Anti-corrosion measures for the urea granulation tower
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The second-phase urea production unit of Sichuan Meifeng Chemical Co., Ltd.’s Deyang branch was put into operation in July 1998, with a designed capacity of 200 t/d. After several technical upgrades, its current production capacity has reached 450 t/d. The specifications of its granulation tower are an inner diameter of 1,200 mm, a wall thickness of 450 mm, and a height of 91,000 mm. The elevation at the bottom of the granulation chamber is 64,000 mm. The entire tower was cast in one go using reinforced concrete slip-forming technology, and it has been in operation for over 8 years to date. 1 Raising of the issue: During operations in recent years, it has been observed that, over time, the inner and outer walls of the urea granulation tower in Phase II have suffered increasingly severe corrosion. By early May 2006, concrete blocks measuring approximately 50 mm × 40 mm × 20 mm were occasionally falling into the lower cone of the granulation tower. At the same time, large chunks of concrete fell off on the inner and outer walls of the north-south walls between the granules, the walls made hollow sounds, and cracks or loosening occurred ； There are various dangerous issues, such as loose or broken corrosion-resistant tiles in the upper part of the floor base, loose and weathered concrete in the lower part of the floor base, and the peeling off of the 3mm thick protective aluminum sheet layer. It not only contaminates the products but also poses a threat to human safety. 2 Analysis of the causes of corrosion: By analyzing concrete samples taken from the north-south walls between the granulation units where corrosion was most severe, as well as from the lower parts of the structures beneath them, it was found that urea dust and crystals penetrated into the concrete, mixing with the concrete particles to form a urea-concrete structure, thereby completely destroying the composition and properties of the concrete. Urea dust possesses strong penetration power and terrible destructive effects, which must be taken seriously; otherwise, the consequences could be disastrous. 3 Solutions 3.1 Treatment methods for the inner and outer walls of the granulation tower First, clean the base layer: after stopping the operation of the tower, wash it several times with a 3% (by mass) zinc sulfate solution, then rinse and clean the tower walls with water repeatedly to remove all urea crystals adhering to the anti-corrosion coating on the walls, followed by natural air drying ； After the tower wall has dried (with a moisture content of ≤6%), a flap tool is used to roughen the existing surface, while simultaneously removing the urea that has penetrated into the surface layer. In areas where bubbling or water accumulation occurs, an abrasive wheel is used to remove the detached or delaminated anti-corrosion coating. Then follow the steps below: ① Clean the surface of the tower wall after grinding ； ②Scrape epoxy putty to level the areas with severe corrosion and uneven surfaces ； ③Apply 1 coat of epoxy primer (20μm thick) ； ④Apply full thickness of epoxy putty (about 1 mm thick), then brush on 1 coat of epoxy topcoat ； ⑤Surface roughness ； ⑥Apply fluorocarbon paint 3 times (100μm thick). The exterior wall is painted with fluorocarbon paint 2 times (85 μm thick). 3.2 Treatment method for the north-south walls between granulation units: (1) First, manually remove the loose concrete layer on the inner and outer surfaces of the walls; then use acid washing/water washing combined with baking with a blowtorch to remove the urea crystals from the concrete over several rounds, until no white urea crystals are practically visible on the walls. Finally, clean the polished surface of the tower wall. (2) Fill the pits created by grinding with cement, and use epoxy putty to level out any uneven surfaces. (3) Apply 1 coat of epoxy primer (20μm thick). (4) Attach it to the wall using 2 mm stainless steel plates with a gap of about 100 mm, and fill it with high-strength cement castable. (5) Seal the old and new joints with epoxy putty, and cover them with fiberglass-reinforced plastic. (6) Apply full thickness of epoxy putty (1 mm thick), then brush on 1 coat of epoxy topcoat. (7) Apply fluorocarbon paint 3 times (100μm thick). 3.3 Treatment methods for the floor surface between granulation layers: (1) First, manually remove the loosened concrete layer, and repeatedly use acid washing/water washing along with heating with a blowtorch to remove the urea crystals from the concrete over several iterations, until no white urea crystals are practically visible; then clean the polished floor surface thoroughly. (2) Scrape epoxy putty to level the areas with severe corrosion and uneven surfaces. (3) Apply 1 coat of epoxy primer (20μm thick). (4) Apply full thickness of epoxy putty (1 mm thick), then brush on 1 coat of epoxy topcoat. (5) Surface roughness. (6) Apply fluorocarbon paint 3 times (100μm thick). (7) Press a 2mm stainless steel plate tightly against the floor surface. (8) Seal the old and new joints with epoxy putty, and cover them with fiberglass-reinforced plastic. 3.4 Treatment for the upper part of the floor between the granulation units: Remove the originally loose corrosion-resistant tiles and corner tiles, reprocess the floor surface, and then lay new corrosion-resistant tiles and corner tiles. 4. Treatment results: Through 3 months of continuous operation, all anti-corrosion tasks for the entire tower were successfully completed, eliminating the potential hazards associated with this granulation tower. The tower, after anti-corrosion treatment, not only looks good but also exhibits excellent granulation performance.
1 Corrosion mechanism of the urea granulation tower: During the granulation process, urea often contains trace amounts of ammonia, carbonates, and water vapor; once urea absorbs moisture, the resulting solution becomes alkaline. When an alkaline urea aqueous solution penetrates into a lining with improperly selected anti-corrosion materials or defective construction, and comes into contact with the concrete matrix of a granulation tower with a porous structure, corrosion of the concrete matrix on the inside of the urea granulation tower becomes quite severe. This directly weakens the strength of the concrete matrix, alters the structure of the granulation tower, affects its service life, and ultimately leads to the destruction of the granulation tower. 2 Protection of the outer wall of the urea granulation tower 2.1 Selection of protective materials By comparing the use of polyurethane coatings with modified high-chlorinated polyethylene (HCPE), and based on years of construction experience, it is recommended that modified high-chlorinated polyethylene (HCPE) be used for the protection of the outer wall of the urea granulation tower, as it yields better results. 2.2 Protection methods: ① Remove the loose cement from the tower wall, clean it with a brush, and roughen the surface using sandpaper. ②Surface treatment with a neutralizing agent and propylene is carried out if necessary. ③General anti-corrosion treatment for modified high-chlorinated polyethylene (HCPE): 1 coat of primer and 2–4 coats of topcoat. ④Modified high-chlorinated polyethylene (HCPE) for heavy-duty corrosion protection: 1 coat of primer, 4–6 coats of high-chlorinated polyethylene (HCPE) glass flake coating, and 1–2 coats of topcoat. 3 Protection of the inner wall of the urea granulation tower 3.1 Cleaning of the inner wall substrate surface The cleanliness, porosity, and roughness of the corrosion-resistant substrate surface have a direct impact on the effectiveness of corrosion protection. Especially in some equipment where there is still an existing corrosion-resistant layer, special surface treatment must be carried out before repairing the corrosion protection; this can be done through the following methods: ① Using flames and chiseling to remove the original corrosion-resistant layer as well as the crystals and residual substances formed in areas heavily damaged by urine corrosion. ②Chisel away the corroded and loose concrete until the hard concrete layer is exposed. ③If the internal rebar corrodes, it should be replaced or the amount of rebar increased. Then, the surrounding concrete that remains firm and not loosened should be rinsed repeatedly with plenty of clean water until no more material emerges. ④After natural drying for 24 hours, the pH value was measured to be neutral, followed by drying at 50–60°C ； Finally, the damaged areas are repaired using new concrete, cement mortar, along with various cement admixtures such as waterproofing agents, water-reducing agents, densifiers, and rust inhibitors; alkali removal treatment is carried out if necessary. ⑤The uneven areas on the surface need to be repaired using cement mortar or resin putty. 3.2 Properties of the inner wall materials in urea granulation towers: Based on the properties of these materials and years of construction experience, it is recommended to select different materials and construction methods according to the characteristics of various sections as well as the properties of different materials. This approach helps to improve the overall protective effect and reduce costs. During the actual construction process, taking into account the project timeline and future repairs, it is best to use vinyl ester resin flake mortar in high-temperature areas; this helps to ensure construction quality and timely completion, while also creating favorable conditions for subsequent repairs. Practice has shown that using vinyl ester resin flake mortar as a material for corrosion protection in the high-temperature areas of urea towers is a preferred choice, and its corrosion resistance has been recognized. 3.3 Construction plans for various sections: The inner wall of the tower from 2 m above the sprinkler heads to 8 m below them, as well as the outer wall of the control room, constitute the high-temperature zone. The temperature in this area ranges from 80 to 120°C, with the corrosive agents being high-temperature materials and steam. Due to the high temperatures, high reactivity of the substances, strong corrosivity, and significant effects of thermal stress, the specific construction method is as follows: Option 1: Add a layer of glass chopped fiber reinforcement at the bottom, using a composite structure of vinyl ester resin high-temperature glass flake mortar and chopped fiber (foundation – one coat of primer – glass chopped fiber – two coats of vinyl ester resin high-temperature glass flake mortar). Option 2: Apply one coat of epoxy primer, one coat of epoxy putty, one layer of backing fabric, apply another coat of epoxy primer, then apply a layer of epoxy resin bonded to glass fiber cloth and a layer of glass fiber mat; finally, apply three coats of topcoat. Temperatures other than those in the high-temperature zone are all below 80°C and are classified as the low-temperature zone; this zone is subject to erosion and corrosion caused by the cooling air currents from the material vapor and dust. Specific method: Option 1: Apply two coats of vinyl ester resin as a base layer, then apply vinyl ester resin medium-temperature glass flake mortar, with a application thickness of 1–2.7 mm. Two coats of vinyl ester resin flake coating are applied, and in the areas with stress concentration at the corners, two additional layers of glass fiber are added on top of the glass flake mortar layer for local reinforcement. Option 2: Treat with two layers of glass fiber cloth bonded with epoxy resin and one layer of glass fiber mat. The bottom of the tower is primarily affected by erosion, as well as the corrosive penetration of alkaline urea. The specific construction methods include: one layer of epoxy varnish (to prevent leakage and enhance adhesion), 4–10 mm of epoxy mortar (for corrosion resistance, leakage prevention, and impact resistance), fiberglass reinforced with two layers of fabric and three coats of resin, an epoxy putty lining, and a 100 mm thick marble surface layer, along with epoxy mortar. The granulation nozzle room is a high-temperature area, with the main corrosive agents being high-temperature materials and steam. This area features high temperatures, high medium activity, strong corrosion, and significant thermal stress effects. The specific construction plans are as follows: Plan ①: Fiberglass lining ； Option 2: Resin cement mortar ； Option 3: The metal cladding lining is constructed by assembling panels made of aluminum sheets with a thickness of more than 3 mm; reinforced steel members treated for corrosion serve as the framework. The aluminum sheets are connected to each other as well as to the steel members using bolts, while the joints are sealed with epoxy putty or AC building structural adhesive.