Conveyors: fluid current – Processes
Reexamination Certificate
2000-10-27
2002-05-07
Ellis, Christopher P. (Department: 3651)
Conveyors: fluid current
Processes
C406S198000, C406S014000, C406S019000
Reexamination Certificate
active
06382881
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the conveyance of fluidizable powder materials. It is a horizontal or inclined conveyance capable of transporting the said materials between a storage area and at least one area to be supplied, the said areas being at different locations.
It is a continuous process for conveyance of a powder product in order to feed a large number of packaging assemblies such as bagging machines, containerization devices, or a large number of production assemblies such as plastic extruding presses or igneous electrolysis cells, from a single storage area.
Powder materials to be conveyed can be fluidized; their size grading and cohesion are such that injecting gas into them at low velocity can eliminate cohesion between particles and reduce internal friction forces. For example, this type of material includes alumina for igneous electrolysis, cements, plasters, quick lime or slaked lime, fly ash, calcium fluoride, magnesium chloride, all types of fillers for mixes, catalysts, coal dust, sodium sulfate, phosphates, polyphosphates or pyrophosphates, plastics in powder form, food products such as powder milk, flour, etc.
DESCRIPTION OF RELATED ART
Many devices have been studied and developed for conveyance of powder materials in fluidized bed. One particular problem is related to the continuous feed of the powder material regulated as a function of consumption requirements of the said material. One of the many examples illustrating this problem is feed of alumina to igneous electrolysis cells for the production of aluminum.
In order to do this, the alumina, which is a powder product conveyed and solubilized in the electrolytic bath, is consumed gradually while electrolysis is taking place, and must be replaced as it is consumed so that the concentration of solubilized alumina remains optimum, encouraging maximum efficiency of the electrolysis cell. It then becomes necessary to adjust the quantity of alumina added into the electrolytic pot, so that its operation is not disturbed by excess or insufficient alumina.
The powder materials conveyance device developed by the applicant and described in European patent EP-B-0 179 055 enables continuous feed of powder solids in their hyperdense phase. It is used particularly for regular and continuous feed to storage and distribution hoppers located in the superstructure of electrolysis vats.
It is a potential fluidization device designed for conveyance of powder materials in a hyperdense bed, in order to transport these materials from a storage area to an area which needs to be supplied. As in conventional fluidization, this device comprises at least one horizontal conveyor called the air-pipe between the storage area and the area to be supplied, composed of a lower duct in which gas circulates, and an upper duct in which the powder material is conveyed, the two channels being separated by a porous wall. Gas is blown into the lower duct through at least one supply tube. Unlike what happens in conventional fluidization, the powder material completely fills the upper duct of the conveyor and this conveyor is fitted with at least one balancing column partially filled by powder material, the filling height balancing the gas pressure. This balancing column creates the conditions for potential fluidization of the powder material. The powder material, which is not disturbed very much due to the very low density of the gas, is present in the air pipe in the form of a hyperdense bed.
In order to make the description of potential fluidization easier to understand, it is worth while repeating the principles of conventional fluidization, normally used for conveying powder materials and described for example in patent U.S. Pat. No. 4,016,053. The device used in fluidization also comprises an air pipe as described above. The fluidization gas is injected into the lower duct at a given pressure p
f
, passes through the said porous wall and then passes between the particles at rest in the powder material forming the layer to be fluidized. Unlike the potential fluidization device described in EP 0 179 055, the thickness of this layer at rest is very much less than the height of the upper duct of the said conveyor, in other words in the absence of any injection of fluidization gas, the powder material only very partially fills the upper duct of the horizontal conveyor.
By imposing a high gas flow, the said particles are moved and lifted, each of them losing its permanent contact points with its neighbors. In this way the interstitial space between the particles increases, internal friction between particles is reduced and these particles are put into a state of dynamic suspension. Consequently, the result is an increase in the initial volume of the powder material and a corresponding reduction in the apparent density, since a suspension of a solid phase in a gaseous phase is formed.
Therefore, the apparent density of the material is lower than the apparent density used in potential fluidization as described in EP 0 179 055, which refers to a hyperdense phase. The term “dense phase” is usually reserved for pneumatic transport at high pressure. The hyperdense phase is characteristic of potential fluidization. To give an idea of the situation, consider the example of the case of alumina Al
2
O
3
in which the solid/gas ratio is of the order of 10 to 150 kg Al
2
O
3
/kg of air in dense phase pneumatic transport and is 750 to 950 kg Al
2
O
3
/kg of air for conveyance by potential fluidization in the hyperdense phase. Therefore, the solid powder can be conveyed at very high solid/gas concentrations in the hyperdense phase, significantly higher than the dense phase in pneumatic transport.
In the case of potential fluidization, even if no gas is injected, the powder material almost completely fills the conveyance device and particularly the upper duct. When gas is injected into the lower duct, the balancing column is partially filled with powder material occupying the upper duct at a manometric head that balances the pressure p
f
and prevents the size of the interstices between the particles from increasing. Consequently, the presence of the balancing column prevents fluidization of the powder material present in the horizontal conveyor and forces the said material to appear as a hyperdense potential fluidization bed. Furthermore, since the interstitial distance between particles does not increase, the permeability of the medium to gas injected at pressure p
f
is very low and limits the gas flow to a very small quantity. We will subsequently refer to this low gas flow that passes through the balancing column “degassing”.
Thus, with a fluidization pressure p
f
equal to 80 millibars, the circulating gas velocity corresponding to the said pressure p
f
and causing fluidization of powder alumina is of the order of 33.10
−3
m s
−1
in the device described in U.S. Pat. No. 4,016, 053, whereas in the potential fluidization device according to EP-B-0 179 055, the circulating gas velocity is only of the order of 4.10
−3
m s
−1
This velocity is too low to cause fluidization of alumina in the conveyor assembly.
No fluidization takes place, but it is possible to talk about potential fluidization; there is no permanent circulation of material in the air pipe, but flow will take place by successive collapsing as soon as the need for any powder material arises, for example when the level of the area to be supplied drops below a critical value. When continuous consumption of the material stored in the area to be supplied is such that the material level drops below the level of the orifice in the supply pipe, a certain quantity of powder material will escape from the pipe creating a “vacuum” which will be filled by falling material, which will create a domino effect and thus continue throughout the air pipe working backwards towards the storage silo.
The potential fluidization device for conveyance in a hyperdense bed, as described in EP-B-179 055, is used on a large scale particularly to supply 300 000 ampere vats in
Cloue Christian
Gasquet Gerard
Aluminium Pechiney
Dennison, Scheiner & Schultz
Dillon, Jr. Joe
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