[Riley_5000]

Hello

How much lime - Ca(OH)2 -  would be needed to effect the same change in pH that a given amount of limestone - CaCO3 - would?

Arbitrary example: If 10ppm of limestone raises water's pH from 2 to 5, how much lime would be needed to raise the pH from 2 to 5?

Sorry, I'm not even sure where to begin. Is there a direct correlation, like 10ppm limestone is generally equivalent to like 5ppm lime?? That would be great.

Any help is appreciated.

[AWK]

This is rather simple stoichiometry if you can neglect evolving CO₂ (it can be easily removed by warming). Note this should concern the acidic final pH.

Show your work!

[Riley_5000]

It's not technically my question. The plant manager wants to know how much lime he needs to dose to get the same pH as the limestone plant is shut down . . .

Anyway, we have lots of H2SO4 in the water which the limestone neutralizes, so I'll hazard:
CaCO3 + H2SO4 -> CaSO4 + CO2 + H2O

Right, now the Lime takes over to remove some hardness:
Ca(OH)2 + HCO3 -> CaCO3 + 2OH?

Right, so I could use the OH to get the pH, sure - I think. But how about lime from the beginning, skipping the neutralising step?

If there is a simple correlation, viz. 5ppm CaCO3: 2ppm Ca(OH)2 then could someone enlighten me?

Thanks

[sjb]

If there is a simple correlation, viz. 5ppm CaCO3: 2ppm Ca(OH)2 then could someone enlighten me?

Thanks

Not really, as ppm is a linear measure of concentration, whereas pH is a logarithmic measure - so some things may depend on your actual required pH change. It would also depend a far bit on what's in your effluent (unless I've misunderstood)

That said, simplistically you may be able to say that limestone has the ability to neutralise two equivalents of acid per mole, as can lime - does that help some?

S

[Riley_5000]

Thanks

But my chemistry sucks. Basically what you're saying is that if I currently use 2kg of limestone then more or less two kg of lime would do it - assuming 100% utilisation?

[sjb]

Not per kilogram, per mole.

What's the molecular weight of calcium carbonate?

Of calcium hydroxide?

[Riley_5000]

Yeah, sorry - my mistake.

I'll also have to adjust the calculated dose to account for relative purities and utilization. Thanks for the help.