Sunday, 25 October 2015

Lime Kilns in Derbyshire

Whilst having a walk along the Monsal Trail in Derbyshire NSB was gobsmacked to see huge concrete structures built into the hillsides...

huge concrete structures built into the hillside, nr Buxton

A helpful sign explained that they were lime kilns that operated from 1880 until 1944 and could convert quarried limestone into 50tonnes of quicklime a day for use in the steel, chemical and agricultural sectors. Quicklime production was (and to a degree still is) a significant industry in the Peak District. The concrete buttresses were added in the 1920s.

A helpful sign

But let's step back a bit and take some time to look at the chemistry and technology behind this process.

Limestone (as well as coral, sea shells and chalk) are all forms of calcium carbonate (CaCO3). Some 300million years ago, in the Carboniferous period, Derbyshire was covered by a warm shallow sea and it was the shells of animals that died and fell to the bottom of the sea that later formed the areas limestone rocks.

Limestone can be converted to Calcium Oxide (CaO), also known as "quicklime", "lime" and "burnt Lime", by heating to 1100-1300C where it undergoes the following reaction:

CaCO3 + heat → CaO + CO2

Generally, the limestone is ground down to the size of pebbles before being heated.The resulting lime is a white solid that has a melting point of 2572°C. The reaction is reversible, however, and to slow this down the lime is often "slacked" by reacting it with water, either partially to form a dry powder, or with an excess of water to produce a putty. Slacked lime has the formula Ca(OH)2.

An extremely helpful video from the "BBC Edwardian Farm" explains the process of lime production in kilns similar to those by the Monsal trail. In particular, it gives some context for the discharge of the final lime product:

Checking that the limestone has converted to lime by its fizzy reaction with water

Edwardian PPE, to protect against the caustic lime

Removing the lime from the bottom of the kiln

Sieving the discharged lime to remove the fine ash and spoil

Lime and slacked lime have a number of important uses, including:

1) As a mortar (lime mortar) that was historically used in bricklaying (now replaced by Portland cement) - this application relies on the lime reverting back to limestone to form the bond.

2) In the formation of soda lime glass when mixed and heated with silicates such as silica sand and soda ash (itself also produced using limestone).

3) In the steel industry, again using limes ability to form high temperature molten solutions with silicates. Lime is added to the ore during melting. The lime forms a solution with silicate impurities in the molten ore. This solution floats on top of the molten iron and is removed. You may know this solution by its common name of "slag".

Whilst the kilns by the Monsal trial had to close due to land instabilities, there were many other kilns in the area and lime production continues as sites such as the Tunstead Quarry, also near Buxton, and one of the largest limestone quarries in Europe, capable of producing up to 6million tons of limestone per year.

Tunstead Quarry from the ground

Tunstead Quarry from the air

The site, has a state-of-the-art Maerz kiln, installed in 2010, and costing some £14million. The washed, high purity stone is then transferred by conveyors to the lime kilns. The limestone is burnt at very high temperatures (1,100- 1,300°C) to induce a chemical reaction that produces quicklime (calcium oxide).

Maerz kiln at Tunstead, under construction, 2009(Copyright Richard Law)

Some fascinating historical images of the Tunstead quarry here Tunstead also has cement kilns, you can find out more about these at the winningly named site and also at

Image Sources
Old kiln and Notice - BFTF own
Calcium Oxide Tunstead Quarry (and here)
Maerz kiln , under this CC licence

Related NSB posts
From the Tate modern to Lanarkshire
Coal Mining in Nottingham
Train Manufacture in Derby
Mining Memories
History of Coal Mining in the East Midlands
Rolls Royce UTC open day

No comments:

Post a Comment