DE2058247A1 - Process for grinding grainy materials at low temperatures - Google Patents

Description

DR. ALFRED SCHON · 8 MUNICH 22 · WIDENMAYERSTRASSE 49/1

OZ 387

Sch / zb

INVENTA AG for research and patent exploitation Zurich,

Zurich, Switzerland

Process for grinding granular materials at low temperatures

Demand when grinding granular materials into powder the material properties in some cases that the grinding process is carried out at low temperatures.

This is true in the milling of plastic granules with greater toughness and relatively low melting points, eg copolymers of the polyamides 6, 6.6, and .12, which must be substantially milled below 0 ⁰ C,

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so that a perfect flowable powder with a uniform grain structure is obtained.

If the heat generated during grinding is not sufficient Measures are dissipated, temperatures are too high, causing the material to partially heat up above the melting point can be, which reduces the grinding effect and the performance of the mill is significantly reduced.

There are various ways of dissipating the grinding heat; For example, pre-ground solid carbonic acid is added to the input product, thus creating the granular material is already pre-cooled to a low temperature before it enters the mill. The grinding heat is then both partly taken from the pre-cooled feedstock as well as from the remaining carbon dioxide snow that evaporates. Although with it good grinding results have been achieved, the process has the disadvantage that a uniform dosage of carbonic acid Offers difficulties, which means that precise temperature maintenance is difficult to achieve. Besides, the handling is the coolant cumbersome and the operation uneconomical.

Another possibility is to use liquid nitrogen as a coolant. If the liquid nitrogen with the feedstock, the extremely low temperature can impair the comminution properties cause.

Both methods are not very flexible with regard to temperature control and either require one to be sent directly to the grinding plant connected plant for liquefied gas production or

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cumbersome transport of these refrigerants over long distances. These processes are therefore more economical Not very attractive.

It has now been found that these disadvantages can be avoided by a method for grinding granular materials "at low temperatures, which is characterized in that the heat generated during grinding by a cooled gas circuit is discharged from the mill, which in turn transfers the absorbed heat via a first heat exchanger to a cold gas circuit, the required cooling capacity through an approximately isentropic expansion a compressed gas is generated in one or more expansion machines, and the expanded gas after the heat absorption in the first heat exchanger, it is heated to approximately ambient temperature in a second heat exchanger is by heat transfer to the same gas compressed in an intermediate compressor, which in countercurrent to the heat-absorbing gas flow on the inlet temp era door of the expansion machines is cooled.

If the cooling capacity is sufficiently large, cold gas turbines can be used as expansion machines, as shown in Gas liquefaction plants are common. The turbines can be braked either with the interposition of a gearbox be done electrically, whereby useful energy is generated, or a brake fan can be coupled directly to the turbine in this way a pre-compression of the gas can already be achieved. For gas compression Both turbo compressors and reciprocating compressors can be used, depending on the size of the required Cooling capacity and the pressure ratio used.

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Air is preferably used as the cycle gas because it is used the effort for system replenishment in the event of leakage losses should be the least. It can of course other gases can also be used which do not liquefy in the operating area.

The inventive method allows the temperature during the grinding process within wide limits and with simple means of Adjust control technology to any level and keep it there exactly constant.

The regulation can take place by changing the pressure gradient in the refrigeration circuit, a change in the gas circuit volume or a change in the effective heat exchanger surface.

The effort for the cold generation according to the new process is much smaller than with the other known processes and extremely easy to use.

In the following example, the method is to be explained with reference to the accompanying figure.

In the expansion turbine 1 5000 kg of air 10 are ata to 1.5 ata expanded, the air from -60 ⁰ C to -130 ⁰ C cooled and then through line 21 to the first heat exchanger 2 is fed, while through heat absorption ^{warmed to -90 0} C from the grinding circuit. In the heat exchanger 3, the air is further heated up to +10 ⁰ C, the heat is extracted from a guided in counter-current air flow, as will be explained below. The air emerging from the heat exchanger 3 is fed via the line 23 to the brake fan, where it is compressed from 1.2 to 1.8 ata. To

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an intermediate cooling in the cooler 5, the air reaches the compressor 6 and is compressed by this to 10 ata. In the final cooler 7 the heat of compression is discharged from the cooling water and the air finally passes through the line 27 back to the heat exchanger 3> in which the circulation air to the turbine inlet temperature -6O ⁰ C is cooled.

The cooled in heat exchanger 2 to -100 ⁰ C air stream passes via line 30 to a fluidized-bed cooler 11, in which the signal supplied from reservoir 10 plastic granulate is pre-cooled. The air-granulate mixture reaches the mill 12 via a line, in which the granulate is comminuted into powder. The resulting grinding heat is taken from the feed stream, which means that the air temperature is heated to -60 ° C. In the separator 13, the powder is separated from the circulating air and brought to a sieve device, while the air is returned to the heat exchanger 2 by the fan 14.

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Claims (1)

20582A7 Claim Process for grinding granular materials, in particular of plastic granules, at low temperatures, characterized in that the resulting during grinding Heat is removed from the mill by a cooled gas circuit, which absorbs the heat in turn delivers to a cold gas circuit via a first heat exchanger, with the required cooling capacity by an approximately isentropic expansion of a compressed gas in one or more expansion machines is generated, and the expanded gas after taking up heat in the first heat exchanger in a second Heat exchanger is heated to approximately ambient temperature by heat transfer to the in an intermediate Compressor compressed the same gas, which in countercurrent to the heat-absorbing gas flow on the The inlet temperature of the expansion machine is cooled. 109825/1307