Most power engineers are familiar with porcelain as an outstanding insulating material used for over a century in myriad overhead line and substation applications. What is much less known, however, is that the performance of electrical porcelain depends as much on the raw materials that make up its mass as it does on maintaining a well-controlled manufacturing process. Too often, there is the perception that these materials are little more than ‘common dirt’ when, in fact, their composition and physical parameters must be tightly regulated and monitored from the moment they are extracted from the ground.
In late 2012, INMR met with experts at Imerys – the French-based industrial minerals giant and one of the world’s major suppliers of specialized raw materials, including ball clays, for the porcelain insulator industry – to examine the important role that raw materials actually play.
Given its extraordinary lifetime, it seems only fitting that the origins of porcelain should be in the distant past. Indeed, the ball clay that makes up a large proportion of a porcelain insulator’s mass is the result of natural forces that have been at work since the Earth’s Eocene Period – some 45 million years ago. Over the millennia, sediments from kaolinite-rich rocks, washed down from hills and were trapped in areas such as coastal deltas where they settled and accumulated underground – only awaiting discovery and extraction.
According to Neil Mifflin, Technical Manager in charge of the quality control laboratory at the UK Ball Clay Operations in the county of Dorset, the confluence of all these conditions is quite rare in the world and accounts for the scarcity of high quality ball clay deposits. He goes on to note, however, that this area of southern England has long been known to contain ball clays with properties considered ideal for applications in the ceramics industry. In fact, the region was mined as far back as Roman times and during the 1700s rose to prominence by supplying raw materials to Josiah Wedgwood, a pioneer in the production and technology of ceramics. The term ‘ball clay’ apparently also has its roots in history since the malleable material was originally sold in the form of small cubes, which during handling became ever more spherical.
When it comes to electrical porcelain used in high voltage line and equipment insulators, the properties required of ball clay are typically very demanding. Here, the focus is on high strength and plasticity as well as good drying behavior, with minimal presence of organic matter. The importance of good drying can better be appreciated if one considers that most of the production lead time for porcelain made by the classical wet process is dictated by how rapidly the extruded ‘green’ body, with relatively high moisture content, dries to the point where it can be turned on lathes and later fired. Other required key ball clay parameters include fine grain size and low residue content. Together, all these characteristics allow it to be shaped into large pieces without deformation and fired in a kiln with no release of gases that might cause unwanted porosity in the body.
Moreover, these essential properties must be consistent from one batch to the next. Says Mifflin, “the most important factor for users is that all the critical parameters of the ball clay remain constant. That’s why we must perform quality control at every step whenever anything is done to the clay either while mining or during subsequent processing.”
Given such considerations, it is not unusual that a quarry used to supply ball clay for certain ceramic applications might not be considered ideal for others. For example, Mifflin notes that Povington Pit is well suited to meet the special needs of electrical porcelain as well as refractories and tiles, whereas nearby Imerys quarries in Devon supply ball clay mainly for sanitary and table ware.