Method and roll mill for drying and grinding moist feed material

Abstract: The invention relates to the drying and grinding of moist feed material in a vertical roll mill, in which the moist feed material is delivered to a grinding table (7) by way of a fresh material inlet, comminuted material is conveyed into a separator (4) by a rising hot gas stream and is dried thereby and from it tailings are separated off and returned to the grinding table. In order to be able to ensure a reliably operating delivery of fresh material (13) to the grinding table (7) substantially without incrustation a predrying of the moist feed material (13) is carried out in the fresh material delivery arrangement (12) in the region before the grinding table by on the one hand introducing hot gas into the fresh material delivery arrangement and on the other hand admixing tailings (16) coming from the air separator (4) with the fresh feed material.

Description: The invention relates to a method according to the preamble to Claim 1 and a roll mill according to the preamble to Claim 7 for drying and grinding moist feed material, particularly moist cement raw material (see EP-A-0 525 423). Methods and roll mills of the aforesaid type are known in many different forms, such as can be seen for example from W.H. Duda, CEMENT-DATA-BOOK. Volume 1, third edition, 1985, for instance pages 235 to 241. The roll mills used for this drying and grinding are constructed with a mill housing having a substantially vertical axis. In this mill housing a mill part (grinding part) is disposed in the lower part for the material comminution and an air separator is disposed in the upper part. The mill part contains a drivable end rotatable grinding table and a plurality of grinding rolls which roll thereon. Moist fresh feed material is delivered to the grinding table approximately in the centre, so that it is comminuted between the upper grinding surface of the grinding table and the grinding rolls. At least a proportion of the comminuted mill feed material is conveyed into the separator by a rising stream of hot gas delivered to the mill part, and is thereby dried. Sufficiently comminuted fine material is drawn off together with the hot gas stream from the air separator, whilst oversize material which is not yet sufficiently comminuted, so-called tailings, is returned to the grinding table in order to be further comminuted there. In the practical use of the known methods and roll mills, difficulties always occur with the delivery of the moist feed material, involving amongst other things on unwanted incrustation of the fresh feed material in the region of the delivery arrangement. The object of the invention is to improve a method according to the preamble to Claim 1 as well as apparatus according to the preamble to Claim 7 in such a way that by relatively simple means, even with relatively moist fresh feed material, it is always possible to ensure a reliably operating delivery of fresh material to the grinding table of the mill part. This object is achieved according to the invention on the one hand (as regards method) by the characterising features of Claim 1 and on the other hand (as regards apparatus) by the characterising portion of Claim 7. Advantageous embodiments and further developments of the invention are the subject matter of the subordinate claims. In the method according to the invention predrying of the moist feed material is carried out in the region of the fresh material inlet, whereby on the one hand hot gas is introduced in a controllable quantity and pressure into the fresh material inlet and on the other hand the tailings coming from the air separator and to be returned to the grinding table are initially mixed with the fresh feed material in the fresh material delivery arrangement, whereupon this predried mixture of fresh feed material and tailings from the separator is delivered to the grinding table. Thus with this procedure the moist feed material delivered to the fresh material inlet can be subjected to a first drying with the aid of a sufficient quantity of hot gas. This can be achieved on the one hand by a direct action of the delivered hot gas on the fresh feed material and on the other hand by the walls of the fresh material inlet which are heated by the through flow of hot gas, and this already leads overall to a very effective predrying of the fresh feed material. When the tailings coming from the air separator and to be returned to the grinding table are also admixed with this fresh feed material at a suitable point in the fresh material inlet, then the fact that these tailings have already been subjected to intensive drying (drying and grinding) whilst being conveyed from the mill part into the air separator and also within the air separator means that due to the admixture of these relatively dry tailings the mixed moisture content of the mixture of fresh feed material and tailings from the separator can be further lowered. Thus these combined predrying measures are almost ideal prerequisites for, on the one hand, delivery to the roll mill of a feed material with a relatively high moisture content (e.g. up to 15% and more) and, on the other hand, ensuring with great reliability that incrustations of material in the region of the fresh material inlet can be avoided is far as possible or completely, so that an extremely reliably operating delivery of fresh material to the grinding table is achieved. Although in many instances it may be sufficient to introduce only a proportion of the tailings coming from the air separator into the fresh material inlet, according to the invention it is generally preferred to introduce all of the tailings coming from the air separator into the fresh material inlet, which brings with it advantages both as regards the lowering of the mixed moisture content and also as regards construction (simplification of the conveying elements). In this invention it is also regarded as advantageous if the tailings are conveyed in a free-flowing state from the air separator to the fresh material inlet and are continuously admixed with the fresh feed material outside the mill housing. This produces a relatively long mixing route for fresh feed material and tailings from the separator. In a further advantageous embodiment of this method according to the invention the fresh feed material and the tailings coming from the separator are passed through the fresh material inlet in a free-flowing state to the grinding table. In this way the fresh material inlet can be kept relatively simple in structural terms, i.e. apparatus parts which have to be driven can be avoided. In the roll mill constructed according to the invention the fresh material delivery arrangement is simultaneously constructed as a predrying arrangement in the region before the grinding table, and this fresh material delivery arrangement has connected to it on the one hand a second hot gas duct for introducing hot gas which is controllable at in quantity and pressure and on the other hand a tailings conveying duct coming directly from the tailings outlet of the air separator. The invention will be explained in greater detail below with reference to the drawing in which - in the single figure - a preferred embodiment of the roll mill according to the invention is illustrated in a vertical section which has been kept largely schematic. Since the basic construction of a roll mill is generally known to the person skilled in the art, it is quite sufficient in the present context to show only the parts of the apparatus which are necessary for explanation of the invention. The roll mill according to the invention is constructed for drying and grinding of moist feed material, particularly moist cement raw material, in approximately the same way, but also for other moist feed materials, such as for example rough coal or the like. This roll mill is constructed with a vertical axis (mill axis) 1 and comprises an outer mill housing 2 in which an actual mill part 3 is disposed - in the lower part - and an air separator 4 is disposed in the region above this mill part 3. A connecting intermediate part 5 is also advantageously constructed or disposed in the region between the mill part 3 and the air separator 4. Accordingly the mill part 3, the intermediate part 5 and the air separator 4 are disposed one above the other coaxially with respect to the mill axis 1. The mill part 3 has, in a manner which is known per se , a grinding table 7 which can be driven rotatably by a suitable drive arrangement 6 and has an upper grinding track 7a on which grinding rolls 8 which are held so as to be stationary and rotatable roll. The air separator 4 can be constructed in any suitable form, i.e. as a so-called dynamic separator (with built-in rotor) or - as illustrated in the drawing - as a static air separator. This air separator 4 contains a collecting hopper 9 for oversize material or tailings with a tailings outlet 9a at the lower end as well as an upper discharge connection 10 for drawing off hot gas charged with fine material. A dead-weight valve 11 can be disposed in the usual manner in the tailings outlet 9a. This roll mill also contains a fresh material delivery arrangement 12 for delivering the moist fresh feed material - indicated by the arrow 13 - to the grinding table 7, over the centre thereof with the distributor cone 7b. At the lower end of the mill part 3, and approximately in the region below and around the grinding table 7 is constructed a hot gas delivery housing 14 to which is connected a hot gas duct 15 coming from a suitable hot gas source. As is indicated in the drawing by broken arrows for hot gas and solid arrows for material, comminuted mill feed discharged from the outer rim 7c of the grinding table is conveyed into the air separator 4 by the rising hot gas stream delivered by way of this hot gas duct 15, and in the air separator the comminuted mill feed is separated into fines and tailings, the mill feed being dried by the hot gas. The fines are discharged from the roll mill through the discharge connection 10 together with the hot gas stream which is flowing out, whilst the tailings from the tailings outlet 9a of the air separator 4 should be returned to the grinding table 7. The latter does not occur directly, however, in the case of the roll mill according to the invention, but rather the tailings separated off in the air separator 4 are first of all introduced from the tailings outlet 9a by way of a tailings conveying duct 16 into the fresh material delivery arrangement 12 in which they are admixed with the fresh feed material before the feed material and tailings are then delivered jointly to the grinding table 7. Apart from the tailings conveying duct 16 there is also a second hot gas duct 17 connected to this fresh air delivery arrangement 12 in order to introduce hot gas in a controllable quantity and pressure into this fresh material delivery arrangement 12. It is regarded as particularly advantageous in construction terms if both the fresh material delivery arrangement 12 and the tailings conveying duct 16 are of approximately chute-like construction with an inclination to the horizontal so that within the fresh material delivery arrangement 12 and within the tailings duct 16 it can be ensured that the particular material conveyed therein flows off or slips downwards freely. The junction 16a of the tailings conveying duct 16 with the fresh material delivery arrangement 12 in provided - when viewed in the material flow direction (arrow 13) - after the junction 17a with the second hot gas duct 17. This junction 17a of the second hot gas duct 17 is advantageously located at the inlet end 12a of the fresh material delivery arrangement 12. Due above all on the one hand to the second hot gas duct 17 connected there and on the other hand to the tailings conveying duct 16 also connected there, the fresh material delivery arrangement 12 is simultaneously constructed in the region before the grinding table 7 as a predrying arrangement for the moist fresh feed material (arrow 13). Although the second hot gas duct 17 and the tailings conveying duct 16 can be connected to the fresh material delivery arrangement 12 at any suitable point, i.e. both inside and outside the mill housing 2, it is generally preferred to connect these two ducts 16 and 17 to the fresh material delivery arrangement 12 outside the mill housing 2. The second hot gas duct 17 can come from any suitable hot gas or hot air source, e.g. a separate hot gas producer, or a kiln exit gas duct (for example from a burning plant for the production of cement and the like). The same also applies to the first hot gas duct 15 which leads directly into the mill part 3. In the illustrated embodiment it may be assumed that the second hot gas duct 17 is branched off from the first hot gas duct 15. It is regarded as particularly advantageous to co-ordinate with the second hot gas duct 17 a hot gas blower 18 which can be installed at any suitable point in the hot gas duct 17 and can be controlled with regard to the quantity of hot gas it introduces into the fresh material delivery arrangement 12. Furthermore, means are also provided here for controlling the pressure of the hot gas to be introduced into the region of the fresh material inlet end 12a of the fresh material delivery arrangement 12. For this purpose there is disposed in the region of this fresh material inlet end 12 a gas pressure gauge 19 which preferably continuously measures the air or gas pressure prevailing therein end supplies measurement signals to a control arrangement 20 which is also connected for control purposes to the hot gas blower 18 or to the drive motor 18a thereof. These control connections are constructed in such a way that the hot gas introduced into the fresh material delivery arrangement 12 - by way of the blower 18 and the second hot gas duct 17 - can be controlled both with regard to its quantity and its pressure so that approximately a zero pressure can be set at or in the region of the fresh material inlet end 12a. These measures reliably prevent unwanted cold air or infiltrated air from entering the fresh material delivery arrangement 12 or passing through this fresh material delivery arrangement 12 into the interior of the roll mill and impairing the desired drying effect.

Claims: Method of drying and grinding moist feed material, particularly moist cement raw material, in a vertical roll mill which contains in a common mill housing (2) a lower mill part (3) with a rotatable grinding table (7) and grinding rolls (8) as well as an air separator (4) above the mill part, wherein a) the moist feed material (13) is delivered by way of a fresh material inlet (12) to the grinding table (7) where it is comminuted, b) at least a portion of the comminuted material is conveyed into the air separator (4) by a rising stream of hot gas and is dried thereby, and the comminuted material is separated into fine material and oversize material, wherein the fine material is drawn off with the gas stream, whereas the oversize material is returned as tailings to the grinding table (7), characterised by the following steps: c) predrying of the moist feed material in the region of the fresh material inlet (12) by introducing hot gas in a controlable quantity into said fresh material inlet and admixing the tailings coming from the air separator (4) with the fresh feed material (13) in said fresh material inlet (12), wherein the introduction of hot gas and tailings into the fresh material inlet (12) is carried out continously outside the mill housing (2); d) introducing the hot gas with a controllable pressure into the fresh material inlet (12) in such a way that a substantially zero pressure is set approximately in the region of the fresh material inlet end (12a); e) delivering the so predried mixture of fresh feed material and tailings to the grinding table (7). Method as claimed in claim 1, characterised in that within the fresh material inlet (12) the hot gas is first of all introduced into the incoming fresh feed material and only thereafter are the tailings from the separator admixed with feed material. Method as claimed in claim 1, characterised in that the tailings coming from the air separator (4) are all introduced together into the fresh material inlet (12). Method as claimed in claim 1, characterised in that the tailings are conveyed in a free-flowing state from the air separator (4) to the fresh material inlet (12) and the fresh feed material together with the admixed tailings are fed through the fresh material inlet in a free-flowing state to the grinding table (7). Vertical roll mill for drying and grinding moist feed material, particularly moist cement raw material, comprising a) a mill housing (2) in which are disposed a lower mill part (3) with a rotatable grinding table (7) and grinding rolls (8) rolling thereon as well as an air separator (4) above said mill part (3), having a fine material discharge (10) and a tailings outlet (9a), b) a fresh material delivery arrangement (12) for delivering the moist feed material (13) to the grinding table (7), c) wherein comminuted material is conveyed by the rising hot gas stream delivered by way of a hot gas duct (15) into the air separator (4) where it is separated into fines and tailings, and wherein the tailings from the tailings outlet (9a) can be returned to the grinding table (7), characterised by the following features: d) the fresh material delivery arrangement (12) is simultaneously constructed as a predrying arrangement, and this fresh material delivery arrangement has connected to it on the one hand a second hot gas duct (17) for introducing a controllable quantity of hot gas and on the other hand a tailings conveying duct (16) coming directly from the tailings outlet (9a) of the air separator (4), wherein said second hot gas duct (17) and said tailings conveying duct (16) are connected to the fresh material delivery arrangement (12) outside the mill housing (2); e) said second hot gas duct (17) is connected in the region of the fresh material inlet end (12a) to the fresh material inlet arrangement (12), and a gas pressure gauge (19) is disposed in this region of the fresh material inlet end, wherein a control arrangement (20) being in control connection with said gas pressure gauge, is provided in such a way that the hot gas introduced in the fresh material delivery arrangement (12) is controllable in quantity and in pressure with regard to setting a zero pressure at the fresh material inlet end (12a). Roll mill as claimed in claim 5, characterised in that a hot gas blower (18) is installed in the second hot gas duct (17) and that the control arrangement (20) is in control connection also with said hot gas blower (18) or to its drive motor (18a) with regard to control both the quantity and the pressure of the hot gas introduced into the fresh material inlet end (12a) of the fresh material inlet arrangement (12). Roll mill as claimed in claim 5, characterised in that both the fresh material delivery arrangement (12) and the tailings conveying duct (16) are of approximately chute-like construction with an inclination which causes the material to flow off freely. Roll mill as claimed in claim 5, characterised in that the junction (16a) of the tailings conveying duct (16) with the fresh material delivery arrangement (12) is provided - when viewed in the material flow direction (13) - after the junction (17a) with the second hot gas duct (17). Roll mill as claimed in claim 8, characterised in that a connecting intermediate part (5) of the outer mill housing (2) is disposed coaxially between the lower mill part (3) and the air separator (4).

Inventors: DOUMET JOSEPH E DIPL-ING, DOUMET, JOSEPH E., DIPL.-ING.
Applicants: DOUMET JOSEPH E DIPL ING, DOUMET, JOSEPH E., DIPL.-ING.
Application year: 1996
Main classification: B02C15
IPC codes: B02C15, B02C23, B02C21