1. Introduction The London Bullion Market Association (LBMA) requires that the allowable error for the assay analysis of refined gold is ±0.005% when the test result is greater than or equal to 99.95%, and ±0.015% when the test result is less than 99.50%-99.95%. Obviously, the current national standard method GB/T 11066.1-89 that we used before cannot meet the requirements of the LBMA. In the past composite gold detection, there were significant defects in equipment and methods: the experimental environment conditions were poor; the accuracy of the detection equipment did not meet the requirements; and the application of the method was not in place. Specifically, the results of the same sample in the same batch varied greatly; the calibration values of the standard samples fluctuated greatly and were unstable; the precision was poor; and the accuracy was also poor. In order to improve the level of gold assay analysis in our laboratory and meet the requirements of the LBMA, we have made great improvements in the gold assay analysis methods and related instruments and facilities.

　　2. Experimental Section

　　2.1 Method Principle The sample is added with an appropriate amount of high-purity silver and a small amount of copper, wrapped in lead foil, and cupelled in a furnace at 950℃ to separate the base metals from the precious metals by absorption in the cupel. The gold-silver alloy is annealed and rolled into a sheet, parted with nitric acid, and the gold is sintered at 800℃. After weighing with an ultra-microbalance, the gold content is calculated, and the gold content is corrected with a gold standard sample tested simultaneously.

　　2.2 Instruments and Equipment

　　2.2.1 Furnace: Maximum heating temperature 1600℃, temperature control accuracy ±3℃ at 1000℃;

　　2.2.2 Ultra-microbalance: Sensitivity 2μg;

　　2.2.3 Rolling machine: Can roll to a thickness of 0.1 mm;

　　2.2.4 Parting basket: Made of 0.5mm～1.0mm stainless steel sheet.

　　2.3 Reagents and Materials

　　Lead foil: Pure lead >99.9%, thickness 0.1mm, and gold-free;

　　Pure silver: >99.99%, and gold-free;

　　Nitric acid (1+1), analytical grade;

　　Nitric acid (2+1), analytical grade;

　　Gold standard sample: Content ≥99.999%;

　　Cupel: Magnesia cupel (main component MgO), size ø×h(mm): 40×35, concave depth 16mm.

　　2.4 Method Steps

　　2.4.1 Weighing:

　　2.4.1.1 After weighing out 0.400 or 0.402 grams of standard gold and sample gold, weigh accurately to 0.001 mg. Standard gold and sample gold must be weighed alternately to prevent weighing deviations caused by air currents and any systematic deviations of the balance itself.

　　2.4.1.2 1.00-1.02 grams of gold-free silver (purity above 9999, sintered beforehand), 7 mg of high-purity gold-free copper, and sample gold are wrapped together in gold-free lead foil. The wrapping method for standard gold is the same.

　　2.4.1.3 The size and weight of each piece of lead foil used should be the same. Before cupellation, the original conical lead foil packaging must be changed to a circular packaging and pressed tightly.

　　2.4.2 Cupellation: Preheat the cupel at 950℃ for 20 minutes. Quickly put the wrapped lead ball (circular) into the cupel. Close the furnace door and slightly open the furnace door after the lead skin is removed. The cupellation temperature is 950℃. After the cupellation is complete, close the furnace door and maintain it at 950℃ for 2 minutes. Then turn off the power and take out the solidified gold-silver alloy when the furnace temperature drops below 750℃.

　　2.4.3 Rolling: Tap one side of the alloy to remove the ash adhering to the cupel and make the flat button-shaped alloy square. The alloy is annealed at 850℃, then rolled into a sheet about 100 mm long, 8 mm wide, and 0.2 mm thick, annealed at 750℃, allowed to cool, and rolled into a crown shape.

　　2.4.4 Parting: Put each rolled gold piece in order into the parting basket. Immerse in preheated hot nitric acid (1+1) at 80℃ for 30 minutes, then take it out and wash it three times with hot water. Then put the parting basket into preheated nitric acid (2+1) at a slight boil for 30 minutes. In the above parting operation, rotate the dish three times every 30 minutes to ensure even heating. Finally, take out the parting basket and wash it ten times with hot water.

　　2.4.5 Drying and Sintering: Dry the gold pieces together with the parting basket on an electric stove, then gently clamp each gold piece into a clean crucible with pointed tweezers, and sinter in a muffle furnace at 800℃.

　　2.4.6 Calculation and Expression of Analysis Results

　　Where: m1 Measured mass of sample gold piece, mg; m2 Mass of sample, mg;

　　m3 Measured mass of standard gold piece, mg; m4 Weighed mass of standard sample, mg

　　D Percentage content of standard gold, %

　　Note: The same balance should be used for weighing gold pieces and samples, and the environmental conditions should remain consistent.

　　3. Results and Discussion

　　3.1 Results

　　The experimental personnel conducted research and exploration on each step of the method, and after a large number of conditional experiments, satisfactory results were obtained. The determination results of the LBMA standard samples tested in our laboratory are summarized in Table 1:

　　Table 1 Analysis Results Comparison and Precision

　　Sample No. Test Result/% Average Value/% Range/% SD/% Standard Value/%

　　A 99.978 99.980 0.004 0.0016 99.980

　　A 99.980

　　A 99.980

　　A 99.982

　　B 99.838 99.842 0.007 0.0033 99.850

　　B 99.840

　　B 99.844

　　B 99.845

　　C 99.502 99.505 0.006 0.0032 99.506

　　C 99.507

　　CC 99.50299.508

　　3.2 Discussion

　　3.2.1 Precision and Accuracy of Analysis Results

　　Fire assay method has high requirements for the operation technology of the inspectors. There are many uncertain factors in the operation process, and the influence of some factors cannot be predicted. If the operation is not up to standard, it will not be possible to accurately determine. The data in the table above shows that the difference between the average value of the results of the three samples and the standard value is within ±0.005% or ±0.015%, and the precision is less than 0.0050%, indicating that the improved method and the analysis level of our laboratory have reached the requirements of LBMA for assaying analysis capabilities.

　　3.2.2 Improvements in Equipment and Environment

　　Before the start of this work, the instruments and equipment and environmental conditions required for fire assay analysis were re-equipped and renovated. This is a prerequisite for doing fire assay analysis.

　　3.2.1.1 Balance Room: Use a one-millionth balance; rebuild the balance platform to ensure the stability of the sample weighing balance; control the environment of the balance room (temperature 25℃, humidity 60%).

　　3.2.1.2 Fire Assay Furnace: Use a fire assay furnace with higher temperature control accuracy (temperature control accuracy is ±3℃ at 1000℃).

　　3.2.3 Method Improvement

　　According to successful foreign experience, the most important steps in the fire assay analysis operation are cupellation and parting. The residual silver content of the standard gold should be controlled between 0.3 and 0.6 mg. Too large or too small indicates that the operation is not stable enough.

　　3.2.3.1 Weighing: Weigh the samples in the order shown in the crucible placement diagram.

　　The crucible placement diagram is as follows:

　　A B P C B

　　P C A P A

　　B P C B P

　　G A P C G

　　P-Gold standard sample, A, B, C-Samples, G-Control sample

　　3.2.3.2 Cupellation: Determine the placement of the standard sample, test sample, and control sample in the fire assay furnace based on the temperature characteristics of the fire assay furnace.

　　3.2.3.3 Rolling: The speed of rolling and thinning should not be too fast, and the width and thickness of the thin sheet should be strictly controlled.

　　3.2.3.4 Parting: Use an adjustable electric furnace with adjustable heating temperature to control the parting temperature; strictly control the density of the reagents used for parting (1+1)HNO3, (2+1)HNO3.

　　4 Conclusion

　　The national standard analysis method for refined gold commonly used in China's gold industry, GB/T 11066.1-89, has a certain gap with advanced similar foreign products. Based on existing conditions, and referring to advanced foreign fire assay methods learned at the 2005 LBMA Precious Metals Fire Assay Seminar and the good experience of several domestic laboratories that do fire assay analysis, the fire assay method for refined gold established through a large number of conditional tests is feasible and has reached or approached the international advanced level.

　　References

　　[1] GB 11066.1-89, Chemical Analysis Method for Gold Fire Assay Method for Determining Gold Content.

　　[2] GB/T 15429.1-94, Chemical Analysis Method for Composite Gold Fire Assay Gravimetric Method for Determining Gold Content.

　　[3] ASTM B562-95, Refined Gold Standard Appendix Fire Assay Method for Analyzing the Chemical Composition of Refined Gold.

　　[4] The LBMA Assaying Seminar 21st&22nd June 2005 The Armourers′Hall，London。

　　[5] Corrected Fire Assay Method for High Gold Alloy. A presentation by Chihiro Nakanishi, Chief engineer, Technical Department, Tanaka Kikinzoku KogyoK.K(TKK) at LBMA Assaying Seminar in London on Wednesday 22 June 2005.

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