Fine, very fine, and microfine comminution of brittle mate¬ rials or, respectively, exhibit brittle behavior to obtain products having maximum particle sizes of between 100 and 300 mm (fine comminution) or between 10 and 50 mm (very fine comminution) or between 1 and 5 mm (microfine comminution) usually is accomplished by means of ball mills, vibration mills, planetary mills, agitator mills, and high-pressure roller mills. Until the beginning of the century also stamp mills were used for fine comminution of mineral raw materi¬ als. Ball mills, vibration mills, and planetary mills are used both for dry and wet milling, while agitator mills and stamp mills are used almost exclusively for wet milling, and high-pressure roller mills for grinding dry and wet materi¬ al. As a rule, a mill is combined" With a classifier to pro¬ vide for grinding in a circuit, with the ground product be¬ ing fed to the classifier where it is divided into fine ma¬ terial and coarse material, the latter being recirculated to the mill for renewed grinding. Ball mills, vibration mills, planetary mills, agitator mills, and especially the known stamp mills have a low degree of efficiency so that the spe¬ cific energy consumption (the energy requirement based on the mill product obtained by comminution) is very high. En¬ ergy consumption for fine comminution (100 to 300 mm) ranges from 10 to 40 k h/t, it is from 50 to 150 kWh/t for very fine comminution (10 to 50 mm) and above 500 kWh/t for microfine comminution (1 to 5 mm) . In addition, the mechani¬ cal expenditure is high. A substance exhibits brittle beha¬ vior if, prior to beginning to crack, a solid particle of it is deformed largely elastically.

It is true, high-pressure roller mills (US patent 4,357,287) operating with a pressure in the roller nip of at least

50 MPa require little energy, but the yield in terms of very

fine product and especially microfine product at pressures which still can be mastered on an industrial scale is rela¬ tively low. This results in a high ratio of coarse material recirculated which in turn requires greater structural units as far as mills, classifiers, and conveyor means are concer¬ ned, all involving high capital investment. In the case of high-pressure roller mills there is an additional difficulty in the case of microfine grinding in that the material to be ground is poorly drawn into the nip between rollers. The peripheral speed of the rollers, therefore, must be reduced and, possibly, the introduction of material into the nip be supported by feed worms in a feed funnel.

It is an advantage of ball mills and stamp mills that the feeding of the material to be ground into the grinding cham¬ ber causes no problems and that they are suitable for wet grinding. They are only little affected by foreign matter and, in principle, sturdier than many other comminuting ma¬ chines. Parts subject to wear are of simple shape and can be exchanged with ease. Stamp mills have a disadvantage in that the throughput per stamp unit is low.

The one-time stressing of material to be ground in a ram press (US patent 4,357,287) also requires a high circulation ratio and considerable expenditure for machinery. Following compression in the ram press, the compacted material can be loosened first by mechanical action exerted by corresponding tools which rearrange the material if it is to be stressed once again afterwards by compression before being conveyed out of the grinding chamber for disagglomeration of the re¬ sulting agglomerates (briquettes) .

It is the object of the invention to provide a method and an apparatus by which fine, very fine, and microfine comminution of brittle materials can be performed with oper¬ ational safety by dry or wet procedures at relatively low energy and mechanical expenditure.

This object is met, in accordance with the invention, by a method of fine, very fine, and microfine comminution of brittle materials, wherein material to be ground in the form of a bed of material, is compressed repeatedly by pistons between two hard, non-yielding surfaces in a grinding cham¬ ber and stressed or compressed, respectively, at a pressure of at least 50 MPa, and agglomerates formed are disagglomerated in a succeeding stage subsequent to the last stressing, and any coarse particles separated by classifi¬ cation, if desired, are subjected to further stressing. For very fine comminution, the material to be ground should be stressed at a pressure of at least 150 MPa. For microfine comminution of the material to be ground a pressure of at least 250 MPa proved to be successful. The pressure in this case is set at such a high value above 50 MPa that it will be especially favorable for the specific material to be ground, the specific average particle size of the material to be ground which is introduced between the surfaces, and the specific degree of communinution by stressing. The num¬ ber of stressing operations performed on the material to be ground in the grinding chamber is selected in the same way.

The successive stressings of the material to be ground by the pistons preferably take place offset by from 60° to 120°, especially 90° with respect to each other. Wet grin¬ ding is effected in a grinding chamber which is closed to¬ ward the outside and from which the liquid or air expelled from the voids between the particles of the material being ground can drain through at least one aperture of narrow cross section.

The method of the invention differs from the methods applied so far in that the material which is to be comminuted is stressed several times in the form of a bed of material be¬ tween two hard, non-yielding surfaces by a plurality of pis¬ tons acting from different directions.

A comminution apparatus suitable to carry out this method comprises at least one grinding chamber to be loaded and unloaded in batches with material to be ground and having hard non-yielding surfaces and further comprises grinding pistons which are adapted to be pushed forward into the grinding chamber and which stress the material to be ground from different directions and at least partly in succession, pressing it against hard surfaces. The hard surfaces can be formed by further pistons, and the pistons are combined in groups in one plane or in different planes and act against each other. The pistons of two adjacent planes may be arran¬ ged offset at an angle of from 60 to 120°, preferably 90° with respect to each other. The opposed pistons of a pair of pistons are moved against each other simultaneously. The other pairs of pistons each stress the material to be ground one after the other.

For wet grinding, the grinding chamber is closed all around and the liquid or air expelled from the voids between parti¬ cles of the material being ground due to the stressing thereof can drain from the same through at least one aper¬ ture of narrow cross section, in particular through a gap between the piston and the piston channel wall. At least one piston is designed as loading piston and one piston as un¬ loading piston having an increased backward stroke as com¬ pared to normal grinding pistons.

The multiple stressing of the material to be ground in a bed of material which is compressed several times in the same direction without rebedding realized in stamp mills or con¬ ventional ram presses, brings no noticeable progress in comminution from the second stressing on because the first stressing already produces a state which is at equilibrium with the prevailing pressure. It is only if the bed of ma¬ terial is loosened between two successive acts of stressing and the particles are rearranged into a new mutual orienta¬ tion that the subsequent stressing can afford further

noticeable progress in comminution. Such procedures and the corresponding equipment are known. They do require additio¬ nal intervention in the compacted bed of material between successive stressing operations involving corresponding ad¬ ditional mechanical expenditure, as was previously realized with a ram press. The invention avoids this disadvantage by effecting successive stressingε in different directions w- hich preferably are offset with respect to each other by from 60° to 120°. The stressing is repeated several times, yet only until the production of further fine material per stressing operation reduces noticeably. The fine comminution of a few hard substances requires but relatively low compac¬ tion at between 50 and 120 MPa and only a few stressing op¬ erations; usually from 2 to 5 will be sufficient. Very fine comminution requires higher compression and a greater number of compressions, usually at least 5 such actions at prefer¬ ably at least 150 MPa. Microfine comminution of very hard substances having a hardness on the Mohs scale of more than 7, on the other hand, requires higher pressures, preferably more than 250 MPa and a greater number of stressing events. The number thereof may be ten or more, depending on the ma¬ terial to be ground and on the desired fineness. The produc¬ tion of fine and very fine and microfine products can be increased manifold by the successive stressing operations from different directions according to the invention, and yet the specific energy requirement does not suffer notice¬ ably. The coarse material circulation ratio is reduced dra¬ matically by this method and, in this manner, the overall need for energy in the milling circuit and the mechanical expenditure are reduced.

The method recited in claim 5 and the apparatus defined in claim 13 are especially well suited for wet comminution and wet grinding. The term wet grinding is used to define a mode of operation with which the material to be ground is contai¬ ned in a liquid which usually is water. The amount of liquid at least must fill all the voids between particles of the

consolidated material to be ground. In each of the mills listed above the material to be ground is stressed by com¬ pacting it due to the mutual approaching of two non-yielding surfaces. Wet grinding has the disadvantage, due to its principle, that a considerable amount of material to be ground is flushed out of the range of influence of those surfaces by the displaced liquid, thus being withdrawn from the stressing. This effect occurs also with dry grinding in the very fine range and especially so in the microfine range. The invention avoids this disadvantage by virtue of the fact that, in wet grinding, the stressing takes place in a grinding chamber which is defined all around, in other words closed, except for a few openings of small cross sec¬ tion to drain the liquid. Consequently only a minor propor¬ tion of the material to be ground can be flushed out. The grinding chamber is opened for loading with material to be ground, then closed prior to the stressing, and reopened for discharge of the material when the stressing is completed. It is convenient to use one pair of pistons or, if desired, a second pair of pistons for loading and unloading of the grinding chamber. A loading piston has a greater backward stroke to take up material to be ground from a feeding chute in front of its face end wall and convey it into the grin¬ ding chamber while partly closing the feeding shaft outlet. After sufficient multiple stressing of the material to be ground the unloading piston is retracted until it opens the inlet into a discharge chute toward which the ground mater¬ ial having been stressed sufficiently is conveyed by the loading piston which has been avanced further.

It was found that sufficiently great clearance between the pistons and the piston channels is sufficient to assure the draining of the liquid expelled while, at the same time, re¬ taining the major portion of the particles to be comminuted of the material to be ground.

Studies in a laboratory installation provided the comminution results below.

Fine comminution of quartz having a maximum particle size xmax of 2000 mm down to a fineness with which the maximum particle size is 80 mm: specific energy consumption 7 to 10 kWh/t. For comparison: a ball mill consumes from 20 to 40 kWh/t.

Very fine comminution of quartz having a maximum particle size x_m_a_x_ of 2000 mm down to a fineness with which the maxi- mum particle size is 20 mm: specific energy consumption from 20 to 30 kWh/t. For comparison: a ball mill consumes from 70 to 100 kWh/t.

Microfine comminution of quartz having a maximum particle size xmax, of 2000 mm down to a fineness with which the maxi- mum particle size is 5 mm: specific energy consumption from 100 to 150 kWh/t. For comparison: an agitator mill consumes from 300 to 500 kWh/t.

Microfine comminution to below 2 mm:

limestone at x = 40 mm: approximately 120 kWh/t quartz at x__ _{aχ = 3Q jm} , _a pp _rox i _ma tely 220 kWh/t zirconium at x__ _χ = 30 mm: approximately 240 kWh/t corundum at _ma = 50 mm: approximately 440 kWh/t.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:

Figs, la and lb show a multi-piston mill in longitudinal section and cross section, respectively;

Figs. 2a and 2b are a first vertical longitudinal and a second vertical longitudinal section rotated through 90° with respect to the first one showing a six-piston

mill which includes a grinding chamber closed all around, especially for wet grinding;

Figs. 3a and 3b are a vertical longitudinal section and a horizontal cross section of a four-piston mill which includes a grinding chamber closed all around, especi¬ ally for wet grinding;

Figs. 4a and 4b are views of a four-piston mill which inclu¬ des a grinding chamber closed all around for wet grinding, designed for discharge of material by feed screw or star-type gate, and

Fig. 5 shows a milling circuit including a multi-piston mill.

In the case of the multi-piston mill 7 illustrated in fig. 1 material to be ground 1 is fed to the mill from the top through a feed funnel 2 and then moves as bulk material from top to bottom in a cylindrical grinding chamber 3 of a mill¬ ing block from which it is discharged at the bottom by a discharge worm 5. It leaves the mill as a ground product 6 having been stressed several times. Grinding pistons 4 are combined in groups of four each in a plane and aligned crosswise in pairs. Several groups of pistons are arranged in parallel planes one on top of the other. The respective opposed pistons each are actuated at the same time, stress¬ ing the material to be ground, which is present as a bed of bulk material, from opposite directions. The two pairs of pistons in one plane become active one after the other. The end faces of the pistons are hard surfaces between which effective stressing of the material to be ground in a bed of particles can take place.

Figs. 2a and 2b show a six-piston mill for use above all in wet grinding, comprising a grinding chamber 3 which is closed all around. All the pistons 4.1 to 4.4 as well as 13.1 and 13.2 are driven in per se known manner by hydraulic power cylinders located outside of the milling block 14 and, therefore, not shown in the drawing. In principle, also

mechanical drive means can be used instead of the hydraulic drives. For reasons of clarity, the seals between pistons and the milling block 14 are not shown either. Pistons 4.1 to .4 and 13.1 and 13.2 move freely in the piston channels of the milling block 14. Expelled liquid can flow through the gap between the pistons and the piston channels. To load the central grinding chamber 3, piston 13.1 which serves as loading piston moves back to plane A marked by a discontinu¬ ous line. The material to be ground slides through a feed chute 15 into the channel of the loading piston 13.1 and is then conveyed by the latter into the grinding chamber 3. A piston 13.2 serving as unloading piston is disposed diamet¬ rically opposite. It is located in the position shown and defines the grinding chamber 3 at its side. The pistons 4.1 to 4.4 arranged in a vertical plane serve for stressing. Piston pair 4.1, 4.2 is disposed at right angles with re¬ spect to piston pair 4.3, 4.4. The piston pairs are advanced alternatingly into the grinding chamber 3, stressing the material to be ground several times in succession. For dis¬ charge of the material from the grinding chamber, the un¬ loading piston 13.2 is moved back into the plane B marked by a discontinuous line in fig. 2a, and the loading piston 13.1 pushes the ground product which has been stressed to a dis¬ charge chute 16 in the milling block 14. Thereafter the loading and unloading pistons are jointly moved back into the starting positions A.

Figs. 3a and 3b illustrate another modification which dif¬ fers from the one according to figs. 2a and 2b in that the loading of the grinding chamber 3 from the feed chute 15 is taken care of by the grinding piston 4.1, while the dis¬ charge is effected by having grinding piston 4.2 move back into the plane marked by the discontinuous line B and having grinding piston 4.1 push the stressed ground product 6 to the discharge chute 16. This modification comprises only four pistons 4.1 to 4.4.

Figs. 4a and 4b illustrate a four-piston mill for wet grind¬ ing which comprises two different kinds of discharge devic¬ es. In fig. 4a the discharge of the stressed ground material 6 is realized by a conveyor screw 17 in an-upwardly inclined tube. In fig. 4b this task is accomplished by a star-type gate 18.

Fig. 5 presents a method diagram of a possible grinding cir¬ cuit system. The material to be ground 1 is supplied to the mill 7 where it is comminuted. Upon stressing, the ground material 6 is conveyed into a disagglomerator 8 which sepa¬ rates or disintegrates the agglomerates that had formed. An impact or ball mill may be used as disagglomerator. The dis¬ agglomerated ground product 9 reaches a classifier 10 for separating coarse material 11 from fine material 12. The coarse material 11 is recirculated to the mill 7, while the fine material 12 is withdrawn from the circuit as the ground product.