Steve,
Here's the equation:
NaNO3 + H2SO4 = NaHSO4 + HNO3
Here's the weights in grams, rounded off, for the reaction to produce 1 liter of 70% nitric acid. This assumes that everything is anhydrous which, of course, it isn't. I have included the 30% water in the nitric on both sides. More water than this will be needed to dissolve the NaNO3 so, the nitric % will be less. Also, there's water in the sulfuric to consider.
1330(NaNO3) + 1533(H2SO4) + 422(H2O) = 1878(NaHSO4) + 986(HNO3) + 422(H2O)
One liter of 70% nitric weighs about 986 + 422 = 1408 grams
The salt, NaHSO4, sodium bisulfate, is strongly acidic when dissolved. It's used for toilet cleaners (Sani-Flush is 75% NaHSO4), lowering the pH of swimming pools, a safer sulfuric substitute in school labs, and an acid pickle for silver castings.
NaHSO4 is very soluble in water, even at 0 deg C. Here's the solubilities: 500 gm/L at 0 C., 670 gm/L at 22 C., and 1000 gm/L at 100 C. If you plot those out, it curves down to the left and seems to hit zero at about -80 C. This happens to be the temp. of dry ice. It seems like, if you used dry ice, you could get all of the salt out. The problem is, nitric freezes about -42 C. and, this figure is probably for 100% nitric. For 70%, it would be higher. There is most likely a low temp to maximize salt dropout without the nitric freezing. You could use a thermometer.
On the net, with the little amount of info that I could find on the process you're doing, they required distillation to get rid of the salt. Of course, they didn't think about chilling it.
The solubilities, above, are for the salt dissolved in water. In the presence of the high nitric acid, the solubilities might be a lot less.
All in all, we don't need 70% nitric acid for most any of our processes. For dissolving silver, copper, etc., a 50/50, nitric/water, mix of commercial 70% nitric, cut by volume, is the most efficient mix for dissolving. This figures out to 41% nitric by weight. For aqua regia, I usually add about 10% water, for effeciency. I could use a weaker nitric, add no water, and end up with the same thing. The nitric for this purpose would be about 53%, by weight, assuming a 3/1, HCl/HNO3 mix.
Also, if the nitric % is high enough, the presence of some of the salt in the solution might not interfere much with our processes. This would require experiments.
I downloaded a couple of old industrial chemical books on Google Books. I found another process. Instead of sodium nitrate, they used barium nitrate. The equation, otherwise, is about the same. It takes about 1.5 times as much weight of Ba(NO3)2 as it does NaNO3. The big difference is that Ba(NO3)2 is only about 10% as soluble in water as NaNO3. Therefore, the nitric yield was only about 10%, by weight. They said you could boil it to raise it to 25%, the point at which HNO3 starts coming off. The 25% would work for our processes but, not as fast. The beauty of using Ba(NO3)2 is that the resulting salt, BaSO4, is zero soluble in everthing and, therefore, could be completely removed at room temp. About the cheapest I found barium nitrate was from pyrotechnic chemical companies, for about $3.30/pound. That was for one pound quantities. They couldn't ship it, so you had to pick it up.
Steve, unless I made mistakes, those are the numbers. I wish I had a lab but, I don't. If you need help or advice, I'm here. Please keep us posted with your progress. As you know, this process could be very important to all the members that can't get nitric acid.
