China clay in your hand

Many years ago I saw a TV segment exploring a china clay pit, and was struck by the infinite landscape and colours. I’ve been wanting to see one with my own eyes ever since. In recent times, I’ve been exploring the stone from my own local quarries. Red rather than white, but my interest persisted.

Then back in January at an Exeter College workshop, Andy-from-Kaleider and I met an employee of the Sibelco china clay quarries near Ivybridge. It was an opportunity not be missed, and Scott was kind enough to offer us a tour. We put it off to warmer weather, which also meant that we collected more interested parties. With his Kaleider hat on, Andy is involved in the SW Creative Technology Network, which runs a number of fellowship programmes. David is a granite sculptor working at the end of Cornwall, and a SWCTN Immersion Fellow last year. Roop is a potter in Exeter and a SWCTN Automation Fellow this year. Both were deeply interested in the potential of the materials on site.

We convened at base camp at 10am. Scott gave us a brief intro to the site layout and history of the workings, and we examined the detailed geological map on the office wall. Then Scott provided hi vis jackets, hard hats and goggles, attached a red flag to the top of a Sibelco truck, turned on the flashing orange lights, and we headed off to the far quarry on Lee Moor.

Digging it out

Sibelco operate two quarries: the larger at Lee Moor (top centre on the zoomed in satellite pic), where kaolin has been worked for over a century, and more recently at Headon and Hemerdon (bottom right of centre). They are right on the edge of the Dartmoor granite outcrop, where the geology is a mix of granite, kaolin, and some feldspar.

The deposits are reasonably well kaolinised, with some variation across the site; the areas with a higher content typically look whiter. But much of the deposit is waste, and most of this is mica.

For the most part, Sibelco dig the deposits out of the ground and truck it to high-pressure washing sites for the first stage of separation. They last undertook rock blasting on the site in 2014. Now, they typically work around the granite, which provides firm foundations for the plant at no risk of undercutting.

Walking around the view point on Lee Moor, we could see the scale of the thing, but also its boundaries.

In front of us was the deep pit created by a century of workings: predominantly greys and whites, and the beautiful turquoise of the pools. Behind that to the left, the landscape turns redder, and we could see the tungsten mine in the distance (bottom left of centre on the zoomed in satellite pic). There’s also still tin in them thar hills.

To our right, there was a lone digger dwarfed in a stark white field of kaolin. Turning slightly further to the right, slap up against it, was the (slightly desiccated) green of the National Park. There was even a public car park, complete with camper van and a few cars, and a standing granite cross.

Sibelco have a few decades left in this deposit. Their options then are to buy more land or close the quarry and make it all good. There is more kaolin towards the National Park, but they relinquished planning permissions in 2001 because of the impact on a sensitive area.

More on the geology:

• Geology of Britain viewer, Royal Geological Survey
• Mineral Planning Factsheet: Kaolin, RGS, September 2009
• Geology, Devon County Council
• Devon County Minerals Local Plan: Lee Moor, DCC, June 2004

Washing it

From Lee Moor, we drove back to another viewing area at Hemerdon quarry. En route there was a nice bit of repurposing of old dumper truck tyres as a roundabout. Again, there was the vista of shades of grey, mingled with some redder outcrops of higher iron content, and turquoise pools.

This time in the background we could see Tarmac’s operations. Sibelco works with both Tarmac and Aggregate Industries, who take much of the aggregate and sand waste. The mica, however, is not strong enough for road aggregate so not useful at the moment. Maybe in the future it could be reused, like power station fly ash.

Then we headed past the conveyor belts taking aggregate up to Tarmac, down to the washing control station. Here we found Paul, sat in his comfy chair, armed with a couple of joysticks, control screens to his left, the intarwebs to his right, and the kettle and fridge behind him.

The quarries operate 24-7, except when extremely cold or extremely wet. So many of the employees work shifts on a pattern of two days, two nights on, four off. Paul will be sat in his comfy chair for a 12-hour shift.

Autopilot is possible while making a cuppa, but for the most part the washing control is manual and involves an element of skill. Paul was directing a high-pressure water jet at two piles of higher and lower content kaolin, aiming for a mix suitable for a ceramics grade end product, and a particular density of kaolin particles held in suspension in the resulting slurry.

Next to the control station, I climbed a low bluff, and looked down on the river of kaolin slurry. Kaolin particles are 2 microns across. Sand is coarser, heavier, and precipitates out, clogging up the flow. So it needs to be dug out from time to time. As we watched, a truck down below zipped back and forth, digging and dumping, digging and dumping, digging and dumping. So it goes, eroding a gully as it flows round the hill down to a pumping station.

Processing it

The second half of our tour was not quite in processing order, and a bit of a whirl.

First up was the lab, where refined kaolin samples are tested. The technicians have x-ray machines to measure chemical content. They have spectrometers to measure whiteness, with barium sulphate as the yardstick. And they bake the samples in kilns to produce thin discs that ching together pleasingly. I think they use this to measure shrinkage. Glancing at the numbers written on the discs, it looks as though higher whiteness correlates with lower shrinkage… possibly something to do with potassium content.

Next up was the control room, full of screens of information. Then the loud building, where the refining happens. Here there are lots of pipes and pumps, and liquid flowing and spinning and gushing and shaking, to remove the mica. At the end of the whole process, the mica content is down to just 0.008%.

Scott also waved at the housing for the superconductor magnet. That was a substantial investment, and operates at a cool -269ºC. Its purpose? A marginal reduction in the iron content so that the Japanese can sit on slightly whiter toilet bowls!

After that, we climbed up to gaze over the settling pool. We visited the room where the refined kaolin is pressed. (Clearly I’ve missed the step of actually separating kaolin from water.) There was another shift worker here, whose job for 12 hours is to detach the pressed kaolin slab from the press and direct it into a hopper. A tough job.

It is then extruded into pellets, or ‘noodles’, and conveyor-belted into the drier to reduce the moisture content by half. This variation on a cakewalk fairground ride shakes the pellets through to the other end. The dust is piling up in drifts, and there are warnings about wearing face masks on the walls. Still, on to another conveyor belt go the pellets, and up and into the store.

Storing it

And this was the Wow! moment. The scale was the first to strike me. The store room is huge, and piled high with pellets as far as the eye can see. The next impression to emerge into my consciousness was the muted colour palette, not just of the product, but the room as a whole. It was calming on the eyes and increased the depth of field. Finally, I noticed the hush. Though I could hear the machinery behind me, and there was a conveyor belt alongside us, the room held a quality of stillness. Here the kaolin pauses on its journey from the ground to the customer.

Sibelco sell five grades of bulk pellets for manufacturing ceramics and paper. From the store, it will eventually go to Plymouth docks for shipping all over world.

On reflection

The whole process was to me a fascinating mix of low tech and high tech, of innovation and ‘the way it’s always been done’.

The processing is a cocktail of lower tech shaking and stirring, spiced up with the high tech superconductor magnet, spectrometer and x-ray analysis.

Getting it out of the ground, on the other hand, has not evolved far beyond panning for gold. Washing out the kaolin is the way it’s been done hundreds of years, although dumper trucks, electricity and a remote-controlled water jet make this a little easier. Taking a positive view, this is part of the local heritage. More negatively, there is inertia. For most of the hundred years that Lee Moor has been in operation, the washing happened at the bottom of the pit. So when it rained and it flooded they had to stop operation. It has only been only in the last six years that they moved the washing up the slope.

On the water supply side, Scott has been working on a new filtration system. At the moment, Sibelco collects and reuses water for washing out. But other processes require clean water, so Sibelco also takes a mains supply. Scott’s plans for introducing filtration could mean that the site will be self-sufficient in the future.

I didn’t dare ask about the site’s carbon footprint, though, and how they are managing that! But later I did a little piece of online research and dug up Sibelco’s climate change agreement with the Government. This is a voluntary agreement to reduce energy use and carbon emissions. In return, they get a Reduced Rate Certificate entitling them to claim a discount on the Climate Change Levy. The upside? They are hopefully reducing consumption and emissions. The downsides? It is only voluntary, so no guarantees it will happen, and any CCL discount gets passed on to higher energy bills – not in itself a bad thing if it results in a fall in domestic consumption, but such price increases generally impact poorer people the most.

Back to my impressions of the site. Unlike many extraction processes, kaolin quarrying is not toxic. It does obviously, however, impact on landuse and ecosystems. Any such large earth movements will trigger ecological succession, and the pioneers have already moved in to start the process.

So it was good to see nature going about its business. Plants are colonising the most unpromising places, from the mica dams to the settling pool. Sheep were happily grazing on the hardy grasses growing through the quarry waste lining the site roads, as well as on the National Park next door. A pied wagtail visited us at the water jet.

These were the details that counterbalanced my overall awe at the scale of the thing. The white scars of the quarries in the landscape stand out on satellite imagery of the region, and dwarf the human scale. In the end, though, the product store created the deepest impression on me. An oasis of simplicity and calm in the midst of industry.