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July, 1938

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Settlingstones occupies the mid position of a mineral belt in the Tyne Valley, and like many lead mines in the North Pennines it commenced its career in the late seventeenth century. Historically, it is of interest to mention that the Roman Wall flanks the northern escarpment of this belt, and whilst local historians are inclined to link the opening of their mines with Roman times, the use of lead in the construction of the wall to fasten iron dowels in their sockets is not evidence that local mining was contemporary with Roman habitation. Some critics assert that the significance of the wall is due to the fact that it defines the northern limit of the Pennine mining field.

Of earlier mining records a parchment-bound folio volume of some 13 pages in the possession of the Newcastle Society of Antiquaries contains statements of sundry accounts from 1687 to 1692, and an old mine plan shows the workings some 30 fathoms deep at the close of the eighteenth century. In the last century the mine came into the possession of Mr. Frederick Hall, and in 1907 the existing company was formed. Settlingstones ceased to raise lead in 1873, but commenced to produce witherite in 1872, and has been a consistent producer of this mineral since that date.

Witherite or natural barium carbonate is a relatively rare mineral. It was first identified by Dr. Withering, a physician of Birmingham, while examining a collection of minerals from an old lead mine on Alston Moor, and the mineral was named in his honour. It crystallises in the orthorhombic system in the form of hexagonal pyramids, which have the appearance of a simple crystal, but in reality are repeated twins. The faces of the crystals are usually rough and horizontally striated. The deposit contains granular, crystalline masses which, in the process of mining, disclose small cavities or vugs lined with shining twinned crystals of pure witherite ; also more rarely it occurs in globular, tuberous and botryoidal forms showing columnar or amorphous structure. Fracturing is uneven and brittle, the hardness ranges from 3 to 3.75, and it has a specific gravity of 4.29 to 4.35. The colour is variable from white, often yellowish or grayish, and occasionally stained with intrusive salts.

Settlingstones vein is clearly one of hydrothermal origin, and is enclosed in a fissure accompanied by a complexity of faulting. It lies in a mass of alternating sandstones, shales and limestones named the Bernician series after Bernicia the ancient name of Northumberland. This series contains the great whin sill, an intrusive quartz dolerite, which is one of the interesting features of local geology. Generally the strata incline eastwards, veering around in a southerly direction toward the Tyne valley. A fault at the west of the mine extensively dislocates the strata vertically and laterally to the south, and a further complication is the presence of a southeast anticlinal axis by which the lower beds are brought up into the belt at the eastern extremity of the mine. Erosion along the crest of the anticline has partly denuded the whin sill and glacier deposits, clay with boulders, cover the strata which in places attain a depth of 150 feet. Throughout the belt the rock mass is disturbed and displaced by major faults and, to a less degree, by minor faulting. The whin sill averages 130 feet thick throughout the mine and has, with the strata, apart from the faulting, a uniform inclination of approximately 1 : 30 to the south west. Above and below the whin sill are beds of whetstone which are apparently metamorphosed shale, altered at the time of the injection of the igneous rock. The interesting feature of the mine is the change in the character of mineral deposition which occurred at the cross vein at the Winter shaft. Here the vein fissure and strata are faulted abnormally and the lead ore is entirely cut off at the fault plane. The vein then becomes a barren stringer for a distance of some 800 ft. in a westerly direction, and from this point the fissure becomes charged with witherite. The witherite vein is practically vertical and varies in width from 4 to 12 ft. Exceptionally, a 30 ft. width has been exposed, but the average is something like 8 ft. Of the vein characteristics, a stringer of zinc blende intercalated in the witherite, known as the "lead" is a feature of interest. This stringer is well defined and continuous throughout the mine and when the vein is impoverished, as happens at times in the softer strata, the lead continues and is followed advantageously. In the wide parts of the vein it shows ramification, and at times mineralisation changes occur between the relative divisions. Gangue minerals consist of lime in the form of calcite and baryto-calcite which frequently fill the capillary interstices of the witherite mass. Cavities are a common occurrence, sometimes of large dimensions, and invariably crystal lined. Iron pyrites is not prevalent, but it occasionally impregnates the vein locally. Galena occurs remotely in pockets or lenses. Barytes is rarely present in the amorphous state its occurrence is associated with the vein cavities which it encrusts with a coral like deposit, and it is inherent in the zeolite Harmotome which permeates the voids of the vein. Rapid changes in the vein matrix constitute a deposition enigma; contrasting features are pure transparent witherite, witherite in granular crystalline form, witherite with a varying disposition of gangue mineral, brecciated country rock and the presence of argillaceous shale are admixtures of frequent occurrence. Of the hydrothermal influence on the walls of the enclosing rocks metamorphic changes are widespread ; the "white whin" is a feature of seritic alteration whilst the witherite contacts frequently show chloritic inclusions ; voids, in the swells of the vein, are often chloritised.

Except in one or two instances the cross veins are barren. They vary in width from a few inches to one or two feet, and are generally open-jointed fissures filled with a matrix of fault breccia and permeated with ferruginous solutions. They make a clean intersection across the vein and their occurrence invariably displaces the vein laterally to a small extent. Post mineral displacement is in evidence, the shear zones on the fissure walls indicate intense movement and slickensiding within the mineral mass has been observed.

The formation of the enclosing rock has an effect on the mineral deposition. The lead ore showed enrichment in the whin sill and became impoverished in the lower shales at a comparatively shallow depth. The witherite exhibits similar fertile features in the whin sill and the corresponding wall of the harder sedimentary rock while the structure of the wide open fissure has lent itself to the environment of ore deposition. In behaviour, the vein in the shale beds at the higher part of the mine is erratic. Swells of pure witherite rapidly contract or break into stringers.

Mining has progressively advanced westward. There are two shafts in commission, and of these the Frederick is the main line of communication. Of circular section, it is cement concreted from the collar to the extent of 170 ft. in the boulder clay, and from here the walls are supported by iron rings, and wood lined. It is equipped for tub-cage hoisting with rope guides. Levels are at 60 ft. intervals, and mining is by flat back shrinkage stoping. Stope supports, at approximately 4 ft. intervals, are hitched into the opposing walls and the span is supported where necessary by vertical props. The structure is covered with a mat of wood reinforced with old tram rails or mild steel tubes. Chutes are erected at some 12 ft. intervals. They are constructed of wood with a discharge lip and removable control board.

Stopes are carried up to the level pillar. The depth of this pillar is determined by the character and the width of the vein. It varies from 6 to 10 or 12 ft., and where the pillar is inclined to crumble it is timber supported from wall to wall. Loose walls are timber supported in the process of drawing the mineral, and when the stopes are drawn down they are usually filled with waste rock from the dressing plant. Generally the walls are firm, but in the shale beds at the higher and lower part of the mine the environment is changed ; they vary in consistency, some are calcareous, others fissile and argillaceous. The soft black variety, when water saturated, breaks down and, caving up the wall of the vein, exposes a mass of witherite. Local conditions determine the height and extent of the horizontal stope cut. Normally 6 ft. slices are broken down but in the soft shale beds the cut and fill method is essential and cross cutting in the vicinity provides the filling material.

Larch timber was formerly employed exclusively, but in recent years mild steel in the form of joists, heavy steel rails and arches are used in the more important passages of the mine ; the introduction of pre-cast cement concrete props, reinforced with old wire rope, make a permanent structure. In the swells of the vein cement concrete walls are built. They afford a stable support and shorten the span of the roof supports.

Rock drilling is accomplished by jackhammer machines, and the system is served by a two-stage compressor, of 650 cu. ft. capacity, housed in the vicinity of the Frederick shaft. The problem is to extract the mineral with a minimum of smalls. In order to avoid a shattering effect, explosive of the powder type is employed 3 oz. cartridges are used and the data of explosive consumption given in Table I, is of interest.

The mine is ventilated by natural means. Air enters via the Ellen and the 80 fm. water level. At the Frederick shaft it is by-passed and fan-boosted when necessary. Circulating the upper workings it returns through the lower mine to upcast at the Frederick shaft.

Water entering the mine is constant and amounts to approximately 400 g.p.m. The limestones are the natural source of percolation and, adjoining the mine, abandoned workings standing full of water contribute to the feeder. The surrounding country is drained by the cross faults which provide free channels for the inlet of water, and at times, in the course of mining, cavities containing water are holed into, and these temporarily increase the regular feeder. Because the mine sumps are relatively small, a stoppage of a few hours' duration to the pumping system would cause the mine to flood. Water entering the Frederick shaft sump is normally pumped to the 80 fm. level. The flow along this level to the Ellen shaft is increased by various feeders, and the peak is attained at the Ellen sump. Consistent with efficiency, the capacity of the pumps normally used is approximately equal to the volume of water. At the Frederick shaft there is a three-throw pump and a pump of the centrifugal type is held in reserve. There is also a 12 stage "Rees Roturbo" pump which lifts the water to the surface when it is required to augment the surface supply. A masonry constructed chamber at the foot of the Ellen shaft contains three pump units of the centrifugal type. One, of normal capacity, is in regular service, and two of much larger size are held in reserve.

The recent conversion of the steam hoist to electrical haulage at the Frederick shaft followed the introduction of the "grid" power supply. Incoming current is transformed to 400 V, and converted to the existing 550 V d.c. system by means of a motor-generator. Equipment at the power station consists of the North Eastern Electric Supply Company's 20,000 kVA transformer, switchgear, 250 kW motor-generator and control panels. The original plant consists of two steam-operated, triple expansion Belliss and Morcom direct-coupled generating sets each of 250 kW capacity served by a Babcock and Wilcox boiler at 180 lb. pressure, superheated to 560° F. Two Lancashire boilers, held in reserve, are steamed when the Babcock and Wilcox boiler is down for cleaning. Distribution is by means of twin busbars. One circuit connects the grid and mainly operates the pumping load. The rest of the plant, including chiefly hoisting, air compressor, haulage and the mineral dressing plant, is operated by the Belliss and Morcom generator. Busbar change over is by the usual switch arrangement and normally the circuits cannot be synchronised. The steam plant operates the working shift, and the boiler is then damped down and held in reserve. In the event of a breakdown on the grid system this plant is brought into commission, and experience shows that the pumps can be re-started within one hour of their stoppage.

The dressing plant is some three quarters of a mile from the Frederick shaft and the mineral is tub-conveyed by endless rope haulage. The dressing process consists of initial wedge bar screening. The minus one inch material passes to side discharge jigs having a screen perforation of ½-in. dia. The bulk gravitates to a conveyor picking belt where the first grade lump witherite is recovered and the waste rock discarded. Inferior grade witherite discharges over the belt and is tub-conveyed, together with the mineral passing the screen of the side outlet jig, to a jawbreaker, passed through crushing rolls and revolving trommels of suitable mesh to a series of end-discharge jigs. The fines of the dressing process gravitate to a settling tank and from there are elevated and conveyed to concentrating tables. There is an economic limit to concentration, and the percentage of commercial witherite recovered from the run of the mine mineral varies from 75 to 80 per cent. Regular analyses of the witherite production are made at the mine laboratory and periodical jig sampling is a useful indication of their efficiency.

Witherite appears on the market in the crude form as lumps or smalls, and also in the ground state, and typical analyses of commercial grades produced are given in Table II above.

Witherite is readily decomposed by all mineral acids and many of the organic acids, with the formation of its corresponding salt, but its application in the chemical field is not of interest here. Its carbonate content has a potential value. In the heavy clay industry the presence of soluble sulphates in combination with other chemical products are rendered inert by barium carbonate which converts these salts into practically insoluble barium sulphate. Cement concrete exhibits similar characteristics. All organic salts attack, more or less, concrete made of portland cement, and the introduction of a pre-determined amount of barium carbonate checks the process of disintegration according to the chemical equation CaSO₄ + BaCO₃ = BaSO₄ + CaCO₃

Of other uses the case-hardening of steel might be mentioned. The carburising is effected at a furnace temperature of 900 to 1,100° C. by means of barium carbonate intermixed with powdered wood charcoal. Carbon monoxide is produced by the reducing agency of the hot carbon on the CO₂ evolved from the witherite. The advantage of the process lies in its ability to "regenerate" spontaneously when exposed to the atmosphere due to the absorption of carbon dioxide from the atmosphere by the barium oxide which is formed during the case-hardening process.

Although witherite has a low solubility it is added in suspension to precipitate soluble sulphates in boiler feed water. Any excess above the quantity required settles with the precipitate and is easily removed. A combination of barium carbonate, soda ash and lime is sometimes used. Because of its poisonous nature it is not applicable to domestic water supplies.

I am indebted to my Directors for their kind permission to publish this article.

Drawings and Photographs accompanying the article