WO2017220167A1

WO2017220167A1 - Process for manufacturing fine and highly porous powdery slaked lime composition and product thereby obtained

Info

Publication number: WO2017220167A1
Application number: PCT/EP2016/064740
Authority: WO
Inventors: Marion Lorgouilloux
Original assignee: S.A. Lhoist Recherche Et Developpement
Priority date: 2016-06-24
Filing date: 2016-06-24
Publication date: 2017-12-28

Abstract

Process for manufacturing a fine and highly porous powdery slaked lime composition comprising a fluidification step for forming said fine and highly porous powdery slaked lime composition having an Alpine flowability greater than 50% and being performed in a dryer-crusher chosen in the group consisting of a pin-mill dryer, a cage mill dryer, a flash dryer de-agglomerator and the combination thereof until the powdery slaked lime composition presents a non-solid residual phase content equal to or less than 3.5 weight % and equal to or higher than 0.3 weight %, and the product thereby obtained.

Description

"Process for manufacturing fine and highly porous powdery slaked lime composition and product thereby obtained"

The present invention relates to a process for manufacturing a fine and highly porous powdery slaked lime composition comprising the steps of feeding quicklime in a feeding zone of an hydrator, feeding water in the feeding zone of the hydrator, slaking said quicklime in a slaking zone of the hydrator by means of a quantity of water that is sufficient to obtain a slaked lime with a non-solid residual phase content between 15% and 55% by weight, preferably between 15 and 35% by weight, drying and crushing said slaked lime to form the powdery slaked lime composition.

By the terms "highly porous powdery slaked lime composition", it is meant in the meaning of the present invention, a powdery slaked lime composition presenting a high BET specific surface area and a high BJH pore volume, meaning a BET specific surface area obtained from nitrogen adsorption equal to or greater than 25 m²/g and a total BJH pore volume consisting of pores with a diameter lower than 1000 A equal to or greater than 0.15 cm³/g.

By the terms "powdery slaked lime composition", it is meant in the present invention, a slaked lime composition composed of loose and individual particles of calcium hydroxide. By the terms "fine powdery slaked lime composition", it is meant according to the present invention, a powdery slaked lime composition presenting a d_9g particle size lower than 200 μηη, in particular lower than 150 μητι. In this perspective, the powdery slaked lime composition differs notably from granules or pebbles which present higher particle size. Calcium oxide, CaO, is often referred to as "quicklime", while calcium hydroxide, Ca(OH)₂, is referred to as "hydrated lime" or "slaked lime", both sometimes being informally referred to as "lime". In other words, lime is an industrial product, based on calcium oxide or hydroxide, respectively.

By quicklime, it is meant a mineral solid material for which the chemical composition is mainly calcium oxide, CaO. Quicklime is usually obtained by calcination of limestone (mainly CaC0₃). Quicklime may also contain impurities, such as magnesium oxide, MgO, sulphur oxide, S0₃, silica, Si0₂ or even alumina, Al₂0₃,..., the sum of which being at a level of some weight %. The impurities are expressed herein under their oxide form, but of course, they might appear under different phases. Quicklime contains generally also some weight % of residual limestone, called unburned residues.

The quicklime suitable according to the present invention may comprise MgO at an amount comprised in the range of 0.5 to 10 weight %, preferably equal to or lower than 5 weight %, more preferably equal to or lower than 3 weight %, most preferably equal to or lower than 1 weight % with respect to the total weight of the quicklime.

Typically, to form slaked lime, quicklime is provided in the presence of water. Calcium oxide in the quicklime reacts quickly with water to form calcium di- hydroxide Ca(OH)₂, in the form of slaked lime or hyd rated lime, in a reaction called hydration or slaking reaction which is very exothermic. In the following, calcium di- hydroxide will be simply called calcium hydroxide.

The slaked lime may therefore contain the same impurities than those of the quicklime from which it is produced.

The slaked lime according to the present invention may also comprise Mg(OH)₂ at an amount comprised in the range of 0.5 to 10 weight %, preferably equal to or lower than 5 weight %, more preferably equal to or lower than 3 weight %, most preferably equal to or lower than 1 weight % with respect to the total weight of the slaked lime.

The slaked lime may also comprise calcium oxide, which might not have been entirely hydrated during the slaking step, or calcium carbonate CaC0₃. The calcium carbonate can be originated from the original limestone (unburned) from which said slaked lime is obtained (via calcium oxide) or being the result of a partial carbonation reaction of slaked lime through the contact with an atmosphere containing C0₂. The amount of calcium oxide in the slaked lime according to the present invention is typically equal to or lower than 3 weight %, preferably equal to or lower than 2 weight % and more preferably equal to or lower than 1 weight % with respect to the total weight of the slaked lime.

The amount of C0₂ in the slaked lime (mainly under the form of CaC0₃) according to the present invention is equal to or lower than 5 weight %, preferably equal to or lower than 3 weight %, more preferably equal to or lower than 2 weight %, with respect to the total weight of the slaked lime according to the present invention.

The slaking reaction is commonly performed in a hydrator, in which quicklime is fed upstream of the slaking direction, meaning the direction along which lime is transported along and into the hydrator. Slaked lime is withdrawn downstream the slaking direction. Transportation means, such as a horizontal shaft equipped with mixing paddles for example, allow the transportation of lime along the slaking direction into the hydrator, from the quicklime feeding until the slaked lime withdrawal. The transportation means allow also homogeneous mixture of lime undergoing hydration and therefore improves the contact between water and lime into the hydrator and avoids the formation of hot spots.

A hydrator can be divided into different consecutive zones. The first one is called the feeding or mixing zone and constitutes the part of the hydrator located upstream the slaking direction, in which quicklime and water are fed and mixed together. The second zone, called slaking zone, represents the part of the hydrator in which the slaking reaction mostly occurs, meaning in which most of quicklime is chemically converted into slaked lime and in which most of the vapor is generated, notably due to this exothermic reaction.

Different kinds of hydration process and hydrator exist, depending on the properties of the quicklime used, but also on the expected yield of the slaking reaction and on the desired properties of the resulting slaked lime.

In order to reach good hydration yield, several parameters shall be taken into account, such as the residence time of the lime in the hydrator, the water reactivity of the quicklime, the location of both the quicklime and of the water feeding along the hydrator, but also the amount of water with respect to the lime amount. The water reactivity of quicklime is generally characterized and measured by the procedure given in the European standard EN459-2 and is often quantified by the value t₆₀, being the necessary time to reach a temperature of 60°C for a water volume of 600 cm³ initially at 20°C, with the addition of 150 g of quicklime.

During hydration of quicklime, more or less fine particles are produced, depending on the particle size of the starting quicklime being fed, but also depending on the speed of the hydration reaction, which latter is explosive and generates cracked and exploded small particles. A well-controlled slaking reaction is therefore difficult to reach while important for producing the whished particle size (from fine particles to generation of lime grains, being particles agglomerated together) as well as the wished porosity. In this perspective, the temperature inside the hydration or slaking zone is also a key factor governing the hydration reaction.

Slaked lime compositions are commonly industrially obtained through different processes depending on the amount of water with respect to the lime used.

In a first manufacturing process called « dry slaking mode », the water is added into the hydrator at an amount limited to the one which is necessary for fully hydrating the quicklime, taking into account that some of which will evaporate during the slaking reaction, due to the exothermic character of such reaction.

At the exit of the hydrator, the resulting slaked iime product is a standard slaked lime composition in a powdery state presenting a BET specific surface area typically between 12 and 20 m²/g and comprising generally less than 2 weight %, even less than 1.5 weight % of moisture (free water).

Standard slaked limes are typically used in a lot of industrial applications like water treatment, sludge conditioning, flue gas cleaning, agriculture, construction, etc.

For some of those applications, the properties of the slaked lime are particularly critical for achieving good performance. For example, in the flue gas cleaning, lime is used as a sorbent of several gaseous pollutants such as HCI, HF, SO_x, NO„. However, such lime, once having captured those pollutants, is a by-product that needs to be treated or recycled. Therefore, the industrials are looking for high performing sorbent in order to reduce the amount of by-product, the treatment of which being expensive.

A way to increase the performance of lime consists in increasing the proportion of the hydrated lime that will actually enter into contact with the pollutants to capture, notably by reducing the particle size and/or increasing the specific surface area and/or the pore volume of the hydrated lime.

Hence, during the past years, more and more products and manufacturing processes were developed in order to control the properties of slaked lime, notably its particle size, pore volume and/or specific surface area, in order to improve its sorption performance.

A first approach consists of producing slaked lime with high specific surface area, by slaking quicklime with alcohol, as notably described in document US5492685, or in the presence of particular additives such as (di-, tri- or poly- )ethylene glycol or (di-, tri- or poly-)ethanolamine, as notably described in document WO9209528.

Another method for producing slaked lime with high specific surface area consists of slaking quicklime with an excess of water so as to obtain, at the exit of the hydrator, a wet slaked lime composition presenting a residual moisture content between 15 and 35 weight %. The wet slaked lime composition is then further dried in a dryer device in order to reduce the moisture content and to form a dried powdery slaked lime composition. Such method is generally called "semi wet process" and is notably disclosed in documents WO 97/14650 and US2894820.

More precisely, in document WO 97/14650, in the name of the applicant, the drying step is followed or simultaneously combined with a grinding step so as to control the particle size of the slaked lime. The resulting powdery slaked lime composition consists essentially of dried calcium hydroxide particles having a residual moisture content of less than 2 weight % of the total composition, a high specific surface area (greater than 30 m²/g) together with a high pore volume (total nitrogen desorption pore volume of at least 0.1 cm /g for pores with a diameter less than 1000 Angstroms). This lime composition further presents an Alpine flowability comprised between 40 and 50% and is disclosed as providing excellent performance for the cleaning of flue gas in installations comprising a bag filter.

However, the document discloses lab or pilot scale manufacturing. Moreover, during the last decades, the environmental legislations have generally been drastically strengthened, in terms of quantity of pollutant authorized in flue gases and in terms of the treatment of the by-product, forcing therefore the industrials to find solutions with improved sorption capacity.

Moreover, it has been discovered that powdery slaked lime compositions obtained with these "semi wet processes" present insufficient flowability for pneumatic transport, especially when presenting high pore volume. This issue has notably been underlined in document JP4341229 which teaches, as a solution, adding organic additive having two or more alcoholic hydroxy I groups, at an amount of 0.05 to 2 weight % with respect to the weight of slaked lime.

The insufficient flowability of the powdery slaked lime composition generates recurring problem of clogging and sticking phenomena during its manufacturing process, its storage but also during its transportation and its further use, which are responsible of additional maintenance leading to additional costs and a decrease of the production yield.

Also, a powder which is not flowable tends to stick on the walls of the container and is thereafter difficult to remove from these walls leading therefore to the loss of a non-negligible quantity of product. In addition, if these deposits are present on the walls of the transportation lines, the application of the powdery slaked lime is compromised due to blockages which are difficult to remove.

Indeed, during its manufacturing process or during its further use in industrial applications, the powdery slaked lime composition is handled and transported, notably by screws, paddles or by air in ducts where the particles are distributed in the gas phase. Subsequently, the powdery slaked lime composition is commonly stored in the compressed state, for example in silos.

The flowability of a powdery compound such as a powdery slaked lime composition depends on multiple parameters, some of which being difficult to control. However, a fluctuation in the flowability of the powdery slaked lime composition is not acceptable in an industrial process because it may lead to variations in the productivity but also to unpredictable clogging phenomena in the installation.

The characteristics of flowability of a powder are, amongst other, governed by the size of the particles composing the powder (see article "Flow properties of powders and bulks solids", Dietmar Schulze htto://dietmar- schulze.de/grdlel.pdfl.

In particular the flowability of a powder generally decreases when the size (for example the diameter) of the particles composing the powder decreases.

The size of the particles composing the powdery slaked lime composition depends on different parameters.

The first parameter influencing the size of the particles is the particle size of the starting quicklime being used to form the slaked lime. Furthermore, the speed of the hydration reaction and the temperature inside the hydrator are also key factors governing the hydration reaction and therefore the size of the particles composing the final powdery slaked lime composition.

Obtaining a process able to produce a powdery slaked lime composition with controlled properties reproducible over time is therefore challenging as it depends on multiple parameters difficult to control during the process of manufacturing. There is therefore a need to reach a way to manufacture highly porous powdery slaked lime, with improved sorption capacities, being industrially feasible and having controlled properties reproducible over time and which is easy to handle in order to avoid clogging and sticking phenomena during the manufacturing process but also during the storage, the transportation and the further use of the powder.

To solve this problem, the invention provides a process as mentioned in the beginning characterized in that said drying and crushing steps are simultaneously performed and are a single fluidification step of the slaked lime for forming said fine and highly porous powdery slaked lime composition having an Alpine flowability greater than 50% and are performed in a dryer-crusher chosen in the group consisting of a pin mill dryer, a cage mill dryer, a flash dryer de- agglomerator and the combination thereof until the fine and highly porous powdery slaked lime composition presents a non-solid residual phase content, measured by a loss on ignition test at 180°C, equal to or less than 3.5 weight %, preferably equal to or less than 3 weight %, in particular equal to or less than 2.5 weight %, notably equal to or less than 2 weight %, and equal to or higher than 0.3 weight %, preferably equal to or higher than 0.5 weight %, with respect to the total weight of the powdery slaked lime composition.

The dryer-crushers suitable according to the present invention are dryer-crusher devices in which the drying and the crushing steps are performed simultaneously and in a period of time comprised between a few seconds and a few minutes (flash dryer-crusher). In this sense, the dryer-crusher devices according to the present invention differ notably from devices performing indirect drying, such as a drum dryer, a disc dryer or a paddle dryer, vacuum drying, freeze drying or fluid bed drying. By the terms "flash dryer de-agglomerator", it is meant according to the present invention, a flash dryer in which there is a rotor or rotating paddles at the bottom of the drying chamber that fluidizes the product and creates turbulences in the hot air flow that is entering the drying chamber tangentially. By doing so, the wet (agglomerated) slaked lime is rapidly dispersed and disintegrated into fine dried particles. The resulting fine particles exit the drying chamber from its top whereas larger particles remain in the chamber for further drying and de-agglomeration.

Examples of flash dryer de-agglomerator suitable according to the present invention include notably the An hydro Spin Flash Dryer* commercialized by SPX FLOW, the Drymeister* Flash Dryer commercialized by Hosokawa Micron Group or the Swirl fluidizer™ commercialized by GEA Group.

The process according to the present invention allows keeping or even improving the sorption properties of the powdery slaked lime composition while making the process easier. Indeed, against all expectations, it has been found that performing the drying and the crushing steps simultaneously in a dryer-crusher chosen in the group consisting of a pin mill dryer, a cage mill dryer, and a flash dryer de-agglomerator as previously defined does not reduce the porosity features of the slaked lime composition, in the contrary. It has also been possible by carrying out the drying step and the crushing step together to improve the flowability properties of the slaked lime composition while reaching the required fineness.

Indeed, dryer and crusher are devices that could notably influence the size distribution but also the shape of the particles of the powdery slaked lime and consequently influence the flowability of the powder. Moreover, due to the high temperature used during the drying of the powder, the inner structure of the powdery slaked lime could also be modified and therefore, the drying step could also damage the porosity features of the powder. Crushing is also a step which can have a negative impact on the porosity features of the powder. By the terms "fluidification step" it is meant, according to the present invention, a step that makes a powdery composition flowable.

Generally, in order to further improve the sorption properties of a powdery slaked lime composition, it is known to decrease the size of the particles. Indeed powdery slaked lime compositions comprising smaller particles present an increased efficiency of the treatment. More precisely, the flue gas treatment is improved due to a better dispersion of the powdery composition in the gaseous phase (flue gas) and a quicker contact between the pollutants and the lime particles of the composition. Moreover, smaller particles present a higher external contact surface, increasing therefore the proportion of hydrated lime that will actually enter into contact with the pollutants to capture.

However, the flowability of a powder generally decreases when the size of the particles composing this powder decreases. Smaller particles are known to cause the decrease of the flowability of the powder because of the important interactions between the particles which generate the cohesion of the powder. This is notably illustrated in Geldart et al. which teaches that the flow properties of a powder measured with different flow behaviour measurement devices indicate more difficult flow behaviour when the particle size is reduced. Indeed, such document illustrated that the so-called Warren Spring Bradford Cohesion Tester (WSBCT), the Johanson cohesion indicator and the poured angle of repose and the Jenike cohesion all show an increase of their value with decreasing particle size (Geldart Geldart, D.; Abdullah, E. C; Verlinden, A. Characterisation of Dry Powders. Powder Techno!. 2009, 190 (1-2), 70-74). Accordingly, up to now, the man skilled in the art has always been forced to make compromise between improved sorption properties and sufficient flowabi!ity of its powdery sorbent.

By the term flowability (fluidite in French), sometimes also called fluidity, it is meant in the present invention the ability of a powder to flow freely, in an even way as individual particles.

The flowability of the powdery slaked iime composition according to the present invention, has been measured on an Alpine air jet sieve device. This Alpine flowability characterises the static flowability of a powder and is determined by the speed of passage of the particles with a diameter of less than 90 microns through a sieve of 90 microns (170 mesh) through the action of a suction. The Alpine flowability expressed in % corresponds to the ratio between the weight of the fraction of less than 90 microns which has passed through the sieve in 15 seconds (with a depression of 100 mm of manometric liquid of density 0.88) and the total weight of the fraction of less than 90 microns which has passed through the sieve after 2 minutes (with a depression of 150 mm of manometric liquid of density 0.88).

The behavior of a powder into a storage silo can be simulated with another method using a powder rheometer such as a Brookfield Powder Flow Tester (PFT) according to the standard ASTM D6128. In this method, a powdery sample introduced in the equipment is subjected to increasing compaction overtime. For each compression step (principal consolidating stress), a specific torque is applied to the powder until failure (unconfined failure stress). The response of the powder to the applied stress is recorded by a computer, which evaluates the static cohesiveness of the tested sample. The results are expressed with a curve, which is compared to ASTM references. The powdery slaked lime composition of the invention is also characterized by a dynamic flowability which can be measured by a Granudrum. In this method, a quantity of the powdery material is placed into a drum having transparent windows, which is rotated and accelerated stepwise from 0 to 20 rpm, than stepwise decelerated. The shape of the rotating powder heap (air/powder interface) inside the drum is analyzed by an algorithm. A dynamic flow angle and a dynamic cohesiveness index are determined for each rotation speed. St has been identified in the present invention that in order to obtain a powdery slaked lime composition with a dynamic flowability (measured by granudrum) sufficient for avoiding clogging and sticking phenomena during transportation and further industrial use, such powdery slaked lime composition must present an Alpine flowability greater than 50%,

It was therefore highly unexpected to reach a highly porous powdery slaked lime composition having an Alpine flowability greater than 50 % while being at the same time a fine composition. According to the process of the present invention, the use of a specific dryer-crusher used under the specific conditions allows to reach a reproducible slaked lime composition having fineness features but also flowability features by controlling the water content and therefore by controlling not only the crushing step but also the drying step while those are performed together in a single fluidification step.

Indeed, the individual particle size as well as the porosity properties of the slaked lime particles are defined by the slaking process. However, due to the high amount of water used during this slaking step, the resulting slaked lime particles, at the exit of the hydrator, present a water content of 15 to 55% by weight, preferably 15% to 35% by weight, which acts as a binding agent and bonds the slaked lime particles together into larger agglomerates. By using specific dryer-crushers under specific conditions, according to the present invention, it has been made possible to de-agglomerate and disperse the particles of Ca(OH)_2, formed during the slaking step, notably by controlling their water content, into individual particles without grinding them and by consequence without deteriorating their properties (individual particle size, specific surface area, pore volume,...).

By the terms "non-solid residual phase content of the slaked lime composition" according to the present invention it is meant the proportion of non- solid residual phase of the slaked lime composition (i.e. the water content, such as the free water content, and/or the content of residual additives from the manufacturing process of the said slaked lime composition, meaning originated from additives added before, during or after the slaking of quicklime) determined by a loss on ignition test. The loss on ignition test consists of heating, at atmospheric pressure, around 20 g of the powdery slaked lime composition at a predetermined temperature, namely 110 °C or 180 "C, and measuring the weight over time of the powdery composition by means of a thermal balance until the weight of the powder does not vary of more than 2 mg during at least 20 seconds. During the heating of the powder, all the components, notably the non-solid components, having an evaporation temperature lower than that applied during the test, are removed from the powder and their content corresponds consequently to the weight loss measured during the test. The non-solid residual phase therefore contents all non-solid components, notably liquid components, having together an evaporation temperature lower than that applied, which will then leave the slaked lime composition during the heating process at the predetermined temperature. The weight % of the non-solid residual phase and of the remaining solid, called the dry extract, are both calculated based on the weight of the product before ignition and after ignition and both expressed with respect to the weight of the product before the ignition test.

The loss on ignition result may therefore vary depending on the temperature used during the test. For example, it may be higher at 180 °C than at 110 °C if additives are used during the slaking process or after, and if such additives or their derived phases present an evaporation point higher than 110 °C and lower than 180 °C, or form with the free water an azeotropic substance or an aqueous mixture that evaporates between those temperatures.

The non-solid residual phase content of the powdery slaked lime composition according to the present invention can be measured through a loss on ignition test at 180 °C. In such a case, the loss on ignition result is equal to or higher than 0.3 weight %, preferably equal to or higher than 0.5 weight %, and equal to or smaller than 3.5 weight %, preferably equal to or smaller than 3 weight %, in particular equal to or smaller than 2.5 weight %, notably equal to or smaller than 2 weight %, and represents the quantity of water and/or substances contained therein having an evaporation point less than or equal to 180 °C.

The non-solid residual phase content of the powdery slaked lime composition according to the present invention can be further measured through a loss on ignition test at 110 °C. In such a case, the loss on ignition value is equal to or smaller than 3.2 weight %, preferably equal to or smaller than 2.7 weight %, advantageously equal to or smaller than 2.5 weight %, in particular equal to or smaller than 2 weight %, notably equal to or smaller than 1.5 weight % and higher than 0 weight %, preferably equal to or higher than 0.2 weight %, advantageously equal to or higher than 0.3 weight %, in particular equal to or higher than 0.5 weight % and represents mainly the quantity of water and/or volatile substances contained therein having an evaporation point less than or equal to 110 T, in particular water.

Advantageously, in the process according to the present invention, the drying-crushing step is performed until the powdery slaked lime composition presents a non-solid residual phase content measured by a loss on ignition test at 180°C equal to the following formula:

LOI 180°C > LOI HOT + 0.2%

wherein

- LOI 180°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 180T with respect to the weight of the powdery slaked lime composition;

- LOI 110°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 110°C and is higher than or equal to 2 weight % and smaller than or equal to 2.5 weight %, with respect to the weight of the powdery slaked lime composition.

In another embodiment according to the present invention, the drying-crushing step is performed until the powdery slaked lime composition presents a non-solid residual phase content measured by a loss on ignition test at 180°C equal to the following formula:

LOI 180T≥ LOI HOT + 0.3%

wherein

- LOI 180T represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 180T with respect to the weight of the powdery slaked lime composition;

- LOI HOT represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at HOT and is either smaller than 0.3 weight %, or higher than 2.5 weight % and smaller than or equal to 3.2 weight %, with respect to the weight of the powdery slaked lime composition. Advantageously, in the process according to the present invention, the drying-crushing step is performed until the powdery slaked lime composition presents a non-solid residual phase content measured by a loss on ignition test at 180T equal to the following formula:

LOi 180T > LOI HOT + 0.4% wherein

- LOI 180°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 180°C with respect to the weight of the powdery slaked lime composition;

- LOI HOT represents the non-soiid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at HOT and is either smaller than 0.3 weight %, or higher than 2.5 weight % and smaller than or equal to 3.2 weight %, with respect to the weight of the powdery slaked lime composition.

Preferably, in the process according to the present invention, the drying-crushing step is performed until the powdery slaked lime composition presents a non-solid residual phase content measured by a loss on ignition test at 180T equal to the following formula:

LOI 180T > LOI HOT + 0.5%

wherein

- LOI HOT represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at HOT and is either smaller than 0.3 weight %, or higher than 2.5 weight % and smaller than or equal to 3.2 weight %, with respect to the weight of the powdery slaked lime composition.

In another particular embodiment according to the present invention, the drying-crushing step is performed until the powdery slaked lime composition presents a non-solid residual phase content measured by a loss on ignition test at HOT equal to or higher than 0.3 weight %, and smaller than 2 weight

%. Depending on the hydration process used during the manufacturing process, the slaked lime at the exit of the hydrator may contain more or less water content. At the exit of the hydrator, the quantity of water contained in the slaked lime composition causes the agglomeration of the particles but also a cohesive composition.

As previously said, in the process according to the present invention, the slaking process is a semi wet process, wherein the quicklime is slaked by means of a quantity of water that is sufficient to obtain a slaked lime at the exit of the hydrator containing a non-solid residual phase between 15% and 55% by weight, preferably between 15 % and 35 % by weight with respect to the weight of the slaked lime.

In the technology used to produce the slaked lime, the water content is reduced by performing a drying step as the final product should be in a powdery state. It is indeed known in the state of the art that to reach a powder with a relatively good flowability, it is important to decrease as much as possible the water content of the powder.

Surprisingly, it has been found that in order to produce a powder presenting a high flowability, the non-solid residual phase content contained in the powder and measured by a loss on ignition test at 180°C, has to be maintained between 0.3 weight % and 3.5 weight % with respect to the total weight of the powdery slaked lime composition. If the non-solid residual phase content is decreased below 0.3 weight %, the flowability of the powder decreases surprisingly and the composition becomes sticky (high adhesion), which is contradictory to the common knowledge of the skilled in the art according to which the non-solid residual phase content of a powder has to be decreased as much as possible if a powder presenting a good flowability is desired.

As already mentioned, the water plays the role of a binding agent between the particles of the powder which may result in the agglomeration of these particles and consequently in the formation of cohesive agglomerates. In order to improve the fineness of the particles of the powder, it is therefore important to remove water from the composition to be able to disperse and de-agglomerate the particles. Despite the fact that the dryer-crusher selected in the process according to the present invention is able to remove the water present in the composition and therefore to disperse the particles, it also allows keeping a good flowability of the powder.

Consequently, the process according to the present invention by using a specific kind of limited dryer-crusher types under conditions to control fineness, flowability and water content, allows the production of a powdery slaked lime composition very efficiently, not being prejudicial for the sorption properties as the porosity properties of the slaked lime composition are maintained and even increased.

Indeed, the particular simultaneous drying and crushing step allows reaching fine particles while avoiding clogging and interruption of the process thanks to the improved flowability.

Further, the particular simultaneous drying and crushing step allows better controlling the properties of the slaked lime which are more constant over time from one production to another.

Without limiting the invention by this interpretation, it is believed that performing the drying and crushing steps simultaneously in a single step allows better controlling the particle size of the resulting slaked lime as well as avoiding deteriorating its pore volume. Moreover, the specific dryer-crushers used according to the present invention also allow performing the drying in a very short period of time (flash drying) avoiding therefore the risk of carbonation of the slaked lime which is detrimental for its porosity properties.

On the contrary, performing the drying step before the crushing step will lead to the formation of very solid and rigid agglomerates of slaked lime particles that will require higher energy to de-agglomerate them, with the additional risk of deteriorating the porosity of slaked lime, while performing the crushing step before the drying step is very difficult to achieve industrially due to the handling problem of the wet slaked lime particles.

The reproducible side of the process according to the present invention, by allowing to reach high flowability despite the fineness of the slaked lime composition without being detrimental for the porosity feature has allowed to reach a very interesting process at an economical point of view since the continuous character of the process is improved and the manufactured product more even in terms of quality, preventing to have to discard some production not meeting the very high standard on the market nowadays. Indeed, according to the present invention, the combination of the improved control of the fineness and of the water content allows to reach a specific and reproducible quality of powdery slaked lime composition with high porosity features, together with a high flowability.

The slaked lime composition, manufactured by the process according to the present invention, when leaving the step of simultaneous drying and crushing still presents a high BET specific surface area, reproducible from a production campaign to another and being comprised between 30 m²/g and 55 m²/g, preferably higher than or equal to 32 m²/g, more preferably higher than or equal to 35 m²/g, more particularly higher than or equal to 38 m²/g such as for instance higher than or equal to 40 m²/g and typically lower than or equal to 50 m²/g, in particular lower than or equal to 48 m²/g .

By the expression "BET specific surface area", it is meant according to the present invention, the specific surface area measured by manometry with adsorption of nitrogen at 77 K after degassing under vacuum at a temperature comprised between 150 and 250°C, notably at 190T for at least 2 hours and calculated according to the multipoint BET method as described in the ISO 9277:2010E standard.

Moreover, when leaving the simultaneous drying and crushing step of the process according to the present invention, the highly porous powdery slaked lime composition further presents a total BJH pore volume reproducible from a production campaign to another and being higher than or equal to 0.15 cm³/g, preferably higher than or equal to 0.17 cm³/g, advantageously higher than or equal to 0.18 cm³/g, more preferably higher than or equal to 0.19 cm³/g, i particular higher than or equal to 0.20 cm³/g, particularly higher than or equal to 0.21 cm³/g and typically lower than 0.30 cm³/g, in particular lower than 0.28 cm³/g-

By the terms "BJH pore volume" according to the present invention, it is meant the pore volume as measured by manometry with adsorption of nitrogen at 77 K after degassing under vacuum at a temperature comprised between 150 and 250°C, notabiy at 190°C for at least 2 hours and calculated according to the BJH method, using the desorption curve.

By the terms "total pore volume" according to the present invention, it is meant the BJH pore volume composed by the pores with a diameter smaller than 1000 Angstroms.

Advantageously, the process according to the present invention is further characterized in that the drying and crushing step is performed in a cage mill dryer, said cage mill dryer being composed of either one, three or five wheels, such as for example without being limited thereto, in a cage mill dryer commercialized by PSP Engineering or by Stedman™.

In another advantageous embodiment, the process according to the present invention is further characterized in that the drying and crushing step is performed in a pin mill dryer, such as for example without being limited thereto, in a Atritor Dryer-Pulverizer commercialized by Atritor Limited.

Alternatively, the process according to the present invention is further characterized in that the drying and crushing step is performed in a flash dryer de-agglomerator, such as for example without being limited thereto, in a Anhydro Spin Flash Dryer* commercialized by SPX FLOW, a Drymeister* Flash Dryer commercialized by Hosokawa Micron Group or a Swirl fluidizer™ commercialized by GEA Group.

In this particular embodiment, a classifier can be advantageously further added, on top of the drying chamber, for better controlling the particle size distribution of the resulting dried powdery slaked lime composition. In a particular embodiment, the process according to the present invention also comprises, before, during and/or after the slaking step and/or before, during and/or after the drying and crushing step, a step of adding an additive to the quicklime, the slaking water and/or to the slaked lime.

For example, the additive added during the process according to the present invention is diethy!ene glycol. In this particular case, diethyiene glycol forms with water a binary aqueous mixture that evaporates at temperatures higher than 110°C.

The quantity of water and the quantity of diethyiene glycol contained in the powdery slaked lime composition can be respectively determined by performing a loss on ignition test both at 110°C, which will substantially indicate the quantity of water contained in the powdery slaked lime composition, and at 180°C, which will substantially indicate the quantity of water and diethyiene glycol contained in the powdery slaked lime composition. In the present invention, it will be considered that the quantity of diethyiene glycol will thus corresponds to the value obtained by subtracting the loss on ignition value obtained at 110°C to the value obtained at 180°C.

Alternatively, the additive added during the process may be an organic additive selected in the group of (mono-) or (poly-)ethylene glycol and (mono-) or (poly-)ethanolamine, in particular triethylene glycol, triethanolamine, and their mixtures.

Advantageously, alkali metal compound selected from the group consisting of alkali metal hydroxides, carbonates, hydrogencarbonates, nitrates, phosphates, persulphates and monocarboxylates, such as alkali metal acetates or formiates, and mixtures thereof, in particular those of sodium, potassium and/or lithium and/or calcium stearate may also be added during the process according to the present invention.

Preferably, the process according to the present invention is characterized in that the drying-crushing step is performed until the powdery slaked lime composition presents a mean particle size d₅₀ equal to or lower than 10 pm, preferably equal to or lower than 8 pm, advantageously equal to or lower than 7 pm, in particular equal to or lower than 6 pm.

The notation d_x represents a diameter expressed in pm, measured by laser granulometry in methanol after sonication, relatively to which X % by volume of the measured particles are smaller or equal. Advantageously, the process according to the present invention is characterized in that the drying-crushing step is performed until the powdery slaked lime composition presents a first fraction of particles having a size of less than 32 pm and a second fraction of particles of a size greater than 32 μιη with the provision that the second fraction being equal to or lower than 50 weight percent, preferably equal to or lower than 40 weight percent, advantageously equal to or lower than 30 weight percent, particularly equal to or lower than 20 weight percent, in particular equal to or lower than 15 weight percent, more preferably lower than 10 weight percent, even equal to or lower than 8 weight percent, with respect to the total weight of the powdery slaked lime composition.

For the purpose of simplicity, the terms "second fraction of particles of a size greater than 32 μιτι" will be also expressed as f½ in the rest of the specification for the fraction retained at 32 μηι.

In a particularly preferred embodiment of the process according to the present invention, hot air is fed during the drying-crushing step at a temperature comprised between 250 °C and SOOT, preferably between 350 and 400°C.

In a preferred embodiment of the process according to the present invention, the powdery slaked lime composition presents, at the exit of the drying- crushing step, a temperature comprised between 80 and 150°C, preferably between 90 and 130°C.

The temperature of the powdery slaked lime composition at the exit of the drying-crushing step can be controlled by adjusting the temperature and/or the volume of the hot air fed during the drying-crushing step and/or the mass flow of the wet slaked lime entering the drying-crushing step.

Advantageously, in the process according to the invention, the drying-crushing step has a duration comprised between a few seconds and a few minutes. Other embodiments of the process according to the invention are mentioned in the annexed claims.

The invention relates also to a fine powdery slaked composition comprising slaked lime particles having a BET specific surface area obtained from nitrogen adsorption equal to or greater than 25 m²/g and a total BJH pore volume equal to or greater than 0.15 cm³/g characterized in that the composition furthermore has an Alpine flowability greater than 50 %, preferably equal to or greater than 51 %, more preferably equal to or greater than 52 %, advantageously equal to or greater than 54 %, in particular equal to or greater than 55 % and said aforementioned composition has a non-solid residual phase content measured by a loss on ignition test at 180°C, equal to or less than 3.5 weight %, preferably equal to or less than 3 weight %, in particular equal to or less than 2.5 weight %, notably equal to or less than 2 weight %, and equal to or higher than 0.3 %, preferably equal to or higher than 0.5 weight %, with respect to the total weight of the powdery slaked lime composition.

The slaked lime may contain the same impurities than those of the quicklime from which it is produced, such as magnesium oxide, MgO, sulphur oxide, S0₃, silica, Si0₂ or even alumina, Al₂0₃,..., the sum of which being at a level of some weight %. The impurities are expressed herein under their oxide form, but of course, they might appear under different phases.

In particular, the slaked lime according to the present invention may comprise magnesium under the form of MgO and/or Mg(OH)₂, at an amount comprised in the range of 0.5 to 10 weight %, preferably equal to or lower than 5 weight %, more preferably equal to or lower than 3 weight %, most preferably equal to or lower than 1 weight % expressed under the oxide form, with respect to the total weight of the slaked lime composition.

The amount of C0₂ in the slaked lime (mainly under the form of CaC0₃) according to the present invention is typically equal to or lower than 5 weight %, preferably equal to or lower than 3 weight %, more preferably equal to or lower than 2 weight %, with respect to the total weight of the slaked lime according to the present invention.

In a preferred embodiment, the amount of available lime present in the powdery slaked lime composition according to the present invention is equal to or greater than 85 weight %, preferably equal to or greater than 87 weight %, preferentially equal to or greater than 90 weight %, advantageously equal to or greater than 92 weight %, and even equal to or greater than 95 weight % with respect to the dry content of the slaked lime composition after LOI at 180°C.

In another preferred embodiment, the remaining weight % are mainly made of limestone-origin compounds and of residues originated from the non-solid residua! phase.

By the terms "amount of available lime", it is meant in the present invention, the quantity of calcium hydroxide and/or of calcium oxide present in the powdery slaked lime composition, measured by a method disclosed in the standard EN-459-2 2010. More precisely, in the present invention, the available lime content present in the powdery slaked lime composition is determined by putting 0.5 g of the powdery slaked lime composition into a sugar solution (15 g of sugar in 150 cm³ of deionized water). The sugar solution will dissolve the available lime (i.e. the calcium oxide and/or calcium hydroxide) contained in the sample. The resulting mixture is agitated during at least 10-15 minutes to insure complete dissolution and then titrated with a solution of hydrochloric acid (HCI 0.5 N), phenolphthalein being used as indicator. The Ca concentration measured by this titration is then expressed as Ca(OH)₂.

In a particular embodiment according to the present invention, the powdery slaked lime composition according to the present invention has a non-solid residual phase content, measured by a loss on ignition test at 110°C, equal to or smaller than 3.2 weight %, preferably equal to or smaller than 2.7 weight %, advantageously equal to or smaller than 2.5 weight %, in particular equal to or smaller than 2 weight %, notably equal to or smaller than 1.5 weight % and higher than 0 weight %, preferably equal to or higher than 0.2 weight %, advantageously equal to or higher than 0.3 weight %, in particular equal to or higher than 0.5 weight %. In one embodiment, the powdery slaked lime composition according to the present invention has a non-solid residual phase content, measured by a loss on ignition test at 180°C, equal to the following formula:

LOS 180T > LOI HOT + 0.2%

wherein

- LOI 180°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 180°C;

In another embodiment according to the present invention, the powdery slaked lime composition according to the present invention has a non-solid residual phase content, measured by a loss on ignition test at 180°C, equal to the following formula:

LOI 180T > LOI 110°C + 0.3%

wherein

- LOI 110°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 110°C and is either smaller than 0.3 weight %, or higher than 2.5 weight % and smaller than or equal to 3.2 weight %, with respect to the weight of the powdery slaked lime composition.

Advantageously, the powdery slaked lime composition according to the present invention has a non-solid residual phase content measured by a loss on ignition test at 180°C equal to the following formula:

LOI 180T > LOI HOT + 0.4% wherein

Preferably, the powdery slaked lime composition according to the present invention has a non-solid residual phase content measured by a loss on ignition test at 180°C equal to the following formula:

LOI 180°C > LOI HOT + 0.5%

wherein

- LOI 180T represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at 180°C with respect to the weight of the powdery slaked lime composition;

- LOI 110°C represents the non-solid residual phase content of the powdery slaked lime composition measured by a loss on ignition test at HOT and is either smaller than 0.3 weight %, or higher than 2.5 weight % and smaller than or equal to 3.2 weight %, with respect to the weight of the powdery slaked lime composition. In another embodiment according to the present invention, the powdery slaked lime composition has a non-solid residual phase content measured by a loss on ignition test at HOT equal to or higher than 0.3 weight %, and smaller than 2 weight %.

Preferably, the powdery slaked lime composition according to the present invention presents a non-solid residual phase comprising water and/or residual additives (mineral and/or organic), free or linked to the lime compound.

Advantageously, the slaked lime particles of the powdery slaked lime composition according to the present invention presents a particle size d₅₀ equal to or lower than 10 μιτι, preferably equal to or lower than 8 μιτι, advantageously equal to or lower than 7 μιη, in particular equal to or lower than 6 μηη.

The notation d_x represents a diameter expressed in μιη, measured by laser granulometry in methanol after sonication, relatively to which X % by volume of the measured particles are smaller or equal.

Preferably, the powdery slaked lime composition according to the present invention comprises a first fraction of particles having a size of less than 32 μιη and a second fraction of particles of a size greater than 32 μιη, the second fraction being equal to or lower than 50 weight percent, preferably equal to or lower than 40 weight percent, advantageously equal to or lower than 30 weight percent, particularly equal to or lower than 20 weight percent, in particular equal to or lower than 15 weight percent, more preferably lower than 10 weight percent, even equal to or lower than 8 weight percent with respect to the total weight of the powdery slaked lime composition.

For the purpose of simplicity, the terms "second fraction of particles of a size greater than 32 μηι" will be also expressed as ₃₂ in the rest of the specification for the fraction retained at 32 μηι.

Advantageously, the powdery slaked lime composition according to the present invention comprises slaked lime particles having a BET specific surface area obtained from nitrogen adsorption equal to or greater than 30 m²/g, preferably equal to or greater than 32 m²/g, advantageously equal to or greater than 35 m²/g.

In a particularly preferred embodiment, the slaked lime particles of the powdery slaked lime composition according to the present invention have a BET specific surface area obtained from nitrogen adsorption equal to or lower than 55 m²/g, preferably equal to or lower than 50 m²/g, in particular equal to or lower than

48 m²/g.

Preferably, the powdery slaked lime composition according to according to the present invention contains slaked lime particles presenting a total BJH pore volume consisting of pores with a diameter lower than 1000 A, obtained from nitrogen desorption equal to or greater than 0.17 cm³/g, particularly higher than or equal to 0.18 cm³/g, preferably equal to or greater than 0.19 cm³/g, in particular higher than or equal to 0.20 cm³/g, advantageously equal to or greater than 0.21 cm³/g. In another embodiment of the invention, the slaked lime particles of the powdery slaked lime composition present a total BJH pore volume consisting of pores with a diameter lower than 1000 A, obtained from nitrogen desorption equal to or lower than 0.30 cm³/g, in particular lower than 0.28 cm³/g.

Preferably, the powdery slaked composition according to the present invention contains slaked lime particles presenting a BJH pore volume consisting of pores with a diameter ranging from 100 to 300 A, obtained from nitrogen desorption equal to or greater than 0.07 cm³/g, preferably equal to or greater than 0.10 cm³/g, advantageously equal to or greater than 0.11 cm³/g, in particular equal to or greater than 0.12 cm³/g and typically lower than 0.15 cm³/g, in particular lower than 0.14 cm³/g.

Advantageously, the powdery slaked lime composition according to the present invention contains slaked lime particles presenting a BJH pore volume consisting of pores with a diameter ranging from 100 to 400 A, obtained from nitrogen desorption equal to or greater than 0.09 cm³/g, preferably equal to or greater than 0.12 cm³/g, advantageously equal to or greater than 0.13 cm³/g, in particular equal to or greater than 0.14 cm³/g and typically lower than 0.17 cm³/g, in particular lower than 0.16 cm /g.

In a particularly preferred embodiment, the powdery slaked lime composition according to the invention further presents an alkali phase characterized by an alkali metal content that is equal to or greater than 0.2 weight % and equal to or less than 3.5 weight % with respect to the total weight of the powdery slaked lime composition.

The alkali phase may be under the ionic form or under a binded form. Different kind of salts can be added during the process, in particular alkali metal compound selected from the group consisting of alkali metal hydroxides, carbonates, hydrogencarbonates, nitrates, phosphates, persulphates and monocarboxylates, such as alkali metal acetates or formiates, and mixtures thereof, in particular those of sodium, potassium and/or lithium.

In another preferred embodiment, the powdery slaked lime composition according to the present invention further contains calcium stearate.

The powdery slaked lime composition is preferably obtained by the process according to the present invention.

Other embodiments of the powdery slaked lime composition according to the invention are mentioned in the annexed claims.

The invention relates also to an industrial sorbent composition comprising at least said powdery slaked lime composition according to the invention. The invention relates also to the use of the powdery slaked lime composition according to the present invention for purifying flue gases. In particular, the powdery slaked lime composition according to the present invention is used in a dry sorbent injection.

Advantageously, the powdery slaked lime composition according to the present invention is used for capturing acidic pollutants from flue gases, such as HCI, HF, SO_x, NO_x,...

In a particular embodiment, the powdery slaked iime composition according to the present invention is used in an industrial sorbent composition, for example in combination with at least one other sorbent typically known for treating flue gas such as a sorbent selected among the list of organic compounds, in particular activated carbon, lignite coke and mixture thereof, and mineral compounds, in particular mineral compounds known for capturing dioxins, furans and/or heavy metals, such as halloysite, sepiolite, bentonite or any sorbent disclosed in the application DE4034417.

Other embodiments of the use according to the invention are mentioned in the annexed claims.

Other characteristics and advantages of the invention will appear more clearly in the light of the following description of a particular non-limiting embodiment of the invention, while referring to the figures.

Figure 1 shows a schematic diagram of a plant for manufacturing a highly porous powdery slaked lime composition according to the present invention.

The device shown in Figure 1 comprises a slaking unit, also called hydrator 1. This hydrator 1 is fed with ground quicklime via a feed line 2, and with water via a feed line 3. If an additive is used in the preparation of the absorbent, said additive is fed in via at least one feed line 4. In one embodiment, said additive is firstly placed in solution in a reservoir (not illustrated), from which it is pumped by means of a pump (not illustrated) and added to the slaking water feed line 3 before entering the hydrator 1. Alternatively, if necessary, the additive can also be added directly to the hydrator 1.

In another alternative, the additive can also be added to quicklime, prior to the slaking. The additive can also be added after the hydrator, namely before the dryer-crusher, but also in the dryer-crusher or after the dryer-crusher.

At the outlet of the hydrator, the non-solid residual phase content of the product is continuously measured by an infrared device 5. This non-solid residual phase content is generally greater than 20 weight %. The product, wet slaked lime, is transferred into a dryer-crusher 6 which is supplied with hot air, around 400 T, via the feed line 7, which makes it possible to de-agglomerate and dry the product. The end product is then separated from the flow of drying air in a bag filter 8, and then directed towards a storage silo 9. The manufacturing plant according to the present invention is characterized in that the dryer-crusher 8 is chosen in the group consisting of a pin mill dryer, a cage mill dryer and a flash dryer de-agglomerator.

The invention will now be described in greater detail by means of non-limiting examples. Example

In the following examples, as in all cases mentioned in the present document, the BET specific surface area is determined by manometry with adsorption of nitrogen at 77 K after degassing under vacuum at a temperature comprised between 150 and 250°C, notably at 190°C for at least 2 hours and calculated according to the multipoint BET method as described in the ISO 9277:2010E standard.

The BJH pore volume is measured by manometry with adsorption of nitrogen at 77 K after degassing under vacuum at a temperature comprised between 150 and 250°C, notably at 190°C for at least 2 hours and calculated according to the BJH method, using the desorption curve for the pores with a diameter lower than 1000 A.

The total pore volume corresponds to the BJH pore volume composed by the pores with a diameter smaller than 1000 Angstroms.

The notation d_x represents a diameter expressed in μιη, measured by laser granulometry in methanol after sonication, relatively to which X % by volume of the measured particles are smaller or equal. The loss on ignition test was performed according to the previously described procedure.

The Alpine flowability was measured on about 50 g of the powdery sample, according to the previously described procedure. Example 1.-

A highly porous powdery hydrated lime with a high flowability according to the invention is produced industrially by mixing water and quicklime in a hydrator (4.5 t/h of quicklime), in such amounts that the product comes out of the hydrator with a non-solid residual phase content, measured by loss on ignition test at 180°C, of 23-24 wt%. This wet hydrated lime is then transported and arrives into a pin mill dryer (Atritor Dryer-Pulverizer commercialized by Atritor Limited) in which hot air is injected (about 20 000 Nm³/h, 370°C). In this pin mill dryer, the product is de-agglomerated and dried simultaneously. Considering the high temperature of the drying air and the rotation speed (850 rpm) of the pin mill, only a very short residence time of a few minutes is necessary within the pin mill to reach the target in terms of particle size and non-solid residual phase content. The drying can therefore be considered as flash. Once dried and de-agglomerated, the slaked lime product is separated from the air by a bag filter. During the transportation of the dry slaked lime product from the pin mill dryer to the bag filter, a 100 % DEG solution is sprayed with an atomizing nozzle in the duct so as to create a DEG fog through which the slaked lime particles have to pass. By this way, a good contact between the particles and the DEG droplets is ensured, which produces a homogeneous product. The amount of DEG corresponds to 0.3 wt% of powdery slaked lime composition. This resulting powdery slaked lime composition has an Alpine flowability of 57 % and a non-solid residual phase content of 0.5 weight %, when measured by a loss on ignition test at 180°C and of 0.3 weight % when measured by a loss on ignition test at 110°C. The powdery slaked lime composition has a d₅₀ of 5 μητι and a fraction of particles of a size greater than 32 μητ> of 10 weight %. Its BET specific surface area and total pore volume are 44.1 m²/g and 0.240 cm³/g_» respectively. Example 2.-

A highly porous powdery slaked lime composition with a high flowability according to the invention is produced industrially by mixing water and quicklime in a hydrator (6 t/h of quicklime), in such amounts that the product comes out of the hydrator with a non-soiid residual phase content, measured by loss on ignition test at 180°C, ranging between 17 and 21 wt%. This wet hydrated lime is then transported and arrives into a cage mill dryer (commercialized by PSP Engineering) in which some hot air is injected (12 500 Nm³/h, 370-400 °C). The drying can here again be considered as flash and the product goes out of the cage mill dryer with a temperature close to 120-125 °C. The cage mill is composed of 5 concentric wheels, 2 being static and the other 3 ones being in rotation (rotation speed up to 900 rpm). As in example 1, once dried and de-agglomerated, the slaked lime product is separated from the air by a bag filter. In this case, there is no addition of DEG after drying, but a very low amount (< 0.1 wt% expressed as % of the quicklime weight) is added in the slaking water prior to hydration. The resulting powdery slaked lime composition has an Alpine flowability of 60 % and a non-solid residual phase content, measured by a loss on ignition test at 180°C, of 0.5 weight %. The powdery slaked lime composition has a d₅₀ of 10 μιη and a fraction of particles of a size greater than 32 μιη of 46 weight %. its specific surface area and total pore volume are 42.0 m²/g and 0.225 cm³/g, respectively.

Example 3.-

A highly porous powdery slaked lime composition with a high flowability according to the present invention is produced industrially by mixing water and quicklime in a hydrator (6.8 t/h of quicklime), in such amounts that the product comes out of the hydrator with a non-solid residual phase content, measured by loss on ignition at 180°C, ranging between 23 and 25 wt%. This wet hydrated lime is then transported and arrives into a cage mill dryer (commercialized by Stedman™) in which some hot air is injected (23600 NrnVh, 260-290 °C). The cage mill is composed this time of 3 concentric wheels (rotation speed close to 520 rpm in standard conditions). Again, once dried and de-agglomerated, the siaked lime product is separated from the air by a bag filter, in this case, like in example 2, there is no addition of DEG after drying, but 0.4 % of DEG (expressed as % of the quicklime weight) are added in the slaking water prior to hydration. The resulting powdery slaked lime composition has an Alpine flowability of 52.3 % and a non-solid residual phase content of 0.7 wt %, when measured by a loss on ignition test at 180°C, and of 0.4 wt % when measured by a loss on ignition test at 110°C. The powdery slaked lime composition has a d₅₀ of 9.3 μητι and a fraction of particles of a size greater than 32 μιη of 34.3 weight %. Its specific surface area and total pore volume are of 41.1 m²/g and 0.209 cm³/g, respectively.

Example 4.- A highly porous powdery slaked lime composition is produced industrially by mixing water and quicklime (2.7 t/h of quicklime) in an hydrator, in such amounts that the product comes out of the hydrator with a non-solid residual phase content, measured by a loss on ignition test (LOI) at 180°C, ranging between 22 and 24 wt%. 0.2 % of DEG (expressed as % of the quicklime weight) is added in the slaking water prior to hydration. The wet slaked lime coming out of the hydrator is then transported to a pin mill dryer (Atritor Dryer-Pulverizer commercialized by Atritor Limited) in which some hot air is injected so as to flash dry the wet slaked lime and produce the highly porous powdery slaked lime composition before storing it in a storage area. A representative sample of about 20 kg of this industrial powdery slaked iime composition has been taken and analysed (BET specific surface area = 41.1 m²/g, total pore volume = 0.214 cm³/g, d₅₀ = 4.2 μηι, R₃₂ = 6.2 %).

Then, a sub sample of about 1 kg has been further dried at laboratory scale in an oven at 180 °C so as to obtain a fully dried slaked lime (non-solid residual phase measured by LOI at 180 °C = 0.03 wt%). Following this full drying, the resulting fully dried slaked lime has been mixed with various given amounts of water and/or diethylene glycol (DEG). The mixture was done by adding the water and/or DEG dropwise onto the fully dried slaked lime which was under agitation in an intensive laboratory mixer (Eirich ELI). When DEG and water were used together, the DEG was added into the water prior to the addition of this liquid solution onto the fully dried slaked lime. The resulting mixture was agitated during 5 minutes and then submitted to a loss on ignition test (LOI) at 180 °C. The results are listed in table 1 below wherein the weight % are expressed with respect to the total weight of the powdery slaked lime composition. The purpose of this methodology is to evaluate whether the non- solid residual phase content measured by LOI at 180°C is representative of the sum of the quantity of water and of DEG present in the powdery slaked lime composition. Table 1.-

As it can be seen from Table 1, the non-solid residual phase measured by LOI at 180 °C following the procedure previously described is a good indicator of the sum of water and DEG added in the laboratory to the fully dried slaked lime since the variation, which is the difference between the theoretical and the measured values, is below ± 0.20 %, most of the time below ± 0.10 %.

Example 5.-

The following example has been performed in order to evaluate the influence of the non-solid residual phase content, in particular the water and/or the DEG content of the powdery slaked lime composition, on its flowability, all other parameters being fixed, like the particle size, the particle shape, the chemical composition, the specific surface area and the pore volume of the slaked lime composition.

In this perspective, various samples of powdery slaked lime compositions have been prepared from the fully dried slaked lime prepared in example 4 and to which different amounts of water and/or DEG have been added by mixing. The Alpine flowability of these powdery slaked lime compositions was measured according to the procedure previously described. The results are presented in Tables 2 and 3 wherein the weight % are expressed with respect to the total weight of the powdery slaked lime composition.

In this table, based on the conclusion of example 4, we have considered that the sum of the amount of water and of DEG added to the fully dried slaked lime corresponds to the non-solid residual phase content that we would have obtained if measured by a loss on ignition test at 180T (theoretical LOI at 180°C).

Similarly, we have considered that the amount of water added to the fully dried slaked lime corresponds to the non-solid residual phase content that we would have obtained if measured by a loss on ignition test at HOT (theoretical LOI at HOT).

Table 2.-

it can be seen from Table 2 that powdery slaked lime compositions having a water content equal to or higher than 0.7 weight % and equal to or less than

1.4 weight % present a good flowability (Alpine flowability higher than 50%), even without the addition of any DEG.

Table 3.-

Amount of water

Amount of DEG Theoretical LOI at Alpine added, i.e. theoretical

added 180 T flowability LOI at HOT

(weight %) (weight %) (%) (weight %)

EXAMPLE 5.4 0.3 0.2 0.5 87

EXAMPLE 5.5 0.8 0.2 1.0 83

EXAMPLE 5.6 1.3 0.2 1.5 83

EXAMPLE 5.7 1.8 0.2 2.0 68 EXAMPLE 5.8 0.5 0.5 1.0 86

EXAMPLE 5.9 1.0 0.5 1.5 88

EXAMPLE 5.10 1.5 0.5 2.0 88

It can be seen from Table 3, that the flowability of powdery slaked lime compositions having a water content equal to or higher than 0.3 weight % and less than 2 weight % can be further improved by the presence of diethylene glycol (DEG).

Comparative Example 1.-

The same procedure as in example 5 was followed except that the water content was changed. The results are mentioned in Tables 4 and 5.

Table 4.-

It can be seen from Table 4, that powdery slaked lime compositions to which no water has been added after full drying at laboratory scale, present an Alpine flowability of less than 50 %. When diethylene glycol (DEG) is added at an amount of 0.2 weight % with respect to the total weight of the powdery slaked lime composition, the flowability is improved but remains below 50 %.

Table 5.- Amount of Amount of DEG Theoretical Alpine water added, added LOi at 180

fiowability (%) i.e. theoretical (weight %) °c

LOI at HOT

(weight %)

comparative 2.0 0.0 2.0 46 example 1.3 comparative 2.8 0.2 3.0 43 example 1.4 comparative 3.0 0.0 3.0 27 example 1.5

It can be seen from Table 5, that powdery slaked lime compositions having a water content equal to or higher than 2 weight % present an alpine flowabiiity of less than 50 % and that this fiowability decreases when the water content increases. For a water content of 2.8 %, adding 0.2 % of DEG improves the flowabiiity, but not sufficiently to ensure an Alpine fiowability above 50 %.

The fiowability obtained with these samples is not sufficient to yield to a powder suitable for an industrial application.

Example 6.-

The same procedure as in example 5 was followed except that different amounts of water and DEG have been added to the fully dried slaked lime with the same method as the one described in example 4. The results are mentioned in Table 6.

Table 6.-

Amount of Amount of Theoretical Alpine water added, DEG added LOI at 180 °C

fiowability (%) i.e. theoretical

(weight %) (weight %)

LOI at HOT (weight %) example 6,1 0.0 0.5 0.5 72

It can be seen from Table 6, that it is possible to obtain powdery slaked lime compositions having a water content close to 0 (since no water has been added to the fully dried sample in this case) presenting however a good flowability (Alpine flowability higher than 50%) by adding diethylene glycol (DEG) so as to obtain a non-solid residua! phase content measured by a loss on ignition test at 180°C following the formula:

LOI 180°C > LOI 110°C + 0.3%

Table 7.-

It can be seen from Table 7, that it is possible to obtain powdery slaked lime compositions having a water content equal to or higher than 2 weight % and equal to or lower than 2.5 weight %, presenting however a good flowability (Alpine flowability higher than 50%) by adding diethylene glycol (DEG), so as to obtain a non-solid residual phase content measured by a loss on ignition test at 180°C following the formula:

LOI 180°C > LOI HOT + 0.2%

Example ?.-

In this example five industrial samples of powdery slaked lime produced according to the present invention, satisfying the Alpine flowability >50% but with different Alpine flowability values, have been selected. The cohesive index was measured with a Granudrum (Aptis Model from GranuTOOLS) using a rotational speed of 2 rpm and data analysis with the Aptis Granudrom software. The results are listed in Table 8 below.

Table 8.-

It can be seen from Table 8 that, generally, an increasing Alpine flowability referring to improved flow behavior of the powdery slaked lime, corresponds to a decreasing cohesiveness index measured with the granudrum.

Although the preferred embodiments of the invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, additions or substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

1. Process for manufacturing a fine and highly porous powdery slaked lime composition comprising the steps of:

feeding quicklime in a feeding zone of an hydrator;

feeding water in the feeding zone of the hydrator;

slaking said quicklime in a slaking zone of the hydrator by means of a quantity of water that is sufficient to obtain a slaked lime with a non-solid residual phase content of between 15 and 55 % by weight, preferably between 15 and 35% by weight;

drying and crushing said slaked lime to form the powdery slaked lime composition;

characterized in that said drying and crushing steps are simultaneously performed and are a single fluidification step of the slaked lime for forming said fine and highly porous powdery slaked lime composition having an Alpine flowability greater than 50% and are performed in a dryer-crusher chosen in the group consisting of a pin-mill dryer, a cage mill dryer, a flash dryer de-agglomerator and the combination thereof until the fine and highly porous powdery slaked lime composition presents a non- solid residual phase content, measured by a loss on ignition test at 180°C, equal to or less than 3.5 weight %, preferably equal to or less than 3 weight %, in particular equal to or less than 2.5 weight %, notably equal to or less than 2 weight %, and equal to or higher than 0.3 weight %, preferably equal to or higher than 0.5 weight %, with respect to the total weight of the powdery slaked lime composition.

2. Process according to claim 1 characterized in that it also comprises, before, during and/or after the slaking step and/or before, during and/or after the drying and crushing steps, a step of adding an additive to the quicklime, the slaking water and/or to the slaked lime.

3. Process according to any of the preceding claims characterized in that the drying-crushing steps are performed until the powdery slaked lime composition presents a mean particle size d₅₀ equal to or lower than 10 μιη, preferably equal to or lower than 8 μητι, advantageously equal to or lower than 7 μηη, in particular equal to or lower than 6 μηη.

4. Process according to any of the claims 1 to 2 characterized in that the drying-crushing steps are performed until the powdery slaked lime composition presents a first fraction of particles having a size of less than 32 μιτι and a second fraction of particles of a size greater than 32 μιη with the provision that the second fraction being equal to or lower than 50 weight percent, preferably equal to or lower than 40 weight percent, advantageously equal to or lower than 30 weight percent, particularly equal to or lower than 20 weight percent, in particular equal to or lower than 15 weight percent, more preferably lower than 10 weight percent, even equal to or lower than 8 weight percent, with respect to the total weight of the powdery slaked lime composition.

5. Process according to anyone of the preceding claims, wherein hot air is fed during the drying-crushing step at a temperature comprised between 250 °C and 500°C, preferably between 350 and 400°C.

6. Process according to anyone of the preceding claims, wherein the drying-crushing step has a duration comprised between a few seconds and a few minutes.

7. Fine powdery slaked lime composition comprising slaked lime particles having a BET specific surface area obtained from nitrogen adsorption equal to or greater than 25 m²/g and a total BJH pore volume consisting of pores with a diameter lower than 1000 A, obtained from nitrogen desorption equal to or greater than 0.15 cm³/g characterized in that the composition furthermore has an Alpine flowability greater than 50 %, preferably equal to or greater than 51 %, more preferably equal to or greater than 52 %, advantageously equal to or greater than 54 %, in particular equal to or greater than 55 % and said aforementioned composition has a non-solid residual phase content, measured by a loss on ignition test at 180°C, equal to or less than 3.5 weight %, preferably equal to or less than 3 weight %, in particular equal to or less than 2.5 weight %, notably equal to or less than 2 weight %, and equal to or higher than 0.3 %, preferably equal to or higher than 0.5 %, with respect to the total weight of the powdery slaked lime composition.

8. Fine powdery slaked lime composition according to claim 7, comprising a first fraction of particles having a size of less than 32 pm and a second fraction of particles of a size greater than 32 xm, the second fraction being equal to or lower than 50 weight percent, preferably equal to or lower than 40 weight percent, advantageously equal to or lower than 30 weight percent, particularly equal to or lower than 20 weight percent, in particular equal to or lower than 15 weight percent, more preferably lower than 10 weight percent, even equal to or lower than 8 weight percent, with respect to the total weight of the powdery slaked lime composition.

9. Fine powdery slaked lime composition according to claims 7 or 8, wherein said slaked lime particles present a particle size d₅₀ equal to or lower than 10 μητι, preferably equal to or lower than 8 μηι, advantageously equal to or lower than 7 μιη, in particular equal to or lower than 6 μπ\.

10. Fine powdery slaked lime composition according to anyone of the claims 7 to 9, wherein said slaked lime particles have a BET specific surface area obtained from nitrogen adsorption equal to or greater than 30 m²/g, preferably equal to or greater than 32 m²/g, advantageously equal to or greater than 35 m²/g.

1 1 . Fine powdery slaked lime composition according to anyone of the claims 7 to 10, wherein said slaked lime particles presents a BET specific surface area obtained from nitrogen adsorption equal to or lower than 55 m²/g, preferably equal to or lower than 50 m²/g, in particular equal to or lower than 48 m²/g.

12. Fine powdery slaked lime composition according to anyone of the claims 7 to 11, wherein said slaked lime particles presents a total BJH pore volume consisting of pores with a diameter lower than 1000 A, obtained from nitrogen desorption equal to or greater than 0.17 cm³/g, particularly higher than or equal to 0.18 cm³/g, preferably equal to or greater than 0.19 cm³/g, in particular higher than or equal to 0.20 cm³/g, advantageously equal to or greater than 0.21 cm³/g.

13. Fine powdery slaked lime composition according to anyone of the claims 7 to 12, wherein said slaked lime particles presents a total BJH pore volume consisting of pores with a diameter lower than 1000 A, obtained from nitrogen desorption equal to or lower than 0.30 cm³/g, in particular lower than 0.28 cm³/g.

14. Fine powdery slaked lime composition according to anyone of the claims 7 to 13, wherein said slaked lime particles presents a BJH pore volume consisting of pores with a diameter ranging from 100 to 300 A, obtained from nitrogen desorption equal to or greater than 0.07 cm³/g, preferably equal to or greater than 0.10 cm³/g, advantageously equal to or greater than 0.11 cm³/g, in particular equal to or greater than 0.12 cm³/g and typically lower than 0.15 cm³/g, in particular lower than 0.14 cm³/g-

15. Fine powdery slaked lime composition according to anyone of the claims 7 to 14, wherein said slaked lime composition further presents an alkali phase characterized by an alkali metal content that is equal to or greater than 0.2 weight % and equal to or less than 3.5 weight % with respect to the total weight of the powdery slaked lime composition.

16. Use of a fine powdery slaked lime composition according to anyone of the claims 7 to 15 for purifying flue gases.