These systems provide a large reduction in the proportion of the electroplating and post-treatment process solutions, which is usually dragged into the rinsing system. The drag-out is instead returned directly to the process tank before the workload in the barrel or centrifuge basket moves to the rinse stages. Note: These should not be confused with drag-out recovery by various means, which should still be applied in conjunction with process drag-out reduction.
The main route by which metal finishing process chemicals enter the environment is via the rinse system. Rinsing is required so that the film of process solution on the surfaces of the barrel or centrifuge basket and its workload is diluted sufficiently to avoid problems in the subsequent process. In the case of final rinsing, the barrel or centrifuge basket and workload must be thoroughly free of residues so that the plated components can be properly post-treated and handled by the machine operators, assembly workers, and the end user of the product.
The amount of rinse water used is directly related to the volume of process solution remaining on the surfaces of the barrel or centrifuge basket and its load during transfer from the process tank to the rinse system. This volume of process solution is known as “drag-out.” Other factors that affect the rinse flow rate and water consumption are the number of rinses used and the dilution ratio necessary for satisfactory processing.
Once they have entered the rinse system, process chemicals are sent to an effluent treatment system for treatment prior to discharge to the sewer. Sludge produced at the precipitation stage can be large and may require dumping in an approved site, or further treatment. Ion exchange systems require regeneration of the resins, which, in turn, produce some sludge and effluent. The physical size, investment, and operating costs are related to the volume and chemical load of the rinse water. The quantity of drag-out is, therefore, a critical factor governing the costs of the electroplating operation, particularly in the following areas:
- The cost of replacing the chemicals and additives from the process solution by drag-out.
- The cost of incoming water used for rinsing.
- The investment required for the electroplating equipment.
- The investment required for the wastewater treatment plant.
- The chemical and additive costs, plus spare part replacement required to operate the wastewater treatment plant.
- The labor costs of operating the wastewater treatment plant.
- The costs of sludge disposal and liquid effluent discharge.
There are a number of techniques available for dealing with specific process solution drag-out. Reclaim of metals by ion exchange or electrolysis; reclaim of solution from drag-out tanks; multiple cascade rinse systems; evaporators; and routing of rinse systems into warm process tanks using automatic control. The size and investment of these techniques is directly affected by the drag-out. Barrel blow down and DSC treatment and transfer systems can significantly reduce drag-out and the investment required in the aforementioned strategies.
SURFACE TREATMENT BARREL BLOW-DOWN SYSTEM
The blow-down method for reducing the quantity of drag-out per barrel and has been successfully installed in Europe and Asia over the last 10 years. How it works: The barrel and its load, upon removal from the process solution, is enclosed in a “blow-down” chamber.1 The chamber consists of a fixed shroud and a moveable shroud. The moveable shroud is mechanically actuated by the barrel during the transporter lift movement so that the barrel becomes enclosed, except for an opening to allow air in and an opening at the bottom to allow the process solution to fall back in the process tank. Air is introduced by a low-pressure fan blower mounted on the transporter, and during blow-down the barrel is indexed to allow trapped solution in the components to be discharged. A typical time for the blow-down is 10–12 seconds. Depending on the component geometry, the drag-out can be reduced by up to 60% when compared to 10–12 seconds of drainage without it.
A more recent development uses an additional 10–12 seconds of blow-down time during which approximately one liter of water is introduced into the air stream as a fine spray. Here, a further reduction of 20% in drag-out can be achieved.
Table 1: Typical Reductions in Drag-Out Volume
| ||Drag-Out (L/hr) ||Rinse Flow Rate (L/hr) ||Chemical Load (g/hr) |
|Barrel without "blow-down" ||40 ||2,828 ||4,800 |
|Barrel with "blow-down" ||20 ||1,414 ||2,400 |
|Barrel with "blow-down" and water spray ||12 ||848 ||1,440 |
After the blow-down time, the barrel is moved to the transporter to the rinse station; the moveable shroud automatically opens when the lower movement commences. Typical reductions in drag-out volume, rinse water usage and chemical usage are shown in Table 1 based on the following machine characteristics:
Barrel size: 1,000 × 360 mm across flats
Barrel load: 100 kg
Barrel drag-out: 2.5 L
Output: 16 barrels/hr (8 twin barrels)
Process chemical concentration: 120 g/L
Rinse system: twin counter flow
Dilution ratio: 5,000:1
DSC POST-TREATMENT SYSTEM
The DSC dip-spin-tilt transfer hoist incorporates centrifuge basket spinning and tilting capabilities. Centrifuge baskets loaded with parts can be immersed in treatment solutions and tilted or oscillated to manipulate the parts per a part process recipe. Actions that can be controlled are the tilting and rotation of the centrifuge basket to reorient parts and the spin speed of the centrifuge basket at any point during the treatment process. After parts are treated, the centrifuge basket can be positioned over the solution tank and spun at a high speed to recover solution directly back to the solution tank, tilted and rotated to reorient parts that tend to hold solution, and then spun at a high speed again to further reduce drag-out.
By spinning the centrifuge basket and reorienting parts, drag-out of solution can be reduced by up to 95% when compared with highly efficient plating barrels. Reducing drag-out of solution is especially important with costly passivation and topcoat chemistries. The DSC dip-spin-tilt transfer hoist addresses the need to reduce the amount of the costly chemicals used by recovering solution rather than dragging it out to the next solution tank. In addition to recovering the treatment solution, a reduction in the use of fresh water minimizes wastewater treatment.
The evaluations listed in Table 2 can be made when comparing DSC to barrel treatment.
Table 2: Reduce Wastewater:
Comparison of DSC vs. Standard Barrel Treatment
Material: M8 × 25 screws
Charges per hour: 16/hr
Load weight per charge: 176 lb
Surface area per pound: 80 in2/lb
Surface area per charge: 97.77 ft2/charge
Capacity per hour: 2,816 lb
Treated surface area per hour: 1,564 ft2/hr
1. Drag out of rinse water for each rinse using an efficient plating barrel
Drag-out per ft2 of surface area treated: 0.628 oz/ft2
Drag-out per charge: 61 oz/charge
Drag-out per hour per rinse station: 7.66 gal/hr/rinse
Rinse water sent to waste treatment to maintain the second rinse at a maximum concentration of 5 mg/l chromium, with the treatment bath maintained at 5 g/l chromium, 243 gal/hr
2. Drag-out of rinse water for each rinse using a DSC Centrifuge System
Drag-out per ft2 of surface area treated: 0.125 oz/ft2
Drag-out per charge: 12.27 oz/charge
Drag-out per hour per rinse station: 1.53 gal/hr/rinse
Rinse water sent to waste treatment to maintain the second rinse at a maximum concentration of 5 mg/l chromium, with the treatment bath maintained at 5 g/l chromium, 48 gal/hr
When utilizing a DSC hoist transfer system vs. a barrel treatment system, there is an 80% savings of rinse water and a 50% reduction of chrome sludge. (Fig. 1). The chemicals required to treat the rinse solutions when dumping the rinse tank are also reduced by 50%.
1. The Pal Barrel Blow-down System is licensed from Hans Henig. Applicable patent numbers: Germany 3830237, Japan 47 379/90, UK 2 241 248, USA 5,015,302.
ABOUT THE AUTHORS
Tony Evans, vice president of PAL Surface Treatment Systems, manages all elements of Process Automation Limited’s PSTS group for the development and supply of surface finishing and electroplating systems, including pollution-abatement equipment.
Aron Lorenz, president of WMV, Inc., has worked for WMV Apparatebau GmbH since 1998. He specializes in the development and integration of automatic painting and surface finishing systems.
Lothar Peukert, global technology and sales manager for WMV Apparatebau GmbH, manages and directs sales, project management, and research projects for cleaning, painting, and surface finishing systems.