Since the presentation at the 1993 VDZ Congress of the first industrial application of the mill, many units have been installed at plants around the world, and we now have the opportunity to assess its performance in a variety of applications.

The principle has been presented so often that it is not the purpose of this article to put forward another description of the machine other than the briefest outline. Horomill is based on the particle bed compression principle of energy saving in which a shell, supported by slide shoes, rotates at a hypercritical speed. Inside the shell, a hydraulic roller exerts a pressure on the grinding bed, often causing material to become attached to the inner face of the shell by hypercritical shell speed. Scrapers are employed to remove this material, which then falls onto a diverting system, which pushes the material against the shell face for regrinding and adjusts the motion of this material inside the mill. The material is ground several times before leaving the mill and being classified by a high-efficiency TSV classifier.

Tables 1 and 2 show which plants are currently operating the Horomill and the typical results. It is clear that the specific energy is low compared to conventional solutions, with the mill consumption for type 1 and type 2 cements being in the range 14 to19kWh per ton (at motor terminals), and the plant auxiliaries consumption generally being from 5 to 7kWh per ton.

The results obtained on the first plants to be commissioned are plotted on Table 3, and give rise to the following points:

* For material bed grinding machines, the water demand of cement on pure paste is equal to or higher than the value found for ordinary tube mills (from 0% to 1%, more rarely to 3%, when compared to open circuit mills).

* This typical mill feature has no influence on the results when considering mortar or concrete, where the parameters are similar to those of tube mills. For example, statistics from one of the countries where the Horomill is being used for clinker grinding show that water demand for concrete was about 205 kg per m3 when cement coming from other plants employing tube mills showed water demands of between 200 and 210 kg per m3. (Average 205 kg per m cubed).

* Similarly, setting times for concrete made with cement ground in the Horomill were about 320 minutes for initial and 450 minutes for final setting. Values for ball mills ranged from 300 to 320 minutes for initial and 405 to 465 minutes for final.

This confirms that the Horomill allows the milling of "non-exotic" cement in a similar manner to conventional machines.

Obviously, a small difference in the water demand always exists on the basis of a pure cement probe, but as there is no physical action in relation to this test, everybody, with the exception of one European country, forgets it.

Finally, the results from Horomill grinding compared to tube mills is equivalent to the difference found by users when third-generation classifiers, giving more Rosin rammler slope, began to be used causing more water demand on the pure cement probe. Third-generation classifier cement is good, even if the grain size is different from cement ground in an old grinding plant.

Raw grinding Initially, many clients were sceptical about using the new system for raw meal grinding with many being unable to understand how the moisture could be removed from the material by a non-airswept mill.

However, extensive tests conducted in Trino (Italy) and industrial tests in Tepetzingo (Mexico) have shown that the Horomill is able to efficiently dry materials with high moisture levels and that the principles involved allow efficient grinding of raw material.

The Moctezuma Tepezingo plant is now using the Horomill for both cement and raw material grinding, and the results for raw material can be seen in Table 4. On the basis of these results, another manufacturer, Galadari in Pakistan, intends to use the Horomill for grinding both cement and raw materials, when commissioning occurs next year.

Other applications Following the presentation at VDZ in 1993, the first client for the machine was a Heidelberg subsidiary, which ordered a Horomill for its SMA Faulquemont plant in France. The mill is currently grinding 12 mtph of anhydrite at 5500 Blaine. Power consumption is claimed to be three times lower than the vibratory mill used before.

Again considering anhydrite, Maxit has ordered a 40-mtph mill for Krolpa (Thuringen), which should be in full operation in the middle part of 1998.

Many suppliers of ground calcium carbonate (GCC) also have expressed an interest in the Horomill solution, and two mills have been ordered so far; one was scheduled for commissioning at the beginning of 1998.

The curves in Table 5 show the results from the Horomill grinding of different products, the substitution ratio is defined as the number of kW consumed when using a tube mill, which can be replaced by one kW using a Horomill.

Conclusion The Horomill has been shown to perform a variety of grinding functions with very low power consumption in the 21 mills sold for cement, raw material, and other applications.