Most power engineers are familiar with porcelain as an outstanding insulating material used for decades in a myriad of 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 just right and carefully monitored from the moment they are extracted from the ground. INMR recently met with experts at Imerys – the French-based industrial minerals giant and one of the world’s major suppliers of specialized ball clays for the porcelain insulator industry – to examine the role that raw materials actually play.
Given its extraordinary lifetime, it is perhaps only fitting that the origins of porcelain should be in the long distant past. Indeed, the ball clay that make up a large portion of the porcelain body is the result of geologic 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, free of metal oxides, washed down from hills and, in rare cases, became trapped in areas such as coastal deltas where they deposited and accumulated underground – only awaiting discovery and extraction.
According to Neil Mifflin, a geologist working at the Povington Pit in the British county of Dorset, certain areas of southern England have met these unusual conditions and long been known to contain ball clays with properties ideal for applications in the ceramics industry. For example, the region was mined as far back as Roman times and during the 1700s first rose to prominence by supplying raw materials to Josiah Wedgwood, a pioneer in local production of porcelain dinnerware. The term ‘ball clay’ apparently also has its roots in history since the relatively soft material was originally sold in the form of small cubes, which over the course of handling became progressively more spherical in shape.
When it comes to electro-porcelain used in HV line and equipment insulators, the attributes required of ball clay are generally quite demanding. Here, the focus is on strength as well as on high plasticity and good drying behavior, with minimal presence of organic matter. Other key parameters include fine grain size and low residue content. Together, these characteristics allow it to be shaped into very large pieces without deformation and later fired with no release of gases that might lead to 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, according to Mifflin, the Povington Pit is well suited to meet the special needs of electro-porcelain as well as refractories and tiles, whereas nearby Imerys quarries in Devon are used mainly to supply ball clay for sanitary hardware or table porcelain. Eugen Alexa has been responsible for Imerys sales of key raw materials to the ceramic insulator industry. He reports that porcelain’s high mechanical and dielectric strength combined with its inertness to the environment have long made it an ideal electrical insulating material. Still, Alexa explains that the composition of electrical porcelain can vary quite significantly, with six common categories of masses known in the industry as C110, C111, C112, C120, C130 and C140. C110 is mainly silicaceous clay rich in SiO2 whereas C112 contains cristobalite, a SiO2 variant with its own molecular structure. The masses most commonly used for MV and HV insulators are C110, C120 and C130, the latter two being rich in content of alumina (Al2O3).
Says Alexa, “the ceramic body is usually prepared according to a strict recipe which involves a compromise in the relative amounts of different ingredients to meet such goals as long service life, ease of production or some combination of low cost and sufficient performance.” For example, a typical formulation for the porcelain mass will contain varying proportions of ball clay, kaolin (for strength and plasticity), feldspar (a flux that helps sintering in the kiln), and fillers such as quartz, alumina or calcined bauxite (intended to impart additional mechanical strength). A variety of secondary materials are also used to facilitate processing including water and additives such as binders – all of which are burned off in the kiln during firing at temperatures of from 1200-1300°C.