Research on the process of producing ammonium fluoride and silica gel from fluorosilicic acid as a by-product of phosphate fertilizer production

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Fluorosilicic acid is a by-product of the phosphate fertilizer industry and can be used to produce ammonium fluoride and silica. The literature often describes the production of ammonium fluoride and silica gel by reacting fluorosilicic acid with ammonium bicarbonate, with carbon dioxide gas being generated during the reaction. The author used ammonia to react with fluorosilicic acid to produce ammonium fluoride and high-quality silica, with both fluorine and silicon being fully utilized ； The ammonia gas generated during the process can be recovered through absorption, preventing waste of resources and avoiding environmental pollution. 1. Research Process 1.1 Raw materials required for production: Fluorosilicic acid: a by-product of the phosphate fertilizer industry, with ω(H2SiF6) ≥ 40% and ω(HF) ≤ 4% ； Ammonia water, ω(NH3)≥25%. 1.2 Process Principle The process mainly includes variety preparation, ammonia neutralization, filtration, ammonium fluoride concentration and crystallization, as well as silica washing and drying. First, fluorosilicic acid is reacted with dilute ammonia water to produce silica fume seeds ； The resulting product is then fed into an ammonia neutralization tank, where it undergoes an ammonia neutralization reaction with a fluorosilicic acid solution, yielding a slurry consisting of an ammonium fluoride solution and silica precipitate ； The slurry is filtered; the clarified ammonium fluoride solution is concentrated, crystallized, and dried to yield solid ammonium fluoride ； After thorough washing, the silica gel ointment is dried to yield the final product. Main chemical reaction equation: H2SiF6 + 6NH3·H2O + nH2O = 6NH4F + SiO2 + nH2O + 4H2O. 1.3 Process flow: Description of the process: 1) Seed preparation: Fluorosilicic acid and dilute ammonia water are mixed in a certain ratio in a mixer to produce seeds. 2) Ammonia neutralization: The obtained seed crystals and concentrated ammonia solution are added to the ammonia neutralization tank, the temperature is raised to 30–50°C, the calculated amount of fluorosilicic acid solution is slowly added, the pH is adjusted to 7–10, and the reaction continues for 15–30 minutes. The entire reaction process is maintained at a constant temperature. 3) Filtration: The reacted slurry is filtered, and the filter cake is washed to obtain silica gel ointment. The filtrate and washing water are sent to a storage tank for concentration. 4) Drying of silica: The washed silica paste is dried to obtain the silica product. 5) Concentration: The ammonium fluoride solution in the storage tank is fed into a concentration device, where it undergoes concentration and crystallization; after filtration, the ammonium fluoride filter cake is obtained. 6) Drying of ammonium fluoride: The ammonium fluoride filter cake is dried under low temperature and vacuum to obtain the ammonium fluoride product. 7) Ammonia recovery: The ammonia gas generated during the reaction process is absorbed in 3 stages, and the dilute ammonia solution is recycled. 1.4 Selection of equipment: 1) The reaction tank used for neutralizing ammonia must be resistant to acids, alkalis, and high temperatures; it should be made of steel lined with PTFE. 2) Before ammonia neutralization, a preparation stage is required; a spray mixer must be installed in front of the ammonia neutralization tank. 3) Since ammonium fluoride reaches saturation at certain concentrations, crystals tend to precipitate and block the equipment, and there is also a specific pH range involved; therefore, in the selection of equipment, concentration devices for ammonium fluoride are crucial, and graphite-based concentration and crystallization units should be used. 4) Ammonium fluoride decomposes more easily at temperatures above 80°C; therefore, low-temperature vacuum drying equipment is used for the dehydration of ammonium fluoride. 1.5 Production cost: To produce 1 t of ammonium fluoride with 250 kg of silica as a by-product, the required raw materials and costs are as follows: 1) 2.70 t of 10 mol/L ammonia solution, at a price of 480 yuan per t, resulting in a total cost of 1,296.0 yuan ； 2) 100% fluorosilicic acid: 0.615 t, unit price of 2,600 yuan/t; total cost: 1,599.0 yuan. The total for the two is 2,895 yuan. 1.6 Product Quality 1) The specifications of the produced silica gel are compared with those specified by industry standards in Table 1. Table 1 Comparison of silica products with standard specifications Item Brightness ω(SiO2)/% Mass loss upon heating (105°C, 2h)/% Mass loss upon calcination (1000°C, 2h)/% pH BET specific surface area/(m2·g-1) DBP oil absorption value/(cm3·g-1) Total iron content/(mg·kg-1) Standards HG/T3063---1999 HG/T3062---1999 HG/T3065---1999 HG/T3066-1999 HG/T3067-1999 HG/T3073-1999 HG/T3072-1999 HG/T3070-1999 Standard specifications Equal to or better than the standard sample ≥90 4.0-8.0 3.0-6.0 6.0-7.5 200-260 2.8-3.5