Wastewater Control and Treatment

Increasing regulations governing discharge water have led to improved techniques for reducing the quantities of wastes that must be treated. These techniques have not only reduced the quantity of wastewater to be treated, but have also reduced the quantity of chemicals used and have lowered water consumption. These methods can be applied to any plating operation. Many references are available, including Ref 11, that cover waste treatment technologies.

The use of counterflow rinses has reduced water consumption and wastewaters while maintaining adequate rinsing between plating operations. Reduced dragout of plating electrolytes can be accomplished by allowing processed parts leaving the plating solution to drain into the plating solution. Drip pans also reduce the amount of electrolyte dragout.

Closed-loop systems have dramatically reduced wastewater, lowered water consumption, and diminished chemical usage. Closed-loop systems allow recovery of rinse waters and chemicals by evaporative, reverse osmosis, or ion exchange recovery methods. Care must be exercised when using closed-loop systems, especially with copper plating, to keep impurities and contaminants from preplate operations out of the copper plating bath where they will be trapped by the closed-loop operation.

In any plating operation, wastewaters must be treated to reduce the hazardous materials to meet regulations. The general procedures for treating copper plating electrolytes and rinse waters resulting from copper plating systems are:

• Cyanide-bearing solutions require oxidation of the cyanide with an oxidizing agent such as chlorine or hypochlorite, followed by precipitation of the heavy metals.

• Noncyanide alkaline solutions are pH-adjusted and have calcium chloride added to precipitate the copper.

• Pyrophosphate wastes require low pH hydrolysis to orthophosphate, followed by precipitation of the heavy metals.

• Acid sulfate and fluoborate wastes are pH-adjusted to precipitate the copper.

Reference cited in this section

11. J.W. Patterson, Industrial Waste Water Treatment Technology, 2nd ed., Butterworth Publishers, 1985 Copper Plating Equipment

Construction materials for equipment are indicated in Table 7. Construction materials for racks and anodes are given in Table 8.

Table 7 Materials of construction for equipment basic to copper plating

Tank linings are of rubber or plastic(a), or Koroseal.

Table 7 Materials of construction for equipment basic to copper plating

Tank linings are of rubber or plastic(a), or Koroseal.

Plating bath

Heating coils

Filters

Filter aids

Dilute cyanide

Low-carbon steel Teflon(b)

Low-carbon or stainless steel; cast iron

Diatomite Cellulose

Rochelle cyanide

Low-carbon steel Teflon1™1

Low-carbon or stainless steel; cast iron

Diatomite Cellulose

High-efficiency cyanide

Low-carbon steel Teflon(b)

Low-carbon or stainless steel; cast iron

Diatomite Cellulose

Pyrophosphate

Stainless steel Teflon1™

Stainless steel Rubber- or vinyl-lined steel

Diatomite Cellulose

Noncyanide alkaline(c)

Stainless steel

Titanium

Stainless steel Rubber- or vinyl-lined steel

Diatomite Cellulose

Acid copper sulfate

Titanium(d) Teflon1™

Rubber- or vinyl-lined steel

Diatomite Cellulose

Fluoborate

Carbon(d) Teflon1™1

Rubber- or vinyl-lined steel

Diatomite Cellulose

(a) Of approved compositions; in the absence of data on bath contamination and effects on deposits, compatibility tests are required.

(b) Dupont trademark.

(c) Polypropylene filter cartridges may be used.

(d) Also for cooling coils, if bath is used below 32 °C (90 °F)

Table 8 Materials for anodes and racks for use in copper plating

Racks are made of copper(a).

Table 8 Materials for anodes and racks for use in copper plating

Racks are made of copper(a).

Plating bath

Anodes

Dilute cyanide

Copper; steel

Rochelle cyanide

Copper™

High-efficiency cyanide

Copper™

Noncyanide alkaline

Copper(d)

Pyrophosphate

Copper™

Acid copper sulfate

Copper(e)

Fluoborate

Copper(d)

(a) Racks are generally coated with an inert plastic coating to prevent plating.

(b) Cast copper, high purity.

(c) Rolled copper, high purity.

(d) Oxygen-free high-purity copper.

(e) Phosphorized copper

Tanks. For cyanide copper solutions, low-carbon steel tanks are suitable. Polypropylene tanks with adequate reinforcing may also be used, provided that the operating temperature is not excessive. Low-carbon steel tanks should be lined with rubber, polyvinylchloride, or another synthetic material that is not susceptible to attack by the cyanide plating solution. This will prevent bipolar effects, which may rob current from significant areas of the work. Tanks for alkaline noncyanide copper, copper pyrophosphate, acid copper sulfate, and copper fluoborate solutions should be of similar construction. Low-carbon steel tanks used for these solutions must be lined with the above materials to prevent the solutions from attacking the low-carbon steel, resulting in short tank life and immersion deposits. New tanks, as well as all other equipment coming in contact with the plating solution, should be leached before use to remove any materials that may leach into the plating solution and cause poor quality deposits. Leaching solutions should be similar to the plating solution to be used, such as a 15 to 30 g/L (2 to 4 oz/gal) caustic solution for copper cyanide or noncyanide copper equipment, or a 5 to 10% sulfuric acid solution for acid copper sulfate. When converting a tank or line that contained cyanide to a noncyanide electrolyte, it is essential to leach out all residual cyanide from the tank lining and any associated equipment.

Barrels. High-speed copper plating solutions for barrel plating are being used in product operations. Polypropylene barrels have been used successfully for prolonged periods.

Anodes. The types of copper anodes used in each of the copper plating solutions are indicated in Table 8. High-purity copper anodes are recommended. Anodes with a lesser purity may form heavy sludges during electrolysis and contribute appreciably to roughness of the deposit. Anodes used for acid copper plating solutions should be phosphorized. These contain a small percentage of phosphorus, which helps to control chemical dissolution and limits the buildup rate of copper in the acid solution. These types should not be used in alkaline cyanide or noncyanide electrolytes, because anode polarization will develop and cause deposit roughness and more difficult copper metal control.

Copper anodes are available in many forms, such as bars, balls, or chips. Bars are suspended from the anode bar. Balls or chips are placed in titanium baskets.

The anode area in a copper plating solution should be controlled and maintained. If the anode area is not maintained, it decreases as the copper is dissolved and the anode current density rises, resulting in increased polarization and formation of undesirable films. These films can restrict current flow or sluff from the anode and cause roughness in the plating solution.

Anode Bags. Bags made of cotton, Dynel, or polypropylene are used in copper plating solutions. Cotton bags are preferred for cyanide copper solutions, and Dynel or polypropylene are used in the acid copper solutions. Bags are used to keep the fine particles formed at the anode from migrating to the cathode, resulting in roughness. The weave and weight of the anode bag are most important. The bag material must be capable of retaining the particles formed at the anode and at the same time allow the plating solution to flow freely around the anode. Anode bags are not generally used in pyrophosphate baths, because they interfere with dissolution of the anode by decreasing the circulation of the solution around the anode.

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