Ready salted – salt mining

Materials World magazine
,
1 Aug 2009

Salt mining is big business in Cheshire, UK. Michael Forrest spoke to Steve Reece, Chief Mining Engineer and Operations Manager at Compass Minerals UK, to find out more.

During the Triassic period, 251 to 199 million years ago, most of what is now England and Wales was located in the centre of the supercontinent Pangea. As with most large continents in tropical latitudes, the centre was a desert whose sands now form a geological horizon, the New Red Sandstone (as opposed to the Devonian-aged Old Red Sandstone found in Scotland).

During this time, the continent began to break up as tensional faults led to the formation of a number of flooded basins resulting from these movements and sea level changes. Evaporation of sea water from these basins left considerable salt deposits in the UK that can be mapped from the Bristol area through Cheshire, Lancashire and Northern Ireland. In Cheshire, salt springs were known to the Romans, and later Medieval ‘wich houses’ were centred on brine pools. Salt, a necessity for diet and food preservation, was recovered in open hearth fired pans which evaporated the brine.

This historic mining industry continues at Winsford, south of Northwich. Here, the Triassic sequence contains two thick salt beds separated by marl, with the lower bed the focus of present-day mining. The upper horizons were worked mainly in the 1970s and 1980s. This is not to say that mining at Winsford is recent. It is the oldest operating mine in the UK, with underground working dating back to 1844 when the first shafts were sunk.

Today the workings extend over an area of five kilometres by three kilometres, bounded by down-throw faults to the west (100m) and east (230m). The lower mined strata have an average thickness of 25m and are covered by impervious Keuper marl. The salt beds are relatively flat lying, although the margins have shallow monoclinal folds that require careful monitoring.

Pillars of salt

One of the most striking features of the mine is the large size of the excavations, both past and present. Up to 2002, the principal mining method was drill and blast and, in many of the older ‘rooms’, the fan of the blast holes can be seen in the rock face. The extraction is room and pillar, whereby a vertical pillar is left during the mining operation to support the roof.

Historically, the extract rate was in some areas 93%, leaving only seven per cent as support. In a number of other mines this resulted in collapse and the ingress of water, an event that was positively regarded by some as it provided brine-rich water to be pumped from the mine. Fortunately this did not happen at Winsford. Today 75% of the salt formation is mined and the remainder is left as support, reducing to 65-68% in deeper parts of the mine.

No support equipment, such as rock bolts, is used in the mine apart from some artificial strengthening around the base of one of the original vertical access shafts, although there are many monitoring points measuring every possible movement. ‘The stability of the mine is remarkable,’ says Steve Reece, Chief Mining Engineer and Operations Manager at Compass Minerals UK.

‘Even in the monoclinal folds no fractures can be detected, and it is thought that the salt “flows” – as witnessed in oil formation salt diapirs – take up any movement caused by void creation during mining. The lack of movement, and the wide spans that can be cut, does not rely on calculation but on empirical research. Rooms cut over 100 years ago show no sign of movement within the rock.’

Market matters

This is in contrast to historic solution mining that took place in the area. Here, water pumped from the salt formations caused unstable cavities that caused many surface buildings to collapse. Too many companies mining resulted in overproduction and a chaotic market provided salt to the open hearth evaporating pans for use mainly in food preservation and later in the chemical industry of the Runcorn area. The Salt Union, formed in 1888 to regulate the market, resulted in the closure of the Winsford mine, only for it to reopen in 1927 after the flooding of the nearby Northwich mines.

Current mining at Winsford is mainly carried out by a continuous miner that cuts tunnels up to 20m wide in multiple passes. Guided by an operator, the remote-controlled machine can rapidly cut rooms and loads directly onto the conveyor system or into large front end loaders that dump to surge piles.

Reece explains, ‘The salt is conveyed to crushers located at the bottom of the shaft for the production of a sized product of six millimetres or, exceptionally, 10mm, before it is hoisted to the surface. Lasers are used to direct the mining operation and align the 42” conveyor to the working areas. The rock salt is homogeneous and careful mine surveying and drilling is required to ensure the continuous miner keeps within the strata, as there are no obvious marker horizons. The lower horizons are of a higher purity than the upper levels and are clear to pink in colour, thought to be the effect of desert dust into the evaporating seawater basins’.

Seasonal demand

All of the production at the Winsford mine is used for de-icing. The market for road de-icing began in the 1950s when ICI owned and operated the mine. The post-war growth of road transport, and especially the motorway network from 1960 onwards, created a demand for de-icing salt.

‘The trade is seasonal, and also cyclical, with major demand in winter followed by low demand in the early spring/summer months,’ notes Reece. ‘During the first half of the year, we replenish the minesite stocks while the second half marks the replenishment of the local authority and other customer stockpiles. This past winter saw a number of authorities with dangerously low stocks due to prolonged winter weather and we operated 24/7 to meet demand. Production at the mine averages around one million tonnes per year, but balancing that against demand, stockpiling requirements, and mine expansion is difficult.’

The scale of underground workings is difficult to imagine. There are some 220km of roadways and millions of cubic metres of mined-out rooms. Ventilation fans are located at the 180m level, pressurising the air and forcing it around the workings. A series of roller doors and curtains maintains the correct airflow as it takes over eight hours for complete circulation.

The temperature underground is a constant 14ºC that is maintained by the mass of rock salt. The air is very dry, with any air moisture absorbed by the salt. These conditions, combined with space and security, make the mine ideal for storage.

In association with Veolia Environmental Services UK, part of a multinational group, the mine is now a major repository for secure waste disposal and has developed a significant records management business called DeepStore which stores mainly paper records for a range of clients. Reece says, ‘The pre-eminent criteria is the underground climate maintained by geology. No warehouse can match the mine conditions - even if we lost all power, nothing would change. The second is security, with access only by controlled shafts’.

The Winsford mine now offers a document storage and retrieval system. ‘We have a nightly trunk service to London that facilitates a 24 hour service provided by 80 employees who work underground,’ explains Reece.