Abstract:
One solute is selectively filtered from a mixture of solutes by making use of the difference between the magnetic susceptibilities of the solutes. 
     Fluid streams of the substances to be filtered are subjected to intense magnetic field gradients to effect the separation or filtration. At points of high magnetic gradient or intense magnetic field, the substance of greater magnetic susceptibility, or the paramagnetic substance distinguished from a non-paramagnetic substance, is deflected laterally through a path of variable resistance to flow so that it separates from non-paramagnetic substance.

Description:
BRIEF DESCRIPTION 
     In carrying out the invention in a preferred form thereof, a separator is provided comprising two parallel channels through which fluids to be separated are passed. Pairs of magnetic coils are mounted between the channels, straddling periodic, flat, tubular passages between the channels. The passages are partially plugged with material substance to form a desired solute such as glass wool to introduce resistance to flow. High current is passed through the coils to produce areas of high magnetic flux gradient at the connections of the passageways to the channels in order to divert the paramagnetic fluid substance or the substance of greater magnetic susceptibility into the passages. The passages are interconnected with a cross duct for receiving the separated fluid. 
    
    
     A better understanding of the invention will be afforded by the accompanying drawings in which 
     FIG. 1(a) is a view of a vertical section through the channels and one of the coils of the apparatus, cut by plane 1(a)--1(a) of FIG. 1(b). 
     FIG. 1(b) is a view of the horizontal cross section represented as cut by the plane 1(b)--1(b) of FIG. 2. 
     FIG. 2 is a fragmentary perspective view of the apparatus with one of the channels removed. 
     FIG. 3 is a graph of the magnetic flux gradient at the axis of the coils when the coils are connected to have magnetic fields aiding, and 
     FIG. 4 is a graph representing the condition with magnetic fields in opposition. 
     Like reference characters are employed throughout the drawings to represent like parts. 
    
    
     DETAILED DESCRIPTION 
     The invention is adapted to the separation of fluids or substances which can be converted to fluids as by solution and which differ in magnetic susceptibility. The invention does not exclude the separation of gases such as oxygen or nitrogen from air, although other methods are available for oxygen and nitrogen separation. 
     A particularly advantageous utilization of the invention is in mineral beneficiation and in processing of ores, especially those of valuable metals such as molybdenum, uranium, cobalt, chromium and nickel. The ore is treated with a solvent that dissolves the desired metal or mineral substance to form a solution of a desired solute together with other unwanted components, also solutes. Specific examples of ore beneficiation are the extraction of uranium from phosphate deposits, the enrichment of chromium from low grade Montana chromite, the extraction of trace elements from United States Eastern Shales and isolation of nickel from Laterite. 
     Other applications in which the invention can be used are: chemical processing and waste recovery. In chemical processing one may have a feedstock consisting of a mixture of liquids and of which one of these liquids is a solution of a paramagnetic salt. The separation of the latter solution by magnetic fractionation can be done more economically than by chemical means. 
     Waste solutions from industrial electroplating, etching and pickling operations contain several valuable metals which can be separated by magnetic fractionation. Specific examples are the separation of chromium and nickel from other metals such as copper and zinc. 
     In the embodiment of the mineral beneficiation apparatus illustrated in the drawings, there are two parallel channels 1 composed of nonmagnetic material such as stainless steel. The upper ends of the channels serve as inlets for a mixture of solutions which flow downward. The channels are preferably of rectangular cross-section. 
     Flat, tubular passages 2 are connected at spaced points between the channels 1. The flat passages 2 are also formed of nonmagnetic tubing. For producing the requisite magnetic field, electric coils 3 are provided which straddle the passages 2. As shown in FIGS. 1(a) and 1(b), the passages 2 are partially plugged with a material such as fiberglass wool 8 which serves to interpose resistance to flow through the passages. High permeability, iron slabs 4 are placed between the magnetic coils 3 to minimize the magnetic reluctance of the return paths of the lines of force outside the coils. 
     For isolation of the coils 3 from the ore solution, a container 5 is provided completely enclosing the coils 3. The glass wool plugs 8 are short enough to leave narrow central passageways or channels 6, interconnected to a cross-duct or trough 7. 
     Suitable means, not shown, are provided as known in the cryogenic art for introducing a very low temperature coolant such as liquid helium into the container 5 and circulating the coolant for maintaining the coils 3 at such low, cryogenic temperature that the coils may carry very large currents and induce flux of such great magnitude as to be effective for deflecting fluid streams of even negligible magnetic susceptibility. 
     The container 5 is lined with super insulation 14 to minimize thermal losses. The insulation may comprise loosely packed reflectorized sheets of suitable material such as mylar in evacuated wrappings as known to those skilled in the cryogenic art. 
     The method of operation of the separator, when the coils 3 are connected to carry electrical current such that their magnetic fields are aiding, is illustrated in the graph of FIG. 3. The curve of FIG. 3(a) represents flux density plotted against distance measured along the flat tubular passages 2 surrounded by the coils 3 having lines of force with direction indicated by the loops 9. The flux density is greatest within the coils 3 and tapers off along the coil axis toward the channels 1. The slope of the flux density curve is consequently greatest at the coil ends, so that positive and negative peaks 11 and 12 in the rate of change dB/dx or flux gradient, FIG. 3(b), occur at the ends of the tubular passage 2 coinciding with the coil ends. 
     The sign of the gradient is opposite ends of the passage 2, and the paramagnetic substance or salt in the fluid stream is acted upon by forces that tend to cause flow laterally inward from the channels 1 through the passageway 6. On the other hand, midway between the coil ends at the passage 6, ##EQU1## The solute, sensing no lateral force there, will flow to the trough 7 by gravity, or if desired, under the influence of a pressure gradient produced by a pump, not shown. 
     Owing to the presence of the fiberglass plugs 8 in the tube 2, the impedance of the channels 1 to flow is considerably less than that in the tube 2 having the fiberglass. Consequently, the fluids tend to flow mostly in the channels 1. If the magnetic force sideways is greater than the resistance of the fiberglass, the magnetic solute, however, will flow laterally and be separated from the main body of the fluid. The height of the coils 3 will be determined by the necessary retention time needed to produce an effective separation. The number of coil pairs is fixed by the width of the separator, which in turn depends upon the mass of material to be processed in a given time. 
     The magnitude of the gradient at the entrance of the passage 6 can be increased if the currents in the coils cause their magnetic field to oppose each other as indicated by loops 13. The corresponding field distribution and gradient is shown in FIG. 4. In this case, the force on the solution of paramagnetic salt is in such a direction as to propel the solution from the passage 6 back into one of the channels 1. In order to prevent the fluid from being subject to reversal of the magnetic forces, part of the region between the coils is blocked. For this purpose a non-magnetic metal bar P of cross-section as illustrated in FIG. 4(c) is inserted in the center of passage 2. 
     Since the region a--a is blocked, the fluid is under no lateral forces in the central passageway 6 and can flow downward toward the trough 7. 
     It will be observed that the selective filtration in fluid form is a continuous process and is adaptable to the treatment of large volumes. 
     The invention does not exclude the provision of fluid columns through which all the fluid to be separated is passed, with the columns located at steep points of the flux density curve to give high flux gradient. In this case, plugs of magnetizable steel wool are mounted in the fluid columns to pass non-paramagnetic substances and retain paramagnetic material until flushed out. The improved arrangement of FIGS. 1(a), 1(b), and 2, however, enables the process to be continuous with no need for intermittent flushing. 
     The feasibility of the process described in connection with the drawings was ascertained by estimating the separation force on the ion. This force, of magnetic origin, is then compared to an opposing force, diffusive in character, and which tends to cause the paramagnetic ions to be dispersed in the fluid. 
     The starting points of the arguments are the governing equations for a two-fluid system viz., the solution of the paramagnetic ions and the rest of the solutes. The equation (of conservation of momentum) for the paramagnetic ions is: 
     
         n.sub.1 m.sub.1 (v.sub.1 ·grad)v.sub.1 =-gradp+n.sub.1 PgradB+n.sub.1 m.sub.1 A.sub.12 v.sub.1                   (1) 
    
     While for the rest of the solutes (not magnetic) 
     
         n.sub.2 m.sub.2 (v.sub.2 ·grad)v.sub.2=grad p+n.sub.2 m.sub.2 A.sub.12 v.sub.2                                          (2) 
    
     in the above 
     
         ______________________________________n.sub.1,n.sub.2 =  number of particles/unit volumem.sub.1,m.sub.2 =  masses of molecules or ions of species (1) and (2)p.sub.1,p.sub.2 =  solute partial pressureB =    flux densityP =    magnetic moment/particleA.sub.12 =  collision frequency of particles (1) with particles (2)v.sub.1,v.sub.2 =  velocity of solute 1 and 2, respectively______________________________________ 
    
     Notice that the Coulomb forces have not been written down. Actually, they have been considered: if we deal with ions then Equation (1) is the sum of two equations; one for the anions, the other for the cations, in which when we add the two, the Coulomb forces cancel each other. The magnetic force is then that on the molecule. Usually, the paramagnetism of the ion is dominant, hence the above statement applies. 
     Filtration becomes effective if one can show that 
     
         (n.sub.1 PgradB) greatly exceeds (n.sub.1 m.sub.1 A.sub.12 v.sub.1) (3) 
    
     Now P=M/N 1  where M is the intensity of magnetization. But M=XH where X=susceptibility of species (1) to be separated and H=magnetic field Hence ##EQU2## Similarly from Einstein&#39;s equation for diffusion ##EQU3## where D=diffusion coefficient 
     k=Boltzmann&#39;s constant and 
     T=temperature 
     For filtration to be possible, therefore we must have ##EQU4## since H=B/P 1 , where P 1  =permeability of solute 
     
         X B/P.sub.1 grad B exceeds n.sub.1 kT v.sub.1 /D           (7) 
    
     It is convenient, however, to use the specific (i.e., per unit mass) susceptibility. The latter denoted by X* is such that ##EQU5## this means that ##EQU6## In the above n 1  /n o  =concentration of solute to be separated while n o  is the density at (STD), i.e., as given by Avogadro&#39;s number 
     Now 
     
         k=1.38×10.sup.-16 ergs/°K. 
    
     
         m=5.31×10.sup.-22 gm (for uranium trioxide) 
    
     
         X*=1.01×10.sup.-6 /gm 
    
     The left side of the inequality (9) is hence ##EQU7## The right hand side, on the other hand, is ##EQU8## 
     If we assume conservatively that grad B is of the order of B/L of the order of B/1 then 
     
         0.574×10.sup.-27 B.sup.2 must exceed 0.41×10.sup.-21 
    
     This immediately points to the need for very high fields; such as produced by superconducting magnets. 
     Therefore 
     if B is of the order of 10 4  ; the inequality is 
     
         0.574×10.sup.-19 exceeds 0.41×10.sup.-21 
    
     if B is of the order of 3×10 4  ; the unequality is 
     
         5.17×10.sup.-19 exceeds 0.41×10.sup.-21 
    
     In the second example, the margin is satisfied with a margin of better than 10 3  ; we thus conclude that selective filtration can be accomplished magnetically. 
     The calculation performed for Uranium can be repeated for other salts. The table shown below gives the relative performance in terms of the parameter X m .sbsb.l *. The higher the value of this number, the more effective the separation. 
     
         ______________________________________Substance    X.sub.m.sbsb.1 * × 10.sup.22 cgs units______________________________________UO.sub.3     5.5Mn.sub.2 O.sub.3        298M.sub.n SO.sub.4        294F.sub.e Cl.sub.2        363F.sub.e SO.sub.4        230N.sub.i Cl.sub.2        127______________________________________ 
    
     It is not difficult to estimate the characteristic time needed to separate the solute of the paramagnetic salt from the mixture of solutes. Using the arguments given by equations (1) to (11) together with the relation for the conservation of mass one finds that for a flux density of 10 4  gauss and for a magnetic field gradient of 10 4  gauss per cm., the time needed is approximately 10 secs. The magnetic separator therefore must be at least 10 cm. long if the liquid flows with a velocity of 1 cm. per sec. Since the characteristic time for diffusion goes inversely with the square of the flux density, doubling the strength of the field would reduce the length by a factor of four. 
     Certain embodiments of the invention have been shown herein and particularly described and certain methods of operation embraced therein have been explained for the purpose of explaining the principle of operation of the invention and showing its application, but it will be obvious to those skilled in the art that many modifications and variations are possible and it is desired, therefore, to cover all such modifications and variations as fall within the scope of the invention.