Gamma-activation analysis (GAA) has the great potential to substitute traditional fire assay analysis in gold mining in the future. The essence of this method is the irradiation of ore samples with gamma quanta of high energies generated by a linear accelerator, and the detection of the excited gold nuclei induced activity by gamma spectrometer.
The first three industrial GAA laboratories were commissioned in Muruntau, Uzbekistan in 1977, in Magadan, Russia in 1979 and in Batagay, Russia in 1986. Of the three commissioned laboratories, only the Muruntau mine laboratory (Uzbekistan) is currently operating. In this laboratory, the gamma-activation method for the determination of gold is actively used due to high speed and low cost of the analysis. In 2020, about 1,700,000 samples were measured on two measurement lines with two linear accelerators. During 2020-21, Baltic Scientific Instruments (Latvia) and Corad (Russia) developed, manufactured and tested a new GAA Au-Isomer setup for Pavlik Gold Ore Company JSC (Russia).
The main technical characteristics of Au-Isomer are listed in the table below:
|Main technical characteristics||Value|
|Gold detection limit||0,04 ppm|
|Measurement accuracy for ore with Au content 1 ppm||8%|
|Performance for samples with gold content ≥1 ppm||70 samples /h|
|Sample volume||300 ml|
|Sample particle size should be not more than||3 mm|
|The energy of electrons at linear accelerator output||7–9 MeV|
|Maximum power of the electron beam||10 kW|
|Radiation background on surface of safety barrier||≤1 μSv /h|
|Facility height with a crane for radiation shield assembly||4.1 m|
|Facility area||(4×7) m|
|Facility weight with shielding||118 t|
|Maximum power consumption||70 kW|
Radiation shielding of the Au-Isomer consists of iron blocks weighing maximum 2.4 tons. A crane, which is part of the installation, is used to assemble the shielding. To service the accelerator, 5 rear blocks are removed and the accelerator is rolled out of the radiation shielding. This procedure takes no more than 20 minutes. The installation is therefore mobile, since it can be disassembled and moved to another place in several 40-foot containers.
This GAA facility works in a fully automatic mode. Containers with 300 ml ore samples are weighed and labeled with a barcode, and then placed on the receiving conveyor of the sample transfer system. At the end of this transport, before the measurement, a barcode is read and then the containers are rolled down the feed channel to the target of the electron accelerator. During irradiation, the containers rotate to ensure uniform distribution of the dose throughout the container volume. After irradiation, the containers are rolled down to the spectrometers to measure the induced activity. After measurement, the containers are lifted with a lifting mechanism for unloading onto the container return conveyor, or loading into a feed channel for re-irradiation. The decision to re-irradiate is carried out by the control program at a low signal level in the energy range of radiation of the gold isomer. The containers measured a second time will alternate in the transport channels with containers measured the first time. The total analysis time is approximately 47 seconds, which provides maximum productivity with a single irradiation/sample reading cycle of up to 76 analyzes per hour.
The Au-Isomer GAA facility demonstrated MDA values for gold between 0.026-0.028 g/t with single irradiation for 10 s and a 15 s spectrum acquisition time. For samples with a gold content of less than 0.5 g/t, averaging was carried out over 2-4 measurements. The facility automatically sends samples for repeated measurements in case of insufficient statistics of the results of one measurement and averages the results of several measurements.
The Au-Isomer GAA facility has been certified by the State Test and Measurement Instrument Certification Center of the D.I. Mendeleev All-Russian Institute for Metrology (VNIIM), with a certificate No 209 / 06-2021 of 15.10.2021. The facility is recognized as suitable for use in testing samples of rocks, ores and products of their processing for gold content by the method of gamma activation analysis using a UELR-8-10A linear electron accelerator. A measuring technique “Determination of gold in rocks, ores and products of their analysis by the method of gamma activation using a UELR-8-10A linear electron accelerator” for the facility has passed the metrological certification, as well (certificate No 2121/209- (RA.RURU10494) -2021, dated 17.12.2021).