BACKGROUND With the wide application of iridium and rhodium in aerospace, electronics, energy and other fields, it has become a very attractive metal in the world. In China, iridium and rhodium ore resources are relatively scarce with low grade, so it is essential to recover iridium and rhodium from secondary mineral resources. Copper anode slime enriches most of the precious metals such as iridium and rhodium in ores, which has high comprehensive recovery value. At present, there is no detection standard for iridium and rhodium in copper anode slime. The migration behavior of iridium and rhodium was not clear, so it was difficult to achieve directional enrichment and efficient extraction of iridium and rhodium metals. Therefore, the development of detection methods for iridium and rhodium in copper anode slime is an important prerequisite for the recovery and extraction of iridium and rhodium.

OBJECTIVES An analytical method for accurate determination of iridium and rhodium in copper anode slime was established to maximize the utilization of mineral resources and the recycling rate of iridium, rhodium and other precious metals. At the same time, it could provide data support for the purification of iridium and rhodium in copper anode slime.

METHODS In this paper, a method for determination of iridium and rhodium in copper anode slime by inductively coupled plasma-mass spectrometry (ICP-MS) with nickel sulphide fire assay was established. In the experiment, the precious metals iridium and rhodium in the sample were captured by nickel sulphide fire assay. The NiS beads were dissolved with 50% hydrochloric acid so that the precipitation of rhodium and iridium was separated from silver and other impurity elements through filtration when it was hot. The precipitates containing iridium and rhodium were effectively separated from silver and other impurity elements. The precipitate of iridium and rhodium with filter film were transferred into a closed digestion tank and dissolved in 50% aqua regia. The contents of iridium and rhodium in the solution were directly determined by ICP-MS.

RESULTS The conditions such as the ingredient of nickel sulphide fire assay, the concentration of hydrochloric acid, tellurium coprecipitation, the sealing digestion time and temperature were studied. The experimental results showed that the molten slag was acidic when the ratio of nickel to sulfur was 4∶1, and it could effectively capture the iridium and rhodium in the sample with good fluidity of molten slag and the separation effect of slag buckle. When the NiS beads were dissolved by 50% hydrochloric acid, the dissolution reaction of NiS beads was suitable and complete. The precipitation containing rhodium and iridium was separated from impurity elements and filtered when hot. The precipitation was sealed and digested by dilute aqua regia (1∶1) at 160℃ for 2-3h. The possible MS interference was eliminated by selecting a suitable determination isotope. The ¹⁸⁵Re was selected as the internal standard of ¹⁰³Rh and ²⁰³Tl as the internal standard of ¹⁹³Ir to eliminate the effect of signal drift, the results of iridium and rhodium had high precision and accuracy. The standard solution series of iridium and rhodium were determined under the optimized experimental conditions. The results indicated that the mass concentration of iridium and rhodium in the range of 10-100μg/L were linear to the ratio of the intensity of iridium and rhodium to the internal standard mass spectrometry. The calibration curves of iridium and rhodium were y=36674.6x+8264.7 and y=45686.7x+288.6, respectively, and the linear correlation coefficient (r) of calibration curves of iridium and rhodium were more than 0.999. The detection limits for iridium and rhodium were 0.007μg/L and 0.011μg/L, respectively, and the lower limits of detection were 0.024μg/L and 0.038μg/L, respectively. The content results of rhodium and iridium in 8 actual samples with the method showed that, the relative standard deviation (RSD, n=7) was between 1.40% and 4.57%, and the recovery was in the range of 95.00% to 103.65%.

CONCLUSIONS The method has high efficiency and accuracy and can meet the detection requirements of copper anode slime samples.