Background of the invention

Field of the invention

The present invention relates to equipment that is suitable for the dry hydration of calcium oxide, i.e. for converting calcium oxide into calcium hydroxide, and for treating the raw materials and the product. The invention also relates to a process of performing the dry hydration, particularly by utilizing said equipment.

Description of the prior art

Lime, i.e.. calcium oxide, is a raw material that is commonly used particularly in the construction industry, but also in for example agriculture. In the construction industry, lime is used, among others in the manufacture of cement and mortar. It is sensitive to reacting with water, whereby, calcium hydroxide is formed, which is also called slaked lime. The reaction is then as follows:

CaO + H ₂ 0→ Ca(OH) ₂ .

Said reaction is vigorous, and enough heat is generated, among others to ignite flammable substances. The formed calcium hydroxide is also sensitive to further reaction, forming calcium carbonate. The reaction is vigorous and heat is generated, whereby the hydration of lime has conventionally been carried out by means of slowly rotating kneaders by simultaneously adding to the reaction vessel burnt lime, i.e. calcium oxide, and water in small batches.

Conventional lime hydration methods include the so-called wet process (wherein burnt lime is added to an excess of water) and the so-called dry hydration method (wherein the amount of water stoichiometrically required by the hydration reaction is added to the burnt lime). The Finnish patent FI 103964 describes a wet hydration process, wherein slaked lime is conducted into excess water into decomposing equipment provided with opposed cylinder rotors, and the aqueous suspension that contains calcium hydroxide exiting the equipment is allowed to settle in a separate container for the time required for the reaction to take place. Thus, calcium hydroxide is formed in the form of fine powder.

A problem of such fine-ground calcium hydroxide powder is, among others the difficulty of transportation due to the volume of the powder. A further problem with wet hydration methods is their poor controllability. Thus, there remains a need for safer methods of hydrating calcium oxide, which are easier to control.

The Finnish patent FI 120733 describes a hydration method, where the amount of water is reduced compared to the previous solution. In this method, calcium oxide is conducted to an impact disintegrator, where also a small amount of water is conducted in order to moisten the particles generated in the disintegration. In addition to the impact disintegrator, the equipment used in the method also contains a screw conveyor used to knead the reacting material. Also in this case the reacting material is conveyed to a separate container, where the reaction is completed. The remaining portion of the amount of water required for the slaking is conducted to this container.

In previous dry hydration methods, one common problem has been the unwanted formation of agglomerates, and slowness. These methods have conventionally been carried out in complex apparatuses containing several reaction chambers, the particular purpose of which has been to decelerate the reaction to increase the controllability. In these solutions, however, no attention has been paid to the quality of the product and the control of the generated thermal energy. Neither is the deceleration of the reaction generally desirable.

EP 1087832 describes a similar dry hydration method, where the hydration is carried out in 3- 20 minutes in an open vessel using a slightly larger amount of water than required stoichiometrically. Consequently, this is a slow reaction, where the end product is able to absorb carbon dioxide from the ambient air, thus deteriorating the quality of the product. Brief description of the invention

An object of the present invention is to provide equipment suitable for the dry hydration of calcium oxide. It is also an object of the invention is to provide a dry hydration process that utilizes this equipment.

A particular object of the invention is to provide equipment and a process that yield homogeneous slaked lime (i.e. calcium hydroxide product) that is not dependent on any variations in the raw material (i.e. burnt lime).

A second particular object of the invention is to provide a process by which calcium oxide can form agglomerates, on the surface of which there is provided calcium hydroxide and by the disintegration of which smaller calcium hydroxide particles are obtained. Said calcium hydroxide product can thus be either in the form of agglomerates or disintegrated particles. Agglomerates are fine-sized and porous and they have a large specific surface area (particularly 15^0m ² /g). If they are disintegrated, particles are obtained having an average diameter of <100nm. The present invention is based on the observation that burnt lime (CaO) reacts most effectively when the raw material containing it is disintegrated into smaller particles simultaneously with reacting it in a fluid formed by water mist and steam. Thus, the fresh surfaces of calcium oxide that are generated in the disintegration immediately receive a protective and reactive aqueous coating after being formed, whereas the original surfaces receive impacts that activate them, causing at least a partial disintegration of the layers that were formed among others on the surfaces of the raw material. Thus, the vigorous nature of the reaction is limited by the formation of new surfaces, and not by the amount of water (or the air humidity) or the mixing speed. The calcium oxide on the fresh surfaces as well as the aqueous coating surrounding these surfaces, thus, immediately react, forming calcium hydroxide (Ca(OH) ₂ ), first in the form of a film, which in turn functions as a reaction substrate for the formation of the crystal. When condensing, water vapour penetrates into the pore structure of the calcium oxide, contributing to the acceleration of the reaction and producing larger agglomerates. Thus, the present invention relates to equipment suitable for the dry hydration of calcium oxide particles, containing a pre-treatment section for calcium oxide, a processing space that is suitable for the completion of the slaking reaction, as well as optionally one or more auxiliary parts, particularly to facilitate the runnability and controllability of the equipment.

The pre-treatment section preferably contains a rotor section, a stator section, and a water mist generator, whereas the processing space preferably contains a screw section formed by two screws with a speed of rotation adjustable according to the process conditions.

The invention also relates to a process, wherein said equipment is utilized, and wherein the calcium oxide particles are reacted into hydroxide, particularly at their surfaces, by using only the stoichiometric amount of water required by said reaction. Thus, the end product of the process is either the calcium hydroxide agglomerate or a smaller hydroxide particle produced from the same by disintegration.

When the amount of water bound to the surface and the pore structure of the calcium oxide particle (CaO particle) corresponds to the amount of water used in the hydroxide- generating chemical reaction, the course of the reaction, the amount of water to be fed, as well as the equipment adjustments can be controlled based on the maximum temperature. Since burnt lime is obtained from a natural product, which is certainly not pure, and since flaws also happen during the burning, the theoretical maximum temperature is not reached, but using the present invention a point very close to this can be achieved. As porous calcium oxide particles are utilized as starting material in the invention, also the calcium hydroxide particles (or agglomerates) are internally porous. When the calcium hydroxide is formed on the surface of the particle, the formed hydroxide crystals at least partly penetrate into the calcium oxide particle, whereby the oxide structure is partly disintegrated and the porosity even increases. Said temperature also indicates this porosity.

When the calcium oxide CaO particle size distribution is abrupt, the process controlled by the control mechanism produces also homogeneous porosity. When measured by nitrogen absorption measurement, the porosity achieved by pilot devices has been 20-40 m /g, more generally -30 m7g. Such agglomerates also constitute a product that is easy to transport based on its particle size. The grinding fineness cannot be taken to nano levels, but particles of the micron level have to be maintained. The target level is c/c 8-10 μιη.

The obtained Ca(OH) ₂ agglomerate forms a powder with a surface area of 15-40 m ² /g. Such a dry powder (-0.6 g/cm ³ ) is easy to transport to the desired destination of use, both in terms of economy and safety. It will also not react uncontrollably or prematurely into calcium carbonate, because no excess water has been added thereto.

The agglomerates of the powder can be disintegrated by active elutriation, whereby particles of the size <100 nm (average diameter) are obtained, expressed in spherical terms.

The equipment of the invention functions so that the calcium oxide (CaO) is optionally first crushed, whereafter it is conducted to the actual pre-treatment where it is ground in a fluid containing aqueous mist and steam. The amount of water is thus the amount required by the chemical reaction of CaO, and this water attaches to the CaO particle surface.

The obtained CaO in fluid form is conducted to the processing space of the equipment, having two screws, by means of which the hydroxide formation can be advanced as desired, particularly by adjusting their speed of rotation in relation to the progression of the process.

To be precise, the dry hydration equipment according to the invention is characterized by what is stated in the characterizing part of claim 1.

The process according to the invention is, in turn, characterized by what is stated in the characterizing part of claim 8.

The invention provides considerable advantages. In the process, the large surfaces of the agglomerates are utilized, making it possible to avoid their disintegration into nano-sized particles until a desired stage, also resulting in the possibility of quick utilization. The agglomerate form and size of the product as well as its bulk density enable economic transportation.

If transportation and storage were carried out with the material in the form of calcium oxide (CaO), there would be a problem of the CaO reacting with air humidity. This problem has been eliminated already for the time of transportation and storage by producing calcium hydroxide agglomerate having a small surface area and containing very small amounts of C0 ₂ gas (about 0.03%), thus not being as susceptible to react into carbonate (CaC0 ₃ ).

The pre-treatment, processing, and auxiliary devices that are included in the equipment of the invention constitute economic production equipment, having a smaller size than conventional equipment, whereby the process implemented by the same is also simpler and quicker to implement.

The combination of the rotor and the control stator is a functional sub-unit, wherein the high rotational speed of the rotor enables the use of rotors with a small diameter, and the turning sides of the stator can be used to control the radial velocities and the magnitude of impact on the particle (see Fig. 2). By means of high-speed motors it is thus easy to reach rotational speeds of 20000 rpm by said rotor. By means of a belt-driven rotor system, even 9000 rpm is economic.

Screw processing has, in turn, been developed so that it does not essentially mix the calcium oxide to be processed, whereby the end product will be an agglomerate composed of thin Ca(OH) ₂ "threads". The double-screw processing space essential for the invention provides good adjustment and agglomeration conditions also for the use of attrition.

When complemented by auxiliary devices, the equipment of the invention can be used for producing several Ca(OH) ₂ -based products.

In the following, the invention is described more precisely by means of the appended drawings and a detailed description.

Brief description of the drawings

Fig. 1 is a diagram of the equipment according to an embodiment of the invention, which is suitable for reacting calcium oxide particles into hydroxide, at least on the surfaces thereof. Fig. 2 is a more detailed diagram of the stator section of the pre-treatment section of the equipment according to an embodiment of the invention (and its connection to the rotor section), wherein Fig. 2A shows the positions of the rotor blade and the turning stator blade with respect to each other; Fig. 2B shows the possible blade positions of the turning stator blade; and Fig. 2C shows the differences between the turning and stationary blades of the stator.

Fig. 3 is a diagram of the equipment according to a particularly preferred embodiment of the invention, illustrating some of the auxiliary parts of the equipment that are described below, and their positions in the complete equipment.

Fig. 4 is a SEM image of the calcium hydroxide agglomerate produced according to the invention, whereby Fig. 4A shows several agglomerates and Fig. 4B shows a close-up of a single agglomerate.

Detailed description of the embodiments of the invention

The present invention relates to equipment for reacting calcium oxide particles (CaO particles) into hydroxide (Ca(OH ₂ )), containing

- a pre-treatment section 1 for calcium oxide, wherein raw material (particles of <20 mm) containing oxide can be treated into particles of a desired size, and which preferably is in the form of an impact disintegrator;

- a processing section 2, where calcium oxide can be reacted into hydroxide; and - optionally, one or more auxiliary parts.

Thus, the CaO particles obtained from the raw material in the pre-treatment section 1 are ground particularly into a micron grade, for example into particles with an average diameter of 1-20 μ ι. At the same time, the CaO particles receive water on their surfaces and, at least partly, inside their pore structure.

The equipment of the invention is operated based on the principle that the calcium oxide particles obtained from the optional raw material disintegration stage are conducted to the first actual processing stage, i.e. to the pre-treatment section 1 of the equipment, where they are ground in a fluid containing aqueous mist (and steam). The amount of water is then the just amount required by the chemical reaction of CaO, and mainly attaches to the surfaces of the CaO particles. The obtained CaO fluid (together with the aqueous mist and steam) is conducted to the processing space 2 of the equipment, its conditions being adjusted so as to advance the formation of hydroxide and to obtain the desired product properties.

The auxiliary parts of the equipment can be used to adjust the particle sizes of the materials, the process conditions, or the properties, such as purity, of the formed product.

The water brought onto the oxide surface immediately reacts with the oxide, forming a film that partly functions as a reaction substrate for the formation of the calcium hydroxide crystal, and partly detaches. The detachment is probably a result of attrition caused by strong turbulence. More surface of CaO is thus revealed for the quick reaction to continue. Due to the water supplied to the particles, the reaction mixture is now in the form of a fluid.

The calcium oxide particles are porous. When the calcium hydroxide forms on the surface of the particle, the formed hydroxide crystals at least partly penetrate into the calcium oxide particle, whereby the oxide structure is partly disintegrated and the porosity of the particle even increases. In this way, hydroxide agglomerates are obtained, having a small surface area with respect to the amount of product.

"Agglomerate" in this context means a porous particle with an average diameter of >2μπι, preferably 2-20μπι, its porosity, measured by nitrogen absorption, being 20-40m ² /g.

Thus, the present invention also relates to such a continuous process of reacting calcium oxide particles into hydroxide, wherein said equipment is used. In said process, calcium oxide particles are added to the pre-treatment section 1 of said equipment, water or water vapour is sprayed into the same space in an amount that is stoichiometric with respect to the amount required by the formation of hydroxide or, more preferably in an amount that is 1.03-1.5-fold (compared to the stoichiometric amount), whereby calcium hydroxide is formed at least on the particle surfaces, and the thus formed particle-water fluid is conveyed to the processing space 2 of the equipment, wherein the hydroxide formation reaction is completed and the hydroxide is allowed to agglomerate.

Thus, the lime slaking is carried out in almost dry conditions in the process, particularly by using the device described above. Optionally, the agglomerates formed in the reaction are thereafter disintegrated into smaller particles in the way described hereinafter. The end product of the process is, thus, either a calcium hydroxide particle or agglomerate.

According to a preferred embodiment of the invention (see Fig. 1).

the pre-treatment section 1 of the equipment contains the following auxiliary parts:

rotor section 11 ;

Stator section 12; and

water vapour generator 13 ;

whereas the processing space 2 of the equipment contains the following auxiliary parts:

- screw section 21.

Thus, the equipment includes one rotor section 1 1 (with rotor blades 1 la), which functions by accelerating the calcium oxide particles (in water vapour) in the outermost rotor of section 11 to a suitable velocity, preferably being 200-900 m/s. Its rotational speed and direction are adjustable, preferably so that the radial velocity (blade velocity) is adjusted to a value of 200-900 m/s, more preferably 200-300 m/s, and most suitably 300 m/s.

Correspondingly, the equipment includes one stator section 12 containing stator blades 12a,b, which preferably have a V-shape and are arranged in circles, particularly so that the outermost circle has a turning blade 12a, by means of which the blade angle can be adjusted, while the other blades 12b are stationary. This stator section 12 operates so that the CaO particles accelerated from the rotor section 1 1 are collided with said stator blades 12a,b at a suitably adjusted collision angle (see Fig 2). The rotor 11 and stator section 12, when combined so that particles can be conducted from the rotor section 1 1 to the stator section 12, are suited for grinding calcium oxide particles. The water vapour generator 13, such as a nozzle, can be used to spray water into this pre- treatment section 1, which thus functions both as a grinder and a pre-hydration space. The large speed of rotation of the rotors enables the use of rotors with a small diameter (such as 250-600 mm), and the turning blades 12a of the stator can be used for controlling the radial velocities and the magnitude of impact on the particle. Thus, high-speed motors are preferably used in the equipment of the invention, by means of which the rotors are accelerated to rotation speeds as high as 9000 rpm, or even 20000 rpm.

The CaO particles remain in this pre-treatment section 1 for approximately <1 s, generally 0.01-0.1 s. The ground and pre-hydrated slurry (i.e. fluid) formed in the pre-treatment section 1 of the equipment is conducted to the processing space 2 with a screw section 21 formed by two screws, preferably a twin-screw screw conveyor 21, particularly a conveyor 21 formed by two helix screws. In this screw section 21, the water bound to the surfaces and pore structure of the calcium oxide particles causes a chemical reaction, whereby the hydroxide formation reaction is completed essentially without mixing the slurry fed thereto.

Optionally, the progress of said reaction can be advanced by raising the temperature, for example to >100°C, preferably to 100-300°C, most suitably to 100-295°C. In the processing space 2, minor attrition is also created between the CaO particles, advancing the reaction. This attrition is adjusted by adjusting the rotation speed of the two screws in the screw section 21.

Thus, the end product is an agglomerate formed by thin fibrils (or Ca(OH) ₂ "threads") (thus corresponding to a threadlike ball). The agglomerates form a powder with a surface area of 15-40 m ² /g. Such dry powder (~ 0.6 g/cm3) is easy to transport to the desired location of use, both in terms of economy and safety. It will not react uncontrollably or prematurely into calcium carbonate, because no water has been added thereto.

If the transportation and storage were carried out with the material in the form of calcium oxide (CaO), as is conventionally the case, there would be a problem of CaO reacting with the humidity in the air. Therefore, CaO particles are generally large, whereby their reactive surface area is as small as possible. The calcium hydroxide (Ca(OH) ₂ ) obtained by the present invention is, however, in an at least partly reacted form, and as an agglomerate powder having a surface area of 20-40 m ² /g and bulk density of 0.6 g/cnv', and the average diameter of the particles contained in it is 2-20 μηι. Because of the variation in the amount of grits in the raw material to be processed (i.e. the particles of other materials in the raw material), inaccuracies are formed in the process, their adjustments (also by the equipment of the invention) having a minor delay. This causes a slight formation of steam. By means of the screws of the processing section 2 of the equipment, the formed steam can be conveyed from the so-called discharge end of the space formed by the screws of the screw section 21 either to its so-called feeding end, or directly to the pre-treatment section 1 of the equipment. The rotation speed of the screws is thus adjusted according to the development of the process temperature. When conducted to this feeding section of the rotor grit, the steam also reduces the amount of air entering the processing space. If the steam is condensed before utilization, this is preferably done at a condensing temperature of <120°C. Underpressure is created when this condensing temperature is <100°C, and overpressure must be used when this temperature is >100°C. Due to the described equipment arrangement and its operation, it is preferred to arrange the volume of the processing space 2 so as to increase toward its discharge end, whereby the equipment does not block up when the material to be processed expands.

To maintain a suitable reaction speed, the temperature in the processing space 2 is preferably allowed to rise to a level of >100°C, more preferably 100-310 °C, particularly 1 10-300 °C, and most suitably about 290 °C.

The formed calcium hydroxide agglomerates (see Fig. 4) are either conveyed from the processing space 2 elsewhere for further processing or are further processed on site, for example by disintegrating them into a smaller size, as described below. They can also be recovered and stored, and used later on as such, for example for the purification of flue gases, as described in EP 1087832.

Also the raw material can be processed. Because a natural product (such as limestone) containing calcium oxide is used as the raw material, being available in different sizes, it is according to an embodiment subjected to crushing before the actual processing. This stage is particularly carried out if the average diameter of the raw material particles is >20 mm. Metals can also be removed therefrom. Thus, according to a preferred embodiment of the invention, auxiliary parts are selected for the equipment from a group that includes:

- raw material treatment means 3, preferably located before the pre-treatment section 1 in the equipment, and in turn containing, for example:

o a particle crusher 31 for crushing >20 mm calcium oxide particles into

smaller (<20 mm) particles, preferably in the form of a notched roller; or o a weighing appliance 32 for adjusting the dosage of raw material; o or both of these (both the crusher 31 and the weighing appliance 32);

- or metal stripping means 4, preferably located before the pre-treatment section 2 in the equipment; however, so that they do not interrupt the flow of material to be processed in the equipment, and which include

o a metal detector 41; or

o a metal separator 42, to which a metal stripping device 43 is optionally

connected;

o or both of these (the detector 41 and the separator 42, to which the stripping device 43 can optionally be connected).

If limestone with a diameter of <20 mm is used, it is, however, processed as such, without separate crushing.

It is also common to place CaO containers 5 required for the storage of the raw material containing calcium oxide specifically in connection with said equipment, whereby they are preferably located right before the particle crusher 31, if used, and in another case, right before the pre-treatment section 1.

The agglomerates formed in the pre-treatment 1 and processing section 2 of the equipment mentioned above can be disintegrated by an active elutriation. The elutriation takes place in a fluid space, preferably with 20-40 g/L of solid matter. This elutriation yields particles with a size of < 100 nm (the average diameter), expressed in spherical terms. These disintegrated particles are in fluid form before an optional separation.

A preferred way of elutriating is the wet grinding by the opposed cylinder rotor method. Another economic alternative is the use of a combination of a pressure roll and said opposed cylinder rotor method for the powder. To prevent agglomeration of the nano-sized Ca(OH) ₂ powder, additives must, however, be added to the material to be processed, their selection and properties being defined by the further use of the Ca(OH) ₂ .

According to a second preferred embodiment of the invention, one of the optional auxiliary parts of the equipment is

- an elutriation section 6 suitable for disintegrating agglomerates formed in the

processing section 2.

The elutriation section 6 preferably works on the opposed cylinder rotor principle and is capable of homogenizing the agglomerates, whereby the re-growth of the particles of the created reaction products is prevented. Thus, the particles are brought to a colloidal size (typically 10-500 nm), whereby they are very stable.

According to a third preferred embodiment of the invention, optional auxiliary parts for the equipment are selected from a group capable of advancing the operation or control of the pre-treatment section 1, these parts including:

- a water dosing device 14 for adjusting the amount of water mist to be fed into the reaction mix, preferably being located in connection with the water mist generator 13 of the pre-treatment section 1; or

- adjusting means 15 to control the rotational speed of the rotors of the rotor section

1 1 of the pre-treatment section 1 and the adjustment of the blade angles of its stator 12, preferably connected to the rotor section 11 and the stator section 12;

- or both of these (the dosing device 14 and the adjusting means 15).

According to a fourth preferred embodiment of the invention, optional auxiliary parts for the equipment are selected from a group capable of advancing the control of the processing space 2, these parts including:

- temperature measuring means 22, preferably located in the processing section 2, more preferably in or below its screw section 21, which are in connection with the rotor 11 and stator section 12 of the pre-treatment equipment 1, most suitably via their adjusting means 1 , so that the temperature of the calcium oxide to be processed can be measured in the screw section 21 and used to control the rotational speed of the rotors and the blade angles of the stator; or

- separate heating means 23, preferably located in the processing section 2; - or both of these (the measuring means 22 and the heating means).

Part of the auxiliary parts of the preferred embodiments described above and their mutual locations are illustrated in Fig. 3.

The durability of the obtained agglomerates was analyzed during a storage period of 6 months, whereby the surface area in the Ca(OH) ₂ powder decreased <20% (about 15%). The decrease in surface area did not, however, decrease the surface area of the nano- particles created in the dry or wet grinding of the agglomerate powder. It remained in an area corresponding to the surface area of < 100 nm particles. The surface area of the powder did, however, decrease 23-19 m ² /g during a longer storage period of 12 months.

The following non-limiting example illustrates the invention and its advantages.

EXAMPLE

The equipment of the invention was used to hydrate calcium oxide (CaO) in laboratory conditions, using 0.32 kg of water / kg of water, whereby calcium hydroxide agglomerates were obtained (Ca(OH) ₂ , see Fig. 4). A part of them was directly subjected to a particle size measurement, whereas a part was disintegrated into smaller particles before the measurement.

The particle size of the Ca(OH) ₂ agglomerate formed by the equipment of the invention:

The particle size was measured by nitrogen absorption measurement, directly, without disintegrating the agglomerates, yielding a result of an average diameter of 50-70nm (converted into a spherical form). The particle size of the particles that were disintegrated from the agglomerates:

Ca(OH) ₂ agglomerate powder (24 m ² /g) was elutriated in an aqueous solution and disintegrated by a laboratory device of the disintegrator type, its rotational speed being 20000 rpm and the collision speed 350 m/s. The solids content of the slurry was about 10% and it remained as a slurry for about 1 h. During this time, no observable amounts of agglomerates or particles were separated.

The particle size of the Ca(OH) ₂ particles was again measured by nitrogen absorption measurement, yielding a result of an average diameter of <100nm (converted into a spherical form).