Conductivity of Rinse Water

A. Some typical specifications are < 5 or < 2 mg/L of dissolved solids. These are common in the automotive industry. It is more customary to use conductivity as the specification since it is easily measured. <5 micro-Siemens is a common specification.

However, it is not the conductivity or dissolved solids in the DI water that is most important, the most important thing is the quality of the last water on your product. Since you are rinsing off acid, there will be some elevated level of contamination in the water, and since DI water has no buffering capacity, it will take a lot of rinsing before the pH is near neutral.

A. Hi, Neysia.

The cleanest water gives the best rinsing, but clean water is expensive from a cost and wastewater volume standpoint. These facts make it very difficult to convince anyone who thinks the rinse rate should be higher, or anyone who thinks it should be lower to change their mind -- there simply isn't any scientifically determinable answer :-)

The best path may be to go back to the very old studies which showed that a good empirical starting point was a dilution rate of 500:1 - 1000:1 in rinses. So take a bit of each solution, dilute it 500:1 and measure its conductivity, and use that as a setting until someone can present a convincing case that it should be raised or lowered. You should consider having Kushner's "Water and Waste Control for the Plating Shop" ⇦[this on eBay, Amazon, AbeBooks affil links] in hand. Good luck.

Regards,

A. Hi Neysia,

Which quality has the feed water from your rinses at starting point and from which source does it come from? Also, what happens with your rinse water, how does it get treated before entering the public sewer? When you already have your conductivity monitoring available then why not using these data by setting a certain set-point by collecting as much data as necessary such as flow rates, surface of parts, rinsing technique etc and measuring in a high frequency the conductivity. Secondly recycle as much as you can and this is with today's technologies absolutely possible and an excellent controllable process to meet always specifications as required.
Water savings starts with Investments and upon requirements (& provided consultancy) payback rate can be hold by a low level.

Try it, it's worthwhile & interesting

Kind regards

A. Sometimes water in rinse tanks after plating or anodizing is controlled by water conductivity. Conductivity meters are not expensive and very simple to operate. I'm not familiar with galvanizing line, but probably it may work. You will have to establish your own baseline and limits.

A. Hi Philip,

I always have problems with measuring for measuring sake. Generating data that benefits no-one is wasted effort and cost.

If you are not having any quality issues and no problems with disposal of waste water, why would you analyse it? What would it benefit you? If you know the pH, total dissolved solids, zinc content or any number of other potential measurements, what are you going to do with the information?

A. Hi Philip!
I agree in theory with rinse tank disposal based upon contamination rather than calendar age, but to set parameters if there isn't anything called out by process specification ... you just need to have a sense of where the first fail point is, and set the dump trigger to be well before that.

Take a look back at old data, and see if Production or QA/QC has any documented failures you can correlate with it that weren't explained by other factors. So for example (and this is just hypothetical) you start to see issues in your product when the rinse tanks are above 1000 ppm TDS. You could say, I will check TDS on a daily basis and when it passes 800 ppm, schedule it to be dumped at the end of the week. There's no point in having it tested for specific contaminants except for environmental compliance, or if you are actively troubleshooting a plating defect... usually TDS is a good indicator of overall rinse health, and it takes only a moment to check. Certainly you could also test pH but I put less trust in that number because the meter will tell you only the raw pH, with no regard as to the total alkalinity or free acid content, and if the overall ionic strength of the solution is quite low... you could find yourself dumping a perfectly good tank that just looks bad to the pH meter! If I were to stick a pH meter in my lab DI water right now, I'll bet it would read about 5.5... pH alone is not nearly so useful to predict end of rinse tank life as TDS/conductivity.

A. We find that changing rinse tank water in galvanizing plants is based more on convenience than on quality, but of course only changed if the tank is "dirty". We find that a titration for HCl is enough to determine that a tank is really bad. (>1% HCl), but pH is also a good easy measure. (<pH 2)
There are two main methods of disposing of dirty rise water: Use some to make up new acid tanks, and dumping.
Dumping is expensive, as this becomes a waste that in the UK must be carried by licensed contractors in special vehicles, etc. Expensive, almost as much as dumping spent acid. Make up in acid tanks is the best, rather than using any fresh water! If the acid tanks and rinses are the same size, then a typical brew for a new acid tank is 1/3 each of rinse water, 28% HCl and spent acid.
The 28% is to get the HCl conc. up, the spent acid to get the pickling working (if a new acid tank has no iron chloride it pickles very slowly until it develops some of its own), and the old rinse water for dilution.
A typical galv plant has about 5-6 pickle tanks, and that means every time you make up a new acid tank, you'll remove about 1/3 the rise tank contents. Make up the rinse tank with fresh clean water.