How to separate sodium carbonate from sodium hydroxide?

I got a drain cleaner in dust form, which consists of $\ce{Na2CO3}$ and $\ce{NaOH}$. I wanted to extract the $\ce{NaOH}$ so I tried to heat the mixture in order to just melt the sodium hydroxide, but it didn't really work as expected. Is there any easy way to do it?




There is [a forum](http://www.sciencemadness.org/talk/viewthread.php?tid=66228) which discusses the similar problem of separating sodium hydroxide and sodium carbonate from drain cleaner. The solution is taken from there(slightly edited): 



> 
> This is relatively easy because sodium carbonate is practically
>  insoluble in 50% sodium hydroxide solution so a glass frit type filter
>  is needed to remover the insoluble sodium carbonate. Most commercial
>  sodium hydroxide contains only a little sodium carbonate from
>  absorption of atmospheric $\ce{CO2}$. So for carbonate free sodium
>  hydroxide solution, make up one litre at a time of say 60% solution.
>  To do this, add about 300g of sodium hydroxide pellets of flakes to
>  500ml of ice-cold water in a large beaker and stirr until practically
>  dissolved, cover and leave to cool . Allow it to cool to about 20-25 C
>  and the add another 300g of sodium hydroxide and stirr until most has
>  dissolved (much less heat is generated on the second addition), dilute
>  to one litre, pour into a 1 L thick-walled polythene bottle and leave
>  it to stand for about 1 month in a cool place and then decant the
>  clear sodium hydroxide solution.
> 
> 
> If the material is say 50% sodium carbonate do the following: 
> 
> 
> Add 500g of mixed sodium salts to 500ml of ice-cold water, stirr the
>  slurry periodically for a couple of hours and then filter through a
>  glass frit funnel (a vacuum filtration set-up will be needed) and wash
>  the cake with a little water, say 50-100ml, to remove most of the
>  remaining caustic soda; some sodium carbonate will dissolve but this
>  will precipitate on the next addition. The cake will be almost pure
>  sodium carbonate but you can always treat it with saturated sodium
>  bicarbonate solution to ensure that it is.
> 
> 
> The clear filtrate should be placed in a calibrated vessel and another
>  500g of material added and again stirred periodically for a couple of
>  hours then filter and wash as before. Hopefully there should be about
>  700-800ml of very strong sodium hydroxide solution. Place it in a
>  polythene bottle and let it stand of a month so the sodium carbonate
>  can crystallize out, decant the clear solution and dilute as required.
> 
> 
> NOTE: 
> 
> 
> * 60% caustic soda solution is rather viscous and will not
>  filter without suction even on a fairly coarse frit, 50% is still
>  pretty viscous and difficult to filter.
> * The sodium carbonate that
>  crystallises out sinks very slowly hence the long standing time.
> * Sodium carbonate dissolves rather slowly in water form some reason so
>  with vacuum filtation the residence time of the wash water will
>  actually dissolve very little sodium carbonate, too much wash water
>  will dilute the solution below 50% and make the carbonate more
>  soluble.
> * Hot 50% caustic soda solution is extremely aggresive
>  towards skin! When handling strong sodium hydroxide solutions, keep a
>  bowl full of dilute citric around in case of emergency.
> 
> 
> There are other method such as treating a more dilute solution with
>  slaked lime, filtering (difficult because the precipitate is fine
>  grained) and evaporating down.
> 
> 
> 


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