Depending on the shop, it shouldn't be hard to dispose of mixed gases. A small general mechanic shop doesn't have the equipment, though.

We have a gas sniffer that tells us what a system contains: R-12, R-134a and air - and what percentage of each. If it's 97-100% R-12 or R-134a, we vacuum it into a tank designated for whatever it is, but if it's mixed and less than 97% pure, we vacuum it into a tank for mixed gases.

The main reason we run 3 separate tanks is because we can sell R-12 or R-134a for a decent price as long as it's at least 97% pure, but we have to pay to get rid of a mixed tank. We do a very good job of identification and recovery, so most of our tanks are 100% pure.

The equipment to do that costs about $2000 - the reason why most small shops don't fool with it.

Frank