Metal recovery process

An electrolytic metal recovery process and apparatus for removing metal from a low metal content solution utilizing a recovery cell where a bed of metal is disposed on a perforate member above the floor of the cell and the cathodes of the cell extend into the bed to provide an essentially infinite area cathode through which the solution treated filters prior to removal from the cell.

FIELD OF THE INVENTION 
This invention relates to electrolytic recovery of metals from solution 
and, more particularly, relates to apparatus and a method which will 
obtain almost complete recovery of a metal from solution in powdered form, 
leaving less than one part per million of the metal in the finally treated 
solution. 
BACKGROUND OF THE INVENTION 
Electrolytic recovery of metals from solution is well known. Such 
electrolytic recovery is disclosed in U.S. Pat. Nos. 3,785,950; 3,535,218; 
1,839,905; and 3,579,431. 
The present invention is particularly suited for the removal of metals from 
solution of low metal content where the metal will deposit on a cathode in 
a powdery form. An example of this is the removal of copper from etching 
solutions used on printed circuit boards and the invention will be 
described in that environment. In the etching of printed circuit boards 
using one common etchant, copper is introduced in the etchant and cupric 
or cuprous ammonium chloride is produced. Copper can be efficiently 
recovered from this used etchant solution and also etchants which contain 
concentrations of cupric or cuprous ammonium salts. By removing the copper 
electrolytically, the etching solution can be substantially regenerated. 
However, the processed etchant still contains some copper in solution and 
the solution may be objectionable as an etchant. In one copper recovery 
technique, a cell containing a plurality of alternate anodes and cathOdes 
is utilized where the electrolyte is ammoniacal ammonium chloride. The 
anodes are made of graphite and the cathodes of copper. Oxygen is 
liberated at the anode and the following effective reaction occurs at the 
cathode. 
EQU Cu(NH.sub.3).sub.4 Cl.sub.2 +H.sub.2 =Cu+2NH.sub.4 Cl+2NH.sub.3 
The outflowing solution, therefore, becomes a regenerated etchant, but, 
however, still contains some copper in solution. 
In the described recovery process, copper is deposited in powdery form on 
the cathode due to the electrolysis occurring within the cell and the 
outflowing solution becomes a regenerated printed circuit board etchant of 
substantially reduced copper content. The copper powder builds up on the 
cathodes, and occasionally, the adhered powder falls to the bottom of the 
cell. The copper powder collected in the bottom of the cell is 
occasionally removed by pumping, or any other conventional method. A 
system of the type described will provide an electrolyte output containing 
approximately two grams/litre copper while the etchant introduced into the 
cell to be treated contains about one hundred and twenty grams per litre. 
The present invention provides a system for secondary treatment of the 
etchant which will remove additional copper in powdered form and provide a 
clarified etchant having less than one part per million copper. The 
finally treated etchant, after NH.sub.3 addition, is essentially 
equivalent to a virgin etchant. 
The metal content of a solution may be reduced by primary treatment using 
other techniques such as ion exchange, hydrogen reduction and solvent 
extraction, as well as electrolysis. 
SUMMARY OF THE INVENTION 
The invention provides a treatment system which may receive the effluent 
output of a primary treatment system as described above. In fact, the 
secondary treatment cell may receive as inputs the output of several 
primary treatment cells, or it may receive previously treated etchant from 
a reservoir. 
The secondary treatment cell comprises the provision of alternate anodes 
and cathodes where the cathodes extend below the anodes into a bed of 
powdered copper or other metal to be removed from solution. The bed of 
powdered copper is supported above the bottom of the cell on a perforate 
plate covered by a filter cloth. This arrangement provides a cathode of 
essentially infinite area through which the treated etchant must filter 
prior to removal from the system. This additional treatment effectively 
removes all copper from the treated etchant and provides an etchant of 
essentially virgin properties for reuse. 
An object of this invention is to provide a new and improved method and 
apparatus for removing metal from solution of low metal content, and 
providing a clarified essentially metal free solution. 
The features of the invention which are believed to be novel are 
particularly pointed out and distinctly claimed in the concluding portion 
of this specification. The invention, however, both as to its operation 
and organization together witn further objects and advantages thereof, may 
best be appreciated by reference to the following detailed description 
taken in conjunction with the drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
A system in which the invention may be embodied is shown in schematic side 
elevation in FIG. 1. This system 10 comprises a primary treatment cell 12 
and a secondary treatment cell 13. The cell 12 comprises a container 
having side, end and bottom walls. Cell 12 may contain a plurality of 
alternate anodes and cathodes (not shown) as hereinafter exemplified in 
cell 13. 
The inside of the walls of the cell are coated with electrical insulating 
material such as polypropylene. The effluent to be treated, which for 
purposes of this example may be considered cupric ammonium chloride, is 
introduced to cell 12 through a conduit 14 and exits cell 12 through a 
conduit 15 to secondary cell 13. Cell 12 is merely representative of any 
cell or system which will reduce a copper rich etching solution to about 
two grams/litre. The etchant is usually considered spent when it reaches 
one hundred thirty grams/litre. 
Under electrolysis, copper is deposited at the cathodes as finally divided, 
loosely adhered, copper powder. Periodically, copper powder which falls to 
the bottom of cell 12, is pumped out, filtered and washed. Cell 12 
operates continuously and regenerated etchant outflows continuously from 
primary cell 12. 
Secondary cell 13 receives the primary processed overflow from primary cell 
12 via conduit 15. The secondary cell, in practice, may receive the 
primary treated etchant from several primary cells. The secondary cell may 
receive the etchant from other sources than primary cell 12, such as a 
storage reservoir for primary treated etchant. The primary treatment may 
be done at a remote location. It is not necessary that the etchant receive 
primary treatment if the etchant has a low metal concentration of about 
two grams/litre. 
The present invention is applicable to clarification of solutions having a 
low metal content regardless of whether or not the solution received 
primary treatment. 
The primary treated etchant is continuously conveyed to secondary treatment 
cell 13 from primary cell 12 via conduit 15. Secondary cell 13 comprises a 
vessel with the inside walls electrically insulated as by means of a 
coating of polypropylene. A positive bus 25 is electrically connected to a 
plurality of anodes 26 depending into secondary cell 13 and a negative bus 
27 is electrically connected to a plurality of cathodes 28 which extend 
below the free end of the anodes 26 into a bed 29 of powdered copper. 
The bed of powdered copper is supported on a perforate plate 30 which is 
covered with a filter cloth 31. The perforate plate 30 is also 
electrically insulated as by coatings of polypropylene on either side. The 
perforate plate 30 is supported above the bottom wall of cell 12 by means 
hereinafter described and defines therewith a reservoir or collecting 
chamber 32. 
A standpipe 33 of insulating material is in communication with reservoir 
32. Plate 30 is cut out to permit standpipe 33 to extend therethrough. 
Reference is now made to FIG. 2 which is a section through secondary cell 
12 showing a cathode 28. Secondary cell 13 has upstanding sidewalls 34 and 
35 extending from a bottom wall 38. The busses 25 and 27 are supported on 
insulating strips 36 and 37, respectively, which are affixed to the upper 
edges of sidewalls 34 and 35. An electrode carrier 38a supports a cathode 
28 as hereinafter described. One end of carrier 38a receives an insulating 
sleeve 39. Thus the cathode carriers 38a, which are electrically 
conductive, are electrically connected to negative bus 27 but electrically 
insulated from positive bus 25. 
The cathodes 28 extend into bed 29 to a position proximate to perforate 
plate 30 and filter cloth 31. 
The perforate plate 30 is supported above the bottom wall 31 on a plurality 
of support and spacing members 40, 41, 42, and 43, which extend 
substantially perpendicular to planes of the electrodes. Members 41 and 42 
are apertured, as hereinafter described, to permit the liquid in reservoir 
32 to move to standpipe 33. Sidewall stiffening members 44 and 45, and 
bottom wall stiffening member 46 are provided as necessary. 
As shown in FIG. 3, each anode 26 comprises a plurality of conducting 
members 47-51 depending from a carrier 52. Carrier 52 is a conducting 
member such as copper having an insulating sleeve 53 thereon which rests 
on negative bus 27. The carriers for the anodes are therefore in 
electrical contact with positive bus 25 and insulated from negative bus 
27. For simplicity of illustration, standpipe 33 is not shown in FIG. 3. 
Reference is now made to FIG. 4 which shows a cell having only three anodes 
and two cathodes, for simplicity of illustration. Each of the anode 
carriers 52 comprises spaced apart conductive rods 54 and 55 which hold an 
anode 26 therebetween and support the depending anode in the cell. Each of 
the cathode carriers 38a comprises spaced apart conductive rods 56 and 57 
which hold the cathodes 28 therebetween while the cathodes depend into the 
cell and into the cathodic bed 29. As shown in FIG. 4, the support members 
40-43 for the perforate plate 30 extend substantially perpendicular to the 
planes of the electrodes. The supporting members 41 and 42 are apertured 
to permit flow of the treated fluid therethrough. Additional plate support 
elements 58 and 59 are provided at either end of the cell 12 to form a 
box-like support structure for plate 30. A conduit 62 extends downwardly 
into standpipe 33 and outwardly through wall 34 to permit siphoning or 
pumping of the solution which has filtered through the cathodic bed 29. 
The provision of the cathodic bed 29 of powdered metal with the cathodes 
extending in thereto provides a cathode of essentially infinite area and 
is effective to decrease the metal in the solution treated in the 
secondary cell to less than one part per million. This results, in the 
case of treatment of printed circuit board etchants, of an essentially 
copperless etchant, which is suitable for reuse. 
The invention has been practiced utilizing a secondary cell 13 in a size of 
four by four feet by five feet deep, which receives the output of four 
primary cells ten by four feet by five feet deep. 
In the treatment of the printed circuit board etchant, the anodes are 
carbon and the cathodes are copper. A spacing of three inches is provided 
between the electrodes in both cells. 
Cell 13 is operated at a current density of approximately one ampere per 
square foot on a clean cathode. The voltage is two and one-half to three 
and one-half volts. Low current density is utilized in the cell 13 because 
of the low concentration of metal in solution. The concentration of copper 
in cell 13 above bed 29 is about fifty parts/million. 
The cathodes above the bed in the secondary treatment cell reduce the 
copper content of the solution to about 50 to 100 parts/million. The 
cathodic bed reduces the copper content of the solution to less than 1 
part/million. It can be envisioned that these operations could take place 
in separated stages, i.e., secondary and tertiary, the secondary stage to 
consist of hanging anodes and cathodes and the tertiary stage to consist 
of anodes and a cathodic bed of copper powder. 
If a solution, which needs to be demetalized contains about or less than 
100 parts/million of metal which will deposit as a powder, then this 
solution could be introduced directly to the equivalent of a tertiary 
stage consisting of anodes and a cathodic bed of the metal powder. 
The cathodic bed 29 in secondary cell 12 is initially established at a 
depth of about six inches and the cathode elements 28 are dimensioned to 
extend almost to the perforate plate 30. When the copper bed 29 builds up 
close to the anodes, copper is pumped from the bed 29 to leave a bed depth 
of approximately six inches. 
In operation, the primary cells 12 are set in view of the flow rate and 
concentration of metal in the electrolyte to remove a predetermined amount 
of metal per day. The current densities of the primary and secondary cells 
are selected in view of the metallic content of the etchant to be treated. 
The current density may be varied as the concentration of metal in 
solution varies. 
While the invention has been described in conjunction with removal of 
copper from printed circuit board etchant of cupric ammonium chloride of a 
two gram/litre copper content, it may be utilized to remove any metal from 
solution where the metal content is relatively low and the metal may be 
deposited on a cathode in a powdered form, as for example nickel and 
cadmium. 
Where the etchant is acidic, such as a copper sulfate, lead anodes will be 
used. The concentration of copper in solution outflowing cell 12 is 
preferably maintained at two grams/litre or less. 
It may thus be seen that the objects of the invention set forth, as well as 
those made apparent from the foregoing description, are efficiently 
attained. While preferred embodiments of the invention have been set forth 
for purposes of disclosure, modification to the disclosed embodiments of 
the invention as well as other embodiments thereof may occur to those 
skilled in the art. Accordingly, the appended claims are intended to cover 
all embodiments of the invention and modifications to the disclosed 
embodiments which do not depart from the spirit and scope of the 
invention.