Due to the stringent demands of energy saving, the use of vertical roller mills for cement grinding now is very common. Even though the reliability and operation stability of modern VRMs are very good, there is still room for further improvements with the use of specific cement additives formulated for the use in this type of grinding systems. In this paper we describe in details an industrial test performed during the production of slag cement in a vertical mill, showing how the use of grinding aids can improve both process parameters and cement quality. The positive effects of chemical addition to particle size distribution and separator efficiency are discussed in details.

In 2015 the worldwide cement production is estimated to have reached 4.3 billion tons. In parallel to this huge increase, the need of cost reduction has become mandatory. As a result, mainly due to lower specific energy consumption (measured in kWh/t of produced material) and higher production (t/h) values,vertical roller mill is slowly, but steadily, outnumbering horizontal ball mills. Vertical cement mills are able to reach production values which are significantly higher than traditional ball mills, in some cases it up to 300 t/h. Empirically, It can be estimated through a simple heat balance that in ball mills only 10% of the energy is used to increase cement fineness, the rest being dissipated basically in form of heat. VRMs allow the exploitation of around 30% of the energy supplied: these results in lower energy consumption, in terms of kWh/t. In addition, a VRM offer a greater versatility than traditional ball mill: the space is reduced, having grinding and separation units gathered in the same machinery, gas flow is higher, allowing the use of material with higher humidity content.

Nevertheless, cement vertical mill also present certain disadvantages

If compared to traditional grinding systems. High pressures of the roller presses are required in case high Blaine values are desired, and a significant amount of water has to be added in the grinding process in order to keep the vibration level of the whole grinding system low at any time. In some cases, an external heat supply is needed to eliminate the water used to stabilize the grinding bed and avoid negative effects of partial hydration of clinker and insufficient dehydration of gypsum.

Cement vertical mill also present certain advantages (Improving grinding in VRM)

There is the possibility of further improvement of cement grinding in VRM. As described elsewhere, the use of grinding aids specifically formulated for the use in vertical roller mill allows the reduction in vibration, which lead to the following advantages: - possibility of fresh feed increasing and higher mill output; - reduction of water, with consequent decrease of gas temperature and hest needed; In addition, the use of a suitable grinding aid can improve the particle size distribution of cement and positively influence clinker hydration, thus improving the cement quality.