China Clay in Cornwall
The Goonbarrow Refinery is part of the Goonbarrow Pit, the name of which (as far as I can tell) first appeared in the 1841 census. According to the census 7 people were employed at the pit, most of them members of the family than ran the pit. However, it should be noted that between 1830 and 1906 a large number of small china clay works, such as the North Goonbarrow clay works, the Imperial Goonbarrow clay works, the Old Beam clay works (1830-1874), the Bugle Clay Works, and the Rock Hill clay works, operated in the area. The modern Goonbarrow pit was born out out several acquisitions by the English China Clay Company Ltd, which purchased the assets of the Old Beam Mine in 1919, the North Goonbarrow and the Imperial clay works in 1927 and the Rock Hill and Bugle China Clay Companies in 1934. It is interesting to note that some of these mines, i.e. the Old Beam mine and the Rock Hill works operated both as a tin mine and a china clay mine at some stage or the other. In 1999 ECC was acquired by the French company Imetal who subsequently changed their name to Imerys, which now own the Goonbarrow Pit and Refinery. The Goonbarrow Refinery mainly produced performance minerals (such as additives for cement or additives for water-based adhesives) and china clay for the paper industry. A restructuring by Imerys Ltd. in 2007 led to the closure of the Goonbarrow Refinery, which is now in the process of being demolished.
Unlike tin or copper, kaolin did not require deep mining and was retrieved using open cast mining. Traditionally, china clay is extracted from the kaolinised granite by “wet mining”. High pressure jets of water (‘monitors’) are used to blast the rock faces of the open pits in order to separate the the soft clay from the granite. The resulting mixture of water, kaolin, sand and mica (silicate minerals in forms of sheets) forms a slurry which gravitates to the lowest point of the pit. It then passes through a series of mechanical classification systems to remove any unwanted sand and mica before being pumped to the surface of the pit by centrifugal gravel pumps for further processing.
As late as the 1970s, the run-off from wet-mining, would enter the local rivers, eventually ending up in St Austell Bay, turning the whole sea white. Moreover, for each ton of clay extracted, there were five tons of waste. The waste or slag was taken and dumped back on the land, in long hills or in large conical structures. These large white hills, referred to as the Cornish Alps or Pyramids, rapidly changed the local skyline.
At this stage the processing of the kaolin consists mainly of separating the clay from other impurities such as sand or mica. This is achieved by using either centrifuges of hydrocyclones. A hydrocyclone ( also simply called cyclone) is a device which allows you to separate or sort particles in a liquid suspension based on the ratio of their centrifugal force to their fluid resistance. This ratio is high for dense and coarse particles and low for light and fine particles.
From the mine the slurry is then pumped or transported to the refinery. Here the clay is fractioned into coarse and fine particles (using another set of hydrocyclones) and different types of clays are blended according to their final purpose. The resulting clay is then thickened in settling tanks and pumped in slurry form to the drying plant. The process of drying is described in the following in the following blogs on Wenford Dries and the China Clay Dries in Par.