Field of the invention The present invention relates to the recovery of lactose from liquids formed as by-products in the dairy industry and related industries.

More specificly, the invention relates to a pro- cess and a plant for converting lactose-containing liquids into easy to handle particulate products hav- ing non-caking and free-flowing characteristics.

In the present specification and the attached claims the term"lactose-containing liquids"is used as comprising solutions, wherein all lactose is in a dissolved state, as well as slurries of lactose cry- stals in lactose solutions.

Background of the invention and prior art Lactose is a disaccharide, which in solid state exists in at least three forms, viz. a-lactose, cry- stallizing as the monohydrate from aqueous solutions at temperatures below 93. 5°C, a-lactose, and amor- phous lactose.

Of these three forms, the a-lactose monohydrate is preferred since it when relatively pure forms stable, hard, and non-hygroscopic crystals.

In contrast thereto, especially the amorphous form of lactose is hygroscopic, and the presence of even minor amounts of the amorphous form on crystals of a- lactose monohydrate may impair the non-caking and free-flowing characteristics of the latter.

Lactose is present in a concentration of approxi- mately 5% in cow milk, which is the dominating source therefore.

Nearly all cheese-making processes have a by- product, whey, which is an aqueous solution, which,

besides nearly all the lactose originally present in the milk, contains some fat, protein, and inorganic salts. The term"whey"is here applied in a broad sense, comprising cheese whey, rennet casein whey, acid whey, and salty whey.

With the primary purpose of recovering protein values from the whey or from milk, this is often sub- jected to an ultra-filtration, whereby the proteins are obtained as retentate, whereas the lactose and most of the minerals remain dissolved in the water passing through the filter membrane. This aqueous so- lution of lactose with highly reduced protein content is simply termed"permeate". This is the lactose source preferred in connection with the present in- vention although other whey-related products come into consideration.

Before used as starting material in the exploita- tion of the present invention, such permeate or re- lated whey products may be subjected to a purifica- tion, e. g. a demineralization, and it is, preferably by means of vacuum evaporation, concentrated to a dry solids content of 40-75% by weight.

The main applications for lactose are as sweetener for ice cream mixes, for baking applications, and as component of animal feed but substantial amounts are also used as nutrient in culture media for micro- organisms in the bio-technical industries. For the baking industry it is, inter alia used with the pur- pose of obtaining a desired brown colour of the bread crust.

Although these various fields of application have different requirements as to purity and visual ap- pearance of the lactose, it is for all applications desired to use the lactose as a particulate non- sticking, non-hygroscopic, and non-caking free- flowing product.

Several processes have been suggested and used for converting lactose in whey or whey-derive products

into a particulate material.

However, a relatively fast drying process as ob- tained by spray drying results in particles, in which only a part of the lactose is present as a-lactose monohydrate, whereas the remaining part thereof is present in a form which makes the particles sticky and hygroscopic.

Thus, a certain rest time is necessary before the cooled product resulting from the drying process can form free-flowing particles.

However, even after such treatment the resulting particles show tendency of hygroscopicity and caking.

This principle of fast drying combined with a sub- sequent rest is utilized in the FILTERMAT process (Niro A/S) disclosed in US Patent No. 4,351, 849 and also in the process disclosed in US Patent No.

5,006, 204 (Assignee: A/S Niro Atomizer). In this last-mentioned process, a pre-crystallization is used before the spray drying, and the spray dried material rests on a rotating disc before after-drying in a fluidized bed.

A different concept is used in US Patent No.

6,335, 045, according to which the lactose-containing concentrate is heated to a temperature above the crystallization temperature and thereafter subjected to a flash process resulting in a concentrate of higher dry solids content than the starting concen- trate, which concentrate is subsequently cooled to induce crystallization. Afterwards, the crystallized concentrate can be dried in a spin-flash dryer. How- ever, even with this rather complicated process, the results are not always satisfactory in terms of cak- ing properties.

In the considerations preceding the present inven- tion, we assumed that if a higher percentage of the lactose in the particulate product were present as the a-lactose monohydrate form, the properties in the above-mentioned respects would be improved, and this

turned out to be correct.

Highly concentrated aqueous lactose solutions have high viscosity, and we assumed that this could be an important reason why the crystallization of a-lactose monohydrate in the prior art processes has been in- complete.

The invention is based on the recognition that it is possible by intensive agitation of the concentrate while it is being further concentrated by evapora- tion, due to the pseudo-plastic and thixotropic prop- erties of the concentrate, to reduce the viscosity thereof to an extent, which highly promotes crystal- lization. The beneficial effect of this decrease of viscosity on crystallization more than compensates for any negative effect on the crystallization pro- cess caused by the intensive agitation. Thereby it is achieved that formation and growth of crystals occur simultaneously with the evaporation of the concen- trate. Thereby, the viscosity is lowered, not only due to the pseudo-plastic and thixotropic character- istics of the concentrate mentioned but also because an excessive viscosity increasing super-saturation would be prevented.

It should be observed that generally the effect of mechanical agitation on the crystallization process is somewhat unpredictable. Reference is made to Mullin and Raven:"Influence of mechanical agitation on the nucleation of some aqueous salt solutions", Nature, Vol. 195, page 35-38 (1962). According to said paper, nucleation, which is a prerequisite for crystallization, is generally higher in a moderately agitated liquid than under quiescent conditions but with increasing intensity of agitation, nucleation decreased and at still further intensities increased again and finally some decrease was observed. In said paper it is proposed that this unpredictable effect is a combined result of the influence of the agita- tion on the diffusion and the attrition.

In most commercial crystallization processes where relatively large crystals are desired, such as in the sugar industry, crystallization is performed at rather gentle agitation of the crystallizing medium, e. g. by using so-called crystallization cradles. An intensive agitation is regarded as harmful, not only due to attrition but also because it may result in the formation of too many crystallization embryos causing too small and uneven crystals.

However, as mentioned above, it has turned out that due to the pseudo-plastic and thixotropic prop- erties of the lactose concentrates under evaporation an improved crystallization result is obtained by us- ing a mechanical agitation of high shear rate.

Summary of the invention The present invention thus relates to a method for evaporative concentration and crystallization of a viscous lactose-containing aqueous liquid, compris- ing: (a) introducing the lactose-containing aqueous liq- uid having a dry solids content of 40-75% by weight into a zone and therein subjecting it to the follow- ing simultaneous measures: (i) progressive heating at a temperature above 40°C but below the maximum temperature for crystalli- zation of a-lactose monohydrate from the liquid; (ii) removal of vapour evaporated from the liquid; and (iii) mechanical agitation able to provide a cry- stallization promoting decrease of the viscosity of the liquid with crystals formed and suspended therein; to progressively concentrate and simultaneously cry- stallize the agitated liquid, and (b) recovering the resulting slurry from said zone at a total solids content above 75% for cooling and optional disintegration and further drying.

By this process, optimal conditions for crystalli- zation of a-lactose monohydrate are achieved.

The mechanical agitation drastically decreases the viscosity of the liquid being evaporated, whereby lactose molecules in the only slightly super- saturated liquid easily diffuse to the surface of crystals and nuclei therefore to promote crystal growth meaning that the content of dissolved lactose and thus the degree of super-saturation of the liquid decreases.

Also the simultaneous progressive evaporation and crystallization ensures the existence of optimal con- ditions for crystallization over a prolonged period and over a broad range of temperature levels of the liquid.

The above measure"removal of vapour evaporated from the liquid"should be construed in a broad sense as covering not only active steps, such as applica- tion of reduced or increased pressure, but also the mere ensuring that such vapour can be vented.

The process is preferably performed as a continu- ous operation, wherein said lactose-containing aqueous liquid is introduced into one end of an elongated essentially horizontal zone; the heating in measure (i) is carried out by supply- ing a heating medium to a jacket surrounding at least a portion of said zone; said vapour evaporated from the liquid is in measure (ii) removed by purging said zone by passing an air flow therethrough; the agitation in measure (iii) is performed by rotat- ing an essentially horizontal shaft carrying agita- tion means to agitate the total volume of liquid pre- sent in said zone; and said slurry is recovered from the end of the elon- gated zone opposite to said one end.

In a typical embodiment of the process, the lac-

tose-containing aqueous liquid used as starting mate- rial is selected from the group consisting of concen- trates of permeate formed by ultrafiltration of sweet or acid whey or milk, concentrates of whey or of demineralized whey, mother-liquor from lactose re- covery, and lactose slush.

In the above-described continuous operation, the heating is preferably performed by supplying steam to the jacket mentioned. If the liquid to be treated is permeate concentrate coming directly from a conven- tional evaporator its content of dry solids can typi- cally be 55-60% and its temperature typically 65°C.

By passage through said zone, the amount of total dry solids increases to e. g. 87% and the amount of crystallized a-lactose monohydrate to 85% of the lac- tose.

Due to the previous intensive agitation of this pseudo-plastic and thixotropic mass, by the recover- ing from said zone it forms a gruel, which is still transportable, e. g. pumpable.

The above-mentioned cooling and optional disinte- gration and further drying can for instance be accom- plished by passing said gruel through an auger appa- ratus having cooling surfaces, and subsequently through a disintegrating and drying device.

However, the cooling may be a natural cooling or a forced cooling.

The invention further comprises a plant suitable for carrying out the above-described process.

Thus, in this aspect the invention deals with a plant for converting an aqueous lactose-containing liquid into free-flowing particles comprising: (a) a first device having (i) an elongated cylindri- cal horizontal housing of essentially circular verti- cal cross-section, (ii) an inlet for lactose- containing liquid at one end of said housing, (iii) a rotatable horizontal shaft axially in said housing, (iv) paddles radially extending from the shaft to-

wards the cylindrical wall of the housing, (v) a heating jacket surrounding at least partially said housing, (vi) openings at each end of the housing for removing vapours and, (vii) an outlet in the housing for lactose-containing gruel near the end opposite to said one end, (b) a cooling device connected to said outlet end de- signed as an auger having at least one screw end be- ing provided with a jacket and/or hollow screw shaft (s) for receiving a cooling medium, and (c) a disintegrating and drying device connected to said cooling device.

The device defined under (a) can be rather similar to an apparatus disclosed in US Patent 3,425, 135. In said disclosure, the apparatus is described as an ap- paratus for processing solids and all the examples illustrate drying of such.

The residence time in this apparatus can be ad- justed by setting the paddles in a backward pitch.

Thereby sufficient time for an almost complete crys- tallization can be obtained when performing the proc- ess of the present invention. An apparatus of similar construction is disclosed in US Patent 5,271, 163, wherein it is described as suitable for flowable ma- terials. However, this last-mentioned apparatus is specific by having means for introduced gas through nozzles in the rotating parts with a view of spread- ing the material to be treated over the walls of the housing. No applications akin to lactose crystalliza- tion seem to be mentioned in said US patent.

The process and the plant according to the inven- tion are further elucidated below by reference to the drawing.

Brief description of the drawing Fig. 1 schematically depicts an embodiment of a plant according to the invention suitable for per- forming the process of the invention.

Fig. 2 schematically shows an alternative suitable embodiment of a detail of the plant of Fig. 1.

Detailed description of embodiments of the invention In Fig. 1 a device wherein the starting liquid is heated and agitated and evaporated and crystallized is generally indicated 1.

This device comprises a cylindrical elongated housing 2 and therein a shaft 3 rotatable by means of a motor 4. On said shaft paddles 5 extend outwardly to a point adjacent to the wall of the housing but the paddles do not contact said housing.

In the embodiment shown, the shaft 3 is axially centred to have all paddles at the same distance from the wall of the housing 2.

The housing is at least partially surrounded by a heating jacket 6, which in the shown embodiment is divided into two sections.

To said jacket, a heating medium, such as hot wa- ter or steam, e. g. at a temperature of 50-150°C is introduced through inlets 7 and condensed water and any surplus of steam is withdrawn from outlets 8.

In contrast to what is shown in the drawing, the inner wall of the jacket will preferably be in line with the cylindrical wall of the housing 2.

The housing has an inlet opening 9 for the concen- trate used as starting material and an outlet 10 for the a-lactose monohydrate crystals containing gruel resulting from the treatment in the device 1.

The device further has an entrance 11 and an exit 12 connected to a fan 13 to provide a draft whereby filtered atmospheric air enters the interior of the housing through 11, passes through and purges the housing and is exhausted through 12 and 13.

Thus, in the embodiment depicted the water vapour formed by the evaporation in the housing is removed by purging with air at ambient temperature. However, it would be possible to supplement or substitute the

heating by means of the jacket 6 by drawing a stream of moderately heated air through the housing.

When operating said device and performing the process of the invention, the starting material liq- uid is introduced through inlet 9 and the shaft 3 with paddles 5 is rotated by means of the motor 4.

Within the housing 2, the liquid is subjected to a high shear agitation by the paddles and is thrown outwards due to the centrifugal force and thus forms an agitated ring along the inner surface of the hous- ing and the heating jacket 7. Thereby an efficient heat transfer is obtained causing a rise of tempera- ture and resulting evaporation of the liquid. Also the shaft 3 may be heated.

The paddles are spaced on the rotor so that all places of the heated inner wall of the housing are affected periodically when the paddles are rotating.

In the embodiment of Fig. 2, paddles 5'on shaft 3'are arranged in mutually displaced rows to ensure a uniform affection of all areas of the heated inner wall of the housing.

The shear rate at the most affected locations is given by the paddle peripheral speed, which is typi- cally 5-15 m/s, divided by the distance from the pad- dle tip to the inner wall of the housing, typically being 5-25 mm. The shear rate provided by the paddles is typically 200-3500 s-1, preferably 300-3000 s-1, and more preferably 400-2500 s-1.

The position of the paddles 5, the rate of intro- ducing starting material through 9 and possibly a derivation of the position of the housing away from the horizontal can be used to obtain a desired resi- dence time within the housing.

Guiding experiments indicate that residence times of 2-15 min will be sufficient.

When reaching the outlet 10 as a hot gruel, the crystallization is substantially completed and the portion of a-lactose monohydrate crystals may typi-

cally be 80-90% of the lactose and the free moisture content 8-18%.

At that stage, the gruel is transportable/pumpable but if a sample thereof is allowed to cool without agitation, the effects due to pseudo-plasticity and thixotropicity disappear and the sample becomes hard.

Therefore, the gruel recovered through 10 is di- rected to a device, which in the depicted embodiment is a double-screw auger 14 having a cooling jacket 15. Preferably, said device also has hollow screw shafts (not shown) for introducing cooling medium.

In the auger 14, the material being treated cools down to a temperature of e. g. 20-40°C and mainly due to thermo-plastic reasons the viscosity increases substantially and a thick non-pumpable mass is formed.

However, the cooling device may be built together with the device 1 as a section having a cooling jacket instead of a heating jacket and, possibly, separate agitation means.

In the shown embodiment, this mass is transferred to a disintegrating and drying device, which may be a fluidized bed apparatus 16 having a gas distribution plate 17 and a stirring means 18 for disintegration of the mass received from 14. Drying air is intro- duced by fan 20 through a heater 19 and fluidizes and dries the material in a bed formed above the plate 17. An outlet 21 acts as a weir and the product is recovered through said outlet.

Optionally, crystallized final or intermediate products may be recycled to the inlet 9 of the device 1, or to the inlet of the cooling device whether this is a section of device 1 having a cooling jacket, or is a separate cooling device.

Materials prepared as described above have been subjected to various tests showing their superiority over conventional products, especially their non- caking and free-flowing properties.

To further elucidate an embodiment of the inven- tion, the following non-limiting example is pre- sented.

Example The pilot plant used in this example comprised a small version of a device marketed under the name ROSINAIRE. This is an apparatus originally designed for drying fibres and other solids. In principle, it is constructed as the device 1 in the drawing.

The horizontal housing of the device had an inside diameter of 254 mm, and the distance from the paddle tips to the interior wall of the housing was approx- mately 6 mm.

The shaft with paddles was rotated at a velocity corresponding to a peripheral speed of the paddles of approximately 9 m/s, corresponding to a shear rate of 1,500 s-1.

In commercial applications, it is foreseen to use substantially larger devices having inside diameters of 1-2 m and shear rates of 300-800 s-1.

The starting material was a demineralized permeate concentrate having a free-moisture content of 45% and a temperature of 65°C.

This starting material was supplied through the inlet 9 at a rate of 130 kg/h. Steam was introduced through 7 at a temperature of 109°C and ambient air was drawn in through 11 at a temperature of approxi- mately 30°C.

This air was removed together with water vapour evaporated from the permeate concentrate through exit 12 at a temperature of 64°C.

The position of the paddles and the amount of starting material introduced into the device was ad- justed to obtain a residence time therein of approxi- mately 4 min.

During the passage through the device, permeate concentrate was subjected to a heating and simultane-

ously to a high shear rate (1,500 s-1 as explained above), and the air flow maintained through the de- vice provided an efficient means for removing water vapour from the extensively agitated liquid, thereby obtaining an efficient evaporation at temperatures substantially below the boiling temperature of said concentrate.

This evaporation caused an increase of the lactose concentration resulting in crystal formation and growth. This formation and growth of crystals were promoted by the viscosity decrease of the slurry, for which reason only a moderate super-saturation was ex- perienced. This is in contrast to other processes in- volving evaporation of aqueous lactose solutions, in which no specific measures are taken to utilize pseudo-plastical and thixotropical capabilities to decrease viscosity by applying a high shear rate.

When reaching the outlet 10, the amount of free- moisture in the crystal slurry had been reduced to 13% and the temperature thereof was 64°C. 85% of the lactose was at that stage present as crystalline cx- lactose monohydrate. The amount of gruel withdrawn per hour through 10 was 83 kg/h having 87% solids.

This material was transferred to the cooling de- vice 14.

To imitate cooling by means of a jacket 15, cool- ing was obtained by introducing liquid nitrogen into the device 14 together with the material from 10.

Thereby the latter was cooled to a temperature of 30°C, and it was transferred to a stirred fluidized bed apparatus for drying to reduce the free-moisture to 2%.

An amount of 75 kg/h of this final product was re- covered as a free-flowing, non-hygroscopic particu- late material.