APPARATUS FOR MIXING A DRY MATERIAL WITH A LIQUID

Abstract: The invention relates to a method and an arrangement for mixing a dry or substantially dry powder material (2) with a liquid to form a slurry (4), while using a vessel (7) which contains the liquid and which is provided with agitating means (5), and in which the powder material (2) is charged to the liquid continuously or intermittently. The powder material (2) is charged to the liquid (9) via a pipe (8). The end (8a) of the pipe through which the powder material is discharged is located at a distance beneath the surface (9a) of the mixture.

Description: TECHNICAL FIELD The present invention relates to an apparatus for mixing a dry, or a substantially dry powder material with a liquid to form a slurry. To this end there is used in accordance with the invention a vessel which contains the liquid and which is provided with agitating or stirring means, the powder material being added to the liquid either continuously or intermittently. BACKGROUND PRIOR ART Various methods and apparatus for mixing a dry or substantially dry powder material with a liquid so as to form a slurry are known to the art, these methods and arrangements generally being adapted to the nature of the powder material and the nature of the liquid used. For example, it is previously known in connection with desulphurizing plants to form a slurry by mixing dust-generating powder with liquid, in the form of water, in a closed vessel, the powder being supplied over the surface of the liquid, which is stirred or agitated with the aid of rotating devices in the form of hooks or propeller-like devices, normally rotating about substantially vertical axes and with the hooks or propellers serving as motion generating means located beneath the surface of the liquid. It is also known in this technical field to mix dust-generating powder with a liquid with the aid of paddle-like blades attached to horizontal shafts in open or closed vessels. The blades are normally arranged to move up and down, through the surface of the liquid. The powder is also here strewn across the surface. It has been observed with both of the aforesaid embodiments that when slurry is taken out from the surface a proportion of undissolved powder accompanies the slurry, while when slurry is tapped from the bottom of the vessel it is accompanied by hard agglomerates or lumps. In desulphurizing plants a slurry containing calcium hydroxide is charged to a contact reactor, where the slurry reacts with the sulphur-containing impurities. When this contact reactor is operated in accordance with the "wet-dry" method there is obtained from the reactor a residual dust which can be said to comprise a mixture of calcium sulphite, calcium sulphate, calcium hydroxide and fly ash. Other arrangements and methods which form part of the present state of this art are those with which the residual dust, or recycle dust, removed from the reactor is recycled for re-use in the contact reactor, in which case there is taken from the contact reactor a dust in the form of a powder material which is mixed with a fresh absorbent, such as calcium hydroxide, and water to form a slurry. When the fresh absorbent is dry slaked lime, the lime, recycle dust and water are mixed together to form a slurry having a dry solids content of ca 35%. When the fresh absorbent added is wet slaked lime, this lime normally has a dry solids content of 20-25% and is mixed in the mixing vessel with recycle dust to a dry solids content of ca 35 %. It is important to obtain a high dry solids content, since the energy losses in the desulphurizing system increase with the amount of water added thereto. The publication US-A-3,175,807 discloses a method and an apparatus for preparing a fiber-reinforced molding composition, in which the used fiber-reinforcing material is added by the use of pressurized air source. The publication US-A-2,890,039 discloses an apparatus for the introduction of substances into liquids of high specific gravity. TECHNICAL PROBLEMS The mixing of dry powder with a liquid is connected with two basically different technical problems. The mixing operation is often creating environmental problems and perhaps even health hazards for the people employed. Moreover the quality of the product, i.e. the suspension or solution, is often uneven and the removed product may contain hard lumps disturbing the function of the equipment where the suspension or solution is to be used later. Therefore a technical problem exists in the provision of an apparatus with which a dry or substantially dry powder can be mixed with a liquid, e.g. water, without creating lumps and deposits in the mixing vessel used. A technical problem resides also in the provision, with the aid of simple means, of equipment, which will enable dry powder material to be mixed with a liquid, in a mixing vessel provided with agitating or stirring means without generating a dusty atmosphere. A further technical problem is one of providing means whereby any lumps or deposits that do form in the mixing vessel, in spite of measures taken to the contrary, are disintegrated or ground to pieces, thus getting a size not causing malfunction in the equipment where the mixed product is to be used. Another technical problem is one of providing simple milling means that also will assist the mixing of the dry powder with the liquid. Finally a technical problem resides in the provision of means which prevent also dust agglomerates or lumps floating on the surface in the mixing vessel from accompanying the mixed product when this is removed from the vessel at or near the surface of the mixture. SOLUTION The present invention provides a solution to one or more of the aforesaid technical problems, and relates to an apparatus of the kind stated in the preamble of claim 1 by means of which apparatus a dry or substantially dry powder material, particularly dry material taken from a contact reactor or calcium hydroxide, can be mixed with a liquid, in the form of water, to form a slurry. Further features related to the invention are stated in the subclaims. ADVANTAGES Those advantages primarily afforded by an apparatus according to the present invention reside in the provision of conditions whereby a dry or substantially dry powder material can be mixed with a liquid without creating a dust laden atmosphere and whereby the material and liquid can be mixed effectively without forming lumps and coatings of dried material on the walls of the vessels or on other components in the vessel. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment at present preferred and exhibiting the characteristic features significant of the present invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 illustrates in side view and partly in section an apparatus constructed in accordance with the invention; Figure 2 illustrates in side view and in section in a somewhat larger scale than in Figure 1 a milling means located in the lower part of a liquid-containing vessel and Figure 3 illustrates in perspective the positioning of a vane used in the milling means. DESCRIPTION OF AN EMBODIMENT PREFERRED The present invention thus relates to an apparatus 1 intended for mixing a dry or substantially dry powder material 2, in the form of calcium sulphite, calcium sulphate, calcium hydroxide and fly ash with a liquid, in the form of water, charged through a pipe 3 such as to form a slurry which can be removed through an outlet 4, while using a vessel 7 which is provided with agitating or stirring means 5 and which contains the liquid 6 introduced thereinto through the pipe 3 and in which the powder material 2 is intended to be charged into the liquid, either continuously or intermittently. In accordance with the invention a pipe 8 having a diameter of 150 mm is arranged to co-act with the vessel 7 in a manner such that one end of the pipe 8 is located at a depth of ca 500 mm beneath the surface 9a of the mixture 9 and that the powder material 2 is passed through the pipe 8 and into a mixing zone 9b with the aid of a mechanical auxiliary device. The mechanical auxiliary device comprises a conveyor, for example a screw conveyor 10, which fills the whole of the interior of the pipe 8. It is proposed in accordance with the invention that the said one end 8a of the pipe and the agitating means 5 are located closely adjacent one another, for instance at a distance apart of less than 50 mm, preferably less than 25 mm. According to the invention the bottom 7a of the vessel 7 tapers downwardly adjacent the mixing zone 9b, such as to co-act with a milling means 11 which is arranged in the narrowest, lowermost part of the vessel and which is intended to crush small lumps formed in the vessel 7 or at some other location, particularly agglomerates or lumps whose density exceeds the density of the mixture 9, or such lumps as those which are urged downwardly by the agitating means 5. Figure 2 is an enlarged view of the milling means 11 illustrated in Figure 1, and it will be seen from Figure 2 that the milling means comprises a circular, lower plate 12 having an irregular or fluted upper surface 12a and diametrically positioned, rotatable vanes 13 and 14 spaced at a small distance above the plate 12. The construction of the vane 13 will be described with reference to Figure 3. Located between the bottom edge surfaces 13a and 14a of the vanes 13 and 14 and the upper surface 12a of the plate 12 is a space 15 which is adapted to correspond to the maximum permitted particle size of the powder material. This distance shall therefore be less than 2 mm. The vanes 13 and 14 are positioned obliquely and directed in the rotational direction, so as to be able to press and crush small lumps against the upper surface 12a of the plate 12. It will also be seen that the screw conveyor 8, the agitating means 5, and the vanes 13, 14 of the milling means are driven by one and the same drive shaft 16, which rotates, for example, at 150 rpm. The slurry mix 9c is removed from the vessel through an outlet 4 located at or adjacent the surface 9a of the mix. Arranged in the vessel 7 adjacent the outlet 4 is a screen 17 which separates the mixing zone 9b in the vessel 7 from the slurry outlet 4. As shown in Figure 1, the pipe 8 extends vertically, although there is nothing to prevent the pipe from being slightly inclined with the screw conveyor 7 filling the interior of the pipe. The other end 8b of the pipe co-acts with a funnel-shaped filling device 20, which includes one or more scraper bars 21 capable of being moved along the inner surface 20a of the filling device. The scraper bars 21 are attached to the shaft 16 via support means, such as to be driven by said shaft. The agitating means 5 comprises two vanes, referenced 5a and 5b, which are located essentially in the centre of the vessel or centrally of the mix 9. The milling means is provided with two vanes, referenced 13 and 14, which are inclined slightly to the vertical plane. The angle at which the vanes are inclined is such as to ensure that a lump entering between the leading part of the vane and the bottom edge 13a thereof will be crushed to pieces against the surface 12a. The pipe end 8a is preferably located beneath the surface 9a at a distance greater than 30cm, but preferably not greater than half the depth of the mix. The agitating means preferably has a diameter such as to cover at least 25% of the diameter of the vessel, but not more than 50%. The screen 17 is arranged to extend beyond the agitating means 5 on the side thereof where it meets the rotation created by the agitating means, but is much shorter on the downstream side. The screen does not extend sealingly against the wall in the vessel. A gap of 10-20 mm affords a certain amount of rotation behind the screen such as to prevent the formation of deposits or coatings deriving from stationary slurry 9c. The relationship between the flows of material entering the mixing vessel is determined by taking density measurements in the outlet 4 or in a subsequent storage tank, not shown. The amount of powderous material charged is controlled through metering means (not shown) located above the funnel 20. The powder material is charged in a quantity such that it only fills a small part of the cross-section of the pipe 8, so that there is effected within the lower part 8a of the pipe an initial mixing of dry powder with a previous mixture in the mixing zone 9b, with the aid of the end part of the screw conveyor, whereafter mixing is effected with the aid of the agitating means 5, which steers the mixture down towards the milling means 11. This initial mixing process takes place partially in a vertical direction within a limited space within the pipe end 8a, therewith preventing the emission of dust. A vane 13 forming part of the milling means 11 is illustrated in Figure 3. The vane consists of L-shaped angle iron which is slightly inclined so that the lower bottom surface 13a of the lower part 13d forms a wedge-shaped space with the surface 12a. This wedge-shaped space may have an angle of "a" of less than 20° perferably less than 10°. This means that small lumps are pressed into the wedge-shaped space and crushed into particles of such small size as to enable them to be used in the final slurry. Larger lumps are pressed outwardly by an oblique surface 13b and glide over the surface 12a and against one another so as to be ground down autogenously. The vessel wall serves to move lumps back towards the surface 12a, so as to be fed away by the next vane. The large lumps are ground down in this way to smaller lumps which can be crushed in the milling means. The upper part 13c is intended to push large lumps in its forward direction, but also contributes towards improving the mixture.

Claims: An apparatus for mixing dry or substantially dry powder material (2) with a liquid (3) to form a slurry (9c) comprises a vessel (7) having outlet means (4) for removing finally mixed slurry (9c), agitating means (5) which is arranged inside the vessel, a pipe (8) extending vertically or substantially vertically into the vessel and a conveyor (10) filling the interior of the pipe for feeding the powder material through the pipe to the agitating means, characterized in that the lower end of the pipe (8) and the agitating means (5) are arranged closely adjacent to one another, that the bottom of the vessel (7) is formed to narrow downwardly and that milling means (11), for crushing lumps formed in the vessel or at some other location, is positioned in the narrowest part of the vessel. An apparatus according to claim 1, characterized in that the conveyor is a screw conveyor (10). An apparatus according to claim 1, or 2 , characterized in that the milling means (11) comprises a plate (12) having an irregular surface, for example a fluted plate, and rotatable vanes (13, 14) located at a small distance above the plate. An apparatus according to claim 3, characterized in that the vanes (13, 14) are positioned obliquely and directed in the rotational direction in order to press and crush small lumps against the upper surface (12a) of the plate (12). An apparatus according to claim 3 or 4, characterized in that the vanes (13, 14) have an L-shaped cross-section including a horizontally directed part which narrows from the center of rotation. An apparatus according to any one of claims 2 - 5, characterized in that the screw conveyor (10), the agitating means (5) and the milling means (11) are driven by one and the same shaft (16). An apparatus according to claim 1, characterized in that a screen (17) for separating a mixing zone (9b) in the vessel (7) from finally mixed slurry (9c) is arranged adjacent the outlet means (4) in said vessel. An apparatus according to claim 1, characterized in that the other, upper end of the pipe (8) co-acts with a funnel-shaped filling device (20). An apparatus according to claim 8, characterized in that the funnel-shaped filling device (20) comprises one or more scraper bars (21) which are moveable along the inner surface (20a) of said device. An apparatus according to claim 6 and 9, characterized in that the scraper bars (21) are driven by said shaft (16).

Inventors: JOHANSSON, BENGT
Applicants: ABB FLAEKT AB
Application year: 1987
Main classification: B01F7
IPC codes: B01F15