Cement Making Equipment in 1924

The year 1924 marked a major transition in the British cement industry. The period 1890 to 1910 had seen massive disruptive change in the technology of manufacture, with the emergence of modern grinding technology allowing a massive improvement in the quality of the product, rotary kilns offering the prospect of massive reductions in cost of labour and fuel, and electrification which made the industry safer and more energy-efficient. These two decades were characterised by revolutionary change, with expensive experimental innovations, both successes and failures, and with successful innovators pushing the more conservative operations out of business.

By 1924, these violent processes had more-or-less played out, and a definite picture was emerging of what a "modern cement plant" should look like. At the same time, the economy was very volatile, and numbers of those at the low end of the efficiency scale were giving up with each periodic downturn. Innovation during the next three decades was slow, and mainly consisted of scaling up existing designs.

The incipient Blue Circle group (consisting of APCM and BPCM), which had 50-60% of national capacity, had been weighed down in its ability to innovate by its wheezy old board of pre-1900 directors. However, its dithering ceased in 1924, when a hostile takeover bid catalysed the clearance of the old lags. A significant part of this process was the preparation of what was essentially a "due-diligence" account of the company's assets (see plant lists for APCM and BPCM). These lists form a remarkable warts-and-all snapshot of the company at that time.

The following articles show an analysis of the information in these lists. They also attempt to include data on the other companies operating at the time, many of which were quite well documented.

Kilns in 1924

By 1924, the transition to rotary kiln production was almost complete; a few large shaft kiln installations still remained, but they all ceased operation by the end of the decade. A few backward plants persisted in use of batch kilns (bottle and chamber kilns). Chinnor had been commissioned as late as 1921 with chamber kilns, while bottle kilns remained in use in the Bridgwater area until the 1940s. A process still very much in progress was the transition from the early, semi-experimental rotary kilns to the mature kiln technology which became available from 1920 onward.

In the table, entries marked with an asterisk (*) were de-commissioned at the time of the survey, mostly permanently, although a few were briefly used later when the market picked up. The totals refer only to capacity in commission.

The total operational capacity was:

Eleven operational plants had less than 40 t/d capacity; all except Chinnor had gone by the end of WWII. Only six plants had over 500 t/d capacity:

Of the 111 rotary kilns listed, only five used dry process, and all those were small. Those at Kirtlington and Premier, and the smaller Coltness kiln, closed before the end of the decade, leaving only the remaining Coltness kiln and Wishaw - small Scottish kilns using slag as a raw material. The universal adoption of wet process was driven by the need for better rawmix homogenisation, while with very low energy prices, the increased fuel usage and polluting emissions were a minor consideration.

Finish Mills in 1924

In the 1890s, the majority of British Portland cement had been ground, as it had been since its invention, by flat stone mills. The major innovation in cement quality, driven mainly by Germany, was the recognition that cement must be very finely ground if its full strength-giving potential is to be realised. Flat stones were quite incapable of producing cement of the required fineness. By 1924, although authorities such as Butler were still maintaining that stone-ground cement was better, flat stone mills had almost entirely disappeared.

Primitive edge-runner mills had for a long time been used to pre-crush the clinker prior to feeding to the flat stones, allowing them to achieve a better output, and this established a standard model of grinding as a two-stage process. Peripheral-discharge ball mills were introduced in Germany in 1876, and after a period of latency, these rapidly became favoured for the pre-grinding stage in Britain in the 1890s. The tube mill for fine grinding emerged in 1891, and by 1900 the more progressive firms were adopting them, having realised that they offered the possibility of almost unlimited product fineness, particularly if the flint media initially used were replaced with steel.

At the beginning of the twentieth century, a ball mill followed by a tube mill became the standard new installation. Combining the two stages in a single unit evolved gradually. Such equipment, with multiple chambers containing successively smaller grinding media, and separated by diaphragms, within a single tube were initially called combination mills. These reached some degree of standard design in the 1920s, and because the industry had invested heavily in still-functional ball-and-tube systems, only new entrants and plants with recent uprates had them by 1924.

A less obvious innovation at this time was the introduction of round-the-clock working. Because of the high labour cost of flat-stone milling, such systems were typically designed for day work with a 72 hour working week. With the advent of ball mills, the work of milling was reduced and de-skilled. This meant that the ratio of capital cost to labour cost increased dramatically, and new milling systems were instead sized to operate continuously during summer high-demand periods.

In the table, mills marked with an asterisk (*) are de-commissioned. The "t/h" column shows the capacity throughput of the system stage in tonnes per hour.

Coal Mills in 1924

With the installation of rotary kilns from 1900 onwards, plants needed coal mills to produce the kiln fuel. This occurred at a time when grinding equipment was still in rapid development. The first generation of coal mills therefore became rapidly obsolete, and later rotary kiln installations had better firing systems. The 1924 plant schedules for APCM and BPCM provide a snapshot of the early installations that were mostly on the point of replacement with newer equipment. There also exists fairly reliable information for most of the other companies in the industry for 1924.

A feature of the firing systems of the time is that they are almost entirely indirect in design. A few direct-firing high-speed mills had been installed in the early 1920s, but major expansion of direct-firing systems only began in the second half of the decade.

In the table, mills marked with an asterisk (*) are de-commissioned. The "duty" column shows the capacity throughput of the system in dry standard tonnes per day.

NOTES

Note 1.

Note 2.