The commonly used processing techniques in metallurgical plants include steam jet vacuum crystallization, evaporation concentration freeze crystallization, polymeric ferric sulfate, iron filings, and diffusion dialysis. There is also the iron oxide red ammonia regeneration method, which is less commonly used due to its multiple equipment and high energy consumption. The capacity of processing equipment is usually expressed in terms of the amount of sulfuric acid consumed per year (concentration of 98%) for pickling steel. Generally, small equipment consumes around 500t/a, while large equipment consumes 1000-3000t/a.
Steam jet vacuum crystallization method: a method of maintaining a certain degree of vacuum in the evaporator and crystallizer through a steam jet and condenser. When the waste liquid with a temperature greater than the evaporation temperature under this vacuum degree passes through, the water in the waste liquid evaporates in an adiabatic state, resulting in a decrease in waste liquid temperature, an increase in concentration, and a corresponding decrease in the solubility of ferrous sulfate. At the same time, new acid is added to the evaporator to increase the acidity of the solution and precipitate the supersaturated ferrous sulfate crystals. The process flow is shown in Figure 1. The advantage of this method is that it can handle a large amount of waste liquid and continuously produce seven crystal water ferrous sulfate (FeSO4 • 7H2O) and regenerated acid (containing 10% to 20% H2SO4 and 44% to 8% FeSO). FeSO4 • 7H2O has a purity greater than 95% and contains less than 1% H2SO4. No secondary pollution. The disadvantage is that new acid needs to be added during the processing, and all acid washing uses regenerated acid. The steam injector nozzle is prone to wear and tear.
Evaporation concentration freezing crystallization method: The waste liquid is first heated in the evaporator, and the water is removed by vacuum evaporation to increase the acidity and concentration of ferrous sulfate. Then, it is sent to the crystallization tank and the temperature of the waste liquid is lowered to 0-3 ℃ with frozen salt water to precipitate ferrous sulfate crystals. The freezing crystallization time is generally 2 hours. The advantages of this method are that it can process a large amount of waste liquid, has high evaporation efficiency, and produces seven crystal water ferrous sulfate. During the treatment process, no new acid is needed, and the new acid is directly used for steel pickling. Regenerated acid contains 16% to 20% H2SO4 and 5.5% to 7% FeSO4. FeSO4 • 7H2O has a purity greater than 95% and contains less than 1% H2SO4. The disadvantage is that it requires a complete set of equipment such as a freezer, which requires a large investment, high cost, and complex operation. Graphite evaporators are prone to blockage by ferrous sulfate crystals. The copper tube in the crystallization tank is corroded by acid and has a short service life, requiring frequent replacement.
Polymeric ferric sulfate method: First, the waste liquid is pretreated, then oxidized with oxygen, and NaNO2 is used as a catalyst to generate basic ferric sulfate polymer, namely polymeric ferric sulfate, which is used as a water purification agent. The process flow is shown in Figure 3. This method can handle waste liquids generated in various situations, with intermittent operation and a production cycle of 2 hours. Polymeric ferric sulfate contains 12.5% to 13.5% Fe2O3, trace amounts of Fe2+, a density of 1.45 to 1.50g/cm3, an alkalinity of 10% to 13%, and a pH value of 0.5 to 1.0. Its process equipment is simple, with low investment and no secondary pollution.
Iron filings method: The free acid in the waste liquid reacts with iron filings to form ferrous sulfate, which is then concentrated by heating and naturally cooled to crystallize and precipitate. This method has serious environmental pollution, high labor intensity, and the discharged residual liquid contains about 0.15% H2SO4 and 421% FeSO, requiring neutralization treatment. Only applicable to the treatment of low waste liquid volume.
Diffusion dialysis method: using ion exchange membrane, diffusion dialysis, to separate free acids and ferrous sulfate in waste liquid. It has low energy consumption and easy operation. Regenerated acid contains 140~180g/L H2SO4 and no more than 10g/L FeSO4. The residual solution contains 20-30g/L H2SO4 and 10-120g/L FeSO4, and requires neutralization treatment. Suitable for the treatment of low waste liquid volume and silicon steel sheet pickling waste liquid.