The present invention relates to a vibratory mill for the treatment of cement and fly ash.

The vibratory mill is of the type in which the mill contains grinding bodies. These grind down the material that, is to be treated and expose the treated material to powerful blows, which lead to the surface energy and chemical reactivity of the ground particles increasing.

The Swedish patent number 524 393 "Method for treatment of fly ash" and the Swedish patent number 524 154 "Process for producing blended cements with reduced carbon dioxide emissions" describe such grinding processes.

Here, the mechanical activation of Portland cement is described, together with that of various other materials such as fly ash, granulated blast furnace slag (GBFS) , and finely divided quartz sand mixed at various proportions, by the use of a vibratory mill with grinding bodies and with ah amplitude of vibration of 2-20 mm and a frequency of 800-2,000 vibrations per minute.

A second vibratory mill is a mill denoted VBM 3034, manufactured by Metso Minerals, Finland.

A major disadvantage of existing vibratory mills is that the ratio of length to diameter is small, and this leads to a short retention time for material that circulates in the mill .

The Canadian patent CA 569381 describes a vibratory mill of the present type. The mill has a grinding chamber with a wall at each end of the chamber. An inlet for material that is to be treated is located at the upper part of the chamber. An outlet is lo ^¬ cated at the opposite end of the chamber at the lower part of the chamber. A wall is present inside the chamber, at the end at which the outlet is located, provided with openings that allow the passage of material that has been sufficiently ground, which material falls down towards the outlet after it has passed the wall that is provided with openings. An im- penetrable wall is present at approximately the centre of the chamber and located in its upper part, which wall prevents material being transported directly from the input through the upper part of the mill to the wall that is provided with openings and through this to the outlet, which, in combina- tion with the above-mentioned ratio of length to diameter, would lead to a very short retention time in the mill. This impenetrable wall, however, is subject to extensive wear by the grinding bodies, and it must for this reason be exchanged approximately every third day. This leads, naturally, to interruption in service and thus costs.

The present invention solves this problem.

The present invention thus relates to a vibratory mill prin- cipally for the treatment of cement and fly ash, which mill has a grinding chamber with a wall at each of the two ends of the chamber, and which mill comprises an inlet for material that is to be treated and an outlet at the rear wall of the grinding chamber, at the lower part of the grinding chamber, and a first wall inside the chamber at the end at which the outlet is located, which wall is provided with one or several openings at its upper part, which openings allow passage of material that has been sufficiently ground from the grinding chamber to the outlet. The invention is characterised in that the inlet is located in the lower part of the grinding cham ^¬ ber at the wall that stands opposite to the location of the outlet .

The invention will be described below, partly in association with an embodiment of the invention shown in the attached drawing, where

- Figure 1 shows a cross-section of a grinding chamber.

Reference number 1 in Figure 1 denotes a grinding chamber, 2 an inlet, 3 an outlet, 4 a central pipe, 5 a wall in order to retain grinding bodies, 6 the front wall of the grinding chamber and 7 the rear wall of the grinding chamber. The reference number 8 denotes an opening in the wall denoted 5.

A vibratory mill, principally for the treatment of cement and fly ash, is shown in Figure 1 which mill has a grinding chamber 1 with a wall 6, 7 at each end of the two ends of the chamber, which mill comprises an inlet 2 for material that is to be treated and an outlet 3 at the rear wall 7 of the grinding chamber, at the lower part of the grinding chamber. A first wall 5 is located inside the chamber at the end at which the outlet 3 is located, which wall is provided with one or several openings 8 in its upper part. The openings allow passage of material that has been sufficiently ground from the grinding chamber 1 to the outlet 3.

According to the invention, the inlet 2 is located in the lower part of the grinding chamber 1 at the wall 6 that stands opposite to the location of the outlet 3. The grinding chamber 1 is cylindrical. A driven shaft, not shown in the drawing, is located in the central pipe 4 and executes eccentric motion such that the drum vibrates without rotating. The eccentric motion is achieved in a conventional manner through the shaft that runs through the pipe 4 being provided with weights outside of the grinding chamber, which weights are asymmetrically located on the shaft.

Conventional grinding bodies of metal are located inside the grinding chamber, which grinding bodies grind down the material that is to be ground. The grinding bodies may be cylin ^¬ drical with a length of 5-7 cm and with a diameter of 1-3 cm. The material that is to be ground is exposed during grinding to many powerful blows from the grinding bodies, and this increases the chemical reactivity of the material. A mass that is similar to a fluidised bed is formed through the vibration of the grinding chamber and the grinding bodies that are located inside it together with the material that is to be ground.

According to one preferred embodiment, the said opening 10 in the second wall 9 is located at a distance from the bottom of the grinding chamber that is 10-30% of the radius of the grinding chamber.

According to a second preferred embodiment, the said opening or openings 8 in the said first wall 5 has or have a total surface area that is 1-60% of the surface area of the first wall 5.

Through the material that is to be ground being fed into the lower part of the grinding chamber and fed out at the upper part of the grinding chamber on the opposite side of the grinding chamber from that of the inlet, the retention time of the material in the grinding chamber will be considerably longer than the retention time when the material is fed into the upper part of the grinding chamber, as is the case in the mill according to the said Canadian patent.

The retention time in a conventional mill of the present type can be 1.5-2 minutes. The comparable retention time in the present mill of the same dimensions can be 3.5-5 minutes.

The line 11 in Figure 1 specifies the level of 80% of maximal load, which is a suitable level of load for grinding. The load in this case then consists of not only grinding bodies but also the material that is to be ground.

Furthermore, material that has been introduced cannot pass directly to the outlet, as has been mentioned above in asso ^¬ ciation with the Canadian patent. For this reason, an impenetrable wall is not necessary to prevent such direct passage.

According to one preferred design, a second wall 9 is present inside the grinding chamber, which second wall is located at the inlet 2 and is provided with an opening 10 at its lower part for the introduction of material that is to be ground.

The said first wall 5 and the said second wall 9 are circular with a central hole for the said pipe 4.

Thus, the present invention solves the problem described in the introduction.

The tables below present results from an experiment with fly ash of Class F according to ASTM C 618, and show that the fly ash acquires a higher degree of fineness and a higher chemical activity after being ground with a vibratory mill according to the present invention.

Table 1. Particle size

Particle size Fraction

Traditional Vibratory mill

vibratory mill according to the invention micrometer 27 55

micrometer 55 67

micrometer 97 95

Table 2. Development of strength according to ASTM

Type of cement-based Compressive strength, MPa

material Curing time, days

1 3 7 28

Portland cement (PC) 10.2 26 .5 30 38.6

50% PC + 50% fly ash ** 8.5 24 .7 31. 2 42.1

50% PC + 50% fly ash * 12.3 28 .7 35. 2 48.1

* Fly ash treated in a mill according to the present invention

** Fly ash treated in a traditional mill

As the tables make clear, fly ash that has been treated in a vibratory mill according to the present invention has a significantly higher fraction of particles in the 1-10 micrometer interval and a significantly higher chemical reactivity, which leads to a significantly higher compressive strength for both a short and a long curing time. This leads to it being possible to replace a greater fraction of Portland cement by fly ash. Various embodiments have been described above. It is, how ^¬ ever, obvious that the design of the inlet can be altered as can also the proportions of the grinding chamber with respect to the ratio of its length to its diameter. The present invention, therefore, is not to be considered to be limited to the embodiments specified above but it can be varied within the scope specified by the attached patent claims .