WO2010091515A1

WO2010091515A1 - Filter bag for bayer liquor filtration

Info

Publication number: WO2010091515A1
Application number: PCT/CA2010/000201
Authority: WO
Inventors: Sandrine Minebois; Jean-Louis Ribet; Serge Tranche
Original assignee: SEFAR FYLTIS une société par actions simplifiée
Priority date: 2009-02-13
Filing date: 2010-02-11
Publication date: 2010-08-19
Also published as: CA2711216C; AU2010213328A1; CN102333895A; AU2010213328B2; BRPI1008790A2; BRPI1008790B1; CA2711216A1; CN102333895B

Abstract

A filter bag for Bayer liquor filtration in an alumina production process, the filter bag comprising a filter sleeve being tubular with an open end and a closed end opposite the open end, and a connector being tubular, made of an anti-scaling material, and affixed to the open end of the filter sleeve.

Description

FILTER BAG FOR BAYER LIQUOR FILTRATION FIELD

The specification relates to filter bags used to separate particles from a liquid slurry, such as iron oxide particles present in the Bayer liquor of alumina production processes, and more specifically, to a filter bag having an anti-scaling connector to ease its replacement.

BACKGROUND

Aluminum is produced by electrolysis in a cell of a molten salt containing alumina. During the process the cell must be fed continuously by fresh alumina powder. In order to produce 1 ton of aluminum, about 2 tons of alumina powder are used. Almost all of the alumina powder in the world is obtained using an alumina production process referred to as the Bayer process. The Bayer process uses an alumina rich ore called bauxite. Typically, the first step is to grind the bauxite and then mix it with a hot and concentrated solution of sodium hydroxide. The resulting slurry is heated under pressure for a few minutes and then cooled to about 100-106 ⁰C. The hot solution is then mixed with a flocculent and pumped into a decanter where most of undissolved particles settle down. Since it is a continuous process, the red mud containing particles is pumped out at the bottom of the decanter while a clear solution containing the alumina, and referred to herein as Bayer liquor, overflows at the top. However, the clear Bayer liquor saturated with alumina trihydrate still contains less than 1 % of iron oxide particles that are to be removed.

The finishing filtration to remove small particles of iron oxide is typically done using one of different types of available filters, such as horizontal or vertical leaf filters, or channel filters such as the Diastar tradenamed filter apparatus developed by Filters Gaudfrin (France). These latter are becoming more and more popular because of their production rate. The above filters use filter bags typically made of polypropylene or polyamide fabrics. A typical alumina plant uses more than 100,000 square meters of fabric per year. Figure 1 schematically illustrates a typical channel filter installation. The Bayer liquor fills a large cylindrical tank 10. A manifold having a number of radially oriented outlet pipes 12 is present in the upper section of the cylindrical tank. A number of filter bags 14a, 14b are suspended from the outlet pipes 12. The outlet pipes 12 are connected to a main outlet 16 where the filtered Bayer liquor is removed. Such applications are provided in a plurality of sizes and configurations. For illustrative purposes, in the installation schematically depicted, filter bags 14a, 14b can have many meters (typically over 10 feet) in length and can have 61 cm (24 inches) in width. A metal frame structure inserted inside the channels of the filter bags prevents the filter bags from collapsing when subjected to the pressure differential between the lower pressure inside the sleeves, and the higher pressure outside the sleeves.

The replacement of filter bags is typically done at a frequency of 45-60 days. To remove the bags, a first step is to remove the layer of alumina trihydrate mixed with iron oxide, referred to herein as scale, that has crystallized on the surfaces in contact with the solution. The scale can reach centimetres of thickness in a few days. The scale can be so hard that removing it is known to require the use a high pressure jet of a caustic solution or a pneumatic hammer. Filter bag replacement is thus a major concern and has even lead to injuries of workers. The problem is even worst for filters composed of many channels like Diastar, for which the connecting part can become heavily scaled.

Henceforth, there remained room for improvement in filter bags used in filtering scaling slurries, of in replacement methods thereof.

SUMMARY

A solution is to provide a filter bag which has an anti-scaling connector at its open end. Connectors of Nitrile rubber or EPDM rubber have been found useful to this effect. Not all rubbers are suitable and some low density rubbers were subject to the occurrence of scaling. Nitrile rubber or EPDM rubber have a relatively high density. The presence of carbon in rubber can be beneficial. Fluorinated elastomers can be unpractical due to their typically relatively high costs. Preferably, the length (or width) of the anti-scaling connector is selected to be as small as possible relative the length of the filter bag, given practical considerations, in a manner that the effect of the connector on the filtering characteristics of the filter bag are minimized. When the time has come to replace the filter bag, it is much easier to remove.

In accordance with one aspect, there is provided a filter bag having a filter sleeve with an open end, and a connector made of an anti-scaling material fixed to the open end of the filter sleeve.

In accordance with another aspect, there is provided a filter bag for Bayer liquor filtration in an alumina production process, the filter bag comprising : a filter sleeve being tubular with an open end and a closed end opposite the open end; and a connector made of a anti-scaling material and fixed to the open end of the filter sleeve.

In accordance with still another aspect, there is provided a filtration bag for removing particles from a slurry, the filter bag comprising : a filtering portion having an open end for receiving a flow of the slurry therein, and a closed end opposite the open end, the filter sleeve being adapted for retaining the particles into a scale as the slurry flows through; and a connector being short in length relative to a length of the filtering portion, fixed to the open end of the filter sleeve, the connector being made of a non-scaling material.

In accordance with another aspect, there is provided a filter bag for removing iron oxide particles from Bayer liquor in an alumina production process, the filter bag comprising : a filter sleeve with an open end, and a closed end opposite the open end, the filter sleeve being made of a material adapted for retaining the iron oxide particles as the Bayer liquor flows through it; and a connector made of a non-scaling material and fixed to the open end of the filter sleeve.

DESCRIPTION OF THE FIGURES In the appended figures, - A -

Fig. 1 is a schematic perspective view showing a typical channel filter installation for Bayer liquor filtration;

Fig. 2 is a front elevation view showing a filter bag with an anti-scaling connector attached to a Bayer liquor outlet pipe; and

Fig. 3 is a perspective view, enlarged, of a portion of the filter bag of Fig. 2.

DETAILED DESCRIPTION

Fig. 2 shows an example of a filter bag 20 for Bayer liquor filtration in an alumina production process. Generally, the filter bag can be seen to have an elongated filter sleeve 22 having a number of adjacent channels 24, and an anti-scaling connector 26. The filter sleeve 22 is made of a material adapted to filter the Bayer liquor from unwanted particles such as iron oxide particles. A polypropylene or polyamide fabric can be used to this end, as known in the art, for example. The filter sleeve 22 has a closed end (not shown) opposite an open end 28. In use, the Bayer liquor enters the channels of the filter sleeve through the fabric of the filter sleeve 22 by seeping through along the length thereof. A metallic frame (not shown) is provided inside the channels 24 to prevent them from collapsing under the pressure applied to the fabric by the Bayer liquor seeping through. The filter sleeve 22 retains unwanted particles such as iron oxide particles, hi addition to caking, such particles can chemically react with alumina trihydrate and crystallize into a very hard material referred to as scale, which eventually renders the filter bag 20 inefficient and in need of replacement. Both the filter sleeve 22 and the anti-scaling connector 26 can be seen to be tubular in shape and are attached in a manner to allow fluid flow communication between the inside of the tubular shape of the filter sleeve 22 and the anti-scaling connector 26

The filter bag 20 is mechanically attached to the outlet pipe 12 using an attachment 30 which holds the open mouth 32 of the anti-scaling connector 26 against a corresponding aperture (not shown) provided in the outlet pipe 12. More particularly, in this specific embodiment, the open mouth 32 of the anti-scaling connector 26 has an outwardly-oriented, surrounding Hp or rim 34 having a greater thickness then the remainder of the anti-scaling connector 26, and which is trapped by the attachment 30. The other open end 36 of the connector 26, opposite the mouth 32, is secured to the fabric at the open end 28 the filter sleeve 22. Sowing has been found effective to this end, for instance. The portion of the connector 26 between the rim 34 and the sleeve 22 will be referred to herein as the neck 38.

The connector 26 is anti-scaling because it is made of an anti-scaling material. A copolymer of acrylonitrile and butadiene, known as Nitrile rubber, or a copolymer of ethylene, propylene, diene, commonly referred to as EPDM rubber, can be used to this end, for example. More generally, some rubbers having a Shore A hardness of between 40 and 90, preferably about 70, and less than 1 % of porosity have been found effective as the anti- scaling material. Further, rubbers having an elasticity (elongation) of at least 30 % can be favoured for ease of installation. Some other materials can also be selected if they are found suitable.

The anti-scaling connector 26 can be moulded, and then affixed to the fabric by sewing, bonding or ultrasonic welding, for example.

To minimize the effect of the anti-scaling connector 26 on the efficiency of the filter bag 20, the length 40 thereof should be small relative to the length of the sleeve 22. However, it should nevertheless have a sufficient length for practicality purposes.

It will be understood that various sizes of filter bags can be provided. Nonetheless, to provide an example for discussion, be it said that the illustrated filter bag 20 has a width 42 of 61 cm (2 feet) and a length of 3.3 meters (11 feet). In this context, it was selected to provide a rim 34 having 8 mm, and a neck 38 having about 7.2 cm (8 cm including the 8mm rim). This allowed placing two sewing lines to securely hold the connector 26 secured to the filter sleeve 22. A first one 44 (see Fig. 3) at about 3 mm from the edge or open end 36 of the connector 26, and a second one 46 about 2.5 cm further. With a rim 34 having 8 mm, it may be highly unpractical to have a neck having less than about 12 mm or even 20 mm, for instance. The neck can be longer than 8 cm, but it can be unpractical to use a neck longer than 120 mm or 135 mm in such an application. Henceforth, a neck 38 having between 70 and 90 mm can be preferred. Other dimensions can be preferred for filter bags having different dimensions.

The selected thickness for the anti-scaling connector 26 depends on the material thereof, on the dimensions of the filter bag, and the particulars of the aimed installation. In the example given above, the thickness of the rubber can be between 2 and 5 mm. The thickness at the rim 34 can be greater, such as between 5 and 10 mm for instance, hi this application, the rim 34 has a thickness of 8 mm. The rim 34 can be omitted in certain applications, depending on the way the filter bags are attached to the outlet pipes for instance.

The exemplary filter bag described above and illustrated was tested for 60 days at 105°C in an installation. This filter bag took significantly less time to remove than a traditional filter bag in the same installation.

It will be understood that the embodiments are given for purposes of providing a complete disclosure of preferred embodiments, but many variants and alternate embodiments are possible. Henceforth, the scope is indicated by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A filter bag for Bayer liquor filtration in an alumina production process, the filter bag comprising : a filter sleeve being tubular with an open end and a closed end opposite the open end; and a connector being tubular, made of an anti-scaling material, and affixed to the open end of the filter sleeve.

2. The filter bag of claim 1 wherein the connector has an outwardly-oriented rim at an end thereof opposite the filter sleeve, and a neck extending from the filter sleeve to the rim, the neck being longer than the rim and having a thickness lesser than a thickness of the rim.

3. The filter bag of claim 2 wherein the neck of the connector is sewn to the filter sleeve.

4. The filter bag of claim 2 wherein the rim has a mechanical resistance sufficient for suspendingly holding the entire filter bag during use.

5. The filter bag of claim 1 wherein the non-scaling material is one of Nitrile rubber and EPDM rubber.

6. The filter bag of claim 1 wherein the anti-scaling material has a Shore A hardness of between 55 and 80, and a porosity of less than 1 %.

7. The filter bag of claim 6 wherein the anti-scaling material has an elongation of at least 30 %.

8. The filter bag of claim 2 wherein the neck has a thickness between 2 and 5 mm, and the rim has a thickness between 5 and 10 mm.

9. The filter bag of claim 1 wherein the connector is affixed to the filter sleeve by one of sewing, bonding and ultrasonic welding.

10. The filter bag of claim 1 wherein the non-scaling material is one of a copolymer of acrylonitrile and butadiene and a copolymer of ethylene propylene diene.

11. The filter bag of claim 1 wherein the filter sleeve is made with a fabric of one of polypropylene and polyamide.

12. A filter bag for removing iron oxide particles from Bayer liquor in an alumina production process, the filter bag comprising :

a filter sleeve with an open end and a closed end opposite the open end, the filter sleeve being made of a material adapted for retaining the iron oxide particles as the Bayer liquor flows through it; and

a connector made of a non-scaling material and fixed to the open end of the filter sleeve.

13. The filter bag of claim 12 wherein the non-scaling material is one of a copolymer of acrylonitrile and butadiene and a copolymer of ethylene propylene diene.

14. The filter bag of claim 12 wherein the non-scaling material is one of Nitrile rubber and EPDM rubber.

15. The filter bag of claim 12 wherein the non-scaling material has a Shore A hardness of between 40 and 90, a porosity of less than 1 %, and an elongation of at least 30 %.

16. The filter bag of claim 15 wherein the Shore A hardness is of between 55 and 80, preferably 70.

17. The filter bag of claim 12 wherein the connector has a rim opposite the filter sleeve.

18. The filter bag of claim 17 wherein the connector has a neck with a thickness of between 2 and 5 mm, and wherein the rim has a thickness between 5 and 10 mm.

19. The filter bag of claim 12 wherein the connector is fixed to the filtering portion by one of sewing, bonding and ultrasonic welding.

20. The filter bag of claim 12 wherein the material of the filter sleeve is a fabric of one of polypropylene and polyamide.

21. A filter bag for Bayer liquor filtration in an alumina production process, the filter bag having a filter sleeve being tubular with an open end and a closed end opposite the open end, the filter bag being characterized in that it further comprises a connector being tubular, made of an anti-scaling material, and affixed to the open end of the filter sleeve in a manner to allow fluid flow communication.