Optimization of a sample preparation method for the determination of arsenic in ferrotungsten and tungstenconcentrates by ICP-AES
A.Belozerova,A.Mayorova, N.Pechishcheva, K.Shunyaev,
N.Nemytova
Institute ofMetallurgy, Ural Branch of the RussianAcademy of Sciences, Yekaterinburg, Russian Federation
Introduction
The quality of metal is largely determined by the presence and amount of undesirable impurities, such as lead, tin, arsenic, which even in small quantities cause cold cracking of the metal[1]. In some cases the impurities content should not exceed 0.001 %. Strict analytical control of the impurities at all stages of processing of raw materials to the final product obtaining is necessary. The ferrous metallurgy uses ferrotungsten, produced from the tungsten concentrate for alloyage of steel and alloys in order to increase the hardness, wear resistance and to improve mechanical properties at elevated temperatures [1].The normative documentation strictly limit the content of impurity elements such as phosphorus, sulfur, bismuth, tin, copper, lead, antimony, and arsenic in the tungsten concentrates andferrotungsten[2-3]. A special place amongharmful impurities takes arsenic, even a small amount of it in the raw material affects on the quality of metal products and the environment. Currently, determination of arsenic in tungsten concentrates in the Russian Federation is regulated by RF National Standard 11884.6-78 [4],in ferrotungsten–by RF National Standard 14638.15-84 [5]. For arsenic determination they recommend photometry (for the range of 0.08-0.5 % in [4], for 0.002-0.10 % in [5]) and titrimetry (for the 0.005-0.5 % in [4]). These proceduresare long and complex; require the use of large amounts of reagents.
Currently, inductively coupled plasma atomic emission spectrometry (ICP-AES) is widely used for chemical analysis of metallurgical raw materials and products [6-9], due to its rapidity, high reproducibility, wide range of determining concentrations and low detection limits.However, ICP-AES determination of arsenic in tungsten materialsis difficult due to the low concentration of this element, a small amount of available emission lines and its low sensitivity [10]. Commonly used methods of the tungsten material sample preparation(dissolving in acid mixture [11], in oxalic acid and hydrogen peroxide mixture [12], alkaline fusion [6, 13], etc.) are not suitable for the subsequent ICP-AES determination of arsenic. Matrix components - tungsten and iron –completely pass into solution and have a significant impact on the emission signal of arsenic [14], particularly due to overlapping of the spectral lines (Table 1).
Table 1. Overlapping lines arsenic and tungsten in the emission spectrum [14]
The spectral lines ofarsenic,nm / Interferingspectral lines oftungsten,nmAs I 289.871 / W I 289.825
As I 286.04 / W I 286.016
As I 278.02 / W I 278.028
As I 245.653 / W I 245.653
As I 237.077 / W I 237.088
As I 236.967 / W I 236.933
As I 234.98 / W II 234.982
As I 228.81 / W I 228.767
As I 193.759 / W I 193.680
As I 193.696 / W I 193.680
As I 189.042 / W I 189.181
For ICP-AES determination of arsenic in tungsten materials it is necessary to separate arsenic from main part of the matrix, there are a number of effective methods [10]. The choice of method depends on the chemical composition of the analyzed material and on the content of arsenic.
For example,in [16] for sample preparationof arsenicoresinteringmaterialsusingsulphidingreagents have been proposed.As thefluxalkaline earth metal oxides(MgO, CaO),andalkali metal carbonates(Na2CO3, K2CO3)are used.The separationof arsenicfrom the matrixoccurs at the stageof water leaching. Largepart of the matrixremains in the formof insolublesulphides;arsenicformssolublethiosalts.
In determining of trace level of the impuritiesinterfering tungsten matrix can be precipitated. Barium acetate is one of the most commercially available and inexpensive precipitants of tungsten [11].
For the separation of small amounts of arsenic from the matrix its co-precipitation, for example, with ferric hydroxide is widely used [10]. For example, for [15] ICP-AES determination of arsenic in the technogenic raw materials (copper anodes, copper production waste) sorption group concentrating on iron, lanthanum and magnesium hydroxides have beenproposed. In the process of sample preparation matrix is separating,impurities (As, Se, Te) are concentrating. However, it is known [10] that a number of other elements - tin, tungsten, vanadium - fully or partially coprecipitated with ferric hydroxide as well as arsenic. To study the degree of separation of matrix components experimental testing of the above methods of sample preparation of tungsten-containing materials is required.
The aim of the present study was selection of the optimal method for sample preparation of tungsten-containing materials for further ICP-AES determination of arsenic.
Experimental
The selection ofthe sample preparation method
Standard reference materials of ferrotungsten and mixtures of two standard reference materials of ferrotungsten were tested following the procedure.
Procedure 1.Sintering.(0.5 ± 0.001) gof the ferrotungsten sample was weighed into the porcelain crucible with a flux: Na2CO3: K2CO3: S (1.8 g: 2.2 g: 4 g) and was mixed thoroughly.The crucible was placed in a muffle furnace at 500 °C for 5 min. Then the crucible with flux was cooled and placed in a heat-resistant glass beaker of 250 cm3, 40 cm3 of distilled water was added and heated. The precipitate was filtered through "blue ribbon" filter, then washed with hot distilled water and discarded. The filtrate was transferred to a 200 cm3 flask, diluted to the mark with water and stirred.
Procedure 2. Sintering is followed by the deposition of tungsten.According to procedure 1 ferrotungsten sample was sintered. Then, after leaching toflux 60 cm3 of hot freshly prepared 10 wt. % barium acetate solution was added portionwise. The resulting precipitate of barium tungstate (BaWO4) was filtered using"blue ribbon" filter, washed with hot distilled water and discarded. The filtrate was transferred to a 200 cm3 flask, diluted to the mark with water and stirred.
Procedure 3.Preconcentration on ferric hydroxide. (0.5±0.001)g of ferrotungsten sample was placed in Teflon beaker, a mixture of hydrochloric acid, nitric acid, hydrofluoric acid (15:15:5 cm3) was added and heated on an electric stove until complete decomposition of the material. 0.2 g of iron nitrate (III) was added to the solution and heated to dissolve. Small portions of ammonium hydroxide were added to form ferric hydroxide. The resulting precipitate was filtered using"blue ribbon" filter and washed with an aqueous solution of ammonium hydroxide (1:20). The filtrate was discarded. The precipitate was dissolved in hydrochloric acid solution (1:1), transferred into a 200 cm3 volumetric flask, dilutedwith water to the mark and stirred.
ISP-AES spectrometer and operating conditions
Emission measurement of solutions after sample preparation was carried out using ICP-AES spectrometer «Optima 2100» (Perkin Elmer) with argonplasma as emission source. Operating parameters: Rf power - 1300 W; carrier gaz flow rate– 0.8 dm3/min; auxiliary argon flow rate- 0.2 dm3/min; plasmagaz flow – 15.0 dm3/min; method of plasma observation- axial (in the case of arsenic), radial (in the case of tungsten); nebulizer flow rate– 1.5 cm3/min; spraying time– 30 c, number of replicas – 2, analytical spectral lines: W II 207.912 nm, As I 189.042 nm, As I 193.759 nm.
Results and discussion
Procedures1-3was carriedout with the samples of certified reference materials of ferrotungsten. Arsenicand tungsten content weredetermined byICP-AES in the resulting solutions.The results are shownin Tables 2and 3. It is shownthat significant amountof tungsten, 350mg/cm3, remainsinthe sample solutionobtainedbyProcedure1.Spectral linesW I 189.181nm and WI 193.680 nm overlap theanalytical linesof arsenicand the determinationof arsenicis impossible.In the samples preparedbyProcedure2 andProcedure3the tungsten contentswere less significant, 150 mg/cm3 and100mg/cm3respectively (Table 2).
Table2. The results ofICP-AESdetermination oftungstenin the solutionsampleaftersample preparation
Sample / Certifiedtungsten content, wt.% (mg/dm3) / The tungsten contentin the sample solution, mg/dm3
Procedure 1 / Procedure 2 / Procedure 3
RF CRM 765-92P / 74.7±0.2
(1850 mg/dm3 after complete dissolution of 0.5 g in 200 cm3) / 350 / 150 / 100
Table 3. The results ofICP-AESdetermination ofarsenicin the solutionsampleaftersample preparation(mean results obtaining using twolines: As I 189.042nm and As I 193.759 nm)
Sample / Certified, wt.% / Found, wt.%Procedure 2 / Procedure 3
RF CRM
765-92P
(500mg) / 0.028±0.001 / 0.0323 / 0.0252
0.0271 / 0.0269
0.031 / 0.0260
mean / 0.0305 / 0.0260
Mix of RF CRM765-92P andRF CRM2853-84
(250 mg:250 mg) / 0.014 / 0.0251 / 0.0139
0.0273 / 0.0143
0.0262 / 0.0146
mean / 0.0262 / 0.0143
The Tables3showsthat the Procedure 2is not suitable forICP-AESdetermination of arseniccontent less than0.02wt.% due tothe effect ofthe overlapping ofthe above-describedtungsten lineson thearsenic lines.The tungsten contentin the sample solutionafterProcedure 3 have notdisturbinginfluence on thedetermination ofarsenicat a concentration below of0,01wt%.
Conclusion
From the threetested procedures to further developof the techniquesof ICP-AESarsenic determination inthe tungstenmaterialsthe most promisingis that including dissolutionin the acid mixture followed by co-precipitation ofarsenic on ferric hydroxide.
Acknowledgements
This work was financially supported by young scientists and graduate students of Ural Branch of the Russian Academy of Sciences project № 14-3-NP-4 and in the frame of the IMET UB RAS theme “Developing of the analyze techniques of the oxide row materials, wastes and products of its treatment”.
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