Modified starch for selectively inhibiting iron ore as well as preparation method and application of modified starch

Abstract

The invention belongs to the technical field of mineral processing engineering, and particularly relates to modified starch for selectively inhibiting iron ore as well as a preparation method and application of the modified starch. The modified starch is prepared by the following steps: by taking soluble starch as a raw material, carrying out ice-water bath ultrasonic pretreatment, reacting with sodium trimetaphosphate within a specific pH range, introducing a sorbitol-xylitol-glycerol ternary compound molecular regulator in the process to control a reaction path, and finally, carrying out precipitation, dialysis and freeze-drying to obtain the product. According to the method, the substitution degree of the product phosphate is accurately controlled to be 0.01-0.04, and the proportion of monoester bonds is not lower than 80%. The modified starch is applied as an inhibitor in reverse flotation separation of iron ore and silicate minerals, a selective adsorption layer is preferentially formed on the surface of the iron ore, efficient inhibition of the iron ore is achieved, meanwhile, excessive inhibition of silicate is avoided, a stable and adjustable separation window is formed in hard water and complex water chemical environments, and the separation efficiency is improved. The method is suitable for efficient separation of iron ore and silicate.

Description

Technical Field

[0001] This invention belongs to the field of mineral processing engineering technology, specifically relating to a modified starch for selectively inhibiting iron ore, its preparation method, and its application. Background Technology

[0002] In the enrichment process of iron ore, especially oxide ores such as hematite, reverse flotation is a key process for removing gangue minerals such as silicates. In this process, the role of depressants is crucial. They selectively adsorb onto the surface of target minerals, inhibiting their interaction with collectors, thereby achieving precise control over the floatability of different minerals. Starch and its derivatives, as widely available, inexpensive, and environmentally friendly natural polymers, have long been used as typical depressants for iron minerals. The starch molecular skeleton is rich in hydroxyl groups, which can bind to exposed metallic hydroxyl sites on the surface of iron minerals through hydrogen bonds and complexation, effectively masking the adsorption sites of collectors, reducing the hydrophobicity of the mineral surface, and retaining it in the slurry rather than floating into the froth product.

[0003] In early industrial practice and research, soluble starch or gelatinized starch derived from corn and potatoes were commonly used as inhibitors. For example, Ge Wencheng et al. used ordinary starch to inhibit hematite in a brominated collector system, achieving preliminary separation from iron-containing silicate minerals. However, due to the high regularity of their molecular structure and the uneven distribution and reactivity of their functional groups (hydroxyl groups), these natural starches often exhibit insufficient inhibitory selectivity in practical applications. They are prone to non-specific adsorption of multiple minerals, leading to "simultaneous inhibition," which reduces the difference in floatability between iron ore and gangue minerals, resulting in low separation efficiency. Furthermore, the inhibitory properties of natural starch affect the composition of the mineral processing water (such as calcium). 2+ Mg 2+ HCO 3- The flotation process is extremely sensitive to changes in ions and pulp temperature. At low temperatures, starch swells slowly and disperses poorly; while at high temperatures, it easily leads to a sharp increase in system viscosity, affecting the rheological properties and foam stability of the pulp, causing difficulties in production operations. Another issue that cannot be ignored is that different batches of natural starch have natural differences in the ratio of linear to branched chains, the degree of gelatinization, and the molecular weight distribution. In addition, they usually require activation with caustic alkali (such as NaOH) to achieve the best inhibitory effect, but the degree of activation and the reaction time are difficult to control precisely, ultimately resulting in large fluctuations in flotation indicators and poor process repeatability.

[0004] To overcome the limitations of natural starch, researchers have explored various physical and chemical modification pathways. Physical modification methods, such as mechanical activation, ultrasonic or microwave treatment, aim to enhance adsorption capacity by disrupting the starch's crystal structure and exposing more active hydroxyl sites. While these methods are environmentally friendly and do not introduce chemical reagents, their modification depth is heavily dependent on the intensity and uniformity of energy input. The modified amorphous structure tends to re-condense or reorder during storage or slurry recycling, leading to a decline in inhibition performance over time, and still struggling to escape the dilemma of "over-inhibition" or "under-inhibition." In terms of chemical modification, introducing charged or strongly coordinating groups (such as phosphate groups, carboxymethyl groups, and hydroxypropyl groups) into the starch chain through phosphorylation, esterification, and etherification is an effective strategy to improve selectivity and adsorption strength. For example, studies have shown that alkylpropyl distarch phosphate esters exhibit preferential inhibition of target minerals in specific systems; others have used starch combined with polycarboxylic acid ethers or utilized inorganic phosphates to co-react with starch to regulate surface charge, improving separation performance in nickel and ilmenite systems. However, these chemical modification methods often tend to introduce higher density functional groups or form cross-linked structures, resulting in increased rigidity and decreased flexibility of starch molecular chains. This can lead to the formation of a dense and excessively large adsorption layer on the mineral surface by the modified product, which not only inhibits the target mineral but also inevitably and strongly inhibits gangue minerals. In complex flotation systems with similar collector polarity and slight differences in mineral surface chemical characteristics, it is still impossible to form a sufficiently broad and stable selective separation window.

[0005] In summary, although starch inhibitors are widely used in iron ore flotation, existing technologies, whether using natural starch or physically or chemically modified starch, have failed to adequately address the balance between inhibitory selectivity, adaptability to operating conditions, and process stability. Developing a novel starch inhibitor system with precisely controllable functional group density, mild inhibitory effect, and a clear selective window has become a critical technological bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a modified starch for selectively inhibiting iron ore, its preparation method and application, aiming to solve the technical problems of poor selectivity, sensitivity to hydrochemical environment and uncontrollable inhibition effect of existing starch inhibitors in the flotation separation of iron ore and silicate minerals.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a method for preparing modified starch for selectively inhibiting iron ore, comprising the following steps: (1) Disperse soluble starch in deionized water and then perform ultrasonic treatment; (2) Add sodium chloride (NaCl) and sugar alcohol molecular regulator to the solution obtained in step (1) to adjust the pH value of the solution to between 10.2 and 10.9, and at the same time raise the temperature to 35-55℃; (3) Add sodium trimetaphosphate (STMP) solution dropwise to the solution obtained in step (2) to carry out the reaction, and maintain the pH between 10.4 and 10.8 during the reaction; (4) Cool the reaction solution obtained in step (3), neutralize it to pH neutral, and then precipitate and wash it with ethanol solution; (5) Dissolve, dialyze and freeze-dry the product after washing in step (4) to obtain the modified starch for selectively inhibiting iron ore.

[0008] Furthermore, in step (1), the ultrasonic treatment is performed under ice-water bath conditions, the ultrasonic power is 300-500W, and the ultrasonic time is 5-15 minutes.

[0009] This invention introduces ultrasonic treatment under ice-water bath conditions before starch phosphorylation modification. Its main function is to provide a mild and controllable physical pretreatment of the starch granule structure. This process utilizes the cavitation effect generated by ultrasound in a liquid medium to apply physical force to the crystalline regions of the starch granules, causing them to loosen and partially dissociate, thereby exposing more encapsulated hydroxyl active sites. The ice-water bath environment effectively absorbs and dissipates most of the heat generated by the ultrasound energy, preventing a sudden increase in system temperature that could lead to uncontrolled gelatinization or mechanical breakage of the starch molecular chains. This operation improves the accessibility and uniformity of the reaction without significantly altering the chemical nature of the starch, contributing to obtaining modified products with a more regular distribution of substituent groups.

[0010] Furthermore, in step (2), the amount of sodium chloride added is 0.05-0.15 times the mass of soluble starch.

[0011] Further, in step (2), the sugar alcohol molecular regulator is composed of sorbitol, xylitol and glycerol, and the mass ratio of the three is 1:(0.2-0.5):(0.1-0.3); the amount of sugar alcohol molecular regulator added is 0.03-0.10 times the mass of soluble starch.

[0012] This invention introduces a compound regulator composed of sorbitol, xylitol, and glycerol into the phosphorylation reaction system. These sugar alcohol molecules contain multiple hydroxyl groups, enabling them to form extensive hydrogen bond networks with water molecules and starch chains in the system. On the one hand, this reduces the activity of water molecules to a certain extent, thereby inhibiting the hydrolysis side reaction of STMP in the aqueous phase and reducing the consumption of ineffective phosphorus sources. On the other hand, through steric hindrance and competitive interactions, they disrupt the tight aggregation of starch molecular chains, making the reaction sites easier for STMP to approach, without triggering violent local reactions. In particular, the rapid permeability and small molecular weight of glycerol, combined with the larger molecular weight and more significant steric hindrance of sorbitol and xylitol, form a dynamic buffer system. This system helps to stabilize the reaction rate and avoid excessive formation of inter-chain cross-linking structures (POPs) caused by excessively rapid local reactions or pH fluctuations. This results in a macroscopic increase in the proportion of phosphate monoester bonds and an improvement in the uniformity of substituent group distribution in the product.

[0013] Further, in step (3), the concentration of the sodium trimetaphosphate solution is (0.08-0.15) mol / L, and the amount of sodium trimetaphosphate added is 3-8 times the mass of the soluble starch.

[0014] Furthermore, in step (3), the dropping time is controlled at 30-60 minutes, the reaction temperature is 35-55℃, and the reaction time is 30-60 minutes.

[0015] Step (3) employs a low-concentration STMP solution and precise pH control to achieve accurate regulation of the product molecular structure. The low concentration of STMP stoichiometry limits the total amount of phosphate groups introduced, providing a basis for obtaining products with low degree of substitution; while strictly controlling the pH within a narrow alkaline window creates an optimal reaction environment that is conducive to the ring-opening formation of monoester bonds (POC) by STMP, while suppressing its hydrolysis and cross-linking side reactions.

[0016] Furthermore, in step (5), the dialysis bag used for dialysis has a molecular weight cutoff of 3-4 kDa, and the dialysis time is 24-48 hours.

[0017] A second aspect of the present invention provides a modified starch for selectively inhibiting iron ore, which is prepared by the above method. The modified starch has a degree of phosphate substitution (DS) of 0.01-0.04, and the phosphate groups therein exist in the form of monoester bonds (POC), with a molar fraction of monoester bonds of not less than 80%.

[0018] The third aspect of the present invention provides the application of the modified starch described above for selectively inhibiting iron ore in the flotation separation of iron ore and silicate minerals.

[0019] Furthermore, the application refers to the use of the modified starch as an inhibitor to suppress iron minerals in a reverse flotation system, added at a dose of 1-80 mg / L to a slurry with a concentration of 20%-35% and a pH of 8.8-10.6.

[0020] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: This invention successfully prepared a starch derivative with a specific low degree of substitution and a monoester-dominated structure by combining a controlled phosphorylation reaction with physical activation. This product exhibits clear site selectivity in the flotation system; its moderate anion density preferentially complexes with active sites on the hematite surface, effectively inhibiting its hydrophobicity. Simultaneously, thanks to its low degree of substitution and non-crosslinked flexible molecular chain structure, its adsorption on silicate surfaces is weak, thus establishing a clear difference in floatability between the two minerals. This structural characteristic also enables it to maintain adsorption stability in hard water environments containing calcium and magnesium ions, reducing sensitivity to water quality. Ultimately, in actual separation processes, stable high-grade iron concentrate and high recovery rates can be obtained over a wide range of process parameters, solving the problems of narrow selectivity and large performance fluctuations of traditional starch inhibitors. Attached Figure Description

[0021] Figure 1 The image shows a comparison of the infrared spectra of the modified starch prepared in Example 1 and the raw soluble starch. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Unless otherwise specified, all raw materials used in the embodiments are commercially available products. The following sources are illustrative examples.

[0024] Soluble starch was purchased from Sinopharm Chemical Reagent Co., Ltd., model number TSP0010.

[0025] Terminology definition: 1: Degree of substitution (DS): The average number of phosphate groups introduced on each anhydrous glucose unit (AGU), preferably in the range of 0.01 to 0.04 in this invention.

[0026] 2: Monoester / Crosslinking: Monoesters are POC bonds; crosslinking is POP bridging between / within starch chains.

[0027] Example 1 This embodiment provides a modified starch for selectively inhibiting iron ore production, the preparation method of which includes the following steps: (1) Accurately weigh 1.00 g of soluble starch, add deionized water, stir and make up to 50.0 mL to form a homogeneous solution. Then, place the beaker in an ice water bath and treat the solution for 10 minutes using an ultrasonic cell disruptor with a power of 400 W.

[0028] (2) Add 0.10 g of sodium chloride (NaCl) and 0.06 g of sugar alcohol molecular regulator (the regulator is a mixture of sorbitol, xylitol and glycerol in a mass ratio of 1:0.3:0.2) to the solution obtained in step (1). Then, adjust the pH of the system to 10.6 using sodium carbonate solution and sodium hydroxide solution, and raise the temperature to 45 °C.

[0029] (3) Take 5.0 mL of 0.10 mol / L sodium trimetaphosphate (STMP) solution and slowly and evenly add it dropwise to the system in step (2), controlling the dropwise addition time to 45 minutes. Throughout the dropwise addition and subsequent reaction, the pH of the system is maintained at 10.6 by adding small amounts of NaOH solution. After the dropwise addition is complete, continue to stir the reaction at 45 °C for 40 minutes.

[0030] (4) After the reaction is complete, the reaction solution is rapidly cooled to below 30 °C and slowly neutralized to pH 7.0 with dilute hydrochloric acid solution. Then, 70% ethanol solution is added to the system, and a white precipitate is obtained by centrifugation (4000 rpm, 10 min). The precipitate is washed.

[0031] (5) The washed precipitate was redissolved in 35 mL of deionized water and transferred into a dialysis bag with a molecular weight cutoff of 3.5 kDa. Dialysis was performed with deionized water for 36 hours. Finally, the solution in the dialysis bag was freeze-dried to obtain the modified starch.

[0032] The modified starch prepared in Example 1 was analyzed by Fourier transform infrared spectroscopy (FT-IR), and the results are as follows: Figure 1 As shown in the figure, the starch with a degree of substitution of 0.01 is the infrared spectrum of the modified starch of Example 1.

[0033] Compared to unmodified soluble starch, phosphorylated starch has a lower content of 1000-1100 cm⁻¹ -1 A slight enhancement of the absorption band in this region, where both the COC vibration and POC bond stretching vibration of starch coexist, indicates the successful introduction of phosphate monoester groups onto the starch molecule. Furthermore, at approximately 930 cm⁻¹... -1A weak shoulder peak can be observed at this point, which is a characteristic absorption of the POP cross-linked structure. However, its peak intensity is extremely weak, indicating that the product obtained in this example is mainly composed of monoesterified structures with very low cross-linked structure content. Meanwhile, at 3300-3500 cm⁻¹... -1 The OH stretching vibration region shows a slight change in the shape of the visible band, indicating that the introduction of the phosphate group disrupts part of the original hydrogen bond network of the starch molecule.

[0034] Because the phosphorylated starch prepared in this embodiment has a low degree of substitution (DS approximately 0.01), the corresponding theoretical phosphorus content is extremely low. Therefore, its POC and POP characteristic peaks are both weak, which is consistent with the spectral characteristics of low-substituted phosphorylated starch reported in the literature. The above infrared spectral characteristics collectively indicate that the present invention has successfully prepared a low-substituted modified starch with a phosphate monoester structure as the main component.

[0035] Example 2 This embodiment provides a modified starch for selectively inhibiting iron ore production, the preparation method of which includes the following steps: (1) Accurately weigh 1.00 g of soluble starch, add deionized water, stir and dilute to 50.0 mL to form a homogeneous solution. Then, place the beaker in an ice-water bath and treat the solution for 8 minutes using an ultrasonic cell disruptor with a power of 350 W.

[0036] (2) Add 0.08 g of sodium chloride and 0.05 g of sugar alcohol molecular regulator (the regulator is a mixture of sorbitol, xylitol and glycerol in a mass ratio of 1:0.4:0.15) to the solution obtained in step (1). Then, adjust the pH of the system to 10.4 using sodium carbonate solution and sodium hydroxide solution, and raise the temperature to 40 °C.

[0037] (3) Measure 4.0 mL of 0.12 mol / L sodium trimetaphosphate (STMP) solution and slowly and evenly add it dropwise to the system in step (2), controlling the dropwise addition time to 35 minutes. Throughout the dropwise addition and subsequent reaction, the pH of the system is maintained at 10.5 by adding small amounts of NaOH solution. After the dropwise addition is complete, continue to stir the reaction at 40 °C for 50 minutes.

[0038] (4) After the reaction is complete, the reaction solution is rapidly cooled to 25 °C and slowly neutralized to pH 7.0 with dilute hydrochloric acid solution. Then, 70% ethanol solution is added to the system, and a white precipitate is obtained by centrifugation (4000 rpm, 10 min). The precipitate is washed.

[0039] (5) The washed precipitate was redissolved in 35 mL of deionized water and transferred into a dialysis bag with a molecular weight cutoff of 3.5 kDa. Dialysis was performed with deionized water for 28 hours. Finally, the solution in the dialysis bag was freeze-dried to obtain the modified starch.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that the ultrasonic treatment in step (1) is performed at room temperature (about 25°C).

[0041] Comparative Example 2 The difference between this comparative example and Example 1 is that in step (2), only sorbitol, a single component, is used as the sugar alcohol molecular regulator, and the total amount added remains unchanged.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that the sugar alcohol molecular regulator in step (2) is composed of sorbitol, xylitol and glycerol in a mass ratio of 0.3:1:0.2.

[0043] Comparative Example 4 The difference between this comparative example and Example 1 is that no sugar alcohol molecular regulator is added in step (2).

[0044] Comparative Example 5 The difference between this comparative example and Example 1 is that sodium tripolyphosphate in step (3) is replaced with sodium tripolyphosphate in the same number of moles.

[0045] Comparative Example 6 The difference between this comparative example and Example 1 is that in step (3), the pH value of the reaction system is controlled at 9.0.

[0046] Performance testing To verify the performance of the modified starch provided by this invention, the following tests were designed. All flotation tests were conducted in an XL-type hanging trough flotation machine with a pulp volume of 40 mL.

[0047] 1. Single mineral inhibition selectivity: Test method: Using pure hematite (purity >98%) and pure chlorite (purity >99%) as research objects, 180 mg / L of fatty acid collector and 20 mg / L of inhibitor (modified starch prepared in the examples or comparative examples) were added under the condition of pH=9.0, and single mineral flotation tests were carried out to calculate the mineral recovery rate.

[0048] 2. Stability in hard water environments: Test method: In a solution containing 100 mg / L Ca 2+ The single mineral flotation test of hematite was repeated in a hard aqueous solution, with an inhibitor dosage of 20 mg / L.

[0049] 3. Actual ore sorting results: Test method: Using an iron ore mine in Shanxi (TFe grade 48.97%) as feed, a laboratory closed-circuit flotation test was conducted under pH=9.0 conditions to evaluate the iron recovery rate when obtaining qualified iron concentrate (grade >63%).

[0050] The results of single-mineral inhibition selectivity and stability tests under hard water conditions are shown in Table 1.

[0051] Table 1. Results of Single Mineral Flotation Recovery and Hard Water Stability Tests

[0052] Based on the above single mineral flotation recovery and hard water stability test results, samples from Examples 1-2 and Comparative Examples 4-5 were further selected for actual closed-circuit flotation tests of ores, and the results are shown in Table 2.

[0053] Table 2 Results of actual closed-circuit flotation tests on ore

[0054] The test results above show that Examples 1 and 2 both exhibit excellent separation efficiency in closed-circuit flotation tests, achieving a high iron recovery rate of over 82% while ensuring a concentrate grade of over 63%. This proves that the modified starch of this invention has successfully established a stable and efficient selective separation window in real and complex ore systems.

[0055] Comparative Example 1, using room-temperature ultrasonication, exhibited poor product structural uniformity, leading to unstable inhibition behavior and making it prone to non-selective entrainment or insufficient inhibition of iron minerals during continuous separation. Comparative Example 2, using sorbitol as a single regulator, showed incomplete regulation, insufficient monoester ratio in the product, weak inhibition ability, and non-specific adsorption of chlorite, resulting in a narrow separation selectivity window and difficulty in achieving both concentrate grade and recovery. Comparative Example 3, with its unbalanced regulator ratio, disrupted the synergistic effect, causing the product to simultaneously exhibit both insufficient and over-inhibited regions, making it impossible to maintain stable foam and slurry phase properties during continuous separation. Comparative Example 4, lacking a regulator, suffered from a cross-linked structure that made the inhibitor molecules overly rigid, resulting in ineffective adsorption on silicate surfaces. A large amount of iron minerals was lost in the tailings due to "synchronous inhibition," leading to a significant decrease in recovery. Comparative Example 5, using STPP instead of STMP, facilitated the formation of cross-linked structures, resulting in a dense adsorption layer on the mineral surface. This severely weakened the floatability difference between iron ore and silicates, leading to low separation efficiency. Comparative Example 6 reacted at low pH, resulting in a severely insufficient reaction degree and excessively low product substitution. It had almost no effective inhibition ability and could not play a separating role in the flotation system.

[0056] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing modified starch for selectively inhibiting iron ore, comprising the following steps: (1) Disperse soluble starch in deionized water and then perform ultrasonic treatment; (2) Add sodium chloride (NaCl) and sugar alcohol molecular regulator to the solution obtained in step (1) to adjust the pH value of the solution to between 10.2 and 10.9, and at the same time raise the temperature to 35-55℃; (3) Add sodium trimetaphosphate solution dropwise to the solution obtained in step (2) to carry out the reaction, and maintain the pH between 10.4 and 10.8 during the reaction; (4) Cool the reaction solution obtained in step (3), neutralize it to pH neutral, and then precipitate and wash it with ethanol solution; (5) Dissolve, dialyze and freeze-dry the product after washing in step (4) to obtain the modified starch for selectively inhibiting iron ore.

2. The preparation method according to claim 1, characterized in that, In step (1), the ultrasonic treatment is performed under ice water bath conditions, the ultrasonic power is 300-500W, and the ultrasonic time is 5-15 minutes.

3. The preparation method according to claim 1, characterized in that, In step (2), the amount of sodium chloride added is 0.05-0.15 times the mass of soluble starch.

4. The preparation method according to claim 1, characterized in that, In step (2), the sugar alcohol molecular regulator is composed of sorbitol, xylitol and glycerol, and the mass ratio of the three is 1:(0.2-0.5):(0.1-0.3); the amount of sugar alcohol molecular regulator added is 0.03-0.10 times the mass of soluble starch.

5. The preparation method according to claim 1, characterized in that, In step (3), the concentration of the sodium trimetaphosphate solution is (0.08-0.15) mol / L, and the amount of sodium trimetaphosphate added is 3-8 times the mass of the soluble starch.

6. The preparation method according to claim 1, characterized in that, In step (3), the dripping time is controlled at 30-60 minutes, the reaction temperature is 35-55℃, and the reaction time is 30-60 minutes.

7. The preparation method according to claim 1, characterized in that, In step (5), the dialysis bag used for dialysis has a molecular weight cutoff of 3-4 kDa, and the dialysis time is 24-48 hours.

8. A modified starch for selectively inhibiting iron ore, characterized in that, The modified starch is prepared by the method according to any one of claims 1-7, wherein the degree of phosphate substitution of the modified starch is 0.01-0.04, and the phosphate groups therein exist in the form of monoester bonds, and the molar fraction of monoester bonds is not less than 80%.

9. The application of the modified starch prepared by the preparation method according to any one of claims 1-7 in the flotation separation of iron ore and silicate minerals.

10. The application according to claim 9, characterized in that, The application refers to the use of the modified starch as an inhibitor to suppress iron minerals in a reverse flotation system, added at a dose of 1-80 mg / L to a slurry with a concentration of 20%-35% and a pH of 8.8-10.6.

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