|Early rotary kilns||Design features of rotary kilns||Wet and semi-wet process kilns||Precalciner kilns|
After the 1920s, the British industry (like that of the USA) entered into another period of technological complacency, due in part to the very low cost of fuel. The development of more energy-efficient Dry Processes was therefore left to Germany and Japan, where fuel was more expensive. These developments involved the use of “preheaters” in which dry raw meal could be heated, and even calcined, in a more efficient way than was possible in a rotary kiln. The modern rotary kiln is solely used for the sintering stage, for which it is uniquely well adapted.
The initial reluctance to employ dry process in Britain was associated with the industry’s concentration in the chalk areas, where processes other than wet must always be questionable. But wet process continued to be used as the industry spread into the hard-rock areas. The cartel arrangement, although exonerated by successive investigations from the charge of stifling innovation, undoubtedly influenced this. Even on wet process, hard-rock plants can be (and often were) more energy-efficient, since it is easy to obtain much lower slurry moisture contents. Nonetheless, the common pricing arrangement allowed a higher base price for plants using limestone, “the limestone being harder and more costly to process”. No one ever seems to have picked up this point, but it effectively locked the industry into the use of less efficient processes. The fforde Committee, charged with identifying the ill-effects of the cartel, reported that rawmix preparation was "usually accomplished, in this country, by a wet process”. So they were persuaded by the cartel that dry process was something that only "foreigners" did.
The development of dry process being only reluctantly implemented in Britain, the industry entered a “post-industrial” stage of continuous decline. 1973 was the all-time peak year of the UK industry, when a cement production of 20 million tonnes was achieved, the vast majority by energy-inefficient production processes. Subsequent improvements have therefore been achieved against the background of a continually declining and volatile market, scarcely capable of financing the necessary high capital costs of modern systems. In the UK, part of the effort that might have been devoted to developing dry process was instead diverted into experiments with semi-wet process which achieved little other than to delay vital innovation.
Despite these diversions, the modern dry process in Britain began to develop, somewhat goaded on by the recommendations of the fforde Committee, in the 1950s, although the installation of the first dry process - at Cauldon was rationalised as being due to a lack of adequate water supply. In the subsequent half century, efficient dry process kilns have almost completely displaced slurry-fed kilns. The pioneering was done by Blue Circle, who had eight dry process kilns in operation before the start-up of Tunnel's Padeswood A3 in 1966.
The types of dry process kilns fall roughly into four categories:
|Long dry kilns|
|Suspension preheater kilns|