Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1985-02-19
1986-07-01
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204106, 204108, 204112, 204114, 204275, C25F 500
Patent
active
045978425
DESCRIPTION:
BRIEF SUMMARY
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.
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
REFERENCES:
patent: 1462421 (1923-07-01), Pearson et al.
patent: 4004994 (1977-01-01), Andrus
Andrews R. L.
Porters Grove Metal Recovery Co., Inc.
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