China Clay in Cornwall
China Clay Dries, Par
As was mentioned before the traditional method of drying the china clay in pan-kilns, such as Wenford Dries was to slow and laborious and more efficient methods for drying the clay were required in order to increase production. Two of these methods can be seen at the China Clay dries in Par. The first method, introduced in 1911 was the filter press (shown in the two pictures below) which was the first meaningful way of speeding up the clay drying process to be invented.
A filter press consists of a feed head and a follow head which stood either side of a bank of press plates. The follow head and the plates are hung from a steel joist above, which is supported by the feed head and the bulk head, and the whole press assembly is supported on a pair of steel joists below. A filter cloth is affixed to each of 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 by pushing 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. 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.
The second method for speeding up the drying process, the Buell dryer, was introduced to the china clay industry by the English China Clays company in 1944 just after WW2. The dryer itself is composed of a large upright cylindrical chamber, inside of which are 25 to 30 layers of trays or “hearths”. Indirectly heated air from an oil-fired (latterly natural gas-fired) furnace or steam heater is distributed throughout the dryer by a series of fans and ducts. At the centre of the dryer is a rotating column, to which the trays are attached and positioned radially within the dryer. Material enters the top of the dryer and lands on one of the top trays. As the central column rotates, fixed arms push the material off the tray, dropping it down onto the one below it. Gradually the material works its way down through the dryer in this manner, and after 45 minutes clay exits the bottom of the dryer onto conveyor belts.
The picture above shows the wet scrubbers which were used to clean the exhaust steam from the dryers. Before the advent of stringent health and safety rules the steam from the dryers, including the dust, was simply exhausted to the atmosphere. Dryers spat out enormous volumes of clay dust, to an extent that almost everything within a 100 yard radius of a dryer was dusted in clay. Eventually, like most industries, the clay companies were forced by law to start reducing dust emissions from their drying plants and one way to do this was the large-scale introduction of wet scrubbers. 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.
As was the case in the pan-kilns after the drying porcess had finished the dried china clay got transferred to the linhay, except that at the China Dries in Par this was done via convetor belts rather than shovels.