EP3102304A1

EP3102304A1 - Method of purifying liquid by use of a particulate sorbent

Info

Publication number: EP3102304A1
Application number: EP15709715.5A
Authority: EP
Inventors: Simo Aho; Ville Hakala; Pertti Rantala
Original assignee: Sofi Filtration Oy
Current assignee: Sofi Filtration Oy
Priority date: 2014-02-04
Filing date: 2015-02-03
Publication date: 2016-12-14
Also published as: FI20145114L; WO2015118223A1

Abstract

The invention relates to a method of purifying liquids, which contain impurities such as oil or dissolved metals. The liquid is first mixed with a finely-divided particulate sorbent to form a suspension, in which impurities are adsorbed or absorbed by the sorbent particles, to form a pre-mixed suspension. A flow of this suspension is then directed substantially tangentially towards a surface of a porous filtering element (7), where- by part of the liquid penetrates through the filtering element and is re- moved through a filtrate outlet as purified liquid filtrate (8). The tangential flow keeps the filtering element (7) unblocked, and the sorbent, together with the rest of the liquid travels along the surface of the filtering element to be removed through a retentate outlet (9). The method is even useful for clarifying the liquid by removal of finely-divided solid impurities.

Description

Method of purifying liquid by use of a particulate sorbent

Technical field

The present invention relates to a method of purifying liquid, by use of a sorbent for adsorbing or absorbing impurities present in the liquid. The liquid is subjected to filtration, in which the sorbent is held on the upstream side of the filtering element, letting pure liquid permeate the filtering element and be obtained as the filtrate.

Background of the invention Known filtering devices on the market include filter cartridges or bags, i n whi ch liquid is forced with high pressure through the filter, whereby solid impurities remain in the filter material. Devices of this type are very simple and their cost of acquisition is low, but they require continuous maintenance, and additionally, the filters are easily clogged and of- ten need be frequently replaced, which brings about high service and maintenance costs.

There are impurities, which pose specific difficulty for ordinary filtering techniques. For removal of such impurities, adsorbing or absorbing materials have been used as a filtering aid, which is coated onto a surface of a filtering element to retain various impurities, while liquid permeates the coat of filtering aid and the filtering element for being obtained as purified filtrate. This precoat technique has been successfully used to remove finely-divided impurities, which tend to escape other conventional purifying methods. As an example of use of filtering aid precoats reference is made to WO 2013/1 17812, which describes rotary drum or disc filters aimed at removal of lime sludge from white and green liquor of chemical pulping processes. As the coat layer gets clogged, scrapers are used to remove the glogged surface, and liquid sprays are employed to wash the filtering element before the used precoat is replaced by a new one.

Another example is WO 2013/054000, in which an autogenous precoat is formed and used for clarifying a liquid containing extremely small solid particles. The technique is intended for use in chemical and process industry, as well as metallurgy plants.

Clarification filtering refers to removing finely divided solids from a liquid flow. Particularly it concerns liquids, where the amount of solids com- pared to the amount of the liquid to be filtered is small (from 0.1 mg/l to 10 g/l) and where also the particle size of the solids is very small (from 0.1 to 50 μιη). This has traditionally been an especially difficult range for filtering. Except for process industry clarification filtering is used for example in mining, in food and pharmaceutical industries, in pretreat- ment of ballast water of ships and in production of clean water.

Cross-flow filtration is another known clarification filtration method. In cross-flow filtration liquid, which contains finely divided particulate material, is fed tangentially across the surface of the filtering element, in other words, not perpendicularly to the surface. Due to the high speed of the liquid flow there is generated a shearing force, which makes part of the liquid to penetrate through the filtering element and separate as a purified filtrate, while the rest, holding the solid impurities, is removed as a retentate and may be recirculated to filtration.

An advantage of the cross-flow method is that any filter cake building up, that might block the surface of the filtering element, is flushed away during the filtering process, thus extending notably the operational life time of the filtering element.

A method and an apparatus operating on the cross-flow filtration principle are disclosed in EP 1 044 713 A1. According to this reference the fil- tering element is brought to rotational movement and the liquid flow to be filtered is fed to the outside of the filtering element in a direction substantially opposed to the direction of movement of the mantle surface, the liquid being filtered through the element mantle to the inside of the element. An improved filtering device utilizing the cross-flow principle is described i n WO 2012/104493. In this case the filtering element is stationary and thus cheaper to manufacture. The liquid to be filtered hits the outer surface of the filter element with an adequate speed and tangentially over the total cylindrical surface of the element, whereby efficient filtering is guaranteed at increased capacity and lower operating costs.

The advantage of cross-flow filtering over the precoat technique is the self-cleaning aspect of the filter element surface, which avoids the re- generation phase of a precoat, which necessitates interrupting the process for the renewals. A drawback is that the cross-flow technique has been limited to clarification treatment of liquids with finely-divided solids. There is even a need to treat liquids, which are oily or carry dissolved metals and which cross-flow techniques have so far been unable to han- die.

Summary of the invention

The purpose of the invention is to solve the above problem by introducing a new method based on the cross-flow principle, which is able to purify liquids with oils, flocculated material as well as metal ions as impuri- ties. The solution according to the invention is a method, which uses a finely-divided particulate sorbent and comprises the steps of (i) mixing the liquid to be purified with the sorbent to form a suspension, in which impurities are adsorbed or absorbed by the sorbent particles, so as to form a premixed suspension, (ii) directing a flow of said suspension to pass by a surface of a porous filtering element, whereby (iii) part of the liquid penetrates through the filtering element and is removed through a filtrate outlet as purified liquid filtrate, and (iv) the sorbent, together with the rest of the liquid travels towards at least one retentate outlet, and is removed through said at least one outlet, while (v) the filtering element is cylindrical, with a circular cross-section, and the suspension is directed substantially tangentially to the surface of the filtering element.

The sorbent used in the invention is generally similar to the filtering aid material used in known precoat techniques, but as distinguished from the prior art it is not coated onto the filtering element surface but cap- tures the impurities before the filtration step, hits the filtering element surface in a substantially tangential flow, and is allowed to run along the surface towards the retentate outlet as a carrier of the impurities adhered thereto. The main categories of impurities the method is particularly fit to treat are oils, which do not dissolve or mix with the liquid that is being purified, flocks, which without use of the sorbent according to the invention would block the cross-flow filtering, and would also clog any precoat prema- turely, and dissolved metals, which previous cross-flow and precoat techniques are both unable to remove. Even organic impurities may be removed by the process.

A further advantage of the invention is that in addition to purification due to the sorbent, any finely-divided particulate impurities present in the liq- uid being treated are removed as well, in accordance with the teachings of e.g. WO 2012/104493, which is incorporated by reference as part of the present description. The result is clarification of the liquid as taught in said reference.

Suitable sorbent materials for use in the invention are porous minerals, such as diatomaceous earth and perlite, which are known as useful precoat materials for filtration, and kaolin, magnesite, talc and ferric precipitates, which are described as particulate adsorbents in WO 201 1/042592, Fl 20120013 A and Fl 122917 B. Diatomaceous earth is particularly effective for removal of oil from an aqueous medium. Active carbon is useful as a sorbent in the invention also, e.g. for separation of Au from cyanide solutions and for removal of organic impurities from drinking water. Suitable particle size of the sorbent is in the range of about 5 to 50 μιτι, and suitable consistency of the premixed suspension is in the range of of 0.5 to 2 %, preferably about 1 %. Another group of useful sorbent materials are solid biophosphonates, which are used to capture dissolved metals in particular from aqueous liquids. Such materials are known and have been described e.g. in WO 2012/131 170 A1 . Metals are complexed with biophosphonates within different pH ranges, for instance Al at pH 1 -2, and several others, e.g. Fe, Co, Ni, Pb, Cu and Ag, at pH 4 or more, allowing Al to be separated from those other metals by adjusting the pH accordingly, to 1 -2 for collecting Al to the sorbent or to at least 4 for keeping Al in the liquid while the other metals are captured by the sorbent. Biophosphonate particles may be very small, in the range of 1 to 5 μιτι, and are difficult to separate from the liquid phase by conventional filtration, but may be effectively used according to the cross-flow filtering techniques of the present invention.

The pore size of the filtering element is preferably about 20 μιτι at most. Generally the particle size of the sorbent should be the same or, prefer- ably, larger than the pore size of the filtering element.

According to the invention the filtering element is cylindrical, preferably a stationary cylinder, with a circular cross-section, and the suspension flow is directed substantially tangentially to the surface of the filtering element. Rotating the cylindrical filtering element according to the teachings of EP 1044713 A1 may be contemplated as an alternative, however. The suspension may be fed centrally with respect to the length of the cylindrical filtering element, for instance through a number of oblique apertures in an outer cylinder as described in WO 2012/104493, and the re- tentate is removed from outlets at the opposite ends of the filtering ele- ment. Thereby the high pressure of the feed flow drives the sorbent to travel as a spiral flow along the periphery of the filtering element towards the outlets, while the cross-flow phenomenon keeps the surface of the element from being blocked by the solids.

The retentate, which consists of the sorbent, the impurities adhered thereto, as well as part of the liquid as carrier, may be circulated back and used for forming the premixed suspension. As the amount of impurities in the sorbent grows, it may be discarded or regenerated for reuse in the process.

As mentioned, adjusting the pH of the premixed suspension may allow the sorbent to adsorb or absorb dissolved metals selectively, so that another metal remains dissolved and ends up in the filtrate. By altering, usually reducing, the pH of the circulated retentate it may then be possible to release the metal(s) to the liquid phase and thereby regenerate the sorbent. For instance, reduction of the pH to 3 or less by addition of strong acid lets bisphosphonate used to capture Fe, Co or Ni be regenerated, or increasing the pH to 3 or more by addition of a base does the same if biophosphonate has been used to capture Al. For the separation of the regenerated sorbent from the liquid phase the same cross-flow filtering equipment may be used as for the liquid purification process prop- er. The liquid phase containing the redissolved metals passes through the filtering element and is removed through the filtrate outlet. The metals can then be recovered, if valuable.

Particularly harmful elements, which can be removed from aqueous liq- uids by use of the invention, include As, Sb and U. Elements removable by use of the invention, which are particularly valuable and therefore desirable for recovery, include noble metals such as Au and Pt, and rare earth metals.

Brief description of the drawings Figure 1 shows schematically an arrangement for purifying impure liquid, including means for mixing the same with a sorbent and subsequent filtration,

Figure 2 shows a cross-sectional view of a filtering apparatus useful in the invention, along the longitudinal axis thereof. Detailed description of the Invention

The arrangement of devices as shown in Fig. 1 is for purifying a liquid, which contains impurities, by means of filtration. Generally, the liquid is first mixed with a finely-divided particulate sorbent to form a suspension, in which impurities are adsorbed or absorbed by the sorbent particles. A premixed suspension is thus formed, and this suspension is directed to pass by a surface of a porous filter element, whereby the cross-flow principle of filtration is being utilized. Part of the liquid penetrates through the filter element and is removed through a filtrate outlet as purified liquid filtrate. At the same time the sorbent, together with the rest of the liquid, travels towards a retentate outlet, and is removed through said outlet and circulated back to the mixing step.

More particularly, with reference to Fig. 1 , an aqueous liquid carrying oily impurities is conducted from a reservoir 1 via line 2 to a mixing tank 3, to be mixed with finely divided diatomaceous earth of a particle size of about 5 to 50 μιτι. Diatomaceous earth works as an adsorbent, which captures the oil and even other impurities that may be present. A premixed suspension having a consistency of about 1 % is formed in the tank 3 and is then driven by a pump 4 via line 5 to a filtering apparatus 6, which comprises a stationary cylindrical filter element 7 of porous material. In Fig. 1 the filtering apparatus 6 is positioned horizontally, but it may also have a vertical positioning, see Fig.2. The suspension is fed tangentially towards the surface of the filter element 7, whereby most of the liquid passes through the porous cylindrical mantle of the filter element 7, to form a purified aqueous filtrate, which is removed through line 8. The rest of the liquid, acting as a carrier for the oily adsorbent particles, travels along the surface of the filter element to outlets 9 at the opposite ends of the filtering apparatus 6, and is removed as a retentate, which is circulated along line 10 back to the mixing tank 3 to be reused as adsorbent and part of the premixed suspension. As the adsorbent particles gradually get saturated by adsorbed impurities, they are periodically removed from the tank 3 and conducted to be regenerated by a washer 1 1 and a dryer 12.

Instead of oil the impurities may comprise flocks or dissolved metals, which like oils are problematic for conventional filtration. Instead of dia- tomaceous earth the sorbent may comprise other porous minerals, e.g. kaolin, magnesite, talc or perlite. The filtering apparatus 6, including the filter element 7, may be of the type described in WO 2012/104493. As shown in Fig. 2, the vertically positioned filtering apparatus 6 has a circular cylindrical shape and comprises at least one inlet connection 13 for feeding liquid to be clarified from line 5 into the apparatus, an outer mantle 14, a circular cylin- drical filter element 7, at least one outlet connection 9 positioned substantially in an upper part of the apparatus and at least one outlet connection 9 positioned substantially in a lower part of the apparatus, and means for feeding the liquid into the apparatus. Further, the apparatus 6 comprises a cross-flow tube (CF tube) 15 having a circular cylindrical shape and being positioned between the filter element 7 and the outer mantle 14, substantially concentric with the filter element 7. The CF tube 15 has apertures directed substantially in tangential direction towards the outer surface of the filter element 7. A mantle space 16 is provided between the outer mantle 14 and the CF tube 15. The incoming liquid from line 5 is fed to the mantle space 16 through one or a plurality of inlet connections 13. In Fig. 1 there are three feed inlets 13 and in the example of Fig. 2 there is shown one feed inlet 13. Most preferably the feeding is effected at three points, whereby the feeding pipes are located in vertical direction so that there is a pipe in both ends of the apparatus 6 and one substantially in the middle. The feed inlets are preferably directed substantially in tangential direction towards the outer surface of the CF tube 15, so that the direction of movement of the liquid to be clarified is the same in the mantle space 16 as in the gap between the filter element 7 and the CF tube 15.

The CF tube 15 is located around the filter element 7 so that there is a gap 17 between the inner surface of the CF-tube 15 and the outer surface of the filter element 7, the gap having a width of 1 to 15 mm, preferably from 3 to 8 mm. The wall thickness of the CF tube can range from 1 to 10 mm. The CF tube 15 may be provided with rectangular apertures, directed substantially in a tangential direction, at an angle of 90 - 1 10^° and preferably of about 90^°, with respect to the outer surface of the filter element 7. The width of the apertures can range from 0.1 mm to 5 mm; preferably it is from 0.3 to 2 mm. The height of the apertures ranges from 1 mm to 150 mm; preferably the height is from 40 to 80 mm. The height and width dimensions are defined as viewed in the direction of the longitudinal axis of the CF-tube. The apertures of the CF tube may be tapering towards the inner wall of the CF tube, whereby tapered channels are provided. Apertures as described above are provided in the CF tube substantially over the total length and around the total circumference thereof.

The cylindrical filter element 7 may be made of porous material, such as ceramic or silicon carbide, or also of sintered metallic mesh. The fil- ter element 7 has a pore size of about 20 μιτι or less. The filter apparatus 6 may comprise a plurality of filter elements 7 on top of each other, in which case the outlets 9 are preferably located at the connection points and at both ends of the apparatus. The liquid is filtered when passing through the mantle of the filter element 7 to a filtrate space 18. The clarified filtrate is discharged through outlet 19 into line 8.

The filtering apparatus 6 works by clarifying the liquid containing the suspended finely divided adsorbent, the liquid being fed from the mixing tank 3 with a pump pressure of 1 to 10 bars to the mantle space 16 of an apparatus. The CF tube 15 directs the liquid to the outer surface of the filter element 7 substantially in a tangential direction over the total cylindrical area of the element, both in the longitudinal and circum- ferential directions. There is new liquid to be filtered hitting against the outer surface of the filter element 7 continuously and with an adequate speed. Preferably all the apertures of the CF tube 15 are directed so that the liquid to be clarified hits against the outer surface of the filter element 7 substantially in parallel direction. In other words, the liquid moves in the gap between the element and the CF tube in the same direction. The outlet connections 9 located at the upper and lower ends of the apparatus 6 maintain a high enough speed of the liquid moving along the outer surface of the filter element 7. Due to the fast movement of the liquid, a shearing force is formed between the surface of the filter element 7 and the liquid, making a part of the liquid pass through the filter element 7 according to the Cross-flow phenomenon. The liquid is filtered when passing through the mantle of the filter element 7 to the filtrate space 18, and i s discharged as clarified filtrate through outlet 19 into line 8. The retentate is circulated from the out- lets 9 through line 10 back to the mixing tank 3.

In the apparatus 6 inside the filter element 7 there is a rod-like ultrasonic element 20 used for purifying the filter element. Even though the filter element 7 is largely self-cleaning, it still must be washable for securi ng a good filtering efficiency of the apparatus. In the purifying cy- cle, the so called backwash, the filtrate filtered by the apparatus is led with high power back to the apparatus, for example by means of compressed air or a pump. At the same time the ultrasonic element 20 inside the apparatus is started.

Claims

1. Method of purifying liquid containing impurities by use of a particulate sorbent, the method comprising the steps of

- mixing the liquid to be purified with a finely-divided particulate sorbent to form a suspension, in which impurities are adsorbed or absorbed by the sorbent particles, to form a premixed suspension,

- directing a flow of said suspension to pass by a surface of a porous filtering element (7), whereby

- part of the liquid penetrates through the filtering element (7) and is re- moved through a filtrate outlet (19) as purified liquid filtrate, and

- the sorbent, together with the rest of the liquid travels towards at least one retentate outlet (9), and is removed through said outlet, characterized in that the filtering element (7) is cylindrical, with a circular cross- section, and the suspension is directed substantially tangentially to the surface of the filtering element.

2. A method according to claim 1 , characterized in that the liquid to be purified contains oily impurities.

3. A method according to claim 1 , characterized in that the liquid to be purified contains flocculated impurities.

4. A method according to claim 1 , characterized in that the liquid to be purified contains dissolved metals as impurities.

5. A method according to any one of the previous claims, characterized in that the liquid to be purified is clarified by removal of fine particulate impurities.

6. A method according to any one of the previous claims, characterized in that the sorbent is porous mineral, such as kaolin, magnesite, talc, diatomaceous earth or perlite.

7. A method according to claim 6, characterized in that the particle size of the sorbent is about 5 to 50 μιτι.

8. A method according to claim 4, characterized in that the sorbent is a solid bisphosphonate, which is reactive with dissolved metals.

9. A method according to any one of the previous claims, character- ized in that said premixed suspension has a consistency of 0.5 to 2 %, preferably about 1 %.

10. A method according to any one of the previous claims, characterized in that the pore size of the filtering element (7) is at most about 20 μιη.

1 1. A method according to any one of the previous claims, characterized in that the particle size of the sorbent is larger than the pore size of the filtering element.

12. A method according to any one of the previous claims, characterized in that the filtering element (7) is stationary.

13. A method according to any one of the previous claims, character- ized in that the suspension is fed centrally with respect to the length of the cylindrical filtering element (7), and the retentate is removed from outlets (9) at the opposite ends of the filtering element.

14. A method according to any one of the previous claims, character- ized in that the retentate is circulated to be used for forming the premixed suspension.

15. A method according to claim 4 or 8, characterized in that the pH of the premixed suspension is adjusted for selective removal of one or more dissolved metals by the sorbent.

16. A method according to claim 15, characterized in that the sorbent circulated as part of the retentate is regenerated by altering the pH and thereby releasing the captured metals to the liquid phase.

17. A method according to claim 16, characterized in that said porous filtering element (7) is used to separate the regenerated sorbent from the liquid phase containing the released metals, the liquid phase passing through the filtering element and being removed through said filtrate out- let (19).