Boosting Grinding Systems

Retrofitting of ball mills is very common to enhance grinding output. During the process, close attention has to be paid to the mill internals, the material of construction and the design aspect.

With focus on increasing productivity of existing grinding systems, modern cement plants are upgrading their mills to high throughput grinding [Ref. 1]. These upgradations or new grinding unit projects are increasingly prescribing pre-crushing of input materials using energy efficient pre-grinders (roller presses or VRMs in semi-finish mode) combined with single chamber ball mills in closed circuit with high efficiency dynamic separator to handle finish grinding.

The single chamber ball mills (mostly converted from previously available twin chamber mills) generally provide larger grinding volume and mainly drive power for high throughput finish grinding, essential to meet high Blaine requirements of additive cement. During this retrofitting exercise, not enough attention is given to optimising the ball mill internals despite significant change in granulometry and grinding characteristics of feed entering the ball mill.

Hence, when mill internals are conventionally designed based solely on ball mill operations alone without considering the changes in the complete grinding circuit, they fail to achieve optimised grinding efficiency, both physically and economically. "Aqua-Duo" shell liners supplied by Aqua Alloys Pvt Ltd are semi-classification type, thin corrugated liners specifically designed to suit grinding in high re-circulation ball mills receiving pre-ground fine material from roller press/re-circulation from separator. Apart from the basic purpose of mill shell protection, the new design ensures uniform mix of grinding media throughout the length to handle higher percentage of finer feed material, provides increased effective mill volume for higher throughput and corrugation for higher torque transmission.

Trials at different cement plants in India have provided insights into the possibility of enhancing ball mill throughput and optimisation of specific power consumption after installation of "Aqua-Duo" type shell liners and maintaining the previous percentage of media filling level. A 5 – 6 per cent increase in mill output is achieved for the same or better overall circuit specific power consumption and product quality after replacing the conventional classifying liners with custom design liners. This article describes the reasons for unconventional liner design, outlines conceptualisation of new liner design, lists the advantages over conventional liners and also summarises the results of trials at four different sites.

Justification for unconventional liner design
The following points provide insights for unconventional thinking as regard to holistic liner design considering changes taking place in modern grinding circuits. It also outlines why media segregation and hence classifying liner design (Ref. Figure 1) is not useful for finish grinding with higher degree of additives and high throughput mills.

• Classifying liners are mainly designed for open circuit systems or low re-circulation closed circuit mills where the percentage of higher size material in feed is higher and media segregation was essential. The media classification concept loses its liner design advantage with mills being upgraded to handle higher throughput and higher percentage of additives, finer than clinker feed.
• New mills can operate with re-circulation factor of nearly 2 to 2.5. Hence actual ball mill feed (pre-ground material from roller press circuit combined with finer reject material and additives) always has mixed granulometry. Only the higher size media near the inlet is sub-optimal for finer re-circulation material (80 per cent <200 micron) or finer additives (fly ash, slag) coming into the mill. Hence classification of media is of lesser relevance in such high re-circulation systems.
• With increased production of high Blaine cement (due to higher percentage of additives) finish grinding is mainly governed by smaller size grinding media distribution (25 mm and below with 17or15 mm being the smallest size). The classification efficiency of conventional liners is severely affected due to smaller piece weight difference in these sizes.
• Ball mills are designed to provide highest productivity at an average volumetric media filling of 25 to 27 per cent [Ref. 2]. Thicker classifying liners with slope profile reduce the mill effective volume and hence accommodate less grinding media charge for the same optimum loading. This limits not only the mill throughput but also the opportunity to harness additional drive loads available during retrofits.

Concept of thin, Semi-Classification, High Torque Transmission Liners
Figure 2 highlights different grinding mechanisms and media action with respect to mill % critical speed and grinding media filling. It shows how the new design contributes for higher productivity in the normal operating region of ball mills by shifting the curve down and increasing the cascading region (good for fine grinding). For the given mill (with fixed % critical speed) the overall thinner cross-section of Aqua-Duo liner design provides scope for higher media charge for same optimum volume filling level (near 25%). Additional grinding media draws higher load at better transmission efficiency (due to optimum filling level) allowing mill to handle higher throughput for same Blaine/residue improvement across the chamber. In between hump liners (used for larger media size distributions) partially contributes to media classification, though with less efficiency. If the same extra grinding media is added with conventional liners the overall mill efficiency will drop as the filling level exceeds optimum level and arm of gravity shifts towards center. The corrugated lining (specially designed to handle 17 to 30mm media) accentuates the tumbling motion increasing the attrition grinding action, The feature also provides higher friction between liner and media and line contact for grinding, enhance the torque transmission contributing more to fine particle breakage.

Field Trial Results
The new design has been implemented and tried in several single chamber cement mills with roller press as pregrinder. The overall performance is encouraging with improvements either in production or quality levels. The best performance improvements are obviously seen in the circuits with roller presses and single chamber mills. The results for four of such mills (before and after replacing the conventional classifying liners) are outlined in Table 2. All the four mill circuits had roller presses as pregrinder setup, single chamber ball mills (over 4 m ID and 25 to 15 mm media charge) in closed circuit with a dynamic separator.

Summary
Re-optimisation of mill liners is the need of the modern grinding systems design. Aqua-Duo design provides effective alternative combining mechanical and process requirements of such systems. The advantages over conventional liners are clearly seen. The field trials on large scale cement mills are positive for output, power efficiency and product quality improvements.

NOTE: Further technical data, detailed results can be shared/discussed on personal request to the author. (email: contact@aqualloys.com)

References
[1]"Innovations in Portland Cement Manufacturing", by J I Bhatty, FM Miller, SH Kosmatka, Portland Cement Association publication, 2004
[2]"Ball charge loading " Impact on specific power consumption and capacity", David S Fortsch, IEEE-IAS Cement Industry Committee, 2006.

This article has been authored by Dr Raghuraj K Rao, Aqua Alloys Pvt Ltd, Belgaum