Note: Descriptions are shown in the official language in which they were submitted.
~7~7~
Industrial wasta waters often contain mercury in metallic
and/or ionic form which, because of its toxlcity, ls very
dangerous for rivers, creeks, lakes etc., because it may be
introduced into the human body via fish and other organisms
serving as food for man. Therefore, the m~rcury emission of
industrial plants must be kept as low as possible. Among the
plants emitting mercury along with their waste waters are
chlorin~-alkali ~lectrolyses operating according to the
amalgam process.
Numerous processes for reducing the mercury content of
industrial waste waters are described in the literature. There
i5 for example a process according to which mercury ions are
reduced to metallic mercury by means of sodium-boron hydride
(US. Patent No. 3,764,528). However, this process is not easy
to operate, because the reducing agent decornposes at a pH
below 9.5 with development of hydrogen, and the mercury metal
which forms in the reaction precipitates in a very finely
distributed form and coagulates with difficulty only. A
further serious disadvantage resides in the fact that hydrogen
is liberatsd which carries mercury vapors to an extent corre-
sponding to the vapor pressure of mercury at the temperature
of the process and thus causes secondary pollution o~ the air,
unless it is liberated from these vapors by a treatment with
d~lute nitric acid. This posterior removal of mercury from the
waste gases is an additional, inconvenient process step ~hich,
due to the cost of the reducing agent, further di~inishes
the pro~itability of the process.
Another process uses sodium sulfide as precipitating
29 agent (Japanese Patent No- 66 7012). The disadvantage of this
~ ' :
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process resides in the fact that -the mercury sulfide is formed
very slowly and that this.formation depends on the pH. Further-
more, formation of a solubl~ complex anion of ~ gS2~ 2 compo-
sition may occur in the presence of an excess of precipitating
agent, thus adversely affecting the efficiency of the process.
Moreover, soluble chlorides which are always present in the
waste waters of a chlorine-alkali electrolysis have a nega-
ti~e effect on the precipitation of mercury sulfide, and the
precipitated mercury sulfide is dispersed i~ the water to be
purified in such a finely distributed form that its elimination
requires an additional flocculant. And finally, excess floccu-
lant has to be car0~ull~ removed from the water in order to
pre~rent secondary pollutionO
Alternatively, thio-urea and salt of hydroxylamine have
been proposed for precipitating mercury and/or mercury sal.ts
from waste waters (Gerrnan Offenlegullgsschrift No. 2,437,779).
Besides relatively long reaction times, flocculants are re-
quired also in this case, and th~ residual mercury content of
the treated waste water is generallly not belaw 100 ppb.
Another process is known according to which mercury salts
contained in waste waters are reduced by means of hydrazine
(German Offenlegungsschrift No. 1,958,169), Starting from pure
HgCl2 solutions, residual mercury contents of less than 100
ppb may ~e attained in some cases, but only with the addition
f flocculants ~for example CaCl2) or of special actiYe
oharcoal as additional adsorbe~t, ~hen this process is applicd
in the industrial practice, ~or example for the work-up of .
waste waters from a chlorine-alkali electrolysis operating ~:
:~ 29 acccrding to tAe amalgam process, filter combinations with
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HoE 76/~' 806
6if~
sand and active charcoal packings consisting partially of ex-
pensive, specially treated coal are required besides the
sedimentation tubes in order to attain acceptable results of
residual mercury ~less than 100 ppb).
Similar processes using tin(II) ions, hypophosphorous acid,
formal~ehyde, metallic iron, zinc, sodium, tin, copper etc.0
as reducing a~ent are mentioned in the literature as well.
However, there are drawbacks such as long reaction times, in-
complete reaction, heating required, inacti~ation of the ~etal
surfaces by amalgam formation, secondary-pollution of the
treated waste waters by residues of agents etcØ
Recently, the technology of ~ercury removal from ~aste
waters has switched over to a large extent to processes using
ion exchangers (for example Swiss Patent No~ 330,863). In
1~ general, it can be stated that this method is dif`ficult and
expensive, because the exchanger compositions are destroyed in
most eases by strong oxidants such as chlorine or hypochlorite
ions, and furthermore because the problem of exchanger regene-
ration has not been solved as yet in a satisfactory manner, so
that the resins have to be discarded after several runs already.
Moreover, some exchanger eompositions lose their activity very
rapidly when the sodium chloride content of the water to be
purifled exoeeds about 10 g /liter, which is often the case in
waste waters o~ chlorine-alkali electrolyses. Furthermore,
only special types of exchangers are appropriate, bscause in
a sodium chloride containing solution the mercury is partially
present a~ ~HgC147 2 anion and the active groups of the ex-
ohangers are o~ten blocked by foreign ions ? for example S03 or -
29 S042 . The residual mercur~ content obtainabLe in these
~ 4~
.: , ,, , . . ~ ... . . . . , .... .... . .. . - . .
~3~
processes is often from 0.1 to 0.~ ppm still (starting values
from 1 to 5 ppm only) despite series-connection of 2 ex-
changer columns, unless the waste water is a~ter-tr~ated in
subsequent absorption towers charged with special acti~e
charcoal which cannot be regenerated.
It was therefore an object of the present invention to
provide a process for reducing the mercury content of in-
dustrial waste waters, especially those from chlorine-alkali
electrolyses operating according to the amalgam process, which
can be carried out in a simple manner, which is economically
interesting and satisfactory in its result. A further object
was to see to it ~hat the process is of technological robust-
ness and adapted to the use of those chemical agents which
are generally present already in chlorine-alkali electrolyses,
that is, sodium chloride and sodium hydroxide, which are con-
tained in such waste ~aters beside6 traces of mercury.
In accordance with this invention, there is provided a
process for reducing the tnercury content of i~dustrial waste
waters, especially those from chlorine-alkali electrolyses
operating according to the amalgam process, which comprisas
converting the total amount of mercury contained in such
waters to the ionic, bivalent oxidation state by adjusting a
content of from 2 to 50 mg of aOtivQ chlorine per liter of
. waste water in a mineral acid medium b~ addition of chlorine,
chlorinated water or sodium hypochlorite, subsequently adding
iron(II) ions until a content of from 0.1 to 1.5 g of such
ions per liter of waste water is attained, and then adjusting
a rédox potential of fro~ -0.1 to -0.8 volt, relative to the
29 normal hydrogen electrode, by addition of chemical agents
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increasing the pH, thus reducing the mercury and precipitatinglt together with the iron oxide hydrates ~ormed, and finally
~liminating the precipitated products.
The process of the invention surprisingly allows the ob-
tention of a satisfactory residual mercury content, even at
a more than 14 000 - fold excess of sodium chloride (relative
to the weight of mercury contained in a m3 ~ waste water),
that is, under extremely unfav~rable conditions. This was not
to be expected because the mercury ion, under the conditions
as described, is bound in complex form as stable tetrachloro-
mercurate(II) an ion. ~urthermore, it was not to be expected
that, for the obtention of a satlsfactory residual mercury
content within the scope of this in~ention, it is nearly un-
important how much mercury is cointained in the waste water to
be treated. Thus, it has been observed that even the waste
water of a chlorine-alkali electrolysis which contained 162 g
of mercury per m3 (adj~sted by addition of HgCl2 solution to
this waste water), could be purified to a residual mercury
content of only 0.05 g/m3, which corresponds to a demercura-
tion degree ef 99.97 ~. The e~ficiency of the process of theinvention i5 therefore ensured also in cases where the waste
water to be treated contains an extremely large amount of
~ mercury or sodium chloride.
Further ad~antages of the operation mode of the invention
25 are the followlng: first, except a superficial clarification
of the waste waters, no preliminary or intermediate puri-
fication steps are required; second, and this is especially
important, the oxidant to be used in excess in the first
29 process step has not to be remo~ed before the reduction of the
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~76Z~
mercury, but is inactivated only with the use of a small ex-
cess amount of cheap reducing agent or alkali lye which latter
one is anyhow at disposal in chlorine-alkali electrolyses.
Thc use of iron(II) salts as reducing agents has the ad-
vantage that the almost insoluble mercury (I~ salt formed
is precipitated in a practically quantitative manner simultan-
eously with the mixture of iron(II) and iron(III) oxide
hydrates precipitating from the solution, so that an addition-
al flocculant is generally no lon~er required.
The demercuration according to this invention o~ waste
wa$ers stemming for example from a chlorine alkali electro-
lysis using a streaming mercury cathode and containing mercury
in metallic as well as ionic form (the amount of metal being
generally from 30 to 80 % of the total mercury content) is
carried out in detail as follows: first, all mercury is con-
verted to the ionic form, that is, the bivalent oxidation
state. This is carried out by acidifying the water flowing
into the apparatus by means of hydrochloric or sulfuric acid
which are generally at disposal to obtain a pH of 4.5 and by
adding chlorine in gaseous form, chlorinated water or sodium
hypochlorite to this acidified solution until a content of
active chlorine of about 2 to 50, preferably 5 to 30 mg/l i9
attained. The pH is not critical; a p~ of ~rom 1 to 3 being
however pre~erred, since higher values require the addition of
larger chlorine amounts, and lower values necessitate a rela-
ti~ely large amount of acid. Because most of the waste waters
of chlorine-alkali electrolyses contain also hypochlorite ions
which, on acidifioation, form chlorine or hypochlorous acid,
29 the redo~ potential necessary for the oxidation of the
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276
metallic mercury is sometimes established in ths wat~r to be
demercurated by the acidification already, ~o that in these
cases ~he addition of an oxidant may be omitted.
The genuine demercuration is carried out by adding an
excess of bivalent iron ions to the mercury ions now being
present in bi~alent form, and by subsequently establishing the
low redox potential in the solution required for the reduction
of mercury ions even in small amounts by addition of agents
increasing the pH, for example sodium hydroxide or calcium
hydroxide, preferably alkali lyes. The anion of the Fe(II)
salt is not critical. An amvunt of iron(II) salt solution,
for example iron(II) sulfate solution or iron(II)chloride
solution, is used which ensures a concentration of from 0.1 to
1.5, pre~erably 0.3 to 1.0 g of iron(II) ions per liter of
waste water, and the redox potential in the solution contain-
ing the mercury and iron ions is adj~sted to -0.1 to -0.8,
pre~erably o.l~ to -0~7, and especially -0.5 to -o.6 9 ~olts,
relative to the normal hydrogen electrode. Higher concentra~
tion of iron(II) ions may be used, but they do not bring
about any further advantages. The Fe~ content may be deter-
mined by titrimetric methods. In order to prevent undesirable
increase of potential by atmospheric oxygen and`possible re-
oxidation of thc mercury, it may be advantageous to carry out
th~ alkalization and the subsequent isolation of the preci-
j 25 pitated products under an atmosphere of inert gas, for example nitrogen.
The precipitated mixture of scarcely soluble mercury(I)
salt; iron(II) and iron(III) oxide hydrates is ~eparated accor-
29 ding to known methods, ~or example by ~iltration, optionally
.- .
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.. . .
with the aid of ~sual flocculants remaining in th~ sludge,
for example polyacrylamides, which are used in amounts of
from 1 to 5 g/m3 of water.
Depending on the mercury content of the sludge which is
determined by the specific conditions of the manufacturing
plant emitting the waste waters, it may be economic to re
cover the mercury, for example by means of distillation pro-
cesses or known dissolution processes using nypochlorite ion
containing solutions. In the case where work-up is not profit-
able the sludge is forwarded to a safe dump~
In the filtrate there are only traces of mercury, andexcept the anions stemming from the iron(II) salt used and
OH ions or alkali ions, no forei~l substances are introduced
into th~ water treated, because the precipitation of iron
in the pH r~nge required for the proces~ of the invention
is nearly quantitative. The filtrate flowing off has an
average iron content of 0.15 mg~l.
The quantitative relation between the redox potential
Eh (measured in ~olts) adjusted in the solution and the re-
sidual mercury content of the treated waste water C~g (mea-
s~red in ppb) is shown in the diagram of ~IGURE 1 of the
accomp~nying drawings.
The following examples illustrate operation and effect
of the process of the invention. All tests were carried out
~5 at normal temperature (about 20 C). For the analytical mercury
examination, an atomic absorption spectrophotometer (Coleman
MAS 50) was used~which had beon specially deQigned for this
purpose.
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~10~ 76/~ 806
E X A M P L E 1:
In a la~oratory apparatus according to FIGURE 2 of the
accompanying drawings, a continuously flowing waste water
current of 2.2 liters/h stemming from ~ chlorine-alkali elec
trolysis operating according to the amalgam process, the NaCl
content of which current had been deliberately increased by
addition of brine, was fed to the oxidation vessel (2) via the
duct (1). The (total) mercury content of the solution fed
in was 8.0 mg/liter, the NaCl content 118.0 g/liter and the
pH 12Ø Via the duct (3), such an amount of 31 % hydrochloric
acid and via the duct (4), such an amount of chlorinated water
containing about 1 g/liter of active chlorine were ~ed to the
duct (1) that the pH of the solution (20) in the oxidation
vsssel (2) was maintained constant at 1.9, and the redox
potential was a constant 1.25 volts, relative to the normal
hydrogen electrode. The feed was automatic and controlled by
the control valve (5) for the chlorinated water, and the con-
trol valve (6) for the hydrochloric acid. Control valve (5~
was operated by the measuring device (7) for the redox poten-
tial via the pneumatic line (8); and control valve (6) b~ themeasuring device (9) for the pH via the pnewnatic line (10).
Thus, a content of 5~4 mg/liter of active chlorine was ad-
~usted. After a mean residence time of about 1 hour, the waste
water (20) left the oxidation vessel (2) provided with an
agitator (22) via the duct (l1), to which duct, leading to
the precipitation vessel (i4), such an amount of an aqueous
iron(II) sulfate solutlon containing 25 g/l of FeS04 .7HzO
was fed that the waste water contained 346 mg/l of Fe2~ ions.
29 ~ia the duct (15), such an amount of 18 % sodium hydroxide
~ 10 ~
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3L~7~
solution was added to the waste water (21) in the precipita-
tion vessel (14) tha$ a constant redox potential of -o.68 volt,
relati~e to the normal hydrogen electrode, was established
therein. This redox potential in (14) was measured by means
of a devioe (not shown) which operated the control valve ( 1~)
via a corresponding pneumatic line (not shown). After a mean
reside~ce ti.~e of about 1 hour, the suspension (21) obtained
left the precipitation vessel ~14) provided with an agitator
(12) via the duct (17) and was thus forwarded to the closed
suction filter (18)~ The gas zone o~ precipitation vessel (14)
and of suction ~ilter ~18) Was filled with nitrogen (not
shown). The filtrate left the suction filter (18) via the
duct ~19). l`he analysis of the filtrate resulted a mean re-
sidual mercury concentration of o,oll mg Hg/liter. The fi.lter
residue contained 0.8 ~ Oe Hg, relative to the dry substance.
E X A M P L E 2:
.
In an apparatus according to FIGURE 2, where the oxi-
dation vessel had a capacity of 0.20 m3 and the precipitation
vessel had a capaoity of 0.11 m3, a continuous waste water
current of 0. 11 m3/h coming from a chlorine-alkali electro-
lysis operating according to the amalgam process and having
an average composition o~ 90.0 g/m3 of Hg (total amount),
29,5 k~r/m3 of NaCl and 3.9 kg/m3 of NaOH was treated in the
manner as described. The iron(II) sul~ate solution used had
a concentration of about 200 g of ~eS04 per liter of solution
anhydrou s ) .
According to the manner described in Example 1, the
dates adjusted in the apparatus were the ~ollowing.
;
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~7~27~
Overflow P~-C~pit~
_. active chlorine[~ F82~ vessel .
Cmg/lite~ h Eg/lite~ Eh Lv~
. . ~
2.8 . 10.1 +1.30 503 -o.68
_~_ __ __
The residual merc~ry conte~t in the filtrate was on the
average 0.05 mg/liter, while the sludge filtered o~f con-
ta.ined 6.3 ~ of Hg9 relative to the dry substance.
E X A M P L E ~-
In a pilot-plant apparatus accordi.ng to ~IGURE 2, where
the oxidation vessel had a volume of 0.50 m3 and the preci-
pitation ~essel had a volume of 0.75 m3, a continuous waste
water current o~ 1 m3/h coming from a sodium chl.oride electro-
lysis operating according to the amalgam process and having
the following composition: ~Ig (total amount) 31.0 g/m3,
NaCl 45.1 kg/m3, NaOH 1.Z kg/m3 was treated. The iron(II)
sulfate solution had the same concentration as in Example 2.
The following conditions were established according to
the man~er as described in the oxidation and precipitation
vessel, respectively:
_ __ ~--
Oxidation vessel Overflow precipitation
pH active chlorine ~h [~ Fe~ vessel ...
Eg/lite~ Cmg/lite~ Eh [~]
_ . _ _ ~ ____
3.3 14.3 ~1.30 726 -o.68
~ ~ ._ ~
The filtrate flowing of~ had a mean content of 0.04 mg/liter
of Hg, and in the sludge, 1,0 % of Hg, relativeto the dry
- 12 -
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" . ' " " ' " ' " ' ' ' ' ": ~ . ," ' . ', " . '' '-' ' - . ' , ' ., '' '.' . ' ' "' . , ,' .. " ." ,"' ', ",, . ' - "' '. , . :' " "'.' ' '': '.
', .'' . ' '.. .' .-' " ' ''' '. ' ' '. ' .': ' " ' ' ' . ' .. " . ' : . .,,, .', ' . .:'". ' ' ' '', ", ., '', '' . ' ' ' '. , :' :':
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HOJS 76/F 806
substance, was detected~
E X A M P L E 4-
In the apparatus according to ~IGURE 2, a continuous
waste water current of 0.11 m3/h containing on the average
11.0 g/m3 of Hg (total amount), 18.~i kg/m3 of NaCl and 0.18
kg/m3 of NaOH was treated. In this case, the reaction con-
dition~s were deliberately chosen in such a manner that the
redox potential in the precipitation vessel exceeded the pre-
ferred range. The corresponding test conditions are listed
in the following Table~
Oxidation vessel Ovër~low Precipitation
, ~ _ Fe vessel
pH active chlorine ~h Cv]
Lmg/l i t e ~ rmg/l i t e ~3 Eh [V~
. . . _ ~ __ ____ .~
2.0 ~ ~ ~I JU 145 -0.33 _
The filtrate contained sti~ 0.13 mg of Hg/liter on the average.
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