US2565097A - Method of crystallizing crystallizable substances - Google Patents

Description

Patented Aug. 21, 1951 METHOD OF CRYSTALLIZING CRYSTAL- LIZABLE SUBSTANCES Paul Francis Sharp, Piedmont, Calif., assignor to Golden State Company, Ltd., San Francisco, Calif., a corporation of Delaware No Drawing. Application July 25, 1946, Serial No. 686,155

6 Claims. (01. 99-55) This invention relates to a process for treating solutions of crystallizable substances which, either, are supersaturated prior to treatment or become supersaturated during treatment, and according to which the solutions treated are conditioned against random growth of large crystals by the utilization of control means and technique which cause the production of a large number of extremely small crystals, the number of which exceeds a predetermined minimum and the sizes of which fall within a predetermined size range.

More particularly, this invention relatesto the crystallization of a portion of the dissolved substance according to novel procedure, which serves to produce crystals of such small size and in such large numbers that all further crystallization subsequently occurring in the stabilization of the supersaturated solution, results in the production only of relatively small crystals, no substantial part of which is greater than a predetermined permissible size. In other words, the practice of this invention involves as an initial step the enforced production from a portion only of the crystallizable substance, of seed crystals so small and so numerous that, when serving as nuclei for additional crystal growth, they will cause the crystallization of all of the remaining crystallizable material to occur in the environment of such a large number of said preformed crystal nuclei that all of the crystallizable material will be exhausted before any substantial to grow to an objectionable size.

This invention is especially adapted for use in v the crystallization of sugars, for example, sugars present in milk products, wherein the portion initially crystallized is so controlled as to sizes and numbers of crystals that the quantity of crystallizable substance present in excess of that productive of a saturated solution, even at low temperatures, is insuflicient to cause the initially formed crystals to grow to a size greater than that desired, or, in the case of food products, greater than the maximum size which evades detection "organoleptically.

The process of this invention comprises the step of crystallizing the crystallizable material from the liquid solution without the addition of extraneous substances. Instead this new method involves the use of a seeding surface of crystalline material, the establishment of a seeding contact between the seeding surface and the material being treated, for a predetermined period of time, and the enforced production of a large number of crystals which, upon being formed,

method the growth of crystal nuclei in contact with the seeding surface is interrupted almost as rapidly as such nuclei are formed. The size and number of these seed nuclei determine the size to which crystals can subsequently grow in the treated solution.

This invention provides a practical and economical method for producing, in a finely divided form, material crystallizable from supersaturated solutions. It is applicable with excellent results to the controlled crystallization of lactose from pure lactose solutions, or from various concentrated milk products such as whole milk, whey, sweetened condensed milk and the like. It also is applicable to the crystallization of sucrose and glucose, and when used it imparts to liquid products a smooth consistency heretofore unobtainable.

This invention is applicable with equal advantage both to products completely processed except for crystallization and to products which may be subsequently processed in any desired manner, such as by drying or further concen trating. In the case of products to be subsequently dried or further concentrated the conditioning provided by the presence of nuclei of the type referred to, will function to prevent the formation of large crystals during all subsequent treatment steps.

It has long been known that crystallization can be induced in supersaturated solutions by the addition to such solution of a quantity of comminuted crystals of like or unlike material, or by the addition of a quantity of the same material from a previous batch in which a large number of crystals has been formed and which serve as nuclei for further crystal formation. However, there are two disadvantages involved in the use of this method. The first disadvantage resides in the fact that random crystal growth occurs during the period when the solution is cooling down to the temperature range in which the seeding action is most effective for enforced crystallization, and the second is found in the fact that in the practice of such method, the only possible means for influencing crystal size over long periods is the uncertain and indirect control afforded by agitation.

Another known seeding method is onewhich has been used in the production of dried whey of a type containing lactose partially in the crystalline form and consists in concentrating whey to approximately 50% solids in an evaporator, further concentrating the whey on the surface of drum driers to a solids content in the neighborhood of 85%, and then removing the tally-like material from the drums by scraping and dropping the same onto a second set of drums mounted below. This taify-like material on passing through the bite between the lower drums, is pressed into a thin layer which almost completely coats the drum surfaces. In the course of one revolution, the taffy-like material is converted into an essentially solid material in which some crystallization is induced by virtue of the fact that the previously dried layer is not completely removed from the drums and, therefore, some crystals of lactose are present. Such crystals act as seed for the tail'y-like material before it passes into a solid state.

In the operation of such a process, which works only with highly concentrated solutions having in the neighborhood of 85% solids, the product is carried substantially to complete dryness during which time it receives a rather severe heat treatment. Furthermore, since the product passes into a solid state with great rapidity, there is no way to interrupt or control the crystallization.

The present invention has as one of its primary objects, the production of a larger number of smaller crystals than heretofore has been possible.

Another object is to provide a method whereby crystallization can be induced under control.

Still another object is to provide a method whereby crystallization is induced without adding extraneous material.

Still another object is to provide .a method whereby all seeding material remains on the seeding surface and only material originally contained in the solution is to be found in the product.

Another object is to provide a method whereby the initial crystallization, may be started as soon as supersaturation is reached thereby preventing an unstable supersaturated condition from building up in the solution before a substantial number of small crystal nuclei are present to control the subsequent crystallization and attendant exhaustion of the remaining crystallizable material.

Another object is to provide a method according to which the crystallization of crystallizable material held in solution may b controlled when such solutions suddenly become heavily supersaturated, which control is provided through the combined effects of the seeding surface and the subsequent growth of the crystals initially formed. The seeding surface serves to induce rapid crystallization and the production of a large and continuously increasing number of minute crystal nuclei and the subsequent crystal growth acts to finally exhaust the supply of crystallizable material in the solution in excess of the saturation requirements.

Still another object is to keep the seeding surface intact so that it can be retained and used for extremely long periods to seed large quantities of new material.

A further object is to provide a method that lends itself readily to continuous operation. Other objects not specifically mentioned will appear to those skilled in this art as the following description proceeds.

In carrying out the present invention, the supersaturated material to be treated is brought into intimate contact with a preformed seeding surface while th same is maintained at a temperature below that which is necessary to dissolve crystals from the seeding surface. At the same time the flow of the material over the seeding surface is so directed that the surface contact is promptly and repeatedly interrupted.

supersaturation of the crystallizable material is of course a prerequisite to the practice of this method. This supersaturated condition may be produced by evaporating the solvent until the desired degree of supersaturation is reached, by cooling a concentrated or unconcentrated solution to a point wher the solubility of the solute in the solvent is exceeded. or by employing a combination 01' both methods. The temperatures employed and the method of attaining the degree of supersaturation utilized preferably should be such that the material is not seriously discolored or otherwise affected to the detriment of the odor and general quality of the material.

The degree of concentration of the solution required to bring it to a point where it is supersaturated with the crystallizable substance will vary with each individual product. For example, lactose-water solutions are supersaturated if the lactose content is greater than about 16 per cent by weight at 68 F.

If the product to be produced is powdered whole milk, the preferred concentration is about 50% to 52% solids content of which approximately 18% is lactose, at which concentration the solution is supersaturated with respect to lactose, even at about F. The maximum concentration gainfully employed if the milk concentrate is to be spray dried is limited by the ability of the atomizer to properly subdivide and spray the somewhat viscous material; The minimum concentration is limited to that at which the solution is just supersaturated when sent to the spray drying operation.

'On the other hand, if the product to be produced is sweetened condensed milk, the milk is usually concentrated to the point where its whole milk or skimmed milk solids content is double that of the original milk employed, and after adding the proper amount of sugar, its constituency comprises about 75% solids, of which approximately 42% to 44% is sucrose and 11% is lactose. This solution is substantially saturated with sucrose and supersaturated with lactose at about F. or higher.

When handling free-flowing, low viscosity liquids, a stationary seeding surface or base which may or may not be scraped has been found to be suitable for use either on a once through or a recycling basis. Also other systems which will be discussed later can be used. One advantage of a closed system, such as a contact tower or a rotating member or members enclosed in some form of housing. is that air may be kept out and oxidation of easily oxidizable substances, such for example as the components of milk products, can be eliminated.

When packed towers are used the preferred method of operation is that of flowing the liquid through the towers at a rate which avoids flooding, and yet will provide sufllcient flow to cause a washing action to occur on the packing surface. The avoidance of flooding, however, is not absolutely necessary if a washing action is provided on the packing surface. Such method causes the liquid containing the crystallizable material to move in contact with the surfaces of the seeding base and at the same time to wash the surface free of crystal nuclei almost as fast as the nuclei are formed. When the flow is too slow, crystal nuclei tend to adhere to the base unless some positive action, such as scraping to remove nuclei as formed, is resorted to. If the undisturbed nuclei are permitted to grow, eventually the seeding base will increase in size until the crystal material plugs the tower. When slow flow conditions must be used and such use will necessarily result in the clogging of the tower, it is better, of course, to use one of the other contact systems hereinafter described.

Such a tower, preferably, is packed with large pieces of crystalline material disposed in such a way as to provide a certain amount of space in the tower lying between separated volumes of the seeding material, whereby the material being treated is successively brought into contact with and removed from the seeding material. If desired, the packed regions of the tower may include, in addition to crystalline material, suitable non-reactive, inert, dilutent material interspersed therebetween.

Instead of large pieces of crystalline material, large individual crystals or fragments of large crystals may be used. By way of example, if lactose is to be crystallized, lactose crystals or crystal fragments as large as 0.5 mm. to 3.0 mm. in length may be used.

Another suitable packing for contact columns consists of pressed pills of previously powdered crystal material. Pills are easier to prepare and do not require special technique for their formation, such as are required to produce oversize crystals.

When treating material of medium viscosity, the flow of which tends to be somewhat sluggish, a moving or rotating seeding base is advantageously employed. The liquid to be seeded may come in contact with the moving seeding surface by any one of a number of ways, for example, by flowing the liquid downwardly from a thin stream funnel onto a roll, plate, or similar device or by just having a revolving member mounted in such manner that at all times it is partially immersed in a trough or receptacle containing a supersaturated solution. Such rotating member picks up a layer of the material being treated. This material may or may not have its contact with the crystalline surface repeatedly interrupted by mechanical means. If the rotational speed is high and the continuous removal of liquid from the crystal contact surface is substantially complete, no additional means for interrupting contact with the seeding base is necessary although complete. seeding may require that the material be recycled for additional contact.

Preferably however, the material is repeatedly removed from and redeposited on the moving member by mechanical scrapers, or by a series of wires which have a rippling effect upon the solution adhering to the contact surface. Such means may be arranged, for example, in parallel relation to the axis of a rotating drum or in certain instances transverse to the drum axis, thus positively insuring the interruption of surface contact of the liquid with the seed crystal surface. The moving base may be mounted for operation with its seeding surface in a horizontal, vertical, or other position. This may be accomplished by having the seeding crystals mounted so as to form at least a part of the surface of a vertical or horizontal cylinder, a conical member or other suitably shaped device.

The moving crystal bearing type of device may be illustrated by referring to two satisfactory constructions. When using, for example, a cylindrical roll, trenches or grooves may be ground or otherwise formed in the roll surface, preferably but not necessarily undercut. These trenches may be cut parallel to the axis of the roll, at an angle thereto, at right angles to the roll axis, or in any other suitable relationship. Grooves cut in the shape of sine waves also may be used. A very desirable type of groove is a continuous one cut helically on the cylindrical surface, to a depth of about of an inch to of an inch and at a width of about A; of an inch to of an inch.' The depth and width, of course, may be varied as desired. In order to have a large crystal surface, such helical grooves are cut with a thread or land, separating the parallel crystal surfaces by a width of only about A; of an inch or less. The grooves of the roll are filled with crystalline material in a manner hereinafter described. The material to be seeded is usually picked up by the grooved drum from a trough or receptacle in which the drum is partially immersed.

Another type of rotating member consists of a truncated conical roll, mounted for rotation in a similarly shaped housing, having small adjustable clearance therewith. The surfaces of both the housing and the conical roll are trenched and filled with suitable crystalline material. The material to be seeded preferably enters this device at the bottom, moves upwardly between the rotating and stationary surfaces, the clearance between which may be as small as /100 of an inch, and finally flows out at the top. Movement of the rotor produces a current which has a washing effect on both the stationary and moving surfaces which prevents the crystal nuclei from remaining in contact with the seeding base for too long a period.

When treating relatively viscous materials, the flow of which is reduced to a point where it is too slow to cause such washing action to occur, it becomes necessary to interrupt the continuity of contact at the seeding surface by some positive action. This does not mean, however, that positive crystal removing action cannot be ured when dealing with less viscous materials but rather that such action is not essential.

One form of apparatus producing a positive action for interrupting the continuity of surface contact is described and claimed in the copending application, Serial No. 697,955, filed September 19, 1946, entitled Apparatus for Proce-sing Substances Containing Crystallizable Material, now Patent No. 2,495,024 of January 17, 1950. Briefly stated, the operation of the device disclosed in said copending application is as follows: Viscous liquid containing crystallizable material flows through the device which comprises a series of discs, each of which is equipped with crystal surfaces disposed on the opposite sides thereof. The device also comprises a series of scraping units mounted in contact with the crystal surfaces of the discs. The discs are ro-- tated by a central drive shaft, and as the material to be treated passes between the discs, the film or layer thereof in contact with the discs is scraped therefrom and the film or layer is allowed to again come in contact with the discs. This action is repeated over and over again a number of times before the material passes out of the seeding apparatus.

The temperature employed for the operation varies with each process and with each product and also with the degree of supersaturation utilized. For example, sweetened condensed milk is usually crystallized at a temperature of between 82 F. and 92 F., whereas a highly concentrated whey product, supersaturated with lactose, which is to be spray dried, may be maintained during seeding at a temperature as high as 110 F. to 130 F., the upper temperature being limited by the degree of supersaturation and the extent of the darkening of the seeding base.

Seeding bases, adapted for use as described above, may be prepared in a number of different ways depending upon the form being produced. For example; the large sugar crystal type may be prepared by lowering crystal nucleus material, such as string, into an undisturbed vat of sucrose solution which is slightly supersaturated. When large crystals, approaching the size of rock candy are formed, the crystals are removed from the solution, dried, and broken into fragments for use as packing for a seeding tower. This process is time consuming since the crystallization is slow.

A more satisfactory and more easily prepared tower packing is the pellet or pill type. This may be prepared by slightly moistening pulverized crystalline material and molding it into any desired shape, preferably in irregular shapes having flat faces and sharp edges.

Satisfactory seeding surfaces for moving elements may be obtained by moistening pulverized crystals until the mass forms into a thick paste, then applying the paste so formed on the surface of a grooved drum or similar member, in such manner as to just fill the grooves or trenches flush with the drum surface and then drying the unit.

Another procedure for producing a. seeding layer may be illustrated by reference to a lactose operation, which is as follows: A hot, concentrated lactose syrup is produced by boiling a lactose solution until free of crystals, at a temperature of about 220 F. to 240 F. The syrup is poured into the trenches cut in the roll or similar device then the roll is wiped to leave the syrup substantially flush with the roll surface. Upon cooling, the syrup crystallizes and hardens in the trenches. Excess moisture present is removed by applying heat, as by rotating the roll in front of a heating device such as an electric lamp, steam coil, or other type of heater.

The methods of producing controlled crystallization described above, vary somewhat in efflciency and ease of operation, depending upon the particular type of operation involved. For example; a seeded roll rotating in a seeded housing is not as satisfactory for seeding sweetened milk as is the roll or disc type of device having scrapers. Such apparatus is perfectly satisfactory for seeding whole milk, however.

In general, for maximum efficiency in the induction of crystallization some positive scraping or rippling action on the seeding surface is desirable.

Whether or not a particular product has been produced efficiently and seeded correctly cannot be determined solely by the number of crystals per cubic centimeter that it may contain as the crystal count per unit volume will vary with different products even though the best possible results have been produced. The most exacting standards and those requiring the exercise of the highest degree of efliciency in the production of a. large number of small crystals are those applied to the production of sweetened condensed milk. Crystals larger than 30 microns in length will impart to sweetened condensed milk a sandy texture which is highly objectionable. Examination of samples of condensed milk obtained on the open market shows crystal content varying from forty million crystals to four hundred million crystals per cubic centimeter, and ranging in size from 10 to 50 microns. In general, the average size of crystal varies inversely as the number of crystals increases.

When the above described method, employing a roll and a positive scraping means, is used for producing condensed milk, truly impalpable crystals are produced, averaging in size from 2 to 5 microns in length and numbering as high as two thousand million per cubic centimeter. Condensed milk prepared by this process, upon ageing, does not develop crystals as large as 50 microns such as are commonly found in commercial condensed milk now on the market.

The following examples illustrate preferred methods of practicing the present invention:

Example I Sweetened condensed milk, containing 74.5% solids of which 8.66% is fat, may be held at a temperature of 80 F. in a trough or other receptacle. Supported in the trough or other receptacle in such manner as to rotate therein with about of its circumferential area immersed in the milk, is a roll approximately 16 inches long and having a diameter of approximately 10 inches. This rotating r011 has a helical ribbon of lactose crystals almost A; of an inch wide and almost a; of an inch thick, with A of an inch of land between adjacent convolutions. Steel wires 3": of an inch in diameter are preferably stretched tautly across the roll supporting frame so as to extend parallel to the axis of the roll and are spaced in such manner as to rub the surface of the roll at approximately inch intervals. Preferably the wires act on an area of the roll surface extending from the roll top to a point on the roll at which a knife or similar device is provided which acts to remove the adhering milk from the surface of the roll. The roll preferably rotates at approximately 13 R. P. M. and picks up an adhering layer of condensed milk which, when brought in contact with the taut wires, is removed from the roll and is passed over the wires by a motion which causes it to roll over on itself as it passes each wire. The contact between the treated material and the seeding crystal base is thus interrupted at half -inch intervals by the taut wires 'when the roll is rotated. 600

pounds per hour of condensed milk can be passed over a roll of the relatively small size referred to, and a highly satisfactory seeding performance results, that is to say, one Producing six hundred and seventy-five million crystals per cubic centimeter. It is preferable, however, to operate the roll at, roughly, half the above-stated capacity, in which event about" eight hundred to eleven hundred million crystals per cubic centimeter are produced. When the apparatus is run at a capacity of five hundred pounds of product per hour and at a rate of one revolution every five seconds and the material being treated is recycled appoximately five times, about two thpusand million crystals per cubic centimeter are produced. Even at this slow rate, seeding is accomplished and the product is ready to can in about five minutes, whereas by standard practice thirty to ninety minutes are usually required for regular seeding and its after-agitation. Furthermore, the process of this invention produces a far more desirable product having more numerous crystals of a more nearly uniform size.

Enample 11 Whole milk may be concentrated in a vacuum evaporator until it is about 51% total solids and then cooled to a temperature of about 68 F. The concentrate may then be passed into a trough about five inches deep in which is rotated a cylindrical roll of the type described in detail in connection with Example I. When operating at a rate of thirteen revolutions per minute, this roll can pick up and remove from the trough, roughly, 700 pounds of concentrated milk per hour. In one pass across the roll surface a batch of whole milk was seeded to an average content of sixtytwo and one-half million crystals per cubic centimaterial, in a solution presentin excess of saturation, into impalpable crystals, which comprises flowing said material in a stream in contact with seed crystal material having a predetermined crystallizing influence per unit area, maintaining said stream inform which produces an area of contact between it and said seed crystal material which has a ratio to the volume per unit area of material in said stream productive of a relatively large number of impalpable crystals without the production of crystals of palpable size, continuing said flow of said stream of material. in contact with said seed material with the formed crystals maintained in substantial association with their own mother liquor and without permitting substantial intermixing of said material and crys-, tals with other portions of said material, and during the course of said flow, repeatedly removing the material of said stream from contact with said seed crystal material and reapplying the same in contact therewith to produce additional impalpable crystals therein, the rate of flow of said stream of material and the extent of said removal and reapplying activity being that which causes the crystallizable potential of each successive portion of said material being treated to be sub-;

stantlally exhausted through the formation of impalpable crystals in a time period insufficient to produce the formation of crystals of palpable size, and flowing said stream out of contact with said seed crystal material after said contact has been maintained for at least a period sufllcient to assure the substantial exhaustion of the crystallizable potential of said material through the formation of impalpable crystals and without the formation of crystals of palpable size.

2. The method of converting crystallizable material', in a solution present in excess of saturation, into impalpable crystals, which comprises flowing said material through a zone of predetermined volumetric capacity containing exposed surfaces of seed crystal material of predetermined area, said predetermined volumetric capacity and predetermined area being of such relative sizes as to produce in the material flowing through said zone a relatively large number of impalpable crystals without producing crystals of palpable size, flowing said material through said zone without substantial intermixing of any portion of said material with any other portion thereof which has a substantially different crystallizable potential and while maintaining the crystals formed therein in substantial association with their own mother liquor, during the flowing of said material through said zone, repeatedly removing material from contact with the surface of said seed crystal material therein and reapplying the same in contact therewith, the rate of, flow of said stream of material through said zone and the extent of said removal and reapplying activity being that which causes the crystallizable potential of each successive portion of said material being treated to be substantially exhausted through the formation of impalpable crystals without the formation of crystals of palpable size during the period of time that said material is flowing through said zone. 1

3. The method defined in claim 1, further characterized in that the material being treated is a lacteal product and the crystallizable and the seed crystal material is milk sugar.

4. The method defined in claim 2, further characterized in that the material being treated is a lacteal product and the crystallizable and the seed crystal material is milk sugar.

5. The method defined in claim 1, further characterized in that the material being treated is milk and the crystallizable and the seed crystal material is milk sugar.

6. The method defined in claim 2, further characterized in that the material being treated is milk and the crystallizable and the seed crystal material is milk sugar.

PAUL FRANCIS SHARP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,814,300 Duschak July 14, 1931 1,857,884 Skogmork May 10, 1932 2,188,907 Lavett Feb. 6, 1940 2,197,804 Lavett Apr. 23, 1940 2,209,328 Dietrich et a1. July 30, 1940 2,369,231 Harding Feb. 13, 1945

Claims (1)

1. THE METHOD OF CONVERTING A CRYSTALLIZABLE MATERIAL, IN A SOLUTION PRESENT IN EXCESS OF SATURATION, INTO IMPALPABLE CRYSTALS, WHICH COMPRISES FLOWING SAID MATERIAL IN A STREAM IN CONTACT WITH SEED CRYSTAL MATERIAL HAVING A PREDETERMINED CRYSTALLIZING INFLUENCE PER UNIT AREA, MAINTAINING SAID STREAM IN FORM WHICH PRODUCES AN AREA OF CONTACT BETWEEN IT AND SAID SEED CRYSTAL MATERIAL WHICH HAS A RATIO TO THE VOLUME PER UNIT AREA OF MATERIAL IN SAID STREAM PRODUCTIVE OF A RELATIVELY LARGE NUMBER OF IMPALPABLE CRYSTALS WITHOUT THE PRODUCTION OF CRYSTALS OF PALPABLE SIZE, CONTINUING SAID FLOW OF SAID STREAM OF MATERIAL IN CONTACT WITH SAID SEED MATERIAL WITH THE FORMED CRYSTALS MAINTAINED IN SUBSTANTIAL ASSOCIATION WITH THEIR OWN MOTHER LIQUOR AND WITHOUT PERMITTING SUBSTANTIAL INTERMIXING OF SAID MATERIAL AND CRYSTALS WITH OTHER PORTIONS OF SAID MATERIAL, AND DURING THE COURSE OF SAID FLOW, REPEATEDLY REMOVING THE MATERIAL OF SAID STREAM FROM CONTACT WITH SAID SEED CRYSTAL MATERIAL AND REAPPLYING THE SAME IN CONTACT THEREWITH TO PRODUCE ADDITIONAL IMPALPABLE CRYSTALS THEREIN, THE RATE OF FLOW OF SAID STREAM OF MATERIAL AND THE EXTENT OF SAID REMOVAL AND REAPPLYING ACTIVITY BEING THAT WHICH CAUSES THE CRYSTALLIZABLE POTENTIAL OF EACH SUCCESSIVE PORTION OF SAID MATERIAL BEING TREATED TO BE SUBSTANTIALLY EXHAUSTED THROUGH THE FORMATION OF IMPALPABLE CRYSTALS IN A TIME PERIOD INSUFFICIENT TO PRODUCE THE FORMATION OF CRYSTALS OF PALPABLE SIZE, AND FLOWING SAID STREAM OUT OF CONTACT WITH SAID SEED CRYSTAL MATERIAL AFTER SAID CONTACT HAS BEEN MAINTAINED FOR AT LEAST A PERIOD SUFFICIENT TO ASSURE THE SUBSTANTIAL EXHAUSTION OF THE CRYSTALLIZABLE POTENTIAL OF SAID MATERIAL THROUGH THE FORMATION OF IMPALPABLE CRYSTALS AND WITHOUT THE FORMATION OF CRYSTALS OF PALPABLE SIZE.