Calcium Content Determination in Tungsten Products

Calcium is one of the harmful elements in tungsten products and must be strictly controlled. The calcium content of tungsten products is generally very low, which is 0.000x% grade. The atomic absorption spectrometry (plane grating spectrography) is used to determine the tungsten content, and the upper limit is 0.020%.

However, the determination of calcium in tungsten related samples or tungsten products, such as the determination of calcium in tungsten ores, determination of calcium in the process of removing calcium from APT (ammonium tungstate) with Scheelite (calcium tungstate) as raw material, determination of calcium in some tungsten products such as tungsten powder, tungsten carbide, tungsten carbide, tungsten strip and tungsten wire in the process of calcium removal in the process of calcium paretrite removal. The content of calcium in some samples is often higher than that of 0.0x - 0.x%, which has exceeded the determination range of atomic emission spectrometry (plane grating perturbation method), and the measurement error is very large by the method of spectrum.

For such samples, the ICP method (plasma direct reading spectrometry) or the classical chemical method—EDTA (disodium EDTA) volume method is generally used. The former is expensive because most companies adopt the latter. The principle is the quantitative determination of calcium in the sample was performed based on quantitative complexation of divalent calcium ions under alkaline conditions. However, the problem lies in this, because under this condition, EDTA can almost quantitatively complex all metal cations. Therefore, the selectivity of this method is very poor, and the anti-interference ability is poor. In order to use this method to obtain accurate results, it must undergo rigorous complexity. Separation of precipitates and other complicated processes to eliminate interference, the results are quite slow, and can not meet the production needs of rapid results. Moreover, once some metal cations, especially divalent metal cations, are not completely separated or masked, the results will be too high or titration impossible.

Recently, a modified EDTA method has been proposed by the researchers, which can be used to determine the content of calcium in tungsten products, including the following steps: using hydrogen peroxide as the main (fast) decomposition sample, boiling the superfluous hydrogen peroxide with ammonia water and adding nitric acid to separate the main tungsten from tungstate precipitation with Ca ions. Chloric acid is further decomposed and determined by atomic absorption spectrophotometer, and the content of calcium in the sample is calculated. At the same time, a few other impurities are decomposed, and a proper amount of strontium salt and lanthanide are added to the sample.

Hydrogen peroxide is the main (fast) decomposition, using the oxidation of hydrogen peroxide and the complexing capacity of tungsten, rapidly decomposing the main tungsten and rapidly complexing with tungsten. The complexation with tungsten can further accelerate the decomposition of the sample, and the oxidation of hydrogen peroxide also decomposes most of the impurities such as calcium. The addition of nitric acid causes the tungsten to precipitate rapidly in tungstic acid form, and calcium ions exist in the solution in ionic form to achieve the purpose of separation. With the high temperature of perchloric acid, high temperature is heated to white smoke, calcium is completely decomposed, calcium content is completely decomposed, calcium content is measured by atomic absorption spectrometry after fixed volume, and the interference of other impurities which are decomposed can be eliminated by adding a proper amount of strontium salt and lanthanum salt. Hydrogen peroxide can quickly decompose samples, the nitric acid can precipitate tungstate and calcium quickly, and perchloric acid decomposes the sample further and thoroughly. Therefore, this method is fast and accurate, the relative error is less than 7.5%, which can fully meet the actual production needs.

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