Catalytic converters are designed to remove toxic gases and pollutants from car exhaust gases. Therefore, they contain the precious metals Pt, Pd and Rh. When the car reaches end of life the converter is scrapped. The precious metals are recovered in a recycling chain comprising several different stakeholders, from scrap yards to toll refineries. Due to their high value, accurate analysis and processing of the precious metal content is critical to ensure a maximum return in recycling. This application note shows a safe, easy and accurate method for providing data giving full insight into the composition of the to-be-processed converter .
Measurements were performed using a Malvern Panalytical Epsilon 4 EDXRF spectrometer, equipped with a 15W, 50 kV silver (Ag) anode X-ray tube, 6 software-selectable filters, a high-resolution SDD30 silicon drift detector and a 10 position sample carousel.
Fourteen car catalyst samples, supplied by several customers, were used to create the calibration. The precious metal content of each calibration sample was determined by fire assay or inductive plasma coupled spectroscopy (ICP). The majors and minors of the calibration samples were certified using one of Malvern Panalytical’s standardless solutions WROXI or Omnian.
The samples were dried for 2 hours before pressing. To make 32 mm pellets, 10 grams of sample was mixed with 1 gram of Ultrawax as a binder. The pressing was done by applying 20 tons for 60 seconds.
Figure 1. Image of one of the prepared pressed pellets.
|Compounds||kV||uA||Filter||Medium||Meas. time (s)|
|Pt, NiO, ZnO, SrO, Y2O3, ZrO, WO3, PbO,||50||80||Ag||Air||60|
|Rh, Pd, BaO||50||300||Cu-thick||Air||180|
|K2O, CaO2, TiO2, MnO, Fe2O3, La2O3, CeO2, Nd2O3||15||370||Al-thin||Air||30|
|MgO, Al2O3, SiO2, SO3||7||2100||Ti||Air||30|
Table 1. Measurement conditions
Four different measurement conditions were used to measure the 3 precious metals and other compounds in the samples. Measuring the total matrix improves the flexibility of the method and makes it more accurate for different catalytic converter materials. The total measurement time was set at 5 minutes per sample.
Figures 2, 3 and 4 show the calibration graphs for Pt, Rh and Pd in the customer-certified calibration samples. The graphs show good correlations between certified concentrations and measured XRF intensities. Detailed calibration results for the precious metals and matrix elements are listed in Table 2.
The RMS (Root Mean Square) value is equivalent to 1 sigma standard deviation.
|Compounds||Concentration range||RMS (ppm)*||Correlation Coefficient|
|Pt (ppm)||566 - 1992||49.5||0.9942|
|Rh (ppm)||64.5 - 265||7.2||0.9949|
|Pd (ppm)||485 - 2407||47.4||0.9954|
|MgO (wt-%)||4.73 - 7.20||0.18||0.96158|
|Al2O3 (wt-%)||27.59- 41.78||1.11||0.9592|
|SiO2 (wt-%)||29.00 - 46.79||1.42||0.9728|
|SO4 (wt-%)||1.05 - 4.89||0.23||0.9874|
|K2O (wt-%)||0.25 - 0.45||0.02||0.9605|
|CaO2 (wt-%)||0.35 - 1.66||0.04||0.9953|
|TiO2 (wt-%)||0.55 - 4.65||0.04||0.9994|
|MnO (wt-%)||0.00 - 0.34||0.02||0.9809|
|Fe2O3 (wt-%)||1.21 - 2.36||0.04||0.9922|
|NiO (wt-%)||0.02 - 0.11||0.001||0.9642|
|ZnO (wt-%)||0.11 - 0.29||0.01||0.9865|
|SrO (wt-%)||0.16 – 0.41||0.02||0.9939|
|Y2O3 (wt-%)||0.01 – 0.13||0.001||0.9997|
|ZrO (wt-%)||1.44 – 6.02||0.12||0.9974|
|BaO (wt-%)||0.22 - 0.81||0.04||0.9853|
|La2O3 (wt-%)||0.14 - 0.73||0.02||0.9928|
|CeO2 (wt-%)||1.59 - 4.83||0.13||0.9947|
|Nd2O3 (wt-%)||0.06 – 0.38||0.04||0.9497|
|WO3 (wt-%)||0.00 - 0.44||0.004||0.9996|
|PbO (wt-%)||0.00 - 2.66||0.07||0.9977|
Table 2. Calibration details (*RMS: The more accurate calibrations have the smaller RMS values).
Figure 2. Calibration graph for Pt in catalytic converters.
Figure 3. Calibration graph for Rh in catalytic converters.
Figure 4. Calibration graph for Pd in catalytic converters.
|Sample name (repeats)||Compounds||Certified conc.||Average conc.||RMS||Rel. RMS (%)|
|NIST 2556 (20x)||Pt (ppm)||697.4 +/- 6.3||692.8||1.3||0.2|
|Rh (ppm)||51.2 +/- 0.5||52.5||0.7||1.3|
|Pd (ppm)||326 +/- 1.6||322.6||2.8||0.9|
|NIST 2557 (20x)||Pt (ppm)||1131 +/- 11||1152.2||2.4||0.2|
|Rh (ppm)||135.1 +/- 1.9||143.5||1.4||1.0|
|Pd (ppm)||233.2 +/- 1.9||225.6||1.3||0.6|
|ERM-EB504 a (5x)||Pt (ppm)||1414 +/- 9.0||1392.0||2.6||0.2|
|Rh (ppm)||210 +/- 2.2||207.8||2.5||1.2|
|Pd (ppm)||1596 +/- 11||1640.9||113.4||0.8|
Table 3. Accuracy and precision testing on three commercially available certified reference materials.
|Compound||Unit||Measured concentration||RMS||Rel. RMS (%)||Compound||Unit||Measured concentration||RMS||Rel. RMS (%)|
Table 4. Result of validation measurement with a car catalytic sample prepared as a pressed pellet.
Accuracy and precision for precious metals
The method accuracy and instrument precision were tested by measuring multiple commercial certified reference materials (CRMs) 20 times (NIST 2556 & NIST 2557) or 5 times (ERM-EB504a) consecutively. The certified and average measured concentrations, RMS and relative RMS values are presented in Table 3, all demonstrating excellent accuracy and precision.
Besides the precious metal content, the Epsilon 4 can help the user to obtain a complete overview of the sample. Knowing the full elemental composition enables you to determine the optimal strategy for efficient processing, and to detect the presence of any rogue elements. An example is shown in Table 4, where a typical sample was measured 20 times.
The results clearly demonstrate the capability of Epsilon 4 to determine the concentrations of valuable precious metals Pt, Rh and Pd and other elements in catalytic converters. The repeatable sample positioning and outstanding sensitivity of the optical components combined with powerful software deconvolution algorithms allows the Epsilon 4 to provide accurate analysis for every measurement in only 5 minutes.