Design of anti-corrosion coating for waste heat boilers in phosphorus chemical industry exhaust gas treatment

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The phosphate chemical industry includes the phosphate fertilizer industry, the yellow phosphorus and phosphides industry, the phosphoric acid and phosphates industry, the organic phosphides industry, as well as the industry of phosphorus-containing pesticides and pharmaceuticals, among others. In the global consumption structure of phosphate rock, about 80% is used in agriculture, while the remainder is used for the production of yellow phosphorus, phosphoric acid, and other phosphate-based products. Phosphorus chemical products are widely used in industry, national defense, advanced science, and people’s daily lives. As phosphorus-based chemical products continue to penetrate more industrial sectors, particularly in advanced science and emerging industries, phosphorus chemistry has become an important industry within the national economy. Phosphorus chemical products are playing an increasingly important role in various aspects of people’s lives, including clothing, food, housing, and transportation. The waste gases generated during the production of phosphorus chemicals mainly include corrosive gases such as carbon monoxide, sulfur dioxide, carbon dioxide, hydrogen fluoride, silicon tetrafluoride, phosphine, and hydrogen sulfide, as well as dust. Among these corrosive gases, hydrogen fluoride and hydrogen sulfide are extremely corrosive, especially hydrogen fluoride gas. In a project I recently took on, the hydrogen fluoride concentration in the exhaust gases from yellow phosphorus chemical processing was 100 mg/Nm3, while the H2S concentration was as high as 2000 mg/Nm3. The temperature of the flue gases on the high-temperature side was 600–700°C, whereas the temperature of the flue gases in the economizer on the low-temperature side was around 200°C. The corrosivity of these high-temperature, high-concentration flue gases is extremely severe; evaporators and economizers made of SUS316 material can only withstand use in such flue gases for 8 months. For such highly corrosive operating conditions, when designing anti-corrosion coatings, we take four types of corrosion factors into account: first, oxidation corrosion caused by high temperatures; second, sulfur corrosion induced by H2S; third, HF corrosion; and fourth, abrasive wear caused by dust. Different composite coating anti-corrosion solutions were designed based on varying temperature levels. For the boiler body and evaporators operating at high temperatures of 600–700°C, a ZS heat-resistant coating with a temperature tolerance of 1200°C is used as the primer to prevent oxygen corrosion; this coating is applied three times. On top of that, a ZS flue gas corrosion-resistant coating with a temperature tolerance of 750°C is used as the topcoat to protect against corrosion caused by corrosive substances and dust in the flue gas, with a designed film thickness of 0.5 mm. The composite anti-corrosion coating made from these two high-temperature resistant coatings exhibits a favorable synergistic anti-corrosion effect, which is well-suited to handle the corrosive conditions associated with phosphorus chemical industry exhaust gases. For the economizer at low temperatures around 200°C, the design calls for using ZS flue gas anti-corrosion coating with a temperature resistance of 750°C as the primer, applied in 3 layers, followed by ZS anti-strong oxidation anti-corrosion coating with a temperature resistance of 250°C as the top coat, applied in 2 layers; the total designed film thickness is also 0.5 mm. The ZS high-temperature sealed anti-oxidation coating uses spinel structures formed through high-temperature and high-pressure treatment as fillers, which enhances the material’s internal corrosion resistance. The coating develops a cross-linked glassy structure at high temperatures, enabling it to effectively prevent various gases and liquids from penetrating into the substrate and thus providing good protection for it. Scales-like graphite is better able to form multiple layers in a fish-scale-like structure; even if a medium manages to penetrate, the path it must take to reach the protected substrate is **lengthened**, thereby ensuring that the coating provides excellent protection for the substrate both at normal temperatures and at high temperatures. ZS flue gas anti-corrosion coating is a widely used general-purpose coating for protecting flue gas; it can be applied to various types of flue gas, flues, chimneys, desulfurization towers, dust collectors, heat exchangers, and other equipment. The coating features resistance to acids and alkalis, high temperature resistance, wear resistance, strong adhesion, no peeling, non-flammability, grindability, a long service life, and easy and rapid application, thereby enhancing the overall corrosion resistance of the flue gas. The ZS high-oxidation-resistant coating is formulated through high-temperature passivation and chelation using materials such as polytetrafluoroethylene, barite, and flake graphene. Once cured, the coating exhibits high inertness and high density, preventing the material molecules from losing electrons easily. It has an extremely strong adhesion to metals, is resistant to thermal vibrations, and can withstand sudden changes in temperature. The ZS highly oxidative-resistant coating can endure for a long time the oxidative corrosion caused by strongly oxidizing materials such as concentrated acid solutions (salts, sulfur, nitric acid), strong intermediates, and polar solvents; it is also resistant to corrosion resulting from the penetration of oils such as aviation kerosene. In a highly polar solution of anhydrous ethanol, after immersion at 100°C for 500 hours, the swelling rate of the ZS high-oxidation-resistant anti-corrosion coating remains below 5‰.