In pyrometallurgical refining what is the driving force behind selective oxidation?

Answer 1

When we set up to win metal in a recovery operation, we frequently choose to remove other materials by simply applying heat to "burn off" or oxidize the things we don't want in our end product. Let's go at this query in two parts. The first part is a no brainer. It's pyrometallurgical refining - we apply heat (fire - the "pyro" part) to a stream of influent to effect the oxidation of stuff we don't want in our desired product. Melt it, blow oxygen through it, burn off dross and presto! good product. Piece of cake. Just burn it off. Remove it by oxidation. But how? Why selectively, or course. Let's look at the "selective" part of oxidation. We need a clear knowledge the "contaminants" in our product stream, their identities and their concentrations. We call the lab folks in off their coffee break and have them find out. Then we weigh these nice folks down with another problem. What is the chemistry of our desired metal and also all of these "contaminants" in our initial product? Everything! And not their "regular" chemistry. We need to know their chemistry at elevated temperatures. In addition, we need to know how all the "contaminants" and our desired metal react with each other (in those proportions calculated in our influent stream) at elevated temperatures. (We don't do that stuff. We're engineers! So we're drinking coffee and filling in crosswords and a sudoku or two while they bust their buns in the lab.) They come back with the data. And the secret to the "selective" part of our "selective oxidation" process. Break is over for us. We grab our hard hats and face shields. The metallurgical chemists have given us a "cook book" to cook up our influent and guide us in winning the desired metal. What happens is that we heat everything up to the "required temperature" per out cook book, and then we add "secret ingredients" to the mix. The material(s) stirred into the molten mix are specifically chosen because they will react with our "undesired materials" and minimally impact our desired metal. The addition of specific materials in specific amounts is the key to making things happen. We force oxygen through the mixture, and the selective oxidation of the dross will occur per our "recipe" courtesy of the guys and gals back in the lab. With the slipping quality of materials in our influent stream (decreasing quality of scrap in the recycling process, lower quality ore - that kind of thing) this process is becoming increasingly important. When calculating costs to recover metals, we see that an already high energy bill will continue to creep up. Energy is increasingly expensive. If there is anything we can do to improve our processes and control our costs, we need to incorporate it. High temperature chemistry acting on a mixture that has had selected materials in controlled amounts added to a molten "base" mix that is then blasted with oxygen is the driving force behind selective oxidation. The application of heat to a mixture and mixing in specific ingredients in specific amounts followed by a big blast of oxygen through the mix to effect specific oxidation of is not new. But its importance in metallurgical recovery processes is now, more than ever, critical to metal markets (and, ultimately, the consumer).