There are a couple of steps you can take to learn more about salaries.
1) Research average salaries. There are two great salary resources I can recommend: salary.com and glassdoor.com. Salaries are based on specific jobs in an industry as well as where the job is located (region). Also, the U.S. Bureau of Labor Statistics has salary information on jobs https://www.bls.gov/bls/blswage.htm. So, first do some research to find out the kinds of jobs people have in the metal refining business (google, look at jobs on indeed.com, glassdoor.com). And then do some research to learn the average salaries.
2) Conduct informational interviews with people who work in this industry. Do you know any people who are working in metal refining or anyone who knows someone working in this industry? Or perhaps someone who runs a business locally? You can also try networking with people in this industry on LinkedIn. Many people are happy to help someone starting out. Politely introduce yourself and explain that you are looking at entering the metal refining business. Then ask them for 20 minutes to talk to you about their experience. Prepare a set of questions to take with you so you make the best use of their (and your) time.
Lisa recommends the following next steps: