Study and mechanism of formation of phosphorus production waste in Kazakhstan

2.1 Instrumental methods of analysis

The following installations were used to carry out instrumental physicochemical analysis methods:

1. Thermodynamic analysis of the mechanism of cottrel dust formation was carried out using a multifunctional software package Chemistry HSC-6, based on the principle of maximum entropy and enthalpy values with Gibbs energy values. For chemical reactions occurring in a heterogeneous system, calculation of the change in the enthalpy ΔH, entropy ΔS, and Gibbs energy ΔG values was implemented in accordance with Eqs. 1–3, respectively:

   (1) Δ H Δ T o = ∑ 1 i ( ν i Δ H f , Δ T i o ) product − ∑ 1 i ( ν i Δ H f , Δ T , i o ) reagent

   (2) Δ S Δ T o = ∑ 1 i ( ν i S Δ T , i o ) product − ∑ 1 i ( ν i S Δ T , i o ) reagent

   (3) Δ G T = Δ H T – T · Δ S

   where T is the temperature and ν i are stoichiometric coefficients.

2. To study the change in cottrel dust’s mass depending on the time and temperature, a thermogravimetric analysis (TGA) of the dust was carried out using a TGA/DSC 1HT/319 analyzer.

3. A JEOL JSM-6490 LV electron scanning microscope (Japan) was utilized to determine the element-weight composition of the sample under study by obtaining micrographs of various materials of inorganic nature and energy dispersion analysis.

4. The infrared spectral analysis of cottrel dust was carried out using an IR Fourier spectrometer, Shimadzu IR Prestige-21, with a frustrated total internal reflection device, Miracle (Pike Technologies Kyoto, Japan). The IR Prestige-21 uses a bright ceramic light source, a high-sensitivity DLATGS detector, and high-throughput optical elements. Optimization of optical/electronics/signal systems minimizes noise and maximizes the S/N ratio (40,000:1 and better).

2.2 Experimental procedure

Determination of phosphates and phosphorus in mineral fertilizers and other wastes was carried out in accordance with the State Standard 20851.2-75 (ISO 5316-77, ISO 6598-85, ISO 7497-84). This standard applies to mineral fertilizers and industrial wastes with a mass fraction of P₂O₅ from 3 to 55% and establishes methods for determining the content of total, assimilable, and water-soluble phosphates with their preliminary extraction from fertilizers, as well as free acidity.

1. Extraction of total phosphates using a mixture of acids. The method was based on dissolving a sample in a mixture of nitric and hydrochloric acids at the boiling temperature. About 2 g of the investigated substance was weighed accurate to the fourth decimal place and placed in a 250 cm³ glass; then 15 cm³ of nitric acid solution and 5 cm³ of hydrochloric acid solution were added. The mixture was heated to its boiling point and boiled in the glass covered with a watch glass until the sample completely dissolved; then water was added to a volume of 50 cm³, and the mixture was boiled for 5 min. The solution was transferred quantitatively into a 250 cm³ volumetric flask, cooled, diluted with water to the mark, mixed thoroughly, and filtered through a dry filter into a dry flask; the first portion of the filtrate was removed.

2. Extraction of total phosphates with citric acid. About 2 g of the investigated substance was weighed accurate to the fourth decimal place and placed in a Shtokhman flask (or volumetric flask) with a capacity of 250 cm³. Then, 200 cm³ of citric acid solution was added into the flask, and the mixture was mixed immediately to avoid the formation of lumps. The flask was stoppered, placed in a rotary apparatus, and mixed for 30 min. After that time, the mixture was diluted with a citric acid solution, the volume was made up to the mark, mixed, and filtered through a dry filter into a dry container, removing the first 30–50 cm³ of the filtrate. Water was used instead of citric acid when bringing the volume to the mark.

3. Extraction of assimilable phosphates with a Trilon B solution. About 1 g of the investigated substance was weighed accurate to the fourth decimal place and placed in a conical or volumetric flask through a dry funnel, washing off the sample with a Trilon B solution preheated in a glass to a temperature of 93 ± 3°C, and the mixture was shaken using a mechanical shaker or mixed by means of a magnetic stirrer.

4. Determination of mobile phosphorus compounds in soil. The test was carried out in accordance with the State Standard 26207-91. Determination of mobile phosphorus and potassium compounds was implemented using the method of Kirsanov modified according to the CINAO/26207-91. This Standard establishes a method for determining mobile phosphorus and potassium compounds in podzolic, soddy-podzolic, gray forest, and other types of soils. The method was based on the extraction of mobile phosphorus (P₂O₅) and potassium (K₂O) compounds from the soil with a hydrochloric acid solution (extracting solution) with a molar concentration of 0.2 mol·dm⁻¹ and subsequent quantitative determination of mobile phosphorus compounds using a photoelectrocolorimeter and potassium ones using a flame photometer.

5. Determination of phosphorus-containing substances in water. The test was carried out in accordance with the State Standard 18309-2014. The method used was based on the hydrolysis of polyphosphates into orthophosphates, with the formation of a phosphorus–molybdenum complex colored blue, and subsequent photometric analysis of the resulting colored compound at a wavelength of 690–720 nm. The content of orthophosphates initially present in the sample was preliminarily determined; their content should be subtracted from the result obtained when determining polyphosphates. This standard applies to drinking (including packaged in containers), natural (ground and surface), and waste water and establishes the following photometric methods for the determination of phosphorus compounds.

2.3 Mechanism of formation of cottrel dust

The reaction gas leaving a phosphorus furnace contains up to 200 g·cm⁻³ of solid particles in the form of dust. Processes associated with dust entrainment take place in various zones of the phosphorus furnace. These processes include the following:

1. Mechanical entrainment of dust by a gas flow;

2. Evaporation of low-boiling components of charge, the condensation of which leads to the formation of dust;

3. Chemical processes in the gas phase leading to the formation of solid particles;

4. Filtration of gases through a charge layer and condensation of sublimates on it. The latter process results in a reduction in dust content. The dispersed composition of dust in furnace gases is shown in Figure 2 and that with mechanical entrainment is given in Table 1.

Figure 2

Dispersed composition of dust in furnace gases.

It follows from Table 1 that 90% of the dust particles have a size of at least 20 µm, and therefore they cannot be the result of mechanical entrainment. The small proportion (10%) of mechanical entrainment is explained by the following reasons: first, the gas velocity in the furnace above the charge layer (under the furnace roof) is small, and for furnaces of this type it is about 0.035 m·s⁻¹; therefore, this part of the furnace is actually a settling chamber. Second, the first zone of the furnace, the height of which is about 2.5 m, has a porosity of 36–39%, and it is a filter zone for the reaction gas. Calculations have shown that the capture coefficient for particles up to 6 µm is insignificant (about 0.1), but particles larger than 10 µm are captured with high efficiency in the temperature range of 327–673°C, that is, over the entire height of the zone [25,26,27].

2.4 Thermodynamic patterns of the formation of cottrel dust

The sequence of processes and the contribution to the formation of cottrel dust are presented in Figure 3.

Figure 3

Sequence of chemical processes and reactions during the formation of cottrel dust.

Experiments were carried out to determine the ratio of initial components and thermodynamic parameters using the Chemistry HSC-6 software package in the temperature interval of 0–2,000°C with an increase in temperature for every 200°C during the formation of cottrel dust which enters the chemical reaction 1 between silicon(iv) oxide and carbon (Table 2) and reaction 2 – silicon(ii) oxide decomposition (Table 3).

Experiments were carried out to determine the ratio of the initial components and thermodynamic parameters in the range of 50–100°C with an increase in temperature for every 10°C during the formation of cottrel dust, which enters into chemical reactions 3 (between silicon(iv) oxide, potassium oxide, and calcium oxide), 4 (interaction of silicon fluoride with water), and 5 (interaction of phosphorus with water) (Tables 4–6).

3.1 Chemical and phase analyses of furnace gases

The chemical composition of dust formed in the furnace during the electrothermal smelting of phosphorites is shown in Table 7.

It follows from Table 7 that the dust is enriched in alkali metal oxides (especially K₂O), silicon(iv) oxide, and phosphates. For the charge’s acidity modulus of 0.85, the SiO₂/CaO ratio in the dust reaches 2.30. As we can see, the bulk of the dust is a consequence of the evaporation of sodium and potassium metaphosphates and silicon monoxide.

To study the change in cottrel dust’s mass depending on the time and temperature, TGA of the dust was carried out using a TGA/DSC 1HT/319 analyzer (Mettler Toledo). The results of the analysis are shown in Figure 4.

Figure 4

TGA of cottrel dust.

Judging from Figure 4, the mass of cottrel dust depends on time and temperature. This is evidenced by the changes in its mass at different temperatures: (1) 99.84°C–100.276 mg, (2) 220.18°C–100.063 mg, (3) 389.58°C–99.748 mg, (4) 544.70°C–99.204 mg, (5) 700.35°C –97.738 mg, and (6) 989.21°C–96.336 mg. According to the results of TGA, it was found that cottrel dust’s mass loss in the temperature interval of 99.84–989.21°C within 70 min is 4.144 mg.

In conclusion, we present the phase composition of dust in the furnace gas in Table 8.

3.2 Results of thermodynamic study of cottrel dust

It follows from Table 9 that with an increase in temperature from 0 to 2,000°C, the Gibbs energy for reaction (1) decreases from 602.5 to −77.5 kJ·mol⁻¹ and becomes negative, and the possibility of this chemical reaction is explained by the negative value of Gibbs energy. The given chemical reaction is possible at a temperature of 1,800°C, when the Gibbs energy value is –13.7 kJ·mol⁻¹. For reaction (2), the Gibbs energy at temperatures from 0 to 2,000°C increases from –614.0 to 36.43 kJ·mol⁻¹. The critical temperature for both reactions is 1,800°C.

Data of Table 10 show that at a temperature of 50–100°C, the Gibbs energy for reaction 3 decreases from 196.6 to 132.1 kJ·mol⁻¹; thus, this reaction does not occur. For reaction 4, the Gibbs energy at a temperature of 50–100°C decreases from −233.0 to −346.4 kJ·mol⁻¹, and for reaction 5 the Gibbs energy in the same temperature interval decreases from −402.4 to −439.2 kJ·mol⁻¹.

3.3 IR spectrum and elemental analysis of cottrel dust

To determine the functional groups in cottrel dust, spectral studies were carried out using an IR spectrometer (Shimadzu IR Prestige-21). The results of studies are shown in Table 11 and Figure 5.

Figure 5

IR spectrum of cottrel dust.

Figure 5 shows the IR spectrum of cottrel dust, from which it follows that:

1. The absorption peak at the wavelength of 1,620.21 cm⁻¹ characterizes the presence of silicate Si–O–Si and Si–O–C groups in cottrel dust;

2. The absorption peak at the wavelength of 1,442.75 cm⁻¹ characterizes the presence of a silicate Si–O–Na with carbon valence bonds;

3. The absorption peak at the wavelength of 1,288.45 cm⁻¹ is characteristic of phosphorus-containing compounds with silicon functional groups (Si₂P₂O₇);

4. The absorption peak at the wavelength in the range of 1,222.87 cm⁻¹ is characteristic of phosphorus and potassium-containing compounds (KH₂PO₄);

5. The absorption peak at the wavelength in the range of 1,095.57 cm⁻¹ is characteristic of phosphorus compounds containing sodium (NaH₂PO₄);

6. The absorption peak at the wavelength of 1,053.13 cm⁻¹ shows the presence of phosphorus compounds containing calcium (CaHPO₄) and magnesium (MgHPO₄);

7. Absorption peak at the wavelength of 968.27 cm⁻¹ is characteristic of phosphorus compounds containing potassium and calcium (KCaPO₄) and fluorine (P-F);

8. Absorption peak at the wavelength of 864.11 cm⁻¹ characterizes the presence of phosphorus compounds containing sulfur (P═S);

9. Absorption peaks at wavelengths in the range of 748.22–478.35 cm⁻¹ are characteristic of compounds containing Al³⁺, Fe³⁺, Mg²⁺, and Fe²⁺.

The results of IR spectral studies confirm the presence of functional groups in cottrel dust’s components. Cottrel dust’s elemental and phase composition, determined using modern instrumental and analytical methods, confirms the correctness of decoding its IR spectra. Based on the research carried out, it can be concluded that cottrel dust is a phosphorus-containing waste and is suitable for producing monocalcium phosphate, which can be used in the production of phosphorus-containing fertilizers.

The elemental composition and micrograph of coal waste (Table 12 and Figure 6) were determined using scanning microscopy (JSM-6490lV, JEOL, Tokyo, Japan). From Figure 4, it follows that the microstructure of potassium humate has mainly an amorphous structure with the addition of potassium, since potassium combines with the functional groups of humic acids. The scale of this microstructure is 40 times the increase from the actual state (600 µm; spectral range: 0–12 keV).

Figure 6

Micrograph of cottrel dust.

It follows from Figure 6 that the surface of cottrel dust is characterized by imperfect cohesion of minerals with the formation of cracks and chips. The main mineralogical phases of the sample are silicophosphates and silicofluorophosphates, which have colorless lamellar crystal structures. Small inclusions of potassium phosphates are characterized by soldered crystals of small elongated irregularly shaped plates. An accumulation of fine-grained chain structures of calcium silicate crystals is observed around phosphate minerals. Intermediate spaces and cracks are filled with the carbonaceous phase [28,29].

When studying the chemical composition of cottrel dust, it was determined that the main part of dust is a consequence of the evaporation of sodium, potassium, calcium metaphosphates, and silicon monoxide. This is evidenced by the elemental and phase compositions of cottrel dust. When yellow phosphorus is produced by the electrothermal method, the dust is formed periodically, although the chemical composition of the starting materials does not change much. For this reason, electrostatic precipitators are purified at the end of each production cycle to remove dust. Regardless of the furnace conditions and the chemical composition of the initial material, the volume of cottrel dust formed varies from 150 to 160 kg per tonne of the resulting yellow phosphorus.

According to the results (Table 12) of instrumental studies, it was found that the chemical composition of cottrel dust contains metals such as Na, Mg, Al, K, Ca, Fe, and Zn. These metals are light and in small amounts. Moreover, the necessary element phosphorus (P) is 13.4%. This phosphorus content is enough to process this waste and then obtain mineral fertilizers.

In addition to elemental analysis, chemical analysis for total and assimilable forms of phosphorus in cottrel dust was carried out. Chemical analysis was performed using 0.2 M Trilon B buffer solutions and 2% citric acid. On the basis of chemical analysis in the composition of cottrel dust: P₂O_5(total) – 30,7%, P₂O_{5(assimilable)} – 17,2%, and light alkali and alkali-earth metals [1,3,7].

3.4 Recycling of cottrel dust

Recycling of cottrel dust using a 10% concentration of sulfuric acid and a temperature of 90°C yields a phosphorus concentration of 11.10%; this phosphorus content is sufficient for its use as a phosphorus-containing component. The filtration properties and moisture content of the sediment were determined by standard methods. The sediment and filtrate were analyzed for the content of P₂O₅ water and P₂O₅ assimilable by double titration. On the finished product, the content of total and assimilable phosphorus oxide was calculated taking into account that transferred to the filtrate and sediment [7,28,30].

Scanning microscopy (JSM-6490lV, JEOL, Tokyo, Japan) revealed the chemical composition of samples of a phosphorus-containing compound, monocalcium phosphate, extracted from cottrel dust from the wastes of the New-Jambul phosphorus plant. The results of the study are given in Table 13.

According to the results of elemental analysis of monocalcium phosphate (Table 13), it was determined that at a sulfuric acid solvent concentration of 10% and a temperature of 90°C, the phosphorus oxide yield is P₂O₅(total) – 25.42% and P₂O₅ (assimilable) – 22.43%. Such a content of phosphorus obtained from waste can be used in medium fertile soils as a phosphorus-containing fertilizer [7,31].

In addition to phosphorus, monocalcium phosphate contains metals such as calcium, magnesium, iron, and aluminum. These metals do not affect the function of phosphorus. Some of the metals remained undissolved in the precipitate. The solubility of phosphorus in monocalcium phosphate was determined to be 96% in Trilon B buffer solutions and 95–96 in 2% citric acid.

The optimal parameters of a phosphorus-containing compound from cottrel dust were determined, and the process of crystallization of monocalcium phosphate was studied depending on the duration of the process and the yield of P₂O₅ with the product [7,32,33] (Figure 7).

Figure 7

Microscopic image of monocalcium phosphate.