Process for manufacturing highly porous slaked lime and product thereby obtained

Abstract

Process for manufacturing highly porous slake lime comprising a feeding step of quicklime, a feeding step of water in a feeding zone of a hydrator, a slaking step of said quicklime in a slaking zone of said hydrator and a maturation step in a maturation zone of said hydrator to form slaked lime.

Claims

1. Process for manufacturing highly porous slaked lime having a high specific surface area of at least 30 m.sup.2/g and a high pore volume, the high pore volume being comprised by pores with a diameter less than 1000 Angstroms, higher than or equal to 0.15 cm.sup.3/g, said process comprising a feeding step of quicklime, a feeding step of water in a feeding zone of a hydrator, a slaking step of said quicklime in a slaking zone of said hydrator and a maturation step in a maturation zone of said hydrator to form slaked lime, characterized in that said feeding step of quicklime and said feeding step of water are performed so as to obtain a water/quicklime ratio comprised between 0.8 and 1.3 by weight, the process further comprising a step of removing vapor generated during said slaking step, said step of removing vapor being performed over at least 90% of the length of said slaking zone to form a raw highly porous slaked lime being a high specific surface area and high pore volume slaked lime.

2. Process for manufacturing highly porous slaked lime according to claim 1, further comprising a drying step of said raw slaked lime to form a dried powdery high specific surface area and high pore volume slaked lime.

3. Process for manufacturing highly porous slaked lime according to claim 1, wherein said slaking step of said quicklime is performed into a single stage hydrator.

4. Process for manufacturing highly porous slaked lime according to claim 1, wherein said step of removing vapor is performed through a fabric filter.

5. Process for manufacturing highly porous slaked lime according to claim 1, wherein said step of removing vapor is performed along the entire length of the hydrator.

6. Process for manufacturing highly porous slaked lime according to claim 1, wherein the feeding step of quicklime is made by a weighing device, said weighing device comprising a conveyor belt, said conveyor belt allowing quicklime to fall into the hydrator.

7. Process for manufacturing highly porous slaked lime according to claim 6, wherein the feeding of water is carried out at a single point at the entry of the hydrator, onto the falling quicklime.

8. Process for manufacturing highly porous slaked lime according to claim 1, wherein said quicklime presents a reactivity towards water t.sub.60, measured according to the European standard EN 459-2, equal to or greater than 15 seconds and equal to or lower than 10 minutes.

9. Process for manufacturing highly porous slaked lime according to claim 1, wherein said quicklime presents a particle size d.sub.98 comprised between 90 m and 10 mm.

10. Process for manufacturing highly porous slaked lime according to claim 1, wherein said water presents a temperature equal to or lower than 60 C.

11. Process for manufacturing highly porous slaked lime according to claim 1, wherein during said slaking step, lime is mixed and lifted by a shaft equipped with mixing paddles.

12. Process for manufacturing highly porous slaked lime according to claim 1, wherein the temperature in the hydrator is kept below 100 C.

13. Process for manufacturing highly porous slaked lime according to claim 11, wherein the manufacturing process of highly porous slaked lime is controlled by measuring the moisture of the raw slaked lime or the motor intensity of the shaft equipped with mixing paddles.

14. Process for manufacturing highly porous slaked lime according to claim 1, wherein the moisture content of the raw slaked lime is ranking between 15 and 30 weight % with respect to the weight of said raw slaked lime.

15. Process for manufacturing highly porous slaked lime according to claim 1, wherein the feeding step of water is a feeding step of water comprising additives, said additives including di-ethylene glycol or an alkali metal compound selected from the group consisting of alkali metal hydroxides, carbonates, hydrogencarbonates, and mixtures thereof.

Description

(1) Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by making reference to the drawings and the examples.

(2) FIG. 1 is a CFD simulation illustrating a single stage hydrator with a small extraction hood section located at the end of the slaking zone of the hydrator.

(3) FIG. 2 is a CFD simulation illustrating a single stage hydrator with a small extraction hood section located at the center of the slaking zone of the hydrator.

(4) FIG. 3 is a CFD simulation illustrating a single stage hydrator with a long extraction hood covering the length of the slaking zone of the hydrator.

(5) FIG. 4 is a schematic illustration of the length and position of the extraction hood onto a single stage hydrator for performing the process according to the present invention.

(6) In the drawings, the same reference numbers have been allocated to the same or analog element.

(7) The present invention aims thus at removing as soon as possible the vapor generated by the slaking reaction of quicklime so as to avoid the contact between said vapor and the lime, said contact being detrimental to the slaked lime porosity. The vapor is mainly generated in the slaking zone of the hydrator. Therefore, in order to optimize the step of removing vapor according to the process of the present invention, the hydrator (single or multi stage) should be equipped with an extraction hood that preferably extends on 100% of the hydrator length.

(8) Alternatively, said extraction hood should extends on a portion of the hydrator length which starts from 35%, preferentially from at least 30%, preferably 20%, more preferably 10%, in particular 0% of the length of said hydrator, upstream the slaking direction, until at least 65%, preferably 70%, in particular 80%, more preferably 90%, notably 100% of the length of the hydrator (see FIG. 4).

EXAMPLES

Example 1

(9) CFD simulations have been performed so as to illustrate the flow path of water steam generated during a slaking reaction of quicklime with a water/lime ratio between 0.8 and 1.3 depending on the size/section and position of the bag filter onto a single stage hydrator (slaking unit).

(10) In these simulations, illustrated on FIGS. 1 to 3, the semi cylinder represents the upper half of the hydrator, namely the part of the hydrator located above the lime bed.

(11) Such upper half of the hydrator is connected to an extraction hood that will evacuate the water steam towards the bag filter (not represented).

(12) The bag filter presents the same section than the extraction hood to which it is connected.

(13) Three situations have been considered. 1. small extraction hood section located at the end of the hydrator (FIG. 1). 2. small extraction hood section located at the center of the hydrator (FIG. 2). 3. extraction hood covering the length of the slaking zone of the hydrator (FIG. 3).

(14) The results show that in situation 1 (FIG. 1), the water steam presents very long flow path which forces the water steam to remain in the hydrator in close contact with the lime bed for a non-negligible period of time before being evacuated through the bag filter. During this period of time, the water steam will interfere with the slaking of quicklime, preventing therefore to precisely control the hydration process.

(15) Situation 2 (FIG. 2) presents better results than situation 1 since it reduces the flow path of the water steam. However, the generated steam, even if less in contact with lime undergoing the slaking reaction, is still in contact with this latter.

(16) Situation 3 (FIG. 3) is the best situation since generated steam is directly extracted before being in contact with lime in the slaking zone.

(17) In conclusion, it is more desirable to dispose a bag filter along the major section of the hydrator so as to evacuate the water steam through a shorter and more vertical flow path as possible, avoiding therefore the presence of transversal flows that would lead to undesirable hydration.

Example 2

(18) Highly porous slaked lime is industrially manufactured according to the present invention in a single stage hydrator measuring about 5.5 m in length and 2.1 m in diameter (only the hydrator tank), producing about 6 t/h of slaked lime and equipped with a bag filter. For this process, quicklime (d.sub.98 of 3 mm) having a t.sub.60 reactivity of less than 1 min is slaked with water at a water/quicklime ratio equal to 1.05 by weight, the water being fed at ambient temperature. The average moisture content of the raw hydrate, meaning the moisture content of the raw slaked lime at the exit of the hydrator and before a drying step, is equal to 21.3 wt. %. The extraction hood of the filter (contact zone between the hydrator and the filter) is located along the slaking zone, i.e. at a central position compared to the length of the hydrator. The raw hydrate is then transported and flash dried during a few minutes into a cage mill dryer in which some hot air is injected. Once dried the slaked lime product is separated from the air by a bag filter.

(19) The resulting dried hydrated lime presents a yearly average BET specific surface area equal to 42.6 m.sup.2/g and a yearly average total BJH pore volume (pores up to 1000 ) equal to 0.255 cm.sup.3/g is produced.

Example 3

(20) A highly porous slaked lime is industrially manufactured according to the present invention in another single stage hydrator, significantly larger than the one of Example 2 as it is producing 9 to 10 t/h of slaked lime. This hydrator is also equipped with a bag filter, the extraction hood of which being also located along the slaking zone, i.e. at a central position compared to the length of the hydrator. For this process, quicklime (d.sub.98 of 3 mm) having a t.sub.60 reactivity of 1.3 min is slaked with water at a water/quicklime ratio equal to 1.0 by weight. The average moisture content of the raw hydrate, meaning the moisture content of the raw slaked lime at the exit of the hydrator and before a drying step, is equal to 24.2 weight %. The raw hydrate is then transported and flash dried during a few minutes into a cage mill dryer in which some hot air is injected. Once dried the slaked lime product is separated from the air by a bag filter.

(21) The resulting dried hydrated lime presents a yearly average BET specific surface area equal to 41.4 m.sup.2/g and a yearly average total BJH pore volume (pores up to 1000 ) equal to 0.3 cm.sup.3/g is produced.

Example 4

(22) A highly porous slaked lime is industrially manufactured according to the present invention in a multi stages hydrator measuring about 5 m in length, producing about 3 t/h of slaked lime and equipped with a bag filter. The hydrator itself is composed of three superimposed stages that all have the same length. The quicklime and the water are both fed at the beginning of the first stage of the hydrator. Di-ethylene glycol is added to the slaking water at an amount of 0.3% by weight with respect to the total amount of quicklime. The extraction hood of the bag filter is located on the whole length of the hydrator. For this process, quicklime having a t.sub.60 reactivity of 1.1 min is slaked with water at a water/quicklime ratio equal to 1.0 by weight, the water being fed at ambient temperature. The average moisture content of the raw hydrate, meaning the moisture content of the raw slaked lime at the exit of the hydrator and before a drying step, is equal to 25 weight %. The raw hydrate is then transported and flash dried during a few minutes into a pin mill dryer in which some hot air is injected. Once dried the slaked lime product is separated from the air by a bag filter.

(23) The resulting dried hydrated lime presents a yearly average BET specific surface area equal to 39.7 m.sup.2/g and a yearly average total BJH pore volume (pores up to 1000 ) equal to 0.195 cm.sup.3/g is produced.

Comparative Example 1

(24) Quicklime hydration trials are performed at laboratory scale in a small laboratory scale pilot single stage hydrator measuring about 80 cm in length, presenting a diameter of about 25 cm and producing about 20 kg/h of slaked lime. In this hydrator, the quicklime and the slaking water are fed upstream of the hydrator and driven along the slaking direction up to the end of the hydrator by a shaft equipped with mixing paddle.

(25) A first trial consists in producing a highly porous slaked lime according to the present invention by slaking quicklime with water at a water/quicklime ratio of 1.1 by weight and by extracting the steam generated during the slaking reaction along the slaking zone with the help of an extraction duct. The experiment goes very well and dried hydrated lime having a BET specific surface area equal to 40.6 m.sup.2/g and a total BJH pore volume (pores up to 1000 ) equal to 0.179 cm.sup.3/g is produced.

(26) Then, the extraction of the steam is shifted toward the end of the hydrator, all the other conditions being kept constant. This experiment has to be stopped due to almost continuous blockages of the extraction duct and lime feeding pipe. Indeed, in these conditions, the steam, that is mainly generated in the central part of the hydrator, has a long path to go to reach the extraction duct. Therefore, some of the steam does not go along this path but rather in the other direction and goes out of the reactor via the quicklime feeding point, leading to a regular plugging of the lime feeding point. Moreover, in order to suck better the steam via the extraction duct, the applied depression has to be increased, which also leads to the extraction of more dust (indeed, there is not only steam in the hydrator, but steam in which a non-negligible amount of slaked lime dust is in suspension), and thus to the plugging of the extraction duct at regular intervals (each 2 minutes approximately). Due to these difficult conditions, it was not possible to pursue the slaking process.

(27) It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.