Mineral composition based on a mixed solid phase of calcium and magnesium carbonates and process for preparing such a composition

Abstract

Process for preparing a mineral composition based on a mixed solid phase of calcium and magnesium carbonates, comprising a preparation, in an aqueous phase, of a suspension of a solid phase containing at least one calcium compound selected from calcium hydroxide, calcium carbonate and mixtures thereof and at least one magnesium compound selected from magnesium oxide, magnesium hydroxide, at least partially hydrated dolomite, and mixtures thereof, the weight of said solid phase being between 5% and 15% of the total weight of the suspension, a carbonation of said solid phase in suspension by injecting a gas containing CO.sub.2 into said suspension that is heated at a temperature of 55 C. to 90 C., at a flow rate of CO.sub.2 of 2.5 to 15 dm.sup.3/min/kg of said solid phase of the suspension, with a reduction of the pH of the suspension to a value of less than 9 and with a stabilization of the electrical conductivity of the suspension, the carbonation being stopped as soon as this stabilization of electrical conductivity is observed.

Claims

1. A process for preparing a mineral composition containing a mixed solid phase of calcium and magnesium carbonates, comprising: i) the step of preparing, in an aqueous phase, a suspension of a solid phase containing at least one calcium compound selected from the group consisting of calcium hydroxide, calcium carbonate and the mixtures thereof and at least one magnesium compound selected from the group consisting of magnesium oxide, magnesium hydroxide, at least party slaked dolomite, and the mixtures thereof, the weight of said solid phase being between 5 and 15% of the total weight of the suspension, the said step excluding a case where dolomite alone is being placed in suspension; ii) conducting carbonatation in a single step of said solid phase in suspension by injecting a CO.sub.2-containing gas into said suspension heated to a temperature of 55 to 90 C. at a CO.sub.2 flow rate of 2.5 to 15 dm.sup.3/min/kg of said solid phase of the suspension, with a drop in the pH of the suspension down to a value below 9 and stabilisation of the electrical conductivity of the suspension, carbonatation being halted as soon as this stabilisation of electrical conductivity is observed; and iii) obtaining a mixed solid phase of synthetic calcium and magnesium carbonates formed of a crystallized calcic portion and a plate-like crystallized magnesian portion, the crystals of the calcic portion and those of the magnesian portion being aggregated in the form of composite aggregates, these aggregates themselves being at least partly agglomerated in the form of agglomerates, said calcic portion comprising at least one carbonate selected from the group consisting of calcite and mixtures of calcite and aragonite, said magnesian portion comprising hydromagnesite in plate-like form, said aggregates being formed of a calcic core on which hydromagnesite plates are aggregated, said mixed solid phase of carbonates of said mineral composition having a thermal conductivity of 25 to 45 mW/K/m for a bulk density of 250 kg/m.sup.3 or lower and of 80 kg/m.sup.3 or higher measured in accordance with standard EN 459.2 and a Ca/Mg molar ratio higher than 1.2 and equal to or less than 4.0.

2. The process according to claim 1, characterized in that the solid phase of the suspension has a Ca/Mg molar ratio of 3.0 or lower.

3. The process according to claim 1, characterized in that said at least one magnesium compound is selected from the group consisting of magnesium oxide, magnesium hydroxide and the mixtures thereof.

4. The process according to claim 1, characterized in that said at least one magnesium compound is at least partly slaked dolomite, optionally in a mixture with magnesium oxide, magnesium hydroxide or a mixture thereof, so as to obtain a Ca/Mg molar ratio higher than 1.2 and of 4.0 or lower in said solid phase of the suspension.

5. The process according to claim 1 characterized in that, at said preparing of a suspension, it comprises slaking by said aqueous phase of calcium oxide (CaO) to said calcium hydroxide.

6. The process according to claim 1 characterized in that, before said preparation of a suspension, it comprises preliminary at least partial slaking of calcium oxide (CaO) to said calcium hydroxide.

7. The process according to claim 6, characterized in that said preliminary slaking is dry slaking or near-dry slaking.

8. The process according to claim 6, characterized in that said preliminary slaking is wet slaking to produce a milk of lime which is then optionally screened, milled and/or diluted for said preparation of the suspension.

9. The process according to claim 6, characterized in that said preliminary slaking is performed with water, water containing at least one additive or water already containing at least a portion of said at least one magnesium compound that is part of said solid phase in suspension.

10. The process according to claim 1 characterized in that, at said preparation of a suspension, it comprises at least partial slaking by said aqueous phase of magnesium oxide (MgO) to said magnesium hydroxide.

11. The process according to claim 1 characterized in that, before said preparation of a suspension, it comprises prior at least partial slaking of magnesium oxide (MgO) to said magnesium hydroxide.

12. The process according to claim 11, characterized in that said prior slaking is dry or near-dry slaking.

13. The process according to claim 11, characterized in that said prior slaking is wet slaking so as to produce a milk of magnesia which is optionally screened, milled and/or diluted for said preparation of the suspension.

14. The process according to claim 11, characterized in that said prior slaking is performed with water, water containing at least one additive or water already containing at least a portion of said at least one calcium compound that is part of said solid phase in suspension.

15. The process according to claim 1 further comprising, after carbonatation, a solid-liquid separation step between said carbonatated solid mixed phase and said aqueous phase of said suspension.

16. The process according to claim 15 wherein said mixed solid phase is subsequently at least partly dried.

17. The process according to claim 1 wherein said mixed solid phase is milled to obtain particles of size 2 mm or smaller.

18. The process according to claim 1 wherein said carbonatation temperature is between 60 and 80 C.

19. The process according to claim 1, characterized in that the gas injected into said suspension contains CO.sub.2 at a concentration of 10 to 100% by volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Appended FIGS. 1, 5, 8 and 10 give the curves of pH and electrical conductivity changes throughout the carbonatation reaction in examples 1, 4, 6 and 7.

(2) FIGS. 2, 3, 4A, 4B, 6, 7, 9 and 11 are scanning electron microscope (SEM) images illustrating the crystalline structure of the compositions obtained in Examples 1 to 7.

DETAILED DESCRIPTION OF THE INVENTION

(3) In the examples, bulk density was measured in accordance with standard EN 459.2. When measuring thermal conductivity, which is dependent on the extent of packing of the sample, a second non-normalised density value is obtained which must be indicated to have knowledge of the measurement conditions of thermal conductivity.

(4) Thermal conductivity was measured on bulk material following the teachings of standards ISO 8301 or NF EN 12664. More specifically, thermal conductivity such as indicated in the present invention was measured on the mixed solid phase of the mineral composition obtained according to the invention i.e. on a powder previously dried at 105 C. and having humidity lower than 4 weight %. If the composition of the invention was in the form of a suspension or paste, said powder was obtained by filtration, drying and grinding of this composition.

(5) Measurement was carried out in a flow meter (Netzsch Heat Flow Meter HFM 463/3IE Lambda Series) on bulk powder placed in a mould of larger size than the flow meter. The preparation of the powder bed in the mould was such that the surface of the powder bed was as flat as possible and the density of the powder bed, on which thermal conductivity was to be measured, was equal to the bulk density previously measured on each powder following the method described in standard EN 459.2 to within 20%, if possible to within 15%. A single measurement was made at 20 C. with a 20 C. temperature difference between the two sides of the sample (i.e. 10 C. for one side and 30 C. for the other).

(6) The specific surface area was measured using the BET method based on measurement of nitrogen adsorption manometry, and <100 m pore volume was measured by mercury intrusion.

Example 1: Carbonatation at 70 C. of a Suspension of Magnesium Hydroxide and Ground Calcium Carbonate with Ca/Mg (Mol) in the Order of 0.5

(7) A/ Raw Materials

(8) The ground calcium carbonate and magnesium hydroxide were industrial products. According to chemical analysis by X-ray fluorescence spectrometry with additional thermogravimetric analysis, the magnesium hydroxide contained 95.00.5% Mg(OH).sub.2, <1.0% MgO, 2.50.5% CaCO.sub.3, and impurities chiefly SiO.sub.2, Al.sub.2O.sub.3 and Fe.sub.2O.sub.3. It was in the form of a suspension containing 53% solids and the remainder water. The ground calcium carbonate contained >95% CaCO.sub.3 and impurities again chiefly SO.sub.2, Al.sub.2O.sub.3, Fe.sub.2O.sub.3. This calcium carbonate was a natural limestone which was simply industrially ground. Laser measurement of particle size distribution gave a d.sub.98 value in the order of 12 m (2 m).

(9) B/ Preparation of the Suspension

(10) 4.9 of ground calcium carbonate and 5.8 kg of magnesium hydroxide (dry equivalent) were placed in suspension in about 80 dm.sup.3 water (water provided by the suspension of Mg(OH).sub.2 completed with mains water) to obtain a mixed suspension. The weights of ground calcium carbonate and magnesium hydroxide were determined to obtain a Ca/Mg molar ratio in the order of 0.5. The dry extract (solid content) of this suspension was 10.7 weight %. This suspension was placed in a reactor of 100 dm.sup.3 capacity equipped with a double jacket and provided with an agitation system and heating system. This reactor was also equipped with a pH probe, electrical conductivity probe and temperature probe. The suspension was kept under agitation and the temperature brought to 70 C.2 C.

(11) C/ Carbonatation

(12) When the suspension reached the temperature of 70 C.2 C., injection of CO.sub.2 was initiated in the form of a gas composed of 15% by volume CO.sub.2 and 85% by volume air. The CO.sub.2 carrier gas was injected via a nozzle located at the bottom of the reactor. The gas flow rates in the suspension corresponded to 55 dm.sup.3/min of CO.sub.2 and 314 dm.sup.3/min of air i.e. 5.1 dm.sup.3/min CO.sub.2 and 29.3 dm.sup.3/min air per kg of solid in the starting suspension (ground calcium carbonate and magnesium hydroxide). Throughout carbonatation the electrical conductivity of the suspension and its pH were recorded. The pH of the suspension before carbonatation was 9.0. Throughout carbonatation the conversion of magnesium hydroxide to carbonate was accompanied by a) a reduction in pH and b) an increase up to a maximum followed by a decrease and then stabilisation of electrical conductivity (FIG. 1). The reaction was halted after a stabilisation time of 1 hour of electrical conductivity at 3.6 mS/cm. After this carbonatation, the pH of the suspension was only 8.0. The increase in electrical conductivity at the start of the carbonatation reaction indicates the formation of soluble magnesium bicarbonate. The following decrease corresponds to precipitation of the hydromagnesite.

(13) D/ Post-Synthesis Treatments

(14) After carbonatation, the distribution of particle size in suspension was measured by laser particle size measurement by placing a few drops of the suspension in methanol. The d.sub.50 of this suspension was 11.6 m and its d.sub.95 26.3 m. The suspension was then filtered through a Bchner, placed in an oven at 105 C. to dry for 48 hours and the dried solid was milled to obtain a powder of particle size smaller than 2 mm and having humidity of less than 3%.

(15) According to X-ray diffraction analysis with additional thermogravimetric analysis the product obtained contained 562% CaCO.sub.3 (calcite), 402% hydromagnesite and 41% brucite Mg(OH).sub.2. Its bulk density measured according to standard EN 459.2 was 173 kg/m.sup.3. The specific surface area was 16.5 m.sup.2/g (BET method based on nitrogen adsorption manometry after degassing a few hours at 190 C.). Thermal conductivity was 37.2 mW/K/m for a density at the time of measurement of 185 kg/m.sup.3. FIG. 2 is a scanning electron microscope image of the product obtained. This photograph shows that the (plate-like) hydromagnesite formed by carbonatation of the magnesium hydroxide has crystallized on the surface of the particles of ground calcium carbonate.

Example 2: Carbonatation at 70 C. of a Suspension of Magnesium Hydroxide and Ground Calcium Carbonate with Ca/Mg (Mol) in the Order of 0.9

(16) This example is fully similar to Example 1, solely the weights of ground calcium carbonate and magnesium hydroxide are different.

(17) This time 6.5 kg of ground calcium carbonate and 4.2 kg of magnesium hydroxide (dry equivalent) were placed in suspension in about 80 dm.sup.3 water (water provided by the suspension of Mg(OH).sub.2 completed with mains water) to obtain a mixed suspension. The weights of ground calcium carbonate and magnesium hydroxide were determined to obtain a Ca/Mg molar ratio this time in the order of 0.9. The dry extract (solid content) of this suspension was 11.3%.

(18) This suspension was placed in the same reactor as used in Example 1 and carbonatation was conducted under exactly similar conditions to those of the carbonatation described in Example 1. As described in Example 1, the electrical conductivity of the suspension increased right at the start of the carbonatation reaction, reached a maximum before showing a distinct decrease and then stabilising. Again the reaction was halted after stabilisation of the electrical conductivity of the suspension for 40 minutes at 4.0 mS/cm (0.1 mS/cm). At this point the pH of the suspension initially of 9.1 dropped to 8.1.

(19) After carbonatation, the particle size distribution in the suspension was again measured and showed a d.sub.50 value of 12.4 m and d.sub.95 value of 30.1 m. The suspension was then filtered, dried and the solid obtained was milled as in Example 1.

(20) According to X-ray diffraction analysis with additional thermogravimetric analysis, the product obtained contained 702% CaCO.sub.3 (calcite), 262% hydromagnesite and 41% brucite. It bulks density measured according to standard EN 459.2 was 211 kg/m.sup.3. The specific surface area was 12.3 m.sup.2/g. Thermal conductivity was 38.1 mW/K/m for a density at the time of measurement of 225 kg/m.sup.3. FIG. 3 illustrates the product obtained. This photograph, as in FIG. 2, shows that the hydromagnesite formed by carbonation of magnesium hydroxide has crystallized on the surface of the particles of ground calcium carbonate.

Example 3: Carbonatation at 70 C. of a Suspension of Magnesium Hydroxide and Ground Calcium Carbonate with Ca/Mg (Mol) in the Order of 1.6

(21) This example is fully comparable with Examples 1 and 2. The weights of ground calcium carbonate and magnesium hydroxide were again different and the ground calcium carbonate was derived from another source this time with a d.sub.98 in the order of 36 m.

(22) This time, 7.9 kg of ground calcium carbonate and 2.8 kg of magnesium hydroxide (dry equivalent) were placed in suspension in about 80 dm.sup.3 of water (water provided by the suspension of Mg(OH).sub.2 completed with mains water) to obtain a mixed suspension. The weights of ground calcium carbonate and magnesium hydroxide were determined this time to obtain a Ca/Mg molar ratio in the order of 1.6. The dry extract (solid content) of this suspension was 12.5%.

(23) This suspension was placed in the same reactor as used in the preceding examples and carbonatation was performed under exactly similar conditions to those described in Examples 1 and 2. As previously, the electrical conductivity of the suspension increased right at the start of the carbonatation reaction, reached a maximum before distinctly decreasing and then stabilising. The reaction was halted when the electrical conductivity was stable at 3.4 mS/cm (0.2 mS/cm) for 30 minutes. At this moment the pH of the suspension, initially 9.4, dropped to 8.3.

(24) After carbonatation the distribution of particle size in suspension was again measured and gave a d.sub.50 value of 9.3 m and d.sub.95 value of 22.7 m. The suspension was then filtered, dried and the solid obtained was milled as in the above examples.

(25) According to X-ray diffraction analysis with additional thermogravimetric analysis the product obtained contained 592% CaCO.sub.3 (calcite), 372% hydromagnesite and about 4%1% brucite Mg(OH).sub.2. Its bulk density measured according to standard EN 459.2 was 214 kg/m.sup.3. The specific surface area was 11.5 m.sup.2/g (BET method based on nitrogen adsorption manometry). Thermal conductivity was 37.6 mW/K/m for a density at the time of measurement of 222 kg/m.sup.3. FIGS. 4A and 4B illustrate the product obtained. These photographs show that the product is homogeneous and that the hydromagnesite plates again cover the particles of ground calcium carbonate.

Example 4: Carbonatation at 60 C. of a Suspension of Dolomite and Ground Calcium Carbonate with Ca/Mg (Mol) in the Order of 1.6

(26) This example was inspired from the preceding examples but this time the ground calcium carbonate was placed in suspension not with a suspension of Mg(OH).sub.2 but with a previously prepared suspension of at least partly slaked dolomite.

(27) At a first step 5.8 kg of dolomitic quicklime of particle size 0-3 mm were placed in 23.0 kg of water heated to 55 C. This suspension was placed under agitation for 30 minutes, time during which the dolomite was at least partly slaked. After a slaking time of 30 minutes, an additional 20.0 kg of water and 3.0 kg of ground calcium carbonate (same GCC as in Example 3) were added and agitation continued for 20 minutes. The weights of dolomite and ground calcium carbonate were determined to obtain a Ca/Mg ratio of 1.5. The suspension obtained was passed through a 200 m screen to remove the coarsest particles (in particular unfired particles initially contained in the dolomitic quicklime). The screened suspension was then diluted to a dry extract (solid content) of 9.4% and 80 dm.sup.3 of this suspension were placed in the 100 dm.sup.3 reactor already used in all the preceding examples.

(28) Carbonatation this time was conducted at 60 C. The CO.sub.2 flow rate was 45 dm3/min and the air flow rate 180 dm.sup.3/min, i.e. a volume concentration of 20% CO.sub.2 in the gas, i.e. also about 6.0 dm.sup.3/min CO.sub.2 and 24.0 dm.sup.3/min air per kg of solid contained in the suspension before carbonatation.

(29) Throughout carbonatation (FIG. 5), the electrical conductivity of the suspension started by decreasing and this decrease was accompanied by a drop in pH from 12.2 to 9.4. These results are proof of carbonatation of the calcic portion of the at least partly slaked dolomite to form a calcium carbonate (PCC) in situ in addition to the starting ground calcium carbonate (GCC). After some slowing of the reaction, the electrical conductivity again slowly increased before finally stabilising for 20 minutes at 8.0 mS/cm (0.1 mS/cm). This second phase of the reaction was accompanied by a second drop in pH down to 8.1 and translated the competition between formation of hydromagnesite and dissolution of Mg(OH).sub.2 (and possibly of already-formed hydromagnesite) to bicarbonate ions of Mg.

(30) After carbonatation, the distribution of particle size in suspension was again measured and gave a d.sub.50 value of 8.8 m. The suspension was then filtered, dried and the solid obtained was milled as in the above examples.

(31) According to X-ray diffraction analysis with additional thermogravimetric analysis the product obtained contained 642% CaCO.sub.3 (calcite), 322% hydromagnesite, the remainder being composed of Mg(OH).sub.2 and Mg.sub.3O.sub.2(OH).sub.2. Its bulk density measured according to standard EN 459.2 was 161 kg/m.sup.3. The specific surface area was 17.7 m.sup.2/g (BET method based on nitrogen adsorption manometry). Thermal conductivity was 36.4 mW/K/m for a density at the time of measurement of 177 kg/m.sup.3. FIG. 6 illustrates the product obtained. This photograph again shows that the crystallization hydromagnesite plates takes place on the surface of the particles of CaCO.sub.3 (PCC formed in situ and GCC).

Example 5: Carbonation at 60 C. of a Suspension of Dolomite and Ground Calcium Carbonate with Ca/Mg (Mol) in the Order of 2.0

(32) This example is fully comparable with Example 4 above but this time with a Ca/Mg ratio in the order of 2.0.

(33) At a first step 4.7 kg of the same dolomitic quicklime used in Example 4 were placed in 19 kg of water heated to 55 C. This suspension was again left under agitation for 30 minutes. After the slaking time of 30 minutes, an additional 20 kg of water and 4.7 kg of ground calcium carbonate (GCC the same as in Example 4) were added and agitation continued for 20 minutes. As previously, the suspension obtained was subjected to 200 m screening. It was then diluted to a dry extract of 11.4%. 80 dm.sup.3 of this suspension were placed in the 100 dm.sup.3 reactor.

(34) Carbonatation was again performed at 60 C. with the same flow rates of CO.sub.2 and air. The dry extract of the suspension being different from that of the suspension in Example 4, these flow rates corresponded this time to 4.9 dm.sup.3/m CO.sub.2 and 19.7 dm.sup.3/min air per kg of solid contained in the suspension before carbonatation.

(35) Throughout carbonatation, the changes in electrical conductivity of the suspension and in pH were similar to those described in Example 5. The reaction was halted when the electrical conductivity stabilised for 20 minutes at 8.0 mS/cm (0.2 mS/cm).

(36) After carbonatation the distribution of particle size in suspension was again measured and led to a d50 value of 7.6 m. The suspension was then filtered, dried and the solid obtained was milled as in the above examples.

(37) According to X-ray diffraction analysis followed by thermogravimetric analysis, the product obtained contained 712% CaCO.sub.3 (calcite), 262% hydromagnesite, the remainder being composed of Mg(OH).sub.2 and Mg.sub.3O.sub.2(OH).sub.2. Its bulk density measured according to standard EN 459.2 was 247 kg/m.sup.3. The specific surface area was 12.6 m.sup.2/g (BET method based on nitrogen adsorption manometry). Thermal conductivity was 38.7 mW/K/m for a density at the time of measurement of 244 kg/m.sup.3. FIG. 7 illustrates the product obtained.

Example 6: Carbonatation at 60 C. of a Suspension Prepared by Slaking a Quicklime with a Suspension of Mg(OH)2 (with Ca/Mg (Mol) in the Order of 1.2)

(38) This example was similar to Examples 4 and 5 above in that the conditions of carbonatation of the suspension were identical. On the other hand, the suspension prepared before carbonatation was different.

(39) In this example the suspension to be carbonatated was obtained, as provided by one embodiment of the invention, by preliminary slaking of quicklime (CaO) in a suspension of Mg(OH).sub.2. No ground calcium carbonate (GCC) was used for preparing the suspension. The suspension of Mg(OH).sub.2 was the same as the one used above in Examples 1 to 3. This suspension of Mg(OH).sub.2 was diluted with water to obtain a dry extract of 20.0%. 4.8 kg of quicklime (obtained by calcining a calcium carbonate) were mixed with 20.0 kg of this diluted suspension of Mg(OH).sub.2. This corresponded to 4.8 kg of quicklime CaO, 4.0 kg of Mg(OH).sub.2 and 16.0 kg of water, i.e. a Ca/Mg molar ratio of about 1.2. This mixture was left under agitation for about 30 minutes during which the quicklime was slaked with the water derived from the suspension of Mg(OH).sub.2. On completion of this step, the solid phase in suspension was mostly composed of Ca(OH).sub.2 and Mg(OH).sub.2 with traces of CaCO.sub.3 (unfired particles in the quicklime and/or in Mg(OH).sub.2) and of non-slaked CaO and impurities. The suspension was passed through a 200 m screen then diluted to a dry extract 6.9%. 80 dm.sup.3 of this diluted suspension were placed in the 100 dm.sup.3 reactor. Carbonatation was again performed at 60 C. with 45 dm.sup.3/min CO.sub.2 and 180 dm.sup.3/min air i.e. 8.2 dm.sup.3/min CO.sub.2 and 32.6 dm.sup.3/air per kg of solid phase in suspension.

(40) As in Examples 4 and 5, throughout carbonatation (FIG. 8) the electrical conductivity of the suspension started by decreasing strongly and this decrease was accompanied by a drop in pH from 12.5 to 9.3. These results were the sign of in situ carbonatation of the calcic portion of the solid phase i.e. of the slaked lime Ca(OH).sub.2 to CaCO.sub.3 (PCC). The electrical conductivity then increased rapidly due to dissolution of the magnesium portion (Mg(OH).sub.2 to bicarbonate ions of Mg. After a certain time, the electrical conductivity again decreased, a sign of hydromagnesite precipitation. Finally, the electrical conductivity stabilised and the reaction was halted after 1 hour at 9.7 mS/cm (0.2 mS/cm). This second phase of the reaction corresponding to the reaction of the magnesium portion was accompanied by a second drop in pH to 7.9.

(41) After carbonatation, the d.sub.50 of this carbonated suspension was 8.1 m. The suspension was then filtered, dried and the solid obtained was milled as in the preceding examples.

(42) According to X-ray diffraction analysis with additional thermogravimetric analysis, the product obtained contained 512% CaCO.sub.3 (calcite), 452% hydromagnesite, the remainder being composed of Mg(OH).sub.2 and Mg.sub.3O.sub.2(OH).sub.2. Its bulk density measured in accordance with standard EN 459.2 was 152 kg/m.sup.3. The specific surface area was 17.0 m.sup.2/g. Thermal conductivity was 36.4 mW/K/m for a density at the time of measurement of 154 kg/m.sup.3. FIG. 9 is a scanning electron microscope image of the product obtained. This photograph shows well crystallized hydromagnesite plates.

Example 7: Carbonatation at 60 C. of a Magnesian Lime with Ca/Mg (Mol) in the Order of 2.9

(43) In this example the raw material was a magnesian limestone containing 75.6% CaCO.sub.2, 22.1% MgCO.sub.3, the remainder being impurities (chiefly SiO.sub.2 but also Al.sub.2O.sub.3 and Fe.sub.2O.sub.3). The molar ratio between Ca and Mg in this case was therefore in the order of 2.9. This limestone (calibre 5-15 mm) was calcined for 2 h at 980 C. in a laboratory electric oven. After calcining, partly calcined magnesian lime was obtained. During calcining, while decarbonatation of MgCO.sub.3 to MgO was complete, that of CaCO.sub.3 to CaO was only incomplete. The calcined product therefore contained MgO, CaO, CaCO.sub.3 and impurities, the CaCO.sub.3 content being in the order of 44% after thermogravimetric analysis of the calcined sampled (950 C. at 5 C./min under nitrogen).

(44) About 6.5 kg of this calcined product were placed in 19.5 dm.sup.3 water previously heated to 55 C. and the slaking reaction was continued for about 1 hour after agitation (300 rpm). After this slaking reaction, the suspension contained Mg(OH).sub.2 and Ca(OH).sub.2 produced by slaking of MgO and CaO, but also non-slaked magnesium oxide MgO, the CaCO.sub.3 derived from incomplete calcining of the starting magnesian limestone and the impurities contained in this source of magnesian limestone. The suspension obtained was passed through a 200 m screen and diluted to obtain a dry extract (solid content) of 7.6% by weight. 80 cm.sup.3 of this suspension were placed in the reactor of 100 dm.sup.3 capacity.

(45) Carbonatation was again performed at 60 C. with 45 dm.sup.3/min CO.sub.2 and 180 dm.sup.3/min air, i.e. 7.4 .sup.dm3/min CO.sub.2 and 29.6 dm.sup.3/min air per kg of solid phase in suspension.

(46) The onset of changes in electrical conductivity of the suspension and in pH occurred in similar manner to Example 6 i.e. with distinct simultaneous decrease in electrical conductivity and drop in pH corresponding to precipitation of calcium carbonate (PCC) from the slaked lime contained in the suspension (FIG. 10). At the end of this first step the pH of the suspension was about 9.5.

(47) Then again in similar manner to Example 6, the conductivity rapidly increased due to production of bicarbonate ions of Mg. While in Example 6 (FIG. 8) the conductivity again decreased before stabilising, here (FIG. 10) no decrease was observed and conductivity gradually stabilised after the increase related to the bicarbonate ions. This difference between this Example 7 and Example 6 is probably related to the reactivity of the Mg sources.

(48) After carbonatation, the d.sub.50 of this suspension was 7.0 m. The suspension was then filtered, dried and the solid obtained milled as in the preceding examples.

(49) According to X-ray diffraction analysis with additional thermogravimetric analysis the product obtained contained 682% CaCO.sub.3 (calcite), 282% hydromagnesite, the remainder being composed of Mg(OH).sub.2 and Mg.sub.3O.sub.2(OH).sub.2. Its bulk density measured in accordance with standard EN 459.2 was 221 kg/m.sup.3. The specific surface area was 17.4 m.sup.2/g. Thermal conductivity was 37.4 mW/K/m for a density measured at the time of measurement of 213 kg/m.sup.3. FIG. 11 is scanning electron microscope image of the product obtained. This photograph shows well-crystallized hydromagnesite plates.

(50) The invention is evidently in no way limited to these examples and numerous modifications can be made thereto without departing from the scope of the appended claims.