If you are a good refiner then you should get all the gold back at the end. Some will be trapped in filters and various liquids, but processing that will get it back later on. See for example stock pot.
Any contamination will be removed along the way, so if you start with impure gold then there will be a weight loss even if you don't lost any gold. This should be quite obvious.
Sure there can be profitability in refining gold. There are a lot of companies that lives on it. You only have to work out how much the refining costs you and see to it that you can buy at low enough price and sell at high enough to cover your costs and a salary.
Finding that margin can be tricky and will probably take you some time and cost before you can establish a working company.
Göran