Calcium Hydroxide Reactor Question

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sculpin01

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Hi,

I built a stirring-type calcium hydroxide reactor (it sits on a stir plate with a rare earth magnet stir bar) that is 40" tall out of 4" diameter PVC with an integral pH probe. I typically add one cup of fresh calcium hydroxide (from BRS, purchased approximately 2 months ago) to the reactor to charge it. The reactor stirs constantly at 300 rpm with a standing 20" column of swirling milky undissolved calcium hydroxide and 20" of clear kalkwasser above that. Fresh water is introduced to the bottom of the reactor and CaOH-saturated kalkwasser is pushed out the top at 1 ml/min when the reactor is on. The reactor runs for approximately 14 hours per day, adding approximately 840 cc of CaOH-saturated water to the tank.

My dilemma is this. When I first fill the reactor with fresh CaOH and RODI water, the pH is 12.3. Given pH probe errors, I assume this is saturated. However, the pH of the effluent begins to drop linearly as soon as it comes online and by 10 days after filling is down to 9.5. Bear in mind, that there is still a 20" high vortex of milky CaOH swirling continuously at 300 rpm at the bottom of the reactor at this point. I feel like 1 cup of CaOH should last a bit longer than 10 days but would appreciate your insight into this matter.

Thanks,

Mike
 
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Randy Holmes-Farley

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I would NOT assume it is saturated. Often, it is not, which can be an issue with such stirrers. pH 9.5 is almost nothing. Each drop of 0.3 pH units from pH 12.5 is a drop of a factor of 2 in potency.

While conductivity is the best way to gauge potency (10.3 mS/cm is saturated at 25 deg C), pH can be crudely used.

Here's a post froma thread yesterday:

You can use a test kit, if you want. . Saturated is about 808 ppm calcium and 114 dKH at 25 deg C.

The easiest way is by conductivity. Saturated limewater is about 10.3 mS/cm at 25 deg C. I monitored it in real time as I mixed my reservoirs.

Crudest way is by pH. Since calibration is poor at this high end, measure pH in truly saturated limewater (say, 1 tablespoon stirred in one cup RO/DI. Cloudy is fine. Then, after rinsing the probe in RO/DI, compare that to the effluent pH. A drop of 0.3 pH units is about half potency.

True pH of saturated limewater at 25 deg C is 12.54. The pH is substantially higher at lower temperature (12.627 at 20ºC and 13.00 at 10ºC), and lower at higher temperature (12.289 at 30ºC; 11.984 at 40ºC)
 
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sculpin01

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Thanks for responding.

So, do you think there is functionally a chemocline in this reactor with essentially unsaturated water floating above the undissolved rotating CaOH column? If so, why does this occur? I would think that diffusion/mixing would drive equilibration/maximum saturation of the column above.

Would adding slow top-to-bottom recirculation to the reactor alleviate this issue?
 

Randy Holmes-Farley

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Thanks for responding.

So, do you think there is functionally a chemocline in this reactor with essentially unsaturated water floating above the undissolved rotating CaOH column? If so, why does this occur? I would think that diffusion/mixing would drive equilibration/maximum saturation of the column above.

Would adding slow top-to-bottom recirculation to the reactor alleviate this issue?

That is apparently sometimes the case.

Vertical circulation will help that issue, but may make the delivered limewater cloudy with undissolved particles.

You are constantly adding fresh water, and if the dissolution is not fast enough to keep up with the water addition, then it becomes unsaturated.
 
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sculpin01

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Based on pH, it stays unsaturated even when not adding water, which I initially assumed was "old" CaOH. I took it offline but still stirring 24 hour ago and the pH has slowly crept up from 9.5 to 10. I am going to add slow vertical circulation to it to see if that affects performance.
 
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