Salt (sodium chloride, NaCl), occurs innature in solid form as rock salt (halite), or

in solution as brine. Rock salt occurs in beds,commonly associated with mudstone, rangingfrom a few centimetres up to several hundredmetres in thickness. The purity of individual saltbeds depends on the extent of mudstoneinterbedding. Salt-bearing strata do not cropout at the surface in the UK because of dissolu-tion by groundwater. Natural brine is producedby the dissolution of salt-bearing strata by cir-culating groundwater. Brine is also producedby solution mining by injecting water into saltbeds and pumping out the resulting salt solu-tion. This may contain up to 26% NaCl whenfully saturated.

Demand

Salt is used in solid form as rock salt and, moreimportantly, as brine. Of total salt production inthe UK, approximately 30% is used as rock salt,principally for de-icing roads, although smalltonnages are used as a fertiliser for sugar beetand as an additive to animal feeds. The remain-ing 70% is consumed as brine. Most (70%)brine production is used directly by the heavyinorganic chemicals industry as an essential

basic feedstock. The remainder is evaporatedusing a vacuum process to produce white salt.

As a chemical feedstock in the heavy inorganicchemical industry, salt-in-brine is used in theelectrochemical process for the production ofchlorine and caustic soda (sodium hydroxideNa0H), and in the Ammonia-Soda Process forthe production of soda ash (sodium carbonateNa2C03). A by-product of the electrolysis ofbrine is hydrogen, which is used as a fuel forpower generation and as a process gas.

Chlorine is essential to the world’s chemicalindustry. Up to 60% of all chemical manufactur-ing in Western Europe depends on the element.It is an essential intermediate in the productionof plastics and polymers, such as PVC, nylonand polyurethane, and is used in sewage andindustrial effluent treatment, water disinfectionand in household and industrial bleaches. Awide range of other chlorine derivative prod-ucts is also produced. Caustic soda is used insoap and detergents manufacture, in aluminaproduction and papermaking, but has also awide range of other uses. Soda ash is usedmainly in the manufacture of glass and deter-gents; other uses include industrial chemicals,aerospace alloys, water purification and efflu-ent neutralisation. Calcium chloride liquor is aby-product of the process, which amongstother uses, is used in the formulation of oil welldrilling fluids.

White salt is sold as a chemical feedstock, forfood processing and table use, for water soften-er regeneration, tanning and in the productionof animal feeds.

Salt-bearing strata are ideally suited for the cre-ation of storage cavities for gas and certain flu-ids. Completed brine extraction cavities areused for storage purposes, although cavitieshave also been specifically created for gas stor-age. The high flow capability of salt cavities isideal for peak sharing and daily balancingneeds. On Teesside completed brine cavities areused for the storage of products such as ethyl-ene, ethane and naphtha, as well as natural gasand hydrogen. Salt cavities have been used foroil storage in the past. Completed brine cavitiesare, however, not ideally shaped or spaced

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Mineral Planning Factsheet

Salt

Brine wellhead.

This factsheet pro-vides an overviewof salt supply in theUK. It is one of aseries on economi-cally importantminerals that areextracted in Britainand is primarilyintended to informthe land-use plan-ning process.

January 2006

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apart for gas storage. Gas cavities should bemore spherical or cylindrical with domed roofsand with a grid spacing related to their size(diameter). At the Warmingham Brinefield inCheshire, smaller cavities have been speciallydesigned for gas storage. The brine created isused for salt production, thus maximising theuse of the salt resource. At the HolfordBrinefield in Cheshire, abandoned brine cavitiesare used for ethylene storage, and one is cur-rently used for natural gas. Permission hasrecently been granted for a new gas storagefacility at the Holford Brinefield. The facility willconsist of eight separate underground storagecaverns, with a total capacity of 165 millioncubic metres. At Atwick, near Hornsea in theEast Riding of Yorkshire, cavities were specifical-ly created for the storage of high pressure natu-ral gas at depths of around 1800 m. Current UKgas storage capacity is, by international stan-dards, small. The UK became a net importer ofnatural gas in 2004. As the UK becomes increas-ingly dependent on imported gas there will be arequirement to develop further storage facilitiesto cope with peak demands. If storage facilitiesare created distant from existing brine con-sumers, then any brine generated will need tobe disposed of into the sea. Plans have beenproposed for a new gas storage facility in thePreesall salt deposits below the FleetwoodPeninsula, Lancashire. The planned facilitywould consist of multiple underground cavernscreated by a washing process, in which pump-ing seawater into the salt deposits dissolves thesalt. Potential also exists for developing gasstorage caverns in offshore salt deposits.

Supply

The UK is a large salt producer with an estimat-ed total output of some 5.8 million tonnes in2004, over 95% of which was produced inEngland. The remainder is rock salt mined inNorthern Ireland. Of total UK output, about 70%was extracted as brine and the remaindermined as rock salt. Following the cessation ofbrine pumping on Teesside in 2002, salt is nowonly produced in two areas in England;Cheshire and the North York Moors NationalPark. The Cheshire Basin accounts for over 85%of the total. In the North York Moors NationalPark, rock salt is mined as an ancillary product

at the Boulby Potash Mine (see Factsheet onPotash).

Brine extraction ceased in Lancashire in 1993,following the closure of the chlorine plant atHillhouse in Fleetwood, and also inStaffordshire in 1970 and in Worcestershire in1971 because of subsidence problems. Veryminor quantities of sea salt are produced by theevaporation of seawater at Maldon in Essex.

Output of salt-in-brine, brine (white) salt, whichis produced by the evaporation of brine, androck salt, has not been disclosed for a numberof years because of the limited number of pro-ducers. However, the BGS has produced esti-mates for the United Kingdom MineralsYearbook (Figure 1). Apart from rock salt pro-duction, output has remained relatively staticbut with an overall declining trend. Salt-in-brine production may have been slightly under-estimated in previous years.

Production of rock salt is largely a function ofthe severity of the weather (demand for de-icing salt increases during cold winters) andoutput is thus variable. Demand for white saltis fairly static, if not declining, at about 1 Mt/y.

Trade

The UK is essentially self-sufficient in salt.Historically exports have exceeded imports buttrade is currently roughly in balance, although

Salt

Mineral Planning Factsheet

Figure 1 UK production of salt, 1980–2004.

Source: UK Minerals Yearbook, BGS.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Thousand tonnes

Rock salt

White salt

Salt-in-brine

exports have a higher value (Table 1). Exportsinclude white salt and rock salt.

Consumption

Total UK consumption of salt has declined fromabout 7 million tonnes in 1980 to about 5.5 mil-lion tonnes in 2004.

Economic importance

The total value of salt production in all forms(rock salt, white salt and salt-in-brine) in the UKwas £233 million in 2004 according to officialstatistics. Salt-in-brine is, however, a criticalraw material for the heavy inorganic chemicalsindustry in north-west England. For example,the Runcorn site operated by INEOS Chlor is anintegrated chemical plant largely producingchlorine and caustic soda. The turnover ofINEOS Chlor is some £550 million, a major pro-portion of which is ultimately derived frombrine production in Cheshire. About 1 450 peo-ple are employed at the Runcorn site, but thecompany estimate that 133 000 jobs are indi-rectly supported by the Runcorn site.

The company is currently undertaking a £390million modernisation programme at its

Runcorn site, mainly to replace existing mercu-ry based cells for chlorine manufacture bystate-of-the-art cellrooms, using environmental-ly-friendly membrane technology.

Brunner Mond, which is also critically depend-ent on brine as a basic feedstock for the manu-facture of soda ash, employs 480 people in itsUK operations, which have a turnover of £104million.

Structure of the industry

Two companies produce rock salt in England;Salt Union Ltd, which operates the WinsfordMine at Winsford in Cheshire, and ClevelandPotash Ltd, which produces rock salt as a by-product of potash mining at the Boulby Mine inthe North York Moors National Park. Salt Unionis a wholly owned subsidiary of CompassMinerals International of the USA. In NorthernIreland, the Irish Salt Mining and ExplorationCo. Ltd has operated the Kilroot rock salt Minesince 1965.

Three companies, all based in Cheshire, pro-duce brine. INEOS Chlor Ltd, a privately-ownedgroup, is by far the largest. The companyacquired the ICI Chlor-Chemicals business in2001 and operates the Holford Brinefield atLostock Gralam in Cheshire and formerlyextracted brine at Saltholme on Teesside. Thisoperation ceased in June 2002 with the closureof the Wilton chlorine plant, thus removing theneed for brine. The company produces some3.2 Mt/y of contained salt-in-brine. Brine fromthe Holford field is supplied to the company’sown plant at Runcorn for the electrolytic manu-facture of chlorine and caustic soda. Permittedchlorine production is some 737 000 tonnesand 831 000 tonnes of caustic soda, althoughoutput is less. In addition, brine is also suppliedto Brunner Mond (UK) Ltd at Lostock andWinnington for the manufacture of soda ash(sodium carbonate) of which the company isthe UK’s sole manufacturer and Europe’s sec-ond largest producer. INEOS Chlor also sup-plies brine to the Salt Union’s Western Pointplant at Runcorn for the manufacture of whitesalt. INEOS announced in late 2005 its intentionto purchase the Salt Union’s vacuum salt busi-ness.

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Mineral Planning Factsheet

Exports Imports

Tonnes £000 Tonnes £000

1996 47 154 21 419 316 796 13 010

1997 284 571 18 639 242 368 11 796

1998 485 815 18 274 237 284 11 807

1999 276 402 22 125 261 434 11 573

2000 307 899 16 548 201 400(a) na

2001 299 607 17 466 234 900(a) na

2002 326 760 20 135 306 488 12 870

2003 537 497 23 202 217 009 10 928

2004 690 990 26 634 219 437 13 713

Table 1 Imports and exports of salt,

1996–2004. Source: HM Revenue & Customs.(a) BGS estimate.

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Brine is also produced by British Salt, a sub-sidiary of US Salt Holdings, at the WarminghamBrinefield in Cheshire for use in the manufactureof white salt at its Middlewich plant. The NewCheshire Saltworks Ltd is a very small producerof white salt at Wincham, near Northwich.

The salt supply chain is summarised in Figure 2.

Resources

The UK has huge resources of salt, whichmainly occur in England, with only limitedresources in Northern Ireland. Salt-bearingstrata of Permian and Triassic age underlieextensive areas (Figure 3). Resources ofTriassic age are economically the most impor-tant and account for some 90% of total produc-tion, most of which is derived from the

Cheshire Basin. The deposits in NorthernIreland are of Triassic age. Permian depositsare only worked at the Boulby Mine.

Triassic

The most important salt resources in Englandoccur within the Triassic Mercia MudstoneGroup, which has a widespread outcrop.However, salt-bearing strata generally onlyoccur where the Mercia Mudstone thickens inmajor depositional basins. The most importantof these, and the source of some 90% of totalsalt output, is the Cheshire Basin, which alsoextends into north Shropshire.

There are two salt-bearing formations in theCheshire Basin, a lower Northwich HaliteFormation and an upper Wilkesley Halite S

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Mineral Planning Factsheet

Chlorine Hydrogen

Caustic Soda (sodium hydroxide)

White salt 1 Mt

Soda Ash (sodium

carbonate)

Brine Electrolysis

1.8 Mt

Ammonia Soda Process 1 Mt

Evaporation

Underground mining 2.0 Mt (NaCl)

Salt Union

Cleveland Potash Irish Salt Mining and

Explooration

INEOS Chlor British Salt

Neww Cheshiree Saltworks

Waste 60,000 t CaSO4,CaCO3,

Mg(OH)2

Solution mining 3.8 Mt (NaCl)

Brine Purification

Rock salt: de-icing roads

SALT 5.8 Mt

Figure 2 Salt supply chain, 2004.

Triassic saltfieldsPermian:Extent of Sneaton, Boulbyand Fordon Evaporites

BrinefieldRock Salt Mine

Figure 3 Distribution of salt-bearing strata and

principle producing sites.

Formation. Production is entirely confined tothe former. The maximum known thickness ofthe formation is some 280 m and the saltoccurs in beds that are virtually pure halite andin others where there are varying amounts ofmudstone and siltstone. It has been estimatedthat some 25% of the formation consists ofmudstone. The Wilkesley Halite Formation iseven thicker and has a known thickness ofsome 405 m. The upper half of the WilkesleyHalite Formation is somewhat purer than theNorthwich Halite.

Triassic saltfields have also been worked in thepast at Preesall in Lancashire, inWorcestershire, Staffordshire, on Walney Islandin Cumbria and in Somerset. Extensive areas ofsalt-bearing strata also underlie Dorset. It ishighly unlikely that any of these deposits willbecome of commercial interest as a source ofsalt in the foreseeable future. However, wherethick (>100 m), relatively pure beds of salt occurthey may be of interest for creating cavities forstorage purposes.

In Northern Ireland salt resources within theCarnduff Halite in the Triassic Mercia MudstoneGroup underlie the area between Carrickfergusand Larne (in south Co. Antrim), and has beenworked for over 100 years. The Kilroot Mineoperated by the Irish Salt Mining andExploration Company Ltd has been workingthis resource since 1965. 15 km to the north ofKilroot a drillhole intersected 400 m of Triassichalite in three seam groups as well as 113 m ofhalite in the older, deeper Permian UpperMarls. Significant salt resources have not beenidentified in any other part of Northern Irelandto date.

Permian

Salt-bearing strata of Permian age extends atdepth from Teesside beneath much of eastYorkshire and into north Lincolnshire.Deposits occur at several horizons, the mostextensive being the Boulby Halite, which isalso the only UK Permian salt of current eco-nomic importance. It was exploited by brinepumping on Teesside until 2002 and is minedat the Boulby Potash Mine. Thick salt depositsalso occur lower in the Permian sequence

within the Fordon Evaporites. At Hornsea ineast Yorkshire these deposits have been usedto create cavities some 100 m high and 100 mwide at depths of between 1710 m and 1840 mfor use in natural gas storage. A stratigraphi-cally higher salt horizon, the Sneaton Halite,occurs above the Boulby Halite but is lessextensive.

Reserves

A figure for total permitted reserves of salt isnot available. At the Holford and Warminghambrinefields in Cheshire there are sufficientreserves with planning permission until at least2042, when the current consents expire.However, new cavities have to be created tosustain brine production. At Holford no newcavities have been created since 1982 and thereis now a requirement for a phased develop-ment, particularly as they take several years toproduce saturated brine. Future developmentsare likely to be linked with the development ofcavities for natural gas storage. The sinking ofnew boreholes and the associated infrastruc-ture requires planning agreement.

Proved reserves of rock salt at the WinsfordMine are sufficient for 70 years, although thecurrent planning consent expires in 2011. Rocksalt reserves at the Boulby Mine are dependenton potash reserves.

In the Kilroot area of Northern Ireland approxi-mately 10 years of reserves remain, at the cur-rent rate of production.

Relationship to environmental designations

The Boulby Potash Mine is located in the NorthYork Moors National Park. Elsewhere opera-tions are not associated with any major envi-ronmental designations.

Extraction and processing

Salt-bearing strata do not crop out at the sur-face, because of dissolution by groundwater,and are absent to depths of about 70 m. Theboundary at which solution is taking place iscalled the ‘wet rock-head’, and the overlyingcollapsed strata may lead to possible subsi-

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dence at the surface. Where salt-bearing strataare too deep to be affected by groundwater cir-culation, the normal contact between the saltand overlying rock is known as the ‘dry rock-head.’

Natural brine springs have been exploited atleast since Roman times. Brine was boiled inopen pans to produce salt. In 1670 rock salt wasdiscovered at depth in Cheshire and this led toconsiderable commercial exploitation both bymining and drilling to the wet rock-head topump natural or ‘wild’ brine. Shallow minessubsequently became flooded and pumping ofthe resultant brine caused the solution of roofpillars leading to catastrophic subsidence anddamage at the surface. Natural brine pumpingalso led to unpredictable subsidence some kilo-metres from the point of extraction. Damagecaused by this method of extraction led to thecessation of salt extraction in Worcestershireand Staffordshire. Remedial work to infill andstabilise the flooded salt mines beneathNorthwich has started.

In Northern Ireland the legacy of uncontrolledbrining operations has had a significant effecton the Carrickfergus area. To date there havebeen three large collapses, with a fourth pre-dicted in the short to medium term.

Both underground mining and solution miningare used to extract salt. Rock salt mining isundertaken at three locations in the UK, at theWinsford Mine in Cheshire, the Boulby Mine inthe North York Moors National Park and theKilroot Mine in Northern Ireland.

Salt mining at the Winsford Mine began in 1844,but the mine was closed between 1892 and1928. Since 1928 it has been the major source ofrock salt in the UK. Mine capacity is about 2.25Mt/y, but averages about 0.9 Mt/y. Extraction isby room and pillar mining and is currently fromthe Bottom Bed of the Northwich HaliteFormation, at a depth of about 140 m. The salt isextracted from galleries 8 m high and 20 mwide. Pillars are 20 m x 20 m giving an extrac-tion rate of 75%. Formerly drill and blast meth-ods were used for salt extraction. A continuousmining machine was, however, introduced in2002, which is used to extract the top lift of 4.5

m, with either bench blasting or the continuousminer being used for the bottom 3.5 m. The rocksalt is crushed to either -6 mm or -10 mmunderground and treated with an anti-cakingagent to keep it free flowing. Rock salt miningproduces no waste.

The mine is dry and stable; room and pillarmining does not create any surface subsidence.However, in 1968 the intersection of a boreholecaused serious flooding. Protection barriers of75 m are now left around boreholes. The saltcontains about 92% NaCl and the presence ofsome mudstone provides a protective coatingto outside stockpiles and prevents dissolution.

At the Kilroot Mine in Northern Ireland the saltbeds vary in thickness from 9 to 27 m andoccur at five separate levels (all of which havebeen worked). Access is via a decline and min-ing is by the room-and-pillar method, output isup to 0.5 Mt/y. At the Boulby Mine, rock salt isa by-product of potash mining (see factsheeton Potash).

Almost all solution mining is now by controlledbrine pumping. The method was introduced byICI in the 1920s and involves the creation of sta-ble cavities in suitable salt strata by the intro-duction of water under carefully controlled con-ditions, thus preventing subsidence. Theprocess recovers up to about 25% of the totalsalt reserve. Brine is extracted from cavities upto 145 m in diameter and up to 200 m in height.The size and shape of the cavities are designedto maintain the stability of the overlying strataand so avoid surface subsidence. Each cavity isdeveloped through a single borehole with atriple tube system. Water is pumped into thecavity and brine is continuously displacedthrough the centre tube. Once the cavity hasenlarged sufficiently, usually after a couple ofyears, this process produces saturated brine,containing 26% NaCl. The position of the waterinjection tube and the depth of a compressedair blanket, which is used to prevent upwarddevelopment, control the area of salt dissolu-tion. By changing the position of these duringdevelopment, the final size and shape of thecavity can be controlled. Cavities are developedfrom the base upwards and during develop-ment their size and shape is monitored by S

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Mineral Planning Factsheet

sonar techniques. Insoluble mudstone falls tothe bottom of the cavity. The brine wells arelaid out on a regular grid with new wells beingdrilled some 200 m apart. Completed solutioncavities are left full of saturated brine, althoughsome are used for both waste disposal andstorage purposes. Controlled brine pumpingtakes place at the Holford and Warminghambrinefields from the Northwich HaliteFormation, at depths of over 250 m. At Holfordup to 50 cavities are currently being used forbrine extraction.

Only very minor quantities of natural brine arenow produced at Wincham, near Northwich.

The brine produced by solution mining requirespurification before it can be used either as achemical feedstock or in the production of whitesalt. The purification process involves precipitat-ing calcium sulphate, calcium carbonate andmagnesium hydroxide, and these insolublewastes are disposed of into worked out salt cav-ities. The waste is subject to the landfill tax atthe lower rate of £2/t applying to inactive orinert waste. Disposal of waste in this way isbelieved to be the best environmental option.Total production of these wastes is estimated tobe of the order of 60 000 t/y.

By-products

There are no currently useable by-products inthe strict sense, although the use of the wastesfrom brine purification have been looked atperiodically and warrant further study.However, salt extraction both by conventionaland solution mining creates large, stable voidsthat are themselves important economic assetsboth for storage purposes and waste disposal.The Winsford Mine, with some 26 million m3 ofspace, has a constant temperature and humidi-ty and is dry and gas-free. Part of the mine iscurrently being used for secure document stor-age. A proposal to use part of the mine for thepermanent storage of hazardous wastes wasgranted planning permission in December 2003and waste disposal commenced in 2005. Strictcriteria will be used for the type of materialstored, which will be dry waste that is non-flammable, non-biodegradable and non-radioactive.

Alternatives/recycling

The chemical uses of salt are directly related toits composition and it is unlikely that any othersource of sodium and chlorine ions could beused as an alternative in the UK. Other materi-als (e.g. urea) have been used for de-icingroads in special circumstances, such as onbridges to avoid corrosion, but on costgrounds, rock salt is unlikely to be replaced forroad treatment.

Salt is valued for its chemical properties and isthus consumed in use. Some recycling of theproducts derived from salt takes place. Forexample, soda ash is an essential componentin the manufacture of soda-lime-silica glass,the most common glass composition.Recycling glass thus also recycles soda (Na2O)in addition to silica and lime.

Transport issues

Brine from both the Holford and Warminghambrinefields is supplied to downstream process-ing operations entirely by pipeline. Rock salt isdelivered by road from the Winsford Mine,although a small proportion (5%) will be subse-quently transferred to rail for movement toScotland. At the Boulby Potash Mine rock salt isremoved from the site by rail for onward trans-fer by lorry and ship from Tees Dock. Rock saltfrom the Kilroot Mine in Northern Ireland isexported by sea, including to the USA.

Planning issues

Modern methods of underground solution androck salt mining do not cause subsidence andraise only modest planning issues at surface.Pumping of natural brine is unlikely to be per-mitted in future, although some old permis-sions are still in operation and there is still alegacy of subsidence from historic working bythis method.

It is likely that in the future, the major planningissue will not be the extraction of salt itself, butthe subsequent use of the void created forwaste disposal and for storage purposes. Ofparticular interest is the scale of the impact ofthese additional operations at surface.

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Underground gas storage requires consider-able surface infrastructure including pipelines,water pumping stations and gas compressorsall of which have an environmental impact. The‘need’ argument will centre around the nationalrequirement for storage of fuel gas/liquidhydrocarbons or safe disposal of hazardouswaste, rather than the demand for brine or rocksalt. The principle issues likely to be consideredin any planning inquiry include the need for thedevelopment, the sustainable use of mineralresources, the impact on the environment ofthe area, particularly the freshwater and marineecology, the suitability of the geology toaccommodate the proposed underground cav-erns, safety and security, ground stability, visu-al and landscape impacts and the effects ontourism and economic development.

Authorship and acknowledgements

This factsheet was produced by the BritishGeological Survey for the Office of the DeputyPrime Minster to support the research project‘Review of Planning Issues Relevant to Some

Non-Energy Minerals other than Aggregates, inEngland’ (2004). It was updated under theresearch project ‘ODPM-BGS Joint MineralsProgramme’.

It was compiled by David Highley, AndrewBloodworth (British Geological Survey) andRichard Bate (Green Balance Planning andEnvironmental Services), with the assistance ofDon Cameron, Fiona McEvoy, Paul Lusty andDeborah Rayner (BGS).

The invaluable advice and assistance of theminerals industry in the preparation of this fact-sheet is gratefully acknowledged.

Mineral Planning Factsheets for a range ofother minerals produced in England are avail-able for download from www.mineralsUK.com

© Crown Copyright 2006.

Unless otherwise stated all illustrations andphotos used in this factsheet are BGS©NERC.All rights reserved.

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Mineral Planning Factsheet