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The development of the roll presses in pre-grinding has shown the interest of compression grinding from the power consumption point of view with respect to the ball mill. The Liming Research Centre has carried out many works in this domain and has particularly studied the interest of this process in finish grinding. The results of these researches conducted on a pilot plant with a throughput of one tonne per hour can be summarised as follows:
• The compression grinding process leads to specific energy values markedly lower than those obtained in a ball mill, even under optimal conditions. The substitution ratios (ratio of the specific energy required by the ball mill to that required by the roll press at the same fineness) vary between 1.4 and 1.8 for cement. These figures obtained by means of the Liming-Index correspond to grinding energy values of ball mills under optimal conditions and therefore are minimum values. For industrial plants, the substitution ratios would rather be in the order of 1.6 - 2.0 in average, under assumption of an average efficiency of 0.9 with respect to the Liming-Index.
• The grinding pressure is high so as to perform a maximum grinding work in only one pass and limit the circulating load, but does not always correspond to the optimal value. In general, grinding efficiency increases with pressure reduction (cements with limestone or pozzolana addition, cement raw mix). Furthermore, the use of high pressures causes lifetime and reliability problems.
• The substitution ratio increases with the fineness roughly proportionally to the square root of the Blaine value; on the other hand, the nip becomes more difficult and less stable and the circulating load increases proportionally to the fineness. It must be noted that the increase of the circulating load is accompanied by an important modification of the finished product size distribution curve.
• For reasons related to the nip dynamics, the grinding speed is limited. Close to this maximum speed, vibration problems are frequent.
The plant is fitted with sensors able to give all the characteristics of the circuit. The mill is fitted with a direct measurement of the mechanical torque in order to assess energy consumption with accuracy.
Reflections and investigations were initiated to find another mining process based on compression in order to keep the energy performance of this process, while grinding the material in several steps to prevent a too high circulating load and grinding pressure. The principle of a cylindrical shell driven in rotation on its horizontal axis, and one or several idle rolls seemed to be a possible solution. The point that seemed to be most difficult was the control of the material circulation for which centrifugal force had to play an important role. The construction of a 1 t/h pilot plant gave birth to this new process.
After the development phase of the machine, complete tests were conducted on raw mixes and different types of cement: Portland cement, cement with limestone or pozzolana. These results were compared with tests carried out on the roll press in integral grinding. Finenesses from 3000 to 5000 Blaine for cements and 15 % to 0.2 % rejects above 80 μm for raw mixes are tested. For the cements, the conventional tests on mortar and concrete were carried out and compared with industrial results.
• For the raw mixes :
* Substitution ratios vary from 1.5 to 2.2 depending on the settings.
* Very high finenesses at 80 μm are easily obtained.
* Water contents up to 20% were successfully tested in using the drying capability of the TSV classifier.
For the cements:
* Substitution ratios vary from 1.3 to 1.75 depending on the settings. The size distribution curves show a course intermediate between the roll press and the ball mill results with however a very marked signature of the TSV for the coarse fraction (larger than d80). But the slope for the fines particles distribution can be adjusted with the setting of the circulating load.
* At the same Blaine fineness, mortar and concrete strengths are higher than those obtained in ball mill and workability is equivalent.
Energy efficiency values are very close to those of the roll press or even higher for cement with a high addition ratio (unless the grinding pressure is considerably reduced in the press and the circulating load consequently increased). The grinding principle based on multiple compressions permits, for a same finished product, to vary the circulating load and to assess the corresponding effect on the size distribution curve and the performances (specific energy, cement quality, ...). With this additional degree of freedom, HOROMILL is, in closed-circuit grinding, more akin to the ball mill than to the press. Therefore, even for very fine cements, it is possible to adopt a reasonable circulating load while using a limited grinding pressure.
Liming has just developed a new mill-horomill, for grinding cement, raw meal and minerals. This new grinding process is designed for finish grinding and also for pre-grinding. The energy saving is similar as roll press, roughly 30% to 50% in respect to the ball mill, but with a moderate circulating load and grinding pressure. After a research phase on a 1 t/h pilot plant, a first 25 t/h industrial plant was achieved in co-operation with European countries. The performances and the cement quality, strength and workability, were checked on this industrial size.
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