[ZAK]

Hello,
I am a purchasing consultant in the UK and one of my clients uses 20% Nitirc Acid in a process called Shaped Tube Electrolytic Machining
A negative charge is applied to Titanium tubes while a positive charge is applied to the work piece. The acid becomes the electrically conductive fluid.
The acid is actually comtaminated with nitrates on receipt to aid the process.
During operation the acid becomes contaminated with dissolved metals such as  Molybdenum, Niobium, Nickel and Tungsten.
I am currently looking in to the possibility of using diffusion dialysis to recover and extend the life of the acid but the investment is high.
One of my client's employees saw a process at a company in the US that in simple terms evaporated the acid solution away from the disolved metals and I am interested to know if this is feasible, the equipment that would be needed and if any organisation could help with a trial experiement.

[Dude]

The boiling point of nitric acid shouldn't be much higher than water.  Thus, one should be capable of distilling the dilute nitric acid solution in a Teflon or glass-lined distillation tower.  The assumptions are that all foulants will remain as bottoms and color won't be a problem.  The key is cost.  Are the costs of building a tower and the energy costs to distill the acid lower than paying to dispose of the nitric acid and replace it with fresh material.  You could do a simple laboratory distillation to establish if the metals are removed and no foreigh species are collected by analyzing the acid by ICP and ion chromatography.  The other issue is your acid concentration.  Your concentration would probably be reduced after use so a higher concentration acid tank would be needed to blend with your recycle acid before the next use to keep a 20 % concentration.  Otherwise, you might run into quality problems.

[Borek]

Nitric acid forms azeotrope at 68% concentration. bp about 120 deg C. Thus there is no problem with acid dilution - on the contrary, after distillation it will be to concentrated.

Seems to me that distillation is not very difficult, as it is proposed as a method of preparation of pure nitric acid - using nitrate and conc. sulfuric acid.

But Dude is right - it is all about cost.