The prior art In present practice, one of necessary steps, which must be carried out to produce the fine crushed material, possibly fine powder, from originally differently grained materials or their mixtures, is the procedure of milling. During this process, the procedures are used, where the milling process is carried out in well-known facilities, advantageously in roller mills in connection with technologically related sorting mechanisms, as are fixed screens, air separators and so on and with mechanisms for transfer of milled material among separate milling, possible sorting stages.

A disadvantage of the present procedures is, that during concurrent milling of several components with different value of grindability, for example in vertical roller mills, the components with higher grindability are over-milled and the components with lower grindability are under-milled. Than, it is necessary to repeat the milling of significant part of under-milled components. Such milling processes necessarily cause higher consumption of energies, what results in lower specific output of the milling system.

The aim of the invention Above mentioned disadvantages of recent procedures are eliminated to a large extent by a subject of the invention-method for milling of material with different grain size and/or with different range of grindability from 30 up to 90 according to Hargrowe (Hg) in more, mutual consequential milling steps, with using of middle-pressure roll presses.

Essence of the invention is, that milled material is gradually in the first milling step formed, in the second milling step it is pre-milled and calibrated as to height and in the third milling step it is after-milled, while the thrust of milling rolls is in separate milling steps gradually increasing and dimensions of rest slots are in separate milling steps gradually decreasing. According to this invention, the value of thrust of milling rolls is variable at least during one milling step.

Further essence of the invention is, that into the first step, at least two materials with the worst grindability are lead, while advantageously the grain size of at least of 50 % of volume of milled material is lower than its maximal grain size.

Further essence of the invention is, that at least between the first and the second milling step, further material is added to the milled material and that type and/or grindability of added material is different from grindability of admission milled material, advantageously material with lower grindability, than is that of material in previous step.

Finally, the essence of the invention is, that between the two milling steps the material is added, which was taken from the following step of milling.

Since the procedure is according to the invention divided into three separate, mutually functionally defined steps, it is possible to adjust the operating conditions in the first one, so that the material with bad grindability, possibly mixture of such materials, is processed preferentially, efficiently from the functional and economical point of view. According to the invention, it is possible advantageously to dose additionally materials with better grindability to further milling step, possibly materials with lower grain size. According to the invention, it is also possible advantageously to

return a part of material, pre-milled in some milling step, for further processing into some of the previous steps and so to reach economically efficient and required re- milling or after-milling of desirable part of the milled material.

Summaroz of Figs on the drawings Possible variants of methods for milling according to the invention are illustrated in block diagram on enclosed drawing, where on Fig. l the algorithm for milling of one type of material is given, on Fig. 2 the algorithm for milling of materials with different grain size and grindability, which are gradually added and Fig. 3 and 4 variant algorithms with return cycle.

Examples of embodiment of the invention Examples of realisation of method for milling according to the invention are described in following examples and are also documented in block diagram on related Figtures.

Example 1 Method of milling according to the invention is in this example used for milling of material with practically the same grindability, for example cement clinker, after its exit from rotary kiln followed by inserted cooler. It is dealing with a material with middle grindability (Hg 80), but with significant different grain size, usually in the range from 0,1 to 45 mm.

The cement clinker is according to Fig. 1 collected in the reservoir 4, from there it is through the piping 40 lead to the milling bodies of the first used milling stage 1, for example in form of a roll middle-pressure press of standard construction, with the rest

slot adjusted to 27 mm. There, supplied cement clinker is pre-formed, especially larger admission grains are being broken.

In the first milling stage 1, the pre-formed cement clinker is transferred by the means of the first transport element 10 to the second milling stage 2, for example again to a middle-pressure press, its not illustrated operating slot is adjusted on value of 15 mm, e. g. lower than that in the previous step and on the contrary, thrust of the milling roll is higher. In this phase of milling, the pre-formed cement clinker, which already doesn't contain grains of significantly different size, is pre-milled and simultaneously calibrated as to height, so that a layer of practically homogenous material, as to grain size and compactness, is produced. The material is than transferred by the means of the second transport element 20 to the third milling stage 3. The operating slot of this stage is adjusted on the value of 10 mm, thrust of the milling roll is on the contrary even higher than the thrust in the previous milling stage. Here, the material is after-milled to the final requested size and the result product is through the outlet 30 lead towards the further technological step.

In case, that in final phase, e. g. on outlet of the third milling stage 3, the milled cement clinker is not sufficiently fine in all its volume and if its contains also certain fraction of undesirable higher grain size, a sorting system 6 is inserted according to Fig.

4 after the outlet 30 from the third milling stage 3, the system contains a separator 60, consisting for example of suitable chosen vibrating screen, air separator and so on.

Sufficiently finely milled cement clinker passes through the separator 60 and is lead through the outlet 61 towards further technological step. Separated undesirable coarser fraction is through the discharge 62 and connected return transport element 620, for example a transporter, lead to the additional reservoir 51 and from there through the piping 510 and the first transport element 10 back to the second milling stage 2.

Example 2 Procedure according to the invention is used for preparation of mixed cement, which consists of 70% till 94% of mixture of cement clinker with average grindability of 75 Hg and with predominantly higher share of grains with larger dimensions, of 6%

till 35% of slag with average grindability of 35 Hg, which has larger share of grains with small dimensions and of 5% of soft gypsum with average grindability of 95 Hg.

Example of procedure of milling of above mentioned components is illustrated in variants on Fig. 2 and is as follows. The first milled component is the cement clinker, dosed from the reservoir 4 through the piping 40 to the first milling stage 1. The slag is dosed into the second milling stage 2 from the additional reservoir 51, e. g. the mixture of slag and cement clinker is entering it. The gypsum with the best grindability is dosed from the additional reservoir 52 through the piping 520 to the third milling stage 3 together with pre-milled mixture of slag and cement clinker.

Example 3 Procedure according to this example is a variant of preparation of mixed cement in composition according to previous example.

The first two materials, e. g. cement clinker and slag, are stored in the first reservoir 41 and the second reservoir 42, as is illustrated on Fig. 3, from there, they are through the pipings 410 and 420 dosed into the common reservoir 4 and consequently <BR> <BR> <BR> through the piping 40 lead to the first milling stage 1 with relatively large operating slot and with low thrust of not illustrated milling roll. The remaining component of prepared mixed cement, e. g. soft gypsum, is through the additional reservoir 51 and to it connected piping 510 dosed into both first admission materials, which were pre-formed before in the previous-the first-milling stage 1, namely after their transport by the means of the first transport element 10 to the second milling stage 2.

In the second milling stage 2, all three components of prepared mixed cement are already together pre-milled, calibrated as to height and consequently by the second transport element 20 they are lead to the final third milling stage 3 and from there, after after-milling, they are through the outlet 30 lead towards further technological step.

As is illustrated on Fig. 3, considering the fact that it is dealing with milling of three components with significantly different mechanical properties, the additional sorting system 21 is advantageously inserted into the milling line yet before the final phase of after-milling. Its separator 210 separates undesirable, so far insufficiently

milled material, which exits the second milling stage 2 and it also transfers sufficiently fine fractions through the transport element 211 to the third milling stage 3. Possible coarser under-milled fraction is, on the contrary, by the return transport element 220 returned to the second milling stage 2, namely through the additional reservoir 51 or directly by the means of the first transport element 10, as is illustrated on Fig. 3 in broken line.

Example 4 This example describes other variant of method for preparation, in this case of blast furnace cement, which consists of mixture of 65% to 80% of blast furnace slag, 20% to 35% of cement clinker and 5% of gypsum, namely according to the procedure illustrated on Fig. 4.

The first milled component is a slag dosed from the reservoir 4 through the piping 40 into the first milling stage. Cement clinker and gypsum with lower grindability is dosed from the additional reservoir 51 to the second milling stage 2 together with slag, exiting the first milling stage 1. Mixture of all three components, necessary for preparation of the blast furnace cement, is than dosed by the means of the second transport element 20 to the third milling stage 3 for after-milling and it exits it through the outlet 30 towards further technological step. Possible application of the sorting system 6 was described in connection with example 1.

Practical applicability in industry Method for milling of materials with different grain size and large range of grindability according to the invention can be advantageously used especially for milling of recycled matters, cement clinker with admixture of slag, puzzolan, ash and so on, possibly for milling of two or more types of minerals with recycling of fractions and during fining of mixtures.