US3372878A

US3372878A - Grinding of materials in grinders having rolls with stepwise-increasing speeds

Info

Publication number: US3372878A
Application number: US396690A
Authority: US
Inventors: Verdier Andre Louis
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-09-20
Filing date: 1964-09-15
Publication date: 1968-03-12
Anticipated expiration: 1985-03-12
Also published as: DE1454884B2; CH434941A; FR1382167A; GB1080782A; DE1454884C3; DE1454884A1

Description

March 12, 1968 A. L. VERDIER 3,372,878 GRINDING OF'MATERIALS IN GRINDERS HAVING ROLLS WITH STEPWISE-INCREASING SPEEDS 4 Sheets-$h eet 1 Filed Sept. 15. 1964 ITH A. L. VERDIER GRINDING OF MATERIALS IN GRINDERS HAVING ROLLS W STEPWISE-INCREASING SPEEDS March 12, 1968 4 Sheets-Sheet 2 Filed Sept. 15. 1964 Fig. 3

MKuw MW w M, M4, Maw

March 1-2,- 1968 GRINDING Filed Sept. l5, 1964 A. L. VERDIER OF MATERIALS IN GRINDERS HAVING ROLLS STEPWISE-INCREASING SPEEDS WITH 4 Sheets-Sheet 5 A. L. VERDIER March 12,1968 4 I GRINDING OF MATERIALS IN GRINDERS HAVING ROLLS W STEPWISE-INCREASING SPEEDS ITH 4 Sheets-Sheet 4 Filed Sept. l5, 1964 u as,

J T5 39F L 5:0 N9 02 g \F v 82 I x J I 5 2K QQ 2 I mm mm United States Patent Oiifice 3,372,878 Patented Mar. 12, 1968 3,372,878 GRINDING OF MATERIALS IN GRINDERS HAV- ING RQLLS WITH STEPWISEJNCREASING SIEEDS Alldl Louis Verdier, 6 Ave. Daniel-Lesueur, Paris, deine, France Filed Sept. 15, 1964, Ser. No. 396,6? Claims priority, application France, Sept. 2%, 1963, 948,254 12 Claims. (Cl. Mfr-16) The invention relates to the grinding or milling of materials in paste form, such as have to be treated in particular in the printing ink, paint, chocolate, pigment, soap, rubber and plastics materials industries.

This grinding process consists not only in reducing the dimensions of the solid particles present in the paste but, according to circumstances, also in, for instance, dispersing them or wetting them, kneading or homogenising the paste or grinding and dispersing the clots or lumps that are formed. One endeavours to obtain at the end of the operation as regular a distribution as possible or pulverulent solid materials in materials in liquid phase.

To this end, grinders or mills having rolls with stepwise-increasing speeds are currently used in the various industries. These machines include a first roll (or slow roll) and at least two other rolls, each of these being pressed against the preceding roll and rotating more quickly than the latter and in the opposite direction. A composition consisting of solid and liquid materials pre viously mixed together is introduced into the hopper of the machine, which is arranged above the first two rolls. The pasty composition enters between the latter, adheres for the greater part to the second roll by reason of its greater speed, passes between the latter and the third roll, to which it mainly adheres, and is thus conveyed, passing in succession between the rolls pressed one against the other, to the outlet of the machine, where it is collected from the last roll by a pick-up knife.

Such grinders generally comprise three rolls, but some machines, in particular those which are used in the soap and chocolate industries, have more.

In some industries there is an increasing dislike for grinders comprising rolls having speeds which increase stepwise from roll to roll, because it is impossible at present to obtain a constant and regular output of a product having the desired properties. It is found in particular that the output depends on the preliminary operation of mixing the composition which is fed into the hopper. In one and the same grinder, without altering either the speed of the rolls or the pressure with which they are applied one against the other, totally different outputs are obtained by changing from one composition to another. It has even been observed that successive operations on one and the same composition do not necessarily give the same output or the same grinding quality. Moreover, the prod not leaving the machine is not homogeneous.

It is an object of the invention to remedy these drawbacks by employing new adjusting means acting on certain phenomena which are inherent in machines having rolls with progressively increasing speeds. The invention makes it possible for reproducible grinding qualities to be obtained with mixtures of solids and liquids, that is to say, it enables a product of constant predetermined quality to be obtained in a manufacturing process.

According to the present invention a batch of paste, which may be previously mixed or not, is introduced into a machine, and is made to pass in closed circuit between the rolls thereof while adjusting the temperature, the speed and the pressure of the rolls one against the other, for a time suflicient for there to be obtained in the hopper at the end of the operation, a paste having the desired quality of dispersion, which paste is then removed by a final passage between the rolls.

This method of grinding can be carried into eifect by means of an improved grinder which also forms part of the invention and which comprises, in combination, means for regulating the wall temperature of the rolls and means for varying the speed of the first two rolls in continuous fashion. According to a further feature of the invention, to this improved grinder there are preferably added judiciously arranged scrapers, a suitable system for adjusting or regulating the speed of the rolls and the pressure thereof one against the other, and/ or a de vice for mixing the paste in the hopper.

The temperature of the rolls, the pressure thereof one against the other and their speed must be adjusted in order to obtain a given temperature within the paste, so as to maintain the viscosity of the paste at a suitable value. In some cases, this temperature must be maintained below a fixed limit, either so as to avoid altering the taste or the colour of the paste-in particular in the case of the manufacture of chocolate or paints respectively or so as to avoid the transformation by heat of certain constituents of the paste, such as of the natural or synthetic resins which are used in the manufacture of certain printing inks, in the case of which it is necessary to avoid starting polymerisation.

By permitting pastes to be worked so as to obtain a very fine and very homogeneous product without excessive heating, the method of grinding according to the invention makes it possible to eliminate the operation of conching in the chocolate industry, this operation consisting in continuously stirring the paste which leaves the grinder for several tens of hours. This method, which thus make it possible to obtain directly a paste containing particles of sugar which are disintegrated to the desired fineness, regularly coated with cocoa butter and intimately mixed with the other constituents, also forms part of the invention, as also do the machines which enable the said method to be carried into effect.

The invention also comprises the application of the new method of grinding to the operation of phase substitution which is known in the pigment industry as the flushing process, this application consisting in introducing a mixture of water and precipitated pigments into the hopper together with a liquid substitution phase and in working the mixture obtained in this way in the manner already described. Thus, the same quality of dispersion which hitherto has been obtained only after a series of complex operations is now obtained directly.

The following description with reference to the accompanying drawings, which is given by way of non-limitative example only, will enable the various features of the invention and the manner of carrying them into effect to be clearly understood, any arrangement appearing either from the text or from the drawings coming within the scope of the said invention.

In the drawings:

FIGURE 1 shows diagrammatically in cross-section, a grinder incorporating the improvements provided by the invention;

FIGURE 2 is a View similar to FIGURE 1, but showing another embodiment;

FIGURE 3 is a diagrammatic elevational view of a grinder;

FIGURE 4 is a cross-sectional view on the line IV-IV of FIGURE 3, showing means for adjusting the tightening forces applied to the rolls and for controlling the distances between them;

FIGURE is a diagram of an alternative constructional form of an adjusting and controlling system for the rolls.

The grinder shown in FIGURE 1 comprises three rolls 1, 2, 3 rotating in the direction of the arrows 1b, 2b, 312 at speeds which increase stepwise from roll to roll, the first two rolls 1, 2 being surmounted by a hopper 4 and the high-speed roll 3 being equipped with a pick-up or collecting knife 5.

In the known grinders, the rolls are hollow and cooled by a flow of water passing through their

shafts

1a, 2a, 3a which are hollow. In the grinder shown, the hollow shaft of each roll is connected to a closed-circuit flow of fluid at controlled temperature and this enables the temperature of the walls of each roll to be regulated independently while avoiding incrustation. The hopper 4 has double walls as indicated at 4a, 4b and between the double walls there flows in closed circuit a fluid at controlled temperature which enters through a connection 40 and issues through a connection 4d. It is thus possible to regulate the temperature of the wall of each of the rolls and of the hopper independently.

A thermometric sensing device 6 is also provided and this enables the temperature of the paste contained in the hopper to be constantly checked. This thermometric device is advantageously supplemented or replaced by a device (not shown) enabling the viscosity of the paste to be continuously checked. Such devices are known per se, there being various types of viscosimeters which can be obtained on the market.

A scraper 7 is fast with two sealing devices 8 which prevent any drop in pressure, at the opposite ends of the line of contact between the rolls 2 and 3, in the paste passing between the latter. Such devices are described in my copending patent application Ser. No. 376,467, Patent No. 3,332,630. The assembly can be moved about an axis 9 so that the scraper 7 can be applied either against the roll 3 in order to throw the layer of paste adhering thereto back on to the roll 2 or, on the other hand, against the roll 2, in order to throw the layer of paste adhering thereto back on to the roll 3.

With these arrangements it is possible, by applying the scraper against the roll 3, to cause the paste to pass in closed circuit between the rolls while independently adjusting the wall temperature of the latter and of the hopper 4, as a function of the temperature or the viscosity of the paste contained in the hopper. The paste which is introduced between the rolls 1 and 2 adheres for the greater part to the roll 2, it passes between the latter and the roll 3, adheres again for the greater part to the latter and, when the scraper 7 is located in the position shown, it is thrown back by the latter on to the roll 2 which returns it to the hopper, the paste passing below the wall 4a, 4b of the hopper, which wall is for this purpose maintained slightly spaced from the roll 2. In order to remove the paste at the end of the operation, the scraper 7 is made to bear on the roll 2 so as to deliver the residual layer of paste still adhering to the roll 2 onto the roll 3; the collecting knife 5 detaches the paste adhering to the roll 3 in known manner.

So that the paste which passes in a closed circuit between the rolls may undergo a uniform treatment throughout,

scrapers

10 and 11 are provided which are applied against the rolls 1 and 2, respectively, so as to detach the layers of paste which adhere thereto and to return them to the mass or main body of the paste. Moving scrapers are also provided and these constantly scrape the walls of the hopper in order to detach therefrom the peripheral layers of paste which would have a tendency to cling thereto. In the drawing, the longitudinal walls of the hopper (that is to say the walls parallel to the axes of the rolls) are shown as having their cross-section in the form of circular arcs whose centre is located at the spindle 13. The scrapers 12 are made to bear on these longitudinal walls and driven with a reciprocating movement about the said spindle 13, by means of arms 12a. Other scrapers (not shown) may be provided, bearing on the end walls of the hopper and set in motion by any suitable means in order to scrape the said end walls.

A rotary mixing device 14 is also provided comprising two worm sections with opposite threads which start from the respective ends of the length of the rolls and end at the middle of the hopper, the said sections revolving on a shaft 15. The threads of the two screws are oriented in a direction such that the rotation thereof constantly shifts the paste located in the vicinity of the ends of the rolls towards the middle of the length of the latter. The worms can be mounted to be freely rotatable on their shaft 15 and the movement thereof is then produced by the ascending current 16 which occurs in the paste above the line of contact of the rolls 1 and 2. The shaft 15 is advantageously mounted at its opposite ends on the end walls of the hopper by means of retractable cups which permit rapid placing in position and removal thereof.

An ascending current such as indicated at 16 occurs in all machines equipped with rolls, whether the rolls have stepwise-increasing speeds or not, and is manifested by a constant rotary movement of the paste in the hopper. This current has not been utilized heretofore. The applicant has found that it is strong enough to drive the mixing device 14-.

The mixing device 14 may also be driven in rotation by a mechanical device (not shown). The arms 12a of the scrapers 12 are then advantageously driven with a reciprocating movement by the mixing device 14 through the medium of a suitable transmission. In the drawing this transmission is represented diagrammatically by a connecting rod 13a which converts the rotary movement of the device 14 into an oscillating movement of the arms 12a about their spindle 13.

The rolls of known grinders are generally driven by gears and the ratio of their speeds is constant. In FIG- URE 1, the rolls 1 and 2 are controlled separately by two continuous speed-changing devices shown diagrammatically at 17 and 18. It is thus possible to control the speeds of the first two rolls independently. The roll 3 is driven by the motor through a train of gears (not shown). This ear train could also be replaced by a continuous speedchanging device.

The arrangement for pressing the rolls one against the other or tightening them, which is not shown in its entirety in FIGURE 1, comprises means enabling the distance between the rolls to be measured and the forces tightening the rolls or pressing them one against the other to be adjusted in order to obtain a desired distance between them. Such means are described in my co-pending patent application Ser. No. 352,158 and will be described again with reference to FIGURES 3 to 5. They comprise very sensitive distance or spacing gauges shown diagrammatically at 19 and 29, which are interposed between the bearings supporting the shafts of the rolls; the tightening forces and the speeds of the rolls are adjusted so that the distances or spacings as read on the gauges assume the desired values. This adjustment is effected by means of a regulating system (not shown in FIGURE 1, but which will be described with reference to FIGURE 4) enabling the distances to be adjusted to a predetermined optimum value, account being taken of the available power of the machine.

I shall describe now with reference to FIGURES 3 to 5 the adjusting and controlling means disclosed in my above-mentioned co-pending patent application Ser. No. 352,158.

The grinder shown in FIGURES 3 and 4 is similar to the grinder shown in FIGURE 1, at least in parts related to the rolls thereof. Three rolls 191, 102, 103 are rotated in the directions indicated by the arrows, at graduated speeds. Portions of paste which are forced between the rolls or are adhering thereto are shown at 104, 104a and 1ti4b. A scraper for removing the paste from the third roll is shown at 105.

The roll 131 is journalled in two fixed side plates 1ti1a and 1411b. The roll 192 is journalled in two

side plates

102a and 102!) which are pivotally mounted on a spindle 1%. The roll 3 is also journalled in two side plates 1h3a and 1031b which are pivotally mounted on a spindle 107. An offsetting device, shown diagrammatically at 110, enables the spindle 6 to be displaced in height.

The rolls are driven rotatively at different speeds by a driving arrangement represented diagrammatically in FIGURE 4 by a train of gears 111.

The tightening system comprises two single-acting jacks 1&8 and 109, the cylinders 108a, 109a of which are connected to the fixed side plates 10112 and 101a respectively, and the piston rods 108b, 10912 of which are connected respectively to the movable side plates and 163a of the collecting roll. A hydraulic fluid drawn from a reservoir 112 is compressed at a constant pressure by a pump 113 and delivered by a pipe 114 into a pressure regulating valve 115 which supplies in parallel, on the one hand, a pipe 116 equipped with a pressure gauge 116a and leading to the chamber 108d of the jack 108 and, on the other hand, a pipe 117 equipped with a pressure gauge 117a and supplying the chamber 1090. of the jack 109 through the medium of a pressure reducing valve 118.

Each of the

valves

115 and 118 enables the pressure to be regulated downstream thereof by delivering a part of the fluid into a discharge pipe 115a and 118a which returns it to the tank 112.

In the operation of this tightening system, when the rolls are at a standstill the tightening or closing action thereof is controlled by regulating, by means of the valve 115, the pressure supplied by the

pipes

116 and 117 to the chambers 198d and 10%., until the

rolls

1111, 102, and 103 are pressed one against the other in the position shown in the drawing. The pressures read on the

pressure gauges

116a and 117a are the same. When the rolls rotate, entraining the paste 1114, the pressure which they exert on the latter is stronger at the non-driven end, that is to say the end where the

plates

101a, 102a and 103a are located, than at the end driven by the gears 111. The pressure in the chamber 109d must therefore be reduced by means of the valve 118 so as to decrease the tightening action at the non-driven end.

In order to obtain regulation of the tightening action which is precise and which can be automatic, the arrangement shown in FIGURE 4 will be used. A differential pressure gauge 119 is connected by

pipes

120 and 121 to the chambers 1118c and 10% which are filled with oil, so that the pressure gauge 119 indicates the difference in the pressures prevailing in these chambers 1080 and 109C.

Between the

ipes

120 and 121 there is arranged a by-pass 122 which can be closed by two cocks 120a and 121:: and which leads to a variable outlet 123 controlled by a cock 1230. An auxiliary pressure gauge 124 is connected by a cock 124a to the pipe 120.

In order to use the system, the machine being at a standstill and the rolls moved apart, the two chambers 1118c and 10% are first filled with oil by means of cocks such as 125 (FIG. 3) and these cocks are closed. When the preliminary tightening is carried out (rolls at a standstill) in the usual manner, the two cocks 120a and 121a are opened and the outlet 123 is controlled by means of the cock 123a in such manner that the pressure in the chambers 163a and 1090 does not reach a value liable to damage the differential pressure gauge 119. The pressure in the respective chambers 1418c and 109s is substantially the same since the pistons of the two jacks advance simultaneously and at the same speed, bringing the rolls nearer to one another, and it is read on the pressure gauge 124. When the three rolls are in contact and are tightened or pressed together with the appropriate pressure, which is read on the

pressure gauges

116a and 117a, the cock 123a is first closed so that the pressures in the chambers 108a and 11190 are strictly equalized owing to the by-pass 121, and then the cocks 120a and 121:: are closed. In this position, the rolls are applied one against the other and the oil pressure is precisely the same in the chambers 1080 and 109C. The pressure gauge 119 therefore indicates a zero differential pressure.

When the rolls are set in rotation, the paste 104 first passes between the rolls 1411 and 1132 and then between the rolls 192 and 1%, moving them apart to a greater extent at the end where the

side plates

101b, 10211, 10% are located than at the end where the

side plates

101a, 102a, 103a are located. The result is that the pistons of the two jacks shift slightly towards the bottom of FIG- URE- 4, reducing the pressure in the chambers 108C and 11190, the pressure decreasing more in the chamber 108s than in the chamber 10%. The differential pressure gauge 119 therefore indicates a lower pressure in the chamber 1118c than in the chamber 10%. By reducing the pressure in the chamber with! of the jack 109 by means of the valve 118, the tightening force applied by this jack to the rolls will be reduced and, consequently, the pressure wh ch the latter exert on the paste at the non-driven end will also be reduced; the reaction of the paste therefore slightly increases the distance between the rolls at this end and the piston of the jack 199 moves slightly towards the bottom of FIGURE 3, thus reducing the pressure in the chamber 1:190. When the differential pressure between the chambers Itiitic and 1096 has reached a zero value, which is indicated by the differential pressure gauge 119, the correction of the regulation by the valve 118 is stopped and, in this way, it is ensured that the sum of the distances between the rolls 1191 and 102 and between the rolls 1% and 1113 is the same on the side where the jack 169 is located and on the side where the jack 108 is located. In one stroke equality has been achieved between the pressures exerted by the rolls on the paste at both ends.

Instead of operating the valve 118 manually in order to bring the readings of the differential pressure gauge 119 to zero, use will preferably be made of the differential pressure between the chambers 1ti3c and 1090 in order to actuate a control device operating the valve 11% in such manner as to maintain this differential pressure at a Zero value. For example, the ditferential pressure gauge 119 may be provided with electrical contacts controlling a servo-motor which actuates the valve 118, but it is also possible to use other types of control using the differential pressure as an imput signal. This control of the valve 118 in response to the differential pressure as indicated diagrammatically in FIGURE 4 by the chain-dotted line 126.

It will be understood that moving apart of the rolls under the action of the paste passing between them has the effect of shifting the pistons of the two jacks 10S and 109 towards the bottom of FIGURE 4.. These movements which, in the embodiment described, are possible because the

valves

115 and 118 maintain fixed pressures in the

chambers

108d and 109d, are very small. The differences in the movement of the two pistons are of the order of one micron in printing ink grinders. However, the ditferences in pressure which result therefrom in the chambers N and 101C are sufficient to be indicated by the differential pressure gauge 119 or to produce a control signal for the valve 113. Any variation in the reaction of the gears 111, due to a change in the speed of the rolls for example, causes a variation of the differential pressure at the pressure gauge 119, enabling equal compression to be re-established throughout the length of the rolls by means of the valve 118.

However, in this connection, it should be noted that a change in the speed of the rolls causing a variation in the ditferential pressure will be desired and called for by, and will therefore be dependent on, the will of the person conducting the operation or the programming thereof. On the other hand, a difference of homogeneity in the paste or a difference in temperature, manifesting itself as a difference in the viscosity of the mixture, cause unexpected variations in the differential pressure. One of the advantages of the invention is that it permits permanent correction of the differential pressure as a function of the variations of the reactions of the control devices; which themselves depend on the variations of the power consumed during the same operation.

In the alternative constructional form shown in FIC- URE 5, the elements performing the same function as in FIGURE 4 are designated by the same reference numerals. The by-pass 122, which in FIGURE 4 is intended to permit the reduction of the volume of the

chambers

108a and 1090 and to establish equality of the pressures in the chambers upon the rolls being tightened when at a standstill, is improved in this case. The cocks 120a and 121a are replaced by electric valves 127 and 128 and the outlet 123 is replaced by a pressure accumulator 129 of the hydro-pneumatic type; moreover, the by-pass 122 is connected to a cock 130 enabling the volume of oil in the whole of the installation to be adjusted on starting up and, consequently, enabling the cocks 125 of FIGURE 3 to be dispensed with.

On the preliminary tightening operation in the rest state, the two electric valves 127 and 128 are opened and the oil expelled by the reduction in volume of the chambers little and 1090 is delivered to the accumulator 129. The preliminary tightening operation having been completed, the electric valves 127 and 128, which are controlled by a single electric circuit (not shown), are closed simultaneously and it is thus ensured that there will be equal pressures in the two chambers 1080 and 1090.

Also shown in FIGURE are the motor 113a of the pump 113 and a filter 113!) located at the intake of this pump, in the reservoir 112.. The valve 118 is an electric valve controlled by the differential manometer 119 through the line 126. The valve 115 is also an electric valve. A manometric contactor 131 is provided which is connected to the pipe 116 and which, by means of electrical connections (not shown) controlling the valve 115 or the pump 113, enables the placing of the installation under pressure to be interrupted in the event of the pressure increasing abnormally.

It will be observed that in FIGURE 4 the pressure gauge 124 measures the pressure in the chamber 108c and consequently gives an indication of the spacing adopted by the rolls under the action of the paste interposed between them. By operating the valve 115 in such manner as to maintain this pressure at a given value, it is therefore possible to give the rolls the desired spacing. In the constructional form shown in FIGURE 5, regulation of the tightening action as a function of the distance between, or spacing of, the rolls is provided, this being represented diagrammatically by a pressure gauge 132 with electrical contacts, which is connected to the pipe 120 and, by a line 133, to the regulation valve 115 and to a control 133a controlling the speed of rotation of the rolls.

Such regulation enables the compression actually undergone by the paste between the rolls to be maintained at the desired value. In machines equipped with rolls and which do not comprise this regulating system, the thickness of the layer formed on the exit or delivery roll such as 103 can be determined by controlling the weight of material supplied during a given time. It is sufficient to know the specific gravity of the material, such as the paste 4-, and to take account of the tangential speed and of the width of the layer recovered form the exit roll. This layer thickness, however, does not give any indication of the value of the distance between the last two rolls or between the preceding rolls. The applicant has found that in a machine having rolls with graduated speeds these distances are distinctly smaller than the thickness of the layer recovered.

The applicant has also found that the reduction of the distances has a considerable influence-for a mixture of a given viscosity-on the one hand on the velocity of How within the paste, which is considerably higher than in contact with the rolls and on the other hand on the maximum pressure within the paste, which increases very rapidly when the distance between the rolls decreases.

Thus, the variations towards an increase or a decrease in the distance have a considerable influence on the qualities of the products obtained.

In known grinders it is therefore possible to control the quality of the product obtained and, by successive experiments, to determine the conditions to be observed for the preparation of the mixture, its viscosity and its other characteristics, as well as for the various adjustments of the machine, so as to obtain a product of the desired quality, but no means are available for knowing and producing the most favourable distances between rolls. Although it is possible to reproduce a given manufacturing operation exactlyprovided that all the factors are strictly identical to those of the reference manufacturing operation there may be considerable variations in the distance between the rolls if one of the factors concerning the engagement or introduction of the material is diiferent. It will be understood that under these conditions it is impossible--if the composition of the mixture or the nature of the manufacturing operation is changed to obtain distance values identical to those which have been given the best results during previous experiments or manufacturing operations.

The regulation represented diagrammatically in FIG- URE 5 by the pressure gauge 132 and the line 133 enables the distance between the rolls to be maintained automatically at the desired value by acting, on the one hand, on the tightening pressure by means of the valve and, on the other hand, on the speed of rotation of the rolls by means of the control 13311 as a function of this distance, which is measured by the pressure in the chamber 108C.

The pressure gauge 132, for example, comprises a plurality of electrical contacts arranged to transmit to the line 133 signals which are used by the valve 115 to supply to the chamber 108d a tightening pressure which will maintain the chamber 108c at the desired pressure, the pressure in the chamber 10% being maintained equal to the pressure in the chamber 1080 by the control device described above. If the valve 115 cannot respond to a call for an increase in the tightening pressure (for instance because the latter is already equal to the pressure of the pump 113), the signal transmitted over the line 133 actuates the control 13311 for slowing down the rolls. The effect of this slowing down is to reduce the quantity of paste which is introduced between the rolls, the result is that the distance between the rolls decreases until the pressure in the chamber 1080, which measures this distance, has reached the desired value. The contacts of the pressure gauge 132 are adjustable and this enables the pressure which is automatically maintained in the chamber 1980 by the regulating arrangement to be adjusted at will.

It will be apparent from the above description with reference to FIGURES 3 to 5, that the chambers 1080 and 109c-together with pressure gauges 124 or 132, and 119-form very sensitive distance or spacing gauges between the rolls. Such gauges make it possible to control the spacing of the rolls by increasing and decreasing the tightening and speeds of the rolls. Tightening of the rolls can be varied by actuating

valves

115 and 113. The speeds of the rolls can be varied in a manner Well known per se, by actuating control means of the driving arrangement shown diagrammatically at 111.

The operator of the grinder machine can check a char- 9 acteristic of the pastenamely the viscosity or temperature thereof-on checking device 6 while the paste is worked in closed circuit by the machine, and adjust the wall temperatures of the rolls and control the tightening and speeds of the rolls to adjust spacings thereof, as a function of the checked characteristic i.e. in the sense that will produce desired evolution thereof.

In my above-mentioned co-pending patent application Ser. No. 352,158 and in the above description with reference to FIGURES 3 to 5, an arrangement is provided such that when all the tightening action of the machine has been used and the spacing must nevertheless be further reduced, the speed of rotation of the motor which drives all the rolls simultaneously is reduced, so as to decrease the volume of paste which is introduced between the rolls, the speed of rotation being reduced by means of a regulating system which receives signals which are a function of the distances between the rolls and which acts on the one hand on the tightening forces and, on the other hand, on the said speed of rotation.

When, in accordance with FIGURE 1, the rolls 1 and 2 are controlled separately by the speed-changing

devices

17 and 18, it is possible, in order to effect reduction of the distance between the rolls when the tightening effect cannot be further increased, to reduce only the speed of the first roll, the other rolls being then slowed down in the event of this reduction in speed not sufficing to obtain the desired distance or gap.

It will be understood that in FIGURE 1 the spacing between rolls 1 and 2 can be controlled independently of the spacing between rolls 2 and 3, since the rotation speeds of the rolls can be controlled independently. Each of spacing gauges 19 and 20 can comprise

pressure chambers

108a and 1090 and pressure gauges like pressure gauges 124 or 132 and 119 of FIGURES 4 and 5.

For example, it is possible to use a regulating system similar to that of FIGURE 5 but acting selectively on the speed-changing device or

devices

17 and 18 and on the motor (not shown) which simultaneously drives these speed-changing devices and the roll 3. In response to a signal calling for reduction of the distance or gap, the regulating system first increases the tightening effect and then, if the demand persists beyond the possibility of tightening, gradually reduces the speed of rotation, be ginning with the slowest roll. In response to a demand for an increase of the distance or gap, the regulating system gradually accelerates the rolls, beginning with the fastest roll, each of the rolls being accelerated to the speed suitable for the operation in progress and then, if necessary, reduces the tightening effect until the said demand is met. However, the power available for driving the rolls must not be exceeded.

It is therefore necessary to provide for the introduction into the regulating system, of parameters which are a measure of the difference between the power consumed by the rotation of the rolls or of each of them and the power available for this rotation, and/or the corresponding differences in torque. Devices emitting signals as a function of these parameters are known to those skilled in the art.

These signals will be fed to the regulating system as prohibition signals. On receipt of such a signal, instead of the regulating system effecting the normal operation which it should have effected if this signal had not been received, it will effect a following operation reducing the power consumed until the said normal operation has become possible. The regulating system will perform a sequence of operations (action on the tightening effect and action on the speed of the rolls) in response to an input signal (demand for a reduction or increase in the distance or gap between the rolls) it being possible for the order of these operations to be changed by means of prohibition signals (risk of exceeding the power). Those skilled in the art will know how to construct such regulating systems.

If the input signal constitutes a demand for the reduction of distance when the first roll, for example, is already consuming all its available driving power, or almost all of its driving power, the regulating system will not perform the normal operation of increasing the tightening effect, but will slow the first roll down until the power consumed has decreased sufficiently to cancel the prohibition signal and to permit the tightening effect to increase. If the regulating system simultaneously receives a demand for an increase in distance and a prohibition signal, it will not perform the normal operation of accelerating the rolls, but will first reduce the tightening effect in order to decrease the power consumed to the point at which the prohibition signal will be cancelled and will only then perform the normal acceleration.

It will be seen that the arrangement described enables the optimum distance to be maintained between the rolls, while causing the rolls to turn at the maximum speed permitted both by the power available for the tightening of the rolls and by the power available for the rotation of each of them.

So as to understand clearly the usefulness and the advantages of the arrangements described, it is necessary to examine in greater detail the phenomena which occur during the working of paste, on the one hand in grinders of the types heretofore known and on the other hand in grinders according to the invention.

In the methods of grinding hitherto employed, it has been found satisfactory to introduce into the hopper of the grinder a composition constituted by solid and liquid components, which were previously mixed, and then to start the machine so that the composition is entrained and passes between the rolls in succession, the product being collected by detaching it from the last roll by means of a pick-up knife. The sole means of adjustment provided on such a machine were its speed of rotation and the tightening force applied to the rolls; there was no awareness of the fact that a gap is produced by the passage of the paste between the various pairs of rolls.

The mixture introduced into the hopper, considered on the scale of the finest particles, is a heterogeneous mass comprising zones of different degrees of denseness (in accordance with the proportions of the constituents present in each zone) and of irregular temperature and viscosity. The paste introduces itself between the first and second rolls, the distance between which is unknown, then between the second and the third, the distance between which is also unknown (and so on between the succeeding rolls if the machine comprises more than three rolls). No means are available for adapting the distance between the rolls to the characteristics of the portion of the paste which passes between them, although in order to obtain uniform grinding of the paste it would be necessary not only to adapt the distance between the first two rolls to the characteristics of the portion of paste passing between them, but also to adapt the distance between the following rolls to the characteristics of the paste when these have been modified by the working to which the paste has been subjected by preceding rolls.

The viscosity of the paste in any given zone depends on the ratio between the volume of liquid and the total surface area of the solid particles present in that zone. The sizes of the solid particles, in each zone of the paste, are distributed over a granulometric range of greater or lesser extent according to the nature of the product, the grinder used and the fineness sought. If the tightening effect on the rolls is increased in order to reduce the dimensions of the largest particles, the finest particles will he reduced at the same time and, consequently, their total area will be increased and the viscosity will be thereby increased. This phenomenon of increase of the viscosity with the fineness of the grinding is demonstrated by, for example, the manufacture of chocolate, in which a paste (cocoa, sugar and cocoa butter) is introduced into the hopper of a five-roll grinder and a pulverulent product is recovered at the outlet of the machine.

It certainly seems, therefore, that the viscosity, like the granulometry of the solid particles in dispersion, the proportions of the various constituents and the other characteristics which result therefrom evolve in the various zones of the paste on each passage between two rolls. Among the characteristics which evolve in this way on each passage, the most important appear to be the viscosity and the adherence or stickiness of the paste, which mainly affect the mechanical action involved in the engagement of the paste between two rolls. It is probably the uncontrollable variation of these characteristics, between two different batches fed into the machine and in the various zones of one and the same batch, that causes the disparities in manufacture which have been found hitherto.

The temperature of the paste has a considerable influence on its viscosity and the friction of the rolls causes considerable heating, which is counteracted in known machines by circulating cold water. This enables excessive heating to be avoided but since it is subject to, in particular, incrustation and irregularities in the supply pressure, it does not permit of controlling the temperature of the rolls. The resultant variable temperature of the rolls, which acts in succession only on one face of the layer of paste entrained by each roll, can cause excessive local heating without the whole volume of paste being affected by this heating. The pastes will therefore be heated in varying degrees and will have higher or lower heterogeneous viscosities and this again has an effect on the distance between the rolls and on the quality of the product obtained, every time that a new operation of grinding a mixture of the same formula is carried out. Submitting the paste to several passages through the machine therefore only accentuates the differences which may exist between the zones which have been worked most and those which have been worked least in one and the same mixture, which differences are not apparent to users of the machine since they have at their disposal for the purpose of monitoring the manufacturing process, only means which make it possible to examine whether the largest agglomerates or the largest grains do not exceed a chosen value. Thus, the upper limit of the range of grain sizes of the solids to be found in the composition of the mixtures is fixed in this way, but there is complete ignorance of what is the lower limit and the distribution of the dimensions within this range.

The grinder described with reference to FIGURE 1, on the other hand, makes it possible to effect the simultaneous grinding and dispersion of solid particles in liquids in successive batches of constant weight for every type of mixture, while affording the possibility of reproducing the cycle of the various phenomena which are necessary for obtaining, at the end of the operation, a constant quality of the mixture worked with the latter always remaining in the pasty state.

In fact, the arrangements described make it possible to substitute controlled phenomena producing a desired result for the uncontrollable phenomena which occur in known machines. The paste, pas-sing in a closed circuit between the rolls and constantly mixed in the hopper, can be maintained at each instant at the viscosity which is most favourable to the stage of the operation in progress, due to the control of the said viscosity (or of the temperature of the paste) and to the simultaneous regulation or adjustment of the temperature of the rolls and the hopper, the tightening effects and the speeds of the rolls (making it possible to influence the heating which the paste undergoes during its passage between the rolls, by reason of the considerable pressures and accelerations brought into play). The concomitant adjustment of the distance between the rolls enables this to be adapted to the viscosity and to the stickiness of the paste (that is to say to its aptitude for introducing 12 itself between the rolls) and to the desired particle finemess.

The following example will enable the possibilities of the grinder described to be appreciated. The example concerns the grinding of a mixture which presents the risk of separation of the liquid phase, either because the ratio of the volume of the liquid to the total surface area of the solid particles is too large, or because the solid material is poorly wetted.

The composition, which may be previously mixed or not, is introduced into the hopper 4 and the flow of fluid inside the rolls 1, 2 and 3 is adjusted so as to provide a relatively low temperature, so as not to reduce the viscosity of the paste which is formed and to ensure adequate surface tension between the solid and liquid components of the paste, and also between the liquid component and the surface of the rolls.

The

speed changing devices

17 and 18 are adjusted so as to give the rolls 1 and 2 almost equal speeds, the roll 2, however, rotating a little more quickly so as to ensure the transfer of the layer to the roll 3. The speeds of the rolls 1 and 2 are as high as possible, having regard to the power of the machine, and the distance between the rolls is adjusted to the greatest value possible, without introducing any risk of the liquid dropping from the rolls (so as to ensure the engagement therewith of both the largest agglomerates and the liquid phase).

As stated above and fully explained in my co-pending patent application Ser. No. 352,158, the distance along which the rolls are forced apart by the material passing between them is a function of the rotation speed of the rolls and the hydraulic tightening pressure which is applied to the rolls. Thus, the tightening pressure is adjusted to the lowest value possible without the liquid material flowing freely through the nip between the first and second rolls.

The working undergone by the paste in this first stage modifies certain physical characteristics of the constituents and the result is a gradual modification of the mechanical action involved in the engagement of the paste between the rolls with a consequent effect on the power consumed. This results in increase or reduction in the speed of the rolls, but at this stage the speeds of the rolls 1 and 2 must remain very close.

In this first stage the solid particles are crushed and broken up; the result is an increase in the ratio of their total surface area to the volume of the liquid and the composition begins to form a paste having a significant viscosity. In the case of particles which are poorly wetted, these are engaged with the liquid between the rolls 1 and 2 by reason of the fact that their speeds are almost equal and, as a result, there is a grinding action and homogenization, which also form a paste having a significant viscosity. In both cases, there comes a point when the viscosity becomes measurable and when it can be checked by means of the viscosimeter located in the hopper 4.

The temperature of the rolls and of the hopper is then increased so as to reduce this viscosity and to enable the distances between the rolls to be reduced gradually. The roll 2 is accelerated to the highest speed permitted by the power of its speed changing device 18; the hydraulic tightening pressure on the rolls 1, 2 is increased and the speed of the roll 1 is reduced until the required distance is obtained between the rolls 1 and 2, which distance is the result of the lowering of the viscosity of the paste combined with the increase in the tightening effect and the increase in the difference in speed between the rolls 1 and 2. The roll 3 is accelerated and its tightening pressure is increased.

At this stage of the operation (there being a small distance between the rolls and a high speed of rotation of the rolls 2 and 3), the phenomenon of heating starts in the very midst of the paste by its passage between the rolls, causing a supply of calories much greater than that which can be supplied by heat exchange with the Walls 13 of the rolls. It will therefore be understood that the heating of the rolls and of the hopper by means of a heating fluid which has originally served to reduce the viscosity serves at the same time to start this phenomenon.

It follows that the temperature of the paste, and therefore the development of its viscosity, is the combined result of heating by the walls of the rolls and heating by the passage of the paste between the rolls (that is to say by consumption of the power developed by the machine). The layers of paste adhering to the rolls are constantly detached by the scrappers 7, It) and 11 and the paste is constantly mixed in the hopper by the mixing device 14 and detached from the walls of the hopper by the scrapers such as 12, so that the temperature indicated by the device 6 or the viscosity indicated by the viscosimeter are actually average characteristics of the paste.

A time comes when this average viscosity or temperature reaches a chosen limit. The temperature of the fluid circulating in the rolls is then gradually reduced and the supply of calories caused by the passage of the paste between the rolls is decreased by increasing the distances between the rolls and/ or reducing their speed, so as not to exceed the said limit.

It is obvious that these adjustments must be adapted to each type of manufacturing process, in accordance with the programme of operations which it is possible to perfect for each type of process, taking advantage of the arrangements described. The duration of each stage of the manufacturing process can be fixed and a control of the times can be arranged.

For example, a program of production can be settled by working a first batch of the materials and recording all particulars of the production process, such as the evolution and timing of the wall temperature and spacing of the rolls and the resulting evolution of the viscosity or temperature of the paste.

Then, the program can be used for working further batches of the materials successively, that is to say the recorded data can be used to control the wall temperatures and spacing of the rolls, whereby to obtain a number of batches of product having like qualities.

It should be noted that as soon as it has been possible to commence the heating of the paste the temperature of the hopper will have been brought as rapidly as possible to a maximum value corresponding either to a theoretical viscosity of the mixture, or to a limiting temperature imposed by the nature of the product. This temperature will be maintained until the end of the operation or until the moment when the manufacturing cycle requires the use of a different temperature; in this case, the fluid supplying the circulation in the hopper has its value brought immediately to the new temperature desired.

The grinder of FIGURE 2 comprises the same arrangements as that of FEGURE l and similar elements are designated by the same references. However, the hopper 21 is larger and surmounts the three rolls 1, 2 and 3. The knife 7 is replaced by a knife 22 which can adopt either of two positions as indicated diagrammatically by the arrows 22a; for the final removal of the paste the knife is brought to bear on the roll 2 so as to detach the paste adhering thereto and to deliver it to the roll 3; during the passage of the paste in closed circuit, the knife 22 is raised and the paste adhering to the roll 3 is detached by another knife 23 bearing on the latter. For the final removal, this knife 23 is raised to the position 2311, leaving between itself and the roll 3 a gap just sufficient to permit the passage of the paste adhering to the roll; this paste is detached by the pick-up or collecting knife 5.

This arangement is advantageous for pasty products such as inks, paints, soap, plastics materials and generally speaking in all those industries where it is sought both to Wet, disperse and grind solid particles in a liquid phase, since it enables the volume of the hopper to be substantially increased without creating dead zones. It also enables the evaporation of solvents to be reduced.

In particular, a machine constructed in accordance with the arrangement of FIGURE 2, which comprises rolls of large diameter and great length so that it can be charged with a considerable tonnage, can be used for carrying out the grinding and conching of chocolate.

What I claim is:

1. A process for the preparation of pigments by the method of liquid substitution known as the flushing process using a grinder having a charging hopper adapted to receive material and a series of rolls rotating 'at speeds increasing stepwise from roll to roll, pressed one against the other and between which the material passes, comprising the steps of: V

(a) introducing into the hopper a composition consisting of a mixture of water and precipitated pi gments together with the substitution liquid,

(b) causing the composition to pass between the rolls and allowing it to return to the hopper in 'a closed circuit While adjusting the temperature, the speed and the pressure of the rolls one against the other in concomitance, for a time sufiicient for the composition to become a paste constituted by a dispersion of the pigments in the substitution liquid and having the desired quality of dispersion, and

(c) removing the paste by causing it to pass between the rolls for the last time and detaching it from the last roll of the series.

2. In a process of wet grinding a batch of material by causing the material to pass through a series of nips defined between consecutive rotating rolls while an urging force is applied to the rolls to press the same against each other for mutual cooperation along adjacent generatriccs thereof, and adjusting the urging force and the rotation speeds and temperatures of the rolls, the improvement comprising ('a) returning the material to the batch to make a mixture therewith and again between the nips and back to the batch in closed circuit to form a paste progressively, while checking a characteristic of the paste in the batch, selected from the group consisting of the temperature and the viscosity of the paste, and adjusting the urging force and the rotation speeds and temperature of the rolls as a function of the said characteristic, and (b) taking off the paste by causing the paste to pass through the nips once again.

3. A process according to claim 2 for the preparation of pigments by the method of liquid substitution known as the flushing process, wherein the batch comprises a composition consisting of water, precipitated pigments, and a substitution liquid.

4. In a grinder having a spread of at least three consecutive rolls with parallel axes, means for applying an urging force to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof, driving means for positively rotating consecutive rolls in opposite directions at speeds which increase from a first roll to a last roll of the spread, a hopper adapted to contain pasty material for feeding the material between the first roll and the second roll of the spread whereby said material is driven and compressed between the consecutive rolls successively, means for adjusting the wall temperatures of the rolls, means for adjusting the urging force, and means for adjusting the speed of the driving means, the improvement comprising means in the hopper for checking a characteristic of the paste, selected from the group consisting of the temperature and viscosity of the paste, means for separately varying and adjusting the speeds of rotation of the first and second rolls, means adapted to return the pasty material from the last roll to the hopper, a helical rotary mixer for returning the paste at the ends of the hopper to the middle thereof, and means adapted to take off the pasty material from the last roll.

5. A grinder comprising a spread of at least three consecutive rolls with parallel axes; means for applying 'an urging force to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof; driving means for positively rotating consecutive rolls in opposite directions at speeds which increase from a first roll to a last roll of the spread; a hopper extending from the first to the last roll, adapted to contain pasty material for feeding the material between the first roll and the second roll of the spread whereby said material is driven and compressed between the consecutive rolls successively; means for adjusting the wall ternperatures of the rolls; means for adjusting the urging force; means for adjusting the speed of the driving means; means in the hopper for checking a characteristic of the paste, selected from the group consisting of the temperature and viscosity of the paste; means for separately varying and adjusting the speeds of rotation of the first and second rolls; a scraper above the last roll, which is applied against the surface of the last roll in order to return the layer of paste adhering thereto to the hopper; means for disengaging said scraper from the last roll and for providing in the wall of the hopper a slit of a height just sufi'icient to allow the layer of paste adhering to the said last roll to pass; and means adapted to take oif the pasty material from the last roll outside of the hopper.

'6. A grinder for manufacturing chocolate without conching, comprising 'a spread of three consecutive rolls with parallel axes; means for applying an urging force to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof; driving means for positively rotating consecutive rolls in opposite directions at speeds which increase from a first roll to a third roll of the spread, the three rolls having upper portions which form a bottom for a hopper adapted to contain pasty material for feeding the material between the first roll and the second roll of the spread whereby said material is driven and compressed between the consecutive rolls successively; means for independently regulating the temperatures of the Walls of the three rolls and the temperature of the inner wall of the hopper; means for adjusting the urging force; means for adjusting the speed of the driving means; means for separately varying and adjusting the speeds of rotation of the first and second rolls; a scraper in the hopper and means for shiftin the scraper between a position in which it is applied against the third roll and a position in which it is spaced from the third roll by a distance just sufiicient to allow a layer of material in paste form adhering to the third roll to leave the hopper; a pick-up knife applied against the third roll outside the hopper; means for detaching the portion of paste adhering to the walls of the hopper and delivering it into the batch of paste; means for mixing the batch of paste in the hopper while returning the portion of paste located in the vicinity of the ends of the rolls to the center of the hopper; and means in the hopper for checking a characteristic of the paste, selected from the group consisting of the temperature and viscosity of the paste.

7. In a process of wet grinding a batch of a composition of solid and liquid materials which tend to segregate, by causing the materials to pass through a series of nips defined between consecutive rotating rolls while an urging force is applied to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof, and adjusting the urging force and the rotation speeds and temperatures of the rolls, the improvement comprising (a) returning the material to the batch to make a mixture therewith and again between the nips and back to the batch in closed circuit to form a paste progressively, with the speeds of rotation of a first on and a second one of the rolls which define between them the first nip of the series that receives the composition being adjusted to almost equal values, the urging force being adjusted to the lowest value possible without the liquid materials flowing freely through the first nip, and the temperatures of the rolls being adjusted to a value low enough for the materials to form a paste having substantial surface tensions between solid and liquid components of the paste and between the liquid components and the surfaces of the rolls, until the paste has a significant viscosity, (b) checking a characteristic of the paste in the batch, selected from the group consisting of the temperature and the viscosity of the paste, and adjusting the urging force and the rotation speeds and temperatures of the rolls as a function of the said characteristic, and (c) taking off the paste by causing the paste to pass through the nips once again.

8. A grinder according to claim 4, wherein the said mixer is disposed above the facing generatrices of the first two rolls, as the result of which the mixer is driven rotatively by an ascending current of paste produced in the hopper above said generatrices.

A grinder according to claim 4, comprising a device for setting the mixer in motion and knives adapted to scrape the walls of the hopper.

it A grinder according to claim 12, comprising means for delivering to the regulating system prohibition signals which are a function of the difference between the power consumed by the rotation of the rolls and the available power of the grinder, said regulatin system being adapted to change in response to said prohibition signals, the order of the operations performed by said regulating system.

11. In a grinder having a spread of at least three consecutive rolls with parallel axes, means for applying an urging force to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof, driving means for positively rotating consecutive rollers in opposite directions at speeds which increase from a first roll to a last roll of the spread, a hopper adapted to contain pasty material for feeding the material between the first roll and the second roll of the spread whereby said material is driven and compressed between the consecutive rolls successively, means for adjusting the wall temperatures of the rolls, means for adjusting the urging force, and means for adjusting the speed of the driving means, the improvement comprising means in the hopper for checking a characteristic of the paste, selected from the group consistin of the temperature and viscosity of the paste, means for separately carying and adjusting the speeds of rotation of the first and second rolls, means adapted to return the pasty material from the last roll to the hopper, means adapted to take off the pasty material from the last roll, and closed-circuit fluid circulating means for independently regulating the temperatures of the walls of each roll and the temperature of the walls of the hopper.

12. In a grinder having a spread of at least three consecutive rolls with parallel axes, means for applying an urging force to the rolls to press the same against each other for mutual cooperation along adjacent generatrices thereof, driving means for positively rotating consecutive rollers in opposite directions at speeds which increase from a first roll to a last roll of the spread, a hopper adapted to contain pasty material for feeding the material between the first roll and the second roll of the spread whereby said material is driven and compressed between the consecutive rolls successively, means for adjusting the wall temperatures of the rolls, means for adjusting the urging force, and means for adjusting the speed of the driving means, the improvement comprising means in the hopper for checking a characteristic of the paste, selected from the group consistin of the temperature and viscosity of the paste, means for separately varying and adjusting the speeds of rotation of the first and second rolls, means adapted to return the pasty material from the last roll to the hopper, means adapted to take off the pasty material from the last roll, and a regulating system receiving input signals which are a function of the distance between the rolls and acting, on the one hand, on the tightening References Cited UNITED STATES PATENTS 801,259 10/1905 Neurnann 241-159 X 1,697,441 1/1929 Walker 241-159 X 2,134,897 11/1938 Theiner M 241-98 Hill 241-67 X Irwin 241-37 X Atkinson 241-37 X Sakata et a1. 241-37 Verdieer 241-159 FOREIGN PATENTS Great Britain.

ANDREW R, JUHASZ, Primary Examiner.

Claims

1. A PROCESS FOR THE PREPARATION OF PIGMENTS BY THE METHOD OF LIQUID SUBSTITUTION KNOWN AS THE "FLUSHING PROCESS" USING A GRINDER HAVING A CHARGING HOPPER ADAPTED TO RECEIVE MATERIAL AND A SERIES OF ROLLS ROTATING AT SPEEDS INCREASING STEPWISE FROM ROLL TO ROLL, PRESSED ONE AGAINST THE OTHER AND BETWEEN WHICH THE MATERIAL PASSES, COMPRISING THE STEPS OF: (A) INTRODUCING INTO THE HOPPER A COMPOSITION CONSISTING OF A MIXTURE OF WATER AND PRECIPITATED PIGMENTS TOGETHER WITH THE SUBSTITUTION LIQUID, (B) CAUSING THE COMPOSITION TO PASS BETWEEN THE ROLLS AND ALLOWING IT TO RETURN TO THE HOPPER IN A CLOSED CIRCUIT WHILE ADJUSTING THE TEMPERATURE, THE SPEED AND THE PRESSURE OF THE ROLLS ONE AGAINST THE OTHER IN CONCOMITANCE, FOR A TIME SUFFICIENT FOR THE COMPOSITION TO BECOME A PASTE CONSTITUTED BY A DISPERSION OF THE PIGMENTS IN THE SUBSTITUTION LIQUID AND HAVING THE DESIRED QUALITY OF DISPERSION, AND (C) REMOVING THE PASTE BY CAUSING IT TO PASS BETWEEN THE ROLLS FOR THE LAST TIME AND DETACHING IT FROM THE LAST ROLL OF THE SERIES.