Method for liquid-solid separation

Abstract

The invention relates to a method for flocculating solid particles in a liquid-solid separation process. The method comprises obtaining a suspension of solid particles suspended in a continuous aqueous phase and obtaining a suspension comprising solid particles having lt; a flocculant solution of cationized starch having an original charge density of 3 meq / g. The flocculant solution is subjected to a high shear treatment, and after the high shear treatment, the flocculant solution is brought into contact with a suspension of solid particles, and the solid particles are flocculated. The flocculated solid particles are separated from the continuous aqueous phase in a dehydration step.

Description

Technical Field

[0001] According to the preambles of the attached independent claims, the present invention relates to a method for flocculating solid particles in a liquid-solid separation process. Background Art

[0002] Various industrial processes include a liquid-solid separation step, in which solid particles are separated from a liquid phase. A suspension of solid materials suspended in a water continuous phase is usually subjected to a liquid-solid separation to provide a solid fraction and a liquid portion that can be further processed, respectively. For example, in a water treatment process, a liquid-solid separation step is used in the manufacture of pulp, paper, board, etc. and in the mining industry. A typical example of a liquid-solid separation step is the dewatering of sludge in any water treatment process. Sludge typically includes various solid particles and / or microorganisms suspended in an aqueous phase. In the liquid-solid separation step, the water content of the sludge is reduced so that the solid portion of the sludge can be further processed, such as deposited for use as a fertilizer, or burned for energy production.

[0003] Typically, the liquid-solid separation step includes flocculating the solid particles suspended in the liquid phase. Flocculation and / or coagulation chemicals are used to improve the formation and / or quality of the flocs formed. For example, in water treatment processes, sludge can be conditioned by adding flocculating agents, such as inorganic compounds of iron and lime, or synthetic organic polymers, before the dewatering step. These flocculating agents are added to the sludge to improve sludge handling and increase the dewatering effect in liquid-solid separation.

[0004] Cationic polymers are usually used as flocculants in liquid-solid separation. Cationic polymers used as flocculants are usually petroleum-based synthetic polymers. Due to the non-degradable nature of synthetic polymers, when using synthetic cationic polymers in liquid-solid separation steps, the solid part of the separation obtained from wastewater treatment may not be used for landfill, composting or for soil improvement. In addition, there is a growing interest in more sustainable industrial processes and for bio-based chemicals and / or biodegradable chemicals at present. The expectation of this use of bio-based chemicals and / or biodegradable chemicals causes a strong interest in finding the substitute of petroleum-based synthetic cationic polymers. In addition, the growing price of oil has reduced its attractiveness as raw material.

[0005] Typically, various bio-based cationic polymers do not provide the same efficiency as synthetic polymers, or the bio-based polymers require extensive processing, e.g., high cationization and / or derivatization, in order to exhibit adequate action as flocculants in liquid-solid separation processes. This naturally reduces performance or increases the costs involved. Therefore, there is a need for alternative ways to obtain adequate results in liquid-solid separations by using bio-based materials as flocculants. Summary of the invention

[0006] It is an object of the present invention to minimize or even eliminate the disadvantages existing in the prior art.

[0007] Yet another object is to provide a more sustainable method for flocculation in liquid-solid separation processes, especially sludge dewatering in water treatment processes.

[0008] Another object of the present invention is to provide a process which provides efficient flocculation and high solids content for the separated solids after the liquid-solid separation step.

[0009] These objects are achieved by the invention having the characteristics presented in the characterizing parts of the attached independent claims. Some preferred embodiments are disclosed in the dependent claims.

[0010] Where appropriate, the embodiments mentioned herein relate to all aspects of the invention, even if this is not always mentioned individually.

[0011] A typical method for flocculating solid particles in a liquid-solid separation process according to the present invention comprises:

[0012] - obtaining a flocculant solution comprising cationized starch (cationized starch) having an initial charge density of <3 meq / g;

[0013] - subjecting the flocculant solution to high shear;

[0014] - after the high shear treatment, contacting the flocculant solution with the suspension of solid particles suspended in a continuous aqueous phase and flocculating the solid particles; and

[0015] - Separation of flocculated solid particles from the continuous aqueous phase in a dewatering step.

[0016] It has now been surprisingly found that when a solution of a cationized starch having an original charge density of <3 meq / g measured at pH 4 is subjected to an appropriate high shear treatment, the cationicity of the starch is increased by the high shear treatment and the flocculation capacity of the cationized starch is unexpectedly increased in the absence of any other chemical treatment. The increase in cationicity caused by the high shear treatment can even be up to two or three times the original cationicity. This unexpected increase in the cationicity and flocculation capacity of the cationized starch can be reflected in: high solids content after liquid-solid separation, short dewatering time and / or improved floc properties, such as floc strength, generally providing more effective liquid-solid separation. The theoretical background of the observed phenomenon is not yet fully understood, but it is believed that the high shear treatment has an effect on the structure of the cationized starch, providing more cationic groups for interaction with solid particles during the flocculation process.

[0017] According to one embodiment of the present invention, a flocculant solution comprising dissolved cationized starch is subjected to high shear treatment, wherein a shear power of at least 5W / kg, preferably at least 20W / kg, more preferably at least 35W / kg is applied to a flocculant solution comprising or consisting of cationized starch. Therefore, high shear treatment is an integral feature of the present invention. According to a preferred embodiment, the flocculant solution can be subjected to a shear power in the range of 5-10000W / kg, preferably 20-5000W / kg, more preferably 35-1000W / kg, even more preferably 35-300W / kg or 35-209W / kg in high treatment. Sometimes the shear power can be up to 500W / kg. The duration of high shear treatment can preferably be at most 600s, preferably 1-600s, more preferably 5-300s, even more preferably 10-180s or 30-150s. Generally, the higher the power used in the high shear treatment, the shorter the duration of the high shear treatment can be, and vice versa.

[0018] The flocculant solution may preferably be subjected to high shear treatment in the absence of an oxidizing agent. This means that the high shear treatment of the flocculant solution comprising dissolved cationized starch is preferably carried out in the absence of an oxidizing agent such as hydrogen peroxide. In this way, the risk of shortening or degradation of the cationic starch molecules may be reduced.

[0019] The concentration of cationized starch in the flocculant solution during the high shear treatment may be 0.05-2 wt-%.

[0020] According to another embodiment, the flocculant solution comprising cationized starch may be subjected to a high shear treatment which provides a temperature increase of 5-15°C to the flocculant solution without external heating.

[0021] The turbidity of the flocculant solution comprising dissolved cationized starch may be ≤100 NTU, preferably ≤50 NTU, such as 1-100 NTU or 5-50 NTU.The low turbidity of the flocculant solution before and after high shear treatment, ie its clarity, indicates that the cationized starch is present in fully dissolved form.

[0022] The high shear treatment is preferably carried out as a continuous in-line treatment. This means that the duration of the high shear treatment is preferably at most 60s. In some embodiments, the duration of the high shear treatment can be 1-60s, preferably 3-45s, more preferably 5-30s. The duration of the high shear treatment can be relatively short, because the cationized starch in the flocculant solution is already in a dissolved form when it is subjected to high shear treatment. Therefore, the high shear treatment is not used to dissolve the cationized starch, but to improve its ability as a flocculant. The short duration of the high shear treatment makes the present invention particularly suitable for industrial use.

[0023] The flocculant solution comprising dissolved cationized starch is preferably subjected to one high shear treatment or to two, three or four consecutive high shear treatments. Preferably, the flocculant solution is subjected to one high shear treatment. Quite surprisingly, subjecting the cationized starch to only one or a small number of consecutive high shear treatments has been able to produce such a significant effect in the flocculation capacity. The limited duration of the high shear treatment also maintains the molecular length of the cationized starch and reduces the risk of significant damage to the starch molecules.

[0024] The flocculant solution containing or consisting of dissolved cationized starch can be subjected to high shear treatment in any suitable high shear device or high shear apparatus that is capable of generating suitable high shear power in an aqueous system. The high shear treatment is preferably carried out or performed in unpressurized conditions or under unpressurized conditions, i.e., carried out or performed by using a mixing device or homogenizer, wherein no premediated overpressure is generated in the mixing device or homogenizer. For example, the flocculant solution containing dissolved cationized starch can be subjected to high shear treatment in a homogenizer, a high-speed mixer, a disperser, a rotor-stator mixer, a mixer with two counter-rotating rotors, a centrifugal pumping device providing direct or counter-rotating flow, a high-pressure device, a shear pump, etc. In some cases, ultrasonic treatment can also be used as a high shear treatment. Suitable high shear mixing devices and homogenizers are well known to those skilled in the art and are commercially available, for example under the trade name Ultra Cavitron TM and Waukesha shear pumps TM . For example, the cationized starch can be dissolved in water, subjected to high shear treatment by high shear homogenization and then contacted with an aqueous suspension containing solid particles to be flocculated. For example, high shear homogenization can be achieved by using a rotation speed of at least 2000 rpm. Homogenization is preferably carried out as a mechanical homogenization without using a pressure difference, for example in a homogenizer using the rotor-stator principle. Alternatively, a suitable shear power can be achieved in the high shear treatment in which the dissolved cationized starch is subjected to high shear power by a centrifugal pump or the like, for example in the process of pumping the cationized starch after it has been dissolved in water.

[0025] The cationized starch used in the present invention is in the form of an aqueous solution, which means that the cationic starch is dissolved in water before the high shear step.

[0026] The cationized starch suitable for use as a flocculant after being dissolved in water and subjected to high shear treatment has an original charge density of <3meq / g, preferably in the range of 1.2-2.7meq / g, more preferably 1.5-2.5meq / g, and even more preferably 1.6-2.0meq / g measured at pH 4 before high shear treatment. As described in the experimental part, the charge density is determined. It was unexpectedly found that when the cationicity of the cationized starch before high shear treatment is within the range, the cationicity and flocculation effect of the cationized starch are greatly enhanced. By high shear treatment, the charge density of the cationized starch can be increased (to) 1.3-2.5 times or 1.5-2.5 times relative to the original charge density. For example, by high shear treatment, the charge density of the cationized starch can be increased by 0.2-1.4meq / g, preferably 0.4-1.0meq / g, and more preferably 0.5-0.7meq / g relative to the original charge density, which is the charge density measured before high shear treatment. If the cationicity of the dissolved cationized starch is above 3 meq / g, high shear treatment does not provide the desired improvement in the cationicity or flocculation ability of the cationized starch.Thus, the present invention provides the possibility to increase the cationicity of starch without increasing the use of cationizing chemicals.

[0027] The starch used in the present invention can be cationized by any suitable method. Preferably, the starch is cationized by using 2,3-epoxypropyltrimethylammonium chloride or 3-chloro-2-hydroxypropyltrimethylammonium chloride, preferably 2,3-epoxypropyltrimethylammonium chloride. Cationic starch can also be provided by using other applicable cationization methods, for example, methods that provide quaternary ammonium esters of starch.

[0028] The cationized starch may have a degree of substitution in the range of 0.32-0.65, preferably 0.33-0.60, more preferably 0.34-0.55.

[0029] The cationized starch can be made from any natural starch, such as potato starch, tapioca starch, corn starch, waxy corn starch, waxy potato starch, wheat starch, barley starch, rice starch, waxy rice starch, pea starch, bean starch, or any mixture thereof.

[0030] The flocculant solution containing the cationized starch can be obtained by dissolving or solubilizing the cationized starch in water. The cationized starch can be dissolved or solubilized by any suitable method such as known to those skilled in the art. The cationized starch used in the present invention is generally directly soluble in water at room temperature (25°C), usually without the need for a cooking step or the like. Therefore, the cationized starch is dissolved in water and forms an aqueous flocculant solution. The flocculant solution preferably does not contain starch granules or starch particles. Preferably, the cationized starch is dissolved in water without using an elevated temperature of more than 40°C, preferably more than 30°C, i.e., without any cooking step involving elevated temperature and / or pressure.

[0031] The flocculant solution preferably contains alkaline earth metal ions, especially Ca, in an amount of <1.5 g / l, more preferably <1 g / l. The amount of alkaline earth metal ions can be from 0 g / l to less than 1.5 g / l or less than 1 g / l. The flocculant solution preferably contains a halide salt, especially chloride, in an amount of <2 g / l, more preferably <1 g / l. The amount of halide salt can be from 0 g / l to less than 2 g / l or less than 1 g / l.

[0032] When measured before high shear treatment, the cationized starch solution used as flocculant solution may have a 3% salt viscosity of at least 3 mPas, preferably at least 5 mPas, more preferably at least 10 mPas. For example, the 3% salt viscosity may be in the range of 3-10000 mPas, preferably 5-7000 mPas, more preferably 10-4000 mPas. The measurement of the 3% salt viscosity is determined as defined in the experimental part of the present application.

[0033] According to a preferred embodiment, the cationized starch used is undegraded starch. In the context of the present invention, the term "undegraded starch" means a cationized starch that has not been treated by oxidation, heat, enzymes and / or acid treatment in a manner that causes hydrolysis of glycosidic bonds or degradation of starch molecules or units. It is believed that the use of undegraded starch provides the starch molecules with a suitable size or length, which is conducive to flocculation.

[0034] Furthermore, the cationized starch used is preferably a non-crosslinked cationized starch. Since the length or size of the dissolved cationized starch molecules is substantially maintained even after high shear treatment, crosslinking of the starch is not required.

[0035] According to one embodiment, a method for flocculating solid particles in a liquid-solid separation process may include

[0036] - obtaining a suspension of solid particles suspended in a continuous aqueous phase;

[0037] - obtaining a flocculant solution comprising a cationized starch having an initial charge density of <3 meq / g;

[0038] - subjecting the flocculant solution to high shear;

[0039] - after the high shear treatment, contacting the flocculant solution with the suspension of solid particles and flocculating the solid particles; and

[0040] - Separation of flocculated solid particles from the continuous aqueous phase in a dewatering step.

[0041] According to one embodiment of the present invention, in addition to the dissolved cationized starch, the flocculant solution may also contain one or more cationic synthetic polymers, preferably cationic polyacrylamide. The flocculant solution may contain 10-100 wt-%, preferably 30-100 wt-%, more preferably 50-99 wt-% of cationized starch calculated based on the active polymer content of the flocculant solution. The use of cationic polymers together with cationized starch increases the flocculation effect of the flocculant solution, especially in demanding applications, while also providing a more sustainable alternative compared to conventional synthetic polymer flocculants.

[0042] The additional flocculating agent may also be contacted with the suspension of solid particles separately from the flocculant solution containing the cationized starch. The additional flocculating agent may be contacted with the suspension of solid particles while the flocculant solution is in contact with the suspension of solid particles, or before or after the flocculant solution is in contact with the suspension of solid particles. The additional flocculating agent may be an inorganic or organic flocculating agent. According to one embodiment, the additional flocculating agent is a cationic synthetic polymer, such as a cationic polyacrylamide.

[0043] The present invention is suitable for flocculating solid particles in any liquid-solid separation process in which solid particles are separated from an aqueous liquid phase. Preferably, the liquid-solid separation process is a process for treating wastewater, preferably municipal wastewater or industrial wastewater. For example, the present invention is particularly suitable for a liquid-solid separation process selected from wastewater treatment, sludge dewatering or conditioning drinking water.

[0044] According to a preferred embodiment, the liquid-solid separation process can be a sludge dewatering step of a water treatment process, such as a sludge dewatering step of a municipal wastewater treatment process or a sludge dewatering step of an industrial wastewater treatment process. Sludge is understood herein to be an aqueous suspension (aqueous suspension, aqueous suspension) comprising a continuous aqueous liquid phase and an organic and / or inorganic solid material and / or particle suspended in the aqueous liquid phase. Sludge to be dewatered, i.e., an aqueous suspension, can be municipal wastewater sludge or agricultural sludge, or it can be derived from a biological treatment process of wastewater and / or from a biological treatment process of sewage. Alternatively, an aqueous suspension, i.e., sludge, can be derived from an industrial process, especially from the wastewater treatment of an industrial process, or from food or beverage production or from food or beverage processing.

[0045] Therefore, the aqueous suspension to be treated with the flocculant solution according to the present invention generally comprises a continuous aqueous liquid phase and organic and / or inorganic solid materials and / or particles suspended in the aqueous liquid phase. The suspension can be rich in materials of bacterial origin, especially if it originates from a water treatment process. The aqueous liquid phase of the suspension can also comprise dissolved organic matter, such as polysaccharides, humus and fatty acids. The suspension can have a biological oxygen demand (BOD) of> 50mg / l, a chemical oxygen demand (COD) in the range of 15-45g / l, preferably 20-40g / l, and / or a dry solid content in the range of 5-80g / l, preferably 10-60g / l, more preferably 20-55g / l. The pH of the suspension can be in the range of from pH 6 to pH 9, preferably from pH 7 to pH 8. The conductivity of the suspension may be in the range of 5-14 mS / cm, preferably 5-10 mS / cm, and / or the charge density may be in the range of from -5.5 to -1.5 μeq / g, preferably from -5.0 to -1.8 μeq / g. The total phosphorus value of the suspension may be in the range of 400-1400 mg / l, preferably 450-1200 mg / l and / or the total nitrogen value may be in the range of 1.2-3.5 g / l, preferably 1.5-3.0 g / l.

[0046] If the present invention is used in a wastewater treatment process, especially in the sludge dewatering step, a flocculant solution containing dissolved cationized starch can be contacted with the suspension in an amount of 1-30 kg cationized starch / ton of dry suspension, preferably 2-20 kg cationized starch / ton of dry suspension, more preferably 3-15 kg cationized starch / ton of dry suspension. DETAILED DESCRIPTION

[0047] experiment

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

[0049] Cationic starch and its characterization

[0050] The cationized starch used in the examples was prepared by using 2,3-epoxypropyltrimethylammonium chloride as a cationizing agent.

[0051] The cationized starch solution used in the examples was prepared by dissolving the cationized starch in deionized water. As described below, the prepared cationized starch solution was characterized by measuring the charge density, salt viscosity and conductivity of the cationized starch solution at pH 4. The charge density of the cationized starch solution was measured before and after the high shear treatment. Other measurements were performed only before the high shear treatment.

[0052] Charge density

[0053] The charge density at pH 4 was determined using BTG's Mütek PCD-04 particle charge titrator. The cationized starch sample was dissolved in deionized water to a 0.20wt-% solution, and then diluted to a 0.01-0.04wt-% solution for measurement according to the charge density of the sample. The pH was adjusted to 4.0 with 0.1M acetic acid and titrated using 0.001N sodium polyvinyl sulfonate (PES-Na) solution as a titrant. During the titration, the pH typically increased by 0.1-0.3pH units. The charge density is expressed as meq / g dry matter.

[0054] 3% Salt Viscosity

[0055] At 25°C, a Brookfield DV-1 viscometer with a small sample adapter was used to measure the 3% salt viscosity (hereinafter referred to as "3% salt viscosity") of a cationized starch solution in the presence of salt in water using a rotor #18 or #31, depending on the viscosity level. The 3% salt viscosity measurement was performed using the maximum possible rotation speed. The LV mode speeds (60, 30, 12, 6, 3, 1.5, 0.6, 0.3 rpm) were typically used, except that the highest speed used was 100 rpm. First, the cationized starch was dissolved in deionized water to a 3 wt-% solution. Sodium chloride (NaCl) was then added to a weight ratio of NaCl: cationized starch of 5:1, and it was dissolved under mixing before measuring the 3% salt viscosity. This means that the 3% salt viscosity of the cationized starch is measured in an aqueous solution containing 13.0 wt-% of NaCl at a cationized starch concentration of 2.6 wt-%. The 3% salt viscosity gives comparable parameters to assess the size of the cationized starch molecule.

[0056] Water Viscosity ca. 2% and Salt Viscosity ca. 2%

[0057] The aqueous viscosity of the cationized starch solution in water is determined using the same equipment and measuring principle as when measuring the 3% salt viscosity 3%. For the measurement of about 2% aqueous viscosity, the cationized starch sample is first dissolved in deionized water to a solution of about 2 wt-%. The cationized starch sample is weighed as such, without considering the water content of the cationized starch sample. The Brookfield viscosity of this cationized starch solution gives a value of about 2% aqueous viscosity. The viscosity is measured for the cationized starch solution as such and for the same cationized starch solution after high shear treatment.

[0058] The approximately 2% salt viscosity is measured by adding sodium chloride (NaCl) to a weight ratio of 5:1 NaCl: weighed cationized starch sample and allowing it to dissolve under gentle mixing before measuring the viscosity. The Brookfield viscosity of this solution gives a value for the approximately 2% salt viscosity. When a high shear cationized starch sample is used, salt is added after the high shear treatment.

[0059] These measurements of about 2% water viscosity and about 2% salt viscosity are comparable and they are used to study the effect of high shear treatment on the solution viscosity of starch solutions. The measurements are comparisons between the same starch solution after different treatments. Therefore, no exact starch concentration is required for the measurements. The water content of the starch samples used varied in the range of 0-8 wt-%.

[0060] Conductivity

[0061] The conductivity of 0.5 wt-% cationized starch in deionized water was measured using a Knick Portavo 902COND conductivity meter equipped with a Knick SE 204 sensor.

[0062] High shear processing

[0063] By using IKAT25 Digital Ultra with blades S25N-25F High shear treatment was performed by mixing 200 ml of freshly prepared 0.2% cationized starch solution at 16000 rpm for 3 minutes. When high shear treatment was used, the samples were marked as "after UT".

[0064] Sludge dewatering test

[0065] Sludge dewatering tests were performed using the capillary suction time (CST) test by using a Triton 319 multipurpose CST (Triton Electronics Ltd, UK). Mixing was performed using a 4-blade stirrer with a total blade width of 30 mm and a blade height of 15 mm with a Heidolph RZR 2021 or IKARW 20 digital mixer.

[0066] In the CST test, the mixing speed was 1000 rpm. The cylinder used had a diameter of 18 mm. The cationized starch sample was added to 100 g of digested sludge in a 250 ml beaker and mixed for 10 s after addition. After 10 s mixing, 4.5 ml of the sample was taken to the cylinder and the CST value was measured.

[0067] The cationized starch samples for CST testing were first dissolved into 0.5% solutions overnight and then diluted to 0.2% solutions for testing. These solutions were used as is or after the solutions were subjected to high shear treatment. The samples subjected to high shear treatment were marked as "after UT".

[0068] Example 1

[0069] Digested sludge was collected from a wastewater treatment plant in Finland. The digested sludge had a dry solids content of 3.0 wt-% and a pH of 7.4. The CST time of the sludge produced without any chemical addition (zero test) was 354 s.

[0070] The properties of the cationized starch (CS) used as flocculant solution are given in Table 1. Table 1 shows the charge density measured for the cationized starch without high shear treatment ("without UT") and after high shear treatment ("after UT").

[0071] The CST test results of cationized starch (CS) used as flocculant solution are given in Table 2. The cationized starch solution was used as is, either after dissolution ("without UT") or after subjecting the cationized starch solution to high shear treatment ("after UT").

[0072] As can be seen from the results in Table 1, when the original charge density of the cationized starch ("without UT") was <3 meq / g, the high shear treatment clearly increased the charge density of the sample ("after UT"). The effect was particularly evident when the cationized starch had a charge density in the range of 1.6-2.2 meq / g. As can be seen from the results in Table 2, the increase in charge density caused by the high shear treatment improved the dewatering properties of the cationized starch. This is demonstrated by the significantly lower CST values ​​obtained in the CST test at the corresponding dosage levels. For the cationized starch CS5 with an original charge density >3 meq / g, the high shear treatment did not seem to improve either the charge density or the dewatering properties.

[0073] It can also be seen from Table 1 that high shear treatment significantly reduces the values ​​of about 2% water viscosity and about 2% salt viscosity. Where the increase in charge density, and hence the improvement in dewatering performance, is minimal or negligible, the reduction in viscosity is also minimal or negligible.

[0074] Table 1. Properties of cationized starch (CS) used as flocculant solution.

[0075]

[0076] Table 2 CST test results of cationized starch (CS) used as flocculant solution.

[0077]

[0078] Example 2

[0079] Digested sludge was collected from a wastewater treatment plant in Finland. The digested sludge had a dry solids content of 2.5 wt-% and a pH of 7.2. The CST time of the sludge produced without any chemical addition (zero test) was 242 s.

[0080] The properties of the cationized starch (CS) used as flocculant solution are given in Table 3. Table 3 shows the charge density measured for the cationized starch without high shear treatment ("without UT") and after high shear treatment ("after UT").

[0081] The CST test results of cationized starch (CS) used as flocculant solution are given in Table 4. The cationized starch solution was used as is, either after dissolution ("without UT") or after subjecting the cationized starch solution to high shear treatment ("after UT").

[0082] The results in Tables 1 and 2 confirm the effect of high shear treatment. High shear treatment increases the charge density of cationized starch and improves the dewatering performance of cationized starch.

[0083] Table 3. Properties of cationized starch (CS) used as flocculant solution.

[0084]

[0085] Table 4 CST test results of cationized starch (CS) used as flocculant solution.

[0086]

[0087] Example 3

[0088] Digested sludge was collected from a wastewater treatment plant in Finland. The digested sludge had a dry solids content of 2.5 wt-% and a pH of 7.3. The CST time of the sludge produced without any chemical addition (zero test) was 254 s.

[0089] The properties of the cationized starch (CS) used as flocculant solution are given in Table 5. Table 5 shows the charge density measured for the cationized starch without high shear treatment ("without UT") and after high shear treatment ("after UT").

[0090] The CST test results of cationized starch (CS) used as flocculant solution are given in Table 6. The cationized starch solution was used as is, either after dissolution ("without UT") or after subjecting the cationized starch solution to high shear treatment ("after UT").

[0091] The results of Tables 5 and 6 further confirm the effect of high shear treatment. High shear treatment increases the charge density of cationized starch and improves the dehydration performance of cationized starch. In particular, as shown in Table 6, when the charge density of cationized starch is in the range of 0.65-2.0meq / g, high shear treatment clearly increases the charge density of cationized starch. At the same time, high shear treatment improves the dehydration performance of cationized starch. When the starch density of cationized starch is close to 3meq / g, even if no increase in charge density is observed, high shear treatment still provides an improvement in dehydration performance, see the results of CS10.

[0092] Table 5. Properties of cationized starch (CS) used as flocculant solution.

[0093]

[0094] Table 6 CST test results of cationized starch (CS) used as flocculant solution.

[0095]

[0096]

[0097] Example 4

[0098] In Example 4 the power input required for high shear processing was investigated.

[0099] The samples used in Example 4 were 1) water as a reference and 2) a 0.2 wt-% solution of cationized starch CS 6 from Example 2.

[0100] For the measurement, a single sample was placed in a 1 liter Dewar flask equipped with a thermometer and a cork cover. An IKAT25 digital Ultra The mixing head of the homogenizer was placed below the sample surface and the bottle was tightly closed with a cap to avoid any heat loss. High shear treatment was performed using 16000 rpm for 4 minutes.

[0101] Calculate shear power using Equation 1:

[0102]

[0103] in

[0104] P is specific power, W / kg

[0105] C p is the specific heat, J / (℃*kg)

[0106] T i is the temperature of the mixture, °C

[0107] t 处理 is the processing time, s

[0108] The sample mixture is considered to have the specific heat capacity of water.

[0109] Table 7 shows the numerical (measured) values ​​used in calculating the shear power.

[0110] Table 7 is used to calculate the values ​​of specific power requirements.

[0111]

[0112]

[0113] The specific power required for high shear processing is 209 W / kg solution.

[0114] The power requirements may also be lower or higher depending on the equipment used, the rotation or pumping speed, the processing time, the sample characteristics and the sample concentration. The sufficient condition is defined by the increase in the charge density of the processed sample and the improvement in its dehydration properties.

[0115] Although the present invention is described with reference to what currently appears to be the most practical and preferred embodiments, it should be understood that the present invention should not be limited to the above-mentioned embodiments, but the present invention is also intended to cover different modifications and equivalent technical solutions within the scope of the attached claims.

Claims

1. A method for flocculating solid particles in a liquid-solid separation process, the method comprising: - obtaining a flocculant solution comprising a cationized starch having an initial charge density of <3 meq / g; - subjecting the flocculant solution to high shear treatment; - after the high shear treatment, contacting the flocculant solution with the suspension of solid particles in the continuous aqueous phase and flocculating the solid particles; as well as - In a dewatering step, the flocculated solid particles are separated from the continuous aqueous phase.

2. The method according to claim 1, characterized in that In the high shear treatment, the flocculant solution is subjected to a shear power in the range of 5-1000 W / kg, preferably 20-500 W / kg, more preferably 35-209 W / kg, wherein preferably, the duration of the high shear treatment is 1-600 s, preferably 5-300 s, more preferably 10-180 s.

3. The method according to claim 1 or 2, characterized in that: Before the high shear treatment, the cationized starch has the original charge density in the range of 1.2-2.7 meq / g, preferably 1.5-2.5 meq / g, more preferably 1.6-2.0 meq / g.

4. The method according to claim 1, 2 or 3, characterized in that: The cationized starch has a degree of substitution in the range of 0.32-0.65, preferably 0.33-0.60, more preferably 0.34-0.

55.

5. The method according to any one of the preceding claims 1 to 4, characterized in that The flocculant solution comprises one or more cationic synthetic polymers, preferably cationic polyacrylamide.

6. The method according to claim 5, characterized in that The flocculant solution comprises 10-100 wt-%, preferably 30-100 wt-%, more preferably 50-100 wt-% of cationized starch calculated on the basis of the active polymer content of the flocculant solution.

7. The method according to any one of the preceding claims 1 to 6, characterized in that The high shear treatment causes an increase in the charge density of the cationized starch, which is 1.5-2.5 times the original charge density.

8. The method according to any one of the preceding claims 1 to 7, characterized in that The liquid-solid separation process is a wastewater treatment process, preferably municipal wastewater or industrial wastewater.

9. The method according to any one of the preceding claims 1 to 8, characterized in that The suspension has a biological oxygen demand (BOD) of >50 mg / l, and / or a chemical oxygen demand (COD) in the range of 15-45 g / l, preferably 20-40 g / l, and / or a dry solids content in the range of 5-80 g / l, preferably 10-60 g / l, more preferably 20-55 g / l.

10. The method according to any one of the preceding claims 1 to 9, characterized in that The flocculant solution is contacted with the suspension in an amount to provide 1-30 kg cationized starch per ton of dry suspension, preferably 2-20 kg cationized starch per ton of dry suspension, more preferably 3-15 kg cationized starch per ton of dry suspension.

11. The method according to any one of the preceding claims 1 to 10, characterized in that A further flocculating agent is contacted with said suspension of solid particles separately from said flocculant solution comprising cationized starch.

12. The method according to any one of the preceding claims 1 to 11, characterized in that During the high shear treatment, the flocculant solution has a cationized starch concentration in the range of 0.05-2 wt-%.

13. The method according to any one of the preceding claims 1 to 12, characterized in that The flocculant solution is subjected to the high shear treatment in the absence of an oxidizing agent.

14. The method according to any one of the preceding claims 1 to 13, characterized in that The liquid-solid separation process is the sludge dewatering step of the water treatment process.

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