Note: Descriptions are shown in the official language in which they were submitted.
363
CASfi 852
This invention relates to a method ~or th~ purification
of mercury-containin~ sludges~ consisting in subjecting the
sludges to heating in a vacuo.
Many chemical and electrochemical processes~ in which
mercury or merc~ry compounds are used, involve pollution problems
since the effluents from these processes ~ waters and exhaust
~ases) contain mercury in such an amount as not to allow dis-
charge without serious hazards
A number of different methods have been suggested for
removin~ metallic mercury from the discharged sludges~ such as
those coming from the cells ~or the electrolysis~ from processes
which use mercury compounds as the catalysts and so on.
The sludges can, for example~ be filtered in rotary fil-ters
under a negative pressure (for the preliminary remo~al of water )
to undergo subsequently heating and dryin~ in multiple ovens~
collecting mercury which is evaporated (distilled~ in condenser
systems (Chemical Engineering Progress7 March 197~ pages 73-80 )~
or it is possible to resort to a treatment with resins which act
as chelating compounds and can be regenerated thus permitting tha~
the separated mercury is recoverede(Chemical ~conomy and ~nginee-
ring Review3 5 1972 page~ 64~65 ~.
No one of the above mentioned procPsses9 however~ succeeds
in removing mercury up to satisfactory levels (it has not been
e~er possible to go, in an appreciable manner~ below 0,1 p~p.
~5 million)
We have now surprisingly ascsrtained that it is possible
further to lower the mercury level or substantially attain the
complete removal thereof
It is thus an object of the present invention to provide
a simple and cheap method for the remo~al mercury ~rom sludges
2.
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containing it, consistlng in hea-tlng the sludges themselves in
a vacuo so ~s ~o vaporize mercury which~ ~hen, under similar
conditions, is condensed anew and collected.
The residual sludges and condensates can thus be
dischargèd without experiencin~ no more hazards of pollution,
inasmuch as the removal of mercury can be pushed so Ear as to
obtain, in such residues, values in the order of 10 8%.
More particularly, the method of the inven-tion
comprises heating a mercury-occluding sludge containing no more
than 50% moisture, to a temperature of from 200 to 350C under
a negative pressure of from 0.06 to 0.1 effective atmospheres to
convert the sludge into a powdered ashy mass, discharging -the
ashes into a mass of water circulated in a closed loop, super-
heating the water vapor evolved from the water mass so as -to
keep all the mercury values in the vapor phase, condensing the
water and mercury vapors and gravimetrically separating the
water and the mercury condensates from one another.
Preferably, the condensation of the water and~mercury
vapors is effected at a temperature between 2nc and 50C and
~0 under a negative pressure of from 0. n6 to 0.1 effective
atmospheres.
Traces of mercury vapor may be removed by a further
c~ndensation effected under a pressure ranging from 2 to 3
effective atmospheres and at a temperature comprised be-tween 5C
and 35C, prior to discharging the inert gases into the a-tmosphere.
In order that the method of the invention may be better
understood reference will be made, in the progress of the
specification, to a particular continuous system which is adapted
to carry -the method into practice.
It is obvious, however, that -this is a mere illus-trative
example and that anyone skilled in the art is capable of putting
the method into practice by using other machinery and wi-thou-t
thereby departing from the scope of this invention.
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With reference to the diagram as shown in the drawing,
the method can be regarded as being carried out in two stages,
which correspond to two discrete sections, that is, an oven for
the continuous roasting of the sludges and a eondensation system
under subatmospherical pressures of the barometric type.
The oven described herein is of the continuous type, which
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is indirectly heated by means of electric resistors. It is eom-
posed by a horizontal cylindrical body 1, which is heated from
the outside by a set of resistors 2, independent from each other;
so as to have several heat degrees, according to the load of the
oven and the concentration of water and mercury.
At one end of the cvvlindrical body~ there is formed the
horizontal sludge inlet mouth 3, on the other~ there have been
formed a "dome" 4, for drawing vapors and, in the underlying por-
tion~ a mouth 5, the diameter of which is equal to that of the
oven, for dumping the exhausted ashes. The sludge loading mecha-
nism as shown in the cross-sectional view A-A is of the pres~ing
screw 6 type (the pitch of the screw grows shorter and shorter)
with an extrusion head equipped with cu~ting blades; the system
is driven by an electric motor with reduction gear having a
speed variator 7
Such an exp,edient is responsible, during processing, of the
perfectly tight seal against the losses of vacuum in the system.
The automatic variation of the rpm of the loading screw is a
function of the temperature of the vapors (t) in the terminal
section of the oven conoerned. The product charged into the oven
must be a sludge having at the most 45% - 50% moisture and which
has been neu-tralized (if and when acidic) at a pH which is certain-
ly alkaline (8 to 9) In the interior of the cylindrical body a
shaft 8 is rotated~ which equipped with a screw 9 for the feeding
of material, which is equipped with special scraping blades 10~
the latter providing to stir the sludges as th0y are being roasted
while ensurin~ moreover, that no danger of jamming is possible
for the screw.
The rotation of the screw is furnished by a speed reducing
gear 11, arranged at either end of the cylindrical body of the
oven, the electric motor of which, 12, (two-speed) is of the kind
which transfers the torque by magnetic expanders so as to guarantee
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the vacuum tightness in an absolute manner.
The reducing unit ll, of necessity, will be arranged in
a vacuum chamber 13 and separated Erom the oven body by a gland
seal. The run of the oven is very simple and wholly automated.
S The sludges~ as they come from the loadirg extruder, are
~ed forward by the screw so that~ after having been first dehydra-
ted~ start to become roasted, the result being the distillation
of the mercury contents.
The working conditions are o.Q6 effective atmospheres at
about 250C, these conditions being such as to ensure the vapo
rization of all the mercury as contained in the sludges concerned.
These conditions as specified above are automatically maintained
by the feeding rate of flow o the slu~ges and by the following
run parameters :
a) speed of rotation of the feeding screw (2 speeds ),
b) insertion of the several sets of electric resistors,
There is thus the possibility of ensuring that the mercury
contents in the ashes emerging from khe screw is virtually æeroi
Inasmuch as in the terminal portion of the oven it is i~pe-
~0 rative to ensure that all the vapors evolved from the sludges
are conveyed towards the condensation system while it is con-
currently required that the exhausted ashes be continually re-
moved from a system which is under a negative pressure, the follow-
ing expedient has been envisaged :
The ashes emerging from the screw fall into a duct 14~ which
is also heated by electric resistors~ which feeds them to a con-
tainer 15, the latter being placed at a barometric level~ a so-
called "dissolver" in which a certain level of water 16 is presentO
The temperature of such water is maintained, by an automa~
tic system~ constantly at a value (about 37C) which ls slightly
above th~ temperature correspondin~ to the vapor pr~sure of
water under the same conditions of normal run of o.o6 effective
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atmosphares (t = 35.8C). This exp~dient a~-t~ in such a way
that~ when the hot ashes fall in water~ a certain amount o~
steam (a direct function of the ash temperature and their speci-
fic heat) which, superheated by the hot walls of the duct to ~50C~
acts in counterflow relative to the ashes and prevents a condensa-
tion of mercury and thus the presence of the metal in the exhausted
ashes.
In order~ then~ to encourage the dissolution of the ashes
in water~ there is a pump 17 which provides a vigorous recircula-
tion through the dissolver.
Lastly, with conventional level-checking systems~ thermo-
statically controlled water is provided and fed in, as the dis-
solved ashes are being withdrawn. The vapors emerging from the
dome 4 of the oven at the temperature of 250C contain, almost
entirely, superheated steam coming, predominantly, from the moisture
contained in the fed in sludges plus the counterflow washing steam
of the exhausted ashes plus~ obviously~ mercury vapors~
All these steam and vapors are conveyed to a vertical tube
bundle condenser 18~ which is water-cooled (processing in the
tubes on account of the possibility of dusts being entrained)
The condensate is collected in the bottom section of an
accumulator 19~ having a hydraulic seal~ which is placed at a
barometric level and the outlet of which directl.y communicate~
at the zero level~ with an atmospherical collectlng vessel 20.
~5 Such a condensate is composed by water and msrcury: water
overflows through an overflow outlet 21~ whereas the mercury is
automatically dumped by a sipho~ 22 from the bottom of the vessel
The vapor phase of the accu~lulat.or 19 is composed by steam
plus traces of mercury vapors plus the unavoidable noncondensed
fractions; these latter ar-e composed almost exclusively by air~
coming from the air dissolved in ash dissolving water and that
occluded in the neutrali~ed loaded sludges.
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These vapors are drawn by a liquid ring vacuum pump 23
which is i~t~ ed to keep the system at a pressure of 0~06
effective atmospheres.
Obviously, the percentage of mercury vapors at the in-
take side of the p~mp is a direct function of the temperature
of the cooling water in the exchanger 18 3 and thus to ensure the
value of 0.1 milligrams per normal cubis meter of Hg in the at -
spherical effluent~ the delivery of the pump is sent to a second
water-condenser 24~ wherein at the pressure of 1 to 3 atmo-
spheres~ as controlled by a pressure adjuster~ an addi~ional
condensation is effected~
As an alternative~ if the specifications o~ the vacuum
pump do not permit a very high pressure drop~ the coolant for
the condenser 24 can also be water cooled at least as low as
1$ 4C - 60C, coming from a refrigeration system which will have
very reduced dimensions on account of the small amounts of
vapor involved.
Summing up~ irrespective of whichever of the two above
mentioned systems is adopted~ the quantity of the atmospherical
effluent is at any rate in the order of a few tens of liters an
hour with a contents of Hg which is always less than 0,1 milli-
grams per normal cubic meter~
7.
