Preparation of anhydrous calcium nitrate, contributed by BASF

Uses: Preparation of anhydrous nitric acid, additive for keeping sth. moist, production of dry nitrogen dioxide gas by decomposition

Calcium nitrate is a fertilizer only for special purposes due to its low nitrogen content, and therefor hard to aquire. This calcium nitrate containing fertilizer is a double salt of the formula 5Ca(NO3)2NH4NO3.10H2O and the NH4NO3 could be removed by repeated recrystallization from boiling water, as NH4NO3 is much more soluble in hot water than Ca(NO3)2.

So in most cases the process of our choice has to be the ammonium nitrate-calcium oxide reaction which is described below as detailed as possible to prevent frustration and waste of time.

The following description benefits from dozens of experimental attempts to optimize and simplify the method.

Reaction equation: CaO + H2O + 2NH4NO3 > Ca(NO3)2 + NH3 + 2H2O

Side reactions: NH3 + H2O > NH3.H2O

Decomposition(undesired): Ca(NO3)2 > CaO + 2NO2 + O2

Step1) get a 50 kg* - bag of KAS 27 EU fertilizer or a similar calnitro product(27%N, mainly ammonium nitrate), preferably in summer, or you will be asked lots of questions - its not that expensive anyway...

Fill the fertilizer into several buckets and seal them airtight for storage.

Step2) get a 40 kg* - bag of lime (>92% CaO). Fill the lime into several buckets and seal them airtight for storage.

*smaller quantities are hard to get

Step3) weigh out 3,4 parts*(by weight or also by volume, if the prilled fertilizer remained dry) of KAS 27 fertilizer and dissolve it with much stirring in the smallest possible quantity of warm water.

Then let sit for a few minutes and carefully decant off the solution from the insoluble CaCO3 and MgO additives.

Rinse the left solid with several ml of hot water, let sit and decant the rest of ammonium nitrate solution.

Prepare a suitable pot for the next step, big enough to own the chosen quantity of reactants while still only be filled by half its volume.

*only in case of using the european union - KAS 27 fertilizer which contains about 77% ammonium nitrate; note: this fertilizer is prilled, and the prills are exactly sized in a way that volume(L) is equal to weight(kg) - and this is very practicable.

Step3) weigh out 1 part(by weight), -more than 2 parts if you want the process to speed up- of lime and place it in the pot.

Step4) Find a nice place outside(good ventilation needed) and add in 4 or 5 portions with much stirring the ammonium nitrate solution.

Attention: use protective glasses!

The reaction will release a lot of heat and the solution will boil.

Don`t breath the harmful fumes of ammonia(ammonia gas intoxications are rare because of its intensive smell, so don`t panic, just be careful).

Step5) Heat the pot until it boils.

Keep the mixture wet by frequent addition of water.

Don`t let the mixture dry out at any time as it would overheat and reach the decomposition temperature of calcium nitrate.

The whole convertion process takes lots of hours, so leave it boiling on your heat source and come back for frequent stirring.

The reaction is finished when after the addition of a small quantity of lime no smell of ammonia can be observed any more or wet litmus paper does not change its colour when placed above the pot.

Step6) add as much cold water as possible and stir.

Allow the calcium carbonate and unreacted calcium hydroxide to settle and decant the calcium nitrate solution carefully into some suitable coffee filters.

You can accelerate the filtering process a great deal by hanging a cord through the filter, this will maintain the flow at any time. You may repeat the process by adding water to the remaining slug to get the last few grams of calcium nitrate out of it.

Step7) first cook the solution down to dryness.

Attention: there is always the possibility that the solution boils over at the moment when the calcium nitrate begins to precipitate.

Then thoroughly dry the calcium nitrate at temperatures between 130 and 400 degrees centigrade. (400C is grey glow, red glow is above 500C) This is important as higher temperatures(red glow) will decompose the calcium nitrate, developing highly poisonous nitrogen dioxide.

Step9) the dried calcium nitrate will be as hard as rock and most often it has to be removed using hammer and chisel. Try to do this while it is still hot, or the calcium nitrate will have to be dried again, as it is very hygroscopic.

Step10) as the calcium nitrate most often will be needed powdered, crush the pieces with a sledge hammer while having them in a suitable plastic bag to prevent them from moisture.

Storage: Totally airtight containers.

The speed of the whole process is determined by the fertilizer/lime-ratio(as mentioned in step3), the stirring and the chosen reaction container(wide opening containers preferred).