Analytica Chimica Acta, 82 (19?6) 439-442 o Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands

Short Communication

RAPID DETERMINATION OF LIME IN MAGNESIA, CHROME- CONTAIMNG MAGNESIA REFRACTORIES AND CHROME ORES BY EDTA TITRATION

SUBRATA BANERJEE and J. B. VIZZINI

General Refractories Research Center, P-0. Box 1673, Baltimore, Maryland 21203 (U.S.A.)

(Received 8th August 1975)

Lime plays a very important role in magnesia and chrome ore-containing magnesia refractory materials; CaO reacts with Si02 and MgO forming various compounds such as calcium silicates, merwinite, monticellite or forsterite depending on the temperature and the amounts and ratios of SiO, and CaO. The formation of eutectics from these compounds influences the properties of the refractory compositions to a great extent.

The existing ASTM methods for lime determination in magnesia [l] and chrome-containing refractories and chrome ores [ 21 are time-consuming and call for considerable technical skill in achieving accurate results; one set of six samples need 8-9 h for magnesia and about 25 h for chrome-containing refractory materials and chrome ores.

Moreover, the coprecipitation of calcium and magnesium ions by ammonium hydrogenphosphate may be unsatisfactory in the presence of a large amount of magnesium, resulting in slightly low values for calcium. A rapid and accurate method for lime determinations is therefore necessary. Fortunately all the calcium compounds formed in these materials are acid-soluble; this is the basis of the rapid method presented below.

EDTA, since its introduction in 1946 by Schwarzenbach for analytical application, has been investigated extensively for the determination of various elements, calcium and magnesium being the most studied ones. Various methods for the EDTA titration of CaO and MgO in cements, silicates, and related materials have been published; a comprehensive review has been given by Jugovic [3]. The R203 group can either be removed or complexed before addition of EDTA.

In the present investigation, the Rz03 group was masked with triethanola- mine. Hydroxynaphthol blue was found to give the sharpest end-point; ca. 0.07 % CaO could be titrated. Polyvinyl alcohol [4] was added during the titration to reduce the adsorption of the dye on the large amount of Mg(OH)* precipitate. For chrome-containing refractory magnesia materials and chrome ores, the acid-insoluble portion (chromite spinel) was filtered off. The total times for lime determinations in sets of six samples in magnesia and chrome- containing magnesia refractory materials (and chrome ores) were 3 h and 7 h,

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respectively, which is a time saving of 60-70 % over the corresponding ASTM procedures.

EXPERIMENTAL

Reagents_ Distilled water was used throughout. All reagents were analytical grade unless otherwise mentioned. The 0.025 M EDTA, prepared in the usual way was stored in a polyethylene bottle.

For the 0.025 M CaCl* solution, reagent-grade CaC03 was dried at 105-110 “C for 2 h and cooled in a desiccator; 2.502 g of this CaCO, was dissolved in 5 ml of 12 M HCl and 25 ml of water by heating to boiling, and then the solution was evaporated nearly to dryness. Water (100 ml) was added and the solution was diluted to 250 ml in a volumetric flask.

For the 0.1 % polyvinyl alcohol solution, an aqueous 1 % (w/v) solution (100 ml) was prepared with boiling water; after cooling, 1.0 g of MgC& - 6Hz0 was added and the solution was diluted to 11.

Procedure_ Boil 0.5 g of finely powdered (-325 mesh) sample with 20 ml of (1 + 2) HCI for 5-10 mm in a 250-ml beaker. Magnesia samples go into solution completely. For chrome-containing magnesia materials and chrome ores, filter the solution through two No. 40 Whatman filter papers, and wash the residue several times with hot water. Add 3 drops of 0.1 % (w/v) methyl red indicator followed by 5 ml of 0.1 % polyvinyl alcohol solution and 5 ml (10 ml in case of materials with high R203) of aqueous 40 % (v/v) trietha- nolamine. Stir and allow to stand for 10 min. Add an appropriate amount (depending on the range of CaO content in the sample) of 0.025 M EDTA solution, and allow to stand for 15 min. Neutralize the solution with 20 % KOH solution, adding 20 ml in excess. Stir vigorously, add about 1 g of hydroxynaphthol blue indicator preparation (1 g of indicator ground with 50 g of KCl) stir again. Titrate the excess of EDTA with 0.025 M CaC1, solution to the color change from blue to the first permanent tint of purple. (% CaO = ml EDTA consumed X molarity of EDTA X 0.05608 X lOO/sample wt.)

Results and discussion

Twelve magnesia samples normally used in the industry, thirteen chrome-containing magnesia materials (including NBS Standard Sample 104) and four chrome ores (including NBS Standard Sample 103a) were anaiysed in duplicate by this method and compared with the values obtained from ASTM methods (Table 1). The average percentage relative deviations from the ASTM results were 2.5 % and 3.5 % for the magnesias and chrome-containing magnesias, respectively; the latter value was obtained when results 11-13 were omitted. The correlation between the proposed method and ASTM method for CaO values was extremely good for magnesia materials, and for the chrome-containing magnesias, except for samples 11-13.

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TABLE 1

Lime determination in standards and other samples

Sample % CaO

ASTM method Rapid EDTAa

Deviation (%) % Relative deviation

Magnesia materials

St96 2.46 2.41 SL-98 0.72 0.70 G-90 2.10 2.07

SC96 2.14 2.05 DB-87 4.18 4.24 SL-98 0.61 0.56 CM-94 0.95 0.92 SC98 0.81 0.82 SL-96 2.95 2.89 SL-88 3.83 3.88

SL-88 4.63 4.61 HBG 2.70 2.62

Chromemagnesia refractory materials

-0.05 2.0 -0.02 2.8 -0.03 1.4 -0.08 3.7 +0.06 1.4 -0.05 8.2 -0.03 3.1 +O.Ol 1.2 -0.06 2.0 +0.05 1.3

-0.02 0.4 -0.08 2.9

1 3.17 3.03 2 3.10 3.09 3 1.35 1.28 4 0.67 0.65 5 0.72 0.73 6 1.01 0.97 7 0.88 0.91 8 0.67 0.70 9 (NBS 104) 3.35b 3.30

10 1.95 1.99 11 2.55 3.25 12 1.42 1.81

12 2.13 2.38

Chrome ores

-0.14 4.7

-0.01 0.3 -0.07 5.5 -0.02 3.1 io.01 1.4 -0.04 5.0 -l-o.03 4.0 +0.03 5.0 -0.05 1.5 •i- 0.04 1.5 + 0.70 27.5 +0.19 11.8 t-O.25 10.5

NBS 103a 0.6gb 0.68 -0.01 1.4 Philippine 0.75 0.68 -0.07 10.4 Transvaal 0.12 0.16 +0.04 33.3 Russian 0.56 0.63 +0.07 12.5

aAverage of duplicate results. bFrom NBS Analysis Certificate.

It was noted that after the addition of triethanolamine at least 10 min was needed for complete reaction; at shorter times, CaO values were low. Also, a minimum of 10 min was needed after the EDTA addition to obtain reproducible results.

It can be seen from Table 1 that the results for the NBS Standard 104 for nL,rr\-r. Wx&.;nn+T\llr 0-J 1 n9., r%rnrna nrn ~-c.n E.aCifC.x,4~vil~7- l-B..+ f-r Cemr\lnc.

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11,12 and 13, and for the Transvaal and Russian chrome ores, the CaO values are considerably higher by the EDTA method than the ASTM method, possibly because of errors arising during coprecipitation as phosphates in the ASTM procedure. The EDTA method seems to be highly reproducible, rapid and convenient for these types of refractory materials.

The authors acknowledge constant inspiration by W. S. Treffner and gratitude to General Rehactories Company management for permitting the publication of this work.

REFERENCES

1 Annual Book of ASTM Sta.ndards, April 1974, Part 17, C 574-71, p. 505-514. 2 Annual Book of ASTM Standards, April 1974, Part 17, C 572-70, p. 484-491. 3 Z. T. Jugovic, Analytical Techniques for Hydraulic Cement and Concrete, Special

Technical Publication No. 395, ASTM, 1965 p. 65-93. 4 A. L Vogel, A Textbook of Quantitative Inorganic Analysis, Longman, 1971, p. 439.