UNIVERSAL METHOD OF DETERMINING BROMIDES IN RAW MATERIALS AND PRODUCTS FORMED IN MAGNESIUM PRODUCTION

Transcription

1 Metallurgist, Vol. 53, Nos. 3 4, 2009 UNIVERSAL METHOD OF DETERMINING BROMIDES IN RAW MATERIALS AND PRODUCTS FORMED IN MAGNESIUM PRODUCTION A. P. Nechitailov, A. G. Suss, A. V. Panov, T. G. Golovanova, and E. A. Belanova UDC This article presents a method of determining bromides in substances that also contain significant quantities of chlorine ions. This problem has become particularly important in analyzing magnesium electrolytes (carnallites, bischofite, etc.) in which the quantity of chlorine salts is significantly greater than the content of bromide ions. A method is developed to determine bromine in electrolytes used in magnesium production when the chloride background in the electrolyte is 10 and 100 times greater than the bromine content. An optimum oxidizing mixture has been found, this mixture being unique in that it selectively oxidizes bromides to elemental bromine without affecting the chlorides that are also present. The occurrence of three successive reactions oxidation of bromides to elemental bromine, replacement of the bromine by an equivalent amount of iodine, and reaction of the iodine with starch makes it possible to determine bromides within a wide range of concentrations: from to 100%. The proposed method is universal it is not tied to any specific type of analytical specimen. Key words: electrolyte for magnesium production, carnallite, bischofite, addition of barium chloride, sodium bromide, bromide ions, chloride ions, selective oxidizing mixture, elemental bromine, universal method. Detecting a halide in a substance that consists mainly of other types of compounds is always a challenging analytical problem. The main method used commercially to produce magnesium is the electrolysis of a molten mixture of the nonaqueous chlorides MgCl 2, KCl, and NaCl. The melt is obtained using dewatered carnallite or bischofite. Manufacturers also use magnesium chloride MgCl 2 obtained either as a product of the chlorination of magnesium oxide MgO or a by-product of titanium production (here, highly pure MgCl 2 is obtained as a result of the reduction of titanium tetrachloride by magnesium). The electrolysis temperature is C and the process is carried out with graphite anodes and steel cathodes [1]. In practice, the choice of electrolyte is very important. The choice is made on the basis of the melts or solid salts fed into the electrolysis bath and the feed rate, which in turn depends on the concentration of MgCl 2 in the electrolyte [2]. If a melt initially has an MgCl 2 content of 5 8% and this is subsequently reduced to 4%, there will also be a decrease in the current yield of magnesium. An increase in the content of MgCl 2 to above 8% will increase the consumption of electric power. To ensure an optimum concentration of MgCl 2, some of the used electrolyte is periodically removed from the bath and fresh carnallite or MgCl 2 is added. Liquid magnesium rises to the surface of the electrolyte and is removed by a vacuum ladle. All-Russia Aluminium-Magnesium Institute (RUSAL VAMI), 86 Srednii Prospekt, Vasilievsky Ostrov, St. Petersburg, Russia; vami@vami.rusal.ru. Translated from Metallurg, No. 3, pp , March, Original article submitted October 29, /09/ Springer Science+Business Media, Inc.

2 The recovered magnesium-bearing raw material has an impurity content of 0.1%. Nonmetallic impurities are removed by remelting the magnesium with fluxes lumps of subdivided used electrolyte chlorides, fluorides, or bromides of calcium, barium, sodium, or magnesium. The product is subjected to further purification [1] by vacuum distillation, zone melting, or electrolytic refining to produce magnesium with a purity of %. The magnesium-bearing raw material cannot be melted and refined in air. Magnesium readily forms compounds with oxygen and nitrogen at high temperatures [3]. The oxide film formed on the surface of molten magnesium is discontinuous, unlike the film formed on other metals (such as aluminum and zinc), so that it does not protect the magnesium from further oxidation. As a result, magnesium should be covered by a layer of a flux when it is being remelted. The fluxes that are used are composed mainly of a mixture of chloride and fluoride (as well as bromide) salts and have two functions: first, protect the liquid metal from air, since melts of these salts wet magnesium well and form a covering layer on it that protects it from oxidation; secondly, remove impurity salts, nitrides, and oxides from magnesium due to the fluxes ability to wash the suspension. The fluxes also refine magnesium chemically, forcing out impurities of alkali metals [4]. The composition and properties of an electrolyte depend appreciably on the electrolysis index. If carnallite is used to supply electrolytic baths, then the electrolyte usually consists of MgCl 2, KCl, and NaCl. If the baths are supplied with magnesium chloride, then in addition to these components the electrolyte may also contain CaCl 2 or BaCl 2 depending on the local conditions [2]. It has been proposed that various salts especially barium chloride be added to electrolytes to increase their density [5]. Factory tests have shown that the addition of 10% BaCl 2 to an electrolyte when the bath is supplied with a carnallite melt increases product current yield from to 86%, i.e., by 6 10% [6]. However, a larger addition of barium chloride would be inexpedient due to the substantial amount of BaCl 2 that would be lost with the spent electrolyte. Under these conditions, the savings realized from the increased yield due to the addition of BaCl 2 would not be great enough to justify the expenditure made on expensive barium chloride. It is better to use NaBr than barium chloride, since the former is less expensive. Magnesium production costs are lower when the electrolytic baths are supplied with an electrolyte of such a composition, with a less costly raw material as one of its components. Sodium bromide is also safer for the environment than toxic BaCl 2. This consideration is especially important for recovering magnesium from electrolytic baths, since the sludge that is formed has to be scooped out from the bath (two or three times a day). Here, a worker must always be positioned next to the bath near the surface of the electrolyte. Hot vaporous HCl and harmful waste gases are released even if the cathodic cell has an upgraded flue system and the hatches in the hoods are left open [4]. In metallurgical production, chemical analysis plays an important role in the monitoring of production processes and raw materials, since any changes in the proportioning of the materials can affect the quality of the finished product [7]. Thus, it is also necessary to monitor the composition of the electrolyte. The electrolyte s composition and properties have a large impact on the course of the operations of magnesium electrolysis and refining, i.e., on the quality of the metal that is obtained. Analytical chemists including the authors of this study have been faced with the problem of developing a method of monitoring the amount of NaBr in electrolytes composed mainly of salts of chlorine. For the electrolytes used in magnesium production and all of the products that are obtained in the process, it is necessary to determine different concentrations of bromides against a chloride background that can be from 10 to 100 times greater than the bromide content. One of the simplest methods of determining bromides is their oxidation to elemental bromine and subsequent measurement of the optimal density of the colored solution, which is proportional to its concentration. This difficulty of using this method to analyze the products which contain large quantities of chlorides of magnesium, potassium and sodium is that oxidation of the bromides is accompanied by partial oxidation of the chlorides. This gives the solution a yellow color that tends to exaggerate the results of the bromine determination. Also, the extent to which the chlorides undergo oxidation varies and depends on many factors (concentration, acidity, the oxidizing agent, etc.) that are difficult to stabilize. Thus, it is not possible to introduce a correction for the oxidation of the chlorides. Testing of numerous oxidizing agents has led to the development of an oxidizing mixture that selectively oxidizes bromides to elemental bromine without affecting the chlorides that are present. The mixture consists of a 0.3M solution of potassium bichromate and a 4N solution of sulfuric acid. 177

3 TABLE 1. Parameters of the Oxidation of Bromides and Results of Their Determination number tested Volumes of the solutions, cm 3 reagents H 2 O H 2 SO 4, 1:1 0.3M K 2 Cr 2 O 7 in 4N H 2 SO 4 Normality of the solution based on sulfuric acid Mass of Br,mg Optical density of the solution TABLE 2. Results of Replacement of Bromine by an Equivalent Amount of Iodine Mass of Br,mg Mass of I, equivalent to Br,mg Optical density of the solution Note. A total of 12 cm 3 of water, cm 3 of a 0.1N solution of H 2 SO 4, and 3 cm 3 of a 1% solution of KI were poured into each specimen, and 15 cm 3 was extracted with CCl. TABLE 3. Results of Determination of Bromides Based on an Equivalent Amount of Iodine with Starch Mass of Br,mg Mass of I, equivalent to Br,mg Optical density of the solution Note. A total of 10 cm 3 of a 0.1M solution of KOH was poured into each specimen and iodine was re-extracted into the aqueous phase. Then 0.1 cm 3 of an H 2 SO 4 solution (1:1) was added and the iodide was oxidized to the corresponding iodate with bromine. A total of 3 cm 3 of a 1% KI solution and 1 cm 3 of a 1% solution of dissolved starch were added after the excess bromine was removed by boiling or bonding with phenol. 178

4 TABLE 4. Check of the Accuracy of the Method of Determining Bromides Based on the Color of Elemental Bromine Mass concentration of Br,% Difference, abs. % determined found Note. The substance analyzed was potassium bromide dried at 110 C. The bromides should be oxidized to elemental bromine inside a dividing funnel closed by a plug to prevent losses of the volatile bromine. The elemental bromine is then extracted with carbon tetrachloride and the optimal density of the solution is measured on a spectrophotometer at a wavelength of 400 nm. Table 1 shows the conditions of oxidation of the bromides and the results of their determination. We used 20 cm 3 of carbon tetrachloride to extract them. It is apparent that to oxidize the bromides and extract the bromine is it necessary to keep acidity within the range from 7.26 to 7.75 of the normal value for sulfuric acid. It was also found that the main law which governs light absorption is observed at least for contents ranging from 5 to 25 mg Br in a volume of 20 cm 3. We used experimental data to calculate the molar coefficient of absorption of the bromine solution in carbon tetrachloride. It is equal to 65. The above method is of practical interest despite the low value of the absorption coefficient because it makes it possible to determine macroscopic quantities of bromides and allows the use of large weighed portions for the determination. The size of the portions is limited only by the solubility of the substance being analyzed, which appreciably expands the measurement range. The above-described method can determine from 1 to 100% of the quantities of bromides that are present. 179

5 TABLE 5. Check of the Accuracy of the Method of Determining Bromides Based on the Color of Elemental Iodine added to nominal weighed portion Mass concentration of Br,% found Difference, abs. % Note. The substance analyzed was potassium bromide, 1 mg/cm 3. The amounts of bromides in magnesium-bearing raw materials and products of their processing are often considerably smaller than discussed above. Thus, subsequent experiments were set up so as to narrow the range of measurement. Since a solution of iodine in an organic solvent has a higher molar absorption coefficient than bromine, when the bromine solution obtained in carbon tetrachloride had a pale color it was replaced by an equivalent amount of iodine. In this case, the extract in the dividing funnel was carefully washed with distilled water and placed in contact with several drops of 0.1N sulfuric acid. We should point out that the bromine solutions in carbon tetrachloride and the dividing funnel were both carefully washed to remove potassium bichromate. Otherwise, both the bichromate and the bromine would have been oxidized in the conversion of the iodides to iodine, and that in turn would have led to results considerably higher than the actual values. The extract was then exposed to an aqueous solution of potassium iodide after being acidified with sulfuric acid. The bromine oxidizes the iodides to elemental iodine, which has a significantly higher molar absorption coefficient than bromine. We then measured the optimal density of the iodine solution that was equivalent to bromine and we calculated its concentration using a graduated diagram. We used the experimental data to calculate the molar absorption coefficient of the solution of iodine in carbon tetrachloride, determining it to be equal to

6 TABLE 6. Check of the Accuracy of the Method of Determining Bromides Based on the Color of an Iodine-Starch Compound added to nominal weighed portion Mass concentration of Br,% found Difference, abs. % Note. The substance analyzed was potassium bromide, 0.01 mg/cm 3. The data in Table 2 show that the main light absorption law is observed at least for concentrations ranging from 0.15 to 1.00 mg Br in a volume of 15 cm 3. Thus, it is possible to determine from 0.01 to 1% of the bromides. The prerequisites exist for further reducing the bromine detection threshold based on the reaction of iodine with starch. To make this possible, it is necessary to re-extract iodine from the organic phase into the aqueous phase and oxidize it to the corresponding iodate. The amount of the latter that will be present will be equivalent to the amount of bromide in the original solution. It is then necessary to add excess potassium iodide to the potassium iodate solution. Here, the iodate will oxidize the equivalent amount of iodide to elemental iodine. The iodine will then react with the starch to form an intensely colored compound having an optimal density that is proportional to the concentration of iodine (i.e., that is equivalent to the content of bromides in the original solution). The above-described operations can be represented as a series of reactions: 1) re-extraction of iodine from solution in carbon tetrachloride: 3I 2 + 6KOH = KIO 3 + 5KI + 3H 2 O; 181

7 2) oxidation of iodide to the corresponding iodate: KI + 3Br 2 + 3H 2 O = KIO 3 + 6HBr; 3) oxidation of iodide to iodine: 5KI + KIO 3 + 3H 2 SO 4 = 3I 2 + 3K 2 SO 4 + 3H 2 O; 4) reaction of iodine with soluble starch: (C 6 H 10 O 5 ) X + I 2 = adsorption compound. Table 3 shows the study results. The final volume of the iodine-starch complex was 25 cm 3. It is apparent from the data obtained here that the main light absorption law is observed at least for concentrations ranging from to % Br in a volume of 25 cm 3. The molar absorption coefficient calculated from the experimental data is and agrees well with the literature data The above method can determine bromides within the concentration range from to 0.01%. Thus, three methods have been developed to determine bromine within a wide range of concentrations: from to 100% against a chloride background. The methods consist of the following: first bromides are oxidized to elemental bromine by an oxidizing mixture composed of a solution of potassium bichromate in sulfuric acid. The chlorides that are present are not oxidized in this case. The elemental bromine is extracted with carbon tetrachloride. If the extract has a marked coloring, it is filtered in a cuvette (to have the water absorbed by the filter) and its optimal density is measured at a wavelength of 400 nm. If the extract has a pale color, it is washed several times with distilled water and then exposed to excess potassium iodide. Here, the bromine is replaced by an equivalent amount of iodine. If the color of the extract is readily apparent, then we proceed as described above but measure optimal density at the wavelength of 490 nm. If the color of the extract is pale, the iodine is re-extracted to an aqueous phase with the use of an alkali solution, acidified with sulfuric acid, and oxidized to the corresponding iodate with bromine water. The iodate is boiled to remove drops of carbon tetrachloride that could enter the solution when it is poured out of the dividing funnel. The boiling is also done to remove excess bromine. For reliable removal (bonding) of the bromine, we add a solution of phenol in icy acetic acid. A freshly prepared solution of potassium iodide and a starch solution are also added, and the optimal density of the resulting main solution is measured at a wavelength of 590 nm. We should point out that the oxidation of the iodine to the corresponding iodate and subsequent use of the iodate to oxidize the iodide added to the solution and convert it to iodine increase the sensitivity of the reaction sixfold. This is apparent from the reaction: IO 3 + 5I + 6H + = 3I2 + 3H 2 O. The data in Tables 4 6 show that all three methods give accurate results that are within the acceptable error range. Conclusion. Studies that were done have led to the development of new methods of determining bromine in electrolytes that are used in magnesium production. The methods can determine bromine in such electrolytes against chloride backgrounds 10 and 100 times greater than the bromine concentration. The proposed methods are universal, since they are not linked to any specific type of specimen. They can thus be used to study different types of materials, which makes them of great practical interest. REFERENCES 1. M. A. Eidensohn, Magnesium, Nauka, Moscow (1969). 182

8 2. Kh. L. Strelets, A. Yu. Taits, and B. S. Gulyanitskii, The Metallurgy of Magnesium [in Russian], GNTI po Chernoi i Tsvetnoi Metallurgii, Moscow (1960). 3. V. N. Tikhonov, Analytical Chemistry of Magnesium [in Russian], Nauka, Moscow (1973). 4. A. I. Ivanov, V. V. Krivoruchenko, and V. A. Il ichev, Electroytic Production of Magnesium [in Russian], GNTI po Chernoi i Tsvetnoi Metallurgii, Moscow (1962). 5. Kh. L. Strelets, Tr. VAMI, No. 39, 471 (1957). 6. V. M. Gus kov and Z. V. Vasil ev, Metallurg, No. 5, 69 (1986). 7. P. P. Korostelev, Chemical Analysis in Metallurgy [in Russian], Metallurgiya, Moscow (1988). 183