stripping IMPORTANT ASPECTS OF STRIP TANK DESIGN BY CRAIG A. BURKART ONDEO NALCO CO., CINCINNATI

This information has been compiled to help in answering strip tank design ques­tions, and in recognizing design problems before and during installation of a paint stripping system. No design is perfect, but for a stripping operation to function properly, certain features must be in place. Diphase systems are particularly de­manding, and the tank design must be sound for a start-up to be successful.

CONSTRUCTION/DESIGN Both strip tanks and rinse tanks should be constructed of stainless steel or mild steel. It is recommended that a lid, which is constructed of stainless steel or mild steel, be in place to reduce evaporation. The lid may be hinged or fitted and liftable. Polyethylene or polypropylene balls can also be used to reduce evaporation, but must be rated for continuous use at 220°F. Local ventilation should consist of a canopy or side-draft hood constructed of stainless steel, mild steel, galvanized steel, or chlorinated polyvinyl chloride (CPVC). (Note: Galvanized steel should be avoided when a high caustic stripping system is used.) A lid will also assist in re­ducing vapors, thereby reducing the amount of ventilation required. Figure 1 is a drawing of a typical strip tank installation.

All pumps should be constructed of stainless steel or cast iron. Piping should be stainless steel or black iron. Valves and fittings may not be constructed of brass, aluminum, or other soft metals. Teflon seals and diaphragms are recom­mended for all pumps that run continuously to circulate the stripping solution. Racks and baskets may be constructed of mild steel to minimize cost.

Tank design should incorporate efficient use of time. When operating a batch process, multiple baskets and a modular design provide for faster throughput. Two 4-6-in.l-beams welded to the floor of the tank and running the length of the tank will support the baskets above the sludge layer. Solution level monitoring equipment should be installed to prevent a boil down. It can be designed to au­tomatically fill or to set off an alarm to warn the operator to fill when the level is low. Over-fill protection should also be installed.

AGITATION METHODS When mixing by recirculation, the pump capacity should have the ability to turn the entire tank over a minimum of five times per hour. The pump intake should be piped 12-24 in. off the bottom of the tank, and the pump discharge should be piped 6-12 in. below the average solution level. This setup is less likely to re­circulate sludge from the bottom and is also less likely to lose efficiency should the solution level drop quickly. Both the intake and discharge manifolds should be on the same side (face) of the tank. This will establish a circular flow and force unwanted solids downward. A shower bar may help to spread the flow across the entire surface of the tank.

When mixing with a propeller or lightning-type mixer, it is very important that the propeller and shaft be proteered from damage by a metal flange or bar.

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STEAM HEATING, COIL

PUMP

PUMP

I-BEAM

EXHAUST

HOOD

Fig. 1. Drawing of a typical strip tank installation.

FLOAT

HOOD

STEAM 1:

Baskets or parts can easily come into contact with the mixer if this simple pre­caution is ignored. The optimum position for one mixer is in the center of one face with the shaft positioned 5-15 0 from vertical and the propeller placed one prop diameter from the bottom of the tank.

With two mixers, the positioning is the same, except that they should be placed in opposite corners of the tank. If the tank is loaded heavily with parts, or the propellers are not allowed adequate clearance, less than adequate agitation can result due ro restricted flow.

Air injection may be used on occasion. More efficient use of the air supplied can be accomplished by using multiple hoses or an attached shower bar with multiple holes. With air injection, the solids tend to be more dispersed through­out the solution. Air bubbles tend to attach to particles of certain sizes and make them float. This gives the solids an upward, instead of the desired down­ward, flow. Air injection cannot be used in all stripping systems due to possible solids precipitation. Injected air contains carbon dioxide, which is acid, and may cause the pH to drop.

Operating a strip tank at a rolling boil provides a natural form of recirculation. No additional equipment is required and, in general, the stripping system will per­form best at the boiling point. The size and placement of the heating element will generally determine the amount of tutbulence in the tank.

HEATING METHOD The preferred heating method is with steam coils. A boiler dedicated to the strip tank will provide constant steam pressure. A nondedicated boiler's pressure may vary based on other system demands, which will cause the strip tank temperature to fluctuate. Gas-fired tubes are a good source of heat, and would be a close sec­ond to steam. A direct-fired burner (one in which the flame is expelled into the solution) is not recommended due to the presence of water-soluble and water­insoluble solvents (in the case of diphase systems). Gas burners placed underneath the tank are very inefficient, and, therefore, are not recommended.

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Electric immersion heaters are not recommended due to their tendency to be­come coated with scale or charred paint and then burn themselves out. If electric immersion heaters are to be employed, make sure they are constructed of stainless steel or are Teflon coated. The placement of any heat source is important to the over­a!! operation of the tank. The heat source, be it electric, gas, or steam, should always be above the sludge layer (6-12 in. above the bottom of the tank). Insulating the tank is a good idea and will make the operation more efficient and safe.

One other uncommon method of heating that can be effective is with a wa­ter- or oil-jacketed tank. The strip tank is surrounded by a water/oil jacket. The water/oil is heated, and that heat is transferred to the stripping solution. This re­duces the scale buildup usually associated with electric heaters because the heater is in the water/oil and not exposed to the stripping solution. This method has one drawback: because water has a limited heating capacity when used in this man­ner, optimum temperatures often cannot be reached.

SOLIDS REMOVAL Unwanted paint solids are the cause of failure of most paint stripping systems and must be removed to extend solution life. Solids consist of undissolved paint pieces or particles and dissolved reaction products of the paint with the caustic. More than anything else, the stripping mechanism dictates what type of solids removal method will be effective.

Solids can be removed continuously by several methods. Overflow screening can be used for large paint pieces. The pieces are forced over a weir into a screen and collected for disposal. Sometimes the basket designed to hold the parts in the tank is of a mesh size suitable to perform this function by itself. Every time the bas­ket is removed, the paint pieces cling to it and are flushed away with the rinse.

The most effective method for removing small pieces and particles is contin­uous filtration. Selection of the correct filter media is important if filtration is to be effective. For example, most powder paint particles are < 150 rm, and a substantial number of them are <50 rm. Therefore, selection of a 250-rm filter will not provide adequate removal. If, however, the paint breaks up into pieces 1 em in diameter, the 250-rm filter is overkill. Also, the pumping system for fil­tration should be separate from the main recirculation system to avoid lapses in agitation when the filter plugs. The filter pump flow must be engineered in such a manner that it works with the main recirculation system to coax solids to the filter pump intake. The previously mentioned I beams on the floor of the tank will guide solids to the intake.

Solids can also be removed via batch processes. A removable screen can be placed in the bottom of the tank to collect large pieces and then be periodically removed, rinsed, and replaced. Filtration can also take place on a batch basis. What was dis­cussed above applies here as well. Larger pieces can also be removed by batch fil­tration. One of the most effective batch solids removal processes is a decant/ desludge procedure, where the solids are allowed to settle and the liquid is decanted. The sludge or paint pieces are removed from the tank, and the liquid is resumed. This method is effective no matter what the condition of the pamt. Solids removal can provide the benefits oflonger tank life and reduced chemical usage.

RINSING METHOD An overflowing dip tank is often employed as a rinse tank in a stripping opera-

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tion, but dip rinsing is not effective when the paint is removed as sheets or pieces. This rinse tank sometimes contains a small amount of rust inhibitor to prevent steel parts from flash rusting. Other times the tank will contain acid (usually phos­phoric) to provide for an "acid-shock rinse" (pH of 4-5) on aluminum parts, or for removing flash rust from steel. There may also be two tanks, one containing acid and one containing rust inhibitor. It is important that the rinse tank have the same (or close to the same) volume as the strip tank if decanting is to take place. During a decant/ desludge procedure, the rinse tank can serve as the holding tank for the stripping solution. Dip rinsing is effective only when paint dissolves.

Pressurized rinsing is usually required when paint is removed as sheets or pieces. Paint that sheets will tend to cling to edges, abraded areas, and in blind holes. One example is epoxy powder, which always requires high pressure rins­ing. Normal household water pressure is 40-50 psi. In stripping terms, mild rinsing pressure is 50- 500 psi, medium pressure is 500- 1,000 psi, and high pressure is 1,000+ psi. So-called water blasting equipment can generate pres­sures up to 20,000 psi, but is not normally necessary.

Generally, an area adjacent to the strip tank that is set up for pressurized rinsing will have a pit covered by a grating for catching the water and paint residues. This pit is cleaned as needed. The water goes to waste treatment or can be recirculated to the strip tank as make-up water. If a dip tank is unavailable, rust inhibitor is sometimes proportioned into the spray rinse. Often, a com­bination of pressurized rinsing, acid dipping, and rust inhibiting is required to produce parts that are ready for repainting.

IN-LINE STRIPPING In-line stripping is used to improve hanger cleanliness and proper electrosta­tic grounding. Both conditions, however, are related to finished part quality. Whether the paint is liquid or powder, clean hangers and good grounding provide good paint transfer efficiency and quality of finish. An in-line stripping system is designed to strip one coat of paint (or one pass through the paint line) from the hangers. Because the hangers never leave the conveyor, and the con­veyor is set at a certain speed based on production, the time the hangers are in the strip tank is fixed. The strip tank must, therefore, be large enough to allow adequate strip time. Moreover, the entire system must be dedicated to remov­ing that one coat/one pass every time.

The design of the system must take the paint removal mechanism into ac­count. If that removal mechanism is sheeting or not completely dissolving, then a spray rinse must be used. In fact, most in-line stripping systems employ a spray rinse. The compartment is very similar to a typical spray washer. It is important to note, however, that the rinse pressure must be enough to knock any undissolved paint off the hangers. Recommended pressures vary from 30-80 psi. Less than 30 psi is not recommended. Maintenance of the rinse sec­tion is just as important as maintenance of the stripping solution because both the stripper and rinse are integral parts of this in-line system. Rinse stages should be inspected like any spray washer, and any plugged or misaligned noz­zles or risers should be adjusted. As with batch paint stripping systems, the rin­sewater can be counterflowed to be used as the make-up water for the strip tank.

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