JPS6051642A - Cement raw material powder calcining process - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently promoting calcination when firing cement raw material powder.

The firing reaction of cement raw materials mainly includes the calcining (decomposition) reaction of limestone, which is an endothermic reaction, and the cement clinker production reaction, which is an exothermic reaction. A calcination device equipped with an independent fuel supply means is installed between the calcination devices, and before supplying the cement raw material powder to the calcination device, the cement raw material powder is suspended in a preheating device using exhaust gas from the calcination device. The calcined material is preheated and then introduced into a calcination device such as an air flow furnace or a fluidized fluidized furnace equipped with an independent fuel supply means, where most of the calcination reaction is completed by the combustion heat of the supplied fuel. The raw material is similarly supplied to a calcination device equipped with an independent fuel supply means, where the remaining calcination reaction and clinker production reaction are carried out.

In such a firing method with a calcination device, the combustion of the supplied fuel is shared between the calcination device and the firing furnace, with the calcination device mainly performing the calcination reaction, and the calcination device mainly performing the cement clinker production reaction. By doing so, we are able to eliminate problems associated with larger firing equipment, such as the shortened lifespan of refractories due to increased heat load in the firing furnace, and at the same time dramatically increase the production volume per internal volume of the firing equipment. It is well known that this firing method is more effective than other firing methods in that it enables stable operation over a long period of time.

In particular, by configuring the calcination process with a plurality of calcination zones each equipped with an individual fuel supply means, and passing the raw material powder through these multiple calcination zones in sequence, the cement raw powder powder is calcined in the calcination zone process. Methods have been proposed to more effectively promote the reaction. Such calcining apparatuses for cement raw powder, which are composed of a plurality of calcining zones, are roughly classified into series type and parallel type when viewed in the flow direction of hot gas.

First of all, Fig. 1 is a diagrammatic system diagram illustrating a cement raw material firing apparatus using serially arranged calcining equipment, in which the flow of hot gas is shown by solid line arrows, and the flow of raw material powder is shown by broken line arrows. . The outline of the apparatus consists of a preheating device 1 which controls the preheating process, a calcination W2 which controls the calcination process, a firing furnace 3 such as a rotary kiln which controls the firing step 1, and a clinker cooling bag W4 which controls the cooling process.

The preheating device l is a powder separator 01 such as a cyclone separator.
~C3, a duct 6, etc., and the firing device 2 is of a series arrangement type in this example, and has a calcining zone 21 on the downstream side and a calcining zone 22 on the upstream side when viewed in the flow direction of hot gas. It is constructed by arranging a plurality of calcining zones (two in the case of FIG. 1) in series, and each calcining zone 21, 22 is individually connected to the fuel supply device 7a.
, 8. A calciner 7.8 equipped with a powder separation ac4.8 attached to the calciner. Consists of C5 etc.

The raw material powder supplied from the raw material input chute 5 sequentially descends through each powder separator C, -C9 that constitutes the preheating device 1.
On the other hand, the high-temperature exhaust gas from the firing furnace 3 and the calcination device 2 is sucked by the induced draft ta13 and rises in the preheating device Wl, so that the heat between the raw material powder and the high-temperature gas is absorbed in the duct 6 and the powder separators 01 to C3. Exchange and separation are repeated. The preheated raw material powder constitutes the calcination device 2 from the lowermost powder separator C3 of the preheating device Wl. It is introduced into the primary calcination furnace 7 on the downstream side, and then passed through the powder separator C4 to the secondary calcination furnace 8 on the upstream side.
will be introduced to

On the other hand, the combustion exhaust gas from the secondary calcination furnace 8 that still contains sufficient oxygen is connected to be discharged to the primary calcination furnace 7 via the powder separator C5. High-temperature combustion air is introduced into the secondary calciner through a high-temperature air conduit 10 and a fuel supply device 7 provided in each calciner 7.8.
The fuel supplied from a and 8a causes combustion in the calciners 7 and 8, and by receiving the combustion heat and the heat of the calciner exhaust gas, the raw material powder is sequentially transferred to the primary calcining zone 21 and the secondary calcining zone. While passing through the band 22, it is calcined.

The calcined raw material powder enters the calcining furnace 3 from the powder separator C6 through the calcining furnace inlet cover 9 and reaches the outlet end ff11 of the calcining furnace 3.
The clinker undergoes necessary heat treatment in the firing furnace 3 by the combustion heat of the fuel supplied together with the primary air for combustion from the fuel supply device 3a installed at 9, and then is cooled in the cooling bag W4.

Note that air for cooling the clinker is supplied by a forced air blower 16, and a part of the high-temperature air that has been heated by exchanging heat with the clinker is distributed and introduced into the calcining device 2 and the calcining furnace 3, but the excess air is It is discharged by induced draft ta17. The clinker coming out of the cooling bag W4 is then carried out to the next process by the conveyor 18.

According to the series arrangement type calcination process as shown in FIG. The gas does not flow into the upstream calcining zone 22, and almost the same amount of gas passes through the upstream calcining zone 22 as the downstream calcining zone 21, so the upstream calcining zone 22 is filled with hot gas. The partial pressure of carbon dioxide gas inside can be reduced, and the calcination reaction of the raw material powder can be efficiently promoted.

At the same time, only raw material powder that cannot be captured by the powder separator C4 attached to the primary calcination zone 21 of the calcination device 2 is circulated from the calcination device 2 to the preheating device 1. , compared to the case of the parallel arrangement type described later, not only is the quantity smaller, but the degree of calcination is also lower, and the amount of raw material powder that causes the recarbonation reaction based on the circulation of these raw materials in the preheating device 1 is also smaller. The calcination reaction can proceed without impairing thermal efficiency.

However, in such a series arrangement type calcination device 2, the entire amount of hot gas passes through the plurality of calcination zones 21 and 22 in series, so the pressure loss in the calcination device 2 is large. The power consumption in the exhaust gas induced draft fan 13 becomes larger than that in the case of . This means that a portion of the high-temperature air drawn directly from the cooling bag W4 to the calcining device 2 is transferred to the high-temperature air branch conduit 10. Even if the short circuit is supplied to the downstream calcining zone 21 through the short circuit, the problem cannot be resolved.

In addition, a hot gas flow is induced from the cooling device 4 to the calcination device 2 via the calcination furnace 3, and a high-temperature air flow is induced directly to the calcination device 2 from the cooling bag W4 through the high-temperature air conduit 10. There is an operational problem in that it is difficult to control the quantitative ratio.

On the other hand, Fig. 2 is a diagrammatic system diagram illustrating a cement raw material sintering device using a parallel arrangement type calcination device, and in this figure, parts having the same functions as those in Fig. 1 are designated by the same reference numerals. The explanation will be omitted.

The calcination device includes an exhaust gas system calcination zone 23 that uses exhaust gas from the calcination furnace 3, and a combustion gas system calcination zone 24 that uses combustion gas produced by high-temperature air from the cooling device 4, as viewed in the flow direction of hot gas. Each calcining zone 23, 24 is equipped with a calciner P33゜34 equipped with combustion supply devices 33a, 34- and a powder separator C84゜CM attached to the calciner. etc., and each calcining zone 2
3 and 24 are connected to preheating devices 11 and 12. Further, the combustion air in the exhaust gas system calcining zone 23 is supplied from the cooling device 4 through the inside of the firing furnace 3, while the combustion air in the combustion gas system calcining zone 24 is supplied via the high temperature air conduit lO. In this conventional example, the exhaust gas system calcining zone 23 is used as the primary calcining zone, and the combustion gas system calcining zone 24 is used as the secondary calcining zone when viewed in the flow direction of the raw material powder.

That is, the raw material powder, which has been separately preheated in the preheating device 11.12 by utilizing the high temperature gas discharged from each calcining zone 23, 24, is transferred to the lowermost powder separator cn, C, of the preheating device 11.12. Both are supplied to the calcination furnace 33 in the exhaust gas system for primary calcination, and then to the powder separator C+.
4 is supplied to the calciner 34 of the flue gas system through 2.
After the next calcining, the powder is supplied to the kiln through the powder separator cm and the inlet cover 9.

Therefore, according to such a parallel calcination device, the hot gas is induced by the induced draft fan 14 or 15 and only passes through one of the calcination zones 23 or 24 arranged in parallel. The pressure loss in the calcination process is small, and the exhaust gas system calcination zone 23 and the combustion gas system calcination zone 24 can be configured to have hot gas flows independent of each other, including the preheating devices II and 12 attached to each. This feature makes it easy to control the amount of hot gas passing through each system.

However, in the conventional parallel calcination process as described in 1, the calcination reaction cannot be efficiently promoted due to the following points.

That is, although the carbon dioxide gas from the fuel and raw material powder is only partially generated in each calcining zone 23 or 24, the amount of hot gas passing through each calcining zone 23 or 24 is also partial.
Unless air for fuel combustion in the calcining zone 23 or 24 is supplied in excess, the partial pressure of carbon dioxide in the hot gas surrounding the raw material powder in the calcining zone 23 or 24 cannot be essentially reduced; If excessive combustion air is supplied, the calcination zone 23
Alternatively, the amount of fuel consumed in step 24 will increase, and in any case, the calcination reaction cannot be efficiently promoted.

In addition, among the raw material powder after primary calcination in the calcination zone 23 of the exhaust gas system, the raw material powder that cannot be captured by the powder separator CI4 attached to the calcination zone 23 is transferred to the preheating device l of the exhaust gas system.
In addition to circulating to the combustion gas system calcination zone 23, the raw material powder captured by the powder separator C4 attached to the exhaust gas system calcination zone 23 is subsequently supplied to the combustion gas system calcination zone 24 for secondary calcination. The powder separator C1 attached to the calcining zone 24 introduces the raw material powder that cannot be captured here again to the preheater 1 of the combustion gas system.
As the number of calcining zones arranged in parallel increases, the amount of raw material powder that is scattered and circulated from the calcining process to the preheating process also increases proportionally, and the raw material powder is gradually calcined to a higher degree. The raw material powder will be circulated.

A portion of such circulating raw material flows into the preheating device 11.12 and, when the temperature decreases, it undergoes a so-called recarbonation reaction in which it reacts with the carbon dioxide components contained in the surrounding hot gas, thus forming The calcined calcium carbonate needs to be circulated and supplied to the calcining zones 23 and 24 to undergo the calcining reaction again, which not only increases fuel consumption in the calcining zones 23 and 24, but also
Since the recarbonation reaction in the preheating device rfIll, 12 generates heat, the preheating device 11. ．． If the exhaust gas temperature from 12 rises, or each preheating device 11. ．． This causes operational troubles such as clogging phenomena in the powder separators Cn and Cn on the high temperature side of No. 12.

In addition, when the order in which the raw material powder passes through each calcination zone 23 and 24 is reversed, and the primary calcination is performed in the combustion gas system calcination zone 24 and the secondary calcination is performed in the exhaust gas system calcination zone 23, has the same problem.

Therefore, the object of the present invention is to make use of the advantages of the above-mentioned series arrangement type and parallel arrangement type calcination devices, and at the same time eliminate their disadvantages, thereby making it possible to efficiently accelerate calcination. The main purpose of this is to provide multiple systems equipped with individual fuel supply means when firing cement raw material powder into cement clinker through a preheating process, a calcination process, a firing process, and a cooling process. In the method for calcination of cement raw material powder, the calcination process is performed using a plurality of calcination zones, an exhaust gas system primary calcination zone using exhaust gas from the calcination process as the plurality of calcination zones, and a cooling step. A combustion gas system primary calcination zone that uses combustion gas from high-temperature air from
A secondary calcination zone is arranged on the gas upstream side of the secondary calcination zone, and the raw material powder is firstly calcined separately in the exhaust gas system primary calcination zone and the combustion gas system primary calcination zone, respectively. , a method for calcination of cement raw material powder in which the primary calcined raw material powder is fed to a secondary calcination zone, calcined to a higher degree, and then discharged to the calcination process, and another method used in this calcination method. primary calcining zone,
A tertiary calcination zone is provided on the upstream side of the gas, and the raw material powder that has been primarily calcined separately in both of the primary calcination zones is sequentially supplied to the secondary calcination zone and the tertiary calcination zone. The object of the present invention is to provide a calcination method that achieves a higher degree of calcination.

Next, embodiments embodying the present invention will be described with reference to the accompanying drawings starting from FIG. 3 to provide an understanding of the present invention. In the following description, the same reference numerals are given to the parts having the same functions as those in the conventional example.

FIG. 3 is a diagrammatic system diagram showing an example of a cement raw material powder unburning apparatus for carrying out the method of the present invention.

In the figure, the calcination device includes an exhaust gas system primary calcination zone 25 that uses exhaust gas from the calcination furnace 3 and a combustion gas system primary calcination zone 26 that uses combustion gas from high-temperature air from the cooling device 4. arranged in parallel in the flow direction of hot gas, and on the gas upstream side of the primary calcining zone 26 of the combustion gas system,
A secondary calcining zone 28 is provided, and each calcining zone 2
5, 26, and 28 are individually fuel supply devices 35. ．． 36. .

38a and powder separators C+4, C24, Cas, etc. attached to the calcination furnaces, each primary calcination zone 25, 26 is equipped with a preheating device 11, 12 are connected.

The raw material powder preheated in the preheating devices 11 and 12 by using the high temperature gas discharged from each of the primary calcining zones 25 and 26 is discharged as exhaust gas from the lowermost powder separators CIi + Cn of the preheating devices 11 and 12, respectively. System primary calcining zone 25
The calcination furnace 35 and combustion gas system primary calcination zone 26 that constitute the
The powder is supplied to the calcination furnace 36 constituting the calcination furnace 36 and subjected to primary calcination separately, and then the powder separators c, 4, and C3 attached to each calcination furnace 35, 36 are supplied to the calcination furnace 36, and the powder separators C, 4, and C3 are supplied to the combustion gas system. After being fed together to the secondary calcination zone 28 and subjected to secondary calcination, the powder separator C2! It is supplied to the firing furnace 3 through the l- and inlet end cover 9.

In this embodiment, the combustion air in the exhaust gas system calcining zone 25 is supplied through the inside of the firing furnace from the cooling device 4, while the combustion air in the secondary calcining zone 28 arranged in the combustion gas system is supplied from the cooling device. 4 to firing furnace outlet end ff119
and the high temperature air conduit 10, and the exhaust gas from the secondary calcining zone 28 still containing sufficient oxygen is supplied as a source of combustion air to the combustion gas system primary calcining zone 26 through the powder separator CtS. It is now possible to do so.

In the example shown in the third figure above, the raw material powders that have been separately primary calcined in the primary calcining zone 25 of the exhaust gas system and the primary calcining zone 26 of the combustion gas system are both in the primary calcining zone 26 of the combustion gas system. After being subjected to secondary calcination to a higher degree in the secondary calcination zone 28 disposed on the upstream side, it is discharged to the firing process.
It exhibits the following unique effects.

That is, since most of the raw material powder that has been primarily calcined is supplied to the secondary calcination zone 28, the amount of carbon dioxide gas generated from the raw material powder in the secondary calcination zone 28 is small; Since combustion air is supplied in excess of the amount of fuel supplied, the partial pressure of carbon dioxide in the combustion gas becomes low, and the secondary calcining zone 2
In step 8, the calcination reaction can be efficiently promoted.

The excess combustion air is supplied to the downstream combustion gas system primary calcination zone 26, where it is effectively used as combustion air, so the amount of fuel consumed in the secondary calcination zone 28 is reduced. It should not be a factor that increases the number of people unnecessarily.

Further, from the calcination zones 25, 26.28 to the preheating device 11.12, there is a powder separator c14 attached to the primary calcination zone 25.26.
Only the raw material powder that cannot be captured by C, 4 is circulated, and not only is the quantity small, but the degree of calcination is also low.
The decrease in thermal efficiency due to the recarbonation reaction in the preheating devices 11 and 12 is slight.

In the combustion gas system, a part of the raw material powder is also circulated from the powder separator CM attached to the secondary calcination zone 28 to the primary calcination zone 26, but the recarbonation reaction is not carried out in the primary calcination zone 26. Since the temperature is higher than the temperature range in which the reaction occurs, the recarbonation phenomenon in the primary calcination zone 26 is negligible. This only causes a difference in the fuel supply ratio in the calcining zone 28, but does not affect the overall thermal performance.

In addition, in the combustion gas system calcining zone, the hot gas passes through a plurality of calcining zones 26 and 28 in series, but in the exhaust gas system calcining zone, the hot gas only passes through one calcining zone 25. Therefore, the pressure loss can be lower than in a series-type calcining process depending on the ratio of gas amounts passing through the combustion gas system and the exhaust gas system.

Furthermore, since the flow of hot gas is independent between the exhaust gas system calcining zone and the combustion gas system calcining zone, it is easy to control the amount of hot gas passing through each system.

In the embodiment shown in FIG. 3 above, the secondary calcining zone 28 is attached to the upstream side of the primary calcining zone 26 in the combustion gas system, but in the present invention, the above-mentioned This also includes the case where the secondary calcining zone is disposed upstream of the primary calcining zone 25 in the exhaust gas system, and FIG. 4 shows a cement raw material firing apparatus according to such an embodiment. That is, a secondary calcination zone 27 is provided on the gas upstream side of the primary calcination zone 25 in the exhaust gas system, and the raw material powder is separately processed in the primary calcination zone 25 in the exhaust gas system and the primary calcination zone 26 in the combustion gas system. At the same time, these primary calcined raw material powders are used together in the exhaust gas system.
After being supplied to the next calcination zone 27 and calcined to a higher degree, it is discharged to the firing process. In this case, since the amount of raw material powder calcined in the calcining zone of the exhaust gas system increases, a portion of the high-temperature air is also directly supplied to the secondary calcining zone 27 from the cooling device 4 through the high-temperature air conduit tab, as shown in the figure. Attraction 1
- It is desirable to

FIG. 5 is a diagrammatic system diagram of a cement raw material sintering apparatus in still another embodiment, in which a secondary calcination zone 27 is arranged on the gas upstream side of the primary calcination zone 25 in the exhaust gas system, and at the same time There is a tertiary calcination zone 29 on the gas upstream side of the gas system primary calcination zone 2G.
The raw material powder is primary calcined separately in the primary calcining zone 25 of the exhaust gas system and the primary calcining zone 26 of the combustion gas system, and is then combusted via the secondary calcining zone 27 of the exhaust gas system. It is made to pass sequentially to the tertiary calcination zone 29 of the gas system to perform calcination to a higher degree.

That is, by supplying the raw material powder in which the calcination reaction has considerably progressed from the secondary calcination zone 27 to the tertiary calcination zone 29, the amount of calcination reaction in the tertiary calcination zone 29 is reduced, and the cooling device By introducing combustion air having a higher temperature than the secondary calcination zone 27 from the fourth to the tertiary calcination zone 29, the amount of fuel used in the tertiary calcination zone 29 is reduced. This design aims to further reduce the partial pressure of carbon dioxide gas in zone 29, and in each system, the calcining zone has a sequential reduction in the partial pressure of carbon dioxide gas by simply passing through two calcining zones. Figure 3 and Figure 3 show that it is possible to carry out the 3rd calcination by passing through the calcination device, and to prevent the amount of raw material powder from scattering and circulating from the calcination device to the preheating device from increasing. This is similar to the embodiment shown in FIG.

The above-mentioned method also applies when a secondary calcination zone is provided on the downstream side of the primary calcination zone 26 in the combustion gas system, and a tertiary calcination zone is provided on the upstream side of the primary calcination zone 25 in the exhaust gas system. Almost the same effect can be obtained.

The technical scope of the present invention is not limited only to the above explanation, and is not limited to, for example, the structure of the preheating device (cyclone type, tower type), the number of series, the number of stages, etc. The exhaust gas from the burning zone and the combustion gas system primary calcination zone is led to a common preheating device, and the preheated raw material from the preheating device is distributed to the exhaust gas system primary calcination zone and the combustion gas system primary calcination zone. It can also be supplied.

In addition, the structure of each calcining zone (spouted bed type, swirling flow type), the type of fuel used in each calcining zone (liquid, solid, or gas), the type, type, mounting position, etc. of the fuel supply means, etc. Can be changed freely.

Furthermore, it is also possible to provide appropriate circulation means for the raw material powder in the calcining zone, or to combine means for denitrifying the exhaust gas flowing from the calcination device into the upstream plate firing zone of the exhaust gas system.

As described above, the present invention uses a plurality of calcining zones individually equipped with fuel supply means when firing cement raw powder into cement clinker through a preheating process, a calcining process, a firing process, and a cooling process. In the method for calcination of cement raw material powder, which performs calcination treatment using
A secondary calcination zone, and a combustion gas system primary calcination zone that uses combustion gas from high-temperature air from the cooling process,
A secondary calcination zone is provided on the gas upstream side of either the primary calcination zone of the exhaust gas system or the primary calcination zone of the combustion gas system, or the secondary calcination zone is further arranged on the gas upstream side of the primary calcination zone of either the exhaust gas system primary calcination zone or the combustion gas system primary calcination zone. A tertiary calcination zone is arranged on the gas upstream side of the zone, whereby the raw material powder is firstly calcined separately in the exhaust gas system primary calcination zone and the combustion gas system primary calcination zone, respectively. , a cement raw material powder characterized in that the primary calcined raw material powder is supplied to a secondary calcining zone, or further supplied to a tertiary calcining zone, calcined to a higher degree, and then discharged to a firing process. Since this is a calcination method, the raw material powder that has been firstly calcined separately in the primary calcination zone of the exhaust gas system and the primary calcination zone of the combustion gas system is transferred to the second calcination zone or further to the tertiary calcination zone. The calcination reaction can be efficiently promoted in a low carbon dioxide atmosphere in the zone.

In addition, from the calcination process to the preheating process, only the raw material powder that cannot be captured in the primary calcination zone of the calcination process is circulated.
Since the amount is not only small but also the degree of calcination is low, the decrease in thermal efficiency due to the recarbonation reaction in the preheating step is slight.

Furthermore, depending on the ratio of passing gas between the combustion gas system and the exhaust gas system, the pressure loss is smaller than that of a series-arranged calciner, and the flow of hot gas between the exhaust gas system and the combustion gas system is independent of each other. This makes it easier to control the amount of hot gas passing through each system.

[Brief Description of the Drawings] Figures 1 and 2 are diagrammatic system diagrams of a conventional cement raw material firing apparatus, and Figures 3 to 5 are system diagrams of a firing apparatus in an embodiment of the present invention, respectively. . (Explanation of symbols) 1.11.12...Preheating device 2...Calcination device 3.
...Calcination furnace 4...Cooling device 7.8.33-39...Calcination furnace 7a, ea, 33a-39a...Fuel supply device 21.
23, 25. 26... Primary calcining zone 22. 24, 27.
28...Second calcining zone 29...Third calcining zone CI'''Ca, Ctl ~Cg, C,, ~C,, -
'Powder separator. Applicant Kobe Steel Co., Ltd. Representative Patent Attorney Takeo Honjo Figure 3 Figure 4

Claims (2)

[Claims] (1) Cement raw material powder is sequentially preheated, calcined,
In a method for calcination of cement raw material powder, the calcination process is performed using a plurality of calcination zones each individually equipped with a fuel supply means when calcination is performed to form cement clinker through a calcination step and a cooling step. An exhaust gas system primary calcination zone that uses exhaust gas from the firing process as a calcination zone, and a combustion gas system primary calcination zone that uses combustion gas from the high temperature air from the cooling process are installed, and these exhaust gas systems 1
A secondary calcination zone is provided on the gas upstream side of either the primary calcination zone or the combustion gas system; The raw material powders are firstly calcined separately in the primary calcination zone, and these primary calcined raw material powders are supplied to the secondary calcination zone where they are calcined to a higher degree and then discharged to the firing process. Semen 1 characterized by
~ Method for calcining raw material powder. (2) Cement raw material powder is sequentially preheated, calcined,
In a method for calcination of cement raw material powder, the calcination process is performed using a plurality of calcination zones each individually equipped with a fuel supply means when calcination is performed to form cement clinker through a calcination step and a cooling step. An exhaust gas system primary calcination zone that uses exhaust gas from the firing process as a calcination zone, and a combustion gas system primary calcination zone that uses combustion gas from the high temperature air from the cooling process are installed, and these exhaust gas systems 1
Secondary calcination zone and combustion gas system A secondary calcination zone is arranged on the gas upstream side of one of the primary calcination zones, and at the same time, a secondary calcination zone is provided on the gas upstream side of the other primary calcination zone. A tertiary calcination zone is provided in the exhaust gas system primary calcination zone and the combustion gas system primary calcination zone to perform primary calcination of the raw material powder separately, and to perform primary calcination of these primary calcined raw materials. A method for calcination of cement raw material powder, characterized in that the powder is sequentially supplied to a secondary calcination zone and a tertiary calcination zone, calcined to a higher degree, and then discharged to a calcination step.