Electrode chamber unit for an electro-chemical cell having a porous percolation electrode

An electrode chamber unit intended for use in an electrochemical cell including at least one porous percolating electrode in the form of a bed of electrically conductive particles, e.g. of graphite. The unit is distinguished in that it includes at least one substantially flat frame (3), preferably of a polymeric material, said frame defining a central opening (4) which is filled with the electrically conductive particles so as to form a porous bed (5), the particles being kept in place in the central opening of the frame by sufficiently dense separators (10), preferably made from a polymer material, which are arranged to cover the central opening on either side of the frame to form an electrode chamber, that the frame is provided with at least one hole (8) for supply and at least one hole (9) for discharge of electrolyte, where said holes are in communication via supply and discharge channels with the central opening of the frame, and that the frame includes current conductors (6,7) for supplying electric current to the conductive particles. An electrochemical cell including the above-mentioned units. A method of producing the cell by placing a separator (13) horizontally, applying at least one frame (3) on top of the separator, filling the central opening (4) of the frame with the particles of the electrically conductive material, placing the next separator (10) thereupon so as to keep the particles in place and repeating the procedure with the number of electrode chamber units which are to be included in the electrochemical cell, and locking the units to each other with conventional locking means (15,16). Use of the cell for purification of water.

TECHNICAL FIELD 
The present invention relates to the field of electrochemical cells, 
particularly electrolytic cells, which contain at least one porous 
percolation electrode in the form of a bed of electrically conductive 
particles. More specifically the invention relates to an entirely new 
structure of an electrode in the form of a bed of conductive grains, or 
rather to a complete electrode chamber unit which can very easily be 
assembled into an electrochemical cell of the filter press configuration. 
By this, the invention can be considered representing a pioneer invention 
within the field of electrodes in the form of beds of conductive grains, 
as, according to the invention, it is simple to construct electrochemical 
cells with the desired number of electrodes, with the desired thickness of 
the respective electrodes, with counter electrodes with the same structure 
as the electrodes according to the invention or counter electrodes of 
another type, e.g. solid plates etc., which in turn enables a simple 
disassembling of the cell, exchange or regeneration of individual 
electrodes, adjustment of cell structure and size to the intended field of 
use etc. 
The invention further relates to an electrochemical cell including the 
above-mentioned electrode chamber units, to a method of producing such an 
electrochemical cell and to the use of said cell for cleaning different 
kinds of contaminated waters. 
BACKGROUND OF THE INVENTION 
The discharge of heavy metals in different wastewaters contaminates the 
surface water and disturbs the biological purification processes in 
municipal sewage plants. The dominating purification technique currently 
used is chemical precipitation of the metals as hydroxides. Hydroxide 
sludge is obtained as a final product, which results in transport and 
deposition costs. Metal ions have been replaced by sodium or calcium ions 
in the wastewater, which further means that this salt-bearing water cannot 
be returned to the process. 
The ideal method of purifying wastewaters containing heavy metals would be 
a precipitation of the metals in a pure form with simultaneous 
desalination of the water to a purity allowing recycling in the process. 
The present invention enables an electrolytic metal precipitation which 
gives these advantages. Electrolytic metal precipitation from diluted 
solutions has previously not come into industrial use, due to the fact 
that conventional electrolysis with planar electrodes requires too large 
electrode areas to give a satisfactory purification effect, with the low 
final contents which are required these days (approximately 1 ppm). Under 
these conditions the electrolysis process is controlled by the transport 
of metal ions to the electrode surface (the cathode). To achieve a high 
material transport rate per volumetric unit, and thereby a small reactor 
volume, the cathode can instead be formed in accordance with the invention 
as a percolated porous body or a stable particulate bed with a high 
specific area. 
The basis principle of using beds of conductive grains as electrodes to 
obtain high specific areas is indeed described in the literature. In this 
connection reference is made to e.g. DOS No. 2,022,497, DOS No. 2,904,539, 
U.S. Pat. Nos. 3,974,049, 3,859,195, 3,459,646, 3,647,653 and 3,650,925 
and DE Nos. 2,424,001, 2,620,702, 2,705,007 and 2,723,708 and U.S. Pat. 
Nos. 4,123,340 and 4,217,191 and SE No. 413,446. However, none of these 
more or less sophisticated structures should be particularly suitable for 
industrial application with the severe requirements now made on 
wastewaters, and the structures are furthermore so fixed in their 
constructions that they cannot be adapted or modified simply and easily to 
different applications or conditions as is the case with the electrodes in 
accordance with the present invention. 
Cell structures of the filter press type are indeed also known, reference 
e.g. being made to U.S. Pat. No. 4,274,939, but in these cases the 
electrodes are of a completely different structure, viz. in the form of 
solid plates, which structure cannot possibly be compared with the beds of 
conductive grains which have so far been used within the field of porous 
percolation electrodes. Other examples of frames with solid electrodes are 
those disclosed in DE No. 3,221,371 A1 and SU No. 619,551. 
Thus, the art of using a bed of conductive grains as a percolation 
electrode must be considered specifically distinct from the art of using 
planar electrodes. Frames with planar electrodes have been known for a 
long time but as far as we know nobody has ever now the idea of utilizing 
percolation beds of distinct grains in frames or even less solved the 
problem of accomplishing this in a useful way. The present invention 
relates to an electrode chamber unit which solves the specific and 
different problems associated with a particulate electrode having good 
purification capacity. For instance, such a structure is extremely 
sensitive to so called channel-formation. Apart from the fact that the 
unit according to the invention does not comprise outer and inner frames 
as in U.S. Pat. No. 4,274,939, the grid structure and scaling properties 
of the frames used in said U.S. patent would not enable the provision of a 
unit in accordance with the present invention. The extremely high degrees 
of purification that have been obtained (up to 99.99%; cf. the Examples) 
represent a major advantage of the unit according to the invention, 
especially in the light of the great versatility of the invention as 
compared to previously known beds of conducting grains, and are not 
obtainable by the known frames. 
The present invention has thus been found particularly usable in 
conjunction with the purification of wastewaters, e.g. washing water from 
the galvanizing industry, or mine water, but the new electrode chamber 
units and cells are by no means limited to just this use, but one of the 
great advantages of the invention is just that electrochemical cells can 
be tailored for practically any kind of electrochemical reaction of the 
electrolysis type. 
A flexible, universally usable cell must meet other demands than those met 
by cells having beds of conductive grains. In common there is the demand 
on small electrode distances and the desirability of some form of package 
principle. However, the electrodes must be easily exchangeable, since 
different processes require different electrode materials. Furthermore, 
the cell must also be designed in a material that withstands corrosion in 
as many conceivable electrolytes as possible. It is also known that many 
metals disturb the electrode processes and cause poisoning of the 
electrodes. It would therefore be desirable with a cell in an inert 
plastics material. If the components of the cell can be injection moulded, 
the precision required for proper sealing can be achieved. Furthermore the 
price can be kept low if the series are reasonably long. Injection 
moulding requires, however, that the number of differently shaped parts 
can be kept down. 
All this is enabled by the new electrode chamber unit in accordance with 
the present invention. 
DISCLOSURE OF INVENTION 
The electrode chamber unit in accordance with the invention is 
distinguished in that it includes at least one substantially flat or 
planar frame defining a central opening which is filled with the 
electrically conductive particles to the formation of a porous bed, the 
particles being kept in place and insulated electrically from the counter 
electrodes with the aid of sufficiently dense separators, which are 
arranged to cover the central opening on either side of the frame to form 
an electrode chamber, that the frame is provided with at least one hole 
for feed of and at least one hole for discharge of electrolyte, said holes 
being in communication via respective inlet and outlet channels with the 
central opening of the frame, and that the frame includes conductors for 
supplying electric current to the conductive particles. 
By the expression "at least one frame" it is to be understood in the 
present case that an alternative to a thick frame is two or more thinner 
frames, which make it possible to work with one and the same frame 
thickness but still vary the thickness of the electrode. In such a case 
the frames are placed against each other and can be regarded in relation 
to the remaining components of the electrode chamber unit as a single 
cohesive frame, the term "frame" being used for the sake of simplicity 
even in the cases where two or more frames are used in one and the same 
electrode. 
However, both the frame and the separators are preferably made in a 
polymeric material, thus enabling the achievement of the above-mentioned 
advantages, particularly if this polymeric material is a thermoplastic, 
injection mouldable material. The material for the frame and the 
separators respectively does not necessarily need to be the same polymeric 
material. Thus, polyethylene and polypropylene can be mentioned as 
examples of suitable materials for the frame, whereas the separator, if 
made from another material than the frame, can be moulded e.g. from 
polyamide or polyester. In this connection it can also be mentioned that 
in the applications requiring ion-selective diaphragms, these diaphragms 
can also function as separators. Thus, in this case the diaphragm has a 
double function. Separate diaphragms can be used of course, which is 
illustrated more closely below. 
Since one and the same frame of the electrode chamber unit in accordance 
with the invention contains holes for the feed as well as holes for the 
discharge of electrolyte, an electrochemical cell made up from the 
electrode chamber units will function in a way where the electric current 
and electrolyte flow are directed in a cross-flow pattern in relation to 
each other, which has been found to give considerably better performance 
than in those cases where the electric current and electrolyte flow are 
directed in a concurrent or countercurrent flow in relation to each other. 
The central opening of the frame, and thereby the electrode bed, can be 
given different configurations, but it has been found that particularly 
favourable flow conditions and performances are achieved if the central 
opening has a substantially rectangular shape. The ratio 
longside:shortside of the rectangle is suitably at least 2:1 and 
preferably at least 4:1, a particularly favourable range being 4:1-10:1. 
Furthermore, if the channels for feed and discharge of electrolyte to and 
from the central opening open out in the respective shortside of the 
rectangle, i.e. at opposing shortsides, advantageous conditions for the 
electrolysis are achieved. 
Particularly in the case of metal recovery from diluted solutions, there is 
a requirement for low ohmic losses and the avoidance of secondary 
reactions, apart from the requirement of a large electrode surface per 
unit of volume and good material transfer, which are met by the electrode 
chamber unit in accordance with the invention. In turn, this means that 
the thickness of the electrode frame, and thereby of the electrode, should 
be kept comparatively small compared with the length and width, 
respectively, of the frame. In absolute numbers this more specifically 
means that the frame preferably has a thickness within the range of 0.2-5 
cm, more preferably 0.5-2 cm and most preferably 0.5-1 cm. By the 
structure of the new units in accordance with the invention, as well as 
the opportunity of assembling electrochemical cells with the desired 
capacities by assembling a desired number of frames, the small bed 
thickness does not involve any disadvantage in relation to the known art, 
but instead the advantage that the electrolysis conditions can be 
optimized in a way which has not been possible previously within the field 
of beds having conductive grains, which in turn opens possibilities for 
new fields of applications for electrodes of this kind. 
With regard to the electrode material, the requirements of the particulate 
electrode are the following: a good electrical conductor, chemically 
inert, usable both as a cathode and an anode and cheapness. The material 
which presently has shown to meet these demands in the best way is 
graphite, which is thus the preferred material for the conductive 
particles, but the invention is of course not limited to the use of this 
material only, since the idea is applicable to all other materials with 
similar properties. 
The size of the electrically conductive particles is easily determined by 
one skilled in the art for each individual case, depending on the 
particular conditions applying to the application in question. In general, 
however, the risk of channel formations and the requirement of reasonable 
pressure drops contradict too small particles or grains in the bed. A 
balance must be struck with the requirement of high specific area, which 
grows inversely proportional to the particle size. A usable range for 
particle size is, however, 0.5-5 mm, preferably 1-2 mm. Furthermore, the 
best purification conditions have been found to occur when the particles 
have an irregular shape, this being often preferable to a homogeneous 
spherical shape. 
An important demand on the electrode chamber units is that they give good 
mutual sealing in the electrochemical cell in which they are incorporated. 
For this reason the frame should preferably be surrounded on either side 
by a separate gasket, which is suitably a planar or flat gasket. Apart 
from its purely sealing function, the gasket can also have the function of 
regulating the packing density of the bed of the electrically conductive 
particles. This means that the gasket is preferably manufactured from a 
soft or elastic material, e.g. rubber, which can be compressed, or allowed 
to expand, respectively, in response to how tightly it is required to pack 
the bed. 
To enable the cell to operate as a continuous concentrator, where large 
volumes of low-concentrated waste solution are refined into small volumes 
of highly concentrated metal solution, there is further required in this 
special application dense and effective ion-selective diaphragms, i.e. in 
this case anion-selective diaphragms. In order that this diaphragm will 
not be punctured by the particles in the bed it is suitably disposed such 
that it is protected by the separator. In accordance with a preferable 
embodiment this is done by selecting as the separator a net or fabric or 
cloth material, preferably a polymeric material which surrounds the 
diaphragm on both sides. The separator net or fabric must be so dense as 
to protect the diaphragm from said puncturing by the electrically 
conductive particles, while at the same time not being so dense as to 
disturb the electrolytic current. 
Current supply via the frame is required to the electrically conductive 
particle bed. The current conductors can be implemented in a number of 
different ways, most important being that they do not disturb the 
electrolyte flow through the bed to any appreciable extent. One embodiment 
of current supplier which has been found to fulfil this requirement is one 
in the form of a substantially flat tongue arranged in the bed of 
conductive particles, the tongue being connectable to a current source via 
bar-like elements, preferably of a substantially circular cross-section 
which are inserted through the peripheral edge of the frame. It has been 
found that the tongue should be thrust a distance into the bed, i.e. with 
a space to the edge of the central opening of the frame, whereby the 
electrolyte flow is disturbed as little as possible. A particularly 
preferred combination of materials in this case is a graphite tongue with 
rodlike elements of titanium. Another embodiment of current conductor 
which has also been found effective is a large-mesh metal net, e.g. 
expanded metal of titanium, arranged inside the bed, its dimensions being 
similar to or smaller than the dimensions of the central opening of the 
frame. 
However, many other configurations of the current conductor are possible, 
as was mentioned above. A variant means that there are no separate current 
supplying means, the current supply function being provided by the frame 
per se, namely by the frame consisting of a composite material with a 
metallic conductor as a core in the frame. 
For the feed of electrolyte from the holes of the frame to its central 
opening there is at least one channel. In accordance with a preferable 
embodiment of the invention, this channel includes an arrangement enabling 
the electrolyte to be distributed over the entire width of the central 
opening of the frame, so that the electrolyte flow will be uniform over 
the entire bed. According to a variant, the arrangement includes several 
smaller channels, but other embodiments of such distribution or throttling 
arrangements are conceivable. In the corresponding way, the channel for 
discharge of electrolyte from the particle bed is suitably provided with 
similar distribution means. 
Another particularly preferable embodiment of the electrode chamber unit in 
accordance with the invention is represented by the case where the frame 
has two holes for the inlet and two holes for the discharge of electrolyte 
with separate channels from both holes to the central opening. If these 
channels are arranged to open out on opposite sides of the central 
opening, this means that one and the same frame is usable both as an anode 
and a cathode in the cell, by rotating the anode or cathode frames by 
180.degree. in relation to each other. The thickness of each electrode can 
be varied discretely by several frames with the same orientations and 
without intermediate separators being assembled into a common thicker 
electrode frame. This makes an extremely valuable contribution to the art 
in this field, since production costs can be kept low as in such a case 
the injection moulding can be carried out by means of one and the same 
tool. 
The invention further relates to an electrochemical cell, which includes or 
is constructed solely from the above-described electrode chamber units. 
The electrochemical cell in accordance with the invention is thus 
constructed according to the filter press principle with electrodes being 
arranged in central openings in frames provided with holes for inlet and 
discharge of electrolyte respectively to and from the electrode. At least 
one kind of electrode (anode or cathode) consequently consists of porous 
percolating electrodes in accordance with the invention, while the counter 
electrodes can be solid or sintered plates arranged in similar or 
identical frames. 
In the cases where the anodes as well as the cathodes are to be porous 
percolation electrodes, the electrode chamber units mentioned above, which 
have two holes for inlet and two holes for discharge of electrolyte are 
suitably used, the frames being turned 180.degree. in relation to each 
other. 
In the electrochemical cell according to the invention the shortside of the 
central opening, in the case where the central opening is a rectangle, is 
arranged along a horisontal plane. By this the electrolyte flow will be 
directed upwardly or downwardly in the vertical plane. 
It will be seen from the above that the expression "electrochemical cell" 
is used in a broad sense, i.e. not in the sense of a single cell with only 
one anode and one cathode, but a cell with the desired number of anodes 
and cathodes. Synonymous expressions in conjunction with the present 
invention are thus "electrochemical reactor" or "electrolysis apparatus". 
The term "separator" in the present case also intends to convey a broad 
meaning which means that the separator is not necessarily arranged between 
two electrodes, but ultimately it may also constitute the end plate of the 
cell and "separate" the outermost electrode from the surroundings. 
According to another aspect of the present invention, a method is provided 
for producing or assembling the electrochemical cell described above. 
Distinguishing for this method is that a separator is placed horizontally, 
the first separator according to the definition above usually consisting 
of the end plate of the cell, that at least one frame is placed on top of 
the separator; that the central opening of the frame is filled with the 
particles of the electrically conductive material; that the next separator 
is placed thereupon so as to keep the particles in place; and then 
optionally an ion-selective diaphragm or membrane; and on top thereof 
another separator, and that the procedure is repeated with the number of 
electrode chamber units which are to be included in the electrochemical 
cell, the units then being locked together with conventional locking or 
clamping means. 
In agreement with what has been discussed above, gaskets of an elastic 
material, e.g., rubber, are used between the frames in accordance with a 
preferred embodiment of the method, the central opening being filled with 
the conductive particles up to the edge of the gasket, and the gaskets 
being compressed to obtain the desired packing density for the conductive 
particles. This compression, together with the abovementioned locking, can 
e.g. be made with through-going metal rods with screwed ends for nuts. 
Another example of the clamping devices are so-called snap-on means, by 
which the plastic frames can be connected to each other, although these 
means can be more difficult to implement as compression arrangements. 
The invention further relates to a special use of the electrochemical cell 
in accordance with the invention, namely for the purification of 
wastewaters, particularly for the processing of water contaminated by 
heavy metals. 
However, the invention is by no means limited to this particular use, one 
of the great advantages of the invention being instead the new 
possibilities of tailoring an electrochemical cell for practically every 
type of electrochemical reaction. Other examples of fields of uses will be 
illustrated below and can moreover be easily worked out by one skilled in 
the art. The size of the cell can be simply adjusted to the needs by 
selection of a suitable number of frames, where each frame is built up for 
a given capacity. To increase the total reliability and reduce the risks 
of leackage it may be advisable not to make the individual modules too 
large. The desired size of the plant can be obtained instead by a 
connection in parallel of several modules or by a connection in series of 
parallel modules. Among further advantages in this connection there can be 
mentioned doubled electrolyte speed in the electrodes for the same total 
residence time in the system, which gives an improved material transport, 
and that the lower current loading in the second step means that the 
electrode thickness can be increased and the total diaphragm area thereby 
decreased. The electrode thickness can be increased in discrete steps by 
putting together several frames for each electrode.

The cell illustrated by an exploded view in FIG. 1 contains a number of 
cathodes 1 and anodes 2, which are of the same principle structures and 
which are thus both representatives of the new inventive idea in 
accordance of the invention. The electrode as well as the counter 
electrode are built up from a frame 3 with a rectangular form defining a 
central opening 4, which is also of a substantially rectangular shape, 
although the rectangle has oblique corners for providing a more 
homogeneous electrolyte flow across the electrode. The central opening 4 
is filled with a porous bed 5 of electrically conductive particles. This 
bed 5 covers substantially the entire opening 4, but for the sake of 
clarity only part thereof is shown in the Figure. The reason for this is 
that the Figure also shows the current conductor 6. In the illustrated 
case this conductor 6 is an expanded metal net embedded in the particle 
bed 5. Rod-like members 7 futhermore project via the edge of the frame 3 
for supply of current to the metal net 6. In the illustrated embodiment, 
the elements 7 from the cathodes are directed towards the viewer, while 
the corresponding elements from the anodes are not visible but are 
directed in the opposite direction, so that it will be simpler to keep 
apart the current conductors to the respective kind of electrode. Each 
frame is furthermore provided with two holes 8 for inlet and two holes 9 
for discharge of electrolyte. In the illustrated case, the electrolyte is 
fed to the cathodes 1 via the right hand hole 8 while it is fed to the 
anodes 2 via the left hand hole 8. The arrow drawn in full thus represents 
the electrolyte flow to and from the cathodes while the arrow drawn with a 
dashed line represents the corresponding flow to and from the anodes. 
On either sides of each cathode 1 and each anode 2 there is a separator net 
10 of the same configuration as the central opening of the respective 
electrode. This net 10 is surrounded by a gasket 11 of an elastic 
material, said gasket enabling regulation of the packing density for the 
particles of the conductive material. 
In the illustrated embodiment the electrode chamber unit also includes an 
ion-selective diaphragm 12 arranged between two separators 10. 
As will be seen from the Figure, the gaskets 11 and diaphragm 12 are also 
provided with electrolyte passage holes in register with the holes 8 and 
9, respectively, in the electrodes. 
The cell from FIG. 1 is illustrated in FIG. 2 in an assembled state, 
additional illustrated details being end plates 13, e.g. of metal, with 
inlet holes 14 and outlet holes (not visible) for electrolyte, rods 15 and 
nuts 16 for clamping the cell together in an assembled condition. As is 
clear from the above description, the end plates 13 are called separators 
for the sake of simplicity and for the purpose of the invention, although 
they have a different structure and partially another function than the 
separators 10. 
FIG. 3 illustrates an embodiment of a current supplier for the electrodes 
in accordance with the invention, more specifically an expanded metal net 
17 with rod-like elements 18. This embodiment of the current supplier is 
the one illustrated in FIG. 1, where the net thus has reference numeral 6 
and the rods the numeral 7. In this case the net is of substantially the 
same shape as the central opening 4 in the frame 3. 
FIG. 4 illustrates another variation of the current supplier with a 
considerably smaller tongue 19 having rod-like elements 20 connected 
thereto, i.e. the tongue is intended to occupy only a portion of the 
central opening in the frame. 
EXAMPLES 
The invention is finally illustrated by the following non-restricting 
working examples. 
Experimental Procedure 
The experiments were carried out while varying the following parameters: 
input concentration, kind of metal ion, flow rate, graphite grain size and 
electrolyte resistivity. Most of the experiments were carried out with a 
single particulate electrode of the type described, closed in on either 
side by an anion-selective diaphragm (Selemion ASV from Asahi Glass, 
Japan) and counter electrodes of lead plates. A polypropylene net was also 
applied as a separator between the lead electrodes and the diaphragms. 
Experiments were also carried out with several particulate electrodes 
(four, every second cathodic and every second anodic) according to FIG. 1. 
The feed solution was stored at room temperature in a polyethylene 
container having a volume of 60 liters. The solution was pumped into the 
cell by a centrifugal pump via a calibrated flow meter (rotameter). The 
flow rate was regulated by a valve on the pump pressure side. The metal 
concentrations in the feed solution and the discharge solution were 
determined with the aid of an atom absorption spectrophotometer (Varion 
AA-275). The pH of said feed and discharge solutions were measured with a 
glass electrode. 
The recirculating anode solution was a 0.1M K.sub.2 SO.sub.4 solution. The 
anode reaction was a generation of oxygen. 
The solution was allowed to pass through the particulate bed in a single 
pass. 
Results 
Copper Precipitation 
Experiments were made with both synthetic solutions made up from K.sub.2 
So.sub.4 (0.1-0.5M) solutions and CuSO.sub.4 to the desired concentration, 
and authentic waste solutions from the surface finishing industry. 
The results are apparent from Table 1 below. 
TABLE 1 
______________________________________ 
c c.sub.out q 
(mg/l) 
(mg/l) pH d.sub.p (mm) 
.rho. (.OMEGA.m) 
(l/min) 
I (A) 
U (V) 
______________________________________ 
67 0.03 1.4 1.0-1.4 
0.30 2.16 14 2.22 
53.7 0.66 2.62 2.0-3.15 
0.23 2.0 13 2.6 
35.5 0.73 2.87 " " 4.0 18 -- 
230* 0.03 0.87 0.7-1.4 
0.18 1.0 24 1.85 
______________________________________ 
*Waste solution from pickling of copper from a surface treatment plant. 
Zinc Precipitation 
Due to the very non-precious nature of zinc (electrode potential=-0.76 V 
relative to a hydrogen gas electrode) an electrochemical precipitation of 
zinc is very difficult. A series of experiments with different pH:s were 
carried out. The results are presented in Table 2: 
TABLE 2 
______________________________________ 
c.sub.in 
pH.sub.in c.sub.out 
pH.sub.out 
q I 
______________________________________ 
60 5.75 3.3-5 12.1 1.0 20 
62 4.2 3.5-5.5 11.9 " " 
70 2.8 .about.4.6 
11.5 " " 
64 2.05 54 2.25 " " 
76 2.4 65 2.75 " " 
______________________________________ 
As expected, the purification effect was not as good as for copper. The 
higher the pH value the better is the purification. 
The effect of the resistivity 
With very dilute solutions the electrolyte resistivity can be very low. A 
trial series was therefore carried out with varying contents of supporting 
electrolyte in the precipitation of copper. 
The results are accounted for in Table 3 (d.sub.p =1.2 mm). 
TABLE 3 
______________________________________ 
[.OMEGA.m] 
[.OMEGA.] 
c.sub.in 
.rho..sub.in 
.rho..sub.out 
c.sub.out 
q (l/min) 
I (A) 
I (V) 
______________________________________ 
116 3.38 3.85 28 2 9 2.15 
90 10 21.1 7 1 9 2.67 
74 5.35 10.6 0.6 1 7 3.24 
______________________________________ 
Experiments were also made with a complete cell consisting of two 
particulate anodes and two particulate cathodes; see FIG. 1. The same 
experimental procedure was used as above. The experiments embraced copper 
precipitation only. The results corresponded substantially to those in 
TABLE 1. 
It is to be understood that the present invention may be embodied in other 
specific forms without departing from the spirit or essential 
characteristics of the present invention. The preferred embodiments are 
therefore to be considered illustrative and not restrictive. The scope of 
the invention is indicated by the appended claims rather than by the 
foregoing descriptions and all changes or variations which fall within the 
meaning and range of the claims are therefore intended to be embraced 
therein.