Highly accurate measurement of filling levels and temperatures in ball mills and vibratory mills The article describes a new, highly accurate, system for measuring filling levels in ball mills by measuring the structure-borne sound directly on the rotating mill shell. Data are transmitted wireless. This direct measurement avoids all the familiar disadvantages of “classical” microphones. There is absolutely no cross-sensitivity to other sources of sound, which means that the filling levels in the first and second grinding chambers can be measured independently of one another. In general, the structure-borne sound provides a great deal more information than emitted airborne sound about the state of a ball mill. A new, non-linear, algorithm for evaluating the structure-borne sound levels can indicate the filling levels in the two grinding chambers with an accuracy of better than 2 % of the maximum value. This has made it possible for the first time to set up control systems that keep the material being ground at a constant level inside the mill. The high precision of the measurement in the two chambers permits new insights into the actual behaviour of a ball mill and also makes it possible to assess the state of the intermediate and outlet diaphragms. If required, the temperature of the cement inside the mill can also be measured by a thermocouple in the intermediate diaphragm and transmitted by a third channel in the wireless link.

1 Introduction In spite of recent developments in grinding technology the ball mill is widely used in various industries and still dominant in the cement industry. Because of rising energy prices and in view of the general trend towards large production units there is increasing discussion about the disadvantages of this mill in respect of its relatively high specific energy consumption and its lack of adaptability to different mill feeds. However, to be set against these disadvantages there are also a number of significant advantages, such as: the simple and proven technical design the robustness and relatively long service life of its components the comparatively simple mode of operation of a grinding plant the relatively low capital costs and, last not least, the favourable particle size distribution of the ground material leaving the grinding chamber. The power consumption of a ball mill is determined mainly by the grinding media filling ratio so for a long time efforts have been made to supply the mill with the maximum amount of mill feed and to operate it as stably as possible at the optimum working point.

2 The “classical approach” to measuring the filling level With ball mills this working point is linked directly to the level of the material being ground in the mill. A mill that is too empty, i.e. one that has been underfed, operates extremely uneconomically from the energy point of view while a mill that is too full, i.e. has been overfed, also grinds very ineffectively as the grinding balls fall on a “soft” bed of mill feed and lessen the progress of comminution along the grinding path. In order to be able to optimize a mill it is therefore necessary to measure the level of material being ground. Direct measurement in the mill is not possible so in most cases microphones, so-called “electric ears”, were used for this purpose. They enable a (very) rough estimate to be made of the level of the material being ground in the mill on the basis of the sound intensity – experience shows that an empty mill produces a loud, high pitched, sound while a full mill produces a somewhat duller and quieter sound

Figure 1 Most experienced plant operators are able to deduce the filling level in the mill from the sound that is picked up, although this requires a certain amount of practice and familiarity in dealing with the particular mill. Human hearing does in fact represent a highly complex “evaluation system” that is still not yet fully understood in all its aspects, which is why the majority of “electric ears” are confined to the measurement of the total intensity of the sound or of a frequency band. One basic difficulty when using microphones is that they can only measure the airborne sound that is emitted from the mill. However, the source of the sound is located inside the mill. The sound then has to pass through the ball charge, the lining and the mill shell before it is emitted to the air. During these transmission processes, particularly during the transition to airborne sound, a considerable amount of the information that was originally available about the true “inner life” of the mill is lost (see Fig. 1). In addition to this the microphone has its own very specific problems as a measuring instrument in a mill building:

It measures both the direct sound from the mill and the indirect reflected sound. It also measures the sound from other machines or mills in the vicinity. It is sensitive to misalignment in the azimuthal or lateral direction. A microphone in a mill building also tends to clog up very rapidly with dust, which adversely affects not only the measuring sensitivity but also the calibration 3 Structure-borne sound The problems of the “classical” approach with a microphone can be avoided if the structure-borne sound is measured directly at the mill shell instead of measuring the airborne sound. A piezoceramic sound sensor is mounted directly on the mill shell, which solves several problems at once: The structure-borne sound signal still carries all the information that is not passed on when emitted to the air, so the measurement is substantially more accurate. The sensor cannot measure airborne sound, so interference noise from other mills or units does not matter. The sensor attached rigidly to the mill housing not only avoids any subsequent misalignment but is also not affected by dust, so there is no need for regular cleaning. The structure-borne sound in the mill shell tends to propagate more in the radial rather than the axial direction, with the result that there is no problem with measuring the first and second grinding chambers in the mill separately. The signals that have been received are then amplified by an electronic system, which is also attached to the mill shell, and transmitted by a microprocessor via a high quality wireless link to a small receiver located a few metres from the mill. The entire electronic system and the transmitter on the mill are supplied with electricity by a cradle dynamometer. The cradle is mounted in the housing of the electronic system on the mill so that it can rotate parallel to the mill axis; it drives a small generator as soon as the mill rotates

Figure 2 The filling level of the material being ground can already be measured just by simple evaluation of the structure-borne sound intensity (Fig. 3):

Figure 3 Fig. 3 shows the raw signal from a structure-borne sound sensor during one revolution for an empty mill, for a mill containing a moderate amount of mill feed and for a full mill. The point of highest intensity, where the falling balls strike the wall, is readily detectable in all the signals. Analysis of the envelope curve, together with a precise angle measurement, also makes it possible to provide accurate information about the location of the bottom end of the ball charge. 4 Non-linear evaluation of the pulse signals The mill used in this example was first operated empty and then full. It is clear that information about the level of mill feed in the mill can be obtained here just from the averaged plain intensity signal. The dynamics of the raw signal for the empty:full ratio is at least 5:1. However, this result is not sufficient for more accurate filling level measurements that would also be suitable for setting up a closed control loop. The question therefore arises as to whether even more precise information about the filling level can be obtained from these signals. Detailed investigations carried out on such signals showed that the “classical” methods, such as filtering out frequency bands with the aid of the Fourier transform followed by calculation of the average intensity in the frequency band, do not lead to any substantial improvement. A Fourier transform presupposes that the signal to be investigated is at least in a steady-state condition over the period covered, which does not apply to the acoustic signals from a mill.

Typical noise spectra from ball mills tend to be composed of different short time “transient” signals that result in a random distribution on the time axis. A different picture is obtained if the noise from a ball mill is investigated on the basis of the occurrence of different categories of such signals.

Figure 4 Fig 4. shows the intensities of different signal categories, plotted against time, during the period when the initially empty mill is being slowly filled while in operation. One category that correlates very well with the mill feed filling level is clearly visible in the diagram. A very precise filling level signal can be derived from this after appropriate statistical analysis and calculation of the line of best fit (yellow line in Fig. 4), see Fig. 5:

Figure 5 Tests on different mills have shown that when, for example, this system is applied to a cement mill it is possible to measure the difference in filling level corresponding to throughputs of 80 and 81 t/h. 5 Automatic calibration As a rule it is always possible to find several signal categories in the structure-borne sound signals from ball mills that have statistically significant correlation with the mill feed filling level. This makes it possible to calibrate the system automatically. The ball mill is operated empty and then full for the calibration. During this period all the signal categories are stored so that the signals with the greatest significance can be determined automatically. This calibrates the system, although the filling level signal obtained is naturally a relative value rather than an absolute one as it is only possible to specify the difference between the two operating states, namely “empty” and “full”. This range is then converted to a mill feed filling level of 0 to 100 %. The quality of the calibration therefore depends on whether the mill was actually operated when empty and full respectively. There are no further requirements. However, there is the problem of having to define when a mill is actually “full”. In practice it has proved appropriate to assume that the optimum working point lies at a mill feed filling level of 100 %. However, it is possible at any time to feed the mill at an even higher level, so the display range of the measuring system is arranged to be 1 to 130 %. In addition to this it is always possible to redefine the current filling level as the new 100 % value for subsequent improvement of the calibration.

6 Areas of use, diagnosis of mill conditions Ca. 500 systems for measuring filling levels have now been installed for ball mills with various sizes and configurations – ball mills with one or two grinding chambers operating in closed circuit with separators, open-circuit mills with two or three grinding chambers, “double rotator” mills with central discharge and ball mills with conical mill cylinders. The measuring system has also been installed for vibratory mills, for which wireless transmission is not required. It has been found that the same method of evaluation can also be applied to these mills with slight modifications. The ball mills listed above are used to grind a wide variety of materials, such as cement clinker and granulated blastfurnace slag, cement raw materials like limestone and sand, coal, ceramic raw materials and mineral raw materials like iron and manganese ores as well as bauxite. With all the applications implemented so far it has been possible to determine a precise filling level signal. Taken as a whole this has given rise to a very large potential resource of data and experience amounting to more than thousand machine years. It has now become apparent that the measuring system can be used not just as a pure filling level sensor but also as a tool for diagnosing the state of the mill and the grinding circuit. Accurate filling level measurements on a series of mills have, for example, shown clearly that the first grinding chamber is often under-supplied and that the second chamber is too “full” during operation. These observations have then usually led to changes in the control strategies, to changes in the grinding media charge grading and to different settings of the intermediate diaphragms. With two-chamber mills it often happens that the intermediate or outlet diaphragm becomes blocked or damaged as a result of the great imbalance between the mill feed filling levels in the two chambers. When a measuring unit has been mounted on a mill for a fairly long period it is also possible to obtain information about the wear to the ball charge from very slow changes to the noise characteristics. However, this evaluation has to be carried out individually for each mill. 7 Temperature measurement in the mill If a very fine product is to be produced or if the feed material is very hot then the material being ground may then reach elevated temperatures because the air flow is no longer able to dissipate the heat that is produced. The air flow can be cooled effectively by injecting an accurately metered quantity of water into the second grinding chamber. This avoids direct contact between the material being ground and the water because the water evaporates into the atmosphere and cannot precipitate in liquid form on the cement particles. There has always been the requirement, especially when grinding cement, of measuring the temperature of the material being ground directly in the mill so that the quantity of water needed can be controlled more accurately. The temperature is measured by inserting an armoured thermocouple directly through a hole in the mill cylinder and into the intermediate diaphragm between the first and second grinding chambers and boosting the signal by a special amplifier that requires very little power. An additional, third, transmission channel in the measuring unit then transmits this signal to the base station. The measurement range covers temperatures from 50 to

200 °C, and the resolution is 1 °C. The thermal capacity and the absolute quantity of material are relatively high so only comparatively slow changes occur. As a rule one measurement per minute is quite sufficient. In spite of this it is ensured that the measurement by the thermocouple always takes place when, through the rotation of the mill cylinder, it is located at the bottom and is completely covered by material.

Figure 6 Fig. 6 shows a fully developed measuring system with two filling level sensors and a thermocouple on a relatively small ball mill. For the installation of such a system the sensors are bonded to the mill shell while the electronic system is attached with liner bolts. This avoids having to weld them onto the mill shell. The very accurate signals from the system then drive a fuzzy controller that keeps the material being ground at a constant level in the mill. 8 Operating experience The operating experience of the users of more than 500 filling level measuring systems that have been installed can be summarized as follows: As a rule an increased, sometimes very significantly higher, throughput is achieved if this increase is permitted by the existing mill control system. The high accuracy of the measurement permits better control strategies that can keep the filling levels constant in the two chambers and are therefore able to react to changes in the grindability of the mill feed. Better and more rapid reactions of the control system as a whole are achieved through the more rapid reaction of the filling level to fluctuations in the mass flows.

The continuous availability of a filling level value leads to less specific wear of the grinding media charge. The additional information contained in the structure-borne sound signal permits more extensive diagnosis of the engineering condition of the ball mill and therefore facilitates improved maintenance. The maintenance costs for the electronic ear are very low as the sensor is placed directly on the mill cylinder. The oldest measuring systems have now been in use for eight years and so far have shown no signs of wear KIMA Echtzeitsysteme GmbH www.kimaE.de