Atomic-Emission Determination of Rhenium in Anthropogenic Raw Materials of Steel Production

The method including microwave acidic dissolution of samples and atomic-emission analysis of rhenium for determination of rhenium in cupper-molybdenum and lead-cupper wares is proposed. The range of determination concentration of metal is 0.05 – 0.5 mass percent, standard deviation of repeatability is 0.002 – 0.01 %.


Introduction
Rhenium is a rare and trace element, which does not form its own minerals and, consequently, its deposits are absent. Therefore, rhenium is obtained as the associated metal during the processing of molybdenum ores and some types of anthropogenic raw materials. Due to the low rhenium content in the raw material its deficit is observed, despite the fact that rhenium is required for the production of heat-resistant alloys in aircraft and rocket engineering, catalysts, thermocouples [1][2][3]. This situation dictates the need for extraction of rhenium from the semi product of metal production and recycling. This, in turn, requires development of methods for determination of rhenium in various kinds of anthropogenic raw materials.
Atomic emission spectrometry with inductively coupled plasma (ICP-AES) is the most promising method for the determination of rhenium in the raw materials among the modern methods of analysis. The first stage in the implementation of this method is the dissolution of the sample. Rheniumcontaining samples are recalcitrant. There are various methods of chemical sample preparation of rhenium-containing raw material samples described in the literature [4][5][6][7]. Sintering with oxides of calcium and magnesium, acid dissolution and alkaline fusion are used for the expansion of copper and molybdenum ores, minerals and concentrates. Matrix elements obtained after leaching the cake remain in the insoluble residue, and the elements passed into the solution do not affect the atomicemission determination of rhenium. During sintering with calcium oxide solution after leaching has a higher alkaline reaction of the medium than during the sintering of magnesium oxide. In addition, the analytical lines of rhenium and calcium overlap each other during atomic-emission determination. So, magnesium oxide is basically used as a reagent for the sintering.
Alkaline fusion with the addition of oxidants is the method that ensures complete transferring the rhenium into the solution. However, the salt content is often too high in the solution obtained, which complicates the subsequent determination of rhenium by ICP-AES.
Dissolution of the mineral acids and mixtures thereof is the most effective and the fastest method of expansion. However, the formation of volatile oxide of rhenium (VII) by heating often leads to an underestimation of the analytical results. The obtained solutions have complex salt background and influence on atomic-emission determination of rhenium.
Most of these methods of decomposition are laborious and time consuming. Therefore, different physical effects are used to intensify them: acoustic and magnetic fields, the temperature increase by carrying out the process in a closed system. Various _________________________ *corresponding author. Email: troeglasovaa@mail.ru methods of expansion the samples of different nature under microwave heating are described in the literature [8][9][10]. The aim of this work is to develop the methods for determining rhenium in the secondary anthropogenic raw materials, including expansion of samples in the analytical autoclaves with subsequent determination of rhenium by ICP-AES.

Experimental
Samples were pre-milled, dried at 105°C to constant weight in Thermo SCIENTIFIC (USA) convection oven. Semi-quantitative analysis of samples was carried out using ARL OPTIM'X Xray fluorescence spectrometer (Thermo Fisher Scientifics).
MKP-05 set of autoclave sample preparation with resistive heating, equipped with AP-3C analytical autoclaves [9], and MARS-5 microwave system (CEM, USA), equipped with HP-500 vessels for the decomposition of samples were used for the decomposition of samples of rheniumcontaining raw materials in a closed system.
Rhenium content in solutions obtained after expansion was determined using Jobin Yvon 38R atomic-emission plasma spectrometer (France). The most sensitive line of rhenium of 227.5 nm which is free from overlaps of accompanying elements was selected experimentally for analysis. Conditions of the analysis are presented in Table 1. Table 1 Operating Three intermediates of copper-molybdenum and lead-copper production were investigated in this work: waste of molybdenum production (S-1), intermediate of molybdenum production (S-2) and leaden cakes (S-3). The results of semi-quantitative analysis of the samples are presented in Table 2. Four measurements of the analytical line intensity were performed for each solution obtained after decomposition of the sample, and then the average value was calculated. The mass concentration of rhenium (µg/mL) was found by average value, using the calibration characteristic. The average of n-parallel determinations, each of which is made from a single sample was taken as the test result ( С ).
Completeness of transferring rhenium into the solution during autoclave expansion was controlled by comparing the results with the results of the analysis of rhenium after the decomposition of the samples in an open system (dissolving in nitric acid solution, alkali fusion with sodium peroxide, sintering with magnesium and calcium oxides). It was established that the expansion in the open system allows fully transferring rhenium into the solution, but does not ensure the complete dissolution of the sample ( Table 2). Additional alkaline fusion of the residue after dissolving the sample in acids was carried out for controlling the completeness of decomposition of the samples.
The exposure time in the autoclave was varied to select the autoclave expansion conditions. Weight of sample was 0.1-0.   Comparative data on the duration of decomposition and consumption of reagents per sample are shown in Table 4.  The technique of atomic-emission determination of rhenium in the intermediate of coppermolybdenum and lead-copper productions in the concentration range from 0.05 to 0.5% by mass was developed basing on these results. Assessment of the metrological parameters of methods was carried out in concordance with the requirements of ISO 5725:1994 [11]. Assessing the uncertainty in measurements was carried out according to guidelines of EURACHEM/CITAC [12]. Error and uncertainty of the analysis technique for various mass fraction of rhenium are presented in Table 5.

Table 5
Metrological characteristics of the technique of analysis of rhenium-containing samples (n -number of measurements, n = 20; С -the arithmetic mean of the results of analysis, % wt., s -standard deviation of repeatability, R s -standard deviation of reproducibility, P = 0.95, u с -the total standard uncertainty, U -expanded uncertainty) Thus, the decomposition of samples of intermediates of copper-molybdenum and leadcopper productions is obtained by dissolving in nitric acid (1:2) at 200°C for 30 min in the analytical autoclaves with microwave heating. Describing method is proposed as an alternative to the laborious and lengthy decomposition in the open system. This method allowed achieving complete decomposition of the samples, reducing the duration of sample preparation, eliminating losses of volatile components and reducing the correction of the blank experiment by reducing the amount of reagents used. The technique of atomicemission determination of rhenium in the intermediates of copper-molybdenum and leadcopper productions was developed on the basis of these studies. Standard deviation of the repeatability ( s ) of the results of analysis by developed technique is within 0.002-0.01% wt. at rhenium content in the range of 0.05% -0.5% wt.