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April, 1953

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Since the war there has been a spate of articles on the potential supplies of lead and zinc in Great Britain, but little has been written on the subject of fluorspar production. This article describes operations at Stanhopeburn Mine, in Weardale, Co. Durham, owned by Fluorspar Limited. An up-todate treatment plant has been installed and the author has recently been able to study operations at first hand.

Occurrence of deposits

Deposits of fluorspar occur in several localities in Great Britain-the northern Pennines, the Matlock area of Derbyshire, Cornwall and Devon, and North Wales. Of these, the Matlock area and the south-west Durham area of the northern Pennines are by far the most important commercially. The spar deposits in both these regions are found in the long-famous "lead measures" of the lower Carboniferous Series, where they are associated with lead and zinc minerals. In fact, for centuries the fluorspar was considered as part of the worthless gangue in mining lead and zinc and either cast out on dumps at the mine entrances or returned to the stopes as backfill.

By the last quarter of the Nineteenth Century the once-flourishing lead mining industry in Great Britain was on the decline. By then the readily-won ore deposits were exhausted and the price of metal had fallen as a result of competition from the newly opening overseas fields.

Demand for fluorspar

The value of fluorspar as an economic mineral dates directly from the introduction of the Basic Open Hearth process for steel-making in the 1880's.

In this process fluorspar is used to give greater fluidity to the slag and so improve its reactivity. Opinions differ as to whether or not the spar also actively participates in the slagging reactions, combining directly with sulphur and phosphorus in the metal^{1, 2} It is certain, however, that fluorspar combines with some of the phosphorus in the slag to give fluorapatitie, Ca₄(PO₄)₃ . CaF, which is insoluble and so reduces the value of the slag as agricultural fertiliser.

In this country, consumption of fluorspar varies from 1 to 19 lb. per ton of steel², the average being 16 lb. in 1943³. On the other hand, in the United States, consumption in 1949 was given as 5.8 lb. per long ton of steel⁴.

Obviously, sulphur is very detrimental in spar for steel fluxing purposes. Silica is not critically harmful but is objectionable in quantities of more than a few per cent, because, as usually computed, 2½ units of fluorspar are required to flux one unit of silica. Consequently an ore containing 85 per cent. CaF₂ with 10 per cent. SiO₂ has only 60 per cent. available CaF₂.⁵

In the U.S.A., fluorspar is sold on the basis of available CaF₂.

In this country top-grade specification for spar for metallurgical work is CaF₂-85 per cent. min.; SiO₂-5 per cent. max.; S-0.3 per cent. max.; though much of the spar sold is of lower grade.

Besides its use in steel making, smaller quantities of fluorspar are used as fluxes in non-ferrous smelting, in ferro-alloy manufacture, and in welding.

For enamelling and glazing, a higher quality of spar is required. K. C. Dunham² gives the specification of "Ceramic" grade fluorspar as 95-96 per cent. CaF₂ with SiO₂ 4 per cent. max. and Fe2O3 less than 0.12 per cent. If present, iron is most objectionable as it leads to staining during firing.

Fluorspar is also the source of fluorine used in hydrofluoric acid manufacture and from this, in turn, springs a whole host of chemicals-recent developments include the Freon gases used in refrigerating plants and the thermoplastic developed by I.C.I. under the name "Fluon." Complex fluoride baths are used in the electrolytic refining of lead and for electroplating whilst another product, of great importance to the aluminium industry, is artificial cryolite, Na₃AlF₆.

Fluorspar for hydrofluoric acid manufacture must be free from carbonates since these would lead to frothing and evolution of CO2.2 Silica should also be very low since it produces fluosilicic acid, H₂SiF₆, while sulphur leads to corrosion of the manufacturers' plant⁶. Hence the specification for "acid grade" spar is very stringent CaF₂ 98.0 per cent. min.; SiO₂ 1.0 per cent. max.; and CaCO3 0.5 per cent. max.

Another use of fluorspar is in the manufacture of high-grade optical glasses. In this connection, handpicked, translucent crystals of fluorspar were sold from Boltsburn mine in Co. Durham between the wars to the German firm of Zeiss. This mine is now flooded.

Just before the last war Great Britain produced 40,000 tons of spar a year. By 1944 production had risen to over 70,000 tons⁷.

The specifications for the various grades of commercial spar are summarised in Table II.

When the demand for fluorspar arose at the end of last century, the lead mine dumps (and to a lesser extent back-filled stopes) presented convenient sources of supply. In many cases all that was needed by way of beneficiation was log washing and perhaps hand picking to get rid of barren lumps of quartz and calcite.

The re-working of old dumps continued as the chief source of supply right up to the recent war. The grade of much of the material sent to the steel makers was poor, but the price was very low and much of the trade was done by small operators who could not afford extensive treatment plant.

However, with the war came the realisation that the dumps were nearly exhausted and that it would be necessary to open-up more of the old mines to satisfy the demands of the expanding steel industry. Fluorspar was placed on the list of strategic minerals.

Stanhopeburn Mine

It was at this point (1941) that the present company took over the semi-derelict Stanhopeburn Mine, in Weardale, Co. Durham.

The accompanying sketch map shows the location of Weardale in the north Pennine region, running from the high fells near Alston in the west, to Bishop Auckland and the south Durham coalfield in the east. Thus, much of the area is in the Carboniferous Limestone series.

Weardale has for centuries been famous as a lead mining area. In mediaeval times the region belonged to the Bishops of Durham who derived considerable revenue from the lead produced. Evidence of the past mining activity is abundantly apparent; the overgrown tip heaps and the scarred hillsides bear silent witness to the scope of bygone operations. The scarred hillsides arise from the practice of "hushing" whereby the surface layers were removed with streams of water so exposing the near-surface vein minerals. By this same method lead ores were upgraded by washing away the lighter gangue.

Sheep are now more important than lead in the dale.

At Stanhopeburn itself the earliest recorded workings were by the Earl of Carlisle and Company in the 17th century. There followed a long period under the London Lead Company who were succeeded by the Beaumont (later the Weardale) Lead Company in 1866³. Fluorspar production began in 1907 and output of this material amounted to 125,000 tons up to the time when Fluorspar Ltd. assumed control.

From 1801 onwards a smelt mill was in operation at the mine, reducing the lead concentrates to metal. A feature of this plant-and of three similar plants in the area-was the great length of flues used to create a draught for the ore hearths; these flues were of dressed stone and had arched stone roofs. They ran up the hillside, from the smeltery by the side of the "burn" to a chimney at the top of the hill.

The most famous of these smelteries was at Rookhope, three miles from Stanhope. It continued in operation till 1902 and the flues were one and a half miles long, rising a vertical distance of 700 feet up the hillside. The lead fume which accumulated in the flues was flushed out periodically with water.

The main entrance to Stanhopeburn mine is a stone-arched adit driven into the hillside from the west bank of the burn. Where it strikes the vein this haulage way or "horse level" turns N.E., following the vein for over a mile.

In passing, it is of interest to note that on the other side of the stream, just opposite the mine portal, is the site of the famous Heathery Burn cave in which was found the earliest evidence of war chariots in Britain, along with a fine and remarkable collection of late Bronze Age cooking implements and weapons⁸.

The first few years of the present company's tenancy were spent in unwatering and reclamation work in the derelict parts of the mine, whilst at the same time an output was maintained by washing and hand-picking ore from the accessible portions.

A diesel compressor was installed on the surface and compressed air was taken down an old rise to the main adit level. More recently, four electric compressors were installed inside the mine and haulage ponies and primitive wagons replaced by up-to-date tubs hauled by two battery locomotives.

The finding of a rod-actuated bucket type pump during unwatering operations has been recorded.

The proved length of oreshoot is 4,000 feet, the orebody being in the form of a ribbon extending over a vertical range of several hundred feet in places. According to Dunham², the great depth of mineralisation is due to the abnormally thick sandstone beneath the Great Limestone at this spot. At one point the orebody is about 50 feet wide, all spar, but in most places it is 5 to 20 feet carrying small amounts of galena-about 2 per cent.

The spar is grey or green in colour, differing in this respect from the other occurrences in Weardale where the predominant colour is mauve or blue. Also, the massive spar at Stanhopeburn is very friable. Crystals are rare.

The present workings lie between 3,000 and 4,000 feet from the adit portal. Stoping is by cut-and-fill methods. 100 feet below the main haulage level is a lower level reached by a small vertical shaft. A second shaft, designed to accommodate two cages, is being sunk to develop ground 200 feet below the main level.

The Weardale Lead Company carried a pair of development levels some 4,000 feet farther, beyond the western end of the main oreshoot, in an unsuccessful search for intersecting lead veins³.

Two miles to the westward the same vein is worked from the other side of the hill at Stotfield Burn mine (Weardale Lead Company).

Drawings and Photographs accompanying the article