I am working for a company who makes a patented product I would like to use in a different way. I will be leaving the company soon to begin working on my idea. The alternative use for this item would be in a completely different industry, and my device would use the company's commercially available item as a main component. Question: Is using an item in a different manner patentable in general?
Generally, existence of a product does not stop someone from obtaining a patent on a method of using the product. The question is whether that use is novel and non-obvious in light of the product's known uses. "In a different way" and "completely different industry" are factors in your favor (i.e., suggesting novelty), but there is no way to predict whether the patent office will agree with you, or will instead take the position that your use is not novel and/or is obvious.
By the way, the fact that the product is already patentED is irrelevant to whether or not your use is patentaABLE. As far as you getting a patent is concerned, what's relevant is the product's known uses.
The fact that the product is patentED means the company can sue infringers. For example, a company that owns a patent having claims to a chair can sue for making, USING, or selling a chair.
Could my company try to claim ownership of my new patent?
Your company can "try" anything, no way to know whether this will happen.
Most employees, as a condition of employment, have already signed an agreement spelling out who owns what inventions. It's possible that your agreement says that your employer owns this "new use of the company's product" invention. If your invention is in a completely different area, was done on your own time, and with your own resources, a court may find that this part of your employment contract is void -- but the company can nonetheless try to enforce the contract and cause a lot of trouble for you.
Before proceeding with your venture, you should read that agreement, and ideally have a lawyer advise you on your rights under the agreement. If you decide not to hire a lawyer for this, you should at least read the agreement very carefully.
Although I have conceived of this idea already while working for the company, I will only prototype and get serious on the design after I leave their employ.
Ideally, you would get a statement, in writing, saying that your employer doesn't contest ownership of the invention.
CAVEAT: The above is NOT legal advice, and you are NOT my client.