The rotary kiln is used in many solid processes, including drying, incineration, heating, cooling, humidification, calcining and reduction, and has many applications in different industries, such as cement manufacturing.
This wide application can be attributed to factors such as the ability to handle varied loads, with large variations in particle size. The rotary kiln is a slightly inclined steel cylinder resting on supports (support rollers) with an inner liner; it is equipped with a rotary motor that induces a turning motion. This type of gas/solid contactor was invented in the cement industry in the early 19th century.
The emergence of the rotary kiln is closely linked to the creation of Portland cement. In 1824, Joseph Aspdin successfully produced an eminently hydraulic lime by firing at high temperature, in a bottle kiln, a mixture of limestone and clays; he referred to his production as “Portland Cement”. However, the bottle kiln had significant shortcomings, including that it only accepted dry materials; its temperature did not exceed that of the lime kiln; its firing was irregular; it operated only intermittently; and its heating consumed a considerable amount of fuel.
In 1872, Johnson proposed combining the firing and pre-drying of the mixture in one kiln, which he called the “chamber kiln or Johnson kiln.” In 1877, the shaft kiln (a vertical flow kiln) was invented. This design was a modification of the earlier kilns: material and fuel were introduced into the kiln from above. The fired clinker exited at the bottom. The great revolution in the continuity of the cement production process ultimately came with the commissioning of rotary kilns in 1880 by Thomas Crampton, despite experiencing operational problems. According to Redgrave, it was difficult to determine exactly how to improve the operation of rotary kilns, as there was still a great deal of uncertainty as to the exact degree of heat required, the amount of atmospheric air required, the effect of the flame, the influence of steam, and many other aspects which were left to the kiln driver (employee operator) due to the absence of precise scientific laws.
In 1898, Hurry and Seaman improved the design of rotary kilns; they added a cooler to facilitate grinding and improved the quality of the clinker. These improvements thus increased the production volumes of the kiln while producing a more homogeneous result. Satellite or planetary coolers appeared in the 1920s, consisting of a set of tubes (usually 7 to 10) attached to the kiln cylinder.
In 1930, in Germany, the first attempts were made to redesign the kiln system to minimize fuel waste, leading to two important developments: the grate preheater and the gas suspension preheater. Finally, in 1970, kilns equipped with a pre-calciner were introduced in Japan, since then they have become the equipment of choice for large installations around the world. This rotary kiln requires a sealing system at the inlet and outlet between the fixed and moving parts to prevent false air intake in the process.
Today, the scarcity and depletion of fossil fuels (which has promoted reliance on alternative energies), as well as the management of gaseous emissions and fine particles in the atmosphere constitute real challenges facing rotary kilns, particularly those related to cement production. Because of this, many companies around the world are increasingly investing in research for technologies and best practices to counteract environmental impact and contribute to sustainability.
Source: https://www.sciencedirect.com/
