Synthesis and characterization of calcium carbonate whisker from yellow phosphorus slag

In this study, a procedure for producing calcium carbonate whisker through yellow
 phosphorus slag carbonation without adding any crystal control agents was proposed.
 The influence of process parameters on the crystal phase and morphology of the
 product was discussed. The content of aragonite in the product was more than 90%
 under optimal conditions. The whiteness of the product was 97.6%. The diameter of a
 single particle was about 1.5–3 μm, and the length of a single
 particle was about 8–40 μm. Various polymorphs and morphologies
 of CaCO3 could be formed by adjusting the production conditions. The
 by-products produced during the whole preparation process could also be reused. The
 whole preparation process of fibrous aragonite from yellow phosphorus slag without
 using any chemical additives was also proposed. These indicated that the production
 strategy had a good application prospect.


Introduction
Because of its unique physical and chemical properties, precipitated calcium carbonate is widely used in chemical [1][2][3], food [4], and pharmaceutical industries [5] and in other fields [6,7]. The properties of materials are determined by their structure, and the crystal type and the morphology are the important factors of materials' structure. For example, carbon materials with different polymorphs and morphologies may show different properties [8]. Hence, the structure of the product that plays a crucial role in the preparation process was investigated. Calcium carbonate has three crystal types. They are spherical vaterite, fibrous aragonite, and rhombohedral calcite. Aragonite has a fibrous morphology, and it can be used in the seal and friction material field [9][10][11][12]. The market price of calcite and vaterite is much higher than that of aragonite. Therefore, a lot of research studies have been carried out by scholars from China and other countries to produce calcium carbonate whisker [13,14].
Currently, the main methods of preparing calcium carbonate whisker are carbonation, decomposition of calcium bicarbonate, urea hydrolysis, double decomposition, and so on [15][16][17][18]. The raw materials used in these methods are mainly pure substances or natural minerals, and chemical additives are usually needed to adjust the morphology of the product during the preparation process. Yellow phosphorus slag is a kind of solid waste produced during the production of yellow phosphorus by the dry process. About 8-10 tons of waste residues may be produced during the production of 1 ton of yellow phosphorus. The main chemical composition of yellow phosphorus slag is amorphous calcium silicate. Hence, the amount of calcium is high in yellow phosphorus slag, and it has good chemical activity. So, it is a good material for producing precipitated calcium carbonate. In China, about 80,00,000 tons of yellow phosphorus slag is produced every year, which can produce 7-10 Mt of calcium carbonate in theory. At present, the slag is mainly used in the field of producing sintered brick [19], glass ceramics [20], and concrete [21]. As far as we know, there are no reports on using yellow phosphorus slag to produce fibrous aragonite.
According to our previous experimental exploration [22], CaCO 3 mixed with calcite and aragonite was successfully synthesized. To obtain high-purity calcium carbonate whisker, yellow phosphorus slag was used as a raw material without any chemical additives at low temperature and normal pressure. Using yellow phosphorus slag to prepare aragonite can not only reduce the preparation economic cost but also realize the resource utilization of the slag. The effect of experimental factors such as reaction temperature, Ca 2+ concentration, ammonia dosage, carbon dioxide velocity, and time on the crystal phase and morphology of the products was systematically discussed in this study. Meanwhile, the whole production process was analyzed, and the economic cost could be reduced by recovering the by-products in theory.

Materials
The raw material of the slag was obtained from China. The main chemical composition was CaO and SiO 2 , and some amount of Fe 2 O 3 , P 2 O 5 , and Al 2 O 3 was also present ( Table 1). The particles were amorphous in nature in the slag (Figure 1a). The microstructure of the slag was granular (Figure 1b). The slag was milled into powder less than 150 μm. The reaction auxiliary was used without further purification, CaCl 2 , HCl and NH 4 OH, and 99.9% CO 2 .

Methods
The whole preparation process was performed according to the experiments of the previous study [22]. During the experiments, the effect of reaction conditions (temperature, concentration of Ca 2+ , ammonia dosage, CO 2 flow rate, and time) on the preparation process of aragonite was systematically discussed. The concentration of Ca 2+ in the acidleaching solution was 1 unit, and it was changed using distilled water and CaCl 2 . The technological parameters of preparation experiments for aragonite are listed in Table 2.
All the chemical reactions related to the experiments are as follows [23]:  (2)

Data analysis
The chemical composition of the slag and the products was tested by X-ray fluorescence combined with chemical titration and inductively coupled plasma. The crystal phase of the raw material and the products was determined by the X-ray diffraction (XRD) spectra (an X-ray diffractometer using Cu Kα radiation [λ = 0.15406 nm] at 40 kV, 40 mA with a scanning speed of 0.02 deg/s). The microscopic morphology of the materials was observed using a scanning electron microanalyzer at an accelerating voltage of 5 kV. The specific surface area of leaching product was tested with an Autosorb-1 MP fully automatic specific surface area analyzer (Quantachrome Instruments, USA). The whiteness of the samples was tested using a whiteness meter (HY-BDY).
According to the previous research [24], the mole fractions (mol%) of calcium carbonate with different crystal forms were determined by the following equations, in which I A 221 , I C 104 , and I V 110 are the XRD intensities of the (221) plane of aragonite, the (104) plane of calcite, and the (110) plane of vaterite, respectively. For a mixture of aragonite and calcite: For a ternary mixture: Ethical approval: The research conducted is not related to either human or animal use.

Results and discussion
3.1 Extraction of calcium from yellow phosphorus slag The effect of reaction factors on the extraction ratio of calcium was studied with several selected parameters.
According to the previous research, the optimal conditions for extracting calcium from yellow phosphorus slag were 3 mol/L, 60°C, 8 mL/g, and 60 min. The main chemical composition of the corresponding reaction product was silicon dioxide. The microscopic morphology of the product was granular, and there were some pores on the surface of the reaction product. The content of silicon dioxide in the reaction product was 98.12%, and the whiteness of the product was 65.4%. The specific surface area of the reaction product was 74.24 m 2 /g, which indicated that the reaction product has good reaction reactivity. Therefore, it can be used directly as a filler, and it can also be used as a raw material to produce white carbon black [22].

Preparation of aragonite
Calcium carbonate has three crystal types. They are spherical vaterite, fibrous aragonite, and rhombohedral calcite. Calcite has the highest thermodynamic stability, aragonite takes second place, and vaterite has the lowest thermodynamic stability [24]. This indicates that the reaction temperature can affect the crystal phase and microscopic morphology of calcium carbonate (Figure 2). Fibrous aragonite mixed with some cubic calcite was synthesized at 80°C (Figure 2a and c). The molar fraction of aragonite and calcite was 68.46% and 31.54%, respectively (Figure 2b). The mole content of aragonite increased to 93.67% and that of calcite reduced correspondingly to 6.33% (Figure 2b) when the reaction temperature was 100°C, and the products were mixture of fibrous aragonite and little cubic calcite (Figure 2a, c and  d). This was because the solubility of calcium carbonate in the solution increased gradually, and the solubility of carbon dioxide and calcium hydroxide in the solution decreased with an increase in reaction temperature. Then, calcium carbonate particles were prone to grow along the c axis [25]. If the reaction temperature was further increased to 120°C and 150°C, the content of aragonite decreased and that of calcite increased correspondingly. Meanwhile, the mole fraction of aragonite was 10.37% and 0%, and the mole fraction of calcite was 89.63% and 100%, respectively, at reaction temperatures 120°C and 150°C (Figure 2b). Single cubic calcite was obtained at 150°C (Figure 2a and f). This indicated that the increase in temperature is favorable for the transition of calcium carbonate to a more thermodynamically stable crystalline form [24]. Figure 3 shows the effect of Ca 2+ concentration on the product phase and morphology. The product comprising spherical vaterite, fibrous aragonite, and cubic calcite was prepared when the Ca 2+ concentration was 1/4 (Figure 3a and c). The molar content was 30.69% for vaterite, 1.9% for aragonite, and 67.41% for calcite (Figure 3b). With the increase in Ca 2+ concentration from 1/4 to 4/4, the content of calcite and vaterite decreased and the content of aragonite increased (Figure 3b). Some calcite and vaterite transformed into aragonite (Figure 3a). With the increase in Ca 2+ concentration to 5/4 and 6/4, the crystal phase of vaterite disappeared. The molar content of calcite changed from 49.07% to 56.78%, and the mole content of aragonite decreased from 50.93% to 43.22% (Figure 3b). If the Ca 2+ concentration was further increased to 7/4 and 8/4, the phase of the product was calcite with cubic morphology, and there were no other phases (Figure 3a, i and j). Thus, it was beneficial to synthesize vaterite when the concentration of Ca 2+ was not high. With the addition of calcium chloride, the concentration of Ca 2+ increased gradually in the solution. Vaterite transformed into thermodynamically metastable aragonite, and aragonite transformed into thermodynamically stable calcite [26,27].
The effect of NH 4 OH dosage on product phases is shown in Figure 4. Pure calcite was obtained with 10 and 15 mL ammonia (Figure 4a and c). When the ammonia dosage increased to 25 mL, the mole content of calcite decreased to 6.33% and the mole content of aragonite increased to 93.67% (Figure 4b). The product was composed of fibrous aragonite and little cubic calcite (Figure 4a and d-f). Further increasing the ammonia dosage to 30, 35, and 40 mL, the content of aragonite decreased gradually (Figure 4a and g-i). Then, 25 mL was the optimized ammonia dosage in the system for producing aragonite. The variation trend of content of aragonite was the same among Ca 2+ concentration, ammonia dosage, and CO 2 flow rate ( Figure 5). The optimized CO 2 flow rate was 100 mL/min. The amount of CO 3 2− increased per unit time when increasing carbon dioxide velocity. The concentration of Ca 2+ was constant, and metastable aragonite was first produced, then it gradually transformed into stable calcite with increasing . Therefore, increasing carbon dioxide velocity in the reaction system was useful for preparing calcite. We also discussed the impact of reaction time on the crystal phase of the product ( Figure 6). The main crystal phase was fibrous aragonite, and there were also some cubic particles in the product when the reaction time was 30 min (Figure 6a and c). The molar content of aragonite was 64.14%, and the molar content of calcite was 35.86% (Figure 6b). With appropriate prolonging of the reaction time, cubic calcite transformed into fibrous aragonite. The product with more fibrous aragonite was produced for the reaction of 60 min, and the content of aragonite in the product was the highest (Figure 6a, b  and d). If the reaction time was further increased to 90 and 120 min, some aragonite transformed into calcite, and the length of the aragonite became shorter ( Figure  6a, e and f). The content of aragonite decreased from 93.67% to 49.56% and that of calcite increased correspondingly from 6.33% to 50.44% (Figure 6b). This indicated that the transformation of aragonite to more stable calcite was inevitable when the reaction time was enough, which was the same as the previous research [29,30].
According to the aforementioned research, aragonite was successfully prepared using yellow phosphorus slag as a raw material without any chemical additives. The content of CaCO 3 in the product was 99.68%, and the content of aragonite in the product was 93.67%. The whiteness of the product was 97.6%. The aragonite samples had a fibrous morphology, and the surface of the sample was very smooth. The diameter of a single particle was about 1.5-3 μm, and the length of a single particle was about 8-40 μm (Figure 6d). The by-product NH 4 Cl can be obtained by evaporating the reaction filtrate, which can reduce the production economic costs. Aragonite samples prepared in the study can be used in packing fields to improve the mechanical strength of composite materials [31] and to replace asbestos for friction and seal materials.

Crystal growth process
The whole process of aragonite production from yellow phosphorus slag is shown in Figure 7. Yellow phosphorus slag was rich in calcium, which laid a foundation for the preparation of aragonite. First, Ca 2+ was leached from raw material with the action of hydrochloric acid. After the reaction solution was cooled to 25°C, the pH value of the reaction solution was adjusted to be neutral using ammonium. Meanwhile, ferric hydroxide was precipitated from the solution. Second, calcium hydroxide was formed in the reaction system under the action of ammonium [23]. Third, gaseous CO 2 was dissolved in water to transform Ca(OH) 2 to CaCO 3 at low temperature and normal pressure. Highly pure aragonite samples can be prepared by adjusting the experimental conditions. During the production process, pure cubic calcite can also be produced. Compared with the previous studies [32,33], no chemical additives were used in the production process. The raw material was industrial solid waste, and the reaction conditions were mild. So, the whole production cost may be much lower than that of the traditional preparation method in theory. The preparation experiment under the optimized conditions will be done in large scale, and the economic cost will be calculated in a follow-up study.

Conclusion
In our research, highly pure calcium carbonate whisker was successfully prepared with yellow phosphorus slag as a raw material under the action of HCl and NH 4 OH. The technological parameters (temperature, concentration of Ca 2+ , and ammonia dosage) have an important effect on the crystal type and microscopic morphology of the product. The products with single calcite, mixture of calcite and aragonite, or mixture of calcite, aragonite, and vaterite were all produced without using any chemical additives in the research. The content of aragonite in the product was 93.67% under optimal conditions. The whiteness of the product was 97.6%. The single particle diameter was about 1.5-3 μm, and the length of a single particle was about 8-40 μm. The by-products, including SiO 2 residue and NH 4 Cl, could be utilized to reduce the economic cost in the whole production process.