Simultaneous Determination of Au and Associated Elements Ag, Cu, Pb, Zn, As and Sb in Gold Ore by ICP-MS with Hypochloric Acid Oxidation

Significance: Gold mines are strategic mineral resources with financial value，which accompanied a amount of Au，Ag，Cu，Pb，Zn，As and Sb. Among them，As and Sb are closely associated with Au and have a certain indicative effect on gold exploration. The content of Ag，Cu，Zn and As directly affects the economic value of gold mines. Therefore，rapid and accurate determination of them is great significance for improving the efficiency of gold exploration^［1-3］. Luo et al^［30］ used potassium permanganate potassium chlorate wet oxidation oxidize carbonaceous matters and sulfides instead of roasting，combined with activated carbon enrichment，and determined Au in ores by AAS，this method can effectively remove carbonaceous matters and sulfides interference，but the oxidation system increases the salt concentration of the solution and is not suitable for determination by ICP-MS. For the direct determination of Au in ore，Ge et al^［31］ used roasting aqua regia dissolution and determine high-grade Au in gold ore by AAS; Rodriguez et al^［32］ used neutron activation analysis to determine Au，Ag，Cu and Zn in two ore samples from Mexico. The existing methods require independent detection of Au and associated elements such as As and Sb throughout the entire process，which makes it extremely cumbersome to simultaneously determine Au，Ag，Cu，Pb，Zn，As and Sb in gold ore.

　　This article used hypochlorous acid instead of potassium permanganate potassium chlorate wet oxidation system，which reduction products do not introduce high salt content such as manganese and potassium. Oxidation with hypochlorous acid avoids the loss of arsenic and antimony caused by sample roasting，effectively eliminating the influence of oxidize carbonaceous matters and sulfides on gold pretreatment without increasing salt content. Combined with offline internal standard method，the error caused by liquid separation and instrument drift is reduced. A method for simultaneous determination of Au，Ag，Cu，Pb，Zn，As and Sb in gold ore by ICP-MS was established，which solved the problem of inability to simultaneously determine gold and associated elements such as arsenic and antimony.

Methods:

Experimental Samples: Select Chinese standard substances (GBW07858，GBW07859，developed by Heilongjiang Institute of Geological and Mineral Testing and Application) and actual samples (No.S1 S2，S3，S4，S5，Particle size ≤0.074µm，provided by Shaanxi Fengxian Sifang Gold Mine Co.，Ltd.) as experimental samples for experimental condition optimization，accuracy and precision verification.

Instruments and Reagents: Select 7700 Series Inductively Coupled Plasma Mass Spectrometer (Agilent，USA) used for the determination of Au，Ag，Cu，Pb，Zn，As and Sb，the main instrument parameters are shown in Table 1. Select nitric acid (1.42g/mL)，hydrochloric acid (1.18g/mL)，and hypochlorous acid (1.00mg/mL) for sample digestion (analytical grade，purchased from Chengdu Cologne Chemical Co.，Ltd).

Experimental Methods: Method steps: (1) Accurately weigh 10.0000g of the sample into a 300mL polytetrafluoroethylene beaker，wet it with water，and add 20mL of nitric acid，5mL of hypochlorous acid，3.0g of ammonium hydrogen fluoride，1.00mL of rhenium internal standard solution with a mass concentration of 10μg/mL，and 10.00mL of rhodium internal standard solution with a mass concentration of 10μg/mL in sequence. Cover the beaker with a surface dish，heat it on an electric heating plate，and maintain a slight boiling state for 10min. (2) Observe if there is a thin layer of black carbon film floating on the surface of the solution. If not，add 1.00mL of hypochlorous acid and heat to near dryness; If available，add 1.00mL of hypochlorous acid and heat for 5min until the black carbon film disappears. Then add 1.00mL of hypochlorous acid and heat until almost dry. (3) Dissolve in 80mL of 50% aqua regia solution，gently boil for 50min，then remove from the beaker. Rinse the measuring dish and cup wall with warm water，cool down，and transfer to a 200mL volumetric flask. (4) Take 500μL of sample solution with a pipette and dilute it 50 times with 2% aqua regia solution (sample dilution factor is 1000) and measure by ICP-MS. The formula for calculating the dilution factor of the sample in this article is: sample dilution factor=liquid-solid ratio of the sample solution (g/mL)×dilution factor of the sample solution.

Quality Control: The detection limit of the method was determined by measuring 11 blank samples，calculating the standard deviation of each element sample blank，and using 3 times the standard deviation to calculate the detection limit of each element. The accuracy evaluation of the measurement results is carried out by measuring Chinese national standard substances and conducting method comparison verification，respectively calculating the relative error and relative deviation to evaluate the accuracy of the results; The precision evaluation of the measurement results was conducted by independently measuring 7 actual samples，calculating the relative standard deviation，and evaluating the stability of the results.

Data and Results: The effects of hypochlorous acid dosage，digestion time，internal standard，and interfering elements on the analysis results were studied. The results showed that added 20mL of nitric acid，5mL of hypochlorous acid，and 3.0g of ammonium hydrogen fluoride to 10.0000g of the sample，covered it with a surface dish，boiled and dissolved it near dryness on an electric heating plate，which can completely oxidize carbonaceous matters. Added 80mL of 50% aqua regia for 50min，dissolved to 200mL，separated 50 times，and calibrate offline internal standards with 100ng/mL rhodium and 10ng/mL rhenium. The detection limits for Au，Ag，Cu，Pb，Zn，As and Sb were 0.03，0.05，0.19，0.26，0.22，0.27，and 0.05μg/g，respectively，with the determination ranges were 0.09-20，0.19-1000，0.74-2000，1.04-2000，0.86-2000，1.07-2000，and 0.17-2000μg/g. This method was applied to conduct 7 parallel determinations of the Chinese national standard substances (GBW07858，GBW07859)，the results were consistent with the recommended values (see Table 6)，meanwhile，analyzed five different types of gold ores，and the results were consistent with the petrochemical analysis of gold mines (GB/T 20899—2019)，with the relative deviations of each element were ≤5.32%.

　　2HClO→2HCl＋O₂ (1)

　　This method used hypochlorous acid to oxidize carbonaceous matters and sulfides. Compared with the potassium permanganate potassium chlorate wet oxidation system，the new ecological oxygen produced by the decomposition of hypochlorous acid has extremely strong oxidizing properties，the decomposition equation of hypochlorous acid is shown in equation (1)，which avoided thigh salt content such as manganese and potassium. Compared with the previous method of directly determining Au，it realized the simultaneous determination of Au and associated elements such as Ag，Cu，Pb，Zn，As and Sb. The operation process is time-saving，cost-effective，and labor-saving. The drawback is that the upper limit for the determination of Cu，Pb，Zn，As and Sb is not high enough. When their content is higher than 2000µg/g，the dilution factor needs to be increased or determine the sample solution by ICP-OES. When the dilution factor is too large，it may cause an increase in the measurement result error，which is relatively reliable for determining by ICP-OES.