Mixed calcium and magnesium compound and method for producing same

Abstract

A method for producing a mixed calcium and magnesium compound comprising the slaking of quicklime with a magnesium hydroxide suspension, forming solid particles, said slaking by non-wet means forming said solid particles comprising a calcium phase and a magnesium phase intimately bonded to each other and of homogeneous volume distribution, and a mixed compound comprising a calcium phase and a magnesium phase.

Claims

1. A method for manufacturing a composition of mixed calcium and magnesium hydrates comprising slaking of quicklime with, an aqueous medium in a hydrator having an inlet and an outlet, with, formation of solid particles, characterized in that said aqueous medium is a suspension of magnesium hydroxide provided before the slaking of the quicklime for forming said solid particles of mixed hydrates of the formula xCa(OH).sub.2.yMg(OH).sub.2.zI in a solid and powdery form having a humidity of less than 4% by weight at the outlet of the hydrator, and comprising a calcium phase and a magnesium phase, intimately bound and with a homogenous volume distribution, a formula in which x, y and a are weight fractions with x+y comprised between 88 and 100% by weight based on the total weight of the mixed hydrates, in which I represents impurities and wherein the proportion of the Ca(OH).sub.2 based on the mass of Mg(OH).sub.2 in the mixed hydrates is comprised between 80 and 25,000%.

2. The method according to claim 1, wherein, before said slaking of quicklime, said magnesium hydroxide suspension is prepared by suspending in water a predetermined amount of magnesium hydroxide comprised between 1 and 55% by weight, based on the weight of said magnesium hydroxide suspension.

3. The method according to claim 1, further comprising drying said solid particles, optionally followed by a de-agglomeration step.

4. The method according to claim 1, further comprising determining a grain size cut-off.

5. The method according to claim 1, wherein said magnesium hydroxide amount is comprised between 5 and 53% by weight, based on the total weight of the magnesium hydroxide suspension.

6. The method according to claim 1, wherein said aqueous medium further comprises an additive which increases the specific surface area of Ca(OH).sub.2 particles of said mixed hydrates.

7. A method for manufacturing a composition of mixed calcium and magnesium hydrates, said method comprising slaking of quicklime with an aqueous medium in a hydrator having an inlet and an outlet, with formation of solid particles, characterized in that said aqueous medium is a suspension of magnesium hydroxide provided before the slaking of the quicklime for forming said solid particles of mixed hydrates of the formula xCa(OH).sub.2.yMg(OH).sub.2.zI in a solid and powdery form having a humidity comprised between 15 and 30% by mass at the outlet of the hydrator, and comprising a calcium phase and, a magnesium phase, intimately bound and with a homogenous volume distribution, a formula in which x, y and z are weight fractions with x+y comprised between 88 and 100% by weight based on the total weight of the mixed hydrates, in which I represents impurities and wherein the proportion of Ca(OH).sub.2 based on the mass of Mg(OH).sub.2 in the mixed hydrates is comprised between 50 and 12,500%.

8. The method according, to claim 7, wherein, before said slaking of quicklime, said magnesium hydroxide suspension is prepared by suspending in water a predetermined amount of magnesium hydroxide comprised between 1 and 55% by weight, based on the weight of said magnesium hydroxide suspension.

9. The method according to claim 7, further comprising drying said solid particles, optionally followed by a de-agglomeration step.

10. The method according to claim 7, further comprising determining s grain size cut-off.

11. The method according to claim 7, wherein said magnesium hydroxide amount is comprised between 5 and 53% by weight, based on the total weight of the magnesium hydroxide suspension.

12. The method according to claim 7, wherein said aqueous medium further comprises an additive which increases the specific surface area of Ca(OH).sub.2 particles of said mixed hydrates.

Description

(1) Other features, details and advantages of the invention will become apparent from the description given hereafter, not as a limitation and with reference to the appended drawings and examples.

(2) FIG. 1 is a graph showing the thermogravimetric curves recorded for the products of Examples 1 and then 3 to 6 (temperature from 20 to 950 C. with a ramp of 2 C./min, in air).

(3) FIG. 2 is a graph showing the thermogravimetric curves recorded for the products of Examples 7 to 9 (temperature from 20 to 950 C. with a ramp of 5 C./min, in air).

(4) In the figures, identical or similar elements bear the same references.

(5) The method according to present invention may be described in different successive steps summarised as follows: i) preparation of a Mg(OH).sub.2 milk, optionally carried out in an independent way; ii) hydration of quicklime with the thereby prepared Mg(OH).sub.2 milk; iii) optional drying and optional de-agglomeration of the product, in the case of slaking with an excess of water iv) grain size control.

(6) Step ii) for hydration of quicklime is carried out in a quite standard hydrator for slaking quicklime via a dry route.

(7) In the method according to the invention, the hydrator may either be a single-stage hydrator, i.e. consisting of a single horizontal cylindrical reactor equipped with a central axis and stirring vanes, this reactor may either be provided or not with another horizontal cylinder used as a pre-mixer, or a multi-stage hydrator for example consisting of three successive horizontal reactors with increasing diameter, in which the product gradually falls during the hydration reaction.

(8) The hydrator is fed with powdery quicklime, having a particle size comprised between 20 m and 10 mm, preferably <5 mm, preferably <2 mm. The quicklimes are customarily characterized by their chemical purity and by their reactivity.

(9) By high purity, is meant a low level of impurities, i.e. generally less than 5%, advantageously less than 4% by weight and preferably less than 3%, or even less than 2% by weight of conventional impurities which are found at the beginning in the limestone (MgO, SiO.sub.2, Al.sub.2O.sub.3, . . . ), but also a high available lime content. The term <<available lime>> commonly represents the fraction of quicklime which is actually in the form of CaO and/or Ca(OH).sub.2 according to the standard EN 459-2: 2010 paragraph 5.8 or ASTM C25 standard of 1994. This excludes all the other possible forms of calcium such as the unfired substances (CaCO.sub.3) or the compounds of the calcium silicate or aluminate types. The quicklimes used in the method according to the invention contain more than 90% by weight, advantageously more than 93% by weight, preferably more than 96% by weight and more advantageously more than 97% by weight of available lime. The proportion of unfired substances in the quicklimes used in the method according to the invention is normally less than 3%, advantageously less than 2%, preferably less than 1% by weight.

(10) The reactivity of quicklime is characterized by the reactivity test described in the EN 459-2: 2010 paragraph 6.6 standard and in particular by the value of t.sub.60. The limes used in the method according to the invention have a t.sub.60 in the range from 0.3 to 8 minutes, preferably comprised between 0.5 and 5 minutes.

(11) The quicklime flow rate used in the method according to the invention is similar to the quicklime flow rate customarily used for slaking in a conventional dry route during which the lime is only slaked with water with the purpose of producing a hydrated and powdery lime.

(12) The Mg(OH).sub.2 milk flow rate used in the method according to the invention varies according to the humidity level of Mg(OH).sub.2 reserved for the final product. At the outlet of the hydrator, the humidity of the product is comprised between 1 and 30% by weight. The more humid is the product which leaves the hydrator, the higher will be its specific surface area and its porous volume.

(13) The final humidity of the product at the outlet of the hydrator is set according to the desired specific surface area and porous volume. The amount of water to be added to the quicklime for obtaining this humidity is then determined on taking into account the evaporation related to the exothermic nature of the quicklime hydration reaction. All the water required for hydrating the quicklime is brought by the Mg(OH).sub.2 milk. Next, depending on the desired respective proportions of Ca(OH).sub.2 and Mg(OH).sub.2, in the final product, the amount of Mg(OH).sub.2 to be added to the quicklime is calculated.

(14) For a given hydration duration, it is then possible to calculate the amount of water and the amount of Mg(OH).sub.2 to be added to the quicklime amount which will be introduced into the hydrator. These amounts of water and of Mg(OH).sub.2 are put into contact before hydration, either by using a pre-existing (either diluted or not) Mg(OH).sub.2 milk in order to have the desired masses of water and of the Mg(OH).sub.2, or by dispersing the desired amount of Mg(OH).sub.2 powder into the desired amount of water. The thereby prepared milk is well homogenized during its preparation and then it is pumped in order to be injected into the hydrator, on the quicklime, either through simple orifices, or through spray nozzles adapted to the diameter of the Mg(OH).sub.2 particles in order to avoid clogging of the latter. During the whole hydration duration, the Mg(OH).sub.2 milk is maintained with stirring in order to avoid any sedimentation of Mg(OH).sub.2 particles. Regular samplings are carried out at the output of the hydrator and the humidity of these samples is measured. If this humidity corresponds to the desired humidity, no additional adjustment is required. If this humidity is too low, it is possible to increase the Mg(OH).sub.2 milk flow rate or to decrease the quicklime flow rate. If the humidity is too high, the Mg(OH).sub.2 milk flow rate may be decreased or that of quicklime may be increased.

(15) If the product which leaves the hydrator has a humidity of less than 2% or even less than 4%, it may directly pass to the grain size control step iv).

(16) On the other hand, if its humidity is comprised between 4 and 30%, the product has to be dried. It has to be de-agglomerated at the same time since such an excess of humidity leads to more or less pronounced agglomeration of the calcium hydroxide particles (Ca(OH).sub.2). This drying and de-agglomeration step is carried out industrially, preferably in a milling machine of the <<cage mill>> type. Other pieces of equipment, notably of the <<flash dryer>> type may be used. After this step, the product should contain less than 2% humidity, preferably less than 1% humidity and its particles should have a size of less than 1 mm, preferably less than 500 m and advantageously less than 200 m, which means that d.sub.97 has to be less than the aforementioned sizes.

(17) In most cases, the hydrated limes or the semi-hydrated or totally standard hydrated dolomites pass through a grain size control before being used in the various applications. This grain size control gives the possibility of removing the coarsest particles (notably the unfired substances) in order to obtain a reactive hydrate for the contemplated application. In the case of the method according to the invention, a similar grain size control step is desirable. This step is carried out industrially in air separators; it may be performed by sifting. The cut-off is generally made with the goal of having a product <250 m, preferably <200 m, preferably <90 m, or even sometimes <60 m.

(18) The product obtained with the method according to the invention is a mixed product based on calcium and magnesium, both of these elements being found in a large majority in their hydroxide forms (Ca(OH).sub.2) and Mg(OH).sub.2). The Mg(OH).sub.2 which is found in the final product from the method according to the invention has the same characteristics as before the hydration when it is put in the form of milk. Indeed, the Mg(OH).sub.2 is not altered during the hydration reaction and therefore retains the same criteria of chemical purity and porosity as those described above. The Ca(OH).sub.2 which is found in the final product as for it is the product of the hydration reaction which occurs between the quicklime and the water brought into the hydrator by the Mg(OH).sub.2 milk.

(19) The final product may be described by a general formula of the type xCa(OH).sub.2.yMg(OH).sub.2.zI, Ca(OH).sub.2 in majority representing said calcium phase and Mg(OH).sub.2 representing the said magnesium phase. Both of these phases are intimately bound and with a homogeneous volume distribution. X, y and z are weight fractions. Y is more particularly comprised between 0.4 and 58%, preferably between 3 and 53%, advantageously between 5 and 44% and more advantageously between 10 and 30%. However, it is important to note that the sum of x and y is comprised between 88 and 100% by weight based on the total weight of the mixed compound and is typically not equal to 100%. Indeed, in addition to the ultra-majority fractions Ca(OH).sub.2 and Mg(OH).sub.2, the final product contains impurities brought by the Mg(OH).sub.2 milk as well as impurities and unfired substances (CaCO.sub.3) brought by the quicklime represented by I. Moreover, it is possible that the hydration of the quicklime is not complete in the method according to the invention, thereby leading to the presence of residual quicklime CaO in the final product. The residual CaO content in the final product is, however, as low as possible and is also comprised in I, which is comprised between 0.1 and 3%, preferably less than 2% and advantageously less than 1% by weight. The MgO content, also comprised in I, in the final product, as for it, is less than 2%, preferably less than 1% and in particular less than 0.5% by weight. Preferentially, the contents of impurities and unfired substances are as low as possible and the sum of x and y is greater than 90%, preferably 92%, advantageously 95%, in particular 97% and extremely preferentially greater than 98%. It should be noted that when the product is obtained through a quasi-dry route, there is partial carbonation of the calcium phase during the drying of the mixed compound according to present invention, which increases the value of the weight fraction z as compared with a mixed compound obtained via a dry route.

(20) In practice, the proportion y of Mg(OH).sub.2 in the final product depends on the humidity of the product at the outlet of the hydrator. Indeed, it has been stated above that all the water required for the hydration reaction is brought by the Mg(OH).sub.2 milk which has a maximum concentration of the order of 55% by weight. Therefore, in order to reach a given humidity at the outlet of the hydrator, a given amount of water has to be added to the quicklime for the hydration reaction and the amount of added Mg(OH).sub.2 is then at most equal to 122% of this amount of water (since the milk contains at most 55% of Mg(OH).sub.2 and 45% of water). The proportions of y expressed in the paragraph above correspond to a product which at the outlet of the hydrator has a humidity comprised between 1 and 30% by weight.

(21) As regards this humidity, it is comprised between 1 and 30% by weight at the outlet of the hydrator, with however two preferred families of products.

(22) The first family of products according to the invention has a humidity at the outlet of the hydrator which is less than 4% by weight, preferably less than 2% by weight and preferably less than 1% by weight. This family corresponds to the family of dry hydrates which do not require drying before the grain size control and packaging steps. Taking into account the remark above pointing out the dependency of the Mg(OH).sub.2 proportion on the humidity of the product at the outlet of the hydrator, the products of this family contain between 0.4 and 48% of Mg(OH).sub.2 and therefore between 40 and 99.6% of Ca(OH).sub.2. Preferably, the Mg(OH).sub.2 proportion in the final product is comprised between 0.5 and 43% by weight, in particular between 0.6 and 40% by weight. That of Ca(OH).sub.2 is therefore preferably comprised between 45 and 99.5% by weight, preferably between 48 and 99.4% by weight (for a sum of x and y ranging from 88 to 100%). The products of this family have a specific surface area and a porous volume which are of the same order of magnitude as those of customary hydrated limes obtained via a standard dry route. After the grain size control step, the whole of the particles of the products of this family have a size of <250 m, preferably <200 m, advantageously <90 m, or even sometimes <60 m. This means that d.sub.97 is less than the aforementioned sizes.

(23) The second family of preferred products according to the invention has a humidity at the outlet of the hydrator comprised between 15 and 30% by weight, preferably greater than 17%, in particular greater than 19%, preferably less than 25%, in particular less than 22%. This family corresponds to the family of products which require a preliminary drying step before the grain size control and packaging steps. At the end of the drying step, the products of this family ideally have a humidity of less than 2% by weight, preferably less than 1% by weight. The drying may be achieved simultaneously or before a de-agglomeration step generally followed by a grain size control step such as has been described in the part relating to the method above, the whole of the particles of the products of this family then have, at the end of the method according to the invention, a size of <250 m, preferably <200 m, advantageously <90, or even <60 m. This means that d.sub.97 is less than the aforementioned sizes.

(24) Taking into account the remark above pointing out the dependency of the Mg(OH).sub.2 proportion on the humidity of the product at the outlet of the hydrator, the products of this family contain, once they are dried, between 0.8 and 58% of Mg(OH).sub.2 and therefore between 30 and 99.2% of Ca(OH).sub.2. Preferably, the proportion of Mg(OH).sub.2 in the final product is comprised between 0.9 and 53% by weight, preferably between 1.0 and 51% by weight, that of Ca(OH).sub.2 is therefore preferably comprised between 35 and 99.1% by weight, preferably between 37 and 99.0% by weight (for a sum of x and y ranging from 88 to 100%). The products of this family have a high specific surface area and a high porous volume. Their specific surface area is greater than 20 m.sup.2/g, preferably greater than 25 m.sup.2/g, in particular greater than 30 m.sup.2/g and less than 50 m.sup.2/g, in particular less than 45 m.sup.2/g, notably less than 40 m.sup.2/g, or even less than 35 m.sup.2/g. Their porous volume is greater than 0.10 cm.sup.3/g, preferably greater than 0.11 cm.sup.3/g, advantageously greater than 0.13 cm.sup.3/g and less than 0.25 cm.sup.3/g, in particular less than 0.20 cm.sup.3/g, notably less than 0.18 cm.sup.3/g, or even less than 0.16 cm.sup.3/g.

(25) For both of these families of products, the specific surface area and the porous volume depend on the proportions x and y of Ca(OH).sub.2 and of Mg(OH).sub.2. Indeed, the specific surface area and the porous volume of Mg(OH).sub.2 are not modified during the reaction described in the method according to the invention and therefore remain low. Regardless of the humidity of the product at the outlet of the hydrator, the specific surface area and the porous volume of Ca(OH).sub.2 formed during the reaction described by the method according to the invention are higher than those of Mg(OH).sub.2 involved in the method. Therefore, the greater the proportion of Mg(OH).sub.2 in the product, the lower are the specific surface area and the porous volume of the final product based on Ca(OH).sub.2 and on Mg(OH).sub.2. This reduction effect of the specific surface area and of the porous volume of the product with the increase of the Mg(OH).sub.2 proportion is all the more pronounced in the case of the second family of products. Indeed, when the humidity at the outlet of the hydrator is high, the specific surface area and the porous volume of the Ca(OH).sub.2 formed during hydration of the quicklime are high and each percent of Mg(OH).sub.2 with low specific surface areas and porous volume added to Ca(OH).sub.2 of this type for this mixture leads to a proportional decrease in the specific surface area and in the porous volume.

(26) In every case, adding Mg(OH).sub.2 in the form of a homogenous suspension in the hydrator during the hydration reaction of CaO into Ca(OH).sub.2 gives the possibility of obtaining an intimate mixture between the Ca(OH).sub.2 and Mg(OH).sub.2 compounds, a clearly more intimate mixture than in the case of simple physical mixtures of Ca(OH).sub.2 and of Mg(OH).sub.2.

EXAMPLES

(27) The laboratory hydrator used for producing all the examples shown in the following is a single-stage hydrator. It appears as a horizontal cylinder measuring about 80 cm in length on 25 cm of diameter. These proportions correspond to the proportions of industrial single-stage hydrators and these dimensions are 6 to 7 times smaller than the dimensions of industrial hydrators. This cylinder is provided with a jacket giving the possibility of controlling the temperature by circulation of a hot or cold fluid. Inside the hydrator, an axis provided with vanes is used for homogenizing the product during hydration but also for pushing it from the inlet (at one end) to the outlet (at the other end of the cylinder). The lime is introduced into the hydrator through a worm screw, calibrated beforehand before adjusting the quicklime flow rate. The Mg(OH).sub.2 milk, as for it, is introduced into the hydrator at two orifices each measuring about 5 mm in diameter, located on the lid of the hydrator, close to the intake of quicklime. When the product has covered the whole length of the hydrator, it leaves it simply by overflowing. Generally, the filling level of the hydrator is of the order of 50% by volume, i.e. that the bed of product reaches about the height of the axis. For all the examples below, the hydrator is pre-heated to 70-80 C. by circulating water at 90 C. in the jacket. This pre-heating gives the possibility of simulating starting conditions, industrially acquired by the continuous operation and avoids condensation of the steam produced by the hydration reaction of quicklime on the walls of the hydrator when the latter are cold. The jacket is then emptied when the hydration reaction begins in the hydrator.

(28) In the case of hydrations with an excess of water (quasi-dry route of Examples 1 to 6), the drilling time of the product in the hydrator is of the order of 15 minutes. In the case of hydrations via a dry route (Examples 7 to 9), the drilling time is longer, of the order of about 25 minutes.

Example 1

Production of a Totally Hydrated Mixed Compound with High Specific Surface Area and Porous Volume, Having a Molar Ratio Ca/Mg Close to 1

(29) A quicklime no. 1 (the properties of which are repeated in Table 1) is hydrated in the laboratory hydrator described above with a Mg(OH).sub.2 milk under particular conditions detailed below.

(30) The quicklime flow rate is of 300 g/min, which corresponds theoretically to 396 g of Ca(OH).sub.2/min, i.e. 5.34 mol of Ca(OH).sub.2/min. 5.34 mol of Mg(OH).sub.2/min have then to be added, i.e. 312 g of Mg(OH).sub.2/min in order to obtain an equimolar mixed compound.

(31) The Mg(OH).sub.2 used in this example is noted as Mg(OH).sub.2 source no. 1 (its main properties are described in Table 2), and appears initially as a milk containing 53% of solid material. Therefore 589 g/min of this milk have therefore to be added into the hydrator in order to obtain a Ca(OH).sub.2/Mg(OH).sub.2 ratio of about 1. However, with the purpose of developing porosity of the hydrate, an excess of water is required during the hydration reaction. In this particular case, the excess of water should give the possibility of obtaining at the outlet of the hydrator a product having a humidity comprised between 16 and 22%. The water introduced during the reaction by the 589 g/min of Mg(OH).sub.2 milk is not sufficient for reaching this humidity level, additional water has to be added. In practice, this water is added to the Mg(OH).sub.2 milk before its introduction into the hydrator.

(32) The hydrator keeps operating under these operating conditions for 30 minutes, thus producing more than 25 kg of powdery humid product. During this production, several samplings were made in order to control the humidity of the product which is measured by the mass loss during fast drying at 150 C.

(33) At the end of the production, the product is dried and de-agglomerated by a brief passage in a hot air current by means of a spin flash dryer. The temperature perceived by the product is of the order of 130 C. Here there is no grain size control step. Once it is dried, the product is characterised by thermogravimetric analysis (from room temperature up to 950 C. with a rise in temperature at 2 C./min) which allows determination of the actual proportions of Mg(OH).sub.2 and Ca(OH).sub.2 in the product after the mass losses observed between 300 and 400 C. and then between 400 and 600 C. and which respectively correspond to dehydroxylations of Mg(OH).sub.2 and of Ca(OH).sub.2. The thermogravimetric curve recorded for this product is shown in FIG. 1. A nitrogen adsorption manometric measurement is also conducted after having carried out degassing of the products at 190 C. for several hours. This measurement allows determination of the specific surface area of the product by the BET method, the BJH method as for it allowing evaluation of the volume of the pores for which the size is comprised between 17 and 1000 .

(34) All the results relative to this product are grouped in the first column of Table 3.

Example 2

Production of a Totally Hydrated Mixed Compound with High Specific Area and Porous Volume, Having a Ca/Mg Molar Ratio Close to 1

(35) This example is quite comparable with Example 1. The only difference is that this time, the Mg(OH).sub.2 milk is diluted beforehand with a greater amount of water than the one added to the Mg(OH).sub.2 milk in Example 1, the goal being here to obtain at the outlet of the hydrator a humid product containing 26 to 28% of humidity and no longer only 16 to 22% as was the case in Example 1.

(36) The thereby obtained product is dried and characterised in a similar way to the product of Example 1 and the results of the measurements appear in the second column of Table 3.

(37) The excess of water used during the hydration reaction, as intended allows development of a high specific surface area and of a high porous volume. The product having a humidity from 26 to 28% (Example 2) has a specific surface area equivalent to that of the product for which the humidity at the outlet of the hydrator was 16-22% (Example 1); but, the porous volume is more developed in Example 2. A minimum amount of excess water is therefore required for developing porosity (humidity 15% at the outlet of the hydrator), but the amount of water does not require accurate control, if only a high specific surface area is sought. However, it should remain less than a limit (humidity 30%) in order to avoid the production of a pasty product and no longer powdery which would stick, notably in the hydrator.

Example 3

Production of a Totally Hydrated Mixed Compound with a High Specific Surface Area and Porous Volume, Containing about 5% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(38) This example is strongly inspired from Example 1.

(39) The quicklime flow rate remains 300 g/min as in the previous examples, which always corresponds theoretically to 396 g of Ca(OH).sub.2/min. About 20 g/min of Mg(OH).sub.2 then has to be added, i.e. about 38 g/min of Mg(OH).sub.2 milk with 53% of initial solid material. The water provided by this small amount of milk is very clearly insufficient for guaranteeing complete hydration of the lime and furthermore the development of the porosity of the hydrate. Water is then added in order to obtain at the outlet of the hydrator a product having a humidity of the order of 16 to 22%. In practice, this water is added to the Mg(OH).sub.2 suspension before its introduction into the hydrator.

(40) The thereby obtained product is dried and characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 1. The first mass loss (300-400 C.) is clearly smaller than the one observed for the sample of Example 1, which expresses, a lower proportion of Mg(OH).sub.2. The second mass loss (400-600 C.) is on the other hand more pronounced and indicates a greater proportion of Ca(OH).sub.2 as compared with the product of Example 1. The results of the different measurements are found in the third column of Table 3.

Example 4

Production of a Totally Hydrated Mixed Compound with High Specific Surface Area and Porous Volume, Containing about 10% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(41) This example is comparable with Example 3, except for the Mg(OH).sub.2 percentage. The quicklime flow rate remains at 300 g/min, like in the previous examples, which always corresponds theoretically to 396 g of Ca(OH).sub.2/min. About 40 g/min of Mg(OH).sub.2, then has to be added, i.e. about 75 g/min of initial Mg(OH).sub.2 milk with 53% of solid material. Water is then added in order to obtain at the outlet of the hydrator once again a product having humidity of the order of 16 to 22%. In practice, this water is added to the Mg(OH).sub.2 milk before its introduction into the hydrator.

(42) The thereby obtained product is dried and characterised similarly to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 1. This curve is very close to the curve corresponding to the product of Example 3. The slightly greater mass loss between 300 and 400 C. for the product of this Example 4 however expresses a higher proportion of Mg(OH).sub.2 than for the case of Example 3. The results of the different measurements are found in the fourth column of Table 3.

Example 5

Production of a Totally Hydrated Mixed Compound with High Specific Surface Area and Porous Volume, Containing about 30% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(43) This example is comparable with Examples 3 and 4, except for the percentage of Mg(OH).sub.2.

(44) The quicklime flow rate remains at 300 g/min, like in the previous examples, which always theoretically corresponds to 396 g of Ca(OH).sub.2/min. About 119 g/min of Mg(OH).sub.2 has then to be added i.e. about 225 g/min of initial Mg(OH).sub.2 milk with 53% of solid material. Water is then added in order to obtain at the outlet of the hydrator once again a product having a humidity of the order of 16 to 22%. In practice, this water is added to the Mg(OH).sub.2 milk before its introduction into the hydrator.

(45) The thereby obtained product is dried and characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 1. The mass lost between 300 and 400 is still greater than in the Examples 3 and 4, that between 400 and 600 C. being on the other hand smaller, which is expressed both by a higher proportion of Mg(OH).sub.2 and a lower proportion of Ca(OH).sub.2. The results of the different measurements are found in the fifth column of Table 3.

Example 6

Production of a Totally Hydrated Mixed Compound with High Specific Surface Area and Porous Volume, Containing about 30% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(46) This example is comparable with Example 5. Nevertheless, the Mg(OH).sub.2 source noted as source no. 1 and used for Examples 1 to 5 is replaced here with a Mg(OH).sub.2 source no. 2 for which the main properties are indicated in Table 2. This time, the Mg(OH).sub.2 no longer appears as a milk but as a dry powder.

(47) Like in Example 5 above, in order to obtain the desired proportions of Ca(OH).sub.2 and Mg(OH).sub.2, 119 g/min of Mg(OH).sub.2 has to be added for a quicklime flow rate of 300 g/min. A production of 30 minutes then corresponds to 3570 g of Mg(OH).sub.2. This amount of Mg(OH).sub.2 is mixed with a well-determined amount of water and this suspension is pumped and then introduced into the hydrator. The amount of water used for preparing this milk is such that the humidity of the product at the outlet of the hydrator has to be comprised as earlier between 16 and 22%.

(48) The thereby obtained product is dried and characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 1. The curve is quite comparable with the curve corresponding to the product of Example 5, expressing close compositions for both of these products in terms of Mg(OH).sub.2 and Ca(OH).sub.2 proportions. The results of the different measurements are found in the sixth column of Table 3.

Example 7

Production of a Totally Hydrated Mixed Compound, Containing about 5% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(49) This time, the product has no specificity in terms of specific surface area and porous volume as compared with standard dry hydrates, whether they are purely calcium (slaked lime) or dolomites.

(50) In order to attain this goal, the quicklime no. 2 (described in Table 1) is used instead of the quicklime no. 1 used in the previous examples. Its flow rate is adjusted this time to 200 g/min, only, which theoretically corresponds to 264 g of Ca(OH).sub.2/min. About 13 g/min of Mg(OH).sub.2 then has to be added. The Mg(OH).sub.2 source is again here source no. 1, i.e. the milk containing 53% by mass of Mg(OH).sub.2. Introducing into the hydrator 13 g/min of Mg(OH).sub.2 amounts to introducing therein 24.5 g of Mg(OH).sub.2 milk with 53% of initial solid material. The water brought by this small amount of milk is very clearly insufficient for guaranteeing complete hydration of the lime even if there is no intention in this example to develop the porosity of the hydrate. Water is then added in order to obtain at the outlet of the hydrator a product having humidity of the order of 1 to 4%. In practice, this water is added to the suspension of Mg(OH).sub.2 before introducing it into the hydrator.

(51) Unlike the previous examples, the product is neither dried nor de-agglomerated; it is characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted on FIG. 2. The measurement was conducted this time with a heating rate of 5 C./min instead of the 2 C./min used for characterising the products of the examples above. Accordingly, the decomposition of Mg(OH).sub.2 occurs this time between 350 and 450 C., that of Ca(OH).sub.2 between 450 and 650 C. The results of the different measurements are found in the seventh column of Table 3.

Example 8

Production of a Totally Hydrated Mixed Compound Containing about 10% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(52) This example is comparable with Example 7, except for the percentage of Mg(OH).sub.2.

(53) The quicklime flow rate is maintained at 200 g/min, which still theoretically corresponds to 264 g of Ca(OH).sub.2/min. About 26 g/min of Mg(OH).sub.2 has then to be added, i.e. 49 g of Mg(OH).sub.2 milk with 53% of initial solid material. The water brought by this small amount of milk is very clearly insufficient for guaranteeing complete hydration of the lime even if there is no intention in this example of developing the porosity of the hydrate. Water is then added in order to obtain at the outlet of the hydrator, a product having a humidity of the order of 1 to 4%. In practice, this water is added to the Mg(OH).sub.2 milk before its introduction into the hydrator.

(54) The thereby obtained product is characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 2. As earlier for the products of FIG. 1, the increase in the first mass loss (350-450 C.) and the reduction in the second mass loss (450-650 C.) as compared with the curve of Example 7, indicate an increase in the proportion of Mg(OH).sub.2 in parallel with a reduction of that of Ca(OH).sub.2. The results of the different measurements are found in the eighth column of Table 3.

Example 9

Production of a Totally Hydrated Mixed Compound Containing about 30% of Mg(OH).SUB.2 .Based on the Mass of Ca(OH).SUB.2

(55) This example is comparable with Examples 7 and 8, except for the percentage of Mg(OH).sub.2.

(56) The quicklime flow rate is maintained at 200 g/min, which still corresponds theoretically to 264 g of Ca(OH).sub.2/min. About 79 g/min of Mg(OH).sub.2 has then to be added, i.e. 149 g of Mg(OH).sub.2 milk with 53% of initial solid material. Water is then added in order to obtain at the outlet of the hydrator a product having a humidity of the order of 1 to 4%. In practice, this water is added to the Mg(OH).sub.2 milk before its introduction into the hydrator.

(57) The thereby obtained product is characterised in a similar way to the product of Example 1. The thermogravimetric analysis curve is plotted in FIG. 2. The observations made in Example 8 remain valid with a still more pronounced increase in the mass loss between 350 and 450 C. and a greater reduction in the mass loss between 450 and 650 C. The results of the different measurements are found in the last column of Table 3.

(58) Generally, Table 3 shows that the proportions of Mg(OH).sub.2 expressed relatively to the proportions of Ca(OH).sub.2 found according to the thermogravimetric curves of FIGS. 1 and 2 are close to the expected values. In the case of hydrations via a dry route (Examples 7 to 9), the actual proportions are quite slightly less than the desired proportions. This deviation may be explained by a significant formation of steam in the case of hydrations via a dry route, greater than in the case of hydration with an excess of water of Examples 1 to 6. Certain fine particles, mainly Mg(OH).sub.2 for which the grain size is very fine, are found in suspension in this steam and are extracted from the hydrator by the steam extraction system, whence the <<loss>> of a small amount of Mg(OH).sub.2 during the reaction. Logically, these <<losses>> are less significant in the case of hydrations with an excess of water (Examples 1 to 6) during which the temperature is lower and the steam production is lower and the actual values are consequently closer in Examples 1 to 6 to the desired values than in the Examples 7 to 9.

(59) Moreover, it is obvious according to the results of Table 3 that the hydrations with an excess amount of water (Examples 1 to 6) lead to products with high specific area and porous volume while hydrations via a dry route lead to products for which the porosity is clearly lower. The specific surface of the products depends on the proportion of Mg(OH).sub.2: the higher the proportion of Mg(OH).sub.2 as compared with Ca(OH).sub.2 the lower is the specific surface area of the product. The differences are more pronounced in the case of hydrations with excessive water (Examples 1 to 6), for which the products may be considered as composites or at the very least as intimate mixtures between the initial Mg(OH).sub.2 and hydrated lime Ca(OH).sub.2 with a high specific surface area (about 40 m.sup.2/g).

Comparative Example 1

(60) Example 1 is reproduced, but this time the Mg(OH).sub.2 source no. 2 which appears as a powder is used instead of Mg(OH).sub.2 source no. 1 which, as for it, appears as a milk. Instead of dispersing the Mg(OH).sub.2 source no. 2 in water in order to prepare a milk like in Example 6, the Mg(OH).sub.2 source no. 2 is this time mixed with quicklime, the quicklime/Mg(OH).sub.2 mixture being introduced into the hydrator instead and in the place of the quicklime and the hydration of this mixture is only ensured with water.

(61) The quicklime no. 1 is used for this example and its flow rate is set to 300 g/min. Like in Example 1, the Mg(OH).sub.2 flow rate required for producing a product having a Ca/Mg ratio close to molarity is then 312 g/min. For a total production of 30 min, these are then 9000 g of quicklime and 9360 g of Mg(OH).sub.2 as a powder which are mixed in a laboratory mixer and then introduced into the metering device customarily used for feeding the laboratory hydrator with quicklime. This metering device is calibrated so as to introduce into the hydrator, 612 g of quicklime+Mg(OH).sub.2 mixture per minute. The amount of water to be added into the hydrator with the purpose of totally hydrating CaO and of producing a calcium-magnesium product with high specific surface area and high porous volume is determined so that the product which leaves the hydrator has humidity of the order of 15%.

(62) At the end of the production, the same drying, de-agglomeration steps and the same characterisations as those described in Example 1, are applied. According to the thermogravimetric results allowing determination of the mass proportions of Ca(OH).sub.2 and of Mg(OH).sub.2 in the final product, this final product after drying has a Ca/Mg molar ratio of 1.21 instead of the ratio of 1 which was desirable. In the example above, the same hydration aiming at producing a comparable equimolar product by introducing Mg(OH).sub.2 into the hydrator as a milk, however, led to a product very close to expectations for which the molar ratio Ca/Mg is 1.01.

(63) Accordingly, it seems that introduction into the hydrator of the Mg(OH).sub.2 source as a milk is clearly better than its introduction as a powder mixed with quicklime. On one hand, the mixture of powders is complicated to prepare for guaranteeing perfect homogeneity thereof. On the other hand, when Mg(OH).sub.2 arrives already dry into the hydrator as a powder, it is possible that the phenomenon described above occurs: actually it may be contemplated that the fine Mg(OH).sub.2 powder particles are found suspended in the steam generated by hydration of the quicklime and are extracted from the hydrator by the steam extraction system, thus leading to the loss of Mg(OH).sub.2. When Mg(OH).sub.2 is brought into the hydrator as a milk, the particles, which are as fine as in the Mg(OH).sub.2 powder are, however, coated with water and have to be dried in order to be found in suspension in the steam, which limits the losses of Mg(OH).sub.2 by the extraction system.

(64) It is quite understood that the present invention is by no means limited to the embodiments described above and that many modifications may be brought thereto without departing from the scope of the appended claims.

(65) TABLE-US-00001 TABLE 1 Quicklime No. 1 No. 2 Particle size <90 m <10 mm Available CaO (mass %.) 97.29 96.92 CaCO.sub.3 (mass %.) 0.57 0.60 MgO (mass %.) 1.12 0.88 Al.sub.2O.sub.3 (mass %.) 0.18 0.22 SiO.sub.2 (mass %.) 0.25 0.67 Fe.sub.2O.sub.3 (mass %.) 0.18 0.20 MnO (mass %.) 0.04 0.00 SO.sub.3 (mass %.) 0.04 0.23 T.sub.60 (min.) averaged over 3 samples 0.7 (0.05) 4.2 (0.05)

(66) TABLE-US-00002 TABLE 2 Mg(OH).sub.2 source No. 1 No. 2 Initial state Suspension, 53% by mass Powder Mg(OH).sub.2 (mass %.) 96.0 99.7 CaO (mass %.) 0.62 0.09 Al.sub.2O.sub.3 (mass %.) 0.10 0.06 SiO.sub.2 (mass %.) 0.11 0.03 Fe.sub.2O.sub.3 (mass %.) 0.37 0.01 MnO (mass %.) 0.07 0.00 S0.sub.3 (mass %.) 0.01 0.11 Cl (mass %.) 0.43 0.02 D.sub.10 (m) 0.88 1.21 D.sub.50 (m) 5.04 3.93 D.sub.90(m) 9.59 19.5 Specific surface area (m.sup.2/g) 8.1 5.5-5.2 Pore volume N.sub.2 0.034 0.023-

(67) TABLE-US-00003 TABLE 3 Ex. No. 1 2 3 4 5 Slaked lime no. 1 no. 1 no. 1 no. 1 no. 1 Source: no. 1 no. 1 no. 1 no. 1 no. 1 Mg(OH).sub.2 Humidity of the 16-22 22-28 16-22 16-22 16-22 product at the hydrator outlet (%) Desired Ca/Mg 1 1 mol. ratio Actual Ca/Mg 1.01 0.02 0.97 0.02 mol. ratio % Mg(OH).sub.2 (78.7) (78.7) 5 10 30 relatively to the desired Ca(OH).sub.2 % Mg(OH).sub.2 (77.5) (80.9) 6.3 0.5 9.7 0.5 31.8 0.5 relatively to the actual Ca(OH).sub.2 Actual % 39.7 0.5 40.6 0.5 5.4 0.5 8.1 0.5 21.9 0.5 Mg(OH).sub.2 in the final product (y) Actual % 51.2 0.5 50.1 0.5 86.1 0.5 83.3 0.5 68.7 0.5 Ca(OH).sub.2 in the final product (x) Sum of actual 90.9 0.5 90.7 0.5 91.5 0.5 91.4 0.5 90.6 0.5 Ca(OH).sub.2 + Mg(OH).sub.2 (x + y) Specific surface 25.0 0.2 25.1 0.2 41.6 0.2 40.4 0.2 32.0 0.2 area by the BET method (m.sup.2/g) Volume of 17-1,000 0.116 0.020 0.145 0.020 00.162 0.020 0.174 0.020 0.150 0.020 pores by the BJH method (cm.sup.3/g) Ex. No. 6 7 8 9 Slaked lime no. 1 no. 2 no. 2 no. 2 Source: no. 2 no. 1 no. 1 no. 1 Mg(OH).sub.2 Humidity of the 16-22 1-4 1-4 1-4 product at the hydrator outlet (%) Desired Ca/Mg mol. ratio Actual Ca/Mg mol. ratio % Mg(OH).sub.2 30 5 10 30 relatively to the desired Ca(OH).sub.2 % Mg(OH).sub.2 29.1 0.5 3.6 0.5 8.4 0.5 29.8 0.5 relatively to the actual Ca(OH).sub.2 Actual % 20.5 0.5 3.3 0.5 7.4 0.5 21.6 0.5 Mg(OH).sub.2 in the final product (y) Actual % 70.5 0.5 90.8 0.5 87.5 0.5 72.6 0.5 Ca(OH).sub.2 in the final product (x) Sum of actual 91.0 0.5 94.1 0.5 94.9 0.5 94.2 0.5 Ca(OH).sub.2 + Mg(OH).sub.2 (x + y) Specific surface 31.6 0.2 16.5 0.2 15.8 0.2 15.1 0.2 area by the BET method (m.sup.2/g) Volume of 17-1,000 0.137 0.020 0.054 0.020 0.056 0.020 0.055 0.020 pores by the BJH method (cm.sup.3/g)