Legally, a utility patent may cover “any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof.” A design patent may cover “any new, original, and ornamental design for an article of manufacture,” and a plant patent may cover a “distinct and new variety of ...
What can be patented? An invention relating either to a product or process that is new, involving an inventive step and capable of industrial application can be patented.
Technically, it is possible to patent a new use of an existing product. Realistically, it might not be worth applying because you would have to show that the new use is nonobvious. Even if you believe your use to spectacularly nonobvious, expect an uphill battle.
A product patent is intended to protect an investor's product. This is facilitated by providing a safeguard to the original innovator to ease competition toward a similar product. Whereas, a process patent is primarily used to protect the procedures via which one produces the product and not the product itself.