If I understand the question correctly, the problem is feeding 1000 kg/hr fine salt and 4000 kg/hr water through a 0.417 cubic meter vessel.  If 5 cubic meters per hour go through the vessel with only a five minute residence time, the vessel must be 5 cubic meters divided by 12 or about 0.4 cubic meters.  At that size a direct drive portable mixer, operating at 1500 or 1800 rev/min depending on the frequency of your power is the most appropriate option.  The mixer should be either 190 or 250 watt mixer, with an appropriately sized axial flow hydrofoil or propeller.  The mixer should be mounted at an angle that limits the depth of the surface vortex to less than 1/4 the distance to the impeller.  If the mixer draws air into the liquid, the mixing efficiency will be reduced.

The problem may be over-specified.  Properly suspending the salt and keeping the vessel well mixed will not assure that all of the salt will dissolve in the desired length of time, especially over the range of temperatures mentioned.  The mixing intensity has less of an effect on the rate of dissolution than the saturation concentration and the particle size.  As long as the mixer keeps the solids in suspension, more mixing intensity will increase only slightly the rate of dissolution.

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