AU2021377502A1

AU2021377502A1 - Percolating container

Info

Publication number: AU2021377502A1
Application number: AU2021377502A
Authority: AU
Inventors: Vincent Warren PRIOR
Original assignee: Wright Brennan Kevin
Current assignee: Wright Brennan Kevin
Priority date: 2020-11-13
Filing date: 2021-11-15
Publication date: 2023-08-10
Also published as: WO2022101875A1

Abstract

There is disclosed a liquid percolating container formed of a composite fabric that includes a layer of woven geotextile and a layer of a non-woven fabric, with the two layers being secured together by means of stitches at least along edges thereof to form a container having an operative inside, an operative outside and an inlet configured to receive an inlet tube, and with the non-woven fabric located on the inside of the container and the woven geotextile located on the outside of the container.

Description

PERCOLATING CONTAINER

FIELD OF THE INVENTION

This invention relates to a container used for percolating liquids, in particular used for dewatering slurries, with the aim of retaining solids in the container and allowing liquid to pass from the inside to the outside of the container.

BACKGROUND TO THE INVENTION

Liquid processing at industrial sites such as mines often involves large volumes of liquids and these often comprise water with mixed-in solids, which are referred to as slurries. The solids typically range in size from relatively large to very small, with latter being called fines or ultrafines. Ultra-fines typically have sizes less than 5 micron.

The processing of liquids from locations such as mines is required to prevent ground contamination and also to recover valuable solids from such liquids. In mines where valuable minerals such as gold and platinum are mined, the recovery of even a small fraction of mineral fines and ultra-fines is potentially very lucrative. This is especially relevant, considering that with underground mining the establishment and operation of infrastructure such as shafts and work faces is very expensive. At the point where mineral components in the form of fines or ultra-fines have made their way into a slurry, all the normal overhead costs have already been incurred, and any economic benefit that can be gained from such recovered fines or ultra-fines minerals represent almost pure profit.

There is a need to improve the effectiveness of the processing of such liquid streams to recover mineral components and more generally to prevent fines and ultra-fines from entering downstream liquid streams, even where such fines and ultra-fines do not have a great economical value in itself. In such instances the benefit from recovering such fines and ultra-fines is that it prevents pollution.

Conventionally such slurries are passed through a barrier made of very strong geotextile fabric with a predetermined mesh or pore size. The fundamental concept is that the pore size is selected to allow water to pass through the mesh, but it prevents the fines and ultra-fines from passing through. With the ultra-fine component of the solids being in the minus 5-micron size range, it means the pore size has to also be minus 5-micron.

An advantage of such woven material is the strength of the geotextile fabric. These materials typically provide filtration and strength.

A major problem with such materials is that woven material, although very strong, blind easily with fines and ultra-fines. Although this means the fines and ultra-fines are retained - which is what is desired - it also means the water does not drain at all or it does not drain fast enough from the slurry - which is not desired. Typically, it is desired that water should drain from a slurry at a rate of more than 50 or 60 l/s/m² for such an application to be economically viable.

The use of woven geotextile material for dewatering applications therefore increases the cost of dewatering containers made from it, at least due to the short lifespan thereof.

Materials that used in civil engineering reinforcing applications are not suitable for such dewatering applications. Generally, civil engineering applications use geotextiles to reinforce specific areas and prevent catastrophic failure, such as landslides or soil collapses.

This generally involves providing an in-situ barrier that retains solids and allows liquid - generally water - to pass through the barrier, under force of gravity and at a reasonable rate. Such applications typically require drainage rates in the region of about 20 to 30 l/s/m², and the civil engineering solutions provide such drainage rates, which are to slow for dewatering applications.

Such applications are not generally concerned with retaining the fine and ultra-fine component of the solids within the liquid (water), and generally such applications are in any event may not presented with a large fraction or soil in this range of sizes. In such applications composite materials that comprise multifilament woven materials may be used. These have a relatively density and high cost. These materials have a very tight weave which limits the drainage rate.

This is not a problem for civil engineering applications where the drainage rate is not critical and where the fraction of fines and ultra-fines is small. Such materials are not suitable for dewatering of liquids that contain a significant component of fines and ultra-fines. It has been observed that where the fraction of ultra-fines, i.e., particles less than 5 micron in size, exceed about 30% of solids in a liquid, such conventional barrier materials blinds very quickly, which drastically limits the drainage rate to even less than the above mentioned 20 to 30 l/s/m². Therefore, slurries cannot be dewatered effectively using such conventional composite barrier materials.

In addition to the above general considerations for civil engineering applications, blinding does pose a problem in some civil engineering applications. To overcome those, composite materials have been developed where a non-woven layer is needle punched to a woven layer, sometimes with a so-called geonet between them. The geonet provides structural integrity and prevents the non-woven material from stretching too much, as it is likely to do in the absence thereof. Stretching of the non-woven material is not preferable and in this context is to be strictly avoided, since it effectively increases the sizes of the pathways through the non-woven material. That allows the liquid that it was supposed to contain to pass through the material with not enough restriction, effectively almost gushing through it without retaining solids.

Although the use of the woven material and especially the geonet prevents this, the composite material still has a blinding problem. To overcome this, one solution that has been used is for the non-woven material to be needle punched to the woven sheet. The needle punching provides specific dedicated liquid pathways through the composite layer that allows liquid, typically water, to move from the non-woven side to the woven side of the composite material.

This works fine under gravity feed systems, but at higher pressures, such as when the liquid is pumped against the barrier, liquid is forced through those pathways with an entrained fraction of fines and ultra-fines that defeat the purpose of dewatering that the present invention seeks to address. This is then similar conceptually to having a stretched non-woven material with larger pathways. Specifically, fines and ultra-fines that should be retained in such processing, instead escape with the liquid when pressure on the liquid is increased to greater than what is present under a gravity feed application.

Generally, dewatering of slurries are done under pressure with the slurry being pumped into a dewatering container and contained under pressure therein. The slurry not just fed under force of gravity into the container, which will take too long for liquid to drain through the barrier, and the fraction of liquid that remains trapped within the slurry will be too large.

The problem of blinding in civil engineering applications have also been addressed in some instances by providing very expensive sheets of materials which include so-called wicking yarn. Such materials may use yarn that have specific cross-sectional shapes, including multichannel, trilobal, and pillow. These materials are suited for gravity fed systems to counteract the capillary action of water which occurs against an inner surface of a composite material. That inhibits the flow of water through such material and destabilises the structure of which it forms part by trapping excess water behind the barrier. Such materials are suitable for applications in which additional drainage is required in unsaturated soil conditions. These materials are too expensive for use in saturated conditions and dewatering applications. They are also not suitable for use in dewatering applications since the flowrates achievable through such materials are still too low.

In this specification the following term has the meanings indicated adjacent each associated with them:

Permittivity - the volumetric flow rate of clean water per unit cross sectional area per unit head under laminar flow conditions, in the normal direction through a geotextile, as determined according to ATSM D4491-99a-13.

OBJECTIVE OF THE INVENTION

It is an objective of the invention to provide a liquid percolating container which at least partly overcomes the abovementioned problems.

SUMMARY OF THE INVENTION

According to this invention there is provided a liquid percolating container formed of a composite fabric that includes a layer of woven geotextile and a layer of a non-woven fabric, with the two layers being secured together by means of stitches at least along edges thereof to form a container having an operative inside, an operative outside and an inlet configured to receive an inlet tube, and with the non-woven fabric located on the inside of the container and the woven geotextile located on the outside of the container. There is further provided for the two layers to be secured together by means of a double row of stitches that extend substantially parallel with the edges of the two layers.

There is also provided for the woven material to comprise a mesh woven from polypropylene, preferably polypropylene tape woven in a 2/2 dredge twill weave, and further preferably for the woven mesh material to be comprised of a woven polypropylene material with a density of about 250g/m², with a warp tensile strength of about 54 kN/m and a warp elongation between 12% and 35%, and a weft tensile strength of about 42 kN/m and a warp elongation between 8% and 25%.

There is still further provided for the woven material to preferably have a permeability of about 66 L/s/m² under a 300mm head of clean water, and a permeability of about 25 L/s/m² under a 100mm head of clean water.

There is further provided for the non-woven fabric to be comprised of a polyester material, preferably with a density of about 350g/m², with a tensile strength of at least 675N, and having a characteristic opening size of between 200pm and 250pm, and preferably about 207pm, and more preferably for the non-woven material to be configured to have a permittivity between 0.700 l/s and 0.9001/s, more preferably between 0.7314 l/s and 0.8931/s, and most preferably about 0.81221/s.

There is still further provided for the liquid percolating container to comprise a dewatering bag.

These and other features of the invention are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a sectional view through a composite fabric according to the invention;

Figure 2 shows the filtrate resulting from using a liquid percolating container according to the invention;

Figure 3 shows how dry the contained material is after 1 week of consolidation following treatment in a liquid percolating container according to the invention; Figure 4 shows dewatering cycle using geotextile material tubes;

Figure 5 shows dewatered material and filtrate using flocculant and high flow geotextile; and

Table 1 shows indicative results of using the fabric of the invention to filter a slurry.

DETAILED DESCRIPTION OF THE INVENTION

Woven geotextile material retains a fair number of solids if a slurry is filtered through it. However, a woven material does not retain most fines and ultra-fines. A non-woven fabric could be used to retain more fines and ultra-fines, whilst still allowing liquid to percolate through the material.

However, the general properties of non-woven material don’t always allow for an entire bag to be made from just the non-woven material. The strength of the non-woven material is not sufficient to support the weight of a large volume of water, and it is prone to failure from tearing. Additionally, in industrial type applications a robust material is required that could withstand environmental impacts. In for example an underground mining application a dislodged piece of rock that impacts the woven material could induce a tear that could lead to the failure of the entire bag. This is potentially catastrophic and could be fatal to people in the vicinity.

The fabric (1) of the invention comprises a composite sheet made of two layers, one being a layer of woven geotextile material (2) and the other being a layer of non-woven fabric (3). The woven geotextile is used for the operative outside of the composite sheet and the inside and the non-woven fabric is used for the operative inside.

The two sheets are stitched (4) together to form a composite fabric sheet (1). The stitches extend along the periphery of the two layers, and the stitches preferably comprises two rows (4A, 4B) of spaced apart stitches, as shown in Figure 1.

To percolate liquid a container in the form of bag is constructed using the fabric (1). The woven geotextile side (2) of the composite sheet (1) is located on the operative outside of the bag and the non-woven fabric side (3) of the composite sheet (1) is located on the operative inside of the bag. The bags can be formed in any applicable shape, for example as a cube or rectangular bag, or as an elongate tube with closed ends.

It is envisaged that a specific combination of woven geotextile and non-woven fabric will give a specific filtering profile for a specific slurry. This includes drainage rate and the filtrate quality over a pumping period, as well as during passive drainage. This data is very specific to the slurry that is treated by passing it through the container.

The preferred configuration of a liquid percolating container according to the invention includes a woven geotextile in the form of a polypropylene tape that is woven in a 2/2 dredge twill weave. The polypropylene material with a density of about 250g/m², with a warp tensile strength of about 54 kN/m and a warp elongation between 12% and 35%, and a weft tensile strength of about 42 kN/m and a warp elongation between 8% and 25%. The woven material has a permeability of about 66 L/s/m² under a 300mm head of clean water (tested according to the BS 6909-3-1989 test method) and a permeability of about 25 L/s/m² under a 100mm head of clean water (tested according to the SANS 10221-2007 test method).

The non-woven fabric is 100% polyester, has a density of about 350g/m², and has a tensile strength of at least 675N. It has a characteristic opening size of between about 207pm, which is the range of between about 200pm and 250pm that is envisaged as optimal for the nonwoven material in this application. The non-woven material has a fine fibre diameter, which is configures the non-woven to have a permittivity between 0.700 l/s and 0.9001/s. In a specific embodiment it had minimum value of 0.7071/s and a maximum value of 0.8981/s. The average permittivity was 0.81221/s with a standard deviation of 0.08081/s. The non-woven was thus configured to have a permittivity of between 0.7314 l/s and 0.8931/s, with an average value of 0.81221/s.

This allows for a material with more fibres available to filter smaller particles more effectively. The fibres are connected to each other by means of a needle punching process. This creates a three-dimensional structure in the non-woven material, which improves the stability and consistency of the non-woven material. Notably, the non-woven material itself is subjected to needle punching, but this is not done in conjunction with the woven material.

In the configuration of the bag, the non-woven fabric does most of the filtering, it being on the inside of the bag. The non-woven fabric filters from large to ultra-fine particles. The woven geotextile fabric on the outside adds the strength and resilience to the bag. The woven geotextile protects the non-woven fabric from elongation (and thus failure) of the non-woven. The woven geotextile remains effectively unloaded with particles.

The positioning of the non-woven fabric on the inside of the container results, over time, in “blinding” of the inner non-woven layer. This does have an impact on filtration efficiency, which is revealed by the filtration profile of the container over time. This serves an indication of when the inner layer of the non-woven fabric is loaded with ultra-fines and fines, and for customers that wish to recover those particles this is then a useful indicator that the container is ready for recovery.

The container can then be decommissioned, and the valuable particles trapped in the inner layer can be recovered from the composite fabric, for example by reverser washing and drainage of the wash-water.

To determine the efficacy of the liquid percolating container, the composite material of which it is manufactured was subjected to tests. An example of test results on a slurry is shown in Table 1. The tests indicate that it is possible to bring the moisture content of a slurry down to 15% with clear filtrate, feeding a PSD with 76% - 25 micron.

In particular, the applicant conducted laboratory-scale hanging bag test work on a sample of sludge. Hanging bag trials are used to determine the dewatering potential of the slurry and the effectiveness of various geotextile in dewatering. A hanging bag with an approximate volume of 12.5 litre is filled with slurry and then allowed to dewater via gravity drainage, whilst the drainage rate and filtrate clarity is monitored. The moisture content of the retained solids is determined after sufficient dewatering has occurred. The hanging bag can then be left to dewater over an extended period to determine the long-term dewatering potential of the slurry.

In addition to hanging bag test work, the applicant also conducted flocculant screening tests in conjunction with gravity drainage tests using a small piece geotextile and a Buchner funnel. This type of test work allows the flocculant dosing rate to be determined, as well as filtrate quality with various geotextiles.

A first test was conducted on a non-woven lined composite geotextile. This resulted in a very clear filtrate, as shown in Figure 2. Figure 3 indicates how dry the contained material is after 1 week of consolidation. It is important to note that dewatering by means of geotextile bags is a cyclical process. A schematic illustrating this concept is shown in Figure 4.

During the initial filling cycle, the dewatering tube is filled to the maximum design height and pumping is terminated. Static drainage then commences. After sufficient dewatering the tube can then be refilled. This process is repeated until the tube is filled. Thereafter further consolidation occurs, and the dewatering process allows for the solids concentration of the dewatered material to increase, as illustrated by the red (lower) line shown in Figure 5. Consolidation in this case is estimated to take 1 week.

A second test was conducted using high flow geotextile in conjunction with flocculant addition. It was determined that a non-ionic flocculant works well at a dosing rate of 40 mg/L. The flocculant allows for agglomeration of the solids, which means dewatering with the standard high flow geotextile, in other words excluding the non-woven liner, is possible. Filtrate clarity is, however, not as good as the composite non-woven lined geotextile. Filtrate quality and the dewatered material (with no consolidation) is shown in Figure 5.

To achieve the necessary degree of dewatering in the absence of the non-woven liner a flocculant is required. This comes at the cost of the flocculant. This is significant, but it is to a large extent balanced by the cost of the non-woven liner of the composite material. However, the use of the flocculant with the exclusion of the non-woven liner cannot deliver the lower clarity of the filtrate that is possible when composite material is used as a liquid percolating container.

It will be appreciated that the embodiment described above is given by way of example only and is not intended to limit the scope of the invention. It is possible to alter aspects of the embodiment without departing from the essence of the invention.

Claims

1. A liquid percolating container formed of a composite fabric that includes a layer of woven geotextile and a layer of a non-woven fabric, with the two layers being secured together by means of stitches at least along edges thereof to form a container having an operative inside, an operative outside and an inlet configured to receive an inlet tube, and with the non-woven fabric located on the inside of the container and the woven geotextile located on the outside of the container.

2. A container the two layers to be secured together by means of a double row of stitches that extend substantially parallel with the edges of the two layers.

3. A container as claimed in claim 1 or 2 in which the woven material comprises a mesh woven from polypropylene.

4. A container as claimed in claim 3 in which the mesh is woven from polypropylene tape in a 2/2 dredge twill weave.

5. A container as claimed in claim 3 or 4 in which the woven material has a density of about 250g/m², a warp tensile strength of about 54 kN/m and a warp elongation between 12% and 35%, and a weft tensile strength of about 42 kN/m and a warp elongation between 8% and 25%.

6. A container as claimed in any one of the preceding claims in which the woven material has a permeability of about 66 L/s/m² under a 300mm head of clean water, and a permeability of about 25 L/s/m² under a 100mm head of clean water.

7. A container as claimed in anyone of claims 1 to 4 in which the non-woven fabric comprises a polyester material.

8. A container as claimed in claim 7 in which the non-woven material has a density of about 350g/m² and a tensile strength of at least 675N.

9. A container as claimed in claim 7 or 8 in which the non-woven material has a characteristic opening size of between 200pm and 250pm, and preferably about 207pm. ntainer as claimed in any one of claims 7 to 9 in which the non-woven material has a permittivity between 0.700 l/s and 0.9001/s, more preferably between 0.7314 l/s and 0.8931/s, and most preferably about 0.81221/s. ontainer as claimed in any one of the preceding claims in which the liquid percolating container comprises a dewatering bag. ewatering container substantially as herein described with reference to Figures 1 to 3.