Simple recycling of biowaste eggshells to various calcium phosphates for specific industries

Egg consumption is very high throughout the world and with it comes enormous amount of waste eggshells. To reduce and utilize these wastes, eggshell wastes were simply transformed to low- or high-purity calcium carbonate grades by washing, crushing, and drying to use as raw materials for producing highly valuable calcium phosphate products. Low-purity calcium carbonate grade was used to prepare triple superphosphate for using in fertilizer industry, whereas high-purity calcium carbonate grade was used to produce dicalcium phosphate dihydrate, monocalcium phosphate monohydrate, and tricalcium phosphate for using in mineral feed and food additive industries. All calcium phosphate samples obtained by simple, rapid, cheap, and environmentally safe method using eggshells and phosphoric acid were identified and their structural phases and impurities were determined by XRF, XRD and FTIR techniques. Thermal behaviors of raw materials and the prepared calcium phosphates excepted tricalcium phosphate were investigated by TG/DTG techniques. The methodologies described here will be useful to manage eggshells by converting them to highly valuable products, which can solve eggshell wastes problem from industries and communities. This finding supports the viewpoint of zero waste operation to produce value-added products for obtaining sustainable development, which may be selected as an alternative way for material recycling and waste management in the future.


Materials and methods
Preparation of raw materials. Eggshell wastes were obtained from commercial food, bakery, and cake shop (Chachoengsao Province, Thailand), which were normally stored 2 tons a day. These wastes were turned into 2 levels of calcium carbonate purity, low-and high-purity grades, as raw materials for producing fertilizer and feed mineral, respectively. Figure 1 shows a general flowchart of the preparation process of the two calcium carbonate grades from eggshells.
Low-purity calcium carbonate (LPC) was prepared by crushing eggshells followed by sieving through a 50 mesh with grinder machine. Thus, coarse powders with purity levels of 94-97% in calcium carbonate that would be used to produce phosphate fertilizers were obtained.
High-purity calcium carbonate (HPC) was prepared by treating eggshells as follow: washed with household detergent, crushed and sieved through a 50 mesh with grinder machine and then calcined at 500 °C for 2 h with a furnace. Subsequently, coarse powders with purity levels of 97-99% in calcium carbonate that will be used to produce animal feed and food additive are obtained.
Commercial-grade phosphoric acid (85% w/w H 3 PO 4 ) was used without prior purification. Two concentrations of phosphoric acid prepared by dilution with water are 50 and 70% w/w H 3 PO 4 . In a typical process, 10 kg of 85% w/w H 3 PO 4 were added to 7 and 2.15 kg of water for preparing 50 and 70% H 3 PO 4 , respectively. The dilution reactions were strong exothermic processes which occurred at temperature range of 50-70 °C.
Production of calcium phosphate compounds. Triple superphosphate (Ca(H 2 PO 4 ) 2 ·H 2 O), fertilizer containing more than 54.62% of available phosphoric acid (P 2 O 5 ) was prepared as follow. At beginning procedure, 3 kg of LPC was placed in a plastic container (20 L). Then 5 L of 70% w/w H 3 PO 4 were slowly added and followed by stirring at 100 rpm for about 15 min. The resulting reaction was strong exothermic and was aged in a plastic container for about 60-120 min to dry spontaneously and then was ground to fine powder. The whiteyellow product (Ca(H 2 PO 4 ) 2 ·H 2 O) ( Fig. 1), triple super phosphate is obtained by a simple, rapid, and low-cost production process according to Eq. (1): , feed and food grade containing 18% of phosphorus and 24% of calcium, was produced as follows: in the starting process, 5 kg of HPC was placed in a plastic container (20 L), followed by the immediate addition of 5 L of 50% w/w H 3 PO 4 and then stirring at 100 rpm for about 20 min. Finally, the resulting reaction which was moderately exothermic was aged in a plastic container for about 50-70 min to get dried product and then was ground to fine powders and the white product of dicalcium phosphate (CaHPO 4 ·2H 2 O) is obtained (Fig. 1). This reaction is simple, quick, and cheap and is shown in Eq. (2).
Monocalcium phosphate monohydrate (Ca(H 2 PO 4 ) 2 ·H 2 O), feed and food grade containing 21% of phosphorus and 15% of calcium, was prepared similarly to triple superphosphate but using HPC as starting agent and the final white powder product was obtained (Fig. 1).
Tricalcium phosphate (Ca 3 (PO 4 ) 2 ) ( Fig. 1), food grade containing 44% of phosphorus and 38% of calcium, was synthesized as follows: in a typical route, 2.52 kg of monocalcium phosphate monohydrate (Ca(H 2 PO 4 ) 2 ·H 2 O) and 1.0 kg of HPC were thoroughly mixed by ball milling. Then the resulting mixture was transferred into a ceramic container and then was calcined at 900 °C for 2 h in a furnace. The calcined product was obtained as white powder corresponding to Eq. (3).
Characterization. The starting materials and as-prepared products were obtained and identified. Four analyses were carried out in this study: X-ray Fluorescence (XRF, Bruker S4 Pioneer), X-Ray Diffraction (XRD, Bruker D8 Advance), Fourier-Transform Infrared Spectrophotometer (FTIR, Perkin Elmer Spectrum GX) and Thermogravimetric Analyzer (TGA, Perkin Elmer Pyris I). The crystal structures of samples were estimated using XRD in 2θ range of 1°-60° with a step size of 0.05 and a step time of 1 s. The room temperature FTIR spectra were recorded in the range of 4000-400 cm −1 with eight scans on a Perkin-Elmer Spectrum GX spectrometer with the resolution of 4 cm −1 . Thermal analysis was used to characterize water content in the as-prepared samples as dicalcium phosphate dihydrate, triple superphosphate, and monocalcium phosphate monohydrate.
(2) CaCO 3 (s) + H 3 PO 4 (50% w/w) + H 2 O(l) → CaHPO 4 · 2H 2 O(s) + CO 2 g .   41 . The XRD patterns of as-prepared triple superphosphate (Fig. 3a) and monocalcium phosphate monohydrate ( Fig. 3c) are very similar but lower and higher peak intensities are observed because the products were obtained from different purities of calcium carbonate grades 36 . The lower peak intensities were observed in the XRD pattern of asprepared triple superphosphate due to the LPC was used as the starting agent. For as-prepared tricalcium phosphate, diffraction lines attribute mainly to beta-tricalcium phosphate (Ca 3 (PO 4 ) 2 , PDF no. 70-2065) 42 . Based on the XRD observation, it can be concluded that no impurity appeared in as-prepared HPC, dicalcium phosphate dihydrate, monocalcium phosphate monohydrate, and tricalcium phosphate powders, and some impurities appeared in as-prepared LPC and triple superphosphate samples. Figures 4 and 5 show FTIR spectra of the starting agents (LPC and HPC) and the as-prepared products. The FTIR spectra of as-prepared LPC (Fig. 4a) and HPC (Fig. 4b) are very similar and assigned according to vibrational modes of CO 3 2− and Ca-O, which are block units of calcium carbonate (CaCO 3 ). The appeared prominent absorption peak of CO 3 2− at 1478 cm −1 corresponds to the asymmetric stretching mode of C-O bond. The appeared vibrational peaks at 1067, 853, and 710 cm −1 are assigned to the symmetric stretching mode of C-O bond, out of plane and in plane bending of CO 3 2− anion, respectively 3 . The FTIR spectra of triple superphosphate (Fig. 5a) and monocalcium phosphate monohydrate (Fig. 5b) are very similar and assigned based on the fundamental vibration of Ca(H 2 PO 4 ) 2 ·H 2 O. For triple superphosphate (Fig. 5a), the strong bands in the region of 1150-950 and 450-600 cm −1 are attributed to the P-O stretching vibrations and the bending OPO vibrations, respectively. The couple bands at 1221 and 865 cm −1 are assigned to the in-plane P-O-H bending (A 2 ), and the out of plane bending (A 1 ) vibration, respectively. A weak band occurred at approximately 669 cm −1 is assigned to rocking mode involving water molecules. In the O-H stretching mode region of the H 2 PO 4 − , which are characteristic for this compound, they appear as the A, B, C trio bands at about 3100-3200 cm −1 (A band), 2300-2400 cm −1 (B band), and 1600-1800 cm −1 (C band). The vibrational bands of water molecule appear in three modes at 1646 cm −1 (ν 2 ), 3192 cm −1 (ν 1 ), and 3459 cm −1 (ν 3 ) and are assigned to bending, asymmetric stretching (O-H), and antisymmetric stretching (O-H) modes, respectively 60 . Vibrational modes of as-prepared dicalcium phosphate dihydrate (Fig. 5b) are assigned according to the units of HPO 4 2− ion, H 2 O, and CaO. The typical intense bands at about 1370 cm −1 is due to the in-plane P-O-H bending, while the out of plane bending vibration is observed at about 893 cm −1 . The strong bands at about 1120, 1060, and 984 cm −1 are assigned to PO 3 asymmetric stretching modes. The weak and broader band at about 569 cm −1 is corresponding to PO 4 and P-OH bending modes. The band at 499 cm −1 is assigned to PO 3 symmetric bending or Ca-O stretching 61 . The FTIR spectrum shown in Fig. 5c is very similar to the FTIR spectrum as shown in Fig. 5a 4 3− and CaO subunits of Ca 3 (PO 4 ) 2 structure are assigned (Fig. 5d). Vibrational bands of PO 4 3− anion of the product are observed in the regions of 450-610 and 900-1000 cm −1 . These bands are assigned to the ν 4 (PO 4 3-) and ν 3 (PO 4 3-) vibrations, respectively. Additionally, Table 1. Compositions (± 0.02%) of starting agents (CaCO 3 obtained from eggshells) and as determined by X-ray fluorescence (XRF). Percentage values less than 0.02% (limit of detection) were omitted for clarity. www.nature.com/scientificreports/ the observation of a strong ν s (POP) bands (725 cm −1 ) is known to be the most striking feature of polyphosphate spectrum. The FTIR results are consistent with X-ray diffraction data. Thermal behaviors of the starting agents (LPC and HPC) and only three as-prepared products (triple superphosphate, dicalcium phosphate dihydrate and monocalcium phosphate monohydrate) investigated by TG/DTG technique are shown in Fig. 6. Tricalcium phosphate is an anhydrous compound, which is not necessary to analyze thermal decomposition. TG/DTG curves of the LPC (Fig. 6a) and HPC (Fig. 6b) samples are quite similar. TG curves of each sample show the mass loss in the range of 600-800 °C, which corresponds to a strong single peak of DTG curves at 770 and 753 °C for the LPC and HPC samples, respectively. Many DTG peaks observed at 297, 512, 540 and 623 °C indicate clearly impurity of the LPC sample. The quantities of mass loss (remaining mass) are found to be 48% (52%) and 44% (56%) for LPC and HPC samples, respectively. The thermal behavior obtained confirms that the LPC sample has a lower purity than the HPC sample. However, the thermal results obtained were well consistent with those of the reference and theory information of CaCO 48,49 . The thermal results indicate that LPC and HPC powders can be transformed to CaO by calcination at above 800 °C, which may be useful for the production of this compound used in specific applications, revealed by: TG curves of triple superphosphate and monocalcium phosphate monohydrate are observed in the range of 30-900 °C. The mass remains occurred at above 700 °C, which are found to be 82.50 and 81.00% for triple superphosphate (Fig. 6c) and monocalcium phosphate monohydrate (Fig. 6e), respectively. The mass losses found to be around 18% (2.57H 2 O) for each sample are related to the loss of three water molecules in Ca(H 2 PO 4 ) 2 ·H 2 O structure. The masses of loss and remain observed from each sample are close to that of theoretical values at 21 (3.0H 2 O) and 79%, respectively 53 . The relative with TG data, five DTG peaks of both samples are observed the same but peak positions are slightly different. Fiver DTG peaks of triple superphosphate and monocalcium       2 ) was found to be the product of the thermal decomposition of both prepared compounds over 800 °C as revealed by the TG curve. This result may be not in agreement with literature 53 , which reported in the range of 500-700 °C of this transition phase. TG curve of dicalcium phosphate dihydrate (CaHPO 4 ·2H 2 O) (Fig. 6d) show the mass loss in the range of 100-800 °C. The total mass loss is 26%, which is closely to the theoretical value (26.16%, 2.5H 2 O). The relative with TG data, three intense DTG peaks are observed at 109, 199 and 288 °C, respectively. The formally reactions of thermal transformations could be: The intermediate compounds including CaHPO 4 ·xH 2 O, CaHPO 4 (s) and mixtures of intermediate have been registered. Calcium pyrophosphate (Ca 2 P 2 O 7 ) was found to be its final decomposition product 800 °C as revealed by the TG curve. This result may be not in agreement with literature 2,4,33,35 , which reported lower temperature (< 700 °C) of this transition phase. Thermal results of each compound indicate clearly that the property dependent on various conditions (preparation technique, synthesis condition, raw material, and so on).
The novelty of this work is the production methods that are simple, rapid, cost-saving and environmentally benign for the obtained calcium carbonate (raw material) and four calcium phosphate compounds, shown in Fig. 1. The eggshell wastes were accumulated about 60 tons/month from only one bakery shop in Chachoengsao Province, Thailand. Transportation of these wastes (60 tons) in the distance of 500-800 km is about 1242 USD, which indicates the logistic cost of eggshell no more than 0.021 USD/kg. The eggshells were prepared by two ways to get particle size of 50 mesh. In the first method, they were crushed only to become low-purity calcium carbonate grade, which was in the cost about 0.0031 USD/kg. In the second method, they were washed with detergent and crushed to become high calcium carbonate grade, which had the cost of about 0.009 baht/kg. The ways of the prepared eggshells turn into low-or high-purity calcium carbonates in this work are similar with previous reports 1, 3 . Calcium carbonate (CaCO 3 ) has three crystal forms as calcite, aragonite, and vaterite and is occurred from two sources: living and non-living sources. Non-living source is found in limestone mineral such as calcite (CaCO 3 ) and dolomite (Ca x Mg y CO 3 , x + y = 1), which may have impurities of other heavy metals such as lead (Pb), cadmium (Cd), arsenic (As), etc. and has limited quantity, which may run out in the future. The living source is found as the main component of pearls and the shells of marine organisms, snails, and eggs. It happens every day and is collected as large volume of wastes and without heavy toxic metal according to theory  www.nature.com/scientificreports/ of natural selection. The low-or high-purity calcium carbonates prepared from eggshells reported in this work were investigated by XRF, XRD and FTIR techniques for confirmation of nontoxic impurities, phases, and structures. Two calcium carbonate grades obtained in this work were used as calcium sources to react with specific phosphoric acid concentration at very short time (< 30 min) and spontaneously dry solid products were obtained as superphosphate for phosphorus fertilizer (56% P 2 O 5 ), monocalcium phosphate, dicalcium phosphate, and tricalcium phosphate for feed animal and food additive. In the production of some calcium phosphates, carbon dioxide (CO 2 ) gas was evolved in the process and was tapped by quicklime to turn into calcium carbonate, which is a similar method to previous reports 62, 63 . The production methods of four calcium phosphates reported in this work were simple, rapid, cheap, and safe methods when compared with methods reported in literatures 2 www.nature.com/scientificreports/ monocalcium phosphate monohydrate, and tricalcium phosphate, respectively. The characterizations of four calcium phosphates were performed using XRF, XRD and FTIR techniques. Impurities, phases and structures were used to confirm that the obtained products were "Green Products" (no metal toxic impurities and according to standard material product) to replace the same products in Thailand markets. All obtained based on this work lead to transforming or recycling of eggshells to four calcium phosphates with lower or near prices and "green products" compared with the same products in the markets. Therefore, eggshell wastes from anywhere can be used to prepare calcium phosphates by methods reported in this work, which will be alternatives to remove these wastes from environmental problem in the future. The authors suggest that pilot projects are required to evaluate their implementation to a commercial scale by using these cleaner production processes to convert waste into industrially viable resources in the future.

Conclusions
Waste eggshells represented one of the most urging problems of agricultural and food industries as well as communities, due to their enormous amount that must be disposed of in some way nowadays. This research described an alternative way to recycle wastes as added-value products to solve eggshell problem of industries and communities. The as-prepared raw materials (low-and high-purity calcium carbonate (LPC and HPC) and products (triple superphosphate, dicalcium phosphate dihydrate, monocalcium phosphate monohydrate, and tricalcium phosphate) were produced by simple, rapid, cheap, and environmentally benign method. Identification of structural phase and impurity of all as-prepared samples was confirmed by XRF, TGA, XRD and FTIR techniques. The findings indicate that all as-prepared samples can be used in industries of fertilizer, animal feed and food additive, which require massive amounts. Therefore, the study represents alternative ways of the environmental benefits resulting from possible transformation of eggshells to value-added calcium compounds needed for wide uses in many industries. If the eggshells are recycled as described above, then a future of these wastes will be used to produce value-added products, which take care of the environment according to the way of zero waste and corresponding to sustainable development.