The present invention relates to a process for purifying a carbohydrate composition and to an apparatus for performing such a process according to the invention. For the general purification of aqueous solutions, various methods are known, for example electrofiltration and electrodialysis.

Electrofiltration is a process for removing or concentrating colloidal substances, for example biopolymers. The principle of electrofiltration is based on superimposition of an electrical field on a standard dead-end filtration. As a result, given skilful polarization, an electrophoretic force acting on the generally charged biopolymers opposes the resistance force of the filtrate flow. As a result, covering layer buildup on the micro- or ultrafiltration membrane is drastically reduced and the filtration time is reduced from several hours in the case of a filtration to a few minutes in the case of an electrofiltration. Compared to cross-flow filtration, however, electrofiltration does not just exhibit a greater permeate flow, but is also a particularly gentle removal process due to the low shear force stress on the frequently sensitive biopolymers. Use in biotechnological product processing is promising, since biopolymers are firstly difficult to filter, but secondly are charged due to the frequent presence of amino or carboxyl groups. The aim in electrofiltration is to counteract cake buildup in order to improve the filtration kinetics of products which are difficult to filter. If an electrical field is superimposed on the filtration process, there is electrophoresis of the particles and electroosmosis. In the case of electrofiltration, an electrical field (DC) is superimposed on the conventional filtration and acts in parallel to the flow direction of the filtrate. If the electrophoretic force F _E opposed to the flow exceeds the hydrodynamic resistance force F _w , particles move away from the filter medium, such that the thickness of the filtercake on the membrane is distinctly reduced. If the solid particles to be removed are negatively charged, these migrate to the anode (plus pole) and are deposited on the filter cloth there. On the membrane on the cathode side (minus pole), only a very thin covering layer is accordingly formed, and so virtually the entire filtrate flows through this membrane.

Electrodialysis, as disclosed, for example, in GB 1 ,296,244, is an electrochemically driven membrane process in which ion exchange membranes are utilized in combination with an electrical potential difference in order to remove ionic species from uncharged solvents or impurities. In an electrodialysis separator, the space between two electrodes is divided by a stack of alternating anion and cation exchange membranes. Each pair of ion exchange membranes forms a separate cell. In industrial systems, these stacks consist of more than 200 membrane pairs. If an electrical direct current is applied to the electrodes, the anions migrate to the anode. The anions can pass through the positively charged anion exchange membranes, but are stopped in each case at the next negatively charged cation exchange membrane. Since the same also happens with the cations, with the reverse sign, the net effect of electrodialysis is an enrichment of the salts in the cells. The solutions with elevated salt concentration are combined to give the concentrate, while the low-salt solutions form the diluate. Moreover, GB 1 ,296,244 discloses a process for desalinifying a sugar solution, wherein the sugar solution is subjected to an electrodialysis between a non-selective ion-permeable membrane having an ion exchange capacity of less than 0.3 meq/gram of dry membrane, and a selective cation-permeable membrane.

Commercial carbohydrates, especially sugars, as used for food purposes, already has comparatively high purity as a result of the industrial processing. In the case of production of an aqueous carbohydrate solution, especially of an aqueous sugar solution, for example by means of ion exchangers, the ions present therein can be converted to a high-purity form. For the production of high-purity silicon, a high-purity sugar source, for example a high-purity sugar solution, is a known necessity.

A disadvantage of this process is that a multistage workup using various specific ion exchangers for various cations and anions has to be selected. Moreover, it is only possible to use dilute aqueous solutions, for example 50% by weight, since concentrated carbohydrate solutions, especially concentrated sugar solutions, have too high a density and too high a viscosity, and are no longer processable in ion exchange processes.

A further disadvantage of processes which work with ion exchanger membranes is that the ion exchangers have to be regenerated, and so continuous operation is possible only using several membrane cells, of which one or more in each case are in operation, while one or more further cells are being regenerated.

It is also known that, for example, carbon obtained in high-purity form from carbohydrates and/or sugars can be used as a reducing agent in order to reduce likewise high-purity silicon dioxide (S1O ₂ ) to still high-purity silicon (Si). Given sufficient purity, the Si obtained can be used as solar Si (99.9999%), and in the case of particularly high purity also as what is called electronics-grade Si.

A further disadvantage is that carbohydrate solutions and/or sugar solutions purified by means of ion exchangers subsequently have to be concentrated again with high energy expenditure in order to be able to convert them, for example together with high-purity silicon dioxide (S1O ₂ ), to shaped bodies from which high-purity silicon (Si) is formed in subsequently pyrolysis and reduction processes. Every component step, including the concentration, as well as the energetically unfavourable aspect, also includes the risk of contamination with extraneous atoms.

In view of the prior art, it is thus an object of the present invention to provide a process for purifying carbohydrate compositions, especially sugar solutions, which lead to particularly pure products with a high yield.

More particularly, the process according to the invention is to enable provision of a high-purity carbohydrate composition, especially of a high-purity sugar, as a suitable carbon source for obtaining high-purity silicon. Moreover, a suitable process is to be provided for removing anions and cations from a concentrated aqueous carbohydrate solution, especially a concentrated aqueous sugar solution.

Also, if at all possible, a process is to be provided which, particularly within a short time, leads to a high amount of purified carbohydrate composition. Accordingly, the process was to be performable very rapidly.

In addition, the process was to be performable in a very simple and inexpensive manner. More particularly, it was to be possible to use a very highly concentrated carbohydrate composition.

Furthermore, the process was to be performable with a minimum number of process steps, and these were to be simple and reproducible.

In addition, the performance of the process was not to be associated with endangerment of the environment or of human health, and so it was to be possible to essentially dispense with the use of substances hazardous to health or compounds which could be detrimental to the environment.

Further objects which are not stated explicitly are evident from the overall context of the description, examples and claims which follow.

These objects, and further objects which are not stated explicitly but are immediately derivable or discernible from the connections discussed herein by way of introduction, are achieved by a process having all features of Claim 1 . Appropriate modifications to the process according to the invention are protected in dependent Claims 2 to 1 1 . Moreover, Claim 12 has for its subject-matter an apparatus for performing such a process according to the invention, while appropriate modifications to this inventive apparatus are protected in dependent Claims 13 to 15. The present invention accordingly provides a process for purifying a carbohydrate composition, comprising an electrochemical treatment of this composition in an apparatus having at least two membranes, one anode and one cathode, the membranes forming at least three compartments, with the anode in an anode compartment, the cathode in a cathode compartment and an intermediate compartment between these compartments, characterized in that the carbohydrate composition is passed through the intermediate compartment and both membranes have an ion exchange capacity of less than 0.3 meq/g.

The process according to the invention can additionally achieve advantages including the following: More particularly, it is possible in an unforeseeable manner to provide a process of the direct type explained above, which has a particularly good profile of properties. It is surprisingly possible to obtain especially carbohydrate compositions, more preferably sugar solutions, having a particularly high purity with a very good yield.

Thus, it has been found that, surprisingly, the process according to the invention can provide a high-purity carbohydrate composition, especially a high-purity sugar, as a suitable carbon source for obtaining high-purity silicon.

A surprising advantage has been found here to be that, in contrast to the two known electrochemical processes described in the literature for processing of solutions or colloidal systems, a suitable process for removal of anions and cations from a concentrated aqueous carbohydrate solution, especially a concentrated aqueous sugar solution, can be provided.

Moreover, the low electrical conductivity of the sugar solution enables a much shorter process duration. The application of a high electrical field with simultaneously low current promotes this surprising advantage of the process according to the invention. This surprisingly allows ion migration to be accelerated, the carbohydrate molecules, especially the sugar molecules, being uncharged and as a result immobile in the electrical field.

The process according to the invention is thus also based on the surprising finding that use of ion exchange membranes, which is necessary in the case of electrodialysis, is not required in the process according to the invention, and high voltages in particular can cause destruction of the membranes. Moreover, the present process does not require any regeneration of the ion exchangers used, and so continuous operation is surprisingly possible as a result, without any requirement for multiple execution of the purifying apparatus.

Furthermore, the process according to the invention can be performed in a very simple and inexpensive manner.

Furthermore, the process can be performed with relatively few process steps, these being simple and reproducible. In addition, the performance of the process is not associated with any endangerment of the environment or of human health, and so it is possible to essentially dispense with the use of substances hazardous to health or compounds which could be detrimental to the environment.

Carbohydrates or components of the carbohydrate composition used in the process according to the invention are preferably monosaccharides, i.e. aldoses or ketoses, such as trioses, tetroses, pentoses, hexoses, heptoses, particularly glucose and fructose, but also corresponding oligo- and polysaccharides based on said monomers, such as lactose, maltose, sucrose, raffinose - to name just a few, or derivatives thereof - up to and including starch, including amylose and amylopectin, the glycogens, the glycosans and fructosans - to name just a few polysaccharides.

It is particularly preferable for the carbohydrate composition to comprise a mono- and/or disaccharide.

In the process according to the invention, it is also possible to use mixtures of at least two of the aforementioned carbohydrates as the carbohydrate or carbohydrate component.

Particular preference is given in the process according to the invention to purifying a carbohydrate composition comprising at least one carbohydrate, especially at least one sugar, in dissolved form in aqueous solution. On completion of the purification, the purified carbohydrate compositions can be obtained in solid, high-purity form from the aqueous solution, for example by means of the evaporation of the aqueous solvent, by the process useable in accordance with the invention.

A pure carbohydrate features, in solid form, a content of: a. aluminium less than or equal to 5 ppm, or preferably between 5 ppm and 0.0001 ppt, especially between 3 ppm and 0.0001 ppt, preferably between 0.8 ppm and 0.0001 ppt, more preferably between 0.6 ppm and 0.0001 ppt, even better between 0.1 ppm and 0.0001 ppt, even more preferably between 0.01 ppm and 0.0001 ppt, even more preference being given to 1 ppb to 0.0001 ppt, b. boron less than 10 ppm to 0.0001 ppt, especially in the range from 5 ppm to 0.0001 ppt, preferably in the range from 3 ppm to 0.0001 ppt or more preferably in the range from 10 ppb to 0.0001 ppt, even more preferably in the range from 1 ppb to 0.0001 ppt, c. calcium less than or equal to 2 ppm, preferably between 2 ppm and 0.0001 ppt, especially between 0.3 ppm and 0.0001 ppt, preferably between 0.01 ppm and 0.0001 ppt, more preferably between 1 ppb and 0.0001 ppt, d. iron less than or equal to 20 ppm, preferably between 10 ppm and 0.0001 ppt, especially between 0.6 ppm and 0.0001 ppt, preferably between 0.05 ppm and 0.0001 ppt, more preferably between 0.01 ppm and 0.0001 ppt and most preferably 1 ppb to 0.0001 ppt; e. nickel less than or equal to 10 ppm, preferably between 5 ppm and 0.0001 ppt, especially between 0.5 ppm and 0.0001 ppt, preferably between 0.1 ppm and

0.0001 ppt, more preferably between 0.01 ppm and 0.0001 ppt and most preferably between 1 ppb and 0.0001 ppt, f. phosphorus less than 10 ppm to 0.0001 ppt, preferably between 5 ppm and 0.0001 ppt, especially less than 3 ppm to 0.0001 ppt, preferably between 10 ppb and 0.0001 ppt and most preferably between 1 ppb and 0.0001 ppt, g. titanium less than or equal to 2 ppm, preferably less than or equal to 1 ppm to 0.0001 ppt, especially between 0.6 ppm and 0.0001 ppt, preferably between 0.1 ppm and 0.0001 ppt, more preferably between 0.01 ppm and 0.0001 ppt and most preferably between 1 ppb and 0.0001 ppt, h. zinc less than or equal to 3 ppm, preferably less than or equal to 1 ppm to 0.0001 ppt, especially between 0.3 ppm and 0.0001 ppt, preferably between 0.1 ppm and 0.0001 ppt, more preferably between 0.01 ppm and 0.0001 ppt and most preferably between 1 ppb and 0.0001 ppt.

A high-purity carbohydrate is notable in that the sum of the abovementioned impurities in the solid form of the carbohydrate is less than 10 ppm, preferably less than 5 ppm, more preferably less than 4 ppm, even more preferably less than 3 ppm, especially preferably 0.5 to 3 ppm and very especially preferably 1 ppm to 3 ppm. For each element, the aim may be a purity in the region of the detection limit.

The determination of impurities in the solid carbohydrate is performed by means of ICP-MS/OES (inductively coupled spectrometry - mass spectrometry/optical electron spectrometry) and AAS (atomic absorption spectroscopy).

An ion exchange capacity of less than 0.3 meq/g, preferably of less than 0.2 meq/g and more preferably of less than 0.1 meq/g, based on the dry weight of the membrane, has been found to be appropriate since there is severe degradation of the membrane in the case of higher anion exchange capacities of the membrane(s) during an electrodialysis, while there is a severe loss of current efficiency in the case of higher cation exchange capacities of the membrane(s). The ion exchange capacity can be measured in a known manner by means of titration methods, and reference is made in this context to standards ASTM D2187-94 and ASTM D2687 inter alia.

The pore diameters of the membrane may be within a wide range, very small pores leading to good retention of the carbohydrate used, but delaying the transfer of the ions through the membranes, such that a longer period is required for the achievement of a given purity. Surprising advantages can be achieved by virtue of the membranes used having a pore diameter of preferably not more than 500 nm, especially not more than 150 nm. In this context, the pore diameter may preferably be at least 2 nm, more preferably at least 10 nm and especially preferably at least 30 nm.

Preferably, the membranes used in the inventive apparatus are formed essentially from carbon, hydrogen, oxygen and nitrogen.

In this case, the membranes may preferably have a thickness in the range from 1 to 1000 μιτι, especially preferably in the range from 10 to 500 μιτι and more preferably in the range from 50 μιτι to 300 μιτι.

Membranes for use in the inventive apparatus may be composite membranes, cellulose membranes, polyamide membranes, especially nylon membranes, polyvinylidene fluoride (PVDF) membranes, membranes of modified or unmodified polyacronitrile (PAN) or membranes of polysulphone or polyether sulphone, preference being given to composite membranes and polyamide membranes, especially nylon membranes. In this case, the membranes may be those suitable for nanofiltration or ultrafiltration, preference being given to membranes for ultrafiltration for the inventive apparatus.

In this case, furthermore, the exclusion limit, called the "molecular weight cut-off' (MWCO) value, of the membranes used, which is defined as the minimum molecular mass of a molecule which is retained by the membranes used, in the inventive apparatus, may be at least 50 daltons, preferably at least 100 daltons, more preferably at least 250 daltons and especially preferably at least 500 daltons. In a further embodiment, a membrane may be used in the inventive apparatus whose minimum molecular mass is preferably at most 10 000 daltons, more preferably at most 5000 daltons and more preferably at most 2500 daltons. These values relate to hydrophilic compounds, especially polyethylene glycols and/or sucrose, the measurement of these values being known from the literature and being reported as standard for commercial products.

The membranes used in the inventive apparatus are preferably essentially commercially available, for example from DMS Desalogics Membrane Supplies GmbH or Pall Corporation. The product datasheets for preferred membranes state the MWCO values; for example, the MWCO value of the GE ultrafiltration composite membrane is stated to be 1000 Da-PEG. The value for the DK nanofiltration composite membrane is stated to be about 150 to 300 Da, measured with sucrose.

It may be preferable in the process according to the invention for the carbohydrate composition to comprise at least 50% by weight, especially at least 60% by weight, preferably at least 70% by weight and especially preferably at least 80% by weight of carbohydrate.

In addition, the flow mechanics of the carbohydrate composition may also be in the laminar range. For the hydraulic diameter, the geometry of the inventive apparatus and the flow rate can thus preferably be used to give a Reynolds number, which represents the ratio of inertial forces to viscous forces, of less than 3000, preferably less than 2500 and more preferably less than 2300.

Should the Reynolds number exceed the above value of 3000, the flow, which has been laminar until that point, will be prone to disruption and could, as a consequence, switch from the desired laminar flow to an unwanted turbulent flow. ln a preferred embodiment of the process according to the invention, an aqueous flush composition comprising a carbohydrate composition may be passed into the anode compartment and/or the cathode compartment. In a preferred embodiment, the concentration of the carbohydrate composition introduced into the anode compartment and/or the cathode compartment may be lower than the concentration of the carbohydrate composition passed into the intermediate compartment. The concentration difference here may preferably be at least 1 g/l, more preferably at least 5 g/l.

According to the invention, the purified carbohydrate composition may have a conductivity less than 1 .0 S/cm, preferably less than 0.5 S/cm and more preferably less than 0.1 S/cm.

Due to the influence that the temperature has on the viscosity of a carbohydrate composition, during the process for purifying the carbohydrate composition, in accordance with the invention, a temperature of at least 20°C, preferably at least 40°C and more preferably at least 50°C may be ensured. The maximum temperature during the purification of the carbohydrate composition results from the maximum thermal stability of the membranes used and should be not more than 95°C, preferably not more than 90°C and more preferably not more than 70°C. The application of a higher temperature can surprisingly distinctly reduce the viscosity of the carbohydrate composition and simultaneously distinctly improve the ion mobility, which leads to an improved purification outcome.

It may also be preferable for no ion exchange membrane having an ion exchange capacity greater than 0.3 meq/g to be arranged between the membranes having an ion exchange capacity of less than 0.3 meq/g. Furthermore, the process according to the invention may be characterized in that a voltage of preferably at least 10 V, more preferably at least 50 V, especially preferably 100 V, particularly preferably at least 500 V and specially preferably at least 1000 V is applied between the anode and the cathode.

The current during the process according to the invention, for the above-detailed reasons of a maximum ion acceleration to be achieved if at all possible through a maximum applied field, should be kept to a minimum. The current density should merely be sufficiently high in order still to be able to perform an electrochemical treatment of the carbohydrate composition. The current density should accordingly preferably be at most 20 mA/cm ² , especially at most 10 mA/cm ² , preferably at most 5 mA cm ² , and more preferably at most 1 mA/cm ² . The current depends on the size of the plant, and is, for example, preferably at most 1000 mA, especially at most 500 mA, preferably at most 20 mA and more preferably at most 5 mA. This achieves improved removal performance for ionic impurities, for example alkali metal, alkaline earth metal, nitrate and/or sulphate ions. In order to be able to achieve particularly good purification of the carbohydrate composition, especially of the concentrated aqueous sugar solution, it may be preferable for the distance over which the electrochemical purification is effected to be preferably at least 0.1 m, more preferably at least 0.3 m and especially preferably at least 0.5 m. The maximum distance over which the purification is effected results particularly from economic considerations, surprising advantages being achievable by a distance over which the electrochemical purification is effected of preferably at most 10 m, more preferably at most 5 m and especially preferably at most 2 m.

In a particular embodiment of the process according to the invention, moreover, the ratio of distance over which the electrochemical purification is effected to the distance between the membranes may preferably be in the range from 1 :1 to 10 000:1 , more preferably in the range from 5:1 to 1000:1 and especially preferably in the range from 10:1 to 500:1 .

In a particular configuration, the purification can be conducted in a loop. In this case, the ratio of the volume supplied and withdrawn in the loop to the volume of the circulation stream may preferably be in the range from 1 :2 to 1 :100, more preferably 1 :3 to 1 :50 and especially preferably in the range from 1 :4 to 1 :20.

The residence time in a clearing system with a circuit may preferably be within the range from 1 minute to 2 days, preferably 30 minutes to 24 hours and more preferably 1 hour to 12 hours. The present invention further provides a preferred apparatus for performance of the process according to the invention. An inventive apparatus has at least two membranes having an ion exchange capacity of less than 0.3 meq/g.

Preference is given here especially to membranes having a molecular weight cut-off (MWCO) value of at least 50 daltons, preferably at least 100 daltons, more preferably at least 250 daltons and especially preferably at least 500 daltons. In a further embodiment, a membrane used in the inventive apparatus may have a minimum molecular mass of preferably at most 10 000 daltons, more preferably at most 5000 daltons and more preferably at most 2500 daltons. The pore sizes of preferred membranes have been detailed above. In a particular embodiment of the present apparatus, it has electrodes which have, as the cathode material, preferably stainless steel or graphite and/or, as the anode material, preferably platinum, and it may be particularly preferable to provide a DSA (dimensionally stable anode) as the anode.

Particularly preferred membranes have been detailed above, and so reference is made thereto.

In addition, the apparatus may preferably be constructed such that the carbohydrate composition to be purified is conducted in a loop.

A preferred embodiment of this apparatus is characterized in that the distance between the membranes is preferably at most 10 cm, more preferably at most 5 cm and especially preferably at most 3 cm. The minimum distance between the membranes arises particularly from economic considerations, surprising advantages being achievable by a distance of preferably at least 0.1 cm, more preferably at least 0.3 cm and especially preferably at least 0.5 cm.

For illustration, Figure 1 shows a schematic section from an illustrative apparatus for purification of a carbohydrate composition.

The apparatus shown schematically in Figure 1 comprises an anode 1 and a cathode 2, which may also be arranged within one cell, such that a further wall may be necessary in order to form a corresponding cell. In addition, the apparatus has two membranes 3, such that at least three compartments are formed, namely an anode compartment 4, a cathode compartment 5 and an intermediate compartment 6 between these compartments.

The present impurities in the form of cations and anions present in dissociated form in a concentrated sugar solution are removed here through the process useable in accordance with the invention, which is shown in Figure 1 , by means of an electrical field. Since the sugar molecules are much larger compared to the ions to be removed, it is possible to use a corresponding membrane 3, also called a diaphragm. Subsequently, as in the electrodialysis, the ions are enriched in the anode compartment 4 or cathode compartment 5. Figure 1 thus shows the migration of anions in the direction of the anode using the example of nitrate (NO3 ^" ) and sulphate anions (SO ₄ ^2" ), while the migration of cations in the direction of the cathode is illustrated using the example of sodium, calcium and boron cations.

Figure 1 shows, using the example of a membrane with an appropriate pore size, that the (ionic) impurities can pass through the membrane. The impurities are removed here via the two outer compartments 4 and 5, also called chambers, while the product is passed through the intermediate compartment 6. This makes it possible to attain, in the final sugar solution, final concentrations of cations and anions much less than 1 ppm. The carbohydrate compositions obtained by the process useable in accordance with the invention, especially the concentrated aqueous carbohydrate solutions obtained in this way, more preferably the sugars and/or concentrated aqueous sugar solutions obtained in this way, consequently have a very high purity.

In addition, it is possible to convert these end products obtained by the process useable in accordance with the invention, for example by a pyrolysis process, to an amorphous carbon pyrolysis product having a high purity, which comprises carbon, especially with graphite components and optionally components of other carbon forms such as coke.

It is thus especially possible to obtain a product which is particularly low in impurities, for example B, P, As and Al compounds. Such an inventive pyrolysis product can advantageously be used as a reducing agent in the production of silicon, especially metallurgical silicon, or even solar silicon, from silicon dioxide at high temperature. More particularly, the inventive graphite- containing pyrolysis product, due to its conductivity properties, can be used in a light arc reactor.

In principle, the pyrolysis product, however, can also be used in all other fields of use in which pure carbon is required, for example in metal carbide production (boron carbide, silicon carbide, etc) or the production of graphite mouldings, preferably electrodes, especially high-purity electrodes, carbon brushes, heating elements, heat exchangers, or as a carburizing agent for steel, or in diamond production, or as a reducing agent in hard metal production (W, Mo, Cr, Ti, Ta, Co, V, etc.), or in zirconium production, or as a blanket for metal melts, or as a substitute for woodchips in metallurgical processes.

These fields of use are described in detail in documents WO 2010/037692 A1 , WO 2010/020535 A2, WO 2010/037699 A2 and WO 2010/037694 A2 inter alia, and the disclosures of these documents are incorporated into the present application by reference thereto.

The invention is illustrated hereinafter by an example, without any intention that this should impose a restriction. Example 1 :

In an apparatus shown schematically in Figure 1 , a sugar solution was purified at room temperature (25°C), and the cathode used was made of stainless steel and the anode used was a DSA (dimensionally stable anode). The carbohydrate composition to be purified was conducted in a loop. In addition, a carbohydrate composition having a concentration of 60% by weight was introduced into the anode and cathode compartments. A membrane was used here which is obtainable from Pall under the Ultipor Nylon NTG name. The individual technical data are listed below: Electrode area 600 cm ²

Number of cells 1

Current 0.13 A

Current density 0.00022 A/cm ²

Voltage 100 V

Electrode separation 3 cm

Power 13 W

Energy input 78 Wh

For the sugar solution to be purified, which has a density of 1 .6 kg/I, the following data also apply:

Volume flow rate 480 l/h

Starting concentration of sugars 70 % by wt

Final concentration 68 % by wt

Residence time 6 h

Initial charge (V) 10 1

Initial charge (m) 1 1 .2 kg

Total mass flow rate 1 .87 kg/h

Sugar mass flow rate 1 .27 kg/h

Sugar loss 0.025 kg/h

MATY 0.076923 m ² /(h ^* A)

(Membrane area-time yield) The successful performance of the purification of this sugar solution is shown by the following data:

Starting concentration (70% by weight sugar solution)

Sodium 2.5 g/g

Nitrate 0.32 pg/g

Nitrite 0.08 pg/g

Chloride 0.41 pg/g

Phosphate 0.1 g/g

Sulphate 1 .25 g/g

Final concentration (68% by weight sugar solution)

Sodium < 1 g/g

Nitrate < 1 g/g

Nitrite < 1 g/g

Chloride < 1 g/g

Phosphate < 1 g/g

Sulphate < 1 g/g

It is clearly evident here that the initial values of the starting concentrations still present in the sugar solution for the different individual ions decline significantly in the course of the process according to the invention, and at the end of the process are all in the region below one microgram per gram.

The process useable in accordance with the invention thus enables the purification of highly concentrated sugar solutions which may have, for example, a concentration in the range of 800-1000 g/l, such that these solutions can be used without a further water depletion in ensuing steps.

The process according to the invention is defined by the characterizing features of the appended claims.