Optimization of zirconia nanopowder precipitation process using Taguchi experimental design methodology

Abstract

The purpose of this research is to investigate the production of ZrO2 nano-powder from the product of the alkaline fusion process of Zircon mineral. The effect of surfactant type, solution pH, and calcination temperature were investigating using L9 orthogonal array in the Taguchi design of experiment method. The experiments related to the Preparation of Zirconia nano-powder were first performed on a synthetic solution prepared by dissolving Zirconium tetrachloride in deionized water. The final tests were performed on the solution obtained from the acidic leaching step in the alkali decomposition of the Zircon mineral. According to the results, the optimal conditions for ZrO2 nano-powder production from synthetic solution involved pH = 7, PVP as a surfactant, and 300 °C calcination temperature, resulting in 10.5 nm ZrO2. Applying these optimal conditions on the real Zirconium solution from the alkaline fusion process of Zircon resulted in the Preparation of 26.6 nm ZrO2 nano-powders

Introduction

With a concentration of around 130 mg/kg in the Earth’s crust and 0.026 mg/L in seawater, zirconium is the eighth most prevalent element in the titanium group and the eighteenth most abundant element in the crust¹. Zircon is the main commercial source of zirconium. Zircon is an orthosilicate mineral in yellow and brown colors, with a hardness of 7.5 and a density of about 4.7 g per cubic centimeter, with diamond and glass polish. In the industry, baddeleyite and zircon are utilized worldwide. Zircon is a mineral found in silicate-rich volcanic, sedimentary, and metamorphic rocks, particularly in granite and pegmatite. Zircon is very useful in high-temperature applications because of its refractory nature, hardness, and resistance to chemical assault. Examples of these uses include ceramic materials and molds for molten metal^2,3. Zirconia (ZrO₂) is a white oxide of zirconium metal. The substance is present in minerals like baddeleyite and zircon. Baddeleyite is a mineral that contains nearly 80% zirconia and doesn’t require pre-processing, making it immediately usable. Zirconium oxide, or zirconia, has three crystal phases that form at temperatures above 1170 ºC in the monoclinic phase, at 1170–2370 ºC in the tetragonal phase, and at 2370–2680 ºC in the cubic phase. The properties of this oxide can be mentioned as low thermal conductivity, high thermal strength, high resistance to thermal shock, high hardness, and rigidity, which makes it useful in the coating of thermal barriers, cutting tools, refractory materials, and catalysts. Zirconium oxide has a wide range of applications.

Zirconia is used in several industries, including fuel cells, refractory materials, optics, electronics, mechanics, and ceramics⁴. Montoya and his colleagues synthesize gadolinium-doped zirconia (Gd-ZrO₂) nanoparticles using a sol-gel method to investigate their structural stability and electrochemical properties⁵. Their research contributes to the development of advanced materials for energy conversion technologies. The findings demonstrate that Gd doping can significantly improve the performance of zirconia-based materials, making them promising candidates for use in next-generation electrochemical devices. Doménech and his colleagues investigate the electrochemical behavior of praseodymium-doped zirconia () materials, focusing on the role of praseodymium (Pr) centers in electrocatalytic and photoelectrocatalytic processes⁶. They found out that the incorporation of praseodymium into the zirconia lattice introduced redox-active centers, which enhanced the material’s electrochemical properties. Also, Praseodymium centers exhibited reversible redox behavior, which was crucial for electrocatalytic applications. In addition, the materials showed significant photoelectrocatalytic activity, indicating their potential for use in photocatalytic processes such as water splitting or pollutant degradation. Consequently, this research highlights the potential of Pr-doped zirconia materials for use in energy conversion and environmental technologies. The findings provide insights into the design of advanced materials with tailored electrochemical and photocatalytic properties. Herrera and his colleagues synthesize iron-zirconia (Fe-ZrO₂) solid solution nanoparticles using a sol-gel technique to investigate their structural, morphological, and electrochemical properties⁷. They discovered that the incorporation of iron into the zirconia lattice improved the material’s electrochemical performance. Additionally, they figured out that the nanoparticles showed potential for use in advanced electrochemical applications due to their stability, conductivity, and catalytic properties. Their research contributes to the development of advanced nanomaterials for electrochemical applications, particularly in energy storage and conversion systems. The sol-gel synthesis approach offers a scalable and efficient route for producing functional materials with tailored properties.

When zirconia is reduced to a size below 100 nm, its specific surface area and crystallinity increase. This enhancement improves its properties and widens its range of applications^8,9.

One kind of inorganic nanomaterial that is regarded as a very promising functional ceramic is nano-zirconia. It has outstanding chemical inertness, thermal stability, hardness, dielectric constant, and refractive index. Nano-zirconia ceramics further enhance the characteristics of zirconia by utilizing nano-powder, which has a uniform and dispersive distribution and small particle size¹⁰. Even though ZrO₂ nano-powder has the same applications as zirconium oxide, such as making fuel cells in military and automotive equipment, the construction industries, and the modern painting industries, it is more efficient than zirconium oxide because of its size. It is critical to recognize the significance of research and production of this substance.

There are various methods for producing nano zirconia, such as the Microwave-assisted method, hydrothermal method, Sol-gel, and precipitation¹¹. The choice of method depends on the intended applications of the final product. The selection of the synthesis method for nano-powder depends on the specific requirements, as each method has advantages and disadvantages. El Agamy and his colleagues prepared and synthesized zirconia nano-powder using the hydrothermal technique by adding ammonium hydroxide to a zirconium oxychloride solution and heating the mixture at 473 K for 7 h. The resulting nano-powder formed at pH 2.4 was filtered, rinsed, dried at 393 K for 24 h, and calcined at 773 K for 2 h. Analysis revealed that the crystallite size ranged from 3 to 15 nm. The nano-powder consisted of monoclinic crystals and tended to agglomerate due to their nanocrystal size and particle proximity³. Behbahani and his colleagues conducted an investigation on ZrO₂nano-powder by subjecting commercial zirconia powder to an alkali treatment through a hydrothermal process, maintained at a temperature of 150 °C for duration of 85 h. The X-ray diffraction analysis indicated that the commercial zirconia was entirely in a monoclinic crystalline phase. However, after the hydrothermal treatment, the molar phase fractions of the product changed to 72% cubic and tetragonal phases and 28% monoclinic phase. The XRD, TEM, and SEM analyses results indicate that the product has a particle size of approximately 15–30 nm¹². Ramachandran and his colleagues synthesized ZrO₂ nano-powder using the co-precipitation method with three different KOH compositions. The resulting products were filtered, washed, and dried in a vacuum oven for 4 h and then milled for 30 min. The sample underwent heating at 900 °C in a muffle furnace for 4 h, leading to the formation of ZrO₂nano-powder. XRD analysis confirmed the monoclinic structure and various analyses confirmed the presence of Zr and O species¹³. Heshmatpour and her colleague used the sol-gel method to synthesize zirconia nano-powder with the help of glucose and fructose as organic additives. The process commenced with Zirconium n-propoxide at a concentration of 70 wt%, which was subsequently diluted to 30 wt% using n-propanol. Pursuing this, the solution underwent hydrolysis through the gradual addition of ammonia and distilled water until the pH level was adjusted to between 9 and 10. The hydrolysis was conducted with a volume ratio of zirconium n-propoxide to water set at 1:4. Additionally, a glucose-fructose mixture in a 1:1 mass ratio was incorporated into the solution while being stirred vigorously, ensuring a mass ratio of 1:15 with respect to water. Upon completion of the polymerization, the gel was dried at a temperature of 110 °C for a duration of 12 h, after which it was calcined at temperatures ranging from 300 to 700 °C. Analysis using various techniques showed that the nano-powder was uniform in size, with both tetragonal and monoclinic phases¹⁴.

Manjunatha and his associate synthesized mesoporous cubic ZrO₂ nano-powder by dissolving 2.3123 g of ZrO(NO₃) _2.H2O and 0.8083 g of L-Serine amino acid in a limited volume of de-ionized water within a 250 mL borosilicate glass beaker. After evaporating the excess water, they obtained a highly viscous transparent gel. They exposed the gel to microwave irradiation for 60 s at 800 W power output. The initial solution boiled vigorously underwent dehydration, and ignited, releasing large amounts of gases, including CO₂, N₂, and water vapor. The process resulted in the production of a highly porous and voluminous white ZrO₂powder that swells extensively. XRD analysis revealed the formation of a cubic phase in the nano-powder synthesized using microwave combustion. The particle size was in the range of 60–65 nm¹⁵. Lim and his colleagues synthesized ZrO₂nano-powder by using the sol-gel technique. The XRD results confirmed the presence of the tetragonal phase. Analyses showed that the product had a particle size of 30–60 nm at 500 ºC for 1 h¹⁶. Qiu and his research team suggested employing a co-precipitation method to produce highly dispersed nano-powders of MgO-Y₂O₃ stabilized ZrO₂. The optimal synthesis parameters were determined to be six hours of high-energy planetary milling followed by calcination at 800 °C in an electric furnace. Under these optimal conditions, the average particle size of the resulting powder was measured at 28.7 nanometers¹⁰. Huang and his colleagues used co-precipitation methods to prepare yttrium-stabilized zirconia (YSZ). A range of precipitants, including oxalic acid, ammonium bicarbonate, ammonium hydroxide, and urea were employed to examine their influence on the co-precipitation synthesis of YSZ nanocrystalline powders. The results indicated that urea was the most effective precipitant, yielding nanocrystalline of YSZ powders with an average primary particle size of approximately 35 ± 6 nm when calcined at 500 ºC, with a pH of 2.93 and a processing time of 2 h¹⁷.

The literature review revealed that the precipitation method is commonly used to produce nano-zirconia due to its simplicity and popularity. The precipitation method is an easy method for the synthesis of nanocrystals. This method enables the production of powder with varying diameters. The likelihood of acquiring the product in grams to kilograms and more is an inordinate advantage of this method. Also, this method enables the usage of cheaper and easily accessible equipment and nano-powder precursors¹⁸. On the contrary, the sol-gel and the hydrothermal methods require an apparatus with a high level of complexity and thereby involve a high-cost process¹⁹.

Transition metal nano-powder is fascinating due to its catalytic, electronic, and optical properties. These ultrafine particles tend to clump together to reduce surface or interfacial energy, forming larger particles. It’s prominent to prevent particles from clumping during synthesis and use. Different stabilizers coat nano-powders, which affect their stability and catalytic activity. Surfactants play a crucial role in enhancing the properties of synthesized powders. This is accomplished by creating electrostatic repulsion and steric hindrance and forming or reversing micelles. This process can be explained through the concept of nucleation and growth. When surfactants stick to the nucleus’s surface, they decrease the tension between solid and liquid interfaces, reducing critical radius. This results in stable nuclei and prevents clumping. The presence of surfactants also enhances the crystallinity of nanostructures and allows for controlling particle growth to achieve the desired size and shape. Ultimately, this enables the synthesis of less aggregated particles with the desired characteristics^20,21.

This study explores the production of zirconia nano-powder using the surfactant-assisted precipitation method. The research investigates the impact of feed solution pH, surfactant type, and calcination temperature on the production of zirconia nano-powder from the synthetic solution using the Taguchi test design method. The study also includes the growth of mathematical models to analyze the influence of different parameters on the nano-powder size. Following the determination of the optimal precipitation process conditions, the production of zirconia nano-powder from the acidic leaching solution of the alkaline zircon fusion process is examined. The results obtained from the developed models are then compared with the laboratory results.

Experimental section

Materials & equipment

Polyethylene glycol-4000 from Fluka and laboratory-grade polyvinyl pyrrolidone-10,000 from Aldrich were employed in this study. Anhydrous zirconium tetrachloride (ZrCl₄) from China was used to prepare the synthetic solution. The ICP analysis result of the ZrCl₄ solution is shown in Table 1.

Table 1.

The ICP result of ZrCl₄.

Elements	Si	Al	Ti	Hf	Fe	Mo	Zr
ppm	1900	650		2250	9600		-
Wt. percent	-	-	-	-	-	-	29

The research used zircon concentrate in 325 mesh (44 microns) containing 53.06% zirconia and 40.59% silica. XRF analysis results are presented in Table 2. The acidic leaching solution for the alkaline zircon fusion process was prepared according to the methods outlined in the reference articles^22–24.

Table 2.

Results of XRF analysis of Zircon sample.

Composition	ZrO2	SiO2	Al2O3	TiO2	HfO2	CaO
Wt. percent	53.06	40.59	5.34	0.10	0.74	0.17

SEM device (Zeiss Ultra 55, Gemini) was used to analyze the surface morphology of zirconia nano-powder. The size and phase composition were analyzed using a Kristalloflex Diffractometer XRD device, model D5000 Siemens. Scherer’s formula was then used to calculate the average crystal dimensions. Elemental analysis was carried out using the ICP-OES device (Perkin-Elmer 2000 DV model).

Experimental procedure

The experiment begins with the preparation of a 100 mL solution of zirconium tetrachloride (ZrCl₄), sourced from China. The results of the ICP analysis for the ZrCl₄ solution are presented in Table 1. Next, a specific surfactant is added to the solution for each test. It is important to note that two types of surfactants in the present study were used: polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP). While using a magnetic stirrer to agitate the solution, we adjusted the pH by adding sodium hydroxide. The zirconium chloride solution, along with the surfactants, was stirred continuously, and its pH was simultaneously adjusted by adding sodium hydroxide. If we had used the reverse method for precipitation—where the zirconium chloride solution containing the surfactant is added dropwise to sodium hydroxide—the high pH of the environment would have facilitated faster particle formation, resulting in larger crystals. Therefore, the direct precipitation method was employed in this research. The surfactants selected in this research were non-ionic. The reason is that the surface of non-ionic surfactants is uncharged, stabilized, and is spread better by spatial force. After the precipitation, a centrifuge separated the solid from the liquid. The separated solid was washed with 300–400 mL of distilled water to remove its sodium hydroxide. Then, after drying, the solid was placed in the furnace to be calcined. Finally, the size of the nano-powder was determined by conducting an X-ray diffraction analysis on the obtained powder. After determining the optimal conditions for the precipitation process, the production of zirconia nano-powder from the acidic leaching solution of the zircon alkaline fusion process is also being investigated.

Design of experiments

The Taguchi method was used for designing partial factorial experiments. It divides process factors or variables into two groups: controllable and uncontrollable factors. Control factors are easily controllable and help in selecting the best design conditions. Uncontrollable variables, on the other hand, are factors that cause changes but are considered constant or fixed due to their difficulty with controlling or lack of knowledge. Compared to other experimental design methods, such as the Classical or Modern method and the Shainin Technique, the Taguchi method has certain advantages. These benefits include running fewer experiments, which reduces costs and saves time, the ability to test interactions and run experiments simultaneously, and the capability to predict the best outcome^25,26. In this research, the Taguchi method was utilized to analyze the production process of zirconia nano-powder. This study focused on investigating the impact of three parameters: surfactant type, pH, and calcination temperature on the particle size. Table 3. provides details of the parameters and their respective levels that affect the particle size.

Table 3.

Selection of parameters affecting the Preparation of nano-zirconia.

symbol	parameter	level
		1	2	3
A	The type of surfactant	PEG	PVP	Without surfactant
B	pH	7	10	13
C	The calcination temperature ()	300	450	600

To identify the optimal conditions for preparing zirconia nano-powder, Taguchi recommended using the L9 orthogonal array. This array involves three parameters, each set at three different levels. Table 4 presents the results of the nine tests and the recorded data. To determine the average size of nano-powder using X-ray diffraction spectrum diagram data, the Scherrer Equation was utilized in our research (Eq. (1))²⁷.

1

Table 4.

Experiments designed by the Taguchi method in the nano-zirconia precipitation stage.

Run	Symbol	A: The type of surfactant	B: pH	C: The calcination temperature (°C)	Nano-powder size (nm)	S/N ratio
1	A1B1C1	PVP	7	300	10.5	−20.4
2	A2B1C2	PEG	7	450	19.2	−25.7
3	A1B3C3	PVP	13	600	23.7	−27.5
4	A2B2C3	PEG	10	600	37.8	−31.5
5	A3B2C1	WITHOUT	10	300	17.4	−24.8
6	A1B2C2	PVP	10	450	13.8	−22.8
7	A3B1C3	WITHOUT	7	600	54.2	−34.7
8	A2B3C1	PEG	13	300	14.2	−23.0
9	A3B3C2	WITHOUT	13	450	24.8	−27.9

In this equation, τ represents the size of the crystallite, λ denotes the wavelength of diffraction, β indicates the adjusted full width at half maximum (FWHM), θ refers to the angle of diffraction, and K is a constant that is approximately equal to one. The experimental runs presented in Table 4 aimed to determine the effect of essential factors such as calcination temperature, pH, and surfactant type on nano-powder size produced from the precipitation method. As a result, a statistical model was derived to predict the optimal conditions for producing nano-powder from zirconium tetrachloride solution.

To effectively assess how much test results deviate from the desired values, it’s essential to calculate the loss function. The Taguchi method uses a statistical measure called the signal-to-noise (S/N) ratio to evaluate the significance of process parameters. This ratio considers the average and the variability by evaluating the mean (signal) against the standard deviation (noise). The ratio is influenced by the quality attributes of the product or process that requires optimization. Commonly utilized signal-to-noise ratios include nominal-is-best (NB), lower-the-better (LB), and higher-the-better (HB). The S/N is determined using one of the Eqs. (2), (3), or (4), depending on the specific conditions of the problem²⁸.

- Taguchi’s S/N Ratio for (LB) Lower-the-better: 

2

- Taguchi’s S/N Ratio for (HB) Higher-the-better:

3

- Taguchi’s S/N Ratio for (NB) Nominal-the-best:

4

In Eqs. (2), (3), and (4), n is the number of repetitions, is the measured output, and is the desired nominal size.

Result and discussion

XRD results

The X-ray diffraction (XRD) technique is a flexible method widely employed for accurately determining the crystal structure, composition, and crystalline grain size of nanoparticles. The XRD diagrams related to nano-zirconia samples prepared in this study are shown in Fig. 1. The XRD results were evaluated using high-score software, and the peak related to zirconium oxide was identified in the samples (Ref. Code 01–079–1768). The average size of zirconia nano-powder is also determined using Eq. (1) and presented in Table 4. As shown in Fig. 1, by increasing the calcination temperature to 600 °C, the peak related to zirconium oxide appears sharply in samples 2, 4, and 7.

Fig. 1.

XRD patterns of the samples prepared in Taguchi-designed experiments.

It appears that the calcination temperature has a significant impact on the size of nano-zirconia, independent of the application of surfactant throughout the nano-powder production process.

ANOVA results

Finding the statistically important process parameters is made easier with the use of an analysis of variance (ANOVA). The ANOVA for the response model is presented in Table 5. The Model F-value of 6142.11 implies the model is significant. There is only a 0.01% chance that an F-value this large could occur due to noise. P-values below 0.05 suggest that the model terms are noticeable. In this case, A and C are significant model terms. P-values greater than 0.1 indicate the model terms are not significant. If there are many insignificant model terms (not counting those required to support hierarchy), model reduction may improve your model. In this case, the model terms with p-values greater than 0.05 were eliminated. Table 6 summarizes the regression coefficients and their significance in the regression models for the reduced model.

Table 5.

Analysis of variance for the model.

	Term	df	Sum of Squares	Mean Square	F-value	Prob > F
Require	Intercept
Model	A-The type of surfactant	2	7.20E-03	3.60E-03	6142.11	0.0002
Error	B-pH	2	3.31E-08	1.66E-08	0.0284	0.9724
Model	C-The calcination temperature (°C)	2	1.63E-02	8.20E-03	14026.70	< 0.0001
Aliased	AB		Aliased
Aliased	AC		Aliased
Aliased	BC		Aliased
Aliased	ABC		Aliased
Error	Lack of Fit	2	1.16E-06	5.824E-07
Error	Pure Error	0	0.0000

Table 6.

Analysis of variance for the selected reduced model.

Source	Sum of Squares	df	Mean Square	F-value	p-value
Model	2.35E-02	4	5.90E-03	19611.46	< 0.0001	significant
A-The type of surfactant	7.20E-03	2	3.60E-03	11944.76	< 0.0001
C-The calcination temperature (°C)	1.63E-02	2	8.20E-03	27278.17	< 0.0001
Residual	1.20E-06	4	3.00E-07
Cor Total	2.35E-02	8
R²	0.9999
Adjusted R²	0.9999
Predicted R²	0.9997

According to the results obtained from ANOVA, the regression model had a high coefficient of determination (R²), Adjusted R², and Predicted R². The results also show that the effective parameters for the preparation of zirconia nano-powder include calcination temperature, surfactant type, and pH respectively. The Box-Cox transformation technique was employed to transform the target variable so that it resembles a normal distribution. According to the Box-Cox plot presented in Fig. 2, the recommended transform for zirconia nano-powder size response is Inverse Sqrt.

Fig. 2.

Box-Cox plot for zirconia nano-powder size response power transforms.

The following semi-experimental expression mentioned in Eq. (5), is obtained using ANOVA analysis to model the zirconia nano-powder size.

5

The equation expressed in terms of coded factors serves as a tool for prediction of the response based on specified levels of each factor. Typically, the high levels of the factors are represented by + 1, while the low levels are denoted by −1. This coded equation is beneficial for assessing the relative influence of the factors through a comparison of their coefficients. Figure 3 presents the validity of the obtained models for zirconia nano-powder size with the real values provided in Table 4. According to this figure, the predicted values for zirconia nano-powder size were in good agreement with the experimental data.

Fig. 3.

Predicted versus actual value for zirconia nano-powder size.

The model’s accuracy was evaluated by analyzing usual probability plots and residual versus run plots. This process involved calculating the residual values, which show the differences between the actual and predicted results. The normal probability and residual versus run plots are shown in Fig. 4(a) and 4(b), respectively. The linearity observed in the typical probability plot for the residuals suggests that the proposed model is accurate. Additionally, the random distribution of residual values about run numbers further supports the model’s accuracy.

Fig. 4.

Residual plots: (a) normal plot of residuals (b) residuals vs. run number.

The effect of investigated parameters on the zirconia nano-powder size

The effect of pH

Changes in the pH level of the environment play a significant role in determining the dispersion of particles in a solution. In the absence of surfactant, the deposition process at higher pH usually increases the size of nanoparticles. In this experiment, based on Fig. 5, the presence of surfactant rendered the effect of pH on the size of nanoparticles ineffective, leading to the selection of pH 7.

Fig. 5.

The effect of pH on the average size of zirconia nano-powder (PVP surfactant).

The effect of surfactant type

Surfactants have two main functions. Firstly, they impede particle growth. Secondly, they prevent particles from aggregating and accumulating in liquid environments by dispersing them. In this study, surfactants like polyethylene glycol-4000 and polyvinyl pyrrolidone-40 were utilized. As shown in Fig. 6, applying surfactant leads to a decrease in the size of zirconia nano-powder. Additionally, the application of PEG surfactant results in larger nanoparticle sizes when compared to those produced with PVP surfactant.

Fig. 6.

The effect of surfactant type on the average size of zirconia nano-powder (pH = 7).

The effect of calcination temperature

Many compounds synthesized via precipitation methods, especially at ambient temperatures, tend to form amorphous structures. This is because the kinetic energy at lower temperatures is insufficient to drive the formation of well-defined crystalline structures. As a result, the particles lack long-range order and are often highly disordered. Calcination is a critical step in transforming these amorphous structures into crystalline phases. During calcination, the material is heated to high temperatures, which provides the necessary energy for atomic rearrangement and crystallization. This process not only improves the crystallinity but also affects the particle size and morphology. In our study, calcination was essential for obtaining zirconia (ZrO₂) with a well-defined crystal structure, as confirmed by XRD analysis.

As shown in Fig. 7, the size of zirconia nanoparticles increased with rising calcination temperatures. This phenomenon can be attributed to Ostwald ripening and sintering. According to Ostwald ripening at higher temperatures, smaller particles tend to dissolve and re-deposit onto larger particles, leading to an overall increase in particle size. In our experiments, the particle size increased significantly when the calcination temperature was raised from 300 °C to 600 °C. For example, in the absence of surfactant, the particle size increased from approximately 17.4 nm at 300 °C to 54.2 nm at 600 °C. This demonstrates that higher temperatures promote particle growth and agglomeration.

Fig. 7.

The effect of calcination temperature on the average size of zirconia nano-powder (pH = 7).

Surfactants play a dual role in nanoparticle synthesis. First, Surfactants adsorb onto the surface of nanoparticles, creating a steric or electrostatic barrier that prevents further growth by limiting the availability of reactive sites. Second, Surfactants stabilize nanoparticles in solution, preventing them from clumping together due to van der Waals forces. PVP was found to be more effective than PEG in reducing particle size. This is likely due to its stronger adsorption onto the particle surface, which provides better stabilization and prevents agglomeration. PVP also has a higher molecular weight, which enhances its steric hindrance effect. While PEG also reduced particle size compared to the no-surfactant case, it was less effective than PVP. This may be due to its lower molecular weight and weaker adsorption onto the particle surface. The effectiveness of surfactants in controlling particle size diminished at higher calcination temperatures. For example, at 300 °C, PVP reduced the particle size to 10.5 nm, but at 600 °C, even with PVP, the particle size increased to 23.7 nm. This suggests that while surfactants can mitigate particle growth, they cannot completely overcome the effects of high-temperature sintering and Ostwald ripening. The 3D surface plot for the effects of calcination temperature and surfactant type on the average size of zirconia nano-powder at pH = 7 is illustrated in Fig. 8.

Fig. 8.

3D surface plot for the effects of calcination temperature and surfactant type on the average size of zirconia nano-powder (pH = 7).

Model optimization and experimental validation

Based on the conducted experiments and the ANOVA results, the optimal conditions for preparing zirconia nano-powder were evaluated by the synthesized solution of zirconium tetrachloride. The process was optimized to obtain operational parameter levels that minimized the size of the zirconia nano-powder. The best conditions include using a PVP surfactant, maintaining a pH of 7, and calcination temperature at 300 °C. The combination of the selected optimal levels (A1B1C1) represents the first experimental run proposed by the Taguchi method. A comparison of the predicted zirconia nano-powder size with the experimental results using the optimal conditions is presented in Table 7. The result shows a good agreement between experimental and predicted values found in the 95% Confidence level.

Table 7.

Experimental and predicted values of zirconia nano-powder size (nm) in the optimal conditions (PVP surfactant, pH = 7, and calcination temperature = 300 °C).

Predicted mean	Std Dev	95% PI low	Data mean (nm)	95% PI high
10.55	0.0375	10.42	10.5	10.68

Zirconia nano-powder Preparation from acidic leaching solution

The objective of this study is to produce nano zirconia from the zircon’s alkali fusion product. Therefore, following the satisfactory results of the experiment on the zirconium tetrachloride synthetic solution, the final test was conducted on the solution obtained from the acidic leaching step in the alkali decomposition of the zircon mineral.

Our previous research²⁴ indicates that the acidic leaching step in the alkali decomposition process of zircon requires sulfuric acid with a concentration of 6 M, heating at 60 °C under reflux, and a liquid-to-solid ratio of 20. For zirconia nano-powder preparation two samples of the acidic solution, similar to the synthesized solution, were prepared. Polyvinylpyrrolidone was added to the first solution with a mass ratio of 0.82. The solution was then stirred, and the pH was adjusted to 7 using sodium hydroxide. Next, the obtained precipitate was washed thoroughly to remove any traces of sodium hydroxide, then dried and calcined in an oven at 300 °C for 2 h. The same process was performed on the second solution without adding surfactant. Finally, the resulting solid powders were analyzed using X-ray diffraction analysis to determine the size and structure of their crystals.

The results of the XRD analysis related to nano-zirconia powders prepared without and with surfactant are presented in Fig. 9a and b, respectively.

Fig. 9.

XRD pattern of the sample prepared from the product of the zircon alkali fusion process (a) without surfactant (b) with PVP surfactant.

The XRD analysis identified the peaks related to zirconium oxide in both samples (Ref. Code 01–079–1768). The average size of the prepared zirconia nano-powders is also determined using the Scherrer Equation and presented in Table 8. According to the results obtained, precipitation in the absence of surfactant leads to the preparation of ZrO₂ crystals with an average size of 45.5 nm. However, using surfactant significantly reduced the size of the crystal to 26.6 nm.

Table 8.

The average size of the prepared Zirconia nano-powders from the product of the Zircon alkali fusion process.

Surfactant type	zirconia nano-powder size (nm)
Without surfactant	45.5
PVP surfactant	26.6

Figure 10a and b present the SEM images of the zirconia nano-powder prepared from the product of the zircon alkali fusion process without and with surfactant, respectively.

Fig. 10.

SEM image of zirconia nano-powder prepared from the product of the zircon alkali fusion process (a) without surfactant (b) with PVP surfactant.

According to the SEM images a lower accumulation of particles is observed due to the use of surfactant (Fig. 10a) and the average zirconia particle size was measured at 29.6 nm. On the other hand, (Fig. 10b) displays larger zirconia particles with an average size of 49 nm. The average zirconia nano-powder size obtained from SEM images is in good agreement with the results calculated using the Scherrer Equation. The results of our investigations showed that the application of the precipitation method with the help of PVP surfactant leads to the production of zirconia nanoparticles with an approximate uniformity and almost spherical morphology. The process flowchart for zirconia nano-powder preparation from alkaline fusion of the zircon mineral is presented in Fig. 11.

Fig. 11.

Process flowchart for zirconia nano-powder preparation from alkaline fusion of the zircon mineral.

The comparison of different methods of zirconia nano-powder preparation and the corresponding average particle size presented in Table 9 shows that the method investigated in this study provides a promising method for the preparation of zirconia nano-powder.

Table 9.

Comparison of different methods for zirconia nano-powder Preparation.

Preparation method	pH	The calcination temperature (C)	Duration time (h)	Average particle size (nm)	Reference
Co-precipitation	> 10	900	4	193	13
Sol-gel	-	500	1	30–60	16
Co-precipitation	2.93	500	2	356	17
Hydrothermal	-	150	85	15–30	12
Sol-gel	9–10	700	3	10–30	14
Co-precipitation	-	800	6	28.7	10
Precipitation	7	300	2	26.6	Present study

Conclusion

Zircon mineral can be converted to zirconium dioxide using the alkaline fusion process, which is a highly effective technique and cheaper than the carbochlorination process of zircon. This research explored the production of zirconia nano-powder from zircon mineral’s alkaline fusion process, which involved three stages: fusion with an alkali, water, and acidic leaching. The production process of zirconia nano-powder was investigated using the Taguchi design of the experiment method. For this purpose, the L9 orthogonal array was employed to explore the impacts of three factors: the type of surfactant (PVP, PEG, without Surfactant), pH (7, 10, 13), and calcination temperature (300, 450, 600 °C). The investigations showed that a pH level of 7, employing PVP as a surfactant, and a calcination temperature of 300 °C are the optimal conditions for ZrO₂ nano-powder production.

The ZrO₂ nano-powder production experiment was performed on the synthetic and genuine zirconium solutions. The findings showed that particles measuring 10.5 nm were obtained from the synthetic solution, while particles measuring 26.6 nm were obtained from the genuine solution under the optimum conditions. The research found that increasing the calcination temperature led to an increase in particle size. It was also discovered that using a larger amount of non-ionic surfactant resulted in smaller particles. Non-ionic surfactants are uncharged substances that can be effectively stabilized and dispersed through steric force, making them a suitable choice for this study. Regarding the effect of pH, it should be noted that there was almost no change in the size of nanoparticles with the pH variation in the range of 7 to 13.

Author contributions

Y.J. wrote the main manuscript and prepared all figures and tables. Y.J. and A.Y conducted programming. All of the authors were involved in data analytics and investigation. K. S. contributed to study planning, project management, and manuscript revision. All authors reviewed the manuscript.

Funding

The study is supported by the Nuclear Science and Technology Research Institute, Tehran, Iran.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Consent for publication

The authors provide consent for publication in this journal.