The invention concerns an apparatus for the electro inetic removal of metals and other charged elements from soil, sludge, slag etc., said apparatus having a built-in positive or negative electrode which is surrounded by a first circulation system demarcated with a charge-selective membrane which transfers ions with the same charge as the electrode, said first circulation system being separated from a second circulation system with a charge-selective membrane arranged for accumulation, and which in turn is demarcated by yet a further charge-selective membrane which transfers ions with a charge opposite to that of the electrode.

Within the field of electrodialysis, use is made of charge- selective membranes called anion-exchange membranes and cation-exchange membranes which principally can only trans- fer anions (negatively-charged ions) and cations (positively-charged ions) respectively. The principle is that of making a sandwich of chambers separated alternately by anion-exchange membranes and cation-exchange membranes, and applying a potential to this stack. Every second chamber is connected together, and thus there is formed a concentration chamber and a demineralisation chamber alternately. From the demineralisation chamber, anions can pass the anion-exchange membrane, which must lie up against the anode, into a concentration chamber, while cations can go through the cation-exchange membrane, which lies up against the cathode, into the next concentration chamber. Once the ions have reached inside the concentration chambers, they will not be able to come back again, the reason being that their direction is determined by the electrical field and they are held back by the membranes.

An apparatus of the kind described by way of introduction is known from WO publication no. 95/32816, which has the English title: "Method and apparatus for the decon¬ tamination of products containing heavy metal".

This known apparatus is not suitable for use in soil be¬ cause the apparatus is cylindrical in configuration, in that the disadvantage here is that it is only with diffi¬ culty that it can be removed or serviced without actually digging it up from the medium. Moreover, the cylindrical configuration does not ensure a good contact surface be¬ tween the earth in which the apparatus is placed and the sides of the apparatus. Furhermore, the known apparatus is not sufficiently robust to withstand the pressure from the surrounding earth.

It is the object of the present invention to provide the known apparatus with a configuration whereby the above¬ mentioned disadvantges are avoided.

This object is achieved with an apparatus which according to the invention is characteristic in that the apparatus is a spear which is built up around a pressure-stable core of plastic and contributes towards the distribution of cir- culation liquids, and in that the spear tapers down towards that end which faces downwards in the medium.

It is the core which creates that rigidity and strength which the apparatus must have in order to last. There is hereby achieved the advantage that the membranes can reach the whole way around inside the apparatus without in¬ convenient and area-consuming stiffeners. The construction around the pressure-stable core also makes the apparatus stable enough to withstand the pressure from the surround- ing earth. Since the apparatus tapers down towards that end which faces downwards in the earth, the weight of the

apparatus and the pointed shape will, in the event of shrinkage or crack-formations in the earth, press itself deeper down in the earth and herewith maintain the good contact. It is hereby ensured that a good contact surface always exists between the sides of the apparatus and the medium in which it is mounted. This is of decisive im¬ portance for the transfer resistance and herewith the operational economy of the apparatus.

In order for the apparatus to work, at least two units must be used, but preferably more, and be configured with at least one anode and at least one cathode whereby a potential can be applied to the medium. Since the migration of ions in the earth is very slow, the use of many units will be preferred.

As disclosed in claim 2, it is advantageous that the medium is separated from the apparatus' second circulation system arranged for collection by an ion-exchange membrane which is arranged to prevent the passage of ions with the same charge as the electrode, in that positive electrodes are mounted with anion-exchange membranes and negative electrodes are mounted with cation-exchange membranes.

It is also advantageous, as disclosed in claim 3, that the electrode for protection against precipitation layers is disposed in the first circulation system, and separated from the second circulation system by an ion-exchange membrane in such a manner that ions with an opposite charge to the electrode are prevented from gaining direct access to the electrode, in that a positive electrode is mounted with a cation-exchange membrane for the separation of the two circulation systems, whereas a negative electrode is mounted with an anion-exchange membrane.

Furthermore, as disclosed in claim 4, it is advantageous

that a net of plastic is inserted in the circulation systems to ensure a turbulent flow of the liquids, while at the same time the nets are arranged to support the membranes so that possible pressure on the apparatus is transferred to the inner, pressure-stable core. There is hereby achieved a good turbulence in the chambers, whereby the effectivity of the ion transfer is increased.

As disclosed in claim 5, it is further advantageous that around the outermost membrane there is mounted a web in such a manner that the web with its outer side is in full contact with the surrounding medium, and with its innner side in full contact with the outermost membrane.

There is hereby achieved a support for the membranes so that they can transfer the pressure from the medium to the pressure-stable core, and so that the membranes do not split due to the pressure.

Finally, as disclosed in claim 6, it is also advantageous that a net is mounted around the outermost membrane in such a manner that a liquid phase is formed between the sur¬ rounding medium and the outermost membrane.

The advantage herewith is that the web distributes the moistness from the medium over a large area, whereby the surface loss due to holes or cavities which are formed by projecting stones can be compensated for. In practice there is thus achieved a contact surface close to 100%, even though there are places where there is no contact between the medium and the web. Moreover, further compensation is achieved for irregularities in the contact surface.

The apparatus according to the invention will now be de- scribed in more detail with reference to the figures in the drawing, where

fig. 1 shows the construction in principle of a charge- selective apparatus according to the invention, and

fig. 2 shows the construction, positioning and configu¬ ration in principle of membranes, seals and plastic net respectively in the apparatus shown in fig. 1.

Fig. 1 shows in principle the construction of a charge- selective apparatus according to the invention. The charge-selective apparatus 16 consists of a core 9 tapered towards the bottom, and in which a series of through-going holes is provided in the top and the bottom. Most of the outer side of the downwardly-tapered core, which is made of plastic, is covered by a perforated metal plate or a metal web 2 which has contact with the top through an electrical¬ ly-conducting connection 1 and a hole 19. On the outside of and completely surrounding the electrically connected, per- forated, downwardly tapered metal plate or metal web, there is placed a cross-sectionally open plastic net 4. On the outside of and lying completely up to the plastic net 4, there is placed a charge-selective membrane 12 which sits tightly in the bottom and the top and which through seals 6 is ensured connection only through some certain and de¬ termined through-going holes 18, 19, 10 and 17. On the outside of and completely surrounding the charge-selective membrane 12, there is also placed a cross-sectionally open plastic net 3 which is similarly tapered towards the bottom. On the outside of and lying completely up to the plastic net 3, there is yet another charge-selective membrane 13 which sits tightly in the top and bottom and which tapers towards the bottom, and which through seals is ensured connection only through some certain and determined through-going holes 8 and 20. On the outside of the outermost membrane 13 there is placed a downwardly-tapered,

flexible textile, plastic or mineral web 11, this being built up in or consisting of an water absorbent, similarly flexible and ion-conveying material. In order to ensure the sealing of the inner membrane at the top and the bottom, it is provided with a top ring 14 and a bottom ring 5. Similarly, the outer membrane 13 is provided with a top ring 15 and a bottom ring 7.

Fig. 2 shows in principle the construction, positioning and configuration of electrically-connected metal plates/webs, membranes, seals and plastic net respectively. All of the parts which are provided on the outside of the downwardly- tapered plastic spear 16 and 9, i.e. the downwardly-tapered metal plate/web 2, the two downwardly-tapered plastic nets 3 and 4 and the outermost, flexible ion-conveying layer 11 can, because of the special construction and sealing at the top and bottom and the described tapering towards the bottom, be assembled and disassembled part by part, which hereby enables the individual components to be replaced. This construction results not only in the top and bottom of the membranes being held away from each other, but also in the membrane surfaces being held fully extended and fully supported over almost the whole of the surface, and also that the parts hold themselves together as an assembled unit which can be mounted and secured on the downwardly- tapered plastic spear by means of just one or possibly some individual spring clips in the top of the apparatus.