3 replies to this topic

[siretb]

Hello.

I have a very severe scaling problem in a plant.
We raise the pH of hard waters (300 mg/l calcium, 30 mg/l CO2/HCO3-, at 72°C) from acidic to more than 10). The flowrate is small (400 l/hr or 1.8 gpm) . Also, we are gypsum-saturated.
Of course the Langelier Index indicates that there is danger. And we do scale, despite the fact that there is a tank with suspended solids in it; scaling occurs downstream)

My question is double:
1) Are magnets a complete bullshit?
2) Besides softening the water, and the use of antiscalants, can I try something else?

Regards. Beranrd

[kkala]

Hello. I have a very severe scaling problem in a plant. We raise the pH of hard waters (300 mg/l calcium, 30 mg/l CO2/HCO3-, at 72°C) from acidic to more than 10). The flowrate is small (400 l/hr or 1.8 gpm) . Also, we are gypsum-saturated. Of course the Langelier Index indicates that there is danger. And we do scale, despite the fact that there is a tank with suspended solids in it; scaling occurs downstream)
My question is double:
1) Are magnets a complete bullshit?
2) Besides softening the water, and the use of antiscalants, can I try something else?
Regards. Bernard

Not having specific knowledge, I will be limited to generalities, most of which you may have already considered or even rejected. Having worked in a fertilizer plant, we had troubles with scales; in the design of an alumina plant we had the chance to know that scales are faced by B. Prevention C. Chemical cleaning D. Mechanical cleaning. Just before prevention one had better A. clarify cause and composition of scales.
A. Potential cause is the pH>10. This precipitates carbonates as CaCO3 (~68 mg/l); but also Ca(OH)2 (~270 mg/l), since its solubility at pH=10 is ~3.7 mg/l. Both precipitated quantities are estimated for 25 oC (no data for 72 oC), see http://www.ktf-split...en/abc/kpt.html for solubility products. Lab analysis of scales could realize to what extent these conclusions are verified.
B. -- Scales downstream settling tank may indicate not enough residence time for their formation up to the tank, so increase its volume. Place a filter just downstream of it, if fine precipitate is observed there.
--Decrease diameter of transfer pipe to increase water velocity, hence reduce scale deposited on the internal pipe walls (at the expense of higher ΔP).
--Inject a chelant additive to convert Ca++ into a non precipitating complex cation. This can replace other antiscalants. According to "Industrial water conditioning" by Betz (7th ed, 1976) most suitable chelant for the case seems EDTA; suppliers may advise modern chelants, probably more effective (they are corrosive though).
C. Diluted H2SO4 (or HCl) can be normally used for periodic chemical cleaning during shutdowns, together with a corrosion inhibitor advised by supplier. Effectiveness vary, it is not satisfactory for hard consolidated (non porous) scales.
D. For lines of such a small size, mechanical cleaning is assumed not applicable. However you can have one working line and one spare, which could be dismantled and cleaned.
Hopefully it offers some help. I know nothing about potential action of magnets. Above proposals aim at further looking into each and probably try some of them.

Edited by kkala, 10 October 2010 - 11:58 AM.

[siretb]

I would like to thank you.

A ) Indeed the high pH is the root cause.
The lab analysis has confirmed thta the solids are 70% CaCO3, a little bit of gypsum, and others (rust, silica, ....) I did not ask for free lime, there has to be some.
B ) I did not consider EDTA; thank you ; There maybe an ecotoxicity issue.
C ) We will do a periodic, acidic cleaning, probably using citric acid.


Thank you again

[kkala]

A ) Indeed the high pH is the root cause.
The lab analysis has confirmed thta the solids are 70% CaCO3, a little bit of gypsum, and others (rust, silica, ....) I did not ask for free lime, there has to be some...

After knowing the lab results, I recalculated solubility of Ca(OH)2 at pH=10 and found it to be ~3.7 g/l (25 oC), not 3.7 mg/l as erroneously written by me in the post of 10-Oct-2010. So no lime precipitates, even though solubility of Ca(OH)2 decreases with temperature (Perry, 7th edition, Table 2.120 - Solubilities, p. 2-122). Sorry for the inconvenience.