[cristiro]

I practically did this reaction and  the alloy started to desintegrate giving a graysh precipitate and a gas which i believe is Hydrogen. The solution remained clear. I believe the precipitate is Mg(OH)2 which is less soluble in water . Al reacted giving Al(OH)3 I guess which disolved in solution. The question is NaOH remained nonreacted as a catalyst?
How can be isolated Mg from AlMg3 by other means?

[AWK]

Al reacts with NaOH solution always, Mg only with hot solutions.
In both cases insoluble hydroxides are formed. In an exces of NaOH Al(OH)3 is soluble to form Na[Al(OH)4].

[cristiro]

So if it is used a cold dilute NaOH solution in excess, even the reaction will be slow and  take days to complete, the precipiate will be Mg and the formed NaAlOH4 will disolve in solution?

[Hispania]

I practically did this reaction and  the alloy started to desintegrate giving a graysh precipitate and a gas which i believe is Hydrogen. The solution remained clear. I believe the precipitate is Mg(OH)2 which is less soluble in water . Al reacted giving Al(OH)3 I guess which disolved in solution. The question is NaOH remained nonreacted as a catalyst?
How can be isolated Mg from AlMg3 by other means?

My experienience with different alloys is that when you react the aluminium alloy with a solution of NaOH, the aluminium is disolved removing H₂; however you´ll have always a dark colour precipitate (I dont´t know what is exactly).
If you want to disolve this dark precipitate you must use H₂O₂, and if there is Si in the composition of the alloy you have to add solid NaOH and heat until 320 ºC. Now you can disolve the paste with HCl.
Then you´ll get a clear solution and you´ll be able to isolated the cations.