Patent Number: 060027347
Section: summary

FIELD OF THE INVENTION The invention relates generally to assaying ore samples for valuable components (assay elements), and more specifically to a technique of assaying large ore samples for gold, silver, and barium content using photon activation analysis. BACKGROUND OF THE INVENTION Classical fire assaying is and has remained the standard assay for gold and silver in precious metal mining. In this technique, a small rock sample, typically 30-150 grams (gm) is mixed with litharge (lead oxide) and silica fluxes and fused in a high temperature furnace. A resultant lead button containing solubilized gold and silver is poured into a conical mold. This lead button is subsequently cupeled in a bone ash or other cupel whereby the molten lead is converted to lead oxide which is absorbed by the substance of the cupel. When the cupeling is completed, a small dore bead of gold and silver remains. This bead is parted in nitric acid to remove the silver after recording the initial weight. The parted bead is weighed again giving the gold weight directly. The silver weight is found by difference of the dore bead weight and the gold bead weight. Alternatively, the dore bead can be dissolved in aqua regia and the resultant solution analyzed on an atomic absorption (A.A.) instrument. The original sample may also be digested in aqua regia to solubilize the gold and silver, and the resultant solution analyzed on an A.A. instrument. In this technique a small sample, typically 10 to 15 grams (gm), is digested in the aqua regia. Some difficulty is encountered with samples that contain very low quantities of gold and silver. Several samples may be fused and the resultant beads added together to obtain greater sensitivity, but this required more time and extra cost. In addition, very great care is required in temperature control and timing to obtain accurate silver assays. While fire assaying is a very sensitive assay technique for gold particularly, which is why it has been used for such a long time, its sensitivity is limited by the size of bead that can be detected and weighed or solubilized in an A.A. finish. There are some organic solvents, such as M.T.B.K., which can extract gold from an aqueous solution and thus concentrate the gold for greater sensitivity on an A.A. instrument. Additionally, all of these techniques suffer from allowing only small ore samples to be analyzed. In order for the sample data to be meaningful, the small sample must be representative of the larger and typically non homogeneous rock sample from which it is obtained. To date, the only technique which has allowed this representativeness has been a meticulous sample preparation wherein a large rock sample is crushed into successively smaller sizes before being split into a smaller sample. The final sample is pulverized typically to less than 150 mesh prior to be digested or fused for assay. This meticulous sample preparation is not only expensive, but it can be very difficult to achieve with native gold metallics present. Gold is usually not homogeneously distributed in a sample but occurs most commonly as metallics frequently alloyed with silver. While silver can occur as a native metallic, it is more frequently present with gold as electrum or combined with sulfur as a silver sulfide. The problems are aggravated when the ore deposits are of low grade. The current price of gold and the development of process and mining technology has allowed the development of large low grade deposits. These deposits typically require much drilling and blasting to break the ore for processing. Large volumes of samples are prepared from these blast holes for assaying to control the mining. The fire assay technique has remained the standard, and low throughput, low sensitivity for low gold content, and the requirement for meticulous sample preparation have been accepted as inevitable. In view of the above discussion, and object of the present invention is to provide a system which can assay relatively large samples of ore for gold and other assay elements of interest. An additional object of the invention is to provide an assay system for certain elements in various environments which requires minimal sample preparation. Yet another object of the invention is to provide an assay system with throughput which is greater than the prior art fire assay method. Another object of the invention is to provide an assay system with sensitivity sufficient to meet requirements for commercial ore testing and production standards. Still another object of the invention is to provide an assay system whose accuracy and precision is not adversely affected by typically non homogeneous ore samples and especially relatively large, non homogeneous samples. Another object of the invention is to provide an assay system that is relatively insensitive to sample geometry. Yet another object of the invention is to provide a non destructive assay or analysis system that can be used with a wide variety of sample types to determine the elemental concentration of any element susceptible to detection by means of photon activation. There are other objects and applications of the invention which will become apparent in the following disclosure. SUMMARY OF THE INVENTION The present invention provides a high throughput technique that permits large sampled of materials to be analyzed for their gold, silver, and other assay element contents, thereby avoiding the time and cost necessary for a meticulous sample preparation. The technique is effective even when the distribution of metallics in the sample is not homogeneous. The present invention departs from the prior art techniques in that it recognizes that certain nuclear physics techniques can yield assay information. Specifically, the invention exploits the properties of certain elements having isotopes with one or more excited nuclear states that are characterized by relatively long half lives (microseconds to minutes). Nuclei in these excited states are referred to as isomers, and cannot be produced directly from the ground state. Rather they must be produced by exciting the nucleus to a higher excited state which quickly decays to the long-lived isomeric state. The isomers decay to the ground state through the emission of a gamma ray having a well defined energy for the particular element. Typical gamma ray energies for isomeric transitions are in the range of 0.05-1.0 million electron volts (MeV) or 50-1000 thousand electron volts (KeV). The technique of the invention entails irradiating the sample with a beam or flux of gamma rays of sufficient energy to excite the nuclei of the assay elements into their isomeric states, ceasing the irradiation, detecting and identifying the gamma rays resulting from the decay of the isomeric states to the ground state, and the analyzing the detected gamma rays to determine the content of assay elements in the sample. In a preferred embodiment, the irradiated sample is rapidly moved to a shielded low background environment in which the gamma rays from the isomeric transition are detected. As mentioned previously, the ore samples are typically non homogeneous. As an example, gold ore can contain gold in highly concentrated "nuggets" which are rather sparsely and non homogeneously distributed through a large volume of non gold bearing material. The average gold concentration of such an ore may, however, be well above the commercial threshold. Any meaningful assay system must be able to accurately obtain results for non homogeneous ore. To address this problem, the present invention is embodied so that portions or "segments" of the ore sample are sequentially irradiated and counted. During each irradiation, the sample is oscillated within the gamma ray flux in order to obtain uniform exposure of each portion. After all segments of the sample have been irradiated and counted, the count results are combined in order to obtain a highly representative assay of the entire sample volume. The technique has a similar sensitivity to gold as fire assay, 0.001 troy ounce per short ton, but can quickly handle large sample weights (typically 10 kg) to give better average assay numbers. The technique makes it possible to process large volumes in a reasonable time (more than 600 samples within 24 hours). While the nuclear physics phenomena exploited by the invention are known, the present discussion of the physics underlying the invention technique is not intended as an admission that the nuclear physics phenomena were recognized in the prior art as having any applicability to a technique for assaying large ore samples, or having applicability to any other stated objects of the invention. The system can also be configured to analyze many types of samples non destructively for any element which is subject to photon activation analysis.