X-ray particle size analyzer

Abstract: An x-ray sedimentation particle size analyzer in which data is taken only at particular positions along the sedimentation cell, and each such position is individually calibrated. Presentation of the data in the form of a particle size distribution curve can be accomplished very accurately using interpolation techniques. The sedimentation cell design is free of the effects of undesirable density gradients, capable of detecting and removing bubbles, capable of attaining a highly uniform dispersion of sample prior to sedimentation, and including a safety interlock device for blocking x-ray projection when the cell is being accessed.

Description: Technical Field The present invention relates to particle size analysis using Stokes' Law sedimentation techniques and x-ray absorption, and more particularly relates to a sample cell of a particle size analyzer. Background Art When using finely divided solids in industrial and scientific applications, it is often necessary to determine the distribution of the sizes of particles making up the sample. It may be critical, for example, that a catalyst be made up of particles at least a certain percentage of which by weight are smaller than a certain diameter. Sample cells for use with sedimentation particle size analyzers are designed to attempt to provide minimum distortion of the x-ray beam by the cell, to provide a bubble-free x-ray path, and to provide maximum dispersion of the sample in the liquid. A problem experienced with current analyzers has been difficulty in obtaining uniform dispersion of sample particles in the suspension prior to the beginning of sedimentation. Sample cells are typically connected via tubing to a mixing chamber, from which the suspension is pumped to the cell. Recirculation of the suspension through the cell and the mixing chamber has been relied upon to maintain a uniform suspension. However, because of interior cell geometry and the location of ports to which the tubing is attached, it has been found that areas of the cell often are not adequately swept by the flow of fluid, and therefore experience some premature settling of larger particles. The problem of settling may occur at the very top of prior closed top cells even when the cell pump is rapidly circulating the suspension. If the liquid's direction of travel is horizontally along the top wall, sedimentation can occur. This can result in an analysis indicating that the sample is finer than it is in fact. Another problem has been a lack of precise reproducibility of the results obtained for the same sample, particularly from machine to machine of the same type. Some relief of this problem was obtained by clamping off the supply tubing to the cell during analysis, and by randomly tilting the apparatus slightly on a trial- and-error basis until an orientation producing more consistent results was found. It is now suspected that the input and output tubing connected to the cell provides horizontal settling channels in which a density gradient can be created. The lighter portion of the suspension at the top of the tubing would have a tendency to rise, and could rise to the top of the cell itself. The presence of less dense material at the top of the cell not resulting from sedimentation in the cell itself would skew the observations taken in that region. This problem is not as significant in cells which have both inlet and outlet at the top of the cell. However, it is important to have a port at the bottom of the cell to provide better flushing action for the removal of sedimentation deposits. The introduction of closed-top cells facilitated accurate analysis of very small particles by eliminating the meniscus at the top of the suspension volume and thereby allowing precise determination of the sedimentation height. However, the closed cell created a greater likelihood that bubbles rising to the top of the cell would become trapped in the path of the x-ray beam. Such bubbles falsely reduced the apparent density of the suspension measured by the x-rays, and were quite capable of rendering an analysis useless. As they often were not detected except in the form of obviously flawed output, such bubbles often resulted in the waste of many hours required to re-run the sample. In prior x-ray particle size analyzers, it is important that the x-ray path through the cell be uniform along the entire scanned height of the cell. The measured transmission of x-rays during a run is used to calculate "percent finer" according to the following equation: Percent finer = ln(I x /I o ) ln(I 100 /I o ) , where I o is the transmitted intensity through the cell containing only the suspending fluid, I 100 is the transmitted intensity through the cell containing the sample fully suspended before sedimentation, and I x is the measured transmitted intensity at a height and time during a run. Calibration of the cell requires a determination of I o and I 100 . If the effect of the cell on the x-rays along the height of the cell is not uniform, I o and I 100 will not have a constant value for all measuring heights. In prior analyzers, the construction of the cell has been made to fine tolerances in order to attempt to provide clear windows mounted in exactly parallel relationship to one another. These attempts have not been entirely successful. Furthermore, the cell windows can become dirty in a non-uniform manner, adding to the problem. Some prior analyzers have made a single measurement of I o and of I 100 at a selected cell height, and assumed that the cell is sufficiently uniform. Another technique has been to measure I o and I 100 at several heights and to accept the average of these measurements as representative of the cell for all calculations of percent finer. All such techniques result in somewhat inaccurate particle size results if there is a significant difference in cell transmission characteristics at different heights along the cell. EP-A-0094339 describes apparatus for use in determining sedimentation rate, and particularly for use with blood. It comprises a vertical column and a pipeline connected to the base thereof for introduction of material for analysis. The sedimentation rate is measured by observing a portion cf the column distal from the base and measuring the time taken for particulates to clear from that portion. Thus, there has been a need in the art for an x-ray particle size analyzer capable of faster analysis, capable of highly uniform dispersion of the sample prior to sedimentation, not affected by sedimentation in tubing, capable of detecting and eliminating bubbles before the start of a run, and capable of compensating in a meaningful way for non-uniformities in x-ray transmission by the cell. Summary of the Invention The present invention provides a sample cell of a particle size analyzer, comprising in use: a sample compartment, and a fluid passageway sharing a common vertical wall, said fluid passageway approaching one side of said compartment from an angle above the horizontal, and communicating with said compartment at an end of said common wall, the edge of said end of said common wall being neither horizontal nor inwardly and upwardly inclined relative to said sample compartment. This result can also be accomplished by having the passageway deliver fluid vertically downwardly into the compartment. The passageway preferably extends from a height above the bottom of the compartment to discharge fluid into the compartment at the bottom of the compartment, and preferably extends downwardly from a height at or above the top of the compartment. A capability for cleaning the cell and removing any bubbles can be provided by including means for tilting the compartment in the direction of the passageway and draining the contents of the compartment through the passageway while venting the compartment through a port at the top of the compartment, and means for filling the compartment through the passageway while the compartment is tilted by reversing the cell pump and venting gas from the compartment through the port. To detect and eliminate bubbles automatically, the apparatus can include means for detecting and eliminating bubbles from the compartment, comprising means for measuring the transmission of electromagnetic radiation through the compartment at a plurality of locations along the height of the compartment; means for determining the degree of uniformity of the transmission among the locations; and means responsive to the uniformity determining means for operating the tilting, draining and filling means to re-fill the compartment. The port at the top of the compartment may be oriented to allow fluid to be injected very rapidly along the top surface of the compartment. A more uniform suspension results because the suspension at the top of the cell is replaced before any significant sedimentation can occur. Since the suspension is more uniform very close to the top of the cell, the x-ray beam can be moved closer to the top of the cell, and this permits faster runs without sacrificing accuracy. The use of a reversible, multi-speed cell pump enables this rapid circulation in one direction as well as the slow filling of the cell in the other direction to eliminate bubbles, as described above. Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the accompanying drawing and the appended claims. Brief Description of the Drawing Fig. 1 is a schematic representation of an x-ray particle size analyzer. Fig. 2 is a pictorial view of the particle size analyzer with portions broken away to show interior detail. Fig. 3 is a partial vertical cross sectional view taken along line 3--3 of Fig. 2, showing positions of the movable cell. Fig. 4 is a partial front plan view of the analyzer taken along line 4--4 of Fig. 3. Fig. 5 is a side plan view of the cell assembly and cell mounting block. Fig. 6 is a top plan view of the cell and mounting block of Fig. 5. Figs. 7 and 8 show an alternate embodiment of the sample cell. Detailed Description Referring now to the drawing, in which like numerals refer to like parts throughout the several views, Fig. 1 shows a schematic representation of a sedimentation particle size analyzer 10. Fig. 2 is a pictorial view of a preferred embodiment of the analyzer 10, which is preferably contained in a housing 12. Sedimentation of sample particles suspended in a liquid takes place in a sample cell 15. The sample and liquid are mixed together in a temperature controlled mixing vessel or chamber 17 by a variable speed magnetic stirrer 20 positioned below the mixing vessel 17. The stirrer 20 rotates a stirrer element 21 positioned within the vessel 17 to thoroughly mix the contents of the vessel. The contents of the mixing vessel can also be recirculated through a recirculation line 24 which includes a mixing pump 25. Operation of the stirrer 20 is preferably controlled concurrently with operation of the mixing pump 25, which can be a uni-directional peristaltic pump. The line 24 includes a liquid inlet valve 28 through which liquid can be drawn from a liquid supply reservoir 27. The line 24 also includes a waste valve 29 through which the contents of the mixing chamber 17 may be routed to waste. In the preferred embodiment, the reservoir 27 and supply valve 28 are located upstream of the pump 25, and the waste valve 29 is located downstream of the pump 25. The mixing vessel 17 is maintained at a temperature adjustable by the operator within a range, preferably 20-50 degrees C., by means of a resistance heater 18 embedded in the wall of the vessel. With input from a temperature sensor 19 positioned beneath the vessel, a conventional analog DC proportional controller maintains the temperature set by the operator within plus or minus 0.5 degree. The cell 15 is connected to the mixing vessel 17 by two supply lines 33 and 34. The line 33 includes a reversible cell pump 35, preferably a peristaltic pump. Forward and reverse pump directions are shown by arrows F and R in Fig. 1. Operating in a forward direction, the pump 35 supplies liquid (sometimes containing suspended sample) from the vessel 17 to the cell 15, whereas in its reverse direction the pump returns liquid from the cell to the vessel through the line 33. When the cell is filled with fluid, further operation of the pump 35 in a forward direction forces fluid out of the cell into the line 34 and back to the vessel 17. Conversely, reverse operation of the pump 35 can draw fluid from the vessel 17 into the cell 15. The line 34 includes an air valve 37 which vents the line and the cell back to the vessel 17 through a fluid line 38. Concentration of the suspension in the cell 15 during sedimentation is measured by passing x-rays from an x-ray source 40 through a first slit 41, the cell 15, and a second slit 42, after which the x-rays are detected by a conventional photodetector 45, such as a geiger counter or scintillator-photomultiplier tube combination. The slits are defined in a U-shaped yoke 43 which surrounds the cell on three sides, as shown in Fig. 2. The x-ray source 40 is powered by a highly regulated DC power supply 39 of a type known to those skilled in the art. The cell 15 is mounted on a U-shaped cell mounting block 48 best shown in Figs. 4-6 and described in detail below. The mounting block 48 is positioned for movement by a cell stepping motor 50. The cell moves up and down relative to the x-ray beam during analysis. Those skilled in the art will understand that this scanning of the x-ray beam can also be accomplished by moving the x-ray source rather than the cell. This apparatus also provides a means for tilting the sample cell, which in the preferred embodiment occurs when the sample cell 15 is lowered by the motor 50 below the x-ray beam. A cam surface 52 defined in the bottom portion of the cell mounting block 48 engages a cam follower pin 53. The force of the cam surface on the pin 53 causes the mounting block 48 and cell 15 to tilt about a pivot axis 54, for purposes of filling, recirculating and bubble elimination as described below. Operation of the sedimentation particle size analyzer 10 is controlled by a microcomputer 60, which can be a general purpose personal computer. Conventional interfaces are provided, preferably on a plug-in expansion board (not shown), to operate and monitor the stirrer 20, the pumps 25 and 35, the valves 28, 29 and 37, the cell motor 50, and a conventional temperature sensor 62 mounted near the cell. The computer is also interfaced with the photodetector 45 through a period averaging circuit 61, which reads the x-ray intensity data detected by the detector 45. The operation of the period averager 61, which may be placed on the same expansion board, is described below. Neither the temperature controllers nor the x-ray source 40 need be interfaced with the computer. Referring now to Fig. 2, the housing 12 defines a temperature controlled cell compartment 70 in which is located an elevator assembly 72. The elevator 72 includes an upper plate 73 and a lower platform 74 held in spaced apart horizontal relation by three columns 76. The columns 76 are hollow and include interior bearings (not shown). Three guide rods 78 pass through the columns 76 and the plates 73 and 74. The guide rods are anchored in the housing 12 at their top and bottom ends. In Fig. 2, the anchor points of the top ends of the rods 78 are not shown. A drive screw 80, shown in Fig. 3, extends downwardly from the motor 50, mounted above the cell compartment, through a drive nut 81 mounted in the upper plate 73 of the elevator unit 72. Rotation of the screw 80 causes the elevator to move up and down along the guide rods 78. The drive nut 81 and screw 80 are fitted with a conventional anti-backlash device designed to create a precise relationship between the degree of rotation of the screw 80 and the actual motion of the elevator, and therefore of the cell 15. The cell compartment 70 is maintained at the same temperature as the mixing vessel 17 by a resistance heater 65 mounted at the back of the compartment. An air circulation fan (not shown) associated with the heater 65 helps to maintain uniformity of the temperature, which is controlled with input from the temperature sensor 62 by an analog DC proportional controller. The cell 15 and mounting block 48 are shown in more detail in Figs. 4-6. The mounting block 48 consists of two U-shaped halves, 83 and 85, connected by screws 84. When secured together, the halves define in the lower edge of the block 48 the cam surface 52. Also defined is a cavity 86 at the lower corner of the block 48 at which is located the pivot axis 54. A pair of openings 87 pass through the halves 83 and 85 of the block 48 along the pivot axis 54 to communicate with the cavity 86. An anchoring foot 88 is attached to the platform 74 adjacent to the front edge thereof and defines a pair of vertical arms 89 extending up into the cavity 86. A pair of bolts 92 extend from the exterior of the block 48 through the openings 87, through a pair of bearings 93, and are secured in the vertical arms 89. The mounting block 48 is thus pivotable about the bolts 92, which lie along the pivot axis 54. A stop 95 extends downwardly from the bottom surface of the block 48 spaced apart from the pivot axis 54, to engage the platform 74 and maintain the block 48 and cell 15 in a level condition when the cell is above the pin 53, as shown in Fig. 3. Referring to Figs. 5 and 6, the cell assembly 15 includes a central plate 100 which has beveled vertical edges 101 on one side and is cut out to define a cell cavity 115. The central plate is sandwiched by a pair of windows 102, of materials well known in the art, and further by a pair of clamping plates 104 which define central cut-outs to expose the portion of the windows covering the cell cavity 115. The assembly is held together by a plurality of screws 105. The cell assembly 15 fits into the mounting block 48 as shown in Figs. 5 and 6. The U-shaped half 83 of the block 48 defines a pair of flat shoulders 107 along the inside vertical edges of the "U". The other half 85 defines a beveled shoulder 109 along one of its inside edges, and a set screw 110 is tapped at an angle through the other arm of the "U". Thus, the assembled halves 83 and 85 form a pocket for matingly receiving one vertical side of the cell assembly 15 with one beveled edge 101 engaging the beveled shoulder 109, and the set screw being tightened against the other beveled edge 101. The cell 15 also sits firmly on the lower horizontal portion of the "U" of the block 48. Therefore, when the set screw 110 is tightened, the cell is in a well defined, reproducible position with respect to the rest of the analyzer 10. The cell cavity 115 defined by the plate 100 has a unique cross sectional shape best shown in Fig. 5. The cavity is generally rectangular, and is connected to the line 33 at the upper corner of the cavity nearest to the pivot axis 54 by a passageway 117, and to the line 34 at the other upper corner by a passageway 118. A notch or recess 116 is formed in the wall of the cavity at the point of entry of the passageway 118. This notch 116 helps to prevent bubbles from collecting at the corner of the cavity. The passageways 117 and 118 may serve as either inlet or outlet passageways depending on the direction of operation of the cell pump 35. The lower corner 119 of the cavity 115 nearest to the pivot axis 54 is rounded. A vertical wall 120 extends downwardly from the top of the cavity 115 and terminates adjacent to the corner 119. The wall 120 is spaced apart a short distance from the side of the cavity to form a channel 122 through which fluid must travel between the main sedimentation compartment of the cavity and the passageway 117. The wall 120 is curved at its end 124 to match the curvature of the corner 119. Fluid entering the cavity 115 from the passageway 117 thus sweeps down the channel 122 and into the bottom of the cavity at a downward angle as shown by the arrow 126 in Fig. 5. The structure of the cell cavity as just described allows fluid entry at both the top and bottom of the cell for circulation and flushing purposes, but provides virtually no horizontal settling channels in which density gradients might form and affect the density in the main compartment in which the analysis occurs. Because of the angled entry of the channel 122 into the main compartment, any lighter density suspension that does form at the end 124 of the wall 120 will tend to rise up the channel 122 rather than within the main compartment, and therefore will not disrupt the analysis. Prior to the analysis, rapid circulation entering through the upper passageway 118 replaces the suspension at the top of the cell so rapidly that no significant settling can occur. The analysis thus begins with a highly uniform suspension. Alternate Cell Construction Figs. 7 and 8 show an alternate embodiment 315 for the construction of the sample cell. The cell 315 is modified at the location at which the passageway 118, as shown in Fig. 5, enters the cell cavity 115. As shown diagrammatically in Fig. 7, a cell cavity 316 defined in a plate 300 (corresponding to the plate 100 of Fig. 5) communicates at the top of the cell cavity with a passage 330. A barrier 332 extends from the top of the passage 330 to a point spaced above the bottom of the passage 330. The barrier 332 extends horizontally across the entire cross section of the passage 330. The vertical side wall 317 of the cavity 316 extends upwardly to form a projection 334 which terminates at a point spaced below the top of the cell cavity 316 and partially blocks the passage 330. The effective width of x-rays passing horizontally through the cell cavity, that is, the width measured by the detector, is designated by the letter A in Fig. 7. The horizontal distance between the inner wall 317 of the cell cavity 316 and the barrier 332 is designated by the letter B. The height of the barrier 332 measured from the top of the passage 330 is designated by the letter C. The height of the same barrier 332 measured from the top of the cell cavity 316 is designated by the letter E. The distance from the projection 334 to the top of the cell cavity is designated by the letter D. The purpose of the barrier 332 is to prevent density gradients that may form in the passage 330 from causing low density material to flow into the top of the cell cavity 316 during sedimentation. In order for the barrier to function effectively, the distance B must be less than A, that is, the extension of the top of the cell to the barrier 332 must be less than the effective x-ray width A. Preferably, B is less than 30 percent of A, so as to minimize the tendency of sedimentation under the dimension B from significantly affecting the analysis. Furthermore, C must be greater than or equal to D, so that as particles settling through the distance C reach the bottom of the barrier 332, particles of the same size have already passed the end of the projection 334. Finally, E must be greater than D, so that the barrier extends down farther from the top of the cell cavity than the end of the projection 334. It is preferable, but not required, that B be minimized, and that the distance between the end of the projection 334 and the bottom of the passage 330 be minimized. It is also preferable to minimize the horizontal run of the passage 330 to minimize the formation of density gradients. Lengthening the horizontal run of the passage 330 would require a lengthening of the downward extent of the barrier 332 to trap low density material outside the cell cavity. As shown in Fig. 7, the top surface of the projection 334 is preferably bevelled, with the bevelled surface facing outside the cell cavity 316. This results in higher density particles settling from under the dimension B moving toward the passage 330 rather than into the cell cavity 316. Similarly, the bottom surface of the barrier 332 is bevelled, with the bevelled surface facing away from the cell cavity. This results in low density suspension forming under the barrier 332 flowing up into the passage 330, rather than toward the cell cavity 316. Fig. 8, shows an example of a cell cavity construction embodying the concepts shown in Fig. 7. The passage 330 is constructed having a minimum horizontal run, by turning the passage 330 upward into a vertical configuration immediately on the outside of the barrier 332. The construction of the port communicating with the cell cavity 316 at the lower end of the cell cavity is similar to the construction shown in Fig. 5. A wall 320 extends from the top of the cell to divide off a vertical channel 322. The bottom 324 of the wall 320 is slanted to provide a channel segment which enters the bottom of the cell at an angle above the horizontal. Thus, any low density suspension forming under the bottom of the wall 320 will tend to rise into the channel 322, rather than into the cell cavity 316, where it could distort the position of the particles from that predicted by Stokes Law. It should be noted that the channel 322 could have any orientation ranging from the vertical orientation shown in Fig. 8 to any angle above the horizontal. It is important that the segment of the channel 322 actually communicating with the cell cavity 316 not have a significant upper surface or boundary 324 that is either horizontal or angled inwardly to permit low density suspension forming thereunder to flow into the cell cavity. Such a surface 324 having a length greater than 10 percent of the width of the cell cavity in which sedimentation takes place is considered significant.

Claims: A sample cell of a particle size analyzer, comprising in use: a sample compartment (115, 316) and a fluid passageway (122, 322) sharing a common vertical wall (120, 320), said fluid passageway (122, 322) approaching one side of said compartment (115, 316) from an angle above the horizontal, and communicating with said compartment (115, 316) at an end (124, 324) of said common wall (120, 320), characterised in that the edge of said end of said common wall being neither horizontal nor inwardly and upwardly inclined relative to said sample compartment. The cell of claim 1, wherein said passageway (122, 322) extends from a height above the bottom of said compartment to enter said compartment level with the bottom thereof. The cell of Claim 1 or Claim 2 and further comprising a fluid port (118,330) communicating with the top portion of said compartment (115,316). The cell of Claim 3, and further comprising means for injecting fluid through said fluid port (118,330) along the top of said compartment. The cell of Claim 3 or Claim 4 further comprising means for filling said compartment (115,316) through said fluid passageway (122,322) while said compartment is tilted in use, said compartment (115,316) being vented through said fluid port. The cell of any of Claims 3-5 further comprising an upper passageway (330) extending away from the top of said compartment (115,316), said upper passageway (330) including a downwardly extending barrier (332), and said compartment including a projection (334) extending upwardly to partially block said upper passageway at the inner side of said barrier (332), said lowermost point of said barrier being below the upper end of said projection (334). The cell of Claim 6, wherein said barrier (332) has a height measured from the top of said compartment (316) greater than the distance from the top of said compartment (316) to the upper end of said projection (334); and said barrier (332) having a height measured from the top of said upper passageway (330) greater than or equal to the distance from the top of said compartment (316) to the upper end of said projection (334); and wherein said compartment (316) includes windows for allowing x-rays to pass through said compartment; the horizontal distance from said compartment to said barrier being less than the effective width of x-rays passing through said compartment. The cell of any preceding claim and further comprising means (72) for tilting said compartment (115,316) in the direction of said fluid passageway (122) and draining the contents of said compartment through said fluid passageway (122). The cell of Claim 8, wherein said means (72) for tilting said cell comprises a cam surface (52) defined in the lower portion of said cell; a cam follower (53) positioned in use under said cell; and means (50,80) for moving said cell downwardly to engage said cam surface (52) with said cam follower (53). The cell of any preceding claim, and further comprising means for detecting and eliminating bubbles from said compartment (115,316) comprising: means for measuring the transmission of electromagnetic radiation through said compartment (115,316) at a plurality of locations along the height of said compartment; means for determining the degree of uniformity of said transmission among said locations; and means responsive to said uniformity determining means for operating tilting, draining and filling means to refill said compartment.

Inventors: KANE MARY F, LAUGHINGHOUSE CHARLES L, OLIVIER JAMES P, ORR CLYDE JR DR, TIDWELL SAMUEL VINCE, WAGNER JACK J, KANE, MARY F., LAUGHINGHOUSE, CHARLES L., OLIVIER, JAMES P., ORR, CLYDE, JR., DR., TIDWELL, SAMUEL VINCE, WAGNER, JACK J.
Applicants: MICROMERITICS INSTR CORP, MICROMERITICS INSTRUMENT CORPORATION
Application year: 1988
Main classification: G01N15
IPC codes: G01N15, G01N1, G01N23