Technical Summary of the Anti-corrosion Construction Plan for Sewage Treatment Ponds

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1 General Overview of the Wastewater Treatment Plant Project: A wastewater treatment plant in Wuhan is used to treat the wastewater generated by the new cold-rolling sheet mill’s production processes, as well as the domestic wastewater from the factory premises. The main production units will generate the following three types of wastewater: R26; acid/alkali-containing wastewater (A/A wastewater); R26; oil and emulsified oil-containing wastewater (O/E wastewater); R26; chromium-containing wastewater (Cr wastewater). All of these wastewaters are highly corrosive, and to ensure the stable operation of the wastewater treatment plant, its various structures must possess strong corrosion resistance. To this end, anti-corrosion treatment is specifically applied to all structures in the wastewater treatment plant. The specific anti-corrosion measures are as follows. 2 Anticorrosion requirements and methods for various structures and buildings in the wastewater treatment plant, as well as the main workshop 2.1 Anticorrosion requirements for various structures Building Area A: The floor shall be made of corrosion-resistant integral mortar floor (MFE-5 vinyl ester resin mortar), with a thickness of not less than 3 mm; acid-resistant bricks shall be used for the sections below 1.2 meters in height on the surrounding walls. Areas B and C: Acid-resistant bricks shall be used for the floor, pipe trenches, and the areas below 1.2 meters in height on the surrounding wall skirts. Zone E: The floor is paved with vinyl ester resin mortar with a thickness of not less than 7 mm; acid-resistant bricks are used for the wall skirts below a height of 1.2 meters, and the drainage channels are made of vinyl ester resin fiberglass with a thickness of not less than 3 mm. Zone F: For areas below 1.5 meters in height, including the floor and baseboards, epoxy mortar with a thickness of not less than 3 mm shall be used. The working environment throughout the wastewater treatment plant is a humid, slightly corrosive chemical environment; other steel components, such as steel ladders and handrails, must have strong corrosion resistance. Chlorosulfonated polyethylene anti-corrosion coating is considered for use in the design. The inner walls of the acid and alkali wastewater treatment tanks, chromium-containing wastewater treatment tanks, and oil-containing wastewater treatment tanks are made of vinyl fiberglass with a thickness of not less than 3 mm. The clarifier for chromium-containing wastewater is made of epoxy fiberglass with a thickness of not less than 3 mm. The wastewater clarifier, final pH adjustment tank, and acid-base sludge concentration tank are coated with epoxy resin, with a thickness of not less than 0.5 mm. Acid-resistant bricks are used for the ultrafiltration circulation tank, the ultrafiltration area trenches, and the oil-containing wastewater sumps. The domestic wastewater tank, contact biological oxidation tank, and inclined plate sedimentation tank are coated with epoxy resin, with a thickness of not less than 0.3 mm. The filtrate collection tank, chromium-containing wastewater pits, and gutters are made of vinyl fiberglass with a thickness of not less than 5 mm. The walkways of various structures are coated with chlorosulfonated polyethylene anti-corrosion paint, in one base coat and one intermediate coat plus two top coats. For specific partitioning details, refer to Figure 2.2, which shows the construction methods for various sections; 3 mm vinyl fiberglass: a coat of vinyl ester resin as a base layer ； Apply a coat of vinyl putty ； Thirteen fabric, fifteen resin vinyl fiberglass ； Apply two coats of vinyl resin topcoat. 5mm Vinyl fiberglass: one coat of vinyl ester resin as base ； Apply a coat of vinyl putty ； 18 fabric, 20% oil vinyl fiberglass ； Apply two coats of vinyl resin topcoat. 0.3 mm epoxy resin coating: one coat of epoxy primer ； Apply a coat of epoxy putty ； Apply the epoxy coating twelve times. 0.5 mm epoxy resin coating: one coat of epoxy resin primer ； Apply a coat of epoxy putty ； Apply the epoxy coating eighteen times. Acid-resistant brick: one coat of epoxy resin primer ； Two-polymer three-oil epoxy resin glass isolation layer ； An acid-resistant brick surface layer of 150×150×20 is laid using epoxy mortar. Resin mortar: one coat of epoxy resin as a base layer ； Two-polymer three-oil epoxy resin glass isolation layer ； Resin mortar finish. 3 Construction techniques for various anti-corrosion methods 3.1 Anti-corrosion resin fiberglass 3.1.1 Substrate preparation and requirements The substrate must be strong and dense ； The strength must be tested and shall meet the design requirements. Underground seepage is strictly prohibited. The surface at the base level must be free from cracks, honeycombing, pitting, and other such defects. For areas with the aforementioned defects, it is permissible to use a 1:2 polymer cement mortar for repair. On the treated surface substrate, phenomena such as peeling and sanding are strictly prohibited. The base layer must be dry and clean, with no signs of water seepage, floating water, or accumulated water on its surface. Before construction, the surface of the base layer should be wiped clean with a soft brush. The corners at the base level should be made into 45° beveled angles or rounded corners with a radius of R=30~50 mm. All pipes, sleeves, pre-drilled holes, and embedded components that pass through the anti-corrosion base layer must be installed or left in place in advance. 3.1.2 Quality requirements for raw materials: 1) Anti-corrosion resins: The resins used in this project are E42 (634) type epoxy resin and MFE-5 type vinyl ester resin. Type E epoxy resin is a new type of synthetic material, which is a synthetic resin obtained by the polycondensation of epichlorohydrin and diphenylpropane in the presence of a basic catalyst. Its quality specifications shall meet the requirements specified in the table below: Parameter E42 (634): Epoxy value, equivalents/100g – 0.38–0.45; Softening point, °C – 21–27; Inorganic chlorine content, equivalents/100g – ≤0.001; Inorganic chlorine content, equivalents/100g – ≤0.02; Volatile matter, % – ≤1; Color index – ≤8; Viscosity, Pa·s (25°C) – --- MFE-5 resin is an acrylate-based vinyl resin modified with isocyanates. It has good resistance to water, acids, and alkalis ； High toughness, low shrinkage, excellent mechanical properties ； The long-term operating temperature is 90°C. The base mass index shall comply with the provisions in the table below. Project MFE-5: Appearance – Pale yellow or yellow transparent liquid. Viscosity, Pa·s (25°C): 0.45±0.10. Acid value, mgKOH/g: 10.0±4.0. Gelation time, minutes (25°C): 15.0±5.0. Solid content, %: 54.0±3.0. Thermal stability (80°C): ≥24. 2) Curing agents – Different resins require different curing agents. For epoxy resins, low-toxicity curing agents should be preferred; amine curing agents such as ethylenediamine and m-phenylenediamine can also be used. For moist substrates, wet-curing epoxy resin curing agents should be employed. In this project, T31 is used as the curing agent. Aminic curing agents such as ethylenediamine are not recommended due to their high toxicity. Curing agents for vinyl resins include initiators and accelerators. Commonly used initiators include a solution of methyl ethyl kymelate peroxide, a cyclohexane dibutyl peroxide paste, and dibenzoyl dibutyl peroxide paste ； The accelerators are a phenylethyl solution of cobalt salts, and an N,N-diaminostyrene solution. The initiator used in this project is a solution of methyl ethyl ketyl peroxide ； The accelerator is a phenylethyl solution of cobalt salts. 3) Toughening agents are used to improve the brittleness after curing, enhance impact toughness, and reduce the viscosity. Common toughening agents for epoxy resins include dibutyl phthalate, triphenyl phosphate, and so on. The toughening agent used in this project is triphenyl phosphate. 4) Diluent: Used to reduce resin viscosity and improve penetration capacity. For epoxy resin thinners, non-reactive thinners such as propane, anhydrous ethanol, and xylene are suitable; non-reactive diluents like n-butyl glycidyl ether and phenyl glycidyl ether can also be used. The solvent for vinyl resin should be styrene. The solvent used for the epoxy resin in this project is propylene, while the solvent for the vinyl resin is styrene. 5) Resin glass reinforcement material: The resin glass reinforcement material used in this project is 0.2 mm thick alkali-free glass fiber cloth. Its chemical composition shall comply with the provisions of the **current standard ‘Alkali-Free Glass Balls’ JC557-1994. 6) Powders: Commonly used powders include quartz powder, porcelain powder, diabase powder, and so on. The powder must be dry, with an acid resistance of not less than 95%; when an acidic curing agent is used, this value should be not less than 98%. It must also be free from impurities such as iron and carbonates. Its volume stability should be satisfactory, and the moisture content should not exceed 0.5%. The strength requirement is that the residue on a 0.15 mm sieve should not be greater than 5%, while the residue on a 0.088 mm sieve should be between 10% and 30%. Quartz powder is not suitable for use in anti-corrosion projects in alkaline media. The powder material used in this project is diabase powder. 3.1.3 Construction method for anti-corrosive fiberglass resin: 1) Preparation of resin paste – A trial preparation should be carried out before preparing the cement and resin paste. The preparation of mortar and adhesive compounds should take into account the progress of the construction site, the skill level of the workers, and the initial setting time of the resin-based adhesives, following the principle of \"small amounts applied multiple times\". For the final application of the cover adhesive, a styrene-paraffin solution corresponding to 3–5% of the resin weight can be added to the adhesive in order to achieve good curing properties and corrosion resistance. The preparation of the rubber compound is described as follows: The solvent and the resin preheated to around 40°C are weighed according to the required amounts and added to a plastic container; after mixing evenly, it is cooled to room temperature for later use. When preparing the back cover, some diluents can be added before the curing agent is introduced. During preparation, it should be determined according to the construction progress, prepared as needed and used immediately, with consumption completed within 30–45 minutes before initial setting. The curing agent should be added gradually while stirring continuously. If the temperature of the glue becomes too high, the container can be placed in a water cooler to cool it down, thereby preventing localized overheating that could affect the quality of the cure. After the epoxy glue has been mixed evenly with the T31 curing agent, it is advisable to let it rest for 5–10 minutes (to mature) before use, in order to ensure good curing properties. The initiators and accelerators used for vinyl resins should be products compatible with those recommended by the manufacturer, and their usage amounts can be referred to in the manufacturer’s instructions. When preparing vinyl rubber compounds, it is strictly prohibited to mix the curing agent and accelerator together. Curing agents and thinners for epoxy resins and vinyl resins should be stored separately to avoid confusion. For the mix ratios of clay and binders, refer to the table below. Construction mix ratios (by mass) for epoxy-based fiberglass adhesives, mortars, and pastes
Material name: Epoxy resin, Diluent (propane), Curing agent (T31), Acid-resistant powder, Styrene-paraffin solution, Mineral pigments, Toughening agent, Sealing agent
Ratio: 100, 40–55, 15–20, —, —, —

For repair materials: 100, 12–16, 16–22, 150–200, —, —
For lining adhesives: 100, 13–25, 25–30, —, —, —, 4–6
For surface coatings: 100, 11–21, 25–35, —, 3–5, 0–2, 4–6

Construction mix ratios (by mass) for vinyl resin-based fiberglass adhesives, mortars, and pastes
Material name: Vinyl resin, Diluent (styrene), Accelerator (styrene solution of cobalt salts), Initiator (methyl ethyl peroxide), Acid-resistant powder, Styrene-paraffin solution, Mineral pigments, Sealing agent
Ratio: 100, 0–10, 1–4, 2–4, 200–300, —, —

For repair materials: 100, 0–10, 1–4, 2–4, —, —
For lining adhesives: 100, 0–10, 1–4, 2–4, —, —
For surface coatings: 100, 0–10, 1–4, 2–4, —, 3–5, 0–2

Note: 1 The construction temperature for the above mix ratios is 0–5°C, and the curing system for the resins has been determined through testing. The mix ratios in the table are formulated based on E42 (634) type epoxy resin and MFE-5 type vinyl resin. The mixing ratio of the styrene paraffin solution is styrene:paraffin = 100:5 ； During preparation, paraffin is first cut into pieces and added to styrene; it is then heated to 60°C using a water bath. Once the paraffin has completely dissolved, it is cooled before being used. 2) Key points for each step in the molding process of hand-poured vinyl ester resin fiberglass: ① Sealing the bottom. Use a brush or roller to apply the topcoating onto the base layer for a second coating; it should cure naturally for at least 24 hours during this time. The thickness of the back cover should not exceed 0.4 mm, and there should be no flowing or bubbles. When applying the first layer of sealant, an appropriate amount of thinner is added to the sealant to enable it to penetrate into the underlying surface. After curing, apply the resin putty by either full application or dot application. If the condition of the substrate surface is poor overall, the resin putty should be applied in full; under normal circumstances, dot application of the resin is preferred.