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Pollution Prevention and Control Technologies for Plating Operations


Section 2 - General Waste Reduction Practices

2.4 DRAG-OUT REDUCTION

2.4.2 Drag-Out Reduction Techniques

2.4.2.2 Direct Drag-Out Return
2.4.2.2.1 Draining/Rinsing Over the Plating Tank
2.4.2.2.2 Other Methods of Direct Drag-Out Return


2.4.2.2 Direct Drag-Out Return

Commercially available equipment for the recovery of plating bath chemicals includes types that apply such principles as ion exchange, reverse osmosis, electrodialysis, and evaporation. These devices usually are applied to a single plating operation where they concentrate the salts in the rinse water, return them to the plating bath, and recycle the purified water to rinse tanks (ref. 305).

Although effective, these recovery technologies are capital intensive. Before the purchases of such equipment, metal finishing facilities usually implement simple methods of drag-out recovery that require much less capital and are simpler to operate. After using these methods and establishing new drag-out conditions, the plater can consider the applicability of additional recovery through commercially available units (ref. 305). This section describes methods that directly return the drag-out to the process tank. In the following section (Section 2.4.2.3) methods are described that recover the drag-out in tanks and then return it to the process bath.

2.4.2.2.1 Draining/Rinsing Over the Plating Tank

After a rack or barrel is removed from a process tank, the drag-out drains from the item and it returns directly to the bath as long as the item is held over the tank. This simple method of direct drag-out return can be maximized on a hand-line by installing a bar over the process line on which the operator can hang a rack or hook. On automatic machines, the unit can be programmed to increase dwell time above the process tank. For barrel operations, the barrel can be rotated over the process tank to help free the drag-out. Two shops gave this pollution prevention method a success rate of 5 (PS 088 and PS 243). Altmayer indicates that continuous barrel rotation is not always the best technique for freeing drag-out. He suggests that cup shaped parts should be rotated several times and then stopped and that free draining parts should not be rotated or rotated once and then stopped (ref. 482). As with many drag-out reduction methods, some experimentation will help to identify the optimal procedure.

The quality of a metal finishing process can be diminished if the drag-out is permitted to dry on the part. This can cause staining, peeling (PS 183), passivation, or it may prevent complete rinsing. To increase the drag-out removal rate over the process tank, rinsing with small amounts of water can be employed. The amount of water that can be used will depend on the water balance for a given process tank. The water balance is affected mostly by evaporation. Process solutions operated at temperatures greater than 120°F often have sufficient surface evaporation such that rinsing can be performed over the tank. However, using this method may reduce or eliminate the potential benefit from other drag-out recovery methods (e.g., use of a drag-out tank).

Rinsing over the tank can be performed by flood rinsing (e.g., hose), spray rinsing, or fog rinsing. The use of flood rinsing is not practical except for very high temperature baths with high drag-out rates. Spray rinsing uses less water than flood rinsing. With the proper selection of spray nozzles this can be a very efficient method of direct drag-out return. Nozzle selection should consider: flow rate, spray velocity, and spray pattern. Air-assisted sprays are also utilized, which are generally more efficient than plain water sprays. Sprays can be hand-held or mounted on the tank rim. For automatic plating machines, the sprays are controlled to operate only when the part exits the bath. For example, PS 193 employs an automatic rinse spray that actuates during vertical lift for a period of 7 seconds.

The use of spray rinsing is not limited to racked parts. It can also be applied to barrel lines. Exhibit 2-11 shows how a barrel plating spray rinse can be employed.

A novel barrel draining method is shown in Exhibit 2-12. This system, which can be retrofit to an existing barrel plating line, makes use of an air purge to push solution out of a barrel. Vendor tests showed that simple rotation removes 37% of the drag-out after 20 seconds (no further dripping) while the new air purge system removes 63% of the drag-out (tests performed with brass plating solution).

Fog rinsing is used at exit stations of process tanks. A fine fog is sprayed on the work, diluting the drag-out film and causing a run-back into the process solution. Fog rinsing is applied when process operating temperatures, high enough to produce a high evaporation rate, allow replacement water to be added to the process in this manner. Fog rinsing prevents dry-on patterns by cooling the workpieces, but it may preclude the use of a drag-out tank as a recovery option. For fog rinsing to be effective, work must be withdrawn from the process tank at a slow rate (ref. 305). PS 295 successfully (rated "5") employs a fog rinse over a drip tank rather than a process tank.

As with any pollution prevention method, fog/spray rinsing over the bath has potential drawbacks and problems. When used in conjunction with a ventilated tank, the spray may increase the pollutant loading to a scrubber or be directly vented to the atmosphere. Fog/spray rinsing may be messy as indicated by one respondent (PS 124), or worse, may cause splashing on nearby workers. One shop indicated that the nozzles required frequent maintenance (i.e., unplugging) and must be occasionally repositioned to point in the correct direction (PS 124).

These methods of drag-out reduction generally work best for automated operations. Manual operations are too dependent on the behavior of the operator. For example, operators "rush the dwell time" when draining over process tanks (PS 086) and "doesn't work due to tired platers...also operators want to increase production and do not take the time to hold the racks to allow extra drainage time" (PS 210).

Survey questions that relate to draining and fog or spray rinsing over process baths include questions 7, 8, 11 and 12 of Exhibit 2-10. The response indicates that 192 (or 60.4%) and 82 (or 25.8%) of the survey respondents implement draining, respectively for manual or automatic plating operations. The average success ratings were 3.44 (manual) and 3.79 (automatic). Fog and spray rinsing over the process tank is less frequently used by the respondents. As shown in Exhibit 2-10, 50 (or 18.9%) respondents use this method with manual operations and 34 (or 10.7%) use this method with automatic operations. The average success ratings were 3.67 (manual) and 3.35 (automatic).

2.4.2.2.2 Other Methods of Direct Drag-Out Return

The following are miscellaneous methods of direct drag-out return that are not discussed elsewhere in the report.

A drain board or drip shield is a tilted surface placed between process and rinse tanks that catches the drips from racks or barrels as they are transferred between tanks, thus preventing the drag-out from falling to the floor. The solution on the drain board returns to its original tank by gravity flow. The drain surface can be plastic or metal. For acid solutions, the best materials are vinyl chloride, polypropylene, polyethylene, and Teflon®-lined steel. Stainless steel should be used for hot alkaline solutions. It is important that the drain surface be positioned at an angle that allows the plating solution to return to the bath (i.e., rather than the subsequent rinse) (ref. 305).

The use of drain boards or drip shields was a frequently used method of drag-out loss prevention by survey respondents. As indicated in question 15 of Exhibit 2-10, 181 survey respondents (or 56.9%) use this method. The average success rating was 3.68.

Another direct drag-out return method, the air knife, is a device that blows an intensive air stream at a part/rack as it exits the bath causing the drag-out to be blown off. The use of air knives is limited due to: (1) the potential to dislodge parts from racks; and (2) the drying effect of the air stream which may cause staining, passivation, etc. Concerning the second limitation, Altmayer suggests that if the air is humidified to near saturation that drying will not occur (ref. 482). One respondent noted that the air supply must be oil-free (PS 257). One modern use of this device is with semi-aqueous cleaning systems, where the air knife is employed to minimize the quantity of solvent drag-out entering the water rinse system.

Air knives are not frequently used according to the survey respondents. Only 7 (or 2.2%) (manual) and 18 (or 5.7%) (automatic) of the shops surveyed employ this method of drag-out loss prevention. These shops rated the success level of air knives at 3.14 (manual) and 3.72 (automatic).


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