Method of recovering mercury

Disclosed is a method of operating an electrolytic cell of the flowing mercury amalgam cathode type. According to the disclosed process, fortified alkali metal chloride brine is fed to the cell while an electrical current is passed through the cell. Depleted brine, diminished in alkali metal chloride content and enhanced in mercury content, is recovered from the cell. The depleted brine is dechlorinated and then refortified with alkali metal chloride. Thereafter, the refortified brine is treated to precipitate sulfate and alkaline earth metal contaminates as well as mercury. The purified, refortified brine is then introduced to a flowing mercury amalgam cathode electrolytic cell. The mercury recovery process is characterized by adding an alkaline hypochlorite solution to the precipitated impurities in order to form a soluble mercury salt, separating the soluble mercury salt from the precipitate at an alkaline pH, and then acidifying the solution of the soluble mercury salt. Thereafter, the mercury salt solution is electrolyzed between an anode and a cathode in order to evolve metallic mercury at the cathode.

DESCRIPTION OF THE INVENTION 
Caustic soda and chlorine may be produced by the electrolytic decomposition 
of sodium chloride salt in the two-step mercury cell process. The mercury 
cell is characterized by having a flowing mercury amalgam cathode with an 
anode suspended from about 1/8 to about 1/4 inch above the mercury cathode 
and a film of aqueous alkali metal chloride brine flowing therebetween. 
Chlorine gas is evolved at the anode according to the reaction: 
EQU Cl.sup.- .fwdarw.1/2Cl.sub.2 +e.sup.-, 
and the alkali metal, typically sodium or potassium, is deposited at the 
surface of the flowing mercury cathode in which it dissolves to form a 
mercury-alkali metal amalgam according to the reaction: 
EQU Na.sup.+ +(Hg)+e.sup.- .fwdarw.Na(Hg). 
The mercury-alkali metal amalgam is withdrawn from the electrolytic cell 
and fed to a denuder, also known as a decomposer. The amalgam is 
decomposed by the action of water in the internally shorted decomposer, 
yielding alkali metal hydroxide and hydrogen gas. The reaction at the 
anode of the denuder is: 
EQU Na(Hg).fwdarw.Na.sup.+ +(Hg)+e.sup.-, 
and at the cathode thereof is: 
EQU H.sub.2 O+e.sup.- .fwdarw.OH.sup.- +1/2H.sub.2. 
the depleted brine is recovered from the flowing mercury amalgam cathode 
electrolytic cell and is dechlorinated. Dechlorination of depleted brine 
is by acidification with hydrochloric acid. The acid reacts with the 
hypochlorous acid present in the chlorinated, depleted brine to evolve 
chlorine. Thereafter, the depleted brine may be treated in various ways to 
remove any remaining chlorine, for example, by blowing compressed air 
through depleted brine in a column or treating the brine with sulfuric 
acid. 
The depleted brine, dechlorinated, having an alkaline pH above about 7 and 
typically containing from about 200 to about 280 grams per liter of sodium 
chloride, is then fortified, for example, to saturation, by passage 
through a bed of salt in dissolving tanks. The fortification introduces 
various impurities into the refortified brine, for example, calcium ions 
and sulfate ions among others. The resaturated brine, i.e., fortified 
brine, is then purified to remove these impurities. Brine purification is 
typically carried out by adding barium carbonate to remove sulfate ion, 
sodium carbonate and sodium bicarbonate to remove calcium ion, and sodium 
hydroxide to remove the excess of sulfide ion. 
In the mercury cell process, only about 10 to about 25 percent of the salt 
contained in a brine feed is electrolyzed and it is necessary to circulate 
large amounts of brine and resaturate the brine as described above. The 
chlorinated brine chlorinates some of the mercury to form mercuric 
chloride. The chlorinated, depleted brine leaving the cell contains 
mercury, both elemental and combined, but usually in the form of mercuric 
chloride. The amount of mercury leaving the cell in the depleted brine is 
usually from about 1.5 to about 15 milligrams per liter of the depleted 
brine although it may be greater when cell upsets occur during operation, 
for example, as much as 200 milligrams per liter or even more of mercury 
in the depleted brine. The amount of mercury potentially lost in this way 
not only represents a considerable amount of mercury but also gives rise 
to potential ecological problems. Depending on the method used for 
separating the precipitates and floc materials from the saturated brine, a 
large amount of mercury may remain in the brine filter sludge and thus 
actually be lost. 
For this reason, sulfide ion, typically in the form of either sodium 
sulfide or hydrogen sulfide, is added to the resaturated or fortified 
brine in order to precipitate the mercury. Normally, the sulfide is added 
as sodium sulfide and normally the brine is at a pH of 8 to 10. The brine 
is then clarified in a settler or other physical separation tank and 
returned to the mercury cell. Nevertheless, some of the mercury is lost 
with the filter cake or the filtrate. 
According to the method of this invention, the mercury values in the brine 
are recovered from the filter cake or precipitate by solubilization with 
hypochlorite, adjustment of pH, and electrolytic recovery.

DETAILED DESCRIPTION OF THE INVENTION 
The method of this invention may be understood by reference to the FIGURE 
appended thereto. The FIGURE shows a flow chart of the process with a 
flowing mercury amalgam cathode electrolytic cell, 1, from which a brine 
line, 3, containing depleted brine goes from the cell, 1, to a 
dechlorinator, 5, where hydrochloric acid is added to the depleted brine 
in order to remove the chlorine therefrom. The dechlorinated, depleted 
brine then passes to a dissolver or resaturator, 7, where solid salt 
addition resaturates the brine up to a level of 315 to about 325 grams per 
liter of sodium chloride, depending on the temperature thereof. The 
resaturation with solid salt typically results in the introduction of 
impurities such as calcium ion and sulfite ion to the brine. This 
resaturated brine then passes from the dissolver or resaturator, 7, to a 
neutralizer, 9, where sodium hydroxide is added. The neutralized brine, 
having a pH of from about 7 to about 10, next goes to a precipitator where 
barium ions typically in the form of barium carbonate, carbonate ion 
typically in the form of sodium carbonate, and sulfide ion typically in 
the form of sodium sulfide, are added to precipitate the calcium ion, 
sulfate ion, and mercury. The resulting slurry is then passed from the 
precipitator, 11, to filter, 13, where it is separated into a solid 
portion, 15, and a liquid portion, 17. The liquid portion, 17, is a 
resaturated, dechlorinated brine reduced in calcium content, sulfate 
content, and mercury content. The brine so treated is typically 
recirculated to the flowing mercury amalgam cathode electrolytic cell, 1. 
The solid material is further treated in a reactor, 19, by the addition of 
an oxidant that is compatible with the flowing mercury amalgam 
electrolytic cell process. A preferred oxidant is sodium hypochlorite, 
NaOCl. According to one exemplification, the sodium hypochlorite is formed 
by the reaction of sodium hydroxide, chlorine, and sodium chloride to 
produce sodium hypochlorite. The sodium hypochlorite reacts with the 
mercury sulfide to form a red to reddish-brown slurry that separates from 
the filter cake. 
The substantially mercury-free filter cake is disposed of. The red to 
reddish-brown slurry has a pH of from about 9 to about 14 and most 
frequently from 9.3 to 9.8. The slurry is treated with an amount of 
hydrochloric acid sufficient to reduce the pH thereof below about 8.5 and 
preferably about 7, whereby to decolorize the red to reddish-brown slurry 
and solubilize the floc as well as to react any excess of hypochlorite ion 
in the solution. The solution, containing mercury ion and having a pH 
below about 8.5 and preferably about 7, is then fed to an electrolytic 
cell, 21, where an electrolytic current is passed through the cell, 
causing the mercury to be recovered as metallic mercury at the cathode 
thereof. 
According to the method of this invention of operating a flowing mercury 
amalgam cathode electrolytic cell, a fortified sodium chloride brine 
containing from about 300 to about 325 grams per liter of sodium chloride 
is fed to the flowing mercury amalgam cathode electrolytic cell while an 
electrical current is passed through the cell from an insoluble anode to 
the flowing mercury amalgam cathode. Chlorine gas and a mercury-sodium 
amalgam cathode are recovered as products from the cell. 
Depleted brine is recovered from the flowing mercury amalgam cathode 
electrolytic cell. The brine is diminished in alkali metal chloride 
content by from about 15 to about 25 percent, that is, to a sodium 
chloride content of from about 200 to about 250 grams per liter. The 
mercury content of the depleted brine is typically from about 1.5 to about 
15 milligrams per liter although it may be as high as 25 or even 50 
milligrams per liter depending upon operating conditions, and may, in the 
event of cell upset, be as high as 200 or more milligrams per liter. 
The depleted brine is dechlorinated, refortified in alkali metal chloride 
content, and purified. Purification typically involves the addition of 
barium compounds, e.g., about 1 to about 4 grams per liter of barium 
carbonate, to precipitate sulfate ion therefrom, carbonate compounds, 
typically about 6 to about 14 grams per liter of sodium carbonate and 
sodium bicarbonate, to precipitate calcium compounds such as calcium 
carbonate therefrom, and sulfide compounds, such as sodium sulfide, e.g., 
about 0.01 to about 0.2 grams per liter of sodium sulfide, to precipitate 
the mercury. The precipitates, generally barium sulfate, calcium 
carbonate, and mercury sulfide, are withdrawn from the fortified brine, 
for example, by filtration. The fortified, purified brine is then 
recirculated to a flowing mercury amalgam cathode electrolytic cell. In a 
multi-cell plant, this may be a different cell than the one from which the 
brine was originally received. 
According to the method of this invention, a compatible oxidizing agent 
such as an alkaline hypochlorite solution is added to the precipitate in 
order to form a red to reddish-brown slurry. Thereafter, the red to 
reddish-brown slurry is separated from the precipitate at an alkaline pH, 
acidified to decolorize and dissolve the floc, e.g., to a pH below about 
8.3 and preferably about 7.0 by the addition of hydrochloric acid, and 
thereafter electrolyzed between an anode and a solid cathode whereby to 
evolve metallic mercury at the solid cathode. 
Generally, the compatible oxidizing agent is an alkali metal hypochlorite 
such as potassium hypochlorite when potassium chloride is being 
electrolyzed and sodium hypochlorite when sodium chloride is being 
electrolyzed. According to a preferred exemplification, the hypochlorite 
is formed is situ by the reaction of the chloride, the hydroxide, and 
chlorine. 
When the oxidizing agent is sodium hypochlorite, the sodium hypochlorite is 
typically added to the mercury sulfide containing filter cake in an amount 
of from about 1.25 moles of hypochlorite ion per mole of mercury to about 
4.6 moles of hypochlorite ion per mole of mercury. The addition is 
generally as an aqueous liquid composition. When the hypochlorite ion is 
added as sodium hypochlorite, the sodium hypochlorite content is generally 
from about 3 to about 8 weight percent. The temperature of the 
hypochlorite solution is not critical. 
The hypochlorite reacts with the mercuric sulfide thereby exothermically 
forming a slurry containing a red to reddish-brown solid. Thereafter, 
sufficient hydrochloric acid is added to the slurry to convert the red 
floc to a white floc and thereafter solubilize the floc. The amount of 
hydrochloric acid required to decolorize the floc and thereafter 
solubilize the floc is an amount sufficient to reduce the pH of the 
composition below about pH=8.3. This is generally at least about 0.2 grams 
of hydrochloric acid, anhydrous basis, per gram of mercury, and preferably 
from about 0.2 to about 0.5 grams of hydrochloric acid, anhydrous basis, 
per gram of mercury. 
The soluble mercury salt, at a pH below 8.3, may then be electrolyzed 
between an anode and a cathode. Typically, the pH is from about 6 to about 
8, with a pH being preferred for high recoveries and high current 
efficiency. At pH's much above 8 or much below 6, the current efficiency 
and percent removal are reduced. The anode and cathode should be 
substantially immune to attack by the mercury-containing acidified 
electrolyte. 
Typical anode materials include coated anodes having a valve metal 
substrate with a suitable electrocatalytic surface thereon, for example, 
platinized titanium substrate or ruthenium dioxide-titanium dioxide coated 
titanium substrate. Suitable cathode materials include silver, copper, 
iron, steel, and the like. 
The cathode current density may be from several amperes per square foot up 
to several thousand amperes per square foot, for example, from about 10 
amperes per square foot to about 15 or even 1,000 or even 2,000 amperes 
per square foot. 
Thereafter, the mercury recovered at the cathode may be removed from the 
electrolytic cell and either returned to the process or otherwise 
collected and controlled. 
The following examples are illustrative. 
EXAMPLES 
A. Preparation of Mercury Sulfide 
Simulated mercury cell plant mercury sulfide was prepared by adding 112 
grams, a stoichiometric excess, of reagent grade 70 weight percent 
ammonium sulfide to 1,150 grams of a 129 grams per liter solution of 
mercuric nitrate, Hg(NO.sub.3).sub.2, in deionized water. The resulting 
black mercury sulfide precipitate was elutriated with deionized water 
until the excess soluble sulfide, ammonium sulfide, was removed from the 
precipitate. 
The washed mercury sulfide, HgS, precipitate was allowed to stand in 
deionized water for five days. Thereafter, the precipitate was filtered 
and a mercury sulfide, HgS, filtrate was obtained. 
In order to determine the effect of drying, the filtrate was divided into 
two portions. One portion was allowed to remain as a wet filter cake 
containing approximately 54 weight percent water. The second portion was 
heated to 110.degree. C. for 24 hours in order to obtain a dry mercury 
sulfide product. 
B. Treatment with Sodium Hypochlorite 
The black mercury sulfide precipitates, both dried and wet, were separated 
into seven portions to which 5.25 weight percent NaOCl was added as shown 
in Table I. Upon the addition of the NaOCl, the solid black precipitates 
to a soluble form containing small amounts of white solids. 
C. Electrolytic Cell 
A glass 250 milliliter beaker was utilized as the electrolytic cell. The 
beaker has a 2.5 centimeter by 0.5 centimeter platinized titanium anode. 
Two cathodes were utilized, a 10 centimeter length of 1.3 millimeter 
diameter silver wire having a 7 centimeter portion immersed in the 
electrolyte, for a cathode area of 2.86 square centimeters, and a 10 
centimeter length of 1.9 millimeter diameter copper wire having a 7 
centimeter portion immersed in the electrolyte for a cathode area of 4.18 
square centimeter. The cathode type and cathode area are shown in Table I. 
The power supply was a Lambda LP-531-FM constant current source with the 
current measured by a Fluke 8000A Digital Multimeter. The current and 
current density are shown in Table I. 
D. Electrolysis 
After formation of the solution of soluble mercury, the pH of the solution 
was adjusted to pH=7 by the addition of 33 weight percent HCl as shown in 
Table I. The solution was then electrolyzed in the beaker cell for the 
time and at the current densities shown in Table I, and mercury droplets 
were observed to form on the wire cathode and to be present on the bottom 
of the beaker. 
Two of the dried samples were soaked in an equal weight of water for 24 
hours prior to treatment with the NaOCl. 
E. Mercury Recovery 
Mercury recovery was determined by weighing the mercury droplets in the 
bottom of the beaker cell and the increase in weight of the wire cathode. 
The weight of mercury recovered and cathode current efficiency are shown 
in Table I. 
TABLE I 
__________________________________________________________________________ 
Current Cathode 
Weight Cathode Density Current 
History 
Weight 
of NaOCl/ 
Type of 
Area Current 
(ampere 
Mercury 
Recovery 
Efficiency 
Run # 
of HgS of HgS 
NaOCl 
HgS Cathode 
cm.sup.2 
Amperes 
per cm.sup.2) 
(gms) 
(%) (%) 
__________________________________________________________________________ 
I Precipitated 
7.46 
4.18 
0.50 silver 
2.86 0.192 
0.067 
6.17 96 26 
(6.43) wire 
II Dried 2.99 
4.43 
1.45 copper 
4.18 4.47 1.069 
1.29 50 52 
(2.58) wire 
III Dried 2.99 
4.22 
1.41 copper 
4.18 7.53 1.802 
1.81 70 33 
(2.58) wire 
IV Dried 2.99 
4.22 
1.41 copper 
4.18 7.53 1.802 
2.58 100 50 
(2.58) wire 
V Dried, 2.99 
3.83 
1.28 
copper 
4.18 5.02 1.201 
2.53 98 45 
Soaked (2.58) wire 
VI Dried, 2.99 
3.68 
1.23 copper 
4.18 4.52 1.081 
2.01 78 67 
Soaked (2.58) wire 
__________________________________________________________________________ 
While the invention is described with respect to certain exemplifications 
and embodiments thereof, the invention is not to be so limited except as 
described in the claims appended hereto.