Positive identification: whether using X-ray or arc/spark metal analysis, technology promises improvements in speed and accuracy - Metal Analyzers

Recycling Today, Nov, 2001 by Deanne Toto

Technological progress continually allows for a variety of improvements in metals analyzers, increasing the speed of analysis and the durability and portability of the instruments.

But technology cannot change one fundamental thing: The alloy detection capabilities of the two most common types of analyzers, X-ray and arc/spark. Therefore, most sources reached for this story agree: The search for a metals analyzer should begin with the question, Which alloys must be detected?

David Jarzinski, regional sales manager of stationary metal analyzers, Spectro Analytical Instruments Inc., Leominster, Mass., says, "Today, mills are lowering allowable limits and tightening down on specifications and allowable tolerances. Therefore, nearly everything needs to be tested sooner or later before it gets re-melted."

Jarzinski says that analysis is especially critical for some alloys. "High-temp alloys come to mind first due to the significant money that can be lost or made with everyday decisions dealing with these metals. Second are aluminum and copper alloys due to the many possible tramp elements or poisons that can quickly devaluate or even make the material worthless. Lastly, titanium stands out due to the many new uses for titanium alloys in safety related products."

THE CRUCIAL CRUCIBLE

For Bob Rappaport of Universal Metal Corp., Worcester, Mass., analysis is absolutely critical. Rappaport says, "as an approved vacuum processor of high temperature alloys, when we sell the alloy, we have to be dead-on. We have to know what we are selling."

In many cases, Rappaport says that Universal Metal Corp. does 100 percent sortation. "Being able to get a reading that is accurate and timely is very important for us to be able to process our metal," he says.

At some locations, a variety of materials are dealt with. At SLC Recycling, a division of Ferrous Processing and Trading, Detroit, vice president of quality control Clarence Watts says, "We purchase and sell recyclable scrap for the steel melting as well as the nonferrous melting business. So, our analyzers have the capability of checking both ferrous and nonferrous materials."

Margo Myers, director of analytical marketing for Thermo MeasureTech, Round Rock, Texas, says, "By making the analysis and assuring the alloy type, the dealer can appropriately price his product. Since we provide XRF (X-ray fluorescence) systems, more of these are sold into non-ferrous, non-aluminum yards. But even aluminum dealers may want an XRF as it will tell them groups of aluminums: 2000 series versus 7000 series."

Jarzinski says, "The analysis requirements must be carefully matched together with the capability of the instruments available before purchasing."

WHICH METHOD?

Optical emission systems (OES), also called arc/spark, and XRF are the two basic methods used for metal analysis.

OES uses an arc or a spark to vaporize the sample, exciting the atoms and ions into emission of radiation. This emitted radiation then passes through an optical fiber and is dispersed into its spectral components. A photo-multiplier tube (PMT) measures the range of wavelengths emitted.

The OES system is essentially two techniques in one machine. Jarzinski explains, "Arc is a very fast sorting tool that typically does not require sample preparation. Many grade/alloy IDs can be performed between 0.5 to 3 seconds. Spark on the other hand is a more finite analysis tool when close alloys need to be separated, such as 6061 and 6063 aluminum, or better chemistry is needed to maximize profits, as with nickel alloys. A spark analysis typically requires some light grinding or sample preparation and has an analysis time of 13 seconds."

Jarzinski adds, "Only OES can determine such elements as carbon, phosphorous and sulfur, which can be important in stainless steels, certain copper alloys and a few nickel alloys." In addition, OES can analyze silicon and magnesium. He also says that the ownership cost for OES is lower because the system does not need to be licensed, therefore saving the associated fees, and that no nuclear sources need to be replaced.

Tom Anderson of Niton Corp.'s Bend, Ore., office, says, "The arc/spark technology really shines for aluminum alloys." However, Anderson says, "With arc/spark, you have to know what the base material is in order to know which analysis program to use." Anderson also says that OES systems are not as adept with high-temperature alloys.

XRF uses X-rays emitted from an X-ray tube or a radioisotope to excite the atoms of the sample material. An electron is ejected, causing a second to fall into its void and energy is released. A detector within the instrument uses the energy level to determine the element.

Anderson says that XRF's strengths lie in high temp and titanium alloys, nickel and cobalts. "That's where their strengths really are--materials with higher concentrations of alloying elements. And that's the limitation of arc/spark."

With regard to aluminum alloys, Anderson says that Niton systems "can easily separate some grades within a series, such as 7050 and 7075. It can also distinguish series in most aspects." Anderson adds that XRF has some limitations with regard to aluminum bronzes and silicon bronzes because of the inability to detect aluminum or silicon.