Methods for cryogenic comminution

Abstract

Method for operating a grinding plant in closed-loop gas operation for the cryogenic comminution of grinding material, characterized in that the grinding material to be ground is cooled by means of a cooling medium, wherein the cooling medium is injected directly into the inlet area of ​​the mill or into the air line to the mill, and additionally the closed-loop gas is cooled down before being returned to the mill of the grinding plant by means of a cooler with an associated cooling unit.

Description

The invention is directed to a method for operating a grinding installation in circulating gas operation for cryogenic comminution of ground material and a grinding installation therefor.When preparing and processing products such as cocoa, spices or seeds, the products or else a presscake produced therefrom are comminuted. In order not to adversely affect the product properties or to adjust product properties for further processing, it is necessary to avoid heating during comminution and to adjust the comminution product to a specific temperature.In the preparation of cocoa beans to the various products therefrom, the cocoa beans are fermented, broken up and separated from the shells. For the usual processing of the fermented cocoa beans, the kernels are pressed out in filter presses or similar apparatuses. The fat contained is solid at ambient temperature and is referred to as cocoa butter. In the warm state, the cocoa butter liquifies, and the process of separation is thereby simplified. This heat is achieved partly by heating the product, partly also by the energy required for pressing. Cocoa butter is widely used in the food industry; presscake is used as starting material for cocoa powder, a major part of which is used for the production of chocolate mass. For further processing, the cocoa powder is required as finely comminuted bulk material in order to enable the following processes of mixing, metering or concentration.During the comminution of cocoa presscake, heat is generated on account of the comminution work required for comminution. In addition, the feed material, i.e. the cocoa presscake from a press, is frequently fed to the grinding plant already warm at temperatures of 50 to 70° C.For the further processing of the ground cocoa presscake, a certain crystal structure of the residues of cocoa butter present is required.This crystal structure is established within a temperature band which is significantly below the outlet temperature of the product from the grinding plant. Furthermore, this crystal structure must be "frozen" by a combination of temperature and residence time at this temperature. This means fixing the crystal structure, which cannot be redissolved by further thermal or mechanical treatment. This treatment is also referred to as stabilization or recrystallization.The process temperature for the formation of the desired crystal structure and for the stabilization thereof is in the range of 20° C. The comminuted cocoa presscake should therefore have this temperature. It is not possible according to the present state of the art to remove the energies of the grinding installation and the preheating from the press by cooling the grinding air with economical utilization of the grinding installation. The low temperature required for this cannot be produced economically by conventional cooling units. Therefore, the mills are operated either in partial load operation or the grinding plant is operated with an increased quantity of air, wherein a part of the quantity of air is led past the mill as a bypass and the ground product is recooled in a filter or cyclone before separation. Since the required temperature at the outlet cannot nevertheless be reached, an after-cooler is used after discharge from the separating element, which after discharge cools the product to the temperature necessary for stabilization.Furthermore, explosion protection should be taken into account in this context. As in almost all organic materials, there is the risk of dust explosion, which requires an explosion-proof construction of the grinding installation resistant to pressure surges.Also, other food products consist of a combination of starch, fat and protein. Therefore, the mechanical and thermal properties as relevant here for the comminution of cocoa presscake are also similar. Typical examples include spices such as coriander.During the comminution of such ground material, which consists to an appreciable extent of fat or oil, this fat or oil will emerge from the comminuted particle collective and stick this to one another and against the machine walls. A certain proportion is also conveyed further as a gas proportion or as microscopically small droplets and condenses at other points of the plant, which is naturally undesirable. The degree of separation of fat and oil, as well as their aggregates and crystal structure, are an important part of the product properties. Furthermore, these properties are defined by the temperature range in which these products can be economically reasonably ground.DE 35 45 828 A1 discloses a method and a device for operating a grinding installation, in which a carrier gas is circulated. An inert gas is used which is cooled before being returned to a mill.U.S. Pat. No. 2,583,697 A and U.S. Pat. No. 3,965,267 A disclose the cryogenic procedure by addition of liquid nitrogen for various products, including cocoa. While DE 2 76 029 A1 describes that a temperature band of 20 to 25° C. is set for cocoa at the outlet of the grinding plant. The reason for this temperature adjustment is given in EP 0 245 549 A1 by the crystallization and the stability of the crystal structure of cocoa butter.The object of the invention is therefore to provide the prior art with an improvement.It is an object of the invention to provide a method and a plant which offer a possibility of economically crushing products such as spices, seeds or presscakes, providing the ground product with an approximately ambient temperature for the subsequent process steps and ensuring explosion protection during the crushing of the product.In a method of the type described at the beginning, the object is achieved according to the invention by the characterizing part of the main claim.The object is achieved by a method and a plant in which the grinding material is comminuted in a grinding plant with the supply of a refrigerant, wherein the grinding plant is designed as a circulating gas plant and the grinding gas is additionally cooled in a cooler with cooling unit before being returned to the mill. The refrigerant serves simultaneously for inerting and thus explosion protection.For comminution of the ground material, a grinding plant with a mill or a classifier mill is used. The plant is designed as a circulating gas grinding plant. The grinding material is introduced into the grinding installation together with the refrigerant. Liquid nitrogen or cryogenic carbon dioxide is used as refrigerant. The refrigerant can be injected directly into the inlet area of the mill or into the air conduit to the mill. Alternatively, the refrigerant can also be supplied completely or partially in a vortex scroll cooler for precooling the warm feed material. The ground material is comminuted in the mill and conveyed into a product separation. Here, the ground product is separated from the gas and discharged. The gaseous nitrogen is guided by means of a circulating gas fan to a cooler with cooling unit, where it is cooled down. The cooled gas is fed back into the mill as recycle gas.The combined cooling of the feed material with the refrigerant and the cooling of the circulating gas with a cooler with a refrigerating unit are necessary in order to economically generate the required low temperatures which are required to discharge the grinding performance and the thermal performances of the warm feed material which comes from the preliminary process. The vaporized refrigerant, e.g. nitrogen, is simultaneously used for explosion protection by inertization. It is recycled in the circulating gas plant. The quantity of the discharged leakage gas results from the gas quantity of the evaporated refrigerant with the deviations of the small quantities of purge gas, gas for filter cleaning and the unavoidable leaks.The refrigerant used for the method according to the invention can be liquid nitrogen and cryogenic carbon dioxide in the form of dry ice for cooling and inertization.Products which can be comminuted by this process include presscakes, but also seeds or spices. As presscake, mention may be made here of, for example, cocoa presscake. Seeds which may be mentioned are, for example, extracted or extruded legumes. Spices such as, inter alia, chili, coriander or cross-crumbs can likewise be ground using this process.The plant for carrying out the process is a circulating gas grinding plant. It comprises a product feed, an addition for the refrigerant, a mill or classifier mill, a product separation, a product discharge, a circulating gas fan, an exhaust air outlet and a cooler with associated cooling unit. Optionally, a vortex scroll cooler can be provided after the product application, in that the product is cooled by means of refrigerant, e.g. liquid nitrogen. The grinding gas, which simultaneously serves for inerting, is circulated by means of a circulating gas fan between the mill, product separation and cooling.This process is used in the comminution of cocoa presscake in the production of chocolate.The method according to the invention makes it possible to reduce the investment costs for a plant for the method. The investment costs for the plant downstream of the grinding for cooling, stabilizing and / or recrystallization of the ground presscake are significantly higher than the additional costs for the grinding plant according to the invention for the presscake. In the absence of downstream plants for product conditioning, not only the investment costs but also the space requirement, the manpowers required for operating the plant and the outlay for cleaning are reduced. This is of particular advantage in the processing of cocoa presscake.Since the refrigerant, such as nitrogen, also serves for inerting the grinding installation in addition to cooling, this enables a lighter and simpler mechanical construction of the installation with its aggregates. Non-pressure shock-resistant systems are used.The cleaning effort is facilitated not only by the elimination of downstream plant parts for product conditioning, but additionally by the easier construction of the non-pressure shock-resistant plant and the lower tendency of the cool product to build up. With a retrofit of existing installations according to this scheme, the temporary re-operation of existing installations is simplified so that these peaks of varying load can be absorbed.Further details, features and advantages of the subject matter of the invention are evident from the dependent claims and from the following description of the associated drawings, in which a preferred exemplary embodiment of the invention is shown by way of example.In the drawing, FIG. 1 shows the basic structure of the grinding plant.FIG. 1 shows a preferred embodiment of the grinding plant for presscakes as a circulating gas grinding plant. It shows the application for cocoa presscakes. The grinding plant comprises a product feed ( 6) followed by a vortex screw cooler with the addition of liquid nitrogen as refrigerant ( 8A). The cocoa presscake cooled by the liquid nitrogen and containing the partly evaporated nitrogen is fed into the mill (1) and comminuted there. The comminuted product is fed to the product separation (2). A filter (as shown here), a cyclone or a combination of both can be used as the product separation. Here, the ground cocoa presscake is separated from the evaporated nitrogen and discharged from the plant via the product discharge ( 7). The gaseous nitrogen is passed through the circulating gas fan, (3) cooled down in the cooler with cooling unit (5) and optionally returned to the feed line to the mill (1) or to the mill (1).Excess gas is discharged from the grinding plant in circulation via the exhaust air outlet ( 4).In an alternative embodiment, no vortex screw cooler is used and the liquid nitrogen (8) in a partial stream of the recycled circulating gas is fed directly to the mill (1).Reference numerals denote reference numerals1 Mill 2 Product separation 3 Circulating gas fan 4 Exhaust air outlet 5 Cooler with cooling unit 6 Metering of the feed material, possibly also vortex screw cooler at nitrogen feed 8A 7 Product discharge 8, 8A Feed of nitrogen liquid

Claims

Method for operating a grinding installation in circulating gas operation for cryogenic comminution of grinding material, characterized in that the grinding material to be ground is cooled by means of refrigerant, wherein the refrigerant is injected directly in the inlet region of the mill or into the air line to the mill and, in addition, the circulating gas is cooled down by means of a cooler with associated cooling unit before being returned to the mill of the grinding installation.Method according to claim 1 characterised in that the ground material is presscake, seeds or spices.The method according to claim 2da characterized in that the press cake is a cocoa press cake.The method according to claim 1 characterized in that the refrigerant is liquid nitrogen, cryogenic nitrogen or cryogenic carbon dioxide.Method according to Claims 1 to 4da, characterized in that the refrigerant additionally serves for cooling at the same time for inerting and thus explosion protection of the grinding installation.Method according to Claims 1 to 5da, characterized in that the grinding installation is not pressure-shock-resistant or not pressure-relieved.Process according to Claims 1 to 6, characterized in that the grinding material is cooled with the refrigerant in a vortex screw cooler before grinding.Grinding installation for carrying out the method according to Claims 1 to 6, characterized in that the apparatus comprises: - a product feed - a refrigerant feed for cooling the grinding material - a mill - a product separation - a product discharge - a circulating gas fan - an exhaust air outlet for excess circulating gas - a cooler with cooling unit, wherein the grinding material and the grinding gas are cooled by means of refrigerant, wherein the refrigerant is injected directly in the inlet region of the mill or into the air line to the mill and, in addition, the circulating gas is cooled by means of cooler and the nitrogen simultaneously serves for inertization.Grinding installation according to claim 8da characterised in that the mill is a classifier mill.Grinding installation according to claim 8da, characterised in that the mill and the associated grinding installation do not have a pressure-shock-resistant or pressure-relieved design.

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