of Copper from Wastewater
Copper can be found in many wastewater sources including, printed circuit board manufacturing, electronics plating, plating, wire drawing, copper polishing, paint manufacturing, wood preservatives and printing operations. Typical concentrations vary from several thousand mg/l from plating bath waste to less than 1 ppm from copper cleaning operations.
Copper can be removed from wastewater by precipitation as an insoluble salt, precipitation as metallic copper and by ion exchange.
Precipitation of the insoluble salt is the most common form of treatment. The most common salt precipitated is insoluble cupric hydroxide which is formed by adjusting the pH of the water to about 9 to form the precipitate. The resulting precipitate leaves about .1 mg/l of dissolved copper in solution if chelates are absent. If chelates are present, the final copper concentration can be much higher.
Other insoluble salts of copper include, cupric carbonate, cupric phosphate and cupric sulfide. The residual copper concentrations of these precipitates are 10ug/l, 1 ng/l and less than 1 fg/l respectively.
When using these salts, it is necessary to maintain the anion concentration at high enough levels for precipitation to occur and adjusting the pH so the anion is available to precipitate the copper. Since the anions are also chelate copper, any residual treatment anion must be removed from solution by an additional treatment step. This step adds another cation to form a precipitate of the residual anion. Calcium is suitable for removing the residual carbonate and phosphate and ferrous ion is suitable for removing residual sulfide. Thus this process is a two step process which is more complicated than the single step copper hydroxide process. However, precipitation by this method can reach very low copper concentrations.
Copper can be also precipitated as insoluble copper metal. By using a reducing agent to adjust the ORP to less than –100 mv, copper levels of less than 1 ppb can be acheived. Typical reducing agents used include, ferrous sulfate, sulfide, sulfite and carbon. The reduction is done best in the presence of a high surface area such as diatomaceous earth, carbon or sand to form sites on which the copper can plate out on. If these surfaces aren’t present, the copper plates out in solution as a fine metallic suspension which is very difficult to settle or filter.
When using a reducing agent, the metallic suspension often is seen as high copper residuals. To remove these residuals, it is necessary to oxidize the copper back to the ionic state and re-precipitate the copper. This method is especially necessary when trying to achieve copper levels of less than 1 ug/l.
Ion exchange can be used to remove copper from wastewater. Normally a weak base cation resin in the sodium form is effective for this treatment. But, anion resin can also be used if the copper is chelated with a soluble anion or the anion resin itself can be doped or spent with the desired anion to remove the copper. This method is useful in selectively removing copper in the presence of other metals.
The equipment normally used for these processes is a batch treatment system for high copper concentrations (greater than 1000 mg/l), continuous precipitation if the copper concentrations are less than 1000 mg/l and the volume is greater than 5000 gpd. Ion exchange or copper reduction is feasible if concentrations are less than 20 mg/l and the desired effluent requires low copper concentrations, less than 20 ug/l.
Contact John Strandberg