Electrolytic cell

In electrolytic apparatus comprising cell chambers through which is passed an electrolyte and in which sets of anode plates are provided, each of which are connected to current-feeding center pins, and the mutually staggered electrode plates protrude into the gaps between plates having the opposite polarity. To ensure a simple, quick and reliable installation and removal of the anode plates, the center pin is provided with contact straps, which are spaced apart in the longitudinal direction of the pin and serve to secure the anode plate. The contact straps are suitably spaced about 180.degree. apart and have at least one opening, which consists preferably of a tapped bore. The electrolytic apparatus is used in processes of producing alkali chlorate by the electrolytic decomposition of aqueous alkali chloride solutions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrolytic cell comprising a plurality of 
parallel anodes and cathodes which are closely spaced apart and have 
confronting surfaces. 
2. Discussion of Prior Art 
The electrolysis of alkali chloride solutions or acid alkali sulfate 
solutions to produce alkali chlorates or alkali persulfates is usually 
carried out in electrolytic cells having steel cathodes and titanium 
anodes. The anodes are usually provided with an activating coating, which 
consists, e.g., of mixed oxides of platinum metals. In the production of 
alkali chlorate by an electrolysis of alkali chloride solutions, the 
titanium of the anode serves to carry electric current and the presence of 
the activating coating reduces the voltage required for a deposition of 
chlorine so that energy is saved. The dimensions of the anode material are 
selected in dependence not only on the current density (kA/m.sup.2) but 
also of the distance to be traversed by the current in the anodes 
themselves. To ensure a uniform distribution of current over the anodes, 
the voltage drop in the anodes must be small compared with the voltage 
drop in the electrolyte. For this reason, the cross-sectional area of the 
anode material must be relatively large. 
It is known to connect titanium anodes at the vertical housing walls of the 
electrolytic cell in such a manner that the total current is uniformly 
distributed over the several sheet electrodes and flows across the cell. 
When the anodes are connected to the cell walls, flanged anodes are 
commonly used, which are secured to the cell walls by connecting screws or 
welded joints. It is known from German Auslegeschrift 26 45 121 that 
current may be centrally fed to the anodes by means of a so-called center 
electrode consisting of a current-feeding pin provided on the vertical 
center line of the anodes. The length of the path along which the current 
must flow is thus reduced to one-half so that the thickness of the 
material can be reduced considerably or to one-half. 
While affording this advantage and resulting in a suitable, compact 
structure, the previously known design involves considerable difficulties 
regarding the assembling of the anode set. Additionally, losses are 
involved in the flow of current from the copper pin over the female screw 
threads of the threaded sleeve and the male screw threads of the sleeve to 
the threaded rings. Each anode plate is individually and loosely fitted on 
current-feeding threaded sleeves and is fixed by means of individual 
threaded rings, which serve also as spacers. The electric contact is 
established by a pressure contact joint. This operation is repeated until 
the desired number of anode plates have been mounted. In that method, 
particularly high costs are due to the need for platinizing the contacting 
surfaces of the anode and of the threaded rings and the threaded sleeves 
carried by the current-feeding pins, in order to ensure that the voltage 
drop at the interfaces will always be low. In another embodiment of the 
known electrolytic cell, the current-feeding threaded sleeves may have 
annular ribs, which have the same thickness as the anode plates. The 
diameter of the ring only slightly exceeds the opening in the anode plate 
so that the ring is almost flush with the anode plate. That ring is fixed 
by a welded joint. 
It is an object of the invention to eliminate the above-mentioned 
disadvantages and to provide for an electrolytic cell an anode assembly 
which can be assembled simply and in an economical manner. 
SUMMARY OF THE INVENTION 
For this purpose, the invention provides an electrolytic apparatus 
comprising cell chambers which are passed through by the electrolyte and 
in which sets of anode plates are provided, each of which is disposed 
between two sets of cathode plates and which are connected to 
current-feeding center pins, and the mutually staggered electrode plates 
protrude into the gaps between plates having the opposite polarity. In an 
electrolytic apparatus of the kind described, the invention resides in 
that the center pin is provided with contact straps, which are spaced 
apart in the longitudinal direction of the pin and serve to secure the 
anode plate. 
The contact straps can have a size of e.g., 20.times.10.times.10 mm and can 
be mounted on, and preferably welded to, the titanium coating of the 
center pin and are spaced at least 90.degree. and suitably about 
180.degree. apart and spaced apart along the pin. To permit the anode 
plates to be secured to the contact straps in a very simple manner so that 
they can easily be replaced, the contact straps have at least one opening. 
The opening or openings are suitably circular holes, which consist 
preferably of tapped bores. The anode plates to be mounted on the contact 
straps have openings adapted to register with the openings or bores in the 
contact straps so that an intimate contact between the anode plate and the 
contact straps can easily be established, e.g., by means of screws. For 
instance, in a so-called four-pin cell each anode plate has, e.g., at 
least four bores so that it can be secured, e.g., by screws, at at least 
the four bores of the associated four contact straps which are carried by 
the four pins and superimposed in a vertical plane. 
Each anode plate is rectangular and has on its vertical center line at 
least one opening and, for instance, in a four-pin cell, four openings. 
These openings consist of slots and have a major diameter that extends, 
e.g., in the vertical center line of the anode plate and is at least as 
large as the diameter of the coated center pin. In a four-pin cell, each 
anode plate is then secured by means of four contact straps to four pins, 
which extend through the slots, so that the anode plates are parallel to 
each other and equally spaced and extend at right angles to the 
longitudinal axes of the bolts. A compact set of anodes is thus obtained. 
The cathodes consist also of sets of cathode plates, which are secured to 
a carrier plate, on one side thereof, at right angles thereto and are 
equally spaced and parallel to each other. The carrier plates constitute 
the side walls of the housing of the electrolytic cell. They are liquid 
tightly connected to and electrically insulated from the remaining parts 
of the cell housing. The leads for feeding current are secured to the 
outside of the carrier plates. All other parts of the cell housing are 
electrically connected to the anodes. 
Whereas all cathode parts consist of steel, the material of all anode 
portions in contact with the electrolyte, inclusive of the contact straps 
provided in accordance with the invention, consists of titanium metal. 
Those surfaces of the contact straps and anodes which form 
current-conducting interfaces are provided with a platinum coating which 
has a high electrical conductivity. This means that the effective surface 
carries a coating of mixed oxides of the platinum metals, particularly the 
oxides of ruthenium and rhodium. 
The current-feeding pin consists of composite material comprising a copper 
core and a shrunk-on sheath of titanium. At one end of the current feeding 
pin the latter is secured to the inside surface of the housing by means of 
an annular flange. For this purpose, the copper core is provided with 
screw threads, which can be unscrewed. A screw body of copper is inserted 
through a suitable opening in the carrier wall and with its screw threads 
is screwed into the screw threads of the copper cores. The free end of the 
screw body is connected by a current-feeding lead to the positive pole of 
a voltage source. To assemble the set of anode plates, each anode plate is 
fitted at the slots over the center pins and the staggered contact straps 
by a reciprocating movement and when it has reached the desired position 
is secured by means of screws to the contact straps. When the screws have 
been fixed, additional plates are installed in the same manner until the 
desired number of anode plates has been mounted on and connected to the 
pins. 
To assemble the electrolytic cell, through which the electrolyte flows in a 
vertical direction, the middle set of anode plates is first secured to a 
carrying grate. The side walls of the cell housing are then removed and 
the cathodes are secured to the side walls. Finally, the cathodes 
consisting of sets of cathode plate side walls are inserted together with 
the side walls to such positions that an anode and cathode lie opposite to 
each other in the cell. 
The electrolytic apparatus according to the invention can be used to 
advantage in electrolytic processes for producing alkali chlorate by an 
electrolytic decomposition of aqueous alkali chloride solutions. 
The advantages afforded by the invention reside in that the design of the 
anode assembly according to invention ensures that the anode plates can be 
installed and removed in a very fast, reliable and economical manner, 
compared with the use of known welded and pressure contact joints for 
connecting anode plates to the current-feeding carrier. This is due to the 
fact that the anode plates are connected only by simple screwed 
connections to contact straps of the center pin. The fast removal is 
significant because the anode plates must be removed from the cell at 
regular intervals in order to be re-activated or re-coated. Because the 
anode plates contact the contact straps only on relatively small surfaces, 
much less platinum is required for the platinizing of contact surfaces. 
The number of current-carrying contacts and, with them, the current 
losses, are minimized too.

DESCRIPTION OF SPECIFIC EMBODIMENT 
The accompanying drawing shows a center pin according to the invention. The 
center pin consists of a copper core 1 and a titanium sheath 2 shrunk 
thereon. Contact straps 3 of titanium have been welded to the titanium 
sheath 2. Each contact strap 3 has at least one bore 5. At these bores, 
the anode plates 12 and 15 are screw-connected to the contact straps. 
Anode plate 12 is screw-connected to contact strap 3 by screw-threaded 
bolt 14 and anode plate 15 is screw-connected to contact strap 3 by 
screw-threaded bolt 16. Anode plate 12 is provided with an oblong hole or 
slot 13. An unscrewed plate (not shown) can be moved to the left over 
contact bracket b and can be lifted and further moved over bracket c. 
The contact strap 3 carries a platinum layer 6 at least on its contact 
surface. The platinum layer can be in the form of an annular flange 6, 
which can be welded to the titanium sheath 2. It has bores 5 for the 
fixation of the anode plate and bores 7 for the fixation of the flange to 
the inside surface of the housing. The platinum coating on the flange 6 is 
designated 8. The threaded portion 10 of the screw body 11 is screwed into 
the tapped hole 9 of the copper core 1.