Phosphogypsum Processing into Innovative Products of High Added Value

Abstract

The paper presents a comprehensive study of the processing possibilities for phosphogypsum, a large-tonnage chemical industry waste, into highly sought-after products, such as ultraviolet pigments, and alkalizing reagents for the preparation of organomineral fertilizers. The materials obtained were characterized by X-ray diffraction (XRD), transmission electron microscopy, and thermogravimetric analysis (TGA). It was found that the phosphogypsum thermal treatment process in the presence of a reducing agent (charcoal, sunflower husk) allowed us to obtain new products with a high added value. For the first time, the possibility of obtaining various products by varying process conditions was established. The process of thermal reduction of phosphogypsum in the presence of charcoal at temperatures of 800–900 °C and an isothermal holding time of 60 min resulted in us obtaining samples capable of glowing when irradiated with ultraviolet light. This effect is due to the formation of a composite material based on calcium sulfide and calcium sulfate in the system. The process of the regenerative heat treatment of phosphogypsum at temperatures of 1000–1200 °C resulted in us obtaining a composite material consisting of calcium oxide and sulfate, which can be used for fractionating liquid waste from livestock farming and to obtain organomineral fertilizer. The technological methods developed allow the usage of chemical industrial waste and agricultural waste in secondary processing to produce highly innovative products that will contribute to the achievement of the sustainable development goals, in particular, “Ensuring rational consumption and production patterns”.

1. Introduction

Current production technologies lead to the formation of large quantities of man-made waste [1,2]. Valuable target product production is often accompanied by the formation of by-products. They are stored in dumps and await processing opportunities. The idea of using chemical industry waste in secondary processing is relevant, widely sought after, and corresponds to the sustainable development goals [3,4].

Phosphogypsum is one of the large-tonnage chemical wastes [5]. It is formed during the apatite ores’ processing, when they are treated with sulfuric acid to produce phosphoric acid. Currently, huge quantities of phosphogypsum have accumulated in the world, creating dumps that resemble white mountains [6,7]. Phosphogypsum dumps occupy valuable agricultural lands and have a negative ecological impact on the environment. Phosphogypsum processing is an urgent issue, the solution to which is being developed by many scientific groups [8,9]. Most often, its use as a building material for road restoration is proposed. However, phosphogypsum is a valuable resource for obtaining several in-demand elements and compounds [7].

Previously, we reported the possibility of processing phosphogypsum into a luminescent material by thermal treatment in the presence of reducing agents, such as sucrose [10] and citric acid [11]. The use of sulfide phosphors in modern society has expanded and includes well-known applications, such as fluorescent lamps, color conversion, constant luminescence, mechanic luminescence, and information storage [12,13,14]. Recently, even LED applications have begun to use sulfide phosphors [15,16,17].

Livestock farming plays a leading role in the agricultural–industrial complex and occupies vast areas [18]. In the process of keeping large farm animals, huge quantities of organic waste are formed, particularly, highly concentrated effluents [19]. The liquid manure produced by large farms and industrial complexes when animals are fed with significant amounts of concentrated feed is distinguished by an increased content of plant nutrients [20] (

Table 1. Chemical composition of pig manure.

Element	Quantity, % (Mass)	Element	Quantity, % (Mass)
Water	72.03	Magnesia (MgO)	0.09
Organic matter	24.74	Silicic acid (SiO2)	1.07
Nitrogen (N) general	0.44	Chlorine	0.17
Ammonia nitrogen	0.19	Sulfuric acid (SO3)	0.08
Phosphorus (P2O5)	0.18	Oxides of A1 and Fe	0.07
Potassium (K2O)	0.58	Magnesia (MgO)	0.18
Lime (CaO)	0.18	Total	100.00

). The manure output from one pig is 55 kg per day [21].

The main geo-ecological problem with the manure produced is its volume: currently, huge areas of lagoons are withdrawn from agricultural use [19]. Manure has a negative impact on the environment, as it causes emissions of ammonia, nitrous oxide, nitrates, and phosphates both during storage and spreading [22]. Pig farming poses a threat due to toxic waste particles, pathogens, bacteria, and heavy metals, which can be dangerous if ingested [23]. The main environmental problems associated with pig farming concern water and air pollution [22].

Phosphogypsum has traditionally been used as a calcium supplement in saline marshy soils in southwestern Spain [24]. The original recommended rates were 25 t per hectare, spread every two years. Current methods involve applying phosphogypsum (after sun drying) to pre-treated soils with additional deep tillage immediately after application, resulting in the erosion of the soil horizon to a depth of 40 cm. In recent years, phosphogypsum has shown promising potential for agricultural applications [25]. It acts as a fertilizer by regulating soil pH, increasing soil permeability, improving soil structure, and reducing soil erosion. It also provides the soil with essential nutrients, such as calcium, sulfur, phosphorus, and micronutrients, promoting crop growth and increasing yields. In addition, phosphogypsum prevents the loss of N and P nutrients by forming ammonium sulfate and calcium phosphate, thereby protecting soil fertility. Only about 30–50% of the nitrogen applied to the soil is effectively taken up by plants [26], which leads to reduced nutrient use efficiency and aggravates agricultural pollution due to nutrient leaching and volatilization [27,28,29].

To process pig manure to obtain organomineral fertilizer, it is necessary to fraction liquid waste from pig farms with an alkalizing reagent [19,30,31,32], which is made up of alkali metal hydroxides [31] and lime [19]. In some cases, electrochemical methods [31] are used to increase the pH value of the suspension. Generally, the methods described are difficult to implement and require the use of chemically pure substances.

For the current research, a large-scale study was conducted to investigate the possibility of phosphogypsum processing using widely available reducing agents. Charcoal and sunflower husks, the latter being agricultural waste, were used to produce innovative products with a high added value, i.e., ultraviolet pigments based on calcium sulfide and an alkalizing reagent containing calcium oxide that is indispensable in the process of obtaining organomineral fertilizer.

2. Materials and Methods

2.1. Materials Characterization

To obtain calcium sulfide, phosphogypsum for agriculture with a calcium sulfate dihydrate CaSO₄∙2H₂O content of 99% (mass.) was used. Charcoal and sunflower husks were used as reducing agents.

2.2. Research Methods

To characterize the obtained composite materials, various methods were used, including X-ray diffraction (XRD), transmission electron microscopy, and thermogravimetric analysis (TGA).

The phase composition was studied on an ARL X’TRA X-ray diffractometer (Thermo Fisher Scientific (Ecublens) SARL, Ecublens, Switzerland) (monochromatic Cu-Kα radiation was used) by scanning points (step 0.01°, accumulation time at a point 2 s) in the range of 2θ values from 5° to 90°.

A Quanta 200 scanning microscope (FEI Company, Hillsboro, OR, USA) was used to obtain microphotographs.

The change in m_s, sample mass, after heat treatment was determined using the following formula:

m_s = (m₁ − m₂) · 100/m₁,

where m₁ is the sample mass after heat treatment, g; m₂ is the practical value of the sample mass, g.

Luminescence spectra were recorded using a CM 2203 spectrofluorometer (Solar, Minsk, Belarus). The sample was pumped with an EPL-375 picosecond pulsed diode laser (Edinburgh Instruments, Kirkton Campus, Livingston, UK) centered at 375 nm.

A thermogravimetric analysis was performed on an STA 449 F5 Jupiter synchronous thermal analysis device (NETZSCH-Gerätebau GmbH, Selb, Germany).

2.3. Synthesis

To study the effects of reducing agent quantities and heat treatment temperatures on the phosphogypsum processing output, a series of experiments were conducted. The quantities of the reducing agent were changed according to the ratios specified in

Table 2. Thermodynamic assessment of some possible reactions.

Equation Number	Reaction	ΔH, kJ	ΔS J/K	ΔG, kJ	t, °C
(1)	CaSO4·2H2O = CaSO4·0.5H2O + 1.5H2O	+83.5	+219.4	+18.1	108
(2)	CaSO4·0.5H2O = CaSO4 + 0.5H2O	+20.6	+70.5	−0.4	19
(3)	CaSO4 = CaO + SO3	+404.7	+188.1	+349.3	>1350
(4)	CaSO4 + 2C = CaS + 2CO2	−172.6	+14.5	−177	-
(5)	CaSO4 + 4C = CaS + 4CO	+517.2	+734.4	+298.4	431
(6)	CaSO4 + 4CO = CaS + 4CO2	−178.4	+14.6	−182.8	-
(7)	C + CO2 = 2CO	+172.5	+175.6	+120.2	709
(8)	2C + O2 = 2CO	−221	+178.6	−274.2	-
(9)	CaSO4 + C = CaO + SO2 + CO	+393.4	+409.3	+273.5	688
(10)	CaSO4 + CO = CaO + SO2 + CO2	+220.9	+233.7	+151.3	672
(11)	3CaSO4 + CaS = 4SO2 + 4CaO	−1056.3	+920.0	−1330.5	-
(12)	CaS + 2O2 = CaSO4	−953.6	359.8	−846.4	-

. The mass of phosphogypsum in the samples was fixed and amounted to 17.2 g. The specified amounts of phosphogypsum and the reducing agent were weighed, homogenized for 10 s in a mixer with an intensity of 1400 rpm, and placed in alundum crucibles in the working space of a muffle furnace. The synthesis was carried out according to the following modes: the samples were heated at a rate of 13 degrees per minute to the treatment temperatures, which were 700, 800, 900, 1000, 1100, and 1200 °C. Upon reaching the specified temperature, the samples were held for 30–90 min.

3. Results and Discussion

3.1. General Characteristics of Phosphogypsum

Phosphogypsum is a light-grey powder (See Figure 1a); the grains have an oblong shape, which is characteristic of calcium sulfate (See Figure 1b).

Figure 2 shows an X-ray diffraction pattern of a sample of the original phosphogypsum. The X-ray diffraction pattern contains reflections belonging to calcium sulfate dihydrate CaSO₄^.2H₂O (PDF Number 010-70-7008) in monoclinic syngony.

When the process of phosphogypsum heat treatment at temperatures above 150 °C is carried out without introducing a reducing agent into the system, the separation of crystallization water according to reactions (1) and (2) (see Table 2: the theoretical values of the estimated temperature of the reaction onset are given) is observed. In this case, anhydrous calcium sulfate is formed (see Figure 3). The X-ray diffraction pattern contains reflections characteristic of CaSO₄ (PDF Number 010-74-2421) in the orthorhombic syngony.

This assumption is confirmed by the results of the thermogravimetric analysis (Figure 4). The DTA curve shows a double peak of the endothermic effect at 150–160 °C with a mass loss of about 20% (mass). There are two peaks, both endothermic, with corresponding mass losses, well identified by peaks on the dTG and dTA curves. The first peak at 147 °C is caused by the separation of a part of the water from the gypsum (reaction No. 1, indicated in Table 2), which usually occurs at a temperature of 120–150 °C. The second peak is associated with the separation of the remaining 1/2 of the water (reaction No. 2) and usually occurs at a temperature of about 163 °C. Further mass changes were not noted in the entire temperature range studied (up to 1100 °C). The obtained data indicate the absence of reaction (3) (Table 2) under these conditions.

Figure 5 shows the results of the thermogravimetric analysis for a pure reducing agent, charcoal.

A broad exothermic effect was observed with a peak at 528 °C with an almost complete loss of mass (93%) in the sample. Cracks associated with the partial destruction of the structure during the separation of crystallization water are formed on the surface of crystals heat-treated without a reducing agent (Figure 6).

3.2. Obtaining Luminescent Pigments

The samples heat-treated at temperatures of 800–1000 °C, when irradiated with ultraviolet light, emitted a yellow glow of varying intensity. The results of measuring the luminous flux are given in

Table 3. Heat treatment results for phosphogypsum mixed with charcoal at different temperatures.

Mass of Reducing Agent, g	Share of Reducing Agent from Stoichiometric Value, %	Relative Luminous Flux at Heat Treatment Temperature, °C
		800	900	1000
0.15	6.3	0.1	0.1	0.1
0.3	12.5	0.12	0.12	0.1
0.45	18.8	0.13	0.13	0.15
0.6	25.0	0.15	0.14	0.25
0.9	37.5	0.18	0.56	0.42
1.2	50.0	0.2	0.88	0.61
1.5	62.5	0.15	0.77	0.57
1.8	75.0	0.11	0.73	0.55
2.4	100.0	0.1	0.68	0.48
3	125.0	0.09	0.59	0.3
3.6	150.0	0.06	0.25	0.16
4.8	200.0	0.05	0.05	0.06

.

Figure 7 shows the dependence of the relative luminosity of the samples on the amount of the introduced reducing agent (the fraction from the stoichiometric value, according to reaction (4), Table 2).

According to the results obtained, the best luminescent ability is exhibited by phosphogypsum samples heat-treated at a temperature of 900 °C in a mixture with charcoal at a molar fraction of the reducing agent of 50% (mass).

Figure 7 shows the X-ray diffraction patterns of the samples after heat treatment. The phosphogypsum sample heat-treated at a temperature of 900 °C without a reducing agent is anhydrous calcium sulfate (Figure 8a). The X-ray diffraction pattern contains reflections belonging to CaSO₄ (PDF Number 010-74-2421) in the orthorhombic modification.

The X-ray diffraction pattern of a phosphogypsum sample heat-treated at 900 °C in the presence of a reducing agent (50 mol.%) is shown in Figure 8b. The X-ray diffraction pattern contains reflections belonging to anhydrous calcium sulfate CaSO₄ (PDF Number 010-70-0909) in the orthorhombic modification and calcium sulfide CaS (PDF Number 000-08-0464) in the cubic modification. When the heat treatment process is conducted at a higher temperature with the same amount of reducing agent, calcium oxide forms in the sample; the X-ray diffraction pattern (Figure 8c) shows CaO (PDF Number 010-77-9574) in the cubic modification.

Replacing the reducing agent (as noted above, sugar [4] and citric acid [5] as reducing agents were previously investigated) with cheaper charcoal demonstrated the possibility of using it to obtain luminescent materials.

The synthesized luminescent materials were used as fillers to obtain paints. It was found that the synthesized pigment can be used to obtain paints and varnishes with both water and non-water bases. Figure 9 shows an example of an object decorated with the obtained paint material: the paint is white in daylight and glows yellow in the dark when illuminated with ultraviolet light.

Thus, the possibility of obtaining an ultraviolet pigment during the thermal treatment of phosphogypsum in the presence of a cheap reducing agent—charcoal—has been established.

3.3. A Study of the Organic Reducing Agent Type’s Effect on the Formation of Calcium Oxide from Phosphogypsum

The possibility of synthesizing an alkalizing reagent for subsequent use in livestock wastewater fractionating and organomineral fertilizer production processes was studied. Charcoal and sunflower husks from agricultural waste were used as reducing agents.

To study the effects of the amount of organic waste introduced, the duration, and the temperature of the heat treatment, the following process was carried out. Phosphogypsum and the reducing agent were weighed with an accuracy of 0.01 g, homogenized in a porcelain mortar for 20 min, and loaded into an alundum crucible. The heat treatment was carried out in a muffle furnace; the rate of the temperature increase in all the experiments was 13 degrees per minute. The heating was stopped after isothermal holding. The cooling of the samples was slow and was carried out with a furnace.

3.3.1. A Study of the Effect of Charcoal as the Reducing Agent

Table 4. The degree of the destruction of calcium sulfate depending on the heat treatment temperature, with charcoal as the reducing agent.

Phosphogypsum–Reducing Agent Ratio	Degree of Calcium Sulfate Destruction Δm, %, at Heat Treatment Temperature, °C
	700	800	900	1000	1200
3.4:1	−3.79	15.28	23.81	32.34	36.47
2.3:1	−6.15	−0.54	9.94	20.41	21.62
1.7:1	−7.84	−1.28	3.70	8.67	10.55

presents data on the study of the introduced reducing agent quantities and the heat treatment temperatures. In all the cases, the heat treatment duration was 60 min.

It was found that as the heat treatment temperature increases, the sample mass loss increases. This dependence was observed for all the studied ratios of phosphogypsum–reducing agent. Also, with an increase in the amount of reducing agent in the sample, regardless of the ratio of the initial substances, the sample mass loss decreased. This experimentally proven fact can be associated with an increase in the proportion of unreacted coal in the sample (particles of unreacted coal were observed in the samples with a high coal content, even at a heat treatment temperature of 1200 °C). The highest degree of calcium sulfate destruction (36.47% (mass)) was observed at the heat treatment temperature of 1200 °C, when the ratio of phosphogypsum–reducing agent was 3.4:1.

Table 5. The degree of calcium sulfate destruction depending on the heat treatment duration with birch charcoal as the reducing agent.

Phosphogypsum–Reducing Agent Ratio	Degree of Calcium Sulfate Destruction Δm, %, During Isothermal Holding Time, min
	30	60	90
3.4:1	33.70	32.34	34.96
2.3:1	21.69	20.41	23.24
1.7:1	9.34	8.67	13.18

presents data on the heat treatment duration effect on the degree of calcium sulfate destruction. In this case, the samples were prepared as described above, placed in a muffle furnace, and heat-treated with isothermal holding for 30, 60, and 90 min at the temperature of 1000 °C. The rate of temperature increase was 13 degrees per minute.

It was found that, regardless of the isothermal holding duration, the mass loss value decreased with the increasing amount of the introduced reducing agent. With increasing duration of the isothermal holding, the mass loss first decreased, then increased. This experimentally established fact can be associated with the preferential formation of sulfide first (according to reactions (4)–(8) (Table 2)), then calcium oxide (according to reactions (9)–(11) (Table 2)), with an increase in the heat treatment duration. With longer isothermal holding, the reoxidation process may occur (reaction (12) (Table 2)), accompanied by an increase in sample mass.

Thus, as a result of the study conducted on the reductive heat treatment of phosphogypsum in the presence of charcoal as a reducing agent, it was established that phosphogypsum can be processed into an oxide-containing material during heat treatment in the presence of charcoal as the reducing agent with a degree of calcium sulfate, the main component of phosphogypsum, destruction of up to 36.47% (mass).

3.3.2. A Study of the Effect of Sunflower Husk as the Reducing Agent

In order to study the possibility of using agricultural waste as a reducing agent, the process was tested in the presence of sunflower husk as a reducing agent.

Table 6. The sample mass loss depending on the temperature of the heat treatment, with sunflower husk as the reducing agent.

Phosphogypsum–Reducing Agent Ratio	Degree of Calcium Sulfate Destruction Δm, %, at Heat Treatment Temperature, °C
	700	800	1000	1100	1200
3.4:1	−0.23	14.50	20.36	23.28	37.36
2.3:1	−0.73	20.56	26.16	28.97	40.03
1.7:1	−1.07	25.89	30.81	33.26	39.32

presents data on the effects of the reducing agent amounts introduced and the heat treatment temperatures. In all cases, the duration of the heat treatment in this series of experiments was fixed at 60 min.

It was found that, unlike the results obtained for charcoal, the sample mass loss increased with the increasing heat treatment temperature for sunflower husk as a reducing agent. This dependence was observed for all the studied phosphogypsum–reducing agent ratios. Also, with an increase in the amount of reducing agent in the sample, regardless of the ratio of the initial substances, the sample mass loss increased. The exception is the result of the heat treatment at the temperature of 700 °C. In this case, no mass loss was observed, and particles of unreacted sunflower husk were visible in the samples. In the case of the heat treatment at the temperature of 1200 °C and a phosphogypsum–reducing agent ratio of 2.3:1, the mass decrease was abnormally high.

It can be assumed that the decrease in the calcium sulfate mass is associated with one of the reactions (4), and (9)–(11) (Table 2). The process described by Equation (4) (Table 2) is thermodynamically possible at room temperature; reaction (10) (Table 2) begins when the system is heated to the temperature of 670 °C. In this regard, the loss of mass by the sample can occur due to the removal of oxygen and/or sulfur from CaSO₄.

The X-ray diffraction pattern analysis shows that at a relatively low heat treatment temperature, calcium sulfate is converted into calcium sulfide by reaction (11) (Table 2). Increasing the heat treatment temperature leads to the possibility of reaction (10) (Table 2). In the X-ray diffraction pattern of the samples heat treated in the presence of a reducing agent at higher temperatures (1100–1200 °C), the reflections characterizing the CaS phase decrease in intensity, while the lines characteristic of CaO, on the contrary, increase. This may be due to reaction (11) (Table 2). In this case, the mass of calcium oxide is significantly less than the mass of calcium sulfide, and the observed tendency of sample mass decrease may be due to an increase in the amount of calcium oxide formed.

Figure 10 and Figure 11 show the schemes of possible processes.

Table 7. The degree of calcium sulfate destruction, depending on the duration of the heat treatment, with sunflower husk as the reducing agent.

Phosphogypsum–Reducing Agent Ratio	Degree of Calcium Sulfate Destruction Δm, %, During Isothermal Holding Time, min
	30	60	90
3.4:1	15.05	20.36	17.34
2.3:1	22.91	26.16	30.72
1.7:1	31.7	30.81	32.26

presents data on the study of the heat treatment duration effect on the formation of calcium oxide or sulfide. In this case, the samples were prepared as described above, placed in a muffle furnace, and heat-treated with isothermal holding for 30, 60, and 90 min at a temperature of 1000 °C. The rate of temperature increase was 13 degrees per minute.

It was found that, as a rule, for the studied samples, the amount of mass loss increased with the increased duration of the isothermal holding and the amount of the reducing agent introduced. The exception is the sample with the minimum amount of reducing agent (phosphogypsum–reducing agent ratio of 3.4: 1) at the maximum isothermal holding—in this case, an increase in the sample mass was observed. This may be due to the process of reoxidation of calcium sulfide into sulfate according to reaction (13) under the condition of an insufficient amount of reducing agent:

CaS + 2O₂ = CaSO₄.

(13)

In this case, the sample mass will increase.

Thus, as a result of the conducted study of the reductive heat treatment of phosphogypsum in the presence of sunflower husks, the following was established:

-

Phosphogypsum can be processed into a sulfide- or oxide-containing material during heat treatment in the presence of sunflower husks as a reducing agent with a degree of destruction of the phosphogypsum’s main substance of calcium sulfate—up to 40% (mass);

-

The degree of calcium sulfate conversion depends on the temperature, the duration of heat treatment, and the amount of the introduced reducing agent. In the case of an insufficient amount of reducing agent with a high duration of heat treatment, the process of the reoxidation of the reaction product back into calcium sulfate occurs;

-

Calcium sulfate decomposition occurs predominantly into sulfide at low heat treatment temperatures of 800–900 °C and into calcium oxide at heat treatment temperatures of 1000–1200 °C;

-

When the process of phosphogypsum heat treatment is carried out in the presence of a reducing agent, a product containing calcium oxide is obtained, which opens up wide possibilities for processing large-tonnage chemical industry waste into popular products.

There are at least two causes of the incomplete burnout of the charcoal introduced as a reducing agent. Firstly, unlike sunflower husk, charcoal has the maximum carbon–oxygen ratio (

Table 8. Ratio (C: O) in carbon-containing reducing agents.

Reducing Agent	Sunflower Husk	Charcoal
Ratio (C:O)	1.24	8.47

).

The second factor contributing to incomplete coal burning is the dimensional characteristics and strength of the reducing agents (Figure 12).

Sunflower husk has a fine lamellar structure, is fragile, and is easily crushed during homogenization. Charcoal forms fairly large dense pieces that are resistant to mechanical impacts. The heterogeneous process occurs on their surface, and as a result of it, a part of the reducing agent remains inaccessible for reaction.

It can be assumed that, in addition to the chemical composition, the morphological features of organic reagents affect the process of phosphogypsum reduction.

3.3.3. A Study of the Possibility of Using Heat-Treated Phosphogypsum in the Presence of Organic Reducing Agents to Obtain an Alkalizing Reagent

To carry out the process of liquid animal waste fractionation, an alkalizing reagent with the highest possible pH value of its aqueous solution is required. To select the most suitable reagent, the prepared samples were placed in a glass reaction vessel, filled with distilled water at room temperature, vigorously mixed, and allowed to settle. A 10% solution was prepared. Then the pH values were measured. As a control experiment, the pH of the unfired phosphogypsum solution was measured: it was 7.14. The experimental results are given in

Table 9. The pH values of a 10% reagent suspension for various heat treatment modes, with charcoal as the reducing agent.

Reaction Conditions	Composition	pH	Turbidness	Notes
700 °C, 1 h	3.4:1	7	Very turbid	Carbon residues
	2.3:1	8	Very turbid	Carbon residues
	1.7:1	8	Very turbid	Carbon residues
800 °C, 1 h	3.4:1	9	Turbid	Carbon residues
	2.3:1	9	Turbid	Carbon residues
	1.7:1	10	Turbid	Carbon residues
900 °C, 0.5 h	3.4:1	11	Transparent grey
	2.3:1	10	Transparent grey	Carbon residues
	1.7:1	10	Transparent grey	Carbon residues
900 °C, 1 h	3.4:1	11	Transparent grey
	2.3:1	10	Transparent grey	Carbon residues
	1.7:1	10	Transparent grey	Carbon residues
900 °C, 1.5 h	3.4:1	11	Transparent grey
	2.3:1	10	Transparent grey	Carbon residues
	1.7:1	10	Transparent grey	Carbon residues
1000 °C, 0.5 h	3.4:1	10	Transparent grey
	2.3:1	9	Transparent grey	Carbon residues
	1.7:1	9	Transparent grey	Carbon residues
1000 °C, 1 h	3.4:1	11	Transparent grey	Carbon residues
	2.3:1	10	Transparent grey	Carbon residues
	1.7:1	9	Transparent grey	Carbon residues
1000 °C, 1.5 h	3.4:1	9	Transparent
	2.3:1	9	Transparent grey	Carbon residues
	1.7:1	9	Transparent grey	Carbon residues
1200 °C, 1 h	3.4:1	10	Transparent
	2.3:1	10	Transparent
	1.7:1	10	Transparent	Carbon residues

and

Table 10. The pH values of a 10% reagent suspension for different heat treatment modes, with sunflower husk as the reducing agent.

Reaction Conditions	Composition	pH	Turbidness	Notes
700 °C, 1 h	3.4:1	11	Transparent grey	Husk residues
	2.3:1	10	Transparent grey	Husk residues
	1.7:1	10	Transparent grey	Husk residues
800 °C, 1 h	3.4:1	10	Transparent grey	Husk residues
	2.3:1	10	Transparent grey	Husk residues
	1.7:1	10	Transparent grey	Husk residues
900 °C, 0.5 h	3.4:1	9	Transparent grey
	2.3:1	9	Transparent grey
	1.7:1	9	Transparent grey
900 °C, 1 h	3.4:1	10	Transparent grey
	2.3:1	10	Transparent grey
	1.7:1	10	Transparent grey
900 °C, 1.5 h	3.4:1	10	Transparent grey
	2.3:1	10	Transparent grey
	1.7:1	10	Transparent grey
1000 °C, 0.5 h	3.4:1	12	Transparent grey
	2.3:1	11	Transparent grey
	1.7:1	11	Transparent grey
1000 °C, 1 h	3.4:1	11	Transparent grey
	2.3:1	11	Transparent grey
	1.7:1	11	Transparent grey
1000 °C, 1.5 h	3.4:1	10	Transparent grey
	2.3:1	10	Transparent grey
	1.7:1	10	Transparent grey
1100 °C, 1 h	3.4:1	11	Transparent
	2.3:1	12	Transparent
	1.7:1	12	Transparent
1200 °C, 1 h	3.4:1	12	Transparent
	2.3:1	12	Transparent
	1.7:1	12	Transparent

. They present the data on the effect of the type and the amount of the introduced organic reducing agents, the temperature, and the duration of the heat treatment on the pH value of the suspension.

According to the obtained results, for suspensions of all the samples heat-treated in the presence of a reducing agent, an increase in their pH values was noted compared to the solution of the original phosphogypsum.

Phosphogypsum heat-treated at a temperature of 700–900 °C in the presence of charcoal as a reducing agent forms a turbid suspension, in which residues of unreacted reducing agent are visible. The grey color of the suspension may be due to the presence of colloidal carbon particles obtained during the destruction of the reducing agent. The suspension stayed grey in color up to the heat treatment temperature of 1000 °C.

The use of sunflower husks as a reducing agent led to the formation of a grey suspension up to a heat treatment temperature of 1000 °C; in the same samples, the presence of reducing agent residues was noted.

Remarkably, an increase in the heat treatment temperature and the duration of the isothermal holding led to an increase in the pH values of the suspension. For the samples heat-treated at relatively high temperatures in the presence of a reducing agent (1000–1200 °C), as a rule, the highest pH values of the suspension were observed.

An important experimentally established fact is a decrease in the temperature of calcium sulfate to oxide decomposition during heat treatment in the presence of agricultural waste as a reducing agent. According to the study data [33], the thermal decomposition of phosphogypsum into calcium and sulfur (IV) oxides occurs in the region of 1670 °C. The experimental data we obtained indicate that the decomposition of calcium sulfate is possible at significantly lower heat treatment temperatures of 1100–1200 °C.

The experimental data indicate that the formation of calcium oxide occurs either at elevated temperatures or with a sufficiently long isothermal holding time.

For use as an alkalizing reagent, the most promising samples are those obtained at a heat treatment temperature of 1100 °C with the maximum pH value of the suspension.

The increased pH values of the solution may be associated with the formation of calcium hydroxide during the hydrolysis of CaO (reaction (14)) and the dissociation of Ca(OH)₂:

CaO + H₂O = Ca(OH)₂.

(14)

The most promising in terms of several indicators—the availability, price, ease of use in the technological process, high pH values of the suspension, and transparency of the suspension—are the phosphogypsum samples heat-treated in the presence of sunflower husks at the heat treatment temperature of 1100–1200 °C. Thus, the developed method allowed us to obtain an alkalizing reagent with improved characteristics in more economically beneficial modes.

4. Conclusions

The following conclusions were reached based on the comprehensive study of the possibilities of processing phosphogypsum, a large-tonnage chemical industry waste, into highly sought-after products.

• When the process of phosphogypsum reductive heat treatment is carried out in the presence of a reducing agent (charcoal, sunflower husk), it produces new products with a high added value. For the first time, the possibility of obtaining various products by varying process conditions was established.
• When the process of phosphogypsum reductive heat treatment is carried out in the presence of charcoal at temperatures of 800–900 °C and an isothermal holding time of 60 min, it produces samples capable of glowing when irradiated with ultraviolet light. This effect is due to the formation of a composite material based on calcium sulfide and sulfate CaSO₄/CaS in the system.
• The synthesized ultraviolet pigment can be included in the composition of water-based and non-water-based paints and varnishes and used to decorate various objects.
• When the process of phosphogypsum reductive heat treatment is carried out at temperatures of 1000–1200 °C, it leads to a decrease and even a loss of the luminesce ability of phosphogypsum.
• When the process of phosphogypsum reductive heat treatment is carried out at temperatures of 1000–1200 °C, it produces a composite material consisting of calcium oxide and sulfate CaSO₄/CaO, which can be used for the fractionation of liquid waste from livestock farming and to obtain organomineral fertilizer.
• To obtain a high-quality alkalizing reagent, it is necessary to carry out the process of phosphogypsum heat treatment at the temperature of 1100–1200 °C for 60–90 min. This allows the significant reduction of the temperature of the thermal decomposition of calcium sulfate (by 300–500 °C).
• The technological methods developed allow the usage of chemical industrial and agricultural waste in secondary processing to produce highly innovative products that will contribute to the achievement of the sustainable development goals, in particular, “Ensuring rational consumption and production patterns”.