Process for removing mercury from brine sludges

The mercury content of electrolysis cell brine sludge is reduced by sequentially: (1) acidifying the sludge to a pH below 2.5 and (2) leaching the sludge with aqueous hypochlorite solvent. The mercury content removed from the sludge may optionally be isolated from the solvent and reused as mercury cathode for brine electrolysis.

BACKGROUND OF THE INVENTION 
Ecologically and economically it is important to prevent loss of mercury 
from industrial processes. Operation of mercury cathode alkali-chlorine 
brine electrolysis cells is a significant source of inductrial mercury 
consumption. Mercury solubilized in the depleted cell brine is normally 
returned to the electrolysis cell. However, some mercury escapes from the 
electrolysis process cycle when insoluble waste products designated 
"sludge" are purged from the electrolysis operation. 
Sludge may originate in the cell brine or the cell apparatus. It is 
particularly troublesome to remove mercury contained in sludges associated 
with perification or resaturation of the brine because the mercury is 
often contained in insoluble form at levels below 100 parts per million. 
U.S. Pat. No. 3,600,285 describes a carbon sorption process for removing 
mercury solubilized in electrolysis cell brine. The patent describes reuse 
of recovered mercury in the electrolysis cell but cautions that recovered 
mercury be reintroduced to the cell after brine alkalization because 
mercury may be lost in the alkalizing mud. 
U.S. Pat. No. 1,637,481 teached the recovery of mercury from cinnabar ore 
by slightly acidifying a paste of the ore to destroy basicity prior to 
reaction with hypochlorite and a metal. 
Mercury removal from ores by leaching with aqueous hypochlorite solvent is 
described in numerous references. For example, G. A. Parks and N. A. 
Fittinghoff's "Mercury Extraction Now Possible Via Hypochlorite Leaching", 
E/MJ, June 1970, Pages 107-109, and U.S. Pat. No. 3,627,482 to R. S. Olson 
et al. describe the desirability of avoiding highly acidic leaching 
solutions because of corrosion and the relative instability of the 
hypochlorite ion. 
The isolation of solubilized mercury may be accomplished by a variety of 
techniques such as precipitation, adsorption, or electrolysis as described 
in U.S. Pat. No. 3,476,552 issued Nov. 4, 1969. 
THE INVENTION 
This invention is a process of removing or reducing the mercury content of 
brine sludges originating from mercury cathode alkali-chlorine brine 
electrolysis cell operation. 
This invention is also a process for recovering and concentrating the 
mercury content of mercury cathode brine electrolysis cell sludge. 
Moreover, this invention is an improved process for making chlorine and 
alkali-metal amalgam wherein mercury lost in sludge is recovered and 
returned to replenish the mercury cathode of a brine electrolysis cell. 
This invention is practiced by subjecting brine cell sludge to the 
essential steps of (1) acidifying the sludge to a pH below 2.5, then (2) 
leaching the sludge with aqueous hypochlorite solvent. 
Unexpectedly, it has been found that by acidifying mercury containing cell 
sludge prior to hypochlorite leaching the mercury content may be reduced 
to levels below 0.75 part per million based on the weight of sludge wet 
cake after leaching. 
The process steps of acidification and leaching may be supplemented by any 
of the additional steps of isolating the mercury content in the leaching 
solvent, separating the isolated mercury from the solvent, and returning 
the separated mercury to the cathode of a brine electrolysis cell. 
DETAILED DESCRIPTION OF THE INVENTION 
Mercury cathode electrolysis cell brine "sludge" is a brine insoluble waste 
product originating from a variety of sources in mercury cathode brine 
electrolysis cell operations. Clarifier sludge and saturator sludge 
comprise the most important types of sludges. Clarifier sludge is a 
precipitate formed when alkali-metal carbonate or hydroxide (e.g., sodium 
carbonate and/or sodium hydroxide) is added to brine to precipitate 
unwanted metal ions. Saturator sludge is a residue resulting from 
reconcentration of depleted brine by dissolving solid alkali-metal 
chlorides (e.g., sodium chloride). Sludges may also arise from other 
process related apparatus or operations such as cell box washings, filter 
backwashes, or solids contained in purge streams. Sludge of different 
types may be combined. Optionally, sludges may be concentrated in settling 
ponds to increase solids or mercury content. 
Sludge is a mixture of particulate solids and brine, and its consistency 
may vary from a fluid slurry to a paste. Normally, the substantial part of 
sludge mercury content is brine insoluble and associated with the solids' 
portion of the sludge. A minor amount of mercury in the sludge is in the 
form of soluble salts normally associated with the brine. 
The word "mercury" refers to any compound, complex, or elemental form of 
mercury. The "mercury content" of the sludge is the weight proportion of 
mercury (calculated as metallic mercury) based on the weight of sludge wet 
cake. "Wet cake" is the filter retained solids obtained by filtration of 
sludge on filter paper until substantially all free liquid is released. 
Mercury occurs in sludge in minor concentrations, usually within the range 
of 10 to 1500 parts per million by weight of sludge wet cake. Typically, 
the mercury concentration in sludge is between 10 to 100 parts per 
million, although settled sludges may accumulate much higher mercury 
concentrations (e.g., up to 3000 ppm.). The process of the invention finds 
particular advantage in reducing the mercury content of clarifier and 
saturator sludge containing 10 to 100 parts per million mercury. The 
process of this invention can reduce the mercury content of sludge below 
0.75 part per million mercury based on the weight of sludge wet cake after 
the steps of acidification and leaching. 
Cell sludge is acidified by mixing with sufficient acid to obtain a pH 
below 2.5. Preferably, the sludge is admixed with sufficient acid to 
achieve a pH between 0 and 2.2 Acidification may be accomplished with any 
strong acid such as nitric acid, sulfuric acid; or hydrochloric acid. 
Hydrochloric acid is the preferred acidifying agent. Waste by-product 
hydrochloric acid is particularly well-suited for sludge treatment. 
Sludges which contain acid reactive materials require correspondingly 
greater amounts of acid to achieve a pH below 2.5. For example, clarifier 
sludge containing a high proportion of precipitated carbonates must be 
reacted with sufficient acid to decompose the carbonates before the 
required pH level can be achieved. 
After the acidification step is completed, the cell sludge is leached with 
aqueous hypochlorite solvent. The leaching operation is an extraction of 
the sludge solids to yield a mercury depleted leached sludge and a mercury 
enriched solvent extract. Examples of suitable hypochlorites useful as 
aqueous solvents are hypochlorous acid and the alkali-metal or 
alkaline-earth-metal hypochlorite salts such as sodium hypochlorite, 
potassium hypochlorite, and calcium hypochlorite. These hypochlorite 
agents are prepared by methods well-known in the art, for example, by 
addition of chlorine to water or chlorine to water solutions of sodium 
hydroxide or calcium hydroxide. 
Since hypochlorites are more stable in neutral or basic solutions, it is 
desirable to maintain the pH of hypochlorite solutions at above moderately 
acidic pH levels, specifically above pH 5. More desirably, the pH of the 
hypochlorite solvent should be between pH 6 to 8. The pH of the 
hypochlorite solvent refers to the pH of the total combined volume of 
hypochlorite solvent after contact with the acidified sludge. For some 
applications, the initial pH of the solvent (before contact with the 
sludge) may be highly alkaline (e.g., pH 9 to 13) to neutralize acid 
contained in the sludge and give a final pH above 5. 
The leaching operation may be conducted by passing the hypochlorite solvent 
through a stationary bed of cell sludge, or alternately the sludge may be 
dispersed throughout the hypochlorite solvent by agitation. Leaching may 
be either intermittent or continuous. 
The quantity of hypochlorite solvent employed in the leaching operation 
should be at least sufficient to solubilize the sludge mercury content, 
presumably as the soluble chloride species HgCl.sub.4.sup.-2. The 
hypochlorite solvent typically is employed in considerable excess because 
it is susceptible to decomposition from a variety of causes. Hypochlorite 
employed at over 100 times the amount sufficient to solubilize the mercury 
content of the sludge is preferred practice. The concentration of 
hypochlorite in the solvent is not critical; however, aqueous solutions of 
hypochlorite solvent having hypochlorite concentrations of from 0.1 to 
30.0 weight percent are generally suitable for leaching sludge. It is 
particularly advantageous to use as solvent waste hypochlorite cell liquor 
having a hypochlorite concentration of approximately 15 weight percent. 
The volume ratio of sludge solids to hypochlorite solvent is not critical, 
although convenient operation will generally require a ratio of at least 
2:1 and preferably from about 4:1 to 1000:1. Time for leaching may vary 
within wide limits but will usually be between 5 minutes and 24 hours. 
Temperature of the leaching operation does not significantly affect 
process results; however, convenient operation will usually be at near 
ambient temperatures. The sequential steps of acidification and leaching 
may be repeated if desired. 
The mercury depleted sludge resulting from the two-step process of this 
invention may be disposed of in a conventional manner such as landfill. 
The conclusion of the process may be determined by analytical methods 
capable of determining the mercury content of the sludge (e.g., 
colorimetric determination with dithizone, atomic adsorption 
spectroscopy). 
The preceding description of the acidification and hypochlorite leaching 
steps constitute one embodiment of the invention. This embodiment is 
suitable where the principle object is to reduce the mercury content of 
electrolysis cell sludge to facilitate its safe disposal. 
Another embodiment of the invention is to recover and concentrate the 
soluble mercury content of used hypochlorite solvent resulting from 
leaching the sludge with hypochlorite. 
Recovery and concentration is accomplished by treating the hypochlorite 
solvent by means which will isolate its mercury content. Most often, the 
mercury content of used hypochlorite solvent is isolated by physical or 
chemical methods which render the mercury content insoluble in the 
solvent. 
A wide variety of methods are suitable for the isolation of mercury from 
the hypochlorite leachate. Among the most suitable methods are (1) 
sorption on materials such as activated carbon, milk proteins, xanthates, 
keratin, scrap rubber, hair, or ion-exchange resins; (2) electrolytic 
deposition; (3) metal replacement; (4) amalgamation; (5) oxidation; and 
(6) sulfidation. 
The mercury isolated from the hypochlorite solvent may then be separated 
from the body of solvent by methods such as filtration or decantation. 
Another embodiment of this invention is to reuse the mercury recovered from 
the sludge in the mercury cathode of a brine electrolysis cell. A 
preferred embodiment of the invention is to incorporate the sequential 
steps of acidification, hypochlorite leaching, and mercury isolation as 
part of a method of electrolyzing brine to give elemental chlorine and 
sodium amalgam. Brine sludge purged from an electrolysis cell is treated 
by acidification and hypochlorite leaching to remove substantially all of 
the mercury content of the sludge as soluble mercury in the hypochlorite 
solvent. The solvent is treated to isolate its mercury content, and the 
isolated mercury is converted to a form suitable for reintroduction to a 
mercury cathode brine electrolysis cell. 
The form of mercury most convenient for reintroduction to the mercury 
cathode is metallic mercury, although chloride of mercury may also be 
used. The mercury isolated from the hypochlorite solvent may be in the 
form of an oxide, sulfide, amalgam, adsorbed cation, or complex. The 
isolated mercury may be converted to forms of mercury suitable for mercury 
cathode introduction by known methods. For example, mercury oxides and 
sulfides may be roasted to give metallic mercury. 
The process of this invention may be performed in either a batch or 
continuous manner. 
Apparatus for conducting the process of the invention may be any suitable 
vessel permitting the transfer of liquids and solids. Particularly 
advantageous in the performance of the leaching step is the use of counter 
current extraction apparatus. 
A process in accordance with this invention may be performed as follows: 
Clarifier sludge and saturator sludge from a mercury cathode brine 
electrolysis cell are combined and acidified with hydrochloric acid to a 
pH of 2.0. The acidified sludge is allowed to set for 8 hours and then 
washed with water. Thereafter, the washed sludge is leached with 50 times 
its volume of 15 weight percent sodium hypochlorite. The sludge solids are 
periodically sampled and analyzed for mercury content. 
The hypochlorite solvent contacted with the sludge is treated with 
sufficient sodium hydroxide to precipitate contained mercury as mercuric 
oxide. The mercuric oxide precipitate is recovered by filtering from the 
leachate and roasted to release metallic mercury. The metallic mercury is 
returned to the cathode section of the brine electrolysis cell apparatus. 
The foregoing procedure can be repeated on a continuous basis. Mercury 
depleted sludge solids are disposed of as landfill. 
The following examples illustrate the advantages accruing from the practice 
of the invention.

EXAMPLE I 
This example illustrates the removal of mercury from electrolysis cell 
brine sludge by the process of this invention. 
One hundred grams of wet cake containing 24.3 parts per million by weight 
of mercury was prepared from the vacuum filtration of saturator sludge. 
This wet cake was reconstituted with 100 ml. of clarifier brine containing 
2.8 ppm. of mercury. The resultant sludge slurry was placed in a beaker 
and a small quantity of concentrated hydrochloric acid added until a pH of 
2.0 was obtained. The slurry was then stirred for two hours. 
The acidified sludge slurry was vacuum filtered, and the sludge wet cake 
residue washed with water. The mercury content of the wet cake after the 
acidification and washing treatment was 12.4 ppm. mercury. 
Thereafter, 50 grams of the acidified and washed wet cake was leached with 
100 milliliters of 5 percent by weight sodium hypochlorite aqueous 
solution (adjusted to pH 7) for a period of 2-3 hours with stirring. 
The hypochlorite leachate was withdrawn from the sludge by vacuum 
filtration and the resultant filter cake washed with water. The filtered 
sludge wet filter cake contained 0.39 ppm. mercury. 
EXAMPLE II 
This example illustrates the removal of mercury from saturator sludge by 
acid treatment alone. 
Part A. The sludge of Example I was treated with a large excess of 
concentrated hydrochloric acid for 2-3 hours. The mercury content 
decreased from 24.3 parts per million to 1.07 part per million (wet cake 
basis). 
Part B. A further treatment of the sludge solids of Part A with 
concentrated nitric acid (70 percent by weight) reduced the mercury 
content to 0.06 part per million. 
Part C. Saturator sludge of Example I was treated with nitric acid by 
taking 20 grams of wet sludge solids and stirring in a beaker with 100 
milliliters of various concentrations of nitric acid for 2-3 hours. After 
nitric acid treatment, the samples were filtered, and the filtered residue 
was washed and analyzed. Results are set out in Table I. 
TABLE I 
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Cell Sludge Leaching with Nitric Acid 
HNO.sub.3 Conc. Wt. (%) 
ppm. Hg. (Wet Solids) 
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5 18.0 
10 13.4 
20 2.5 
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EXAMPLE III 
This example illustrates the removal of mercury from clarifier sludge by 
acid treatment alone. 
Clarifier sludge was treated with concentrated hydrochloric acid to a pH of 
4. Half of the sludge solids went into solution as a result of 
acidification. The undissolved sludge solids were recovered by filtration 
and the filtered cake washed with water. The mercury content of the 
resultant filter cake was 26 parts per million (wet cake basis). 
EXAMPLE IV 
This example illustrates the removal of mercury from clarifier sludge by 
hypochlorite leaching alone. One hundred grams of wet clarifier sludge was 
suspended in 2 liters of 5 percent sodium hypochlorite solution with 
stirring for various lengths of time. The leached sludge was filtered and 
washed with water. Experimental conditions and results are shown in Table 
II below. 
TABLE II 
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ppm. Hg. in 
Filter Wet Cake After 
Run No. pH 2 Hours 24 Hours 
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1 7 1.7 1.4 
2 11 4.5 3.7 
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EXAMPLE V 
This example illustrates the leaching of saturator sludge with hypochlorite 
solvent alone. 
Saturator sludge was subjected to multistage hypochlorite leaching by the 
following procedure: 
About 600 grams of wet saturator sludge were placed in a glass beaker and 
leached by stirring with 5 percent sodium hypochlorite on a 1:1 volume 
basis for one hour. Thereafter, the sludge was filtered, washed, and the 
filter cake again leached with the same volume of hypochlorite solution. 
This leaching was repeated three times. In each leaching the pH of the 
sodium hypochlorite, initially 11, was adjusted to a pH of 8. The results 
of the sequential hypochlorite leach are shown in Table III below. 
TABLE III 
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MERCURY REMOVAL BY MULTI-STAGE LEACHING 
WITH SODIUM HYPOCHLORITE 
pH % Avail. Cl.sub.2 
ppm Hg. 
Stage Start .fwdarw. 
1 hr. 
Start 
.fwdarw. 
1 hr. 
(Wet Filt. Cake) 
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1 7.8 7.2 3.0 1.3 4.4 
2 7.9 6.5 5.7 1.8 3.2 
3 7.9 6.0 6.2 2.0 1.5 
4 7.8 6.9 5.5 1.4 1.6 
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INTERPRETATION OF EXAMPLES 
Example I shows that mercury content of saturator is reduced below 0.75 
part per million on a wet cake basis when a two-step operation involving 
(1) acidification and (2) hypochlorite leaching is used. Examples II and 
III show that acid leaching alone is not sufficient to remove mercury 
below the level of 0.75 part per million on a wet cake basis unless very 
severe and corrosive conditions (e.g., 70 percent nitric acid in extreme 
excess) are employed. 
Example IV and Example V show that hypochlorite leaching alone or repeated 
hypochlorite leaching is not effective in removing mercury below 
concentrations of 0.75 part per million (on a wet cake basis). 
Although the invention has been described with reference to particular 
specific details and certain preferred exemplifications thereof, it is not 
intended to thereby limit the scope of this invention except insofar as 
the details are recited in the appended claims.