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April, 1948

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Fluorspar

Fluorspar or Fluorite, CaF₂, with a specific gravity of 3 to 3.2, is quite a common gangue mineral in metallic lodes, particularly in lead-zinc deposits, whilst Barite and Calcite are common accessory minerals.

The mineral is usually transparent, translucent and often tinted green, yellow, blue, purple or pink. A particularly deep blue spar known as Blue John occurs in the Castleton district of Derbyshire and no similar deposits of this deeply tinted variety are known in any part of the world. It has a most pleasing appearance and has been worked for ornamental purposes for centuries.

Economic deposits occur in many places, being worked in the States, England, Germany, the U.S.S.R., France, Italy, Spain, Newfoundland and China.

The principal use is for a flux in basic open-hearth steel manufacture, whilst considerable quantities are now being used for the manufacture of hydrofluoric acid. Other uses include the ceramic industry and the smelting of aluminium, lead and silver ores.

Commercial grades are classified into acid, ceramic and metallurgical "spar." The acid grade contains 97 to 98 per cent. CaF₂, generally with under 1.0 per cent. CaCO₃ or SiO₂ but up to 1.5 per cent. SiO₂ is often accepted at the present. The ceramic grade is slightly lower with about 3.0 per cent. SiO₂ and 1 per cent. CaCO₃ permitted provided it is over 95 per cent. CaF₂ and does not contain more than 0.12 per cent. Fe₂O₃ (according to U.S. specifications).

The steel trade in the U.S. usually demands a metallurgical spar carrying over 85 per cent. CaF₂ and not over 5 per cent. SiO₂ and 0.3 per cent. Sulphur, but lower grades are often acceptable, material with 8 per cent. Silica being readily saleable.

The average metallurgical spar in this country carries about 83 to 85 per cent. CaF₂, 5 to 6 per cent. SiO₂ 4 to 5 per cent. CaCO₃ and up to 0.7 per cent. PbS. Picked lump spar which is mainly of "acid grade" contains 96 per cent. CaF₂, under 1.0 per cent. SiO₂ and about 1 to 1½ per cent. CaCO₃.

A few deposits and old dumps are simply picked to obtain lump spar, the finer material being washed, but, generally, jigging is also used. A more recent development is the application of flotation, which is usually capable of producing higher grade concentrate than gravity treatment, whilst Heavy Media concentration or Sink and Float has been introduced for upgrading feed to flotation plants and for the production of "gravel" or metallurgical spar.

Other methods such as decrepitation followed by screening have been tried at a few plants, such as at the Rock Candy Mill, Traill, B.C.

Crushing is usually a very simple operation employing jaw crushers for primary duty followed by stage reduction in rolls to avoid the production of excessive fines if gravity treatment is to follow.

Gravity concentration is usually a delicately controlled operation as the density differential between Fluorspar and the gangue is small (i.e. Barite, 4.4, Fluorite 3.1, Calcite 2.7 and Quartz 2.65). In consequence, close sizing is desirable and careful control is necessary. Furthermore, as the particle size becomes smaller, so the efficiency of separation is reduced and frequently the recovery on tables is poor to the extent of being uneconomic.

When jigging is employed, it is common practice first to withdraw a lead concentrate, then a Barite rich fraction and finally a Fluorite rich product from the third or fourth compartment.

Flotation is now being employed with considerable success for the production of acid grade spar, containing less than 1 per cent. SiO₂ and is particularly successful where the ore is siliceous. In general, fatty acids or soaps of these acids are employed and the pulp conditioned with soda ash with or without sodium silicate. In some cases, however, the silicate concentration necessary for quartz depression produces too high a pH value when commercial sodium silicate is used, but may be reduced by the judicious use of sulphuric acid.

Flotation in the presence of Calcite is more difficult, but this mineral may be crowded into the middling by using warm water whilst chromium salts and tannin act as depressors. Usually sodium dichromate and/or Quebracho extract is now used to depress Calcite.

If there is an appreciable quantity of Barite present, it may be partially removed by prior table treatment or floated using special collectors, such as Emulsol X-1 with citric acid or aluminium sulphate, which tend to depress Fluorspar.

One of the most important factors is to condition in a warm pulp, above 60 deg. C., giving good results, whilst the pH should be between 8 and 9.5, 8.5 being a good average value. The water should be softened and any iron salts in the water precipitated or complexed before use.

Although the collector is usually oleic acid or reagent "708," reagent "825," a new member of the anionic class, is also stated to be effective, whilst Eigeles oxidised paraffin was found to be successful. Holman-Michell "table flotation" has also been applied successfully on material ranging from in. to 60 mesh and a commercial plant is now in operation in this country separating fine gravel spar from Calcite, Quartz and Barite, whilst a second plant is in the course of erection.

Plants treating Fluorspar usually fall into three classes, viz.:—

1. Jig plants.
2. Sink-and-float units.
3. Flotation plants.

In many cases, however, a combination of jigs and/or sink-and-float is employed before flotation to avoid excess of fines in the concentrate when metallurgical spar is the main product. It seems likely that briquetting or pelletising of the flotation concentrate will be used more extensively, and this development has been pioneered by the Mahoning Mining Company in the United States. Various binders including cement, bitumen and sulphite lye have been used, but as yet the process is rather expensive, although a reduction in costs is likely with the developments in this field.

A typical jig plant is that of Williamson, situated at Jamestown, Colorado. Here the crushed ore is sized into five grades, jigged and tabled as indicated in flowsheet No. 1, whilst the middling is retreated by flotation after fine grinding.

In the flotation section, the sulphides are first removed in Morse-Weinig cells, being cleaned in Denver units. The tailing from the sulphide rougher passes to a set of ten 42-in. Morse-Weinig cells for Fluorspar flotation. Three stages of cleaning are given to the rougher froth, which is finally dried in a Merton roaster, where, due to fairly high temperature, some of the silica is slagged and probably volatilised as Silicon tetrafluoride, thereby improving the grade of the concentrate.

In the German jig plants, close sizing is the rule. At the Erna Anna mine, Harz machines are used and concentration is made on the sieves which are bedded down with steel punchings. Barytes is drained from the first compartment and a Fluorspar concentrate, containing 75 to 80 per cent. CaF₂ with 5 to 10 per cent. Silica and some 3 to 4 per cent. Barite is made from ore running 65 per cent. CaF₂, 10 to 15 per cent. SiO₂ and 8 to 10 per cent. BaSO₄, is drawn from the last compartments. When acid grade is desired, the jig concentrate is ground and treated by flotation, using oleic acid and sodium silicate. No depressors are used for the Calcite. The purpose of previous gravity treatment appears to be twofold, firstly to lower the Barite content and secondly to raise the grade of flotation feed by eliminating some silica and wall rock. Flotation concentrate assays 84 to 98 per cent. CaF₂, 0.5 to 1.5 per cent. SiO₂ and 2 to 5 per cent. BaSO₄. The flowsheet is shown in Figs. 2 and 3. In Great Britain, jig plants are the rule on most of the small producers and are comparatively simple in layout. A large gravel-spar producer's plant in Derbyshire has been selected as a typical example, in which the ore receives little crushing, being soft and friable, and is subsequently concentrated by double jigging, yielding a gravel spar for the steel trade. The ore carries Galena, Barite and Calcite, some iron oxide as well as a little Quartz. Galena and Barite are recovered in the first compartment and dressed up as a by-product, whilst Fluorspar is drawn from the last two compartments and rejigged, eliminating further Barite as well as slime. A high-grade gravel spar is also produced by treating minus 1/8-in. jig concentrate and middling on one "Holman" sand table, which is equipped for "Holman-Michell" table flotation, after the pulp has been conditioned in a screw conveyor type of mixer. Here Fluorspar is floated, depressing the remaining Barite, Calcite and silica. The complete flowsheet is given in Fig. 4.

A number of heavy media plants are now in operation, there being one in this country and four in the United States.

In Derbyshire, a Huntington & Heberlein sink-and-float plant has been installed where a double cut is made, the first sinking Barite and some Galena whilst the Fluorspar, Calcite, etc., float, and the second sinking the Fluorspar, allowing it to be separated from the Calcite and the silica at a density of around 2.8.

The product from the Fluorspar cone will presumably be a gravel spar and also serve as a feed to the existing flotation plant for the production of acid grade spar.

At North Gate, Colorado, the ore is washed in a rotary scrubber with a in. opening, then crushed to 1 in. and screened on 10 mesh, the undersize being discarded and the oversize treated in a sink-and-float unit, when a finished metallurgical spar is removed.

Ferrosilicon medium is used with a density of 2.75 at the top of the cone and 2.95 at the bottom. Costs are not divulged, but the loss of medium is said to be 2 lb. per ton and costs $0.05 per lb., delivered. To this must be added a royalty, based on the tonnage fed, to the separatory unit.

At the Gila plant, New Mexico, a similar heavy media treatment is used after washing at 10 mesh. In this case, however, the minus 10 mesh material is treated in jigs. Crushing is done in two stages, using a 9 in. x 16 in. jaw and an 18 in. cone crusher set at in., the product being washed on a rotary scrubber, the oversize from which goes to the heavy media unit, whilst the undersize at minus 10 mesh passes to the jigs. Ferrosilicon, ground to 100 mesh, is used as a medium with a sp. gr. of 2.75 and is recovered in the usual manner. The direct loss of ferrosilicon, not including losses due to shut-down in intermittent operation, are 0.716 lb. per ton of cone feed. In the jig section, the washed undersize is classified at about 60 mesh, the overflow being discarded and the rake treated in two 18 in. x 32 in. Ellis Denver jigs and one 16 in. x 22 in. Denver jig. The jig feed carries about 75 to 76 per cent. CaF₂ and 7.7 per cent. SiO₂, the concentrate averaging 85.0 per cent. CaF₂ and 6.0 per cent. SiO₂, whilst the tailing carries 50 per cent. CaF₂, although the cone tailing only carries 20 per cent. CaF₂ and 41.5 per cent. SiO₂. The overall recovery is reported to be 78.2 per cent.

Drawings and Photographs accompanying the article