Introduction to Pyrolysis Technology for the Resource Utilization of Waste Phosphogypsum

After years of dedicated research and development, Henan Jufeng Technology Co., Ltd. has successfully developed a resource‑utilization and harmless treatment technology for waste phosphogypsum through thermal pyrolysis. This technology targets the byproduct of the phosphate industry—waste phosphogypsum—and employs an oxygen‑free pyrolysis process. By precisely controlling temperature (200–750°C) and the reaction environment, it achieves efficient conversion and resource recovery of solid waste. Taking waste phosphogypsum—primarily composed of CaSO₄·2H₂O—as the feedstock, the process involves multiple sequential steps, including closed‑feed input, low‑temperature drying, medium‑temperature pyrolysis, high‑temperature cracking, and exhaust gas treatment. Under oxygen‑free conditions, crystalline water and organic impurities are decomposed, while valuable components such as calcium oxide are recovered. Pyrolysis gas is co‑fired with natural gas to provide heat, and the resulting waste heat is recycled and reused. Coupled with a dual‑alkali exhaust gas treatment system, this process effectively removes harmful gases like HF and SO₂, produces no dioxins throughout the entire operation, and operates under sealed, negative‑pressure conditions, equipped with multiple explosion‑proof measures and a nitrogen‑purging system. The pyrolysis technology boasts low costs—only 65–90 yuan per ton—and can handle waste phosphogypsum with varying moisture contents and organic matter levels, with a processing capacity ranging from 6 to 14 tons per hour. The pyrolysis products can be further processed into high‑value-added items such as eco‑friendly packaging boxes and industrial raw materials, offering significant advantages in environmental protection, economic viability, and safety. This technology represents a critical solution for solid waste management in the phosphate chemical industry.

Process Flow of Waste Phosphogypsum Resource Utilization Pyrolysis Technology

1. The production workshop is a dust-free facility, and the material buffer room features a closed design. Automated overhead cranes are used within the workshop to evenly transport materials to the automatic feeding system. The dust generated during the feeding process is purified and then introduced into the natural gas combustion system via air to replenish oxygen.

2. The automatically metered waste phosphogypsum material is conveyed via a closed system to the oxygen‑free extrusion feeding system.

3. Oxygen‑closed extrusion feeding uses a conical screw to convey the material downward in an oxygen‑closed manner into the anaerobic drying and volatilization system.

4. Waste phosphogypsum is fed into a low-temperature drying and volatilization system. This equipment utilizes the waste heat from combustion exhaust gases as its heat source to dry the waste phosphogypsum at a low temperature of approximately 200°C, thereby volatilizing the moisture and organic compounds contained within. If the oxygen content inside the equipment exceeds 3%, nitrogen is introduced to dilute the air and reduce the oxygen level, preventing the occurrence of combustion or explosion accidents.

5. The anhydrous dried phosphogypsum feed is introduced into a medium‑temperature volatilization pyrolysis system, where the heating is provided by utilizing the waste heat from combustion exhaust gases to preheat the material, thereby causing the phosphogypsum to undergo pyrolytic volatilization and decomposition at around 350°C.

6. The pyrolyzed phosphogypsum is reintroduced into a high‑temperature, oxygen‑free thermal cracking system, where, under high‑temperature conditions ranging from 400–750°C, organic matter and readily decomposable substances in the phosphogypsum are thoroughly subjected to oxygen‑free cracking and gasification. Both the drying and volatilization off-gases from the initial drying stage and the cracking off-gases are passed through high‑temperature filtration; the filtered high‑temperature off‑gases are then co‑fired with natural gas, and the waste heat from combustion is utilized to supply heat to this production line, gradually raising the temperature of the phosphogypsum from low to high, thereby achieving complete volatilization and cracking.

7. The pyrolyzed material is conveyed via oxygen‑closed discharge, using an insulated double‑layer screw conveyor. The outer wall of the screw is equipped with a heat exchange system that preheats the residual heat from the high‑temperature pyrolysis material while simultaneously preheating the make‑up air and natural gas, thereby enabling the reuse of waste heat and achieving the goals of energy conservation, emissions reduction, and decreased natural gas consumption.

8. The cracked phosphogypsum, after being conveyed via oxygen‑closed heat exchange, enters the paddle‑blade cooling system, where cooling water reduces its temperature to below 60°C. The cooled phosphogypsum is then transported to the enclosed buffer bin system using a conveyor elevator. A dust collection system is installed during the lifting and feeding process to ensure zero dust pollution.

9. The flue gas generated from combustion heating is treated using the double-alkali method: First, the flue gas is sprayed and washed with a sodium hydroxide alkaline solution, dissolving and absorbing the harmful components into the alkaline liquid. After the flue gas is treated to meet emission standards, it is discharged, while the wastewater is subjected to the addition of calcium hydroxide, causing the harmful components to undergo chemical reactions and form calcium salt precipitates.

Schematic Diagram of Resource-Based Pyrolysis Treatment Equipment for Waste Phosphogypsum

Components for the Treatment of Tail Gas from Drying and Pyrolysis of Waste Phosphogypsum

When waste phosphogypsum (main component: CaSO₄·2H₂O) undergoes pyrolysis under anaerobic conditions at 500°C, its pyrolytic reactions are more pronounced than at 420°C, with both dehydration and reductive decomposition occurring simultaneously. Due to the absence of oxygen, no oxidation reactions take place; the primary process involves the decomposition of calcium sulfate (CaSO₄), which may be accompanied by side reactions involving impurities.

During the oxygen‑free pyrolysis of waste phosphogypsum at 500°C, the off-gas primarily consists of CO₂, CO, and H₂O, and also contains trace amounts of SO₂, H₂S, HF, VOCs, as well as flammable gases (H₂/CH₄).

The specific reactions are as follows:

1. Dehydration Reaction: Waste phosphogypsum first loses its crystal water, forming anhydrous gypsum (CaSO₄). CaSO₄·2H₂O→CaSO₄+2H₂O↑(1)

2. Reduction and Decomposition Reactions: Under anaerobic conditions, CaSO₄ is reduced to produce CaO, SO₂, CO, and CO₂, among others. CaSO₄ + C → CaO + SO₂↑ + CO₂↑ (2) CaSO₄ + 2C → CaS + 2CO₂↑ (3) CaSO₄ + 4C → CaS + 4CO↑ (4)

3. Impurity Reactions: Impurities in waste phosphogypsum—such as fluorides and organic compounds—may also undergo reactions during thermal decomposition, generating HF, H₂S, VOCs, and other byproducts. CaF₂ + SiO₂ → CaSiO₂ + 2HF↑ (5) Organic phosphorus + heat → H₂S + VOCs + other products (6)

Waste Phosphogypsum Drying and Pyrolysis Off-Gas Treatment Reaction

Principle: The exhaust gases generated during the drying and thermal pyrolysis of waste phosphogypsum do not need to be treated using costly RTO regenerative thermal oxidation systems. After being processed through a comprehensive combustion and heat‑supply system, the combustion rate of combustible gases in the exhaust can be increased to over 99%. The exhaust gas, once heated by combustion, is then cooled in a heat exchanger and passed through two stages of alkaline liquid spraying—using sodium hydroxide and calcium hydroxide as the alkaline solutions. Sodium hydroxide is used solely as an intermediate medium that is recycled, while calcium hydroxide reacts with fluorine and sulfur to form salts, which are subsequently removed via precipitation and filtration.

The reaction equation is as follows:

Exhaust Gas Combustion Heating System: 2CO + O₂ → 2CO₂↑, 2H₂S + 3O₂ → 2SO₂↑ + 2H₂O, 2H₂ + O₂ → 2H₂O, CH₄ + 2O₂ → CO₂↑ + 2H₂O

Combustion of Volatile Organic Compounds (VOCs): CnHm + (n + m/4)O₂ → nCO₂ + 2mH₂O

Wastewater Treatment for Flue Gas Using the Double-Alkali Method

The HF gas produced by the oxygen‑free thermal cracking of waste phosphogypsum cannot be burned and must be absorbed via alkaline spray at the backend.

1. Defluorination reaction: HF + NaOH → NaF (sodium fluoride) + 2H₂O

Ca(OH)₂ (calcium hydroxide) + NaF (sodium fluoride) → CaF₂ (calcium fluoride) + 2NaOH

2HF + Ca(OH)₂ (calcium hydroxide) → CaF₂ (calcium fluoride) + 2H₂O

2. Desulfurization Reaction: SO₂ + 2NaOH → Na₂SO₃ + H₂O

Na₂SO₃ + Ca(OH)₂ → CaSO₃ + 2NaOH

3. Avoid directly spraying calcium hydroxide Ca(OH)₂ liquid, as it can easily cause scaling and clogging in the spray tower and equipment.

4. No Dioxins: The four basic conditions for dioxin formation are the presence of chlorine, oxygen, metal compound catalysts, and temperatures ranging from 260 to 860°C. Since none of the exhaust gases contain chlorine, and neither chlorine nor metal catalysts are present during the combustion process, dioxins will not be generated.

5. During the oxygen‑free thermal cracking of waste phosphogypsum at 500°C, the main components of the offgas are CO₂, CO, and H₂O, along with trace amounts of SO₂, H₂S, HF, VOCs, and flammable gases (H₂/CH₄). Consequently, the offgas does not contain furan‑type gases, and the combustion temperature is approximately 750°C, resulting in virtually no nitrogen oxide emissions (if nitrogen oxide removal is required, urea can be sprayed).

Various types of exhaust gases do not require RTO combustion exhaust gas treatment equipment; instead, the volatile pyrolysis exhaust gases are combined with natural gas and subjected to comprehensive combustion for use as heat source in the drying and pyrolysis system, thereby enabling the recycling of tail gas generated after oxygen‑free cracking of waste phosphogypsum. After combustion and utilization, the tail gas undergoes rapid cooling, multi‑stage spraying, and gas–mist separation to effectively absorb harmful gases from the exhaust stream; subsequently, the gas—after adsorption treatment—is discharged through an induced draft fan and chimney in compliance with emission standards.