described in U.S. Pat. No. 4354942
to Kaczur et al. The method includes treating the deposits of mercury
containing material with a stabilizing amount of an inorganic sulfur
compound. Such suitable inorganic sulfur compounds include sulfides,
alkali metal thiosulfates, alkaline earth metal thiosulfates, iron
thiosulfates, alkali metal dithionites, and alkaline earth metal
dithionites. These inorganic sulfur compounds react with soluble
mercury in the deposits to convert it to insoluble mercury compounds.
The process substantially inhibits the elution from the deposits
of soluble mercury compounds with water. The process is described
as useful for treating land areas, landfill deposits, or submarine
sediment deposits to reduce the soluble mercury concentration. The
patent describes the application of inorganic sulfur compounds to
the deposits of mercury-containing materials by, for example, spreading
or blowing inorganic sulfur compound on the surface of mercury-containing
areas such as land areas, landfill deposits, the water covering
submarine deposits with mercury-containing material or the land
areas surrounding these water-covered sediments. The stabilizing
agent can be carried into the deposits by water in the form of rainfall
or water applied to those areas, for example, by spraying or irrigation.
Preferred embodiments for surface application of mercury-containing
material includes potassium sulfides, sodium thiosulfide, calcium
thiosulfide, and sodium dithionite as inorganic sulfur compounds
having a solubility in cold water of greater than 1 grams per liter.
The patent describes that it is desirable to minimize the formation
of mercury polysulfide complexes which can be eluted or leached
from deposits in effluent which contain Hg concentrations higher
than desired. It describes that mercury polysulfide formation may
also be minimized or eliminated by the addition of polysulfide inhibitor
such as inorganic sulfites and bisulfite compounds. In addition,
at column 5 the patent describes that it is desirable to prevent
or minimize the formation of hydrogen sulfide by limiting the amounts
of hydrogen-containing sulfur compounds present as impurities in
the inorganic sulfur compounds. In addition, hydrogen sulfide formation
can also be minimized by limiting the exposure of the stabilizing
inorganic sulfur compounds to air or sunlight. The examples describe
the treatment of a mercury containing sludge by the surface application
of various stabilizing agents. The Kaezur et al. patent does not,
however, describe a method for stabilizing elemental mercury present
in spent molecular sieve desiccants prior to their disposable in
landfill, it does not even recognize the problem.
A process for the stabilization of mercury-containing waste is
described in U.S. Pat. No. 4844815 to Ader et al. The process
includes adding elemental sulfur and cement kiln dust to the waste/sludges
in amounts effective to reduce the amount of leachable mercury to
an environmentally acceptable level. Preferably, a strong base such
as a strong caustic, e.g. sodium hydroxide, is also added to the
waste with the sulfur and cement kiln dust. The types of wastes
treated are described as sludge wastes generated from various industrial
processes, in order to reduce the amount of leachable mercury in
these wastes to below 20 ppb, described as ten times the drinking
water standard. There is no description or suggestion in the Ader
et al. patent of treating spent molecular sieves containing elemental
mercury for their safe disposal.
The treatment of sorbants, such as molecular sieves, with sulfur
containing compounds to improve the sorbants' ability to adsorb
mercury is well known. For example, see U.S. Pat. Nos. 3873581;
4233274 (background); 4474896; 4708853; 4786483; 4814152;
4834953; 4877515; and 4985389. None of these patents, however,
address the fixing of mercury in spent molecular sieve desiccants,
to allow for safe disposal after their use. For example U.S. Pat.
No. 4786483 to the inventor herein discloses a process for removing
hydrogen sulfide and mercury from gases. The process utilizes porous
granular sorbant material such as silica, alumina, silica-alumina,
molecular sieves and mixtures of any of these materials. The sorbant
material is impregnated with a chemical compound which is capable
of converting mercury to the oxide form and simultaneously hydrogen
sulfide to elemental sulfur. The reactive compound disclosed includes
alkali metal peroxomonosulfate salt, e.g. potassium peroxomonosulfate
(KHSO.sub.5). Alternatively, the gas can be contacted directly with
an aqueous solution of the chemical compound, such as alkali peroxomonosulfate
salt. This patent, however, does not disclose a method for treating
spent molecular sieve desiccants which contain elemental mercury
so that they can be safely disposed.
U.S. Pat. No. 4834953 to the inventor herein, describes a process
for removing residual mercury from treated natural gas by contacting
the gas with an aqueous solution of alkali polysulfide, such as
sodium polysulfide, and subsequently contacting the effluent gas
with a cobalt salt on a suitable support such as calcium sulfate.
The effluent gas can then be treated, as desired, to remove moisture
and any other contaminants with an adsorbent. Copper sulfide may
be present on various supports, such as activated carbon or molecular
sieves. In addition, the prior art section describes a procedure
where natural gas is contacted with a fixed bed of copper sulfide
on an alumina-silica support to remove the mercury present in natural
gas. Also, U.S. Pat. No. 4877515 and its sister patent U.S. Pat.
No. 4985389 both to the inventor herein describe the use of polysulfide
treated molecular sieves to remove mercury from gaseous or liquid
hydrocarbon streams. The molecular sieve is pretreated with an aqueous
solution of polysulfide and subsequently dried under conditions
calculated to dry, but not decompose, the polysulfide present on
the molecular sieve. The treated molecular sieve is then placed
in contact with a stream of gaseous or liquid hydrocarbons. There
is no description or suggestion in any of these patents of a method
for fixing the elemental mercury present in the spent molecular
sieves, allowing for their safe disposal.
Methods describing the removal of heavy metal pollutants from aqueous
solutions and suspensions by precipitation are disclosed in U.S.
Pat. Nos. 3740331; 3790370; 4260494; 4333913; 4731187
and 4814091. For example, U.S. Pat. No. 3790370 to Lalancette
describes a process for removing metal ions, such as mercury and
other metal ions, from polluted waters. Their process utilizes a
sulfur-containing precipitating agent for diffusing with the contaminated
water through peat. The process yields a water insoluble sulfide
of the metal, which is adsorbed on the peat, allowing recovery of
metal-free water. In the process, the metal oxide may be recovered
by burning the metal sulfide-containing peat. The precipitating
agents utilized may include sodium sulfide, ammonium polysulfide
or sulfidrhyl bearing compounds of the formula R-SH where R is an
alkyl phenyl, i.e. hexanethiol. None of these patents, however,
describe or suggest the treatment of spent molecular sieves containing
elemental mercury to allow for their safe disposal.
In summary, none of the related art provides a method for fixing
elemental mercury in spent molecular sieve desiccants to allow for
their safe disposal.
Accordingly, one purpose of the present invention is to provide
a process for fixing elemental mercury in spent molecular sieve
desiccants, to allow for the safe disposal without the danger of
the mercury leaking out to the environment.
SUMMARY OF THE INVENTION
This and other purposes are achieved by the present invention,
which provides a process for fixing elemental mercury (Hg.degree.)
in spent molecular sieve desiccants. The process includes isolating
spent molecular sieve desiccant containing elemental mercury, and
contacting the spent desiccant in a dilute aqueous solution of an
alkaline metal salt. The reaction traps the mercury within the spent
desiccant as a fixed heavy metal salt. The alkaline metal salt can
include, for example, sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.3),
sodium polysulfide (Na.sub.2 S.sub.x), or potassium peroxomonosulfate
(KHSO.sub.5). The reaction with an aqueous solution of any of these
salts acts to fix the elemental mercury (Hg.degree.) as an insoluble
mercury compound within the solid desiccant.
If an aqueous sodium thiosulfate or sodium polysulfide solution
is utilized, then the process also includes introducing hydrochloric
acid (HCl) in a suitable quantity to liberate sufficient elemental
sulfur to react with the elemental mercury (Hg.degree.) to form
an HgS salt. The formation of HgS fixes the elemental mercury (Hg.degree.)
in the form of a heavy metal salt in the solid desiccant. If, however,
an aqueous potassium peroxomonosulfate (KHSO.sub.5) solution is
utilized, then HCl is not needed. The aqueous potassium peroxomonosulfate
solution is used in a suitable concentration to convert the elemental
mercury (Hg.degree.) in the spent desiccant to HgO. Accordingly,
elemental mercury (Hg.degree.) is fixed as an insoluble heavy metal
oxide in the solid desiccant.
For a better understanding of the present invention, reference
is made to the following description, taken in conjunction with
accompanying equations, the scope of which is pointed out in the
appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A process for fixation of elemental mercury (HG.degree.) in a spent
molecular sieve desiccant is disclosed. The process allows for the
safe disposal of the desiccant with the elemental mercury (Hg.degree.)
chemically fixed as an insoluble compound within the desiccant.
In order to carry out the present invention, the quantity of elemental
mercury (Hg.degree.) present in the desiccant is first determined.
This determination may be done utilizing conventional techniques
well known to those of ordinary skill in the art. A dilute aqueous
solution is then prepared, composed of either sodium thiosulfate
(Na.sub.2 S.sub.2 O.sub.3), potassium peroxomonosulfate (KHSO.sub.5),
or sodium polysulfide (Na.sub.2 S.sub.x) in molar quantities sufficient
for converting the elemental mercury (Hg.degree.) in the spent desiccant
molecular sieve to an insoluble mercury oxide or salt.
If an aqueous sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.3) solution
is used, the necessary concentration for fixing the mercury is determined
in accordance with equations 1 and 2 as follows: ##STR1##
The aqueous solution is then mixed with the spent desiccant containing
the elemental mercury (Hg.degree.). The treated spent desiccant
is then mixed with a quantity of HCl sufficient to liberate elemental
sulfur ("S.degree.") in accordance with equation 1. The
HgS salt is the reaction product of the elemental sulfur, generated
by the action by HCl on Na.sub.2 S.sub.2 O.sub.3 and elemental
mercury (Hg.degree.) present in the desiccant. Furthermore, since
the elemental mercury (Hg.degree.) is entrapped within the solid
desiccant and the aqueous Na.sub.2 S.sub.2 O.sub.3 /HCl reaction
occurs in the mixture of the aqueous/desiccant mixture, the HgS
formed by the reaction "fixes" the elemental mercury (Hg.degree.)
in the form of an insoluble heavy metal salt in the solid desiccant.
If an aqueous sodium polysulfide (Na.sub.2 S.sub.x) solution is
utilized, then a most preferred concentration of the aqueous solution
is about 100 ppm sodium polysulfide (Na.sub.2 S.sub.x), which is
mixed with the spent molecular sieve desiccant containing the elemental
mercury (Hg.degree.). The aqueous solution of sodium polysulfide
may preferably range in concentration from about 10 to about 1000
ppm sodium polysulfide. The mixture of the sodium polysulfide treated
desiccant is then treated with a sufficient concentration of HCl
to liberate elemental sulfur in accordance with equations 3 and
4. ##STR2##
These reactions provide sufficient elemental sulfur required to
react with the elemental mercury (Hg.degree.) in accordance with
equation 2. In addition, the H.sub.2 S generated in accordance with
equation 4 reacts with elemental mercury (Hg.degree.) as follows:
##STR3##
As discussed above, the elemental mercury (Hg.degree.) is entrapped
within the desiccant and the aqueous reaction of sodium polysulfide
and hydrochloric acid occurs in the aqueous/desiccant mixture. Thus,
the HgS formed, according to Equation (5), fixes the elemental mercury
(Hg.degree.) in the form of an insoluble heavy metal salt within
the solid molecular sieve desiccant.
If an aqueous potassium peroxomonosulfate (KHSO.sub.5) solution
is selected, then a dilute aqueous solution containing KHSO.sub.5
is prepared by dissolving an amount of KHSO.sub.5 sufficient to
convert the elemental mercury (Hg.degree.) in the spent desiccant
to HgO, in accordance with equation (6): ##STR4##
The aqueous potassium sulfate solution is mixed well with the mercury
containing spent molecular sieve desiccant. The potassium peroxomonosulfate
in the aqueous solution allows the reagent to convert the elemental
mercury (Hg.degree.) directly to HgO. The reaction fixes the elemental
mercury (Hg.degree.) in the form of an insoluble heavy metal oxide
(HgO) in the solid desiccant.
Each of these mercury conversion steps can be carried out separately,
to fix the elemental mercury (Hg.degree.) in the interior of the
desiccant, depending on the availability and convenience of the
particular reagents or processing apparatus. Other desiccants besides
molecular sieves could advantageously be treated by the method of
the present invention to fix elemental mercury absorbed by such
desiccants. Additionally, these steps may be combined to sequentially
convert any remaining trace amounts of mercury at various commercial,
processing or disposal stages.
Thus, while I have described what are the presently contemplated
preferred embodiments of the present invention, further changes
and modifications could be made by those of ordinary skill in the
art without departing from the spirit and scope with the invention
and it is contemplated to claim all such changes and modifications.
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