Lawrence K Wang Nazih K Shammas Donald B Aulenbach and William A Selke

CONTENTS

6.1 Introduction 232

6.2 The Nickel-Chromium Plating Process 232

6.2.1 Nickel Plating 232

6.2.2 Chromium Plating 233

6.3 Sources of Pollution 233

6.3.1 Environmental Impact of Nickel 234

6.3.2 Environmental Impact of Chromium 234

6.4 Waste Minimization 235

6.4.1 Assessment of Hazardous Waste 235

6.4.2 Improved Procedures and Segregation of Wastes 235

6.4.3 Material Substitution 236

6.4.4 Extending Process Bath Life 236

6.4.5 Dragout Reduction 236

6.4.6 Reactive Rinses 237

6.5 Material Recovery and Recycling 237

6.5.1 Dragout Recovery 237

6.5.2 Evaporative Recovery 238

6.5.3 Reverse Osmosis 238

6.5.4 Ion Exchange 239

6.5.5 Electrodialysis 239

6.5.6 Electrolytic Recovery 240

6.5.7 Deionized Water 240

6.6 Chemical Treatment 240

6.6.1 Neutralization 240

6.6.2 Hexavalent Chromium Reduction 241

6.6.3 pH Adjustment and Hydroxide Precipitation 243

6.6.4 Reduction and Flotation Combination 245

6.7 Conventional Reduction-Precipitation System 246

6.8 Modified Reduction-Flotation Wastewater Treatment System 247

6.9 Innovative Flotation-Filtration Wastewater Treatment Systems 249

6.9.1 Flotation-Filtration System Using Conventional Chemicals 249

6.9.2 Flotation-Filtration System Using Innovative Chemicals 249

6.9.3 Flotation-Filtration Systems 250

6.10 Summary 253

References 255

6.1 INTRODUCTION

Applicable local, state, and federal environmental laws require that the waste generated by the nickel-chromium plating process be pretreated to provide a discharge acceptable to the public wastewater treatment system.

The specific purpose of this chapter is to describe the chemical and physical pretreatment methods required for nickel-chromium plating wastewater, to describe the upgrades needed by a municipal wastewater treatment system to manage this waste, and to relate the methods and upgrades to the operation of the total treatment system. Special emphasis is placed on presentation of the following:

1. The chemistry of nickel-chromium plating and waste generation

2. The type of pollutants and their sources

3. Waste minimization

4. Recovery and recycling

5. Conventional reduction-precipitation treatment systems

6. Modified reduction-flotation treatment systems

7. Innovative flotation-filtration treatment systems

6.2 THE NICKEL-CHROMIUM PLATING PROCESS

The nickel-chromium plating process includes the steps in which a ferrous base material is electroplated with nickel and chromium. The electroplating operations for plating the two metals are basically oxidation-reduction reactions. Typically, the part to be plated is the cathode, and the plating metal is the anode.

6.2.1 Nickel Plating

To plate nickel on iron parts, the iron parts form the cathodes, and the anode is a nickel bar. On the application of an electric current, the nickel bar anode oxidizes, dissolving in the electrolyte:

The resulting nickel ions are reduced at the cathode (the iron part) to form a nickel plate:

Nickel plating can also be accomplished by an electroless plating technique involving deposition of a metallic coating by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited. A special feature of electroless plating is that no external electrical energy is required. The following are the basic ingredients in electroless plating solutions:

1. A source of metal, usually a salt

2. A reducer to reduce the metal to its base state

3. A chelating agent to hold the metal in solution so the metal will not plate out indiscriminately

4. Various buffers and other chemicals designed to maintain stability and increase bath life

Nickel electroless plating on a less noble metal is common.1-7 For example, the source of nickel can be nickel sulfate. The reducer can be an organic substance, such as formaldehyde. A chelating agent (tartrate or equivalent) is generally required. The nickel salt is ionized in water:

There is then a redox reaction with the nickel and the formaldehyde:

The base metal nickel now begins to plate out on an appropriate surface, such as a less noble metal.

6.2.2 Chromium Plating

In chromium plating, the chromium is supplied to the plating baths as chromic acid. For example, plating baths can be prepared by adding hexavalent chromium in the form of either sodium dichromate (Na2Cr2O7 • H2O) or chromium trioxide (CrO3). When sodium dichromate is used it dissociates to produce the divalent dichromate ion (Cr2O^-). When chromium trioxide is used, it immediately dissolves in water to form chromic acid according to the following reaction8-15:

Chromic acid is considered a strong acid, although it never completely ionizes. Its ionization has been described as follows:

Moreover, the dichromate ion (Cr2O^-) will exist in equilibrium with the acid chromate ion as follows:

Theoretically, HCrO- is the predominant species between pH 1.5 and 4.0, HCrO- and CrO4- exist in equal amounts at pH 6.5, and CrO^- predominates at higher pH values. Chromium plating wastewater is generally somewhat acid, and the acid chromate ion HCrO- is predominant in this wastewater.

Chromating is one of the chemical conversion coating technologies. Chrome coatings are applied to previously deposited nickel for increased corrosion protection and to improve surface appearance. Chromate conversion coatings are formed by immersing the metal in an aqueous acidified chromate solution consisting substantially of chromic acid or water-soluble salts of chromic acid, together with various catalysts or activators.

6.3 SOURCES OF POLLUTION

A conceptual arrangement of the nickel-chromium plating process can be broken down into three general steps:

1. Surface preparation involving the conditioning of the base material for plating

2. Actual application of the plate by electroplating

3. The posttreatment steps

The major waste sources during normal nickel-chromium plating operations are alkaline cleaners, acid cleaners, plating baths, posttreatment baths, and auxiliary operation units.

The wastestreams generated by the plating process can be subdivided and classified into eight categories1-5'6-15:

1. Concentrated acid wastes

2. Concentrated phosphate cleaner wastes

3. Acid rinsewater

4. Alkaline rinsewater

5. Chromium rinsewater

6. Nickel rinsewater

7. Concentrated nickel wastes

8. Concentrated chromium wastes

In the above categories, there are seven major types of aqueous pollutants that must be pretreated and removed5,15:

1. Acidity

2. Alkalinity

3. Nickel

4. Chromium

5. Iron

7. Suspended solids

The environmental impact of the two most toxic pollutants, nickel and chromium, is briefly presented in the following.11617 Significant concentrations of these elements pass through conventional treatment plants.

6.3.1 Environmental Impact of Nickel

Nickel is toxic to aquatic organisms at levels typically observed in POTW (publicly owned treatment works) effluents:

1. 50% reproductive impairment of Daphnia magna at 0.095 mg/L

2. Morphological abnormalities in developing eggs of Limnaea palustris at 0.230 mg/L

3. 50% growth inhibition of aquatic bacteria at 0.020 mg/L

Because surface water is often used as a drinking water source, nickel passed through a POTW becomes a possible drinking water contaminant.

A U.S. Environmental Protection Agency (U.S. EPA) study of 165 sludges showed nickel concentrations ranging from 2 to 3520 mg/kg (dry basis).18 Nickel toxicity may develop in plants from application of municipal wastewater biosolids on acid soils. Nickel reduces yields for a variety of crops including oats, mustard, turnips, and cabbage.

6.3.2 Environmental Impact of Chromium

Chromium can exist as either trivalent or hexavalent compounds in raw wastewater streams. The chromium that passes through the POTW is discharged to ambient surface water. Chromium is toxic to aquatic organisms at levels observed in POTW effluents15:

1. Trivalent chromium significantly impaired the reproduction of Daphnia magna at levels of 0.3 to 0.5 mg/L.

2. Hexavalent chromium retards growth of chinook salmon at 0.0002mg/L. Hexavalent chromium is also corrosive and a potent human skin sensitizer.

Besides providing an environment for aquatic organisms, surface water is often used as a source of drinking water. The National Primary Drinking Water Standards are based on total chromium, the limit being 0.1 mg/L.19

A U.S. EPA study of 180 municipal wastewater sludges showed that municipal wastewater sludge contains 10 to 99,000 mg/kg (dry basis) of chromium. Most crops absorb relatively little chromium even when it is present in high levels in soils, but chromium in sludge has been shown to reduce crop yields in concentrations as low as 200 mg/kg.18

6.4 WASTE MINIMIZATION

All metal finishing facilities have one thing in common—the generation of metal-containing hazardous waste from the production processes. Reducing the volume of waste generated can save money and at the same time decreases future liabilities. Typical wastes generated are as follows:

1. Industrial wastewater and treatment residues

2. Spent plating baths

3. Spent process baths

4. Spent cleaners

5. Waste solvents and oil

This section identifies areas for reducing waste generation. It also suggests techniques available to metal finishers for waste reduction and is intended to help metal finishing shop owners decide whether waste reduction is a possibility.

Both state (Health and Safety Code) and federal (40 CFR, Part 262, Subpart D) regulations require that generators of hazardous waste file a biennial generator's report. Among other things, this report must include a description of the efforts undertaken and achievements accomplished during the reporting period to reduce the volume and toxicity of waste generated. The Uniform Hazardous Waste Manifest requires that large generators certify that they have a program in place to reduce the volume and toxicity of waste generated that is determined to be economically practicable. Small-quantity generators must certify that they have made a good faith effort to minimize waste generation and have selected the best affordable waste management method available.

As waste reduction methods reduce the amount of waste generated, and also the amount subject to regulation, these practices can help a shop comply with the requirements while also saving money. The shop's owner or manager must be committed to waste reduction and pass that commitment on to the employees, establish training for employees in waste reduction, hazardous material handling and emergency response, and establish incentive programs to encourage employees to design and use new waste reduction ideas. The following is a list of some common waste reduction methods for metal finishing electroplating shops.20,21

6.4.1 Assessment of Hazardous Waste

Waste assessments are used to list the sources, types, and amounts of hazardous waste generated to make it easier to pinpoint where wastes can be reduced.

Source reduction is usually the least expensive approach to minimizing waste. Many of these techniques involve housekeeping changes or minor inplant process modifications.

6.4.2 Improved Procedures and Segregation of Wastes

These may be summarized as follows:

1. Good housekeeping is the easiest and often the cheapest way to reduce waste. Keep work areas clean.

2. Improve inventory procedures to reduce the amount of off-specification materials generated.

3. Reduce quantities of raw materials to levels where materials will be used up just as new materials are arriving.

4. Designate protected raw material and hazardous waste storage areas with spill containment. Keep the areas clean and organized and give one person the responsibility for maintaining the areas.

5. Label containers as required and cover them to prevent contact with rainfall and avoid spills.

6. Use a "first-in, first-out" policy for raw materials to keep them from becoming too old to be used. Give one person responsibility for maintaining and distributing raw materials.

7. Use bench-scale testing for samples rather than process baths.

8. Designate one person to accept chemical samples and return unused samples to suppliers.

9. Limit bath mixing to trained personnel.

10. Segregate wastestreams for recycling and treatment, and keep nonhazardous material from becoming contaminated.

11. Prevent and contain spills and leaks by installing drip trays and splash guards around processing equipment.

12. Conduct periodic inspections of tanks, tank liners, and other equipment to avoid failures. Repair malfunctions when they are discovered. Use inspection logs to follow up on repairs.

13. Inspect plating racks for loose insulation that would cause increased dragout.

14. Use dry cleanup where possible to reduce the volume of wastewater.

6.4.3 Material Substitution

In summary:

1. Use process chemistries that are treatable or recyclable on site.

2. Use deionized water instead of tap water in process baths or rinsing operations to reduce chemical reactions with impurities in the tap water, which would increase sludge production.

3. Use nonchelated process chemistries rather than chelated chemistries to reduce sludge volume.

4. Replace cyanide process baths with noncyanide process baths to simplify the treatment required.

5. Use alkaline cleaners instead of solvents for degreasing operations; they can be treated on site and usually discharged to the sewer with permit authorization.

6.4.4 Extending Process Bath Life

This may be achieved with the following procedures:

1. Treatment of process baths can extend their useful life.

2. Bath replenishment extends the useful life of the bath.

3. Monitoring (using pH meters or conductivity meters) the process baths can determine the need for bath replenishment.

6.4.5 Dragout Reduction

Dragout reduction is achieved using the following steps:

1. Minimize bath concentrations to the lower end of their operating range.

2. Maximize bath operating temperatures to lower the solution's viscosity.

3. Use wetting agents (which reduce the surface tension of the solution) in process baths to decrease the amount of dragout.

4. Withdraw workpieces from tanks slowly to allow maximum drainage back into process tank.

5. Use air knives or spray rinses above process tanks to rinse excess solution off a workpiece and into the process bath.

6. Install drainage boards between process tanks and rinse tanks to direct dragout back into process tank.

7. Use dedicated dragout tanks after process baths to capture dragout.

8. Install rails above process tanks to hang workpiece racks for drainage prior to rinsing.

9. Use spray rinses as the initial rinse after the process tank and before the dip tank.

10. Use air agitation or workpiece agitation to improve rinse efficiency.

11. Install multiple rinse tanks (including counterflow rinse tanks) after process baths to improve rinse efficiency and reduce water consumption.

6.4.6 Reactive Rinses

The following steps should be applied:

1. Reuse the acid rinse effluent as influent for the alkaline rinse tank, thus allowing the fresh water feed to the alkaline rinse tank to be turned off (reactive rinsing). This can also be applied to process tank rinses.

2. Treat rinsewater effluent to recover process bath chemicals. This allows the reuse of the effluent for rinsing or neutralization prior to discharge.

3. Reuse the spent reagents from the process baths in the wastewater treatment process.

4. Recycle spent solvents on site or off site.

5. Use treatment technologies to recycle rinsewaters in a closed loop or open loop system.

6. Some recycling and most treatment processes require a permit. Be sure to contact the local Department of Health Services regional office to determine if there is a need for a permit to treat or recycle the wastes.

7. Pretreat process water to reduce the natural contaminants that contribute to the sludge volume.

8. Use treatment chemicals that reduce sludge generation (e.g., caustic soda instead of lime).

9. Use sludge dewatering equipment to reduce sludge volume.

10. Use treatment technologies (such as ion exchange, evaporation, and electrolytic metal recovery) that do not use standard precipitation/clarification methods that generate heavy metal sludges.

6.5 MATERIAL RECOVERY AND RECYCLING

Unlike the 1970s and 1980s when waste management costs were relatively inexpensive, today's metal finishers are facing increasingly higher disposal costs. This change is due in part to a decrease in the volume of available landfill space, which has resulted in escalating landfill fees and more stringent federal and state environmental regulations that mandate treatment prior to landfilling.

Metal finishers are seeing their profits shrink as waste management costs increase. To control waste disposal costs, metal finishers must focus on developing and implementing a facility-wide waste reduction program. In other words, as discussed in Section 6.4, metal finishers must consciously seek out ways to decrease the volume of waste that they generate.

One approach to waste reduction is to recover process materials for reuse. Materials used in metal finishing processes can be effectively recovered using available technologies such as dragout, evaporation, reverse osmosis, ion exchange, electrodialysis, and electrolytic recovery.22-26

6.5.1 Dragout Recovery

Dragout recovery is a simple technology used by metal finishers to recover plating chemicals. It involves using drain boards, drip tanks, fog-spray tanks, or dragout tanks separately, or in combination, to capture plating chemicals dragged out of plating tanks from parts being plated.

Drain boards are widely used throughout the metal industry to capture plating solutions. Boards are suspended between process tanks and are constructed of plastic, plain or teflon-coated steel. Solutions drip on the boards and drain back into their respective processing tanks.2227

In contrast, a drip tank recovers process chemicals by collecting dragout into a separate tank, from which it can be returned to the process as needed.

In a fog-spray tank, plating chemicals clinging to parts are recovered by washing them with a fine water-mist. The solution that collects in the fog-spray tank is returned to the process tank as needed. The added water helps to offset evaporative losses from the process tanks.

Dragout tanks are essentially rinse tanks. Dragout chemicals are captured in a water solution, which is returned to the process tank as needed.

The presence of airborne particles and other contaminants in recovered plating chemicals may necessitate treatment of the collected solution to remove the contaminants prior to solution reuse.

There are advantages and disadvantages to dragout recovery. Depending upon the solution, up to 60% of the materials carried out of a plating tank can be recovered for reuse; thus dragout can affect metal deposition and surface finish quality. Impurities can concentrate in the solutions causing a deteriorating effect on the plating process when returned to the plating bath.

6.5.2 Evaporative Recovery

A widely used metal salt recovery technique is evaporation. With evaporation, plating chemicals are concentrated by evaporating water from the solution. Evaporators may use heat or natural evaporation to remove water.2228 Additionally, evaporators may operate at atmospheric pressure or under vacuum.

Atmospheric evaporators are more commonly used. They are open systems that use process heat and warm air to evaporate water. These evaporators are relatively inexpensive, require low maintenance and are self-operating. Under the right conditions, they can evaporate water from virtually any plating bath or rinse. A packed-bed evaporator is an example of an atmospheric evaporator.

Vacuum evaporators are also used to recover plating chemicals. They are closed systems that use steam heat to evaporate water under a vacuum. This results in lower boiling temperature, with a reduction in thermal degradation of the solution. Like atmospheric evaporators, they require low maintenance and are self-operating. A climbing file evaporator is an example of a vacuum evaporator.

A typical evaporative recovery system consists of an evaporator, a feed pump, and a heat exchanger. Plating solution or rinsewater containing dilute plating chemicals is circulated through the evaporator. The water evaporates and concentrates the plating chemicals for reuse. In open evaporator systems, the water evaporates and mixes with air and is released to the atmosphere. It may be necessary to vent the contaminated airstream to a ventilation/scrubber treatment system prior to release. In enclosed evaporators the water is condensed from the air and can be reused in rinses, which further increases savings. Water reuse is preferred whenever possible.

As with all process equipment, the design size of an evaporator system is dependent upon volumetric flow, specifically the rinsewater flow rate required and the volume of process solution dragout. When operated properly, a commercial evaporator can attain a 99% material recovery rate.

There are drawbacks to using an evaporator to recover plating chemicals. For instance, impurities are concentrated along with recovered plating chemicals. These impurities can alter desired deposited metal characteristics, including surface finish quality. Vacuum evaporation can be used to avoid degradation of plating solutions containing additives that are sensitive to heat.

The evaporative recovery is a very energy-intensive process. Approximately 538 chu (970 Btu) are required to evaporate 1 lb of water at standard atmospheric pressure. Additional energy is required to raise the temperature of the solution to its boiling point.

6.5.3 Reverse Osmosis

Reverse osmosis (RO) recovers plating chemicals from plating rinsewater by removing water molecules with a semipermeable membrane. The membrane allows water molecules to pass through, but blocks metallic salts and additives.29

Like evaporators, RO works on most plating baths and rinse tanks. Most RO systems consist of a housing that contains a membrane and feed pump. There are four basic membrane designs: plate-and-frame, spiral-wound, tubular, and hollow-fiber. The most common types of membrane materials are cellulose acetate, polyether/amide, and polysulfones.29

Diluted or concentrated rinsewaters are circulated through the membrane at pressures greater than aqueous osmotic pressure. This action results in the separation of water from the plating chemicals. The recovered chemicals can be returned to the plating bath for reuse, and the permeate, which is similar to the condensate from an evaporator, can be used as make-up water. RO units work best on dilute solutions.30

The design and capacity of an RO unit is dependent upon the type of chemicals in the plating solution and the dragout solution rate. Certain chemicals require specific membranes. For instance, polyamide membranes work best on zinc chloride and nickel baths, and polyether/amide membranes are suggested for chromic acid and acid copper solutions. The flow rate across the membrane is very important. It should be set at a rate to obtain maximum product recovery. RO systems have a 95% recovery rate with some materials and with optimum membrane selection.22

There are advantages to using RO. Energy usage is much lower than for other recovery systems and plating chemicals can be recovered from temperature-sensitive solutions. However, RO also has limitations. The membrane is susceptible to fouling, which is often caused by the precipitation of suspended and dissolved solids that plug the membrane's pores. Also, as with evaporators, RO can concentrate impurities along with plating chemicals, which degrade plating quality.

6.5.4 Ion Exchange

Ion exchange is a molecular exchange process where metal ions in solution are removed by a chemical substitution reaction with an ion-exchange resin.31 Ion exchange can be used with most plating baths. Metal cations exchange sites with sodium or hydrogen ions and anions (such as chromate) with hydroxyl ions. The exchange resin can generally be regenerated with an acid or alkaline solution and reused. When a cation exchange resin is regenerated, it produces a metal salt. For example, copper is removed from an ion exchange resin by passing sulfuric acid over the resin, producing copper sulfate. This salt can be added directly into the plating bath.2332

The required size of an ion-exchange unit is dependent upon the composition and volume of plating dragout. Each ion-exchange resin has a maximum capacity for recovery of specific ions. The ion-exchange unit's size (volume of resin) is determined by the amount of metal to be removed from the recovered solutions.

Ion exchange has its drawbacks. Most commercially available resins are nonselective and, therefore, similarly charged ions can be exchanged by a given resin whether desired in the process or not. This means that certain contaminants cannot be removed by ion exchange and are returned to the plating tank with the metal salt.22 The metal salt solution produced after regeneration is often a dilute solution that can only be put back into the process bath if evaporation is used to make room in the process tank. In addition, ion exchange is not a continuous process and system sizing must take into account resin regeneration time.

6.5.5 Electrodialysis

Electrodialysis units recover plating chemicals differently from the recovery units discussed thus far. In electrodialysis, electromotive forces selectively drive metal ions through an ion-selective membrane (in RO, pressure is the driving force; in ion exchange, the driving force is chemical attraction). The membranes are thin sheets of plastic material with either anionic or cationic characteristics.33

Electrodialysis units are constructed using a plate-and-frame technique similar to filter presses. Alternating sheets of anionic and cationic membranes are placed between two electrodes. The plating or rinse solution to be recovered (electrolyte) circulates past the system's electrodes. Hydrogen and oxygen evolve. Positive ions travel to the negative terminal and negative ions travel to the positive terminal. The electrolyte also provides overall electrical conductivity to the cell. In some units, the current is periodically reversed to reduce membrane fouling.

Electrodialysis is compatible with most plating baths, and the design size of a unit is dependent upon the rinsewater flow rate and concentration.22

Electrodialysis has advantages and disadvantages. For instance, the process requires very little energy and can recover highly concentrated solutions. On the other hand, similarly to other membrane processes, electrodialysis membranes are susceptible to fouling and must be regularly replaced.

6.5.6 Electrolytic Recovery

Electrolytic recovery (ER) is the oldest metal recovery technique. Metal ions are plated-out of solution electrochemically by reduction at the cathode.34 There are essentially two types of cathodes used for this purpose: a conventional metal cathode and a high surface area cathode (HSAC). Both cathodes can effectively plate-out metals, such as gold, zinc, cadmium, copper, and nickel.22

Electrolytic recovery systems work best on concentrated solutions. For optimal plating efficiency, recovery tanks should be agitated ensuring that good mass transfer occurs at the electrodes. Another important factor to consider is the anode/cathode ratio. The cathode area (plating surface area) and mass transfer rate to the cathode greatly influence the efficiency of metal deposition.

Electrolytic recovery can be used with most plating baths. The amount of metal to be plated per square meter of cathode determines the electrolytic recovery unit's design capacity. Therefore, the volume and concentration of plating dragout greatly influences system design and size.2235

There are advantages to the electrolytic recovery process. For instance, ER units can operate continuously, and the product is in a metallic form that is very suitable for reuse or resale. Electrolytic units are also mechanically reliable and self-operating. Very importantly, contaminants are not recovered and returned to the plating bath. Thus, electrolytically recovered metals are as pure as "virgin" plating raw material.

The major disadvantage to electrolytic recovery is high energy cost. Energy costs will vary, of course, with cathode efficiencies and local utility rates.22

6.5.7 Deionized Water

Using deionized water to prepare plating bath solutions is an effective way of preventing waste generation. Some groundwater and surface waters contain high concentrations of calcium, magnesium, chloride, and other soluble contaminants that may build up in process baths.22 By using deionized water, buildup of these contaminants can be more easily controlled. Technologies such as RO and ion exchange can also be used to effectively remove soluble contaminants from incoming water.36

6.6 CHEMICAL TREATMENT

Treatment for the removal of chromium and nickel from electroplating wastewater involves neutralization, hexavalent chromium reduction, pH adjustment, hydroxide precipitation, and final solid-liquid separation.15,37-48

6.6.1 Neutralization

Excess acidity and alkalinity may be eliminated by simple neutralization by either a base or an acid. This is a simple stoichiometric chemical reaction of the following type5,15,49:

Examples of this include the following:

1. Alkali

Base Acid Salt Water

2. Acid

Acid Base Salt Water

A slight excess of base may be titrated in the previous reactions to shift the pH to a slight basic condition. This is important for the precipitation of certain metal salts (such as nickel, iron, and trivalent chromium) as hydroxides.

6.6.2 Hexavalent Chromium Reduction

Chemical treatment of chromium wastewater is usually conducted in two steps. In the first step hexavalent chromium is reduced to trivalent chromium by the use of a chemical reducing agent. The trivalent chromium is precipitated during the second stage of treatment.15

Sulfur dioxide (SO2), sodium bisulfite (NaHSO3), and sodium metabisulfite (Na2S2O5) are commonly used as reducing agents.15,50 All these compounds react to produce sulfurous acid when added to water, according to the following reactions:

It is the sulfurous acid produced from these reactions that is responsible for the reduction of hexavalent chromium. The reaction is shown in the following equation:

The typical amber color of the hexavalent chromium solution will turn to a pale green once the chromium has been reduced to the trivalent state. Although this color change is a good indicator, redox control is usually employed.

The theoretical amount of sulfurous acid required to reduce a given amount of chromium can be calculated from the above equation. The actual amount of sulfurous acid required to treat a wastewater will be greater than this because other compounds and ions present in the wastewater may consume some of the acid. Primary among these is dissolved oxygen, which oxidizes sulfurous acid to sulfuric acid according to the following reaction:

Each part of dissolved oxygen initially present in the wastewater produces 6.1 parts of sulfuric acid.

Undissociated sulfurous acid is responsible for the reduction of hexavalent chromium. Consequently, the reduction reaction is strongly pH-dependent because of the effect of pH on acid dissociation:

The dissociation as a function of pH and the effect of pH on reaction rate is shown in Figure 6.1 and Figure 6.2, respectively.15 Obviously, the reaction proceeds much faster at low pH values, where the concentration of undissociated sulfurous acid is highest. As a result, chromium reduction processes are generally conducted at pH values of 2 to 3 to maximize reaction rates and minimize the volume of reaction vessels. Sulfuric acid is generally added to reduce the pH of the wastewater to the desired level and to maintain it at that level throughout treatment. If the pH is not maintained at the desired level but is allowed to increase during treatment, the reaction may not go to completion in the retention time available, and unreduced hexavalent chromium may exist in the effluent. The amount of acid required to depress the pH to the level selected for chrome reduction will depend on the alkalinity of the wastewater being treated. This acid requirement can be determined by titrating a sample of wastewater with sulfuric acid to the desired pH in the absence of a reducing agent.

In addition to the sulfuric acid required for pH adjustment, some amount of acid is consumed by the reduction reaction (Equation 8.15). If sulfur dioxide is used as the reducing agent, it will provide all the acid consumed by this reaction, and additional acid will not be required. However, if sodium bisulfite or sodium metabisulfite is used, additional acid must be supplied to satisfy the acid demand. This acid requirement is stoichiometric and can be calculated from Equations 6.19 to 6.22.

3 NaHSO3 + 1.5 H2SO4 + 2 H2CrO4 ^ Cr2(SO4)3 + 1.5 Na2SO4 + 5 H2O (6.19)

1.5 Na2S2O5 + 1.5 H2SO4 + 2 H2CrO4 ^ Cr2(SO4)3 + 1.5 Na2SO4 + 3.5 H2O (6.20)

3 NaHSO3 + 2 H2SO4 + 2 H2CrO4 ^ Cr2(SO4)3 + Na2SO4 + NaHSO4 + 51 H2O (6.21) 1.5 Na2S2O5 + 2 H2SO4 + 2 H2CrO4 ^ Cr2(SO4)3 + Na2SO4 + NaHSO4 + 3.5 H2O (6.22)

FIGURE 6.1 Relationship between H2SO3 and HSO3 at various pH values. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

FIGURE 6.1 Relationship between H2SO3 and HSO3 at various pH values. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

50 45 40 35

50 45 40 35

Retention time (min)

50 100 200

FIGURE 6.2 Rate of reduction of hexavalent chromium in the presence of excess SO2 at various pH levels. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

Retention time (min)

50 100 200

FIGURE 6.2 Rate of reduction of hexavalent chromium in the presence of excess SO2 at various pH levels. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

Similar equations can be developed for pH values between 2 and 3 as a function of the SO^ and HSO- distribution.

6.6.3 pH Adjustment and Hydroxide Precipitation

Wastewater pH is adjusted by addition of an acid or an alkali, depending on the purpose of the adjustment. The most common purposes of wastewater pH adjustment are the following:

1. Chemical precipitation of dissolved heavy metals, as illustrated by Figure 6.3

2. Pretreatment of metal-bearing wastewater before sulfide precipitation so that the formation of hazardous gaseous hydrogen sulfide does not occur

3. Neutralization of wastewater before discharge to either a stream or a sanitary sewer37-48

To accomplish hydroxide precipitation, an alkaline substance such as lime or sodium hydroxide is added to the wastewater to increase the pH to the optimum range of minimum solubility at which the metal precipitates as a hydroxide51:

2 M(III)3+ + 3 Ca(OH)2 ^ 2 M(III)(OH)3 + 3 Ca2+ (6.24)

where M(II) = divalent metal and M(III) = trivalent metal.

FIGURE 6.3 Solubility of metal hydroxides and sulfides. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

FIGURE 6.3 Solubility of metal hydroxides and sulfides. (Taken from Krofta, M. and Wang, L.K., Design of Innovative Flotation-Filtration Wastewater Treatment Systems for a Nickel-Chromium Plating Plant, U.S. Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB-88-200522/AS, January 1984.)

The precipitated metal hydroxide can then be removed from the wastewater by clarification or other solid-water separation techniques.52

As a practical example, following the reduction of hexavalent chromium, sodium hydroxide, lime, or sodium hydroxide can be added to the wastewater to neutralize the pH and precipitate the trivalent chromium, nickel, iron, divalent, and other heavy metals. If lime is used, lime will react with heavy metals and with any residual sodium sulfate, sulfurous acid, or sodium bisulfite. The following reactions apply:

2 Fe2(SO4)3 + 6 Ca(OH)2 ^ 4 Fe(OH)3 + 6 CaSO4 (6.27)

Chromium hydroxide is an amphoteric compound and exhibits minimum solubility in the pH range of 7.5 to 10.0. Effluents from chromium reduction processes should be neutralized to the range of zero solubility (pH 8.5 to 9.0) to minimize the amount of soluble chromium remaining in solution.

It should be noted that if sodium hydroxide is used instead of lime, the chemical cost will be higher, less sludge will be produced, and effluent sulfate concentration will be higher.15

6.6.4 Reduction and Flotation Combination

Alternatively, hexavalent chromium can be reduced, precipitated, and floated by ferrous sulfide. By applying ferrous sulfide as a flotation aid to a plating waste with an initial hexavalent chromium concentration of 130 mg/L and total chromium concentration of 155 mg/L, an effluent quality of less than 0.05 mg/L of either chromium species can be achieved if a flotation-filtration wastewater treatment system is used.15

Ferrous sulfide acts as a reducing agent at pH 8 to 9 for reduction of hexavalent chromium and then precipitates the trivalent chromium as a hydroxide in one step without pH adjustment.5162 So, the hexavalent chromium in the nickel-chromium plating wastewater does not have to be isolated and pretreated by reduction to the trivalent form. The new process is applicable for removal of all heavy metals. All heavy metals other than chromium are removed as insoluble metal sulfides, M(II)S.

6 Fe2+ + Cr2O2- + 14 H+ ^ 2 Cr3+ + 6 Fe3+ + 7 H2O (6.34)

M(II)S, Cr(OH)3, and Fe(OH)3 are all insoluble precipitates, which can be floated by dissolved air flotation (DAF).

This new method can eliminate the potential hazard of excess sulfide in the effluent and the formation of gaseous hydrogen sulfide. In operation, the FeS is added to wastewater to supply sufficient sulfide ions to precipitate metal sulfides that have lower solubilities than FeS. Typical reactions include the following51,62:

FeS + Ni2+ ^ NiS + Fe2+

(6.37)

FeS + Zn2+ ^ ZnS + Fe2+

(6.38)

FeS + Pb2+ ^ PbS + Fe2+

(6.39)

FeS + Cd2+ ^ CdS + Fe2+

(6.40)

FeS + Cu2+ ^ CuS + Fe2+

(6.41)

FeS + 2Ag+ ^ Ag2S + Fe2+

(6.42)

Ferrous sulfide can also react with metal hydroxide to form insoluble metal sulfide:

Ferrous sulfide itself is also a relatively insoluble compound. Thus, the sulfide ion concentration is limited by its solubility, which amounts to only about 0.02 g/L, and the inherent problems associated with conventional sulfide precipitation are significantly minimized.

The newly developed flotation-filtration process involving the use of ferrous sulfide as a flotation aid offers a distinct advantage in the treatment of nickel-chromium plating wastewater that contains hexavalent chromium, nickel, iron, and other metals.

6.7 CONVENTIONAL REDUCTION-PRECIPITATION SYSTEM

A conventional system for treatment of nickel-chromium plating wastewater involves the use of the following unit processes37-48:

1. Neutralization

2. Chromium reduction

3. pH adjustment and hydroxide precipitation

4. Clarification (either sedimentation or DAF)

5. Sludge treatment (filter press and final disposal)

Figure 6.4 shows an example of an existing plating facility and its conventional reduction-precipitation wastewater treatment system in New Britain, TN.15

Initially the nickel-chromium plating process is designed to minimize the liquid loading to the waste treatment system. Counterflow rinsing, spray rinsing, and stagnant rinse recovery methods are employed in order to minimize the amount of wastes to be treated and allow as much treatment or retention time in the waste treatment system as is possible.

In the application of the previous chemical methods, a certain amount of steady-state continuity has been built into the system. To accomplish this, initial concentrated alkaline and acid rinse wastewaters are retained after dumping in the waste holding tank [T-91] (Figure 6.4) and acid chromium plating wastewater is stored in the waste holding tank [T-51]. Extremely concentrated chromium plating wastewater from rinse step No. 1 is sent to an evaporation tank [T-40] for

Polymer coagulant

Sodium hydroxide

Sulfuric acid

Sodium bisulfite

1 1 / \ 1

T-98

- T-3l

Acid-alkali wastewater sump

Chromium wastewater sump

Sewer '

Acid-alkali wastewater sump

T-3G

Hot water

Wastewater tank T-91

City water rrt]

Surge tank T-99

Filtrate

T-lGG

Sedimentation clarifier

T-lGl

Chromium wastewater sump

Filter press T102

Sludge disposal

T-2G

T-4Ü

Deionized water

Chromium evaporator

1

Sewer

Wastewater tank T-51

Hot vapor

Deionized water sv h

Ap in

To acid air control

To chromium air control

FIGURE 6.4 Conventional reduction-precipitation wastewater treatment system.

chromium recovery. In the case of the wastewater tank [T-51], the waste is slowly bled into the chromium wastewater sump [T-20] to minimize overloading of the total system. The alkaline and acid wastes in [T-.91] are neutralized and slowly bled directly to an acid-alkali wastewater sump [T-30]. It should be noted that the concentrated alkaline wastes are the result of alkaline cleaner replenishment and do not contain heavy metals.

Hexavalent chromium wastes resulting from rinsewater and the concentrated acid bleed accumulate in the chromium waste sump [T-20]. The chromium wastes are then pumped into the chromium treatment module [T-21] for reduction to the trivalent form. This pump is activated only if the oxidation-reduction potential (ORP) and pH are at the proper levels and if the level in the chromium wastewater sump [T-20] is sufficiently high.

Liquid flowing into the chromium treatment module [T-21] is monitored by a pH instrument that controls a feed pump to add the required amount of sulfuric acid from a storage tank. The sulfuric acid is needed to lower the pH to 2.0 to 2.5 for the desired reduction reaction to occur. An ORP instrument controls the injection rate of sodium metabisulfite solution from a metering pump to reduce hexavalent chromium (Cr6+) to the trivalent state (Cr3+).

The acid and alkali wastes are pumped from the acid-alkali wastewater sump [T-30] into the acid-alkali treatment module [T-31]. Metering pumps controlled by pH instruments feed either acid or caustic to the module as required to maintain an acceptable alkalinity for the formation of metal hydroxides prior to discharge to the precipitator consisting of a mixing tank [T-98], a surge tank [T-99], and a sedimentation clarifier [T-101]. The pH is adjusted to a value of 8.5 for optimum metal hydroxide formation and removal.

An ultrasonic transducer is installed on the pH probe mount in the acid-alkali treatment module [T-3l]. This prevents fouling of the electrodes and provides a more closely controlled pH in the effluent discharged to the precipitator.

The first step in the precipitator is the addition of polyelectrolyte solution in the flash mix tank [T-98], surge tank [T-99], and then into the slow mix unit [T-100] containing a variable speed mixing paddle. The purpose of this unit is to coagulate and flocculate53 the metal hydroxide precipitates.

From the slow mix unit [T-100], the waste flows into the lamellar portion of the sedimentation clarifier [T-101].5455 The lamella in the clarifier concentrates the metal hydroxide precipitates. Clarified effluent can be discharged to the sewer.

Concentrated metal hydroxide sludge is pumped from the clarifier to a polypropylene plate filter press [T-102]. The plate filter press56 is of sufficient capacity without any buildup in the lamellar portion of the unit. This also prevents any overflow of precipitate to the sewer system. The metal hydroxides form a dense sludge cake suitable for disposal in an approved landfill. The liquid effluent from the plate filter is returned to the surge tank [T-99].

A sampling station is provided on the rear exterior wall of the facility for flow measurement and monitoring of the effluent stream.

6.8 MODIFIED REDUCTION-FLOTATION WASTEWATER TREATMENT SYSTEM

A modified reduction-flotation system (Figure 6.5) is very similar to the existing conventional reduction-precipitation system (Figure 6.4), except that a DAF clarifier [T-101F] is used for clarification1557 instead of using a conventional sedimentation clarifier (Tank T-101, Figure 6.4).

The flotation system consists of four major components: air supply, pressurizing pump, air dissolving tube, and flotation chamber.5758 According to Henry's Law, the solubility of gas (such as air) in aqueous solution increases with increasing pressure. The influent feedstream can be saturated at several times atmospheric pressure, 1.8 to 6 kg/cm2 (25 to 85psig), by a pressurizing pump. The pressurized feedstream is held at this high pressure for about 0.5 min in an air dissolving tube (i.e., a pressure vessel) designed to provide sufficient time for dissolution of air into the stream to be

Polymer coagulant

Sodium hydroxide

Sulfuric acid

Sodium bisulfite

1 1

/ \

1

T-98 +

- T-3l *

- T-2l

ii n

Acid-alkali " wastewater sump T-30

Chromium wastewater sump

T-20

T-100

t 1

Surge tank T-99

Flotation clarifier

Filtrate

Filtrate

Filter press T102

Acid-alkali " wastewater sump T-30

Chromium wastewater sump

SlUdge disposal

T-20

Hot water

Wastewater tank T-91

City water n

T-40

Deionized

Chromium

water

Sewer

Wastewater tank T-51

Hot vapor

Deionized water

CN

CO

s

o

O

o

.2

£

£

£

b

c

OJ

OJ

OJ

•B M

u

US

X

1

To acid air control

To chromium air control

FIGURE 6.5 Modified reduction-precipitation wastewater treatment system.

treated. From the air dissolving tube, the stream is released back to atmospheric pressure in the flotation chamber.15,57

Most of the pressure drop occurs after a pressure reducing valve and in the transfer line between the air dissolving tube and the flotation chamber so that the turbulent effects of the depressurization can be minimized. The sudden reduction in pressure in the flotation chamber results in the release of microscopic air bubbles (average diameter 50 pm or smaller) which nucleate on suspended or colloidal particles in the process water in the flotation chamber. This results in agglomeration that, due to the entrained air, gives a net combined specific gravity less than that of water, causing the flotation phenomenon. The vertical rising rate of air bubbles ranges between 15 and 60cm/min (0.5 to 2.0 ft/min). The floated material rises to the surface of the flotation chamber to form a floated layer. Specially designed flight scrapers or other skimming devices continuously remove the floated material. The surface sludge layer can in certain cases reach a thickness of many inches and can be relatively stable for a short period. The layer thickens with time, but undue delays in removal will cause a release of particulates back to the liquid. Clarified subnatant water (effluent) is drawn off from the flotation chamber and either recovered for reuse or discharged.

The retention time in the flotation chamber is usually about 3 to 5 min, depending on the characteristics of the process water and the performance of the flotation unit. The process effectiveness depends upon the attachment of air bubbles to the particles to be removed from the process water.57 The attraction between the air bubbles and particles is primarily a result of the particle s urface charges and bubble size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation unit can be.

A high-rate DAF unit with only 3 min of retention time can treat water and wastewater at an overflow rate of 2.4 L/s/m2 (3.5 gal/min/ft2) for a single unit and up to 7.2 L/s/m2 (10.5 gal/min/ft2)

for triple stacked units. The comparison between a flotation clarifier and a settler shows the following59,60:

1. The DAF unit floor space requirement is only 15% of the settler.

2. The DAF unit volume requirement is only 5% of the settler.

3. In DAF, higher biosolids densities are obtained than in sedimentation. Even in shallow flotation clarifiers a satisfactory biosolids density is attainable.

4. The degrees of clarification of both clarifiers are the same with the same flocculating chemical addition.

5. The operational cost of the DAF clarifier is slightly higher than that for the settler, but this is offset by the considerably lower cost of the installation's financing.

6. DAF clarifiers are mainly prefabricated in stainless steel for erection cost reduction, corrosion control, better construction flexibility, and possible future upgrades, contrary to in situ constructed heavy concrete sedimentation tanks.

It should be noted that the chemical reactions of the conventional reduction-precipitation system (Figure 6.4) and the modified reduction-flotation system are identical.

Comparatively, the modified reduction-flotation system will have lower annual total cost (amortized capital cost plus O&M cost) and will require less space, because the flotation unit is very shallow in depth and thus can be elevated. It is expected, however, that the treatment efficiency of the modified system will be higher due to the fact that the DAF clarifier can separate not only the suspended solids but also organics such as oil and grease, detergent, and so on.57,58,61 Conventional sedimentation clarifiers can separate only insoluble suspended solids.

6.9 I NNOVATIVE FLOTATION-FILTRATION WASTEWATER TREATMENT SYSTEMS

6.9.1 Flotation-Filtration System Using Conventional Chemicals

There are two innovative flotation-filtration wastewater treatment systems that are technically feasible for the treatment of the nickel-chromium plating wastewater.

The first system, shown in Figure 6.6, is identical to the conventional reduction-precipitation in chemistry (i.e., neutralization, chromium reduction, pH adjustment, metal hydroxide precipitation, and so on). However, a flotatio

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