chipster1234 - 5-7-2011 at 17:27
Im trying to make calcium nitrate in the lab. Based on customer requirements they would like to synthesis calcium nitrate using nitric acid as their
base raw material
1) im using nitric acid to react with calcium carbonate
unfortunately im experiencing a huge amount of sludge/precipitate formation.
based on the equation : CaCO3 + 2HNO3 = Ca(NO3)2 + CO2 + H2O. (i just cant see where the sludge would come from???)
can anyone help me please?
2) is there any other material that i can use where sludge doesnt appear? i have tried using Ca(OH)2 but there is huge amounts of precipitation
3) could the precipitate actually be calcium nitrate itself?
barley81 - 5-7-2011 at 17:56
Are you sure that your reactants are pure? Calcium nitrate is very soluble, so this shouldn't happen. Did you add any water to the reaction?
chipster1234 - 5-7-2011 at 18:22
the HNO3 is ~54% purity
whereas the Calcium Carbonate is >90% pure
so to answer your question, yes we do have water coming from the HNO3 source as it is only 54%. (*smile*)
may i know why you ask if i have added water?
From what im doing is that, i will react the reactants and then allow the sludge to settle down before decanting/seperating the liquid from the
sludge. the liquid would be calcium nitrate of approximately 55% strength.
the sludge would be disposed off. my main task would be to eliminate this sludge if possible. any ideas?
barley81 - 5-7-2011 at 18:37
I have no idea then. I asked if you added water because if the nitric acid were 70% calcium nitrate would ppt. Maybe you could test the sludge for its
composition? I don't have very much lab experience thus far, I'm only thinking of some possibilities.
hkparker - 5-7-2011 at 18:44
>90% pure isn't saying much. My first thought is that because nitric acid is an oxidizing agent it could oxidize the CaCO3 to CaO but if that was
possible it would probably need to involve RFNA.
Probably impurities. Wikipedia says calcium nitrate can be produced this way without problem (followed by neutralization with ammonia).
Try dissolving some of you CaCO3 in HCl and see if you can get it all to dissolve. If not, its almost certainly an impurity.
Bot0nist - 5-7-2011 at 18:48
Are you using a stoichiometric excess of calcium carbonate? Have you neutralized the precipitate with NH<sub>3</sub>(aq)?
What are the characteristics of the sludge and of the decanted liquid and its residue after evaporation? (mass, solubility, ph, melting point, etc.)
Calcium nitrate tetrahydrate solubility in water:
102 g/100 mL (0 °C)
129 g/100 mL (20 °C)
363 g/100 mL (100 °C)
edit: damn, beat me to it HK! 
[Edited on 6-7-2011 by Bot0nist]
hkparker - 5-7-2011 at 18:59
Hey this aint no race Bot0nist
I would agree more info in needed.
chipster1234 - 5-7-2011 at 19:21
thank you so much for your response
HK: will be sure to test out the CaCo3 with HCl But wont this give us a ppt as well seeing as CaCl2 is produced?
Botonist:
1) i am using HNO3 in excess about 2.2 moles excess to CaCO3.
2) will be sure to test out the sludge and its properties.
3) the whole reaction was not neutralised with NH3. may i know why is the neutralisation of HNO3 necessary? Can we use Ca(OH)2 instead of NH3?
hkparker - 5-7-2011 at 19:53
CaCl2 is soluble.
CaNO3 might be much less soluble in acidic conditions due to Le Chatelier's principle. As nitric acid produces NO3- ions, the solution will adjust by
precipitating Ca(NO3)2, but I'm not sure if that would be true at these dilutions.
EDIT:
I would use ammonia or sodium carbonate solution as they are liquid. Sodium carbonate will let you know when the solution is neutral by no longer off
gassing. See if the solid dissolves in a neutralized or very slightly acidic solution.
[Edited on 6-7-2011 by hkparker]
AndersHoveland - 5-7-2011 at 20:44
Yes, I rather suspect the "common ion effect" is at work. For example, sodium chloride is not very soluble in 20% concentrated hydrochloric acid. The
best solution would be to dilute the nitric acid to a 5% concentration with water before reacting.
Intergalactic_Captain - 5-7-2011 at 23:49
Sounds like you're in an industrial setting and someone's trying to use waste HNO3 from some other process. If that's the case, was it used in a
nitration process?
The only contaminants in your CaCO3 should be CaO and Ca(OH)2. If it's raw, both should react just fine - If it's been calcined, you might have some
fused chunks that seem like "sludge" but in reality are just reacting incredibly slowly.
If the acid is waste from a nitration process, it may contain some amount of H2SO4 or some soluble sulfates. If that's the case, your sludge is most
likely precipitated CaSO4.
...Without knowing more about your raw materials, those are the two most plausible possibilities I can think of...
497 - 6-7-2011 at 04:01
CaSO4 seems very likely. It forms some of the sludgiest precipitates I've ever seen. It's terrible.
Megamarko94 - 6-7-2011 at 05:33
it will take forever to cristalize it out since its hygroscopic.