Spellbinding Grinding Improvements

The cement industry is, like every other industry, driven by the best solutions providing efficiency, sustainability, and digitalisation. For the grinding of raw materials, solid fuels, and cements the vertical roller mill (VRM) system is one of the most energy efficient systems available. The combination of three process steps in one system – drying, grinding, separating – makes it very versatile in regard to handling dry and moist feed materials, and grinding these to very high fineness, creating the product properties required by the different market areas. With the introduction of the MVR mill to the market in 2010 a new concept was available for very high throughput rates. By using a larger diameter, a larger contact area resulting from roller size and/or number of rollers, a faster rotational speed, and higher hydraulic force, the mill capacity can be increased.

Whereas the MPS mills for cement grinding and raw material grinding have been equipped with a power range between 500 kW up to 5400 kW resp. 6400 kW, the MVR mill covers a range of installed power between 60 kW and up to 11 700 kW.

With the MultiDrive^® there is sufficient scope to go up to 18 000 kW.

Performance

The performance of a vertical roller mill is defined by a required throughput at a required fineness paired with a low specific thermal and electric energy consumption. For cement grinding, the required product quality is the most important target together with the above-mentioned points. Some areas in general need special attention: feed uniformity, metal detection and extraction, and preventive maintenance to name just a few. The levers to pull for a well performing vertical roller mill are operational parameters such as table speed, gas flow, working pressure, and mechanical adjustments such as dam ring height and covering the nozzle ring.

The success of the VRM in cement grinding was determined by its production of the same or better cement quality. This can be traced back to not only adjusting to the same or similar Particle Size Distribution, but also to the adjustment of feed material properties (clinker C3A) and especially sulfate agent proportioning.

The sulfate agent needs a balanced proportion of di-hydrate, hemi-hydrate and anhydrite. As a VRM has a significantly higher energy efficiency than a ball mill, much less heat is put into the grinding process. As a result, the dehydration degree of the sulfate agent is lower. The lower hemi-hydrate or plaster content can be compensated for by the addition of more gypsum (within the limit according to relevant standards), by the addition of a more reactive form of gypsum, or by the addition of more heat to the system. By installing the G4C® system with a separate mill and hot gas generator to partially calcine the gypsum, the hemi-hydrate content can be controlled exactly for each clinker that is used. This is made possible by setting the outlet temperature of the mill to adjust the proportion of hemi-hydrate. This system is installed with an MVR 6000 C-6 in Australia and supports a tailor-made sulfate agent for the finished product.

An important factor in the characterisation of cement properties is strength development in combination with setting times. National standards define the procedure for testing. Due to differences in those standards, the results of compressive strength development are not comparable to each other. Gebr. Pfeiffer has its own mortar laboratory and collects samples from the operation of MPS and MVR mills to characterise cement product properties. To ensure the reliability of results, the laboratory participates in annual round robin tests (ATIHL and BE CERT).

Concerning the procedure for sample preparation, the proportions of cement, sand, and water are in accordance with EN 196-1. The properties of several cements produced in MPS and MVR mills are listed in Figure 1. Nearly all OPC/CEM I products have developed a 28-day strength of 60 MPa or higher. High 28-day strength figures are achieved as well with a product fineness less than 4000 cm2/g. Early strength after two days is clearly impacted by clinker quality and, for composite cements, the type of composite is an additional factor.

The composite cements achieve high 28-day strengths. Products need to be ground fine enough to enable composites to reach the required strength level. The given normal consistency (NC) figures have been determined in accordance with EN 196-3 and demonstrate that the workability of products from MVR mills meets the demands of industry. Custom adjustments have been made in many cases during the commissioning of MVR mills, for example: PSD adaptation, sulfate agent selection, or the use of the G4C system to get precise plaster-content. These results show that cements that are ground in vertical roller mills are clearly on the same level as cements ground in ball mills.

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