Corrosion prevention measures for phosphorus fertilizer non-metallic materials

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2.jpg In the production of phosphate fertilizers, commonly used non-metallic materials include graphite, rubber, fiberglass, plastics, synthetic fibers, various types of fibers, various kinds of pastes, and coatings. These non-metallic materials are mainly used in static equipment such as various storage tanks, exhaust gas systems, gas-liquid separation systems, pipelines, etc., and they can basically meet these requirements. List some devices in which the use of non-metallic materials has been successful. 　　1. Wet phosphoric acid extraction tank – also known as acid hydrolysis tank. The conditions inside this tank are quite harsh, with substances such as H2SO4, H3PO4, HF, HCl, and acid-insoluble materials present. The reaction temperature is around 80°C; it is highly corrosive, and due to the flow of the medium, there is also a certain degree of abrasion. The basic structure of an extraction tank is usually a reinforced concrete or steel shell, with a non-metallic material lining inside. The lining structures include single lining and composite lining, with graphite brick slabs used for the outermost layer. In terms of application, both impermeable graphite and permeable graphite offer excellent corrosion resistance. Currently, there are four relatively mature anti-corrosion measures for extraction tanks in use domestically. 　　(1) Using epoxy phenolic mortar as the formation layer, followed by two layers of permeable graphite sheets (δ=12mm), this single-layer lining structure is both economical and relatively reliable. It is merely because epoxy phenolic mortar and graphite sheets have a certain degree of brittleness; if not installed or used properly, it is likely to affect the substrate of the extraction tank, thereby reducing its service life. Therefore, to extend the service life of the extraction tank, the key lies in maintenance; any damaged or weak areas should be repaired as early as possible. 　　(2) An epoxy phenolic fiberglass reinforced plastic layer is used as the impermeability barrier layer; an epoxy phenolic paste layer is employed as a transition layer beneath the fiberglass reinforced plastic. On the surface of the fiberglass reinforced plastic, there are two alternating layers of phenolic paste and permeable graphite sheets, with another layer of phenolic paste applied on top as the final surface layer. This approach effectively prevents leaks caused by the brittleness of the phenolic paste. 　　(3) Use lead plates as the impermeable layer, followed by a layer of acid-resistant tiles and two layers of impermeable graphite plates; apply a layer of phenolic plaster on the surface. However, since lead lining is harmful to workers’ health, the lead plates do not fit tightly against the casing, it is difficult to ensure construction quality, and corrosion is severe; therefore, it is no longer used. 　　(4) A steel shell lined with rubber can be used; this may involve a layer of hard rubber, a layer of soft rubber, followed by acid-resistant tiles and impermeable graphite rings, with phenolic mortar applied on the surface ； It can also be lined with three layers of rubber (soft rubber + hard rubber + soft rubber). The effectiveness of this method in terms of corrosion prevention depends crucially on the quality of the rubber lining; if it is damaged, it is quite difficult to repair. As an impermeable layer, rubber lining can withstand large temperature gradients and has a good bond with the substrate; if its quality can be ensured, it is worth promoting. 　　2. Stirring paddle This is a component that suffers significant corrosion (mainly abrasion). In terms of metal selection, K alloy, Mo2Ti, and UB6 (a steel specifically used for phosphate production abroad) can be used; however, the CL-concentration must be strictly controlled (it should be less than 0.1%), otherwise corrosion resistance drops significantly and honeycomb-like defects appear. The impellers can also be made of steel or 18-8 stainless steel, or covered with rubber made from Mo2Ti (mainly neoprene), or with nitrile rubber – the latter offering good corrosion resistance but poor wear resistance. Phosphorus ores vary significantly across different regions in the country, and it is difficult to meet the requirements using ordinary metal materials. The method of lining with carbon steel followed by rubber coating, although less costly, requires frequent maintenance and replacement. 　　3. Filters Inclined filter types are the most common. The misalignment disc is made of K alloy (Cr25Ni20Mo2Cu3Ti) ； The slurry distributor, gas-liquid separation cylinder, and filter disc frame are made of rigid PVC or PP; the filter cloth is polyester-based, while the idler wheels and support wheels are made of engineering plastic nylon, or they can also be made of rubber-lined steel or alloys. A fine-wire woven polypropylene filter cloth can also be used to overcome the problem of severe permeation. 　　4. Concentration equipment The main components of concentration equipment include flash chambers, vacuum evaporators, and heat exchangers. Flash chambers are mostly made of metal materials; lining methods can also be used, such as graphite plates lined with steel, stainless steel lined with butyl rubber, or carbon steel lined with butyl rubber – all of which yield good results. Vacuum evaporators can use a double-layer composite lining with hard rubber at the bottom layer and soft rubber at the top layer. Heat exchangers can be graphite tube heat exchangers or PTFE heat exchangers, among which the HP-type porous graphite heat exchanger is the most common. 　　5. Gas-liquid separation system and exhaust gas absorption system: The temperature of the medium in the gas-liquid separation system does not exceed 80°C; the phosphoric acid concentration is low, resulting in weak corrosivity. The system operates at low pressure, so an integral PVC structure can be used, with rubber or PVC pipes available for the conduits. For the exhaust gas system, the corrosion issue caused by hydrogen fluoride is a major concern; it is reasonable to use carbon steel lined with rubber for the exhaust gas scrubber, exhaust gas exhaust fans, and fluorine absorption tower.