May/June 2015 While hand-held X-ray fluorescence spectrometers are a mature technology well-suited to the needs of the scrap industry, new laser-based analyzers also show promise.
By Kenneth A. Hooker
Scrap dealers can’t rely solely on their eyes and hands to know for sure what they’re buying and selling. To sort your scrap properly and set prices fairly, you need to determine the actual composition of the scrap you’re buying.
For years, optical emission spectrometers were the simplest and most convenient instruments available to analyze and determine the chemical composition of metal objects. OES units direct an electric spark onto the surface of a prepared sample, burning off a tiny section of the material and exciting atoms that reflect light back to the OES. The unit matches the wavelengths of the light received to those stored in its library to identify the elements in the sample and measures the intensity of the light to determine the concentration of the alloying elements.
OES instruments still have a role to play in the scrap industry: They currently provide the most precise analysis of the elements in a piece of metal, and they are the only tools that are able to recognize carbon in steel. If that’s a capability you need, OES is the way to go, but perhaps less than 1 percent of scrapyards ever need to analyze for carbon content, according to one analyzer dealer who also teaches analyzer training courses.
He points out some other drawbacks of OES compared with hand-held analyzers: “OES can be very quick if it’s a rough sort, but it takes maybe 30 to 40 seconds to get careful chemistries,” he says. “Also, there’s no such thing as a hand-held OES machine. They’re ‘luggable,’ but you’re talking 30 to 40 pounds, as compared with 3 or 4 pounds for a hand-held analyzer.”
XRF: Today’s Standard
The most commonly used hand-held analyzers in the scrap industry today are X-ray fluorescence spectrometers. Introduced to the scrap industry in the late 1990s, these instruments were the first to offer a lightweight and easily portable way to analyze the elements in a piece of metal. Tubes housed within these analyzers generate X-rays that excite atoms on the sample’s surface, sending fluorescent X-rays back to the unit for analysis. The unit compares the reflected element’s characteristic X-rays with stored elemental profiles, then it displays the sample’s chemical breakdown on the analyzer’s readout screen. If the alloy matches one of the hundreds stored in the tool’s alloy grade library, the screen also will provide the alloy’s name.
Early versions of the XRF analyzer became popular because they were lightweight, relatively fast and easy to operate, and able to accurately detect the presence of nickel, chromium, molybdenum, and other heavy elements. They could quickly distinguish among types and grades of stainless steel, allowing scrap recyclers to separate materials of higher and lower value and thus charge appropriate prices.
These early XRF instruments relied on silicon PIN diode detectors, most of which were unable to measure lighter elements such as magnesium, aluminum, silicon, phosphorus, and sulfur without using a cumbersome helium purge or vacuum attachment. The introduction of high-resolution silicon drift detectors about 10 years ago gave XRFs the ability to measure light elements as well as heavy elements at much lower concentrations than had previously been possible. SDDs allow for the use of multiple internal filters, which change the excitation energy of the X-rays you aim at the sample and allow the instrument to detect additional elements.
One potential downside is that the more demanding light-element analysis requires a little more time to register, and the time extends depending on the number and amounts of light elements in the sample. A typical analysis using an SDD-equipped instrument might take three to five seconds, according to the analyzer dealer/trainer, but analyzing certain low-magnesium-content aluminum alloys could take as long as 30 seconds per sample. For a scrap recycler testing thousands of samples a day, those additional seconds become a significant consideration.
Many scrapyards use XRF analyzers mostly for rough sorting and are willing to forgo some precision in the analysis in favor of greater speed. To meet those varying needs, XRF manufacturers offer models with different capabilities at different price points, most ranging from the mid-$20,000s to the mid-$30,000s.
Selecting an XRF Analyzer
Many industry experts view the XRF analyzer as a mature technology, undergoing only minor refinements over the past few years. This makes it harder for competitors to differentiate their products to the customer, one manufacturer observes. All XRF makers are working to produce analyzers as small, easy to use, and ergonomic as possible, rendering them more like a tool and less like a scientific instrument, he says. Two factors that scrap recyclers should consider in XRF analyzers in addition to their performance are durability and safety.
The first question you might ask is whether a unit you’re considering buying can withstand the rugged scrapyard environment. It should be waterproof, dustproof, and padded internally to resist damage if it’s dropped or handled roughly. Of course, everyone wants a drop-proof instrument, the analyzer dealer/trainer says, but no one makes one yet. The detector at the front end of an XRF analyzer has a very thin beryllium window that lets the X-rays through; if it is too thick, the X-rays can’t get through. If it breaks, he says, the detector and the X-ray generator might have to be replaced together, then the analyzer needs a week’s worth of calibration at the factory. These repairs are expensive—from $5,000 to $10,000 or more—so it’s important to train operators to use the units carefully. He also recommends that the operator use a lanyard or wrist strap with the analyzer to keep from dropping it during testing.
In recent years, most XRF manufacturers have come up with shields that can help protect the detectors against punctures. One company claims the shield it introduced just two years ago has yet to allow a detector failure due to punctures. As a result, the company quickly made the shields a standard feature rather than an option. This shield also doesn’t interfere with X-rays, so it doesn’t affect the unit’s accuracy or its light-element performance, the company representative says. With some competitors’ versions, he says, you might have to either change the calibration to measure light elements or remove the shield.
Despite these improvements, accidents can happen, so it’s important to consider an analyzer manufacturer’s warranty policies: What’s covered? Can you purchase extended warranty coverage? Are loaner units of the same model available if your analyzer needs to be repaired?
Because they produce X-rays, XRF analyzers can potentially create a radiation hazard if you fail to use them properly. Manufacturers provide training for operators on how to use the instruments safely. Though today’s XRF analyzers use sealed X-ray tubes and don’t have transportation restrictions, they are subject to certain registration requirements. The requirements vary by state, so you should understand your own state’s rules to be sure you are in compliance.
Laser Analyzers Show Promise
The technology most recently adapted for hand-held metal analyzers is laser-induced breakdown spectroscopy. LIBS analyzers use a focused laser beam to vaporize a tiny amount of a sample’s surface, which ionizes to create a plasma. This process excites atoms, which emit photons characteristic of the element. A device within the LIBS instrument disperses the emitted light into wavelength bands that identify the sample’s composition. Like XRF, LIBS is essentially a nondestructive testing method, and it requires little or no sample preparation if the sample’s surface is representative (i.e., if it’s not blocked by paint, scale, or other coatings).
Laboratory LIBS equipment has been in use since the 1960s, but the first hand-held device only recently appeared, in 2013. The scrap industry currently uses relatively few LIBS analyzers, but proponents say they offer some considerable potential benefits.
One manufacturer that is just launching its LIBS analyzer commercially outlines three areas where LIBS offers advantages over XRF: safety; speed, particularly in analyzing light elements; and ruggedness.
“With LIBS, you’re moving from a potentially hazardous technology—X-rays—to a nonhazardous one,” this manufacturer says, noting that regulatory burdens are a bigger factor with XRF than the potential for being exposed to radiation, which is low if they’re used properly.
LIBS analyzers also offer speedier response times when analyzing light elements, he says. “An XRF unit might take 30 to 60 seconds to get a measurement of a piece of aluminum; with LIBS, you can get a measurement of the same piece, with lower levels of detection and greater precision, in one or two seconds.”
The analyzer dealer/trainer agrees that the measurement speed of LIBS instruments is a big advantage. “The new LIBS analyzers are very quick: It’s always about one second and one shot,” he says. “Their downside is that they’re not quite as accurate on the heavier elements. They’re very good on the lighter elements—good on coppers and aluminums—but not as good on the high-value materials, like [austenitic] stainless and nickel-based alloys.”
LIBS also is able to analyze some alloys that XRF can’t, such as beryllium in copper alloys, according to the product manager of a manufacturer of all types of analyzers. Another example is lithium in aluminum alloys: “Lithium is very challenging in recycling aluminums, because if it ends up in the furnace, it can damage the walls of the furnace so badly that it can cost several hundred thousand dollars to repair,” he says. “Therefore, it’s very important to identify any lithium contained in aluminum scrap before it gets to the melting process.”
Lasers also are not fragile, which gives LIBS analyzers an important advantage over the XRF, the manufacturer says. “The laser itself and the internal spectrometer are made with no moving parts and all solid-state construction, so they’re much more rugged for use in a scrapyard environment.”
LIBS analyzers are making inroads: They now represent an estimated 20 percent to 30 percent of new analyzers purchased in the scrapyard market, the dealer/trainer says.
Some in the industry see XRF and LIBS as complementary products, at least in certain settings. “For big scrapyards, LIBS is an excellent tool because it’s a really fast tool that lets you sort any alloy, regardless of the type,” says the product manager. “It can be stainless, or copper-based, or titanium-based, or even aluminum-based material, and LIBS can handle it, all with one-second measuring time. On the other hand, if you need to get very accurate results with elements like nickel, molybdenum, chromium, and so forth, then the XRF is the most accurate tool for that.” Thus, while smaller scrapyards might opt for just a LIBS analyzer for fast screening, bigger yards might supplement it with XRF whenever it needs highly accurate results, he says.
While acknowledging that XRF analyzers generally provide somewhat more accurate results on heavy elements, LIBS manufacturers say their products’ results improve if given a couple of extra seconds of measurement time. As a first-generation product, LIBS analyzers are sure to become more accurate with further development, they point out.
What to Look For
When buying a hand-held analyzer for your yard, consider these factors.
Alloy and grade libraries. Hand-held metal analyzers typically come preloaded and calibrated for a suite or library of materials they can recognize and measure. The standard libraries vary somewhat with the manufacturer and model of the instrument. When considering a purchase, compare the analyzer’s standard library against the materials and grades you expect to measure. If you have special requirements that the standard offerings don’t meet, you can modify an existing library, add specialty or exotic alloys, or create your own custom libraries.
Data storage and transfer capabilities. Hand-held metal analyzers are generally capable of storing and displaying large quantities of data, which you can transfer to a PC, a wireless network, or a mobile device using a USB memory stick, Wi-Fi, or Bluetooth technology. Instrument manufacturers typically provide software you can use to generate standard or custom reports and records.
Most customers in the scrap recycling industry make only limited use of the data storage and reporting features, according to one XRF manufacturer. They “are looking to take measurements on the fly, so they’ll take a measurement, look at the screen, and then make a decision,” he says. “If they’re sorting things into different piles or analyzing a load that’s just come in, they may set the instrument for an averaging feature … take 10 or 20 measurements, get an average, and then make a decision.”
Another analyzer manufacturer agrees, but he says that larger scrap recyclers who sell directly to scrap consumers will use the reporting software to build their blends. “Suppose they have so many tons of this material and so many of that, and they’re selling to a 304 [stainless] mill. They can store their data in the analyzer, then bring it back into the computer, either hardwired in or by Bluetooth. The reports give them a pretty accurate assessment of what they’re sending out in the rail cars to the mill,” he says.
What’s Next?
Each of the three main types of metal analyzers has its advantages: OES provides the most precise measurements but also is the most demanding to operate; hand-held XRF analyzers offer a consistently high level of performance; and the new LIBS instruments can perform speedy light-element analysis. So scrap recyclers now have a variety of sorting tools at their disposal.
Some in the industry think that LIBS analyzers will make significant inroads in the market as their technology matures. One equipment seller who deals in used analyzers expects that LIBS will outstrip XRF hand-held machines before long, but he sees more integrated systems taking hold in the future. “We’ll be seeing sensor arrays incorporated as part of automated sorting systems that are already under development,” he says.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.