This whole process sounds like an extremely bad idea and potentially a very hazardous situation.  Heating the wax to a temperature above the flash point of the solvent means that all oxygen must be purged from the vessel and vapors must not be allowed to escape.  If the wax must be heated to remove it from a drum, directly transferring it into the solvent would create a seriously dangerous situation.
 
If the flammable solvent will actually dissolve the wax, then the best suggestion is to melt the wax in the drum and pour it into small blocks.  Then allow the blocks to cool, before adding them to the solvent and mixing until the wax dissolves.  Even under those conditions, a closed tank, with a nitrogen purge, and pressure relief / fire suppression system may be needed.  This question is much less about mixer design and much more about a hazards analysis.

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