Phosphogypsum (PG) is wasting slag of phosphate fertilizer industry, which is normally disposed of by dumping on land or by pumping into rivers or seas for a long time. It is well known that there are high concentrations of calcium sulfate, heavy metals, and rare-earth elements (REE) in PG. In the final stage of last century, many researchers studied how to exploit and recover them, and more important research achievements have been reported and put into practice. S. Al-Thyabat et al [1,2,3]. reported REE could be extracted indirectly from solutions of PG by ion exchange resin using both direct and counter current methods or adsorption technology. As soil amendments, PG was applied in cultivation of canola plants , sunflowers , tomatoes, green pepper plants  and so on. Results show that PG can immobilize heavy metals, thereby reducing the heavy metal content of fruit. Building material is another important application of PG treated to reduce activity concentrations of radioactive elements , such as binder , hardened tiles . In addition to the aforementioned applications, PG was also used in the production of sulfuric acid , cement [11,12], salts , etc.
Calcium fluoride (CaF2) is a colorless or white crystalline powder. Natural calcium fluoride is known as fluorite, and is an important initial material for the fluorine chemical industry. In the near future, the preparation of calcium fluoride will be a research focus due to the reducing of grades and reserves for fluorite . Not only was calcium fluoride used in the chemical industry, but it is also widely used in metallurgy, building materials, light, optics, defense and other industries. Calcium fluoride was mainly used in different industries as a flux , an optical element , a novel fluorinating reagent and support of catalyst , an anticaries agent  and the like. In the reference , Calcium fluoride was fabricated in solution using PG, it is a great discovery, but the PG solubility is pretty small and there are many impurities in it, so it difficult to obtain pure and high concentration calcium sulfate solution for production of calcium fluoride.
Various methods containing precipitation, micro emulsion, hydrothermal, flame and so on can be used to prepare calcium fluoride. But in this work, the aim of this work is to investigate the preparation of calcium fluoride from PG and ammonium fluoride (NH4F) by direct precipitation method.
2 Material and methods
PG obtained by a Chinese fertilizer company in Sichuan, and the components are presented in Table 1. The other chemicals were purchased from Kelong Chemical Co., Ltd. (China), which are analytically pure (A.R.). All chemicals would be formulated into different concentration solutions by specific requirements to experiment.
2.2.1 Pretreatment of PG
Firstly, the block PG was broken down in tubular furnace (KTL1600) and grinded into powder. Then the 10 g PG powder was washed respectively with 200 mL distilled water, 50 mL 0.1 mol/L dilute alkali solution, 50 mL 0.1 mol/L dilute acid solution and 200 mL distilled water, respectively, to remove soluble impurities. Finally, the wet powder was dried in a drying oven (M331518) at 120°C for 120 min, and loaded into a clean reagent bottle to reserve.
2.2.2 Experiment method
According to the report by Yang , the decomposition temperature of PG was significantly lowered, when calcining with sulphur. So in this work, five grams of PG powder treated was mixed with activated carbon using an SO42-/S molar ratio of 0.1, calcined at 800°C for 120 min in the nitrogen atmosphere, and the overflow gas was absorbed by sodium hydroxide solution. The residues were dissolved with 100 mL HCl (1+9), and stirred for 30 min at 300 r/min. Then the mixture was separated by percolation, and NH4F was added in filtrate with the different Ca/F molar ratio to react at room temperature at 300 r/min. After reaction, the suspension was separated by centrifuge at 8000 r/min for 5 min, and the residue was washed with HCl (1+9) and distilled water for 3~5 times, respectively. Finally, the wet product was dried at 120°C. Meanwhile, the calcium fluoride preparation process was expressed in Figure 1.
2.2.3 Analysis method
According to the references reported by Zhang  and Nian , and the chemical analysis for CaF2 describe as follows: 0.5000 g CaF2 was weighted and moistened with ethanol in 250 mL beaker, with that 50 mL mixed acid (hydrochloric acid (38%) /boric acid (2 mol L-1) / sulfuric acid (98%) = 250/100/25(v/v/v)) was used to treat this sample. The mixed solution was heated in beaker covered with a watch glass to boil slightly for 30 min, and cooled at room temperature. After that, the watch glass and beaker were rinsed by distilled water, and the mixture solution was diluted to 100 mL and heat to boil again. After heating, the solution was filtered with a rapid filter paper in a 250 mL volumetric flask, and the hot distilled water containing a few drops of hydrochloric acid was used to rinse beaker and residue for 5 times and 10 times, respectively. When the solution cooled, the test solution was diluted with distilled water to 250 mL. After dilution, 25 mL sample solution was pipetted in a 250 mL beaker, and diluted to 100mL with distilled water, and then 5mL triethanolamine, 20 mL 200 g/L potassium hydroxide solution and the appropriate amount of mixing indicator were added respectively. The EDTA standard solution was used to titrate sample solution until the green fluorescence of the test solution disappeared (observed on the black background liner). Calculation formula as follows:
T is the titer of EDTA standard solution for calcium fluoride (g/mL); V, V1, V2 are the total volume of sample solution, sample solution volume for titration and the consumed EDTA standard solution volume, respectively (mL); m is the weight of the sample (g); A is the mass fraction of calcium carbonate in the sample which converted into the equivalent of calcium fluoride (%), A= mCaCO3×0.7808.
The phase of the obtained nano powders was determined by powder X-ray diffraction (D8 Advance, BRUKER AXS). Scans were performed between 10 ° < 2θ < 90 °. The estimated standard uncertainty of the 2θ measurement is 0.01 °. Phase and element content were determined by SEM (MERLIN Compact, Carl Zeiss AG).
2.2.4 Ethical approval
The conducted research is not related to either human or animals use.
3 Results and discussion
3.1 Preparation of nano-calcium fluoride
Orthogonal experimentation was exploited in this work, and the experiment factors of Ca/F molar ratio, reaction and drying time were presented in Table 2. The results of calcium fluoride preparation according to L9(34) orthogonal table were also listed in Table 3, and the variance analysis results were shown in Table 4. From table 3 and 4, it can be found that the order of calcium fluoride preparation effect follows the order of A > C > D (the reaction time > the reaction temperature > Ca/F molar ratio), which further confirmed that reaction time as a very significant effect factor for calcium fluoride preparation, the Ca/F molar ratio and reaction temperature had also significant effect on preparation.In conclusion, the optimal processes were A3C2D3, which the Ca/F molar ratio was 0.4, the reaction time and temperature were 70 min and 40°C, respectively.
3.2 Verification experiment
As reaction initial materials, PG powder and NH4F were used for nano-calcium fluoride synthesis by the method previously described in section 2.2.2, according to the optimized preparation condition at reaction time 70 min, reaction temperature 40°C, and Ca/F molar ratio 0.4. An amount of upper product were taken to detect the content of CaF2, the results were shown in Table 5. Under this condition, the average content of CaF2 in samples and RSD was 98.15% and 0.02%, respectively. This result showed good consistent with the orthogonal experiment. Therefore, the results of orthogonal test were stable and credible.
3.3 XRD analysis
The XRD analysis of nano-calcium fluoride was shown in Figure 2. From present figure, it can be found that the powder was indeed pure CaF2. The diffraction peaks of nano samples were highly consistent with CaF2 standard card (PDF: 35-0816), and there are no impurity peaks. This indicates the feasibility of synthesizing CaF2 nanoparticles using direct precipitation method from PG and NH4F. Calcium fluoride average particle diameter was around 70nm, which could be obtained by calculating using Scherrer formula L=kλ/βcosθ (where L is grain size, k is Scherrer constant, λ is incident X-ray wavelength, β is full width at half maximum value and θ is diffraction angle), according to the XRD data for crystal planes (111), (220), (311), respectively. The CaF2 nano-powder had a good crystalline that was 69.36% on the basis of calculating using MDI jade 5.0.
3.4 SEM analysis
The SEM images of CaF2 nano-powder was shown in Figure 3 at a higher magnification. These images indicated that particles ranging from < 20 nm to about 150 nm in size and more uniformly dispersed. The larger particles exhibited numerous spherical protuberances on the surfaces, suggesting that the smaller CaF2 granules could adhere to them, and lead to a more serious agglomeration.
Direct precipitation method and orthogonal experiment were used to prepare nano-calcium fluoride from the PG decomposition residue with activated carbon. The CaF2 content of sample reached about 98.15%. The purified product was synthesized by the reaction time 70 min, the reaction temperature 40°C, and the Ca/F molar ratio 0.4. The analysis of XRD and SEM showed that the average particle diameter was around 70nm and the crystalline of product was 69.36%. And moreover, this simple method could increase the industrial chain of PG application and apply a method to solve the fluorite resource shortage.
This project is supported financially by the program projects funded of Sichuan University of Arts and Science (Nos. 2014Z015Y, 2018SCL001Z), the opening project of Key Laboratory of Green Catalysis of Sichuan Institutes of Higher Education (No. LYJ1605, LZJ1803), the program of Science and Technology Department of Sichuan Province (No. 2018JY0061), the program of Education Department of Sichuan Province (No. 18CZ0038).
Al-Thyaba S., Zhang P., REE extraction from phosphoric acid, phosphoric acid sludge, and phosphogypsum, Mineral Processing and Extractive Metallurgy, 2015, 124, 143-150. CrossrefWeb of ScienceGoogle Scholar
Lokshin E.P., Tareeva O.A., Elizarova I.R., Ruling out accumulation of thorium in sulfuric acid solutions for leaching of phosphogypsum, Russian Journal of Applied Chemistry, 2015, 8, 719-723. Web of ScienceGoogle Scholar
Lokshin E.P., Tareeva O.A., Elizarova I.R., Sorption of rare-earth elements from phosphogypsum sulfuric acid leaching solutions, Theoretical Foundations of Chemical Engineering, 2015, 49, 773-778. CrossrefWeb of ScienceGoogle Scholar
Mahmoud E., El-Kader N.A., Heavy Metal Immobilization in Contaminated Soils using Phosphogypsum and Rice Straw Compost, Land Degradation & Development, 2015, 26, 819-824. Web of ScienceCrossrefGoogle Scholar
Elloumi N., Zouari M., Chaari L., et al., Effect of phosphogypsum on growth, physiology, and the antioxidative defense system in sunflower seedlings, Environ Sci Pollut Res Int., 2015, 22, 14829-14840. PubMedWeb of ScienceCrossrefGoogle Scholar
Al-Hwaiti M., Al-Khashman O., Health risk assessment of heavy metals contamination in tomato and green pepper plants grown in soils amended with phosphogypsum waste materials, Environmental Geochemistry and Health, 2015, 37, 287-304. CrossrefWeb of SciencePubMedGoogle Scholar
Al-Hwaiti M.S., Assessment of the radiological impacts of treated phosphogypsum used as the main constituent of building materials in Jordan, Environmental Earth Sciences, 2015, 74, 3159-3169. CrossrefWeb of ScienceGoogle Scholar
Zhou J., Sheng Z., Li T., et al., Preparation of hardened tiles from waste phosphogypsum by a new intermittent pressing hydration, Ceramics International, 2016, 42, 7237-7245. CrossrefWeb of ScienceGoogle Scholar
Wang Y., Liu M., Sun H., et al., Preparation of sulfuric acid from phosphogypsum by ammonium-transferred method: Technical principle and process evaluation, Chemical Industry And Engineering Progress, 2015, 34, 196-201. Google Scholar
Yoon S., Mun K., Hyung W., Physical Properties of Activated Slag Concrete Using Phosphogypsum and Waste Lime as an Activator, Journal of Asian Architecture and Building Engineering, 2015, 14, 189-195. CrossrefWeb of ScienceGoogle Scholar
Zhao H., Lia H., Bao W., et al., Experimental study of enhanced phosphogypsum carbonation with ammonia under increased CO2 pressure, Journal of CO2 Utilization, 2015, 11, 10-19. CrossrefWeb of ScienceGoogle Scholar
Zhou L., Tang T., Qian Y., et al., Research Progress in Preparation of Calcium Fluoride, Contemporary Chemical Industry, 2015, 44, 2254-2256. Google Scholar
Li Y., Xv X., Wang B., et al., Research on the fluxed effect of LiF and B2O3 Journal of University of Science and Technology Beijing, 2002, 24, 429-431. Google Scholar
Su L., Xun J., Yang W., et al., Growth and application of CaF2 single crystal, Journal of the Chinese Ceramic Society, 2003, 11, 1203-1206. Google Scholar
Quan H., Tamura M., Gao R., et al., Preparation and application of porous calcium fluoride-a novel fluorinating reagent and support of catalyst, Journal of Fluorine Chemistry, 2002, 116, 65-69. CrossrefGoogle Scholar
Koeser J., Carvalho T.S., Pieles U., et al., Preparation and optimization of calcium fluoride particles for dental applications, Journal of Materials Science: Materials in Medicine, 2014, 25, 1671-1677. Web of SciencePubMedGoogle Scholar
Douahem H., Hammi H., Hamzaoui A. H., et al., Modeling and optimization of phosphogypsum transformation into calcium fluoride using experimental design methodology, Journal of the Tunisian Chemical Society, 2016, 18, 106-113. Google Scholar
Yang J., Liu J., Wang X., et al., Study on preparation of calcium sulfide by using sulphur to reduce phosphogypsum, Inorganic Chemicals Industry, 2015, 3, 45-48. Google Scholar
Zhang X., Zhang Q., Liu H., The determination of calcium fluoride in fluorit, Geology of Chemical Minerals, 2017, 1, 58-60. Google Scholar
Nian J., Zhang L., Zhu C., et al., Determination of calcium oxide and calcium fluoride in steel slag powder deoxidizer by EDTA complexometric titration, Metallurgical Analysis, 2014, 6, 20-27. Google Scholar
About the article
Published Online: 2018-09-21
Conflict of interest: Authors state no conflict of interest.
Citation Information: Open Chemistry, Volume 16, Issue 1, Pages 864–868, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2018-0093.
© 2018 Lvshan Zhou et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0