# Sulfuric acid used to neutralize Lime Slurry

I began my co-op position at a certain industrial company 2 days ago. A boss of mine asked me to calculate the amount of sulfuric acid required to neutralize a Lime-slurry mixture. We are going to run some test before we actually go through with the operation, but his plan is to apply sulfuric acid to a 1.06 mill capacity pool, which I’m told is kept at a natural pH of 8.5. He wanted to add the appropriate amount of H2SO4 first, mix the pool, and thereby inject the lime slurry, such that the pool is brought back to 8.5 pH. The lime slurry may be expected to be 20% lime by weight, and remainder water.

CaO(s) + H2SO4(aq) ----> H2O(aq) + CaSO4(s)

He suggested this method as he reasoned that the mixed H2SO4 will react better, and quicker, with the slowly injected lime slurry as it enters the water, making it so there is less chance of lime falling to the bottom of the pool and forming concrete.

The boss told me just to figure that the reaction will go 1 to 1 (as suggested in the stoichiometry shown above). However, I wonder how the dissociation of the acid in the pool will affect things. From wiki, the K values are the following:

``````H2SO4 + H2O → H3O+ + HSO4−    K1 = 2.4 x 106   (strong acid)

HSO4− + H2O → H3O+ + SO42−     K2 = 1.0 x 10-2
``````

The lime also reacts with the surrounding water:

CaO(s) + H2O(aq) → Ca(OH)2(aq)

My question is, what should I expect to be the pH change throughout this process? How should I model it? We are adding the sulfuric acid in first, which is strongly acidic for one of it’s protons, but not for the other. And I’m not sure how the lime behaves in an aqueous form. From what my boss has suggested the process should be simple, but I just want to understand all the details.

Thanks to anyone for any tid bits or suggestions!

In general, when dealing with sulfuric acid, I ignore the second proton, but since Calcium is divalent, treating it as a 1:1 mixture takes into account the second proton neatly.

This sounds like a complex system and most of the knowledge is going to be with people experienced in working with it. I can tell you that, since the solution is aqueous, there is exactly zero CaO in there. On mixing with water it all turns to Ca(OH)2. But that doesn’t mean you can’t run stoichiometry as CaO.

Yeah I was a bit confused why my boss felt so confident treating as a 1 to 1, but your comment makes sense. Thank!

what’s the daily throughput? if it’s big, you do have to resort to chemical means but would this sound crazy:

make a secondary circuit where a small and manageable amount is diverted and neutralized naturally through a series of canals containing water hyacynths and lilies? the fast hyacynth growth will form a reducing and acidic environment than can filter even “black water.” lime isn’t really that environmentally polluting. it occurrs in nature. just course it through a slightly acidic natural envionment.