METHOD OF SEPARATING A SUBSTANCE FROM A LIQUID BY MEANS OF A PARTICULATE MATERIAL

Abstract: In connection with separation of a substance from a liquid by binding said substance to small particles, for instance solids, a body of liquid is kept in rotation together with the particles having a density lower than that of the liquid. Liquid from which said substance is to be separated is brought into contact with the particles in a way such that these are maintained suspended in the rotating liquid body.

Description: Method of separating a substance form a liquid by means of a particulate material The present invention relates to a method of separating a substance from a liquid by binding the substance to a particu¬ late material. By a particulate material is meant in this connection any suitable material in a finely divided form, thus comprising not only solids but also particles of a more or less liquid material, for instance gel particles. The particles may be porous or impervious to liquid. Preferably, they are chemically stable, and in certain applications of the invention they have to be inert to the liquid, with which they are to get into contact, and/or to the substance or substances which they are to separate from the liquid. Even if the particles may have any suitable form they are preferably spherical and substan¬ tially of the same size. Separation by means of so called liquid chromatography tradi¬ tionally means that a liquid, from which a substance is to be separated, is caused to pass through a treatment chamber filled with a lot of small particles retained closely packed together in the treatment chamber between two perforated end walls. The liquid thus has to flow through the interspaces between the immobilized particles, the substance to be separated being retained by means of the particles by adsorption at their surfaces or in some other way. A problem in connection with this separation technique is that the flow resistance for the liquid is very large in the inter¬ spaces between the particles. The smaller the particles are, the larger this flow resistance will be. The liquid thus has to be highly pressurized to be able to pass through the interspaces between the particles. The problem is accentuated by the fact that the particles preferably are made as small as possible, since a certain volume of packed particles exposes a larger total particle surface to a through-flowing liquid the smaller the particles are. For these reasons it is difficult to provide for acceptable costs a separation plant having a large separation capacity, and. the described separation technique, therefore, is mainly utilized in a small scale, e.g. for analysing purposes or for the separation of very small amounts of very expensive substances. As an. alternative to the above described separation technique it has been suggested that a liquid, from which a substance should be separated, should be supplied to the lower part of a treat¬ ment chamber containing a bed of particles, which are free to move relative to each other, and then be caused to flow through the bed of particles from below and upwardly in a way such that the particles are maintained in a fluidized or suspended state within the treatment chamber. A fluidizing technique of this kind would require a smaller overpressure of the supplied liquid than the previously described separation technique. However, the capacity of a separation plant in this case would instead be limited by the fact that the flow velocity of the liquid through the bed of particles could not be too large. This flow velocity must not be larger than that the particles are retained in the bed by gravity and are not entrained by the upwardly flowing liquid out of the treatment chamber. One further technique having been suggested to bind a certain subtance in a liquid to particles resides in keeping the particles suspended in the liquid by agitation during some time and then - af er the particles have attracted the said substance in the liquid - separating the particles from the liquid by gravity separation or centrifugal force separation. A disadvan¬ tage of this technique is that it will take a relatively long time to create the required contact between the particles and all parts of the liquid for the separation of the substance therein. An object of the present invention has been to provide a new method of separating a substance from liquid by binding the substance to small particles, which method makes it possible to obtain a substantially larger separation capacity than the above described methods and, thus, can be used in much a larger scale than these methods. Another object of the invention has been to enable use of substantially smaller particles than could be used earlier. These objects may be obtained according to the invention by keeping a body of liquid having a predetermined density in rotation together with small particles having a density lower than that of the liquid, and by bringing liquid, from which said substance is to be separated, into contact with the particles in a way such that the particles are maintained suspended in the rotating liquid. The invention consequently resides in keeping particles lighter than a liquid in a suspended state in a rotating liquid body, while the liquid, from which a substance is to be separated, is caused to flow past the suspended particles. If desired, the particles may be kept suspended in the whole of the rotating liquid body. Preferably, however, the particles are kept suspended only in a layer of the liquid body, the liquid to be freed from said substance being caused to flow through the suspension layer. The main part of the liquid should flow bet¬ ween the suspended particles in a direction from the rotational axis of the rotating liquid body towards the radially outermost part of the liquid body. Since the particles are lighter than the liquid, the particles suspended in the liquid will try due to the centrifugel force to move radially inwards, the force influencing the particles in this direction being larger the longer from the rotational axis the particles are situated. Therefore, the supply of liquid from which said substance is to be separated may be made very large without any risk that the particles should leave the treatment chamber together with radially outwardly flowing liquid. The force to which the particles are subjected by the flow of the supplied liquid is independent of the distance between the particles and the rotational axis- If the particle size would vary somewhat in the suspension layer, this does not matter very much, since extremely small particles which are easily entrained by the supplied liquid will still be retained in vicinity of the other particles as a consequence of the radially outwardly increasing centrifugal force. By the same mechanism a particle is immediately returned radially inwardly, if it has been influenced by occasional forces and been transferred too far out from the rotational axis. The liquid body kept in rotation may have any desired shape. If it has a shape such that the through-flow section for the supplied liquid increases radially outwardly, the balancing effect on the suspended particles will be even larger, since the force acting on the particles and caused by the flow of the liquid in this case decreases with an increasing distance from the rotational axis of the liquid body, whereas the centrifugal force, acting in the opposite direction, increases. An increasing through-flow section for the supplied liquid may be desirable, particularly in the radially outer part of the suspension layer, for safe retainment of the particles therein. In the main part of the suspension layer it may be desirable, however, to have instead a radially outwardly decreasing through-flow section for the supplied liquid. Thereby, namely, it may be accomplished if desired a substantially uniform distribution of the particles in the suspension layer along the flow way of the liquid therethrough. If the particles have a certain size distribution, this has to be considered in connection with the planning of the flow way of the liquid through the suspension layer for establishing a desired distribution of the particles therein. As would be obvious, any desired distance between the particles in the suspension layer may be accomplished by a suitable choice of centrifugal force within and liquid flow through the suspen- sion layer. This distance may even be changed by controlling of the centrifugal force and/or the liquid flow during the course of the separation process. If desired, it is possible by using particles of different sizes to obtain different (separate or overlapping) layers of suspen¬ ded particles in the rotating liquid body. This could be useful in connection with batch-wise use of particles during a separa¬ ting operation in order to limit the radial movement of the particles within the suspension layer of the rotating liquid body. Advantageously, a conventional centrifugal separator being only insignificantly modified may be used for the performing of the method according to the invention, both said liquid and the particles being kept in rotation in the form of a substantially annular body. The invention may be used in connection with substantially all separation processes of a kind in which conventional liquid chromatography could come into question. Furthermore, the invention makes it possible by means of the separation technique here in question to resolve many separation problems which at present cannot be resolved in an economically acceptable way. Thus, the invention makes it possible in an industrial scale to separate very small amounts of valuable or harmful substances from large amounts of heavily diluted solutions. For instance, the invention may be used for extracting valuable metals, such as gold and silver, from solutions, or for extracting proteins, polypeptides or aminoacids from biotechnical process liquids. Also carbohydrates and lipides can be extracted. Furthermore, the quality of certain drinks, such as beer or wine, may be improved by removal of certain substances. Another example is that waste water may be purified from undesired substances, such as heavy metals, phenoles or cyanide, before it is dumped in the nature. The choice of material for the particles to be used has to be made with regard to the liquid from which a substance should be separated and to said substance. Since many process liquids have a density of about 1,0 g/cm , particles having a density lower than 1,0 g/cm often have to be chosen. Then, different kinds of plastic material may come into question. For instance poly- ethene or polypropene may be used as a basic material in the particles. However, a material heavier than 1,0 g/cm may be used, if particles of such material are expanded by means of previously known technique, such that they will contain a gas of one kind or another. Particles of this kind having a density as low as 0,04 g/cm are available on the market. In the same manner as is generally known in connection with conventional liquid chromatography the particles to be used for the method according to the invention normally have to be prepared such that they can attract or bind a substance to be separated from a liquid. In one way or another - of several known ways - the particles are given an active surface which for instance chemically or physically may bind said substance to the particles, when liquid gets into contact therewith. For instance, it is possible to give the particle surfaces a certain charging, so that they can attract and bind particulate substan- ces having the opposite charging (ion exchange technique) . Even well known principles for so called affinity-chromatography, hydrofobic interaction, etc, may be used in connection with the present invention. It is also possible to use particles having the ability of absorbing a substance in a liquid. In such cases the substance penetrates into the particle body in question, where it is retained. As in connection with conventional liquid chromatography it is often desired that the particles used at the separation method according to the invention can be reused many times. Correspon¬ ding techniques may be used as in connection with conventional liquid chromatography for freeing the particles from substances having been bound to the particles during a separation process. The particle size should be as small as possible but has to be adapted to the chosen combination of the accomplished centri¬ fugal force and the magnitude of the liquid flow in the centri- fugal field in question. Preferably, particles in the magnitude of 0,1 - 10 um are used. The invention is described in the following with reference to the accompanying drawing. In the drawing fig 1 shows schemati- cally a so called open centrifuge rotor of a kind that can be used upon application of the invention. Fig 2 shows a part of the centrifuge rotor in fig 1 somewhat modified. Fig 3 shows a so called closed centrifuge rotor that alternatively can be used upon application of the invention. Fig 1 shows a part of a centrifuge rotor comprising a rotor body 1 supported by a vertical drive shaft 2. The rotor body 1 defines a separation chamber 3, in which a stack of conven¬ tional frusto-conical separation discs 4 is arranged coaxially with the rotor. The stack of separation discs 4, which have axially aligned so called distributing holes 5, rests upon a conical bottom plate 6. This plate has corresponding holes 7 aligned with the distributing holes 5. The bottom plate 6 forms together with the lower part of the rotor body 1 a chamber 8 which communicates-with the separation chamber 3 through the holes 7 in the bottom plate 6. A channel 9 extending axially through the drive shaft 2 opens into the chamber 8. A conical partition 10 delimiting together with the upper part of the rotor body 1 a number of channels 11 rests upon the stack of separation discs 4. The channels 11 extend from a radially outer part of the separation chamber 3 towards the centre of the rotor and open in a first outlet chamber 12 formed by the radially inner parts of the rotor body 1 and the partition 10, respectively. Radially inside the first outlet chamber 12 the radially inner part of the partition 10 forms a second outlet chamber 13. The separation chamber 3 communicates through an overflow outlet 14 with said second outlet chamber 13. Both of the outlet chambers 12 and 13 are formed as radially inwardly open annular groves, and into these groves there extend stationary outlet members 15 and 16, respectively, which may have the form of conventional so called paring discs. Both of the outlet members are supported by a stationary inlet pipe 17 extending from the outside of the rotor into a central inlet chamber 18 in the rotor, formed radially inside the frusto- conical separation discs 4. The part of the inlet pipe 17 that is situated in the inlet chamber 18 is perforated, so that a liquid supplied through the inlet pipe may be sprayed radially outwardly and be distributed along the axial extension of the inlet chamber. At the radially outermost part of the separation chamber 3 the rotor body 1 has a number of outlet openings 19 evenly distri¬ buted around the rotor axis. In a manner known per se these outlet openings may be arranged to be opened and closed during operation of the rotor. During operation the above described centrifuge rotor will contain both liquid and very small particles having a density less than that of the liquid. The liquid will form a rotating body filling the largest part of the separation chamber 3 comprising the spaces between the separation discs 4 and part of the central inlet chamber 18, and the particles will be suspended in a cylindrical sleeve formed layer 20 of the liquid body situated closest to the rotor axis. The layer 20 extends radially inwardly to the level of the overflow outlet 14, where¬ by liquid supplied to the rotor through the inlet pipe 17 will be sprayed against the inside of the layer 20. The layer 20 may be divided in separate sectors by means of radially and axially extending entrainment wings (not shown) supported radially inside the separation discs 4 by the bottom plate 6 and the partition 10. The centrifuge rotor in fig 1 is intended to operate in the following manner. Liquid from which a certain substance is to be separated is supplied through the inlet pipe 17 and is sprayed towards the inside of the rotating sleeve formed liquid layer 20 containing particles. By this liquid supply the particles will be main- tained in a suspended state in the rotating liquid body. The larger the liquid supply through the inlet pipe 17 is made, the larger distance will be obtained between the particles in the liquid body. During the passage of the supplied liquid through the layer 20 it is brought into effective contact with the particle surfaces which are prepared in a known manner to attract the substance to be separated from the liquid. When the liquid has passed through the layer 20 and been freed from all or part of said substance, it flows further on radially outwardly through the spaces between the separation discs 4 to the radially outer part of the separation chamber 3. From there the liquid flows through the channels 11 radially inwardly to the outlet chamber 12, from where it is removed by means of the stationary outlet member 15. During the separating operation described above new particles may be supplied to the rotor and used particles may be removed from the rotor either continuously or intermittently. New particles can be supplied through the channel 9 in the drive shaft 2 and, through the chamber 8 and the distributing holes 5, be pumped into the spaces between the separation discs 4. Supplied new particles will thus move countercurrently relative to the liquid flowing radially outwardly in the rotor. Hereby, a corresponding amount of particles having separated the above mentioned substance from the supplied liquid is displaced out of the inlet chamber 18 through the overflow outlet 14 to the outlet chamber 13. By means of the stationary outlet member 16 the particles are removed from the outlet chamber 13. According to an alternative way of Intermittently displacing particles out of the separation chamber 3 the outflow of liquid through the stationary outlet member 15 may be interrupted temporarily, while new liquid continuously is supplied through the inlet pipe 17. Then, the liquid surfaces in both the outlet chamber 12 and the separation chamber 3 (or the inlet chamber 18) will move radially inwardly. New particles are preferably supplied to the centrifuge rotor suspended in a liquid of a suitable kind, for instance a part of the liquid having left the centrifuge rotor after having been freed from the above said substance. In a known manner particles having been used in the described separating operation, i.e. having been discharged from the centrifuge rotor through the outlet chamber 13, may be reconditioned and be used anew. Upon need the particle suspension supplied to the rotor through the channel 9 may be more diluted, i.e. contain less particles per unit of volume of the particle suspension, than the particle suspension discharged from the rotor by means of the outlet member 16. The composition of the discharged particle suspension is determined by the particle density that is used in the layer 20 of particles fluidized in liquid. By use of conical separation discs 4 of the described kind particles having for its object to attract the said substance in the supplied liquid may be effectively prevented from being entrained by liquid too far on its way radially outwardly in the rotor. However, it may be suitable to dimension the separation discs in a way such that a large part of the particle suspension is formed radially inside the separation discs in order to avoid that the particle suspension will become too dense. It has been assumed above that the centrifuge rotor has a separate inlet for new particles. Alternatively, new particles may be supplied to the centrifuge rotor through its ordinary inlet for the said liquid, i.e. they may be mixed with the liquid before it is supplied to the centrifuge rotor. Fig 2 shows an embodiment of a centrifuge rotor arranged for supply of new particles in this way. Fig 2 shows a perforated inlet pipe 17a supporting an outlet member 16a. The outlet member extends radially outwards in an outlet chamber 13a, which through a number of holes 21 in a partition 22 communicates with the separation chamber of the rotor. The holes 21 are placed at a level radially outside the free liquid surface formed in the inlet chamber 18a by the liquid body rotating therein. In the radially innermost layer of the liquid body, which layer is designated 20a in fig 2, the particles are maintained in a suspended state by supply of liquid through the perforated inlet pipe 17a. As can be seen, the partition 22 extends radially inwardly to a level inside the layer 20a, so that the particle suspension cannot flow into the outlet chamber 13a any other way than through the holes 21. These are situated at the level of the radially outer part of the layer 20a. In the embodiment shown in fig 2 the centrifuge rotor has frus o-conical separation discs 4a, which at their radially innermost edges support annular members 23 extending substan¬ tially horizontally. The members 23 have a radially outwardly increasing thickness, whereby the interspaces between them converge radially outwardly. This is a design of the members 23 enabling that a substantially unchanged or even decreasing throughflow section may be provided for liquid flowing radially outwardly through the layer 20a of the particle suspension. The members 23 have holes 24 axially aligned with each other and with the holes 21, whereby particles having separated a certain substance from the passing liquid may leave the layer 20a at the actual radial level and flow out into the outlet chamber 13a. Therein, if desired, a free liquid surface may be maintained by means of the outlet member 16a radially outside the free liquid surface in the inlet chamber 18a. It is presumed that the holes 21 form a throttle for the flow of suspension out into the outlet chamber 13a. In an embodiment of the centrifuge rotor according to fig 2 new particles preferably are supplied to the layer 20a together with the liquid supplied through the inlet pipe 17a. This enables the new particles at the beginning to be kept suspended in the radially innermost part of the layer 20a and - as they gradually separate said substance from the liquid and thereby in certain applications of the invention get heavier - to move radially outwardly and finally leave the layer 20a through the holes 24 and 21. Fig 3 shows an alternative embodiment of a centrifuge rotor by means of which the method according to the invention may be performed. Parts of the centrifuge rotor in fig 3 corresponding to parts of the centrifuge rotor in fig 1 have the same refe¬ rence numerals as the latter with the addition of a letter b. The centrifuge rotor in fig 3 has an inlet pipe 25 for liquid from which a substance is to be separated, extending into the rotor through the channel 9b in the drive shaft 2b. The inlet pipe 25, that is rotatable together with the rotor, is per¬ forated along its extension in the inlet chamber 18b. The rotor body lb supports at its upper part a pipe 26 and a pumping wheel 27 connected with the pipe. The pipe 26 and the pumping wheel 27 are surrounded sealingly by a stationary pumping house 28 having an outlet conduit 29. The interior of the pipe 26 communicates with the channels lib in the rotor and with the interior of the pumping wheel 27, whereby the latter is arranged to pump liquid from the separation chamber 3b through the channels lib out into the pumping housing 28 and further through the outlet conduit 29. In a similar manner the conical partition 10b at its upper part is connected with a pipe 30, which extends coaxially through the pipe 26, and supports a pumping wheel 41. The upper part of the pipe 30 and the pumping wheel 31 are surrounded sealingly by a stationary pumping housing 32 having an outlet conduit 33. The interior of the pipe 30 communicates with the inlet chamber 18b and with the interior of the pumping wheel 31, whereby the latter is arranged to pump particle suspension from the inlet chamber 18b out into the pumping housing 32 and further through the outlet conduit 33. A centrifuge rotor according to fig 3 operates in substantially the same manner as a centrifuge rotor according to fig 1. The only difference is that the whole inlet chamber 18b is filled with particle suspension during operation and that the separa- ting operation within the rotor may be perfored at a super- atmospheric pressure and without the liquid and particles coming into contact with the atmosphere surrounding the rotor.

Claims: Claims 1. Method of separating a substance from a liquid by binding the substance to small particles, c h a r a c t e r i z e d i that a body of liquid having a predetermined density is kept in rotation together with small particles having a density lower than that of the liquid, and that liquid from which said substance is to be separated is brought into contact with the particles in a way such that these are maintained suspended in the rotating liquid body. 2. Method according to claim 1, c h a r a c t e r i z e d i n that the main part of the liquid from which said substance is to be separated is caused to flow between the suspended particles in a direction from the rotational axis of the rota¬ ting liquid body towards the radially outer part of the liquid body. 3. Method according to claim 1 or 2, c h a r a c t e r i - z e d i n that the liquid from which said substance is to be separated is caused to flow through a layer (20) of the rotating liquid body, in which the particles are maintained suspended. 4. Method according to claim 3, c h a r a c t e r i z e d i n that liquid is continuously removed from the radially outer part of the said layer (20). 5. Method according to any of the preceding claims, c h a ¬ r a c t e r i z e d i n that new particles are supplied and used particles are removed from the liquid body, while this is kept in rotation. 6. Method according to any of the preceding claims, c h a ¬ r a c t e r i z e d i n that the liquid body and the particles are kept in rotation as a substantially annular body. 7. Method according to any of the preceding claims, c h a ¬ r a c t e r i z e d i n that liquid is conducted from the rotating liquid body radially outside the suspended particles to a space (12) , in which it is allowed while being rotated to form a free, preferably annular, liquid surface which is maintained at a predetermined radial level. 8. Method according to any of the preceding claims, c h a ¬ r a c t e r i z e d I n that a free surface of the rotating body of liquid and particles is maintained at a predetermined level in a chamber (3), that is delimited by a rotating body (1), by means of an overflow outlet (14) from said chamber (3). 9• Method according to any of claims 3-7 , c h a r a c - t e r i z e d i n that new particles are supplied to said layer (20a) of the rotating liquid body at a first level and that used particles are removed from the layer (20a) at a second level (21) radially outside said first level. 10. Method according to any of the preceding claims, c h a ¬ r a c t e r i z e d i n that the liquid from which said substance is to be separated is caused to flow past the suspen¬ ded particles along a flow way, the through-flow section of which decreases and in which the centrifugal force increases, seen in the direction of flow of the liquid.

Inventors: EHNSTROEM LARS, EHNSTROEM, LARS
Applicants: ALFA LAVAL AB, ALFA-LAVAL AB
Application year: 1989
Main classification: B01D15
IPC codes: B01D15, B04B5, G01N30