Patent Number: 054141958
Section: summary

BACKGROUND OF INVENTION This invention relates to a system and method for monitoring and controlling the level of heavy metal contamination in the slurry of a soil or other particulate material washing process. BACKGROUND OF THE INVENTION Contamination of soil and other particulate materials, such as for instance sludges, sediments, scrap yard dust and the like, is becoming a more common environmental problem. Often the particulate material is contaminated with heavy metals such as, for instance, cadmium, copper, lead, mercury, radium, uranium and thorium. Various methods and systems have been developed for reducing the concentration of these heavy metal contaminants in the particulate material to acceptable levels, typically well below 1000 ppm and generally below 100 ppm. One such technique is disclosed in commonly owned related U.S. patent application Ser. No. 07/529,092 filed May 25, 1990 now U.S. Pat. No. 5,128,068 entitled "Method and Apparatus for Cleaning Contaminated Particulate Material." In this process, the soil or other particulate material is first washed with a leachate or surfactant to mobilize soluble and dispersible contaminants. Large particles are mechanically separated, washed and returned to the site. The fines in which the heavy metals are concentrated, are together with the leachate and solubilized contaminants, then separated from the intermediate sized particles by a counterflow of washwater to produce a waste slurry which is disposed of or further treated. The slurry of separated intermediate sized particles and wash water is dewatered to produce additional recovered particulate material. The size of the fines separated from the intermediate sized particles can be varied (more fines recovered with the intermediate sized particles or more diverted in the waste slurry) to adjust the level of contamination remaining in the recovered particulate material. There is a need for a reliable on-line system and method for monitoring the concentration of the heavy metal contaminants in the recovered particulate material for assuring that the required reduction in contamination level has been realized and to control the process to achieve such a result. One of the difficulties is that it is the residual concentration of the heavy metal contaminants in the recovered particulate matter which must meet the required standards, not the slurry. There is a further need therefore for such a system and method which can determine on-line the concentration for residual heavy metal contaminants in the solids fraction of a slurry. SUMMARY OF THE INVENTION These and other needs are satisfied by the invention which is directed to a system and method for on-line measurements and control of the concentration of heavy metal contamination in a soil or other particulate material washing process in which a slurry of washed particulate material having residual heavy metal contamination is produced. In order to determine the concentration of the residual heavy metal contamination in the particulate material in the slurry, energy is applied to the slurry at a level to produce electromagnetic radiation which is characteristic of the constituents of the slurry including at least the residual heavy metal contaminants. The electromagnetic radiation is analyzed to measure the amount of heavy metal contaminants and the amount of particulate material in the portion of the slurry to which the energy has been applied and from these amounts the concentration of residual heavy metal contaminants can be determined. In accordance with one embodiment of the invention, the energy is applied to the slurry in the form of x-rays at a wavelength which causes secondary emission or x-ray fluorescence of the heavy metal contaminants. The Compton scatter at wavelengths adjacent the characteristic wavelength emitted by the heavy metal contaminant is also determined and used as a measure of the mass of the particulate material irradiated which in turn is used to calculate the concentration of the heavy metal contaminant in the recovered particulate material. In another embodiment of the invention, employing prompt neutron activation, energy is applied to the slurry by a thermal neutron field. Thermal neutrons captured by nuclei of elements in the slurry transmute the element to another isotope in an excited state. These nuclei de-excite promptly emitting gamma rays. The residual heavy metal contaminants emit gamma rays of readily identifiable characteristic energies. The magnitude of gamma rays at the characteristic energy levels is a measure of the number of atoms of the heavy metal contaminant present in the slurry within the neutron field. The hydrogen in the water phase of the slurry also emits gamma rays of characteristic energy levels, so that the water content of the slurry can be determined. The mass of particulate material is determined using a density measurement and the amount of water present. This prompt neutron activation technique measures the concentration of residual contaminants in a larger volume than the x-ray fluorescence technique which can only measure contamination near the surface of the slurry. A third embodiment of the invention employs laser induced breakdown in which energy is applied to the slurry through a laser beam causing dielectric breakdown of the elements in the slurry which in turn emit light of characteristic energies. Characteristic light emitted by the heavy metal contaminant and reference elements which have been established statistically from samples to be substantially uniformly present in the particulate matter being treated, is measured and used to calculate the concentration of the heavy metal in the particulate material. As another aspect of the invention, a neural network trained with slurry samples with varying concentrations of contaminants and varying water content can be used to analyze the characteristic emitted radiation. When used to control the soil washing process, the network need only output a signal indicating whether the concentration of contaminant in the particulate material fraction of the slurry is within or not within, with an appropriate margin, a specified required level.