Recycled metal purity determination is the analytical process of quantifying the base metal content and complete impurity profile of secondary aluminum, copper, zinc, lead, precious metals, and ferrous scrap recovered from end-of-life products and industrial waste streams. Accurate purity testing underpins fair trade settlement, alloy specification compliance, and regulatory conformance across the circular metals economy. This article explains the sample types, testing items, analytical methods, and international standards used by accredited third-party laboratories to certify the purity of recycled metals.
Technical Overview
Recycled (secondary) metals are recovered from diverse scrap streams, including end-of-life vehicles, electronic waste, cables, packaging, machining swarf, and post-industrial offcuts. Unlike primary metals, recycled materials accumulate tramp elements such as Fe, Ni, Pb, Sn, Sb, and Bi through successive recycling loops, and they may carry coatings, solders, embedded plastics, or other contaminants. Determining the purity of recycled metal is therefore essential for grade classification, alloy formulation, trade settlement, customs classification, and compliance with environmental regulations such as RoHS and the End-of-Life Vehicle (ELV) Directive.
Purity determination follows two complementary strategies. The direct method measures the base element itself, for example copper by electrogravimetry or silver by potentiometric titration. The indirect method, universally applied to high-purity grades above 99.9%, calculates purity as 100% minus the sum of all quantified impurities, using high-sensitivity techniques such as ICP-OES, ICP-MS, and glow discharge mass spectrometry (GDMS). Both approaches require rigorous sampling, because heterogeneity is the single largest source of error in recycled metal analysis.
Testing Samples
Recycled Aluminum and Aluminum Alloys
Common samples include remelt aluminum ingots, secondary die-casting alloys (such as ADC12 and A380-type alloys with Si contents of roughly 9.6–12% and Cu of 1.5–3.5%), extrusion and rolled scrap, and dross. Purity of remelt ingot is typically certified against grades such as Al 99.70 per GB/T 1196 or equivalent ASTM and EN specifications, with impurity profiling per the GB/T 20975 series.
Recycled Copper and Copper Alloys
Samples include copper cathode (grade A cathode, Cu-CATH-1, with Cu+Ag content ≥99.9935% per GB/T 467 and ASTM B115), copper wire granules, brass and bronze ingots, and No.1/No.2 scrap. Electrolytic cathode requires trace-level determination of more than a dozen impurity elements including Se, Te, Bi, Sb, As, Pb, Sn, Ni, and Fe using the GB/T 5121 series of methods.
Recycled Zinc, Lead, and Tin
Zinc die-cast alloys (Zamak-type), zinc dross, refined secondary lead from battery recycling, and tin-based solder alloys are tested against grades such as Zn 99.995 (GB/T 470), Pb 99.985 to Pb 99.990 (GB/T 469), and cast zinc alloy ingots per GB/T 8738, using the GB/T 12689 and GB/T 4103 analytical series.
Recovered Precious Metals
Gold, silver, and platinum-group metals recovered from printed circuit boards, catalytic converters, and photographic or industrial residues are assayed by fire assay cupellation (ISO 11426 for gold), potentiometric titration (ISO 11427 for silver), and ICP techniques after acid digestion.
Ferrous and Stainless Scrap
Shredded steel, stainless steel scrap, and alloy steel revert are characterized for C, S, P, Mn, Cr, Ni, Mo, and residual elements per the GB/T 223 series, GB/T 11170, and ASTM E415.
Testing Items
Depending on the metal and commercial purpose, recycled metal purity programs typically cover the following items:
- Base metal content and calculated purity across the range of 99.0% to 99.999% (3N to 5N grades)
- Major alloying elements: Si, Mg, Zn, Cu, Mn, Fe, Ni, Sn, and others relevant to the alloy family
- Trace and tramp impurities at ppm to ppb levels, including Pb, Cd, As, Sb, Bi, Se, Te, Sn, and Fe
- Interstitial gases: oxygen, nitrogen, and hydrogen by inert gas fusion; carbon and sulfur by combustion infrared detection
- Restricted substances for regulatory compliance: Pb, Cd, Hg, Cr(VI), and brominated flame retardants per RoHS Directive 2011/65/EU and ELV Directive 2000/53/EC limits
- Radioactivity screening of imported scrap, including gamma dose-rate measurement and radionuclide identification of common orphan sources such as Cs-137, Co-60, and Am-241
- Foreign matter, oil, and moisture assessment for trade-weight correction of scrap consignments
Testing Methods
Spark Discharge Optical Emission Spectrometry (Spark OES)
Spark OES is the workhorse for rapid, simultaneous multi-element analysis of solid metallic samples, delivering detection limits at the 1–10 ppm level for most elements. Standard methods include ASTM E1251 for aluminum alloys, ASTM E415 for carbon and low-alloy steel, GB/T 20975.25 for aluminum, and GB/T 11170 for stainless steel. Samples must be presented as flat, machined discs or chill-cast pucks of adequate thickness.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
Following acid digestion, ICP-OES provides fully quantitative determination of impurity elements from sub-ppm to percent levels with a wide linear range, and it is the principal technique for the impurity-summation (difference) method used to certify 3N and 4N recycled copper, aluminum, zinc, and lead.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS extends detection to ppb and sub-ppb levels, enabling reliable purity assignment of 4N to 5N grades where individual impurities fall below the practical limits of ICP-OES. It is also used for toxic elements such as Cd and Hg in regulatory compliance testing.
X-Ray Fluorescence Spectrometry (XRF)
Both benchtop and handheld XRF (guided by ASTM E1621) provide non-destructive compositional screening of ingots, scrap, and finished goods within seconds. Detection limits of roughly 10–100 ppm for mid-Z elements make XRF ideal for grade sorting and pre-shipment screening, with quantitative confirmation performed by OES or ICP methods.
Combustion and Inert Gas Fusion Analysis
High-frequency combustion infrared detection quantifies carbon and sulfur, while inert gas fusion with infrared and thermal conductivity detection measures oxygen, nitrogen, and hydrogen, with methods such as ASTM E1019 applied to steel and nickel-bearing recycled alloys.
Classical Wet Chemistry
Electrogravimetry determines copper content directly with excellent accuracy (ASTM E53, GB/T 5121.1), while EDTA titration, redox titration, and gravimetric procedures support certification of zinc, lead, and tin products and serve as referee methods for disputed results.
Fire Assay and GDMS for High-Purity and Precious Metals
Cupellation fire assay remains the definitive technique for gold fineness determination (ISO 11426), complemented by potentiometric titration for silver (ISO 11427). For ultra-high-purity recycled metals of 5N and above, glow discharge mass spectrometry (GDMS) surveys virtually the entire periodic table at ppb to ppt levels.
Sampling and Sample Preparation
Because recycled metal is inherently heterogeneous, representative sampling is critical. Accredited laboratories apply increment sampling across the consignment, size reduction by riffling or coning-and-quartering, and, where necessary, remelting under protective atmosphere followed by chill casting of spectrometry discs, ensuring that analytical results reflect the entire lot rather than a single fragment.
Applications
- Trade settlement and customs classification of imported and exported scrap and secondary ingots
- Incoming inspection and charge formulation at secondary smelters, refineries, and die-casting foundries
- Valuation of electronic waste and catalytic converter streams for precious metal recovery
- Certification of compliance with RoHS, ELV, and REACH substance restrictions
- Verification of recycled content and purity claims in sustainability reporting and green procurement
- Process development and mass-balance studies for new recycling technologies
- Dispute resolution and referee testing between buyers and sellers
Quality Assurance and Reporting
Tests should be performed by laboratories accredited to ISO/IEC 17025, with methods validated against matrix-matched certified reference materials and participation in interlaboratory proficiency schemes. A compliant test report identifies the sampling protocol, sample preparation route, analytical techniques and their standard references, results with measurement uncertainty and detection limits, and the calculated purity statement, providing legally defensible documentation for trade and regulatory use.
FAQ
Q: How is the purity of a metal above 99.9% determined?
A: Direct measurement of the base element cannot resolve differences at the fourth decimal place, so purity is calculated as 100% minus the sum of quantified impurities, determined by ICP-OES and ICP-MS, or by GDMS for 5N-plus grades.
Q: What sample forms and quantities are required?
A: Spark OES requires a solid block or disc at least 20 mm thick with a machined flat surface; wet chemical analysis requires 5–20 g of clean drillings or millings. Heterogeneous scrap is remelted and chill cast by the laboratory before analysis.
Q: Can XRF alone certify recycled metal purity?
A: XRF is excellent for rapid, non-destructive screening and sorting, but its detection limits and surface sensitivity mean that formal purity certification is normally issued from spark OES, ICP, or classical referee methods.
Q: Is radioactivity screening included?
A: Gamma dose-rate screening and radionuclide identification are strongly recommended and, in many jurisdictions, mandatory for imported scrap, ensuring consignments remain below clearance levels before processing.
Q: What is a typical turnaround time?
A: Routine purity certification generally takes three to seven working days from receipt of a representative sample, with expedited 24–48 hour services available for time-critical shipments.