How to implement a "circuit breaker" for an air cylinder
The term that you are most likely looking for is: "limit switch".

There are many options. 

1) You can purchase pneumatic cylinders with internal switches that activate when the rod reaches the end of travel.

These switches can handle only a small amount of current and you will need to buffer the signal in some fashion. A relay will work well. 

2) Use an external limit switch. Again, there are many choices - it might be a microswitch (with roller lever) or a magnet attached to the mechanism that then operates a magnetic reed switch. 

Depending on the physical layout of the machine and how easy it is for someone to make accidental contact with the limit switch, you may need to use a low-voltage circuit that then activates a relay or contactor that controls the mixer.