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LIBS for Melt Diagnostics

Energy Research Company (ERCo), with funding from DOE's Industrial Technologies Program, has installed a full-scale system to measure in-situ and in real time the elemental concentrations of Commonwealth's (a secondary aluminum plant) aluminum melt. Termed LIBS for Laser Induced Breakdown Spectroscopy, the concept employs a laser and spectrometer as shown in Figure 1. A probe is placed inside the melt and a laser is repetitively fired through a fiber optic cable and through the probe. A small amount of melt, at the probe tip, absorbs the laser light producing temperatures sufficiently high to heat and vaporize it into a gaseous plasma state.

 

 

The resulting plasma emits a radiative signal that is picked up by the fiber optic cable and transferred to the spectrometer. This characteristic line radiation is spectrally resolved which uniquely identifies the elements in the melt. In addition, the signal amplitude provides the concentration of each element present.

ERCo has installed a full-scale commercially operating LIBS System at Commonwealth Aluminum in Uhrichsville, OH. The photo on the left of Figure 2 shows the LIBS probe installed in Commonwealth's filter bowl and the right side of the figure shows a close-up of the probe inserted in the melt. Figure 3 shows the cabinet which houses the laser, spectrometer, gas flow controllers, and ancillary components. It is located on a mezzanine overlooking the filter bowl.

The LIBS System is designed to be a single push button operation with no training required. The operator presses the on-button and, if all the interlocks are satisfied, the probe automatically extends into the melt and begins collecting data. Similarly, a single button ends the measurements and retracts the probe. Further, the LIBS System is self-calibrating so that the operator does not need to be trained to calibrate it. 

In addition, the LIBS System has been certified as being eye safe, so neither specialized safety training nor safety equipment are required.

 

Figure 2 - LIBS Probe Installed at Commonwealth
(Click on image to enlarge)

Figure 3 - Instrument Cabinet Located on the Mezzanine
(Click on image to enlarge)

 

Test Results

Table 1 shows a summary of the LIBS data for a typical day, November 6, 2003. Also shown, as a comparison, are samples periodically collected by Commonwealth and analyzed using a conventional spark spectrometer.

For the elements with concentrations of 1% or higher, the difference between the LIBS measurements and the Commonwealth button samples is from 0.0 to 7.1%.

 

Table 1- 11/6/03 Data for 3105 Alloy

  Al Cu Fe Mg Mn Si
LIBS Average 97.87% 0.17% 0.65% 0.47% 0.52% 0.28%
Commonwealth Average 97.56% 0.18% 0.65% 0.49% 0.56% 0.30
% Difference 0.32% 5.6% 0.0% 4.1% 7.1% 6.7%

 

 

The plots in the slide show on the right show the LIBS measurements over time as compared to the Commonwealth sample results.

 

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