Economic and regulatory factors make metal recovery and wastewater reuse attractive for the electroplating, semiconductor and printed circuit board industries. Cadmium, zinc, copper, chromium, mercury, lead, gold, silver and cyanide complexes of these metals are the common metals requiring control. Ion exchange processes, evaporators, chemical precipitation, reverse osmosis and combinations of various processes are extremely effective tools for treating industrial wastewater streams and meeting current metal discharge requirements.
Ion exchange resins were originally developed to remove naturally-occurring ions found in most city water supplies such as calcium, magnesium, sodium, sulfates, chlorides, bicarbonates, carbonates and silica.
Common ions found in most city water include:
- Calcium (Ca2+)
- Magnesium (Mg2+)
- Sodium (Na+)
- Alkalinity (HCO3-)
- Sulfate (SO42-)
- Chloride (Cl-)
- Silica (SiO2)
A cation resin (CR) in the hydrogen form exchanges or removes cations, and an anion resin (AR) in the hydroxide form exchanges or removes anions according to the following chemical reactions:
Ion exchange is a viable method for ion removal or metal removal when treating high volumes of relatively dilute solutions. In other words, the resins are used as concentrators of the contaminants, minimizing the volume of waste.
Just as common ions found in most city water supplies can be removed by ion exchange, so too can metal ions be removed from waste streams; however, developing an effective ion exchange system for waste treatment is usually more difficult because of the complexity of the stream. For example, metals in waste streams can exist as cations or complexed anions, be monovalent or polyvalent or may not exist as ions. Additionally, waste streams often have oxidizing agents, oils, greases and detergents that harm ion exchange resins, and should be removed prior to any ion exchange system. Therefore a complete understanding of the wastewater stream chemistry is needed to properly design the ion exchange system.
The following is important information about wastewater stream chemistry:
- TDS (or conductivity) with min. and max. if variable
- pH and temperature with min. and max. if variable
- Basic inorganic analysis of ions (Ca, Mg, Na, Cl and SO4)
- Presence or absence of oxidants; in order to determine if a chelating resin is appropriate to use, it is necessary to know the basic details of the water to be treated
- Presence or absence of complexing agents
- Level and type of organic molecules
- Level of suspended solids
Electroplating can cause environmental contamination because of rinse water discharge and disposal of spent plating bath solutions. Because the plating baths contain such high levels of metals, reclamation by precipitation followed by filtration is already being practiced. The problem is that the majority of metals being discharged to the environment come from the plating operation’s rinse waters. The rinse waters have been a challenge for the industry because the level of metals is too low for effective precipitation methods and meeting discharge requirements. An application perfectly suited for an ion exchange process, the best available technology is treating high volumes of solution with low concentrations of contamination. Although ion exchange processes can be implemented to treat most rinse waters, it must be noted that the greatest obstacle to applying this technology appropriately is when components other than the metals of interest are not considered. To design the most effective ion exchange process for removing metals from plating operation rinse water, a full analysis must be available.
As an example, let’s evaluate a copper plating rinse water using city water as the make-up for the rinse tanks.
The question is, which resin to use? A standard strongly acidic cation exchanger in the sodium form (softener resin) such as ResinTech’s CG-8 (Na) can be used to remove the copper but will also remove the calcium and magnesium. If you use an exchange capacity of 30 kilograins of CaCO3/cu ft, you can treat 1,350 gal/cu ft.
An alternative would be to use an iminodiacetic chelating resin such as ResinTech’s SIR-300, which is selective for the copper and other heavy metals. If you use an exchange capacity of 15 kilograins CaCO3/cu ft, you can expect to treat up to 42,000 gal/cu ft because you are only removing the copper.
Iminodiacetate Functional Group
Although the iminodiacetic chelating resin performs better as a concentrator, there are other considerations. The cost of an iminodiacetic chelating resin is higher than a standard strongly acidic cation resin. The regeneration of an iminodiacetic chelating resin is more complicated using a two-stage regeneration of acid followed by caustic, whereas the standard strongly acidic cation resin is typically with just brine.
Another option worth investigating is to redesign the system to remove competing ions from the city water feeding the rinse tanks by installing a softener, demineralizer or reverse osmosis. With no competing ions present, a less expensive, standard cation resin can be used for removal of the copper with high throughputs similar to the iminodiacetic chelating resin.
In addition to iminodiacetic chelating resins, there are other types of chelating resins available whose chelating functional groups can be aminophosphonic, thiouronium and picolyamine. Although the removal mechanisms of these resins are similar, their unique functional groups make them selective for different metals and able to perform under different conditions.
Chelating resins are more selective than standard resins because the removal is not only based on an ion exchange mechanism, but by also forming a chelate.
Aminophosphonic Functional Group
Aminophosphonic chelating resins are able to form chelate divalent metal ions in the absence of calcium. They are generally used for removing hardness from brine but have occasionally been used instead of iminodiacetic resins when the TDS is greater than 1,000 ppm or conditions are slightly basic.
Thiouronium Functional Group
Thiouronium resins are not true chelating resins but are highly selective for mercury, gold, silver, platinum and palladium. Their primary use is for removing mercury to low levels in the presence of high levels of heavy metals.
Picolyamine Functional Group
Picolyamine chelating resins are highly specialized resins used to remove heavy metals. What makes them unique is their ability to operate at extremely low pH levels. They are typically used to remove transition metals from plating baths, not rinse water.
Often overlooked, weakly acidic cation resins in sodium, such as ResinTech’s WACMP, should be considered as an economic alternative to iminodiacetic chelating resins for heavy metal removal. In most cases, determining the use of either a weakly acidic or iminodiacetic type is entirely based on pH and its effect on the operating capacities of these resins. A weakly acidic cation resin will outperform an iminodiacetic cation resin on soft water with a neutral or slightly basic pH.
As mentioned already, most of the hazardous waste produced in metal
finishing comes from the wastewater generated during the rinsing operations that follow cleaning and plating operation. Efforts to increase rinse efficiency are vital. Ion exchange processes for treating make-up water to rinse tanks and for reuse should be used. Another process change that can optimize wastewater management is waste segregation. Older operations typically have a common trench to which all rinses drain. The wastewater chemistry for these combined rinses make metal recovery and water reuse more difficult.
When evaluating an ion exchange process in your facility, it is vital that you have complete knowledge of the wastewater chemistry (this requires more than simply identifying the metals of concern and their concentrations); effluent requirements (the goal to meet discharge, recover or reuse); flow rate requirements; and regenerable (onsite or offsite) or sacrifical requirements.