GB1601022A

GB1601022A - Method and a device for the manufacture of cement clinker

Info

Publication number: GB1601022A
Application number: GB5688/78A
Authority: GB
Original assignee: Kloeckner Humboldt Deutz AG
Current assignee: Kloeckner Humboldt Deutz AG
Priority date: 1977-02-26
Filing date: 1978-02-13
Publication date: 1981-10-21
Also published as: FR2381725A1; JPS53106724A; DK84078A; DE2708486A1; FR2381725B1; BR7801136A; DE2708486C2

Description

(54) A METHOD AND A DEVICE FOR THE MANUFACTURE OF CEMENT CLINKER (71) We, KLOCKNER-HUMBOLDT DEUTZ AKTIENGESELLSCHAFT of Deutz-Mulheimer-Strasse 111, 5 Koln 80, Federal Republic of Germany, a German Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of manufacturing cement clinker which is low in alkali and made of alkali-containing raw material by thermally treating it in stages with the aid of a preheating stage, a calcining stage, a sintering stage and a cooling stage, whereby heat from fuel of any type is supplied in the calcining stage and in the sintering stage. The invention also relates to a device for carrying out the method.

It is known that fairly high alkali contents in raw cement materials have an influence both on the mechanism of the burning process and on the quality of the clinker. The disadvantageous influence on the burning process consists in the fact that circulation of alkali takes place in the material and the gas flow between the sintering stage and the calcining stage, the circulation leading to high alkali enrichment and caking of alkali compounds. The high quantities of alkali entering the clinker lead to difficulties in processing the cement and to blistering of the cement. The resistance of the cemented concrete made therefrom to expansion is not ensured either owing to expansion resulting from reactions with aggregates.

It is known from German Patent Specification No. 298 179 to burn the clinker in two stages in order to avoid the disadvantages described. The alkali-containing waste gases of the second burning stage - the sintering stage are removed from the entire process in this type of method so that alkalies do not condense in the colder calcining stage nor go back into the sintering region with the material and so that they cannot form a progressively alkali enriching circulation. This method does however have the extreme disadvantage that the considerable heat content of the waste gases of the sintering stage are lost to the entire process without being used again, whereby the manufacturing costs of the cement are increased until they are completely uneconomical.Therefore, proposals have been made consequently which should avoid this disadvantage at least partially.

The U.S. Patent Specification No. 3 235 239 deals with a method in which the waste gases of the sintering stage are also supplied partially to the calcining stage. However the feared circulation of alkali occurs again with this alkali-containing part of the flow, between the sintering stage and the lower temperature calcining stage, but the larger this part of the flow is for reasons of economizing on energy, the greater is the alkali enrichment. Thus this method is not satisfactory either with respect to avoiding the disadvantages of a high alkali content in the raw material or with respect to necessary low consumption of heat of the burning process.

Another proposal for solving the problem of the alkali is contained in German Patent Specification No. 1 471115 according to which the alkali-containing waste gases of the second burning stage - the sintering stage - are to be cooled by using them to preheat at least a part of the raw material in such a way that substantial proportions of alkali are not deposited on the raw material. In order to be able to fulfil this claim an indirect exchange of heat between the alkali-containing waste gases and the raw material is proposed. This proposal is in practice, with respect to costs and functional reliability, disadvantageous.

The disadvantages described above arise even with another known method of thermal treatment of raw cement material. There the material is subjected to thermal treatment in a heat exchanger system comprising two cyclone preheaters arranged in parallel. The waste air from the material cooler is supplied to a cyclone preheater which has the calcining device and the waste gases from the sintering kiln are supplied to the other cyclone preheater.

With a high alkali content of the raw cement material, a part of the waste gases from the kiln are constantly removed in order to remove alkali from the system. Even this measure cannot be sufficient with a raw cement material having a high alkali content, since the alkalies remaining in the system can still be fully condensed in the high-temperature waste gas of the sintering kiln and deposited on the particles of raw powder. Circulation of alkali cannot be avoided by these measures. The not unimportant heat losses due to discarding kiln waste gases of a very high thermal value to the atmosphere are also detrimental to heat constimption of the burning process.

The present invention seeks to avoid or reduce the described disadvantages of the previously known methods and of improving the method of thermal treatment of raw cement powder, and particularly raw cement powder containing alkaline chloride, so that optimum heat economy of the burning process can be achieved while substantialy avoiding circulation of alkali.

According to a first aspect of the invention, there is provided a method of manufacturing cement clinker comprising preheating the raw material in a preheating stage, calcining the preheated material in a calcining stage, sintering the calcined material in a sintering stage, removing the waste gases directly from the sintering stage, mixing the removed gases with a cooler gaseous medium, feeding the gas mixture directly to the preheating stage as a heat emitting medium and cooling the sintered material in a cooling stage.

Thus practically all of the alkalies present in the whole of the waste gas of the sintering stage are condensed from the gas phase in finely distributed form. Preheating of the raw material or at least of a predominant part thereof can then take place subsequently by direct contact with the total quantity of waste gas from the sintering stage since, because of the finely distributed state of the alkalies, they do not remain with the raw material in cyclone separators but are removed with the gas. Thus the preconditions are created for preheating alkali-containing raw cement material without any substantial loss of heat and for carrying out the burning process with optimum economy of heat. The damaging circulation of alkali previously considered to be unavoidable are entirely eliminated.It is advisable here for the cooling of the waste gases of the sintering stage to be controlled so that the resultant mixture temperature is below approximately 700"C, preferably about 500"C. Owing to this large reduction in temperature of the mixed gases the hot waste gases of the sintering stage are chilled to such an extent that the alkalies contained up till then in the gases in volatile form are sublimed directly into dust fine alkaline salts and all of the kiln waste gases have become inert with regard to alkali transmission to the raw material.

In refinement of the invention provision may be made for the waste gas of the sintering stage to be largely cleared of dust before mixing so that the danger of alkalies condensing on dust particles is reduced further and the alkalies in the waste gas can be passed almost totally into dust fine alkaline crystals.

It is advisable here for the colder gaseous medium to be cleared of dust before mixing with the waste gas. The advantages achieved by removing dust from the kiln waste gases can also be achieved if a medium containing dust is used for cooling.

In further refinement of the invention provision may be made for waste air from the cooling stage to be used as a fairly cold gaseous medium for cooling off the waste gases of the sintering stage. This is particularly advantageous from a heat technology viewpoint since the waste air from the cooling stage is collected at a temperature which is substantially higher than the ambient temperature and in an advantageous manner is at a temperature level within the range of the temperature of the waste gas from the heat exchanger at which the alkalies are finely precipitated after preheating the raw material. Furthermore waste gas from the calcining stage may be used as a fairly cold gaseous medium for cooling of the waste gases from the sintering stage.This is particularly suitable as a cooling medium when, in special cases, there is no need to use the waste gas heat in any other way after the calcining stage.

In special refinement of the invention further provision may be made for the part of raw material preheated in the preheater stage to be supplied to the calcining stage at that point where the part of raw material fed direct to the calcining stage has reached approximately the same temperature as the preheated part of the raw material. This is of special importance, particularly with respect to even heat treatment of the total quantity of raw powder in the calcining stage, since optimum supply of heat can take place in the calcining stage without heat losses arising owing to large temperature differences between parts of the raw material to be calcined.

Acccrding to a further refinement of the invention provision may be made for the waste gas of the preheating stage to be effectively cleared of dust in two successive steps whereby the fairly coarse dust separated in the first step is supplied back to the burning process and the fairly fine dust separated in the second step is rejected. This measure reduces the loss of raw material with the waste gas very economically without leading to undesirable circulation of alkali. After the waste gas of the sintering stage and the gaseous medium, which is fairly cold, have been freed of dust and thereafter mixing, the fairly coarse dust in the waste gases of the preheating stage can originate in practice only from the preheated raw material and therefore in accordance with the above is free of alkalies condensed thereon.The fairly fine dust from the waste gases of the preheating stage on the other hand originates to a considerable extent from the waste gas of the sintering stage and the admixed and fairly cold gaseous medium and is therefore loaded to a considerable extent with alkalies condensed on it. Therefore it must be rejected. According to a second aspect of the invention, there is provided a device for the manufacture of cement clinker comprising a heat treating apparatus for the raw material including a preheating stage, a calcining stage, a sintering stage and a cooling stage, means for removing the waste gases directly from the sintering stage, means for mixing the removed gases with a cooler gaseous medium and means for feeding the gas mixture directly into the preheating stage as a heat emitting medium.

Owing to these constructional measures, even with relatively high alkali contents in the raw cement materials, a clinker quality having a small alkali content is ensured whereby the costs of investment in systems can be low and an optimum low heat consumption can be achieved.

It is advisable for the sintering device to be in direct connection with the mixing device at the waste gas end so that the volatile alkalies in the hot waste gases from the sintering stage are chilled immediately they come out of the sintering kiln and are turned into a fine dust so that this waste gas containing alkali dust cannot transmit the alkali to the raw material to be preheated.

In refinement of the device provision may be made for the device for mixing waste gases from the sintering device with a fairly cold gaseous medium to have a suction line for fresh air and to be in connection with the waste gas end of the calcining device via further pipes and to be in connection with the waste air end of the cooling device. As a result, the temperature of the mixed gases may be controlled at very low construction costs so that the volatile alkalies in the kiln waste gas are safely sublimed to form fine-grain alkaline salts. At the same time heat economy of the burning process is improved since gaseous media having a considerable heat content from the process itself can be used in the process almost without loss.

In further refinement of the device, provision may be made for the calcining device to comprise a preheating part for the raw material operating preferably in counterflow and a part for the calcining itself connected after the preheating part in the direction of material flow and operating preferably with unidirectional flow. Thus the gas line running between the cooler and the preheating part may be used as a calcining region by means of this measure and the length of the reaction path may be set simply to the respective prevailing conditions of the method.

In preferred refinement of the device, provision may be made for a rotary tube kiln serving as a sintering device to be constructed in its inlet region with a larger diameter than in its remaining region, so that the flow speed of the kiln waste gases to the inlet head of the kiln is reduced and is preferably approximately 5 m/sec so that the proportion of dust carried along out of the rotary tube kiln by the hot gases is considerably reduced by this.

In further advantageous refinement of the device provision may be made for the gas flow through the sintering device to be conveyed subsequently through the mixing device and thereafter through the preheating device with the aid of a waste gas blower and for the gas flow through the preheating and calcining device to be conveyed with the aid of another waste gas blower, so that the quantities of gas in the individual heat exchanger branches can be set exactly and the smallest possible waste gas heat losses can be achieved. It is advisable that the waste gas blowers be equipped with drives which can be controlled with respect to speed.

The invention will now be described in greater detail, by way of example, with refer ence to the drawings in which: Fig. 1 is a schematic view of the method in accordance with the invention without giving the specific embodiments of the individual method steps, and Fig. 2 shows a schematic view of a cement manufacturing plant for carrying out the method in accordance with the invention.

In Fig. 1 the method in accordance with the invention is shown schematically. A part 3 of the raw material is supplied direct to the calcining stage 4 without being preheated. The other part 1 of the raw material, preferably the larger part, after being preheated in the preheating stage 2, is almost completely decarbonated together with the part 3 of the raw material which has not been preheated, in the calcining stage 4 by adding fuel 11. The whole of the decarbonated material 5 is burned in the sintering stage 6 to form cement clinker 7.

After cooling off in the cooling stage 8, the clinker leaves the process as a finished product.

The fresh air 9 supplied to the cooling stage is subdivided into three outlet flows of different temperature level as air heated by the cooling process. The hot air flow 10 gives off its heat in the calcining stage 4 and serves as combustion air for the part 11 of the fuel. Combustion is controlled in the calcining stage so that the temperatures are sufficiently high for decarbonation of the raw material but are safely below the vaporization temperature of the alkaline compounds contained in the raw material. The waste gas 12 from this treatment stage, which is free of alkali, is therefore suitable for further use without any disadvantages, for example for drying the raw material by direct contact.

The hot air flow 13 of the cooling stage 8 with a part 14 of fuel causes conversion of the decarbonated material 5 from the calcining stage 4 into cement clinker 7 in the sintering stage 6. Owing to the high sintering temperature of approximately 1,400"C the alkalines contained in the material are substantially vaporized and, together with the combustion gases from the sintering stage 6, form the waste gas flow 15. The latter is intimately mixed in a mixing device 16 with the flow 17 of cooling air from the cooling stage 8 and mixing takes place so that a mixing temperature of less than approximately 700"C is provided, preferably about 500 C. Thus almost all of the alkali in the gas is condensed and in practice is divided up like fine dust in the solid aggregate condition.The waste gas alkali aerosol 18 thus formed enters the preheating stage 2 and gives off its heat, in direct contact, to the raw material 1 wihch is preheated for further treatment Since all of the alkali is already in solid form it can no longer be deposited on the raw material by condensation. During separation of gas and material which is necessary after heat exchange, the alkali suspended in the form of smoke and gas remains in the gas phase and leaves the process as a part of the aerosol 18.

The devices for carrying out the separate method steps shown schematically in Fig. 1 basically comprise a device for preheating the raw material, for heating it to a high degree and calcining it, a device for sintering the material to form cement clinker and a device for cooling the burned clinker. These devices should operate, for reasons of optimum heat economy of a cement manufacturing plant, in counterflow and the device for calcining the raw cement material should also operate in counterflow at least in that part which heats up to a high degree. However, devices operating in crossflow or unidirectional flow may be used for the method steps in accordance with the invention and finally combinations of all of these are of course possible.

A preferred refinement of a cement plant for carrying out the method in accordance with the invention is shown in Fig. 2. This cement plant comprises two adjacent cyclone heat exchangers 19 and 20 which are connected in front of a rotary tube kiln 21, a grate cooler 22 being connected after this. The heat exchanger 19 is connected to the sintering kiln 21 at the gas end and comprises two cyclone stages arranged above each other while the heat exchanger 20 comprises four cyclone stages arranged above each other and is connected to the grate cooler 22 at the gas end via a calcining region 33 and a pipeline 34.

A mixing chamber 23 for cooling the waste gases of the rotary kiln is arranged between the rotary tube kiln 21 and the heat exchanger 19 in the waste gas line 24 leading from the rotary kiln to the lowermost cyclone stage of this heat exchanger. A waste air line 25 provided with a control element 26 leads into this mixing chamber 23, the waste air line being in connection with the grate cooler 22 via a device 27 constructed as a cyclone separator for dust removal. A suction inlet 28 arranged on the mixing chamber and having a closable control element 29 permits admixture of fresh air. The heat exchanger 19 has a feedline 30 for cold raw powder in its upper region and an outlet line 31 for the preheated raw powder in its lower region. Finally, at the waste gas end, a controllable blower 32 is connected after the heat exchanger 19.

A feedline 37 for the other part of the quantity of cold raw powder is arranged in the upper region of the heat exchanger 20. The two heat exchangers 19 and 20 are connected via the outlet line 31 for the material for the raw powder preheated in the heat exchanger 19.

Directly adjacent the raw powder inlet 35 from the last but one cyclone stage of the heat exchanger 20, a supply line 36 for fuel opens into the calcining region 33. The lowermost cyclone stage of the heat exchanger 20 is in connection with the rotary tube kiln 21 via a line 38 for passing on the calcined powder.

Finally, at the gas end, a controllable waste gas fan 39 is connected after the heat exchanger 20.

The rotary tube kiln 21 has a larger crosssection in the region of material inlet than in the outlet region and at the material outlet end is in connection with the grate cooler 22 via the kiln head 40. In the kiln inlet head is provided a throttle element 41 for control of the draft conditions. Firing of the rotary tube kiln takes place with the aid of a fuel supply line 42. The combustion air for the fuel is removed from the grate cooler 22 at the point at which the cooling air is at its hottest.

The plant shown schematically in accordance with Fig. 2 is provided with the measurement and control techniques known from modern method technology for manufacturing cement and these techniques serve to monitor and control the manufacturing process also by using programmed electronic computers. The measurement value generators and adjusting elements necessary for this are not shown in detail.

The mode of operation of the plant described is as follows: A fairly large quantity of raw powder, preferably with a moisture content below 1%, is fed to the heat exchanger 19 via the supply line 30 and it is fed to the heat exchanger in counterflow to the waste gases from the kiln rising upwards. The preheated quantity of raw powder is supplied to the heat exchanger 20 via the outlet line 31. A fairly small quantity of raw powder is fed to the heat exchanger 20 via the supply line 37. This quantity of raw powder also passes into the two upper cyclone stages in counterflow to the hot burning gases of the calcining region 33 and is brought together with the preheated quantity of raw powder from the heat exchanger 19 in the outlet line 31.Both quantities of raw powder have reached approximately the same temp era- ture there. The entire quantity of raw powder then passes into the penultimate cyclone stage of the heat exchanger 20 and is passed into the calcining region from this cyclone stage via the raw powder inlet 35 and is calcined with the aid of supplied fuel from the fuel supply 36.

It is then separated from the gas flow in the hottest cyclone stage of the heat exchanger 20 and is passed into the rotary kiln 21 via the line 38 and burning is completed there. Finally it is passed out of the outlet head 40 of the kiln into the grate cooler 22 as cement clinker 7 for cooling. More than one cooling stage may be provided.

The waste gases 15 from the kiln, which have speeds of below 10 m/sec, preferably approximately 5 m/sec, as a result of the enlargement of the cross-section of the rotary kiln in the inlet region, flow through the waste gas line 24 into the mixing chamber 23 controlled by the throttle element 41. Waste cooler air 17 is introduced into this mixing chamber through the waste air line 25 and possibly fresh air is introduced through the suction connecting piece 28 such that the mixed gases have a temperature below 700"C, preferably about 500"C, so that the alkalies are sublimed in crystalline form.

The mixing gases used to preheat the larger proportion of raw material in the heat exchanger 19 have then become inert with respect to the possibility of transmission of alkali to the raw material. The alkali suspended in the gas in the form of smoke is then drawn off from the heat exchanger 19 as a part of the aerosol by means of the fan 32, preferably via an electrostatic dust removal device (not shown).

Hot waste air 10 from the cooler is supplied to the calcining region 33 approximately from the central region of the grate cooler 22 through the waste air line 34. The combustion gases of this calcining region, fired with fuel, then flow, as a heat-emitting medium, in counterflow to the quantity of raw powder fed to the heat exchanger 20 and are drawn off therefrom by the waste gas fan 39, preferably, via an electrostatic dust removal device (not shown).

Numerous variations are possible with regard to the construction of the cement plant within the framework of the invention. Thus the mixing chamber 23 which is shown can comprise one or more mixing cyclones for example. The heat exchanger 19 can also have one or even three cyclone stages instead of two, whereby its material emission to the heat exchanger 20 is connected to the material extraction of the first or last but one cyclone of this heat ex changer 20.

The method in accordance with the invention according to Fig. 1 can of course also be used if the raw material is present in the form of slurry or granulated material. Then it is only necessary to see that the waste gas of the preheating stage which contains the alkali particles does not come into contact with the still moist particles of raw material.

WEIAT WE CLAIM IS: 1. A method of manufacturing cement clinker comprising preheating the raw material in a preheating stage, calcining the preheated material in a calcining stage, sintering the calcined material in a sintering stage, removing the waste gases directly from the sintering stage, mixing the removed gases with a cooler gaseous medium, feeding the gas mixture directly to the preheating stage as a heat emitting medium and cooling the sintered material in a cooling stage.

2. A method according to claim 1, wherein the cooling of the waste gases of the sintering stage is controlled so that the mixture temperature is below 700"C, preferably about 500"C.

3. A method according to claim 1 or 2, wherein the waste gas of the sintering stage is largely cleared of dust before mixing.

4. A method according to claim 1, 2 or 3, wherein the colder gaseous medium is cleared of dust before mixing with the waste gas.

5. A method according to any one of claims 1 to 4, wherein the waste gas of the sintering stage and the colder gaseous medium are cleared of dust during mixing.

6. A method according to any one of claims 1 to 5, wherein the cooling of the waste gases of the sintering stage takes place within the shortest possible time after outlet of the waste gases from the sintering stage.

7. A method according to any one of claims 1 to 6, wherein waste air front the cooling stage is used as a fairly cold gaseous medium for the cooling of the waste gases of the sintering stage.

8. A method according to any one of claims 1 to 7, wherein waste gas from the calcining stage is used as a fairly cold gaseous medium for cooling off the waste gases of the sintering stage.

9. A method according to any one of claims 1 to 8, wherein a fairly large part of the cold raw material is fed to the preheating stage and a fairly small part is fed directly to the calcining stage.

10. A method according to claim 9, wherein the moisture content of the proportion of raw material fed to the preheating stage is below 1%.

11. A method according to claim 9 or 10, wherein the part of the raw powder preheated in the preheating stage is supplied to the calcining stage at that point where the part of raw material fed directly to the calcining stage has

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. stages in counterflow to the hot burning gases of the calcining region 33 and is brought together with the preheated quantity of raw powder from the heat exchanger 19 in the outlet line 31. Both quantities of raw powder have reached approximately the same temp era- ture there. The entire quantity of raw powder then passes into the penultimate cyclone stage of the heat exchanger 20 and is passed into the calcining region from this cyclone stage via the raw powder inlet 35 and is calcined with the aid of supplied fuel from the fuel supply 36. It is then separated from the gas flow in the hottest cyclone stage of the heat exchanger 20 and is passed into the rotary kiln 21 via the line 38 and burning is completed there. Finally it is passed out of the outlet head 40 of the kiln into the grate cooler 22 as cement clinker 7 for cooling. More than one cooling stage may be provided. The waste gases 15 from the kiln, which have speeds of below 10 m/sec, preferably approximately 5 m/sec, as a result of the enlargement of the cross-section of the rotary kiln in the inlet region, flow through the waste gas line 24 into the mixing chamber 23 controlled by the throttle element 41. Waste cooler air 17 is introduced into this mixing chamber through the waste air line 25 and possibly fresh air is introduced through the suction connecting piece 28 such that the mixed gases have a temperature below 700"C, preferably about 500"C, so that the alkalies are sublimed in crystalline form. The mixing gases used to preheat the larger proportion of raw material in the heat exchanger 19 have then become inert with respect to the possibility of transmission of alkali to the raw material. The alkali suspended in the gas in the form of smoke is then drawn off from the heat exchanger 19 as a part of the aerosol by means of the fan 32, preferably via an electrostatic dust removal device (not shown). Hot waste air 10 from the cooler is supplied to the calcining region 33 approximately from the central region of the grate cooler 22 through the waste air line 34. The combustion gases of this calcining region, fired with fuel, then flow, as a heat-emitting medium, in counterflow to the quantity of raw powder fed to the heat exchanger 20 and are drawn off therefrom by the waste gas fan 39, preferably, via an electrostatic dust removal device (not shown). Numerous variations are possible with regard to the construction of the cement plant within the framework of the invention. Thus the mixing chamber 23 which is shown can comprise one or more mixing cyclones for example. The heat exchanger 19 can also have one or even three cyclone stages instead of two, whereby its material emission to the heat exchanger 20 is connected to the material extraction of the first or last but one cyclone of this heat ex changer 20. The method in accordance with the invention according to Fig. 1 can of course also be used if the raw material is present in the form of slurry or granulated material. Then it is only necessary to see that the waste gas of the preheating stage which contains the alkali particles does not come into contact with the still moist particles of raw material. WEIAT WE CLAIM IS:

1. A method of manufacturing cement clinker comprising preheating the raw material in a preheating stage, calcining the preheated material in a calcining stage, sintering the calcined material in a sintering stage, removing the waste gases directly from the sintering stage, mixing the removed gases with a cooler gaseous medium, feeding the gas mixture directly to the preheating stage as a heat emitting medium and cooling the sintered material in a cooling stage.

achieved approximately the same temperature as the preheated proportion of raw material.

12. A method according to any one of the preceding claims, wherein the combustion air for the sintering stage is removed from the cooling stage at that point where the cooling air obtained is at its hottest.

13. A method according to any one of the preceding claims, wherein the cooling of the hot clinker passing out of the sintering stage takes place in more than one cooling stage.

14. A method according to any one of the preceding claims, wherein gas and air flows are produced by a fan, control of these flows being achieved by control of the fan.

15. A method according to any one of the preceding claims, wherein removal of dust from the waste gases of the preheating stage on the one hand and the calcining stage on the other hand takes place so that the removed dust is not mixed together.

16. A method according to any one of the preceding claims, wherein the waste gases from the preheating stage are cleared of dust in two successive steps whereby the dust removed in th first step is supplied back to the burning process and the dust separated in the second step is rejected.

17. A device for the manufacture of cement clinker comprising a heat treating apparatus for the raw material including a preheating stage, a calcining stage, a sintering stage and a cooling stage, means for removing the waste gases directly from the sintering stage, means for mixing the removed gases with a cooler gaseous medium and means for feeding the gas mixture directly into the preheating stage as a heat emitting medium.

18. A device according to claim 17, wherein the sintering stage is in direct connection with the mixing means at the waste gas end.

19. A device according to claim 17 or 18, wherein the mixing means for mixing waste gas from the sintering device with a fairly cold gaseous medium has a suction line for fresh air and is in connection with the waste air end of the cooling stage or with the waste gas end of the calcining stage via further lines.

20. A device according to claim 17, 18 or 19, wherein devices for removing dust are connected into the connecting lines between the cooling stage and the calcining stage on the one hand and the mixing means on the other hand.

21. A device according to any one of claims 17 to 20, wherein a blower is provided for conveying the fairly cold gaseous medium for mixing with the waste gases of the sintering device to the mixing device.

22. A device according to any one of claims 17 to 21, wherein the preheating stage is a cyclone heat exchanger operating in counterflow.

23. A device according to any one of claims 17 to 22, wherein the calcining stage comprises a preheater part for raw material, operating in counterflow and a calcining part for calcining, operating in counterflow and connected after the preheater part in the direction of material flow.

24. A device according to claim 23, wherein the preheater part of the caicining device is a multistage cyclone heat exchanger.

25. A device according to any one of claims 17 to.24, wherein the sintering device is a rotary tube kiln.

26. A device according to claim 25, wherein the rotary tube kiln acting as a sintering device has an inlet region, of larger diameter than the diameter of the remainder of the kiln.

27. A device according to any one of claims 17 to 26 wherein the mixing means is a mixing cyclone with dust separation.

28. A device according to any one of claims 17 to 27, wherein the cooling stage is a grate cooler the whole of whose waste air being supplied completely for cooling of the waste gases of the sintering device by mixing.

29. A device according to any one of claims 17 to 28 wherein the cooling stage is a grate cooler from which air is taken near its clinker outlet for cooling the waste gases of the sintering stage by mixing.

30. A device according to any one of claims 17 to 29, wherein a first waste gas fan is provided for producing gas flows through the sintering device and subsequently through the mixing device and then through the preheating device and a second waste gas fan is provided for producing gas flows through the calcining stage.

31. A device according to claim 30, wherein the waste gas fans are equipped with speedcontrolled drives.

32. A device according to claim 30 or 31, wherein electrostatic dust removal devices are connected in the dust flow in front of the waste gas fans.

33. A method of manufacturing cement clinker substantially as described herein with reference to the drawings.

34. A device for the manufacture of cement clinker substantially as described herein with reference to the drawings.