Aluminium (and other conductive but non-magnetic metals) can be separated by using eddy currents. Basically a high frequency electrical field creating electrical currents and with it a magnetic field in the metal, which shoots the material away from the rest. That's the common way of sorting trash.
On the other hand, sorting plastics by type (can be done reasonably reliably by looking at the product) is still very much a human task. For example bottles come in four kinds of plastic. HDPE, commonly used for milk bottles (both the semi-transparent ones and the white ones), PP (often used for shampoo bottles), PET (the crystal clear water and soda bottles) and PVC (still used for water - also crystal clear - the difference is that PVC will turn milky white when creased and PET bottles have a little teat at the bottom because of the different way they're produced).
OP didn't specify the source and quality (cleanliness, etc) of the bottles, not sure if they're looking at sorting trash or just distinguishing between two bottles which is a very different thing.
Place a bottle between a light source and a light sensor and you'll be able to tell the difference between a transparent (PET or PVC) and a non-transparent bottle (aluminium, HDPE, PP), but you can't say whether the non-transparent bottle is aluminium or plastic that way.
Anyway, OP, please clearly define your problem.
What exact kind of bottles?
In what form? (neatly placed or trash on a conveyor)
How many/how fast?