Pyrolytic Separation and Determination of Fluoride in Raw Materials M. J. NARDOZZI and L. L. LEWIS Applied Reseurch laboratory, United States Steel Corp., Monroeville, Pa.
Pyrolytic separation has been applied to the problem of determining the fluoride content of diverse inorganic mixtures. With this technique a sample is mixed with tungstic oxide as a reaction accelerator and placed in a quartz tube heated to 1000' C. Moist oxygen is passed over the mixture, and the fluoride, which is liberated quantitatively, is swept through the tube and collected for the determination. Separation time has been reduced to 15 minutes for samples of iron ore, slag, glass, and certain minerals. The separated fluoride is determined titrimetrically or spectrophotometrically.
T
HE FLUORIDE CONTENT of the diverse inorganic mistures used as raw materials in industry has been determined conventionally by titration after separating the fluoride by distillation (6). satisfactory results are obtained with the proper precautionsnamely, acid-insoluble samples must be fused with sodium carbonate before the distillation, and distillation conditions must be controlled closely to effect complete separations. Un,fortunately, separation by distillation requires about 3 hours, and a second distillation may be necessary. Once separated, the fluoride is determined by titration with thorium in the presence of alizarin as indicator; although the color end point of this titration is indistinct, it can be obtained precisely with experience. Because fluoride separations from raw materials by distillation are difficult as well as lengthy, another separation technique was evaluated. This technique, pyrolysis, consists of passing moist oxygen (or other suitable carrier gas saturated with moisture) over the sample in a heated tube, generally of quartz, sweeping the fluoride released through the tube, and collecting the fluoride in a suitable medium for determination. There is no undue dilution of the condensate by pregenerated steam because the latter has been eliminated in favor of a carrier gas saturated with moisture. Since the introduction of steam pyrohydrolysis in 1954 (9, 11) and moist oxygen pyrolysis in 1957
I
I OXYGEN 2 GAS REGULATOR 3 FLOWMETER 4 TIGON TUBING 5 WATER FLASK 6 PYROMETER
Figure 1 . pa ratus
7 8 9 IO I1 12
RUBBER STOPPER WARTL TUBE RESISTANCE FURNKE. loodc SAMPLE BOAT FLUORIDE RECEIVER, PLASTIC GAS BURNER
Fluoride pyrolysis ap-
(7, 9), fluorine has been separated readily from the fluorides of thorium, zirconium, uranium, neptunium (7), and aluminum (10). Powell and Menis ( 7 ) demonstrated that a quartz reaction tube can be used to advantage and that tungstic oxide is one of the better accelerators for the evolution of fluoride. A further development of the pyrolysis technique has been reported (8) using a fluxing solvent for the accelerator,
WO,. h recently developed spectrophotometric method for deternlining fluoride has fewer interferences than the titration method that uses thorium with alizarin as indicator. In this spectrophotometric method ( I - d ) , fluoride ion reacts with the insoluble metal chloranilate to form the insoluble metal fluorochloranilate and the soluble chloranilate ion, which is highly colored. The absorbance of the violet chloranilate color that develops is proportional to the amount of fluoride present. The work reported herein describes an extension of the pyrolytic technique to the separation of fluoride from raw materials, and the application of spectrophotometric or titrimetric methods to the determination of the separated fluoride. EXPERIMENTAL
Reagents. Ethylene glycol monomethyl ether, No. E-182 (methyl Cellosolve); p H 4.63 buffer, No. 11-506-86; thorium chloranilate, No. T-401; tungstic .oxide, No. A-325; all from Fisher Scientific Co. Ilundum Combustion Boats. Lab-
oratory Equipment Co., Style KO. 528-53. Standard Fluoride Solution. Dissolve 4.4210 grams of sodium fluoride in water and dilute to 2000 ml. in a polyethylene bottle. This standard contains 1.00 nig. of fluoride per 1.00 ml. of solution. Apparatus (Figure 1). Use a quartz combustion tube that is 46 cm. long and 3.2 cm. in outside diameter. Seal to one end of the tube a t a right angle a smaller quartz tube 10 cm. long and 0.6 cm. in outside diameter. Heat the combustion tube with a 12-inch, split-type resistance furnace that will operate continuously a t 1000' C. Provide for temperature monitoring of the furnace with a thermocouple. Gas Supply and Fluoride Collector. Obtain and set up a cylinder of oxygen with a floumeter and appropriate controls so that oxygen can be passed through the apparatus a t controlled and known flow rates. Connect the gas cylinder to the flowmeter, the mater flask, and the combustion tube with Tygon tubing. Fit the large end of the combustion tube with a 1-hole rubber stopper and a short length of glass tubing. Lay the tube in the furnace so that about 4 cm. of the reduced end extends beyond the end of the furnace. Obtain a receiving flask made of a fluoride-resistant material and of a size and shape that may be easily placed a t the reduced end of the combustion tube. (A plastic 50-ml. graduated cylinder that has been cut off a t the 35-ml. mark serves Tell.) Sample Preparation. Weigh up to 1 gram of sample t h a t has been ground to pass a 100-mesh sieve. The boat capacity limits the sample size to about 1 gram. Divide about 1.5 grams of tungstic oxide accelerator into three approximately equal fractions and line the bottom of a sample boat evenly with one fraction. Thoroughly mix the second fraction of the accelerator with the accurately weighed saniple and spread the mixture over the lining in the bottom of the boat. Cover the mixture in the boat with the third fraction of the tungstic oxide. Pyrolytic Separation. Raise the furnace temperature to 1000" C. and pass oxygen through the system at a rate of about 1200 ml. per minute. Heat the water in the flask to about 75' C. to increase the water vapor VOL. 33, NO. 9, AUGUST 1961
1261
content of the oxygen, thereby decreasing recovery time. Before treating the first sample,,purge the system for a t least 5 minutes after the furnace has reached the operating temperature. After the system has been flushed thoroughly, reduce the oxygen flow to about 600 ml. per minute. Place the plastic receiving flask, which cont&ins 25 ml. of water, a t the end of the delivery tube and adjust the receiver so that the end of the delivery tube is about 2.5 cm. below the surface of the water in the receiving flask. If any water has condensed within the inlet end of the combustion tube, dry the tube end before inserting the sample. If the water is not removed, the sample may be blown from the boat and water may be forced from the ieceiving flask. With a rod, push the boat containing the sample into the center of the furnace and quickly replace the oxygen delivery tube. Limit the oxygen flow rate to about 600 ml. per minute when the sample is added to decrease the possibility of the sample being blown from the boat. After the sample is inserted, slowly increase the flow rate to 1200 ml. per minute. After 15 minutes of pyrolysis, remove the receiving flask and withdraw the boat from the furnace tube. Flush the system with oxygen for 4 minutes before inserting a new sample. Clean the tube daily by closing the small end of the tube and pouring into the tube a hot solution of about 100 grams of sodium hydroxide dissolved in 400 ml. of water. After 5 minutes, drain and rinse the tube. If any oxide remains, scrub it loose with a brush and rinse again. Fluoride Determination. Because fluoride can be determined more rapidly by titration than by spectrophotometry, the former is preferred. However, the slower spectrophoto-
metric method must be used for samples that contain acidic substances that are collected with fluoride. Also, the spectrophotometric method should be used for samples that contain less than 0.1% fluoride because the titration blank is significantly large a t low fluoride levels. When the fluoride level is low, the receiving solution is weakly acidic and the relatively large titration blank arises presumably from the absorption of carbon dioxide. If the fluoride is to be determined by titration, follow one of the recommended procedures (6). For colorimetric analysis, transfer the solutions to 100-ml. volumetric flasks containing 10 ml. of pH 4.63 buffer and the proper volume of methyl Cellosolve. For samples with less than 1 mg. of fluoride, add exactly 50 ml. of methyl Cellosolve; for samples with 1 to 6 mg. of fluoride, add exactly 25 ml. of methyl Cellosolve. Dilute to 100 ml. and proceed in the same manner as outlined below for the preparation of the calibration curves. Calibration Curves. LOW-CONCENTRATION CALIBRATION CURVE. This curve covers fluoride concentrations below 1 mg./100 ml. To 100-ml. volumetric flasks containing exactly 50 ml. of methyl Cellosolve and 10 ml. of p H 4.63 buffer, add standard fluoride solution so that a range of from 0.1 to 1.0 mg. of fluoride is covered. Prepare a blank in the same manner. Dilute to exactly 100 ml. Add 0.15 gram of thorium chloranilate and shake the flasks occasionally. Allow 1 hour for full color development. Filter through a dry Whatman No. 42 filter paper and collect about 25 ml. of filtrate in a dry container. Read the absorbance of the aolutions in 5cm. cells on a Beckman Model B spectrophotometer a t a wave length of 540 mp and a sensitivity of 2, using the blank as the liquid in the reference cell. HIGH-CONCENTRATION C.4LIBRATION
Table 1.
Spectrophotometric Determination of Flu0 ride with Thorium Chloranilate after Pyrolytic Separation Fluoride, % ' Sulfur, % Detd. Reptd. Sample0 0.028 0.20 0.18 Iron ore, GS-1 0.34 0.055 0.38 2 4
0.028
0 . 12
0 038 0.034 0 009
0 28
0.010 0.027
11 12
Ore concentrate, G S 3 GS9
0.38
0 094 0 053
0.003
0.30 0.48 0.005
0.043 0.&4
1.27
0.028
0.50
0.11 0 25
0 39 0 089 0 045
0.31 0.47
0.004
0.67
1.29 0.63 N.D.b 0.60 0.50 N.D. 0.45 0.072 23.0 0.090 3 . 4 2 N.D. 3.4 5.72 N.D. 5.79 0.066 0.072 N.D. 0.020 0.027 N.D. w-1 GS samples from U. S. Steel Corp.; SRI, from Stanford Research Institute; NBS, from National Bureau of Standards; G-1 and W-1, from Geological Survey, Department of the Interior. b X.D. = not determined.
Concentrator tailin& GSIO Steel-mill slag, SRI-5372 Flue dust, SRI-5088 Copper ore, SRI-7803 Phosphate rock, NBS 56b Opal-glass, NBS 91 Silicate rock, G-1 0
1262
ANALYTICAL CHEMISTRY
'
CURVE. This curve covers the range of fluoride concentration from 1.0 to 6.0 mg./100 ml. To 100-ml. volumetric flasks containing exactly 25 ml. of methyl Cellosolve and 10 ml. of pH 4.63 buffer, add standard fluoride solution so that a range of from 1 to 6 mg. of fluoride is covered. Prepare a blank in the same manner. Dilute to exactly 100 ml. Add 0.3 gram of thorium chloranilate, and shake occasionally during the 1 hour required for color development. Filter, and collect data for the calibration as described above. Use cells with a 1-cm. light path. RESULTS AND DISCUSSION
Determination of Fluoride. Thorium chloranilate is a very useful reagent for the spectrophotometric determination of fluoride because acetate, sulfate, sulfite, nitrate, nitrite, chloride, bromide, iodide, and thiosulfate ions do not interfere, and phosphate and molybdate interfere only when present in amounts equal to or in excess of the fluoride concentrations (3). Fortunately, phosphate is not collected with fluoride by pyrolysis; tests for phosphate were negative on the fluoride-containing solutions from the pyrolysis of phosphate rock (31.6% P205). No study was made of the volatilization of molybdenum because it occurs only as a trace constituent in iron ores. In Table I are listed the results of fluoride determinations made on various samples in which fluoride was determined spectrophotometrically. The data indicate acceptable accuracy. Reported values were determined by thorium-alizarin titration after separation by distillation. Interference of Iron and Aluminum. Interference arose unexpectedly with the spectrophotometric method when, one lot of slag sampleq was analyzed, Table 11. This interference was traced to small amounts of iron and aluminum that were volatilized with the fluoride and led to positive errors. The erratic results are assumed to be due to the varying amounts of iron and aluminum carried over with these samples. Why iron and aluminum were volatilized from these slags is not apparent. There is no relationship between interference and the iron and aluminum concentrations in samples, as samples that contained higher percentages of iron (iron ore, Table I) and aluminum (AIFs) (7) have been analyzed with no interference. Any tendency of the iron and aluminum to volatilize is undoubtedly related to how aluminum and iron are associated chemically with fluorine in the sample. To overcome the interference of iron and aluminum in these spectrophotometric determinations, the samples were treated prior to pyrolysis to cause hydrolysis of any iron-fluorine and alu-
minum-fluorine bonds in the substances present and thus reduce the volatilization of the metals. When samples were pretreated by adding a n alkaline solution, evaporating, and baking at a low temperature, results were improved; however, the tediousness of the pretreatment and sample transfer makes the technique impractical. The interference of iron and aluminum was circumvented by determining fluoride titrimetrically, Table 11. The iron-fluorine and aluminum-fluorine substances that were volatilized from the slags presumably hydrolyzed in the receiving solution to yield hydrofluoric acid (a relatively strong acid) and the weakly basic hydrous oxides of iron and aluminum. The hydrofluoric acid from these slags was thus titrated selectively with a strong base in the presence of the small quantities of iron and aluminum without interference. Data on a phosphate rock are included in Table I1 t o indicate the degree of precision normally obtainable by the two methods of determination. Interference of Sulfur. Because fluoride can be determined considerably faster titrimetrically than spectrophotometrically, the titration procedure was checked furthrr by analyzing the samples listed in Table I. All results (not shown) %ere satisfactory, except for the sample of copper ore, which contained 23% sulfur. Titrimetric determinations are not mtisfactory when the eolution being titrsted contains, in addition to fluoride, other strong acids, such as those of chloride, nitrate, or sulfate. Raw materials including ores, however, are generally free of chloride and nitrate, although they do contain sulfur. The interference level of sulfur was therefore studied by adding sodium sulfate to various materials prior to pyrolytic separation and acid-base titration. The error that arises when sulfur is present is indicated in Table 111. When less than about 1.5% fluoride was present, it appeared that sulfur did not interfere in the titrimetric fluoride determination if the sulfur content of the sample did not esceed the fluoride content. When the fluoride content was as high as 3 or 4%, sulfur interfered if it equaled the fluoride content, an interference that led to a positive relative error of about 10%. Of course, when sulfur interferes, the thorium-chloranilate method may be used, provided there is no interference from iron or aluminum. Interference from sulfur would be even greater in the titrimetric method except that sulfur is volatilized for the most part as sulfur dioxide, and this compound is swept from the receiving solution by the carrier gas. At the pyrolysis temperature of 1000" C.,
Table 11.
Comparison of Spectrophotometric and Titrimetric Determination Fluoride in Certain Slags after Pyrolytic Separation
of
Fluoride, % Sample Slag, M-1596-A B M-1597-A
B M-1598-A B
Phosphate rock, NBS 561,
Detd. Spectrophotometrically 3.60, 3.79, 4.08, 3 . 1 5 , 3.35, 3 . 7 9 3.64, 3.36, 3.59, 3.47 4 . 9 1 , 5.27, 4 . 4 8 , 4.20 4.59, 4.93, 4.38 4.48,4.74,4.22, 4.46 3.84, 4 . 4 7 , 4.32 3.36, 3.34, 3.44, 3 46, 3 . 5 4 , 3 . 3 9 , 3.35, 3.45
the equilibrium of sulfur trioxide with sulfur dioxide and oxygen is such that the sulfur dioxide concentration in the sulfur oxides is greater than 90% (6). The sulfur oxides that remain in the receiving solution (sulfur trioxide plus any remaining sulfur dioxide) are titrated as acids and lead t o positive error. Error from sulfur was thought to arise in part from the oxidation in the receiving solution of sulfur dioxide to the triox
