Cement Kiln - Early History

Early History

Portland cement clinker was first made (in 1824) in a modified form of the traditional static lime kiln. The basic, egg-cup shaped lime kiln was provided with a conical or beehive shaped extension to increase draught and thus obtain the higher temperature needed to make cement clinker. For nearly half a century, this design, and minor modifications, remained the only method of manufacture. The kiln was restricted in size by the strength of the chunks of rawmix: if the charge in the kiln collapsed under its own weight, the kiln would be extinguished. For this reason, beehive kilns never made more than 30 tonnes of clinker per batch. A batch took one week to turn around: a day to fill the kiln, three days to burn off, two days to cool, and a day to unload. Thus, a kiln would produce about 1500 tonnes per year.

A kiln is basically an industrial oven, and although the term is generic, several quite distinctive designs have been used over the years. Although perhaps more normally associated with pottery making, both ‘Bottle’ and their very close relatives ‘Beehive’ kilns, were also the central feature of any cement works. Early designs tended to be updraft kilns, which were often built as a straight sided cone into which the flame was introduced at, or below, floor level. Reaching heights of up to 70 ft, the dome or bottle shape of the kiln, known as the ‘hovel’, would be quite a prominent landmark. As well as protecting the inner kiln or ‘crown’, the opening at the top of the hovel also acted as a flue, to remove the smoke and exhaust gases that were produced during the production process. There was a three to four foot gap between the outer wall of the hovel and inner shell of the crown. Due to the fact that the 1-foot-thick (0.30 m) crown wall would expand and contract during firing, it was strengthened with a number of iron bands, known as ‘bonts’. These were set twelve inches apart and ran right around the circular oven. The development of downdraft kilns in the early 20th Century proved to be much more fuel efficient and were designed to force the heated air to circulate more around the kiln. The design incorporated a gentle curve at the 'shoulders' of the kiln, which served to reflect the rising heat from the fire at the bottom of the kiln, back down again over the material. The smoke and exhaust was then sucked out through holes at the bottom of the kiln via a flue, which was connected to a nearby chimney. The chimney would also serve a number of neighbouring kilns as well. The kiln would be fired for several days to achieve the high temperatures required to produce cement clinker, and although the above methods were successful, the problem with any batch kiln was that it was intermittent and once the product had been produced, the fire had to be extinguished and the contents allowed to cool. This not only wasted a lot of the heat, but also added to the expense of the finished product.

In order to save money on fuel, a kiln was required that could run almost continuously, whilst the raw material was somehow fed through it. It was this scenario that lead to the development of the ‘Chamber’ kiln in the late 1850s. This particular kiln comprised a number of individual chambers, which were arranged so that the hot flue gases from one chamber, were drawn off and used to pre-heat the material in the following chambers, before they were drawn up the chimney. Once the first chamber had been filled with raw material, coal was added through the roof holes of the chamber and was then set alight. At the same time, the second chamber was being filled with raw material. The airflow from the first chamber was then adjusted, using a number of dampers, to funnel the hot air through to the second chamber to pre-heat the material. More coal was then poured into the second chamber and ignited, as the third chamber was being filled and so on. This process continued along the length of the kiln, so that by the time the last chamber had been fired, the first chamber had already been cleared and re-filled with more raw material so that the process could start again. Although such chamber kilns were still being installed as late as 1900, the development of the rotary kiln was already starting to have a major impact. The rotary kiln was a major advancement for the industry as it provided the continuous production of a much more uniform product in larger quantities.

Around 1885, experiments began on design of continuous kilns. One design was the shaft kiln, similar in design to a blast furnace. Rawmix in the form of lumps and fuel were continuously added at the top, and clinker was continually withdrawn at the bottom. Air was blown through under pressure from the base to combust the fuel. The shaft kiln had a brief period of use before it was eclipsed by the rotary kiln, but it had a limited renaissance from 1970 onward in China and elsewhere, when it was used for small-scale, low-tech plants in rural areas away from transport routes. Several thousand such kilns were constructed in China. A typical shaft kiln produces 100-200 tonnes per day.

From 1885, trials began on the development of the rotary kiln, which today accounts for more than 95% of world production.