Radiant energy – Calibration or standardization methods
Reexamination Certificate
2002-03-15
2003-12-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Calibration or standardization methods
C250S368000, C378S044000
Reexamination Certificate
active
06657189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of bulk material handling. More precisely, it relates to devices employing nuclear radiation to measure amounts of specific constituents in bulk materials in a continuous process.
2. Description of the Prior Art
A Prompt Gamma Neutron Activation Analyzer, or PGNAA device bombards bulk amounts of material, such as limestone, coal, sand, mineral ores, wheat and the like with neutrons causing specific constituents in these materials to respond by issuing gamma rays that are subsequently measured to indicate concentrations of these constituents. Constituents such as calcium oxide (CaO) in limestone, sulfur (S) in coal, moisture (H
2
O) in sand, iron (Fe) in mineral ores, and proteins containing nitrogen (N) in wheat, are determined by using such a PGNAA. The bulk materials are delivered to the PGNAA on a rubber conveyor belt and passed through a radiation chamber where the materials are exposed to the neutron radiation. The gamma radiation caused thereby is registered on instruments that provide a direct readout of these constituents. Such a real-time analysis is needed in order to insure accurate tracking of concentrations of these constituents to provide a basis for delivering bulk materials carrying an accurate amount of whatever constituent is desired.
Depending on the type of process or application, the flow rate or mass loading of material to be measured in the PGNAA is not constant, resulting in variable material loads per unit length inside the radiation chamber. Variations of material loading can be continuous and can extend from empty to full PGNAA capacity.
The typical practice in the prior art of setting up and calibrating a PGNAA is to load the conveyor belt with a sample of the bulk material, in the particle size distribution and average tonnage rate (translated into the mass loading per unit length of the bed of material on the conveyor belt) expected in operation, where the amount of the constituent to be measured is accurately known. This “standard” bulk material is usually mixed extremely well and many samples of the mix are taken and evaluated by chemical and other means to determine the exact concentration of the specific constituent. A large quantity of this mix, in sufficient size to emulate a real-time pass-through of the bulk material, is then tested under the PGNAA and the measurement instruments adjusted to indicate the amount of constituent that is already known in the “standard” material.
This “standard” material is expensive to make, difficult to keep isolated, costly to store, and the numerous tests run on it are expensive and time-consuming. In addition, constant tonnage flow rate through the PGNAA (constant mass loading per unit length) is difficult if not impossible, to achieve and maintain and surges in product create changes in tonnage and flow rates. It has been shown that these departures from desired optimum flow rate causes deterioration in the accuracy of the measurements. This leads to sales or quality of bulk material too rich or too lean in one or major specific constituents.
If the PGNAA is calibrated with a “standard” of a given material mass loading, it will measure elemental composition accurately only when analyzing that same material mass loading. The PGNAA will produce significant measurement errors when analyzing materials of a different mass from that used in calibration. In general, PGNAA devices produce larger measurement errors when the mass loading is lower, and smaller errors when the mass loading is higher than the mass loading contained in the “standard” during the calibration. The technical reasons for the PNGAA measurement errors at non-calibrated tonnage or mass loadings are described below. The consequence of this phenomenon is that analysis measurements of variable material streams are not accurate and not reliable enough for process control.
The amount of material flow is measured by a conventional weigh scale or flow meter and reported continuously and instantaneous as F in units of tons per hour (TPH). Given the conveyor belt speed B in units of meters per second (m/s), the instantaneous material loading L in mass units of kilograms per meter (kg/m) can be determined by:
L=F
/(3.6
×B
) Equation I.
Using Equation 1, if the belt speed B=1.95 m/s and the F=400, 800, and 1200 TPH, then loadings L=56.98, 113.96 and 170.94 kg/m respectively. Conversely, the tonnage flow rate through the PGNAA can be calculated by:
F=
3.6
×B×L
Equation 2.
The technical reasons for the PNGAA measurement errors at non-calibrated tonnage or mass loadings are caused by non-constant amounts of constituent signal emanating: (1) from: the conveyor belt, and (2) from the walls, irradiating, shielding, detectors and construction materials used inside the PGNAA device itself. Constituent signals emanating from any source other than the bulk material to be measured are referred to as “background signal”.
Conveyor belts used in the coal, cement, and mineral ore industries are one source of constituent PGNAA background. These belts are primarily Styrene Butadiene Rubber (SBR), in approximately a 1:4 blend of Styrene and Butadiene respectively. Styrene is C
8
H
8
and Butadiene is C
4
H
6
. Additives to the SBR rubber include ~0.5% sulfur for vulcanization, nylon or polyester cords for reinforcement, 10-30% oil and 10-15% CaCO
3
for flexibility, and a few percent SiO
2
and Al
2
O
3
for improved wear resistance.
The materials of the conveyor belt and the walls and internals of PGNAA itself, referred to as “background materials”, will capture neutrons and emit gamma rays and produce PGNAA signal just as the bulk material itself, producing constituent background signals. Compounding this problem, the portion of the gamma ray spectra captured by the detectors that is attributable to the background materials, is not a constant signal because the amount of bulk material inside the measurement zone influences the magnitude of neutron flux impinging on the background materials. Therefore, the errors associated with the unknown magnitude of background signal from the constituents such as H, C, S, N, Ca, Al, Si, and others in the background materials prevent a prior art PGNAA device from accurately reporting only the analysis of the constituents in the bulk material itself. Furthermore, variable amounts of both neutron and gamma ray attenuation caused by variations in the thickness of the bulk material bed also contribute to PGNAA errors because the relative magnitudes of the constituent signals emanating from the bulk material itself are not constant with variable belt loading. In summary the measurement errors are a function of a multitude of parameters, each in some way caused by and related to variations in tonnage or flow rate. For these reasons, the prior art method of simply subtracting constant values of background from each measured constituent will not achieve measurement accuracy in PGNA analyzer applications operating under variable material flow conditions.
In prior applications of PGNA analyzers, considerable cost and effort has been applied in the industry to achieve near-constant flow rate and mass thickness. Such means have included the use of surge hoppers and constant flow feeders, variable speed conveyor belt drives, and vertical, plug-flow type PGNA analyzers, all so as to deliver a constant mass of bulk material (kg/m) or material cross-section to the measurement region.
In prior art, a PGNA analyzer is conventionally calibrated using a set of two or more unique and well-known mixtures of (1) high-purity base materials the amounts of which are carefully weighed prior to mixing, or (2) well-homogenized mixtures of representative unknowns that have been blended, sampled and analyzed for element or compound analysis by conventional laboratory means. In either case the chemistry of each standard is known quite accurately and allow the set of standards to be utilized as a gene
Atwell Thomas L.
Lanz Victor W.
Lucchin Anton M.
Analyser Systems AG
Gabor Otilia
Hannaher Constantine
Higgs, Fletcheer & Mack LLP
Reidelbach, Jr. Charles F.
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