There are neutralization method, ferric sulfate method, organic solution extraction method, dialysis method, ion exchange method and so on. The latter three methods are still in the experimental research stage, and the industrial application is more ferrous sulfate method to produce ferrous sulfate, polymerized ferric sulfate and pigment.

Ferric sulfate process

The characteristic of this method is that the iron in the waste liquid can be reused. Therefore, researchers pay attention to it and gradually form a more mature practical technology.

The following six methods have been put into production.

Concentration filtration natural crystallization method: also known as iron filings method, first put sulfuric acid waste liquid and iron filings into a pickling tank for full reaction, then heat the solution to 100 ℃ for 2 h, then heat and concentrate it, then cool it naturally to make ferrous sulfate crystallize and precipitate, and then dehydrate and dry it by a dryer.

This method can recover low, medium and high grade ferrous sulfate from pickling wastewater for industrial, agricultural, pharmaceutical and chemical reagents. It has the advantages of simple operation, less investment and low cost, but it can only recover ferrous sulfate, not sulfuric acid, with small treatment capacity, poor product quality and long production cycle, which is more suitable for small-scale production of township enterprises.

Ferrous sulfate coprecipitation method: this method combines neutralization method and ferrous sulfate method to treat acid wastewater.

The basic principle is: adding alkali liquor or lime into the wastewater to neutralize the acid wastewater to produce sodium sulfate. The sodium sulfate still has a certain solubility, so polyacrylamide flocculant is needed to make the metal ions gather and settle.

Two stage treatment is adopted. The sediment obtained from the first stage treatment is stirred by agitator to destroy the balance between precipitation and ions in liquid phase. Then, it goes through neutralization tower to make full reaction, and then goes through the second stage settlement treatment to obtain better effect. This method is suitable for the treatment of acidic wastewater with high sulfur content.