The object of the present invention is a fine grinding apparatus as presented in the preamble of claim 1 and a method as presented in the preamble of claim 10 for stabilizing calcium oxide that is in steel slag.

The fine grinding apparatus and method according to the invention, hereinafter more concisely solution, is well suited e.g. for grinding crushed steel slag into a pulverized fraction and for stabilizing the particles containing calcium oxide in said pulverized material.

One fine grinding apparatus to be used in the solution according to the invention can be a plane grinder, which comprises an essentially planar grinding plate rotating on the horizontal plane, onto the top surface of which the material to be ground can be fed, and on the top surface of which grinding plate is a plurality of grinding discs, which rotate from the effect of the rotating motion of the grinding plate and press the material to be ground against the top surface of the grinding plate by means of the compression forces of the compression means acting on the grinding discs.

Grinder devices known per se in the art, or more concisely mills, applicable to grinding steel slag are disclosed e.g. in Finnish utility model no. FI12742 and Finnish patent no. FI128329. Tests and use of the mill have demonstrated that the crushed material is of exactly the right quality for what is desired for the mill. These mills, however, do not have a grinding plate rotating around a vertical axis on the horizontal plane according to the present invention nor grinding discs rotating on its surface, in which case the separating capability and quality of the calcium oxide in the end result required from the ground material in order for it to be used as a substitute for cement powder, either partly or wholly to replace cement powder, is not necessarily achieved.

One plane grinder provided with grinding discs is disclosed in the Finnish utility model FI13080 of the same applicant, but the use of carbon dioxide and/or vapor for stabilizing calcium oxide in the ground material is not presented in this solution.

One grinder device provided with grinding discs or rolls rotating along with a grinding plate rotating on the horizontal plane is disclosed in patent specification no. CN106861824 A. The grinder device comprises a grinding plate rotating on the horizontal plane, on the outer edge of which is a separate inclined grinder groove, in which the individual grinding discs are disposed to rotate from the force of the rotating motion of the grinding plate. Each grinding disc is on its own rotating shaft, which is in an oblique attitude, i.e. at an angle with respect to the horizontal plane of the top surface of the grinding plate. In addition, a hydraulic cylinder is fastened to the shaft of each angled grinding disc, with which cylinder the pressing of the grinding disc on the grinding plate and the angle of the shaft with respect to the horizontal plane can be adjusted. The structure of the CN specification differs substantially from the structure according to the invention, in which the rotating shaft of the grinding discs can revolve around the plane of rotation that is on the vertical plane of the grinding disc, i.e. can simultaneously revolve the plane of rotation of the grinding discs around a vertical axis of rotation. This cannot be done in the solution according to the CN specification, in which case the material to be ground cannot be pulverized to be as fine-grained as with the solution according to the invention. The CN specification also does not disclose stabilization of the calcium oxide with carbon dioxide and/or water vapor being performed in conjunction with grinding inside the frame part of the grinder device.

The plane of rotation in this context refers to the plane that is perpendicular in relation to the axis of rotation of the grinding disc and that runs in the direction of the plane of the sides of the grinding disc via the center point of the grinding disc. In practice, this type of plane passes through the center point of the width of the grinding disc. The plane of rotation can also be referred to as the rotation plane. When the rotating shaft of a grinding disc is on the horizontal plane, the plane of rotation of the grinding disc is on the vertical plane.

Also US patent specification no. US3951347 discloses a grinder device for crushing hard to pulverize material. According to the patent specification, the device comprises a customary rotatable grinding plate and a plurality of rotatable grinding discs or rolls in a groove on the surface of the plate. Each disc package has two grinding discs and they are connected to each other with a common horizontal shaft, as also in the solution according to the invention. However, there is no mention in the patent specification that the grinding discs would revolve around their plane of rotation, nor are such structures visible in the drawings of the patent specification. In this case the material to be ground cannot be pulverized to be as fine-grained as with the solution according to the invention, which occurs also from kneading the material to be ground. The CN specification also does not disclose stabilization of the calcium oxide with carbon dioxide and/or water vapor being performed in conjunction with grinding inside the frame part of the grinder device.

With the grinder solution according to prior art intended for processing steel slag there is no possibility of achieving an end result of sufficient quality in terms of its grain size and composition for use in substituting cement powder. In this case, with the grinder solutions according to prior art, ground steel slag cannot necessarily be successfully used as a substitute for cement in the manufacture of concrete, because the calciferous material, mainly calcium oxide, to be obtained as a result of grinding with them is not, according to tests, suited as such for use as a cement substitute because it is too reactive.

Of the solutions according to prior art presented above, not a single one discloses stabilization of the calcium oxide with carbon dioxide and/or water vapor being performed in conjunction with grinding inside the frame part of the grinder device. Stabilization with carbon dioxide of calcium oxide separated from steel slag, i.e. carbonization, is per se known in the art, but in the solutions known according to prior art it is an entirely different technique that is generally a long-lasting ageing process and requires its own separate premises. The carbon dioxide reacts with the calcium oxide, calcium hydroxide and calcium silicate contained in the steel slag, forming calcium carbonate, and in this case, amongst other things, the calcium oxide stabilizes by binding water with itself, i.e. the calcium oxide hydrates.

The aim of the present invention is to eliminate the aforementioned drawbacks and to provide a simple and inexpensive fine grinding apparatus and method for getting the steel slag to such a grinding fineness and quality and to such a stabilized state that the ground fine-grain material can be used, inter alia, as an additive or as a substitute for cement. Additionally, an aim of the invention is to further develop a plane grinder and to improve its efficacy and desired result. The fine grinding apparatus according to the invention is characterized by what is disclosed in the characterization part of claim 1. Correspondingly, the method according to the invention is characterized by what is disclosed in the characterization part of claim 10. Other embodiments of the invention are characterized by what is disclosed in the other claims.

For realizing the aim of the invention, the fine grinding apparatus according to the invention for stabilizing the calcium oxide in steel slag comprises an essentially closed frame part and inside it a grinding plate rotating around a vertical axis of rotation on the horizontal plane, onto the top surface of which the granular steel slag material to be ground is feedable, and on the top surface of which grinding plate is a plurality of grinding discs suspended on a vertical drive shaft, which rotate from the effect of the rotating motion of the grinding plate and compress the material to be ground against the grinding plate. Preferably the apparatus comprises one or more gas input channels leading to inside the frame part for carbonizing the ground material containing calcium oxide (CaO) inside the frame part.

The method according to the invention for stabilizing calcium oxide in steel slag comprises at least the following method phases: a) the steel slag material to be ground is ground and kneaded to become smaller in grain size between the top surface of the grinding plate and the grinding discs rotating on the top surface and rotatable on their plane of rotation; b) the dust-like, finely ground and kneaded material containing calcium oxide is allowed to rise and float into the inside space of the frame part; c) the material containing calcium oxide floating inside the frame part is exposed to a flow of carbon dioxide and/or a flow of water vapor, after which the material thus exposed is guided out of the frame part via the extraction apparatus. Additionally, for enhancing the efficiency of kneading, the plane of rotation of the grinding discs is revolved around the vertical axis of rotation during grinding.

One great advantage of the solution according to the invention is a simple, compact, operationally reliable and inexpensive fine grinding apparatus e.g. for grinding steel slag and for improving the quality of the grinding result. With the solution according to the invention the properties of ground steel slag can be improved and brought to a such a fineness and quality that the dust-like finely-ground material containing calcium can be used when suitably stabilized, inter alia, as an additive or as a substitute for cement. One great advantage, among others, is that stabilization of calciferous fine-grained material, mainly calcium oxide, can be performed in conjunction with grinding inside the frame part of the fine grinding apparatus. In this case expensive external stabilization plants are not needed.

The fine grinding apparatus according to the invention, preferably an improved plane grinder, is intended for grinding residual slag produced from a steel mill, in which case steel residue can be separated from the residual lime, and the residual lime can be ground with the solution according to the invention to be so fine-grained that it is lightweight dust-like material carried along in an air flow, the calcium oxide contained in which can be stabilized directly in the frame part of the fine grinding apparatus by means of carbon dioxide and water vapor to become of such a composition that the fine-grained stabilized lime material carbonated in this way is suited for the cement industry to partly or wholly replace cement powder. The residual slag is exposed in the fine grinding apparatus to a kneading treatment between the top surface of the grinding plate and the grinding discs, in which case heat is also produced that assists the grinding. If the ground lime is not ground to be sufficiently fine, it is conducted back into the grinding cycle again until the grain size of the residual lime is less than 100 pm, preferably between 10-50 pm, which is suitable in terms of its size for the cement industry as a substitute for cement.

In the following, the invention will be described in more detail by the aid of some preferred embodiments with reference to the simplified and diagrammatic drawings attached, wherein

Fig. 1 presents partly diagrammatically a simplified and partially sectioned side view of one fine grinding apparatus, e.g. a plane grinder, according to the invention,

Fig. 2 presents a simplified top view of the fine grinding apparatus according to Fig. 1 sectioned along the line A-A of Fig. 1, and also the grinding discs in their first revolving position,

Fig. 3 presents a simplified top view of the apparatus according to Fig. 1, sectioned along the line A-A of Fig. 1 and also removed from the frame part of the apparatus, and the grinding discs in their second revolving position, and

Fig. 4 presents a simplified, magnified, and sectioned side view of an input aperture and a conical grinder of a fine grinding apparatus according to the invention.

Figs. 1 and 2 present one simplified and partially cross- sectioned fine grinding apparatus to be used in the solution according to the invention. In Fig. 1 the fine grinding apparatus is presented as viewed from the side, and in Fig. 2 as viewed from the top and sectioned along the line A-A of Fig. 1 as well as the grinding disc units 4 in their first revolving position.

The fine grinding apparatus according to the invention comprises an enclosed, casing-type and essentially airtight frame part 1 and inside it an essentially planar grinding plate 2 rotating on the horizontal plane around its vertical axis of rotation and mounted on one or more support bearings 2a, onto the top surface 8 of which grinding plate the material 3 to be ground is feedable via the input aperture la. Preferably the material 3 to be fed into the input aperture la is crushed and prescreened steel slag, of a grain size of less than 6 m, preferably less than 4 mm, e.g. converter slag.

In Figs. 1 and 2 the grinding plate 2 rotates counterclockwise as viewed from above, i.e. in the direction of the arrow C. The material to be ground is preferably steel slag crushed into lumpy material, which contains inter alia, residual lime containing abundant calcium oxide.

On the top surface 8 of the grinding plate 2 is a plurality of grinding disc units 4 with their grinding discs 4a, which rotate essentially around their horizontal axis of rotation R2 by the effect of the rotating motion of the grinding plate 2. Each grinding disc unit 4 is fitted by means of its vertical drive shaft 6 to its own support means 5, preferably to a hollow vertical shaft, which is fixed immovably to the cover lb of the frame part 1. Each drive shaft 6 is fastened at its bottom end to its grinding disc unit 4.

In addition, the apparatus comprises a pressing mechanism 6a for each grinding disc unit 4. The pressing mechanism 6a can preferably be inside a hollow, tubular support means 5 and arranged to press the grinding discs 4a against the grinding plate 2. The pressing mechanism 6a can comprise e.g. one or more power means, such as a hydraulic cylinder, which is adapted to press the vertical drive shaft 6 in the vertical direction towards the grinding plate 2. The grinding discs 4a press the material 3 to be ground against the grinding plate 2 from the effect of the compressive force acting on the grinding discs 4a.

By revolving the drive shaft 6 around its vertical center axis, the grinding disc unit 4 simultaneously revolves around its vertical center axis, i.e. around its vertical axis of rotation 5a. Preferably the vertical center axis of the support means 5, the vertical center axis of the drive shaft 6, and the vertical axis of rotation 5a of the grinding disc unit 4 are all on the same straight vertical line. When the grinding disc unit 4 revolves around its vertical axis of rotation 5a, the plane of rotation of each grinding disc 4a simultaneously pivots. This function can also be called revolving of the plane of rotation around the axis of rotation 5a.

When the drive shaft 6 rotates around its vertical axis of rotation 5a, the friction between the top surface 8 of the grinding plate 1 and the grinding discs 4a increases and a kneading friction force is produced in the material to be ground between the grinding discs 4a and the surface 8 of the grinding plate 2, which force improves the grinding result. Preferably the compressive force of the pressing mechanism 6a acts on the grinding disc unit 4 and grinding discs 4a via the drive shaft 6.

The revolving of the drive shafts 6 around their center axes 5a, e.g. in the manner presented by the arrow B, in either rotation direction whatsoever, is realized with the revolver mechanism 17 of the solution, which preferably comprises e.g. a pressure cylinder and an articulation mechanism suited to the purpose. The revolver mechanism 17, which is presented in conjunction with only one grinding disc unit 4 and diagrammatically with a dot-and-dash line in Figs. 2 and 3, can be disposed e.g. on the cover lb of the frame part 1 or in some other suitable location on the frame part lb. The revolver mechanism 17 can be e.g. inside the frame part 1 fastened to the cover lb or side wall of the frame part.

The material 3 to be ground is fed via the input aperture la into the fine grinding apparatus, preferably to the center of the grinding plate 2. The material 3 drops from the input aperture la onto the conical grinder 7 at the center point of the grinding plate 2, where it is pre-ground, after which the material to be ground drops onto the top surface 8 of the grinding plate 1.

The material ground between the grinding plate 2 and the grinding discs 4a forms as a result of kneading a fine grained, dust-like lime containing mainly calcium oxide, which is very lightweight and which is lifted to the top part of the frame part inside the frame part 1 by means of either suction or positive pressure conducted to inside the frame part 1. The coarser material, which contains ground steel slag containing particles of steel and larger particles of lime, is conducted via rotary feeders 16 in the bottom part, e.g. in the base, of the frame part 1 to a conveyor below, from where the material is again conducted into the input aperture la of the fine grinding apparatus for regrinding. The base structure of the frame part 1 is preferably shaped in such a way that the coarser material dropping from the grinding plate 2 percolates by itself into the rotary feeders 16.

Fig. 1 further presents a gas input unit 9 coupled into connection with the fine grinding apparatus according to the invention, or belonging to the apparatus. The input unit 9, which is presented only in simplified form and diagrammatically, comprises a gas input channel 12 extending preferably from outside the closed enclosure-type frame part 1 to inside the frame part 1, which channel can be provided with one or more nozzles, and which is preferably connected to a carbon dioxide source 10 as well as to a water vapor source 11 for feeding carbon dioxide and/or water vapor to inside the frame part 1 for treating the ground fine-grained calciferous material to be suitable for use instead of cement powder, either totally or as a part of the cement material. Preferably the carbon dioxide source 10 and water vapor source 11 are pressurized. This phase can be called the accelerated carbonization of the floating calcium oxide particles of a grain size of less than 100 pm, which phase stabilizes the otherwise over-reactive calcium oxide. The apparatus can comprise one or more gas input channels 12 and they can be connected either to a carbon dioxide source 10, or to a water vapor source 11, or to both of these. The floating of the finely-ground calcium oxide particles in the upper space of the frame part 1 can be maintained by means of the gas flows of the feeder channels 12.

Fig. 1 also presents an extraction apparatus 13 for dust like lime, the outlet pipe 14 of which apparatus is situated in the top part of the frame part 1, preferably in the cover lb of the frame part 1. An extractor 15 is connected to the outlet pipe 14, which extractor is adapted to suck the dust like lime material treated with carbon dioxide and/or water vapor and floating in the top part of the frame part 1 out of the frame part, and to displace the material along the transfer channel 18 into a suitable collection container 19 for further processing. Preferably the collection container 19 is a closed container, the air space and humidity of which can be adjusted, and in which the carbon dioxide is allowed to continue acting on the fine-grained lime material for stabilizing the calcium oxide. The extraction apparatus 13, with the means connected to it, is presented only in simplified form and diagrammatically.

Fig. 3 presents a simplified top view of a fine grinding apparatus according to Fig. 1, sectioned along the line A-A of Fig. 1 and also removed from the frame part of the apparatus, and the grinding discs 4a in another revolving position.

In the situation presented by Fig. 3 each grinding disc unit 4 is revolved around its vertical axis of rotation 5a with its own drive shaft 6 to the extent of the desired angle of rotation D. In this position the axis of rotation R2 of the grinding discs 4a of the grinding disc unit 4 is at an angle D with respect to the line R1 in the direction of the radius of the grinding plate 2 at the point of the grinding disc unit 4. In such a case the aforementioned line R1 in the direction of the radius crosses the vertical axis of rotation 5a of each grinding disc unit 4, i.e. center axis, preferably at a right angle to it.

Correspondingly, the plane of rotation RP of each grinding disc 4a, e.g. the plane in the direction of the side of the disc, is at an angle of 90°-D with respect to the line R1 in the direction of the radius of the grinding plate 2 at the point of the axis of rotation 5a of the grinding disc unit 4. The angle D can be any angle whatsoever and can be negative or positive with respect to the line R1 in the direction of the radius of the grinding plate 2. Furthermore, the angle D can be equal or also unequal always simultaneously for each grinding disc unit 4, in which case the grinding result of each grinding disc unit 4 is different.

A dual disc grinding disc unit 4 is fastened to its drive shaft 6 preferably at the center of the horizontal axis 4b. There can also be just one grinding disc 4a in a grinding disc unit, in which case the fastening of the disc to the drive shaft 6 can be done from just one side of the horizontal shaft 4b or from both sides of the disc. Preferably in this case the center axis of the drive shaft 6 is on the same line, as viewed from above, as the center point of the disc 4a, in which case the vertical axis of rotation 5a of the disc and the axis of rotation, i.e. center axis, of the drive shaft 6 coincide. The center axis of the drive shaft 6 in a single-disc grinding disc 4 can also be on a different vertical line to the axis of rotation 5a of the grinding disc 4a.

There can be more than two grinding discs 4a attached to the same horizontal shaft 4b on which the compressive force of the pressing mechanism 6a acts via the drive shaft 6. Also a dual-shaft or multishaft bogie structure is possible .

Fig. 4 presents a simplified, magnified and partially sectioned side view of an input aperture la and a conical grinder 7 of a fine grinding apparatus according to the invention. Preferably the conical grinder 7 comprises a cone 7a narrowing upwards, rotating along with the grinding plate 2, and a non-rotating counter-cone 7b opening downwards, between which cones is a narrow, inclined ring-shaped gap 7c, through which the material to be ground is able to drop onto the top surface 8 of the grinding plate 1. Preferably the downward opening countercone 7b is on top of the upward narrowing cone 7a in such a way that the top part of the cone 7a is above the ring-shaped gap 7c.

The gap 7c is dimensioned to be so narrow that most of the material flow fed into the input aperture la blocks the gap 7c during operation of the apparatus in such a way that the gas, such as carbon dioxide and/or water vapor, fed into the frame part 1 cannot get out via the gap 7c. In this case e.g. the pressure produced by the material in the gap 7c together with air pressure is greater than the pressure inside the frame part 1. The gap 7c closed in this way by the material to be ground prevents the gas and vapor fed into the frame part 1 from exiting via the gap 7c. Thus the inside space of the frame part 1 is closed and essentially airtight during operation of the apparatus, although the casing-type frame part 1 is not structurally fully airtight. This enables the success of the accelerated carbonization, which further improves the grain size of the lime particles containing calcium oxide to less than 100 pm, in which case the lime particles have a lot of surface area on which the carbonization can act.

Typically the steel slag to be treated with the solution according to the invention can contain in percentages by mass mainly, inter alia, calcium oxide (CaO) 35-60 m-%, iron (Fe) 10-30 m-%, ferrous oxide (FeO) 10-35 m-%, silicon dioxide (SiCh) 10-20 m-%, magnesium oxide (MgO) 1-15 m-% and aluminium oxide (AI2O3) 1-5 m-%. The applicant has conducted numerous tests for grinding and further treating steel slag so that ground steel slag could be used to partly or wholly replace cement powder, as well as for both the purposes of the construction industry and for other less demanding purposes. The tests analyzed, inter alia, the drying time and strength of the types of concrete products in which cement powder was replaced either partly or wholly with the ground material produced with the solution according to the invention from steel slag.

For example, one such test, which exceeded the limit values needed for the requirements of construction concrete, related to steel slag with a composition of components in percentages by mass as follows: calcium oxide (CaO) 37.9; iron (Fe) 14.0; metallic iron (Fe _m ) 13.3; magnesium oxide (MgO) 2.6; manganese oxide (MnO) 1.4; phosphorus pentoxide (P2O5) 0.4; silicon oxide (S1O2) 14.3; titanium oxide (T1O2)

0.9; divanadine pentoxide (V2O5) 1.1; aluminium oxide (AI2O3) 2.6; sulphur (S) 0.2.

In the aforementioned test the steel slag was ground with the fine grinding apparatus according to the invention and the material to be ground was kneaded, with grinding discs 4a revolving around their plane of rotation, against the top surface 8 of the grinding plate 2 to become such fine ^¬ grained dust-like material that the particles of a grain size below 100 pm, suitably below 75 pm and preferably below 50 pm containing calcium oxide that were separated from the coarser grains could rise to float into the top part of the closed frame part 1 of the apparatus. In addition to their lightness, the carbon dioxide and/or water vapor fed to inside the frame part, and producing positive pressure in it, facilitated the rising, as also the output aperture in the top part of the frame part, which output aperture acted to displace the ground and dust-like material from the top part of the frame part 1 into the collection container.

The apparatus was adjusted in such a way that the ground dust-like material containing calcium oxide was exposed to a flow of carbon dioxide and a flow of water vapor in the closed frame part 1 for an average time of approx. 10-15 minutes, during which time the material particles were floating in the frame part 1. After this the carbonated material particles were conducted into a collection container 19, where the carbonization process was allowed to continue until the particles containing calcium oxide were stabilized, from the effect of the carbon dioxide and of the water sucked into them, to become fit for use as substitutes for cement powder.

The method according to the invention comprises one or more of the following phases:

- steel slag 3 preferably pretreated to a grain size of less than 100 pm is fed via the input aperture la into the fine grinding apparatus

- the gap 7c between the input aperture la and the frame part 1 of the fine grinding apparatus is closed to be essentially airtight with the flow of pretreated steel slag, which is guided via the gap 7c into the conical grinder 7 of the apparatus;

- the pretreated steel slag 3 is preground in the bottom part of the input aperture la and is dropped inside the frame part 1 of the fine grinding apparatus, the frame part being closed and essentially airtight, to a rotating grinding plate 2 for further grinding of the preground steel slag; in conjunction with the further grinding, the steel slag material to be ground is ground and kneaded to become smaller in grain size between the top surface 8 of the grinding plate 2 and the grinding discs 4a rotating on the top surface; the efficiency of the kneading is enhanced by revolving the plane of rotation of the grinding discs 4a around the vertical axis of rotation of the grinding discs 4a; by means of grinding and kneading the material to be ground fed into the frame part is transformed into dust-like material containing calcium oxide, the grain size of which is less than 100 pm, suitably less than 75 pm, and preferably less than 50 pm; the dust-like, ground and kneaded material containing calcium oxide is allowed to rise and float into the inside space of the frame part 1; the floating is made more effective by at least one flow of carbon dioxide and/or flow of water vapor directed to inside the frame part 1; with which flow the particles containing calcium oxide floating inside the frame part 1 are simultaneously stabilized; the particles containing calcium oxide floating inside the frame part 1 are exposed to a flow of carbon dioxide and/or a flow of water vapor for an average approx. 10-15 minutes, after which the particles thus exposed are guided out of the frame part 1 via the extraction apparatus 13 and stored in an enclosed collection container 19 to await further processing; stabilization of the particles containing calcium oxide is allowed to continue in the enclosed collection container 19. Thus, according to the method the frame part 1 is closed in the conical grinder 7, to become airtight or gastight, with the steel slag material to be ground. Since the combined total area of the extremely fine-ground, dust-like material containing calcium oxide on which the carbon dioxide and water vapor can react is large, the carbonization to be performed by means of carbon dioxide and water vapor acts quickly and efficiently. In this case there is no need to wait for stabilization of the calcium oxide to become adequate as a substitute for cement powder to occur over a long time normally by ageing, but instead it can be used immediately after grinding and also is even better after brief storage in an enclosed collection container 19, where the conditions, including air contact, can be kept favorable for progressing the stabilization process to an end result of the highest possible quality.

The strength of the carbon dioxide flow and water vapor flow inside the frame part 1 is adjusted with respect to the suction of the extraction apparatus in such a way that the ground and kneaded particles containing calcium oxide floating in the top part of the frame part 1 float in the frame part for the average desired exposure time, e.g. the recently aforementioned 10-15 minutes, but depending on the conditions the time can also be shorter, e.g. 2-10 minutes, or longer, e.g. 15-30 minutes. Thus the total range of the exposure time can be 2-30 minutes. The ratio of carbon dioxide to water vapor as percentages by volume to be fed into the frame part can vary according to, inter alia, the desired exposure time and the properties of the steel slag to be ground. The carbon dioxide to be fed in is e.g. approx. 2-15%, suitably 4-10% and preferably 5-7% of the total volume of the carbon dioxide and water vapor.

Since there are many variables in the process the aforementioned exposure times, adjustments and conditions can be changed, e.g. for a dissimilar steel slag. For this reason it is advantageous to test, before the start of a large-scale production process, each steel slag on site separately, at least in terms of with which carbon dioxide and water vapor mixture ratio, and with which exposure time, a test result adequately suitable for purpose is obtained. Thus, the final correct carbon dioxide and water vapor mixture ratio, the pressure balance prevailing inside the frame part, and the carbonization time are identified only in conjunction with a test run. In such a case attention is paid to, inter alia, the strength of the prepared test pieces and the hardening time.

It is obvious to the person skilled in the art that the invention is not limited solely to the examples described above, but that it may be varied within the scope of the claims presented below. Thus, for example, the fine grinding apparatus can differ in its structural solution to what is presented above.

It is also obvious to the person skilled in the art that the gas input channels can be situated elsewhere than on the cover of the apparatus. Preferably the carbon dioxide feeder channels can also be situated in the bottom part, or even in the base, of the frame part, in which case the gas flow coming via them assists in raising the fine-grained material containing calcium oxide towards the top part of the frame part. Correspondingly, the feeder channels for water vapor can preferably also be on the flank of the frame part, in which case they are nearer the floating lime particles containing calcium oxide. It is also obvious to the person skilled in the art that the suspension structure of the grinding disc units can be different to what is described above. In this case e.g. the pressing of the grinding discs against the surface of the grinding plate could have been realized with other solutions than with a power cylinder concurrent with the direction of the vertical axis of the support means. The grinding discs can be pressed against the surface of the grinding plate by some suitable power means e.g. from the side of the grinding disc unit.

It is also obvious to the person skilled in the art that the suspension of a grinding disc unit can be realized otherwise than with one drive shaft rotatable around its center axis. What is essential is that the structure of the fine grinding apparatus comprises means for turning the plane of rotation of the grinding discs to different angles with respect to the line between the position of the grinding discs and the center point of the grinding plate.