Method for the dry slaking of calcium and magnesium oxides from calcomagnesian compounds

Abstract

The invention relates to a method for the dry slaking of calcium oxides and magnesium from calcomagnesian compound containing preferably at least 10 wt. % of MgO in relation to the total weight of said calcomagnesian compound, in which calcomagnesian compound is supplied to a slaking vessel, a slaking aqueous phase is supplied to the slaking vessel, followed by slaking the calcomagnesian compound delivered to the slaking vessel, by means of the slaking aqueous phase, and forming hydrated solid particles of calcium hydroxides and magnesium, in the presence of an additive. The invention also relates to the compound produced in this way.

Claims

1. A method for the dry slaking of calcium and magnesium oxides of a calco-magnesian compound, comprising the steps of: feeding a calco-magnesian compound containing MgO into slaking equipment; feeding an aqueous slaking phase into the said slaking equipment; and slaking the said calcium and magnesium oxides of the said calco-magnesian compound fed into the said slaking equipment, with the said aqueous slaking phase, leading to the formation of slaked solid particles of calcium and magnesium hydroxides, wherein the said slaking is performed at ambient pressure in the presence of an additive selected from the group consisting of water-soluble metal hydroxides, water-soluble metal silicates, water-soluble aluminates, water-soluble metal halides, water-soluble metal nitrates, water-soluble ammonium salts, ammonia and the mixtures thereof and wherein the slaked solid particles are in the form of solid particles containing less than 30% water by weight, the said calcium and magnesium oxides of calco-magnesian compound having a magnesium oxide content of at least 10 weight % and lower than 50 weight % relative to the weight of the said calco-magnesian compound, and having a calcium/magnesium molar ratio of between 0.8 and 1.2.

2. The slaking method according to claim 1, wherein the said calco-magnesian compound is selected from the group consisting of dolime, or semi-hydrated dolime, combined calco-magnesian compounds, and the mixtures thereof.

3. The slaking method according to claim 1, wherein the said water-soluble metal hydroxides are selected from the group consisting of sodium, potassium or lithium hydroxides, and the mixtures thereof.

4. The slaking method according to claim 1, wherein the said water-soluble metal silicates are selected from the group consisting of sodium and lithium silicates, alkaline-earth silicates and the mixtures thereof.

5. The slaking method according to claim 1, wherein the said water-soluble aluminates are selected from the group consisting of potassium aluminate, sodium aluminate, lithium aluminate, ammonium aluminate and the mixtures thereof.

6. The slaking method according to claim 1, wherein the said water-soluble metal halides are selected from the group formed by metal chlorides, metal bromides, metal fluorides and the mixtures thereof.

7. The slaking method according to claim 1, wherein the said metal nitrates and the said metal halides comprise at least one atom of a metal selected from the group consisting of aluminium, calcium and magnesium.

8. The slaking method according to claim 1, wherein the said metal nitrates and the said metal halides comprise at least one atom of a metal selected from the group consisting of aluminium and magnesium.

9. The slaking method of claim 1, wherein the said additive is added to the said aqueous slaking phase prior to the said feeding of the said aqueous slaking phase to form an additive-containing aqueous slaking phase.

10. The slaking method according to claim 1, wherein the said additive is added to the said aqueous slaking phase inside the said slaking equipment or in the said feed of the said aqueous slaking phase.

11. The slaking method according to claim 1, wherein the said additive is added to the said MgO-containing compound or in the said feed of the said calco-magnesian compound.

12. The slaking method according to claim 1, wherein the said additive is supplied at a content of between 0.1 and 20 weight %, relative to the total weight of MgO.

13. The slaking method according to claim 1, wherein the said calco-magnesian compound has a conversion rate of MgO to Mg(OH).sub.2 of at least 10% as measured by a simplified conversion rate determination test.

14. The slaking method according to claim 1, wherein the said slaking equipment is a dry process hydrator.

15. The slaking method according to claim 1, wherein the MgO conversion rate to Mg(OH).sub.2 is improved by 30% relative to the conversion rate obtained without an additive.

16. The slaking method according to claim 15 wherein the said slaking has a reaction time is less than 5 hours.

17. The slaking method according to claim 1, wherein the said calco-magnesian compound is a powdery compound.

18. The slaking method according to claim 1, wherein the said slaked solid particles have a maximum humidity of 5%.

19. The slaking method according to claim 18 further comprising a step to de-agglomerate or mill the said slaked solid particles.

20. The slaking method according to claim 1, wherein the said aqueous slaking phase has a temperature before slaking lower than 90° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the reactivity curves of the dolimes from Example 1.

(2) FIG. 2 illustrates the effect of additives on hydration of the MgO part of the dolimes from Example 2.

(3) FIG. 3 gives the reactivity curve of the dolimes from Example 3.

(4) FIG. 4 gives the reactivity curve of the dolime from Example 7.

DETAILED DESCRIPTION OF THE INVENTION

(5) The method of the invention therefore applies dry or nearly-dry hydration that is controllable, rapid and simple of dolime, dolomitic quicklime and magnesian lime inter alia or of any compound containing at least 10 weight % MgO which, in standard hydration equipment and through the use of an additive in preferred proportions of 0.1 to 20% preferably 1 to 10% and particularly advantageously 2 to 5 weight % relative to the total weight of MgO, allows a conversion rate of MgO to Mg(OH).sub.2 to be obtained that is significantly higher than that obtained under the same conditions but without an additive.

(6) The additives of the present invention are selected from the group formed by water-soluble metal hydroxides, in particular alkaline hydroxides (Na, Li or K hydroxides and particularly NaOH), water-soluble metal silicates in particular water-soluble alkaline or alkaline-earth silicates (particularly the alkaline silicates of Na, Li or K, and more particularly Na silicates), water-soluble aluminates, water-soluble metal halides in particular chlorides, bromides or fluorides, (particularly metal chlorides), water-soluble metal nitrates, water-soluble ammonium salts, ammonia and the mixtures thereof.

Example 1

(7) In this example, 4 different dolomites were considered. These dolomites had different reactivities such as illustrated in FIG. 1 showing the temperature curves recorded during the reactivity test according to ASTM C110 of these dolomites. Under the conditions of this test all these dolomites reached 60° C. starting from 40° C. in less than 5 minutes.

(8) Without considering impurities (e.g. traces of CaCO.sub.3) dolime 1, according to thermogravimetric and X-ray fluorescence analyses, had a weight percentage of MgO of 39.5 weight %, a weight percentage of CaO of 51.9 weight % and a ratio x/y(mol) i.e. Ca/Mg of 0.94. Dolime 2, according to thermogravimetric and X-ray fluorescence analyses, had a weight percentage of MgO of 36.8 weight %, a weight percentage of CaO of 47.9 and ratio x/y(mol) i.e. Ca/Mg of 0.93. Dolime 3, according to thermogravimetric and X-ray fluorescence analyses, had a weight percentage of MgO of 37 weight %, a weight percentage of CaO of 56.9 and a ratio x/y(mol) i.e. Ca/Mg of 1.11. Dolime 4, according to thermogravimetric and X-ray fluorescence analyses, had a weight percentage of MgO of 40.8 weight %, a weight percentage of CaO of 58.0 and ratio x/y(mol) i.e. Ca/Mg of 1.02.

(9) These dolimes were all hydrated in a Lödige ploughshare mixer of 5 dm.sup.3 capacity acting as hydrator. For this hydration 1 kg, of dolime was gradually added to the mixer under rotation using a laboratory powder-metering feeder. Simultaneously water (demineralised water) was added to the mixer using a peristaltic pump connected to a spray nozzle. Since the dolimes did not all have the same particle size and were more or less fluid, the powder-metering feeder could not be used with the same settings for all the dolomites. Therefore the time needed to add the entirety of the kilogram of dolomite was of greater or shorter length for the different tested dolomites. However, for each setting of the dolomite metering feeder the water pump was also adjusted so that at all times the entirety of the dolime and the entirety of the water were added simultaneously during one same addition time, the time varying between 10 and 20 minutes according to dolomite. The water/dolime weight ratio for all the dolomites was set at a value of 0.4.

(10) After this feeding phase into the mixer, mixing was continued for 1 h after which the product was removed from the hydrator, dried at 150° C. and analysed by thermogravimetry to determine the conversion rate of the MgO part of each dolime under these hydration conditions. Since the samples were analysed as soon as hydration was completed in the mixer, the reaction time in this case was comparable to the residence time of the product in the mixer i.e. 1 h (about 1 h 20 including the addition time of the dolomite and water to the hydrator).

(11) For each of the 4 dolimes considered, hydration was initially conducted without additive and then with the addition of 3% sodium hydroxide (NaOH) relative to the weight of dolime. When the additive was NaOH this additive was placed in the hydration water at least 1 hour before the start of dolime hydration so as to guarantee complete dissolution of NaOH in the water and allow the NaOH solution thus prepared to return to ambient temperature.

(12) All the results are given in Table 1 and indicate a significant positive effect during the hydration reaction of the use of NaOH on the extent of hydration of the MgO part of these dolomites. The use of NaOH allows an increase in MgO conversion rates by 200% up to over 630% compared with the conversion rate without additive, and this effect is most significant and visible after a hydration time of only 1 h.

(13) TABLE-US-00001 TABLE 1 increase final % initial % tC.sub.MgO tC.sub.MgO Humidity* Mg(OH).sub.2 MgO e.sub.1 (%) e.sub.2 (%) (%) (%) Dolomite 1 10.5 6.0 39.5 1.8 13.1 12.3 — no additive Dolomite 1 + 7.5 17.0 5.2 13.2 36.5 197 3% NaOH Dolomite 2 14.0 5.6 36.8 1.7 11.4 12.0 — no additive Dolomite 2 + 10.4 19.3 5.9 11.0 43.6 263 3% NaOH Dolomite 3 11.9 5.4 37.0 1.7 13.7 11.9 — no additive Dolomite 3 + 8.2 18.3 5.7 12.7 41.9 252 3% NaOH Dolomite 4 10.2 2.5 40.8 0.8 13.5 4.9 — no additive Dolomite 4 + 5.7 17.3 5.3 12.9 35.8 631 3% NaOH *Humidity of the product leaving the mixer after hydration time of 1 h, determined by measuring weight loss of the sample on drying at 150° C.

Example 2

(14) In this example only one dolime was considered. This was dolomite 1 already used in Example 1. This dolomite was hydrated following the same operating mode as described in Example 1 also keeping to a water/dolime weight ratio of 0.4. On the other hand, hydration was conducted this time for only 30 minutes after the addition time of the dolime and water to the mixer instead of 1 hour as in Example 1. At the end of these 30 minutes (t=30 minutes) the product was removed from the mixer, a first sample was taken and the remainder of the product was placed in a closed plastic bucket. Other samples were then taken from this bucket at t=60, 90, 200 and 500 minutes i.e. a total of 5 samples at each time. In this example the reaction time therefore corresponds to the residence time (30 minutes) to which is added a storage time in the bucket.

(15) The dolomite was hydrated without additive and then with 3% NaOH as in Example 1, and thereafter with KOH and LiOH. For KOH and LiOH the weights of KOH and LiOH were calculated so as to add to the mixer in which hydration took place the same number of moles of OH as the amount added with 3% NaOH. In the particular case of LiOH the additive used was hydrated (LiOH.H.sub.2O). These additives were added to the hydration water following the same procedure as described in. Example 1. All the results are grouped together in Table 2. These values clearly show the positive effect of the different additives tested on the hydration level of the MgO part of the dolime under consideration, and this effect was visible as early as after 30 minutes i.e. as soon as the product was removed from the hydrator (FIG. 2).

(16) TABLE-US-00002 TABLE 2 increase Time final % initial % tC.sub.MgO tC.sub.MgO (min) Humidity.sup.a Mg(OH).sub.2 MgO e.sub.1 (%) e.sub.2 (%) (%) (%).sup.b No 30 11.5 3.6 39.5 1.1 13.2 7.4 — additive 60 11.5 3.3 1.0 13.3 6.7 — 90 11.5 3.0 0.9 13.2 6.1 — 200 11.4 4.5 1.4 13.3 9.2 500 11.4 4.7 1.4 13.6 9.7 — +NaOH 30 5.0 13.3 39.5 4.1 13.2 28.1 283 60 4.7 13.2 4.1 13.0 27.9 314 90 5.1 13.5 4.2 12.9 28.5 366 299 5.3 14.3 4.4 13.2 30.4 229 500 4.9 14.4 4.4 13.1 30.5 216 +KOH 30 8.8 15.1 39.5 4.7 13.0 32.1 337 60 9.4 14.7 4.5 13.1 31.2 363 90 9.3 16.0 4.9 13.0 34.1 458 200 9.2 15.7 4.8 12.9 33.4 262 500 9.2 15.6 4.8 12.9 33.2 243 +LiOH.sup.c 30 8.5 14.8 39.5 4.6 13.6 31.7 331 60 8.3 15.1 4.7 13.2 32.2 378 90 8.4 15.3 4.7 13.3 32.7 434 200 8.4 15.8 4.9 13.1 33.7 265 500 8.5 15.9 4.9 13.4 34.1 252 .sup.ahumidity of product on leaving the mixer after hydration time of 1 h determined by measurement of sample weight loss when dried at 150° C.; .sup.bin comparison with product obtained under the same conditions without additive .sup.cin fact LiOH•H.sub.2O

(17) In addition to a significant effect on the conversion rate of MgO to Mg(OH).sub.2 the additives used in this example also had a significant effect on specific surface area and pore volume of the hydrated products as illustrated by the results in Table 3.

(18) TABLE-US-00003 TABLE 3 Specific surface Time (min) area.sup.a Pore volume.sup.b No additive 500 22.4 0.105 +NaOH 500 12.9 0.067 +KOH 500 8.7 0.057 +LiOH.sup.c 500 13.6 0.072 .sup.aBET specific surface area obtained by nitrogen manometry after degassing at 190° for at least 2 h; .sup.bvolume of pores measuring 17 to 1000 Å such as calculated using the BJH method on the basis of nitrogen adsorption/desorption measurement on degassed sample; .sup.cin fact LiOH•H2O.

Example 3

(19) Example 3 below was conducted starting from dolime 5 having a d.sub.98 of 3 mm or less and containing 39.7% by weight MgO according to analysis by X-ray fluorescence spectrometry.

(20) The dolomite 5 used in this example (excluding impurities such as CaCO.sub.3 for example) according to thermogravimetric and X-ray fluorescence analysis contained a weight percentage of MgO of 39.7%, a weight percentage of CaO of 55.1 and had an x/y(mol) ratio i.e. Ca/Mg of 1.00.

(21) This dolomite was initially subjected to the simplified conversion rate determination test at 70° C. as previously described and allowing a description of the reactivity of the materials to be hydrated. The conversion rate of the MgO in this dolomite to Mg(OH).sub.2 throughout this test was 22.6% as illustrated by the results in the Table below and calculated using formula 1.

(22) TABLE-US-00004 TABLE 4 e.sub.1 e.sub.2 tc.sub.MgO % Mg(OH).sub.2final % MgO.sub.initial (%) %) (%) 10.8 39.7 3.3 13.5 22.6

(23) In addition, the reactivity test such as described in the foregoing and in ASTM C110 indicated relatively high reactivity for this dolomite: the t.sub.70 was 3.5 minutes and only 1.2 minutes were required to reach 60° C. starting from 40° C. as illustrated by the reactivity curve in FIG. 3.

(24) This dolime was then hydrated in a laboratory pilot hydrator. This hydrator is a single-stage hydrator. It is in the shape of a horizontal cylinder measuring about 80 cm in length with a diameter of 25 cm. These proportions correspond to the proportions of industrial single-stage hydrators conventionally used for the hydration of quicklime or dolime at atmospheric pressure by dry or nearly-dry process. These dimensions are 6 to 7 times smaller than the dimensions of industrial hydrators. This cylinder is equipped with a double jacket allowing temperature control by circulating a hot or cold fluid. Inside the hydrator a shaft fitted with paddles is used to homogenise the product during hydration but also to move the product from the inlet (at one end) towards the outlet (at the other end of the cylinder). When the product has travelled the entire length of the hydrator it leaves the hydrator by simple overflow. In general the filling level of the hydrator is in the order of 50% by volume i.e. the height of the bed of product reaches about the height of the shaft.

(25) The hydrator is pre-heated to 70-80° C. by circulating water at 90° C. inside the double jacket. This pre-heating allows simulation of a steady state that is obtained industrially by continuous operation and prevents condensation of water vapour produced by the hydration reaction of the dolomite on the walls of the hydrator if they are cold. The double jacket is drained when the hydration reaction is initiated in the hydrator.

(26) In this example, the dolomite was placed in the hydrator using a worm-screw that was previously calibrated to adjust the flow rate of dolime at 200 g/min.

(27) The dolomite was hydrated with a water/dolime weight ratio of 0.4 in the presence of 1% sodium aluminate of formula NaAlO.sub.2 relative to the weight of dolime. Hydration was performed continuously and the mean residence time of the hydrate in the reactor was in the order of 15-20 min. Overall, the hydration under these conditions lasted 60 minutes. To pay heed to these conditions, a solution was prepared prior to hydration by mixing 5160 g of demineralised water and 120 g of sodium aluminate of formula NaAlO.sub.2. This solution was held under continuous agitation. When the operation to add dolime to the hydrator was commenced, the injection of this solution was also initiated. This injection into the hydrator was performed via two orifices each measuring about 5 mm in diameter located on the cover of the hydrator close to the inlet for the dolime, using spray nozzles. The flow rate of this solution was set at 88 g/min.

(28) The first 40 minutes of hydration corresponded to the start-up of the hydrator. Over the following 20 minutes samples were regularly taken and dried in a thermal balance at 150° C. in less than 30 minutes. Their humidity was recorded. These dried samples were then subjected to thermogravimetric measurement allowing calculation of the MgO conversion rate to Mg(OH).sub.2. In this example, since the samples were taken directly at the outlet of the hydrator the reaction time can be considered comparable to the residence time in the hydrator.

(29) The same hydration procedure was also carried out using this same dolime under exactly similar conditions but without additive (200 g dolime per minute and 86 g/min of tap water i.e. a water/dolime ratio of 0.43 as previously). This allowed a comparison between the results obtained with and without sodium aluminate under similar operating conditions. These results are compared in Table 5. The additive allowed a significant increase in the percentage of MgO hydration under the conditions of this example, this percentage being more than doubled in the presence of 1% sodium aluminate relative to the weight of dolomite.

(30) TABLE-US-00005 TABLE 5 increase final % initial % e.sub.1 e.sub.2 tC.sub.MgO tC.sub.MgO Humidity.sup.a Mg(OH).sub.2 MgO (%) (%) (%) (%) Water/dolomite = 4.6 3.6 39.7 1.1 13.8 7.4 — 0.4 (g/g) No additive Water/dolomite = 5.1 7.7 39.7 2.4 13.7 16.0 115 0.4 (g/g) 1% NaAlO.sub.2 .sup.ahumidity of product on leaving the mixer after hydration time of 1 h, determined by measuring weight loss of the sample on drying at 150° C.

Example 4

(31) This example was very similar to Example 3 in that the same dolime was used and hydrated in the same hydrator. However this time the dolime, again added to the hydrator at the rate of 200 g/min, was hydrated with a water/dolime weight ratio of 0.8. Initially the dolomite was hydrated with demineralised water without the addition of an additive (160 g water/min). The water was then replaced by a solution of sodium aluminate of formula NaAlO.sub.2 previously prepared by mixing 9600 g of tap water and 600 g of sodium aluminate NaAlO2. During the 60 minutes of hydration (the first 40 minutes to reach steady state) this solution was injected into the hydrator at a rate of 170 g/min. The results obtained under these new conditions with and without additive are compared in Table 6. In this example as in Example 3 the reaction time can be considered comparable to the residence time in the hydrator since the samples were characterized as soon as they left the hydrator.

(32) The additive allowed a very significant increase in the percentage of MgO hydration under the conditions of this example, this percentage more than tripled in the presence of 5% sodium aluminate relative to the weight of dolomite. The value of 28.5% reached for the conversion rate of MgO in the presence of 5% sodium aluminate is a high conversion rate compared with the conversion rates usually obtained without additive under industrial conditions of dolomite hydration by dry process, these generally being considered as not able to exceed 25%.

(33) TABLE-US-00006 TABLE 6 increase final % initial % e.sub.1 e.sub.2 tC.sub.MgO tC.sub.MgO Humidity.sup.a Mg(OH).sub.2 MgO (%) (%) (%) (%) Water/dolomite = 20.1 4.3 39.7 1.3 14.0 8.8 — 0.8 (g/g) No additive Water/dolomite = 22.0 13.5 39.7 4.2 13.4 28.5 225 0.8 (g/g) 5% NaAlO.sub.2 .sup.ahumidity of product leaving the mixer after hydration time of 1 h, determined by measuring weight loss of the sample on drying at 150° C.

Example 5

(34) In this example the dolime 2 described in Example 1 was hydrated in the laboratory pilot hydrator already used in Examples 3 and 4 above. This time the dolime flow rate was set at 150 g/min but all the other settings were maintained constant in relation to the protocol described in Example 3 (pre-heating, filling level . . . ).

(35) This dolime was first hydrated with demineralised water previously heated to 70° C. and added to the hydrator at a rate of 55 g/min. This water was then replaced by a sodium hydroxide solution (NaOH) at a concentration of 10% by weight also pre-heated to 70° C. and added at a flow rate of 85 g/min. This corresponds to adding 5.7% NaOH relative to the weight of dolime.

(36) As previously in Example 3, the mean residence time in the hydrator was in the order of 20-30 minutes. As soon as it left the hydrator, the hydrated product was characterized (again the reaction time was equivalent to the residence time). The results are given in Table 7. In this case the effect of the use of the hydroxide during the hydration reaction of the dolime was very strongly marked.

(37) TABLE-US-00007 TABLE 7 Final Initial % % e.sub.1 e.sub.2 tc.sub.MgO Humidity.sup.a Mg(OH).sub.2 MgO (%) (%) (%) No additive 2.0 <0.5 36.8 0 10.8 <1 +5.7% NaOH 2.9 21.7 6.7 11.3 49.7 .sup.ahumidity of product leaving the mixer after hydration time of 1 h, determined by measuring weight loss of the sample on drying at 150° C.

(38) The product obtained in the presence of sodium hydroxide and having a conversion rate in the order of 50% showed a BET specific surface area of 11.0 m2/g.

Example 6

(39) This time it was dolime 3 described in Example 1 which was hydrated in the pilot hydrator already used in Examples 3 to 5. It was fed into the hydrator at a rate of 150 g/min.

(40) This dolime was first hydrated with demineralised water previously heated to 70° C. and fed into the hydrator at a rate of 59 g/min. This water was then replaced by a sodium hydroxide solution (NaOH) at a concentration of 7.5 weight %, also pre-heated to 70° C. and fed at a rate of 71 g/min. This corresponds to adding 3.5% NaOH relative to the weight of dolime.

(41) As previously in Example 5, the mean residence time in the hydrator was in the order of 20-30 minutes. On leaving the hydrator the products were placed in a bucket for storage in the laboratory. A first series of samples produced with and without sodium hydroxide was taken directly at the outlet of the hydrator (reaction time=residence time in the hydrator). A second series of samples produced with and without sodium hydroxide was taken from the buckets 1 hour after being removed from the hydrator (reaction time=residence time+1 h). The results are given in Table 8. While the sodium hydroxide already allowed the conversion rate of MgO to Mg(OH).sub.2 to be doubled during the time spent in the hydrator (reaction time=residence time) it had a much stronger effect when the products were stored for 1 hour before being characterized (reaction time=residence time+1 h). In the presence of sodium hydroxide a conversion rate of almost 76% was able to be reached after a storage time of only 1 hour after leaving the hydrator. On the other hand, the product obtained without additive did not react during this hour of storage. The product obtained in the presence of sodium hydroxide and having a conversion rate in the order of 76% had a BET specific surface area of 9.2 m.sup.2/g.

(42) TABLE-US-00008 TABLE 8 Reaction Final % Initial % tC.sub.MgO time Humidity.sup.a Mg(OH).sub.2 MgO e.sub.1 (%) e.sub.2 (%) (%) No additive Residence 1.6 1.9 37.0 0.6 13.2 4.1 time Residence 1.2 1.8 0.6 13.6 3.9 time + 1 h +3.5% Residence 11.0 4.2 1.3 13.5 9.2 NaOH time Residence 1.5 31.3 9.7 13.1 75.7 time + 1 h .sup.ahumidity of product leaving the mixer after hydration time of 1 h, determined by measuring weight loss of the sample on drying at 150° C.

Example 7

(43) In this example dolime 6 was considered. This time it was hydrated in a Hobart epicyclic, planetary mixer of 5 dm.sup.3 capacity.

(44) Calcined dolomite 6 used in this example (excluding impurities such as CaCO.sub.3) according to thermogravimetric and X-ray fluorescence analyses contained a weight percentage of MgO of 40.9%, a weight percentage of CaO of 54.1 and the x/y (mol) ratio i.e. of Ca/Mg was 0.95.

(45) The main difference between this mixer and the ploughshare mixer in Examples 1 and 2 is that this mixer is much less closed thereby leading to release of energy from the hydration reaction and hence to a lower temperature during hydration than the temperature measured for the same dolomite and the same water/dolomite ratio in the more closed ploughshare mixer. However the water and dolomite were added in exactly the same manner as in Examples 1 and 2.

(46) This time 800 g of dolime and 320 g of demineralised water (water/dolomite=0.4) were gradually fed into in the mixer over a time of about 20 minutes. On completion of this feed phase the mixture was left under mixing for a hydration time of 60 minutes. The product was then removed from the mixer, dried at 150° C. and analysed to determine the conversion rate of MgO to Mg(OH).sub.2 under these conditions (reaction time=residence time).

(47) In this example hydration was first performed with water without additive to be used as reference, then with 3% sodium silicate relative to the weight of dolomite. The sodium silicate was used in the form of water glass i.e. in the form of a sodium silicate having a molar ratio SiO.sub.2/Na.sub.2O=3.5 in solution in the water at a concentration of 37%.

(48) The results are given in Table 9. They show a low percentage of dolomite 6 hydration without additive compared with the percentages obtained without additives for dolomites 1 to 6 in the preceding examples, probably due to the lower hydration temperature as explained above. On the other hand, the hydration with sodium silicate allowed a much higher MgO hydration rate to be obtained under these same conditions.

(49) TABLE-US-00009 TABLE 9 Final % Initial % tC.sub.MgO increase Humidity.sup.a Mg(OH).sub.2 MgO e.sub.1 (%) e.sub.2 (%) (%) tC.sub.MgO (%) No additive 2.7 1.5 40.9 0.4 13.8 2.9 — +3% Na 5.0 4.8 1.5 12.4 9.4 224

Comparative Example 1

(50) This comparative example is similar to Example 2, but this time it was not an alkaline hydroxide which was used as additive for hydration of dolime 1 but hydrochloric acid. As in Example 2, the water/dolomite weight ratio was kept at 0.4 and the amount of additive at 3% relative to the weight of dolomite. For this purpose, following the protocol in Example 2, a solution prepared with 81 g hydrochloric acid at 37 weight % and 349 g of demineralised water was gradually added to 1 kg of dolomite. After a residence time of 30 minutes in the mixer where hydration took place the product was dried and characterized (reaction time=residence time). The results of this test are given in Table 10 and show a negligible effect of hydrochloric acid on the percentage hydration of the MgO portion of this dolomite (FIG. 2).

Comparative Example 2

(51) This second comparative example is exactly similar to comparative Example 1, but this time the hydrochloric acid was replaced by diethylene glycol (DEG) in liquid form (1 kg of dolime+400 g demineralised water previously mixed with 30 g of liquid DEG). The results are given in Table 10 and again indicate a negligible effect of DEG on the MgO conversion rate of this dolomite (FIG. 2).

(52) TABLE-US-00010 TABLE 10 Initial Final % % e.sub.1 e.sub.2 tc.sub.MgO Mg(OH).sub.2 MgO (%) (%) (%) No additive 3.6 39.5 1.1 13.2 7.4 3% HCl 3.9 1.2 12.9 7.9 3% DEG 4.0 1.2 11.6 8.0

(53) The present invention is evidently in no way limited by the embodiments described in the foregoing and numerous modifications can be made thereto without departing from the scope of the appended claims.