This question does not provide much useful information about the existing mixer that does not work.  Without an impeller diameter, impeller type (number of blades, blade shaper or angle, etc.), and rotational speed the existing mixer cannot be evaluated.  In any case, the important limitation is the maximum torque of the magnetic drive.  The most effective mixer will be the one that most nearly loads the torque capability of the magnetic drive.

Without any additional information, the best advice is to use a properly sized (for maximum drive torque) hydrofoil impeller located about 100 mm from the bottom of the drum.

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