This invention refers to a method for increasing the ef¬ ficiency of grinding finely divided material, wherein primary material is ground and dried in a counter jet mill and the formed product is classified into a fine fraction and a coarse fraction, and the coarse fraction obtained is returned to the counter jet mill for regrinding.

The counter jet grinding technique is suitable in grinding a lot of different materials. With the above disclosed method very high quality end products will be obtained to moderate costs, but for some materials the coarse fraction part will increase rapidly and a lot of energy will be consumed to recirculate this increased part. The specific weight of the coarse fraction returning to the counter jet mill is also often remarkably lower than the specific weight of the primary feed material, and for this reason the grinding efficiency of the counter jet mill will dec¬ rease remarkably due to the changes in the specific weight of the materials to be ground. The specific weight changes of the material to be ground will thus increase the grin¬ ding costs.

The purpose of the present invention is to eliminate these problems and to increase the efficiency of the grinding process. This is achieved by the method, which is charac¬ terized in that the coarse fraction is led via a pressure roller crusher to the feed funnel of the counter jet mill.

By this method the efficiency of the grinding process can be increased remarkably, because new micro cracks and cracks will be formed into the coarse fraction particles when they pass through the pressure roller crusher, which cracks will facilitate further grinding of these particles in the counter jet mill during the next grinding step. Due

to the method according to the invention the weight ratio between the the coarse fraction and the fine fraction will decrease remarkably, i.e. the efficiency of grinding pro¬ cess will increase.

At the same time the feeding of material into the counter jet mill will be stabilized, because the material mat lea¬ ving the pressure roller crusher is in the form of pres¬ sed, easily breaking material clods. These material clods will at least be broken by the influence of a screw feeder and a rotator of the feeding device of the counter jet mill.

Materials suitable for the counter jet ginding technique can be chared into three different groups due to their physical properties. The particles of materials belonging to group one are brittle and hard, and possibly hardened by water such as water granulated blast-furnace slag. The particles of materials belonging to group two are undama¬ ged and undisintegrated, such as natural minerals, quartz and zirconium sand. Materials belonging to group three ha¬ ve their bulk density decreased dramatically during the grinding step. Such materials are for instance talc and syntetic quartz.

The further characteristics of the invention will appear from the enclosed claims 1 to 5.

Below the invention will be disclosed in more detail refe- ring to the enclosed drawing, wherein

fig. 1 shows a flow sheet of a base device for carrying out the method according to this invention,

fig. 2 shows a flow sheet of an alternative device and

fig. 3 shows a flow sheet of a prefered device for car-

rying out the method according to the invention.

According to this invention finely divided primary materi¬ al is fed from a silo 1 into a feed funnel 2 of a counter jet mill, and therefrom it is fed by means of a double valve feeder 3 into a pressurized equalizing tank 4 and therefrom by the influence of a srew feeder 5 further-as an uniform material flow into a fluidizing tank 6, wherein the material is fluidized by means of a working gas flow. From the fluidizing tank 6 a working gas material mixture is accelerated by means of the super pressure maintained in the fluidizing tank 6 through a bisecting device 7 and two substantially oppositely directed, long accelerating nozz¬ les 8 into a grinding chamber 9, wherein the material par¬ ticles rushing from the oppositely directed accelerating nozzles 8 collide against each other at a very high speed and are ground due to the strong collision influence. The ground material-working gas mixture rushes from the grin¬ ding chamber 9 further to a pneumatic centrifugal classi¬ fier 10 wherein the solid particles of the material wor¬ king gas mixture are classified into a fine fraction and a coarse fraction. The fine fraction i.e. the end product is removed from the classifier 10 as a continuous material working gas flow through the outlet pipe 11 for the fine fraction. The coarse fraction is accumulated into a pocket 12 outside the pheripheral face of the classifier 10, whe- refrom it is removed batchwise into a feed pipe 14 of a pressure roller crusher 13 beneath the pocket 12. When the solid particles of the coarse fraction pass through the pressure roller crusher 13 as a thick material mat, they will be at least partly crushed and at the same time new micro cracks and cracks will be formed in the particles. The pressed material clods formed in the pressure roller crusher 13 are returned or dropped into the feed funnel 2 of the counter jet mill in order to be regrinded. The micro cracks and cracks formed in the solid particles will remar¬ kably facilitate the further grinding of the material

particles in the counter jet mill 9. Further the feeding of the material into the counter jet mill 9 will be more effective, because the specific weight of the material clods coming from the pressure roller crusher 13 is often many times higher than the specific weight of untreated coarse fraction.

Primary materials belonging to group one, having brittle and hard particles, can preferably be ground in the device shown in fig.l _^

If the material to be ground belongs to group two having substantially undamaged and undisintegrated particles it will be prefered that primary material is led through a pressure roller crusher 15 of their own, as shown in fig. 2, to the feed funnel 2 of the counter jet mill 9, whereby these particles will be partly crushed and at the same ti¬ me new cracks and micro cracks facilitating grinding will be formed in the particles.

Because the particle size of the primary material oftenly is many times larger than the particle size of the coarse fraction which is to be returned to the counter jet mill 9, a common pressure roller crusher cannot be used at least not without any auxiliary devices.

In order to increase the efficiency of the feeding of the coarse fraction pressure roller crusher 13, especially when the primary material belongs to group three, the specific weight of the coarse fraction can preferably be mechani¬ cally increased by pressing away the air which is contained in the spaces between the solid particles of the coarse fraction during the feeding of the coarse fraction. This could be achieved for instance by means of a pressure screw 16 and a pair of rollers 17 cooperating with this screw or alternatively by means of one or several pressure pistons.

When the specific weight of the coarse fraction has been increased in this way its physical properties will be more similar to those of the primary material, whereby such a pressurized coarse fraction and primary material can be treated together without problems i a common pressure roller crusher, as disclosed in figure 3.

When studying the influence of pressure roller crushing on the capasity of a counter jet mill, especially a PU VA FP jet mill, three different tests were carried out. During all tests the energy consumption of the jet mill was kept constant. In all tests water granulated blast-furnace slag was used as primary material and the fineness of the end products of these tests was held constant. In test number one no pressure roller crusher was used at all, but the primary material was fed directly into the feed funnel of a jet mill acting in a closed loop with a FPC-classifier. In test number two the primary material was predisintegra- ted with a pressure roller crusher before feeding into the jet mill used in the previous test. In test number three primary material was fed directly into the feed funnel of the jet mill used in the previous tests but the feedback of the FPC-classifier i.e. the coarse fraction was fed via a pressure roller crusher back to the feed funnel in ac¬ cordance with figure 1 of this application. The results of these test are disclosed in the table below.

Test No. Circulation load/% Capacity ratio

1 600 1

2 400 1,3

3 250 1,7

Circulation load means the ratio between the feedback or Coarse fraction and the end product in per cent. When the capasity of test number 1 was given ratio 1 the capasities of the other tests received the ratios disclosed in the

table. As will appear from the table, the capasity of the jet mill increases remarkably when a pressure roller crus¬ her is connected to it in accordance with the present in¬ vention.