A method of this kind is known from US-A-4,634,583. The disadvantage of this known method is its low rate of efficiency. This is due to the fact that the particle size of the calcined raw meal which is introduce into the exhaust gases is relatively large, causing its surface area to be reduced correspondingly. Therefore, this known method requires much more raw meal than is theoretically required, leading to an unnecessary waste of heat since the extracted raw meal is cooled from a temperature range between 850 and 950° to a temperature range of 220-250°C.

It is the object of the present invention to provide a method and apparatus by means of which the mentioned disadvantage is reduced.

According to the invention this object is achieved by a method of the kind mentioned in the introduction and being characterized in that the extracted raw meal is slaked and ground prior to being brought into contact with the exhaust gases.

An improvement of the thermal economy is thereby obtained by reducing the quantity of raw meal required for the desulphurisation process. This is ascribable to the fact that by subjecting the raw meal to slaking and grinding, the fineness of the raw meal will be improved, thereby enhancing the efficiency of the desulphurisation process.

Tests conducted have indicated that a reduction of the particle size of the reactant will enhance the rate of efficiency of the desulphurisation process.

It is preferred that a substantial amount of the calcium oxide (CaO) contained in the extracted raw meal is converted into calcium hydroxide (Ca (OH) 2), due to the greater capability of the latter to bond sulphur dioxide (S02). According to the invention it is therefore preferred that the extracted, calcined raw meal is subjected to minimum 80 per cent slaking, and preferably full-scale slaking. The percentage of slaking is determined to be equal to (1-free CaO after slaking/free CaO before slaking) x 100, where free CaO is reported on an ignition-loss free basis as is conventional.

Since a reduction in the particle size of the reactant will raise the efficiency of the desulphurisation process, it is further preferred that the extracted, calcined raw meal is ground to an average particle size of less than 5 um, preferably of less than 3 pm.

The invention also includes apparatus for desulphurisation of exhaust gases from a cement making plant, having means for slaking and for grinding the extracted raw meal prior to bringing it into contact with the exhaust gases.

It is particularly in cases where sulphides, such as pyrite (FeS2), are incorporated in the raw materials that the exhaust gases from the cement making plant contains Su2.

SO2 is formed by partial oxidation for example of FeS2, at around 550°C. Furthermore, the highest efficiency of desulphurisation when using extracted raw meal is achieved at high temperatures. It is therefore preferred that the extracted, calcined raw meal is introduced into the exhaust gas stream at a location immediately after, viewed in the direction of flow of the exhaust gases, SO2 has been formed by partial combustion of the sulphide constituents in the raw materials. In a traditional cement making plant comprising a preheater with five cyclone stages, the formation of S02 from sulphide- containing raw materials will typically take place in the inlet duct for the third cyclone stage and in that cyclone stage itself. In such a plant it is preferred that the extracted raw meal is introduced into the exhaust gases in the exhaust gas discharge duct from the third cyclone stage or at a subsequent location. In actual practice, SO2 measuring equipment will be used to establish exactly where S02 is formed, with the decision concerning the location of the point of introduction being made accordingly.

It is preferred that the extracted, calcined raw meal in a flash pipe is introduced into a stream of mixed air and water causing it to be slaked, that the slaked raw meal is separated in a separating mechanism, and is subsequently conveyed to and ground in a mill.

It is further preferred that the airstream from the flash pipe is used to convey the slaked and ground, calcined raw meal from the mill to the point of introduction.

The overriding objective of the present invention is to remove the SO2originating from the raw materials because the S02 formed from the fuels in the hotter parts of the kiln system is effectively bonded to CaO and converted into CaSO4which is a solid which will be a constituent component of the finished clinker.

In the following the invention will be explained in further detail with reference to the accompanying diagrammatical drawings, in which:- Figure1 shows a traditional cement making plant where the method according to the invention is used; and Figure. 2 shows an especially preferred embodiment of the invention.

In Figure 1 is seen a cement making plant which comprises a cyclone preheater 1 with five cyclones 1a to 1e, a calciner 4, a rotary kiln 3 and a clinker cooler 5. The

plant is run in accordance with traditional operating principles which involves raw material being introduced at an inlet 9 in the inlet duct to the first cyclone stage 1 a of the cyclone preheater and heated, calcined and burned into clinker by first being directed through the preheater 1, the calciner 4 and subsequently through the kiln 3 in counterflow with hot exhaust gases which are formed at, respectively, a burner 11 in the rotary kiln 3 and burners 13 in the calciner 4. The burned clinker is subsequently cooled in the clinker cooler 5.

In accordance with the method of the invention, some of the hot, calcined raw meal is extracted from the fifth stage separating cyclone 1 e of the preheater 1 by means of a dosing mechanism 15 which is designed for extracting a controlled stream of material. The stream of material thus extracted is directed to a slaking and grinding apparatus 17 of any suitable kind. For example, this could be a ball mill, a vertical mill, a ring roller mill or a roller press to which water (H2O) is added, possibly in vaporized form. In an especially preferred embodiment of the invention the apparatus 17, as shown in Fig. 2, comprises a riser pipe or flash pipe 17a with a subsequent separating cyclone 17b and a mill 17c. In this embodiment the extracted, calcined raw meal is introduced into an ascending stream of mixed air and water, thereby causing the raw meal to be slaked. The air is supplied by means of a fan 16 and the water is supplied via an opening 20. The slaked, but still substantially dry, calcined raw meal is subsequently separated in the separating cyclone 17b from where it is conveyed, if convenient via an intermediate bin (not shown) to the mill 17c wherein it is ground to the desired particle size.

From the apparatus 17 the now slaked and ground, calcined raw meal is conveyed, possibly via a scoop device, to an entrance opening 19 which is provided in the outlet duct 21 of the third cyclone stage 1 c. In the embodiment shown in Fig. 2, the raw meal is conveyed pneumatically via a duct 18 by means of the airstream from the flash pipe.

After being introduced into the exhaust gases, calcium oxide (CaO) and calcium hydroxide (Ca (OH) 2) will react with the SO2 present in the exhaust gases and will be converted into calcium sulphite CaS03 and/or calcium sulphate CaS04. These solids may be a constituent part of the clinker in similarity with other constituent components of the raw materials.

The slaked and ground, calcined raw meal may either be conveyed direct to the entrance opening 19 or it may be conveyed to an intermediate bin, not shown, wherefrom it may subsequently be led by means of a dosing means to the entrance opening. The actual introduction of the material may take place using a special lance which is capable of dispersing the slaked and ground, calcined raw meal across the entire cross-sectional area of the duct 21 in order to optimize the efficiency of the desulphurisation process.