The steel industry has used most of the fluorspar imported.

An interesting fact is that the distribution of fluorspar consumption by uses in Germany before the war closely followed that in the United States. The distribution was reported to be as follows: 10 Iron and metal smelting industry, 79 to 84 per cent; glass industry, 10 to 15 per cent; chemical industry, 5 per cent; enamel industry, 5 per cent; optical industry, 5 per cent.

One large Illinois producer states that in recent years production has averaged about as follows: Gravel grade, 90 per cent; ground grades (glass and enamel), 7.5 per cent; acid lump, 2.5 per cent.

METALLURGICAL USES

The metallurgical uses of fluorspar depend upon its relatively low melting point (1,270 to 1,387° C.); upon its fluidity when melted; upon its ability to flux or form eutectics with silica, calcium and barium sulphates, alumina, and other refractory materials, making easily fusible and very fluid slags; and upon its ability to volatilize or slag off sulphur, phosphorus, and other deleterious impurities in iron and other metals. The chemical reactions which occur when fluorspar is used as a flux are not well understood, and authorities differ not only as to the chemical reactions but also as to the rôle fluorspar plays in smelting and the nature of the results obtained. In an effort to clarify the situation, metallurgists representing the best practice in this country were consulted and the literature was reviewed, but the differences of opinion have not yet been reconciled.

BASIC OPEN-HEARTH STEEL

Both lump and gravel fluorspars are employed in the manufacture of steel by the basic open-hearth process, but use of the latter is much more common. Gravel spar usually reaches steel mills in open-top railroad cars which may be dumped or unloaded with a crane and clamshell bucket. It is generally stored in open stock piles or large open or covered bins near the open-hearth building. Fluorspar is often stored at steel mills in large quantities, especially when the demand and prices are high. It is dumped in piles, kept at 800 to 1,000 pounds, in front of each furnace.

An open-hearth furnace has a rated capacity of 15 to about 75 tons of metal per heat (average amount usually 50 to 60 tons). Limestone in the proportion of about 10 per cent of the weight of the metal charge is first spread over the bottom; then the charge of pig iron and scrap is added and the heat is started. When the charge

10 Commerce Reports, "Fluorspar industry active in Germany": Bureau of For. and Dom. Commerce, No. 33, Feb. 9, 1921, p. 788,

is melted the limestone rises to the top, often in large lumps, and floats on the surface of the bath.

This layer of limestone gradually becomes a thick spongy mass through which the gases from the molten metal rise with difficulty. When this slag becomes too thick it must be thinned or the viscosity reduced with fluorspar. R. B. Bostwick, superintendent of the open-hearth department, Duquesne Steel Works, Duquesne, Pa., describes the process as follows:

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During the period known as working the heat," that is, from the time the lime has risen from the bottom to the surface of the bath, to the time of tapping the heat, fluorspar is added in varying amounts. In this period the elimination of carbon or the regulation of its content and the working of the slag are the duties of the first helper, and in the manipulation of this slag, fluorspar renders its greatest assistance. The addition of the spar is made by shoveling it onto the surface of the slag by the helpers, in amounts which are determined by the viscosity of the slag, the temperature of the slag, and the judgment of the first helper.

It is our practice to add fluorspar in amounts not exceeding 6 pounds per ton of steel produced, or from 200 to 600 pounds per heat. In many heats it is unnecessary to add fluorspar, particularly where considerable ore (hematite) must be added to eliminate carbon. In such heats the oxide of iron in the slag confers sufficient fluidity on the slag.

The chief purpose of the fluorspar is to render the slag sufficiently fluid so as to hasten the transfer of heat from the flame to the steel beneath the slag, which reduces the time or duration of the heat, and that the slag may flow from the furnace without difficulty at tapping. Fluorspar accomplishes this by lowering the melting point of a portion of the slag, depending upon the amount added. With calcium silicate, the spar forms a eutectic of about 38 per cent CaF2, permitting of carrying more lime in the slag in order to increase its basicity.

In addition to the above, fluorspar is held to serve another purpose by virtue of its chemical activity; namely, the elimination of sulphur through volatilization from the slag. The importance of this, however, is in question, and nothing conclusive has been proved.

Fluorspar is sometimes added in small quantities throughout the melt and sometimes just before tapping. Some operators believe that it is unwise to add fluorspar less than one-half hour before the heat is tapped. J. C. Davis, vice-president, Inland Steel Co., says:

When we use it at all, it is shortly after an open-hearth heat is melted and the lime is all up off the bottom. We use fluorspar only in case of an excessive amount of free lime in the slag or if the slag is too viscous. In our practice our standard heat has a weight of 26 tons, and in cases of too great viscosity of slag, or an excessive amount of free lime therein, we make an addition of from 20 to 50 pounds of fluorspar. If such an addition is not sufficient to bring about the desired fluidity or fusion of free lime, a further addition of like amount is made. In no case, however, do we add fluorspar within one-half hour prior to the time of tapping the heat. This precaution is taken to guard against the possibility of fluorides entering the metal as a result of reactions between CaF2 in the slag and certain elements in the metal. It

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might be stated here that there has been considerable speculation as to the reactions involved when fluorspar is added to molten basic slag. The fluidity conferred to the slag may be explained by the following:

We know that the base and acid of a slag are alone relatively infusible. Cao melts at 1,900° C. and SiO2 at 1,780° C. A combination of the two, however, CaSiO3, melts at 1,512° C. Mixtures of silicate slags or minerals form eutectics. Thus CaSiO melts at 1,512° C., MgSiO, at 1,524° C., but a mixture with 30 per cent of the latter and 70 per cent of lime silicate melts at 1,350° C. Furthermore, a silicate with more than one base fuses at a lower temperature than a one-base silicate. It is also known that CaSiO forms a eutectic with 38.2 per cent CaF, (melts of 1,300° C.), which freezes at 1,130° C. It can therefore be assumed that under ordinary conditions CaF2 does not enter into chemical combination with basic open-hearth slag, but rather is held in solution by basic silicates. Under gently oxidizing conditions, however, which sometimes exist in working a heat, it is entirely possible that some CaF2, if present in the slag, is split up to form fluorides with silicon or phosphorus from the metal.

The possibility of such reactions taking place and the danger of fluorides remaining in the molten metal has caused us to greatly restrict the use of fluorspar for several years.

In summing up the rôle of fluorspar as a dephosphorizing agent in basic slags, Howe drew the following conclusions:

Fluorspar appears to favor dephosphorization: (1) By liquefying the slag, thus enabling it to assimilate the lime present, part of which might otherwise remain unmolten and inert and thus render the slag effectively basic; (2) probably by volatilizing Si from the metal, thus diminishing the formation of SiO2 and thereby increasing the basicity; (3) in certain cases when conditions are not strongly oxidizing by volatilizing phosphorus as fluoride.

On account of its great influence, and an influence contrary to theory, we are convinced that fluorspar is a catalytic reagent in this case, or, in other words, an agent of transformation. The action, we believe, taking place may be shown by the following equations:

In No. 1 the fluorspar reacts with the silica, giving us calcium silicate, a siag that is easily melted, and silicon fluoride, a gas. A part of this silicon fluoride gas no doubt escapes, but our theory is that the greater part of it reacts with the lime as in No. 2, giving us more calcium silicate and our fluorspar back again to complete No. 1 reaction a second time, and so on until the silicon fluoride has all escaped, when the reaction will cease. If this catalytic theory is correct, we have the silicon fluoride gas generated at intervals for some period, and in order to make sure that none of it is held in solution by the steel, no fluorspar should be added to a heat within 30 minutes of the tapping time.

CONSUMPTION

The average quantity of fluorspar used per ton of steel varies within wide limits from as low as 4 pounds to as high as 25 pounds. Statistics gathered by the Bureau of Mines in 1924, from steel

companies which produce about two-thirds of the output of the United States, show that the consumption in that year varied from 3.3 to 24.9 pounds and averaged 7.8 pounds per ton of steel. One large steel company in the West uses 15 to 20 pounds of fluorspar per ton of steel because instead of limestone it employs dolomite, which makes a very thick, viscous slag. One of the largest fluorspar producers stated that the average consumption of his customers was about 10 pounds per ton of steel.

FUNCTION OF FLUORSPAR

The function of fluorspar has been partly explained above. Its action in removing sulphur is not well understood. In the past it has been generally assumed that the active agent in removing sulphur and phosphorus was limestone and that fluorspar aided only by allowing a more highly basic slag to be carried by increasing slag fluidity. However, Schleicher 11 contends that fluorspar itself is an effective agent in removing sulphur from steel. He shows that in nine heats without fluorspar the average sulphur content of the finished steel was 0.08 per cent and that the slag without fluorspar was more basic than with fluorspar. The ratio of the oxygen of the bases to that of the acids was 1.67 without fluorspar and 1.30 with fluorspar.

From his many tests he concluded: (1) If fluorspar is added to an open-hearth slag it is only decomposed and reduced to a certain extent, namely, 2 to 22 per cent of the CaF, content; (2) that silica is first removed from the slag as silicon fluoride, but this silica is replaced from the furnace lining; (3) that fluorspar aids in volatilizing sulphur from the slag, enabling the slag to take up more sulphur from the bath. His conclusions regarding silica can only apply, of course, when an acid (silica) lining is used in the furnace, and not to the basic open-hearth process. He cites the following example to prove his theory of desulphurization: A 40-ton heat was very high in carbon and had a very thick slag. Ten minutes after about 10 per cent of fluorspar was added to the slag the metal, after a good deal of foaming, was soft. Before the spar was added there was 0.048 per cent of sulphur in the steel and 0.34 per cent in the slag. Ten minutes later it amounted to 0.064 per cent in the steel and only 0.09 per cent in the slag. Careful calculation showed that 10.8 kg. (23.8 pounds) of sulphur had volatilized. The finished metal showed 0.05 per cent sulphur, so the slag had reabsorbed sulphur from the metal. On account of this resulphurization of the slag, Schleicher suggests that after the last fluorspar

11 Schleicher, S., ["Fluorspar in open-hearth steel practice"]: Stahl u. Eisen, Jahrg. 42, Mar. 17, 1921; abstract, Iron Age, vol. 102, Mar. 23, 1922, pp. 783-784.

is added the heat should remain in the furnace long enough to allow this action to take place.

Regarding the action of fluorspar on refractory linings, Jones 12 says: "No bad effects of the spar on the walls or roof of the openhearth furnace have been known and it has been found as time goes on that open-hearth superintendents are increasing the amount of fluorspar used per ton of steel."

Fluorspar, then, performs the following functions in the basic open-hearth steel process:

1. It lowers the melting point of the slag, thus allowing lower furnace temperatures and increased operating speed.

2. It increases the fluidity of the slag, thus permitting escape of gases from the metal and better "working" of the metal, and making the slag easier to handle.

3. It aids in the removal of sulphur and phosphorus by volatilization and by slagging, either by direct action or by enabling a more highly basic slag to be used.

EFFECT OF SIZE OF GRAVEL FLUORSPAR

A screen

The particles of most gravel fluorspar range in size from a maximum of about three-fourths inch down to fine dust. analysis of a typical gravel spar follows:

Because fluorspar from some deposits or some methods of concentration can only be recovered in fine sizes-20 mesh and fineran effort was made to find whether such fine material would be acceptable to the steel industry. Opinions on this subject differ greatly.

One operator stated that "Finely ground fluorspar can not be used successfully, as it will not sink through the slag, and consequently will be wasted." Another operator said: "Material as fine as 20 mesh is not satisfactory unless mixed with coarser material. Consumption of the fine material usually shows an increase over the ordinary gravel size.”

12 Jones, G. H., Fluorspar and its uses: Advance print of paper read before Am. Iron and Steel Inst. at New York, Oct. 27, 1922, p. 6.