WO2025114647A1

WO2025114647A1 - Method for producing cationized starch, cationized starch and its use

Info

Publication number: WO2025114647A1
Application number: PCT/FI2024/050649
Authority: WO
Inventors: Jouni Touronen; Jussi Nikkarinen; Jonni Ahlgren; Pia WILLBERG-KEYRILÄINEN
Original assignee: Kemira Oyj
Current assignee: Kemira Oyj
Priority date: 2023-12-01
Filing date: 2024-11-29
Publication date: 2025-06-05
Anticipated expiration: 2026-06-01

Abstract

The present method relates to a method for producing cationized starch having a cationic charge density of at least 2 meq/g. A reaction mixture comprising water, starch and an amount of cationization agent is formed and adjusted to a starting temperature, which is >40 °C and below a gelatinization temperature of the starch. At least 1 weight-% of an alkaline agent is added for catalysing a cationization reaction between the cationization agent and the starch, the reaction mixture comprising at most 40 weight-% of water and 30 – 50 weight-% of starch. The reaction mixture is kneaded at a reaction temperature of >40 °C during the cationization reaction, wherein the starch is dissolved and the reaction mixture is transformed into a highly viscous mass. The viscous reaction mixture is then dried for obtaining dried cationized starch.

Description

METHOD FOR PRODUCING CATIONIZED STARCH, CATIONIZED STARCH AND

ITS USE

The present invention relates to a method for producing dry, cationized starch, cationized starch and its use according to the preambles of the enclosed independent claims.

Cationized starch is widely used in industry, for example in paper and board manufacture. The current trend to reduce the use of petroleum-based chemicals and to replace them with renewable biobased alternatives has increased the interest in cationized starch, and there is a general desire to find new industrial applications for cationized starch. Especially in water purification and in treatment of sludges, there is a growing need for biobased treatment chemicals, which are easily degradable in soil and thus result in treated sludges suitable for landfills. There is a special interest in highly cationized starch, which is easy to dissolve in water, and thus easy to use industrially.

There are many different ways of producing cationized starch, of which a wet process and a dry process are commercially the most common. In the wet process starch is kept in slurry form, i.e. as dispersed particles in a liquid phase, throughout the cationization reaction. The presence of large amount of water reduces the yield of the wet process and leads to complicated separation of the produced cationic starch from other process components, e.g. through filtration and washing. In the dry process, the starch remains in dry powder form throughout the cationization reaction and all the chemical components used for the reaction and its activation are in powder form. Dry process for starch cationization does not involve filtration and washing steps, but the obtained cationized starch may contain residual salts, unreacted reagents and other impurities, which is disadvantageous.

The cationization process influences the properties of the obtained cationized starch, most notably the cationization degree. It has been observed that depending on their production history cationized starches behave differently in the end-use applications. There is a need for a method for producing cationized starch which yields a high cationization degree and effective functional characteristics in end-use applications for the produced starch.

An object of this invention is to minimise or possibly even eliminate the disadvantages existing in the prior art.

Another object of the present invention is an effective and uncomplicated method for producing dry, cationized starch with high cationization degree.

Another object of the present invention is a method for producing dry, cationized starch, which is effective as flocculant in liquid-solid separation.

Yet another object of the present invention is a dry, cationized starch, which is effective as flocculant in liquid-solid separation.

These objects are achieved by the features disclosed in the independent claims. Some preferred embodiments of the present invention are presented in the dependent claims. The features recited in the dependent claims are mutually freely combinable unless otherwise explicitly stated.

The exemplary embodiments presented in this text and their advantages relate to all aspects of the present invention, even though this is not always separately mentioned.

A typical method according to the present invention for producing cationized starch having a cationic charge density of at least 2 meq/g, measured at pH 4, comprises

- forming a reaction mixture comprising water, starch and an amount of cationization agent;

- adjusting the reaction mixture to a starting temperature, which is >40 °C and below a gelatinization temperature of the said starch,

- adding to the reaction mixture at least 1 weight-% of an alkaline agent, calculated from the amount of the cationization agent, for catalysing a cationization reaction between the cationization agent and the starch, the reaction mixture comprising at most 40 weight-% of water and 30 - 50 weight-% of starch, calculated from a total weight of the cationization agent, alkaline agent, water and starch;

- kneading the reaction mixture at a reaction temperature >40 °C, preferably >50 °C during the cationization reaction, wherein the starch is dissolved and the reaction mixture is transformed into a highly viscous reaction mixture,

- subjecting the highly viscous reaction mixture to a drying step for obtaining dried cation ized starch.

A typical cationized starch according to the present invention has a cationic charge density of at least 2 meq/g, measured at pH 4, and is obtained by the method according to the present invention.

A typical use according to the present invention of the cationized starch obtained by the method according to the present invention is as a flocculation agent in a liquidsolid separation, such as a treatment of water and/or biological sludge.

Now it has been surprisingly found that by producing dry, cationized starch under a selected set of reaction conditions, it is possible to produce highly cationized starch in an efficient manner. The obtained cationized starch shows good or even excellent properties as a flocculation agent in liquid-solid separation. Especially, the selected elevated temperature throughout the cationization reaction together with the effective mixing, provides the obtained cationized starch with high cationization degree and appropriate viscosity behaviour for industrial use. The theoretical background behind the present invention is not yet fully understood. However, the method according to the present invention provides an effective and uncomplicated way of producing cationized starch with high cationization degree. The gist of the invention is thus to produce cationized starch with optimal performance properties, especially for use as a flocculation agent in liquid-solid separation, such as dewatering of biological sludge.

Any available starch, such as potato starch, waxy potato starch, rice starch, corn starch, waxy corn starch, wheat starch, barley starch, pea starch or tapioca starch, can be used for cationization in the present method. According to one preferable embodiment the used starch is selected from potato starch, corn starch or tapioca starch. Even more preferably the starch is potato starch. Preferably the used starch is a non-degraded starch, either a native starch or a low cationic starch. Starch to be cationized may be provided in a form of dry particulate material, i.e. aggregates or powder, having a water content of 5 - 25 weight-%, preferably 10 - 20 weight-%.

First, a reaction mixture comprising at least the starch, a cationization agent and water is formed for the production of cationized starch. The components of the reaction mixture, i.e. starch, cationization agent and water, can be mixed together in any order. For example, the cationization agent can be added to the water, followed by the addition of starch in particulate form, or vice versa. The obtained reaction mixture is in a form of a slurry or a dispersion. This means that solid starch particles are uniformly dispersed in a liquid phase, which comprises water and the cationization agent. The reaction mixture is kept under mixing in order to provide and maintain the uniform dispersion of the starch particles in the liquid phase.

The cationization agent may be any cationization agent conventionally used for cationization of polysaccharides, especially starch. According to one preferable embodiment the cationization agent is selected from 2,3-epoxypropyltrimethyl- ammonium chloride (EPTAC) or 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) or any of their mixtures.

After formation of the reaction mixture, the temperature of the reaction mixture is adjusted to a starting temperature, which is >40 °C, preferably 45 °C or higher (i.e. >45 °C), more preferably 50 °C or higher (i.e. >50 °C), sometimes even 52 °C or higher, but below a gelatinization temperature of the used starch. Gelatinization temperature denotes here the temperature or temperature range where the intermolecular bonds of the starch break down in the presence of water. The gelatinization temperature depends on the used starch, and the gelatinization temperatures of different starch types are known for a person skilled in the art. This means that the starch in the reaction mixture is not subjected to the gelatinization or cooking before the addition of the alkaline agent alkaline agent and catalysation of the cationization reaction, i.e. the method is free of gelatinization and/or cooking of starch present in the reaction mixture. According to one embodiment of the present invention, before the addition of the alkaline agent the reaction mixture may be adjusted to a starting temperature, which is >40 °C, preferably >45 °C, more preferably >50 °C or >52 °C, but below the gelatinization temperature, which is in a range of 53 - 72 °C, typically 55 - 72 °C. Taking into account the gelatinization temperature of the used starch, the starting temperature of the reaction mixture may be adjusted, for example, to a temperature in a range from >40 to 75 °C, preferably in a range of 45 - 72 °C, more preferably in a range of 50 - 60 °C or 52 - 60 °C.

After the adjustment of the temperature of the reaction mixture, as described above, at least 1 weight-% of the alkaline agent (given as active agent), calculated from the weight of the cationization agent in the reaction mixture, is added into the reaction mixture for catalysing the cationization reaction between the cationization agent and starch. The alkaline agent may be selected from sodium hydroxide, potassium hydroxide, lithium hydroxide or calcium hydroxide, preferably sodium hydroxide, potassium hydroxide or calcium hydroxide, more preferably sodium hydroxide or calcium hydroxide. The alkaline agent may be added in amount of 1 - 15 weight-%, preferably 1 - 10 weight-%, more preferably 1 - 5 weight-%, calculated from the weight of the cationization agent in the reaction mixture.

The reaction mixture, after the addition of the alkaline agent, may comprise at most 40 weight-% of water, calculated from total weight of cationization agent, alkaline agent, water and starch. The reaction mixture may comprise <40 weight-%, preferably <38 weight-%, more preferably <36 weight-%, of water, and/or >25 weight-%, preferably >30 weight-%, of water, calculated from the total weight of the cationization agent, alkaline agent, water and starch. According to one embodiment, after the addition of the alkaline agent and at the beginning of the cationization step, the reaction mixture may comprise 25 - 40 weight-%, preferably 30 - 38 weight-%, more preferably 32 - 36 weight-%, of water, calculated from the total weight of the cationization agent, alkaline agent, water and starch in the reaction mixture. The water originally present in the components of the reaction mixture, at the time of the formation of the reaction mixture, i.e. the water content of starch and the cationization agent, is taken into account in calculation of the water amount of the reaction mixture. The low amount of water makes it possible to effectively produce dry cationized starch and to minimize the amount of undesired side product residuals. Further, it is assumed, without wishing to be bound by any theory, that the low water amount has a positive impact on the properties of the cationized starch.

The reaction mixture may comprise, after the addition of the alkaline agent, 30 - 50 weight-%, preferably 33 - 47 weight-%, more preferably 35 - 45 weight-% or 36 - 43 weight-%, of starch, calculated from the total weight of the cationization agent, alkaline agent, water and starch.

The water-to-starch ratio in the reaction mixture may be from 0.6 to 1 .2, preferably from 0.7 to 1.1 , when the temperature of the reaction mixture is maintained at a temperature of >40 °C, preferably 45 °C or higher, more preferably 50 °C or higher.

Furthermore, the reaction mixture may comprise, after the addition of the alkaline agent, 15 - 40 weight-%, preferably 20 - 35 weight-%, more preferably 25 - 33 weight-%, of the cationization agent, calculated from the total weight of the cationization agent, alkaline agent, water and starch.

The reaction mixture is kept under mixing during and after the addition of alkaline agent. The alkaline agent catalyses the cationization reaction which may lead to an occurrence of a temperature peak, where the temperature of the reaction mixture may be temporarily raised over the gelatinization temperature of the used starch. The temporary temperature peak occurs after the addition of the alkaline agent and may reach up to a temperature of 95 °C, for example to a temperature of 70 - 90 °C, or to a temperature of 72 - 80 °C. The temperature of the reaction mixture, however, is kept under the boiling point of the reaction mixture, i.e. <100 °C. The duration of the temperature peak, i.e. the time when the temperature exceeds the starting temperature, and preferably the gelatinization temperature of the used starch, may be relatively short, for example 1 - 120 min, preferably 10 - 60 min, more preferably 20 - 40 min. It is speculated, without wishing to be bound by any theory, that the temperature peak promotes the simultaneous cationization and dissolution of the starch in the reaction mixture. After the temperature peak, the temperature of the reaction mixture is decreased to the reaction temperature, and the reaction mixture may be maintained at this reaction temperature of >40 °C, preferably >45, more preferably >50 °C or >52 °C, until the cationization reaction has proceeded to the completion or until at least 80 weight-%, preferably at least 90 weight-%, even more preferably at least 95 weight- % or at least 99 weight-%, of the cationization agent has reacted and/or until the amount of the unreacted cationization agent in the rection mixture has decreased on a level <100 ppm, preferably on a level of 10 - 100 ppm . Preferably, after the temperature peak, the reaction mixture temperature is maintained at a temperature level in a range from 40 to 75 °C, preferably in a range of 45 - 70 °C, more preferably in a range of 50 - 60 °C or 52 - 60 °C.

In the present context, it is considered that the cationization reaction starts when the alkaline agent is added into the reaction and the cationization reaction ends when the amount of the unreacted cationization agent in the rection mixture has decreased on a level <50 ppm.

As the cationization reaction proceeds, the viscosity of the reaction mixture starts to increase as the starch starts to dissolve, and the reaction mixture is transformed into a highly viscous mass or highly viscous reaction mixture. The reaction mixture is thus changed from the slurry or dispersion comprising starch particles into a homogenous reaction mixture, where no individual starch particles can be visually observed. At this stage the highly viscous reaction mixture can be described as dough-like mass. After the addition of the alkaline agent, the reaction mixture may have a viscosity of 50 000 - 1 000 000 mPas, preferably 75 000 - 850 000 mPas, more preferably 100 000 - 700 000 mPas, measured with Anton Paar MCR302e CC27, at shear rate 10 1/s and at temperature of 50 °C. In the present invention, the high viscosity of the reaction mixture is advantageous, as it enables effective drying of the obtained cationized starch, e.g. by using contact drying as described below. The reaction mixture is kneaded or mixed, preferably continuously, at the reaction temperature >40 °C, preferably >45 °C, more preferably >50 °C or >52 °C, during the cationization reaction. As described above, the reaction mixture is transformed in highly viscous form, as the starch is dissolved and the cationization reaction proceeds.

According to one preferable embodiment, the reaction mixture may be subjected to kneading or mixing before the end of the cationization reaction with a mixer tip speed of >1 m/s, preferably >1 .5, more preferably >2 m/s, for example in a range of 1 - 5 m/s, preferably 1.5 - 4.5, more preferably 2 - 4 m/s, sometimes 2.5 - 4 m/s. The kneading or mixing with the specified high mixer tip speed enhances the cationization reaction. The kneading or mixing with the high mixer tip speed can preferably be carried out before the end of the cationization reaction, i.e. preferably before 99 weight-%, more preferably before 95 weight-%, even more preferably before 90 weight-% of the cationization agent has reacted and/or before the amount of the unreacted cationization agent in the rection mixture has decreased on a level <50 ppm, preferably <100 ppm. It is possible that the speed of the kneading or mixing of the reaction mixture varies during the cationization reaction. For example, it is possible that the reaction mixture is first kneaded or mixed with a low speed, i.e. with a mixer tip speed <2 m/s or <1 m/s, and then before the end of the cationization reaction the mixer tip speed is increased to the values defined above. Alternatively, it is possible that the reaction mixture is kneaded or mixed with a constant speed through the cationization reaction, with a mixer tips speed as defined above.

The effective mixing or kneading of the reaction mixture is preferably ensured by using a high shear mixing device, capable of mixing viscous pastes. Suitable mixing devices are for example Lodige™ mixers, such as Ploughshare™ mixers, Sigma™ mixers, and the like. Suitable mixing devices include also extruders, such as single screw extruders and twin screw extrudes. According to one embodiment, the high- shear mixing device may comprise a rotating mixing tool, such as mixing blade(s), impeller(s), agitator(s), screw(s), or the like, wherein the mixing tool may have a tip speed >1 m/s, preferably >1 .5 m/s, more preferably >2 m/s, for example 1 - 5 m/s, 1 .5 - 4.5 m/s, more preferably 2 - 4 m/s. According to one embodiment, the reaction mixture may be subjected to kneading or mixing with a mixer tip speed of >1 m/s, preferably >1 .5 m/s, more preferably >2 m/s, for example in a range of 1 - 5 m/s, preferably 1 .5 - 4.5, more preferably 2 - 4 m/s, sometimes 2.5 - 4 m/s, after the end of the cationization reaction. It has been observed that kneading or mixing with a high mixer tip speed improves the properties of the cationized starch, for example increases the charge density and/or reduces the viscosity.

It is possible to add additional water to the reaction mixture during the cationization reaction after the temperature peak has been passed. It is also possible to add additional water to the viscous reaction mixture at or after the end of cationization reaction, but before the drying step. Additional water may be added in amount of 0.5 - 35 weight-%, preferably 5 - 30 weight-%, more preferably 10 - 25 weight-%, sometimes 15 - 25 weight-%, calculated from the total weight of the reaction mixture. The addition of additional water improves the handling properties of the reaction mixture, especially if the reaction mixture viscosity increases significantly. However, in view of the efficiency of the following drying step, the amount of added additional water during and/or after the cationization reaction should be kept as small as possible, irrespective of the origin. According to one embodiment, the method is free of addition of water to the reaction mixture after the addition of alkaline agent.

According to one embodiment of the invention, the reaction mixture may be neutralized with an acid before the drying step, after the completion of the cationization reaction. Any suitable acid can be used for neutralization, for example an organic acid, such as citric acid, adipic acid, formic acid, acetic acid, fumaric acid, or their mixtures, and/or an inorganic acid, such as hydrochloric acid. Citric acid is preferred, as it provides optimal control of colour and odour of the obtained cationized starch. The pH of the viscous reaction mixture is preferably adjusted to a pH value in a range of 5 - 10, preferably 5 - 9. After the cationization reaction is completed, the viscous reaction mixture is subjected to a drying step for obtaining dried cationized starch. Any suitable drying technique can be used in the drying step. According to one preferable embodiment, the reaction mixture is dried in a drying step which has a drying severity value <5000 h, preferably <1000 h, more preferably <500 h, wherein the drying severity value is calculated as

Preferably the drying severity value is in a range of 10 - 5000 h, preferably 10 - 1000 h, more preferably 10 - 500 h. The drying severity value may even be in the range of 10 - 400 or 15 - 350 h. It has been observed that the drying severity has an impact on the properties of the obtained dried cationized starch. If the drying severity value is too high, the properties of the dried cationized starch are significantly deteriorated. It is assumed, without wishing to be bound by any theory that the structure of the cationized starch may be damaged if the drying step is too harsh.

According to one embodiment of the invention the reaction mixture may be subjected to a drying step, where the drying is performed as contact drying, freeze drying or vacuum drying. According to one preferable embodiment, the drying step may be performed as contact drying, such as drum drying or roll drying. Contact drying is here understood as an indirect drying technique, such as drum drying or roll drying, where the heat required for removal of moisture is transported by conduction. The reaction mixture is in form of highly viscous mass or paste after the cationization reaction, which makes it especially suitable for contact drying. In contact drying, the highly viscous reaction mixture may be applied as a layer on a drying surface, such as an outer surface of a heated rotating drum or roll, where the moisture is evaporated from the reaction mixture. A layer of dried cationized starch is thus obtained and can be removed from the drying surface. The contact drying may employ an arrangement of a plurality of heated rotating drums or rolls, on surfaces of which the viscous reaction mixtures is dried. The contact drying, e.g. drum drying or roll drying, may be performed by using a drying surface temperature of 100 - 200 °C, preferably 120 - 180 °C, more preferably 140 - 165 °C. In case the contact drying is performed as drum or roll drying with an arrangement comprising a plurality of heated drums, the surface temperatures of the individual drums may be the same or different from each other. The arrangement may have, for example, increasing or decreasing temperature profile. The drying time in the contact drying may be 0.5 - 15 min, preferably 1 - 10 min, more preferably 1 - 5 min. Preferably the drying time is as short as possible to ensure the desired drying severity value. The contact drying produces a continuous strip or band of cationized starch, which is then easy to comminute, e.g. by milling or grinding, to appropriate particle size.

In general, after the drying step the obtained dry cationized starch may be comminuted, e.g. by cutting, crushing and/or grinding, into flakes, particles, granules or powder. Any suitable cutting, milling, crushing or grinding apparatus may be used. The dry cationized starch may be comminuted into a particle size in a range of 0.1 - 2.5 mm, preferably 0.125 - 1.5 mm. The comminuting method and particle size may be selected on basis of the intended end use of the dry cationized starch.

After comminuting the obtained particulate dry cationized starch may be further subjected an optional secondary post-drying step, if needed. The secondary postdrying may be performed in a rotary dryer, in a fluidized bed dryer or in any other suitable dryer apparatus available. The secondary post-drying is fully optional, but it may be used to adjust the moisture content of the dry cationized starch, especially if very low moisture content of 1 - 9 weight-% or 2 - 5 weight-% is desired for the dry cationized starch.

Preferably, the method is free of addition of other functionalization agents for starch functionalization than cationization agent.

The dry cationized starch obtained with the method according to the invention has a cationic charge density of at least 2 meq/g, preferably at least 2.1 meq/g, more preferably at least 2.4 meq/g, measured at pH 4. The cationic charge density of the obtained cationized starch may be in a range of 2.0 - 3.5 meq/g, preferably 2.1 - 3.3 meq/g, more preferably 2.4 - 3.0 meq/g, measured at pH 4. Charge density at pH 4 can be determined by using AFG Analytics’ particle charge titrator, as described in the experimental part. The present method is thus able to provide dry cationized starch which a high cationicity. This kind of starch is especially suited for liquid-solid separation, such as treatment of water and/or dewatering of biological sludge.

The cationized starch obtained by the present method may have a degree of substitution of at least 0.45, preferably at least 0.5, more preferably at least 0.6. The degree of substitution may be in a range of 0.45 - 1 .2, preferably 0.5 - 1.1 , more preferably 0.6 - 0.9 or 0.7 - 0.8. In the present context, the degree of substitution defines how many substituted groups are contained in the cationized starch, calculated per one anhydroglucose unit of starch. The degree of substitution can be calculated from the charge density of the starch or by measuring nitrogen content as Kjeldahl nitrogen and calculating the charge density and degree of substitution from the measurement results. These calculation and determination methods are known for a skilled person. The method according to the present invention is thus intended for production of cationized starch with a high cationicity or very high cationicity. With the present method it is possible to produce, for example, cationized starch which is water-soluble at temperature of <50 °C, preferably <40 °C, even <30 °C, as described below.

In the present context, the “dry cationized starch” denotes cationized starch having the cationic charge density as defined above, and a moisture content of <18 weight- %, typically <10 weight-%, obtained with the method disclosed in the present application. The moisture content of the dried cationized starch may be 1 - 18 weight-%, more typically 1.5 - 10 weight-% or 2 - 9 weight-%. The obtained cationized starch in dry form is easy to transport and store. Due to the high cationicity, the starch according to the present invention is, however, easily dissolved in water at a temperature < 50 °C, i.e. without cooking, as explained above. This makes the use of the dry cationized starch according to the present invention in industrial processes uncomplicated. The cationized starch obtained by the method of the present invention is water- soluble. Preferably the cationized starch is soluble in water having a temperature of 30 °C or lower, for example in water having a temperature of 10 - 25 °C. Watersoluble means that the cationized starch is fully miscible with water, providing transparent or nearly transparent solution. When mixed with excess of water, the cationized starch is dissolved and the obtained solution of cationized starch is preferably essentially free from visible discrete solid particles, granules or coagulated clots of cationized starch. Excess of water means that the obtained solution is not a saturated solution. No cooking of cationized starch obtained by the present method is required for dissolving. The cationized starch may have a turbidity value less than 1000 NTU, preferably less than 500 NTU, more preferably less than 250 NTU. The cationized starch may have a turbidity value in a range of 1 - 150 NTU, preferably 1 - 100 NTU. The turbidity values are measured from a water solution of the cationized starch, at 1 weight-% concentration, by using HACH, 2100 AN IS Laboratory Turbidimeter.

The dry cationized starch obtained by the present invention may be used as a flocculation agent or as a dewatering agent in an aqueous liquid-solid separation processes. For example, the dry cationized starch may be used as a flocculation agent in purification of water or as a dewatering agent in dewatering of sludge from municipal or industrial wastewater treatment.

The cationic starch obtained by the method according to the present invention is suitable for increasing a solid content of various sludges, especially biological sludge. Sludge is here understood as an aqueous suspension, which comprise a continuous aqueous liquid phase and organic and/or inorganic solid material and/or particles suspended in the aqueous liquid phase. The sludge may be rich in material of bacterial origin. The aqueous liquid phase of the sludge may contain also dissolved organic substances, such as polysaccharides, humic substances and fatty acids. Due to the content of variable organic material, e.g. bacteria, other organic substances, the treatment of sludges, as well as wastewater, especially municipal and agricultural wastewater, is different from treatment of inorganic metallurgical suspensions or cellulosic fibre suspensions, such as pulp for paper or board manufacture. The cationized starch obtained with the present method is especially and unexpectedly suited for treatment of sludge which may be municipal wastewater sludge or agricultural sludge, or it may be biological sludge originating from a biological treatment process of wastewater and/or sewage. Alternatively, the aqueous suspension for treatment with the cationized starch obtained by the present method may originate from an industrial process, especially from wastewater treatment of an industrial process, or from food or beverage production or from food or beverage processing.

EXPERIMENTAL

Some embodiments of the present invention are described in the following, nonlimiting examples.

The following characterisation methods are used in the examples:

Cationic charge density was determined at pH 4 using AFG Analytics’ CAS-II touch! Charge Analyzing System. Sample to be analysed was dissolved in deionized water, to give a 0.025 - 0.05 weight-% concentration. pH of the sample was adjusted to 4.0 with 0.1 M acetic acid and titrated using 0.001 N sodium polyethylenesulfonate (PES-Na) solution as the titrant. During the titration, pH of the sample was normally increasing 0.1 - 0.2 pH units. The charge density, as meq/g dry sample, was calculated from the titration result.

Dry content was measured by drying about 2 g of the (cationized starch) sample overnight in an oven at 105 °C. Dry content is expressed as weight-% of the sample.

Turbidity of 1 weight-% solution was measured using HACH TL2360 Laboratory Turbidimeter.

Viscosity of 3 weight-% cationized starch solution in water is measured using a Brookfield LV viscometer with a small sample adapter at 25 °C, using either spindle #18 or #31 , depending on viscosity level. Viscosity at the maximum possible rotational speed, selected from speeds of 0.3, 0.6, 1 .5, 3, 6, 12, 30, 60 and 100 rpm, was taken as the result value.

Example 1

117 kg of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 42 kg of deionized water and 127.9 kg of potato starch were added in 600 liter reactor of a Lodige ploughshare mixer to form a reaction mixture. The reaction mixture was in a form of a slurry, which was agitated at agitation speed of 50 rpm and heated to a temperature of 50 °C. Mixer tip speed was 2.6 m/s. When the temperature of the reaction mixture had stabilized, 4.56 kg of 50 % NaOH solution was added to start the cationization reaction. After about 30 minutes the temperature of the reactor had increased to about 70 °C. The viscosity of the reaction mixture increased during the increase of the temperature and the reactor mixture transformed into a highly viscous mixture, i.e. dough-like mass.

When the temperature reached 70 °C, an automatic control begun to cool the reactor and to remove the reaction heat. In about 30 minutes the temperature was back to 50 °C. The reactor mixture was kept in 50 °C for 6 hours. Then the agitation speed was set to 20 rpm (mixer tip speed 1 m/s) and the reactor mixture were agitated overnight.

After 24 hours (from initial formation of the reaction mixture), 66 kg of water was added to the reaction mixture to reduce the viscosity, and the reaction mixture was agitated further 30 minutes. The viscous reaction mixture was pumped with a piston pump to a roll dryer and dried. The roll dryer surface temperature was about 130 - 140 °C and the contact time was about 60 seconds. The resulting sheet of dry, cationized starch CS1 was milled to powder and sampled. The characteristics of the cationized starch CS1 are given in Table 1 .

Example 2

1932 g of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 255 g of deionized water and 2131 g of potato starch were added in 10 liter reactor of a Lodige ploughshare mixer to form a reaction mixture. The obtained reaction mixture was in form of a slurry and it was agitated at agitation speed 150 rpm. Mixer tip speed was 2.0 m/s. The temperature of the reactor mixture was heated to 50 °C. When the temperature had stabilised, 54 g of 50 % NaOH solution was slowly added to the reaction mixture during 10 minutes to start the cationization reaction. After about 30 minutes the temperature of the reactor had increased to about 70 °C. During the temperature rise the viscosity of the reaction mixture increased and the reaction mixture became a highly viscous, i.e. dough-like mass.

When the temperature of the reactor had reached the temperature about 70 °C, an automatic control begun to cool the reactor and remove the reaction heat. In about 30 minutes the temperature was back to about 50 °C. The reactor contents were kept in 50 °C. After 6 hours the agitation speed was set to 20 rpm (mixer tip speed 0.3 m/s) and the reactor contents were agitated overnight.

After 24 hours (from initial formation of the reaction mixture), the viscous reaction mixture was removed and a sample was taken. The sample of cationized starch CS2 was dried in vacuum oven at 40 °C, milled to powder and analysed. The characteristics of the obtained cationized starch CC2 are presented in Table 1 .

Example 3

117 kg of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 42 kg of deionized water and 127.9 kg of potato starch were added in 600 liter reactor of a Lodige ploughshare mixer to form a reaction mixture. The obtained reaction mixture was in form of a slurry and it was agitated at agitation speed of 50 rpm. The mixer tip speed was 2.6 m/s. The temperature of the reactor mixture was heated to 50 °C. When the temperature of the reaction mixture had stabilised, 4.56 kg of 50 % NaOH solution was added to the reaction mixture to start the cationization reaction. After about 30 minutes the temperature of the reactor had increased to about 70 °C. During the temperature rise the viscosity of the reaction mixture increased and the reaction mixture became a highly viscous, i.e. dough-like mass.

When the temperature of the reactor had reached the temperature of about 70 °C, an automatic control begun to cool the reactor and remove the reaction heat. In about 30 minutes the temperature was back to about 50 °C. The reactor contents were kept in 50 °C for 6 hours. After 6 hours the agitation speed was set to 20 rpm (mixer tip speed 1 m/s) and the reactor contents were agitated overnight.

After 24 hours (from initial formation of the reaction mixture), the viscous reaction mixture was removed from the reactor. The viscous reaction mixture was spread on plastic sheets and dried in vacuum oven overnight at temperature of 40 °C and pressure of 200 mbar(abs). Thus obtained dried cationized starch CS3 was milled and analysed. The characteristics of the cationized starch CS3 are presented in Table 1 .

Example 4

2116 g of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 732 g of deionized water and 1946 g of potato starch were added in 10 liter reactor of a Lodige ploughshare mixer to form a reaction mixture. The obtained reaction mixture was in form of a slurry and it was agitated at agitation speed of 150 rpm. The mixer tip speed was 2.0 m/s. The temperature of the reactor mixture was heated to 50 °C. When the temperature had stabilised, 45 g of 50 % NaOH solution was added to the reaction mixture to start the cationization reaction. After about 30 minutes the temperature of the reactor had increased to about 70 °C. During the temperature rise the viscosity of the reaction mixture increased and the reaction mixture became a highly viscous, i.e. a dough-like mass.

When the temperature of the reactor had reached the temperature of about 70 °C, an automatic control begun to cool the reactor and remove the reaction heat. In about 30 minutes the temperature was back to about 50 °C. The reactor contents were kept in 50 °C for 6 hours. After 6 hours the agitation speed was set to 20 rpm (mixer tip speed 0.3 m/s) and the reactor contents were agitated overnight.

After 24 hours (from initial formation of the reaction mixture), the viscous and sticky reaction mixture was removed from the reactor and a sample was taken. The sample of obtained cationized starch CS4 was dried in oven and analysed. The characteristics of the obtained cationized starch CS3 are presented in Table 1 . Comparative Example 5

59.8 g of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 20.7 g of deionized water and 55.0 g of potato starch were added in a cylindrical vessel. The mixture was vigorously agitated and a slurry was obtained. The vessel with the slurry was placed in a shaker in a thermostated bath, at 20 °C. When the temperature had stabilised, 1 .26 g of 50 % NaOH solution was dropwise added to the reaction mixture to start the cationization reaction. The vessel was tightly closed with a cap and left in the shaker in the bath (20 °C) for 168 hours. After couple of days the slurry had turned into a moist powder, and at the end of the shaking period the powder had solidified into a moist cake. A sample of the obtained moist cake was dried in a vacuum oven at 40°C, milled to powder and analysed. The characteristics of the obtained cationized starch CCS are presented in Table 1 . It is seen that when the temperature (20 °C) and the mixing intensity throughout the reaction were low, the obtained cationized starch CCS had a lower charge density and extremely high viscosity as 3 weight-% solution. These properties make the cationized starch CCS difficult to use in practical applications, especially in liquidsolid separation, such as dewatering of sludge.

Table 1 Characteristics of starches prepared in Examples 1 - 4, Comparative

Example 5.

* measured from 1 weight-% solution

** measured from 3 weight-% solution

Example 6: Performance of cationized starch in sludge

The performance of cationized starch CS3, prepared in Example 3, for sludge dewatering was compared to the performance of commercial cationic starches. The cationized starch CS3 was dissolved into water, 0.2 weight-% concentration.

As a reference, two commercially available cationic starches RCS1 (Vector IC 42280, Roquette, starch solution with dry content of 43 weight-%) and RCS2 (Hi- Cat 1574A, Roquette, dry starch powder) were used. Both of these reference starches were used as 0.2 weight-% water solutions: RCS1 was diluted with water to 0.2 weight-% concentration and RCS2 was dissolved into water at 0.2 weight-% concentration.

The efficiency of the cationized starch CS3 according to the present invention and the commercial reference starches RCS1 and RCS2 in sludge dewatering was tested as described in the following. The sludge used in the tests was digested sludge taken from a Finnish municipal wastewater treatment plant. The dry content of the sludge was 2.7 weight-% and its pH was 7.9. Without any chemical addition the sludge gave CST time of 199 s.

Sludge dewatering tests were made by using capillary suction time (CST) test. The tests were carried out using the Triton type 319 Multi-purpose CST with a type 317 Stirrer-Timer (all from Triton Electronics Ltd, UK). The used cylinder had a diameter of 18 mm. The mixing speed used in the tests was 1000 rpm. The studied starch sample was added to 100 g of sludge. The sludge was mixed 10 s after dosing of the starch sample, after which a 4.5 ml sample was taken to cylinder and the CST value was measured.

The dosages of the starch samples are given as kg active starch per ton dry sludge. For CS3 and RCS2 it was assumed that the active starch content was equal to the starch/product as such. For RCS1 the active starch content was calculated from the dry content of the starch.

The CST results are shown in Table 2. Table 2 CST time results (s) of Example 5.

It is seen from Table 2 that the cation ized starch CS3 prepared according to the present invention gives clearly better results than the commercial cationic starches at every dosage level. The better results are seen as low values for the measured CST time, which indicates that the cationized starch CS3 effectively forms flocs or the like and promotes dewatering of the sludge.

Example 7: Performance of cationized starch in sludge dewatering

The performance of cationized starch CS3, prepared in Example 3 and cationized starch CS4, prepared in Example 4, was compared to the performance of cationized starch CCS, prepared in comparative Example 5, for sludge dewatering.

The sludge dewatering tests were carried out in the same manner as in Example 6. The sludge used in the tests was digested sludge taken from a Finnish municipal wastewater treatment plant. The dry content of the sludge was 2.8 weight-% and its pH was 7.5. Without any chemical addition the sludge gave CST time 425 s.

The CST results are shown in Table 3.

It can be seen from Table 3 that the cationized starches CS3 and CS4 obtained by the method of the present invention give clearly better performance in sludge dewatering than cationized starch CCS obtained at a low temperature and with low mixing intensity. Table 3 CST time results (s) for Example 6.

Example 8

18.9 kg of glycidyltrimethylammonium chloride (Raisacat™ 151 , Chemigate Oy, Finland), 6.8 kg of deionized water and 20.6 kg of potato starch were added in 133 liter Winkworth sigma blade mixer to form a reaction mixture. The obtained reaction mixture was in form of a slurry and it was agitated at agitation speed of 20 rpm. Mixer tip speed was 0.3 m/s. The temperature of the reactor mixture was heated to 50 °C. When the temperature had stabilized, 0.7 kg of 50 % NaOH solution was added to the reaction mixture to start the cationization reaction. After about 1 hour the temperature of the reactor had increased to about 63 °C. During the temperature rise the viscosity of the reaction mixture increased and the reaction mixture became a highly viscous, i.e. a dough-like mass. In about 3 hours the temperature of the reaction mixture was back to about 50 °C. No cooling of the reaction mixture was necessary or performed. After 6 hours the agitation speed was set to 2 rpm (mixer tip speed 0.03 m/s) and the reactor contents were agitated overnight.

After 24 hours (from initial formation of the reaction mixture) the viscous and sticky reaction mixture was removed from the reactor and a sample was taken. The sample was dried in oven and analysed. The characteristics of the cationized starch at this stage, before mixing with tip speed of 1.5 m/s, are presented in Table 4 as “cationized starch dough”.

4410 g of the viscous and sticky reaction mixture was then added into 5 liter reactor of a Lodige ploughshare mixer. The temperature of the reactor mixture was heated to 50 °C. The reaction mixture was agitated at agitation speed of 150 rpm. Mixer tip speed was 1 .5 m/s. Samples of cationized starch from the reaction mixture were taken after 1 , 2, 3 and 4 hours of mixing. The samples were dried in oven for 4 hours at 60 °C as thin sheets, comminuted, dissolved and analysed as aqueous solutions. The characteristics of the obtained cationized starches are presented in Table 4.

Table 4 Characteristics of starches prepared in Example 8.

** measured from 3 weight-% solution

It is seen from Table 4 that the further shearing of the reaction mixture in the ploughshare mixer with a tip speed of 1.5 m/s produced a significant decrease in the viscosity as well as an increase in charge density of the obtained cationized starch.

Example 9: Performance of cationized starch in sludge dewatering

The performance of the cationized starches prepared in Example 8 were compared with each other in sludge dewatering.

The sludge dewatering tests were carried out by using capillary suction time (CST) test in the same manner as in Example 6. The sludge used in the tests was digested sludge taken from a Finnish municipal wastewater treatment plant. The dry content of the sludge was 4.1 weight-% and its pH was 7.7. Without any chemical addition the sludge gave CST time of 174 s.

The CST results are shown in Table 5.

Table 5 CST time results (s) for cationized starches of Example 8.

It can be seen from Table 5 that the starch samples subjected to a further shearing in a Lbdige reactor gave clearly better performance in sludge dewatering than the cationized starch which was noy subjected to further shearing. It is assumed, without wishing to be bound by ant theory, that the effective mixing stage before the completion of the cationization reaction induces an increase in charge density and a structure which is beneficial in dewatering applications.

Although certain embodiments and examples have been described in detail above, those having ordinary skill in the art will clearly understand that many modifications are possible in the embodiments and examples without departing from the teachings thereof. All such modifications are intended to be encompassed within the below claims of the invention.

Claims

1. Method for producing cationized starch having a cationic charge density of at least 2 meq/g, measured at pH 4, the method comprising

- adjusting the reaction mixture to a starting temperature, which is >40 °C and below a gelatinization temperature of the said starch;

- subjecting the highly viscous reaction mixture to a drying step for obtaining dried cationized starch.

2. Method according to claim 1 , characterized in that 1 - 15 weight-%, preferably 1 - 10 weight-%, more preferably 1 - 5 weight-%, of alkaline agent, calculated from the amount of the cationization agent, is added to the reaction mixture.

3. Method according to claim 1 or 2, characterized in that after the addition of the alkaline agent, the reaction mixture comprises 25 - 40 weight-%, preferably 30 - 38 weight-%, more preferably 32 - 36 weight-%, of water, calculated from the total weight of the cationization agent, alkaline agent, water and starch.

4. Method according to claim 1 or 2, characterized in that after the addition of the alkaline agent, the reaction mixture comprises

- 15 - 40 weight-%, preferably 20 - 35 weight-%, more preferably 25 - 33 weight- %, of the cationization agent, - 25 - 40 weight-%, preferably 30 - 38 weight-%, more preferably 32 - 36 weight- %, of water,

- 30 - 50 weight-%, preferably 33 - 47 weight-%, preferably 35 - 45 weight-%, of starch, calculated from the total weight of the cationization agent, alkaline agent, water and starch.

5. Method according to any of preceding claims 1 - 4, characterized in that the reaction mixture is subjected to kneading with a mixer tip speed of >1 m/s, preferably >1 .5 m/s, more preferably >2 m/s.

6. Method according to any of preceding claims 1 - 5, characterized in that the starting temperature is adjusted to a temperature in a range of 40 - 75 °C, preferably 45 - 72 °C, more preferably 50 - 60 °C.

7. Method according to any of claims 1 - 6, characterized in that after the addition of the alkaline agent the reaction mixture has a viscosity of 50 000 - 1 000 000 mPas, preferably 100 000 - 700 000 mPas.

8. Method according to any of claims 1 - 7, characterized in that the temperature of the reaction mixture is temporarily raised up to a temperature of 95 °C, preferably 70 - 90 °C, after the addition of alkaline agent.

9. Method according to any of preceding claims 1 - 8, characterized in that the drying step is performed as contact drying, freeze drying or vacuum drying.

10. Method according to claim 9, characterized in that the drying step is performed as contact drying, preferably as drum or roll drying.

11 . Method according to any of preceding claims 1 - 10, characterized in that the drying is performed at 100 - 200 °C, preferably 120 - 180 °C, more preferably 140

- 165 °C, and/or under a drying time of 0.5 - 15 min, preferably 1 - 10 min, more preferably 1 - 5 min.

12. Method according to any of preceding claims 1 - 11 , characterized in that the drying step has a drying severity value <5000 h, preferably <1000 h, more preferably <500 h, wherein the drying severity value is calculated as

13. Method according to any of preceding claims 1 - 12, characterized in that the obtained dry cationized starch is comminuted after the drying step.

14. Cationized starch having a cationic charge density of at least 2 meq/g, measured at pH 4, obtained by the method according to any of claims 1 - 13.

15. Cationized starch according to claim 14, characterized in that the cationized starch has

- the cationic charge density in a range of 2.0 - 3.5 meq/g, preferably 2.1 - 3.3 meq/g, more preferably 2.4 - 3.0 meq/g, measured at pH 4, and/or

- a turbidity value of 1 - 150 NTU, preferably 1 - 100 NTU.

16. Use of cationized starch according to claim 14 or 15 or obtained by the method according to any of claims 1 - 13 as a flocculation agent in a liquid-solid separation, such as a treatment of water and/or biological sludge.