Method for producing cationic starch, cationic starch and use thereof

Abstract

This invention relates to a method for producing cationic starch having a cationic charge density of at least 2 meq / g. A reaction mixture comprising water, starch, and a certain amount of a cationic agent is formed and adjusted to an initial temperature >40°C and below the gelation temperature of the starch. At least 1 wt% of an alkaline reagent is added to catalyze a cationic reaction between the cationic agent and the starch. The reaction mixture comprises up to 40 wt% water and 30 wt%–50 wt% starch. During the cationic reaction, the reaction mixture is kneaded at a reaction temperature >40°C, wherein the starch dissolves and the reaction mixture transforms into a highly viscous agglomerate. The viscous reaction mixture is then dried to obtain dried cationic starch.

Description

Technical Field

[0001] According to the preamble of the appended independent claims, the present invention relates to a method for producing dried cationic starch, cationic starch, and its uses. Background Technology

[0002] Cationic starch has wide applications in industry, such as in paper and paperboard manufacturing. The current trend of reducing the use of petroleum-based chemicals and replacing them with renewable, bio-based alternatives has increased interest in cationic starch, with a general desire to find new industrial applications. Particularly in water purification and sludge treatment, there is a growing demand for bio-based treatment chemicals that readily degrade in soil, resulting in treated sludge suitable for landfill. Highly cationic starch is of particular interest due to its high water solubility, making it easy to use industrially.

[0003] There are many different methods for producing cationic starch, with wet and dry processes being the most commercially common. In the wet process, the starch remains in slurry form throughout the cationization reaction, i.e., as dispersed particles in the liquid phase. The presence of a large amount of water reduces the yield of the wet process and complicates the separation of the produced cationic starch from other process components, for example, through filtration and washing. In the dry process, the starch remains in dry powder form throughout the cationization reaction, and all chemical components used for the reaction and its activation are in powder form. The dry process of starch cationization does not involve filtration and washing steps, but the resulting cationic starch may contain residual salts, unreacted reagents, and other impurities, which is disadvantageous.

[0004] The cationization process affects the properties of the resulting cationic starch, most notably the degree of cationization. It has been observed that cationic starches perform differently in their end-use applications based on their production history. A method for producing cationic starch that exhibits high cationization and effective functional properties in the end-use applications of the produced starch remains needed. Summary of the Invention

[0005] The purpose of this invention is to minimize or even eliminate the drawbacks of the prior art.

[0006] Another object of the present invention is to provide an efficient and simple method for producing dried cationic starch with a high degree of cationization.

[0007] Another object of the present invention is a method for producing dried cationic starch, which is effective as a flocculant in liquid-solid separation.

[0008] Another object of the present invention is a dried cationic starch that is effective as a flocculant in liquid-solid separation.

[0009] These objectives are achieved through the features disclosed in the independent claims. Some preferred embodiments of the invention are given in the dependent claims. Unless otherwise expressly stated, the features cited in the dependent claims may be freely combined with each other.

[0010] The exemplary embodiments and advantages provided herein relate to all aspects of the invention, even if they are not always mentioned in isolation.

[0011] A typical method according to the present invention for producing cationic starch with a cationic charge density of at least 2 meq / g (measured at pH 4) includes:

[0012] - A reaction mixture containing water, starch, and a certain amount of cationic agent is formed; - Adjust the reaction mixture to an initial temperature greater than 40°C but lower than the gelation temperature of the starch. - Based on the amount of cationizing agent, at least 1 wt% of an alkaline reagent is added to the reaction mixture to catalyze the cationization reaction between the cationizing agent and starch. Based on the total weight of the cationizing agent, alkaline reagent, water, and starch, the reaction mixture contains up to 40 wt% water and 30 wt%-50 wt% starch. - During the cationization reaction, the reaction mixture is kneaded at a reaction temperature >40°C, preferably ≥50°C, wherein the starch dissolves and the reaction mixture is transformed into a high-viscosity reaction mixture. - A drying step is performed on the high-viscosity reaction mixture to obtain dried cationic starch.

[0013] The typical cationic starch according to the 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 invention.

[0014] The typical use of the cationic starch obtained by the method according to the invention is as a flocculant in liquid-solid separation, for example, in water treatment and / or biological sludge treatment.

[0015] It has now been surprisingly discovered that highly cationic starch can be efficiently produced by manufacturing dried cationic starch under selected reaction conditions. The resulting cationic starch exhibits good to excellent performance as a flocculant in liquid-solid separation. In particular, the selected high temperature and effective mixing throughout the cationization reaction provide the obtained cationic starch with a high degree of cationization and suitable viscosity behavior suitable for industrial applications. The theoretical background behind this invention is not yet fully understood. However, the method according to the invention provides an efficient and simple method for producing cationic starch with a high degree of cationization. Therefore, the key point of this invention is the production of cationic starch with optimal performance, particularly suitable as a flocculant in liquid-solid separation, such as in the dewatering of biological sludge.

[0016] Any available starch, such as potato starch, waxy potato starch, rice starch, corn starch, waxy corn starch, wheat starch, barley starch, pea starch, or cassava starch, can be used for cationization in the method of this application. According to a preferred embodiment, the starch used is selected from potato starch, corn starch, or cassava starch. More preferably, the starch is potato starch. Preferably, the starch used is undegraded starch, either natural starch or low-cationized starch. The starch to be cationized can be provided in the form of dry granular material, i.e., aggregates or powders with a moisture content of 5 wt%-25 wt%, preferably 10 wt%-20 wt%.

[0017] First, a reaction mixture comprising at least starch, a cationic agent, and water is formed for the production of cationic starch. The components of the reaction mixture, namely starch, cationic agent, and water, can be mixed together in any order. For example, the cationic agent can be added to water, followed by granular starch, or vice versa. The resulting reaction mixture is in the form of a slurry or dispersion. This refers to solid starch granules being uniformly dispersed in a liquid phase containing water and the cationic agent. The reaction mixture is maintained under mixing to provide and maintain a uniform dispersion of starch granules in the liquid phase.

[0018] The cationizing agent can be any cationizing agent commonly used for polysaccharides, particularly starch cationization. According to a preferred embodiment, the cationizing agent is selected from 2,3-epoxypropyltrimethylammonium chloride (EPTAC) or 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) or any mixture thereof.

[0019] After the reaction mixture is formed, the temperature of the reaction mixture is adjusted to an initial temperature, which is >40°C, preferably 45°C or higher (i.e., ≥45°C), more preferably 50°C or higher (i.e., ≥50°C), and sometimes even 52°C or higher, but below the gelation temperature of the starch used. The gelation temperature, as referred herein, means the temperature or range at which the intermolecular bonds of starch break down in the presence of water. The gelation temperature depends on the starch used, and the gelation temperatures of different starch types are known to those skilled in the art. This means that the starch in the reaction mixture has not gelled or gelatinized before the addition of the alkaline reagent and the catalytic cationization reaction; that is, the method does not gelatinize and / or gelatinize the starch present in the reaction mixture. According to one embodiment of the invention, before the addition of the alkaline reagent, the reaction mixture can be adjusted to an initial temperature, which is >40°C, preferably ≥45°C, more preferably ≥50°C or ≥52°C, but below the gelation temperature, which is in the range of 53-72°C, typically 55-72°C. Taking into account the gelation temperature of the starch used, the initial temperature of the reaction mixture can be adjusted to, for example, a temperature in the range of >40 to 75°C, preferably 45 to 72°C, more preferably 50 to 60°C or 52 to 60°C.

[0020] As described above, after adjusting the temperature of the reaction mixture, at least 1 wt% of an alkaline reagent (provided as an activator) is added to the reaction mixture based on the weight of the cationizing agent in the reaction mixture to catalyze the cationization reaction between the cationizing agent and starch. The alkaline reagent 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. Based on the weight of the cationizing agent in the reaction mixture, the alkaline reagent may be added in an amount of 1 wt%-15 wt%, preferably 1 wt%-10 wt%, more preferably 1 wt%-5 wt%.

[0021] After the addition of the alkaline reagent, the reaction mixture may contain up to 40 wt% water, calculated based on the total weight of the cationizing agent, alkaline agent, water, and starch. Based on the total weight of the cationizing agent, alkaline agent, water, and starch, the reaction mixture may contain ≤40 wt%, preferably ≤38 wt%, more preferably ≤36 wt% water, and / or ≥25 wt%, preferably ≥30 wt% water. According to one embodiment, after the addition of the alkaline reagent and at the start of the cationization step, the reaction mixture may contain 25 wt%-40 wt%, preferably 30 wt%-38 wt%, more preferably 32 wt%-36 wt% water, calculated based on the total weight of the cationizing agent, alkaline agent, water, and starch in the reaction mixture. When calculating the amount of water in the reaction mixture, the water initially present in the components of the reaction mixture at the time of its formation, i.e., the water content of the starch and cationizing agent, is taken into account. Low amounts of water can efficiently produce dry cationized starch and minimize the amount of unwanted byproduct residues. Furthermore, without being bound by any theory, we assume that low water content has a positive effect on the properties of cationic starch.

[0022] After the addition of the alkaline reagent, the reaction mixture may contain 30 wt%-50 wt%, preferably 33 wt%-47 wt%, more preferably 35 wt%-45 wt% or 36 wt%-43 wt% of starch, calculated based on the total weight of the cationizing reagent, alkaline reagent, water and starch.

[0023] When the temperature of the reaction mixture is maintained at >40°C, preferably 45°C or higher, more preferably 50°C or higher, the water / starch ratio in the reaction mixture can be 0.6-1.2, preferably 0.7-1.1.

[0024] Furthermore, after the addition of the alkaline reagent, the reaction mixture may contain 15 wt%-40 wt%, preferably 20 wt%-35 wt%, and more preferably 25 wt%-33 wt% of the cationic reagent, calculated based on the total weight of the cationic reagent, alkaline reagent, water, and starch.

[0025] The reaction mixture is maintained under mixing during and after the addition of the alkaline reagent. The alkaline reagent catalyzes the cationization reaction, which may result in a temperature peak, whereby the temperature of the reaction mixture may temporarily rise above the gelation temperature of the starch used. This temporary temperature peak, occurring after the addition of the alkaline reagent, can reach a maximum of 95°C, for example, 70-90°C, or 72-80°C. However, the temperature of the reaction mixture is maintained below the boiling point of the reaction mixture, i.e., <100°C. The duration of the temperature peak, i.e., the time during which the temperature exceeds the initial temperature and preferably the gelation temperature of the starch used, can be relatively short, for example, 1-120 min, preferably 10-60 min, more preferably 20-40 min. It is not desired to be bound by any theory, but it is presumed that the temperature peak promotes the simultaneous cationization and dissolution of the starch in the reaction mixture.

[0026] After the temperature peak, the temperature of the reaction mixture is reduced to the reaction temperature, and the reaction mixture may be maintained at this reaction temperature >40°C, preferably ≥45°C, more preferably ≥50°C or ≥52°C, until the cationization reaction is completed, or until at least 80 wt%, preferably at least 90 wt%, more preferably at least 95 wt% or at least 99 wt% of the cationizing agent has reacted, and / or until the amount of unreacted cationizing agent in the reaction mixture is reduced to ≤100 ppm, preferably 10-100 ppm. Preferably, after the temperature peak, the temperature of the reaction mixture is maintained in the range of 40-75°C, preferably 45-70°C, more preferably 50-60°C or 52-60°C.

[0027] In the context of this application, it is considered that the cationization reaction begins when a basic reagent is added to the reaction, and ends when the amount of unreacted cationizing reagent in the reaction mixture decreases to ≤50 ppm.

[0028] As the cationization reaction proceeds and the starch begins to dissolve, the viscosity of the reaction mixture begins to increase, and the reaction mixture transforms into a high-viscosity agglomerate or high-viscosity reaction mixture. Thus, the reaction mixture changes from a slurry or dispersion containing starch particles to a homogeneous reaction mixture in which no individual starch particles are visible to the naked eye. At this stage, the high-viscosity reaction mixture can be described as a dough-like agglomerate. After the addition of an alkaline reagent, the reaction mixture can have a viscosity of 50,000-1,000,000 mPas, preferably 75,000-850,000 mPas, more preferably 100,000-700,000 mPas, measured using Anton Paar MCR302e CC27 at a shear rate of 10 1 / s and a temperature of 50°C. In this invention, the high viscosity of the reaction mixture is advantageous because it enables, for example, efficient drying of the obtained cationized starch by contact drying as described below.

[0029] During the cationization reaction, the reaction mixture is preferably kneaded or mixed continuously at a reaction temperature >40°C, preferably ≥45°C, more preferably ≥50°C or ≥52°C. As described above, as the starch dissolves and the cationization reaction proceeds, the reaction mixture transforms into a high-viscosity form.

[0030] According to a preferred embodiment, the reaction mixture may be kneaded or mixed at a mixer tip speed in the range of ≥1 m / s, preferably ≥1.5, more preferably ≥2 m / s, for example, 1-5 m / s, preferably 1.5-4.5, more preferably 2-4 m / s, and sometimes 2.5-4 m / s, before the cationization reaction is completed. Kneading or mixing at a specified high mixer tip speed can enhance the cationization reaction. Kneading or mixing at a high mixer tip speed can preferably be carried out before the cationization reaction is completed, i.e., before 99 wt%, more preferably 95 wt%, even more preferably 90 wt% of the cationizing agent has reacted and / or before the amount of unreacted cationizing agent in the reaction mixture has decreased to ≤50 ppm, preferably ≤100 ppm. During the cationization reaction, the kneading or mixing speed of the reaction mixture may vary. For example, the reaction mixture may initially be kneaded or mixed at a low speed, i.e., a mixer tip speed <2 m / s or <1 m / s, and then the mixer tip speed is increased to the value defined above before the cationization reaction is completed. Alternatively, the reaction mixture may be kneaded or mixed at a constant rate via a cationization reaction, with the mixer tip speed as defined above.

[0031] 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 include, for example, Lödige™ mixers, such as Ploughshare™ mixers, Sigma™ mixers, etc. Suitable mixing devices also include extruders, such as single-screw extruders and twin-screw extruders. According to one embodiment, the high-shear mixing device may include a rotary mixing tool, such as mixing blades, impellers, agitators, screws, etc., wherein the mixing tool may have a tip speed of ≥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.

[0032] According to one embodiment, after the cationization reaction is completed, the reaction mixture can be kneaded or mixed at a mixer tip speed in the range of ≥1 m / s, preferably ≥1.5 m / s, more preferably ≥2 m / s, for example, 1-5 m / s, preferably 1.5-4.5 m / s, more preferably 2-4 m / s, and sometimes 2.5-4 m / s. It has been observed that kneading or mixing at a high mixer tip speed can improve the properties of the cationized starch, for example, increasing charge density and / or reducing viscosity.

[0033] During the cationization reaction, additional water may be added to the reaction mixture after the temperature peak has passed. Additional water may also be added to the viscous reaction mixture at or after the cationization reaction but before the drying step. Based on the total weight of the reaction mixture, additional water may be added in amounts of 0.5 wt%-35 wt%, preferably 5 wt%-30 wt%, more preferably 10 wt%-25 wt%, and sometimes 15 wt%-25 wt%. Adding additional water, especially when the viscosity of the reaction mixture increases significantly, improves the processability of the reaction mixture. However, considering the efficiency of the subsequent drying step, the amount of additional water added during and / or after the cationization reaction, regardless of its source, should be as small as possible. According to one embodiment, the method does not add water to the reaction mixture after the addition of the basic reagent.

[0034] According to one embodiment of the invention, after the cationization reaction is completed and before the drying step, the reaction mixture can be neutralized with an acid. Any suitable acid can be used for neutralization, for example, organic acids such as citric acid, adipic acid, formic acid, acetic acid, fumaric acid, or mixtures thereof, and / or inorganic acids such as hydrochloric acid. Citric acid is preferred because it provides optimal control over the color and odor of the obtained cationized starch. The pH of the viscous reaction mixture is preferably adjusted to 5-10, preferably in the range of 5-9.

[0035] After the cationization reaction is completed, the viscous reaction mixture is subjected to a drying step to obtain dried cationized starch. Any suitable drying technique can be used in this drying step. According to a preferred embodiment, the reaction mixture is dried in the drying step to have a drying severity value of ≤5000 h, preferably ≤1000 h, more preferably ≤500 h, wherein the drying severity value is calculated as follows:

[0036] Preferably, the drying intensity value is within the range of 10-5000 h, more preferably 10-1000 h, and even more preferably 10-500 h. The drying intensity value can even be within the range of 10-400 or 15-350 h. It has been observed that the drying intensity affects the properties of the obtained dried cationic starch. If the drying intensity value is too high, the properties of the dried cationic starch will deteriorate significantly. Without being bound by any theory, it is assumed that if the drying process is too harsh, the structure of the cationic starch may be damaged.

[0037] According to one embodiment of the invention, the reaction mixture may undergo a drying step, wherein the drying can be performed as contact drying, freeze drying, or vacuum drying. According to a preferred embodiment, the drying step can be performed as contact drying, for example, drum drying or roller drying. Contact drying is understood herein as an indirect drying technique, such as drum drying or roller drying, in which the heat required to remove moisture is transferred by conduction. After the cationization reaction, the reaction mixture is in the form of a highly viscous agglomerate or paste, making it particularly suitable for contact drying. In contact drying, the highly viscous reaction mixture can be applied as a layer to a drying surface, such as the outer surface of a heated rotating drum or roller, where moisture evaporates from the reaction mixture. This yields a layer of dried cationized starch, which can be removed from the drying surface. The contact drying can employ an arrangement of multiple heated rotating drums or rollers to dry the viscous reaction mixture on their surfaces. The contact drying, for example, drum drying or roller drying, can be performed using a drying surface temperature of 100-200°C, preferably 120-180°C, more preferably 140-165°C. If contact drying is performed using a drum or roller drying method and employs an arrangement including multiple heated drums, the surface temperatures of the individual drums can be the same or different from each other. For example, the arrangement can have increasing or decreasing temperature profiles. The drying time in the contact drying can 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 intensity value. The contact drying produces continuous strips or bands of cationic starch, which can then be easily, for example, pulverized to a suitable particle size by grinding or crushing.

[0038] Typically, after the drying step, the obtained dried cationic starch can be pulverized into flakes, microparticles, granules, or powder, for example, by cutting, crushing, and / or grinding. Any suitable cutting, grinding, crushing, or grinding equipment can be used. The dried cationic starch can be pulverized to a particle size in the range of 0.1-2.5 mm, preferably 0.125-1.5 mm. The pulverization method and particle size can be selected according to the intended end use of the dried cationic starch.

[0039] After pulverization, if necessary, the obtained dried cationic starch granules may undergo an optional secondary drying step. This secondary drying can be performed in a rotary dryer, fluidized bed dryer, or any other suitable available dryer equipment. The secondary drying is entirely optional, but it can be used to adjust the moisture content of the dried cationic starch, particularly if the dried cationic starch requires an extremely low moisture content of 1 wt%–9 wt% or 2 wt%–5 wt%.

[0040] Preferably, this method does not add any functionalizing agents other than cationic agents used for starch functionalization.

[0041] The dried cationic starch obtained by the method of the present invention has a cationic charge density of at least 2 meq / g, preferably at least 2.1 meq / g, and more preferably at least 2.4 meq / g, as measured at pH 4. The cationic charge density of the obtained cationic starch, measured at pH 4, can be in the range of 2.0-3.5 meq / g, preferably 2.1-3.3 meq / g, and more preferably 2.4-3.0 meq / g. The charge density at pH 4 can be determined using a particle charge titrator from AFG Analytics, as described in the experimental section. Therefore, the method of this application can provide dried cationic starch with high cationicity. This starch is particularly suitable for liquid-solid separation, such as water treatment and / or biological sludge dewatering.

[0042] The cationic starch obtained by the method of this application can 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 can be in the range of 0.45-1.2, preferably 0.5-1.1, more preferably 0.6-0.9 or 0.7-0.8. In the context of this application, the degree of substitution defines how many substituent groups are contained in the cationic starch, calculated per unit of dehydrated glucose of starch. The degree of substitution can be calculated based on the charge density of the starch or by measuring the nitrogen content, such as Kjeldahl nitrogen, and calculating the charge density and degree of substitution based on the measurement results. These calculation and determination methods are known to those skilled in the art. Therefore, the method according to the invention aims to produce cationic starch with high or very high cationicity. The method of this application can produce, for example, cationic starch that is water-soluble at temperatures <50°C, preferably <40°C, or even <30°C, as described below.

[0043] In the context of this application, "dried cationic starch" refers to cationic starch obtained by the methods disclosed in this application, having the cationic charge density as described above, and a moisture content of <18 wt%, typically <10 wt%. The moisture content of dried cationic starch can be 1 wt%-18 wt%, more typically 1.5 wt%-10 wt% or 2 wt%-9 wt%. The obtained dried form of cationic starch is easy to transport and store. However, as mentioned above, due to its high cationicity, the starch according to the invention is readily soluble in water at temperatures <50°C without the need for cooking. This makes the use of the dried cationic starch according to the invention in industrial processes uncomplicated.

[0044] The cationic starch obtained by the method according to the invention is water-soluble. Preferably, the cationic starch is soluble in water at a temperature of 30°C or lower, for example, in water at a temperature of 10-25°C. Water solubility means that the cationic starch is completely miscible with water, providing a clear or nearly clear solution. When mixed with excess water, the cationic starch dissolves, and the resulting cationic starch solution is preferably substantially free of visible discrete solid particles, granules, or agglomerates of cationic starch. Excess water means that the resulting solution is not a saturated solution. The cationic starch obtained by the method of this application does not require gelatinization to dissolve. The cationic starch can have a turbidity value of less than 1000 NTU, preferably less than 500 NTU, more preferably less than 250 NTU. The turbidity value of the cationic starch can be in the range of 1-150 NTU, preferably 1-100 NTU. The turbidity value is measured by using a HACH, 2100 AN IS Laboratory Turbidimeter at a concentration of 1 wt% from the cationic starch aqueous solution.

[0045] The dried cationic starch obtained by this invention can be used as a flocculant or dewatering agent in aqueous liquid-solid separation processes. For example, the dried cationic starch can be used as a flocculant in water purification or as a dewatering agent in sludge dewatering for urban or industrial wastewater treatment.

[0046] The cationic starch obtained by the method according to the invention is suitable for increasing the solids content of various sludges, particularly biological sludge. Sludge is to be understood herein as an aqueous suspension comprising a continuous aqueous liquid phase and organic and / or inorganic solid materials and / or particles suspended in the aqueous liquid phase. The sludge may be rich in bacterial-derived materials. The aqueous liquid phase of the sludge may also contain dissolved organic matter such as polysaccharides, humic substances, and fatty acids. Due to the variable content of organic matter (such as bacteria) and other organic matter, the treatment of this sludge, and wastewater, particularly municipal and agricultural wastewater, differs from the treatment of inorganic metallurgical suspensions or cellulose fiber suspensions, such as the treatment of pulp used in papermaking or paperboard manufacturing. The cationic starch obtained by the method of this application is particularly and unexpectedly suitable for treating sludge that can be municipal wastewater sludge or agricultural sludge, or biological sludge that can be derived from biological treatment processes of sewage and / or domestic sewage. Alternatively, the aqueous suspension treated with cationized starch obtained by the method of the present invention may be derived from industrial processes, particularly from wastewater treatment of industrial processes, or from food or beverage production or processing. Detailed Implementation

[0047] experiment

[0048] Some embodiments of the present invention are described in the following non-limiting examples.

[0049] The following characterization methods were used in these embodiments: The cation charge density was determined at pH 4 using AFG Analytics' CAS-II touch! Charge Analyzing System. The sample to be analyzed was dissolved in deionized water to obtain a concentration of 0.025 wt%–0.05 wt%. The pH of the sample was adjusted to 4.0 with 0.1 M acetic acid, and titration was performed using a 0.001 N sodium polyvinyl sulfonate (PES-Na) solution as the titrant. During titration, the pH of the sample typically increased by 0.1–0.2 pH units. Based on the titration results, the charge density was calculated and expressed as meq / g dry sample.

[0050] The dry content was measured by drying approximately 2 g (cationized starch) of the sample overnight in an oven at 105°C. The dry content is expressed as a percentage of the sample weight.

[0051] The turbidity of a 1 wt% solution was measured using a HACH TL2360 Laboratory Turbidimeter.

[0052] The viscosity of a 3 wt% cationic starch aqueous solution was measured at 25°C using a Brookfield LV viscometer with a small sample adapter, with either rotor #18 or #31 depending on the viscosity level. The viscosity at the highest possible rotational speed selected from 0.3, 0.6, 1.5, 3, 6, 12, 30, 60, and 100 rpm was used as the result value.

[0053] Example 1

[0054] 117 kg of glycidyltrimethylammonium chloride (Raisacat™ 151, Chemigate Oy, Finland), 42 kg of deionized water, and 127.9 kg of potato starch were added to a 600-liter reactor of a Lödige plowshare mixer to form a reaction mixture. The reaction mixture was in slurry form and was stirred at 50 rpm and heated to 50°C. The mixer tip speed was 2.6 m / s. After the temperature of the reaction mixture stabilized, 4.56 kg of 50% NaOH solution was added to initiate the cationization reaction. After approximately 30 minutes, the reactor temperature rose to approximately 70°C. The viscosity of the reaction mixture increased during the temperature rise, and the reactor mixture transformed into a high-viscosity mixture, i.e., a doughy agglomerate.

[0055] When the temperature reaches 70°C, automatic control begins to cool the reactor and remove the heat of reaction. Within approximately 30 minutes, the temperature returns to 50°C. The reactor mixture is maintained at 50°C for 6 hours. The stirring speed is then set to 20 rpm (mixer tip speed 1 m / s), and the reactor mixture is stirred overnight.

[0056] After 24 hours (from the initial formation of the reaction mixture), 66 kg of water was added to the reaction mixture to reduce viscosity, and the reaction mixture was stirred for another 30 minutes. The viscous reaction mixture was pumped to a roller dryer using a piston pump and dried. The surface temperature of the roller dryer was approximately 130-140°C, and the contact time was approximately 60 seconds. The resulting flake-like dried cationic starch CS1 was ground into powder and sampled. The properties of the cationic starch CS1 are provided in Table 1.

[0057] Example 2

[0058] 1932 g of glycidyltrimethylammonium chloride (Raisacat™ 151, Chemigate Oy, Finland), 255 g of deionized water, and 2131 g of potato starch were added to a 10-liter reactor of a Lödige plowshare mixer to form a reaction mixture. The resulting reaction mixture was in slurry form and was stirred at 150 rpm. The mixer tip speed was 2.0 m / s. The temperature of the reactor mixture was heated to 50°C. Once the temperature stabilized, 54 g of 50% NaOH solution was slowly added to the reaction mixture over 10 minutes to initiate the cationization reaction. After approximately 30 minutes, the temperature of the reactor was raised to approximately 70°C. During the heating process, the viscosity of the reaction mixture increased, and the reaction mixture became a high-viscosity, e.g., dough-like lump.

[0059] When the reactor temperature reaches approximately 70°C, automatic control begins to cool the reactor and remove the heat of reaction. Within approximately 30 minutes, the temperature returns to approximately 50°C. The reactor contents are maintained at 50°C. After 6 hours, the stirring speed is set to 20 rpm (mixer tip speed 0.3 m / s), and the reactor contents are stirred overnight.

[0060] After 24 hours (from the initial formation of the reaction mixture), the viscous reaction mixture was removed and sampled. The cationic starch CS2 sample was dried in a vacuum oven at 40°C, ground into powder, and analyzed. The properties of the obtained cationic starch CC2 are provided in Table 1.

[0061] Example 3

[0062] 117 kg of glycidyltrimethylammonium chloride (Raisacat™ 151, Chemigate Oy, Finland), 42 kg of deionized water, and 127.9 kg of potato starch were added to a 600-liter reactor of a Lödige plowshare mixer to form a reaction mixture. The resulting reaction mixture was in slurry form and was stirred at 50 rpm. The mixer tip speed was 2.6 m / s. The temperature of the reactor mixture was heated to 50°C. After the temperature of the reaction mixture stabilized, 4.56 kg of 50% NaOH solution was added to the reaction mixture to initiate the cationization reaction. After approximately 30 minutes, the reactor temperature was raised to approximately 70°C. During the heating process, the viscosity of the reaction mixture increased, and the reaction mixture became a high-viscosity, e.g., dough-like lump.

[0063] When the reactor temperature reaches approximately 70°C, automatic control begins to cool the reactor and remove the heat of reaction. Within approximately 30 minutes, the temperature returns to approximately 50°C. The reactor contents are maintained at 50°C for 6 hours. After 6 hours, the stirring speed is set to 20 rpm (mixer tip speed 1 m / s), and the reactor contents are stirred overnight.

[0064] After 24 hours (from the initial formation of the reaction mixture), the viscous reaction mixture was removed from the reactor. The viscous reaction mixture was spread on a plastic plate and dried overnight in a vacuum oven at 40°C and 200 mbar (abs). The resulting dried cationic starch CS3 was ground and analyzed. The properties of the cationic starch CS3 are provided in Table 1.

[0065] Example 4

[0066] 2116 g of glycidyltrimethylammonium chloride (Raisacat™ 151, Chemigate Oy, Finland), 732 g of deionized water, and 1946 g of potato starch were added to a 10-liter reactor of a Lödige plowshare mixer to form a reaction mixture. The resulting reaction mixture was in slurry form and was stirred at 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 stabilized, 45 g of 50% NaOH solution was added to the reaction mixture to initiate the cationization reaction. After approximately 30 minutes, the reactor temperature was raised to approximately 70°C. During the heating process, the viscosity of the reaction mixture increased, and the reaction mixture became highly viscous, e.g., a doughy lump.

[0067] When the reactor temperature reaches approximately 70°C, automatic control begins to cool the reactor and remove the heat of reaction. Within approximately 30 minutes, the temperature returns to approximately 50°C. The reactor contents are maintained at 50°C for 6 hours. After 6 hours, the stirring speed is set to 20 rpm (mixer tip speed 0.3 m / s), and the reactor contents are stirred overnight.

[0068] After 24 hours (from the initial formation of the reaction mixture), the viscous reaction mixture was removed from the reactor and sampled. The obtained cationic starch CS4 sample was dried in an oven and analyzed. The properties of the obtained cationic starch CS3 are provided in Table 1.

[0069] Comparative Example 5

[0070] 59.8 g glycidyltrimethylammonium chloride (Raisacat™ 151, Chemigate Oy, Finland), 20.7 g deionized water, and 55.0 g potato starch were added to a cylindrical container. The mixture was vigorously stirred to obtain a slurry. The container containing the slurry was placed in a shaker in a constant temperature bath at 20°C. When the temperature stabilized, 1.26 g of 50% NaOH solution was added dropwise to the reaction mixture to initiate the cationization reaction. The container was tightly sealed with a lid and placed in a shaker in the bath (20°C) for 168 hours. After several days, the slurry turned into a moist powder, and at the end of the shaking period, the powder solidified into a moist cake. The obtained wet cake sample was dried in a vacuum oven at 40°C, ground into powder, and analyzed. The characteristics of the obtained cationic starch CCS are provided in Table 1. It can be seen that when the temperature (20℃) and the mixing intensity throughout the reaction process are low, the obtained cationic starch CCS has a low charge density and extremely high viscosity, such as a 3 wt% solution. These characteristics make cationic starch CCS difficult to use in practical applications, especially in liquid-solid separation, such as sludge dewatering.

[0071] Table 1. Characteristics of starches prepared in Examples 1-4 and Comparative Example 5 Measured from a 1 wt% solution Measured from a 3 wt% solution

[0072] Example 6: Performance of cationic starch in sludge

[0073] The performance of the cationic starch CS3 prepared in Example 3 for sludge dewatering was compared with that of commercial cationic starch.

[0074] Cationic starch CS3 was dissolved in water to a concentration of 0.2 wt%.

[0075] For reference, two commercially available cationic starches were used: RCS1 (Vector IC 42280, Roquette, starch solution with a dry content of 43 wt%) and RCS2 (Hi-Cat 1574A, Roquette, dry starch powder). Both reference starches were used as 0.2 wt% aqueous solutions: RCS1 was diluted with water to a concentration of 0.2 wt%, and RCS2 was dissolved in water to a concentration of 0.2 wt%.

[0076] The efficiency of the cationic starch CS3 according to the present invention and commercial reference starches RCS1 and RCS2 in sludge dewatering was tested as follows. The sludge used in each test was digested sludge obtained from a municipal wastewater treatment plant in Finland. The sludge had a dry content of 2.7 wt% and a pH of 7.9. The CST time of the sludge was 199 s without the addition of any chemicals.

[0077] Sludge dewatering was tested using capillary time of absorption (CST). These tests were conducted using a Triton 319 multi-purpose CST unit with a Type 317 stirrer-timer (both from Triton Electronics Ltd, UK). The cylindrical tank used had a diameter of 18 mm. The mixing speed used in these tests was 1000 rpm. The starch sample under study was added to 100 g of sludge. The sludge was mixed 10 s after the starch sample was added, and then 4.5 mL of the sample was placed in the cylindrical tank and the CST value was measured.

[0078] The dosage of the starch sample is given in kg active starch / ton of dry sludge. For CS3 and RCS2, it is assumed that the active starch content is equal to starch / product itself. For RCS1, the active starch content is calculated from the dry content of the starch.

[0079] The CST results are shown in Table 2.

[0080] Table 2 CST time results for Example 5

[0081] As can be seen from Table 2, the cationic starch CS3 prepared according to the present invention significantly outperforms commercial cationic starch at each dosage level. The better results are considered to be due to the lower measured CST times, indicating that the cationic starch CS3 effectively forms flocs and promotes sludge dewatering.

[0082] Example 7: Sludge dewatering performance of cationic starch

[0083] The performance of cationized starch CS3 prepared in Example 3 and cationized starch CS4 prepared in Example 4 for sludge dewatering was compared with that of cationized starch CCS prepared in Comparative Example 5.

[0084] Sludge dewatering tests were conducted in the same manner as in Example 6. The sludge used in these tests was digested sludge obtained from a municipal wastewater treatment plant in Finland. The sludge had a dry content of 2.8 wt% and a pH of 7.5. The CST time of the sludge was 425 s without the addition of any chemicals.

[0085] The CST results are shown in Table 3.

[0086] As can be seen from Table 3, the cationic starches CS3 and CS4 obtained by the method of the present invention have significantly better performance in sludge dewatering than the cationic starch CCS obtained at low temperature and low mixing intensity.

[0087] Table 3 CST time results for Example 6

[0088] Example 8

[0089] 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 to a 133-liter Winkworth Sigma blade mixer to form a reaction mixture. The resulting reaction mixture was in slurry form and was stirred at 20 rpm with a mixer tip speed of 0.3 m / s. The temperature of the reactor mixture was heated to 50°C. When the temperature stabilized, 0.7 kg of 50% NaOH solution was added to the reaction mixture to initiate the cationization reaction. After approximately 1 hour, the temperature of the reactor was raised to approximately 63°C. During the heating process, the viscosity of the reaction mixture increased, and the reaction mixture became highly viscous, such as a doughy lump. After approximately 3 hours, the temperature of the reaction mixture was returned to approximately 50°C. Cooling of the reaction mixture was not required or performed. After 6 hours, the stirring speed was set to 2 rpm (mixer tip speed 0.03 m / s), and the reactor contents were stirred overnight.

[0090] After 24 hours (from the initial formation of the reaction mixture), the viscous reaction mixture was removed from the reactor and sampled. The sample was dried in an oven and analyzed. The characteristics of the cationic starch at this stage, prior to mixing at a tip speed of 1.5 m / s, are provided in Table 4, referred to as “cationic starch dough”.

[0091] 4410 g of the viscous reaction mixture was then added to a 5-liter reactor of a Lödige plowshare mixer. The temperature of the reactor mixture was heated to 50°C. The reaction mixture was stirred at 150 rpm. The mixer tip speed was 1.5 m / s. Samples of cationic starch were collected from the reaction mixture after mixing for 1, 2, 3, and 4 hours. These samples were dried in an oven at 60°C for 4 hours to form thin flakes, pulverized, dissolved, and analyzed as an aqueous solution. The properties of the obtained cationic starch are provided in Table 4.

[0092] Table 4. Properties of the starch prepared in Example 8 Measured from 3 wt% solution As can be seen from Table 4, further shearing of the reaction mixture at a tip speed of 1.5 m / s in a plowshare mixer resulted in a significant decrease in the viscosity of the obtained cationic starch and an increase in its charge density.

[0093] Example 9: Sludge dewatering performance of cationic starch

[0094] The sludge dewatering performance of the cationic starch prepared in Example 8 was compared.

[0095] Sludge dewatering tests were conducted using the same method as in Example 6, employing a capillary time of absorption (CST) test. The sludge used in these tests was digested sludge obtained from a municipal wastewater treatment plant in Finland. The sludge had a dry content of 4.1 wt% and a pH of 7.7. Without the addition of any chemicals, the CST time of the sludge was 174 s.

[0096] The CST results are shown in Table 5.

[0097] Table 5. CST time results of cationic starch in Example 8

[0098] Table 5 shows that the starch samples that underwent further shearing in the Lödige reactor performed significantly better in sludge dewatering than the cationized starch that did not undergo further shearing. Without being bound by any theory, it is assumed that an effective mixing phase prior to the completion of the cationization reaction leads to an increase in charge density and the formation of structures favorable for dewatering applications.

[0099] Although certain implementation methods and embodiments have been described in detail above, those skilled in the art will clearly understand that many modifications may be made to the implementation methods and embodiments without departing from their teachings. All such modifications are intended to be included in the appended claims of this invention.

Claims

1. A method for producing cationic starch having a cationic charge density of at least 2 meq / g as measured at pH 4, the method comprising: - A reaction mixture containing water, starch, and a certain amount of cationic agent is formed; - Adjust the reaction mixture to an initial temperature >40°C and below the gelation temperature of the starch; - Based on the amount of the cationizing agent, at least 1 wt% of an alkaline reagent is added to the reaction mixture to catalyze the cationization reaction between the cationizing agent and the starch. Based on the total weight of the cationizing agent, the alkaline reagent, water, and starch, the reaction mixture contains at most 40 wt% water and 30 wt%-50 wt% starch. - During the cationization reaction, the reaction mixture is kneaded at a reaction temperature >40°C, preferably ≥50°C, wherein the starch dissolves and the reaction mixture is transformed into a highly viscous reaction mixture. - The highly viscous reaction mixture is subjected to a drying step to obtain dried cationic starch.

2. The method of claim 1, wherein, Based on the amount of the cationizing agent, 1 wt%-15 wt%, preferably 1 wt%-10 wt%, more preferably 1 wt%-5 wt% of an alkaline reagent is added to the reaction mixture.

3. The method according to claim 1 or 2, characterized in that, After the addition of the alkaline reagent, the reaction mixture contains 25 wt%-40 wt%, preferably 30 wt%-38 wt%, and more preferably 32 wt%-36 wt% water, calculated by the total weight of the cationizing agent, alkaline reagent, water, and starch.

4. The method according to claim 1 or 2, characterized in that, After the addition of the alkaline reagent, the reaction mixture comprises, based on the total weight of the cationizing agent, alkaline reagent, water, and starch, the total weight of these components. - 15 wt%-40 wt%, preferably 20 wt%-35 wt%, more preferably 25 wt%-33 wt% of the cationic agent, - 25wt%-40wt%, preferably 30wt%-38wt%, more preferably 32wt%-36wt% water, - 30 wt%-50 wt%, preferably 33 wt%-47 wt%, more preferably 35 wt%-45 wt% starch.

5. The method according to any one of claims 1-4, characterized in that, The reaction mixture is kneaded at a mixer tip speed of ≥1 m / s, preferably ≥1.5 m / s, more preferably ≥2 m / s.

6. The method according to any one of claims 1-5, characterized in that, The initial temperature is adjusted to 40-75°C, preferably 45-72°C, and more preferably 50-60°C.

7. The method according to any one of claims 1-6, characterized in that, After the addition of the alkaline reagent, the reaction mixture has a viscosity of 50,000-1,000,000 mPas, preferably 100,000-700,000 mPas.

8. The method according to any one of claims 1-7, characterized in that, After adding the alkaline reagent, the temperature of the reaction mixture is temporarily raised to 95°C, preferably 70-90°C.

9. The method according to any one of claims 1-8, characterized in that, The drying steps are carried out by contact drying, freeze drying or vacuum drying.

10. The method according to claim 9, characterized in that, The drying step is carried out by contact drying, preferably by drum drying or roller drying.

11. The method according to any of the preceding claims 1-10, characterized in that, The drying is carried out at 100-200°C, preferably 120-180°C, more preferably 140-165°C, and / or for a drying time of 0.5-15 minutes, preferably 1-10 minutes, more preferably 1-5 minutes.

12. The method according to any of the preceding claims 1 - 11, characterized in that, The drying step has a drying intensity value of ≤5000 h, preferably ≤1000 h, and more preferably ≤500 h, wherein the drying intensity value is calculated as follows: 。 13. The method according to any one of claims 1-12, characterized in that, The obtained dry cationic starch is pulverized after the drying step.

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

15. The cationic starch according to claim 14, characterized in that, The cationic starch has - The cation charge density measured at pH 4 in the range of 2.0–3.5 meq / g, preferably 2.1–3.3 meq / g, more preferably 2.4–3.0 meq / g, and / or - Turbidity values ​​of 1-150 NTU, preferably 1-100 NTU.

16. Use of the cationic starch according to claim 14 or 15, or the cationic starch obtained by the method according to any one of claims 1-13, as a flocculant in liquid-solid separation, for example, in water and / or biological sludge treatment.

Images