If I understand your question correctly, you want a seal that will work for 6 bar, but not a mechanical seal.  In that case, your only real option is a stuffing box seal (a couple of examples are shown here: Stuffing Box Drawing and Stuffing Box).  Construction of a seal housing with gland and gland follower is not complicated.  The packing material must be cut to the proper dimensions and several rings (at least 3 and probably 6 rows) will be required to seal 6 bar pressure.
 
You may have some difficulty sealing a 48 mm diameter shaft running at 160 rpm.  That diameter and speed will generate considerable heat running against a packing.  The bigger, safety issue with your current tank is whether it can handle the pressure.  No practical tank can have a flat top or bottom at those pressures.  The thickness of the material for a flat bottom or top would have to be 50 mm, at least.  The vessel needs to have a dished bottom and dished top head, along with sufficient thickness in both the heads and shell to handle the pressure.  The welds holding the parts together must also be of sufficient design and strength to handle the pressure.