Road Flares

What is in road flares?

Different companies use different compositons when manufacturing road flares, so it is impossible to give a specific answer for the flare in your hand.  However, they do tend to have similar compositions, and so a generalized answer is possible.

Examine the following table.
Strontium nitrate 69 61 60 62 75 63 31 43
Potassium nitrate   15 23 15        
Potassium perchlorate 8       10   44 9
Sulfur 13 6 7 19 10 16    
Charcoal   5 2 4        
Sawdust 10       4      
Aluminum flake           16    
Magnesium 50-100                29
Red gum   13 7          
Epoxy resin             25  
Shellac           5    
Dextrin     2          
Parlon               12
Source "Railway"
"Wing tip"
"Low reside"

Note that every composition contains strontium nitrate (Sr(NO3)2).  That is because the strontium is responsible for either a red or red-orange color.   However, strontium nitrate is not a great oxidizer - a mixture of Sr(NO3)2 and a fuel will hardly burn.   So a better oxidizer such as potassium perchlorate or potassium nitrate or an energetic fuel such as aluminum or magnesium is added to give the extra energy needed for a fast combustion.

The rest of the components are either fuels, or a compound that serves as acombination fuel and binder, or parlon which is a binder but not really a fuel.

Some older flare formulations also had things such as pitch, asphalt, wax, tallow, potassium chlorate and black powder.   Those are not likely to be part of modern flare formulations.

The flares are pressed dry.

Can I extract the strontium nitrate from the flare?

Yes, but... But, if you INSIST, you can get a long way by noticing that out of the list above, the only water soluble things are strontium nitrate, potassium intrate and potassium perchlorate, and to some extent dextrin.  One could break up the road flare, crumble it, and soak it in water.   Filtering the road flare would then give a mixture of those 4 compounds assuming that the road flare is similar to those in the table above. Note that soaking magnesium in aqueous solutions of oxidizers can lead to highly exothermic reactions which can cause a steam explosion.

Separation of the strontium nitrate from the other compounds is not trivial.    In the following analysis let us assume we have extracted a kilogram (2.2 pounds) of road flares,  and we have 1 liter (about a quart) of a concentrated solution of the salts.    Since dextrin is not a common ingredient, let us assume that there is no dextrin.  If dextrin is present it will make the separation much more difficult.    If one cooled the solution to 0° C, up to 600 grams of the strontium nitrate would remain in solution, and a lot of the other salts would precipitate out.  Only 7.5 grams of the potassium perchlorate would remain in solution, but a full 130 grams of potassium nitrate would remain in solution.   If the crystals were removed by filtration, and the solution evaporated, one could obtain a material that might have the following composition:

Note that we don't know how pure the material is!   The amount of strontium nitrate could be less than 600 grams - it could be as low as 400 grams (or less).  But commonly one would expect to obtain about 600 grams.   There may be a substantial amount of potassium nitrate...   since some road flare compositions contain 15 to 23 % of potassium nitrate, the final mixture could have a full 130 grams.  So, this mixture could have 130/(600+130) = 18% potassium nitrate as contamination, and it would only be 82% pure.    Purifying the strontium nitrate from potassium nitrate is not an easy thing, and would lose a LOT of the strontium nitrate.      On the other hand, if the flare contained only potassium perchlorate as the added oxidizer, then it might contain only 1.2% of contaminant, and be almost 98% pure strontium nitrate.

Please note that the analysis above assumed that the liter of solution was properly concentrated to maximize seperation and retain strontium nitrate.   Imagine what would happen if one used sub-optimal conditions.   For example, if one used a flare with a composition similar to flare #3 (60% strontium nitrate and 23% potassium nitrate) and didn't make a properly concentrated solution.  

Let us say the solution is under-concentrated:  If the solution contained 500 grams of oxidizers, then it would contain about 361 grams of strontium nitrate and about 139 grams of potassium nitrate.   When that would be cooled to 0°, then 9 grams of the KNO3 would come out of solution, and the final solution would contain 361 grams of strontium nitrate and 130 grams of potassium nitrate.   The product would be about 73% strontium nitrate and 26% potassium nitrate (rounding errors).

On the other hand, if the solution is too-concentrated:   If the solution contains 1100 grams of oxidizers, then it would contain about 790 grams of strontium nitrate, and 310 grams of potassium nitrate.  When that solution is cooled, it will crystallize out BOTH strontium nitrate and potassium nitrate, giving a final solution with about 650 grams of strontium nitrate, and 130 grams of potassium nitrate.  Than means the salt obtained by filtration and evaporation would be about 83% pure,  BUT you have lost 140 grams of strontium nitrate (it's mixed with 180 grams of potassium nitrate crystals which you filtered out).  Recovering that is possible, but very difficult.

So, the bottom line -

You will never get a pure product though a simple single step procedure.   If you don't do every thing right, you will get a very impure product or you will end up losing some of the strontium nitrate.