EP0878236A1 - Method and apparatus for dissolving granular material in a fluid jet mill - Google Patents

Abstract

The coarse grains which are to be ground are supplied continuously to a closed injector hopper (6) attached to the injector (3) which is operated by propellant air. The pressure (p) is measured at a product-free spot in the injector hopper and by adjusting the volume flow of propellant air at the injector the pressure (p) in the hopper is kept constant at a preselected underpressure ideal value (po). The injector hopper is biased with inert gas and a pressure gauge (16) is mounted on the lid (9) of the injector hopper. The ideal value is set so that the upper boundary value ensures no return flow from the injector to the injector hopper.

Description

The invention is based on a method for metering in granular coarse material into an air jet mill, in which the coarse material to be ground is mixed with a propellant operated injector is blown into the air jet mill. In addition, the Invention an apparatus for performing the method, which consists of a Propellant powered injector, an injector funnel attached to the injector and a dosing device connected to the injector funnel for loading the injector funnel with the powdery coarse material to be ground.

State of the art

Jet mills are operated with compressed air, inert gas or steam as the blowing agent. For the sake of simplicity, compressed air is always used as a blowing agent in the following talked. When entering products using injectors in jet mills and in particular Blow-back can occur in spiral jet mills, e.g. if the Injector catch nozzle (Venturi nozzle) a too low back pressure compared to Internal pressure builds up in the mill or is occupied by product caking. The inside pressure of the mill depends on the grinding chamber hold-up. A high idle Internal pressure can be overcome. In many cases this requires capture nozzles with small ones Mixing section diameter. The internal pressure in the mill drops sharply as soon as the product is in the grinding chamber is. In this state, an injector catch nozzle could well be used larger diameter can be selected. In fact, in practice it often becomes Jet mill operated with the largest possible capture nozzles, uni poorly fluidizable or to be able to enter baking products more easily. It must be a constant Product supply must be ensured, otherwise a backflow into the injector and in uses the product feed device (blow back). This requires careful Supervision by the operating personnel. Manufacturer of jet mills and in particular of spiral jet mills have so far not offered automated monitoring against blowing back the product entry at upstream injector dosing devices on.

task

The invention has for its object to take apparatus precautions to safe and trouble-free operation of a jet mill without monitoring To enable personnel and the product dosage in the optimal range drive. In addition, the start-up and shutdown process from a control room can be done automatically.

solution

a) daß das Grobgut einem am Injektor angebrachten, geschlossenen trichterförmigen Injektortrichter kontinuierlich zugeführt wird,

b) daß der Druck p an einer produktfreien Stelle im Injektortrichter gemessen wird und

c) daß durch Nachstellung des Treibluftmengenstroms am Injektor der Druck p im Injektortrichter auf einem vorgewählten Unterdruck-Sollwert p₀ konstant gehalten wird.

According to the invention, this object is achieved by

a) that the coarse material is continuously fed to a closed funnel-shaped injector funnel attached to the injector,

b) that the pressure p is measured at a product-free point in the injector funnel and

c) that the pressure p in the injector funnel is kept constant at a preselected negative pressure setpoint p ₀ by adjusting the motive air flow at the injector.

The setpoint p ₀ is preferably specified within a pressure range p ₁ ≤ p ₀ p p ₂ , the upper limit value p _{2 being} characterized in that, at this pressure, there is just yet no backflow from the injector into the injector funnel. In this way, a minimization of the amount of air required to operate the injector can be achieved.

In practical operation, the setpoint p _{0 is} expediently within a range from 1 mbar to 0.2 mbar negative pressure (compared to normal pressure).

The control is advantageously carried out as a proportional control; ie the propellant air flow is adjusted proportionally to the pressure deviation Δp measured in the injector funnel from the setpoint p ₀ .

According to a further embodiment of the invention, the grinding air for the air jet mill and the product feed into the injector funnel are switched off when the pressure in the injector funnel exceeds a limit value p _{3 which is} above the limit pressure p ₂ . In this way, in the event of unforeseen malfunctions, the metering device can be switched off and blow-back of the pulverulent ground material from the metering device into the environment can be reliably avoided.

In the case of sensitive products to be ground, it may be advisable to use To apply an inert gas to the injector funnel. In this way e.g. at hygroscopic products avoid the entry of room air or air humidity will. There have also been cases where the grinding process during the Grinding activated surfaces of the regrind were attacked by atmospheric oxygen. In these cases there is also a covering of the ground material in the injector funnel and an admission of inert gas to the air jet mill is advantageous.

For the implementation of the control method, a device has proven to be effective which consists of an injector fed with propellant air, an injector funnel attached to the injector, and a metering element connected to the injector funnel for loading the injected funnel with the coarse material to be ground, and is characterized according to the invention by a control circuit which is characterized has a pressure sensor for measuring the pressure p in the closed injector funnel, a motor-adjustable valve in the injector as an actuator for changing the injected air flow rate, and a controller that compares the actual pressure value p with a predetermined target value p ₀ and a control signal for actuating the Actuator and thus to minimize the difference vacuum setpoint minus actual vacuum value. The pressure sensor is preferably arranged in the product-free space on the lid of the injector funnel.

The dosing device for loading the injector funnel with the regrind consists advantageous from a dosing screw.

advantages

• Aufgrund der geregelten und gleichzeitig minimierten Injektorluftzuführung wird die Sichtwirkung der nachgeschalteten Luftstrahlmühle verbessert, da die Injektorluft eine Störung der Spiralströmung in der Mahlkammer darstellt. Dieser störende Einfluß wird auf dem geringstmöglichen Niveau gehalten.
• Außerdem können aufgrund der geregelten Injektorluftminimierung neue Anwendungen für die Einbindung von Strahlmühlen in Regelkonzepte erschlossen werden.
• Da der Injektor jeweils nur mit der gerade eben notwendigen, minimalen Luftmenge arbeitet, wird das Produkt schonend in die Mahlkammer eingetragen. Bei stark klebenden oder schleißenden Produkten verlängert sich dadurch auch die Standzeit der Fangdüse am Eintritt in die Strahlmühle.
• In geringem Umfang wird auch noch Treibmittel (Druckluft, Stickstoff oder Dampf) eingespart.
• Der Injektoreintrag ist geschlossen. Eine Kontamination durch eingesaugte Partikel aus der Umgebung ist ausgeschlossen; ebenso der Eintrag von Luftsauerstoff bei inertisierter Fahrweise (Einspeisung von Inertgas in den Injektortrichter).
• Die Mühle ist abgesichert gegen Rückblasen. Dadurch wird eine Kontamination der Umgebung mit Produkt sicher vermieden (Verbesserung der Arbeitssicherheit).
• Als Nebeneffekt ergibt sich eine starke Geräuschminderung
• Weiterhin kann die Mühle mit mit einer Fangdüse betrieben werden, deren Mischstreckendurchmesser für einen sicheren Betrieb bei ungeregelter Fahrweise zu groß wäre.
• Der An- und Abfahrvorgang der Strahlmühle kann automatisiert durchgeführt werden.

The following advantages are achieved with the invention:

• Due to the regulated and at the same time minimized injector air supply, the visibility of the downstream air jet mill is improved, since the injector air represents a disturbance in the spiral flow in the grinding chamber. This disruptive influence is kept at the lowest possible level.
• In addition, the regulated injector air minimization opens up new applications for integrating jet mills into control concepts.
• Since the injector only works with the minimal amount of air that is just necessary, the product is carefully introduced into the grinding chamber. In the case of strongly adhesive or abrasive products, this also extends the service life of the catching nozzle at the inlet into the jet mill.
• To a small extent, blowing agents (compressed air, nitrogen or steam) are also saved.
• The injector entry is closed. Contamination by sucked-in particles from the environment is excluded; also the entry of atmospheric oxygen during an inertized driving style (feeding of inert gas into the injector funnel).
• The grinder is protected against back blowing. This reliably prevents contamination of the environment with the product (improvement of occupational safety).
• As a side effect, there is a strong reduction in noise
• Furthermore, the mill can be operated with a collecting nozzle, the mixing section diameter of which would be too large for safe operation with an uncontrolled driving style.
• The startup and shutdown process of the jet mill can be carried out automatically.

Detailed description

In the following, an embodiment of the invention is described with reference to one in the Drawing schematically illustrated control device explained.

Air jet mills are widely used in the chemical industry today Particle size reduction used. These mills are characterized by their simple Structure, good cleanability and ease of maintenance. The entry the ground material is carried out by an air jet injector dosing device upstream of the jet mill.

The disadvantage is that the operation of jet mills is often very labor intensive. So e.g. in the case of strongly adhesive products, caking often occurs in the injector entry. In extreme cases, this can "blow back" and contaminate the Lead environment with product. It was therefore for operational solutions for Control and regulation concepts sought, the handling and operational safety of the mills in connection with an air jet injector metering device improve.

The basic drawing shows the air jet mill 1 with the injector metering device 2. The latter consists of the air jet injector 3, with a valve 4 and one Servomotor 5 for adjusting the valve opening cross section. Instead of a valve an adjustable bezel could also be used. Downstream of the valve 4 an injector funnel 6 is attached, from which the pulverulent ground material from the injector 3 is sucked in and then entered into the jet mill 1 with the injector air flow becomes. The injector funnel 6 is fed with the regrind by means of a dosing screw 7 charged from the feed container 8. The closed with a lid 9 Air or an inert gas can be applied to the injector funnel 6 via the nozzle 10 will. The injector 3 and the jet mill 1 are connected via lines 11 and 12 or the main line 13 is supplied with compressed air. In line 12, in injector 3 and in the injector funnel 6 pressure sensors 14, 15 and 16 are installed. With the Sensor 14 is the grinding air operating pressure at the jet mill 1 and with the sensor 15 the injector admission pressure was measured. The pressure sensor 16, the product-free Space on the lid 9 of the injector funnel 6 is used for measurement the gas pressure in the injector funnel. All measured variables are a control computer 17 fed. With the valve 18, the operating pressure for the jet mill 1 and the injector 3 can be set.

The injector 3 first produces a so in the closed injector funnel 6 strong negative pressure, that uncontrolled ground material is sucked out of the dosing screw 7 would. This is avoided by feeding in inert gas or additional air the injector funnel 6 is supplied via the nozzle 10. The additional air can the pressure level is raised to such an extent that no product is sucked in uncontrollably becomes. The additional air is set manually.

Naturally, the injectors work product-laden with a large excess of air. A large amount of air is only required when idling. The empty mill will approached with maximum injector pressure. The high form is necessary to Enter regrind against the internal pressure of the mill. If there is grist in the mill, the internal pressure in the mill drops. The injector pre-pressure can then be withdrawn be, since only a lower back pressure has to be overcome. The form is withdrawn so far that there is always a minimal vacuum in the injector area is maintained, which should be just big enough to blow back of regrind from the injector funnel 6 safely avoided becomes. The regulation described below was developed for this purpose.

The sensor 16 forms, together with the control computer 17 and the servomotor 5 for adjusting the valve 4 in the injector 3, a control circuit, with the aid of which the pressure p in the injector funnel 6 is kept constant. The servomotor 5 brings about a reduction or increase in the opening cross section of the valve 4 and thus a decrease or increase in the motive air flow rate in the injector 3. The decrease in the motive air flow rate leads to a decrease and the increase leads to an increase in the vacuum p in the injector funnel 6. The setpoint p ₀ the vacuum p is set in the range p ₁ = 1 mbar to p ₂ = 0.2 mbar. Experience has shown that a negative pressure of 0.2 mbar can still reliably prevent backflow from the injector 3 into the injector funnel 6.

If there is a reduction in the supply of regrind, the internal pressure in the mill increases. An increase in the injector admission pressure is then necessary for trouble-free operation necessary. This is done via the control algorithm stored in the control computer 17, which increases the injector admission until there is negative pressure again. Similarly, the injector admission pressure can be reduced if the supply of regrind increases becomes.

Tests have shown that it is possible to control the air jet mill with this scheme to operate automatically. The regulation was able to make changes in the grist feed, or in the pressure control system. It is therefore possible to Air jet mill to operate with the minimum amount of injector air without the risk enter that regrind is blown from the injector area into the environment. Minimizing the amount of injector air also improves the visibility the jet mill, because the quantity of false air entered via the injector adversely affects the visual effect. In addition, the The grinder can be started up safely with a large catch nozzle without Need to accept unnecessarily large quantities of false air.

The encapsulation of the entry area and resulted in a positive side effect the injector air minimization a significant reduction in the operating noise Air jet mill. The noise emission was 105 dbA when idling. With regrind in the Grinding chamber the value dropped to <90 dbA. In the closed version with Injector air minimization reduced the noise emissions to approx. 70 dbA.

If the deviation from the target value p ₀ for the negative pressure in the injector funnel 6 can no longer be corrected via the injector air quantity, an emergency shutdown of the jet mill is provided. This emergency shutdown works in such a way that when a limit value p _{3 is} exceeded in the injector funnel 6, a solenoid valve 18 in the central pressure supply line 13 is closed and the supply of regrind is prevented by switching off the regrind dosing screw 7. In practice, the threshold value for the shutdown is set to a value between 0.2 mbar and 0 mbar. This ensures that even in critical accidents no regrind can get into the environment by blowing back from the injector area.

Claims (9)

Method for metering granular coarse material into an air jet mill (1), in which the coarse material to be ground is blown into the air jet mill (1) by an injector (3) operated with propellant air, characterized in that a) that the coarse material is continuously fed to a closed injector funnel (6) attached to the injector (3), b) that the pressure p is measured at a product-free point in the injector funnel (6) and c) that the pressure p in the injector funnel (6) is kept constant at a preselected negative pressure setpoint p ₀ by adjusting the flow of propellant air at the injector (3). Method according to Claim 1, characterized in that, in accordance with operation with a minimal amount of injector air, the setpoint p _{0 is} specified within a range p ₁ ≤ p ₀ ≤ p ₂ , the upper limit value p _{2 being} characterized in that at this pressure there is just none Backflow from the injector (3) into the injector funnel (6). Method according to Claim 2, characterized in that the setpoint p _{0 is set} within a range from p ₁ = 1 mbar to p ₂ = 0.2 mbar negative pressure. Method according to Claims 1 to 3, characterized in that the propellant air flow is adjusted in proportion to the pressure deviation Δp measured in the injector funnel (6) from the setpoint p ₀ . Method according to Claims 1 to 4, characterized in that the grinding air for the air jet mill (1) and the product feed into the injector funnel (6) are switched off when the pressure in the injector funnel (6) exceeds a limit value p _{3 which is} above the limit pressure p ₂ . Method according to claims 1 to 5, characterized in that the injector funnel (6) with inert gas. Device for carrying out the method according to claims 1 to 6, consisting of an injector (3) fed with propellant air, an injector funnel (6) attached to the injector (3) and a metering member (7) connected to the injector funnel (6) for loading the injector funnel ( 6) with the pulverulent coarse material to be ground, characterized by a control circuit with a pressure sensor (16) for measuring the pressure p in the closed injector funnel (6), a motor-adjustable valve (4,5) in the injector (3) as an actuator for the change in the amount of propellant air fed in and a controller which compares the actual pressure value p with a predetermined target value p ₀ and supplies a control signal for actuating the actuator ( _4, 5). Device according to claim 7, characterized in that the pressure sensor (16) is arranged on the lid (9) of the injector funnel (6). Device according to claims 7 to 8, characterized in that the dosing member consists of a dosing screw (7).

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