MXPA00011633A - Extrusion product based on hydrated lime - Google Patents

Abstract

The invention concerns an extrusion product containing hydrated lime, in the form of a calcined extruded product containing at least a binding agent capable of withstanding the extrusion product calcination temperature. Said product is prepared by dry process mixing of hydrated lime, atleast said binding agent and a plasticising agent, gradually incorporating water in said mixture so as to obtain a paste capable of being extruded, extruding said paste into said extruded product, drying and calcining the dried extruded product at a calcination temperature higher than the plasticising degradation temperature and lower than the hydrated lime decomposition temperature.

Description

EXTRUSION PRODUCT BASED ON HYDRATED LIME DESCRIPTIVE MEMORY The present invention relates to an extrusion product comprising hydrated lime. Such products, which are obtained by extruding a paste of slaked lime with an organic compound, such as an alcohol of the methyl alcohol type or a solution from the fermentation of alcohol or a solution of lignin sulfonate, are known. The extruded materials which are obtained afterwards are air-dried or dried in a dryer, in order to make possible the evaporation of the organic compound and then, optionally, these are graded (see the abstract of the patent JP-A- 60081021, Derwent, Accession No. 85-149035, and the abstract of the patent JP-A-59152219, Derwent, Accession No. 84-252992). These products whose purpose is the agricultural enrichment of soils, are designed to disintegrate in the presence of moisture and therefore to disperse, in the best possible way, in the soil to be treated. Therefore, they do not have a high structural strength and their water resistance is as low as possible. In addition, it is known to purify gases, in particular flue gases, using calcium hydroxide. For example, a provision has been made to mix zeolite, sepiolite or bentonite, slaked lime and colloidal silica, as a binder, and thereafter granulate and dry the mixture obtained in order to use the granules as a product to neutralize the acid gases ( see JP-62071534 and JP-61293546). These products have the disadvantages of a fairly low lime content and insufficient mechanical strength. Provision has also been made, for example, to mix coal, lime and volatile ash or pozzolanic material, and agglomerate the combined mixture as particles whose purpose is to produce the flue gas desulfurization (see US-A-4 230). 460). Agglomerated pellets from lignite ash and calcium hydroxide have already been provided for the same purpose (EP-A-307 928). Generally these products are designed to be introduced directly into the combustion chamber, mainly for the desulfurization of fuels containing sulfur. In accordance with a technique, known as dry purification, provision has already been made for treating gases comprising acids using agglomerated calcium hydroxide as grains of any form. The combustion gases are passed through a stationary or fluidized bed of these grains (BE-A-1 000 726). This treatment requires that the gases are passed through the bed of calcium hydroxide grains at an elevated temperature, in particular between 150 and 350 ° C. In the summary of JP-A-58143837, Derwent, Accession No. 83-779759, a description is given of products from the granulation of slaked lime, limestone and water, which, after drying, are used to purify gases rich in SO2- which additionally comprise, optionally, a halogenated gas. Finally, provision has been made, in WO-A-97/14650, for the use of a Ca (OH) 2 powder with an excellent pore volume and specific surface properties for purifying flue gases. This powder is injected into the exhaust gas with difficulty to optimize the control of Ca (OH) 2 consumption and with the need to provide a filtering device at the outlet of the plant, in order to recover the desulfurization products. The control of lime consumption depends on the content of impurities in the gases to be treated, whose content frequently varies considerably with time, for example, in the particular case of gases resulting from incinerators. The result of this is that any of the lime based products are injected in excess with respect to the SO2 content of the gases to be purified, in order to make sure to solve the problem of SO2 peaks that occasionally appear, or the product is injected in a form corresponding to the average SO2 content in the gases to be purified and in this case, there is a risk that SO2 will be released in the presence of said peaks. Finally, it is known to use, in a stationary bed, a material with the shape of spheres, cylinder or rings, based on an oxide of aluminum, silicon, calcium and other metals, to purify the dust from the exhaust gases (see US-A-4 042 352). However, this document does not give details regarding the actual composition of these products or their manufacture. A goal of the present invention is to solve the problems presented by the purification of gases, in particular waste gases, while at the same time avoiding those presented by the use, nowadays, of lime dust or agglomerates. To solve these problems, the invention provides an extrusion product comprising hydrated lime, which is provided in the form of a calcined extruded material comprising at least one binder that can withstand a calcination temperature of the extrusion product. The term "calcined extruded material" is to be understood, according to the invention, as a product which, after extrusion, has reached a calcination temperature. The term "Calcination temperature" should be understood as a temperature higher than a simple drying temperature, the latter corresponding to that with which the moisture in the product is removed. Generally, at the calcination temperature, at least one chemical component present in the starting material is decomposed, for example a plasticizer whose purpose is to facilitate extrusion, and therefore the extruded calcined material in accordance with the invention no longer contains this component. An extruded material as such offers the advantage of forming, in the calcined state, a product in which the lime and the binder together produce a porous and mechanically strong structure which, on the one hand, will withstand crushing and wear by rubbing and , on the other hand, it will not decompose in the presence of water. In this way, it is possible to prepare beds of extruded products through which the gases to be purified can pass without having to overcome the problems of obstruction and without having to supply, downstream of the purification bed, a filter to recover the powder resulting from the purification. In addition, the use of an absorber in the form of extruded materials enables complete exhaustion of the absorbent before it is replaced. Mention may be made, as the appropriate binder, of substances having hydraulic setting properties, in particular cement, calcined gypsum or calcium aluminate. It is also possible to imagine aluminosilicates, such as bentonite, montmorillonite or kaolinite. Aluminosilicates that do not swell or swell to a slight degree with water will be preferred. It is also possible to contemplate mixtures of these various materials. The calcined extrusion product advantageously comprises at least 50% by weight, preferably at least 80%, advantageously 85% or even 90% and more, of hydrated lime. The lime content of the product is then optimal, while at the same time maintaining excellent lime reactivity and a prolonged shelf life for the absorbent product. According to one embodiment of the invention, the product is provided in the form of cylinders with a length equal to 1 to 6 times its diameter, preferably from 2 to 4 times, and the cylinders have a base diameter of the order of 1 mm at 30 mm. This configuration makes possible, as a stationary or mobile bed, an excellent production by gas flow velocity to be purified. According to an improved embodiment of the invention, in a gaseous medium, the product has an absorption degree of SÜ2, HCl and HF similar to that of hydrated lime, in a pulverized state, under identical operating conditions. Therefore, while solving the problems presented by the powders, the purifying power of the powder forming the clogged product is not affected in a detrimental way or is only affected to a very slight degree in the calcined extruded product. Other embodiments of the product of the invention are indicated in the appended claims. The invention also relates to a process for preparing an extrusion product according to the invention. According to the invention, a preparation process is provided, which comprises dry mixing the hydrated lime, said binder and a plasticizing agent, gradually incorporating water so as to obtain a paste that can be extruded, extrude this paste in said extruded product, drying and calcining the dry extruded product at a calcination temperature higher than the decomposition temperature of the plasticizing agent and less than the decomposition temperature of the hydrated lime.

Such a process has the advantage of using a plasticizing agent that facilitates the extrusion of the formed mixture, while at the same time making possible the disappearance of any trace of this agent, as well as any trace of water, in the final product. In this way a product is obtained that is mechanically very strong but also extremely porous and highly resistant to any decomposition with water. The calcination temperature can conveniently be of the order of 150 to 350 ° C, preferably of 200 to 300 ° C, in particular of about 250 ° C. According to an advantageous embodiment of the invention, an additive chosen from the group consisting of polysaccharides, natural or synthetic cellulose, cellulose derivatives, polyvinylpyrrolidone, a polyethylene or polyvinyl derivative or a mixture of these materials is used as the plasticizing agent. According to an improved embodiment of the invention, the process comprises adding to the dry mixture activated carbon, lignite coal, zeolite or a mixture of these materials. The extrusion product then makes possible the purification of flue gases with respect to dioxins, furans and other typical impurities in trace forms. The hydrated starting lime in an advantageous manner by itself has exceptional absorption qualities with respect to SO2, HCl and HF, such as those described in particular in WO-A-97/14650. The invention also relates to the use of an extrusion product in the treatment of gases and vapors. Provision is made for a stationary bed which is located downstream of the combustion chamber and which comprises a predetermined amount of the extrusion product according to the invention. An arrangement as such can be provided after a normal treatment of the gases by a powder, followed by filtration, using the stationary bed to purify the peaks of SO2, HCl or HF presented by the gases to be treated. This results in a remarkably more controlled consumption of lime powder consumption [sic], which can be determined in accordance with the common average for SO2, HCl or HF in the gases to be treated. Provision can also be made for the formation of a fluidized bed, which moves, for example, in accordance with a vertical direction under the effect of gravity. The gases to be purified are moved countercurrent to the bed, and the bed base is discharged regularly after depletion of its absorbent powder. In this case it is possible to eliminate the treatment with dust and the expensive filtering device, because this has to be changed frequently. In the following the invention will be described in greater detail using the given implementation examples without implicit limitation.

EXAMPLE The hydrated lime mentioned in the following examples is a hydrated lime with a high specific surface area (> 40 m2 / g) and with a high total pore volume per nitrogen desorption (> 0.2 cm3 / g). The obtained extrusion products will be subjected to the tests. The conditions of the measurements and tests that will be used are specified below.

Measurement of crushing strength Each cylindrical extruded product is individually subjected to an increasing load, together with its generator, until a failure is obtained; the force applied to the moment of crushing is determined in kilograms. These measurements are made by an automatic press equipped with a mobile piston. The results, expressed in kg / mm, correspond to the average value of the measurements in 10 to 20 cylinders, divided by the average length of these cylinders. The extrusion product advantageously has a crushing force of at least 0.8 kg / mm.

Measurement of wear resistance by rubbing 100 g of extruded cylinders are rotated in a cylindrical drum with a diameter of 305 mm and a length of 260 mm, rotating the drum on its axis at the speed of 55 revolutions / minute. At the end of the test, that is, after 1500 rotations, the percentage of fine powders or less than 850 microns is determined. The wear resistance by rubbing is, advantageously, less than 15%.

Test with boiling water 15 extruded products are immersed in boiling water for 15 minutes. The number of extruded products that pass the test is then examined and the result is expressed as a percentage of extruded products that did not disintegrate or that did not explode. A test resistance with boiling water of at least 85% is preferably provided.

Measurement of the pore volume with carbon tetrachloride A weight of 50 grams of extruded cylinders is immersed in refluxing carbon tetrachloride for 2 minutes. After cooling and filtering, the weight of absorbed carbon tetrachloride is determined by capillary action and then the volume of liquid absorbed. The results are expressed in cm3 / g solids. Advantageously, a pore volume calculated with carbon tetrachloride greater than 0.2 cm 3 / g is advantageously provided.

Determination of the specific surface area (BET) and the pore volume (BJH) The specific surface area is determined from the nitrogen absorption isotherm (multiple point BET method); the pore volume is calculated by the BJH method during nitrogen desorption starting from a relative pressure of 0.95 (pore size dimensions between 20 and ± [sic] 1000 Á). These measurements are made with a Micromeretics ASAP 2010 device. Advantageously, a BJH pore volume greater than 0.01 cm3 / g and a specific surface area greater than 15 m2 / g is provided.

EXAMPLE 1 13.3 kg of hydrated lime, 0.7 kg of bentonite (Bentonil C2 T from Société Francaise des bentonites et derives) and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6 from Hoechst) are mixed dry for 10 minutes in a "Z" mixer. . Then 5.1 liters of water are gradually incorporated into the above mixture over the course of 10 minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an extruder Alexander- Werke. The extruded cylinders are dried for 3 hours at 110 ° C and then calcined for 3 hours at 250 ° C.

The characteristics of the calcined extruded products are shown in table 1.

TABLE 1 As can be seen, these extruded materials have very good properties of mechanical resistance and resistance to water, while at the same time retaining a specific surface area and a pore volume of the order of [sic] that of the powder used at the beginning.

EXAMPLE 2 13.3 kg of hydrated lime are mixed dry, 0.7 kg of cement based on calcium aluminate (SECAR 71 from Lafarge) and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6 from Hoechst) for 10 minutes in a "Z" mixer. Then 5.7 liters of water are gradually incorporated into the above mixture over the course of 10 minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an extruder Alexander- Werke. The extruded cylinders are dried for 3 hours at 110 ° C and then calcined for 3 hours at 250 ° C. The characteristics of the calcined extruded products are shown in table 2.

TABLE 2 The capacity of these extruded cylinders to absorb SO2 was measured in the laboratory in the following way: A sample weight equivalent to 4.5 g of Ca (OH) 2 is placed [sic] in a cylindrical reactor with a diameter of 35 mm. A gas comprising 3000 Vpm of SO2, with a moisture content of 8% and a temperature of 300 ° C, is then passed through the absorbent bed for 6 hours. The flow velocity of the gas is 90 l / h so that the amount of SO2 passing through the bed is the stoichiometrically necessary to convert all the calcium hydroxide to calcium sulfate. The concentration of S02 [sic] at the inlet and outlet of the reactor is measured by an infrared cell. The degree of absorption is determined by the following relationship: (A-B) x 100 / A in which: A: is the total amount of SO2 that enters during a period of 6 hours. B: is the total amount of SO2 that comes out during this 6-hour period. For the cylindrical extruded products prepared according to this example, an SO2 absorption degree of 48% is obtained (a value close to the degree of absorption of pure powdered calcium hydroxide, which is of the order of 55% under the conditions of comparable test).

EXAMPLE 3 13.3 kg of hydrated lime, 0.7 kg of commercial grade calcined gypsum and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6 from Hoechst) are mixed dry for 10 minutes in a "Z" mixer. Then 5.1 liters of water are gradually incorporated into the above mixture over the course of 10 minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an extruder Alexander- Werke. The extruded cylinders matured for 24 hours at room temperature, are subsequently dried for 3 hours at 110 ° C and calcined for 3 hours at 250 ° C. The characteristics of the calcined extruded materials are shown in table 3.

TABLE 3 EXAMPLE 4 13.3 kg of hydrated lime, 0.7 kg of calcium aluminate (CA 25 from Alcoa) and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6 from Hoechst) are mixed dry for 10 minutes in a "Z" mixer. Then 4.5 liters of water are gradually incorporated into the above mixture over a period of ten minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an extruder Alexander- Werke. The extruded cylinders are matured for 72 hours at room temperature and at 80 ° C for 24 hours in a sealed container; these are then dried for 3 hours at 110 ° C and calcined for 3 hours at 250 ° C. The characteristics of the calcined extruded cylinders are shown in Table 4.

TABLE 4 EXAMPLE 5 11.97 kg of hydrated lime, 1.33 kg of activated carbon (GL 50 from Norit), 0.70 kg of bentonite (Bentonil C2 T from Société Francaise des bentonites et derives) and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6) are mixed dry. from Hoechst) for 30 minutes in a "Z" mixer. Then 5.1 liters of water are gradually incorporated into the above mixture over a period of ten minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an extruder Alexander- Werke. The extruded cylinders are brought to 80 ° C for one hour; these are then dried for 3 hours at 110 ° C. and calcined for 3 hours at 250 ° C. The characteristics of the calcined extruded cylinders are shown in Table 5.

TABLE 5 EXAMPLE 6 Dry mix 13.3 kg of hydrated lime, 0.35 kg of calcined gypsum, 0.35 kg of Bentonite (Bentonil C2 T from Société Frangaise des bentonites et derives) and 0.014 kg of methylhydroxyethylcellulose (Tylose MH 15000 P6 from Hoechst) for ten minutes in a "Z" mixer. Then 5.1 liters of water are gradually incorporated into the above mixture over a period of ten minutes while continuing to mix and the paste is then extruded into cylinders with a diameter of 3.2 mm and an average length of 6 mm using an Alexander-Werke extruder . The extruded cylinders are kept at 80 ° C for one hour; then they are dried for 3 hours at 110 ° C and calcined for 3 hours at 250 ° C. The characteristics of the calcined extruded cylinders are shown in Table 6.

TABLE 6 The degree of absorption of SO2 of the extruded cylinders, measured in accordance with the test protocol described in example 2, is 50%. The ability to absorb HCl in the laboratory was also measured as follows: A sample weight equivalent to 3.4 g of Ca (OH) 2 [sic] is placed in a cylindrical reactor with a diameter of 25 mm. A gas comprising 3000 Vpm of HCl, with a moisture content of 8% and at a temperature of 250 ° C, is subsequently passed through the absorbent bed for 6 hours. The flow velocity of the gas is adjusted so that the amount of HCl passing through the bed is the stoichiometrically necessary to convert all of the calcium hydroxide to calcium chloride. The concentration of HCl at the outlet of the reactor is determined by thermostatically controlled bubbling at 10 ° C and conductometric measurements. The degree of absorption is determined by the following ratio: (A-B) x 100 / A in which: A: is the total amount of HCl entering during a period of 6 hours. B: is the total amount of HCl that comes out during this 6-hour period. Under the test conditions described above, an absorption degree of 44% is obtained.

EXAMPLE 7 kg of extruded cylinders were prepared in accordance with the formula and protocol described in example 6. In a pilot plant, a comparative test was carried out with a porous limestone absorber sold for the purification in a stationary bed. This plant consists of a chamber comprising 4 plates with a diameter of 35 cm on which an absorbent layer is placed at the rate of 5 kg per plate for the extruded cylinders and 10 kg per plate for the limestone grains. This layer is connected to a diverter for an industrial gas comprising on the order of 800 mg / Sm3 of SO2 (oxygen content of 18%). The temperature of the gas is 65 ° C. The gas, at the speed of 18 Sm3 / h passes successively through the four plates and the SO2 content is determined after it has passed through each plate. The analysis of the composition of the gas after each plate after operating for 1 hour, is shown in table 7.

TABLE 7 It is to be understood that the present invention is in no way limited to the implementation examples indicated above and that many modifications may be made thereto without departing from the scope of the invention set forth in the appended claims. It is possible, for example, to imagine the extrusion product being provided not only in cylindrical form but also in any form that can be extruded, for example an empty cylinder, a grooved cylinder, prism with any base and the like.

Claims (17)

NOVELTY OF THE INVENTION CLAIMS

1. - An extrusion product comprising hydrated lime, characterized in that it is provided in the form of a calcined extrudate comprising at least one binder that can withstand a temperature for the calcination of the extrusion product.

2. The extrusion product according to claim 1, further characterized in that the binder is selected from the group consisting of substances having hydraulic setting properties and aluminosilicates.

3. The extrusion product according to claim 2, further characterized in that the binder is selected from the group consisting of bentonite, montmorillonite, kaolinite, cement, calcined gypsum, calcium aluminate or mixtures of these materials.

4. The extrusion product according to any of claims 1 to 4 [sic], further characterized in that it comprises at least 50%, preferably at least 80%, advantageously at least 90% by weight of hydrated lime.

5. The extrusion product according to any of claims 1 to 3, further characterized in that it additionally comprises activated carbon, lignite coal, zeolite or a mixture of these materials.

6. The extrusion product according to any of claims 1 to 4, further characterized in that it is provided in the form of cylinders with a length equal to 1 to 6 times its diameter, preferably from 2 to 4 times, and because the cylinders have a base diameter of the order of 1 mm to 30 mm.

7. The extrusion product according to any of claims 1 to 6, further characterized in that, in a gaseous medium, it has a degree of absorption of SO2, HCl and HF similar to that of hydrated lime, in low pulverized state identical operating conditions.

8. The extrusion product according to any of claims 1 to 7, further characterized in that it has a crushing strength of at least 0.8 kg / mm and a frictional wear resistance of less than 15%.

9. The extrusion product according to any of claims 1 to 8, further characterized in that it has a resistance to the test with boiling water of at least 85%, preferably at least 90%.

10. The extrusion product according to any of claims 1 to 9, further characterized in that it has a pore volume calculated by nitrogen desorption of at least 0.05 cm3 / g, preferably of at least 0.1 cm3 / g, advantageously at least 0.2 cm3 / g, and a specific surface area greater than 15 m2 / g, preferably greater than 25 m2 / g and advantageously greater than 30 m2 / g.

11. A process for preparing the extrusion product according to any of claims 1 to 10, characterized in that it comprises dry mixing hydrated lime, at least one of the aforementioned binders and a plasticizing agent, gradually incorporate water in This mixture, so as to obtain an extrudable paste, extrude this paste as said extruded product, dry and calcined the dried extruded product at a calcination temperature higher than the decomposition temperature of the plasticizing agent and lower than the decomposition temperature of the hydrated lime.

12. The process according to claim 11, further characterized in that an additive selected from the group consisting of polysaccharides, natural or synthetic cellulose, cellulose derivatives, polyvinylpyrrolidone, a polyethylene or polyvinyl derivative is used as the plasticizing agent. a mixture of these materials.

13. The process according to any of claims 11 and 12, further characterized in that it comprises adding to the dry mixture activated carbon, lignite coal, zeolite or a mixture of these materials.

14. The preparation process according to any of claims 10 [sic] to 13, further characterized in that the hydrated lime used has a specific surface greater than 30 m2 / g, preferably greater than 40 m2 / g, and a pore volume per nitrogen desorption greater than 0.2 cm3 / g, preferably greater than 0.3 cm3 / g.

15. The use of the extrusion product according to any of claims 1 to 10 for the purification of gases and vapors.

16. The use according to claim 15, which comprises passing said gases or vapors through a stationary bed of the extrusion product.

17. The use according to claim 15, further characterized in that it comprises passing said gases or vapors countercurrent with respect to a moving bed of the extrusion product.