Limestone: the Tata Shapfell kilns

The Tata Shapfell limekilns

This blogpost is part of my ‘limestone lockdown’ project. For an Introduction to the project, and a guide to the growing list of related posts, see Limestone in the Lake District: an Introduction – and the ‘categories’ list in the right-hand bar.

The motorway sweeps down past the smooth rounded ‘sleeping elephants’ of the Howgill fells, down into the valley by Tebay, and then up again onto the moorland heights of Shap. Suddenly, incongruously, you see a tall vertical array of cylinders and truncated cones, pale greyish-green, streaked with rust, with steam drifting from the chimneys. You pass so quickly that there is barely time to register how bizarrely out of place is this industrial machinery amongst the grazing sheep and the moorland of the distant hills.

The imposing structure is a bank of four kilns, for ‘burning’ limestone to produce quicklime, and they are the remaining active processes of the once-busy Shapfells or Hardendale quarry. These metal Maerz kilns couldn’t be more different from the stone-built kilns at Wardhall, not only in their construction but also in the very efficient way they ‘burn’ – calcine – the lime.

Calcining (a quick reminder) is the process of heating limestone, calcium carbonate CaCO3, to change its chemical composition by driving off carbon dioxide CO2, as gas, to leave ‘quicklime’, calcium oxide CaO. This requires temperatures in excess of 800oC which, in ‘old-fashioned’ limekilns, are produced by burning coal in the presence of air. In the Maerz kilns, heat comes from the burning of natural gas which is injected into the upper cylinder.

John Baird, the Operations Manager, is infectiously enthusiastic about the kilns. He has worked on the site for more than 30 years, “in most roles, in the aggregate plant … drove dumpers …” and has overseen great changes, including the closure and now restoration of the quarry area as a nature reserve. About three years ago, a new control and monitoring system was put in place for the kilns themselves –  the SCADA system (Supervisory Control and Data Acquisition) which is a system of networked computers and monitors, where a single operator can see – and control – what is happening in the kilns in real time. We go into an office to watch a short powerpoint presentation about how the kilns work, and John switches into the SCADA system – and I’m impressed how we can watch a live overview with coloured graphs and flows, and then focus in on minutiae such as the changing pressure of the gas from minute to minute, the temperature, the weight of stone in a hopper that’s feeding a particular kiln, and so on. The operator, watching this, can dictate the length of burn, the amount of air, the intensity of the flame, and more. Later we have a look at the control room, where there is indeed just a single operator, sitting at a bank of keyboards in front of an array of monitors.

Some basic details about the Maerz kilns at Shap: Each active kiln uses up to 520 tonnes of limestone per day, and each tonne of stone gives about a half-tonne of lime (quicklime). Stone, about 4.5 tonnes per hopper, is loaded every 11 mins into each of the parallel shafts continuously throughout the day, and lime is continuously taken out of the bottom. The kilns are Parallel Flow Regenerative, PFR, kilns – three of the kilns have two parallel shafts, and No.1 kiln (the first on the left as you head North on the motorway) has 3 shafts.

Maerz PFR kilns: see their website for more details

Burning is carried out at 850-1100oC, and what is so special about the Maerz kilns is that the parallel arrangement of shafts linked by a cross-channel means that the hot exhaust from one cylinder is passed up through the stone in the second, a contra-flow that pre-warms the stone waiting there to be burnt. This ‘counter-current’ is exactly the system by which penguins’ feet are kept warm, and how the kidney is so efficient at removing waste products in the urine – biology got there first! This makes me happy. And John – who laughs a lot and is unendingly friendly and helpful – says, “I think Shapfell is the most efficient lime-maker in the world.”

He has kitted me out with helmet, hi-vis jacket, gloves and eye-protectors, and I assume we are going to wander around at ground level. But now comes a very welcome surprise: “Are you okay with heights?” John asks. We are going to climb up between the kilns, almost to the top.

We pass the hoppers at the base, from which the lime is decanted and taken by conveyor belt to be stored then transferred to waiting railway wagons. The railway is a siding from the nearby West Coast mainline – one of the sleek passenger trains speeds by during our tour – and wagons bring limestone to the kilns and take lime away. From here at ground-level we watch a large yellow lorry tipping out its load of limestone, which is rapidly bull-dozed to the edge of a waiting pile, from which it will be conveyed to a crusher, and eventually loaded into a hopper that will take it to the top of a kiln. There is limestone of a pinkish shade, which comes from Hanson’s quarry just 5 miles to the North; and there is whiter stone which is brought by rail from a Tarmac quarry at Buxton, Derbyshire. At one time the stone would have been dug out here, at Shapfell/Hardendale, but local ‘good’ stone, the best for quicklime and steel-making, has been exhausted, and the land is being restored.

Limestone from different sources

Around us all the hard surfaces are white: the handrails, the outside of the hoppers, the gantries, cranes and platforms – but it is not powdery, the whiteness is hard with a pimpled surface, smooth to touch. It must have built up over the years, but surprisingly, there is no dust.

We cross a metal bridge, beneath a conveyor belt, and onto a platform from which the first flight of steps leads upwards, perforated metal stairs through which the ground below is visible; but there are sturdy handrails each side. The cylindrical wall of this bottom section of the kiln is cool but up on the second platform I can feel warmth radiating from the wall. The weather today is perfect, with little wind, but I can imagine how pleasant that warmth would be when the wind is howling across from the Howgill Fells, funnelling up the valley by Tebay.

There is a complex scenery surrounding us, of pipes, supports, metal stairs, pressure valves, and the smooth bulk of the kilns, with intriguing glimpses of sheds and moving hoppers. Number 2 kiln has been decommissioned, and John says it will not be removed – I can see that the resulting gap would be as ugly to the symmetry as a missing tooth – and Number 3 has just been shut down for the day.

But there is a busy-ness on the ground and lower levels around Number 4, and up on the third platform, now about 100 feet above the ground, there is a smell of hot new paint. This kiln has been completely refurbished over the past two years. “It was a £3.8 million project,” John explains, “and we started heating it again about 5 days ago.” Apparently it takes this time to re-light – the refractory lining eventually becomes hot enough, between 600-700oC, that the gas will ignite itself when the gas-lances are turned on. Here by Number 4 there are pressure gauges and flexible gas pipes, and everything looks clean and new and glossy. Below us, in an enclosure on the ground there is a vast storage tank for natural gas; the works are supplied by their own mainline pipe. The recent hike in gas prices has quadrupled the cost of calcination …

Gas pipes for Number 4 kiln

Climbing another airy flight of steps, we return along the platform to kiln Number 1 to watch a hopper laden with stone being pulled up the rails. Arriving at the top, it tilts; John Baird warns “Cover your ears!”, and the hopper tips its load into the kiln with a rattle and crash of tumbling stone, then creeps back down to be refilled. In 11 minutes another laden hopper will tip its load into a parallel shaft to be pre-warmed by the exhaust gases before being fired.

So what is coming out of the chimneys? I’d been reading about old types of commercial kiln in David Johnson’s book [1] and photos showed the land surrounding white with lime-dust. Johnson notes, “A former public house close to Swinden Quarry, Grassington, and all nearby fields were noted for their covering of white lime dust” from the Spencer steel kilns, built in the early 1900s. Here at the Shapfell works a small amount of dust comes out in the steam, but John assures me that the filtration processes are very efficient. Tata own the land surface around the site and use it for local tenancy grazing, and we look down on the fields at the northern boundary and it is true, there is no trace of whiteness; but whiteness has accreted, slowly, on the metal structures and walkways.

Depending on the weather, the drifts of steam coming from the chimneys may appear white, grey, or even be invisible. A system of Venturi ‘scrubbers’ cleans up some of the emissions, such as sulphur dioxide and particulates. There is water-vapour – steam – from the heating of the washed limestone; there is carbon dioxide from the burning of the methane-rich natural gas, and there is of course also a very large amount of CO2, approximately 40% of the initial weight of stone from the breakdown of CaCO3 to form CaO.

When mixed and heated with metallurgical coal (such as that which would be produced by the projected and controversial West Cumbria coal-mine) and iron ore (haematite), the quicklime collects the impurities, floating on top of the molten iron as slag (see ‘Volcanoes of Workington’). Steel-making itself, by burning coal, also produces CO2. But until new methods for steel-production are devised, at scale, there will continue to be the need for lime and coal as accessories. Carbon-capture would seem to be the only way forward if CO2 emissions are to be reduced.

The Shapfell kilns produce 4,000 tonnes of quicklime per week and nearly all of this goes for making steel – formerly to plants at Teeside and Scunthorpe, but now more than 90% of the output going to Port Talbot: Britain still does have a (much-reduced) steel-making industry. The quicklime produced here at Shapfell is of the high specification required for steel. We look around the lab later, where the quicklime is tested. The lab is small, bench-tops clear, and with surprisingly little equipment, although most of what is there is apparently expensive. A pyrex desiccator, a piece of lab equipment that I probably haven’t seen since research student days, contains a pile of small white ceramic crucibles for heating lime. The quicklime is sampled every four hours throughout the production sequence to check the amount of unburnt material, known as the Loss of Ignition, LoI, percentage: for the steel industry this must be less than 4%, in other words the lime should be at least 96% pure. Reactivity with water is also tested by mixing 600ml water at 20oC with 140 gm lime – the temperature of the reaction should reach between 60-70oC in two minutes. To demonstrate the exothermic reaction between CaO and water, John mixes some powdered quicklime with water straight from the tap, and the beaker is immediately swathed in steam. (For more on the effects of mixing quicklime with water – the ‘hot lime’ process – see article about Tim Wells: coming soon). Positive results in these lab tests mean the lime produced at Shapfell can be labelled as conforming to the British Standard, a new certification required since we left the EU, the CE (Conformité Européen) certification being no longer deemed relevant by the government.

For steel-making, the calcined lime needs to be crushed to the optimal size of about 60mm. But for use in the renovation and repair of old buildings, as lime mortar and lime plaster, the lumps of quicklime need to be crushed much smaller. It was through Tim Wells, of Eden Hot Lime, that I got an introduction to Angela Wilson, the Commercials Office Manager, who very cheerfully and willingly set up my visit to the kilns – Tim buys in dumpy-bags of 2mm pellets and he then adds water to the quicklime, converting it to slaked lime, calcium hydroxide Ca(OH)2, the basis of lime plaster.

The agri-lime shed

After leaving the kilns, the labs and the control room, we walk back to the offices, past a large open-fronted shed piled with white powder. This is ‘agri-lime’, finely-crushed limestone – not quicklime – for spreading on the fields. Although no longer a lucrative part of the business, production is kept going mainly for the benefit of local farmers (agrilime article coming soon).

Limestone: to be used in making steel, in renovating old buildings, and to fertilise the fields – there are several stories here about the uses of the stone that comes from the quarries. Other stories are of the extraction of the stone, and of the restoration of the land.

Both Tata Shapfell (Hardendale) and Hanson’s (Shap Beck) quarries are on land owned by Lowther Estates. (Lowther Estates own land and foreshore, and have mineral rights, to vast swathes of the county – and have already been mentioned in the context of the small quarry near Great Asby.) Hardendale, opened in 1960, was leased from Lowther Estates by the Messrs Colville, who owned the Ravenscraig steelworks on the Clyde. Colvilles became part of the nationalised British Steel, and through a series of buy-outs and reorganisations, became Corus, which eventually was partially taken over by the Indian company Tata. Shap Beck was leased from Lowther by Harrisons Limeworks, and originally sent lime to Colvilles’ Ravenscraig. Harrisons had their own kilns, but these were decommissioned in about 1980, and the quarry – now owned by Hanson’s Aggregates, who have consent from the Lowther Estates until 2042 – now supplies the characteristic pinkish limestone for the Tata kilns.

Wherever there is limestone in Cumbria there are quarries, either small indentations in a hillside outcrop to supply stone for drystone walls or for a small local kiln, or the larger industrial excavations that supply stone for cement or roads, such as the Bredon quarry at Moota, and the currently (and hopefully, forever) disused quarries at Clints Crags, both to the North of Cockermouth. In recent times, when the extraction of stone has become uneconomic or the quality of extractable material has declined, it has been recognised that remediation of the landscape and environment is necessary. Thus, at Moota, the disused sections of the quarry are being re-modelled and improved for plant and animal recolonisation, and an explanatory ‘geology trail’ is being developed.

So too at Hardendale, where landscaping began as early as the 1970s. John Baird explains that they now have a new restoration plan (you can download details and maps here) . From up on the northern platform we had looked down on large rectangular piles of pale material and an improbably milky-turquoise pool of water: this was where the slurry of clay and small limestone particles from washing was allowed to settle and drain. As we watched, a yellow JCB was scooping up buckets of the soft residue and loading it onto an enormous truck; it would then be tipped into the quarry – where it would recombine with atmospheric CO2 and harden, to form new stone (see diagram of ‘The Lime Cycle‘ in ‘Quicklime: Hot Mix‘)

Settling ponds and quarry re-fill

The quarry’s land is also on and bordered by SSSIs and Special Areas of Conservation, so Natural England are involved in design and maintenance of the new reserve, and RSPB volunteers help record the birds. ‘There’s a big newt area too,” John says, “with hibernacula” and, laughing, he goes on to explain the intricacies of newt-protective fencing. “We’re aiming to reduce the footprint of the works on site, we’ll be removing redundant buildings, like the crushing plant. There’ll be strategic planting of trees to screen the works. The former settling pools are full of wildlife.” He’s clearly as enthusiastic about the long-term restoration work as he is about the management of the kilns.

Looping back onto the M6 again to drive North, I saw that there was white exhaust coming from the chimney of the newly-lit Number 4. Now, each time I drive across that moorland limestone country and see that tall, incongruous bank of grey cylinders, I’ll say to myself, with some amazement, “I was up there!”.

[1] David Johnson (2018) Lime kilns, history and heritage: Amberley Press

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