Note: Descriptions are shown in the official language in which they were submitted.
1 1 ~3958
CAS~ 1304
IMPROVEMENTS IN THE ELECTROLYl IC_CELI S,
MORL PARTICULARL`( FOR T~IE PRODUCTION Ol- ZINC . -
... . . ....... . _,_
This invention relates to the industrial electro-
lytic cells and more partieularly -to the electroly-tie
eells used for the produetlon of zinc. Thi.s invention
is for an improvemen-t in such electrolytic cells which
ean be applied to a number of elec-crolytic proeesses
in which gases are evolved, but whieh is par-tieularly
suitab].e for the proeesses of electrolytie production
of zine.
In this specification, special reference will be
had to the production of zinc, being it understoocl that
the invention can be applied with advantage also to
other industrial electroly-tie proeesses
It is known that the process for producincg pri-
mary zinc is eomprised of a number of stages in which
there is obtained a hi.gh-purity acidic solution of a zinc
salt, generally zinc sulphate, which i.s reduced in the
final stage to metallic zinc by eleetrolysis.
Such electroly-tic cells consist of va-ts in which
: there are numerous con-frontingly positioned anocles and
eathodes. As a rule, the anodes are of lead and the
cathodes of alumlnium. Metallic zinc is deposited on the
cathode Ind makes up a tl);n sheetJ wllich is removed f~om
1 3 ~3958
time to time by unripping, qenerally requiring the inter-
vention of operators. The inter-electrode gaps bet-
ween the adJoiningly arranged electrodes must be re-
. .
duced as far as practicable in order to limit the volt-
age drop through the solution, which is proportional to
the inter-electrode gap: the electrodes, otherwise, must
be rigorously palallel to each other to obtain an even
deposit on the entire cathode surface and thus a well
-formed sheet.
The primary and secondary reactions which take
place in such an electr~chemical reactor are as follows:
zn504 + H2 ----~~~~~~ ~n + H2S4 + 2 2 (1)
H20 _______-_ H + Io (2
Zn + H2S4 ~~~~~~ - ZnS04 ~ H2 (3)
The first two reactions take place concurrently and the
definite predominance of the reaction (1), i.e. the
primary reaction, over the reaction (2) determines the
electrical yield of the reactor.
The redissolution reaction (3) may take place
accidentally when the supply of electricity is disconti-
nued. The reactions (2) àncl (3) may take place either
due to chemical reasons, such as the inadequate purifi-
cation of -the electrolyte, and/or for physical reasons,
such as temperature.
An effect of the evolutlon of gas, be it consider-
ed as "normal"-procluc-tion of gaseous oxygen according
to the reaction (1), or "abnormal"-production of hyclroge
according to tlle reactions (2~ and (3), is the forma-
,,
~ ~ ~39~
.
tion of acidic mists.
This phenomenon is the result of the formdtion
of tiny droplets ol` acidic solution which, especially
under "abnormal" electrolysis conditions, are entrain-
ed from the bath surface and convectively airborne in
the environment.
Another undesirable effect is experienced whell
the working conditions are "abnormal" and this is the
formation of localized explosive mixtures.
; 10 Under "normal run" conditions, the content of mists
in an electrolysis cell room is around a few milligrams
per normal cubic metre of air, in terms of sulphuIic
acidity. Under "abnormal" conditions, this value is
raised to de-finitely higher values.
The stripping of the zinc cathodes tal~es place
at preselected time intervals, generally 24 to 48 hours,
and is carried out mechanically. This operation requi-
res the attendance of operators near the cells so that
th. operators are e~posed to the aggressive environment.
This exposure, in the long run, brings about al-
teration of the nasal sept, the oral cavity, the teeth
and the respiratory tràct in general.
Such an environmen-t, moreover, is aggressive for
the rnachinery, the implements and the building structure
of the cell rooms, so that these must be built with
special precautions.
It has been attempted to overcome these short-
comingis by purifying the work environment with forced
`~ ~ B395~
ventilation, with a total renewal of air from 5 to 10 times in
an hour. This procedure, however, involves a number of draw-
backs, such as :
- production of strong air streams
- difficulty of air-conditioning, especially for
installations located in cold areas, and
- considerable waste of power.
It would be advantageous to be able to offset these
defec-ts thus preventing the pollution of the environment in
the electrolytic cell room.
It would also be advantageous to be able to directly
withdraw the polluting gases just as they are being produced.
The present invention provides an improved electro-
chemical cell comprising an electrolyte-containing vat and a
set of anodes and cathodes arranged in alternate sequence and
serially connected, characterized in that the containing vat is
equipped with suction manifolds having suction ports to draw
the gases which are present in the space immediately overlying
the electrolyte level; shielding baffles being provided on
the anodes so as to leave a transversal space in correspondence
with the position of the cathodes within which the cathodes
themselves are positioned, said baffles taking the entire anode
width and the cell width, a suction system providing a negative
pressure in the manifolds to draw out the gases evolved in the
electrochemical reaction together with the possible mists
generated thereby while preventing their diffusion into the
cell room.
The invention will be described hereinaf-ter with
reference to the accompanying drawings, given by way of example
without limitation, wherein,
Figure lA is a cross-sectional view of a vat;
Figure lB is a plan view of the vat of Figure lA,
., ~ .
~ 3 ~3958
Figure 2 illustra-tes an assembled se-t of electrodes,
and
Figure 3 is an exploded view of an assembly in
accordance with the invention.
The electrolytic cells according to the invention
consist of a vat made with suction manifolds formed through
the top edges of the vat at a level immediately above the
free surface of the electrolyte. These manifolds have suction
ports which directly withdraw from the air chamber overlying
the free level of the electrolyte the gases which are evolved
therefrom.
In Figure lA there is diagrammatically shown the
cross-sectional view of the vat, wherein the manifolds 1 are
indicated together with the ports 2, as formed through an
enlargement 3 of the top portion of the sidewall 4.
- 4
a
.
5 ~
The electrolyte leve:L is shown at 5. As an alter-
native, the manifold can be provicled on one si.de of the
vat only. An overflow is indicated at 6.
In FIGURE lB the vat is shown in plan v;e~Y.
The sets of anodes and cathodes are ilnmersed in
the vat in al-ternate sequence and rest on -the supporting
members provided on the top edge of -the ce~l. rhe cell
is covered by a set of ba-ffles9 which are secured along
the entire anode leng-th, and are cut out in correspondence
of the centre line of the inter-anode gaps, in correspon~
dence with the working positions o-f the cathodes.
The empty space so provided, in adclitiorl to making
it possible to remove ancl replace the cat.hodes for per-
forming the operations of stripping the deposited zinc
layer, provides a reference which is useful for a correct
positioning and an immedlate check of the electrode aligrl-
ment.
The configuration of -the assemblecl electrode set
~ is shown in F]GURE 2, wherein 7 indica-tes thc anodes ancl
; 20 8 the rclative bùs bar whicJi rests on its supports,
whereas 9 and 10 are the cathocles and their relative bus
bar, respectively.
The shi.elding baffles are indicated at 11 : tlley
are secured to the bars 8 by spring mennbers 12 and/or
with screws 13.
The baffles are preferably made of a resilicnt ma-
terial so as to provide a satisfactory ti~htness while
being no hindrance -to the ca-thode-movillg opcrations. The
~ :~ 63958
.
6.
baffles in question can also be fas-tened othcrwise such
as by forming -through the horizontal top surfaces of
the bars ~ a set of grooves while the baffles 11 shall
have proJections matching wi-th such grooves. The as-
S sembly of all the baffles isolates a space above theelectroly-te and it becomes thus possible to draw ~rom
such a space the gas which has evolved together with the
acidic mists formed concurrently therewith above -the
electrolyte.
The suction manifolds are connected to a main
suction duct f to which in turn a conventional suction
sys-tem is connected equipped with electric exhaus-ters
andtor eJectors.
The electrolytic cells according to the inver-tion
permit to overcome the drawbacks aforementioned since
the withdrawal of the acidic mists is effected with small
volumes of sucked air and thus with a small consumption
of power so that the air-conditioning in the cell room
is no problem.
- The cells according -to the invention in addition
permi-t to recover lf so desired an oxygen-enriched
atmosphere. The drawn out gases and mists can be dispers-
ed into the atmosphere through a chimney in a dilu-ted
state so that the exhaus-t gases are harmless : as an
alternative a gas-abating system can be provided to be
used a-t ~eàst when "abnormal" reaction conditions are
under way.
If electric cxhausters are used those kinds are
39~
7.
preferred, in which the moving members haYe anti-explo-
sion bearings.
The neyative ~ressure used for drawing the air
over~ying the electrolyte is up to 15 mm of water co-
lumn. This value is correlated to the -tight seal provided
by the baffles, as too low absolute values of the nega-
~ tive pressure do not afford an adequate exhaustion, where-
as to high values originate a useless air draw through
the suction gaps, which dilutes the drawn out gases with-
out any advantage while requiring a higher power consump-
tion. Whenever a system for abating the drawn out gases
is provided, the suction system is subJec-ted to addition-
al pressure drops.
In order that the significance ard the advantages
of the invention may be better understood, the TABLE re-
ported below gives a few working data as obtained in a
test cell.
Run Composition of the atmosphere within
conditions the cell
Increase 2 H2conten-t H2S04 contcnt
% by vol.mg/nor.m3mg/nor.m3
,
normal ~ 1.0 300 1.0
normal -~ 2.0 250 0.8
irregular ~ 2.0 550 3.0
irregular + 1.2 700 7.0
irregular ~ 2.0 1000 15.0
irregular + 3.0 400 5 . O
i ~ ~3958
By maintaining ~ suction of lSO nor.m3 an hour
in the device ].ocated in the internal edge of the vat
no detectable alterations of the composi-tion of the
air in the cell room have been experienced.
The cells according to the invention can be built
with conventional materials such as coated or protected
cement, resistant plastics materials, such as reinforced
polyvinyl chloride PVC~, or in resin~glass. Air por.ts,
manifolds and suction fittings can be insertc~d in the
body of the cell during construction or thty can be pre-
pared separatcly and subsequently applied to the cell.
In the former case, however, the ribs deriving
from the insertion of such members in tlle structure of
said elements cooperate to increasin~ the sturdiness and
the stiffness of the structures themselves.
The suction manifolds can also form the support-
ing structure for the electrodes.
An overall exploded view of the assembly is shown
in FIGURE 3.
2~ A possible alterna-tive to the suction system Wittl
negative pressures is that of pressurizing the entire
; cell room to a value slightly above atmospherical so as
to prc~vent t.l,e fl.ow of gas from the space overlying the
elec-trolyte level to the environment, but in such a way
that thc reverse flow from the cell room environment to
the spacc above the electrolyte be prevented and thus
the f].ow towards the mani.folds is also prevcnted.
.
Such an approach, willch is doubtless more intri--
1 3 6395~ 1
cate, might be more prudential and thus it cou].d be
preferred for particular electrochemical processes
in -the electroplating industry.
