GB2111193A - Method and apparatus for assessing the quality of cement clinker - Google Patents

Abstract

The quality of cement clinker 11 is assessed with regard to its reflectance by using a photoelectric cell arrangement 10 which receives light from a pulsed infrared source 13 irradiating a bed of clinker transported on a conveyor-belt 12. The clinker quality is related to its colour (reflectance), which in turn is related to a high or low free calcium oxide content. Arrangements are disclosed to eliminate errors due to background illumination. To compensate for changes in optical path, due to changes in the height and disposition of the clinker bed on the conveyor, the field of illumination of the light source overlaps the field of view of the photoelectric cell arrangement by a predetermined amount. <IMAGE>

Description

SPECIFICATION Method and apparatus for assessing the quality of cement clinker This invention relates to a method and apparatus for assessing the quality of cement clinker. The invention may be applied, for example, in a cement works where cement clinker is being transported on a conveyor belt past a testing station, and an indication or control signal is required so that corrective action can be taken when the quality of the cement clinker falls below an acceptable level.

In the process of making cement, the output from a cement kiln, which is known as cement clinker, is fed to a cement mill. It is essential to ensure that the quality of the cement clinker which is fed to the mill is sufficient to ensure the production of good quality cement. Good quality or well "burned" clinker, i.e. having a low free calcium oxide content, is dark grey/black in colour.

Underburned, poor quality clinker, which results from kiln lightups, stoppages, etc., can range from a "dirty yellow" colour to light grey/black. We have found that this colour change, which is related to relatively high or low free calcium oxide contents in the clinker, can be used to give an indication of clinker quality. The invention provides a method and apparatus for assessing the quality of cement clinker based on this colour change.

According to the invention, a method of assessing the quality of cement clinker comprises the steps of: (a) irradiating the clinker, at a testing station, with light from a light source, (b) receiving light reflected from said clinker by receiving means which provide a corresponding detection signal, and (c) processing the detection signal to provide an indication and/or control signal, with respect to the reflectance of the clinker, the latter signal being utilised to distinguish between clinker having relatively high or low free calcium oxide contents.

Preferably, a pulsed infra-red light source is used to irradiate the clinker, as this provides the advantages of (a) a high intensity of illumination, (b) exclusion of background illumination (due to the use of a suitable bandpass filter), and (c) a proportional relationship between the amplitude of the detection signal and the colour change of the clinker. The processing also preferably employs AC amplification further to exclude variations in background illumination.

Typically, the clinker is transported on a conveyor-belt past the testing station and the light source provides a given field of illumination on the clinker bed. Light reflected from the clinker is received by a photoelectric cell arrangement.

Preferably compensation is made for changes in the optical path length of light which is first transmitted from the light source to the clinker bed, and is then reflected from the clinker bed to the light receiving means. Such changes can be caused by variations in the clinker bed height and disposition of the clinker on the conveyor-belt. The compensation is affected by arranging for (a) the field of irradiation of the clinker by the light source to intersect with the field of view of the light receiving means of the clinker, and (b) the zone of intersection to decrease when the clinker height increases.

The invention also provides apparatus for assessing the quality of cement clinker, the apparatus comprising means for transporting the clinker past the testing station which includes a light source for irradiating the clinker; photoelectric means for receiving light reflected from the clinker and for thereby generating a detection signal; and processing means for processing said detection signal, said processing means including means to compare the amplitude of the detection signal with a predetermined threshold value, which threshold value is related to a given reflectance of cement clinker having a predetermined free calcium oxide content, the comparison means providing an output which is utilised as an indication or control signal.

The apparatus may include a warning device, or a control device, which receives the signal from the processing means whereby corrective action is taken when the quality of the clinker falls below a predetermined level.

An example of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of apparatus for assessing the quality of clinker, Fig. 2 schematically illustrates an infra-red light generating circuit, Fig. 3 schematically illustrates a circuit having a photoelectric cell for receiving the infra-red light and for processing a corresponding signal, Figs. 4a and 4b form Fig. 4 and schematically illustrate a timing and comparison circuit for processing the signal generated by the circuit of Fig. 3, Fig. 5 schematically illustrates an indicator arrangement for use with the circuits of Figs. 3 and 4, Fig. 6 schematically illustrates the optical arrangement of the apparatus with respect to a bed of clinker on a conveyor belt, Fig. 7 is a plan view for explaining the optical system, and Fig. 8 is a graph illustrating an approximate relationship between % free lime (calcium oxide) in cement clinker to the amplitude of a photoelectrically-detected signal.

Referring to Figs. 1 and 6, the apparatus generally comprises an infra-red light source 1 which is driven by a power amplifier 2 connected to an oscillator 3 having a low duty cycle. This circuitry provides a high-intensity, pulsed beam of infra-red light which is directed by tube 10 onto a bed of cement clinker 11 which is transported by a conveyor belt 1 2. Some of the infra-red light, which is reflected from the upper surface of the clinker bed 11, is received by a photoelectric cell 4 mounted in a tube 13. The photoelectric cell 4 is connected to a selective amplifier 5 which supplies a signal to an output stage 6 for driving a visual display 7 and alarm circuits 8. The necessary requirements are met by a power supply unit 9 connected as shown in Fig. 1.

Referring to Fig. 6, the light source 1 and photoelectric cell 4 (mounted in respective tubes 10, 13) are located in a plane which is about 45 cm from the upper surface of the conveyor belt 12. This distance is limited to a maximum determined by the power of the infra-red light source and to a minimum by the need to provide sufficient clearance to allow for belt overload. The tubes 10, 13 are open (i.e. there are no lenses or filters etc. between the respective ends of the tubes and the light source 1 and photoelectric cell) and the tubes are air-purgd to eliminate or reduce dust build-up (as explained below).The tubes are also arranged so that the field of illumination of source 1 slightly overlaps with the field of view of photoelectric cell 4 (Fig. 7) to provide a kind of "parallax" effect to compensate for changes in the height of cement clinker on the belt 12 (as will be explained in more detail below).

The reflectance of the clinker, which is largely due to its free calcium oxide content, causes a proportional amount of infra-red light to be reflected into the photoelectric cell 4. As the amplitude of the infra-red detection signal is substantially proportional to the colour change of the cement clinker, as shown by the experimental readings plotted in the graph of Fig. 8, a measure of this amplitude gives an approximate measure of clinker quality.

The selective amplifier 5 (Fig. 1) has predetermined bandpass characteristics designed to exclude signal pick-up, by the photoelectric cell 4, caused by local mains-operated illumination, and to accept a narrow band of frequencies centred on the pulse frequency (or a harmonic thereof) of the pulsed infra-red source (which differs significantly from the mains frequency).

Variations in ambient daylight illumination are further excluded by the use of AC amplification in the selective amplifier 5.

The output stage 6 includes a comparator (not shown) which compares a threshold signal (representing a given or acceptable free calcium oxide content in the clinker) with the infra-red detection signal. If the amplitude of the detection signal rises above that of the threshold signal, the output stage 6 causes visible and audible warnings to be given by display 7 and alarm 8, so that corrective action can be taken. The output of stage 6 can also be used to provide a control signal whereby corrective action is automatically taken if the detection signal rises above the threshold signal.

The centre frequency of the infra-red light from the source 1 is far removed from the frequency band of any low-level infra-red emission from hot clinker on the belt 12 and so such low-level emission does not affect the apparatus.

A more detailed description will now be given by the circuitry.

Fig. 2 shows the infra-red light generating circuit in more detail. The source 1 comprises an array of 12 light emitting diodes (LED), which are connected by terminals T1 -T3 to an output stage including transistors TR 1, TR2 and TR3, TR4 connected in Darlington pairs. These act as constant peak current generators to provide a constant emitter current of about 300 mA. A unijunction transistor U1 is resistance/ capacitance coupled to act as the oscillator 3 (Fig. 1) and thereby provide pulses of 0.2 mS duration with an interval controlled by a potentiometer P1.These pulses are supplied to one input of an operational amplifier (OPAMP) Al, the output of which is clipped to a consistent peak amplitude of 6.2 V by a zener diode Z1 These amplified and clipped pulses drive the bases of transistors TR1, TR3. A potentiometer P2 may be used to adjust the emitter current for the purpose of calibration.

Fig. 3 illustrates the signal detector circuitry in more detail. The photoelectric cell 4, which is connected to the circuit by terminals T5, T6, generates pulses as a result of receiving the pulsed infra-red light generated by the source 1 and reflected from the clinker bed. These pulses are AC coupled to OPAMPS A2 and A3 which have a frequency response designed to exclude 100 Hz signals that can be introduced by local fluorescent lamps. The gain of amplifier A3 is variably controlled by potentiometer P3 in order to adjust the sensitivity of the receiving circuit. The output of amplifier A3 is AC coupled to a further OPAMP A4, which has a gain of 1 and which is used to introduce zero shift into the final output. A potentiometer P4 enables adjustment of the zero point of a meter M1 used in the circuit of Fig. 5 (see below).A diode D2 restores the signal to a zero reference level, because some differentiation of the signal occurs in the previous stages. A diode D3 connects the output of amplifier A4 to the input of a further OPAMP A5. Diode D3 rectifies the output pulses, as the load presented by A5 is very high. Reverse leakage of diode D3 mainly controls the time constant of a charging circuit which includes capacitor C1, the time constant being of the order of 10 seconds. Resistor R1 is included in series with diode D3 in order to minimise interference transients which may be introduced by the mains supply.

The output of amplifier A5, is supplied to an opto-isolator I (which is of known construction) in order to isolate the output stages and thereby prevent earth loops on site. It gives a voltage transfer of about 1:1 with a zero shift of about 2.5 volts. The base of a transistor TR5 is required to be at 2.1 volts above ground to give a zero output level of 4 mA. Therefore, the operating levels of amplifiers A4, A5 are about 4.8 V above ground to remove the necessity for dual voltage supplies for these amplifiers. Transistors TR5, TR6 form a load independent output stage producing 4-20 mA, or 1--5 volts into a load which may vary from zero to 300 ohms. References SR designate integrated voltage control circuits.

Fig. 4 illustrates a further part of the signal processing circuitry in more detail. The output from terminal T4 of the receiver circuitry shown in Fig. 3 is supplied to terminal T4 of the circuit shown in Fig. 4b. An output of 1--5 volts is thereby fed to a non-inverting input of a differential amplifier (OPAMP) A6 via an integrating network including resistor R2 and capacitor C2. Amplifier A6 acts as a comparator to compare the output voltage, generated by the photocell 4, with a standard reference voltage, adjusted by potentiometer P5 connected to the inverting input of amplifier A6. Amplifier A6 is operated in an open-ended mode giving a very high gain and rapid switching.The output of amplifier A6 switches driver transistors TR7, TR8 which drive green and red LED indicators LED 16, LED 1 7 (shown in Fig. 5). Transistor TR8 also drives a relay having a coil W to provide remote alarm facilities.

A resistor R3, diode D6 and potentiometer P6 provide a 1 volt reference level for zero suppression of a meter M1 (Fig. 5) which is connected to terminals T4, T5. The meter M1 (Fig. 5) is used to monitor the output signal and the alarm set point (see below).

An integrated timing circuitTiM1 is used to introduce a calibrated signal into the output at regular intervals, such as every four hours, to check correct performance of the instrument, e.g.

to indicate the need to clean the optics (see below). An integrated timing circuit TIM2 provides a 40 sec calibration interval, during which the contacts of a relay REL1 are changed over and the potential at terminal T9 (Fig. 2) - which is usually at zero volts -- rises to a value determined by the resistance of potentiometer P2 and thermistor TH 1 (Fig. 2). Since the latter resistance is much higher than that of the emitter circuit of the output stages TR1-TR4, the emitter current falls to a negligible value effectively turning off the main source 1 of infra-red radiation (i.e. during the calibration interval).During the calibration interval, the calibration signal may be recorded on a chart recorder so as to serve as a reference for the detection signal (which is also recorded at intervals outside of the calibration interval).

As shown in Fig. 4a, an LED 13 is connected via terminals T7, T8 to the circuit. LED 13 is housed in tube 13 (Fig. 6) so as to directly irradiate the photoelectric cell 4 and thereby provide a given level of radiation during the calibration interval.

The performance of the apparatus can thereby be checked, e.g. once every 4 hours, to ensure that the optics are not obscured by dust.

Fig. 5 schematically illustrates the meter M1 and three LED's 15-1 7. LED 1 5 indicates that the calibration interval is in progress and LED's 1 6 and 1 7 indicate respectively low and high threshold values of the received signal, i.e.

corresponding to cement clinker having a low or a high free calcium oxide content. The threshold values can be set up experimentally, for example, by allowing the photoelectric cell to view different samples of cement clinker having known percentages of free calcium oxide.

Fig. 5 also schematically illustrates a pressure responsive switch assembly PS1,the contacts of which are connected in a circuit including an LED 14. This circuit merely provides a series connection, with the power supply, through the switch contacts, to energise the LED 14, when there is a drop in pressure in a compressed air supply (not shown). The compressed air supply is connected to the tubes 10, 1 3 shown in Fig. 6 so that there is a constant outflow of air from the ends of the tubes. Such air-purging of the tubes minimises inaccuracies due to any build up of dust layers which might obscure either the infra-red light source 1, or the photoelectric cell 4, or both.

If the air pressure of the supply falls below a predetermined value, the switch contacts of PS1 complete the circuit with the power supply and LED 14 thereby giving an appropriate warning.

The power supply circuitry is of standard design and has not been shown in the drawings so as to simplify the description. Moreover, only the main features of the circuitry have been described above, since the detailed construction and operation of the components illustrated in Figs.

2--5 will be clear to those skilied in the art.

Turning now to Fig. 6, tubes 10 and 13 are both cylindrical. Tube 10 thereby provides a circular cone 1 Oa of radiation, from the infra-red source 1 housed in the tube, which results in a corresponding circular field of illumination on the surface of the clinker bed 11. Similarly tube 13, which houses the photocell 4, has a circular field of view on the surface of the clinker bed 11. These two circular fields are shown, in plan view, in Fig. 7 wherein the field of illumination 1 Ob overlaps the field of view 1 3b by a zone 14 of intersection. In the particular arrangement used, the cylindrical axes of tubes 10, 1 3 were (a) parallel to one another, and (b) separated by distance of about 23 cm.The tubes 10, 13 were spaced from the clinker bed such that the zone of intersection had a maximum width w of about 5 cm, with the ends of the tubes 10, 1 3 located at a distance of about 45 cm from the usual surface of the clinker bed 1 The reason for this arrangement will now be explained.

The clinker 11 on the conveyor-belt 12 has a non-uniform reflecting surface. For example, there may be changes in the height of the clinker as well as its disposition. The absorptivity/emissivity of indented surfaces (such as that of a pile of lumps) is partly dependent on the degree of indentation.

Extreme indentation effectively leads to black surfaces and the effect is more pronounced at low emissivities. Since these effects can lead to changes in the optical path length between source 1 and the photoelectric cell arrangement 4, a kind of "parallax effect" is utilised to reduce error.

According to this parallax effect, the light source 1 and the photoelectric cell arrangement 4 are mounted so that they have overlapping fields on the surface of the clinker bed 11, as shown in Fig. 7. If the height of the clinker bed 11 reaches the level of the broken line 11 a (Fig. 7), whereby the surface of the clinker bed is nearer to both the infra-red source 1 and the photoelectric cell 4, there will be an increase in the intensity of radiation received by the photoelectric cell 4 due to the reduction in optical path length. However, the "parallax effect" compensates for this because there is then a reduction in the area of the zone of intersection 14, as can be seen by comparing the triangular bases 15a and 15 in Fig. 7.In other words, if the level of clinker on the conveyor belt 1 2 rises, the width w of the zone of intersection is reduced and hence less but more intense reflected radiation is received by the photoelectric cell 4 in tube 1 3. Conversely, if the level of the clinker bed 11 falls, the zone of intersection 14 becomes larger and hence more reflected radiation, of a lower intensity, is received by the photocell 4 in tube 13. The geometry of the arrangement can be experimentally determined so that small changes in the height of the clinker bed do not make any substantial change in the level of detected radiation.

Fig. 8 is a graph showing an approximately linear relationship between the percentage of free lime (calcium oxide), plotted on the x axis, to the amplitude of a received signal (in arbitrary units), plotted along the y axis. Although there is a scatter of points, the correlation is reasonably linear.

As mentioned above, good quality clinker, having a low free calcium oxide content, is a dark grey/black in colour. On the other hand, poor quality clinker has a light grey or whitish colour.

Therefore, poor quality clinker will reflect more infra-red radiation from the source 1 onto the photoelectric cell 4. The presence of good quality clinker will result in a substantially zero output on meter M1 and LED 16 will be illuminated. On the other hand, if the clinker quality deteriorates to a poor value, the meter M1 will show a large scale reaching and LED 17 will be illuminated. During a calibration interval, when LED 13 alone illuminates the photoelectric cell 4, the deflection of meter M1 should be, for example, about 0.75 maximum deflection. LED 13 is housed in tube 13 so that it directly illuminates the photoelectric 4 during a calibration interval when the source 1 is effectively switched off.

In setting up the apparatus, potentiometer P3 is adjusted to provide adequate sensitivity.

Potentiometer P6 is adjusted to give a suitable back-off voltage and potentiometer P6, is adjusted to give a substantially zero reading on meter M1.

Potentiometer P6 provides a suitable back-off voltage to enable adjustment of an optimum zero point. A threshold value is then selected, by adjusting potentiometer P5. This threshold value represents a given or acceptable free calcium oxide content.in the clinker. A suitable threshold may be found, for example, by placing individual samples of cement clinker having known free calcium oxide contents beneath the ends of the tubes 10, 13 and by noting the meter readings. P5 is then adjusted accordingly. If the amplitude of the detection signal rises above that of the threshold, visible and/or audible warnings are given so that corrective action can be taken.

The detection signal could be used too as a control signal for automating corrective action, i.e.

if the detection signal rises above the threshold.

Sources other than infra-red sources may be used, as long as a change in colour of the clinker can be detected.

A gated receiver, synchronised, with the pulsed light source, can be used (as an alternative) to give high discrimination against reflected ambient radiation and intrinsic thermal radiation.

Claims (12)

1. A method of assessing the quality of cement clinker, comprising the steps of: (a) irradiating the clinker, at a testing station, with light from a light source, (b) receiving light reflected from said clinker by receiving means which provide a corresponding detection signal, and (c) processing the detection signal to provide an indication and/or control signal, with respect to the reflectance of the clinker, the latter signal being utilised to distinguish between clinker having relatively high or low free calcium oxide contents.

2. A method according to claim 1 where the clinker is irradiated with a pulsed beam of infrared light and the reflected light is received by photoelectric means which provide the detection signal.

3. A method according to claim 1 or 2 wherein the clinker is transported on a conveyor belt past the testing station.

4. A method according to claim 3 wherein compensation is made for changes in the optical path length of light which is first transmitted from the light source to the clinker, and is then reflected from the clinker to the receiving means, said changes being due to variations in the height of the clinker on the conveyor belt, said compensation being effected by arranging for (a) the field of irradiation of the clinker by the light source to intersect with the field of view of the receiving means of the clinker, and (b) the zone of intersection of said fields to reduce when the clinker height increases.

5. A method of assessing the quality of cement clinker according to claim 1 and substantially as herein described.

6. Apparatus for assessing the quality of cement clinker, the apparatus comprising means for transporting the clinker past the testing station which includes a light source for irradiating the clinker; photoelectric means for receiving light reflected from the clinker and for thereby generating a detection signal; and processing means for processing said detection signal, said processing means including means to compare the amplitude of the detection signal with a predetermined threshold value, which threshold value is related to a given reflectance of cement clinker having a predetermined free calcium oxide content, the comparison means providing an output which is utilised as an indication or control signal.

7. Apparatus according to claim 6 wherein the light source is a pulsed infra-red source.

8. Apparatus according to claim 6 or 7 wherein the light source and the photoelectric means are arranged to compensate for changes in the optical path length of light which is first transmitted from the light source to the clinker, and is then reflected from the clinker to the photoelectric means, said changes being due to variations in the height of clinker on said transporting means, the arrangement being such that the field of irradiation of the clinker by the light source intersects with the field of view of the receiving means of the clinker, and that the zone of intersection of said fields are reduced when the clinker height increases.

9. Apparatus according to claim 8 including respective tubes for transmitting light from the light source to the clinker and for receiving the light reflected from the clinker; the spacing between the tubes and the spacing between the ends of the tubes and the transporting means being such as to produce said zone of intersection of said fields.

10. Apparatus according to claim 9 wherein said tubes are continuously air-purged, the air being supplied by means including a pressure responsive switch for actuating an alarm if the air pressure drops below a predetermined value.

11. Apparatus according to any one of claims 6-10 including means for checking the calibration of the apparatus at repeated intervals.

12. Apparatus according to any one of claims 6-11 1 wherein the processing means includes AC amplification means and a predetermined band pass characteristic designed to exclude signal pick-up caused by local main-operated illumination.

1 3. Apparatus for assessing the quality of cement clinker substantially as herein described with reference to the accompanying drawings.