By Joseph McDonald
With tightening restrictions on discharge, alternatives to conventional methods of precipitation must be examined.
Today, all firms engaged in manufacturing printed circuit boards (PCBs) discharge their spent process water to either waterways or municipal treatment plants. The heavy metal residuals contained in the discharge are potentially toxic, and therefore must comply with the Environmental Protection Agency's standards for regulating the quality of plant effluent. New restrictions have substantially lowered the amount of heavy metals allowed for discharge and have forced many plants to seek alternative means of wastewater treatment.
The purpose of this article is to explore the use of organic precipitants metals removal techniques, as compared to existing conventional treatment methods, on the basis of effectiveness and cost.
CONVENTIONAL METAL HYDROXIDE PRECIPITATION
Until recently, the most widely applied system for removing soluble metals in wastewater was precipitation of the soluble metal ions as insoluble metal hydroxides. This is a relatively simple method requiring addition of lime or caustic to the pH of minimum solubility for a particular metal ion. This method, although widely practiced, has several limitations, and can result in an effluent which does not comply with discharge standards as set today or in the near future.
One of the limitations of hydroxide precipitation is the relatively high cost of pH adjustment chemicals, coupled with the associated sludge-hauling and disposal fees. This is partially the result of the current shortage of caustic and the corresponding increase in price to over 163% of 1988 year-end figures. Although lime can be substituted and is readily available at an inexpensive price, it also has an inheritent problem in the amount of sludge that is generated by its use. The steady increase in cost per pound for hazardous waste hauling and disposal is fast making the addition of lime for pH adjustment prohibitive.
The second drawback to the hydroxide precipitation method is the inability to precipitate complex metal compounds. During the electroplating process, stripping and cleaning solutions act as chelating and sequestering agents, causing metal complexes to form. These metal complexes are unaffected by simple changes in pH and are resistant to insoluble hydroxide formation. In the PCB "caper" process, ammonium persulfate can cause these metal complexes to form.
Last, even under ideal conditions in the absence of chelated compounds and with sufficient control over pH adjustment, several cases exist where the minimum solubility of the metal ion is not low enough to meet newer discharge restrictions. Many plants have a variety of metals to remove, several of which have differing optimum pH points ( see Figure 1). In those cases, a compromise must be selected at the expense of leaving some metals still in solution. It is due to the aforementioned restrictions in the effectiveness of the hydroxide precipitation that recent crossover technology has been developed to replace this once-accepted norm.
ORGANIC PRECIPITANTS
The use of organic precipitants, such as polythiocarbonates and dithiocarbamates has been identified as an effective metal removal technique when used in conjunction with coagulant and flocculant technology. This new method should be viewed as a three-step approach involving the following:
- Producing insoluble metal compounds using organic precipitants
- Coagulating these precipitates for more effective removal rates
- Flocculating the fine particles to promote rapid and complete settling, resulting in a fully acceptable plant discharge
Step 1: Organic Precipitants
A recent innovation in metal reduction has been the introduction of thiocarbamate and its sister organo-sulfur compound polythiocarbonate to completely or partially replace hydroxide precipitation.
Organic precipitants, when added to the waste stream, form a dense stable floc that is neither dependent on pH nor affected by chelated or other complexed metals. While experiments have show metals removal to trace amounts using organic precipitants alone, it may be more economical to adjust the pH of the wastewater to allow hydroxide precipitation of the most abundant metal and then further reduce the remaining metal ions using the organic precipitant product.
Organic precipitant addition is controlled by the use of a stoichiometric ratio for each particular metal ion to be removed. It is strongly recommended that laboratory analysis determine the exact amount of metal ions present. The sludge formed by organic precipitants is both stable and capable of passing most E.P.A. leachate testing. Also, the quantity produced is a mere fraction of that produced by lime, and disposal therefore is less costly.
In response to the fungicidal effect of dithiocarbamates, a new generation of compounds which result in non-toxic thiocarbonate complexes is emerging as the alternative to the carbamate-based products. These alternative products exhibit all of the organic precipitant’s effectiveness while at the same time contributing their own additional benefits. One such benefit is the ability to treat small amounts of metal ions (less than ten parts per million ppm) to trace amounts as a final polisher. Carbamate-based products, on the other hand, normally need metal ions in excess of ten ppm to establish a driving mechanism. Both products produce a fine dense precipitate, which requires the use of a coagulant and flocculant to aid in formation and settling of floc particles.
Step 2: The Use of Specialty Polymers to Aid Metals Removal
The use of coagulants and flocculants to enhance floc formation and settling in the area of metal removal, including hydroxide precipitation, is both well-known and well-documented. Coagulants help to agglomerate small metal precipitates into larger microflocs that are readily separated from solution.
Until recently, coagulants were either inorganic salts or organic polymers, each of which had its own limitations. The recent introduction of specialty blended polymers should be of particular interest to the metal finisher. These products are synergistic blends of organic polymers with inorganic coagulants, which result in may unique properties conducive to the treatment of metal wastes.
Coagulation
Coagulants are cationically (positively) charged formulations whose performance is described by two basic physical mechanisms: charge neutralization and bridging by molecular weight. Charge neutralization is the process that occurs when the cationic coagulants are combined with the anionically (negative) charged matter in suspended wastewater.
The molecular weight of the compound is a function of the number of repeating monomer units. High molecular weight helps in floc formation by bridging smaller flocs together into larger, more settable sizes. Organic coagulants possess higher molecular weights as compared to their inorganic counterparts.
In the past, aluminum in the form of aluminum sulfate or polyaluminum chloride has proven to be effective as an aid in extracting soluble metal ions, but by itself is pH-dependent and has a tendency to form a voluminous sludge. The use of a specialty blend product greatly increases the effective range of treatment. Specialty blends are affected by pH to a much lesser degree. The also provide a higher molecular weight, which produces a dense compact floc more favorable for treatment.
It is due to these innovative products that carbamate and thiocarbonate products are able to remove metals to trace amounts at a comparable cost to older methods.
Step 3: Flocculants
The final step in the process of metals treatment is the separation of the precipitated flocs from the discharge stream. This is accomplished by a separation device, such as a settling or an upflow lamella-type clarifier. The key to effective treatment is how rapidly the flocs are able to settle and how efficiently the fines are removed from the effluent. This is where a high molecular weight flocculant is utilized. These products are normally nonionic (no charge) to highly anionic in nature, and bridge the microflocs formed by the cationic coagulants into larger microflocs. The effects of the ultra-high molecular weight are evident in extremely rapid settling times, coupled with the removal of small fines from the overflow discharge water.
*Figure 1 to be added soon.
Figure 1. Metal Solubility as a function of pH.
Joesph McDonald is a senior product specialists (polymers) with the Drew Industrial Division of Ashland Chemical Company, Boonton, NJ
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