Rawmill - History

History

The history of the development of the technology of raw material grinding defines the early history of cement technology. Other stages of cement manufacture used existing technology in the early days. Early hydraulic materials such as hydraulic limes, natural cements and Parker's Roman cement were all based on "natural" raw materials, burned "as-dug". Because these natural blends of minerals occur only rarely, manufacturers were interested in making a fine-grained artificial mixture of readily available minerals such as limestone and clay that could be used in the same way. A typical problem would be to make an intimate mixture of 75% chalk and 25% clay, and burn this to produce an ”artificial cement". The development of the "wet" method of producing fine-grained clay in the ceramics industry afforded a means of doing this. For this reason, the early cement industry used the "wet process", in which the raw materials are ground together with water, to produce a slurry, containing 20–50% water. Both Louis Vicat and James Frost used this technique in the early 19th century, and it remained the only way of making rawmix for Portland cement until 1890. A modification of the technique used by the early industry was "double-burning", in which a hard limestone would be burned and slaked before combining with clay slurry. This technique avoided the grinding of hard stone, and was employed by, among others, Joseph Aspdin. Early grinding technology was poor, and early slurries were made thin, with a high water content. The slurry was then allowed to stand in large reservoirs ("slurry-backs") for several weeks. Large, un-ground particles would drop to the bottom, and excess water rose to the top. The water was periodically decanted until a stiff cake, of the consistency of pottery clay, was left. This was sliced up, discarding the coarse material at the bottom, and burned in the kiln. Wet grinding is comparatively energy-efficient, and so when good dry-grinding equipment became available, the wet process continued in use throughout the 20th century, often employing equipment that Josiah Wedgwood would have recognized.