by Rudy Sedlak
Recently one of the forum users brought up the fact that one of the vendors is pushing Liquid cleaners combined with a feed and bleed system, and wondered about the wisdom of these approaches. My principle and most recent experience is in printed circuit board (PCB) production, and the PCB industry is ahead of the metal finishing industry in switching to liquids, and to feed and bleed, and the lessons learned here might be helpful.
Liquid cleaners vs dry are a difficult call, except on a case by case basis. I do not know what rationale they are being sold on, but it is important to recognize that they are frequently more expensive per unit of activity, in spite of the fact that the "filler" is free. Liquid cleaners are very difficult to formulate in concentrated form, and the alkaline liquid cleaners frequently require the use of Potassium salts, instead of Sodium salts, because of solubility reasons. Potassium salts are much more expensive per unit of activity, in that they are not only more expensive per pound, but they are less active per pound.
If you look at the economics of the use of Potassium Hydroxide (KOH) vs. Sodium Hydroxide (NaOH), you can get an idea of the problem. If my memory serves me correctly, KOH is 2-3 times as expensive as NaOH per pound. However since we are only interested in the [OH-] part of the caustic, and the Sodium or Potassium is merely along for the ride as a neutralizer for that part of the molecule, we have to analyze for the economics per pound of [OH-], and things get worse. In NaOH, the [OH-] is 17/40 parts of the molecule, or 42.5% of the molecule. In KOH the [OH-] is 17/56.1 or 30.3% of the molecule. So if the KOH is, let's say, 2.5 times the cost per pound of the NaOH, the [OH-] part is actually 2.5 X 42.5/30.3 = 3.5 times as costly.
This is complicated by the fact that many surfactants and especially the cheap ones are not real soluble in concentrated liquid systems. So the surfactants may have to be chosen not on whether they are optimum for cleaning, but on the basis of solubility/compatibility with a concentrated salt solution. Further if the cleaners have organic anti-tarnish or corrosion inhibitors, the good ones, that wash off with the rinse waters are also not real soluble in the presence of concentrated salt solutions.
As you can see, I am no great fan of changing to liquid systems just to have a liquid system. There are some products that inherently are more appropriate as liquids, naturally, and obviously a solvent or liquid acid based cleaner would be, but jumping to a liquid just to get ease of feeding, may compromise both economics and performance. Now let us address Feed and Bleed, also known as the "vendors guaranteed bonus" system. My experience to date, has it that Feed and Bleed, set up where there is an attempt to approach a steady state, no-dump mode, increases chemical usage about 2 to 3 fold. There may be a rationale for this in PCB manufacturing, where they get a lot of money per unit area, but general metal finishing does not have this luxury.
There are some interesting possibilities, that improve the economics of feed and bleed, at least at first glance, such as feed and bleed, combined with a cascading multiple tank cleaner system. However, this concept requires a lot of floor space, and ignores the evaporation heat loss effects on the power bill.
Overall the best compromise I have ever seen for this situation is a two tank, old/new cleaner system. This is the situation where two tanks are used in line, with the first tank in the line being the one that is dumped regularly, and the second tank is then transferred into the first tank, rt may require divine influence, at least in California.
Hope this sheds some light, Rudy Sedlak
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