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bjwingf

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Zinc removal?
Jun 03^rd, 2011, 4:16pm

Hi all,

I have some questions about zinc removal for an industry.

Currently they are using a inclined plate clarifier, but are not getting the flow through the clarifier that they need. There are two sides to the clarifier with 26 plates each. The plates are 8'x3' and spaced about 3 inches apart. Right now they are only getting 50 gpm through and my calculations show they should get more than that. Thoughts?

My other question is what pH is required to precipitate zinc? They bring it up to around 9.0 right now and I think that may need to go higher. Could that possibly create a larger floc that will settle easier in the clarifier?

Thanks for your help!

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Jeff Naumann

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Re: Zinc removal?
Reply #1 - Jun 3^rd, 2011, 5:16pm

The following is "borrowed" from a company's website:

To convert dissolved (ionic) metals into insoluble particles, the traditional method is to adjust the pH of the solution using caustic, such as sodium hydroxide. The precipitating reaction forms metal hydroxides. The results obtained are effected by several conditions - one of which is the pH of the solution.

Every dissolved metal has a distinct pH at which the optimum hydroxide precipitation will occur. Here are a some examples:
Cadmium pH 11.0
Copper pH 8.1
Chromium pH 7.5
Nickel pH 10.8
Zinc pH 10.1

Metal hydroxides are amphoteric, i.e., they are increasingly soluble at both low and high pH, and the point of minimum solubility (optimum pH for precipitation) occurs at a different pH value for every metal. At a pH at which the solubility of one metal hydroxide may be minimized, the solubility of another may be relatively high. Since metal hydroxides are quite soluble, many such hydroxides will start to go back into solution if the pH changes even slightly.

Wastewaters from industrial processes usually contain several metals. For example. the typical process wastewaters from printed circuit board manufacturing contain copper, tin, lead and nickel. Such mixed metals create a problem when using hydroxide precipitation since the ideal pH for one metal may put another metal back in solution.

In addition, chelators, sequestering agents, bath additives, cleaners and electroless formulations will interfere with the reaction of hydroxide precipitation. From a practical point of view, it is impossible to eliminate such components from a waste stream. Good treatment practices, such as segregation and pretreatment of some process waste streams, will minimize the anti-precipitant effects of such ingredients. Otherwise, an alternative precipitant may be required in conjunction with pH adjustment.

If the wastewater stream contains other metals, a "compromise" pH is often chosen.

Can you provide more information on the waste stream?

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bjwingf

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Re: Zinc removal?
Reply #2 - Jun 3^rd, 2011, 5:33pm

The wastestream is from a plating operation rinse water that contains zinc. To my knowledge the only metal is zinc and they raise ph to 9.0 and add polymer to floc the metals in the clarifier. Flowrate is approx 50 gpm.

It appears to me that they need to raise ph. The clarifier appears to be large enough unless it is channeling all water through only a few plates.

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Jeff Naumann God Member Offline I love Water and Wastewater.com Posts: 1133 Torrance, California Gender: Company or Organization: Jeff Naumann & Associates	Re: Zinc removal? Reply #3 - Jun 3^rd, 2011, 5:51pm In my experience, few "plating shops" have only one metal in their wastewater. You should find out what the complete metals analysis is for their waste.
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grrun

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Re: Zinc removal?
Reply #4 - Jun 3^rd, 2011, 11:17pm

I assume that the discharge analysis is reported as Zn. To decide if the Zn is soluble or insoluble; compare discharge samples unfiltered and filtered through a millipore filter. If the unfiltered and filtered samples are approximately the same concentration; then do some jar testing by increasing the pH (as Jeff points out Zn is amphoteric and may possibly be complexed).

If the unfiltered sample is much higher than the filtered sample; the the Zn hydroxide is floating out of the clarifier and you need to do some jar testing to find something to settle the Zn(OH)₂ better.

If flowrate or flow distribution is a problem, that is best discovered by observation of the surface of the clarifier. Don't forget to calibrate the pH meter or controller (too low or too high of a pH will increase the solubility of Zn).

grrun

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bjwingf YaBB Newbies Offline Posts: 17 Gender: Company or Organization: Pollution Management	Re: Zinc removal? Reply #5 - Jun 4^th, 2011, 8:05am Is there a calculation based on plate surface area to determine flow? I have a spreadsheet but would like to double check.
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grrun

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Re: Zinc removal?
Reply #6 - Jun 6^th, 2011, 2:45pm

pH may determine the solubility of the Zn(OH)₂, but other factors may determine the settling rate (polymers) and that is why I inquired about the characteristics of the Zn in the discharge. I suspect that there are calculations for maximum flow rates for inclined plate clarifier; but not for minimum flowrates. Also, I doubt that there are settling rates for many individual compounds because this is usually determined empirically.

Therefore, the upward velocity probably is not great enough to float the Zn(OH)₂. However, the plate inclination may not be great enough to cause the settled solids to slide downward and the plates could possibly be plugged with sludge (in my estimation, unlikely). Jar tests may help you determine the approximate settling rate for the treated sludge at various pHs and also the effect of various polymers. It may be that your pH limit is 9.0 and your plant does not (or will not) want to add another neutralization operation to the discharge.

The critical factor is the distance between plates rather than the surface area of the plates.

grrun

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SludgeHound Senior Member Offline Water and Wastewater.com is the best! Posts: 388 Gender: Company or Organization: Nalco Company	Re: Zinc removal? Reply #7 - Jun 12^th, 2011, 8:57am In my experience Zn precipitates better at about 10.5 pH. A small amount of anionic flocculant will build particle size and give good settling.
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Dedalus Full Member Offline Science is a business of empiricism. Posts: 238 NY Gender: Company or Organization: Dedalus Environmental	Re: Zinc removal? Reply #8 - Jun 21^st, 2011, 3:16pm When I used to treat cyanide Zn wastewater, after CN destruct, I used pH 9.2 as my standard to drop the Zn. I also added some ferrous sulfate. An old contributor to this board, 'Catman', who works in a captive plating shop that does chloride Zn plating for Caterpillar Tractor, thought 9.5 worked better.
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bjwingf YaBB Newbies Offline Posts: 17 Gender: Company or Organization: Pollution Management	Re: Zinc removal? Reply #9 - Jun 21^st, 2011, 4:00pm I've got the problem solved...we adjusted the ph to 10.2 and were able to get the flow we needed through the clarifier. We even lowered to 9.5 and still got favorable results. Thanks for the tips everybody. This is a great place and source of info.
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watermeter_sh@yahoo.com YaBB Newbies Offline I love Water and Wastewater.com Posts: 6 Company or Organization: Shang Hai Tian Shen Copper Inc	Re: Zinc removal? Reply #10 - Jun 23^rd, 2011, 9:50pm i really have learned a lot here by reading your posts. Tks a lot!
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maxxypane Ex Member	Re: Zinc removal? Reply #11 - Mar 31^st, 2012, 12:07am The solubility of zinc will depend on temperature & pH of the water. When the pH is fairly natural, zinc in water insoluble. Solubility will increases with increasing level of acidity. Above pH 11, solubility also increases.
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