[curiouscat]

Is it practically feasible to process NH4Cl industrial scale to yield NH3 & HCl. Is this reaction quantitative? To get reasonably fast rates how high do I need to heat it.

Context: I have a waste stream containing NH4Cl dissolved in water. Trying to think how to reprocess it.

I could send it to a multi effect evaporator. Then get solid NH4Cl & heat it. Upstream process has use for both NH3 (aq.) & HCl (32%).

Or could I somehow just heat the aq. solution & get NH3 + HCl without bothering to ever go to the solid NH4Cl?

Once I get the mixed gases how do I separately dissolve them in water?

Comments? 

[Hunter2]

I think this will not work, ammonia and hydrochloric gas immediately form ammonium chloride by re-sublimation.  Experiment: open a bottle of each component and stay them together you will see white smoke development.

[Enthalpy]

Instead of heating, could you displace NH₃ from NH₄Cl brine? With a strong nonvolatile base like CaO maybe? That would be half of the work.

Then, reprocess CaCl₂ by molten electrolysis? I'm not quite sure...

Maybe HCl can be displaced from CaCl₂ by H₂SO₄, and then you dump CaSO₄, but this may not make economical sense.

What would the electrolysis of aqueous NH₃Cl lead to?

[curiouscat]

Instead of heating, could you displace NH₃ from NH₄Cl brine? With a strong nonvolatile base like CaO maybe? That would be half of the work.

Then, reprocess CaCl₂ by molten electrolysis? I'm not quite sure...

Maybe HCl can be displaced from CaCl₂ by H₂SO₄, and then you dump CaSO₄, but this may not make economical sense.

What would the electrolysis of aqueous NH₃Cl lead to?

Thanks @Enthalpy. Those ideas would work in principle.

But at that point I might as well use NaOH / Ca(OH)2 in the upstream process directly.

Basically, I have an upstream reaction that can be carried out with either NaOH or NH3 or Ca(OH)2. More prefrably either of the first two.

Using NaOH is the default but that generates a large amount of NaCl (aq.) getting rid of which is a nuisance.

I was wondering if using NH3 might let us reprocess & recycle. The key challenge is how to do this:

NH4Cl  NH3 + HCl

[curiouscat]

I think this will not work, ammonia and hydrochloric gas immediately form ammonium chloride by re-sublimation.  Experiment: open a bottle of each component and stay them together you will see white smoke development.

Yes, but that's at room Temp. I'm suspecting (hoping?) that at elevated T the equilibrium will favor the gases.

Then it'd be a question of separating them while still hot. Not easy by any means, but is there a fundamental chemistry problem?

[Enthalpy]

I have misued CPropepShell (meant to compute chemical equilibria in a rocket engine), mixing ionized helium (to bring neutral heat) with NH₄Cl at 1atm, and:
- The lowest temperature to get an answer is 792K.
- Then, all Cl makes HCl, but N and H make N₂ and H₂, only 1/1000 make NH₃.
- At 2375K, atomic Cl climbs to 0.01* of HCl, Cl₂ is trace, the rest is essentially the same.
- At 2830K, atomic Cl climbs to 0.1* of HCl, the rest is the same.
- At 3400K, atomic Cl is 0.5* HCl, NH₃ stays as traces

Tried again at 500bar: the smallest T for an answer is 912K, then
- NH₃ moles are half as abundent as N₂ moles.

These are strict equilibria, which make sense in a hot flame, not necessarily at lower temperatures where some reaction paths aren't allowed. He is in small mount here, no big equilibrium shift.

Do I remember that NH₃ is synthesized at a lower temperature with a catalyst (and big pressure) to favour it over N₂ and H₂? At least CPropepShell predicts very little of it. Logically, NH₃ is destroyed at a lower temperature than the salt.

I still don't grasp how you could separate HCl from NH₃ even if this latter did form. It would require some form of chemical separation, wouldn't it?

But what looks credible is to recycle HCl and maybe H₂, release N₂. Just heat at 1atm does that. Upon cooling, all the Cl₂ fraction will form HCl, N₂ and the remaining H₂ won't react. You plant would buy NH₃ and destroy it cleanly, cycle HCl.

Or make ammonium chlorophosphate, sell it as a fertilizer?

[curiouscat]

Or make ammonium chlorophosphate, sell it as a fertilizer?

If it comes to that then might as well sell Ammonium Chloride itself.

[curiouscat]

I still don't grasp how you could separate HCl from NH₃ even if this latter did form. It would require some form of chemical separation, wouldn't it?

Agreed. I don't know of a good way yet. I'm just brainstorming for now.

[unsu]

In the Solvay process, NH₄Cl is treated with Ca(OH)₂ to recover NH₃. CaCl₂ is a waste, it is either dumped into the sea or used in winter road clearence.

[Enthalpy]

To my opinion, and unless someone has an astounding idea, the proper way is to decompose NH₄Cl at enough heat and atmospheric pressure. Dissolution in water separates HCl, the N₂ and H₂ left over can be burnt together if no better use. Clean, cheap.

Use successive water-gas contacts so the gas meets clean water in the last step, leaving well washed N₂ and H₂.

[curiouscat]

To my opinion, and unless someone has an astounding idea, the proper way is to decompose NH₄Cl at enough heat and atmospheric pressure. Dissolution in water separates HCl, the N₂ and H₂ left over can be burnt together if no better use. Clean, cheap.

It will be cheaper for me to just sell the NH4Cl I think. Unless I can recycle the NH3 the idea is an economic dud.

[curiouscat]

I have misued CPropepShell (meant to compute chemical equilibria in a rocket engine), mixing ionized helium (to bring neutral heat) with NH₄Cl at 1atm, and:
- The lowest temperature to get an answer is 792K.

I don't understand what you wrote here. What  is the equilibrium prediction of your code at say, 200 C or 300 C.

To me that seems the interesting region.

[Enthalpy]

CPropepShell failed to compute an equilibrium below 500°C. Equilibria of solid+gas products are difficult to compute, and 300C isn't the normal use of this software.

[Enthalpy]

More in detail: Propep's aim is to compute equilibria in rocket combustion chambers (and nozzles if supposing they shift during the expansion). Because of 3000-4000K, it neglects all chemical structure, all reaction details, and needs only to know the atomic composition and heat of formation of the propellants. Then it checks which of its known combustion products can apply (including OH, gaseous MgO, Cl*, even C₂) and solves all equilibria.

As a sidenote, it was programmed in Fortran for I dont' know what computer, ported much later to Ms-dos and never serviced since. Several graphical user interfaces were added, none is perfect nor really easy, one package being CPropepShell. Rpa (Rocket Propellant Analysis) was written recently and is far better than Propep plus interface, except that it lacks flexibility for my horrible tinkering.

You get it: what you ask at 200 or 300°C is cute lace, what Propep does is sheet stamping. Propep answers the best thermochemical equilibrium even if no reaction path leads to it. Sometimes it also fails to converge.

With ammonium chloride at 1atm and +200°C I get no answer, at +300°C I get the molar fractions
HCL                  3.3650e-001
H2                   4.9050e-001
NH3                  9.5031e-003
N2                   1.6350e-001
and at +274°C, NH₃ climbs to 1.54e-2,
but I suspect Propep ignores NH₄Cl as a combustion products.

Maybe an other equilibrium software suits the needs of chemistry better.

[curiouscat]

Maybe an other equilibrium software suits the needs of chemistry better.

Indeed. I'd love recommendations.

I guess I could always do it the old school way. With NIST data & by hand.