Method for Reclaiming Paint Overspray

Technical Field

This invention relates generally to a method for reclaiming paint overspray and more particularly to a method of separating paint overspray into reusable liquid and solid components.

Background Art

Water wash ^' paint booths are broadly used to capture excess paint particles in the form of overspray and prevent the discharge of such particles into the atmosphere. The treatment of waste water from spray paint booths has been a long standing problem. Increasingly stringent environmental control laws require that waste water containing suspended paint particles must be disposed of only at approved chemical waste sites. The number of approved and licensed disposal sites has steadily decreased over the past few years while the demand for such sites has increased. Accordingly, the cost of disposing of paint sludge materials has risen sharply and, in some cases, is now approaching the original cost of the paint.

Various chemical treatments have been proposed for removi.ng paint overspray particles from paint booth waste water. For example, U.S. Patent 4,071,449 issued to Tuyosi Inoue, et al, on April 15, 1977 discloses a chemical treatment for paint booth waste water containing water-soluble paint particles. The Inoue, et al process includes the addition of both an acidic agent and an alkaline agent to form an insoluble salt, resulting in the disposition of the paint particles as a non-adhesive coagulation product. U.S. Patent 3,736,277 issued to Howard S. Eender on May 29, 1973

discloses a method for reclaiming a solvent-based paint overspray by chemically treating paint booth wash water containing the overspray to form a sludge product, skimming the sludge, and subsequently dredging the skimmed sludge. U.S. Patent 4,096,061 issued to Thomas P. Brennan on June 20, 1978 discloses a process for chemically treating paint booth waste water containing dispersed latex paint particles to produce a reusable sludge product. U.S. Patent 4,102,303 issued to Andre Guy Cordier, et al, on July 25, 1978 discloses a method for continuously chemically treating a portion of the paint booth waste water to separate paint overspray particles from the waste water which, after treatment, is returned to the paint booth for reuse.

Chemical treatment of paint booth waste water, while meeting with some success, also has a number of inherent disadvantages. The cost of the chemicals used in any of the various treatment processes is often high and are generally consumed, or used up in the course of treatment and must be continuously replaced. Also, some chemical treatments incorporate the use of highly acidic compounds in the process, increasing the cost of equipment to transfer and treat the waste. In addition, residual elements or compounds incorporated into the treated waste products may render the solid component unusable for reclamation

More recently, alternative methods to the chemical treatment of paint booth waste water have been developed. An electrostatic separation process for paint booth waste water is described in an article titled "Turn A Paint Booth Profit Ey Treating

Overspray", Production, November 1980, pp 90-91. Although the electrostatic separation process overcomes many of the problems inherent in chemical treatment processes, it is sensitive to the type of paint particles contained in the waste water. Electrostatic

treatment is dependent upon both the physical and electrical properties of the material being treated, and is generally limited to use with solvent-based paint systems.

It is therefore highly desirable to have a process for effectively separating the liquid and solid components of paint booth waste water that is not dependent on chemical reaction or is limited to only certain types of paint. It is also desirable that the o resultant components from such a process would be immediately useable for reclamation or recirculation.

The present invention is directed to overcoming one or more of the problems as set forth

I above. 5

Disclosure of the Invention

In accordance with one aspect of the present invention a method for reclaiming paint overspray particles includes collecting an effluent containing o the overspray particles, separating the particles in the effluent and forming a slurry containing the particles, and centrifugally separating the slurry into solid and liquid components.

There has been a long-standing need for an economical and efficient single method for reclaiming a wide variety of paint overspray particles. Electrostatic separation of paint particles contained in paint booth waste water is generally limited to use in solvent-based paint systems. A multitude of chemical processes have been developed for specifically treating particular paint compositions. However, chemical treatments are generally costly, use consumable materials in the treatment process, and often yield a product that is contaminated as a result of reaction with the treating agents. The present

invention employs mechanical separation techniques to separate paint booth waste water into solid and liquid components. Both of the components produced are reusable and the process is broadly applicable to all types of paint particles.

Brief Description of the Drawings

The single figure of the drawing is a schematic view of a system for reclaiming paint overspray according to an embodiment of the present invention.

Best Mode for Carrying Out the Invention

A system for reclaiming paint overspray particles according to the method of the present invention is generally indicated by the reference numeral 10. The system 10 is shown in conjunction with a paint spray booth 12 having a spray gun 14 for directing atomized paint particles 16 towards a workpiece 18, and a water curtain 20 cascading down at least one wall of the booth 12. Water for the curtain 20 is pumped into the booth 12 from a supply tank 21. Alternate paint booth configurations may additionally use water, in the form of a curtain or spray, as a filter medium for air exhausted from the paint booth. In all cases, the purpose of the water is to capture and hold in_suspension the paint particles 16, known as overspray, which are not deposited on the workpiece.

The water curtain 20 containing the suspended paint overspray particles 16 forms an effluent 22 which flows downwardly and is discharged into a collection tank 24 positioned elevationally below the paint booth 12. The collection tank 24 includes a traveling screen filter 26 disposed at one end of the tank 24 for separating material from the effluent into low and high solids content fractions and transferring the high

solids content fraction as a slurry 27 to a holding tank 28. A conduit ^' 30 extends upwardly from the holding tank 28 and includes an in-line pump 32 for pumping the slurry 27 from the holding tank 28 to a mixing tank 34. The mixing tank 34 typically has a capacity of about 7500 litres (2000 gallons) and includes a motor-driven mixer 36 positioned within the mixing tank 34. A supply line 38 extends between the water supply tank 21 and the mixing tank 34. A pump 40 is disposed in the supply line 38 so that water from the supply tank 21 may be selectively pumped into the mixing tank 34 to controllably dilute the slurry contained within the mixing tank 34 to form a diluted slurry 41.

A transfer pump 42 is positioned at a lower discharge end of the mixing tank 34 and is connected to a conduit 44 extending between the transfer pump 42 and the inlet end of a decanter centrifuge 46. The decanter centrifuge 46 is well known in the art and is typically used for continuous clarification, dewatering ^or de-oiling of suspensions in various industries. One such decanter centrifuge suitable for the method of the present invention is an Alfa-Laval Model No. 207 liquid-solids centrifuge available from the DeLaval Separator Company, Poughkeepsie, N.Y. The decanter centrifuge has a first outlet 48 for discharging a separated solid component 50 and a second outlet 52 for discharging a separated liquid component 54.

The receiving station of a conveyor 56 is positioned elevationally below the first outlet 48 of the decanter centrifuge 46. A container 58 is positioned elevationally below the discharge station of the conveyor 56. The second outlet 52 of the decanter centrifuge is in communication with a pump 60 positioned between the second outlet 52 and a liquid component return line 62 extending between the pump 60

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and the water supply tank 21. An additional pump 64 is positioned between an intake line 66 terminating in an upper portion of the collection tank 24 and a return line 68 extending between the pump 64 and the water supply tank 21.

Industrial Applicability

In operation, the effluent 22 containing paint particles 16 suspended in water drains from the paint booth 12 into the collection tank 24. In the collection tank 24, the suspended particles are partially separated from the water by any one, or combination, of several known methods. In the system shown, a traveling bed filter 26 is used to separate the overspray particles 16 from the water carrier and transfer the resultant slurry 27 typically containing approximately 10% solid particles into the holding tank 28. Alternatively, the overspray particles may be separated by settling out onto the bottom of the collection tank. The upper clarified water is first pumped through the return line 68 and into the water supply tank 21 by the pump 64. The slurry 27 remaining in the lower portions of the collection tank typically contains approximately 50% solid ^' particles, and is manually removed from the collection tank 24 by shoveling or pumping into the holding tank 28. Although not required, the effluent 22 may be chemically treated with any one of several well known commercial flocculating agents to aid in the above-described separation procedures. The slurry 27 is pumped from the holding tank

28, through the conduit 30 and into the mixing tank 34. If required, water is pumped into the mixing tank 34 from the water supply tank 21, through the supply line 38, and mixed with the slurry 27 to form a diluted slurry 41 desirably containing solid particles in the

range of about 5% to 30%. It has been found that pumping and processing difficulties will occur if the solid content of the slurry 41 is greater than 30%. Operating inefficiencies will be encountered if the slurry 41 is over-diluted to the point where it contains less than 5% solid particles.

The slurry 41 is pumped from the mixing tank 34, through the conduit 44, to a central inlet port of the decanter centrifuge 46. As is well known in the art, the slurry 41 is distributed by centrifugal force onto the wall of a cavity inside the decanter centrifuge 46. A solid component 50 is deposited as a layer on the rotor wall thus leaving the liquid component 54 to form a radially inner ring, the depth of which is adjustable. A screw conveyor in the decanter centrifuge 46 transports the solid component 50 towards the first outlet 48 where it discharged by centrifugal force through the first outlet 48 and onto the conveyor 56. After centrifugal separation, the solid component 50 typically contains greater than about 75% by weight solid particles and is acceptable for use as a base for reclaimed paint products. As an added benefit, the solid component 50, being condensed and having a much smaller volume than the slurry 27 from which it was derived, generally qualifies as a "dry" disposable product. If desired for handling and future processing expediencies, the solid component 50 may be easily air dried or heated in an oven to reduce the moisture content to less than about 1% by weight.

The processed liquid component 54 in excess of that required to maintain the predetermined adjustable depth in the decanter centrifuge is discharged by gravity into the second outlet 52, and is then pumped through the return line 68 the water supply tank 21. After separation in the decanter centrifuge 46, the liquid component 54 typically contains less than 1% by weight solid particles and is highly acceptable for reuse in the paint booth water curtain 20.

The process of the present invention is particularly effective for reclaiming water-borne paint overspray particles. In one test, an effluent 22 having a 10% solids content was collected from a spray paint booth 12 using water-borne paint. After processing by the method of the present invention, the liquid component 54 contained only 0.2%, by weight, solids and the solid component 50 contained about 80%, by weight, solids. Drying the solid component resulted in a 20% weight reduction but no noticeable volumetric change. In a second test, an effluent 22 having a 50% solids content was collected from a spray paint booth 12 using water-borne paint. The effluent 22 was mixed with an equal amount of water to form a dilute slurry 41 containing about 25% solid particles. After processing by the method of the present invention, the liquid component 54 contained about 1.0%, by weight, solid particles and the solid component contained about 75%, by weight, solid particles.

Other aspects, objects and advantages of this invention can be obtained from a study of the drawing, the disclosure and the appended claims.