How to Analyse Blast Furnace Slag

From May 1951 to March 1967, the Bureau of Mines, as part of its iron and steelmaking research program, operated a series of experimental blast furnaces. The first furnaces were located in Pittsburgh; later furnaces were constructed and operated at the Bruceton, Pa., facility just outside of Pittsburgh. With these furnaces, because of the shorter residence time of the charge, as compared with that of a commcrcial-size furnace, knowledge of slag composition was important. W. W. Scott points out,

“The slags made in metallurgical operations consist of complex mixtures and solid solutions of silicates, oxides, aluminates, fluorides, and many other compounds. In the molten condition some of these components are probably in igneous solution in each other, their exact composition and the nature of their association depending upon the state of equilibrium at the time the slags were produced. The composition of slags is important both because of its influence upon furnace operations and because of its effects upon the character of the metallic products. For these reasons it is necessary to make frequent slag analyses in all metallurgical plants.”

If these “frequent slag analyses” are to be of value in a blast furnace operation, they must also be rapid and relatively accurate. In recent years, several important time saving instruments have been added to the analytical chemistry laboratories that permit more rapid and often more accurate analysis of materials. These instruments have been a welcome addition, especially to the many control laboratories throughout the world. X-ray fluorescence (XRP) spectrographic analysis has helped considerably to provide better control of mining operations, furnace products, and process plants. In the mid 50’s, when XRP was coming of age and establishing itself as an accepted method of analysis, Walsh published the first paper on atomic absorption spectrometry (AAS). The application of these two techniques, XRF and AAS, are relatively new, but both have contributed tremendously to the field of analytical chemistry, and particularly to rapid chemical analysis.

In the latter stages of the blast furnace program, some of the control analyses were performed by XRF and AAS. During the earlier stages, however, there was little or no instrumentation available to facilitate the task of rapid control analysis of blast furnace slags. As a result of this, a series of rapid, reliable wet chemical methods was developed.

Most of the methods are variations of the classical methods; they have been altered where possible to reduce the time requirement without greatly reducing the precision and accuracy of the procedure. These methods are not presented as the most precise nor the most rapid, but rather as a combination of speed, accuracy, and simplicity that produces results that fall well within a satisfactory range of precision and accuracy for most control work. Such methods are also useful for quick checks on more sophisticated methods.

Another area of consideration for rapid analyses is laboratory hardware. Many time saving devices, such as auto burets and aliquoters, cooling baths, and cooling blocks, can be employed. Some of these items will be mentioned with the applicable method.

Sample Preparation

The taking and handling of the gross sample will not be discussed here for various reasons; primarily because this task usually is not the responsibility of the analyst. The chapter by J. B. Barnitt in Scott’s Standard Methods of Chemical Analysis may be consulted for methods of obtaining a gross sample. However, of concern to the analyst is the reduction and preparation of the laboratory sample. This could be a rather coarse sample of several hundred grams that is reduced to a minus 200-mesh sample of less than 25 grams.

If the analysis is to be run on a recently tapped slag, which is the case in control analysis, there is no need to dry the slag. If, on the other hand, the slag has been exposed to the air for a long period of time, or if the history of the slag is not known, it should be dried. Slags retained as standards and/or reference materials should be dried before using, and any change in analysis due to slag aging should be noted.

The laboratory sample is crushed in a jaw crusher or other suitable device to minus ¼ inch. The minus ¼-inch sample is riffled to obtain a working sample of approximately 25 grams. This smaller sample is reduced to minus 200 mesh by one of two methods: (1) an impact mortar (fig. 1), or (2) a rotation mill (fig. 2). The pulverized sample is transferred to a blending cloth. Any metallic iron that may be present due to metal drops in the slag or from the abrasive action of the grinding equipment is removed with a horseshoe magnet wrapped in paper. The sample is rolled and blended, then transferred to an adequate sample container.

If it is desirable to retain additional quantities of the slag sample, it is advisable to save the larger pieces of the original sample. With the larger pieces of slag, moisture pickup and slag aging is not as great

a problem as with the finely ground material.

Analytical Methods

Total Iron

Iron is determined by titration with K2Cr2O7, after reduction with SnCl2, and barium diphenylamine sulfonate is used as an internal indicator. This method has several advantages over that of using KMnO4. It has fewer interferences by other elements and it is less affected by organic matter. Also, dichromate solutions are very stable. Carey reported that a 0.1 N solution of potassium dichromate did not change titer appreciably during a period of 24 years.

Reagents

Hydrochloric Acid.– Concentrated.

Potassium Permanganate.-Five-percent aqueous solution; store in dropper bottle.

Stannous Chloride.– Dissolve 150 grams of SnCl2·2H2O in 300 ml of HCl, and dilute to 1 liter with water.

Mercuric Chloride.– Saturated aqueous solution.

Sulfuric-Phosphoric Acid Mixture.– Mix 150 ml of H2SO4 with 300 ml of water, cool, add 150 ml of phosphoric acid and then dilute to 1 liter with water.

0. 1.N-K2CR2O7.– Weight exactly 4.904 grams of the pure salt (NBS-136) and transfer quantitatively to a calibrated liter flask. Dilute with water.

Barium Diphenylamine Sulfonate Indicator.– One-tenth-percent aqueous solution.

Procedure

1. Weigh a 1.000-gram sample and transfer to a 400-ml beaker, add 50 to 100 ml of water. Bring to a vigorous boil and add 15 ml of HCl (concentrated). The addition of acid is made in this manner to avoid or minimize agglutination or clumping of the slag. Even so, it is sometimes necessary to police the bottom of the beaker before proceeding further.
2. Digest at a low boil until the sample is in solution and then wash the beaker, cover, and sidewalls with a minimum of water. Bring to a boil and add potassium permanganate (5-pcrcent aqueous solution) dropwise until any organic matter has been destroyed and the iron has been oxidized to the ferric state.
3. Add stannous chloride carefully, dropwise, until the ferric iron has been reduced to the ferrous state-indicated by a change from yellow to colorless or light green-and add 1 to 2 drops excess, no more.
4. Cool, preferably in ice water, and add 10 ml of saturated mercuric chloride solution, stir until a faint cloud of mercurous chloride appears. Let stand 1 to 2 minutes to insure complete precipitation of the Hg2Cl2.
5. Dilute to about 300 ml with water and add 20 ml of phosphoric-sulfuric acid mixture, and about 10 drops of the barium diphenylamine indicator. Titrate with the 0.1 N K2Cr2O7, to a violet-blue end point that persists for at least 1 minute.

Silica and Mixed Oxides (SiO2 and R2O3)

These two analyses, silica and the mixed oxides, are made on the same sample. The mixed oxides are precipitated from the filtrate of the silica determination. For this reason, the two procedures are presented together.

Silica

Willard and Cake (20) recommended the use of perchloric acid (HClO2) in place of hydrochloric acid as the dehydrating agent in the determination of silica. After studying nine different methods for the determination of silica, Meier and Fleischman preferred the perchloric acid method over all other methods that were investigated. They pointed out, as have others, that a single dehydration with HClO4 recovers about 98 to 99 percent of the silica. This is also true of the other commonly used dehydrate agents.

One of the major advantages of using HClO4 is the formation of soluble perchlorates of heavy metals during dehydration. With H2SO4, and to a lesser extent with HCl, insoluble salts may be formed and the silica is not as pure as that formed with HClO4. In classical methods, the silica is determined by the loss of weight following treatment of the ignited precipitated silica with hydrofluoric acid, and the effect of insoluble salts and other contaminants is minimized. However, such a procedure is too time consuming for control analyses, so the silica is weighed direct. In so doing the formation of soluble perchlorate salts and the absence of other contaminants in the silica is of considerable importance.

Many of the advantages of HClO4 are bypassed by some laboratories because of the danger of explosion. There are precautions that must be exercised with the use of HClO4, but if it is properly handled, the advantages of the acid far exceed the disadvantages.

Reagents

Hydrochloric Acid.-Concentrated.
Nitric Acid.-Concentrated.
Perchloric Acid (70 to 72 percent).-Concentrated.
Sodium Carbonate.-Anhydrous, granular, reagent grade.

Procedure

1. Weigh a 0.5000-gram sample and transfer to a 400-ml beaker. Add 15 ml of hot water, bring to a boil and add 10 ml of concentrated hydrochloric acid. Digest for a few minutes. If the sample is completely dissolved proceed with step 2. If there is an undissolved residue (light or dark), the sample must be started over in the following manner:
   1a. Weigh 0.5000-gram sample and transfer to a 15-ml-capacity platinum crucible containing about 3 to 5 grams of sodium carbonate. Mix sample and flux with a platinum rod and cover mixture with no more than 1 gram of sodium carbonate. Cover crucible with platinum lid and heat the crucible and contents at fusion temperature for 10 minutes over a high-temperature burner. Cool crucible and lid by placing in a platinum dish resting in a tray of cold water. After cooling for several minutes, place crucible and lid in a properly identified 400-ml beaker and add about 30 ml 1-1 hydrochloric acid. After the melt has been leached from the crucible and lid, wash and remove them.
2. Add 10 ml of HNO3 and 30 ml of perchloric acid. Boil down rapidly until fumes of perchloric acid appear. Continue fuming for at least 10 minutes, but do not take to dryness. Cool beaker somewhat, wash down cover and beaker sidewalls with a small amount of water and add about 15 ml concentrated hydrochloric arid. Add about 150 ml of hot water, and filter through Whatman 541 paper. Police beaker and wash residue and filter paper with hot water. Reserve filtrate for mixed oxide determination.
3. Ignite filter paper and residue in a ceramic crucible. After complete ignition transfer residue quantitatively to tared weighing pan, and weigh SiO2, direct.
4. Multiply weight so obtained by 200 to obtain percent silica.

Mixed Oxides

The filtrate from the silica determination may contain iron, aluminum, phosphorus, titanium, vanadium, manganese, chromium, and many other trace constituents including the rare earths. Of these, under normal conditions, iron, aluminum, manganese, and perhaps titanium and phosphorus are present in significant amounts. These elements, except manganese, are all precipitated with ammonium hydroxide in the filtrate from the silica determination.

Reagents

Ammonium Hydroxide.-Concentrated.
Methyl Red.-Dissolve 0.5 gram of the acid hydrochloride in 300 ml of ethanol and dilute to 500 ml with distilled water. Filter if necessary.
Ammonium Chloride Solution (2 percent).-Add 20 grams of the salt to 1 liter of distilled water, and make alkaline to methyl red with ammonium hydroxide.

Procedure

1. Adjust volume of filtrate reserved from the silica filtration to approximately 200 ml and heat to about 60° C.
2. Add several drops of methyl red indicator. From a buret add ammonium hydroxide until the indicator changes from red to yellow, then add 3 to 5 drops excess.
3. Heat rapidly to boiling and filter through Whatman 541 filter paper. Police the beaker and wash filter paper and contents with a hot 2-percent solution of ammonium chloride.
4. Transfer filter paper and contents to a platinum crucible and ignite at 1,000° C until completely burned.
5. Cool, and transfer residue quantitatively to a tared weighing pan and weigh as quickly as possible to minimize moisture pickup. Multiply weight of residue by 200 to obtain percent R2O3.

Calculation of Crude Alumina

Multiply the result obtained for percent total iron by 1.43 to obtain equivalent ferric oxide. Subtract this value from that obtained for percent R2O3, and report the difference in percent as crude Al2O3 (Al2O3 plus any P2O5, and TiO2). Subtract the percent TiO2 from this value (if TiO2 has been determined) and report Al2O3 plus any P2O5.

Manganese

Marshall was first to describe the oxidation of Mn(II) to Mn(VII) by ammonium persulfate with silver nitrate as a catalyst. This principle was employed by Walters when he developed a colorimetric method for manganese in steels. Later, Smith developed a volumetric method using ammonium persulfate-silver nitrate, and titrated the permanganic acid with sodium arsenite. The persulfate method, although it does not have the concentration range of the bismuthate method, is more applicable for rapid control work. With the persulfate method there is no need for filtration or back titration, and the oxidation step takes place at elevated temperatures, rather than below room temperature.

Hillebrand, Lundell, Bright, and Hoffman state, “The per-sulfate method has been improved to the point where as much as 100 mg of manganese can be determined with an accuracy that compares quite favorably with values obtained by the bismuthate method.” This quantity of manganese is well within the range found in blast furnace products.

Reagents

Dissolving Acid Mixture.– Add 100 ml of H2SO4 to 525 ml of distilled water. Cool, and add 125 ml of H3PO4, and 250 ml of HNO3.

Silver Nitrate Solution (0.8 percent).– Dissolve 8 grams of AgNO3 in 1 liter of distilled water.

Ammonium Persulfate Solution (25 percent).- Dissolve 25 grams of (NH4)2 S2O8 in 85 ml of distilled water. This solution is to be made up fresh daily.

Standard Sodium Arsenite Solution.– Dissolve 3.7 grams of sodium arsenite in 1 liter of distilled water.

Hydrofluoric Acid.– Concentrated.

Procedure

1. Transfer to a 500-ml Erlenmeyer flask 1.000 gram of the slag for a manganese range of from 0.1 to 1.0 percent, 0.5000 gram for a range of 1.0 to 2.0 percent or 0.2500 gram for a range of 2.0 to 4.0 percent.
2. Add 50 ml of water and heat. Add 25 ml of the dissolving acid mixture and bring to a boil.
3. Add 8 to 12 drops of HF and digest until sample is in solution.
4. Add sufficient water to bring the volume to about 125 ml. Then add 10 ml of silver nitrate solution and again bring to a boil.
5. Add 10 ml of a freshly prepared 25 percent aqueous ammonium persulfate solution and boil for no less than 1 minute or no more than 1½ minutes.
6. Remove from hotplate and cool quickly in a water bath to room temperature.
7. Add about 75 ml cold water and titrate with standard sodium arsenite solution until all reddish or pink tints disappear.
8. Multiply the volume of arsenite solution required by the previously established factor for the sample weight used to obtain percent manganese.

Alternate Procedure for Slags Insoluble in Acid Mixture

1. Transfer appropriate weight of sample into a 500-ml Erlenmeyer flask and add about 10 ml of water.
2. Bring to a boil and add 10 ml HCl, 10 ml HNO3, and 10 to 20 drops of HF.
3. When dissolved add 10 ml of HClO4 and fume lightly. Remove flask from hotplate, cool, and wash down flask sidewalls with jet from a wash bottle.
4. Fume again and continue heating until most of the perchloric acid has been vaporized, but do not take to dryness.
5. Add 25 to 50 ml of water, add 20 to 30 drops of sulfurous acid, and boil for about 5 minutes.
6. Add 25 ml of the dissolving acid mixture and sufficient water to bring the volume to about 125 ml and proceed as in step 4 of the regular procedure.

Calcium and Magnesium

The classical gravimetric methods of precipitating calcium as the oxalate and then precipitating the magnesium as magnesium ammonium phosphate, is too time consuming for consideration in control analyses. The method of redissolving the calcium oxalate precipitate and titration with KMnO4 is beset with problems, and is not practical due to the low limit of magnesium that is permissible.

Cheng, Kurtz, and Bray, determined calcium and magnesium in limestone with a method which employed the chelating reagent ethylenedinitrilo tetra-acetic acid (EDTA). Kusler, Hattman, Zellars, and Jefferson adapted this method to the analysis of blast furnace slags. In this method calcium and magnesium are determined on the sample following removal of the ammonium hydroxide group and manganese. Jeffery describes a similar method using triethanolamine to complex iron, aluminum, and manganese. Jeffery’s method is valid only with trace amounts of the interfering elements.

Reagents

EDTA Titrating Solution.– Four grams of the disodium salt of ethylene-dinitrilo tetra-acetic acid dihydrate (EDTA) per liter of water.

KOH Buffer Solution.- Twenty-percent aqueous solution.

Calcium Indicator.- Mix thoroughly in a mortar 20 grams potassium chloride, 0.20 calcine, and 0.12 grams thymol-phthalein.

NH4OH Buffer Solution.– Dissolve 60 grams of ammonium chloride in about 200 ml of water, add 570 ml of concentrated ammonium hydroxide, and dilute to 1 liter with water.

Potassium Cyanide.– Ten-percent aqueous solution.

Lime Plus Magnesia Indicator Sulution.– Dissolve 0.15 gram of Eriochrome Black T powder and 0.5 gram of sodium borate in 75 ml of absolute methanol.

Ammonium Persulfate Solution.– Twenty-five-percent aqueous solution.

Hydrochloric Acid.– Concentrated.

Procedure

1. Weigh a 0.5000-gram sample and transfer to a 400-ml beaker. Add 25 ml of hot water, bring to a boil and add 15 ml of concentrated hydrochloric acid. Digest for a few minutes. If the sample is not completely dissolved, a new sample must be taken and started as in step 1a of the silica determination.
2. Adjust volume to about 200 ml, add several drops of methyl red indicator and a small amount of paper pulp, and heat to about 60° C. Add ammonium hydroxide until indicator changes color and then about 6 to 8 ml excess. Cover and bring to boiling. At this point, add 25 ml of a freshly prepared ammonium persulfate solution (25 percent), and continue boiling for 1 to 2 minutes. Filter through a rapid filter paper into a 600-ml beaker and wash precipitate with hot ammonium chloride wash solution prepared as described under “Mixed Oxides.” This treatment separates iron, aluminum, manganese, and most of the silicon (precipitate) from the lime and magnesia (filtrate). A single separation is sufficient for slags associated with blast furnace operation.
3. Discard precipitate and boil the filtrate for ½ hour (final volume about 150-200 ml). The insertion of a small (1-inch-square) piece of acid-hardened filter paper under the stirring rod and the use of a ribbed cover will minimize bumping and aid in rapid evaporation.
4. Transfer the solution quantitatively to a 500-ml volumetric flask and adjust volume to the index at room temperature. Transfer two 25-ml aliquots to two 250-ml beakers. One aliquot will be required for the lime plus magnesia titration and one will also be required for the lime titration.
5. To determine calcium oxide (lime) add about 10 ml of the KOH buffer solution and about 10 mg of the calcium indicator powder (enough on the end of a small spatula to give a definite green fluorescence), and several drops of the KCN solution. Titrate with the standardized EDTA solution to a violet end-point with no green present. Multiply the volume of EDTA required by the factor for calcium oxide to obtain percent lime.
6. To determine magnesium oxide (magnesia), add to the aliquot reserved for magnesia about 8 to 10 ml of NH4OH buffer solution, several drops of the cyanide solution and from 6 to 12 drops of the lime plus magnesia indicator solution until a wine-red color of suitable intensity is obtained. Titrate to a pure blue end point with standard EDTA solution. Under these conditions both lime and magnesia are titrated. Subtract from this volume the volume required for the lime titration and multiply the difference by the magnesia factor to obtain the percent magnesia.

Titanium

Titanium forms a yellow complex in acid solutions with hydrogen peroxide. This is the basis for the most generally used colorimetric method. It is applicable to samples with few or small quantities of regularly occurring, interfering elements. Blast furnace slags usually meet these requirements and the method may be applied to their analysis with some general precautions which may not be of concern, except in special cases. (In many situations an actual analysis for TiO2 may not be necessary.)

The standards used should be of similar composition in order to minimize any variation of minor quantities of color-contributing elements. The presence of iron, chromium, and vanadium comprises the biggest sources of error. Small additions of phosphoric acid to the samples and standards prior to reading them will correct variations due to great differences in iron content. Chromium can be removed by the addition of NaCl or HCl while the sample is being fumed with HClO4 the chromium forms chromyl chloride and is volatilized Vanadium can be corrected for by first reading the sample normally and then reading a portion of the sample after an addition of HF. The fluoride will bleach the TiO2 color complex, but it has no effect on the V2O color complex.

Reagents

Sodium Carbonate.– Anhydrous, grandular.
Perchloric Acid.– Seventy-two percent.
Hydrogen Peroxide.– Three percent.
Hydrochloric Acid.– Concentrated.
Nitric Acid.– Concentrated.

Procedure

1. Fuse 0.500 gram of slag in a large (30-ml) platinum crucible with about 7 grams of sodium carbonate. Maintain fusion for at least 10 minutes after initial melting.
2. Pour melt directly into a 400-ml beaker containing 200 ml of cool distilled water, add crucible and cover quickly. This enables rapid disintegration of the melt. Wear a face shield during this operation. Alternatively the melt may be poured into a cooled platinum dish and the solidified melt along with the crucible and lid, transferred to the 400-ml beaker containing the water. Digest on a moderate hotplate, using a ribbed cover glass and an etched rod to minimize bumping. Continue digestion until melt has been completely disintegrated.
3. Remove crucible and lid. Filter and wash residue with hot water. Discard filtrate. Place filter and residue into the original beaker and pulp the paper with about 15 ml of a solution of equal parts of concentrated hydrochloric and nitric acids. Heat moderately several minutes and then add 30 ml of perchloric acid and fume for 10 min. Cool and add about 75 ml of hot water
4. Filter into a 250-ml volumetric flask, wash with hot water, cool, and make solution up to volume. Discard filter plus residue. Take two 25-ml aliquots and transfer to 50-ml volumetric flasks, marked A and B. Add 5 ml of 3 percent hydrogen peroxide to B and then make both A and B up to volume. Mix well.
5. Read absorbance at a wavelength of 410 nu on a spectrophotometer. Use solution A to obtain the “blank” setting on the instrument and solution B to obtain absorbance reading.
6. A standard curve is established with a series of standards that cover the range anticipated in the samples.

Total Sulfur

Direct combustion in oxygen was the standard method of analysis for carbon long before, such methods were accepted for the determination of sulfur. Although Vita reported very good results for the technique when applied to iron, steel, and slags as early as 1920, the method did not come into general use until about 1940. The method is rapid. Usually, determinations can be made in less than 15 minutes, depending on the type of material and the sulfur content.

Combustion of sulfur in oxygen does not always result in all of the sulfur being converted to SO2, but the amount that is converted is constant provided the materials are similar and the combustion conditions are the same ( The heat source for this procedure is an induction furnace capable of temperatures in excess of 3,000° F. The SO2 produced by combustion of the sample in oxygen is absorbed in a dilute HCl-starch-iodide solution and automatically titrated with a standard solution of potassium iodate.

Reagents

HCl Solution.- Add 15 ml HCl to 500 ml of R,0 and then dilute to 1 liter.
Starch Solution.– Mix 2.0 gram arrowroot starch in 50 ml water until dispersed. Add the resulting solution to 150 ml of boiling water. Boil for two minutes. Cool to room temperature and add 6.0 grams KI. This solution should not be kept over a couple of days. It should be discarded if it turns blue.
KIO3-KI Solution.– Dissolve 0.4440 gram of KIO3 and 5.0000 gram KI in water and dilute to a liter. If the resulting solution is acid, add KOH (about 2 pellets/l) until the solution is alkaline. Standardize the solution with material similar to the types of samples that will be analyzed.

Procedure

1. Weigh appropriate weight of slag (0.1000 to 1.0000 gram) and transfer to a combustion crucible.
2. Add approximately 1.0 gram of iron chips and 0.5 gram copper turnings
3. Cover crucible with a porous ceramic lid and place into furnace.
4. Add HCl solution to titration chamber and turn oxygen at 1 liter/minute.
5. Add starch and enough KIO2-KI solution to give a light blue color to the solution.
6. Refill the KIO3-KI buret and turn on the furnace.
7. After the combustion cycle is complete, record the buret reading and deduct any previously determined blank. Multiply the reading by the appropriate factor for the percent sulfur.