GB2039229A - Method and apparatus for concentrating trace materials from water - Google Patents

Abstract

For the concentration of materials such as uranium, present in trace quantities in water, especially sea water, carrier bodies able to take up the material from the water are released into it so that their density causes them to sink downwardly or float upwardly to a collection region from whence they are transferred to a chemical plant for removing the trace material concentrated on them. The bodies can then be recycled for release once again from the original discharge point. To avoid abrasion of material providing sites for the uptake of the trace material to be concentrated, this material can be located on surfaces set inwardly from the periphery of the carrier bodies. A ring shape for the carrier bodies gives advantageous flow of water around the bodies. <IMAGE>

Description

SPECIFICATION Method and apparatus for concentrating trace materials from water The invention relates to a method and apparatus for concentrating trace materials from large quan tities of water, especially but not exclusively from sea water, by accumulation on carrier bodies.

Effecting concentration of trace materials dissol ved in water may be useful for decontamination of the water, but is of primary interest for the extraction of raw materials from large water masses, for exam ple for the extraction of uranium from sea water.

Sea water contains a number of economically important raw materials, including for example the metals uranium, copper, tin and silver as well as other metals, in dissolved form. Although they mostly occur in only small concentrations, they con stitute a large potential supply because of the enormous quantities of water in the oceans.

Uranium, for example, is present in sea water in an average concentration of only 3.3 ppb, but neverthe less the quantity of uranium in the oceans of the world is about a thousand times greater than the quantity presently estimated to be available from known terrestrial deposits.

As a general principle, it is assumed that, for the purpose of collecting dissolved raw material, sea water must be brought into continuous contact with more or less fixed solid bodies which have a capacity for concentrating the dissolved raw material, such as special adsorbers or ion exchangers. Most dissolved raw materials, like uranium which is present in the form of a carbonate complex, are-with only a few local differences in concentration-distributed rela tively uniformly throughout the world's sea water and can thus be extracted at any point which is con sidered suitable. However, the extremely low con centrations present a considerable obstacle to any industrial extraction method.Very large quantities of sea water must be brought into contact with the adsorber in order to extract an economically worth while quantity of the desired raw material(s). In the case of the extraction of uranium, about 109 m3 of sea water are required per day for one tonne of uranium.

A number of methods have been proposed for bringing large quantities of water into contact with solids which are capable of effecting concentration.

In a method proposed by Atomic Research Establ ishment, Harwell, 1968, (Davies R. V. et al, Nature (London)203 (1964)1110), there is let into a dam an adsorptively acting fixed bed (for example a filling of granular material), which so divides a natural bay that the tidal flow let into the bay through a lock must pass through the adsorber bed in the dam in order to be able to flow back into the sea through the other part of the bay. The natural factors necessary for this purpose, such as a sufficiently high tide rise or a suitable shape, size and geology of the bay, are to be found only at a few places of the earth. Even if these pre-requisites are satisfied, the flow resistance of the solid bed is so great that it can be offset only by an appropriately large cross-section of the bed.

For an installation capacity of about one tonne per day, the gigantic dimensions of about 50 m dam width and 20 km dam length would be necessary, and this would involve difficult problems of construction and unacceptable total cost.

In addition, the extraction of uranium from the stream of cooling water from power plants has been considered, but the quantities of uranium which can thus be extracted are too small. Also, there exist disadvantages similar to those encountered in the above-discussed tidal method.

In accordance with another concept, a current of sea water is caused to pass through-an array of adsorber-coated plates or strips disposed at intervals of 0.1 to 10 mm, in a direction parallel to their faces (Japanese Patent specification 51-67217 and C.

Bettinali and F. Pantanetti, Proceedings IAEA AG/33-4Vienna 1976). In order to achieve a sufficient concentration of the raw materials present in sea water, there would here be required a number of aligned plates forming a plate system having a total length estimated at up to 200 m. A substantial disadvantage of this arrangement is-as in the case of the fixed bed-the very high flow resistance. This naturally applies also when the plates are replaced by endless belts in order to improve the continuous adsorber exchange (US Patent specification 3763049). Owing to the high water pressures occurring, there also arise considerable problems in fixing, as well as a danger of the plates or belts being set into vibration and being damaged or coming into contact with one another, which would result in abrasion of the adsorber.

Finally, there has been considered a fluidised bed method in which sea water flows up through a long adsorber bed from below and the granulated adsorber material is whirled up. In order to stabilise the granular material in the bed, the grains must be maintained in suspension by accurate selection of their sinking speed and of the speed of flow of the sea. As a result of the supply of fresh material, the granular material can flow transversely to the direction of flow of the sea towards a point at which it is withdrawn from the bed, and can thus be continuously exchanged. However, considerable pumping must take place in order to bring about the necessary upward flow of the sea in the adsorber bed.If natural currents of sea water are utilised, and are deflected in a vertical upward direction of flow, great difficulties are presented both by the monitoring of the flow and the stabilisation of the bed, as also by the necessary anchoring against the considerable dynamic forces of the flowing water. A further disadvantage is that natural abrasion will lead to a reduction of the particle size of the granular material, and hence of its sinking speed, with the consequence that some of the adsorber is washed away.

The object of the invention is to provide a method of extracting raw material from natural waters, espe cially sea water, wherein even very large quantities of water can be brought into contact with solid bodies capable of effecting concentration, with minimum technical expenditure and in continuous operation, and wherein, it is possible to avoid disadvantages of existing methods, while on one hand unhindered access of the water to the effective surfaces is made possible, and on the other hand the dynamic pressure and flow resistance are kept low.

According to one aspect of the present invention there is provided a method of concentrating trace material from water, characterized in that carrier bodies by which the material is taken up and whose density is different from that of the water, are discharged into a water layer (especially a natural current of sea water) so that their density causes them to traverse the layer, are thereafter collected again and transferred, laden with trace material taken up by them, to processing means at which trace material accumulated by the carrier bodies is removed from them, and optionally then redischarged into the water layer.

In this method, therefore, there may be used carrier bodies which sink in free fall through the water and are collected after a predetermined period of fall. They may in particular be discharged below the surface layer of a current of sea water, in which case they will be carried laterally as they fall.

Alternatively carrier bodies having lower density than water, which enables them to rise through the water in an appropriate time, can be discharged at a predetermined depth into a water layer-which may in particular be a current of sea water-and can then be collected again after they have risen. Cage-like collecting devices can be used for this. The rising carrier bodies need not necessarily be allowed to reach the surface, but it may be convenient for them to do so.

Concentration of trace material on the carrier bodies can occurthrough any desired manner of uptake by the bodies, notably as adsorption resulting from physical or chemical forces, or as accumulation resulting from bonding, especially complex formation or ion exchange. The sites active to take up the material (effecting concentration of it) may be secured to or secured in the carrier body-which itself may consist substantially of material active to effect uptake-especially as a thin layer anchored to (or in) a surface accessible to the water, so that during the period of fall or period of rise of the body the water has access to the sites for the uptake of material.

It is therefore desirable that the carrier bodies should have a surface of maximum possible size, while the surrounding water should as far as possible have unhindered access to the surface.

On the other hand, however, loss of concentration-effecting material by mechanical abrasion should be prevented. Therefore, the carrier body should be of such nature that the surfaces provided with (concentration-effecting) sites for uptake of trace material is/are not subjected to any friction by contact with any materials such an neighbouring bodies.

The carrier bodies must have different density from the water, so that the particles have a suitable tendency to rise orto sink, with the aid of which automatical passage through the water course under appropriate conditions (i.e. in times which are correlative with the accumulation of trace material from the surrounding water) can be achieved. Subject to this they may be of any desired, appropriate shape and size, the latter preferably being in the order of magnitude of 1 to 10 cm. However, larger or smaller carrier bodies may of course be employed. Criteria for the selection of particular carrier bodies are more particularly a large ratio of uptake effecting surface to volume, ready "filterability" or "retainability" of the carrier bodies and minimum thickness of the hydrodynamic boundary layers at the uptake effecting surfaces.The thickness of these layers is dependent upon the dimensions of the carrier bodies.

A particularly suitable form for the carrier bodies for use in this invention is that of a ring. If the material effecting uptake is resistant to mechanical abrasion, the ring may be, for example, in the form of a torus. Otherwise cylindrical rings are preferable, which are coated only on their inner peripheral surface with material to effect uptake of trace material from the water, so that abrasion of the active layer by mechanical contact with neighbouring bodies is prevented. A ring has the advantage that the thickness of the stagnant hydrodynamic boundary layer at the surface of the carrier body can be kept small even with relatively large carrier bodies. Since this boundary layer is a factor in the speed with which the carrier bodies become loaded with the trace materials to be concentrated, it should be as thin as practicably possible.In the case of cylindrical rings, the measurement which is characteristic of the thickness of the boundary layer is given by the shell height or ring thickness, and in the case of a torus by the diameter of the generating circle. On the other hand, the ability of the carrier bodies floating upwardly, for example to the surface to be retained by a cage, or the ability of the sinking bodies to be retained by a generally horizontal screen or plate, and their filterability during subsequent chemical processing (which may be on board ship) is determined by the (greater) ring diameter, which may be so chosen that the apertures (meshes) of the respective retaining device (cage or screen or filter) need not be made too small. This serves to keep low the flow resistance of the cages or screens or filters.

Such rings exhibit a particularly favourable behaviourforthe concentration of trace materials when their (polar) axis remains substantially in the direction of movement. In order to ensure this, the centre of gravity of the ring should lie on its axis but at a particular distance from the centre of symmetry.

A further improvement can be afforded by forcing the ring or carrier body to undergo an automatic "spinning motion" as it moves through the water.

A number of examples of the form of carrier bodies are sketched in Figures 6 to 13 to be described hereinafter.

The carrier bodies having lower density than water which may be employed in accordance with the invention are discharged at a particular depth into a water layer to be depleted, and, as a result of their buoyancy, they float up, conveniently to the surface of the water where they can be collected, and thereafter eluted or treated in some other appropriate mannerforthe extraction ofthetrace material concentrated on their surface or otherwise accumulated by them. Likewise, bodies which will sink are caught at a depth depending upon the desired time of descent and conveyed to the processing means, for example a chemical factory situated on the surface of the sea.

The water layer present between the outlet for carrier bodies and the catching point is subjected (by reason of concentration of trace material on the carrier bodies) to a continuous depletion and should therefore be constantly renewed, which can be achieved by a continuous relative movement between the outlet and the catching point on the one hand and the water layer on the other hand. Such a relative movement may be brought about either by towing the carrier body outlet and the catching point through the Water by means of a ship, or disposing the devices in a natural water current.

Sea currents in coastal regions have flow velocities which are sometimes far above 1 m/s. Sea currents of 0.5 m/s are very common, and the design of such installations should preferably be adapted thereto. However, adaptation to any flow velocity can readily be effected by appropriately varying the distance between the outlet and the catching point, and where necessary, the rate of discharge of the carrier bodies.

In further aspects this invention embraces apparatus for carrying out the method, and carrier bodies for use therein.

In the following, the invention is further explained with reference to embodiments illustrated by way of example in the accompanying drawings in which: Figures 1 and 2 diagrammatically illustrate an installation for the extraction of uranium from sea water by the sinking method, as seen from the side and in plan view; Figures 3 and 4 diagrammatically illustrate an installation for using the buoyancy method, as seen from the side and in plan view; Figure 5 is a sketch illustrating the further symbols employed, and Figures 6 to 13 illustrate various forms of carrier bodies.

Referring to Figures 1 and 2, carrier bodies 1 are discharged from an outlet 2 in free fall into a sea current 3. The outlet 2 is situated at a depth (for example about 5 metres) below the water surface at which surface disturbances caused by wind orthe like cannot affect the flow profile, which is usually 'constant on average up to a depth of (at least) 50 m.

The carrier bodies 1 are carried away from the discharge point by the current as they descend and, after passing through a distance 4, during which heavy metal, notably uranium, is concentrated on them, they enter a catching and collecting device 5 from which they are directed onto a conveying path 6. The plant which affects the conveying is diag rammatically indicated by 7 but its position and manner of operation need not be as indicated. Pref erably it operates hydraulically in accordance with the principle of communicating tubes that is to say, the conveyance is effected by a hydraulic pumping arrangement in which water and entrained carrier bodies are pumped upwardly together using a system of tubes with the upward flow tube(s) sealingly connected to tube(s) for a complementary downward flow of water which has a counter-balancing action and offsets the work of upward pumping.

With such an arrangement the pumping work done can be restricted essentially to that needed to overcome frictional forces and perform a certain residue of work arising from differences in density. The plant 7 conveys the "laden" carrier bodies to a chemical plant 8, which effects further processing of the bodies 1 to free them from accumulated heavy metal. When this has been done they return through a duct 9 to the outlet 2.

The bodies 1 discharged from the outlet 2 in free fall into the current 3 are subjected to substantially no irregular influences which would cause dispersion or spreading-out of the carrier bodies, but are collected without any appreciable loss in the catching device 5.

In the foregoing description, the essential parts of the installation (carrier body outlet and catching point, collecting devices and the conveying path, as well as the chemical plant) are shown as each lying substantially symmetrically with respect to a single plane extending in the direction of the current. In practice, this would broadly be true of the outlet and the catching point, but the conveyor could lie elsewhere for example, at the edge of the flow zone.

(Also parts of the chemical plant could conceivably be provided in the lower region of the installation).

An impression of the possible dimensions of such an installation is given by the following rough calculations for one exemplary installation: The volume flow Q is given by the equation:

in which Muconcis the quantity of uranium/time to be accumulated (670 kg/d); ?7cone is the degree of accumulation or extraction (30%), and {UJMw is the uranium concentration in the sea water (3.3 ppb).

For the inlet cross-section F, there is then obtained: F = rn 15700 m2 s with s (velocity of flow of the sea water) = 0.5 m/s.

The vertical height or depth H (see Figure 5) of the installation is obtained from the sinking speed, Ssink, of the bodies of, for example, 0.08 m/s and from the necessary transit time tconc during which trace material is taken up, for example 10 min, in accordance with the following equation H ='cone ~ Ssink = 50.m.

The width B and the length L of the installation are obtained by the following equations: B F H L = S.tconc=300m.

The conveying equipment at the end of the transit path L conveys the carrier bodies to the surface of the water. This is preferably hydraulic pumping equipment by which water and entrained carrier bodies are pumped upwardly together using a system of tubes with the upward flow tube(s) sealingly connected to tube(s) for a complementary downward flow of water. Such an arrangement is preferred because it requires the least outlay on construction and has the lowest energy requirement.

In accordance with the example described here, about 50,000 m3 of carrier bodies are conveyed per hour. Since the bodies must not become jammed in the conveying equipment, about twice the quantity of water is necessary, i.e. 100,000 m3/h.

The energy requirement for this conveyance to the surface is calculated in accordance with the following: Lth V AP AP=#--#-w2 L 2 If it is presumed that conveyance is to take place through 10 tubes each having a diameter of 1.4 m, there is obtained with a conveying speed of V = 2.8 m3/s a theoretical energy requirement of Lth = 80 kW.

Including the pump efficiency of71P = 0.7 and the efficiency of the hydraulic conveyance NHP = 0.35, the actual energy requirement Lpr is 228 kW. This is very low as compared with other conveying systems.

The conveyance of the carrier bodies to the chemical plant and to the point of discharge may also take place hydraulically.

An installation in accordance with the invention operates under optimum concentration conditions, because the environment of each body descending into the current is constantly automatically renewed.

The costs of installation and operation are lower than when the environment of concentrating bodies undergoes forced renewal. Finally, the installation may be so designed that it can be readily modified and adapted to differing conditions.

In the embodiment employing the buoyancy method according to the invention which is illus trated in Figures 3 and 4, two ships 10 hold a net-like cage 11 which floats on the surface of the water and terminates in the form of a constricted end 12. Two ships 13 hold a carrier body outlet 14 which is sus pended in the water and from which carrier bodies can be discharged into the water at the desired depth.

As a result of relative movement of the ships and the outlet and catching devices with respect to the water, the water layer into which the carrier bodies are discharged is constantly renewed, and there is superimposed upon the vertical component of the carrier body movement a horizontal component relative to the outlet and the catching point, which horizontal component is produced by a corresponding velocity of flow of the water or a corresponding speed of travel of the ships.

The carrier bodies reach the water surface in the region 15 within the surface cage 11. Finally, the relative movement causes the carrier bodies which have risen to the surface to collect in the region 12, from which they can be continuously extracted, for example by a ship 16. When the carrier bodies have become sufficiently laden with trace material after traversing the water layer, they can be regenerated in the ship 16. They can thereafter, or immediately (without regeneration), be returned to the ships 13 and to the outlet 14 for example by a pipeline 17 extending underwater.

The following are examples of possible methods of taking up the adsorber particles which have arrived in the region 12: (1) The particles are sucked out of the closed cage on the surface.

(2) The cage is open at the end of the region 12, so that it acts only as a narrowing surface funnel. The stream of particles floating on the surface passes directly into a receiving hatch situated at the bow of a ship.

As a numerical example, for the extraction of uranium from sea water by the buoyancy method with a desired rate of production of 1 tonne or uranium per day, it would be necessary (with a starting concentration in the natural sea water of 3.3 ~ 109 tonnes of uranium/tonne of sea water and a desired depletion of the water of about 30%) to bring 109 tonnes of sea water into intimate contact with suitable adsorber particles per day. The water volume which is daily contacted with the adsorber particles employed in accordance with the described method is V = B - T- v - where B is the width of the outlet aperture 14, Tthe depth of this aperture below the water surface, v the relative velocity between the aperture 14 and the surrounding water, and t the time (1 day). With B = 150m,T= 150m,v=0.5m/sandt~ 105s(1 day),the target of log t/day is achieved.

If the adsorber particles are made such that their rising speed is about 0.1 m/s, their time of rise is about 1,000 seconds. For the lateral drift or diffusion in the surface water of the sea, an effective "diffu sion constant" between 0.1 and 1 (m2/s) can be assumed. This would mean that the diameter of the surfacing zone 15 undergoes on both sides an additional widening of about 20 m as compared with the extent of the outlet 14. If the collecting cage 11 has an aperture width of about 500 m, there should be no loss due to particles coming to the surface outside the collecting device. Any leakage from the first col lecting cage could be met by providing a second catching cage further outside.

The quantity of carrier bodies which must be delivered by the outlet per unit time depends primarily upon the kinetics of uptake by the carrier bodies, assuming that, within the time of rise (for example 1,000 seconds), a sufficient depletion of the water is to take place in the water layer concerned. In principle, there may be employed as carrier bodies any chemically suitable adsorbers, or concentrationeffecting substances which satisfy the buoyancy conditions either in themselves or when processed with appropriately light carrier material.

The regenerated carrier bodies are conveyed from the processing unit through pipelines or by ship to the outlet ship 13.

Figs. to 13 show examples of carrier bodies.

A simple ring-shaped body as illustrated in Figure 6 may have, in accordance with Figure 8 or 10, an additional ring or a conical cross-sectional profile for stabilising the ring axis in the direction of movement. Additional surfaces disposed within the ring, more particularly grids (Figure 6), concentric rings (Figure 7), optionally with radial stays (Figure 9) or rosettes (Figure 10) thin material coated with reactive material on both sides, enlarge the area of surface active to effect concentration. The inclusion of spin-directing surfaces extending substantially radially, but not in axis-parallel relationship (see Figure 11) produce an automatic spinning motion in the case of a rising carrier body.

An advantageous configuration differing from the ring shape is illustrated in Figure 12. A perforated hollow sphere here forms the envelope for spongelike or convoluted reactive or reactively-coated material. A further form of construction may be formed interengaging discs (in the simplest case two or three discs extending perpendicularly to one another) so as to produce a body structure of which the rebounding surfaces may be protected from contact with neighbouring particles by additional arcuate members (Figure 13). The periphery of this structure is a spheroidal skeleton. These carrier bodies are more particularly described in Federal German Patent Application P 29 14 079).

Claims (22)

1. Method of concentrating trace material from water, characterized in that carrier bodies by which the material is taken up and whose density is different from that of the water are discharged into a water layer so that their density causes them to traverse the layer, are thereafter collected again and transferred, laden with trace material taken up by them, to processing means at which trace material accumulated by the carrier bodies is removed from them.

2. Method according to claim 1 characterized in that the carrier bodies have higher density than the water and sink through the water layer in free fall, are collected at an appropriate depth and are conveyed therefrom to the processing means.

3. Method according to claim 1 characterized in that the carrier bodies have lower density than the water and rise through the water layer after discharge and are collected after rising.

4. Method according to claim 2 or claim 3 characterized in that the water layer traversed by the carrier bodies has a vertical measurement of up to about 100 metres.

5. Method according to any one of the preceding claims wherein the water is sea water.

6. Method according to claim 5 wherein the water layer is provided by a region of the sea with a natural current through it.

7. method according to any one of the preceding claims characterized in that after said removal of trace material accumulated by the carrier bodies, the carrier bodies are again discharged into the water layer.

8. Method according to any one of the preceding claims characterized in that the carrier bodies are mechanically stable bodies which have an area of surface freely accessible to the water but protected from contact with neighbouring bodies, on which sites for the uptake of the material to be concen trated are provided.

9. Method according to claim 8 characterized in that the carrier bodies are spheroidal skeletons.

10. Method according to claim 8 characterized in that the carrier bodies comprise fibrous, filamentary or spongy material.

11. Method according to claim 10 characterized in that the said material is surrounded by an envelope which is of lattice-form or otherwise has a multiplicity of apertures therein.

12. Method according to claim 8 characterized in that the carrier bodies consist of inert material of which the protected surface is coated with a thin layer of material providing sites for the uptake of the trace material to be concentrated.

13. Method according to claim 3 characterized in that the carrier bodies comprise material of high porosity.

14. Method according to any one of claims 1 to 8, 12 or 13 characterized in that the carrier bodies are ring shaped.

15. Method of concentrating trace material from water according to claim 1 and substantially as herein described with reference to any of the accompanying drawings.

16. Apparatus for carrying out the method of claim 3, characterized by a carrier body outlet duct having a carrier body outlet at a particular depth below the water surface, which outlet is preferably in horizontal relative movement in relation to the water mass surrounding it, and catching and collecting means situated on the water surface.

17. Apparatus according to claim 11, character ized in that the catching and collecting means is a conically tapered surface edge which collects the carrier bodies floating on the surface.

18. Apparatus for carrying outthe method of claim 2, characterized by a carrier body outlet lying below the water surface, a carrier body catching means to be disposed at the end of the path of the sinking carrier bodies and comprising collecting devices arranged to deliver into conveying means for delivering the carrier bodies to processing means for removing accumulated trace material from them and thereafter returning the carrier bodies to the said carrier body outlet.

19. Apparatus according to claim 18 wherein the conveying means operates by pumping water and carrier bodies contained therein, within a closed tube system so that the work of upward conveyance of water and carrier bodies is at least partially offset by downward transport of water.

20. Apparatus for concentrating trace materials from water substantially as herein described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

21. A carrier body for use in the method of any one of claims 1 to 15 having a surface providing sites for taking up the trace material from the water, characterized by a geometrical form in which the said surface is freely accessible to water when the body is immersed therein but is protected from coming into mechanical contact with neighbouring bodies.

22. A carrier body substantially as herein described with reference to any of Figs. 6 to 13 of the drawings.