US3454425A - Refining and recrystallizing of sugars using aqueous alcohols - Google Patents

Description

United States Patent Oifice 3,454,425 Patented July 8, 1969 3,454,425 REFINING AND RECRYSTALLIZING OF SUGARS USING AQUEOUS ALCOHOLS Bruce Smythe, 47 Gritfith Ave., Roseville, New South Wales, Australia, and Robert Staker, 26 Millwood Ave., Chatswood, New South Wales, Australia No Drawing. Continuation-impart of application Ser. No. 208,171, June 15, 1962. This application Oct. 10, 1966, Ser. No. 585,322

Int. Cl. C13f N US. Cl. 127-64 2 Claims ABSTRACT OF THE DISCLOSURE A method of producing refined sugars from a sugar consisting of crystals bearing a film of molasses, wherein the sugar bearing molasses is washed at certain temperatures in a first solvent that contains appreciable quantities of dissolved sucrose until the molasses film is substantially removed from the crystals and subsequently dissolving the clean crystals in a second selective solvent at a temperature between 70 and 150 degrees C. so as to dissolve the crystals, cooling the hot liquor to crystalize the sucrose and subsequently separating the refined crystalized sugar from the mother liquor.

The present invention is a continuation-in-part of United States Patent application Ser. No. 208,171, filed June 15, 1962.

The invention relates to the refining and recrystallizing of sugars. More particularly, it relates to the removal from raw sugar of colour factors and various other impurities, and to the preparation therefrom of a refined sugar in crystalline or liquid form.

The different varieties of raw sugar (e.g. mill white, cane raws, beet raws) all consist of crystals of sucrose surrounded by a viscous film of molasses. For a given raw sugar, the ratio of total impurities to total sucrose may be 1:99, but the ratio of impurities occluded in the crystals to impurities in the molasses film may be 1:9.

Since most of the non-sucrose components of raw sugar are contained in the external molasses film, it is standard practice to perform a preliminary afiination by washing the molasses film from the crystals. The separated crystals are then further purified by a procedure (I) terminating in the production of refined sugar, while the molasses raw washings are treated by a procedure (II) terminating in the production of additional raw sugar and the final rejection of a very impure waste molasses.

According to a known method currently favoured, the washing fluid used for the preliminary affination of raw sugar is hot aqueous syrup, e.g. impure recycled raw washings. When this is mixed with raw sugar, a magma is obtained comprising sucrose crystals dispersed in a stable colloidal suspension of (essentially) diluted molasses. The greater part of this diluted molasses is then removed from the crystals by centrifugation.

By procedure (I), the separated crystalline material coated with residual molasses-is mixed with hot water to give an aqueous sucrose solution containing dissolved or colloidally suspended impurities not removed in the preliminary affination step. This solution is then treated with selected materials (e.g. lime, carbon dioxide, superphosphate, sulphur dioxide, bone char, activated charcoal, or selected combinations thereof) to elfect the removal of certain impurities; and sucrose is recovered from the purified syrup by steps including evaporation, crystallization and centrifugation. If required, sucrose of still higher purity may be obtained by redissolving the recovered sucrose and repeating the last-mentioned steps.

By procedure (II), the raw washings material is submitted to chemical and/or physico-chemical treatment, and a low-grade sugar is recovered from the treated washings. This sugar is used to augment the supply of raw sugar to the preliminary affination stage, and a Waste molasses liquor is finally rejected containing approximately equal parts by weight of impurities and sucrose.

This known method suffers from many disadvantages of which the more obvious ones are noted below.

(1) The preliminary affination of raw sugar with hot aqueous syrup results not only in the dilution of the molasses film but also in the dissolution of a certain fraction of the crystalline sucrose. Since it is considerably more difficult to recover refined sugar from the raw washings than from the crystalline sucrose, this dissolution process can only occur at the expense of refining efficiency. While the dissolution of sucrose can be greatly reduced by further increasing the sucrose content of the washing syrup toward saturation level, the viscosity of such a syrup is so high that (i) the entire atfination process becomes tediously slow, and (ii) the efficiency of raw washings separation by centrifugation becomes much reduced. In practice, a compromise is sought between these opposing factors.

(2) In many crystal manufacturing processes it has been found economically desirable to employ a continuous crystallizer of the type incorporating a first region for crystal growth and a second region from where crystals of desired size may be recovered by gravitysettling. However, by virtue of the high viscosity of aqueous sugar solutions generally and by virtue of the small density differential between such solutions and crystalline sucrose, it has not been found commercially practicable to employ such continuous crystallizers for the purpose of recovering crystallized sucrose from mother liquor in the terminal steps of procedure (I). For this reason, the recovery of crystallized sucrose from mother liquor has hitherto been performed by employing an inherently less eflicient batch process (involving sugarboiling), the success of which is determined in part by operator skill.

Additionally, the high viscosity of sugar solutions impedes the separation of recrystallized sucrose during centrifugation. This problem is particularly pronounced when the recrystallized material consists of crystals of small size (e.g. 0.1 mm. diameter).

(3) The waste molasses liquor finally rejected in the terminal steps of procedure (II) is an aqueous mixture containing an unsatisfactorily high ratio of sucrose to impurities (as noted previously, about 1:1 by weight).

It is an object of the present invention to select solvents and conditions for their use which overcome (inter alia) the above disadvantages in the refining and recrystallizing of sugars.

It is a particular object of the invention to select a versatile solvent in which,

(i) 'by the selection of appropriate conditions, untoward dissolution of crystalline sucrose does not occur during the preliminary aflination of ,raw sugar, but in which,

(ii) by the selection of appropriate different conditions, washed raw sugar can be readily dissolved to give a solution of such low density and viscosity that sucrose can be recrystrallized and separated therefrom in a com mercially attractive process employing a continuous crystallizer of the described known type.

We have ascertained by experiment that many of the components of raw sugar which are substantially soluble in concentrated aqueous sucrose solutions, are substantially insoluble-or at least controllably solubleeither in selected organic solvents or in selected aqueous mixtures containing such organic solvents.

The selectivity of a solvent for any given component of a mixture refers hereinafter to the ratio of the weight dissolved of that component to the weight dissolved of the remaining components when a quantity of the mixture is added to a quantity of the solvent.

The solvent selectivity of water for the components of raw sugar is relatively poor: firstly, because many of the components (sucrose and impurities alike) are inherently soluble in water; secondly, because other components-even if not inherently soluble in water per se are soluble, or form stable colloidal suspensions, in aqueous solutions containing sucrose. On the other hand, the solvent selectivity of some organic liquids for the sucrose component of raw sugar is conspicuously greater than that of water; further, many of the raw sugar impurities are not only inherently insoluble in such organic liquids per se, but are also substantially insoluble in their sucrose-containing solutions.

We have now found that this high solvent selectivity is also characteristic of some binary solutions consisting of water in admixture with an organic solvent selected from those organic solvents having a solvent selectivity for sucrose in raw sugar greater than that of water.

The criteria by which suitable organic solvents can be selected for incorporation in these sucrose-selective binary solutions are:

(i) the organic solvent must be freely miscible with water at all concentrations to give a single phase binary solution, and

(ii) dissolution of sucrose in the resulting binary solution must not result in the separation therefrom of a second phase.

Thus, on the basis of the first criterion, sucroseselective combinations of water and n-butanol must be rejected as unsuitable since they comprise a single phase binary solution only at very low or very high concentrations of one component. Again, on the basis of the second criterion, sucrose-selective combinations of water and i-propanol must be rejected as unsuitable sincewhile they comprise a single phase binary solution at all concentrations-separation of a second liquid phase occurs when a significant quantity of sucrose is dissolved therein. On the other hand, sucrose-selective combinations of water and certain other alcohols (e.g. methanol, ethanol, furfuryl alcohol), or combinations of water and certain substituted alcohols (eg. Z-methoxy-ethanol, 2- ethoxy ethanol), satisfy both these criteria. Combinations of water and alcohols (or substituted alchools) which satisfy all the criteria are hereinafter referred to as aqueous alcohols.

By manipulating variables of concentration and temperature, we have discovered that aqueous alcohol combinations may be selected for use in the refining and recrystallizing of sugars by methods which overcome the disadvantages of the prior art.

When hot aqueous alcohol syrup is substituted for hot aqueous syrup as a washing agent in the preliminary aflination of raw sugar, comparative tests with different aqueous alcohol combinations indicate that a relative increase in the alcohol content of the combination is associated with the following effects:

(a) a relative reduction in the dissolution of crystalline sucrose;

(b) a relative increase in the quantity of precipitated impurity;

(c) a relative improvement in the ease of separating precipitated impurity by centrifugation;

(d) a relative reduction in the raw washings viscosity;

(e) a more unfavourable distribution of colour factors between those remaining in the washed crystals and those removed by the raw washings.

These effects are, of course, not independent of each other. Thus, the effects (b) and (d) are at least partly determined by the low sucrose concentration in the washing solution, this being directly attributable to effect (a). Effect (c) is related to the fact that organic insolubles are increasingly less hydratedhence, less bulky-in aqueous alcohols of increasing alcohol content.

When hot aqueous alcohol is substituted for hot water as a solvent for washed raw sugar in purification procedure (I), comparative tests with different aqueous alcohol combinations indicate that a relative increase in the alcohol content of the combination is associated with effects similar in kind to those noted under (b), (c), (d) and (e), and is additionally associated with:

(f) a relative increase in the density differential between recrystallized sucrose and supersaturated mother liquor.

When an aqueous alcohol syrup washing agent is used for the preliminary affination of raw sugar, or when an aqueous alcohol solvent is used for the dissolution and recrystallization of sugar, comparative tests indicate that a relative increase in the temperature of the operation is associated with the following effects:

(g) a relative increase in the dissolution rate of crystalline sucrose;

(h) a relative increase in the absolute solubility of sucrose;

(i) a relative increase in the rate of crystallization of sucrose from supersaturated solution;

(j) a relative increase in the rate of inversion of sucrose to reducing sugars;

(k) a relative reduction in the quantity of precipitated impurity.

pendent increase in the concentration of dissolved sucrose.

Having regard to the above trends, we have been able to define optimum conditions for using selected aqueous alcohol combinations in the refining and recrystallizing of sugars; and we have established that, by the use of these conditions, the noted disadvantages of the prior art can be overcome and considerable economies can be effected.

The invention provides a method of producing refined sugar from a sugar consisting of sucrose crystals bearing a film of molasses, said method including the steps: washing the sugar at a temperature of between about 20 and 40 C. in a selected solvent until the molasses film is substantially removed from the crystals, and separating the cleaned crystals from the washings; said solvent containing sucrose dissolved in a combination consisting of 15-40% by weight of water and -60% by weight of a selected alcohol; said alcohol having a solvent selectivity for sucrose in raw sugar greater than that of water, and being selected from the group consisting of those alcohols and substituted alcohols which are freely miscible with water at all concentrations to give a single phase binary solution from which a second liquid phase is not separated by the dissolution therein of sucrose.

It has been noted previously that the dissolution of crystalline sucrose by hot aqueous syrup can be greatly reduced by further increasing the sucrose content of the washing syrup towards saturation level, but that such syrups must be avoided because of their high viscosity.

This viscosity restriction does not apply when aqueous alcohol syrup is used for washing purposes, and the dissolution of crystalline sucrose (for which aqueous alcohols are highly selective solvents) may therefore be very effectively inhibited by ensuring that the aqueous alcohol syrup is nearly sucrose-saturated. As will be illustrated in subsequent examples, very satisfactory aqueous alcohol washing syrups can be made by mixing a quantity of raw sugar with the selected aqueous alcohol combination.

Aqueous alcohols usable according to the invention consist of 15-40% by weight of water and 85-60% by weight of alcohol, preferred combinations for particular Reducing sugar, Ash,

percent percent Colour 1 1 Colour units by transmittanoy of a standard solution using light of wavelength 420 millimicron my.

2 Unwashed raw sugar.

3 Sugar washed with aqueous syrup (refinery conditions).

4 Not estimated.

Very poor.

Poor.

We have found that no significant advantage is to be gained by washing raw sugar with aqueous alcohols at temperatures above 40 C.; accordingly, since it is always preferred to operate at substantially ambient temperatures, we have limited ourselves in the above statement of the invention to washing Within the temperature range 20-40 C.

The ratio by weight by solvent to raw sugar is not critical, but preferred values lie between 04:1 and 1:1. In practice, the upper limit is set by economic factors while the lower limit is set by the need to form an easily worked slurry.

In performing this aspect of the invention sugar crystals are mixed with the solvent until the film of surface molasses is substantially removed. The period required for this condition to be achieved is short and is easily determined by observation. We have found generally that washing for a minute is as effective as washing for an hour.

The molasses washings (containing insoluble and precipitated impurities) are separated from the crystals by methods known per se, e.g. involving centrifugation. If desired, crystal quality can be upgraded at this stage by rinsing the crystals, e.g. during centrifugation, with a fresh quantity of the selected solvent. It will be appreciated that water or dilute non-alcoholic aqueous syrups cannot be used satisfactorily for rinsing in this manner, since they are proved to efiect excessive dissolution of crystalline sucrose; however, aqueous alcohols or aqueous alcoholic syrups are not subject to this disadvantage and can be used for rinsing with great success.

The separated molasses washings are treated (e.g. by steps including additional centrifugation or by filtration) to recover a solvent from which suspended impurities have been removed. This solvent may of course be recycled or used again in some other suitable stage of the refining process.

A method has been described previously (D. F. Othmer and A. H. Luley: Sugar 43,7,26) wherein raw sugar is washed with anhydrous alcohols (e.g. methanol) for the purpose of avoiding excessive dissolution of crystalline sucrose; and it has also been known according to this previously described method to use such alcohols at high temperatures (e.g. the boiling point of methanol, 64.7 C.) for the purpose of softening the molasses film. We have shown, however, that the use of anhydrous conditions is undesirable, since it leads to a precipitation of sucrose from the molasses film adhering to crystals of raw sugar (such precipitated sucrose passes intothe molasses washings from which it cannot be easily recovered). Furthermore, as noted above, we have established that washing can be conducted satisfactorily at temperatures not in excess of 40 C.

The following examples illustrate the preliminary afii nation of raw sugar by washing with aqueous alcohol solvents according to the method of the invention. The yields are consistent with a crystalline sucrose dissolution of between about 2.56%.

EXAMPLE I An aqueous ethanol washing syrup was made by dissolving 101 g. of a cane raw sugar with a mixture of 195 mls. water and 574 mls. ethanol (i.e. an aqueous ethanol containing 70% ethanol by weight). This solution was then mixed with 2,000 g. of a raw sugar at 25 C. for one minute. Washed sugar crystals were recovered from the slurry by centrifugation and the insoluble residue was filtered out of the raw washings. Analyses of the raw sugar, washed sugar and insoluble residue are given below.

Raw sugar: Percent Reducing sugar 0.39 Ash 0.24

Yield (a) washed sugar, 1,933 g.:

Reducing sugar 0.03 Ash 0.09 Yield (b) insolubles, 1.2 g. ash 35 EXAMPLE II A sample of beet raw sugar was treated as in Example I. Analyses of the raw sugar, washed sugar and insoluble residue are given below.

Raw sugar: Percent Reducing sugar 0.008 Ash 0.62

Yield (a) Washed sugar:

Reducing sugar 0.004 Ash 0.08 Sucrose recovery 94 Yield (b) Insolubles: Substantially no insolubles.

EXAMPLE III An aqueous ethanol washing syrup was made by dissolving 75 g. of a cane raw sugar in a mixture of 150 g. water and 350 g. ethanol (i.e. an aqueous ethanol containing 70% ethanol by weight). This solution was then mixed with 1,060 g. of the raw sugar at 22 C. for one minute. Washed sugar crystals were recovered from the slurry by centrifugation and the insoluble residue was filtered out of the raw washings. Analyses of the raw sugar, washed sugar and insoluble residue are given below.

Raw sugar: Percent Reducing sugar 0.51 Ash 0.46

Yield (a) Washed sugar, 1005.5 g.:

Reducing sugar 0.06 Ash 0.15 Yield (b) Insolubles: 1.06 g. ash 35 The invention also provides an alternative or ancillary method of producing defined sugar from a sugar consisting of sucrose crystals bearing a film of molasses, said method including the steps: dissolving the sugar in a selected solvent at a temperature of between about 70 and 150 C. under a pressure suflicient to prevent boiling, and separating insoluble impurities from the hot liquor; said solvent consisting of 15-40% by weight of water and -60% by weight of a selected alcohol; said alcohol having a solvent selectivity for sucrose in raw sugar greater than that of water, and being selected from the group consisting of those alcohols and substituted alcohols which are freely miscible with water at all concentrations to give a single phase binary solution from which a second liquid phase is not separated by the dissolution therein of sucrose.

Depending on the desired quality of the product, the sugar material processed by the above methods of dissolution may be a crude raw sugar (bearing a viscous film of highly impure molasses) or a raw sugar that has already been washed (bearing a diluted film of molasses). Sugar of highest refined quality cannot of course be recovered from crude raw sugars by applying merely the dissolution method of the invention; nevertheless, such products-even if somewhat inferiorare useful for many purposes.

When raw sugars are refined by a two-stage process involving both washing and dissolution according to the invention, it is preferred to employ the same aqueous alcohol combination in both stages of the process. Alcohol to water ratios preferred for dissolution are the same as those already noted for washing, viz, in the concentration range 7565% by weight in the case of unsubstituted alcohols and in the concentration range 85-75% by weight in the case of substituted alcohols.

Loss of alcohol by evaporation is at least minimised according to the invention by ensuring that dissolution is carried out under non-boiling conditions. If this cannot be achieved under atmospheric pressure at the selected dissolution temperature, higher pressures are imposed. The absolute solubility of sucrose in aqueous alcohols rises sharply with increased temperature, hence it is preferred to employ temperatures as near to the defined upper limit as practicable. However, selected temperatures usually do not exceed 120 C., since otherwise adverse colour formation and undue inversion may occur. Dissolution may often be performed at temperatures higher than 120 C., if the period of retention at these temperatures is suitably short. The required time period for dissolution may of course be reduced by carrying out the process on a sugar that has been ground to a small particle size.

At any given temperature, the ratio of Weight of sol vent to raw sugar required for dissolution is of course determined only by solubility consideration.

It will be appreciated that the refined sugar produced by the above dissolution method may be recovered, as desired, in liquid or crystalline form. A liquid sugar may be obtained from the purified hot liquor by steps including distillation to remove the alcohol content; alternatively, a crystalline sugar may be obtained from the hot liquor by steps including cooling, crystallization and separation of sugar crystals from the resulting magma. The hot sugar solutions may be treated with a discolourising agent (eg. activated charcoal) prior to recovering the product in either of these forms.

When forming a solid product, the hot liquor is preferably cooled by counter-current heat exchange and is then allowed to crystallize. A controlled degree of cooling ensures a steady rate of crystal growth and an even-sized product. As is well known, the average crystal size of the product may also be controlled by deliberately seeding the solution with a selected quantity of small crystals.

The supersaturated mother liquor has a low viscosity and the crystalline product can be readily separated from it by centrifugation or filtration.

The crystallization step may be split into two or more stages in series, so that no stage operates under such a large temperature reduction that false grain becomes troublesome or heat exchange surfaces become fouled by crystal growth.

The separated crystals may be dried without further treatment; alternatively, they may be redissolved in a further quantity of aqueous alcohol and a more refined product can then be obtained by recrystallization.

The crystals are dried preferably by means of a hot air stream, and provision is made for alcohol recovery from the hot air stream. Alternatively, a vacuum drying technique may be employed. The dried crystals when cool are ready for packing.

The mother liquor separated from the crystallized sugar can be further treated to recover additional sucrose (e.g. by cooling to sub-ambient temperature or by diluting with extra alcohol). Alternatively, it may be used as an aqueous alcohol syrup in the described Washing process of the invention.

The alcohol may be recovered from the exhausted mother liquor by stripping in a distillation column, and the reclaimed alcohol is retained for re-use. If sufliciently pure, the aqueous syrup remaining after distillation may be used as a basis for liquid sugar manufactured; if not sufiiciently pure, it may be further exhausted of its su crose content by conventional recrystallization techniques or may be discarded as conventional molasses having an impurities to sucrose ratio of 1:1.

The following two examples illustrate the purification of raw sugar by steps including dissolution in hot aqueous alcohol solvents according to the method of the invention.

EXAMPLE IV Solution was made by dissolving 250 g. of a Washed cane sugar at 99 C. in a mixture of g. of water and 320 g. of Z-methoxy-ethanol (i.e. an aqueous 2-methoxy-ethanol containing 80% by weight of the alcohol). After centrifuging to remove insoluble matter, the solution was reheated; 10 g. of 48/ 65 sugar was added for seeding purposes and the mixture was allowed to crystallize by cooling slowly, with stirring, to 25 C. The product was recovered by centrifugation.

Yield (a) Product sugar, 195 g.:

Size of crystals, diameter mm 0.5 Reducing sugar percent 0.004 Ash do 0.006 Yield (b) Insolubles 0.30 g.:

Ash do 35 EXAMPLE V A solution was made by dissolving 260 g. of a washed beet raw sugar at 70 C. in a mixture of 120 g. of water and 280 g. of ethanol (i.e. an aqueous ethanol containing 70% by weight of ethanol). The solution was filtered to remove precipitated impurities, small seed sugar crystals were added and the mixture was allowed to crystallize by cooling slowly, with stirring to 30 C.

Yield (a) Product sugar, 17 g.

Size of crystals, diameter mm 0.5 Reducing sugar percent 0.002 Ash do 0.005

Yield (b) Insolubles, negligible.

The invention additionally provides a method of recrystallizing a sugar consisting of substantially pure sucrose crystals, said method including the steps: dissolving the sugar in a selected solvent at a temperature of between about 70 C. and 150 C. under a pressure sufficient to prevent boiling, cooling the liquor to recrystallize the sucrose, and separating sugar crystals from the liquor; said solvent consisting of 1540% by weight of Water and -60% by weight of a selected alcohol; said alcohol having a solvent selectivity for sucrose in raw sugar great er than that of water, and being selected from the group consisting of those alcohols and substituted alcohols which are freely miscible with water at all concentrations to give a single phase binary solution from which a second liquid phase is not separated by the dissolution therein of sucrose.

The following example illustrates the application of this aspect of the invention to obtain a sucrose product of fine particle size.

EXAMPLE VI A solution was made by dissolving 1,200 g. of refined white sugar at C. .(40 p.s.i.g.) in a solvent consisting of 700 g. ethanol and 300 g. Water (i.e. an aqueous alcohol containing 70% alcohol). A fondant slurry was added to seed the mixture as it was cooling to 40 C. The product was recovered by filtration.

Product sugar yield, 940 g.

Size of crystals mm. diameter 0.1

Insoluble sludge materials resulting from the practice of the invention may be treated with hot aqueous alcohol solvent to recover the free sucrose occluded therein. Residual sludge materials after solvent removal (e.g. by distillation) are found on analysis to have an impurities to sucrose ratio of between :1 and 1. Such materials are discarded.

The final insoluble fraction comprises organic materials and mineral matter in the proportions roughly 2:1, the organic materials portion being a potential source of such materials as higher saccharides, proteins, waxes, etc.

The herein described process of refining and recrystallizing sugars by means of selected aqueous alcohols may be applied (independently or in combination) to yield products having greater purity and/or smaller crystal size than was hitherto feasible using the non-alcoholic solvents and methods known to the prior art. The invention has enabled closer control in the process of dissolution and recrystallization (continuous crystal separation now being possible) and all manufacturing costsboth capital and operating-can be reduced.

We claim:

' 1. A method of producing refined sugar from a sugar consisting of sucrose crystals bearing a film of molasses, said method including the steps: washing the sugar at a temperature of between about 20 and 40 C. in a first selected solvent comprised of water and an alcohol selected from the group consisting of methanol, ethanol, furfuryl alcohol, Z-methoxy-ethanol, and Z-ethoxy-etha- 1101, until the molasses film is substantially removed from the crystals, separating the cleaned crystals from the washings, dissolving the cleaned crystals in a second selected solvent comprised of water and an alcohol se lected from the group consisting of methanol, ethanol, furfuryl alcohol, Z-methoxy-ethanol, and 2-ethoxy-etha- 1101, at a temperature of between about 70 and 150 C. under a pressure sufficient to prevent boiling, separating insoluble impurities from the hot liquor, cooling the hot liquor to crystallize the sucrose, separating the refined crystallized sugar from the mother liquor and drying the separated crystals; said first selected solvent being substantially saturated with sucrose dissolved in a combination consisting of 15-40% by Weight of water and 85- -60% by weight the selected alcohol; said alcohol having a solvent selectivity for surcrose in raw sugar greater than that of water, and being selected from the group consisting of those alcohols and substituted alcohols which are freely miscible with water at all concentrations to give a single phase binary solution from which a second liquid phase is not separated by the dissolution therein of sucrose.

2. A method of producing refined sugar according to claim 1 in which the solvent used to wash the crystals is recycled mother liquor from the crystallization step.

References Cited 824,929 12/ 1951 Germany.

MORRIS O. WOLK, Primary Examiner.

DAVID G. CONLIN, Assistant Examiner.

US. Cl. X.R. 12758, 63