EP0362525B1 - Cold-grinding method and apparatus - Google Patents

Abstract

Fluidized-bed counter-jet mills are gas-jet mills with which extremely fine milling results can be produced with very little wear and tear. An improvement in the milling results can be achieved by cooling the gas jets with a refrigerant, but the improvement that can be achieved is not substantial and the consumption of refrigerant is very high. In order to lower the consumption of refrigerant while considerably improving the fineness of the milled material, the coarse material which is separated by the grader of the mill is cooled by the refrigerant within the fluidized-bed counter-jet mill and fed back into the milling chamber. Liquid nitrogen is the preferred refrigerant with which the sump of the mill is cooled.

Description

The invention relates to a fluidized bed counter jet mill according to the preamble of claim 1.

Jet mills have long been known comminution machines in which the particles to be comminuted are accelerated by gas streams and comminuted by collision. There are a number of different jet mill designs. They differ in the type of gas flow, the type of Impact of the particles against each other or on an impact surface and whether the particles to be crushed are carried in the gas jet or whether the gas jet hits the particles and entrains them. Air or superheated steam is usually used as the grinding gas.

In the fluidized bed counter-jet mill, as is known for example from EP-A-0139279, free-expanding gas jets meet in a grinding chamber in which the material to be ground is in the form of a fluidized bed. The grinding is carried out practically exclusively by the grinding material particles colliding against one another, the grinding is thus almost wear-free. The fluidized bed counter-jet mill is assigned a classifier in which the fine material obtained is separated from the coarse material which has not yet been comminuted sufficiently. The coarse material is returned to the grinding chamber.

Many materials, for example plastics, are difficult or impossible to grind to fine grain sizes due to their toughness. The supply of cold and the resulting embrittlement of the materials can improve the grinding properties of such tough materials. In jet mills, the propellant gas stream is therefore cooled, as described, for example, in DE-A-21 33 019. The cooling of the propellant gas stream makes it possible to grind materials that would not be grindable in jet mills under normal conditions. Despite intensive cooling, for example with liquid nitrogen, and despite the self-cooling of the propellant gas stream due to its expansion, the achievable improvement in grindability leaves much to be desired. Although fine Reach grain sizes, but only with an extremely high expenditure of time and energy.

From SOVIET-INVENTIONS-Illustrated 84-194530 / 31, week 8431, SEP 12. 1984 & SU-A-1 060 199 a jet mill working with an impact surface is known, in which to reduce the oxidation of the ground material and the rotating coarse material and to increase the general performance of the mill, the ground material blasted onto the impact surface is cooled with a cryogenic refrigerant . Apart from the wear of the impact surface and the resulting possibility of contamination of the product, only the increase in general performance is described; nothing is said about an improvement in grain fineness.

The invention has for its object to provide a fluidized bed jet mill for cold grinding, which requires little energy and refrigerant and at the same time enables the fine grinding of products to finest grain sizes that were previously unattainable with a significant increase in throughput.

Starting from the prior art taken into account in the preamble of claim 1, this object is achieved according to the invention with the features specified in the characterizing part of claim 1. Advantageous developments of the invention are specified in the subclaims.

The invention is therefore based on the consideration of cooling the circulating coarse material with a cryogenic refrigerant rather than the propellant gas flow. The invention This measure leads to an abrupt improvement in the grinding results, as can be seen from the operating results below.

It is surprising that this can be achieved solely by the measure according to the invention, not to cool the propellant gas flow, but rather the coarse material circulating. The inventors concluded this from operational tests with a fluidized bed counter-jet mill, in which despite the intensive cooling of the propellant gas stream with liquid nitrogen, the coarse material circulating in the bottom of the mill only a slight cooling resulted. The inventors concluded from this that cooling on the coarse material itself had to take place in order to improve the grinding performance.

Later theoretical considerations confirmed the correctness of this conclusion.

It is essential here that when grinding with a cooled propellant gas stream, the impact energy of a particle converted into heat is only incompletely transferred to the cold gas stream. There are several reasons for this. The time for heat transfer from the particle to the gas is extremely short. Since the heat capacity decreases with falling temperature, the temperature increase due to a shock at higher temperatures is greater than at higher temperatures. The relative movement between particles and cold propellant gas is low, so that the values for the heat transfer also drop. In addition, the thermal conductivity of many regrinds is already low and gets even worse with falling temperature. Furthermore, the acceleration energy of cold gas is worse than that of warm gas. The finer the desired grain sizes, the worse the grinding conditions become when grinding with a cooled propellant gas stream. This is particularly true of thermoplastics that have a very low crystallization range. Their grinding becomes completely uneconomical.

The invention enables a considerable increase in throughput to be achieved on fluidized bed counter-jet mills. There are particles of the finest fineness with a corresponding surface enlargement and smooth surface structure. The end product is easy to pour and has a high bulk and bulk weight. In particular, materials that are tough, rubber table, sticky or greasy can be excellently ground using the method according to the invention. These are primarily natural products, many pharmaceutical products, thermoplastics, waxes and high-molecular plastics. Liquefied gases, especially nitrogen, are primarily considered as refrigerants, but also carbon dioxide. In the simplest and in many cases the most practical case, these can be fed directly into the bottom of the mill. Indirect cooling of the coarse material is of course also possible. Indirect cooling can also take place using other refrigerants, for example brine baths.

Some embodiments of the invention will be explained with reference to the accompanying drawings.

Fig.1

eine Fließbett-Gegenstrahlmühle in schematischer Form,

Fig.2

die Kühlung des Sumpfes der FließbettGegenstrahlmühle von Fig.1,

Fig.3

eine Mischform aus direkter und indirekter Kühlung des Sumpfes,

Fig.4

eine Ausführungsform ähnlich Fig.3, jedoch mit ausschließlich direkter Kühlung.

Show it:

Fig. 1

a fluidized bed counter jet mill in schematic form,

Fig. 2

cooling the sump of the fluidized bed counter-jet mill of Fig. 1,

Fig. 3

a mixed form of direct and indirect cooling of the swamp,

Fig. 4

an embodiment similar to Figure 3, but only with direct cooling.

In the following description, the same reference numerals have been used for the same parts in all figures.

1 shows a fluidized bed counter jet mill in schematic form. The mill consists of a housing 1 which comprises the grinding chamber 2 and the sump 3. The propellant gas enters the grinding chamber 2 through the nozzles 4. The classifier 5 connects to the housing 1. The coarse material to be ground is in the form of a fluidized bed 6 in the grinding chamber. The regrind is metered in through the lock 7. The fine material 10 separated in the sifter 5 is drawn off through the fine material outlet 8, as indicated by the arrow 9, and fed to the filter system 15. This has a connection 16 for the exhaust gas and a removal lock 17 for the fine material 10 obtained. The coarse material 11 flows from the classifier 5 back into the grinding chamber 2. The propellant gas, with which the nozzles 4 are acted on, is brought in through the supply line 14.

According to the invention, the coarse material located in the sump 3 of the mill is cooled by liquid nitrogen. This is introduced through the line 12 and the porous entry body 13. Porous feed grains are particularly suitable for small mills. For grinders with larger diameters, other feed systems, for example nozzle plates, are preferable in order to be able to feed the nitrogen in as finely divided as possible. The nitrogen supply through line 12 and the porous Entry body 13 takes place as a function of the temperature control 18. The regrind feed through the lock 7 can also take place directly into the sump 3. The fine fraction of the fine material 10 is determined by the speed of the classifier 5. The coarse material 11 flowing back from the classifier 5 forms, together with the grinding material entering from the lock 7, the fluidized bed 6. The liquid nitrogen entering through the porous feed body 13 evaporates and cools the sump of the mill, ie the coarse material 11 flowing back from the classifier 5 and possibly freshly applied grinding material . The vaporized cold nitrogen is drawn upwards through the material and enters the grinding zone. Cold gas, coarse material and regrind form a first fluidized bed zone below the grinding chamber 2 in the sump 3.

2 shows in schematic form the lower part of the fluidized bed counter-jet mill from FIG. 1, but with the lock 7 for the regrind arranged directly on the sump 3. The arrows 19 illustrate the mixture of cold gas, propellant gas, coarse material and fine material which is drawn off towards the classifier.

3 shows a variant with indirect and direct heat exchange between the nitrogen supplied and the regrind. The liquid nitrogen is supplied through lines 20 and 21. The liquid nitrogen entering through line 21 enters a double-walled tube 22 which is closed at the end faces. This double-walled tube 22 has outlet openings 23 directed inwards. The entire lower part of the mill housing is also designed as a double-walled chamber 24. The line 20 opens into it. The chamber 24 has outlet openings 25 arranged in the sump 3 for the nitrogen supplied through the line 20. The coarse material 11 flowing back from the classifier is therefore initially indirectly cooled in the area between the double-walled tube 22 and the chamber 24. Then, there is direct cooling by the nitrogen emerging from the outlet openings 23 and 25. Depending on the mode of operation, this can still be liquid or already gaseous.

4 shows an embodiment similar to FIG. 2, but with an elongated sump 3. A certain flow form is impressed on the coarse material 11 and the cold gas by means of a tubular apron 26. The apron 26 separates the grinding chamber into a central shaft 37, where the grinding process takes place, and into an annular shaft 38 for the coarse material flowing back. The liquid nitrogen is supplied at two points, namely through line 12a directly into the sump 3 and through line 12b into an injection system 39 in the annular shaft 38. The nitrogen introduced through line 12b therefore cools the coarse material flowing back from the classifier.

The invention is not limited to the embodiments described above, since there are numerous other possibilities for cooling the coarse material flowing back from the classifier by means of a refrigerant. Several grinding zones with sump in the form of a cascade can also be connected in series. Here, the mixture of fine and coarse material emerging from the grinding zone is separated from the exhaust gas in a filter and fed to the next grinding zone. The one under each grinding zone located sump is cooled according to the invention. A sifter is only assigned to the last stage.

The following operating results show the progress of the method according to the invention compared to the conventional mode of operation. Product name Grain sizes Product throughput Kg / h Gas flow Feed End product Air m³ / h N₂ m³ / h d₁₀ µm d₅₀ µm d₉₀ µm d₁₀ µm d₅₀ µm d₉₀ µm Hostalen® GUR 200 30.3 82.2 127.3 17.5 32.1 85.3 2.0 850 without Hostalen® GUR 200 30.3 82.2 127.3 14.3 23.6 38.6 13.0 850 200 polyamide 70 130 200 no grinding effect 850 without Polamide 70 130 200 10.2 26.5 41.6 3rd 850 300

Hostalen® GUR 200 is a high molecular polyethylene from Hoechst AG, Frankfurt. As the operating results show, the two investigated materials could not be ground to the finenesses that can be achieved with the method according to the invention under economic conditions.

The grain size analysis was determined using a commercially available laser granolaometer (Cilas). From the printed curve, the d₁₀, d₅₀ and d₉₀ values were selected as representative values. For example, the d₁₀ value of 10.2 µm in line 4 of the table means that 10% of the end product are grains with a size below 10.2 µm.

The increase in product throughput for Hostalen®GUR 200 from 2.0 to 13.0 kg / h with a much narrower grain band is particularly surprising. The associated significant increase in the specific surface area opens up new application possibilities.

Claims (5)

1. Fluidized-bed counter-jet mill having a grist delivery device, a sifter (5) for the separation of coarse material (11) with fine material (10) and a cylindrical grinding chamber (2), which can be acted upon by gas jets, and having in its lower region a sump (3) for additionally meted grist and for coarse material flowing back from the sifter, characterized by means, disposed in the grinding chamber, for the supply of a cryogenic coolant for cooling delivered grist and coarse material flowing back from the sifter.
2. Fluidized-bed counter-jet mill according to Claim 1, characterized by a double-walled tube (22) which is disposed in the sump and is closed at the end faces, runs in the axial direction at a distance from the walls of the grinding chamber, possesses a conduit (21) for the supply of cryogenic coolant and exhibits discharge openings (23), directed at the axis of the tube, for the coolant.
3. Fluidized-bed counter-jet mill according to Claim 2, characterized in that the walls of the grinding chamber, which walls are assigned to the double-walled tube, are of double-walled configuration, can be acted upon by cryogenic coolant and possess discharge openings (25), directed into the sump, for the coolant.
4. Fluidized-bed counter-jet mill according to Claim 1, characterized by a concentric, tubular apron (26), which divides the grinding chamber into a central shaft (37) and an annular shaft (38).
5. Fluidized-bed counter-jet mill according to Claim 4, characterized by a spray-in apparatus (39) for the cryogenic coolant in the annular shaft.

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