The use of process baths for the batch processing of metallic items is common, and brings with it the potential for inefficient resource use, risks to human health and the pollution of water resources. The minimisation of drag-out losses is an important part of any strategy to mitigate these impacts.
The types of processes where drag-out is of relevance are operations such as degreasing, electroplating and surface treatment upstream of further processing, as examples. Workpieces are either mounted on jigs or placed inside a basket, and then dipped into chemical solutions, where processing occurs. Often the solutions are heated, using either steam or electrical heating elements.
Processing units like these typically comprise a number of individual process baths, each containing different solutions, and often with rinse tanks between operations where the contact between individual chemicals is to be avoided, or where the chemical treatment has to be followed by soil removal. The workpieces are dipped into each bath, held there for a specific period (sometimes with agitation) and then removed, before being transferred to the next step in the process. As the workpieces are removed from each bath, a small portion of the solution in that bath is “dragged out” with the workpieces, and this is where the term “drag-out” originates. So what makes “drag-out” so bad and what can we do about it?
The first and most obvious problem is that there is an economic loss associated with the chemicals that are removed with the workpieces, since these have to be replenished. This loss depends on the volume of drag-out and the concentration of the chemicals in the bath. In the case of heated solutions, energy is removed with the drag-out, and this energy has to be compensated for also.
Chemicals carried over to subsequent baths can have a negative impact on the operation of those baths due to interactions between the chemicals “dragged in” and the chemicals in the receiving bath. These interactions can impact on pH, cause precipitation and also result in any number of modes of interference. This can lead to product quality problems or require an increase in chemical concentration in the receiving bath, multiplying the economic loss.
Where the receiving bath is a rinse tank, drag-out can lead to effluent problems and increased rinse-water consumption. For industries dealing with heavy metals, which could even include carcinogens such as hexavalent chromium, the long-term environmental and human health impacts can be dire, particularly if the site concerned does not have an effluent treatment facility. Effluent treatment is in any case not a panacea, since waste/sludge requires safe disposal, and the life cycle risks still remain for the polluter.
The ability to deal with drag-out losses depends in part on the equipment employed, and this should be modified where it is not appropriate. In many cases, particularly in the case of smaller facilities, the process plant is manually operated. The jig or basket may therefore be moved from one bath to the next by hand. From a human health perspective this is not the best option, given the types of fumes that may arise and the dangers of skin contact with harmful chemicals. There are ways to deal with that, such as lip extraction systems and the use of the correct PPE, but I digress. Whether manually controlled or handled using robots, the tactics commonly used for drag-out loss reduction are the same, and are as follows:
- Employ a pause period once the workpieces have been lifted out of the bath to allow the solution to drip back into the bath before moving the workpieces to the next bath. This “drip time” can be optimised for different types of workpieces and for each individual bath. Its important to appreciate that an infinite drip time does not equate to zero drag-out, since the shape of individual workpieces will determine how much liquid is trapped and cannot be removed by drip time alone.
- Use mechanical action to dislodge trapped fluid. This can be achieved by tilting the basket or jig from side to side. Obviously the idea is not to make this so vigorous that surface damage occurs through scratching – this should not really happen if workpieces are mounted securely.
- Employ water sprays to rinse trapped solution from the workpieces. Care must be taken to ensure that bath contents are not excessively diluted with this approach. For small operations this can be done manually.
- Install drip trays between process baths. These trays should be designed to capture drag-out that drips from workpieces and then to direct this fluid back into the bath from which the workpieces have been removed. Drip trays also help with general plant housekeeping by minimising spills.
Employing any one of these approaches will help, but if you employ more than one approach or all of them together, you are bound to see a significant improvement in chemical usage, effluent quality and probably product quality as well.
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