AU2012302693A1 - Cement manufacturing method - Google Patents

Abstract

This cement manufacturing method is a cement manufacturing method for manufacturing cement clinker by burning a cement starting material charged from a kiln back part (2) into a cement kiln (1) the interior of which is held under a high-temperature atmosphere of 1400°C or higher by a flame from a main burner (5) provided at a kiln front (4) while sending the cement starting material to the kiln front (4) side. By charging an amount of dry sludge that compensates for the amount of change of P

Description

1 METHOD FOR PRODUCING CEMENT TECHNICAL FIELD [0001] The present invention relates to a method for producing cement. More particularly, the present invention relates to a method for producing cement which includes a method of producing a cement clinker by burning cement raw materials in a cement kiln maintained in a high temperature environment, and which can thereby control an amount of P 2 0 5 within a desired range in the cement clinker. Priority is claimed on Chinese Patent Application No. 201110256138.4 , filed August 31, 2011 and Japanese Patent Application No. 2012-032365, filed February 17, 2012, the content of which is incorporated herein by reference. BACKGROUND ART [0002] In a conventional cement-manufacturing facility, a cement clinker is manufactured by firstly preheating cement raw materials with a preheater to provide a predetermined temperature, then introducing the same into a cement kiln, and firing preheated cement raw materials by using flames from a main burner provided on a kiln outlet part in the cement kiln under in a high temperature of approximately 1,4500 C. Then, production of the cement is conducted by grinding cement clinker to which calcium sulfate dihydrate for preventing a rapid setting has been added. [0003] Here, a phosphorus component (P) included in the cement raw materials or a fuel is incorporated into the cement clinker which is obtained according to the method described above. Further, the phosphorus component (P 2 0 5 ) incorporated into the cement clinker is known to have an effect on some properties of the cement. In order to retain a quality of the cement, the amount of P 2 0 5 in the cement clinker is maintained in a predetermined value, while controlling an amount of P 2 0 5 in the cement raw materials and 2 the fuel. [0004] Traditionally, coal and coke are used as the fuel for being burned in the main burner in the cement kiln. However, recent years, a mixture in which an auxiliary fuel as waste oil such as an industrial waste is added to the coal and the like has been used as part of the fuel due to rising fuel prices. Furthermore, in addition to a waste plastic and a waste tire which are other industrial wastes, Patent Document 1 indicates that a wet organic sludge such as a sewage sludge, a residue sludge in food processing, and a sludge in papermaking process is effectively used as a supplemental fuel for producing the cement, after a granulation process and a drying process. [0005] However, the organic sludge frequently includes 5 wt % or more of P 2 0 5 in a solid content, thereby, a retard setting of cement and a decrease in compressive strength are possibly caused depending on a used amount thereof. [0006] Patent Document 2 discloses a method of producing cement including a step of extracting phosphorus to decrease the content of the phosphorus component in a wet sludge, a step of drying to heat and dry the wet sludge which is obtained from the step of extracting phosphorus and which is decreased the content of the phosphorus component, and a step of producing a cement to produce the cement by using dried sludge obtained from the step of drying as a fuel and a raw material to produce the cement in the cement-manufacturing facility. In this method of producing cement, it is possible to control the content of phosphorus in the solid content of the dried sludge at 4 wt% or less. [0007] In this method of producing cement, a great deal of wet sludge can be treated in the cement-manufacturing facility than before, since the amount of P 2 0 5 in the sludge is decreased to 4 wt% or less. However, in this method, when a charged amount of the wet sludge in the step of producing a cement is in a state of excess- charge or an amount of

P

2 0 5 in the cement raw materials are increased, the amount of P 2 0 5 in the cement clinker 3 increase, there is a problem in that the retard setting of cement and the decreasing of compressive strength are caused. [0008] Furthermore, in addition to the retard setting of cement and the variation of compressive strength, the grindability of the cement clinker varies with the amount of

P

2 0 5 in the cement clinker. Thus, it is required that the amount of P 2 0 5 in the cement clinker is maintained within a predetermined range, from a cement quality control perspective, when the organic sludge such as the wet sludge is charged in the cement-manufacturing facility and effectively used as part of the fuel. [Citation List] [Patent Document] [0009] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H 11-217576 Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2004-203662 DISCLOSURE OF INVENTION [Technical Problem] [0010] The present invention has been made in view of such circumstances, and an object is to provide method of producing cement in which the quality of the cement can be stably retained by maintaining the amount of P 2 0 5 in the cement clinker within the predetermined range, when the amount of P 2 0 5 in the cement raw materials varies. [Solution to Problem] [0011] [1] A method of producing cement, by which a cement clinker is produced by burning cement raw materials in a cement kiln maintained at a temperature of 1400 'C or higher by flames from a main burner provided on a kiln outlet part, while sending the 4 cement raw materials supplied from a kiln inlet part to a kiln outlet part, includes: a step of controlling an amount of P 2 0 5 included in the cement clinker in a range of 0.3 to 1.0 % by mass, by adding a dry sludge to the cement kiln, wherein an additive amount of the dry sludge is a compensation amount used to compensate for a variation of an amount of P 2 0 5 in the cement clinker or a variation of an amount of P 2 0 5 in the cement raw materials. [0012] [2] The method of producing cement according to [1], wherein the dry sludge is a heat-treated sewage sludge, and after grinding the dry sludge, dry sludge having a diameter of equal or less than 5 mm is charged into the cement kiln from the kiln outlet part. [0013] [3] The method of producing cement according to [1] or [2], wherein the dry sludge is charged into the cement kiln, after measuring a variation amount of P 2 0 5 in the cement raw materials, and measuring the amount of the dry sludge used to compensate for the variation amount. [0014] [4] The method of producing cement according to any one of [1] to [3], wherein the dry sludge is charged into the cement kiln, after measuring a variation amount of P 2 0 5 in the cement clinker, and measuring the amount of the dry sludge used to compensate for the variation amount. [0015] [5] The method of producing cement according to any one of [1] to [4], wherein a fuel for the main burner is a pulverized solid fuel which is obtained by grinding a solid fuel using a mill, and the dry sludge is charged into the cement kiln with the solid fuel from the main burner after feeding the dry sludge into the mill and ground along with the solid fuel. [0016] [6] The method of producing cement according to any one of [1] to [4], wherein the 5 dry sludge is injected into flame of the main burner from an auxiliary injection pipe provided on an upper side of the main burner. [Advantageous Effects of Invention] [0017] In the method of producing cement according to any one of [1] to [6], the dry sludge is charged into the cement kiln under at a high temperature of 1,4000 C or more. When the dry sludge is burned and decomposed at a high temperature, P 2 0 5 in an incombustible is incorporated into the burned cement raw materials. [0018] Therefore, when an amount of P 2 0 5 in the cement clinker is poor, the amount of

P

2 0 5 in the cement clinker can be rapidly and certainly controlled within a predetermined range of 0.3 to 1.0 wt% by increasing the charged amount of the dry sludge in the cement kiln [0019] In this case, since the dry sludge is charged into the cement kiln at a high temperature of 1,4000 C or more, the dry sludge is completely burned and decomposed, thereby P 2 0 5 is produced, and the amount of P 2 0 5 in the cement clinker can be efficiently controlled. [0020] As the result, even if the amount of P 2 0 5 in prepared cement raw materials varies and the amount of P 2 0 5 in the cement clinker decreases, the amount of P 2 0 5 in the cement clinker can be maintained in the range of 0.3 to 1.0 wt%, thereby the quality of the cement can be stably retained. [0021] Here, a reason for maintaining the amount of P 2 0 5 in the cement clinker in the range of 0.3 to 1.0 wt% will be described below. Generally, it is known that in a cement after a final production step to produce the cement, an effect of a strength expression is obtained by improving hydration reactivity of alite with increases in the amount of P 2 0 5 in the cement, however the strength is decreased according to a change of constituents of the 6 cement clinker such as an increasing amount of belite with an extra amount of P 2 0 5 . [0022] Accordingly, the inventors of the present invention have performed a substantiative experiment to confirm a variation value of a strength of a cement hardened body when the amount of P 2 0 5 in the cement (=the cement clinker) is changed. Fig. 4 is shown the results of the substantiative experiment, a compressive strength of the cement hardened body at after 3, 7, and 28 days in which the amount of P 2 0 5 in the cement is changed are showed. [0023] As shown in the same figure of the substantiative experiment, more than or equal to 0.3 wt % of the amount of P 2 0 5 may lead to increase the compressive strength of the cement hardened body, and 0.5 wt % of the amount of P 2 0 5 may lead a maximal value thereof. However, the compressive strength of the cement hardened body decrease with further increasing of the amount of P 2 0 5 . The value of the compressive strength of the cement hardened body at 1.0 wt % of the amount of P 2 0 5 is the same as the value of the compressive strength of the cement hardened body at the 0.1 wt % of the amount of P 2 0 5 . [0024] According to the above result, the amount of the dry sludge charged into the cement kiln is controlled to maintain the amount of P 2 0 5 in the cement clinker in the range of 0.3 to 1.0 wt%. As the result, it is possible to consistently produce cement which is improved the strength expression by controlling the amount of the dry sludge which is charged into the cement clinker so as to be the amount of P 2 0 5 within the range of 0.3 to 1.0 wt%, while the amount of P 2 0 5 is varied in the cement clinker. [0025] When the control of the amount of P 2 0 5 by charging the dry sludge is not performed, the amount of P 2 0 5 in the cement is less than or equal to 0.1 wt%, unless other wastes having a lot of P 2 0 5 are used as a raw fuel. [0026] Therefore, if the amount of P 2 0 5 in the cement clinker decreases, it is possible to 7 improve the strength expression in the cement by charging the dry sludge into the cement kiln, thereby the amount of P 2 0 5 is controlled at least 0.3 wt% or more. [0027] The method of producing cement according to [3] or [4] is adopted as a method of determining a charged amount of the dry sludge. However, a time delay occurs from a step of charging the dry sludge into the cement kiln to a step of incorporating the P 2 0 5 which is generated by burning and decomposing the dry sludge. Therefore, the method disclosed in [3] in which the dry sludge is charged into the cement kiln in order to compensate for the variation amount after comparing an amount of P 2 0 5 in the new cement raw materials and the amount of P 2 0 5 in the used cement raw materials when changing a raw materials is more preferable than the method disclosed in [4] in which the amount of the dry sludge charged into the cement kiln is determined in accordance with a variation amount of P 2 0 5 in the cement clinker. [0028] In the recent years, industrial wastes such as a dephosphorization slag including

P

2 0 5 is charged into the cement kiln as a part of the raw materials. These industrial wastes have a low impact on the variation amount of P 2 0 5 in the cement clinker, since the amount used is limited according to the amount of other elements. However, it is possible to further control the amount of P 2 0 5 in the cement clinker within the above predetermined range, when the charged amount of the dry sludge is determined by the variation amount of P 2 0 5 in the cement raw materials and the variation amount of P 2 0 5 in industrial waste. [0029] Further, as the method of producing cement according to [2], the charged dry sludge is preferably a heat-treated sewage sludge, and has a diameter of equal or less than 5 mm by grinding. By using the dry sludge, it is possible to prevent the burner from blocking the dry sludge from charging into the cement kiln from the kiln outlet part. [0030] By use of the dry sludge containing no aqueous, there is no reduction of thermal 8 efficiency occurring by heat of vaporization when the dry sludge is burned. Further, since the dry sludge is previously ground at a diameter of equal or less than 5 mm, the dry sludge is completely thermally decomposed and the desired amount of P 2 0 5 can be produced under at a high temperature in the cement kiln. [0031] Further, in the method for charging the dry sludge into the cement kiln, as disclosed in [5], the dry sludge can be charged into the cement kiln with the fuel from the main burner, and as disclosed in [6], the dry sludge can be injected into flame of the main burner from the auxiliary injection pipe provided on an upper side of the main burner. By use of these methods, the dry sludge can be charged near the flame of the main burner at the highest temperature, and the dry sludge can be reliably burned and decomposed. [0032] If the fuel of the main burner is coal, it is preferable that the dry sludge is charged into the cement kiln with the coal from the main burner after feeding the dry sludge into the coal mill which grinds the coal to obtain the pulverized coal, and the dry sludge is ground and mixed along with the coal, as disclosed in [5]. Thereby the dry sludge can be reliably thermally decomposed by the flames from the main burner. [0033] Furthermore, prior to feeding the sludge into the coal mill, the sludge does not have to be extremely dry. The water content ratio of the sludge is preferably 50 % or more. When the water content ratio of the sludge is more than 50 %, there is a problem in that the sludge adhere to inside the coal mill, and when the water content ratio of the sludge is less than 30 %, there is a problem in that heat energy waste, as described below. Therefore, the water content ratio of the sludge is preferably in the range of 30 to 50 %. [0034] Furthermore, while the coal is ground by the coal mill, the temperature of pulverized coal which is discharged from the mill is preferably maintained at 85 'C or less to prevent safety problems. If the sludge having the water content ratio in the range of 30 to 50 % is fed into the mill, a reduction of an amount of a cooling air in the mill can be 9 performed, and a cost reduction effect is expected to be performed, with the above heat energy for drying the sludge. BRIEF DESCRIPTION OF THE DRAWINGS [0035] FIG. 1 is a schematic block diagram illustrating a cement-manufacturing facility for carrying out a first embodiment of the present invention. FIG. 2 is a schematic block diagram illustrating a cement-manufacturing facility for carrying out a second embodiment of the present invention. FIG. 3 is a graph illustrating a variation amount of P 2 0 5 in the cement clinker when charging the dry sludge into the cement kiln, the result of the experiment is obtained by the use of the method in the first embodiment. FIG. 4 is a graph illustrating a relationship between an amount of P 2 0 5 in the cement clinker and a compressive strength of the cement hardened body from the results of the experiment. BEST MODE FOR CARRYING OUT THE INVENTION [0036] [First embodiment] Fig. 1 illustrates a cement-manufacturing facility which is used in the first embodiment of a method of producing cement according to the present invention, reference number 1 denotes a cement kiln 1 used for burning cement raw materials. The cement kiln 1 is a rotary kiln provided to be freely rotatable about an axis. In the cement kiln 1, a preheater 3 which preheats the cement raw materials is provided on a kiln inlet part 2 which is shown on the left of the figure, a main burner 5 which heats the inside of the cement kiln 1 is provided on a kiln outlet part 4 which is shown on the right of the figure. In Fig. 1, reference number 6 denotes a clinker cooler cooling a cement clinker after burning [0037] 10 The preheater 3 is constituted by a plurality of stages of cyclones 21 which are serially arranged in a vertical direction (perpendicular direction). In the preheater 3, a feed pipe 7 which feeds the cement raw materials connects to the cyclone 21 in the uppermost stage, and transfer pipe 3a which transports the cement raw materials in the cyclone 21to the kiln inlet part 2 of the cement kiln 1 connects to the bottom of the cyclone 21 in the lowermost stage. [0038] Further, an exhaust gas pipe 3b which feeds a burned exhaust gas discharged from the cement kiln 1 to the cyclone 21 in the lowermost stage is provided on the kiln inlet part 2. The exhaust gas discharged from the cyclone 21 in a top of the uppermost stage is discharged by passing through an exhaust line 8 using a distributing exhaust fan (not shown). [0039] A feeding device which feeds the pulverized coal (a pulverized solid fuel) used as a fuel is provided on the main burner 5 in the cement kiln 1. The feeding device is constituted by a coal silo 10 which stores a mixture (solid fuel) of coal, coke, and an auxiliary fuel used as raw materials of the fuel, a coal mill 11 which grinds the coal and the like fed from the coal silo 10 and obtains the pulverized coal having a predetermined grain size, and a fuel feeding pipe 12. The fuel feeding pipe 12 injects the pulverized coal obtained by the coal mill 11 into the cement kiln 1 passing through the main burner 5 by using compressed air supplied from a distribution supply pipe (not shown). [0040] Furthermore, a measuring device 13 which measures a dry substance obtained from a sewage sludge and the like is provided in the cement-manufacturing facility, and a dry sludge measured by the measuring device 13 is fed to the coal mill 11 with the coal supplied from the coal silo 10. The measuring device 13 measures an amount of the dry sludge fed into the cement kiln 1, the amount of the dry sludge is determined by an amount of P 2 0 5 in prepared cement raw materials (14) or an amount of P 2 0 5 in the cement clinker (15) 11 discharged from the clinker cooler 6 which is measured by, for example, a well-known fluorescent X-ray analysis device (not shown). [0041] Next, a method of producing cement in the first embodiment which uses the cement-manufacturing facility having the above described constructions will be described below using Fig. 1. First, in the cement-manufacturing facility, cement raw materials obtained by grinding and mixing limestone, clay, silica stone, and a raw material of iron oxide are fed into the cyclone 21 in the top of the uppermost stage of the preheater 3 passing through the feed pipe 7. Then, the cement raw materials fed into the cyclone 21 in the top of the uppermost stage is preheated by a high-temperature exhaust gas which is sent from the cement kiln 1 and rises from the lower part, as the cement raw materials falls down sequentially into the cyclones 21 of the lower part, and finally the cement raw materials is fed into the cement kiln 1 from the cyclone 21 in the lowermost stage passing through the transfer pipe 3a. [0042] On the other hand, coal and the like which is used as raw materials of the fuel and is stored in the coal silo 10 is fed to the coal mill 11 to grind, thereby the pulverized coal is obtained, and the pulverized coal is fed into the main burner 5 provided on the kiln outlet part 4 in the cement kiln 1 from the fuel feeding pipe 12. Then, since the pulverized coal fed into the cement kiln 1 from the main burner 5 is burned, the kiln outlet part 4 in the cement kiln 1 is maintained at a high temperature of 1,4500 C or more. [0043] Therefore, the cement raw materials which is fed into the cement kiln 1 from the kiln inlet part 2 is heated at about 1,4500 C by the burned exhaust gas discharged from the main burner 5 while the cement raw materials are sent to the kiln outlet part 4 gradually, and then the cement clinker is produced by burning the cement raw materials. Next, the cement clinker sent to the kiln outlet part 4 falls down to into the clinker cooler 6, and sent to a right half portion in the figure. The cement clinker is cooled at predetermined 12 temperature by the air fed into the clinker cooler 6, and finally discharged from the clinker cooler 6. [0044] Further, in parallel with the above step, the sludge such as the sewage sludge which is fed into the cement-manufacturing facility is previously dried in a heated atmosphere at a temperature of 100 to 3000 C, thereby a dry sludge having the water content ratio in the range of 30 to 50 %, preferably in the range of 35 to 44 % is produced. Then, an amount of P 2 0 5 in prepared cement raw materials (14) is compared to an amount of P 2 0 5 in the cement clinker, and the dry sludge which is equivalent to a deficiency amount of P 2 0 5 in the cement clinker is measured by using the measuring device 13, then the measured dry sludge is fed into the coal mill 11. As a result, the amount of P 2 0 5 in the cement clinker is always controlled, and the amount of P 2 0 5 in the cement clinker is maintained in the range of 0.3 to 1 wt%. [0045] In the above measurement of the dry sludge, the dry sludge is allowed to be fed into the coal mill 11, by measuring the dry sludge which is equivalent to a decreased amount of P 2 0 5 in the cement clinker, when the amount of P 2 0 5 in the cement clinker (15) discharged from the clinker cooler 6 decreases. [0046] Then, the dry sludge which is mixed with the coal and the like supplied from the coal silo 10 is ground at a diameter equal or less than 5 mm, preferably equal or less than 3 mm, and more preferably equal or less than 0.1 mm by the coal mill 11, as a result the dry sludge powder is obtained, the dry sludge powder is fed into the cement kiln 1 with the pulverized coal from the main burner 5, and is thermally decomposed at a temperature of 1400 'C or higher by flames from a main burner. [0047] [Examples] Fig. 3 shows the result of the experiment which was obtained by the use of the above method and the above cement-manufacturing facility.

13 In the cement-manufacturing facility, when 130 tons of the cement clinker was produced per hour, the amount of P 2 0 5 in the cement clinker was expected to be 0.1 wt%. [0048] Therefore, in order to compensate for the amount of P 2 0 5 in the cement clinker, a dry sludge including 6 wt% of P 2 0 5 was measured by the measuring device 13, and 6500 kg/hour of the dry sludge (50 kg-the dry sludge/t- clinker) was charged into the cement kiln 1 with the pulverized coal from the main burner 5. As a result, as shown in Fig. 3, the amount of P 2 0 5 in the cement clinker discharged from the clinker cooler 6 hovered at around 0.4 wt%. [0049] Because of the above description, by the above method of producing cement, the amount of P 2 0 5 in the cement clinker could be controlled when the amount of P 2 0 5 in the cement raw materials varied and the amount of P 2 0 5 in the cement clinker decreased. Specifically, since the amount of the dry sludge which was charged into the cement kiln 1 at a high temperature of 1,4000 C or more from the kiln outlet part 4 increased, the amount of P 2 0 5 in the cement clinker could be rapidly and reliably controlled in the predetermined range of 0.3 to 1 wt%. [0050] In particular, in the present invention, with the sludge such as the sewage sludge was dried by the heat-treatment, the dry sludge was previously ground to a diameter equal or less than 5 mm, and the dry sludge was charged into the cement kiln 1 with the pulverized coal (a pulverized solid fuel) from the main burner 5. That is, there was no reduction of the thermal efficiency in the cement kiln 1 by the aqueous included in the sludge, and the dry sludge could be completely decomposed, thereby the amount of P 2 0 5 in the cement clinker was controlled efficiently. [0051] Therefore, when the amount of P 2 0 5 in the prepared cement raw materials vary and the amount of P 2 0 5 in the cement clinker decrease, the amount of P 2 0 5 in the cement clinker can be reliably controlled in the range of 0.3 to 1 wt%, and then the quality of the 14 cement can be stably retained. [0052] In addition, when the sludge having the water content ratio in the range of 30 to 50 % is fed into a coal mill, a reduction of an amount of a cooling air in the mill can be performed, and a cost reduction effect is expected to be performed with the above heat energy for drying the sludge. [0053] [Second embodiment] Fig. 2 illustrates a cement-manufacturing facility which is used in the a method of producing cement in the second embodiment according to the present invention, the same portions shown in Fig. 1 are denoted by the same reference numerals, and the description thereof will be simplified. In the cement-manufacturing facility, a grinder mill 16 is provided in order to grind the dry sludge measured by a measuring device 13 and to obtain dry sludge powder having a diameter equal or less than 5 mm, preferably in the range of 1 to 3 mm. Further, in a kiln outlet part 4 of a cement kiln 1, an auxiliary injection pipe 17 is provided on an upper side of the main burner 5, the dry sludge powder ground by the grinder mill 16 is injected into flame of the main burner 5 and into the cement kiln 1 with a compressed air from the auxiliary injection pipe 17. [0054] In a method of producing cement in the second embodiment which uses the cement-manufacturing facility having constructions above described will be described below. A required amount of dry sludge is determined based an amount of P 2 0 5 in prepared cement raw materials (14) or an amount of P 2 0 5 in the cement clinker (15) discharged from the clinker cooler 6, the dry sludge measured by the measuring device 13 is supplied to the grinder mill 16. Then, the dry sludge powder ground by the grinder mill 16 is injected into flame of the main burner 5 from the auxiliary injection pipe 17, and is thermally decomposed by the flames from a main burner 5 at a temperature of 1,4000 C or more.

15 [0055] Therefore, in the method of producing cement disclosed in the second embodiment as with the first embodiment, when the amount of P 2 0 5 in the prepared cement raw materials (14) vary and the amount of P 2 0 5 in the cement clinker (15) decrease, the amount of P 2 0 5 in the cement clinker can be reliably controlled in the range of 0.3 to 1 wt%, and then the quality of the cement can be stably retained. [0056] In addition, in the first embodiment and the second embodiment, the dry sludger is charged into the cement kiln 1 with the pulverized coal (a pulverized solid fuel) from the kiln outlet part of the cement kiln 1 passing through the main burner 5, or the dry sludger is injected into flame of the main burner 5 from the auxiliary injection pipe 17. However, the present invention is not limited to the configuration of these embodiments. For example, it is possible to charge the dry sludger into the cement kiln 1 from the preheater, a pre-combustion reactor, or the kiln inlet part 2. INDUSTRIAL APPLICABILITY [0057] The method of the present invention is available for a control of an amount of

P

2 0 5 in a cement clinker in a predetermined range in producing the cement. REFERENCE NUMBER [0058] 1: Cement kiln 2: Kiln inlet part 4: Kiln outlet part 5: Main burner 10: Coal silo 11: Coal mill 12: Fuel feeding pipe 16 13: Measuring device 16: Grinder mill 17: Auxiliary injection pipe

Claims (6)

1. A method of producing cement, by which a cement clinker is produced by burning cement raw materials in a cement kiln maintained at a temperature of 1400 'C or higher by flames from a main burner provided on a kiln outlet part, while sending the cement raw materials supplied from a kiln inlet part to the kiln outlet part, comprising: a step of controlling an amount of P 2 0 5 included in the cement clinker in a range of 0.3 to 1.0 % by mass, by adding a dry sludge to the cement kiln, wherein an additive amount of the dry sludge is a compensation amount used to compensate for a variation of an amount of P 2 0 5 in the cement clinker or a variation of an amount of P 2 0 5 in the cement raw materials.

2. The method of producing cement according to claim 1, wherein the dry sludge is a heat-treated sewage sludge, and after grinding the dry sludge, the dry sludge having a diameter equal or less than 5 mm is charged into the cement kiln from the kiln outlet part.

3. The method of producing cement according to claim 1 or 2, wherein the dry sludge is charged into the cement kiln, after measuring a variation amount of P 2 0 5 in the cement raw materials, and measuring the amount of the dry sludge used to compensate for the variation amount.

4. The method of producing cement according to any one of claims 1 to 3, wherein the dry sludge is charged into the cement kiln, after measuring a variation amount of P 2 0 5 in the cement clinker, and measuring the amount of the dry sludge used to compensate for the variation amount.

5. The method of producing cement according to any one of claims 1 to 4, wherein a fuel for the main burner is a pulverized solid fuel which is obtained by grinding a solid fuel using a mill, and the dry sludge is charged into the cement kiln with the solid fuel 18 from the main burner after feeding the dry sludge into the mill and ground along with the solid fuel.

6. The method of producing cement according to any one of claims 1 to 4, wherein the dry sludge is injected into flame of the main burner from an auxiliary injection pipe provided on an upper side of the main burner.