Your mixer design is somewhat unusual with large-diameter impellers and narrow blade widths.  Your description is vague with respect to how many blades (extensions) are at each impeller location on the shaft.  My design estimates recommend that with four blades at each impeller location, the blades should be at least 25 mm thick.  If only two blades are at each impeller location the blades need to be 32 mm thick.  In other words, the blades need to be 1/4 to 1/3 as thick as they are wide, for them not to bend.  Thinner blades require operational limitations or they may bend or break due to normal operation.  Bent blades may be an indication that they are hitting lumps or other things in the tank.  Fatigue failures, due to normal operation, often result in the blades breaking off completely.
 
Editor's Note: The questioner sent in additional information after this post. Our expert, David Dickey, senior consultant, MixTech Inc., offered an expanded answer. The Q & A is below:

Q: Based on my question above:
• blade position in angle 45 degree with width 100 mm, thickness 15 mm and provide the 10 mm stiffener along the length of blade
• bottom position has two blades, it is about 33 mm
• top position is about 840 mm from the center of bottom position, it also has two blades
• diameter of both position of blade = 1590 as mentioned before.

A: The additional information does not change my previous answer.  The most likely cause of bent blades is either that they are not thick enough for the applied loads or they are hitting lumps or other objects in the fluid.  The additional information about two blades, regardless of angle, at each location and the bottom location within 33 mm of the bottom, only confirms the need for 32 mm thick blades.  A 15 mm thick blade with a 10 mm stiffener does not meet that thickness requirement.  This problem is much less about the details of the design and more about fixing the consequences.
 
If increasing the thickness of the blades is considered to be impractical, then reducing the speed of the mixer will also reduce the loads on the blades.  A reduced speed may not have an adverse effect on processing capabilities.