US2204063A - Method of producing clean wetground mica - Google Patents

Description

June 11, .1940. .F. c. ATWOOD ,2

urn-non OF raonucme 0mm WET-GROUND MICA Filed July 17, 1937 -INVENTOR ATTORNEY Patented June 11, 1940 v 7 2mm amnion or raono'cmc GROUND HICA Francis M Atwood to Atlantic- Beaearc mingtan. DeL, a corporationrof Dela Newton, Mala, assignmh A-ociates,

Ine., Wil- Application Jill! 17, 1937, No. 154,133

2 Claim.

This invention relates to a method of producing clean wet-ground mica.

One object of the invention is to provide a novel and improved method by which cleaner, more .5 brilliant and finer wet-ground mica can be produced from any given raw material than by other p'rior methods of which I am aware.

A further object of the invention is to provide a novel economical and efficient method of producing wet-ground mica by which a character of mica may be produced which, because of the thinness of the individual flakes and the freedom from dirt and foreign particles, lends itself particularly for use in paints and similar coating materials for the production of reenforced coat- "ing fllms in which, as a practical matter. the mica is undetectable or at least can be detected only upon minute examination, and consequently does not detract from the value or appearance of the coating film. v

With these general objects in view, and such others as may hereinafter appear, the invention consists in the method of producing clean wetground mica hereinafter described and particularly defined in the claims at theendof this specification.

In the drawing the single figure is a sectional view of a sufficient portion of a centrifuge to enable the centrifuging step in the present process 7 to be understood.

The use of mica and particularly of comminuted mica flakes for various industrial uses has been growing rapidly and at the present time substantial quantities thereof are used in the production of coating materials such as paints and the like. The use of mica in paints for most decorating work has met with substantial resistance because 'of the characteristic lustre and light reflection resulting from the visibility of the individual mica particles when embodied in the paint film. Following my introduction of the desirability of using mica flakes as a reenforcing medium for a paint film, many attempts have been made to overcome this difficulty. I have discovered, however, that it is possible to produce a character of mica particle which is extremely thin or, in other words, whose dimensions in one direction are many times that in another direction and which mica particles are free or substantially free result, when ground mica of this character is embodied in a paint film, the absence of dirt particles between the individual overlapping portions of adjacent mica flakes enables the flakes to lie down and reenforce the paint film without being from dirt or other-foreign particles, and as a substantially visible to, or capable of detection by the naked eye, and particularly where high index pigments are used in the paints.

' In accordance with the invention the grinding of the mica is carried on under conditions con- 5 ducive to the formation of mica flakes of extreme thinness even to the extent of a few microns in thickness, and in view of the extreme thinness of the mica flakes, the presence of any substantial amount of'dirt or foreign matter in association with the mica would operate to prevent the overlappingmica flakes from lying in substantially the same plane and as a result of the presence of small dirt particles the brilliance and sparkle characteristic of mica has been imparted l5 to the paint or other coating film, a result which in many instances has been regarded as objectionable. In accordance with the present invention, the complete process is such as to enable the desired character of mica to be produced economically so as to enable the mica product to be embodied in paints and when brushed into -a film, to beentirely free from any of the disadvantages above mentioned.

For a better understanding of the invention, reference may be had to the accompanying drawing, in which the singlefigure is a front view, partly in section, of a centrifuge used in practicing my invention.

,In accordance with the preferred process the mica, such as scrap mica, is first partially split and mechanically freed from as much as possible of the adhering dirt and foreign matter. This may be accomplished in various ways but it is preferred to pass the mica through the cleaning 35 apparatus shown in my Patent No. 2,177,358.

' After the cleaned mica flakes have thus been obtained they are, in accordance with the present invention, wet-ground preferably in the presence of deflocculating agents which may comprise one or more of the following: sulphite waste liquors and modifications thereof, glucose, pentose, arabinose, dextrose and any of the other polyoses, lignin, tannin liquors, and in general those liquids which possess the ability of wetting the mica to 45 facilitate delamination, and which in addition tend to prevent the flocking of the delaminated mica flakes and to prevent the formation of agglomerates. The grinding operation may be conducted in any usual or preferred form of grinding mill.

After the mica has thus been wet-ground the mill or mills are emptied by flushing with water and the slurry is next subjected to a special centrifugal treatment in order to separate the mica from the water with which it has been mixed during the washing or flushing from the mill. In the drawing I have illustrated a commercial form of centrifuge which may be utilized in practicing this step orprocess and, as therein shown, the centrifuge consists of a solid bowl suspended upon a shaft 82 rotated in the usual manner by a motor 84 mounted upon a framework 90.

The slurry is introduced into the centrifuge through the pipe line 92 at one end. The liquid leaves the centrifuge through the opening 94 at the same end. During operation of the centrifuge and rotation of the bowl the mica is thrown against the bowl by centrifugal force and the liquid flows over the edge of the opening 94 at the discharge end of the centrifuge. When the level of the mica in the bowl approaches the lip of the opening 94 at the discharge end of the centrifuge, the operation of the centrifuge is terminated and the mica dug or otherwise removed from the bowl of the centrifuge in the form of a meal. The operation of the centrifuge, therefore, is intermittent.

I practice, a centrifuge of the structure illustrated having a bowl 40 inches in diameter has been advantageously used rotating at a speed of 1200 R. P. M. The slurry was fed into the centrifuge at about the rate of 25 gallons per minute for a period of about 10 minutes, and the slurry contained approximately 20% of solids and 80% of water. The discharged mica cake contained about 30% of water.

In addition to the foregoing I may operate the centrifuge in such manner as to effect a wet classification of the coarser and finer mica particles. For this purpose by running the suspended mica through the centrifuge at higher rates of fiow it is possible to take out or separate the coarser and larger particles of the mica while leaving the finer particles suspended in the water. I may then proceed to pass the water containing the finer particles of mica in suspension through the centrifuge again at a substantially lower rate of fiow and am able to effectively separate the finer particles and thus substantialiy completely remove all of the mica from the water. Because of their differences in settling rates, I am able to separate the finer mica particles from clay and other fine contaminations by the operation of the centrifuge in the manner described.

The mica cake, produced as above described, has been reduced in moisture content to such an extent that it will now dry very readily as contrasted with the drying rate of the very wet mica hereto resulting from prior grinding methods. In some instances I have found it possible to reduce the moisture content of the mica by the centrifuging action to 30 to 40% as distinguished from the 50-75% water content ordinarily contained in the ground mica particles as result of gravity settling.

It has heretofore been the practice in wet grinding mica to run the slurry of mica and water into settling tanks preparatory to the separation of the mica from the water. A great deal of difficulty has been experienced in the satisfactory separation of the water from the mica, and attempts to filter the mica have met with little success, because of the flake-like character of the mica. This is particularly true when there are certain inclusions of phlogopite and biotite mica which will neither settle nor filter with any degree of success. Even with muscovite mica the settling is slow and a certain amount of fine mica is always left in the supernatant liquid. and is lost. Moreover the mass of settled mica still carries from to of water which has to be dried out by expensive drying methods. Heretofore clays and other dirt associated with the mica. or in the water could be reduced in amount only by repeated washing and settling and such procedure has been highly uneconomical. The dried mica therefore, has heretofore always contained a substantial amount of clay and dirt associated with the water remaining in the mica.

From the description thus far it will be observed that the present method is particularly adapted for the production of clean finely divided mica flakes of extreme thinness and which are suitable for use as a reenforcing medium for coating films such as paint films and while the preferred apparatus and the method of mechanically cleaning the mica has been herein illustrated, it will be understood that other mechanical cleaning operations may be used with advantage if desired in order to mechanically clean the mica from substantially all the dirt, quartz and any other foreign matter. For example, the scrap mica may be passed through corrugated rolls moving at differential speeds and the action of the rolls utilized to crush the dirt to dislodge it from the mica flakes while permitting the latter to bend or fiex and become partially separated from the laminae due to the differential speed.

While the above specification is directed particularly to the method of obtaining clean mica flakes from muscovite mica, it is understood that it is also applicable to the cleaning and grinding of biotite and vermiculite scrap, which products are sometimes known as chlorite and jeifersite, but they are intended to be classed in this application as sources of micaceous products.

The centrifuge suitable for use in practicing the present process comprises a centrifuge of the solid bowl type. In operation the mica water slurry is passed through the bowl and the centrifugal force developed operates to cause the mica particles to pass outwardly and accumulate against the wall of the bowl while permitting the water of the slurry to continue to pass through and be discharged from the bowl. This method of separating the mica overcomes difficulties which have been experienced in attempting to centrifuge mica in a bowl of the perforate type by reason of the sealing of the perforations by the overlapping mica. In thepresent instance the mica is caused to move outwardly while the water component continues to pass through the bowl.

As a further description of the action, the force due to the centrifugal action acting on the mica is greater than the force acting on the water due to the greater specific gravity of the mica particle. As soon as a body of mica is formed in the bowl of the centrifuge, this force begins to act to actually squeeze the water out of the body of the mica thereby assisting in further removal of water and mica. By the same token the clay in suspension is probably somewhat in a colloidal form and somewhat hydrated which would account for the fact that it stays in suspension or is squeezed out with the water rather than being retained in the body of the mica.

The present application constitutes a continuation in part of my application, Serial No. 90,960, filed July 16, 1936.

While the preferred apparatus for practicing the present invention has been herein illustrated and described, it will be understood that if found advantageous, other apparatus may be used.

Having thus described the invention, what is claimed is:

1. In a process of preparing finely divided clean mica particles of a few microns in thickness suitable for reinforcing paint films without being substantially visible therein, in which process mica and a relatively small amount of impurities adhering thereto are ground in the presence of a carrier liquid and an organic deflocculating agent to facilitate the delamination of the mica to said thickness and to place said impurities in colloidal suspension in the carrier liquid, the step comprising separating said finely divided mica from the slurry containing the same as the major solid ingredient by a batch centrifuging operation in which a quantity of the slurry is subjected in a solid wall centrifuge to centrifugal force of a magnitude and for a sufficient time to produce and retain a compact solid layer of the finely divided mica and to effect an expulsion from said layer of carrier liquid and impurities colloidally suspended therein, said centrifuging being unaccompanied by agitation within the liquid, continuously discharging liquid and impurities from the centrifuge bowl, and removing the separated mica from the centrifuge prior to the centrifuging of an additional quantity of the slurry.

2. In a process of preparing finely divided clean mica particles of a few microns in thickness suitable for reinforcing paint films without being substantially visible therein, in which process mica and a relatively small amount of impurities adhering thereto are ground in the presence of a carrier liquid and an organic deflocculating agent to facilitate the delamination of the mica to said thickness and to place said impurities in colloidal suspension in the carrier liquid, the step comprising separating said finely divided mica from the slurry containing the same as the major solid ingredient by a batch centrifuging operation in which a quantity of the slurry is subjected in a solid wall centrifuge to centrifugal force of the order produced in a 40 inch bowl rotating at 1200 revolutions per minute and for a sumcient time to produce and retain a compact solid layer of the finely divided mica against the solid wall and to eifect an expulsion from said layer'of carrier liquid and impurities colloidally suspended therein. discharging the liquid and impurities continuously and removing the separated mica from the centrifuge prior to the centrifuging of an additional quantity of the slurry.

FRANCIS CLARK] A'I'WOOD.

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