Site Records


Site Name: Boulby Potash Mine - a site visit

Boulby
Redcar & Cleveland
TS13 4UZ
OS Grid Ref: NZ7618

Sub Brit site visit 26th October 2007

[Source: Paul Sowan]

LOCATION

The mine is at Boulby is a kilometre inland from the Yorkshire coast on the A174 road between Saltburn-by-the-Sea (10 kilometres) and Whitby (16 kilometres.) The coast here is dramatic, the cliffline being punctuated by deep gorges conveying streams to the sea. Boulby Head is, at 203 metres, said to be the highest sea-cliff in England. There is much of interest to geologists, and there is a published excursion guide (Rawson, 1992) which includes details, inter alia, of the Staithes to Port Mulgrave

section. There are numerous relics of former industries including ironstone mining, jet mining, and sites where alum was once manufactured from shale. The Tom Leonard Mining Museum is at an old ironstone mining adit at Skinningrove nearby. This seems to be a most promising area for a future Subterranea Britannica Study Weekend.

THE WHITBY REDCAR & MIDDLESBROUGH UNION RAILWAY
The mine is served by a freight terminus on what remains of the former Whitby, Redcar & Middlesbrough Union Railway. This line, with a very scenic coastal route, was authorised by Act of Parliament in 1866 but not completed and opened until 1883. The stretch from Middlesbrough via Redcar to Saltburn-by-the-Sea remains open to passengers, although the remarkably handsome station building at Saltburn is now occupied by commercial concerns, trains now stopping some metres short of it at a single platform with a ‘bus shelter’ style structure. Beyond this the line was closed to passenger services in May 1958, but re-built and re-opened as far as Boulby to serve the mine in 1974.

Photo: After closure of the line to Skinningrove the embankment and bridge over Carlin How Bank were removed so that the road could be realigned. When the line to Boulby Mine was reinstated in 1973, a new viaduct had to be built to 'bridge the gap'. The old road can be seen on the left.
Photo by Ken Mell

This freight-only section runs from a junction just short of the Saltburn terminus via Loftus and Brotton (where the Corus steelworks has a terminal), thence the 992-yards Grinkle tunnel (NZ 7417) to Boulby. At one point the line is extremely close to the cliff top edge. Beyond Boulby the line is closed, but ran via Staithes and the Kettleness (NZ 8315) and Sandsend (NZ 8513) tunnels, and Sandsend to Whitby. Several imposing iron viaducts, such as that at Staithes, have been demolished. Potash trains commenced running from Boulby to Tees Dock near Billingham in May 1974.

GEOLOGY
The Yorkshire coast is a classic locality for geological exposures and fine scenery. The geology of the region has been described by Vernon Wilson (1948), Dorothy Rayner and J.E. Hemingway (1974), Sir Peter Kent and others (1980), and Peter Rawson and John Wright (1992), and related to the overall context of English geology by Duff and Smith (1992.) Details specifically on the beds of Permian age (containing the potash) have been published by Smith (1974b)

Middle and Lower Jurassic beds (including the ironstones formerly mined in this area) outcrop southwards from Redcar, overlying Triassic and Permian strata at depth. The thickest evaporite beds are in the Upper Permian, the generalised succession being as follows:

Upper Permian Beds Composition

Carnallitic Marl

Marl containing magnesium potassium chloride

Potash Beds

Up to 45% sylvinite (potassium chloride) – the potash mine panels are excavated in this bed

Rock-salt Beds

Up to 95% sodium chloride – the access roadways are driven in this bed

Billingham Main Anhydrite

Anhydrous and hydrated calcium sulphate – this bed is not exploited

The evaporites mined here were deposited near the western shore of the former landlocked Zechstein Sea, and extend offshore most of the way eastwards to mainland Europe.  The former sea occupied much of the present North Sea basin, extending to the Baltic, and to Germany and Poland where rock-salt and potash are also mined. The soluble minerals in this ancient sea-water crystallised, in order of increasing solubility, as the ancient sea dried up and shrank.  A smaller-scale version of the same process now occurs at the Dead Sea.

The commonest salts deposited were firstly anhydrite (calcium sulphate) and gypsum (calcium sulphate 2-hydrate); then fairly pure halite or rock salt (sodium chloride); and finally potash or sylvinite (potassium chloride) mixed with halite and some clay.  The high value potash is the object of the mine, although for reasons explained below rock salt has also to be mined to reach it.  At this locality, it would be uneconomic to mine the underlying anhydrite or gypsum: these more widespread minerals are extracted from shallower mines in the Derbyshire / Nottinghamshire, East Sussex, and Vale of Eden (Cumbria) . 

Sivinite
areas. Other salts also occur but are of no economic interest, including carnallite (a hydrated double salt of magnesium and potassium chlorides) and polyhalite (a hydrated triple salt of calcium, magnesium and potassium sulphates.)  Another mineral species present, somewhat problematic, is boracite, a magnesium borate.

The evaporite beds, progressively thicker, extend out below the bed of the North Sea.  In the opposite direction, progressively thinner, they extend inland for some distance, but (as the evaporites are by definition soluble in water) they are replaced at outcrop by breccias resulting from overlying beds collapsing as the salts were dissolved away.  Much detail concerning the evaporites (Smith, 1974a) has been published.

The Permian System (named by the British geologist R.I. Murchison in 1841, from his work in the Perm Region of Russia) extend from about 280 to 225 million years before the present.  The landlocked Zechstein Sea was drying up, leaving salt flats, towards the end of that period.

HISTORY OF MINING IN THE DISTRICT

Lumpsy Pit, Brotton 1920 - From The Key magazine
Historically, the earliest mines in the district were probably the rabbit-burrow-like mines made in searching for jet (a hard polishable anthracite which could be carved into ornaments.)  This was especially popular in Victorian times for costume jewellry, particularly at funerals.  There were numerous workshops in Whitby devoted to this trade.  Alum (hydrated aluminium potassium sulphate) was also manufactured here, from shale dug at the surface.  Both the alum shales and the jet were derived from the Upper Lias (Lower Jurassic) rocks outcropping in coastal cliffs.

On a more industrial scale, there were in the Cleveland Hills and along the coast numerous drift mines for ironstone.  Port Mulgrave, south of Boulby, was built in the 1850s to handle the ironstone trade.  The Cleveland ironstone mines were producing some two million tons per annum by 1939.  Some of these mines are reputedly still accessible, although problems with bad air (carbon dioxide) have been reported.

A little coal was once mined at Danby in Eskdale. Investigations (Wilson, 1948) before World War II, primarily to locate possible oil reserves in Britain, led to boreholes proving the existence of Upper Permian beds containing bedded evaporite minerals (principally halite, gypsum, and anhydrite) at considerable depth below the Cleveland Hills.


Last shift at North Skelton ironstone mine 17.1.1964 - From The Key
magazine

Billingham Anhydrite Mine during a recent inspection
The first really deep shaft mines were to exploit the seven-metres thick anhydrite at the Billingham mine, worked on the pillar-and-stall system from 1930 to 1971 when the mine closed.  It has also been mined at the Warren Cement Works mine at Hartlepool.  The mineral was used in the manufacture of sulphuric acid, and of ammonium sulphate fertiliser.  A shallow pillar-and-stall mine for gypsum was operated for some time at Sherburn-in-Elmet.

Salt was extracted by the evaporation of sea-water (on sunny days!) on the tidal flats of the Tees and nearby from very early times (as indeed suggested by place-names such as Saltburn.)  Rock-salt was first detected in a deep borehole at Middlesbrough in 1862, with production starting at Port Clarence (north of the Tees) in 1874, and at Middlesbrough, North Ormesby and Eston at various dates from 1886 – 89.  Production at these three places peaked at about 63,130 tonnes in 1894, but had declined to about 21,000 tonnes by 1918, and has subsequently ceased.  Uncontrolled solution mining of common salt, predictably, gave rise to some ground stability problems.  It was followed by more controlled brine-pumping designed to leave carefully planned and spaced solution cavities, some of which have been used subsequently for the storage of natural gas.  By 1974 solution mining was restricted to the coastal flats some four kilometres north of Middlesbrough.  It was the common salt, however extracted, that led to the concentration of heavy chemical industries in the Middlesbrough – Billingham area, where amongst other materials chlorine, caustic soda (sodium hydroxide) and sodium metal were manufactured, not to mention numerous derivatives.

HISTORY OF POTASH MINING
Potash was first discovered locally in 1939 during the drilling of a bore in search of oil at Aislaby, near Whitby.  Subsequent exploration from about 1948 - 1955 demonstrated substantial reserves at depths of 1,100 to 1,200 metres, but these investigations ceased as it was assumed that the material could not be exploited economically at such depths.  It was established that the potash beds were somewhat variable, containing from 26% up to 45% of potassium chloride.  From 1962 consideration was given to recovering the potash by solution mining – pumping water down to dissolve the material, then pumping the solution back to the surface.  This option was not adopted.

The first deep shafts were sunk at Boulby by Imperial Chemical Industries in 1968 / 69, with potash production commencing in 1973 / 74. The beds, up to seven metres thick, were mined initially by pillar-and-stall mining. Production at the outset was of the order of 7,100 tonnes per week, rising by 1976 to around 1,000,000 tonnes per annum.

Subsequently the mine has been worked by Anglo-American, and by Tarmac, but is now operated by British Potash Ltd, a subsidiary of Israel Chemicals

Ltd, which concern also has deep mines near Barcelona in Spain.  The mining method now in use is described below

A general overview of potash mining and technology has been provide by Garrett (1996.)

THE MINE SURFACE BUILDINGS AND SITE
The mine buildings stand in open farmland, and straddle the sites of two earlier, and much shallower, ironstone mines.  The buildings comprise a winding-house; two shaft-tops; extensive processing plant sheds with a tall chimney discharging ‘steam’ from the drying plant; and separate loading stations where potash and rock-salt are separately discharged to rail wagons for transfer to Tees Dock, where much is exported by sea.  There are also numerous support buildings. Some stockpiled rock-salt is stored on site, as the material is applied almost exclusively for de-salting icy roads in winter.

Photo:Boulby Mine
Photo by Nick Catford

The mine is worked in three shifts, 24 hours a day, seven days a week.  Of the 930 staff, 620 (including two women) work underground.  Women also work in the separately administered Dark Matter Laboratory.  Workers’ ages range from 25 to over 60.

The mined area extends at present over 96 square kilometres, the greater part of it northwards below the bed of the North Sea.

Click here for further information and pictures of Boulby Mine

[Source: Paul Sowan]

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