RESEARCH INTO THE PROCESS OF CARBONATE CONVERSION OF PHOSPHOGYPSUM IN THE WATER MEDIA

For a long time, the problem of phosphogypsum disposal was not given due attention, which led to the accumulation of huge reserves of this environmentally hazardous waste. Transportation of phosphogypsum in dumps and its storage requires significant capital investment and operating costs, as well as the allocation of large land areas. The storage of phosphogypsum harms the environment, as it leads to the entry of significant amounts of toxic substances into groundwater and the atmosphere. The object of the study was phosphogypsum, which is formed during the production of phosphoric acid from apatites and phosphorites. To date, numerous studies have focused on finding effective ways to use phosphogypsum, but its composition usually limits its reuse. The research aims to study the scientific prerequisites for creating an effective control scheme for phosphogypsum by converting it to calcium carbonate. For this purpose, the chemical composition and technological parameters of phosphogypsum conversion from the dumps of «Dnipro Mineral Fertilizer Plant» (Kamianske, Ukraine) were studied. The proposed conversion method involved a reaction between phosphogypsum and aqueous NaOH solution to form Ca(OH)2, which was converted to CaCO3 by carbonization in a CO2 stream. The influence of temperature on the conversion of phosphogypsum into Ca(OH)2 has been established. The conversion was investigated at 25 °C, 40 °C, and 70 °C for 3 hours. It was found that at the first stage of the process the impurities contained in phosphogypsum were transferred to the precipitate of Ca(OH)2. As a result of carbonization, low-quality CaCO3 was obtained, which contained more than 10 % of impurities. Increasing the duration of carbonization to 1 hour allowed to obtain CaCO3 with content of Ca(OH)2<4 %. The results of the study show that the method of carbonate conversion of phosphogypsum requires further optimization of technological parameters to improve the purity of the finished product.


Introduction
Production of uranium products on the territory of the former Production Association «Prydniprovsky Chemical Plant» (PA «PChP») in Kamianske (Dnipropetrovsk region, Ukraine) has led to a threatening scale of accumulation of phosphogypsum [1]. As a result of the imperfect construction of phosphogypsum storages on the industrial site of «Dnipro Mineral Fertilizers Plant» (Kamyanske, Ukraine), today there is a gradual release of a significant amount of toxic impurities in the tributary of the Dnieper River [2].
In recent years, numerous researches work in the field of technologies for the direct use of phosphogypsum and its conversion into other products [3][4][5]. The technical feasibility and technological feasibility of using phosphogypsum in the national economy instead of traditional types of phosphorus fertilizers have been experimentally proven [6]. Despite the wide range of applications of phosphogypsum, the huge volume of its formation and significant costs for cleaning, as well as competition with industrial counterparts significantly limits the market for phosphogypsum and its products [7].
Among the works that consider methods of conversion of phosphogypsum into marketable products, the most promising should be considered those that involve the integrated use of raw materials and the extraction of all valuable components present in phosphogypsum [8][9][10]. In particular, in [10] the process of phosphogypsum conversion to CaCO 3 was developed by treatment with sodium hydroxide followed by carbonization in an aqueous medium. The obtained calcium carbonate contains the main part of impurities (rare earth elements, radionuclides), and the mother liquor serves as a source for obtaining valuable sodium sulfate, which can be used in the production of synthetic detergents and glass. Reactions describing the process are expressed by the following chemical equations: Thus, the object of research is selected phosphogypsumwaste generated during the production of phosphoric acid from apatites and phosphorites.
The aim of research is a study the effectiveness of carbonate conversion of phosphogypsum to optimize process conditions.

Methods of research
2.1. Phosphogypsum conversion technique. 200 cm 3 of the initial NaOH solution with a concentration of 3 mol/dm 3 was poured into a 500 cm 3 flask with 5 necks and placed in a water bath at the selected temperature. A pH electrode, a temperature probe, and a mechanical stirrer were inserted into the flask through three of the five available necks. The other two mouths of the flask were closed with plastic plugs. The NaOH solution was stirred continuously until a constant pH of the solution and the selected temperature. 50 g of the phosphogypsum sample was added to 200 cm 3 of NaOH solution at the selected temperature. The suspension was stirred at 750 rpm for 3 h, after which the liquid phase was separated by filtration using «blue tape» filter paper. The precipitate was then washed 4 times with 50 cm 3 of distilled water to remove unreacted salts and dried at 75 °C for 3 hours.
2 g of the obtained Ca(OH) 2 was dispersed in 40 cm 3 of running water at room temperature. The solution was stirred at 750 rpm for 1 h to obtain a homogeneous solution of Ca(OH) 2 and to ensure constant pH and temperature of the solution 25 °C inside the flask. After stirring, the solution of Ca(OH) 2 in the flask was purged with gaseous CO 2 at a flow rate of 285 cm 3 /min for 30 minutes. The resulting precipitate was washed three times with distilled water in portions of 50 cm 3 , dried at 75 °C for 3 hours, and weighed before further analysis.

Methods of research of chemical composition of raw materials and products.
To determine the main oxide elements that may be present in the samples, analysis was performed by mass spectrometry with inductively coupled plasma (ISP-MS) using the device Agilent-7500CE 3M (USA) according to standard methods [11].
X-ray phase analysis was performed on a diffractometer DRON-3M (RF), the identification of mineral phases was carried out following the files of the American Society ASTM using software DIFFRACplus TOPAS for quantification with an accuracy of ±1 %.

Results of research and discussion
3.1. Investigation of the effect of temperature on the yield of Ca(OH) 2 . For each temperature, the reaction was performed in duplicate, the average values were plotted against time.
Reactions between phosphogypsum and NaOH were studied at selected temperatures (25 °C, 40 °C, and 70 °C), and pH changes over time are shown in Fig. 1.
The pH of the NaOH solution at 25 °C before the addition of phosphogypsum was 13.5. After the addition of 50 g of phosphogypsum, the pH rapidly decreased to 13.38, indicating that OHions in solution began to react with Ca 2+ ions due to the dissociation of CaSO 4 contained in phosphogypsum to form a precipitate of Ca(OH) 2 . After the addition of phosphogypsum, the pH value was constantly reduced throughout the experiment (3 h). This may indicate that the process was not completed after 3 h of reaction at room temperature and that some phosphogypsum remained unreacted.
At 40 °C, the pH of the NaOH solution before the addition of phosphogypsum was equal to 13.26. After the addition of phosphogypsum, there was an immediate decrease in pH and even after 3 hours of the reaction, the stabilization of pH values did not occur. This may mean that the conversion has not yet been completed.
At 70 °C, the pH of the NaOH solution before the addition of phosphogypsum was 11.90. After the addition of phosphogypsum after 3 h of interaction, the pH decreased and reached a stable value of 11.23 at the 165th minute of the reaction. However, 15 min is not enough to claim that the reaction between phosphogypsum and NaOH at 70 °C was complete.
The precipitates formed at different temperatures between the gypsum and NaOH samples were filtered, washed, dried, and weighed. The mass of each precipitate is shown in Table 1.  obtained amounts of precipitates formed from phosphogypsum slightly exceed the expected theoretical mass required for the formation of pure Ca(OH) 2 . Because the molecular weight of CaSO 4 •2H 2 O is greater than Ca(OH) 2 , the increased weight of the product, compared with stoichiometrically calculated, may indicate incomplete conversion of gypsum to Ca(OH) 2 . This confirms the results obtained during the pH measurement, indicating that part of the phosphogypsum did not react after 3 h of reaction. The content of the main elements in the precipitates of Ca(OH) 2 formed at different temperatures is summarized in Table 2.
The calcium content (expressed as CaO) in each of the samples Ca(OH) 2 (64-77 %) is twice the amount of CaO contained in the initial phosphogypsum before conversion (33.3 %).
Other elements are present in the form of impurities that were previously contained in the untreated phosphogypsum and passed into the precipitate of Ca(OH) 2 . It was previously reported [10] that in the process of conversion there is a transfer of impurities from phosphogypsum to calcium hydroxide.  Fig. 2 shows the change in pH of the precipitates of Ca(OH) 2 obtained by the interaction of phosphogypsum and NaOH at 25 °C, during carbonization.
After dispersing Ca(OH) 2 in water, the pH of the solution before purging with CO 2 was 12.3. After the introduction of CO 2 into the suspension, the pH value began to decrease rapidly, and after 30 min of the process reached 8.47. The decrease in pH is the result of the formation of a precipitate of CaCO 3 . However, as can be seen from Fig. 2, after 30 min of bubbling, the carbonization of Ca(OH) 2 is still not complete.
The mineralogical composition of the carbonized sludge was analyzed by radiography. The results of the analyzes are expressed in mass percent (wt. %), Summarized in Table 3. As can be seen from Table 3, calcite is the main mineral phase of the sediment. Today, it is believed that low-quality CaCO 3 has a purity below 90 %, and high-qualitymore than 95 % [4]. An insufficiently high content of CaCO 3 , confirms that the process of carbonization of Ca(OH) 2 precipitates derived from phosphogypsum has not been completed.
Therefore, an attempt was made to convert 5 g of phosphogypsum into CaCO 3 using 20 cm 3 of NaOH solution; the ratio of solid to liquid (S:L) = 1:4. Radiographic analysis revealed the formation of stable calcite, with a small amount of unreacted portlandite. No gypsum crystals were found in the sediments, which suggested its complete transformation.
The CaCO 3 precipitate formed at room temperature using 5 g of phosphogypsum and 20 cm 3 of NaOH solution was further characterized because the complete conversion of phosphogypsum to CaCO 3 was obtained. Calcite was found to be the dominant mineral phase (> 96 %) of the precipitate. Subsequently, a study of mass balance was performed using three samples of the formed precipitates to determine the percentage of calcium phosphogypsum, which was transformed into calcite. The results of this study are summarized in Table 4.
From the Table 4 shows that from 5 g of phosphogypsum (containing 1.97 g of calcium) dispersed in 20 cm 3 of NaOH solution, 2.75 g of CaCO 3 is formed, which corresponds to the average degree of conversion of phosphogypsum to CaCO 3 56.08 %. This means that a significant part of calcium (43.92 %) remains unreacted.

Conclusions
In the process of carbonization in an aqueous medium, phosphogypsum was converted to Ca(OH) 2 in 3 h of interaction with a solution of 3M NaOH. The degree of conversion at 25 °C of phosphogypsum was 96 %. It was found that the impurities contained in phosphogypsum are converted into Ca(OH) 2 .
The main phase of the sediment was calcite. However, 30 min of carbonization was not enough for complete carbonization of Ca(OH) 2 derived from phosphogypsum resulting in low-quality CaCO 3 (impurity content -13 %).
To increase the efficiency of the conversion process, it is recommended to increase the reaction time to 4 h for a more complete conversion of phosphogypsum to Ca(OH) 2 , as well as to increase the duration of carbonization to 1 h.