FLUORSPAR-ITS CHEMICAL and INDUSTRIAL APPLICATIONS LENHER SCHWERIN General Manager, Victory Fluonpar Mining Company, Elizabethtown, Illinois
The histury and properties of fluorspar are briefly outlined. The occurrence of &orspar and the genesis of bedding replacement deposits are discussed. The w e of fluorspar as a metallurgical flw because of its remarkable effect in decreasing the viscosity of basic slags .is described. The results of the author's resenrch i n that field are outlined.
It i s pointed out that fluorspar i s the chief economic source of Xuorine in hydrojZuoriG acid and all flum'ne compounds used in or produced by the chemical industry. The uses of the principal compounds are given. Another important use of fluorspar, i n ceramics, is presented. A nm rapid method for the chemical analysis of fluorspar i s suggested.
PROPERTIES OF FLUORSPAR
from warm aaueous solutions. Octahedral and more complicated forms apparently form a t higher temperatures by reaction of the vapor phase, known geologic all^ as pneumatolytic processes. The writer has obtained, by slow crystallization of a melt of the pure material, dendrites exceedingly rich in crystal faces which have ~ "Ot been Observed in nature. Fluorite has the prop$ty of decrepitating or violently flying to pieces when heated to 200' or 300'~. his -led the writer some years ago.to suspect an allotropic transformation accompanies by volume change. However, accurate density determinations showed an average decrease from 3.182 to 3.180, and spectra showed that lattice expansion was too slight to be sipificant. It was determined that fluorite crystals usually contain water which is dtiyen off with disruptive force when heated
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LUORSPAR has long been used as a flux in metallurgical operations, its name being derived from the fl ti^ flue, fiere,to flow, in recognition of its low point and liquidity when ~ ~(1)as early ~ as 1556 i recognized ~ ~ ,.lasses l of three "stones which easily melt in the fire" called fluores or schdnefEiisse by the miners of that day, who added them to the ores which they smelted, "for by the heat of fire, like ice in the sun, &ey liquefy and flow ~ 1 thoueh minerals had not then been scientificallv classi~ ~ ~ ~ ~ , which fied, from ~ g r idescription ~ ~ l of ~ the ~ first ~ "are not only transparent, but are also resplendent, and have the colors of gems, for some resemble crystal, others emerald, heliotrope, lapis lazuli, amethyst, sapphire, ruby, and so forth, ai,d other gems, but they differ from them in hardness." i t is evident that he referred to fluorspar. It has an unusual characteristic of occurring in all colors met with in the mineral kingdom, but its relative softness and cleavability make it of little value as a gem. ~
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FLUORESCENCE
The phenomenon of fluorescence derives its name from fluorite, the first mineral in which this effect was observed. As Wooster (3) expresses it, "The colour of CRYSTAL PROPERTIES OF FLUORITE a.given specimen often varies with the way in which In this countrv i t is usual to refer to the mineral bv licht falls on it. Thus manv preen-blue snecimens are itself as fluorite and to the ore as fluorspar. f he green by transmitted ligh< and blue b; reflected." chemical composition of fluorite is calcium fluoride, Furthermore, certain colored varieties of fluorite glow CaFa, and its melting point is 1378'C. It crystallizes with a vivid green-blue light when irradiated with inin the isometric system, with face-centered cubic lattice, visible ultra-violet light. The character of the emitted each fluorine atom surrounded by four calcium atoms light depends upon the crystal specimen and the waveand each calcium atom by eight fluorine atoms (2). It length of the irradiation. The fluorescence and color of exhibits perfect octahedral cleavage. Its crystal habit fluorite have been determined to be due to impurities is apparently influenced by the mode or temperature of such as manganese, the rare earths, and radioactive its origin (3). Usually it is found in the form of material. Pure fluorite will not fluoresce. Exceedsimple c u b s or interpenetration twins in which the ingly minute quantities of these impurities suffice to cubes are arranged symmetrically about a face of the give the above effects, by imparting their characteristic octahedron (Figure 2). The simple forms seem to be colors as pigments, by scattering action resulting from derived from hydrothermal origin, that is, precipitation their fine state of division and dispersion (4) much as 160
colloidal gold may appear either red or blue, or by radioactive radiations of p- and yrays. Fluorite also exhibits thermoluminescence, that is, when heated to 40-80°C. the crystals glow, the light being visible in a darkened room.
The above reactions seem to explain satisfactorily the metasomatic replacement of the limestone, whereby a molecule of CaFI is deposited in place of a molecule of
OCCURRENCE OF FLUORSPAR AND GENESIS OF BEDDING REPLACEMENT DEPOSITS
Although fluorspar is widely distributed in igneous rocks, its occurrence in commercial quantities is distinctly limited. In this country it is mined on a small scale in Colorado, New Mexico, and Arizona, but the
large center of production is a small area in southern Illinois and across the Ohio River in Kentucky, where it occurs in bedding replacement deposits and in veins in sediments (limestone, sandstone, and shale) of Carboniferous age. The deposit operated by Victory Fluorspar Mining Company is a replacement of the limestone, the banding of the ore, typically illustrated in Figure 3, being a relict structure, due to physical and chemical differences in the layers of the original limestone resulting from seasonal changes while it was being laid down. The comb structure of pure bands is due to a decrease of volume, the CaFz molecule being more dense than the CaC03 molecule which is replaced. The presence of inclusions of iron sulfide (Figure 2) in the form of orthorhombic marcasite rather than isometric pyrite indicates that the solutions which brought in the fluorine were acid and a t low temperature. Currier (5) has proposed the following chemical equations to explain the precipitation of CaFz in situ and the reorganization of the silica:
CaC03, preserving the texture of the original limestone, but where crystals are found to have grown out into cavities, an explanation must be given to account for the evident migration of the calcium. It seems reasonable that the solutions contained carbonic acid from the release of COZas indicated above, which dissolved limestone, and calcium bicarbonate coming in contact with fluoride iohs a t some distance from the limestone would account for the growth of the crystals.
USE AS A METALLURGICAL FLUX
It will be readily understood that the limitations of a brief paper such as this will not permit a discussion of all the uses of fluorspar. Hatmaker and Davis (6) have written an excellent statistical review of the fluorspar industry, to which the readel is referred for further details. The largest consumption of fluorspar is in metallurgy as a flux and depends upon its almost unique property of decreasing the viscosity of slags when added to them, principally in making steel by the basic open hearth (Siemens) process. Briefly, in this process certain of the constituents of pig iron, chiefly C, Si, P, and S, are removed by oxidation, the agent being iron ore. The latter is dissolved in a basic silicate slag which floats on the bath of molten metal. The reactions occur The present writer has found that an aqueous ten per a t the slag-metal interface toward which the reactants cent. solution of NaF a t room temperature for three move by Musion and convection, the oxidation proddays will react with limestone and coat it with CaF2 ucts dissolving in the slag or escaping as gas. Ueat and suecests.. therefore. that the reaction be made more must also be transferred to the metal by the slag, congeneral as involving the fluoride ion, as follows: vection being much more effective than conduction
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alone. It is evident that the control of the refining process depends to a considerable extent upon the fluidity of the slag. Fluorspar is added when the slag becomes too viscous, the condition being thus promptly corrected. Smaller amounts of fluorspar are used for similar purposes in electric furnaces making alloy steels and ferro-alloys; in cupola furnaces melting iron; and, to a lesser extent, in the smelting of nickel, refractory ores of copper, gold, silver, and various rare metals. EFFECT OF FLUORSPAR ON THE VISCOSITY OF BASIC SLAGS
Several years ago the writer conducted an investigation of the effect of fluorspar on the viscosity of basic slags (7, a), since little was quantitatively known about this important question. The absolute viscosities of molten fluorspar and various slags with and without varying additions of fluorspar were determined through the range of steel-making temperatures, that is, up to 1650' or 1700°C. The viscosity of molten fluorspar, having the composition SiOa 0.05 per cent., CaO 1.20 per cent., and CaF298.75 per cent. is shown in Figure 4. The sharp break in the curve a t 1420°C. and the fluidity at higher temperatures are striking when compared with that of the slags whose viscosities were measured. In Figures 5 and 6 the viscosity curves of four typical slags are plotted in two ways. Slag Number 1 contained no fluorspar and the additions to Slags Numbers 2, 3, and 4 increased in that order in equal increments. The curves in Figure 5 show almost equal displacements to the left for each increment in the amount of fluorspar added. The investigation led to the following conclusions: "(1) Regarding the viscosity of a basic slag, the effect of adding fluorspar is analogous to increasing the temperature. "(a)For a particular molten dag, the lower its temperature the more pronounced*s the effect of a given flnorspar addition. "(b) At a given temperature the effect of flnorspar is a direct function of the amount added. "(2) The decrease in viscosity of a slag caused by the addition of fluorspar is not a temporary effect, but lasts an hour or two a t least, provided no other materials enter the slag to change its composition meanwhile. "(3) The presence of silica in fluorspar, within the limits investigated, does not lower the efficiency of the calcium fluoride, per unit added, in decreasing the viscosity of the slag." The effect of fluorspar was discussed from purely theoretical considerations and three possibilities were recognized. First, the fluidity conferred on slags by fluorspar may be an additive one. Second, since a liquid having a structure may be expected to have a high viscosity, fluorspar may inhibit the formation of molecular aggregates, or (third) by lowering the freezing point i t may displace their formation to a lower temperature.
USE OF FLUORSPAR IN THE PRODUCTION OF HYDROFLUORIC ACID
The production of hydrofluoric acid accounts for the second largest consumption of fluorspar, which is the chief economic source of fluorine. Finely ground fluorspar is mixed with sulfuric acid in iron retorts, the following reaction taking place: CaR
+ H.S04 +H2F2+ CaSO,
The trade requires that the fluorspar be of very high purity, a t least ninety-eight per cent. CaF2 with not more than one percent. of either SiOz or CaCO,, for the reason that silica wastes both fluorine and sulfuric acid by forming hydrofluosilicic acid, and calcium carbonate neutralizes the acid and causes foaming in the retort. The volatile hydrogen fluoride passes out of the retort and is collected by one of two methods, depending upon whether aqueous or anhydrous acid is required. The aqueous acid is collected in lead cooling and absorbing towers. I t is shipped in lead carboys or rubber-lined barrels, although for laboratory use it is packed in wax
bottles. The anhydrous acid is condensed by refrigeration and is shipped in containers made of iron, although copper, brass, and even magnesium are unattacked. Hydrofluoric acid is used in the chemical industry for the synthesis of all fluorine compounds. Limited space permits the mention of only some of the more important ones. For further details the reader is referred to a paper by Reed and Finger (9). ORGANIC FLUORINE COMPOUNDS
The organic fluorine compounds are stable and possess many unique properties. For example, some new refrigerants of suitable thermodynamic properties, containing both chlorine and fluorine, are finding widespread use because they are nonexplosive, nonflammable, noncorrosive. and nontoxic. Thev are known as Carrene, the two compounds n&ally referred
reo on or
to by these terms being dichlordifluormethane (CC12F2) aluminum salts in ceramics, and the zinc, magnesium, and trichlormonofluormethane (CClaF), respectively. and aluminum salts to harden concrete. A great numOther Freons are also chlorine and fluorine derivatives ber of other inorganic fluorides are useful in many ways. of methane or ethane. The aluminum industry accounts for a large portion of the fluorspar which is converted into hydrofluoric INORGANIC FLUORIDES acid. Synthetic cryolite, 3NaF.AlFa, is produced from The salts of hydrofluoric acid, find many uses indus- it and used to replace in part the natural mineral in the trially. The alkali, ammonium, aluminum, and zinc molten bath from which aluminum is produced electrofluorides are used as preservatives or insecticides, and lytically. Cryolite is found in commercial quantities in zinc fluoride is also used as a wood preservative. Anti- only one place in the world, Iviatut, Greenland, and while i t is-imported into the united States in considerable quantities, the synthetic material is successfully competing with it. CERAMICS
Another large use of high-grade fluorspar is in the field of ceramics, in the manufacture of opal glass and enamels. The fluorspar for this purpose must be finely ground, and contain ninety-five per cent. or more CaF2, with iron, lead, zinc, and sulfur restricted to very small amounts, as they affect the color. Opal glass is molded into a great variety of shapes with which we come in daily contact. The most common are lamp shades, containers for toilet preparations, rods for towel racks, liners for fruit jar caps, and table, soda fountain, and counter tops. The enamels on sheet steel and cast iron usually contain fluorspar, added as a flux and opacifier. Articles so enameled include plumbing fixtures such as bath tubs, wash basins, and kitchen sinks, refrigerator linings, cooking stoves, table and counter tops, and signs. The glaze on pottery, tile, and earthen ware might also be .. mentioned. OPTICAL FLUORITE A limited but extremely important use of fluorite is in apochromatic objectives for microscopes. Fluorite has a very low index of refraction, ND = 1.4339, which is lower than that of any other material available for lenses. For this purpose clear, flawless portions of crystals are required, which, due to their scarcity, command high prices. The fluorite apochromatic objectives are superior, fcrst, because they have less chromatic aberration. Three optically significant colors mav be broueht to a focus; whereas with crown and flint glass pairs only two differently colored rays can be so united, and second, because they have improved resolving power, due to higher numerical aperture. For most of the uses to which fluorspar is put, no satisfactory substitutes have been found.
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I400
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I450 1600 1550 1600 TLMPLRATURC %.
F I O ~ VI E VISCOSITY
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mony and boron fluorides are used in the synthesis of organic fluorine compounds, the former in the manufacture of Freon and the latter as a polymerizing agent. The alkali and ammonium acid fluorides are used as antiseptics and laundry sours and in the etching of glass. The fluorides of zinc, barium, sodium, magnesium, and aluminum are useful glass and enamel opacifiers. The silico-fluorides are used in electrolytic lead refining (Betts process), the sodium, calcium, and
ANALYSIS O F FLUORSPAR:
A NEW RAPID METHOD
The old and well-known procedures for the analysis of fluorspar are either inaccurate or long and tedious. Probably the best precise method for the complete analysis of the ore has been developed by Lundell and Hoffman (10). The work of Schrenk and Ode (11) on the determination of silica in the presence of fluorite suggests a new and more rapid method for carbonates, calcium fluoride,
and silica in ores containing no sulfides. In th&presacid, capamay be decomposed by, acids ence of without the SiOz being attacked, the reason being that the liberated HzFz immediately reacts with the boric acid to form boron trifluoride. Therefore, the reaction outin glass beakers, without danger of may be etching. The acid favored for the decomposition is perchloric, because calcium perchlorate is very soluble and easily removed from th; residue. The CaFz is completely decomposed and when tested on synthetic mixtures and U, S, B,S, sample of fluorspar ~~~b~~ 79, the method was found to be precise and rapid. The distinction between CaCOa and MgCOa in fluorspar is ordinarily of no significance. The procedure suggested by the present writer based on the above considerations, is as follows: 1. Digest 0.5%. sample (finely ground) with ten
cc. of ten
per cent. acetic acid in a glass beaker on the water bath for one and ane-half hours. Filter through ashless paper, wash residue and paper three times with hot water, ignite, and weigh. Report loss of weight as carbonates. ateen cc, of twenty 2. Add to the residue in a per cent. perchloric acid saturated with boric acid a t 50°C.
Heat gently until fumes of perchloric acid come off for four or five minutes. Add a few cc. of water and repeat the fuming for four or five minutes. Dilute to fifty to seventy-five cc. and afterheatthe solution, filter through ashleu paper. Wash first with a dilute solution of perchloric acid and then with hot water until free of calcium perchlorate. Transfer the residue in the filter Paper to a tared platinum crucible, -add two drops concentrated H801, and ignite. Weigh and rsport loss of weight as CaF2. on a sand 3, Add five cc, hydroAuoricacid and take to bath in the hood. 1f the silica was high. - . as would be aooarent .. from inspection of the previous residue, this treatment should he repeated to insure complete decomposition. Add a few drops HdO4. to convert Ba t o sulfate. Ignite t o bright redness. cool in a desiccator, and weigh. Report losS pf weight as SiO*. 4. The residue may contain aluminum and iron oxides and BaSO,, which may be determined by usual methods, if required.
In case the above method is used as a control in production when quick results are necessary and as freauentlv is the case. knowledne of the silica content is of as much significance as of the calcium fluoride, Steps 2 and 3 may be exchanged, so that the silica is determined before the fluoride, and the latter may be estimated by difference, provided the silica is free and not in solid solution in the fluoride, which would partly protect the silica from attack by hydrofluoric acid.
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LITERATURE CITED
Acruco~~. "De re metallica," vanslated from the first Latin 8
cdilion of 1550 by Herbert Hoover. Mining hlagazine, London. 1912,. -pp. 11.; and 380-1. B R A ~"Atomic , structure of minerals," Cornell University Press. Ithaca. New York. 1937.. PD. - - 57-8. WOOSTER in "Encyclop;edia Brittanica," 14th ed., Vol. 9, 1937. n rn- 428-II. ---., r --- - . DOELTER, "Ueher kalloide f;irhemittel im mineralreich," Kolloid-2.. 26,23-7 (1920). CURRIER, "Origin of the bedding replacement deposits of
fluorspar in the Illinois field," E c o n a i c Geology, 32, 364U"
//1(127\ ,A"".,.
HATMAKER AND DAVIS, "The fluorspar industry in the
U. S.," Bulletin No. 59, Illinois State Geological Survey, Urbana. Illinois (1938). ~ c n w r ~ ~" ~ ' .r ~ i o " ' ofltkspar l a n open hearth basic slags," .Ue=l~ls&Alloyr. 5,Gl-6.83-8(1934).
SC~XWER "The ~ N , rtTcct of fluorspar on the viscosity of basic slaes." ihld . 5. 118-!XJ 11!)3.1). REED FINGER. "F~U&SPBT'~S a chemical raw material." C h m . Indudries, 39,577-81 (1936).
AND
(10)
BRIGHT,"Chemical analysis of iron and steel." John Wiley &Sons. Inc.. New York City,
LUNDELL, HoPPMAN, AND
1931.
(11) SCHRENK AND ODE. '*Determinationof silica in the presence of fluorspar," Ind. Eng. Chem., Anal. Ed., 1, 201-2 (1929).
