Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 1



# 306 EMISSIONS CONTROL
TECHNIQUES

TABLE OF CONTENTS

306 EMISSIONS CONTROL TECHNIQUES

306.1 FUME SUPPRESSANTS 306.1.1 Chemical Fume Suppressants 306.1.2 Mechanical Fume Suppressants

306.2 ADD-ON CONTROL DEVICES 306.2.1 Capture Techniques
306.2.2 Pack-Bed Scrubbers

306.2.3 Mist Eliminators

306.2.4 HEPA Filters 306.2.5 Combination Systems 306.2.6 Encapsulating Tank Covers



## 306 EMISSIONS CONTROL TECHNIQUES

To comply with the State ATCM regulation, a plating or anodizing facility must either

1. Meet a surface tension limit using chemical fume s uppressants containing a wetting agent or;

2. Meet an emission limit (mg/amp-hr or mg/dscm) demo nstrated by a performance test.

Techniques to control hexavalent chromium emissions from chrome electroplating and anodizing tanks include fume suppressants and add-on control devices. In the first, an anti-mist additive is added to the plating bath to reduce the amount of m isting that takes place. In the second, the mist is captured, and the entrained chromium is extracted f rom the exhaust air stream prior to discharge to

the stack.

In California, decorative platers typically use an anti-mist additive to achieve compliance with the
State's hexavalent chromium emissions limit. About 25% of decorative platers in the State uses addon control equipment in addition to an anti-mist ad ditive. About 5% use only add- on control equipment

to reduce hexavalent chromium emissions.

Hard chrome platers in California use add-on contro ls to achieve compliance with the State's hexavalent chromium emissions limit. However, about 50% of hard chrome platers in the State use an anti-mist additive in conjunction with add-on contr ols to reduce hexavalent chromium emissions from

their tanks.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 2 About 40% of chromic acid anodizing operations in t he State use an anti-mist additive alone to achieve compliance. Another 40% use an anti-mist ad ditive in conjunction with add-on controls. The remaining 20% use only add-on controls to reduce he xavalent chromium emissions from their tanks. 6



### 306.1 FUME SUPPRESSANTS

Fume suppressants are mist suppressant or physical barriers at the surface of the plating bath used to control chromic acid mists. There are two catego ries of fume suppressants: chemical fume suppressant and mechanical fume suppressant.



### 306.1.1 Chemical Fume Suppressant

A chemical fume suppressant is any chemical agent t hat reduces or suppresses fumes or mists at the surface of an electroplating or anodizing bath. Ano ther term for chemical fume suppressant is mist suppressant. Fume suppressants can be temporary or permanent depending on the mechanism of depletion from the baths. Temporary fume suppressan ts are depleted mainly by the decomposition of the active chemical components. Permanent fume supp ressants are depleted by drag-out of the

solution.

Chemical fume suppressants are added directly to th e bath to reduce or inhibit misting. Chemical fume suppressants include: wetting agents, foam bla nkets (Figure 306-1), and combinations of both (Figure 306-2). Most chemical fume suppressants in use today contain wetting agents. The difference between wetting agents and foam blankets is the way in which they reduce emissions. Wetting agents control misting by lowering the surface tension of the bath, while foam blankets physically suppress the mists produced on the surface of the bath.

Fume suppressants are manufactured in liquid, powde r, or tablet form. Some of the main advantages of using chemical fume suppressants include:

1. Minimization of plating solution evaporation lo sses; 2. Low cost of chromium emission control; 3. Very little energy consumption; and 4. No solid waste generation.

Limitations in using chemical fume suppressants inc lude:

1. Some fume suppressants have a tendency to a ggravate gas pitting and defects in base metals;

and 2. Foam blankets that are too thick can entrap hydrogen gas, which poses a risk of explosion.

Chemical fume suppressants typically reduce chromiu m emissions between 93 and 99 percent. 1

306.1.1A Foam Blankets

A foam blanket is a type of chemical fume suppressa nt that generates a layer of foam across the surface of a solution when current is applied to th at solution. Foam blankets do not prevent the formation of chromic acid mist, but instead trap th e mist under a blanket of foam. The foam blanket isChrome Plating and Anodizing Operations

Cal/EPA – January 2006 3 formed by the agitation produced by hydrogen and ox ygen gas bubbles generated during electroplating. Once formed, the foam blanket is us ually maintained at a thickness of 1.3 to 2.5 centimeters (0.5 to 1.0 inches) and covers the enti re surface of the plating bath (see Figure 306-1). The ability of the foam layer to contain mist is gr eatly reduced if proper thickness is not maintained . If the foam blanket is too thin, less than 1.3 centime ters (0.5 inches), it will no longer trap the chrom ium containing mist. Conversely, a thickness greater th an 2.5 centimeters (1.0 inches) will cause hydrogen gas to build up in the foam layer, creatin g a hydrogen explosion hazard should a spark be generated from the contacting equipment. These prob lems tend to occur more frequently with higher current densities and longer plating times. 1

Figure 306-1. Foam Blanket

The characteristics of foam blanket solutions are d etermined by the surface area of the solution, amount of current applied, and temperature and chro mic acid concentration of the plating bath, (e.g. less suppressant is required at lower temperatures) . Visual monitoring of the thickness of the foam blanket is the most common method for determining w hen to add foam blanket solution to the bath. The frequency of adding foam blanket solution, also , depends upon the drag-out rate of the solution and the amount of work being performed in the plati ng tank. Chemical decomposition, to a greater extent than drag-out, is the primary mechanism for depletion of foam blankets.

306.1.1B Wetting Agents

A wetting agent is a type of chemical fume suppress ant that reduces the surface tension of a liquid. When wetting agents lower the surface tension of a plating bath, gases escape at the surface of the solution with less of a "bursting" effect, causing less mist formation. In essence, a wetting agent reduces bubble size, and smaller bubbles burst with less impact on the surface.

Plating bath typically have a surface tension of 70 dynes/cm. With the addition of sufficient wetting agent, the surface tension of the plating bath can be reduced to 45 dynes/cm or less where it has been shown to create a rapid reduction in chromic a cid emissions. 1 The two methods used to measure the surface tension of the plating solution are the stalagmometer (the drop weight method)Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 4 and the tensiometer (the ring method). Most plating facilities use a stalagmometer to measure the surface tension. Go to the Photo Gallery to view pictures of the stalagmometer and the tens iometer.

Figure 306-2. Fume Suppressant - Combination of Wet ting Agent and Foam



### 306.1.2 Mechanical Fume Suppressant

A mechanical fume suppressant is any device that re duces fumes or mist at the surfaces of an electroplating or anodizing bath by direct contact with the surface of the bath. Polyballs (Figure 306 -3) are the most commonly used mechanical fume suppress ant. Polyballs are hollow, plastic floating spheres with typical sizes range from 3/4 to 2 inch es in diameter. They are fabricated from plastics

that are resistant to chemical attack and are used to reduce heat loss, bath evaporation, and misting. In order to be most effective, balls must cover the entire surface area of the bath.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 5 Figure 306-3. Polyballs

The U.S. EPA has examined the use of these floating spheres for air pollution control purposes, and found them to be of limited use. In one such test, a two to three inch layer of 1.5 inches in diameter spheres achieves a removal efficiency of only about 75 percent. 1 The effectiveness of polyballs is also reduced by the fact that the evolution of hydrogen gas at the workpiece tends to push the spheres away, exposing a portion of the plating bath surfac e to the air above.



### 306.2 CONTROL DEVICES

This section discusses the capture techniques to ca pture the chromium misting at the surface of the

tank and the add-on control devices available to ex tract the entrained chromium from the exhaust air stream prior to discharge to the stack. An add-on a ir pollution control device is any equipment installed in the ventilation system of chrome elect roplating and anodizing tanks for the purposes of collecting and holding chromium emissions from the tank(s). Add-on control devices include: packbed scrubbers, mesh pad mist eliminators, fiber bed mist eliminators, and combination systems. Also discussed is the encapsulating tank cover which doe s not require an exhaust stack to vent process air outside of the facility.



### 306.2.1 Capture Techniques - Lateral Exhaust System

Local exhaust ventilation is the most common method used to capture chromic acid mist from chrome plating and chromic acid anodizing baths. Typical l ocal exhaust ventilation equipment includes lateral (slot) and push-pull exhaust hoods. Canopy hoods or overhead exhaust hoods and enclosures are normally not used because they restrict access to t he plating or anodizing tank. In addition, overheadChrome Plating and Anodizing Operations

Cal/EPA – January 2006 6 hoods would draw mists from the tank surface into t he breathing zone of anyone who was working at the tank. However, there is a control system called the encapsulating tank lid or cover used by hard chrome platers that contains the mist while allowin g other gasses to vent. This control system is further discussed in Subsection 306.2.6, Encapsulat ing Tank Covers.

A tank is said to be "laterally exhausted" if the hood does not project over the entire tank and the

direction of the air movement into the hood is subs tantially horizontal. Lateral exhaust systems typically used to control chromic acid mist from op en-surface tanks include both single-sided and double-sided slot hoods. The slot velocity affects the uniformity of airflow distribution along the le ngth

of the hood.

Single-sided hoods have a maximum reach across the width of the tank of about 30 inches (2 and 1/2 feet) while double-sided hoods have a maximum reach of about 60 inches (5 feet). In this system, the chromic acid mists are collected by slot vents whic h usually run along the long side (single-sided) or along both long sides of the tank (double-sided) (s ee Figure 306-4).

Figure 306-4. Slot Vents on a Lateral Exhaust Syste m

Push-pull ventilation is recommended for tanks grea ter than 42 inches (3 and 1/2 feet) in width. In aChrome Plating and Anodizing Operations

Cal/EPA – January 2006 7 push-pull system (Figure 306-5), one slot vent is a n intake ("pull") while the other supplies air unde r pressure ("push"). Push air supplied through slots, orifices, or nozzles is blown across the width of the

tank to direct contaminated air toward the exhaust hood, the "pull" vent, located on the opposite side

of the tank.

Figure 306-5. Push-Pull System



### 306.2.2 Packed Bed Scrubbers

A packed-bed scrubber (PBS) consists of a single or double packed bed that contains packing media on which the chromic acid droplets attach. First, t he gas stream is wetted by spraying water against the flow of the gas to make the droplets larger. Th en the gas stream passes through the packed bed(s) where the droplets attach onto the packing m edia. A mist eliminator section follows the packed

bed section of the scrubber. The mist eliminator re moves any water entrained from the packed bed section. Treated gases then pass through an induced draft fan and out a stack of exhaust vent. The scrubber water is recirculated and periodically tap ped and discharged to the electroplating tanks as makeup solution. Figure 306-6 shows a packed-bed sc rubber.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 8 A PBS has a typical efficiency of 97 percent for de corative chromium electroplating and anodizing tanks and 99% for hard chromium electroplating tank s. Factors influencing efficiency include: liquidto-gas ratio (the flooding or starving of the media with scrubbing liquor), gas velocity, surface area and type of packing media, distribution of the medi a in the bed, and effectiveness of washdown procedures. 3

Figure 306-6. Packed-Bed Scrubber

(Courtesy of Midwest Air Products Co., Inc. (MAPCO) , adapted)



### 306.2.3 Mist eliminators

Mist eliminators generally remove droplets from gas streams by inertial impaction of droplets on these devices and centrifugal force applied to the gas st ream. Many types of mist eliminating devices have been developed, but most are categorized within the following types: chevrons, mesh pads, and fiber

beds.

Mist eliminators may be used in a horizontal or ver tical gas flow configuration. When mist eliminators are used on a vertical gas stream, liquids removed from the gas will drip from the eliminator from the force of gravity on the liquid against the flow of the gas stream. Liquids collected by mist eliminato rs used on horizontal gas flows will not drip against the flow of the gas, so horizontal mist eliminators can handle higher gas velocities and liquid loading s.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 9 Mist eliminators are usually equipped with liquid s prays to prevent the accumulation of solids. An accumulation of solids can plug part of the elimina tor, reduce the area of flow, create excessive pressure drops and increase liquid entrainment. Pro perly operating mist eliminators usually have a pressure drop between 0.5 and 4.0 inches of water. Type, size, and composition of the pads are dependent on the system they are incorporated in (i .e. gas density, flow velocity, etc.).

306.2.3A Chevron-Blade Mist Eliminators

Chevrons are devices composed of zigzag baffles whi ch force a gas stream to turn sharply. Liquids

collect on the blades of the chevron and drain down ward by gravity. Basically, a series of parallel zigzag blades (see Figure 306- 7) are positioned in the body or housing of the dev ice, in order to direct the air flow through narrow passageways that repeat edly change its direction by approximately 30 degrees. At regular intervals, the extruded blades have narrow vertical channels which serve to capture some of the chromium droplets by inertial f orces. Since the channels are oriented in a vertica l plane, condensed chrome is transported to a drain a t the bottom of the housing. Common blade designs are the overlapping and sinusoidal wave des igns (see Figure 306-7).

Figure 306-7 Blade Configurations Used in Chevron M ist Eliminators (EPA Capsule Report)

Blade-type horizontal mist eliminators have demonst rated a wide efficiency range. Removal efficiencies ranged from about 83 to 99 percent. So me of the factors influencing efficiency were: gas velocity, blade spacing, seal integrity, and wash d own frequency. 1, 3

306.2.3B Composite Mesh Pad Mist Eliminators

A composite mesh pad mist eliminator (CMP) consists of several layers of interlocked fibers densely packed between two supporting grids. The composite mesh pad was developed to remove small particles (< 5 micrometers or 0.2 mils) that were n ot effectively controlled by other control devices. There are usually at least three mesh pad stages (s ee Figure 306-8). The first stage removes large particles. The second stage removes smaller particl es using a composite mesh pad. The final stage may remove any reentrained particles not collected by the composite mesh pad in the second stage.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 10 A reentrained particle is a particle which was caug ht by the mesh pad, but some force removed it from the mesh pad and returned it to the air stream

.

The layers of material in composite pads are arrang ed with the smallest diameter fiber layer located i n the center of the pad and progressively larger diam eter layers located on both sides of the center. Particles collide with the fibers in the pad and at tach to the fiber surfaces. These captured particle s combine into larger particles as they travel throug h the small diameter fiber layers in the center of the pad. These larger particles either drain to the bot tom of the unit or are reentrained in the gas strea m. The reentrained particles are then captured by the large diameter fiber layers in the back of the pad.

CMP systems incorporate a larger particle removal s ystem located upstream from the composite mesh pad, to reduce the plugging potential of the p ad. The large particle removal system can either be a series of larger diameter mesh pads or a packe d bed scrubber section. Typical removal

efficiencies associated with this control device ar e greater than 99 percent. 1, 3

Figure 306-8. Mesh Pad Mist Eliminator (Courtesy of Midwest Air Products Co., Inc. (MAPCO) , adapted)Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 11 306.2.3C Fiber Bed Mist Eliminators

A fiber bed mist eliminator (FBME) removes contamin ants from a gas stream through the mechanisms of inertial impaction and Brownian diffu sion. FBMEs have most often been used to reduce acid mists from sulfuric, phosphoric, and ni tric acid plants. An FBME consists of one or more fiber beds. Each bed consists of a hollow cylinder formed from two concentric screens (see Figure 306-9). The fiber between the screens may be made o f glass, ceramic, plastic, or metal. Fiber bed units are designed for horizontal, concurrent gas l iquid flow through the fiber bed. The contaminated gas stream flows toward the downstream face of the bed. The acid mist in the gas stream impacts on

the surface of the fibers and drains down the outer face of the bed to the sump while the clean gas flows up and out the top of the unit.

Figure 306-9 Fiber Bed Mist Eliminator (EPA Capsule Report)

A FBME device is typically installed downstream fro m another control device that can prevent plugging of the FBME device. The upstream device re moves the majority of the emissions and thus prevents plugging of the fiber bed.

To date, there is no adequate test data available t o accurately quantify the control efficiency of FBM E. The U.S. EPA believes that these systems can achiev e the same emission limits as CMP systems and chemical fume suppressants. 1, 3Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 12 

### 306.2.4 HEPA Filters

A High Efficiency Particle Arresting or Particulate Air (HEPA) filter is defined by the Department of Energy (DOE) as a throwaway, extended-media, dry ty pe filter with a rigid casing enclosing the full length depth of the pleats. The filter shall exhibi t a minimum efficiency of 99.97% when tested at an

aerosol of 0.3 micrometers diameter. 5 These filters were originally designed to trap mic roscopic particles such as radioactive dust in atomic plants and are commonly used for critical environments like hospitals, clean rooms, and lead abatement pro jects. Figure 306-9 illustrates a HEPA filter system.

Figure 306-9. HEPA Filter SystemChrome Plating and Anodizing Operations

Cal/EPA – January 2006 13 

### 306.2.5 Combination Systems

Combination systems are various arrangements of mul tiple control devices, as discussed above, in a common exhaust system. Some of the variations inclu de:

1. Mesh pads installed in exhaust hoods at the tank, f ollowed by an end-of-line mesh pad or packed bed scrubber;

2. In-line mesh pad mist eliminators, located after th e exhaust hoods, followed by an end-ofline mesh pad or packed bed scrubber;

3. Blade-type horizontal mist eliminators followed by an end-of-line mesh pad or packed bed scrubber;

4. Mesh pad mist eliminator system followed by a high efficiency particulate air (HEPA) filter as a final polishing device; and

5. Mesh pad mist eliminator system followed by a filte r bed as a final polishing device.

Some of the advantages in using multiple devices in clude: segregating chemicals, keeping duct work clean, and reducing the inlet loading on the end-of -line unit. 1



### 306.2.6 Encapsulating Tank Covers

An encapsulating tank cover involves the use of a s pecial tank hood or cover that contains the mist while allowing other gasses to vent (see Figure 306 -10). The system does not require an exhaust stack to vent process air outside of the facility. The hydrogen and oxygen gas, generated during the chrome plating process, pass through a selective me mbrane system to the plating room environment. Water vapor and chromic acid mist do not penetrate the membrane, but will condense within the confines of the tank's cover. At the end of the pla ting batch, an evacuation cycle removes latent fumes or mists prior to opening the cover for parti al retrieval. This approach can reduce the building 's energy requirements significantly, since the unit d oes not exhaust conditioned air outdoors, and there are no large fan motors. An additional benefit is t hat no chromium-bearing fluids are generated, as with the washdown cycles of mesh pad mist eliminato r systems or the recirculated liquor in packedbed scrubbers.Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 14 Figure 306-10. Encapsulating Tank Cover (EPA Capsule Report, PF Online)

Because water vapor is not removed from the plating bath through surface evaporation, reclaim rinsing strategies and cooling systems may be adver sely affected. Also, parts should either have identical start/stop times (i.e., batch plated) or be capable of multiple plating cycles, since all pl ating in the tank must be stopped before any parts can be ad ded or removed. 1, 4Chrome Plating and Anodizing Operations

Cal/EPA – January 2006 15 

## References

1.U.S. EPA, 1998, Capsule Report-Hard Chrome Fume Suppressants and Co ntrol Technologies

, Office of Research and Development, Washington DC, Document No. EPA/625/R-98/002, December 1998, Available online at: http://www.epa.gov/ORD/NRMRL/pubs/625r98002/625r980 02.pdf

2. U.S. EPA, 1993, Chromium Emissions from Chromium Electroplating and Chromic Acid Anodizing Operations-Background Information for Proposed Stan dards , Document No. EPA 453/R-93-030b, p. 4-1 - 4-70

3. Colorado Department of Public Health and Environ ment (CDPHE), 1999, CDPHE Chromium
MACT - Control Measures , Last updated April 21, 1999, Accessed January 20, 2005, http://www.cdphe.state.co.us/ap/chromium/html/contr ol/index.html

4. PF Online an online component of Products Finish ing magazine, 1997, Reducing Hexavalent Chromium Emissions, December 1997, Accessed on May 25, 2004, http://www.pfonline.com/articles/129705.html

5. U.S. Department of Energy, 1997, DOE Standard Specification for HEPA Filters Used By DOE
Contractors , Document No.: DOE-STD-3020-97, Washington, D.C., January 1997, p. 4, Accessed on September 21, 2005, http://www.eh.doe.gov/hepa/docs /std3020.pdf

6. California Air Resources Board, telephone discus sion with Shobna Sahni, Lead staff on ATCM for Chrome Plating and Chromic Acid Anodizing Operation s, September 2005

7. California Air Resources Board, 2000, Toxics Enforcement Manual, Part B: NESHAP/MACT Guideline Documents, Volume 2, Chapter I, Chrome El ectroplating and Anodizing Operations

, Compliance Assistance Program, Sacramento, CA, Nove mber 2000, p. 200-12 - 200-18