Tramp elements that typically show up in lower concentrations, or trace amounts, are generally defined as those that are not deliberately added and that cannot be completely removed by typical metallurgical processes. Manufacturers must be aware of tramp elements in their incoming raw materials because even a minute presence of some incorrect metals in an alloy would change the composition and could result in undesired qualities, like surface defects or brittleness in finished products. For example, according to a National Physical Laboratory report, “Copper and copper-nickel alloys tend to become less noble with increasing flow rate and to corrode more….” That would be quite a problem for manufacturers in the automotive, aerospace, marine and medical device industries who need to ensure their finished products don’t corrode easily.
It’s not the actual element that is entirely the problem, though. Copper is an alloying element in some grades of stainless steel and aluminum. It can be a valuable metal when used as the principle alloying element in aluminum as it can be strengthened significantly through solution heat-treating. However, as we learned in When is the Value of Copper Less than a Penny?, it is not a desirable element for mills producing grades of aluminum that are supposed to be copper-free. Also, copper, and tin, can be harmful tramp elements in steels and can directly influence the mechanical properties of steel products.
A Problem for Scrapyards and Metal Recyclers
Because tramp elements can be a problem for manufacturers, they can also be a problem for metal recyclers. Scrap metal recycling is a global industry that takes any metal product at the end of life and turns it into a usable feedstock for creating new metal and new products. According to the latest USGS Mineral Commodity Summaries, in 2015, aluminum recovered from purchased scrap in the United States was about 3.61 million tons, of which about 54% came from new (manufacturing) scrap and 46% from old scrap (discarded aluminum products). Aluminum recovered from old scrap was equivalent to about 30% of apparent consumption. Beryllium was recovered from new scrap generated during the manufacture of beryllium products and from old scrap. Beryllium manufactured from recycled sources requires only 20% of the energy as that of beryllium manufactured from primary sources. Bismuth-containing new and old alloy scrap was recycled and thought to compose approximately 10% of U.S. bismuth consumption, or about 80 tons. Recycled chromium (contained in reported stainless steel scrap receipts) accounted for 34% of apparent consumption.
The report also noted that old scrap, converted to refined metal and alloys, provided 160,000 tons of copper, equivalent to 9% of apparent consumption. Purchased new scrap, derived from fabricating operations, yielded 670,000 tons of contained copper. Of the total copper recovered from scrap (including aluminum- and nickel-base scrap), brass mills recovered 79%; copper smelters, refiners, and ingot makers, 15%; and miscellaneous manufacturers, foundries, and chemical plants, 6%. Copper in all scrap contributed about 32% of the U.S. copper supply.
Worldwide, about 30% of the total germanium consumed is produced from recycled materials. During the manufacture of most optical devices, more than 60% of the germanium metal used is routinely recycled as new scrap. Germanium scrap is also recovered from the window blanks in decommissioned tanks and other military vehicles.
We’ve only scratched the surface of where recycled metals come from, how they are used, and what they contain. But just as important is knowing how much of that recycled metal contains tramp elements. Recyclers may not be able to control the amount of undesirable elements that show up in the recycling process. Thus it is of utmost importance to analyze the scrap before it goes out the door and verify that outgoing shipments meet customer requirements. Scrap metal recyclers have discovered that handheld x-ray fluorescence (XRF) analyzers are the best way to provide the elemental analysis needed to positively identify numerous alloys, rapidly analyze their chemical composition at material transfer points, and guarantee the quality of their product to their customers.
A Problem for Manufacturers
Why go through the bother to do elemental analysis several times in one operation? Manufacturers and mills can reject or severely downgrade the load from the originally agreed upon price if contaminants or tramp minerals are found in the load. With growing concerns about the occurrence of tramp elements in incoming raw materials as recycled scrap metal use increases, and the catastrophic effect tramp elements can have if they change the overall composition and qualities of the final product, manufacturers are becoming more diligent in inspecting all incoming metals to ensure they meet the specifications on the paperwork. Many manufacturers, especially ISO 9000 facilities, utilize portable XRF analyzers throughout the facilities for quality checks when the metal comes in, after the metals are sorted, and for final analysis before the finished product leaves the facility.
Determining the purity of metal and percentages of any alloying elements present is a very important quality control step for both the scrap metal recycler, the mill, and the manufacturer. Metals that aren’t composed of the right alloys in the correct percentages, or alloys that contain undesirable tramp elements, not only mean unhappy customers but can actually cause expensive and dangerous material failures. So in this competitive environment, if a scrap recycler thinks they don’t need to verify their material shipments, they could potentially lose business, and drop out of the industry “without a trace.”