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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
2.4.2.1 Minimizing Drag-Out Formation
2.4.2.1.1 Controlling Plating Solutions
2.4.2.1.2 Position on Rack
2.4.2.1.3 Workpiece Withdrawal
2.4.2.1.4 Design and Maintenance of Racks and Barrels
2.4.2.1 Minimizing Drag-Out Formation
The most obvious source of pollution in a metal finishing shop is the drag-out of various processing baths into subsequent rinses. The amount of pollutants contributed by drag-out is a function of factors such as the design of the racks or barrels carrying the parts to be plated, the shape of the parts, and plating procedures. As previously discussed, several interrelated parameters of the process solution, including the concentration of process chemicals, temperature, viscosity, and surface tension also impact pollution levels.
Many devices and procedures can be used successfully to reduce drag-out. These techniques are usually employed to alter those important and interrelated process solution parameters.
2.4.2.1.1 Controlling Plating Solutions
As a rule, as the chemical content of a solution is increased, its viscosity increases. Increased viscosity contributes not only to a large volume of drag-out, but also to a higher chemical concentration of drag-out. The consequent need for more rinse water creates additional pollution control problems. Plating baths can often be operated at significantly lower concentrations than those recommended by chemical manufacturers.
Of the 318 plating shops responding to the Users Survey, 110 (or 34.6%) indicated that they have lowered the concentration of a plating bath to reduce drag-out losses. The average success rating given by the respondents for this pollution prevention tool was 3.33 (see question 16 of Exhibit 2-10).
As mentioned in Section 2.4.1, temperature is interrelated with viscosity and surface tension. A total of 57 survey respondents (or 17.9%) indicated that they have tried increasing bath temperature to reduce viscosity and hence decrease drag-out formation. The average success rating for this method was 3.14 (see question 17 of Exhibit 2-10), which was the second to the lowest success rating of all pollution prevention options listed in the survey form.
For years wetting agents have been used in process solutions to aid in the plating process. These substances are used, for instance, in bright-nickel plating to promote disengagement of hydrogen bubbles at the cathode. They are also used as an aid to drag-out reduction. A wetting agent is a substance, usually a surfactant, that reduces the surface tension of a liquid, causing it to spread more readily on a solid surface. A typical plating bath solution has a surface tension close to that of pure water at room temperature or about 0.0050 lb/ft. The addition of very small amounts of surfactants can reduce surface tension considerably-to as little as 0.0017 to 0.0024 lb/ft (ref. 1, 39, 305). Further additions of the wetting agent will not lower the surface tension appreciably beyond this point (ref. 521).
Kushner (ref. 1) estimates that the use of wetting agents will reduce drag-out loss by as much as 50 percent, although no test data or other quantitative information are presented. He recommends the use of non-ionic wetting agents that are not harmed by electrolysis in the plating bath. Platers contemplating the use of a wetting agent for drag-out reduction should conduct experiments to determine their potential benefit before implementation. Also, platers should investigate the compatibility of a wetting agent with the bath chemistry before use. Some process baths (e.g., hard chromium) can only tolerate certain products (ref. 482).
In addition to reducing drag-out, wetting agents are used to reduce chromic acid mist formation during plating. The lower surface tension achieved with wetting agents reduces the effect that oxygen and hydrogen bubbles (generated at the tankÕs electrodes) have when they reach the surface of the bath (ref. 521).
Of the 318 plating shops responding to the Users Survey, 103 (or 32.4%) indicated that they use a wetting agent to reduce drag-out. The average success rating given by the respondents for this pollution prevention tool was 3.12 (lowest rated method).
One survey respondent that uses wetting agents made an interesting point, "I have no way of telling if wetting agents are effective." That shop rated this method at a success level of two. Shops that would like to quantify the effect of wetting agents on drag-out can perform simple drag-out tests like those described in the literature (ref. 1, 20, 305) using a sample of their standard bath and one to which a wetting agent has been added. Also, the surface tension of a bath can be measured with an inexpensive device called a stalagmometer.
Kushner further suggests keeping the concentration of all dissolved salts at the minimum needed for proper operation. To follow this recommendation, the plater should not permit substances to buildup in the plating bath, if it is possible to control and maintain them at the proper level. For example, cyanide baths are often permitted to buildup very high carbonate concentrations even though the concentration level could be controlled by treatment (e.g., carbonate freezing). Such a buildup could increase drag-out by as much as 50 percent (ref. 305).
The metal finisher's primary consideration in the positioning of workpieces on a rack is proper exposure of the parts to the anodes for optimal coverage and uniform thickness of the electrodeposit. Drainage and rinsability are secondary, but very important considerations to pollution control. Also, damage to the workpiece surface can be caused by insufficient or inefficient rinsing, and succeeding process solutions can be contaminated by drag-in of unremoved chemicals from the previous solution (ref. 305).
Several rules apply to the position of work on plating racks for drag-out minimization. The basic principle, however, is that every object can be positioned in at least one way that will produce the minimum quantity of drag-out. This position could be determined by experiment, but unless a significant number of similar items are to be plated, it may be advisable to follow the suggestions of Kushner and Wallace (ref. 1, 305):
- Tilt all solid objects with plane or single-curved surfaces so that drainage is consolidated, that is, twist or turn the part so that the clinging fluid will flow together and off the part by the quickest route.
- If possible, avoid racking parts directly over one another to prevent lengthening the drainage path of the plating solution.
- Avoid table-like surfaces by tipping the part, but not at the expense of forming solution "pockets."
- Orient parts so that only a small surface area comes in contact with the liquid surface as it leaves the plating solution.
From the Users Survey, 165 (or 51.9%) shops indicated that they concern themselves with workpiece position in an effort to reduce drag-out formation. The average success rating given by the respondents for this pollution prevention method was 3.75 (see question 19 of Exhibit 2-10).
2.4.2.1.3 Workpiece Withdrawal
The velocity at which work is withdrawn from the process tank has a major effect on drag-out volume. The faster an item is pulled out of the tank, the thicker the drag-out layer will be because viscosity forces do not have a chance to operate and a much larger volume of liquid will cling to the surface (ref. 305). The effect is so dramatic that Kushner (ref. 1) suggests that most of the time allowed for withdrawing and draining the item should be used for withdrawal. An automatic machine that performs smooth, gradual withdrawal usually will drag out less solution per item racked than will manually operated equipment.
Questions 5 and 6 in Exhibit 2-10 indicated that reducing the speed of withdrawal is practiced by 69 respondents (or 21.7%) with automatic plating equipment and 121 respondents (or 38.1%) with manual lines. The success ratings were 3.61 and 3.23 respectively.
2.4.2.1.4 Design and Maintenance of Racks and Barrels
The transport of chemicals inside loose rack coatings from one process to another is not uncommon. For example, chromium may appear in rinse waters that are discharged into a plant's industrial sewer some distance from the chromium process tanks. The chromium often reaches these remote areas by way of loose rack coatings. Increased attention to rack maintenance not only will eliminate this potential problem but also will contribute to a welcomed reduction in the number of workpieces rejected because of poor contact (ref. 305).
Improving the design of racks, baskets, etc. to reduce the amount of solution they can trap will also reduce drag-out. For example, one shop reported great success by replacing old baskets with ones that had larger holes (PS 55). Many shops indicated that they have redesigned their racks to reduce pollution (e.g., PS 025, PS 058, PS 089, PS 159, PS 173, PS 192, PS 196, PS 229, PS 301, PS 316). Similarly, many barrel plater reduced drag-out by increasing the hole size of the barrels or making other design changes (e.g., PS 132, PS 214, PS 229).
As a group, the survey respondents indicated that regular rack/barrel maintenance was an important pollution prevention method. A total of 207 respondents (or 65.1%) indicated that they implement this method and the average success rating was 3.77 (see question 16 of Exhibit 2-6).