Ultra fine milk of lime composition

Abstract

A composition of milk of lime comprising particles of slaked lime suspended in an aqueous phase, characterised in that said particles of slaked lime have a particle size described by a particle size distribution profile that is narrow and monomodal and the method of production thereof.

Claims

1. A method for the production of a milk of lime having a high reactivity defined by a t.sub.90 less than or equal to 10 sec, comprising the following consecutive steps of: a. the slaking of a quicklime by applying a proportion by mass of quicklime relative to water which equates to a ratio by weight in grams of quicklime relative to water greater than 1 to 8, and less than 1 to 3 so as to form a lime suspension, b. particle size cutting of said lime suspension, with the obtaining of at least one coarse first fraction to be ground and a fine second fraction; c. particle size reduction by wet ball grinding of said at least one coarse first fraction to be ground and d. obtaining of said milk of lime having a high reactivity.

2. The method according to claim 1, wherein said particle size reduction by ball grinding is carried out in a ball mill containing grinding balls which are less than 1.4 mm in size.

3. The method according to claim 1, wherein said particle size cutting comprises a screening by size on a vibrating sieve with mesh size of 250 m, and wherein said at least one coarse first fraction to be ground is the fraction passing through the screen with mesh size of 250 m, and said fine second fraction is the retained fraction consisting of inert particles or impurities to be removed.

4. The method according to claim 1, wherein said particle size cutting comprises of a double screening by size on a first vibrating screen with mesh size of 250 m, and on a second vibrating screen with mesh size less than or equal to 100 m, with the obtaining of said at least one coarse first fraction to be ground, said fine second fraction and a third fraction, wherein said at least one coarse first fraction to be ground is the fraction passing through the meshes of the 250 m screen and retained on the screen with mesh size less than or equal to 100 m, said fine second fraction is a fraction of residue retained at the 250 m mesh screen, consisting of inert particles or impurities to be removed and said third fraction is a fraction passing through the meshes of the screen with mesh size less than or equal to 100 m.

5. The method according to claim 4, wherein said at least one ground coarse first fraction and said fine second fraction are mixed in a proportion of between 20% and 75% for the at least one coarse first fraction and 25% and 80% for the fine second fraction.

6. The method according to claim 1, wherein the said particle size cutting is carried out by hydrocycloning of said lime suspension, with the obtaining of a fraction of particles whose particle size is less than 10 m as the fine second fraction and a fraction of particles having a particle size greater than 10 m as the least one coarse first fraction to be ground and wherein said milk of lime having high reactivity to water is obtained by mixing of said fine second fraction and said at least one ground coarse first fraction.

7. The method according to claim 1, wherein said particle size cutting comprises of a first step of screening by size on a vibrating screen with mesh size of 250 m, to obtain an accepted fraction passing through the meshes of the screen with 250 m mesh size, and a retained fraction composed of inert particles or impurities to be removed, and a second step of hydrocycloning of said accepted fraction, with the obtaining of a fraction of particles whose particle size is less than 10 m as the fine second fraction and a fraction of particles having a particle size greater than 10 m as the at least one coarse first fraction to be ground and wherein said milk of lime having high reactivity to water is obtained by mixing of said fine second fraction and said at least one coarse first ground fraction.

8. The method according to claim 1, wherein the said particle size cutting comprises of a first step of double screening by size on a vibrating screen with mesh size of 250 m, and on a vibrating screen with mesh size less than or equal to 100 m, with the obtaining of an accepted fraction passing through the meshes of the screen with 250 m mesh size but retained on the vibrating screen with mesh size less than or equal to 100 m, of a retained fraction retained on the vibrating screen with 250 m mesh size, composed of inert particles or impurities to be removed, and of a fraction accepted at the vibrating screen with mesh size less than or equal to 100 m, as well as a second step of hydrocycloning of at least one of the accepted fractions, with the obtaining of at least one fraction of particles whose particle size is less than 10 m as the fine second fraction and at least one fraction of particles whose particle size is greater than 10 m as the at least one coarse first fraction to be ground and wherein said milk of lime having high reactivity to water is obtained by mixing of said fine second fraction and the at least one ground coarse first fraction.

9. The method according to claim 1, wherein said quicklime is a quicklime having a reactivity t.sub.60 measured according to the reactivity test described in the standard EN459-2 within a range of 0.5 to 20 minutes.

10. The method according to claim 1, wherein the slaking is carried out with water.

11. The method according to claim 1, wherein the said slaking is carried out in a lime paste slaker.

Description

(1) Other characteristic features, details and advantages of the invention will become apparent from the description given here below, without limitation and with reference being made to the accompanying drawings.

(2) FIG. 1 is a graph representing the distribution of sizes of particles that is narrow and monomodal identified by a set of characteristic diametres for the milk of lime according to the invention depending on the grinding energy applied.

(3) FIG. 2 is a graph representing the measured viscosities of suspensions of milk of lime according to the invention having various grades of fineness, produced under different conditions of grinding (grinding energy applied).

(4) FIG. 3 is a graph representing an example of monomodal distribution of particles of a milk of lime, as measured by laser diffraction.

(5) In the figures, identical or similar elements have the same identifying references.

(6) The present invention therefore relates to a method of producing a suspension of milk of lime in three steps that make it possible to produce an ultra fine milk of lime with high reactivity wherein the particle size distribution profile is narrow and monomodal.

(7) The first step consists of slaking the quicklime in a paste slaker in order to produce an aqueous suspension of lime, the second step consists of an operation of particle size cutting also known as particle size screening which makes it possible amongst other things, to eliminate the inert particles and impurities such as silica or limestone from the lime suspension, possibly diluted. The third step is a step of wet ball milling which makes it possible to obtain a particle size distribution profile that is narrow and monomodal along with a reduced inert particles content.

(8) This particular narrow and monomodal particle size distribution makes it possible to obtain an ultra fine milk of lime that is homogeneous in terms of particle sizes and has high reactivity that is also homogeneous.

(9) In one embodiment according to the present invention, the accepted fraction of milk of lime output from the 250 m vibrating screen is processed in a second step of particle size cutting like for example in a hydrocyclone or with one or more vibrating screen/s having smaller mesh size, in order to obtain a fraction the particle size of which is already as expected and at least one coarser fraction to be ground.

(10) Each coarser fraction or one or more of them are then ground according to the present invention in a step of wet grinding in a ball mill in order to form particles of slaked lime having a narrow and monomodal particle size distribution profile that is well defined in the context of the present invention. The accepted fine fraction, the particle size of which is already as expected and at least one of the coarser ground fractions are then mixed or used separately.

(11) In one variant according to the invention, the quicklime is slaked in a stirred reactor, in a ball mill or in a high shear stress slaker, instead of a lime paste slaker, provided that the slaked lime particles are not too small in size, which would hamper any further particle size cutting to eliminate the inert fraction, on account of the possible clogging of the screen caused.

(12) As it may be noted, the choice of conditions for slaking and particle size cutting depends largely on the characteristics of the milk of lime resulting from the source of quicklime. The object of the invention is to produce, regardless of the characteristics or qualities of the source of lime, a composition having a milk of lime base, of which the particle size distribution profile is narrow and monomodal. The flexibility achieved by the method according to the invention with respect to the source of lime is not the only advantage. In fact, the method also allows for the use of different sources of slaking water unlike in the previous documents where water had also to be of high purity. In addition, the loss during screening is reduced, which offers significant economic benefits.

(13) The milk of lime according to the present invention, having said narrow and monomodal particle size distribution profile enables the use thereof in applications with short processing periods such as rapid neutralisation processes, synthesis of chemical products consuming slaked lime, rapid softening of water or mineralisation of the latter or even the precipitation of calcium carbonates.

(14) The composition of milk of lime according to the present invention is thus characterised by the fact that said particles advantageously have a particle size d.sub.98<10 m, a particle size d.sub.50<1.5 m and a particle size d.sub.30<1 m as measured by sedimentation (for example with a Micromeritics Sedigraph apparatus), and/or a particle size doe less than or equal to 10 m, a particle size d.sub.50 less than or equal to 3 m, preferably less than or equal to 2.5 m, in particular less than or equal to 2 m and a particle size d.sub.10 less than or equal to 1 m, preferably d.sub.25 less than or equal to 1.5 m, preferably less than or equal to 1 m, as measured by laser diffraction by means of using a device of the type Beckmann-Coulter LS 13 320 or Horiba LA950.

(15) The viscosity of the milk of lime according to the present invention is less than 350 mPa s, preferably less than 250 mPa s, and in a more preferential manner less than 200 mPa s, in an advantageous manner less than 100 mPa s, as measured by the standard Brookfield Rheometer DV-III with a speed of rotation of 100 revolutions per minute.

(16) The solids content of the milk of lime according to the present invention is in addition greater than or equal to 2%, advantageously greater than or equal to 5%, in a preferential manner greater than or equal to 10%, in particular greater than or equal to 12%, In a particularly preferential manner greater than or equal to 15%, relative to the total weight of the suspension. In general, the composition according to the invention is a suspension of slaked lime having a solids content less than or equal to 30%, in particular less than or equal to 25%.

(17) Finally, the rate of dissolution in distilled water of the lime milk according to the present invention, as measured by the KIWA procedure, is advantageously such that 90% of the particles of slaked lime are dissolved in less than 8 seconds, preferably in less than 5 seconds and in a more preferential manner in less than 3 seconds.

EXAMPLES

Example 1

(18) 3 samples of lime B, D, E, from three different geographical sources (respectively north of France, Portugal and central France) having a behaviour with respect to formation of milk of lime that is similar during slaking are slaked in a vertical hydrator measuring 15 dm.sup.3 equipped with a propeller rod stirrer, that is 70 mm in diametre, by adding hot water at a temperature of 40 C. to quicklime, based on a lime/water proportion of by weight. The processing time period is 30 minutes with a speed of rotation of 400 rpm. The milks of lime thus obtained are screened on a vibrating sieve with mesh size of 90 m. Each fraction accepted is diluted until a suspension of 15% by weight of solid material is obtained and is wet ground in a ball mill filled up to 85% with glass balls that are 0.8 mm to 1.2 mm in size, at a speed of rotation of 2200 rpm for a period of about 2.5 min until the obtaining of a d.sub.98<10 m and a d.sub.50<3 m as measured by laser diffraction.

(19) The suspensions obtained have a particle size distribution profile that is narrow and monomodal and a viscosity less than 350 mPa s as mentioned in the table.

Comparative Example 1

(20) Two samples of lime, A and C, from two geographically different sources (southern Poland and south of France) having a behaviour with respect to formation of milk of lime that is slightly different compared to the three limes of Example 1 are hydrated, screened and ground under the same conditions as in Example 1. Before grinding, suspension A has a viscosity that is significantly higher than that of suspensions B, D and E. For suspension C, the viscosity is much higher. After grinding, suspension A has a moderately higher viscosity and suspension C is far too viscous for proper industrial handling (see table). In both cases, the high viscosity of the suspension can be avoided by adjusting the conditions of slaking in particular by reducing the temperature of the slaking water by around 15 C. and/or by decreasing the stirring by about 200 rpm, in order to obtain particles the breaking energy of which is higher or by using grinding conditions which provide less energy per collision (see Example 2).

Example 2

(21) Lime samples A and C are hydrated and screened under the same conditions as in Example 1, but ground at 2100 rpm with finer balls that are between 0.5 mm and 0.7 mm in size, that is to say, which transmit about 60% lower energy by collision. Even though the fraction of unground slaked lime (A1 and C1) is similar to that in Comparative Example 1, and despite the fact that the ground slaked lime (A1 and C1) is finer than the ground lime in Comparative Example 1 (A and C), the viscosity of the suspension C1 is significantly reduced (see table) and the viscosity of the suspension A1 is not very strongly increased, in spite of the extreme fineness of the milk of lime obtained having a d.sub.98 of about 5 m, a d.sub.50 of about 1.5 m and du of about 1 m.

(22) TABLE-US-00001 TABLE analytical results pertaining to the suspensions in Examples 1 and 2 and in Comparative Example 1. The distribution of particle size was measured by means of the device Beckmann-Coulter Laser Diffraction Particle Sizer LS 13 320 Distribution of particle sizes of a milk Viscosity KIWA of lime that is screened but not ground Sample % Solid (mPa .Math. s) T.sub.100 (s) T.sub.90(s) d.sub.98 (m) d.sub.95 (m) d.sub.90 (m) d.sub.50 (m) d.sub.25 (m) A 15.1 140 65.4 9.4 61.0 42.2 28.3 5.7 2.5 B 15.1 10 92.5 20.9 78.2 58.0 39.0 7.3 3.1 C 15.2 350 55.3 9.6 61.9 46.6 30.9 5.4 2.4 D 15.2 45 100.5 17.4 80.1 57.9 33.4 5.6 2.7 E 15.1 21 64.7 15.5 73.6 55.6 35.4 6.4 3.1 A1 15.0 140 135.4 5.0 62.1 46.2 27.9 4.7 2.2 C1 14.8 410 49.5 4.6 59.2 42.0 28.4 4.8 2.0 Distribution of particle sizes of a milk Viscosity KIWA of lime that is ground Sample % Solid (mPa .Math. s) T.sub.100(s) T.sub.90 (s) d.sub.98 (m) d.sub.95 (m) d.sub.90 (m) d.sub.50 (m) d.sub.25 (m) A 15.3 390 1.8 1.6 8.9 7.7 6.5 2.3 1.4 B 14.9 190 4.2 3.0 9.1 7.9 6.6 2.4 1.4 C 15.2 1760 4.8 2.8 8.9 7.7 6.4 2.2 1.3 D 14.9 265 13.7 0.8 9.2 8.0 6.7 2.4 1.4 E 14.8 125 5.4 4.0 8.7 7.6 6.5 2.5 1.5 A1 15.0 530 5.6 4.6 6.1 5.1 4.1 1.6 1.0 C1 14.7 660 5.0 4.6 4.9 4.0 3.0 1.4 0.9

Example 3

(23) In order to illustrate the influence of the conditions of slaking on the grinding of milk of lime and the properties of the ultra fine milk of lime thus obtained, another sample (B1) from the same geographical source as the sample B is hydrated under two different sets of conditions.

(24) In the first case, one portion of the sample is hydrated by adding hot water at a temperature of 40 C. to the quicklime based on a lime/water proportion of by weight.

(25) In the second case, another portion of the sample is hydrated by addition of warm water at a temperature of 20 C. based on the same lime/water proportion of by weight. The milks of lime thus obtained are then screened on a vibrating screen with mesh size 250 m and the accepted fractions are then sufficiently diluted so as to obtain a solid content of 13% by weight and divided into different sub samples. These sub samples are then ground under different conditions in a ball mill, in particular by varying the rotation speed of the mill and the processing time period, but generally otherwise, under the same overall conditions as in Example 1, that is to say, with the same size of balls, the same kind of balls, the same fill rate, etc.

(26) The distribution of particle sizes obtained is presented in FIG. 1. The measured viscosities of the suspensions obtained are shown in FIG. 2.

(27) In an expected manner, the characteristic diameters decrease when the grinding energy applied increases. However, slaking with water having a temperature of 20 C. results in a fraction of milk of lime having particles which are coarser, that is to say having higher particle sizes d.sub.50, d.sub.95 and d.sub.98. The fractions of milk of lime with larger particles lead to a milk of lime that is coarser post grinding by applying the same grinding energy in comparison with milk of lime hydrated with water having a temperature of 40 C.

(28) While the suspension slaked by addition of water having a temperature of 40 C. may be ground under the conditions presented in FIG. 1 in order to achieve the particle size distribution profile in accordance with the present invention, this was not the case for the milk of lime obtained with water having an initial temperature of 20 C. The milk of lime slaked with water having a temperature of 20 C. is a bit too coarse in terms of d.sub.98.

(29) Surprisingly, the milk of lime obtained with water initially at a temperature of 20 C. has a higher viscosity in spite of the presence of a fraction of particles of larger size in this milk of lime as compared to that obtained with the slaking water initially having a temperature of 40 C.

(30) In both these two cases, the viscosity increases with the grinding energy applied, that is to say, when the particle size decreases for the respective milks of lime.

(31) It may therefore be concluded that the conditions for slaking, screening or particle size cutting (coarse fraction before grinding) and the conditions for grinding may be optimised in relation to each other in order to produce an improved milk of lime with higher reactivity and reduced viscosity.

(32) It is indeed understood that the present invention is in no way limited to the embodiments described here above and that modifications may well be made thereto without departing from the scope of the appended claims.