Electrolytic-cell and a method for electrolysis, using same

In an electrolytic cell, an electrolyte is caused to flow between an anode and a cathode. The conventional separator between the anode compartment and the cathode compartment can be eliminated, thus allowing circulation of a dispersed electrode throughout the electrolytic cell and obviating the drawbacks resulting from the use of a separator. The dispersed electrode is constituted by a suspension of conductive particles in said electrolyte.

The present invention relates to techniques involving electro-chemical 
reactions. It is more specifically related to a method involving 
oxido-reduction reactions and to the design of an electrolytic-cell 
suitable for practising said method. 
The present invention is likely to have applications as varied as the 
possible uses of electro-chemical reactions, in various industrial fields. 
However, the following description will more specifically refer to two 
broad fields, viz. the electrolysers for converting chemicals in an 
electro-chemical synthesis, and those used for treating industrial 
effluents in order to eliminate toxic or contaminating organic compounds 
from said effluents. 
The present invention results from the observation that the opposite 
reactions (direct and reverse reactions) occurring at the level of the 
electrodes of each polarities have quite often highly different rates, and 
that this generally apply to fast all usual oxidation or reduction 
reaction. 
Up to this day, for fear that the undesirable one of the two opposite 
reactions should annihilate the desired one, one has been led to separate 
the electrodes by means of a diaphragm, or separator, adapted to divide 
the electrolytic bath. It is the usual practice to have the electrolytic 
operations carried out in electrolysers divided, by means of a separator, 
into two or more compartments, respectively containing one of the 
electrodes of opposite polarities immersed in an electrolyte. Such a 
separator should be both porous enough so as to be pervious to the ions 
providing the flow of electric current from one compartment to the other, 
and capable, however, to act as an efficient barrier with respect to the 
reactants or products involved in the electro-chemical reactions in each 
of said compartments. 
Such a separator has a number of drawbacks : in particular, its very 
presence is sufficient for inducing an ohmic drop between the electrodes, 
such a drop being all the more important as the electrolyser is operating 
at higher current-densities. 
The object of the invention is to obviate these drawbacks. 
According to the invention the electrolytic process is performed by 
contacting the electrolyte containing the reactants simultaneously with 
the cathode and the anode at different polarities, with no diaphragm 
between them. 
By eliminating the separator, it further becomes possible to resort to a 
dispersed electrode circulating through the whole electrolyte and, 
therefore, capable of being in contact with both the fixed cathode and the 
fixed anode. However, the polarity of said dispersed electrode can be 
considered as unambiguously defined, and actually determined by the 
direction of that of the reactions the rate of which is the faster. A 
so-called "dispersed electrode" is constituted, as known per se, by 
conductive particles in suspension in an electrolyte. The use of such a 
dispersed electrode permits to improve further the efficiency of the 
operation. 
The method of the invention can be used in all cases in which the 
components in an aqueous solution have oxido-reduction properties such 
that one of the two opposite reactions (viz. oxidation or reduction) 
develops with a faster kinetics than the other. In such a case, the 
electrolytic solution can be brought into contact simultaneously with the 
positive electrodes and the negative ones, without, however, the risk that 
those reactions taking place in the vicinity of the electrodes with a 
given polarity should be annihilated by those taking place in the vicinity 
of the electrodes with the opposite polarity. With each of the compounds 
to be transformed or eliminated, only one reaction (viz. either an 
oxidation-reaction at the positive electrode, or a reduction-reaction at 
the negative electrode) is likely to occur with a substantial rate. This 
effect can be favourably amplified, if need be, through an appropriate 
selection of the relative areas of the electrode with opposite polarities. 
Such a method is in a position to be applied in a specially advantageous 
manner, either in the field of electro-synthesis operations, or for 
treating industrial waste effluents and especially aqueous effluents 
containing organic compounds which are to be destroyed by oxidation. These 
represent particular instancy of the more general case of the electrolysis 
methods, according to which an electrolyte containing soluble reactants is 
treated with a view to converting such soluble reactants into products 
which are soluble in the electrolyte too. 
Since it is not required to provide between the electrodes any separator 
preventing the cathodic reaction products from coming into contact with 
the anode, and conversely, the proposed method leads to a simplification 
in the design and operation of the electrolysis cells, which makes the 
latter specially suitable, in particular, for treating industrial 
effluents in their very exhaust circuit, in a continuous process wherein 
the effluent flows through an electrolytic cell and constitutes there the 
electrolyte submitted to electrolytic oxidation of its organic components. 
As a possible example of the applications of the method according to the 
invention, the following description will put stress on the advantages of 
said method over the usual methods and devices for the treatment of 
effluents, and describe a basic diagram with no restrictive intent. 
According to usual practice, the treatment of waste effluents requires 
either the physical elimination of toxic or contaminating organic 
compounds, or the chemical transformation of the latter with a view to 
render same harmless. In the first case, either ion-exchanging resins, or 
active coal are resorted to, while, in the second case, use is made of 
chemical products with oxidizing or reducing effects, adapted to transform 
contaminants into readily recoverable and separable substances. 
These two modes of treatment have a number of drawbacks, and in particular 
: a high cost-price in view of the use of costly substances and large 
cumbersome devices; the need of a permanent maintenance of the equipment 
by a skilled personnal; an ever unsatisfactory answer to the problem of 
continuous operation; a low energetic efficiency; a treatment with a 
duration likely to induce a decrease of the operation rhythm of the 
manufacturing unit. 
The present invention, in its application to the treatment of waste 
effluents, permits to obviate the above drawbacks, through the 
installation of an electro-chemical device that can be directly mounted in 
the exhaust circuit of contaminating waste effluents.

Electrolytic cell 1 is directly fed through the effluent-exhaust line 2. In 
particular, similarly to the case of an electrolytic cell adapted to 
electro-chemical synthesis operations, the cathode 3 and the anode 4 can 
be arranged in the following two ways, supposing a dispersed electrode is 
admixed with the electrolyte constituted by the effluent : 
either on both sides of the cell, said cathode and anode forming, for 
instance, the very walls of section of the conduit; the suspension 
constituted by the electrolyte and the dispersed electrode circulating 
between said cathode and anode; 
or transversely to the effluent flow, in which case the suspension has to 
pass through the fixed electrodes. Said fixed electrodes, in such a case, 
can be of various shapes and, in particular, can be constituted by plates 
provided with holes through which the necessary migration of the 
suspension takes place; grids acting in the same manner; or else a network 
of wires, tubes or plates, or generally any structure disposed 
transversally to the flow and provided with holes of a size greater than 
the size of particles of the dispersed electrodes to let them pass 
through. 
Once purified, the effluent leaves the device at 5, after having passed 
through a filter 6, in particular a rotary filter, adapted to recover the 
particles of the dispersed electrode which are sent back to 7 for being 
recirculated through the cell, the re-introduction thereof into the 
circuit being controlled by a pump 8. 
The efficiency of the method can be evaluated by measuring the reduction in 
the oxygen chemical demand, which is the amount of oxygen that is required 
to complete the oxidation reaction of the organic compounds in the 
effluent. This oxygen demand, or consumption, is expressed as the weight 
of oxygen per unit volume of effluent. The figures obtained after the 
treatment according to the invention, for a given feeding effluent, are 
much lower than with the conventional methods, although the time the 
effluent stays in the electrolytic cell is reduced. In a typical example, 
considering a waste effluent from a gun-powder factory, with an oxygen 
demand of 15,000 to 20,000 mg/1, the oxygen demand is reduced to 100 to 
500 after the treatment according to the invention, while no method was 
known up to now which could reduce it to less than some thousands mg/1. 
Quite obviously, and as appears from the above description, the invention 
is not restricted to the embodiments disclosed, given merely by way of 
examples. In particular, the invention covers the application of any other 
method permitting to perform an electrolysis reaction in the absence of a 
separator between the anode compartment and the cathode compartment. 
Moreover, as appears from the figure, several anode-cathode pairs can be 
arranged in series along the circuit of the effluents forming the 
electrolyte, to which are added the conductive particles of the dispersed 
electrode.