Technical Data  
  Printed Circuit Etching


The following document has been written and supplied courtesy of the FMC Corp. It suggests a treatment method that might be used with spent Sodium Persulfate solutions.


Technical Data

Revised 4/96

Removal of Copper and Persulfate from
Spent Sodium Persulfate Etchant by Precipitation

Chemical displacement by aluminum metal is useful for reducing the concentration of copper, and persulfate ions in spent sodium persulfate etching and cleaning solutions to a disposable level.

Operating Conditions

a. Reaction Components

1. Aluminum metal: Aluminum displaces copper from the spent solution and reacts with persulfate according to the following equations:
2 Al + 3 Cu+2 = 3 Cu + 2 Al +3
3 S2O8 -2 + 2 Al = 2 Al+3 + 6 SO4-2 + Heat
Scrap aluminum turnings or chips can be used as the source of aluminum ions. Aluminum alloys 1100, 2017 and 7075 were tried and found to work satisfactorily. Other aluminum alloys were not available, but it is believed that all aluminum alloys will work.

The size and form of the aluminum scrap will affect the heat evolved. Powdered or foil aluminum reacts violently with the chloride catalyzed waste etchant.

2. Chloride ion: It was found that chloride ion acts as a catalyst in the reaction. As little as 0.01% by weight of sodium chloride (table salt) is sufficient to start the reaction. Addition of 0.1% by weight of sodium chloride crystals is recommended.
The concentration of chloride ion affects the time required for the reaction to start.
3. Dilution water: There is a large amount of heat evolved, therefore, dilution water is used to limit the total temperature rise. If the reaction tends to get too vigorous, the addition of cold water will promptly bring the reaction under control.
4. Copper precipitate: Copper precipitates as a spongy reddish-brown mass. When the copper is precipitated from concentrated waste solutions (containing more than 4 oz of copper/gallon) the copper deposits adherently on the aluminum metal. This can slow the reaction down to an uneconomical rate and precipitation from more dilute solutions is recommended. As little as 0.004 oz of copper/gallon will produce a visible precipitate of copper.
After the reaction has been completed, many finely divided copper particles are in suspension. Either time must be allowed for these particles to settle or the solution be filtered before dumping.
Since the copper sponge or sludge is in the powdered form, it is possible that when exposed to air, oxidation could occur with the evolution of a considerable amount of heat. Precautions must be taken when storing the copper sludge.
5. Clarified liquor: The clarified liquor contains dissolved sulfate and aluminum ions. Their exact concentrations will depend on such factors as the etchant's initial persulfate concentration, the amount of copper in the spent etchant and the dilution ratio.
For example: Treatment of etchant containing 4.5 oz/gal. Cu produced a clarified liquor containing:

Al +3   5g/l
SO4-2  = 70 g/l
Cu  =  0.6 parts per million
S2O8-2  =  None Detectable

b. Temperature

The displacement reaction is accompanied by the evolution of heat. A means for heat absorption is necessary. It is recommended that dilution water of twice the volume of spent etchant be used. If no tank of this size is available, the spent etching solution can be doled out in portions into a smaller tank.

c. Agitation

Agitation is important to prevent localized overheating. Also, the cooling effect of good agitation is vital in this reaction.

d. Time of Reaction

This reaction is extremely rapid, between ten minutes to one hour being required for complete copper precipitation. The time of reaction varies with the chloride content, form of copper deposit, concentration of copper ions and temperature. The effect of the chloride ion concentration has been mentioned before. Copper may deposit on aluminum in the form of an adherent film reducing the rate of aluminum dissolution. The greater the quantity of cupric ions in solution the longer the time to precipitate them out. At higher temperatures the reaction proceeds at an accelerated rate. However, speeding up the reaction by heating the solution is not recommended.

e. Tank Materials and Auxiliary Equipment:

Reinforced concrete tanks have been used successfully. It is suggested that the tank be located outside, rather than in the building.

Recommended Procedures :

The following are suggested methods for treating 100 gallons of waste solutions containing 4 oz copper/gallon:

Direct dilution:

1. Add 200 gallons of water to a 360 gallon tank.
2. Add 16 pounds of aluminum metal as small sheets or turnings.
3. Add 3 pounds of sodium chloride, (0.1% by weight of total solution).
4. Add 100 gallons of waste solution to be treated. Agitate vigorously until reaction has ceased and solution is free of copper ions.
5. Let settle and decant or filter the solution is free of copper ions.
6. Let settle and decant or filter the solution to remove suspended copper particles.
7. Scoop out copper sponge.
Stepwise dilutions:
1. Add 100 gallons of water to a 270 gallon tank.
2. Add 13 pounds of aluminum metal.
3. Add 3 pounds of sodium chloride.
4. Add 25 gallons of spend etchant, agitating vigorously.
5. Add another 25 gallons of etchant when the first batch has started to turn a light gray color and the temperature starts to drop.
6. Repeat Step 5 until entire 100 gallons has reacted.
7. Let settle and decant or filter the solution to remove suspended copper particles.
8. Scoop out copper sponge.
If the reaction slows down while copper ions are still in solution, the addition of more aluminum metal and/or sodium chloride will reactivate the copper precipitation.
One modification of the above procedure which has been found effective in the field makes use of a concrete septic tank or burial vault which should have a capacity of about fifty gallons for each pound of copper etched daily. As spent etchant is produced it is added to the tank which is preferably located outdoors. The treatment of the first quantity of spent etchant to be added to the tank can be conducted as described in Steps 1-4 under "Direct dilution". Following this the aluminum turnings (13 pounds per 100 gallons of spent etchant) can be added along with additional spent etchant as it becomes available. When the tank becomes almost full (about once each month) addition of more spent etchant is halted to allow any dissolved copper to be discharged completely from the solution. The liquor may then be sewered and the precipitated copper (contaminated somewhat with aluminum) scooped out. From time to time, a handful of additional sodium chloride may be added to the tank to raise the rate of deposition of copper. The rate of reaction obtained using this procedure is much lower than that obtained as described above but relatively little attention is required to keep this system operating around the clock.


The information contained herein is, to our knowledge, true and accurate. However, we make no warranty or representation, express or inferred and nothing contained herein should be construed as permission or recommendation to infringe any patent. No agent, representative, or employee of this company is authorized to vary any of the terms of this Notice.


The above FMC bulletin
"Removal of Copper and Persulphate from Spent Sodium Persulfate Etchant by Precipitation"
was provided by the FMC Corporation


A SMALLER RECIPE ---by the Gallon/Pound---this may be expanded as required

The TREATMENT --- time requirement -- several hours
    1. Use plastic containment.
    2. Agitate until reaction is complete. ( indicated by clear solution, no blue cast )
    3. Allow fine copper particles to settle to the bottom of plastic container.
    4. Decant upper solution and filter as necessary to capture suspended copper particles.
    5. Neutralize remaining acid solution to meet local EPA standards.
    6. Before disposal, be sure to check local EPA standards for compliance.