A composite purification method for ultra-low iron content zirconium sulfate solution

doi:10.21203/rs.3.rs-2959734/v1

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A composite purification method for ultra-low iron content zirconium sulfate solution

https://doi.org/10.21203/rs.3.rs-2959734/v1

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It is critical to remove Fe removal from the titanium dioxide coating solution. In this study, we combined potassium ferricyanide precipitation with ultrafiltration for deep Fe removal. The effects of the potassium ferrocyanide dosage, reaction temperature, reaction time, stirring speed, and ultrafiltration membrane specifications on Fe removal were systematically studied. Furthermore, the experimental conditions were optimized to achieve the lowest Fe content of 8.1 mg/L in the solution. The size distribution and zeta potential of the precipitate particles were determined, and the results revealed that the effective diameter of the particles was 16.581 μm and the zeta potential of the surface groups was negative. A cationic flocculant was added to eliminate the static repulsion between precipitate particles and promote their growth. The Fe content reached 3.8 mg/L after 0.1 mL of flocculant solution was added to the solution. Lastly, the Fe removal mechanism was determined.

Fe removal

potassium ferrocyanide

precipitation

ultrafiltration

Titanium dioxide is a critical white inorganic chemical pigment with remarkable photocatalytic activity^{[1, 2]}, which can be used as light-sensitive oxidation or reduction catalyst^{[3, 4]}. Because of its high surface energy and excellent photocatalytic activity ^{[5, 6]}, titanium dioxide should be coated before application^[7–9]. Coating alumina, silicon, and zirconium on the surface of titanium dioxide are the most common industrial method ^{[10, 11]}, and the most common titanium dioxide coating solutions are aluminum sulfate, zirconium sulfate, and sodium silicate ^[12–14]. In particular, coating zirconium on the surface of titanium dioxide can enhance its light resistance and anti-powdering property. Because the high Fe content will cause color fading and diminish the quality of the cladding layer, the Fe content is a critical indicator of the coating solution quality. Coating solutions with high Fe content can only be used in medium- and low-grade industries^{[15, 16]}. Therefore, Fe removal is a required step for obtaining a high-quality titanium dioxide coating solution.

Significant attention has been focused on methods for Fe removal from industrial coating solutions ^[17–27], including recrystallization, solvent extraction, and precipitation( and adsorption). To date, solvent extraction has been the most commonly studied Fe removal method ^{[28, 29]}, and the most widely studied extraction systems are based on phosphonic acid, amines, and carboxylic acids. However, the extraction systems using phosphonic acid extractant present poor selectivity; moreover, the Fe-loaded organic phase is difficult to strip^{[30, 31]}. Because carboxylic acid extractants easily dissolve in the aqueous phase, extractant loss is significant ^{[32, 33]}. When amines are used as extractants, a third phase typically forms during extraction, limiting their application ^[34]. Recrystallization is used for Fe removal because of the difference in solubility between Fe and other elements at the same temperature during cooling crystallization^{[35, 36]}. But Fe removal efficiency decreases after several recrystallization cycles. Precipitation is also commonly applied for Fe removal^[37], particularly oxidation-hydrolysis precipitation methods and the formation of insoluble precipitates^[38]. Precipitation methods are facile, inexpensive, and highly efficient ^[39]. Commonly used precipitants include potassium permanganate^[40], hydrogen peroxide^[41], potassium ferricyanide^[42], potassium ferrocyanide^{[43, 44]}, and sodium diethyldithiocarbamate ^[45]. The thorough removal of Fe using the precipitation method with negligible coating element(Al, Si, and Zr) loss is difficult; moreover, the residual Fe content of the solution is usually higher than 15 mg/L. The size of the precipitate particles is often too small to allow separation using conventional methods. For example, the Fe precipitate particles produced using the potassium ferricyanide precipitation method are small and form a colloidal suspension which is difficult to separate ^{[46, 47]}; Therefore, this method cannot be used to thoroughly remove Fe.

Membrane filtration is a highly efficient, low-energy, and facile liquid separation technology with broad applications in wastewater treatment^[48–50]. Membrane separation differs from conventional filtration because it can separate particles with sizes in the molecular range; moreover, the process is based on a physical mechanism and does not involve phase changes ^[48]. Ultrafiltration, which is a membrane separation method that requires a differential static pressure of 0.1–0.5 MPa, can be used to separate particles with diameters in the range of 0.01–0.1 µm^{[51, 52]}. Combinations of ultrafiltration and precipitation methods can be used to effectively separate precipitates from the solution. In our previous study^[53], an organic precipitation-ultrafiltration process has been successfully used to remove iron from aluminum sulfate solutions in depth.

To obtain high-quality low-Fe titanium dioxide coating solutions, in this study we used a commercially available zirconium sulfate solution as the raw material. Moreover, the potassium ferrocyanide precipitation method combined with ultrafiltration was used for thorough Fe removal to obtain a solution with a Fe content lower than 5 mg/L.

The experimental raw material was a commercially available zirconium sulfate solution used for titanium dioxide coating (Chongqing Titanium Industry Co.,Ltd of Pangang Group, Chongqing 404100, China). The Fe content and Zr contents of the solution were 18 mg/L and 2.9 mol/L, respectively. The pH value of the solution is about 2.7 and the viscosity is about eight times the viscosity of water. All the reagents used in this study were of analytical grade. As described in Section 3, pre-established amounts of precipitant (potassium ferrocyanide, K₄[Fe(CN)₆]) were added to specific volumes of zirconium sulfate solution. After stirring for a predetermined time at a specified temperature, the solid and liquid phases of the mixtures were separated using an ultrafiltration cup. Subsequently, the Fe contents and Zr losses were detected and the results were analyzed.

The 1,10 phenanthroline spectrophotometric method was used to determine the Fe contents of the samples, and the ascorbic acid was used to reduce Fe³⁺ to Fe²⁺. First, 1,10 phenanthroline reacted with Fe²⁺ at pH 2–9, and then, the absorbance of the thus-obtained orange-red solution was measured at the maximum absorption wavelength. Next, the absorbance was converted into Fe content using the Beer-Lambert law. The Zr content of the solution was determined using the ethylenediaminetetraacetic acid (EDTA) complexation titration method. The absorbances of the solutions at different wavelengths and the standard Fe content curve are presented in Fig. 1. The maximum absorbance was observed at 510 nm, which was consistent with the data in the literature ^{[54, 55]}. The R-square of the standard Fe content curve was 0.99982 (Fig. 1b), indicating that the method was sufficiently accurate for Fe content detection.

3.1. Reaction Mechanisms

3.1.1. Iron valence analysis

First, the Fe(II) and Fe(III) contents of the solution were determined. The total Fe in the solution was determined using the 1,10 phenanthroline spectrophotometric method, and the Fe(II) content of the solution was determined using the 1,10 phenanthroline spectrophotometric method without the addition of ascorbic acid. The results indicated that the absorbance of the Fe(II)-containing solution in the absence of ascorbic acid was the same as that of the blank solution. Moreover, the Fe contents of the solution with and without added hydrogen peroxide were the same. In summary, the primary Fe species in the solution was Fe(III), and K₄[Fe(CN)₆] was selected as the Fe removal agent. The Fe removal effect was investigated at different K₄[Fe(CN)₆] dosages, reaction temperatures, reaction times, stirring speeds, and ultrafiltration membranes with different specifications.

3.2. Results of iron removal under different conditions

The effects of K₄[Fe(CN)₆] dosage, temperature, reaction time, stirring speed, and film characteristics on iron removal were investigated separately.

Fig 2 (a) shows the effect of different K₄[Fe(CN)₆] dosages (the mole ratio of K₄[Fe(CN)₆] to Fe is 0.9, 1.0, 1.1, 1.2, 1.3, and 1.4) on iron removal. The reaction conditions were as follows: temperature of 25 °C, stirring speed of 300 rpm, and reaction time of 1.0 h. In addition, a 50,000 Da membrane was used for ultrafiltration. As the dosage of K₄[Fe(CN)₆] was increased, the Fe content of the solution decreased at first and then increased. By contrast, the Zr loss increased with increasing K₄[Fe(CN)₆] dosage. Upon the addition of K₄[Fe(CN)₆] to the Fe-containing zirconium sulfate solution, the Fe in the solution reacted with it to produce a precipitate. Consequently, the Fe content of the solution decreased gradually. However, because K₄[Fe(CN)₆] contains Fe, the Fe content of the solution gradually increased as the K₄[Fe(CN)₆]-to-Fe molar ratio was increased beyond 1.0. The gradual increase in the Zr loss was ascribed to the gradual increase in the amount of precipitate generated because Zr was adsorbed and entrapped in the precipitate. The residual Fe content of the solution was the lowest when the number of moles of K₄[Fe(CN)₆] matched the number of moles of Fe ions in the solution, that is, at a K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0. Therefore, a K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0 was selected for the subsequent experiments.

Fig 2 (b) shows the influence of different temperatures (5, 25, 45, 65, and 85℃) on iron removal. The experimental conditions were as follows: K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0, stirring speed of 300 rpm, and reaction time of 1.0 h. Moreover, a 50,000 Da membrane was used for ultrafiltration. As the temperature increased, the Fe content first decreased and then reached a plateau. By contrast, the Zr loss increased significantly with temperature and then reached a plateau. As the temperature was increased, the mass transfer in the system was promoted and precipitates were more easily generated. Furthermore, the collision and growth of the small precipitate particles were favored, improving membrane separation performance. Moreover, the Zr loss increased with the increasing amount of precipitate because Zr was adsorbed and entrapped in the precipitates. At temperatures higher than 45 °C, upon slowly cooling the system to room temperature, a large number of zirconium sulfate crystals were produced. This led to an increase in the Zr loss, which reached approximately 6%. Considering the operation cost and crystallization behavior of the precipitates, 45 °C was selected as the optimal temperature for the subsequent experiments.

Fig 2 (c) shows the effect of different times (0.5, 1.0, 2.0, 4.0, 8.0, 24.0, 48.0 h) on iron removal. The reaction conditions were as follows: K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0, stirring speed of 300 rpm, and reaction temperature of 45 °C. Moreover, a 50,000 Da membrane was used for ultrafiltration. As the reaction time was increased, the Fe content of the solution first decreased sharply and then increased slowly until it reached a plateau. Conversely, the Zr loss increased sharply and then decreased slowly until it reached a plateau. For short reaction times, the precipitate particles were small and could not be separated via ultrafiltration. Upon increasing the reaction time, the precipitate particles grew and the Fe content decreased. As the reaction time was further increased, a fraction of the agglomerated large particles disintegrated into small particles, which formed a suspension and increased the Fe content of the solution. The Zr loss was inversely correlated with the Fe content. Considering the Fe removal results, a reaction time of 2.0 h was selected for the subsequent experiments.

Fig 2 (d) shows the effect of different stirring speeds (100, 200, 300, 400, 500, and 600 RPM) on iron removal. The experimental conditions were as follows: K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0, a reaction time of 4.0 h, and a reaction temperature of 45 °C. In addition, a 50,000 Da membrane was used for ultrafiltration. As the stirring speed was increased, the Fe content first decreased and then reached a plateau. Because stirring promoted mass transfer, it also promoted the precipitation reaction and the growth of the small precipitate particles via interparticle collisions. Considering the operation cost and Fe removal results, a stirring speed of 300 rpm was selected for the subsequent experiments.

Fig 2 (e) shows the effect of different molecular weight cut-offs (MWCO; 500,000, 200,000, 100,000, 50,000, and 20,000 Da) on the removal. The experimental conditions were as follows: K₄[Fe(CN)₆]-to-Fe molar ratio of 1.0, a reaction time of 4.0 h, a reaction temperature of 45 °C, and a stirring speed of 300 rpm. As the MWCO of the membranes was decreased, the ability of the membranes to separate small particles increased and the Fe content of the solution decreased linearly and reached a plateau, which was related to the size of the precipitate particles. Moreover, as the MWCO was decreased, the filtration rate decreased. In particular, at an MWCO of 20,000 Da, because of the small membrane pore size, which provided a large solid-phase surface area, zirconium sulfate crystals precipitated on the membrane surface, hindering filtration and causing Zr loss.

3.3. Physical and chemical properties of the system

The precipitation on the membrane is blue (Fig 3(a)). Precipitation is made up of smooth surface particles (Fig 3(b)), and the size of the particles is about dozens of microns. The particle size of the particle was measured using the Nano Measurer (Fig 3(c)), indicating that the particle size of the particle was mainly in the 2-32.9μm, and was accompanied by a very small number of large particles, with an average diameter of 14.85μm. In addition, the surface charge and particle size of the precipitated particles directly affect the removal of iron, so the ZetaPALS(Brook Havenus, US) and Laser particle size analyzer(Mastersizer 2000)were used to analyze the Zeta potential and particle size distribution of the precipitate, as shown in Fig. 4.

The size of the particle is between 6.6-30.2μm, and the average particle size is 16.581μm, the results of the laser particle size analyzer and Nano Measurer were similar. At present, the speed filter paper aperture of the market is 10-15μm, which can not effectively filter the precipitation, so the choice of ultrafiltration is better. To increase the particle size, the surface potential of the particle is one of the key factors of particle growth^[60]. As shown in the particle's potential diagram (Fig. 4 (b), the zeta potential on the particle surface was negative in the range of -20 mV to -55 mV. The electrostatic repulsion could be eliminated and the particle should be gathered together, according to DLVO theory. Therefore, the addition of cationic flocculants can reduce the electrostatic repulsion between particles and promote the growth of particles. In this study, cationic polyacrylamide was selected as the flocculant.

3.4 Iron removal with flocculant

The effect of different flocculant dosages (0, 0.1, 0.2, 0.5, and 1.0 wt%) on the efficiency of iron removal was investigated with the K₄[Fe(CN)₆] dosage of 1.2 g, the reaction time of 4.0 h, reaction temperature of 25 °C, the membrane of 50,000 Da, and stirring speed of 300 rpm. The results are shown in Fig. 5.

With the flocculant addition, the iron content in the liquid after iron removal begins to decrease, and the zirconium loss increases. At 0.1wt % flocculant dosage, the iron content reached the lowest at 3.8 mg/L and then began to increase, while the zirconium loss reached the maximum at 0.1wt % flocculant dosage and then gradually decreased. This is because, with the addition of a flocculant, zirconium sulfate and precipitated particles grow up together. When the dosage is greater than 0.1wt %, the flocculant acts as a dispersant^[61], impeding the growth of precipitated particles and reducing zirconium loss, ultimately leading to a poor iron removal effect.

3.5. Physical and chemical properties of the system upon flocculant addition

The particle size distribution and zeta potential of the precipitate in the presence of the flocculant are shown in Fig. 6.

Upon adding polyacrylamide cationic flocculant to the post-Fe-removal mixture, the zeta potential of the precipitate particles shifted from negative to approximately zero, which effectively prevented electrostatic repulsion. Consequently, the effective particle size was increased to 133.137 µm and was better for filtration.

The precipitated particles after flocculation were measured using Nano Measurer, and the particle size is shown in Fig. 7.

The particle size distribution is between 22.7-220.3μm, and the average particle size is 87.45μm. Particle size has several times or even tens of times of growth after adding flocculant. In addition, SEM images before and after adding the flocculant are shown in Fig. 8.

Before the adding flocculant, the precipitated particles were relatively dispersed, with the particle size mainly distributed within 2-15μm, and a few particles above 40μm (Fig. 8 (a)). After the addition of the flocculant, small particles gather together to form dozens or even hundreds of micron aggregates (Fig. 8 (b) (c) (d)), which is easier to filter and separate. The composition of Fig. 8(c) particles was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), the result is shown in Fig. 9.

As can be seen from the Fig. 9, iron and zirconium are evenly distributed. Zr₂(SO₄)₃ crystals precipitate with Fe₄[Fe(CN)₆]. It also proved that the decrease in iron content in Fig. 8 would increase the loss of zirconium sulfate. In addition, atomic force microscopy (AFM) of the membrane surface before and after adding the flocculant is shown in Fig. 10.

Before adding the flocculant, the filter membrane roughness was low (mean roughness (Sa) 1.31 nm), and the particles were small. The peak height and peak width of the sampled particles were about 20nm and 0.5 μm, respectively (Fig. 10 (a)). After flocculant addition, the filter membrane roughness increased considerably (mean roughness (Sa) 30.15 nm) and the number of particles decreased. The height of the particle peak reached 240 nm, and the width was approximately 2.5 μm (Fig. 10 (b)).

3.6. Mechanism of the precipitation—flocculation—ultrafiltration process

The above results can be summarized to ascertain the mechanism of deep iron removal, as shown in Fig. 11.

When a precipitation–ultrafiltration process is used, small precipitate particles with negatively charged surfaces are produced. Owing to electrostatic repulsion, the particles cannot grow, and the small particles cannot be separated via ultrafiltration, as illustrated in Fig. 11(a). Upon adding a cationic flocculant to the system, the surface charge of the particles was neutralized, and the electrostatic repulsion between them was eliminated, allowing the particles to grow and promoting their separation via ultrafiltration, as presented in Fig. 11(b).

(1) As the K₄[Fe(CN)₆] dosage and leaching time were increased, Fe removal increased and then decreased. By contrast, decreasing the membrane MWCO and increasing the temperature and stirring speed promoted Fe removal.

(2) The effective diameter of the precipitated particles was 16.581 nm, and their zeta potential was negative, indicating that negatively charged groups were present on their surfaces. The cationic flocculant eliminated the electrostatic repulsion between particles, effectively promoting particle aggregation and growth and improving Fe removal.

(3) The Fe content of the commercially available zirconium sulfate solution was decreased from 18 to 3.8 mg/L, and the obtained solution can be used for high-end titanium dioxide coatings.

Ethical Approval

There existed no human or animal studies in this study.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors' contributions

Y. Xin. conceived of the presented idea. L. Zhao and K. Song carried out the experiment. M. Hu verified the analytical methods. Y. Xin supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Funding

This work was carried out with the financial support of the National Natural Science Foundation of China (No. 51904048), the National Key Research and Development Program of China (No.2022YFC3901001-1), and the Fundamental Research Funds for the Central Universities (Grant No. 2022CDJXY-011).

Availability of data and materials

The data are available upon request to the authors. Supporting data for this study can be obtained from the corresponding author upon reasonable request.

Conflict of interest All authors declare no confict of interest. The funders had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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A composite purification method for ultra-low iron content zirconium sulfate solution

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Abstract

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1. Introduction

2. Experimental

3. Results and Discussion

3.3. Physical and chemical properties of the system

3.4 Iron removal with flocculant

3.6. Mechanism of the precipitation—flocculation—ultrafiltration process

4. Conclusions

Declarations

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