China Clay Processes: How they work.

[Stoker]

The Rotary Dryer

At one time the most widespread mechanical drying process in Cornwall's clay industry was the rotary dryer. The first installation at Rock Hill Kiln on the Goonbarrow branch fed rail traffic from a small store on the East Caudledown Mill siding from 1949. By the mid to late 1950's the now very familiar gigantic clay stores were being constructed to be fed by rotary dryers. By the time of the large scale industry layoffs around 2008/9, most of them had been shut down, having operated continuously for some 50 years.

 

This is a sectional diagram that I have drawn of the standard type of rotary dryer that ECLP used. They consisted of three slightly inclined concentric tubes. The innermost and outermost tube carried the hot gasses from the burner. The clay was fed into the space between these tubes, so as to keep the clay separate from the gas, thus avoiding contamination. Steam is drawn out of the dryer through the duct on the right hand side. The flow of the gas is shown colour coded on the diagram, with the red hot gas emerging from the burner, flowing down the central tube, becoming cooler as it reaches the end, whereupon it transfers to the outer tube, and makes it's way to the exhaust duct. To rotate the dryer, just off the centre is the large girth gear, which is driven by a gearbox, in turn driven by an electric motor. The weight of the dryer rests on rings sat on rollers mounted to large concrete plinths.

 

Shredded filter press cakes of clay arrive on the conveyor on the right hand side of the diagram, and make their way to the pelletiser extruder on the upper left. A small amount of dried clay is portioned off using a device called a bifurcated chute, to be backmixed here in the pelletiser. This reduces the moisture content of the clay, preventing the pellets from sticking together in the dryer. The aim of pelletising the clay is to increase the surface area for drying - a rotary dryer of this type would be quite ineffective at drying large chunks of moist clay. Pelletised clay is fed into the dryer on the upper right hand side of the diagram, and makes it's way to the lower left, where the dried clay would fall through tubes between the inner and outer tube of the dryer.

These dryers varied in size from 1m to 2m in diameter, and 10m to 20m in length. Their most advantageous characteristic was their relatively small size, and the requirement for minimal additional equipment with the exception of a cake shredder and pelletiser. This small footprint and simplicity, meant that they could be retrofitted into any old coal fired kiln that had a filter press house, which by the 1950s was the majority, with very few muck wagon kilns still in use. Output could then be significantly increased, with filter pressed clay being completely dry within a few hours, rather than a few days. Manual handling was also almost completely eliminated, with the exception of peeling filter cakes out of the presses. Also eliminated was the need for increasingly expensive coal, with the dryers now running on oil. ECLP were finding significant savings and output increases in the mechanisation of their dryers. With individual dryers now capable of increased outputs, many older sites were shut down.

Sadly, this meant that a few men were put out of a job, but since the industry was expanding at the time, most were trained on new machinery and moved off to other posts.



Pictured above was the rotary dryer installation at Treviscoe.



Blackpool dryers housed a pair of large rotary dryers.

[Stoker]

Milling.

As technology improved, new uses and markets opened up. Customers who required clay of an extremely low moisture content would receive milled clay in paper sacks.

This is a two part diagram I have drawn of a standard milling process for china clay, showing both side-on and top-down views. A clay mill consisted of an upright cylinder containing various shapes and sizes of rotor blades that would act to disintegrate the clay pellets. Clay would be fed into the bottom of the mill, along with hot air from an indirect air heater. Clay entering the mill would be drawn by air pressure through the vanes of the first rotor, whilst impacting the blades - any clay lumps small enough would be able to pass into the next set of more closely spaced blades, anything too large would fall back down to the set below. In this way, the mill was "self classifying". The action of the spinning rotors, air pressure, and rising heat would draw the clay up through the mill, with the resulting fine dust and hot air leaving through a duct at the top. The clay is then blown into a cyclone, where the heavier clay particles separate and fall to the bottom, and the lighter hot gas and steam rises and ducts out the top. The hot air and steam then passes through a second cyclone, where any residual clay dust has another chance to separate out. The steam and hot air then leaves through an exhaust stack. Clay is collected from the bottom of the cyclones by an auger, and is fed into a hopper. An auger at the base of the hopper can then be operated manually by the person in charge of bag filling.

Clay mills were sometimes installed within large modern clay stores, where the bagged clay could be stored on pallets awaiting shipment. The equipment itself required a minimal amount of space, and would sometimes be installed in it's own separate building as at Drinnick and Ponts Mill.

[Stoker]

Filter Press

The first meaningful form of widespread mechanisation of china clay drying was the installation of filter presses in the 1920s. Until then, it had been necessary to allow china clay to fall out of suspension and form sediment within tanks behind the kiln, until the tank was completely full of clay. At this point, it would be shoveled out into a small wagon, known as a muck wagon, giving rise to the name "muck wagon kiln", which would be wheeled out onto a traveling bridge, it's contents being deposited on the desired section of the heated "pan" floor. This process of sedimentation could take well over a week depending on the size of the tank.

A filter press would be installed in a press house, which would almost always occupy the space of one of the middle settling tanks. The presses themselves were on a floor known as the press deck, on the floor below wagons would be positioned on tracks underneath the presses, ready to catch the falling press cakes. Clay slurry from the neighbouring settling tanks would be pumped to the press house reception tank, from which it would be pumped into the filter presses. What once took weeks, would now only take hours, producing a cake of much lower moisture content than the mud-like slurry that came from the wagon-tanks of the old method. A particularly unpleasant and labor intensive job of mucking out the tanks was now eliminated, pictured below:





Above is a section diagram I have drawn of a standard filter press. A filter press consists of a few elements. A feed head, at the left, a follow head at the right, stood either side of a bank of press plates. These plates have been shaded grey for clarity. The follow head and the plates are hung from a steel joist above, which is supported by the feed head and the bulk head on the right, and the whole press assembly is supported on a pair of steel joists below. A filter cloth is affixed to the press plates. Each press plate has a hole in the middle, allowing clay slurry to pass through. Clarified water leaves the press through a small tube at the bottom of each plate.

During operation, the press is closed tight using the giant screw on the right, which pushes the follow head onto the press plates. The valve is then opened allowing clay being pumped at pressure to enter the feed head and the press in the direction of the arrows. The slurry fills the gap between each pair of plates. A pressure gauge on the feed head gives an indication of current pumping pressure - after 1-2 hours, the gauge will begin to increase, indicating that the press has reached capacity. The feed is shut off, the follow head is retracted, and the plates are separated one by one. The solid cake has a habit of sticking to the filter cloth, therefore it usually has to be freed by hand. A good press operator will be able to judge when it is the right time to shut off the press - shutting it off too early will result in wet cakes, shutting it off too late risks equipment damage and dangerous pressure leaks.

In a mechanical dryer, the finished cakes would fall onto a slow moving conveyor belt, which would take them to a shredder. The clarified water, also known as filtrate, would trickle out into a trough and be drained off to be pumped to flooded disused quarries, used as reservoirs. This was just one of many sources of recycled water in the industry.

[queensquare]

A really useful and fascinating thread. Many thanks for taking the trouble to post.

 

Jerry

[Fat Controller]

Fascinating stuff; thanks for posting. I'd worked at the other end of the supply chain in Stoke in the late 1970s/early 1980s; things were somewhat less 'high-tech' there. The semi-dried clay would be tipped into a device called a 'blunger' (an over-large food mixer would be the best description) and returned to a semi-liquid state for use in the various companies' moulding machines. Each company had its own specific viscosity for particular types of ware. Some clay was mixed until it was a really thin (single-cream) consistency for use as glaze. These blungers were the only time I've encountered wooden bearings, used to avoid metallic contamination, and because they could work submerged without additional lubrication; I believe the material was called 'lignum vitae'.

[jamie92208]

 I believe the material was called 'lignum vitae'.

Lignum vitae, which I believe is a very hard and dark wood that's heavier than water, was used for propellor shaft bearings on ships in the 19th Century.  I got hold of some to make bearings for a model boat i built.   It's facinating to her that it's still in use.

 

Jamie

[Stoker]

Fascinating stuff; thanks for posting. I'd worked at the other end of the supply chain in Stoke in the late 1970s/early 1980s; things were somewhat less 'high-tech' there. The semi-dried clay would be tipped into a device called a 'blunger' (an over-large food mixer would be the best description) and returned to a semi-liquid state for use in the various companies' moulding machines. Each company had its own specific viscosity for particular types of ware. Some clay was mixed until it was a really thin (single-cream) consistency for use as glaze. These blungers were the only time I've encountered wooden bearings, used to avoid metallic contamination, and because they could work submerged without additional lubrication; I believe the material was called 'lignum vitae'.

Blungers were also used in Cornwall in the slurry plants at Blackpool dryers and Par docks, but by that time (1950s) lignum vitae bearings had been superseded by sealed bearings. I suspect that the blunging units you worked with in Stoke had been in use for a very long time indeed, given that it's such an old industry. Even to this day the potteries of Stoke are one of the longest standing, if not the longest standing customer of the clay industry in the UK, having received shipments from Cornwall for over 200 years.

 

Needless to say, ECLP and other companies spent a good amount of time and effort training their employees on how not to accidentally contaminate the clay. The most frequent offender, as it happens, was lubricating oil from processing machinery. There was a specific quantity that customers could tolerate, to be measured in parts per million. One of Maurice Dart's jobs as a chemist working for ECLP, before he became the published author that we all know, was to test for the presence of hydrocarbon lubricants in samples.

[Stoker]

Wet Scrubbing

 

Dubious name aside, another familiar structure standing beside the modern mechanical dryer buildings was the wet scrubber - a large upright cylinder mounted up on framework, with a steam exhaust chimney stack emerging from the top of it. Before the days of "health and safety". DEFRA, and people giving a monkeys about the environment, the steam from the dryers was simply exhausted to the atmosphere - harmlessly, one might add. Harmless except for the dust. Dryers spat out enormous volumes of clay dust, to an extent that is hard to mentally picture these days given how well controlled and relatively green the clay mining landscape is now. Suffice to say, almost everything within a 100 yard radius of a dryer was dusted in clay. There was some concern that this may be bad for the environment and the health of people in the area. Eventually, like most industries, the clay companies were forced by law to start reducing dust emissions from their drying and quarrying sites.

 

One of the ways the clay companies were able to do this, was by the widespread implementation of wet scrubbers.

 

In this simple diagram I have drawn of a standard wet scrubbing unit, we can see how the steam from the dryer enters the base of a large cylindrical vessel, and passes through a fine mist of china clay slurry. Clay dust particles suspended in the steam get caught in the mist of clay slurry. The slurry drains out at the bottom, while the "scrubbed" steam leaves through the stack. The clay slurry is then pumped to the filter presses.

 

Wet scrubbing had the side effect of pre-heating the clay slurry as well as slightly reducing it's moisture content. This in turn slightly reduced the moisture content of the resulting filter cake, and consequently made drying marginally more efficient. In smaller dryers, this made a fairly significant impact, sometimes requiring the addition of another bank of filter presses.



The gigantic Buell type dryers had equally large wet scrubbers. This one stood at Rocks dryers before it's demolition replaced it with Europe's largest tube press installation.

[Stoker]

Mechanically Raked Settling Tanks

The Dorr Oliver company was responsible for this now very familiar process. Prior to these circular tanks with their familiar corrugated sheet wind breaks, it was necessary to manually push settled china clay toward the drain using a flat plank of wood on the end of a long handle - a tool somewhat akin to the "shyver" used in the mica drags. The work was physically demanding and time consuming. To improve this, the clay industry adopted Dorr Oliver's mechanically raked settling, refining, thickening and storage tanks. A circular tank fitted with rotating rake arms, suspended above a gently sloping conical floor - at it's centre, a drain channel from whence thickened slurry was drawn out through centrifugal pumps. At the periphery of the tank wall, a drain channel allowed clarified water to overflow. A circular baffle in the centre, known as the feedwell, retarded the slurry feed, encouraging it to separate.



The settling tanks outside of a dryer were known as the reception tanks. For the sake of practicality and pumping costs, clay slurry was pumped from the refinery to the dryer with a water content not suitable for drying. Upon reaching the drying plant, it would be necessary to thicken the slurry by settling it. It would then be pumped to holding tanks, which could cope with and buffer the batch stop-start nature of the filter presses.



Here we see the reception tanks at Treviscoe. Some of these tanks have lost their feedwell baffle, presumably because they are being used as holding tanks rather than settling tanks.

[jamie92208]

Mechanically Raked Settling Tanks

 

The Dorr Oliver company was responsible for this now very familiar process. Prior to these circular tanks with their familiar corrugated sheet wind breaks, it was necessary to manually push settled china clay toward the drain using a flat plank of wood on the end of a long handle - a tool somewhat akin to the "shyver" used in the mica drags. The work was physically demanding and time consuming. To improve this, the clay industry adopted Dorr Oliver's mechanically raked settling, refining, thickening and storage tanks. A circular tank fitted with rotating rake arms, suspended above a gently sloping conical floor - at it's centre, a drain channel from whence thickened slurry was drawn out through centrifugal pumps. At the periphery of the tank wall, a drain channel allowed clarified water to overflow. A circular baffle in the centre, known as the feedwell, retarded the slurry feed, encouraging it to separate.

 

settling tank process.jpg

 

The settling tanks outside of a dryer were known as the reception tanks. For the sake of practicality and pumping costs, clay slurry was pumped from the refinery to the dryer with a water content not suitable for drying. Upon reaching the drying plant, it would be necessary to thicken the slurry by settling it. It would then be pumped to holding tanks, which could cope with and buffer the batch stop-start nature of the filter presses.

 

trevtankssmall.jpg

 

Here we see the reception tanks at Treviscoe. Some of these tanks have lost their feedwell baffle, presumably because they are being used as holding tanks rather than settling tanks.

Were those continously stirred or was it only done once after the slurry had time to settle on a batch process.

 

Jamie

 

PS Keep the info coming.  I went on a tour of various sites organised by a retired ECC employee that I found out about after visiting Wheal Martin Museum.

 

Jamie

[Stoker]

Were those continously stirred or was it only done once after the slurry had time to settle on a batch process.

 

Jamie

 

PS Keep the info coming.  I went on a tour of various sites organised by a retired ECC employee that I found out about after visiting Wheal Martin Museum.

 

Jamie

The tanks were continuous - the storage tanks had an overflow that pumped to the next tank and so on. The waterline in the tanks would rise and fall with the flow from the filter presses. The filter press pumps would always be running, but pressure release valves would return the slurry back to holding tanks if none of the filter presses were in their filtration cycle. Usually there would be one pump per 2 - 4 banks of presses, so at any given time they might all be in the filter cycle, or they might all be discharging. The greater the total number of filter presses a dryer had, the more this could be staggered to prevent ebb and flow at the feed end.

 

Also, the information will keep coming - I've got a few more installments planned. I was originally going to publish all of this information in a book; having spoken to publishers about the cost involved, as well as getting all the necessary permissions for printing photographs, etc. I have decided against the idea, and will instead post all the information here. This has the advantage that the information will be freely accessible on the largest UK railway modelling forum on the internet, so in theory reaching a wider audience.