88    A CONTINUOUS CLOSED LOOP REGENERATION
SYSTEM FOR A CHROMIC/SULFURIC ACID
ETCHANT BATH
Dennis J. Kavanaugh, Chemical Engineer
CH2M Hill Industrial Design Corporation
Portland, Oregon 97208
Allen R. Boyce, Facilities Process Engineer
Tektronix, Inc.
Forest Grove, Oregon 97116
INTRODUCTION
Many of the products in the electronics industry require the technology of electroless plating on
plastics, such as acronitrile-butadiene-styrene (ABS), polyphenylene oxide, polysulfone, polyimide,
and polycarbonate.
This technology includes the electroless plating of nickel and copper. The initial step in the process
involves the etching of the plastic. The etchant is a solution of chromic acid of 420 mg/L and 4N
sulfuric acid. The etchant becomes "spent" by the reduction of hexavalent chromium Cr + 6 to trivalent
chromium Cr + 3. Typically, the spent etch solution is then treated in a reduction reaction with
metabisulfite followed by hydroxide percipitation.
This is the application of an existing technology, the utilization of an electrolytic purification cell
with a porous ceramic diffusion barrier, for regeneration of the etchant.
LITERATURE REVIEW
Three processes for chromium regeneration have been developed: chemical oxidation, electrolytic
regeneration using a large anode to cathode surface area ratio and electrolytic regeneration utilizing a
semipermeable membrane to limit the rate of diffusion between the anolyte and catholyte.
Sharai1 proposed chemical regeneration using Na2S208 with Ag2S04as a catalyst. Perceived limitations are solution saturation leading to insolubility and preciptation, and chemical costs.
Seegmiller2 in 1948 pioneered early electrolytic regeneration of chromic acid using a 30:1 anode to
cathode surface area. Recent refinements, in 1976, by Snyder3 have yielded current efficiencies of 50
to 85% for a chromic etchant (900 g/L) and a chromic (900 g/L)/phosphoric (57 g/L) etchant.
Chidambaram4 in India in 1969 demonstrated the technical and economic feasibility of electrolytic
regeneration of chromic acid used in the oxidation of p-nitrotoluene to p-nitrobenzoic acid.
Kappel5 discovered during experimentation in 1963 on full-scale systems in Germany that incorporation of a diffision-limiting barrier between the anolyte and catholyte decreased energy requirements
from 4 kwh/kg Cr03 produced to 2.5 kwh/kg Cr03. A sulfuric acid catholyte was used. Tirrell6,
Lancy7, and Fujii8 filed for United States patents in the early 1970's addressing contaminants removal
from etchants using anion selective or cation selective permeable membranes. Gussack9 in 1973
received the first United States patent specific to the oxidation of chromium in an electrochemical cell
using a semipermeable membrane to separate the catholyte and anolyte. This patent was later transferred to AMJ Chemicals. In application some membranes have been found to be susceptible to leaks
around seals and degredation from high strength, oxidizing etchants.
Saito10 received a Japanese patent in late 1975 for an electrolytic diaphragm cell for chromium
oxidation. In 1976 Kobayashi" developed a teflon membrane for application in an electrolytic diaphragm cell for regeneration of chromate plating solutions in Japan.
Korenowski and Lancy12 refined Lancy's earlier research and developed a regeneration system to
recover both spent chromic/sulfuric acid etch and rinsewater. In addition to oxidation of chromium,
organic contaminants are partially removed by maintaining a temperature of at least 60 C in the anode
873