The diffusion spot diameter with one crystal at 15 min (mm) is 59. The rate of diffusion with one crystal So how would I convert this 59 to (mm/h) ?

This is straight forward. If there are 60 minutes in one hour, then to convert minutes to hours as requested, simply divide the number of minutes given by 60. Thus if you divided the 59 by 60 it gives you what? round the answer to whole number.