Title of Invention

A PLANT FOR DRY CLEANING BY REMOVING FLUORINE CONTAINING WASTE GASES

Abstract

A plant (1) for dry cleaning, by removing fluorine containing waste gases, from a reduction process in several electrolytic cells for aluminum, comprising: a central storing place (2) for aluminum oxide from which a piping system (3) distributes the aluminum oxide from the central storing place (2) to a plurality of decentralized cleaning plants (1), each decentralized cleaning plant (1) comprising a storage portion (2) to store aluminum oxide before and after use in the decentralized cleaning plant (1), in flow communication with a filter portion (4), the filter portion (4) using aluminum oxide to remove fluorine containing waste gases from a waste gas flow, and an exhaust fan (5) integrated with a top part of the filter portion (4) of each decentralized cleaning plant (1), wherein a separate silo is not employed, and each decentralized cleaning plant (1) is adapted to serve groups of between 5 and 40 electrolytic cells.

Full Text

The present invention is related to a plant for dry cleaning, by removing fluorine containing waste gases from a reduction process in several electrolytic cells for aluminum. Known embodiments of plants for dry cleaning of waste gases developed during electrolytic processing of aluminum, are centralized plants servicing one or several halls comprising ovens with cells whereby each hall may comprise between 70 and 150 electrolytic cells or ovens and which are arranged centrally between the halls or outside. The plant is connected with each of the cells by means of a comprehensive and costly effluent channel. The aluminum oxide used as adsorbent during the cleaning process is stored in separate silos before or after use in the cleaning process and thereafter is transported back to each cell by means of transportation vehicles, cranes or other transportation systems for aluminum, such as a system for transportation in a compact phase. The invention to which the subject matter of GB 1416344 is directed is a process for the recovery of fluorine values from the waste gas from an electrolytic reduction cell for the production of aluminum in which waste gas is contacted with suspended alumina particles for a contact time sufficient to react at least 90% of the hydrogen fluoride of the waste gas with the alumina particles, alumina particles above a predetermined size and constituting a major proportion of the alumina brought into contact with the gas being separated from the waste gas in a first separation stage and the minor remaining proportion of fine alumina particles being collected in a second preparation stage, the size above which alumina particles are separated in the first separation being selected so that the first separation stage being selected so that the separated alumina particles contain less than 0.1 & by weight of benzene-soluble hydrocarbon tar. The invention to which the subject matter of US 3780497 is directed is a complex apparatus for adsorbing fluorine on alumina particles whereby the alumina particles are introduced at the base of an adsorption column through which a stream of gas containing fluorine flows upwardly. The top of the adsorption column is connected to a separator. The stream of gas charged with particles is kept flowing through the adsorption column and the separator by a fan, which discharges a stream of scrubbed gas through a chimney. Alumina, which has been partially charged with fluorine, is recovered in hopers and is at least partly recycled to the column. In order to recycle the alumina, the hoppers are served by a conveyor, which supplies an intermediate hopper into which fresh alumina can be directly introduced through a supply duct. The bottom of the hopper can be connected to the spout supplying the column via an evacuation duct equipped with a valve, or may be connected to a silo supply bath via an evacuation duct equipped with a valve. The invention to which the subject matter of WO 9615846 is directed is the treatment, through a dry adsorption process, of a gas from a hot electrolytic process for aluminum production comprising at least two stages. Particulate aluminum oxide (the adsorbent) passes through the at least two stages of the adsorption process counter currently to the gas. Thus, the gas is treated with a partly spent adsorbent in a first dry adsorption stage, whereupon the particulate adsorbent is separated from the gas downstream from the first adsorption stage. Part of the separated particulate adsorbent is removed from the adsorption process with a view to recycling adsorbed fluorine-containing substances to the electrolytic process. The remainder of the separated adsorbent is recirculated in the first adsorption stage in order to optimize the adsorption of fluorine-containing substances and the adsorption of sulphur dioxide in this first adsorption stage. Simultaneously, the gas is transferred to a second dry adsorption stage. In this second stage, the gas is treated with essentially unspent, reactive particulate aluminum oxide so that any gaseous fluoride remaining in the gas is very efficiently adsorbed, while at the same time a substantial part of the sulphur dioxide in the gas is also adsorbed. Finally, this particulate aluminum oxide is separated from the gas downstream from the second dry adsorption stage, before the gas is discharged into the surrounding atmosphere. The separated aluminum oxide is transferred to the first adsorption stage, optionally after passing a desorption stage for the removal of adsorbed sulphur dioxide in order to reduce the discharges of sulphur dioxide from the aluminum production. The seperation of sulphur dioxide in the second adsorption stage is improved by recycling, to the second adsorption stage, aluminum oxide having undergone the desorption treatment. In contrast to the teachings of the prior art, GB 141 6344, US 3780497 and WO 9615846, which have been discussed above, the present invention is directed to a decentralized cleaning plant for dry cleaning by removing fluorine containing waste gases from a reduction process in several electrolytic cells for aluminum comprising : a storing place for aluminium oxide; a transportation system to distribute aluminum oxide from the storing place to the decentralized cleaning plant; a filter portion to store aluminium oxide before and after use in the decentralized cleaning plant; and an exhaust fan integrated with a top filter part of the plant. With the device according to the present invention for cleaning of waste gases from the reduction process of aluminum, substantial savings are achieved as to the volume or area needed, as well as savings in the equipment and time savings during the process and the building. This is achieved with the plant according to the present invention. Accordingly the present invention provides plant for dry cleaning, by removing fluorine containing waste gases from a reduction process in several electrolytic cells for aluminum, comprising; a central storing place for aluminum oxide from which a piping system distributes the aluminum oxide from the central storing place to a plurality of decentralized cleaning plants, each decentralized cleaning plant comprising a storage portion to store aluminum oxide before and after use in the decentralized cleaning plant, in flow communication with a filter portion, the filter portion using aluminum oxide to remove fluorine containing waste gases from a waste gas flow, and an exhaust fan integrated with a top part of the filter portion of each decentralized cleaning plant, wherein a separate silo is not employed, and each decentralized cleaning plant is adapted to serve groups of between 5 and 40 electrolytic cells. By using a decentralized plant according to the present invention major parts of the traditional stores and transportation systems for aluminum oxide as can be found in known plants for dry cleaning, are eliminated. Likewise a substantial part of the channel system for waste gases as well as the connection channels between the cleaning plant and the main chimney is made redundant. Even the need for main fans for waste gas is eliminated. With the present invention a substantial reduction on the space requirement is achieved, furthermore the processing time is reduced and a much more simpler construction and operation is achieved. There is no demand for a storing silo for treated aluminum oxide and likewise the demands for fundamentations are substantially reduced. As opposed to known embodiments of plants, no chimney is required furthermore no silos and no lifting devices. Furthermore large flexibility is achieved as to starting up the operation and there is a short distance between the places for storing of the aluminum oxide and the ovens. The method of storing and the place of storing for the aluminum oxide may be chosen depending on local conditions and suitability. With the present invention suitable groups of between 5 and 40 electrolytic cells may be served in each pant, preferably between 10 and 20. The present invention provides further substantial reduced energy consumption and thereby achieves substantial cost savings. As a result of the simplified plant, the need for spare parts also is substantially reduced. Outlet dampers for the filters are not necessary and there also is no need for a compressor room as the case is with known plants. The installation time is strongly reduced and the design well adapted for adjustment to specific products. The drawing discloses schematically a plant according to the present invention where figure 2 discloses a ground view of a plant according to the invention and figure 1 is a sectional view of the ground view of figure 2. Fresh aluminum oxide is transported in a compact form from a central tank 2 to each decentralized cleaning plant 1 by a transportation system, such as a piping system 3. Further storing of aluminum oxide before or after use in the plant 3 is made in storing tanks integrated in the filter portion 4 of the cleaning plant. The waste gas fan 5 which is serving each cleaning plant 1, is integrated with the top filter part and makes the traditional outlet dampers on top of the filter redundant. The new embodiment enables recirculation of fluorides and other raw materials from a specific group of cells, in return to the same cells. The modules for removing impurities as well as SO2 also can be integrated in a very simple way as part of the decentralized plant 1 for dry cleaning, possibly at a later time. WE CLAIM: 1. A plant (1) for dry cleaning, by removing fluorine containing waste gases, from a reduction process in several electrolytic cells for aluminum, comprising: a central storing place (2) for aluminum oxide from which a piping system (3) distributes the aluminum oxide from the central storing place (2) to a plurality of decentralized cleaning plants (1) each decentralized cleaning plant (1) comprising a storage portion (2) to store aluminum oxide before and after use in the decentralized cleaning plant (1), in flow communication with a filter portion (4), the filter portion (4) using aluminium oxide to remove fluorine containing waste gases from a waste gas flow, and an exhaust fan (5) integrated with a top part of the filter portion (4) of each decentralized cleaning plant (1), wherein a separate silo is not employed, and each decentralized cleaning plant (1) is adapted to serve groups of between 5 and 40 electrolytic cells. 2. The plant as claimed in claim 1, wherein the plant {]) is adapted to serve groups of between 10 and 20 electrolytic cells. 3. The plant as claimed in claim 1, wherein the storing place (2) is a central tank. 4. A plant for dry cleaning, by removing fluorine containing waste gases, substantially as hereinabove described and illustrated with reference to the

Documents:

1371-mas-1998 abstract.pdf

1371-mas-1998 claims-duplicate.pdf

1371-mas-1998 claims.pdf

1371-mas-1998 correspondence-others.pdf

1371-mas-1998 correspondence-po.pdf

1371-mas-1998 description (complete)-duplicate.pdf

1371-mas-1998 description (complete).pdf

1371-mas-1998 form-19.pdf

1371-mas-1998 form-2.pdf

1371-mas-1998 form-26.pdf

1371-mas-1998 form-4.pdf

1371-mas-1998 form-6.pdf

1371-mas-1998 others.pdf

1371-mas-1998 pct-search report.pdf

1371-mas-1998 pct.pdf

1371-mas-1998 petition.pdf