Facilitating cement grinding in vertical mills

Mainly due to lower specific energy consumption (measured in kWh/t of material produced) and higher production (t/h) ­values, vertical cement mills are slowly, but steadily, outnumbering “traditional”, horizontal ball mills. The immediate advantages of such mills can be easily summed up in the following key points:

– ­vertical cement mills are able to reach production values which are significantly higher than the ones achievable with traditional ball mills. As the latter are able to reach an indicative maximum of 180 –200 t/h, vertical mills reach up to 300 t/h.

– lower energy consumption, resulting in a lower specific cost (€/kg).

– while traditional mills have to be first assembled by the manu-facturer and then transported to the industrial site with all the resulting costs as well as logistical problems, vertical mills are usually built “on-site”. This aspect permits reaching mill sizes (and therefore production values) which are not achievable in the case of traditional ball mills.

– vertical mills offer a greater versatility than traditional ones: for example, intermediate silos for cleaning are not necessary, as the transition times between different cement types/compositions are significantly shorter or nonexistent.

– additionally, vertical mills are less sensitive to the eventual high-humidity content of raw materials (given that a sufficient energy source is present).

­Nevertheless, vertical mills also present certain disadvantages, if compared to traditional grinding systems. These aspects can be summed up in the following key points:

– high pressures of the roller presses are required in case high Blaine values are desired.

– ­vertical mills are more sensitive to materials which have a very high fineness (the threshold is set around a maximum of 50  % material < 4 mm).

– ­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 all times. This aspect has major repercussions on the cement temperature. Therefore an external heat supply is needed to guarantee a certain exit temperature, thus increasing the specific production costs.

Grinding aids are polar compounds or mixtures thereof which reduce the surface charges resulting from the grinding of cement raw materials. Grinding aids for traditional mills, also used in ­vertical mills, permit the achievement of performance targets (e. g. fineness, production, compressive strengths) that can ­hardly be achieved without them. The positive effects of ­grinding aids are such that nowadays, the manufacturers of cement mills ­themselves pre-install insertion systems for these products. This ­recently happened to be the case also for vertical grinding ­systems.

Nevertheless, practical experience in the field have highlighted the fact that traditional grinding aids (i.  e. grinding aids developed for tubular ball mills) have shown themselves to be dramatically less efficient, if not completely inefficient, when introduced in vertical grinding mills. This also happened to be true in the case of the very same product utilized in two different industrial scenarios.

After careful observations and investigations performed by the Mapei Technical Assistance Group, it was concluded that this significant difference in behaviour has to be related to a series of causes, where the most determining one is, with all probability, the different ventilation concept applied in vertical mills. In more detail, ventilation (expressed as an air/material ratio) in vertical mills is significantly stronger in vertical mills than in traditional ones. As a result, when traditional grinding aids are introduced at standard insertion points, such as on the raw material conveyor belt and/or directly inside the cement mill, a “stripping” phenomena takes place, which physically eliminates a major amount of grinding aid before it can become effective during the grinding process itself. Additionally, the higher ventilation in vertical mills also contributes significantly to the early evaporation of the grinding aid in the case of traditional introduction systems.

Taking into account the increasing proliferation of vertical mills in the cement production industry as well as the above-mentioned technical issues which derive from utilizing traditional grinding aids in vertical mills, the Mapei Cement Additives Division R&D has developed a novel class of grinding aids especially for use in vertical systems. These particular products contain, in addition to “classic” compounds like ethanolamines, glycols, inorganic salts, etc., a certain amount (ranging from 3 up to wt.-10  %) of a special blend of high-boiling compounds and thickening agents.

The characteristic low-volatility of such compounds ensures that these grinding aids are not affected by the high ventilation standards of vertical mills and remain unchanged until they can participate in the grinding process itself. Additionally, the special chemical activity of ethanolamines, glycols, inorganic salts and their effect on the compressive strengths and cement fineness remains completely unaffected by the presence of such substances.

For the reasons mentioned in the previous paragraph, the insertion point of the grinding aid plays a crucial role in the case of vertical mills. During several industrial trials in various cement plants, Mapei has developed an innovative introduction system which is able to prevent the stripping phenomena caused by the high ventilation, thus maximizing the positive effects of the Mapei VM additives. As is widely known, cement grinding in vertical mills requires the presence of a continuous water jet in order to stabilize the bed of grinding material and therefore prevent the development of excessive vibrations. The water required for this task is stored in normal tanks and pumped directly inside the mill on the grinding table, through a corresponding pipeline. The particular technology developed by Mapei consists of introducing the grinding aid in the water pipeline before it enters the mill. In this way the flux of grinding aids, together with water, is forced directly under the rollers. Thus, the additive is able to be immediately effective in the grinding process. The Mapei introduction system is described in Figures 1–3.

In order to demonstrate the efficacy of the new technology described in the present article, i.  e. the combination of additives from the Mapei VM series and the new introduction system, we report on the results of one particularly representative industrial trial which was carried out in a major Middle Eastern cement plant during April 2010.

The average running conditions during the industrial trial with the MA.G.A./VM 12 grinding aid were as follows: Vertical mill model: LM 56 3+3 C/S; nom. diam.: 5600 mm; number of rollers: 3+3; power installed: 5300 kW; power absorbed: 3500 kW; ventilation: 680 000 m³/h; mill ΔP: 50 mbar; water ­injection: 2.1 %; roller pressure: 75 bar.

According to the European standard DIN EN 197-1, a CEM I-type cement (92  % clinker; 4  % gypsum; 4  % limestone)
was produced under four different grinding conditions: blank (no additive has been introduced), with a reference product (a traditional grinding aid), with the same reference product dosed directly onto the grinding table (in accordance with
the introduction system described in the following paragraph) and finally with an additive from the VM series, the
MA.G.A./VM 12. This product is both a grinding aid and a performance enhancer on the early as well as late compressive strengths (Fig. 4). Pure grinding aids, performance enhancers, workability enhancers and hexavalent chromium reducing agents, specifically developed for the use in vertical mills, are also already available. Data about cement production parameters as well as cement quality are reported in Table 1.

As can be clearly seen, the reference product dosed at traditional insertion points (in this case on the clinker conveyor belt) shows little or no improvement on the production process and cement quality. The insertion of the same reference product at the same dosage together with the water jet directly under the rollers already leads to a certain improvement, demonstrating the validity of this new introduction technology. Nevertheless, the MA.G.A./VM 12 grinding aid clearly stands out and shows strong improvements of the mill production as well as of the cement quality. A comparison between the different compressive strengths which have resulted from the four different scenarios is reported in Figure 4.

A novel class of grinding aids specifically for vertical mills has been presented. The development of such additives has been carried out based on the technical difficulties experienced by cement producers worldwide when using traditional grinding aids. Additives from the MA.G.A./VM series, coupled with the innovative introduction system developed in the field by Mapei technicians, have proved to be able to overcome these particular problems. The resulting performances, in terms of compressive strength enhancement, fineness/production increases etc, are comparable, if not superior, to the ones characteristic of traditional cement additives utilized in a horizontal mill-based production scenario. The efficacy of this new grinding technology has been demonstrated through the data collected during one of the many industrial trials which were performed with these particular products.