EP1664734A1

EP1664734A1 - Method for continuously controlling the particle size distribution of powders used in the ceramics industry, and plant for its implementation

Info

Publication number: EP1664734A1
Application number: EP04764963A
Authority: EP
Inventors: Francesco Fontana; Davide Foschi; Pietro Rivola; Silvano Valli; Bruno Zauli; Enrico Primo Tomasini; Gian Marco Revel; Nicola Paone; Fabio Ragazzini
Original assignee: Sacmi Imola SC
Current assignee: Sacmi Imola SC
Priority date: 2003-09-12
Filing date: 2004-09-07
Publication date: 2006-06-07
Also published as: WO2005026699A1

Abstract

Method for continuously controlling the particle size distribution of ceramic powders leaving the powder forming plant, comprising at least the following steps: withdrawing at least one powder sample at predetermined intervals; measuring the particle size distribution of the withdrawn sample; comparing the measured particle size distribution with a reference particle size distribution by means of a processor; regulating at least one parameter of said powder forming plant influencing the particle size distribution, on the basis of the differences between the measured particle size distribution and the reference particle size distribution; and plant for its implementation.

Description

DESCRIPTION

METHOD FOR CONTINUOUSLY CONTROLLING THE PARTICLE SIZE DISTRIBUTION OF POWDERS USED IN THE CERAMICS INDUSTRY, AND PLANT FOR ITS IMPLEMENTATION

TECHNICAL FIELD

The present invention relates to a plant and relative method for controlling and determining the particle size distribution of powders used in the ceramics industry, particularly of ceramic powders obtained by spray drying or atomisation, also known as atomized powders, or powders obtained by dry grinding.

PRIOR ART

The plant which produces such atomized powders is commonly known as an atomizer.

It is known to determine the particle size distribution of powders using methods such as sieving, laser diffraction, acoustic spectroscopy with ultrasound and analysis by microscope.

A rapidly executed method of good accuracy and low cost is that based on processing the images obtained by suitable video cameras or photo cameras or by known optometric means; this method is known as CAIA

(computer-aided image analysis).

The known methods enable the particle size distribution (in weight or volume) of ceramic powders to be analysed by comparison with the required distribution. Such analysis is made in order to avoid using ceramic powders with a particle size distribution outside the specified ranges, which would give rise to the formation of tiles with such porosity as not to satisfy the conformity criteria defined in quality procedures. It of common use in practice for a technical expert to analyze the particle size distribution values of said powders and to directly adjust the parameters which regulate the operation of the powder production plant, for example the atomizer, to maintain the particle size distribution curve within a predetermined range. Such adjustments hence require the presence of an expert able to effect said measurements, process the data and then suitably vary the values of said parameters.

These operations give scope for a high percentage of human error with consequent statistical uncertainty regarding the data obtained, a low measurement rate and a delay in adjusting the powder forming plant, and finally high costs to the firm resulting from monitoring by at least one technical operator.

DISCLOSURE OF THE INVENTION The object of the present invention is to overcome the aforesaid drawbacks within the framework of a simple and rational solution. In particular, the invention proposes to automate the measurement and recording of the particle size distribution curve and to effect its control by functional connection to a system which determines the dimensional distribution of the granules leaving the powder production plant, in particular from the atomizer, by means which regulate at least one operating parameter of the plant.

This object is achieved by the invention, by virtue of the characteristics defined in the claims. The invention is applicable to any type of powder used in ceramics, also including for example powders obtained by dry grinding. Specific reference will be made hereinafter to powders obtained by atomization, but the description can also refer to powders obtained by other systems, by changing the regulated parameter. With regard to powders obtained by dry grinding, the parameter for regulating the particle size distribution is the grinding time and, in continuous processes, the rotational speed of the mill. In atomization plants it has been found in practice that the particle size distribution curve can be conveniently regulated by choosing as parameter the slip feed pressure into the atomizer, in the sense that higher pressure produces smaller powder granules.

The invention comprises a control method with feed-back, defined as online, consisting of: withdrawing at least one powder sample at predetermined intervals; determining and measuring the particle size distribution of the withdrawn sample; comparing said measured particle size distribution with a reference particle size distribution by means of a processor; and regulating at least one parameter of the powder manufacturing plant influencing the particle size distribution, on the basis of the differences between the measured curve and the reference curve. In the case of the atomizer plant, the parameter regulated is the slip feed pressure into the atomizer, as stated. In the case of a continuous grinding plant, the parameter is the mill rotational speed.

The invention also comprises a plant for implementing the method, in accordance with claim 13. Claims 14 to 23 define advantageous embodiments of said plant.

In particular, the plant according to the invention comprises at least one withdrawal means for withdrawing powders leaving the powder forming plant; means for calculating the particle size distribution of said powders; and a processor arranged to compare said calculated particle size distribution with a reference particle size distribution, and able to regulate at least one operating parameter of the powder production plant, which parameter is able to significantly affect the granule dimension of the powder manufactured by the plant. The advantages of the invention are immediately apparent on noting that the plant regulation to obtain powders of desired particle size distribution takes place substantially in real time, at predetermined time intervals, without the use of labour.

The constructional characteristics and merits of the invention will be more apparent from the ensuing detailed description given with reference to the figures of the accompanying drawings, which illustrate by way of non- limiting example a particular preferred embodiment of the plant according to the invention.

The illustrated example relates to an atomization plant, but is also suitable for clarifying the invention in relation to other powder manufacturing systems, such as dry grinding. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic side view of a first embodiment of the plant according to the invention and of the powder forming system;

Figure 2 is a perspective view of the plant of Figure 1 ; Figure 3 is a view of Figure 2 from above;

Figure 4 is a front view of Figure 2;

Figure 5 shows the section V-V of Figure 3;

Figure 6 is an enlarged view of a detail of Figure 5;

Figure 7 is an enlarged view of a different detail of Figure 5; Figure 8 is a schematic side view of a second embodiment of the plant according to the invention and of the powder forming system;

Figure 9 is a view in the direction of the arrow A of Figure 8;

Figure 10 is a view in the direction of the arrow B of Figure 8;

Figure 11 is a view of Figure 8 from above.

BEST MODE FOR CARRYING OUT THE INVENTION

Said figures show the plant 1 for continuously controlling the particle size distribution of powders 2 produced by a suitable system 3.

The system 3 essentially comprises a known atomizer 30 into which the slip to be processed is injected by nozzles connected to a feed pump 32. The pressure at which the slip is delivered by the feed pump 32 is operated and controlled, via suitable regulator means, by a processor 33 on the basis of particle size distribution values for the powders 2 measured by the plant 1 at the exit of the atomizer 30. When the slip has been processed the atomizer 30 discharges the powders 2 onto a first conveyor belt 34 which then deposits the powders 2 onto an underlying second belt 35 arranged to convey them to a storage area, not shown.

As can be seen from the figures, the plant 1 for controlling the powder particle size distribution comprises a frame 4 which upperly supports withdrawal means 5 for withdrawing powders 2 leaving the atomizer 30. Said withdrawal means 5 comprises a cylinder-piston unit 50, the rod of which carries a cup-shaped body 51 for collecting the powder samples to be analysed. The cylinder 50 is provided at its rear with a pneumatic motor 52 the purpose of which is to rotate said body 51 through 180° to position it between a loading and discharge position respectively.

With reference to Figure 1 , the cylinder-piston unit 50 translates said cup- shaped body between at least one advanced position for withdrawing the powders 2 and a retracted position for discharging the powders 2. When in its advanced position the body 51 withdraws a powder sample while the powders fall from the belt 34 to the underlying belt 35.

In particular, during translation into its advanced position the cup-shaped body has its mouth facing downwards, and only when it lies below the powder cascade does the pneumatic motor 52 rotate the body 51 so that its mouth faces upwards. In contrast, when in its retracted position the body 51 lies above a conduit 6, which in a first embodiment of the present invention is vertical and arranged to convey the powders 2 into an underlying distributor device 7. When the body 51 lies in its retracted position the pneumatic motor 52 rotates it through 180° to discharge the powder sample contained therein into the conduit 6. It should be noted that in order to ensure that the powder quantity of each sample is always substantially the same, a scraper 53 is used positioned in front of the vertical conduit 6, its purpose being to scrape the upper edge of the body 51 to remove excess powder. The conduit 6 also supports two devices 54 for blowing compressed air, their purpose being to direct the air stream into the body 51 while it translates towards its loading position. The use of the two blower devices 54 enables any powder residues remaining in the body after discharge of the withdrawn powder sample to be completely removed. With reference to Figures 6 and 7, the distributor device 7 consists of a rotary valve 70 driven by a pneumatic motor 71 (Figure 2). Above the valve a hopper 72 is present to receive the entire powder sample withdrawn by the cup-shaped body 51. The valve 70 comprises a rotating cylindrical body 73 provided with a flattened part 74 and a cavity 75, which is dimensioned to receive only a portion of said powder sample.

The distributor 7 is associated with a cylinder-piston unit 76 (Figure 3) arranged to cause it to translate between an advanced loading position in which it lies below the conduit 6, and a retracted position (Figure 7) in which it feeds said powders to means for measuring their particle size distribution.

In particular, when in its retracted position the distributor 7 lies above an acquisition plate 8 on which it deposits the powder portion received in the cavity 75. The acquisition plate 8 is supported by the frame 4 via four elastic springs 80, each of which is fixed to the top of a support rod 81 branching from the frame 4. A usual vibrator device 82 is associated with the lower surface of the acquisition plate 8 to vibrate the plate 8 in order to uniformly distribute over its surface the powders discharged by the distributor. Above the acquisition plate 8 the frame 4 supports an optical system 9 comprising optoelectronic means, in the example a digital photo camera 10, for acquiring digital images of the samples of powder 2 deposited onto the acquisition plate 8.

A lighting system, which in the described example comprises a circular neon lamp, is associated with said photo camera 10 to illuminate the powders on the acquisition plate 8 in such a manner as to prevent the formation of shadows which could prejudice the quality of the images received.

To the side of the acquisition plate 8 there is positioned a suction system 12 comprising a suction fan, not shown, connected by a conduit 16 (Figure 2) to a movable spout 13 (Figure 7) arranged to translate above the acquisition plate 8 to remove powders after the photo camera 10 has taken their image. The spout 13 is operated by a cylinder-piston unit 120 supported by the frame 4 of the plant. It should be noted that the conduit 16 is provided with a movable shutter 160 to interrupt the suction action by the suction fan.

At its front the spout 13 carries a blower device 14, the purpose of which is to blow compressed air to clean the acquisition plate 8 after passage of the spout 13 in order to be certain that powder granules 2 do not remain on the plate. The plant 1 is controlled by the processor 33 in accordance with the following operating procedure. It should firstly be noted that as the powders 2 deposited by the atomizer 30 onto the belt 34 tend to stratify, the samples are preferably withdrawn from at least two different points of the powder cascade by suitably regulating the advancement of the cup-shaped body 51. According to a second embodiment, when in its retracted position the body 51 lies above a flexible conduit 6' (Figure 8) arranged to convey the powders 2 directly onto the acquisition plate 8, without the aid of the distributor 7 present in the first embodiment. In this second embodiment (Figures 8-11), the plate 8 is supported by a structure 60 which can be orientated; said structure 60 is hinged by suitable connections to a bracket beam 62 rigidly fixed to the frame 4 of the plant 1 by known fixing means. The fixing of the bracket beam 62 to the frame 4 by said fixing means enables the acquisition plate 8 to be positioned at a predetermined height from the ground. The acquisition plate 8 can be orientated by suitable regulator means 63, positioned between the structure 60 and the beam 62, such as to enable said plate 8 to be disposed substantially horizontal. At that side opposite the side at which the conduit 6' is positioned, said plate 8 terminates with an inclined plate piece 64 which engages the mouth of a suction system 12' of known type.

The suction system 12' is connected to the inclined plate piece 64 by a flexible tube 16' which terminates with a shutter 160' able to open and close communication between the suction system 12' and the tube 16'. The operations described hereinafter are repeated for each withdrawal point. Initially the body 51 withdraws the powder sample and overturns it into the conduit 6, 6' which transfers it into the distributor 7 and from there onto the acquisition plate 8 in the first embodiment, or transfers it directly onto the acquisition plate 8 in the second embodiment. In the first embodiment, the distributor 7 is caused by the processor 33 to position itself above the acquisition plate 8. Once in position the rotary valve 70 deposits onto said acquisition plate 8 only a portion of the powders of the withdrawn sample. At this point the processor 33 causes the distributor to move into its advanced position distant from the plate 8, while at the same time the vibrator device 82 vibrates the plate 8 to distribute the powders 2 uniformly thereover. The action of the vibrator device is then interrupted and the photo camera 10 takes a photograph of the powders 2 which it makes available to the processor 33. When the image has been taken, the suction fan draws the powders through the movable spout 13. When the powders have been drawn in, the distributor 7 is repositioned over the acquisition plate 8 to discharge a subsequent portion of powders from the same sample. At this point the cycle is repeated a predefined number of times, until the entire powder sample withdrawn by the body 51 has been analysed. Once the sample has been analysed the processor calculates the arithmetic mean of the particle size distribution values obtained. The aforedescribed operations are repeated for a determined number of samples withdrawn from said different withdrawal points. The processor then calculates the arithmetic mean of the average values obtained for each sample and compares it with a reference value. Based on the result of the comparison, the processor 33 varies the pressure of the slip feed pump 32.

In the second embodiment, the distributor 7 is absent, as are therefore the steps involved therewith, the rest of the operating cycle proceeding as heretofore described.

Claims

1. A method for continuously controlling the particle size distribution of ceramic powders leaving the powder forming plant, characterised by comprising at least the following steps: - withdrawing at least one powder sample at predetermined intervals;

- measuring the particle size distribution of the withdrawn sample;

- comparing the measured particle size distribution with a reference particle size distribution by means of a processor;

- regulating at least one parameter of said powder forming plant influencing the particle size distribution, on the basis of the differences between the measured particle size distribution and the reference particle size distribution.

2. A method as claimed in claim 1 , characterised in that said step of measuring the sample particle size distribution consists of measuring the particle size distribution of a predefined number of powder portions of the same sample.

3. A method as claimed in claim 2, characterised by calculating the arithmetic mean of the measured particle size distribution values for each predefined number of powder portions of the same sample.

4. A method as claimed in claim 2, characterised by analysing a plurality of samples and using as the measured particle size distribution value the average of the mean particle size distribution values determined for each sample.

5. A method as claimed in claim 1 , characterised in that the particle size distribution measurement comprises the determination of a particle size distribution curve.

6. A method as claimed in claim 1 , characterised by comprising the step of depositing said powder sample in a single layer of powder granules on an acquisition plate.

7. A method as claimed in claim 6, characterised by comprising the step of measuring said layer by optometric means.

8. A method as claimed in claim 1 , characterised by comprising the step of processing said measurement to obtain a curve which represents the particle size distribution of the powder sample.

9. A method as claimed in claim 1 , characterised in that the powder forming plant is an atomizer plant.

10. A method as claimed in claim 9, characterised in that said parameter is the ceramic slip feed pressure into the atomizer plant.

11. A method as claimed in claim 1 , characterised in that the powder forming plant is a dry grinding mill.

12. A method as claimed in claim 11 , characterised in that said parameter is the rotational speed of the dry grinding mill.

13. A plant (1 ) for continuously controlling the particle size distribution of atomized ceramic powders (2) produced by a system (3) comprising means (63) for regulating their particle size distribution, associated with at least one withdrawal means (5) for withdrawing at least one sample of powders leaving said system (3), and means (8) for measuring the particle size distribution of the powders (2) of said powder sample, characterised by comprising a processor (33) arranged to compare the measured particle size distribution with a reference particle size distribution, in order to regulate, by regulator means (63), at least one operating parameter of the powder (2) production system (3) which is able to significantly affect the dimension of the powder granules produced.

14. A plant (1) as claimed in claim 13, characterised in that said processor (33) determines the particle size distribution curve of the powders (2).

15. A plant (1 ) as claimed in claim 13, characterised in that the particle size distribution measurement means comprise a plate (8) for acquiring at least one portion of the powders (2) of said powder sample, above which there is positioned an optometric system (10) for obtaining images of said powders (2).

16. A plant (1) as claimed in claim 15, characterised in that the optometric system comprises a photo camera (10) or a video camera.

17. A plant (1 ) as claimed in claim 15, characterised in that said acquisition plate (8) is associated with a vibrator device (82), the operation of which facilitates uniform distribution of the powders (2) on said plate (8).

18. A plant (1 ) as claimed in claim 15, characterised by comprising a distributor device (17) provided to deposit on said acquisition plate (8) at least one portion of the powders (2) of said at least one powder sample.

19. A plant (1) as claimed in claim 18, characterised in that said distributor device (7) comprises a rotary valve (70) above which there is positioned a hopper (72) for receiving the powders (2).

20. A plant (1 ) as claimed in claim 19, characterised in that said valve (70) comprises a cavity (75) able to receive only a portion of the powders (2) received by said hopper (72).

21. A plant (1 ) as claimed in claim 18, characterised by comprising a withdrawal means (5) arranged to deposit said powders (2) into said distributor device (7).

22. A plant (1 ) as claimed in claim 21 , characterised in that the powder sample withdrawal means (5) comprise a cup-shaped body (51) connected to the end of the rod of a cylinder-piston unit (50), and a pneumatic motor (52) to rotate said cup-shaped body (51) through 180°.

23. A plant (1 ) as claimed in claim 13, characterised in that said regulator means (63) act at least on the value of the slip feed pressure to said system (3).