Metal finishing industry
4.1 metAL FInISHInG InDUStrY .1 overview
4.1.4 Wastewater characterisation
Due to a wide range of process activities and the great variety of chemicals and materials used in the unit operations, there is a long list of pollutants existing in the metal finishing industry wastewaters. The list covers metals, cyanide, organic wastes, total suspended solids, and other inorganic pollutants such as sulphate, chloride, phosphates, fluoride and ammonia (US EPA 1995; EC 2006).
Common and precious metals are important groups originating from the metal finishing industry facilities. These groups include over thirty metals.
These metals are classified with respect to their toxic effects on the environment and/or human health. Some of them (e.g., iron, magnesium, manganese, and titanium) are sorted as non-toxic and are not regulated as they are effectively treated or recovered by suitable processes. The metals having adverse effects are classed as toxic and are regulated by imposing stringent discharge limitations.
Depending on their speciation cadmium, lead, nickel, hexavalent chromium, copper, and zinc are of most concern for their environmental and/or health effects. Therefore, their uses in the industry have been restricted by European
Commission (EC) regulations (EC 2006). Addition to these six metals, silver has also been recognised and regulated as a toxic metal existing at high concentrations in the metal finishing industry wastewaters by US EPA (1983;
1984).
Cyanide is a well-known hazardous compound but is commonly used as a complexing agent for cadmium and precious metals plating and for other solutions such as copper and zinc baths. In the form of strongly complexed metal such as nickel cyanide complex species, cyanide creates several treatment problems. As cyanide concentrations may reach several thousand mg/L in stream segregated flows of the metal finishing industry, this pollutant is regulated by EC and US EPA.Organic wastes resulting from the metal finishing industry operations vary in a wide spectrum and are principally classified into two main groups: toxic organics, and oil and grease. The origins of toxic organics are complicated.
Hence, for ease of evaluation, they can be sorted out into three groups: solvents;
complexing agents; and surfactants. Some degreasing methods necessitate organic solvent usage. These applications are the primary sources of priority pollutants in metal finishing industry. Since spent organic solvents contain priority pollutants, they are segregated to haul or reclaim on-site (US EPA 1984). Therefore, their controls rely on in-plant control practices. Complexing agents such as ethylene diamine tetraacetic acid (EDTA), tartaric acid, nitrilo triacetic acid (NTA) etc. are commonly used in the plating baths. They retain metals in solution as complexes. Some of them such as EDTA have low biodegradability. As the function of complexing agents is to prevent metals from separating out of solution, the flows containing complexed metals necessitate specialised control and treatment processes. Hence, these flows are conceived as a separate group on the basis of metal removal processes. Surfactants are also widely used in many operations such as degreasing, wetting metal surfaces and assisting other operations, and as brighteners by promoting finely divided metal deposition employed in metal finishing industry (EC 2006). Some surfactants have low biodegradability and their biodegradation by-products may have adverse effects. Owing to the bio-accumulative and toxic characteristics, some surfactants such as nonyl phenyl ethoxylate have been listed as priority pollutants and their usages are banned by EC regulations (EC 2006). Oil and grease is a traditional pollutant parameter causing several aesthetic and material transport problems in receiving media and having toxic effects on aquatic organisms. As oil and grease is found at significant concentrations in the metal finishing wastewaters, it has been commonly regulated. Total suspended solids is a conventional parameter used for the control of organic and inorganic toxic
pollutant discharges as well as the performance of the treatment. Hence, this parameter has also been limited by US EPA and EC regulations (US EPA 1983;
EC 2006).
The main pollutant groups most commonly present in wastewaters generated by the unit operations employed in the metal finishing industry are tabulated in Table 4.3 (US EPA 1983). The table includes thirty unit operations bearing more than one pollutant groups. The other unit operations which are not covered in the table, generate only common metals bearing wastewaters.
Flow segregation is the basic approach used for the treatment, control and management of the metal finishing industry wastewater. It serves to conduct an appropriate treatment application for each class of waste adequately.
The main reason for segregation is that combination of some waste classes may result in failure of some of the treatment applications. Furthermore, the mixture of wastewater groups brings about health hazards and economic losses. Within this context, seven waste groups corresponding to wastewater streams have been defined by US EPA on the basis of characters and treatment requirements of wastewaters (US EPA 1983; 1984). These groups are given as follows:
· common metals;
· precious metals;
· complexed metals;
· hexavalent chromium;
· cyanide;
· oils;
· solvents.
The main components of the common metals bearing wastewaters are the metals and acids. Metal concentrations present in the flows vary from several mg/L to several hundred mg/L.
The major constituents of precious metals bearing wastewaters are gold and silver. They are much more commonly used than other precious metals such as palladium, platinum, osmium, ruthenium, and rhodium in the metal finishing industry facilities. Due to the high cost of precious metals, the precious metals bearing flows are wasted only after efficient recovery applications. In the recovered flows, gold concentration reduces in a few mg/L while silver concentration may reach as high as one hundred mg/L.
table 4.3 Waste characteristic distribution by unit operation (US EPA 1983)
Potential wastewater pollutants ® Unit operations
↓
Common metals Precious metals Complexed metals Chromium (VI) Cyanide Oils Solvents
Electroplating √ √ √ √
Electroless plating √ √ √ √
Anodising √ √ √ √
Conversion coating √ √ √ √
Etching (Chemical Milling) √ √ √ √ √
Cleaning √ √ √ √ √ √ √
Machining √ √
Grinding √ √ √
Polishing √ √
Tumbling √ √ √ √
Burnishing √ √ √ √
Impact Deformation √ √
Pressure Deformation √ √
Shearing √ √
Heat Treating √ √ √
Thermal Cutting √ √
Abrasive Jet Machining √ √
Electrical discharge machining √ √
Electrochemical machining √ √ √
Salt Bath Descaling √ √
Solvent Degreasing √ √
Paint Stripping √ √ √
Painting √ √ √
Electrostatic Painting √ √ √
Electropainting √ √ √
Assembly √ √ √
Calibration √ √
Testing √ √
Mechanical Plating √ √
Printed Circuit Board Manufacture √ √ √ √ √ √ √
Complexed metals bearing flows are generated from a number of unit operations employed in the metal finishing industry. The main sources of these flows are the electroless and immersion plating baths and their subsequent rinses.
Nickel, copper and tin are the most commonly used metals in these operations.
The complexed metals bearing wastewaters must be segregated and treated so as to prevent further complexing of metals present in the free form in the other flows (US EPA 1983).
Wastewaters with high cyanide concentrations commonly originate in electroplating and heat treatment operations. Many other unit operations, as listed in Table 4.3, contribute to cyanide wastes. Cyanide concentrations may reach up to 500 mg/L. The flows containing cyanide are subjected to stream segregation for treatment before being combined with other wastewaters.
Hexavalent chromium bearing flows are generated in many unit operations such as electroplating, anodising, conversion coating, and acid treatment. The hexavalent chromium may be present at high concentrations in these flows.
Owing to its high toxicity, hexavalent chromium bearing flows are strictly segregated and undergone separate pretreatment applications.
In the metal finishing industry, oily wastewater flows consist of a mixture of free and emulsified oils, grease, and other assorted organics. These flows are characterised as concentrated and diluted streams. Oil concentrations may reach several hundred thousand mg/L in the concentrated flows. The selection of treatment methods of oily wastewaters is based on the concentration levels of the wastes and their physical state as well.
Since they are classified as hazardous wastes by EC and US EPA regulations, spent solvents are separately collected to reclaim or dispose off as a hazardous waste. Therefore, they are not considered as a wastewater flow.
4.1.5 In-plant control and clean technologies
US EPA has described pollution prevention as any practice which minimises the amount of any hazardous substance, pollutant, or contaminant entering the waste stream or otherwise released to the environment prior to recycling, treatment, or disposal, and reduces or eliminates the hazards to public health and the environment (US EPA 1992; 1997; 2003). The issue of the integrated pollution prevention and control (IPPC) for the metal finishing industry has been also well defined in “Best Available Techniques (BAT-EC) Reference Document (BREF), Surface Treatment of Metals and Plastics (STM)” by EC (2006). Similar to those of US EPA, the implementations of IPPC described in BREF for STM encompass the effective management systems, efficient raw material, energy and water usage, the substitution by less harmful substance, as well as minimisation, recovery and recycling of wastewater and wastes (EC 2006).
The pollution prevention applications are based on source control. Source control is a process of reducing the pollution and health hazards that originate from a certain production by taking appropriate measures from the selection of production method to the good operating practices. Every effort for pollution prevention ultimately, at least legally, targets to avoid banned applications, to satisfy occupational health measures, and to fulfil discharge standards for water pollution control, within the economical constraints of the facilities and the market (Tünay et al. 2004).
Waste reduction practices include several activities such as training, awareness and participation of the employees, the improvement of process schedules, and the improvement of procedures used in the processes. The specific approaches proposed for the waste minimisation for the metal finishing industry facilities encompass source reduction and recycling/resource recovery (US EPA 1992;
EC 2006). Source reduction technologies are designed to minimise the amount of waste initially generated. Many source reduction practices necessitate only simple housekeeping changes or minor in-plant process modification.
Source reduction techniques are also applied to process baths and rinsing operations. At the process bath level, source reduction can be realised by material substitution, extending bath life, and drag-out reduction. Within this context, the substitution of toxic plating solution with non or less toxic constituents are the recommended practices. The replacement of hexavalent chromium with trivalent chromium is a good example of the material substitution.
In so doing, drag-out is minimised and an extra treatment step necessary to reduce the chromium from hexavalent to trivalent state before precipitation is eliminated. Similar to hexavalent chromium replacement, the use of non-cyanide plating baths omits non-cyanide oxidation step. The substitutions of EDTA and the other strong complexing agents with weaker and biodegradable ones such as those based on gluconic acid are also recommended as BAT-EC to avoid their usage and to eliminate the extra treatment step required for their removals. Rinse water reduction can be achieved by improving rinse efficiency, and rinse water flow control. Spray rinsing, rinse water agitation, increased contact time, rinse elimination, and counter flow multiple tank rinsing can be used to minimise the volume of rinse water and to enhance the rinsing system efficiency.
Resource recovery and recycling technologies either directly use waste from one process as raw materials for another process or recover valuable materials from a waste stream before disposal (US EPA 1992; EC 2006). Some spent process baths and much more rinse water can be reused in other operations, and process ingredients can be recovered from rinse water in order to return to process baths
or to sell. Multiple-use rinse water is a common reuse application. After rinse water becomes too contaminated for its original purpose, it may be used for other rinse processes. Rinse water can be recycled in a closed loop or open loop system.
While treated effluent is returned to the rinse system in the closed loop operation, it is also reused in the rinse system in open loop operation but the rinsing is completed by final rinse being fed by fresh water to ensure high quality rinsing.
Metal and their salts can also be recovered from spent baths and rinse water by suitable treatment methods in order to return to baths as makeup and to sell or return to suppliers. Evaporation, reverse osmosis, ion exchange, electrolytic recovery and electrodialysis can be applied individually or in combination for the recovery of metals and their salts.
4.1.6 Conventional end-of-pipe wastewater treatment As mentioned in Subsection 4.1.4, general treatment approach for the wastewaters originating from the metal finishing facilities is commonly based on the flow segregation. The flow segregation allows the reduction of hexavalent chromium, the destruction of cyanide, the recovery of precious metals, and the removal and/or recovery of oils prior to the removal of common metals. Stream segregation minimises the flow rate of wastewater to be treated in each component and accordingly reduces the cost of these pretreatment applications. A general flow segregation and wastewater treatment scheme are depicted in Figure 4.1 (US EPA 1983).
The segregated flows are subjected to their specific pretreatment applications before combining with the other wastewaters for end-of-pipe treatment. As shown in Figure 4.1, complexed metals bearing wastewaters are segregated and separately treated, solvents are hauled or claimed. The segregated flows containing more than one pollutant such as cyanide and hexavalent chromium bearing streams, and oily wastewaters receive the specialised treatment before passing on to common metal removal. A typical treatment scheme is terminated by metal removal units for the metal finishing industry wastewaters.
Physical and physico-chemical treatment methods based on chemical precipitation, oxidation and separation technologies are commonly used for the treatment of the metal finishing industry wastewater. Table 4.4 highlights the potential treatment methods applied to segregated flows, and covers resource recovery/recycling techniques used in-plant control (updated upon US EPA (1983)). This table also includes promising novel techniques compiled from the related literature.
Figure 4.1 A typical wastewater treatment scheme based on flow segregation for the metal finishing industry
“The best available technologies economically achievable (BAT-US)” have been described by US EPA for the treatment and control of process wastewater produced in the metal finishing industry facilities (US EPA 1983). The factors taken into consideration in assessing BAT-US cover the cost of achieving BAT-US effluent reductions, the age of equipment and facilities involved, the processes employed, potential process changes, and non-water quality environmental impacts including energy requirements.
In BAT-US definition for the metal finishing industry, treatment systems are described under three options. “Option 1” includes emulsion breaking and oils separation by skimming for oily flows, cyanide oxidation, hexavalent chromium reduction prior to hydroxide precipitation and sedimentation for common metal removal; complexed metals bearing flow segregation and separate treatment by high pH hydroxide precipitation and sedimentation; and sludge treatment by drying beds. “Option 2” treatment system encompasses all of “Option 1” together with polishing filtration. “Option 3” treatment system covers “Option 1” end-of-pipe treatment system plus the addition of in-plant controls for cadmium
table 4.4 Potential treatment methods for segregated flows
Potential wastewater pollutants ®
Treatment methods ↓
Common metals Precious metals Complexed metals Chromium (VI) Cyanide Oils Solvents Recyclying/recovery
Carbon adsorption √ √ √ √ √
Centrifugation √ √
Chemical reduction √ √
Coalescing √
Diatomaceous earth filtration √ √ √ √
Electrocoagulation √ √
Electrochemical oxidation √
Electrochemical reduction √
Electrochemical regeneration √ √
Electrodialysis √ √
Electrolytic recovery √ √ √
Emulsion breaking √
Evaporation √ √ √ √ √
Flotation √ √
Granular bed filtration √ √ √
High pH precipitation √
Hydroxide precipitation √ √ √
Insoluble starch xanthate √ √ √
Ion exchange √ √ √ √ √
Membrane filtration √ √ √
Oxidation by Cl2 or H2O2 or O3 √
Oxidation by O3/UV or H2O2/UV √
Peat adsorption √ √ √
Pressure filtration √ √ √ √
Resin adsorption √ √
Reverse osmosis √ √ √ √ √ √
Sedimentation √ √ √
Skimming √
Sulphide precipitation √ √ √ √
Ultrafiltration √ √ √ √ √ √
including evaporative recovery, ion exchange, and recovery rinses to minimise toxicity. The similar treatment systems for process wastewaters and recovery/
recycling techniques used for in-plant control practices are also defined in BREF
published for STM by EC (EC 2006). The BAT treatment systems recommended by both US EPA and EC comprise the unit processes that are proven and effective techniques such as precipitation, filtration, and sedimentation. Information about these unit processes is extensive and readily accessible in the related literature (EC 2006; US EPA 1983; 1984; 1997; 2003; Tünay et al. 2004) and need not to be reiterated here, since the intention of this book is to provide more detailed information about the oxidation and reduction processes used in treatment of the metal finishing industry wastewaters. Therefore, the oxidation and reduction processes are introduced and discussed in more detail in the following subsections.
4.2 oxIDAtIon AnD reDUCtIon ProCeSSeS
As mentioned in the previous subsections, the oxidation and reduction processes are commonly used for the treatment and recovery/recycling practices of the metal finishing industry wastes. Since implementations of these processes are strongly related to pollutant type to be treated or recovered/recycled, these processes are introduced and discussed on the basis of pollutant type in this chapter. Within this context; the pollutants to be treated by oxidation or reduction process are classified as follows:
· cyanide;
· hexavalent chromium;
· complexed metals;
· precious metals;
In the following subsections, special emphasis is given to well-proven and common treatment methods in terms of their efficiencies, design criteria, and costs, while promising novel techniques are introduced considering their basic features.
4.2.1 Cyanide destruction