# Patent

## Title
Laser ablation for the environmentally beneficial removal of surface coatings

## Abstract

A laser-based method of removing a coating from a surface comprises directing a laser pulse to a first position on the surface, removing the coating from the first position by rapidly elevating a surface temperature of the first position using the laser pulse and thereby disassociating the coating from the surface and collecting the disassociated coating. In some embodiments, the coating comprises an environmentally harmful substance such as Hexavalent Chromium. In some embodiments, the coating comprises Diamond-Like Carbon (DLC), Vitrified Contaminant Material (CMAS). The disassociated coating is collected by a waste collector.

## Background
RELATED APPLICATIONS
This application claims priority of U.S. provisional application, Ser. No. 61/604,368, filed Feb. 28, 2012, and entitled “Laser Ablation for the Environmentally Beneficial Removal of Three Substances; Diamond-Like-Carbon (DLC), Vitrified Contaminant Materials (CMAS), and Paints/Primers containing Hexavalent Chromium,” which is incorporated herein by reference.


FIELD OF THE INVENTION
The invention relates to ablating a coating using a laser. In particular, the invention relates to a laser apparatus that operates at conditions for removal of coatings such that the process is environmentally beneficial in comparison to other removal mechanisms.
BACKGROUND OF THE INVENTION
Laser-based coating removal systems use pulses of light from high power lasers to ablate or vaporize the paint or other coating from a surface. Ablation is the thermochemical disassociation, vaporization, and removal of material because of the incident convective and radiant energy. FIG. 1 illustrates the surface action phenomena during a typical ablation process.
Commercially-available lasers have the capability to generate pulses of radiant energy for suitable duration to achieve ablation conditions without significant convective heat transfer. Specifically, laser-induced ablation can be limited to the material surface through the appropriate selection of laser wavelength and active control of the radiative heat transfer rate and duration. The important application is the removal of the coatings and finishes from substrate structures that must be protected and reused. The radiative energy input must be significant, but the heat conduction into the substrate must be minimized. The amount of heat conduction into the substrate is affected by the radiant energy and the material surface of the substrate. The wavelength, irradiance, and pulse width of the radiant energy impact the amount of heat conduction into the substrate. The substrate properties, including transmissivity, absorptivity, emissivity, density, specific heat, thermal conductivity, and ablation onset thermochemistry (i.e. melt, vaporization, and surface chemical reaction temperatures), also affect the heat conduction into the substrate.
Wavelength is an important parameter because it must be selected to minimize the transmissivity of the radiant energy into the substrate. Thus, the radiant energy is absorbed at the surface. A high absorptivity of the material surface is also valuable, as this reduces the amount of laser energy needed to achieve a desired ablation rate. In many applications, such as metal substrates below a surface coating, low substrate absorptivity is also advantageous, because this limits the potential for ablation after the surface material is removed. Alternatively, substrate ablation for highly absorbing substrates may need to be limited by the use of active surface recognition prior to the initiation of the laser radiant energy pulse.
SUMMARY OF THE INVENTION
Embodiments of the present invention are directed to a laser-based coating removal method and system to remove an identified coating in a process that is environmentally beneficial in comparison to other removal mechanisms. The method comprises directing a laser pulse to a first position on a surface having an identified coating. The laser pulse rapidly elevates the surface temperature at the first position and causes the coating to disassociate from the surface. After the coating has disassociated from the surface it is able to be collected and safely disposed.
In one aspect a laser-based method of removing a coating from a surface comprises directing a laser pulse to a first position on the surface, removing the coating from the first position by rapidly elevating a surface temperature of the first position using the laser pulse and thereby disassociating the coating from the surface and collecting the disassociated coating. In some embodiments the coating comprises an environmentally harmful substance such as Hexavalent Chromium. In some embodiments the coating comprises Diamond-Like Carbon (DLC) or Vitrified Contaminant Material (CMAS). In some embodiments, the surface temperature of the first position is elevated to the oxidation temperature of the DLC and it is converted to vapor-phase carbon dioxide before it is collected. In further embodiments, the surface temperature of the first position is elevated so that the components of the CMAS are disassociated from the surface by disrupting the chemical bonds and the admixture is disaggregated. In still further embodiments, the surface temperature of the first position is elevated to a temperature at which the hexavalent chromium is disassociated from the surface and reduced to a particulate form of trivalent chromium. In some embodiments, the disassociated coating is collected by a waste collector. In further embodiments, the heat conduction into the surface from the laser pulse is limited. In some embodiments, the disassociation occurs essentially independent of the laser wavelength. In some embodiments, the laser pulse is moved to a second position in order to remove a coating from the second position.
In another aspect, a laser-based coating removal system comprises a surface comprising a coating and a laser based removal apparatus comprising a laser source to provide a laser pulse to a first position on the surface, wherein the laser source removes the coating from the surface by rapidly elevating a surface temperature of the first position using the laser pulse and thereby disassociating the coating from the surface. In some embodiments, the coating comprises an environmentally harmful substance such as Hexavalent Chromium. In some embodiments, the coating comprises Diamond-Like Carbon (DLC) or Vitrified Contaminant Material (CMAS). In some embodiments, the surface temperature of the first position is elevated to the oxidation temperature of the DLC and it is converted to vapor-phase carbon dioxide before it is collected. In further embodiments, the surface temperature of the first position is elevated so that the components of the CMAS are disassociated from the surface by disrupting the chemical bonds and the admixture is disaggregated. In still further embodiments, the surface temperature of the first position is elevated to a temperature at which the hexavalent chromium is disassociated from the surface and reduced to a particulate form of trivalent chromium. In some embodiments, the disassociated coating is collected by a waste collector. In some embodiments, the heat conduction into the surface from the laser pulse is limited. In further embodiments, the disassociation occurs essentially independent of the laser wavelength.
In a further aspect, a method for removing Diamond-Like Carbon (DLC) from a surface comprises directing a laser pulse to a first position on the surface, elevating a surface temperature of the first position using the laser pulse to an oxidation temperature of the DLC thereby converting it to vapor-phase carbon dioxide and collecting the disassociated DLC from the surface.
In still a further aspect, a method of removing Vitrified Contaminant Material (CMAS) from a surface comprises directing a laser pulse to a first position on the surface, elevating a surface temperature of the first position using the laser pulse to disassociate the components of the CMAS from the surface and disaggregating the admixture and collecting the disassociated CMAS from the surface.
In another aspect, a method for removing Hexavalent Chromium from a surface comprises directing a laser pulse to a first position on the surface, elevating a surface temperature of the first position using the laser pulse to a temperature at which the hexavalent chromium is disassociated from the surface and reduced to a particulate form of trivalent chromium and particulate effluent, and collecting the trivalent chromium and particulate effluent from the surface.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the surface interaction phenomena during an ablation process.
FIG. 2 illustrates a laser-based method for removing an identified coating from a surface according to some embodiments.
FIG. 3 illustrates a laser-based method for removing an identified coating from a surface according to some embodiments.
FIG. 4 illustrates a laser-based method for removing an identified coating from a surface according to some embodiments.
FIG. 5 illustrates a laser-based system for removing an identified coating from a surface according to some embodiments.



Embodiments of the invention are described relative to the several views of the drawings. Where appropriate and only where identical elements are disclosed and shown in more than one drawing, the same reference numeral will be used to represent such identical elements.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
Reference will now be made in detail to implementations of a laser-based coating removal system and method as illustrated in the accompanying drawings. The same reference indicators will be used through the drawings and the following detailed description to refer to the same or like parts. In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will also be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions be made to achieve the developer's specific goals, such as compliance with application and business related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.
Embodiments of the present invention are directed to a laser-based coating removal method and system to remove an identified coating from a surface in a process that is environmentally beneficial in comparison to other removal mechanisms. The system comprises a laser source to provide a laser light, a routing element coupled to the laser source and configured to direct the laser light onto a target region of the surface thereby removing the coating from the target region. In some embodiments, the system comprises a detection means to detect the coating as the coating disassociates from the target region of the surface. In some embodiments, the system comprises a waste collector for collecting the coating that has been removed.
A laser-based method for removing an identified coating from a surface is able to remove the identified coating in an environmentally beneficial manner. A laser pulse is directed to the surface in order to raise the temperature on the surface of the coating. When the temperature of the surface reaches a critical point, the coating disassociates from the surface and is able to be collected and safely disposed. Using a laser-based method avoids media-blast or chemically based processes that are time consuming and generate significant toxic waste. The laser-based method rapidly raises the temperature of the surface to its critical point under strictly controlled operating parameters in order to induce the desired effects.
In some embodiments, the system and method is used to selectively remove the identified coatings Diamond-Like Carbon (DLC), Vitrified Contaminants (CMAS), and primers and paints containing hexavalent chromium in an environmentally beneficial manner. However, the system is able to selectively remove other environmentally harmful chemicals as desired.
Diamond-Like Carbon (DLC)
DLC exists in different forms of carbon atoms that display some of the typical properties of diamond. DLC is applied as coatings to other materials in order to benefit from the diamond-like properties. Particularly, the value of a DLC coating accrues from its ability to provide properties of diamond to the surface of almost any material. These properties include hardness, wear, resistance, x-ray transparency, high thermal conductivity, and low friction. For example, a DLC film friction coefficient against polished steel ranges from 0.05-0.20. DLC is often applied in order to reduce the abrasive wear in high-load bearing surfaces, cutting tools, and press molds.
Removal of the DLC coating for substrate inspection and repair poses a significant challenge because of the coating's hardness and chemical resistance. Consequently, DLC film removal is sometimes attempted using an oxygen-dominated plasma beam to generate an appropriate ionized oxygen gas flow rate that etches the DLC film in a manner proportional to the treatment time. Alternatively, DLC may be removed by electrolyzing the film in an acid solution. However, these methods are limited by the processing rate, incomplete removal of the DLC film, access to confined spaces, field operability and/or waste chemical generation.
A laser-based method for DLC removal is able to remove DLC by impinging high irradiance laser pulses on a target DLC surface in an oxygen-rich environment. The laser pulse contacts the surface and immediately elevates the DLC to its carbon dioxide temperature, thereby rapidly and cleanly converting the DLC to vapor phase carbon dioxide. When the DLC has been converted to the vapor phase carbon dioxide it is able to be collected and safely disposed. Additionally, the limited heat conduction into the substrate provided by the laser process protects the coated surface from damage. In some embodiments, closed-loop feedback control immediately suppresses the beam when the DLC coating is completely removed.
FIG. 2 illustrates a laser-based method of removing DLC from a surface in accordance with some embodiments. The method begins in the step 210. In the step 220, a laser pulse is directed to a position of the DLC on a surface of the substrate. In the step 230, a surface temperature of the position is elevated using the laser pulse to an oxidation temperature of the DLC. When the DLC reaches the oxidation temperature it is converted to vapor phase carbon dioxide and disassociates from the surface of the substrate. After the DLC disassociates from the surface it is able to be collected in the step 240 and disposed. As described above, the laser pulse contacts the surface and immediately elevates the DLC to vapor phase carbon dioxide, disassociating it from the surface of the substrate. The limited heat conduction into the substrate protects the coated surface from damage.
Vitrified Contaminant Material (CMAS)
Operation of gas turbine engines in typical environments leads to ingestion of significant mounts of sand, dust, volcanic ash, and other atmospheric contaminants and containing oxides of Calcium, Magnesium, Aluminum, and Silicon (CMAS). In the high temperature, high pressure sections of these engines, the contaminants vitrify and accumulate on the turbine flow surfaces. The accumulations cannot be readily removed with conventional media blast or chemical exposure methods. Consequently, the conventional cleaning methods entail dismantling the engine, removing the contaminated component and then exposing the surfaces to a highly-concentrated acid bath for a protracted period. The acid baths are a toxic exposure hazard to personnel and the environment while only partially removing the contaminants.
A laser-based method is able to remove the CMAS quickly and completely from exposed surfaces and small openings such as bleed air holes, pores, cracks, and crevices. In some embodiments, laser-based cleaning may be accomplished in situ or on subassemblies removed from the engine. A high irradiance laser pulse produces a rapid temperature rise of the surface of the CMAS and simultaneously disassociates the CMAS components, disrupts the amorphous chemical bonds and disaggregates the admixtures. Once the CMAS is disassociated from the surface, purge air is able to be directed towards the point of ablation and adjacent vacuum collectors result in a rapid and thorough cleaning of the surfaces within the focal range of the laser delivery device.
FIG. 3 illustrates a method of removing vitrified contaminant material (CMAS) from a surface in accordance with some embodiments. The method begins in the step 310. In the step 320, a laser pulse is directed to a position of the CMAS on the surface of the substrate. Then, in the step 330, a surface temperature of the position is elevated using the laser pulse to a temperature at which the CMAS disassociates from the surface and disaggregates. After the CMAS disassociates from the surface it is able to be collected in the step 340 and disposed. In some embodiments, the CMAS is collected by utilizing purge air and vacuum collectors to direct the CMAS to a waste collector. As described above, the laser pulse produces a rapid temperature rise of the surface of the CMAS and simultaneously disassociates the CMAS components, disrupts the amorphous chemical bonds and disaggregates the admixtures.
Hexavalent Chromium Removal and Conversion to Trivalent
Paints and primers such as used in numerous applications include compounds containing hexavalent chromium, often in the form of zinc chromate (ZnCr04). ZnCr04 is a chemical compound containing the chromate anion, appearing as an odorless yellow solid powder and is used industrially in chromate conversion coatings. Its use as a corrosion resistant agent was applied to aluminum alloy parts first in commercial aircraft, then in military aircraft. During the 1940's and 1950's it was used as paint in the wheel wells of retractable landing gear of military aircraft to protect the aluminum from corrosion. It is also used on many systems as a primer.
Chromates such as ZnCr04 have been linked to occupational diseases, specifically dermatitis, nasal irritation, and lung cancer. As a result, numerous organizations have directed abatement programs and with varying degrees of success. Removal of ZnCr04 coatings using standard abrasive methods such as media blast or sanding, or the use of solvents results in an unacceptable level of operator exposure and generates large toxic waste streams.
A laser-based ablation method for zinc chromate rapidly removes the zinc chromate coat without damaging the substrate material while simultaneously converting the hexavalent chromium to less toxic particulate forms of trivalent chromium (Cr2O3). The less-toxic particulate effluent is then able to be collected by conventional filtration techniques. Impingement of a high irradiance laser pulse on the surface of a coating with ZnCr04 rapidly raises the surface temperature of the coating to a value at which the hexavalent chromium is reduced to its triavalent form. For example, in some embodiments, the elevated temperature created by the laser pulse drives the following equilibrium chemical reaction completely to the right.

4ZnCr04+ . . . →4ZnO(solid)+2Cr2O3(solid)+3O2+ . . . .

FIG. 4 illustrates a method of removing hexavalent chromium from a surface of a substrate in accordance with some embodiments. The method begins in the step 410. In the step 420, a laser pulse is directed to a position of the hexavalent chromium on a surface of a substrate. In the step 430, the temperature at the position is elevated using the laser pulse to a temperature at which the hexavalent chromium is disassociated from the surface and reduced to trivalent chromium and byproducts. After the hexavalent chromium is disassociated and reduced it is able to be collected in the step 440 and safely disposed. In some embodiments, the less-toxic trivalent chromium and byproducts is collected by conventional filtration techniques. As described above, the laser-based ablation method for zinc chromate rapidly removes the zinc chromate coat without damaging the substrate material while simultaneously converting the hexavalent chromium to less toxic particulate forms of trivalent chromium (Cr2O3).
FIG. 5 illustrates a laser-based coating removal system for the environmentally beneficial removal of surface coatings in accordance with some embodiments. The system 500 comprises a laser source 502, a routing element 504, and a controller 506 for removing a coating 98 from a surface 99. The laser source 502 is directed to the coating 98 on the surface 99 by the routing element 504. Specifically, the routing element 504 directs the laser pulse 97 to the surface 99 in order to remove the coating 98 from the surface 99, as described above. When the laser pulse 97 contacts the surface 99, the temperature of the surface is immediately elevated to a critical temperature at which the coating 98 is disassociated from the surface 99. In some embodiments, the routing element 504 comprises one or more beam splitters and/or scanning optics which direct the laser pulse 97 to the surface.
As shown in FIG. 5, in some embodiments, the system 500 comprises a detection means 508 for detecting the removal of the coating 98 from the surface 99 and a waste collector 510 for collecting the waste byproducts resulting from the laser pulse 97 contacting the surface 99. In some embodiments, the waste collector 510 includes a local storage for storing the collected ablated waste byproduct. Alternatively, the waste collector 510 acts as a waste removal apparatus and is coupled to an external waste receptacle.
In some embodiments, the routing element 504 and the detection means 508 are of the type as described in the co-owned U.S. Pat. No. 7,633,033 and entitled “Color Sensing for Laser Decoating”, which is hereby incorporated by reference. In some embodiments, the waste collector 510 of the type as described in the co-owned U.S. Pat. No. 7,009,141 and entitled “Laser Scanning Head with Rotary Scanning Coaxial Refractive Optics”, which is also hereby incorporated by reference.
The laser pulse is able to comprise a fluence in the range of 1-10 Joules per square centimeter (J/cm2) depending on the coating and the substrate. In some embodiments, the irradiance of the laser pulse is in the range of 6-60 MegaWatts per square cm (MW/cm2). Particularly, the laser pulse power is sufficient in combination with the laser optics and beam quality to achieve the desired irradiance. Additionally, the laser ablation is achieved essentially independent of the laser wavelength because Diamond-Like Carbon (DLC), Vitrified Contaminants (CMAS), and chromate-containing coatings have low transmissivity for the typical industrial laser wavelengths in the range of 0.3 μm to 10.6 μm. Further, the method is minimally affected by the laser pulse duration. Specifically, in some embodiments, the method uses a laser with a delivered average power of 350 W providing a fluence of 5 J/cm2 and irradiance of 35 MW/cm2.
In its application, a laser-based method of removing a coating from a surface is able to remove an identified coating from the surface and in an environmentally beneficial manner. Particularly, a laser pulse is able to be directed to a position on a surface in order to elevate the surface temperature to a critical temperature at which a coating is disassociated from the surface. Once the coating disassociates from the surface it is able to be collected and safely disposed. Thus, it is clear that the coating removal system described herein has numerous advantages. Specifically, the coating removal system is able to efficiently remove environmentally harmful coatings on a surface in a process that is environmentally beneficial in comparison to other removal mechanisms Particularly, the laser-based method rapidly raises the temperature of the surface to its critical point under strictly controlled operating parameters in order to induce the desired effects. As a result, the laser-based method avoids media-blast or chemically based processes that are time consuming and generate significant toxic waste.
The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention.

## Claims

1. A laser-based method of separately removing one of a plurality of different coatings from a surface comprising:
a. directing a laser pulse comprising a delivered power of 350 W, a fluence of 5 J/cm2 and an irradiance of 35 MW/cm2 to a first position for removing the coating on the surface;
b. removing the coating from the first position by using the laser pulse to rapidly elevate a surface temperature of the first position to the critical temperature as determined by the coating, wherein at the critical temperature the coating separates and is removed from the surface; and
c. collecting the coating that has been separated and removed.

2. The method of claim 1 wherein the coating comprises an environmentally harmful substance.

3. The method of claim 2 wherein the surface temperature of the first position is elevated to a temperature at which hexavalent chromium is separated from the surface and reduced to a particulate form of trivalent chromium.

4. The method of claim 2 wherein the coating comprises hexavalent chromium.

5. The method of claim 4 wherein the surface temperature of the first position is elevated to a temperature at which at least a portion of the hexavalent chromium is separated from the surface and reduced to a particulate form of trivalent chromium.

6. The method of claim 1 wherein the coating comprises diamond-like carbon or vitrified contaminant material comprising a mixture of calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide.

7. The method of claim 6 wherein the surface temperature of the first position is elevated to an oxidation temperature of the diamond-like carbon and the diamond like carbon is converted to vapor-phase carbon dioxide before it is collected.

8. The method of claim 6 wherein the surface temperature of the first position is elevated so that the calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide are separated from the surface by disrupting one or more chemical bonds.

9. The method of claim 1 wherein coating that has been separated and removed is collected by a waste collector.

10. The method of claim 1 wherein heat conduction into the surface from the laser pulse is limited to prevent damage to the surface.

11. The method of claim 1 wherein separation of the coating occurs at a wavelength of the laser pulse with a low transmissivity with respect to the coating.

12. The method of claim 1 further comprising moving the laser pulse to a second position in order to remove a coating from the second position.

13. A laser-based coating removal system for separately removing one of a plurality of different coatings from a surface, the system comprising:
a. a surface comprising one of the plurality of coatings; and
b. a laser based removal apparatus comprising
i. a laser source to provide a laser pulse comprising a delivered power of 350 W, a fluence of 5 J/cm2 and an irradiance of 35 MW/cm2 to a first position of the coating on the surface, wherein the laser source removes the coating from the surface by using the laser pulse to rapidly elevate a surface temperature of the first position to the critical temperature determined by the coating, wherein at the critical temperature the coating separates and is removed from the surface.

14. The system of claim 13 wherein the coating comprises an environmentally harmful substance.

15. The system of claim 14 wherein the surface temperature of the first position is elevated to a temperature at which the hexavalent chromium is separated from the surface and reduced to a particulate form of trivalent chromium.

16. The system of claim 14 wherein the coating comprises hexavalent chromium.

17. The system of claim 13 wherein the coating comprises diamond-like carbon or vitrified contaminant material comprising a mixture of calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide.

18. The system of claim 17 wherein the surface temperature of the first position is elevated to an oxidation temperature of the diamond like carbon and the diamond like carbon and it is converted to vapor-phase carbon dioxide before it is collected.

19. The system of claim 17 wherein the surface temperature of the first position is elevated so that the calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide are separated from the surface by disrupting one or more chemical bonds.

20. The system of claim 13 wherein coating that has been separated and removed is collected by a waste collector.

21. The system of claim 13 wherein heat conduction into the surface from the laser pulse is limited to prevent damage to the surface.

22. The system of claim 13 wherein separation of the coating occurs at a wavelength of the laser pulse with a low transmissivity with respect to the coating.

23. A method for removing diamond-like carbon coating from a surface comprising:
a. directing a laser pulse comprising a delivered power of 350 W, a fluence of 5 J/cm2 and an irradiance of 35 MW/cm2 to a first position on the surface;
b. elevating a surface temperature of the first position using the laser pulse to an oxidation temperature of the diamond-like carbon thereby converting the diamond-like carbon to vapor-phase carbon dioxide;
c. collecting separated diamond-like carbon from the surface; and
d. using feedback control to immediately suppress the laser pulse when the diamond-like carbon coating is removed.

24. A method of removing vitrified contaminant material comprising a mixture of calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide from a surface comprising:
a. directing a laser pulse comprising a delivered power of 350 W, a fluence of 5 J/cm2 and an irradiance of 35 MW/cm2 to a first position of the coating on the surface;
b. elevating a surface temperature of the first position to a critical temperature using the laser pulse at which point the calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide are separated and removed from the surface; and
c. collecting the separated calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide from the surface.

25. A method for removing hexavalent chromium from a surface comprising:
a. directing a laser pulse comprising a delivered power of 350 W, a fluence of 5 J/cm2 and an irradiance of 35 MW/cm2 to a first position of the coating on the surface;
b. elevating a surface temperature of the first position using the laser pulse to a critical temperature at which the hexavalent chromium is separated and removed from the surface and reduced to a particulate form of trivalent chromium and particulate effluent; and
c. collecting the trivalent chromium and particulate effluent from the surface.

26. The method of claim 25, wherein separated hexavalent chromium, trivalent chromium, and particulate effluent is collected from the surface.