To answer all of these questions with associated calculations is not a simple task, since the mixer design involves many variables and design decisions.  However, some basic information and general rules may be sufficient to give direction for this design.
In general, these densities are at the moderately high end of powder properties handled in ribbon blenders.  The 0.85 specific gravity corresponds to about 50 lbs/ft^3 and 1.5 tons in a 60 ft^3 blender is high.  The length of 98 inches with a width of 37 inches is long and narrow, indicating that the blender will be used for continuous processing.  The typical pitch for a helical ribbon is about 0.5 times the diameter, or about 18 inches for this blender, although that pitch can be adjusted to give an even number or fractional number of turns for the blender length.  Blade widths are typically about 1/10 of the diameter or about 3.5 inches for this case.  Because of the ribbon diameter, one pair (double flight) of outside ribbons and one pair of inside ribbons should be sufficient.  The blade thickness needs to be 1/8 to 3/16 inches, depending on the number of supports.  The motor power needs to be 15 to 25 hp depending on the rotational speed and to some degree on how easily the powder flows or if liquid is being added.  A typical operating speed might be between 35 rpm and 40 rpm, with the speed having a significant effect on the power requirement.  The selection of the operating speed will depend in part on the selection and method of speed reduction.
The entire mechanical design depends on the maximum torque provided by the motor power at the rotational speed.  The number and strength of the supports must be able to handle the torque, along with sufficient rigidity to minimize fatigue of the metal and welds.  The blade thickness must also be able to handle the forces associated with the drive torque and will depend on the pitch of the ribbons.  These guides along with detail design selections and calculations should provide design guidance for a ribbon blender.

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