Abstract:
In the production of meal-fine products by a milling process the milling material generated in a comminuting device is ordinarily separated at the desired grain size in a downstream sifter and the fine material (meal-fine product) obtained here is then separated in the fine material separating device from the sifting air. According to the invention it is proposed to construct the sifter as a V-sifter and to connect this latter, without interposition of the otherwise ordinarily used pipelines, directly with the fine material separating device, preferably an electro-filter.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a milling installation for the generation of meal-fine products, in particular for the generation of cement raw meal and/or of cement from cement clinkers, with a comminuting device for the comminution of the raw materials, a sifter for the separating-off of the fine material generated in the comminution from the still-present coarse material and with a fine material separating device for the separating-off of the fine material from the sifting air of the sifter. 
     As a comminuting device in known milling installations for the comminution for meal fineness there are predominantly used impact hammer millers, tube mills, or a roller press, when raw material is to be milled into the fineness range of cement raw meal or of cement and when a relatively great throughput is desired. 
     Downstream of these comminuting devices there is then ordinarily engaged a sifter, in which the comminuted raw material is sifted at the desired grain size, with the aid of sifting air, i.e., is separated. The sifters used there are, according to the problem definition, dynamic or static sifters. In the sifter the still too coarse comminution material is separated from the sufficiently comminuted fine material, conveyed out of the sifter over a sluice and then returned to the comminuting device. The fine material leaves the sifter with the sifting air and is thereupon separated from the sifting air in a corresponding device. This device is ordinarily a further sifter with a downstream-engaged dedusting device. 
     In order to keep the dedusting gas amounts as low as possible, it is a known practice there to return a major portion of the purified sifting air to the sifter in a closed circuit. 
     In the so-called milling drying in which, with the aid of the sifting air, moist raw substances are dried during their comminution and/or their sifting, hot gases are used as sifting air such as, for example, furnace exhaust gas, cooler exhaust gas or hot gases of a hot-gas generator. In these cases then the amount of the returned sifting air for thermal reasons—fresh hot gas is needed for the drying—is decreased or altogether no sifting air is returned any longer. 
     Especially for the generation of cement raw meal and for the comminution of cement clinkers in known milling installations in recent time a so-called V-sifter has been used which, as a static sifter without driven components, is distinguished by a low specific energy requirement. 
     Thus in EP 0650763 BI a circulating milling installation is described in which raw material is milled with the aid of a roller press to cement raw meal fineness, in which process the sifter used in the milling installation is a static cascade sifter designated as a V-sifter, with two sifting zone-boundary walls enclosed by the shaft-form sifter-housing, and that form a sifting zone between them and are also being traversed by the sifting air in transverse flow, which (walls) have guide plates in cascade or Venetian blind manner inclined obliquely downward in the direction to the discharge opening for the sifted-out coarse fraction, in which system the two guide plate walls, and therewith the interlying sifting zone are arranged to lie obliquely at an angle deviating from the vertical. 
     The coarse fraction separated off in this V-sifter is led back to the roller press, while the fine material fraction together with the sifting air enters into a cyclone dust separator, which is connected with a second-circulating mill-installation with tube mill and sifter of its own, upon which there follows a dedusting device. 
     In a publication of the firm of KHD Humboldt Wedag AG (“Improvements in raw meal and cement generation by V-sifter and roller press,” Zement-Kalk-Gips No. 3, March 1997, pp. 140-147) there is presented a further example of an application for the use of the V-sifter in the cement raw meal and cement generation. In this example, in the cement raw meal generation the limestone comminuted in a roller press is fed to a V-sifter, in which there occurs a separating of the coarse material from the generated fine material. While the coarse material is returned to the roller press, the fine material with the sifter air passes into an after-engaged cyclone sifter, in the after-engaged cyclone separators of which the finished cement raw meal is separated off in the desired fineness. 
     Although with the static V-sifter there is available a simply constructed device with low wear characteristics and a low specific energy requirement, still, for example in the milling drying for the generation of a cement raw meal for the furnace feed, the usual specific energy requirement for the milling and dedusting blower remains at 4 to 9 kWh per ton of generated raw meal, which is still relatively high. 
     SUMMARY OF THE INVENTION 
     It is the problem of the invention, therefore, to further develop known milling installations with an integrated V-sifter so that without cost-intensive constructive measures there is obtained a clear lowering of the specific energy requirement for the entire milling process. 
     By means of the invention, by connecting the V-sifter directly to a fine material separating device, preferably to an electro-filter, to form a homogeneous component, without using the otherwise usual pipelines, there is yielded first of all a clear reduction of the specific energy requirement for the entire milling installation, and namely primarily by the elimination of the flow resistance otherwise caused by the pipelines, for the sifting air, which otherwise must be compensated for by an increase in electric drive performance for the blower. 
     If according to the invention an electro-filter is used, a tube-form filter is likewise possible, then moreover, additional dedusting of the sifting air is no longer necessary, whereby likewise a saving in specific energy is made possible. 
     Simultaneously with the lowering of the energy costs, additional installation costs are advantageously saved by the elimination of the otherwise usual pipelines and the elimination of the otherwise necessary dedusting device downstream from the separation of the fine material. 
     By causing the constructive dimensions and attuning of the V-sifter to correspond to those of the fine-material separating device, it becomes possible, for example in the cement raw meal generation in which the meal is dried to maintain the operation of the cyclone heat exchanger furnace, of the milling installation and of the dedusting with only one suction blower. 
     So that the V-sifter can be directly connected with the fine-material separating device without interposed pipelines, it is necessary to arrange the outlet opening of the V-sifter for the sifting air laden with fine material not as otherwise usual at the upper end of the sifting housing, but laterally on the sifting housing, so that the sifting air at least can also be fed laterally into the fine-material separating device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages, details and features of the invention are explained in the following in a schematic flow diagram of an exemplary installation for the production of cement raw meal by milling drying. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The milling installation represented in this schematic flow diagram is a milling drying installation in which with the aid of hot gas  20 , for example furnace or cooler waste gas, moist raw material  1  is dried and simultaneously comminuted, there, to cement raw meal fineness. 
     Comminution is performed in the example of execution represented in a roller press  31  which is dedusted by fresh air feed  24  and connection to the hot-gas pipeline  23 . Downstream of the roller press  31  there are arranged a V-sifter  32  with a directly connecting electro-filter  37  for the separating off of the generated meal-fine finished material  9  with the aid of sifting air  25 . 
     The sifting air  25  is formed from hot gas  20  injected into the milling drying installation as waste gas of an unrepresented cyclone heat-exchanger furnace, and from an admixed partial amount of purified sifting air  22 ; it enters first into the V-sifter  32 , over a hot-gas pipeline  46  through a laterally arranged entry opening  34 . 
     In the V-sifter  32  there now occurs a separation of the material  4  to be sifted, which is introduced into the V-sifter  32  from above, via a material entry opening  33 , at the desired separating grain size. The coarse material  10  obtained is discharged via a coarse material discharge opening  35  downward from the V-sifter  32 , and the separated fine material  11  flows in common with the sifting air  25  through a lateral outlet opening  36 , out of the V-sifter  32  and through a directly following lateral entry opening  39  into an electro-filter  37 , which is constructed with a preliminary separating chamber  38 . In this electro-filter  37  there now takes place an extensive separating-off of the fine material  11 , which is discharged downward over discharge openings  41  out of the electrofilter  37 , and the purified sifting air  25  emerges as dedusted waste gas  21  from a lateral outlet opening  40  out of the electro-filter  37 . Through a pipeline  44  this waste gas  21  is now discharged from the milling installation via a suction blower  43  which also provides for the necessary sub-pressure for the operation of the heat exchanger furnace and for the operation of the V-sifter  32 . A partial amount  22  of this waste gas  21  is branched off to the flow pressure side of the blower  43  and led over a return line  45  to the hot-gas pipeline  46  and mixed there with the hot gas  20 . With the aid of the separated-off partial amount  22  of the waste gas  21 , therefore, a gas circulation through the V-sifter  32  is maintained, which can be controlled in respect to gas temperature and gas amount over the amount of the recirculated partial amount  22  of the waste gas  21 . 
     So that in the represented milling installation ultimately the entire raw material is comminuted to cement raw meal fineness and is dried sufficiently for placing in the furnace, various material circulations paths are possible between roller press  31 , V-sifter  32  and electrofilter  37  for the materials fresh material (moist raw material  1 ), sifter coarse material  10 , and coarse material  5  from the separation material  12  of the preliminary separating chamber  38  of the electrofilter  37 . In the following the possible material circulations are described. 
     The total moist raw material  1  is first fed into the milling installation, over the raw material charging device  30 . Depending on moisture and coarseness quality of the raw material, this raw material  1  then represents a partial amount  2  of the sifter load  4  and/or a partial amount  3  of the roller press load  14 , i.e., the raw material can be sluiced into the milling circulation at the comminuting device  31  and/or at the sifter  32 . 
     In the feeding into the V-sifter  32 , the partial amount  2  of the raw material, together with the total roller press output  15 , constitutes the sifter load  4 . In the feeding into the roller press  31  the partial amount  3  of the raw material  1  with the coarse material  10  of the V-sifter and possibly with coarse material  5  of the separation material from the preliminary separating chamber  38  of the electrofilter  37  constitute the roller press load  14 . The coarse material  5  is separated here from the separating material  12  by a smaller second sifter  42 , for example a dynamic sifter. The fine material  6  obtained here has raw meal fineness, and it is fed to the finished material  9 . In an electro-filter without preliminary separating chamber, alternatively all of the separation material  7  can be supplied to this second sifter  42 . In sufficient fineness of the materials separated off in the electro-filter  37 , the discharge  13  of the preliminary separating chamber  38  can be directly brought together with the discharge  8  of the other chambers of the electrofilter  37 , to the finished material  9 . 
     The coupling according to the invention of a V-sifter with a fine material separating device to form a compact component is not restricted to the milling installation or milling drying installation for cement raw meal generation using an electro-filter as fine-material separating device; on the contrary, it is usable everywhere in milling installations in which meal-fine products are to be generated by interaction of a comminuting installation with a sifter and a fine-material separating device, and where the technical and local conditions are given for this coupling according to the invention. 
     As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of our contribution to the art.