1 Introduction silicon mainly present in solid solution in the pig iron, in the form of FeSi, Fe 2 Si or FeMnSi. It is the main indicator for judging the grade specification of pig iron, and it is also the main basis for determining the temperature condition of the blast furnace. Accurate determination of silicon is of great significance for timely and accurate guidance of blast furnace production and accurate determination of product specifications.
The determination methods of silicon in the twin iron are mainly gravimetric method, volumetric method and photometric method. The first two are cumbersome to operate and less used in production analysis. Among the photometric methods, representative analytical methods are silicon molybdenum yellow and silicon molybdenum blue [1 ~ 3].
Among them, the silicon molybdenum yellow method is rarely used because of its poor sensitivity and selectivity. The practical application of the silicon molybdenum blue light method is also different, mainly because low silicon uses dilute nitric acid to decompose samples, and high silicon (Si ≥ 1.5%). The sample was decomposed using a non-oxidizing acid (dilute sulfuric acid). The disadvantage of this method is that the dissolved acid cannot be correctly selected for measurement without knowing the extent of the silicon content.
 Through a large number of tests to improve the determination method of silicon in pig iron, the mixed method of dilute H 2 SO 4 —HNO 3 is used to measure low silicon and high silicon by the same method, which overcomes the above disadvantages and the sensitivity of the method (molar absorption) The coefficient ε720 reaches 1.31×10 3 L/mol•cm, and the accuracy and precision are good.
2 test part
2.1 Principle
Sample dilute acid (sulfuric acid - nitric acid mixture) was dissolved with potassium permanganate, as metasilicate, orthosilicate oxide, carbide and destruction, followed by addition of ammonium molybdate at an appropriate acidity, and silicic acid generating silicomolybdenum Heteropoly acid, and coordination of iron with oxalic acid, make the solution transparent and destroy the heteropoly acid formed by phosphorus , arsenic and the like with ammonium molybdate, eliminate the interference, and reduce to molybdenum blue with ammonium ferrous sulfate. Determined with a luminometer.
2.2 Instruments and reagents
721 spectrophotometer  dissolved acid (sulfur, mixed acid): 50mL of sulfuric acid (specific gravity 1.84g / mL) was slowly injected into 950mL of water, after cooling, add nitric acid (specific gravity of 1.42) 8mL.
Potassium permanganate: saturated. 
Sodium nitrite: 3%.
Ammonium bismuth molybdate: 5%, weigh 5g ammonium molybdate dissolved in 100mL water and add 2~3 drops of concentrated ammonia water.
Shikimic acid: 5%.
Ammonium ferrous sulfate: 6%, 6 g of ammonium ferrous sulfate is dissolved in 100 mL of water, and 1 mL of concentrated sulfuric acid is added.
2.3 Operation steps  Weigh 0.0800g of sample in a 100mL steel measuring flask and add 20mL of dissolved acid. After heating at low temperature, the temperature should not be too high. The time should not be too long. If necessary, a small amount of water can be added to prevent silicon. Acid dehydration.), add potassium permanganate to a stable red, boil for 30s, add sodium nitrite until the solution is clear, slightly boil for 1min, remove, cool the water to room temperature, dilute with water to the mark, shake.
 Pipette 10 mL of the upper supernatant into a 250 mL Erlenmeyer flask, accurately add 5 mL of ammonium molybdate from the burette, shake well, heat in a water bath for 30 s, and immediately add 10 mL of oxalic acid and 60 mL of water (these can be mixed together before operation). Immediately after clearing, add 4 mL of ammonium ferrous sulfate and shake for 1 min. Use a 1 cm cuvette (0.5 cm cuvette with absorbance greater than 0.8), use water as a reference, and measure the absorbance at 720 nm. The content was found on the curve.
2.4 Drawing of the working curve
Low-silicon and high-silicon, respectively, take 4 to 5 different silicon-containing pig iron standards and draw them in the same procedure.
3 Discussion of results
3.1 Elimination of interfering elements Phosphorus and arsenic are the main interfering elements. Silicomolybdic acid has higher stability after formation under lower acidity. After the formation of silico-molybdenum heteropoly acid, the complexing agent oxalic acid is added due to phosphorus. The arsenic complex is a pentavalent complex ion, which is relatively unstable and rapidly decomposes, thereby eliminating interference. Although the silicon-based tetravalent complexation is relatively stable, oxalic acid can slowly decompose silicon molybdenum yellow. Therefore, in actual operation, ferrous iron is added immediately after the solution is clear to avoid decomposition.
3.2 Accuracy test
The accuracy test results are shown in Table 1. 
It can be seen from Table 1 that the absolute value of the relative error of the measurement results of the method is less than 1.00%, and the absolute error is much smaller than the allowable error range specified by the national standard GB223-81, and the result is reliable.
3.3 Precision test  Precision test is shown in Table 2.
It can be seen from Table 2 that the standard deviation of the method is less than 0.014%, the coefficient of variation is less than 1.2%, and the precision is good.
3.4 Stability test
The stability test is shown in Table 3.
It can be seen from Table 3 that the results of the method can be stably stabilized within 10 min, and the stability is good.
4 Conclusion 
In summary, the proposed method for the analysis of silicon in pig iron is good in accuracy and precision. More importantly, it solves the contradiction between different methods for high silicon and low silicon in the original method, and realizes low silicon. Consistent with the high silicon measurement method.
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