Sulfated modified sodium alginate, and preparation method and application thereof

Abstract

This invention discloses a sulfated modified sodium alginate, its preparation method, and its applications. Urea is heated and melted to obtain urea liquid. Sodium alginate powder is added to the urea liquid to form a dispersion. The mixture is then cooled to 50-70°C, aminosulfonic acid is added, and a protective atmosphere is introduced at a flow rate of 50-200 mL / min. The temperature is then raised to 100-140°C, and the reaction yields sulfated modified sodium alginate. When this product is used as a flotation depressant for magnesium silicate gangues, it utilizes the specific bidentate chelating effect and strong hydration ability of the sulfate ester groups on the talc surface to form a dense molecular shielding film. Simultaneously, it effectively avoids non-specific adsorption on the surface of valuable metal minerals (such as molybdenite) through strong electrostatic repulsion. This invention not only significantly improves the flotation separation efficiency of valuable minerals but also features a simple, environmentally friendly synthesis process with broad industrial application prospects.

Description

Technical Field

[0001] This invention belongs to the fields of polymer material synthesis, mineral processing engineering and flotation reagent technology, specifically relating to a sulfation-modified sodium alginate, its preparation method and application. Background Technology

[0002] Natural high-molecular-weight polysaccharides (such as sodium alginate, starch, and cellulose) have attracted much attention as green inhibitors of gangue minerals in mineral flotation separation due to their wide availability, biodegradability, and environmental friendliness. However, unmodified natural sodium alginate molecules, due to their single functional groups (containing only hydroxyl and carboxyl groups), readily and non-selectively adsorb onto target valuable minerals (such as molybdenite and chalcopyrite) in complex slurry systems through non-specific hydrophobic interactions or hydrogen bonds. This results in "competitive adsorption" or "dragging effect," severely reducing the recovery rate of valuable metals.

[0003] To overcome the technical bottleneck of poor selectivity in natural polysaccharides, introducing specific chelating ligands (such as sulfate ester groups -OSO3⁻) onto their molecular chains through chemical modification has become a current research hotspot. The sulfate ester group possesses extremely strong electronegativity and hydration ability, which can significantly enhance the selectivity of polymeric inhibitors at specific metal sites (such as Mg²⁺). 2+ Ca 2+ The chemical complexation ability and electrostatic repulsion effect of the surface of sodium alginate are important. However, traditional sodium alginate sulfation modification processes generally use strong acids or highly toxic reagents (such as chlorosulfonic acid, sulfur trioxide-pyridine complexes, or concentrated sulfuric acid). These traditional processes have fatal flaws: the reaction conditions are harsh, which easily triggers the β-elimination reaction of glycosidic bonds in the sodium alginate molecular chain, leading to severe degradation of the polysaccharide backbone and a sharp decrease in molecular weight, thus losing the steric hindrance effect necessary for polymer inhibitors; in addition, traditional solid-phase or heterogeneous reaction systems have problems such as low mass transfer efficiency, uneven degree of substitution, and serious environmental pollution.

[0004] Therefore, there is an urgent need to develop a green, mild synthesis process without the addition of toxic organic solvents to achieve high-substitution sulfation modification of sodium alginate while maintaining the integrity of the sodium alginate molecular skeleton to the greatest extent possible. This would provide high-performance green reagent support for its efficient flotation separation of complex silicate gangues (especially natural hydrophobic talc). Summary of the Invention

[0005] To address the technical problems of high reagent toxicity, easy degradation of the polysaccharide backbone, and unstable product performance in existing methods for preparing sulfated modified sodium alginate, the first objective of this invention is to provide a method for preparing sulfated modified sodium alginate. This invention is based on a green, mild melt method to prepare sulfated modified sodium alginate, achieving a high degree of substitution with sulfate modification.

[0006] The second objective of this invention is to provide a sulfated modified sodium alginate prepared by the above-described preparation method.

[0007] A third objective of this invention is to provide an application of the sulfated modified sodium alginate prepared by the above-described method. When the sulfated modified sodium alginate provided by this invention is used as a flotation inhibitor for magnesium silicate gangues (such as talc), it utilizes the specific bidentate chelating effect and strong hydration ability of the sulfate ester groups on the talc surface to form a dense molecular shielding film. Simultaneously, it effectively avoids non-specific adsorption on the surface of valuable metal minerals (such as molybdenite) through strong electrostatic repulsion, thus significantly improving the flotation separation efficiency of valuable minerals.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] This invention discloses a method for preparing sulfated modified sodium alginate. Urea is heated and melted to obtain urea liquid. Sodium alginate powder is added to the urea liquid to form a dispersion. The mixture is then cooled to 50-70°C, aminosulfonic acid is added, a protective atmosphere is introduced, and the temperature is raised to 100-140°C, preferably 100-130°C, and more preferably 120-130°C. The reaction yields sulfated modified sodium alginate.

[0010] The preparation method of this invention employs a green and mild solvent-free "urea-sulfamic acid" melting method. Experiments have shown that "urea-sulfamic acid" forms a deep eutectic solvent (DES) system at the temperature controlled by this invention. In the deep eutectic solvent system, the strongly electronegative, large-volume sulfate ester groups (-OSO3) are... - Covalent grafting of urea onto the sodium alginate molecular chain yields a highly substituted and structurally stable sulfated modified sodium alginate. In this invention, urea not only acts as a catalyst to reduce the activation energy of the sulfation reaction, but also acts as a highly polar melting medium to promote the stretching of the rigid polysaccharide chain and improve mass transfer efficiency. The excellent buffering effect of this system effectively inhibits the β-elimination degradation of the sodium alginate polysaccharide skeleton under high temperature and strong acid conditions. While avoiding the use of toxic organic solvents, it achieves directional modification with a high degree of substitution (DS up to 0.769).

[0011] In the urea-sulfamic acid melt system of this invention, the order of addition is crucial, as it has a decisive influence on the structural uniformity and skeletal integrity of the product. In this invention, urea is melted first, followed by the addition of sodium alginate powder. The high-temperature molten urea is fully utilized as a highly polar solvent to disrupt the strong hydrogen bond network between sodium alginate molecules, allowing its rigid polysaccharide chains to fully expand and uniformly disperse in the liquid phase before the reaction. Subsequently, the temperature is lowered to 50-70°C before adding sulfamic acid. This avoids the high-temperature thermal decomposition of sulfamic acid and ensures that the subsequent sulfation reaction proceeds smoothly in a uniform "quasi-deep eutectic solvent" microenvironment during heating, thereby obtaining a high and uniform degree of substitution.

[0012] If aminosulfonic acid is added first, it will rapidly activate or even violently decompose in the urea melt at temperatures above 130°C, generating a large amount of gas and causing overflow, resulting in the ineffective loss of the sulfonating agent. When solid sodium alginate powder is added subsequently, because the concentration of the active intermediate in the reaction system is already extremely uneven, the sodium alginate powder does not have enough time to fully swell and expand in the urea before reacting, leading to extremely uneven sulfate substitution in the product. Furthermore, excessively high local acidity and heat can cause direct carbonization and severe degradation of the undispersed polysaccharide powder.

[0013] Furthermore, the final reaction temperature must be effectively controlled. If the temperature is too low, sulfamic acid and urea will have difficulty effectively colliding to form a highly reactive sulfation intermediate, resulting in extremely slow esterification kinetics and a very low degree of sulfate substitution in the product. This makes it impossible to form an effective, water-based, high-molecular-weight inhibitor with targeted inhibition function. If the temperature is too high, although it can accelerate the sulfation reaction, it will trigger extremely serious side reactions. High temperatures and acidic environments will rapidly catalyze the β-elimination reaction of the sodium alginate backbone glycosidic bonds, leading to irreversible and severe breakage and degradation of the polysaccharide backbone. The sharp decrease in molecular weight will directly cause the product to lose the steric hindrance effect necessary for a high-molecular-weight inhibitor. At the same time, high temperatures can easily cause localized carbonization and blackening of the reactants and products, destroying the flocculation and inhibitory properties of the agent.

[0014] In a preferred embodiment, the temperature at which urea is heated to melt is 130~150℃.

[0015] In a preferred embodiment, the sodium alginate is first subjected to a drying pretreatment under vacuum conditions, the temperature of which is 35-45℃ and the time of which is 4-10 hours.

[0016] In actual operation, a certain amount of sodium alginate (SA, analytical grade) is weighed and placed in a vacuum drying oven for drying pretreatment to completely remove adsorbed water from the raw materials and prevent water from hydrolyzing aminosulfonic acid, thereby reducing reaction efficiency.

[0017] In a preferred embodiment, sodium alginate powder is added to urea liquid and stirred at 200-800 rpm for 15-20 minutes to obtain a dispersion.

[0018] In actual operation, sodium alginate powder is slowly added to molten urea in batches, and continuous stirring is carried out to make sodium alginate uniformly dispersed in the urea melt, forming a homogeneous system.

[0019] In a preferred embodiment, the mass ratio of urea to sodium alginate powder is 1~20:1, more preferably 6~8:1, and even more preferably 7:1.

[0020] In a preferred embodiment, the mass ratio of aminosulfonic acid to sodium alginate powder is 0.5~5:1, preferably 2~4:1, and more preferably 3:1.

[0021] In this invention, in the urea-sulfamic acid molten system, urea not only acts as a catalyst but also serves as a highly polar molten medium. Therefore, the amount added needs to be controlled. If the amount of urea added is too small, the system cannot form a molten medium environment of sufficient volume, which will cause the rigid sodium alginate macromolecular chains to be unable to fully extend, resulting in extremely poor mass transfer efficiency. At the same time, the lack of sufficient urea to effectively catalyze and activate sulfamic acid leads to slow sulfation reaction kinetics, ultimately resulting in a very low degree of sulfate substitution in the product. Excessive urea will greatly dilute the reaction system, significantly reducing the probability of effective collisions between sulfamic acid and the hydroxyl sites of sodium alginate, thus hindering the reaction process. In addition, a large amount of urea at high temperatures will decompose... The release of excessive ammonia during decomposition leads to an overly alkaline local microenvironment, which severely exacerbates the β-elimination degradation reaction of the sodium alginate backbone, destroying the polysaccharide's polymer backbone. As the core sulfonating agent in this invention, the concentration of aminosulfonic acid directly determines the thermodynamic driving force of the reaction and the integrity of the product structure. If too little aminosulfonic acid is added, the system lacks a sufficient concentration of sulfonating agent to provide the thermodynamic driving force needed to overcome the steric hindrance of the secondary hydroxyl groups in sodium alginate. This results in a limited number of sulfate ester groups grafted onto the polysaccharide backbone, failing to impart the required high electronegativity and strong hydration ability to the inhibitor. Excessive aminosulfonic acid leads to the excessive accumulation of free strong acids and a sharp increase in ionic strength within the system. This extremely acidic, high-temperature microenvironment produces a synergistic destructive effect, violently catalyzing the hydrolysis and β-elimination cleavage of sodium alginate glycosidic bonds. This not only fails to further increase the effective substitution degree but also causes severe breakage of the macromolecular backbone, leading to a decrease in the product's molecular weight and loss of the essential steric hindrance effect and flocculation properties required for a polymer inhibitor.

[0022] In a preferred embodiment, the protective atmosphere is nitrogen, and the gas flow rate is 50~200 mL / min.

[0023] In the preferred embodiment, the reaction time is 2-6 hours, preferably 3-5 hours. During the reaction, the mixture is observed to gradually change from a white suspension to a pale yellow or brownish-yellow high-viscosity liquid.

[0024] In the preferred embodiment, after the reaction is completed, the product is cooled and dissolved in water to obtain a solution. The solution is then added to a precipitant to precipitate the product. After filtration, washing, and vacuum drying, the sulfated modified sodium alginate is obtained.

[0025] In a further preferred embodiment, the precipitant is selected from anhydrous ethanol or methanol.

[0026] In a further preferred embodiment, the detergent used during washing is anhydrous ethanol.

[0027] In a further preferred embodiment, the vacuum drying temperature is 35~45℃.

[0028] The present invention also provides a sulfated modified sodium alginate prepared by the above preparation method.

[0029] This invention also provides an application of the sulfated modified sodium alginate prepared by the above method, using the sulfated modified sodium alginate as a flotation inhibitor for silicate gangue minerals (especially magnesium-containing silicates such as talc).

[0030] Further preferably, the silicate gangue mineral is talc.

[0031] The beneficial effects of this invention are as follows:

[0032] 1. The synthesis process is green and mild, with an advanced mechanism. This invention utilizes a deep eutectic solvent (DES) system formed by urea and sulfamic acid at a specific temperature. Urea not only acts as a catalyst to lower the activation energy of the sulfation reaction, but also acts as a highly polar melting medium to promote the unfolding of rigid polysaccharide chains and improve mass transfer efficiency. The excellent buffering effect of this system effectively inhibits the β-elimination degradation of the sodium alginate polysaccharide backbone under high temperature and strong acid conditions, achieving directional modification with a high degree of substitution (DS up to 0.769) while avoiding the use of toxic organic solvents.

[0033] 2. Novel structure and optimized conformation. After modification by the method of this invention, large-volume, highly electronegative sulfate groups were successfully covalently grafted onto the sodium alginate backbone (mainly at C-2 and C-3 positions), significantly enhancing the polarizability of the molecular chain. Simultaneously, the steric hindrance and electrostatic repulsion of the sulfate groups caused the compact sodium alginate framework to reconstruct a more extended topological conformation, exposing more active binding sites and greatly enhancing its chelating ability and spatial shielding effect on target minerals.

[0034] 3. Excellent selectivity, adaptability, and application potential. When used as a flotation depressant, this product's high-density sulfate groups not only exhibit strong bidentate chelation coordination with Mg²⁺ at the polar edge of magnesium silicates (such as talc), forming a dense, strongly hydrated molecular shielding film, but also effectively prevent non-specific adsorption on negatively charged valuable minerals (such as molybdenite) by utilizing its strong electrostatic repulsion. This completely solves the industry problem of "poor selectivity" in traditional polysaccharide depressants, possessing extremely high industrial application value. The modified SAS maintains high activity and solubility over a wide pH range (especially weakly acidic to weakly alkaline environments), overcoming the defect of natural sodium alginate easily forming alginate gel and becoming ineffective under acidic conditions, thus enabling it to adapt to flotation conditions of molybdenum ores with different properties. Attached Figure Description

[0035] Figure 1 The Fourier transform infrared spectroscopy results are shown for the SAS and SA samples in Example 1 of this invention.

[0036] Figure 2 The results are the Raman spectra of the SAS and SA samples in Example 1 of this invention.

[0037] Figure 3 The results are the proton NMR spectra of the SAS and SA samples in Example 1 of this invention. Detailed Implementation

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0039] This invention first implements the optimal preparation and characterization of sulfated modified sodium alginate (SAS).

[0040] Example 1

[0041] (1) Take sodium alginate (SA) and vacuum dry it at 40℃ for 8 hours.

[0042] (2) Then, assemble a mechanical stirrer, a spherical condenser and a thermometer on a 250 mL three-necked round-bottom flask. Place the three-necked flask in an oil bath with precise temperature control. Connect a nitrogen venting device above the condenser. All glassware has been dried before use and kept in a water-free state. Add 7.0 g of urea to the three-necked flask and turn on the oil bath to heat to 130°C so that it completely melts and becomes a transparent liquid.

[0043] (3) While stirring, 1.0 g of sodium alginate powder was divided into 3 batches and urea liquid was added within 30 s to disperse evenly. Then the dispersion system was cooled to 60℃.

[0044] (4) Add 3.0 g of aminosulfonic acid in 3 batches to the dispersion system within 30 s, then introduce nitrogen gas, control the gas flow rate to 150 mL / min, raise the temperature to 130℃, and react at a constant temperature for 4 hours.

[0045] (5) After the reaction mixture is cooled, water is added to dissolve it, and the precipitate is formed in anhydrous ethanol. The precipitate is washed with anhydrous ethanol and then dried under vacuum at 40°C to obtain the SAS product.

[0046] Example 2

[0047] The other steps are the same as in Example 1, but the isothermal reaction temperature in step (4) is 100°C.

[0048] Example 3

[0049] The other steps are the same as in Example 1, but the isothermal reaction temperature in step (4) is 120°C.

[0050] Example 4

[0051] The other steps are the same as in Example 1, but the isothermal reaction temperature in step (4) is 140°C.

[0052] Comparative Example 1

[0053] The other steps are the same as in Example 1, except that the constant temperature reaction time in step (4) is 1 hour.

[0054] Example 5

[0055] The other steps are the same as in Example 1, but the isothermal reaction time in step (4) is 6 hours.

[0056]

[0057] Figure 1 The SAS Fourier transform infrared (FTIR) spectrum prepared in Example 1 is shown below: 1240–1260 cm⁻¹ -1 800~840 cm -1 The characteristic peaks of S=O and COS stretching vibrations are clearly visible at this location; Figure 2 In the SAS Raman spectrum prepared for Example 1, at 874 cm⁻¹ -1 A sharp COS symmetrical stretching peak appears at 1033 cm⁻¹. -1 and 1130 cm -1The characteristic peak of S=O appears at this point, and the CH stretching peak splits at 2904 cm⁻¹. -1 and 2966 cm -1 The double peaks indicate that the introduction of strongly electronegative sulfate groups promoted the rearrangement of the skeletal structure; furthermore, in the high-resolution proton NMR spectrum (…), the double peaks suggest that the introduction of strongly electronegative sulfate groups promoted the rearrangement of the skeletal structure. 1 In H-NMR, due to the steric hindrance of the sulfate group, the terminal proton (H-1) exhibits a significant multiply splitting characteristic in the region of δ 4.64~5.40 ppm.

[0058] Elemental analysis revealed that for SAS preparations with different reaction times, the product DS was only 0.355 after 2 hours of reaction, while it decreased to 0.637 after 6 hours. In Example 1, the reaction was carried out at 130°C for 4 hours, and the mass fraction of sulfur in the obtained SAS corresponded to a high degree of substitution (DS) of 0.769 for the sulfate ester group, which is the optimal parameter point for the reaction. This result indicates that the DES system established in this invention reaches kinetic and thermodynamic equilibrium at around 4 hours. Excessive reaction time will lead to irreversible β-elimination cleavage of the polysaccharide backbone, proving the criticality and scientific nature of the process parameter settings in this invention.

[0059] This invention further implements a basic evaluation of the flotation application of SAS in the separation of magnesium silicates (talc) / molybdenite.

[0060] Example 6

[0061] Molybdenite with a purity >95% and talc are mixed in a 1:1 mass ratio to construct an artificial mixed mineral.

[0062] The pulp pH was adjusted to 9.0, the inhibitor dosage was 100 mg / L, and the collector dosage was 50 mg / L.

[0063] Molybdenite and talc with a purity >95% were taken separately, crushed, ground in a ceramic ball mill, and sieved through a particle size of 0.035-0.074 mm. They were then mixed at a mass ratio of 1:1 to construct an artificial mixed mineral sample.

[0064] During the flotation process, weigh 2.0 g of the mixed mineral sample, place it in a 40 mL flotation cell, add an appropriate amount of deionized water, and stir to adjust the slurry for 1 minute.

[0065] Add an appropriate amount of NaOH to adjust the pH of the slurry to 9.0.

[0066] Add inhibitor: Add the sodium sulfated alginate (SAS) solution prepared in Example 1 to a concentration of 100 mg / L in the slurry and stir for 3 minutes.

[0067] Add collector: Add sodium dibutyldithiocarbamate (CNS) as collector (50 mg / L) and stir for 3 minutes.

[0068] Add foaming agent: Add methyl isobutyl methanol (MIBC) as foaming agent (dosage 10 mg / L) and stir for 1 minute.

[0069] Open the air valve to perform skimming and flotation. The skimming time is 3 minutes.

[0070] The concentrate (foam product) and tailings (tank product) were collected separately, dried, weighed, and tested for molybdenum (Mo) and silicon (Si) grades. The recovery rates were calculated, as shown in Table 2.

[0071] Comparative Example 2

[0072] Unlike Example 2, no inhibitor was added during the flotation slurry preparation process in Comparative Example 2, which served as a blank control for the experiment. The remaining operation steps, reagent dosages, and raw ore composition were the same as in Example 1, and the results are shown in Table 1.

[0073] Comparative Example 3

[0074] Unlike Example 1, Comparative Example 3 used unmodified sodium alginate (SA) instead of SAS in Example 2 during the flotation pulp conditioning process. The remaining operation steps, reagent dosage and raw ore composition were the same as in Example 1. The results are shown in Table 2.

[0075]

[0076] As shown in Table 2, in the reverse flotation system without inhibitors, the concentrate Mo grade was 48.41%, and the Si grade was as high as 20.61%, with corresponding recoveries of 65.77% and 59.46%, respectively. This result indicates that molybdenite and talc gangue minerals cannot be effectively separated by flotation without inhibitors. In Comparative Example 3, after adding 100 mg / L of inhibitor SA, the concentrate Mo grade was 49.83%, and the Si grade was 8.31%, with corresponding recoveries of 97.18% and 34.43%, respectively. This indicates that the selective inhibition performance of inhibitor SA is limited. When 100 mg / L of the inhibitor SAS of this invention was added, the concentrate Mo grade increased to 50.98%, and the Si grade decreased to 5.30%; the Mo recovery remained at an extremely high level of 98.25%, while the Si recovery dropped sharply to 21.69%. This confirms that the inhibitor has excellent selective inhibition performance on the molybdenite / talc system.

[0077] Comparative experiments clearly show that, compared with the non-selective physical adsorption of unmodified SA, the SAS synthesized in this invention achieves specific chemical adsorption on the surface of magnesium silicate gangue by utilizing high-density sulfate groups, constructing a very strong three-dimensional hydration steric hindrance, and achieving more efficient targeted inhibition and removal of talc gangue without affecting the floatability of the target metal mineral (molybdenite).

Claims

1. A method for preparing sulfated modified sodium alginate, characterized in that: Urea is heated and melted to obtain urea liquid. Sodium alginate powder is added to the urea liquid to form a dispersion. The temperature is then lowered to 50-70°C, aminosulfonic acid is added, a protective atmosphere is introduced, and the temperature is raised to 100-140°C. The reaction yields sulfated modified sodium alginate.

2. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: The temperature at which urea is heated to melt is 130~150℃.

3. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: The sodium alginate is first subjected to a drying pretreatment under vacuum conditions. The drying pretreatment temperature is 35-45℃, and the drying pretreatment time is 4-10 hours.

4. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: Sodium alginate powder was added to urea liquid and stirred at 200-800 rpm for 15-20 minutes to obtain a dispersion.

5. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: The mass ratio of urea to sodium alginate powder is 1 to 20:1; The mass ratio of aminosulfonic acid to sodium alginate powder is 0.5~5:

1.

6. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: The protective atmosphere is nitrogen, and the gas flow rate is 50~200 mL / min.

7. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: The reaction time is 2 to 6 hours.

8. The method for preparing sulfated modified sodium alginate according to claim 1, characterized in that: After the reaction is complete, the product is cooled and dissolved in water to obtain a solution. The solution is then added to a precipitant to precipitate the product. After filtration, washing, and vacuum drying, the sulfated modified sodium alginate is obtained. The precipitant is selected from anhydrous ethanol or methanol; The detergent used in the washing process is anhydrous ethanol; The vacuum drying temperature is 35~45℃.

9. A sulfated modified sodium alginate prepared by the preparation method according to any one of claims 1-8.

10. The application of a sulfated modified sodium alginate prepared by the preparation method according to any one of claims 1-8, characterized in that: The sulfated modified sodium alginate was used as a flotation inhibitor for silicate gangue minerals.

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