US3649328A - Process for forming luminescent screens - Google Patents

Abstract

A fine grain low-noise luminescent screen suitable for use in high-resolution image display devices such as cathode-ray tubes, photodisplay tubes, flying spot scanners, etc., is formed on an internal surface of a transparent image display panel by settling agglomerating particles from a carrier solution to form a coating having crevices and spaces between adjacent particle agglomerates and then combining a decanting and rotating process whereby rotational movement of the panel causes the solution to wash the entire surface of the coating throughout the decanting process to deposit finer individual particles in these crevices and spaces whereby the undesirable wave patterns normally obtained during decanting are eliminated and a coating of much improved surface smoothness and uniformity of thickness and density is obtained.

Description

United States Patent 1 Mar. 14, 1972 Neis [54] PROCESS FOR FORMING LUMINESCENT SCREENS [72] Inventor: Donald A. Neis, Lombard, Ill.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: June 22, 1970 [21] Appl. No.: 48,463

[52] US. Cl. ..117/33.5 CS, 117/101 [51] Int. Cl. ..H0lj 31/12 [58] FieldofSearch ..1l7/33.5 C, 33.5 CL,33.5 CP,

117/335 CS, 101; 96/36.l

[56] References Cited UNITED STATES PATENTS 2,970,930 2/1961 Windsor ..ll7/33.5 CS 3,077,389 2/1963 Schulze et a1. ....1l7/33.5 CS 3,143,435 8/1964 Martyny ....1l7/l0l X 3,313,643 4/1967 Branin ....1 17/101 X 3,364,054 1/1968 Weingarten ..1 17/101 X 3,467,059 9/1969 Korner et al ..1 17/101 X Primary Examiner-Alfred L. Leavitt Assistant Examiner-Wayne F. Cyron Att0rneyVincent Rauner and L. Arnold [57] ABSTRACT A fine grain low-noise luminescent screen suitable for use in high-resolution image display devices such as cathode-ray tubes, photodisplay tubes, flying spot scanners, etc., is formed on an internal surface of a transparent image display panel by settling agglomerating particles from a carrier solution to form a coating having crevices and spaces between adjacent particle agglomerates and then combining a decanting and rotating process whereby rotational movement of the panel causes the solution to wash the entire surface of the coating throughout the decanting process to deposit finer individual particles in these crevices and spaces whereby the undesirable wave patterns normally obtained during decanting are eliminated and a coating of much improved surface smoothness and uniformity of thickness and density is obtained.

12 Claims, No Drawings PROCESS FOR FORMING LUMINESCENT SCREENS BACKGROUND This invention relates to a process of forming luminescent screens by depositing luminescent material on an image display surface, and, more particularly, to a combination settling, decanting and rotating process for forming fine grain lownoise luminescent screens suitable for use in high-resolution image display devices.

The resolution of an image screen is determined by many variables such as phosphor efficiency, persistence and granular characteristic, screen thickness, density and surface smoothness, and electrical noise level. Particle size affects many of these variables such as screen thickness and density, surface smoothness, and noise level; and therefore, particle size is very important in producing high-resolution screens. In the formation of high-resolution luminescent screens, it is known to be desirable to have a particle size of small diameter when compared to the diametrical size of the excitation beam, and for high-resolution screens employing electron beams of less than mils in diameter, particle size becomes increasingly critical.

A common method of forming high-resolution screens is by conventional gravitational settling of the particles from a particle-carrying solution onto a surface of an image display device, commonly referred to as a scanning surface, which solution has been elutriated until a desired range of particle sizes is obtained from which to form the screen. Common methods of elutriation yield a normal distribution of particle sizes wherein the majority of the particle sizes lie within the middle region of a normal distribution curve. The larger heavier particles of the particle size range significantly afiect screen resolution, and thus for high resolution screens utilizing relatively small dimensional electron beams, lengthy elutriation processes and long settling times are presently required in order to obtain a solution with the desired particles sizes suspended therein, and thereafter, to settle these finer lighter particles from the solution. Successful elutriation done for any appreciable length of time also requires the addition of a stabilizing agent to the solution.

Previously, attempts to form high-resolution particles using only very extremely fine light particles have resulted in very dense screens with poor resolution, in addition to the disadvantage of an inordinate amount of elutriation and settling time necessary to obtain the finer particles. Also, the gravitational settling process as commonly used does not give an even dispersion of the finer lighter particles over the screen, thus leaving relatively rough areas on the surface of the screen, and dense areas of accumulated finer particles in other screen surface areas.

Of the more commonly used techniques, such as draining, siphoning and decanting, for pouring off the supernatant liquid from the screening surface, decanting leaves ridges of undesirable residues known as wave patterns on the surface of the luminescent screen resulting from wave motions set up in the solution during the tilting motion of the decanting process. It is thus desirable to employ a tilting mechanism which is relatively free of vibration. Presently, luminescent screens formed by gravitational settling have percentage peak-to-peak electrical noise levels of 8 to 10 percent. Other forming methods such as cataphoretic depositions yield screen noise levels of 6 to 7 percent, as compared to screen noise levels of 2 to 5 percent obtained by the present invention employing relatively vibration free tilting equipment.

SUMMARY It is therefore an object of this invention to provide a novel process of forming fine grain low noise luminescent screens suitable for use in high resolution image display devices wherein maximum settling of the finer lighter particles from the solution is obtained in a minimum time.

It is another object of the invention to provide a novel process of decanting wherein an additional washing and scrubbing action of the supernatant liquid completely eliminates the undesirable wave patterns previously obtained.

It is still another object of the invention to provide a relatively simple and economical way to form luminescent screens having much improved electrical noise levels and high resolution characteristics and particularly for screens excited by electron beams of less than 3 mils diameter.

Further, it is an object to provide a combined decanting and rotational process closely following a brief settling period wherein the rotational movement during the decanting process causes a washing motion of the supernatant liquid which actually enhances the settling process by redistributing and depositing the settled particles to give a smoother coating surface than heretofore obtainable by standard gravitational settling.

In a preferred practice of this invention, a container-shaped cathode ray tube having a faceplate panel with its internal surface constituting a screening surface, is oriented so that its central longitudinal axis which passes perpendicular to the faceplate panel, becomes a vertical axis. A particle-carrying solution containing suspended luminescent phosphor particles of a desired size and color combined with a binder agent, is poured into the tube to at least cover the faceplate panel. Next, the suspended phosphor particles are allowed to settle onto the screening surface for a set time, to form a coating whereby substantially all of the faster settling, larger heavier particles are settled from the solution leaving only the slower settling, finer lighter particles suspended therein. Thereafter, the decanting of the supernatant liquid begins by slowly tilting the container to pour off the supernatant liquid, while simultaneously rotating the container about a rotational axis in spaced and parallel alignment with the vertical axis of the container, whereby the supernatant liquid is washed in a recycling motion over the entire face of the coating surface to both deposit the finer lighter particles of phosphor among certain crevices and spaces formed in the coating surface by particle agglomeration and to redistribute agglomerated particles to give a smooth coating surface and a uniform thickness and density to the deposited luminescent screen.

Other objects and advantages of the invention will occur to those skilled in the art as the invention is set forth by the following description.

DETAILED DESCRIPTION In one particular embodiment of this invention, a commercially available P-l6 phosphor powder is used to manufacture a flying spot scanner tube having a circular faceplate panel of approximately 3 inches in diameter and a suitable funnel portion as is well known in the art. This type of image display device requires high screen resolution and utilizes small dimensional electron beams. The 3-inch tube whose screen is to be formed in accordance with the present invention to be set forth with particularity hereinafter, is intended to employ an electron beam of less than 3-mils diameter.

The Pl6 phosphor is a calcium magnesium silicate with an additive of cerium and is responsive to electron bombardment to emit a bluish purple near violet light. The P-l6 phosphor powder is mixed with a proportionate amount of distilled water, gelling agent and a binder agent to form a solution wherein the phospho. particles are suspended. The particlecarrying solution is then prepared by standard elutriation procedures until the solution contains particle sizes within a desired range to yield the required screen resolution when considering the diameter of the particle size as compared to the diametrical size of the electron beam to be used with a preselected cathode ray tube.

In the manufacture of the 3-inch diameter flying spot scanner tube, 6.0 grams per liter solution of l6 l6 phosphor powder and 12.0 grams per liter solution of a gelling agent such as barium acetate, barium nitrate, sodium bicarbonate, or ammonium carbonate (or a suitable mixture thereof), are mixed in an elutriation vessel with deionized or distilled water.

The prepared mixture of solution is placed in an elutriation vessel having a side drain cock located 1% inches from the bottom thereof. Optionally, any convenient elutriation vessel could be used which has a side drain cock. After proper mixing, the phosphor solution is elutriated for approximately 5 minutes and then a supernatant liquid, as well known in the art, is drained off through the drain cock. Because of the short elutriation times employed herein, no stabilization agent is required as would be the casein elutriation procedures of long duration.

Next, the supernatant liquid is again properly mixed, elutriated for approximately minutes and then drained off. Thereafter, 36 milliliters of either potassium or sodium silicate solution containing approximately 28 percent solids are added to the twice-hence elutriated solution, and the mixture elutriated again for approximately minutes and then drained off, whereby a particle size of less than 5 microns is obtained for the phosphor particles suspended in the solution. The potassium or sodium silicate is a well known binder agent which gives the desired adhesion of the settled particles to each other and to the internal surface of the faceplate panel.

With the elutriation process completed, the phosphor-carrying solution is now ready to be used to form the luminescent screens of the present invention. Approximately 100 milliliters of the prepared solution are poured into the 3-inch diameter tube to at least cover the internal surface of the faceplate panel, which surface is commonly known as a screening surface. The solution is allowed to settle for approximately 45 minutes to form a coating during which time it is found that the settling process is enhanced by maintaining the settling solution at a temperature differential of 10 to 20 F. cooler than the temperature of the screening surface.

It is to be understood that the thickness of the deposited coating is independent of the quantity of the solution used to settle out the phosphor particles so long as settling times are not so inordinately long as to completely settle out the suspended phosphor particles. More importantly, when settling times are relatively short, screen thickness is largely determined by the settling time and amount, e.g., grams per liter solution, of the phosphor and the other ingredients such as gelling and binder agents. For this 3-inch diameter tube, thicknesses of from 0.5 to 1.0 thousandth of an inch are obtained. These same thicknesses are desirable for any given size and shape of tube or screening surface to give the required high resolution.

During the settling process, the tube is positioned on a tilt table, preferably having a flat surface, of a tilting mechanism; or alternatively, the settling process can be accomplished elsewhere and the tube placed on the tilt table thereafter. The tilting mechanism has the capacity provided by suitable gears and drives to both tilt through an angle of l 80as is commonly known in the art, and also to simultaneously rotate or revolve about a certain rotational axis. The rotational axis is perpendicular to the surface of the tilt table and preferably comprises its central transverse axis; and the 3-inch diameter tube is oriented so that the tubes central longitudinal axis which passes perpendicular to the faceplate panel is now a vertical axis.

it is known that any given tube may be used as a container to hold a liquid solution either when the faceplate panel has skirts which can act as retaining walls or the panel is joined to its funnel portion which can serve as the retaining walls for the container. Thus oriented as a container, the tube is positioned on the surface of the tilt table with its vertical axis aligned parallel to the rotational axis and spaced therefrom any convenient distance. Optionally, the vertical axis of the tube can be spaced at any convenient distance from the rotational axis of the tilt table so long as the two axis are not in concentric alignment. By spacing the two axes apart, a complete washing motion of the phosphor-carrying solution is obtained over every region of the coating surface. It is obvious that if the two axes are placed in concentric alignment that no washing action or rotational movement of the solution will occur in the center of the luminescent screen.

Following the above-recited settling period, the 3-inch diameter tube is slowly revolved about the rotational axis of the tilt table while simultaneously decanting the supernatant liquid remaining from the settling process by tilting the table and thus the tube through an angle of approximately In this way, the supernatant liquid is caused to wash or scrub the entire surface of the deposited coating of phosphor particles while the supernatant liquid is being poured off, i.e., throughout the decanting process. it is this washing action provided by the rotational movement of the tube and thus the supernatant liquid that is thought to give the much improved noise levels and high resolution for the luminescent screens formed by this invention.

It is commonly known that during the settling process, the suspended phosphor particles tend to form agglomerates of particles partially because of their electric charges and partially due to the amount of gelling agent and binder agent in the solution. Also, once the particles settle from the solution, the tendency to agglomerate is even more pronounced until, for the most part, the coating is substantially comprised of agglomerated particle groups, rather than individual particles. These agglomerated particle groups within the coating are of various sizes and shapes, but for the purpose of better illustrating the present invention, can be divided into two groups at the point of settling from the solution, namely, larger heavier particle agglomerates and smaller lighter particle agglomerates. It is to be understood that the faster settling, larger heavier particles referred to hereinbefore are actually the larger agglomerates and a portion of the smaller agglomerates; and that the previously referred to slower settling, finer lighter particles are actually the finer individual particles which have failed to be joined in agglomerated form and the other portion of the smaller agglomerates.

This particle agglomeration, both in the solution and in the coating, creates crevices and spaces between adjacent ones of the settled agglomerates on the surface of the coating and creates voids between adjacent ones of the settled agglomerates within the coating due to the nonuniform shape of the agglomerates. Thus, particle agglomeration tends to provide higher noise levels, poorer resolution, nonuniforrnity of screen density and thickness, and surface roughness.

During standard gravitational settling, these disadvantages are ameliorated somewhat by the slower settling, finer lighter particles which have remained in the solution either in individual form or as smaller size agglomerates. These smaller agglomerates and finer lighter individual particles settle into the crevices and spaces but do not uniformly settle over the entire surface. Thus, without the washing action of the present invention, this nonuniform settlement produces unfilled relatively rough areas and other screen areas that are overly filled to provide nonuniform thickness as well as areas of varying density.

The rotational movement of the supernatant liquid avoids the above-noted disadvantages of the nonuniform settlement of the phosphor particles by washing the surface of the coating much like that of a household washing machine with the exception that the screening surface does not rotate about its central traverse axis due to the spacing between the vertical axis of the tube and the rotational axis of the tilt table. The smaller agglomerates and finer lighter individual particles are circulated and recycled over the coating surface, and are caused to be deposited in the crevices and spaces between the larger agglomerated particle groups so as to fill them to the level of the surface.

It is readily obvious that the washing action prevents overfilling or a build up of phosphor particles over the level of the coating surface as the excess particles would be subjected directly to the agitation of the supernatant liquid and thus swept on to be deposited elsewhere. Also, any agglomerated particle groups that are formed above the average level of the coating surface would be subjected directly to the agitation of the washing supernatant liquid so as to be dislodged and carried on to lower level areas and under-filled crevices and spaces of the coating surface. If no lower levels or underfilled areas are available, the unattached or loose particles are poured off in the decanting of the liquid. The average level of the coating surface is, of course, determined during the settling of the agglomerating particle groups, and the final level of the coating surface closely approximates this average level due to the scrubbing effect of the suspended phosphor particles in the washing liquid.

Upon close examination of the final coating after the combined decanting and rotating step, it is to be noted that there are no voids among the finally agglomerated particle groups, or crevices on the interface of the coating and the internal surface of the faceplate panel which would not have been accessible for receiving settled particles during the washing action of the supernatant liquid. It is felt that this advantageous effect is due to the shifting of the agglomerated particle groups of the coating during the washing action to achieve a better fit between adj acent ones of the particle groups as well as to open passages to the coating surface through which the finer lighter particles settle.

The tilting rates and the rotational rates of the tilt table are relative to each other and will depend upon the type of image display device, the shape and size of the screening surface of the faceplate panel, the amount of screening solution being used, etc. In the one particular embodiment, the 3-inch diameter tube is tilted at a rate of approximately minutes for 180 tilt, and rotated about the rotational axis at a rate of approximately revolutions per minute (rpm). The luminescent screen obtained by the above-recited steps was examined and found to be a fine grain screen having a very smooth surface free of the common wave motion lines as would be left on the surface of the screen during a standard decanting process, and also having a measured electrical noise level of approximately 2 percent peak-topeak. The spacial response was measured by using a 60cycle sweep rate and a standard bar chart in a well-known manner; and the spacial response, which is an approximation of screen resolution, was found to be 12 line pairs per millimeter.

In another embodiment of the invention, a containershaped tube with a 3-inch diameter faceplate panel and intended for use as a high-resolution image display device with an electron beam less than 3-mils diameter, was covered with approximately 100 milliliters of the same phosphor-carrying solution containing phosphor particles less than 5 microns in diameter, and then subjected to a settling period of approximately 45 minutes to form a coating on the screening surface. Thereafter, the tube was subjected to the combination decanting and rotating process as explained hereinbefore using a tilting rate of approximately minutes for 180 tilt and a rotational rate of approximately 2 rpm. The spacial response of the screen was found to be comparable with the above-recited 3-inch diameter tube; and the noise level was measured at approximately 4 percent peak-to-peak.

In still another embodiment of the screen, a containershaped cathode ray tube having a generally rectangular faceplate panel of l0inch diagonal dimension, was covered with approximately 1,500 milliliters of the same solution and allowed to settle for approximately 30 minutes; then, the tube was subjected to the combination decanting and rotating process having a tilting rate of approximately 5 minutes for 180 tilt and rotational rate of approximately 3.5 r.p.m. The spacial response of the deposited rectangular screen was found to be comparable with the above-recited 3-inch diameter tubes, and the noise level was measured at approximately 5 percent peak-to-peak.

The elutriation process, while being set forth with some degree of particularity herein, is not critical to the invention and other gelling agents or binder agents in various amounts may be used. However, it has been found that the fine grain low noise high resolution luminescent screens are better formed from a particle size distribution of less than 5 microns in diameter. Also, the process is not limited to any particular phosphor material and may be successfully accomplished with other commercially available phosphors, such as P-15,P-

24,P37,and others. It may also be found useful in some screening applications which are accomplished by this invention to use a stabilizing agent during elutriation where longer settling times are desired. Additionally, an electrolyte may be utilized to enhance the settling of the phosphors during the settling process; the faceplate panel may be pretreated to reduce the repulsion between the screening surface and the suspended phosphor particles, as both the use of the electrolyte and the pretreatment of the panel are well known in standard gravitational settling processes.

The tilting rates and rotational rates are preferably applied as a constant rate throughout the combined decanting and rotational steps, and are selected for a given size and shape of tube and faceplate panel. It has been found that a satisfactory range of tilting rates is from 5 to 20 minutes for of tilt; and for rotational rates, from 1 to 20 rpm. At points immediately above or below these ranges some degree of screening success is obtainable, of course, but increasing degradation of resolution and noise level are experienced as the ranges are extended. Within these tilt and rotational rates, noise levels of from 2 to 5 percent peak-to-peak are obtained, with a spacial response varying from 8.5 to 20.5 line pairs per millimeter.

The type of image display is as well noncritical to the present invention; and any high-resolution tube is susceptible to manufacture by this novel technique of settling, decanting and rotating. The size and shape of the screening surface is noncritical as the washing action of the supernatant liquid is active over surfaces of all sizes and shapes.

While the present invention has been described for only a few embodiments, it will be obvious to those skilled in the art that it is not so limited and that various modifications and variations can be made without departing from the spirit and scope thereof, and to this extent the appended claims are intended to cover such modifications and variations.

I claim:

1. A process for forming fine-grain low-noise luminescent screens on the internal surface of the faceplate panel for a cathode ray tube used as a high resolution image display device wherein the tube acts as a container and the internal surface of the panel acts as a screening surface when the tube has its central longitudinal axis which passes perpendicular to the panel positioned as a vertical axis, said method comprising the steps of: covering the screening surface of the container with a carrier solution containing suspended luminescent phosphor particles responsive to electron bombardment to emit light of a preselected color, substantially settling the suspended particles from the carrier solution onto the screening surface to form a coating of phosphor particles whereby a supernatant liquid is formed above said coating of settled particles, decanting the supernatant liquid from the container while simultaneously rotating the screening surface and the container about a rotational axis spaced from and parallel to said vertical axis whereby the supernatant liquid washes the entire region of the surface of said coating to redistribute said settled particles until said coating is substantially free of voids and the surface of said coating is substantially free of crevices and spaces and unevenly distributed particles to provide a high resolution luminescent screen of uniform thickness and density.

2. A process as claimed in claim 1 wherein the phosphor particles are suspended in said carrier solution and settle therefrom partially in larger heavier agglomerated particle groups of various sizes and shapes, partially in smaller lighter agglomerated particle groups of various sizes and shapes, and partially as finer lighter individual particles.

3. A process as claimed in claim 2 wherein during said settling period said larger heavier particle agglomerates largely settle from said solution to form the coating whereby some voids are created within said coating between adjacent ones of the larger agglomerates and some crevices and spaces are created in said coating surface between adjacent ones of the larger agglomerates, and said smaller lighter particle agglomerates partially remain suspended in said solution and partially settle from said solution with said larger agglomerates to partially fill said voids, crevices and spaces, and said finer lighter individual particles largely remain suspended in said solution.

4. A process as claimed in claim 3 wherein during the combined decanting and rotating period the washing of said supernatant liquid over said coating surface caused the suspended smaller agglomerates and finer particles to be deposited in said voids, crevices and spaces so as to substantially fill some, overfill others, and underfill still others, said washing shifts the relative orientation of all the adjacent particles of said coating whereby a particle orientation is obtained that is relatively free of voids, and scrubs said coating surface to redistribute excess smaller agglomerates and finer particles from overlyfllled crevices and spaces to underfilled crevices and spaces between adjacent ones of the particle agglomerates whereby a generally smooth coating surface is obtained that is relatively free of crevices and spaces.

5. A process as claimed in claim 1 wherein said luminescent phosphor particles are less than 5 microns in diameter.

6. A process as claimed in claim 5 wherein said carrier solution is a mixture of proportionate amount of distilled water,

phosphor particles, gelling agent, and a binder agent, and said mixture is prepared by an elutriation process until the desired phosphor particle size is obtained.

7. A process as claimed in claim I wherein said screening surface is generally circular.

8. A process as claimed in claim 1 wherein said screening surface is generally rectangular.

9. A process as claimed in claim 1 wherein said rotational axis is slightly spaced from said vertical axis of the container.

10. A process as claimed in claim 1 wherein the decanting of said supernatant liquid includes tilting the container through at a constant rate of between 5 to 20 minutes to pour off said liquid.

11. A process as claimed in claim 1 wherein the rotating of said screening surface and container about said rotational axis includes continuously rotating through 360 at a rotational rate of between 1 to 20 revolutions per minute.

12. A process as claimed in claim 11 wherein the rotating of said screening surface and container about said rotational axis and the tilting during decanting is accomplished by a substantially vibration free means.

Claims (11)

1. 2. A process as claimed in claim 1 wherein the phosphor particles are suspended in said carrier solution and settle therefrom partially in larger heavier agglomerated particle groups of various sizes and shapes, partially in smaller lighter agglomerated particle groups of various sizes and shapes, and partially as finer lighter individual particles.
2. 3. A process as claimed in claim 2 wherein during said settling period said larger heavier particle agglomerates largely settle from said solution to form the coating whereby some voids are created within said coating between adjacent ones of the larger agglomerates and some crevices and spaces are created in said coating surface between adjacent ones of the larger agglomerates, and said smaller lighter particle agglomerates partially remain suspended in said solution and partially settle from said solution with said larger agglomerates to partially fill said voids, crevices and spaces, and said finer lighter individual particles largely remain suspended in said solution.
3. 4. A process as claimed in claim 3 wherein during the combined decanting and rotating period the washing of said supernatant liquid over said coating surface caused the suspended smaller agglomerates and finer particles to be deposited in said voids, crevices and spaces so as to substantially fill some, overfill others, and underfill still others, said washing shifts the relative orientation of all the adjacent particles Of said coating whereby a particle orientation is obtained that is relatively free of voids, and scrubs said coating surface to redistribute excess smaller agglomerates and finer particles from overlyfilled crevices and spaces to underfilled crevices and spaces between adjacent ones of the particle agglomerates whereby a generally smooth coating surface is obtained that is relatively free of crevices and spaces.
4. 5. A process as claimed in claim 1 wherein said luminescent phosphor particles are less than 5 microns in diameter.
5. 6. A process as claimed in claim 5 wherein said carrier solution is a mixture of proportionate amount of distilled water, phosphor particles, gelling agent, and a binder agent, and said mixture is prepared by an elutriation process until the desired phosphor particle size is obtained.
6. 7. A process as claimed in claim 1 wherein said screening surface is generally circular.
7. 8. A process as claimed in claim 1 wherein said screening surface is generally rectangular.
8. 9. A process as claimed in claim 1 wherein said rotational axis is slightly spaced from said vertical axis of the container.
9. 10. A process as claimed in claim 1 wherein the decanting of said supernatant liquid includes tilting the container through 180* at a constant rate of between 5 to 20 minutes to pour off said liquid.
10. 11. A process as claimed in claim 1 wherein the rotating of said screening surface and container about said rotational axis includes continuously rotating through 360* at a rotational rate of between 1 to 20 revolutions per minute.
11. 12. A process as claimed in claim 11 wherein the rotating of said screening surface and container about said rotational axis and the tilting during decanting is accomplished by a substantially vibration free means.