Whiteness is the main technical indicator of kaolin products. Magnetic separation + chemical bleaching is a commonly used whitening method for kaolin. Kaolin chemical bleaching using sodium dithionite oxidation - reduction of ferric iron in an acidic medium a reducing kaolin divalent iron dissolved in water, so as to achieve the purpose of whitening. Chemical agents such as sulfuric acid and sodium dithionite are added during the bleaching process.
1. Main pollutants of kaolin bleaching wastewater
The main pollutants of waste water produced by chemical bleaching of kaolin are H+, Fe2+, solid suspended matter SS and color, etc., and the most harmful effects on recycling and reuse are harmful ions such as hydrogen ions and ferrous salt ions.
The impact of ferrous ions in bleaching wastewater on production is mainly reflected in the following aspects:
(1) Serious accelerated corrosion effects on various steel equipment and pipelines;
(2) It has serious corrosive effect on magnetic medium of high gradient magnetic separator
Basic metal magnetic medium bristles generally 1Cr17 ferritic stainless steel. Stainless steel is a very thin and strong and stable chromium- rich oxide film (protective film) formed on the surface to prevent the oxygen atoms from continuing to infiltrate and continue to oxidize, thereby obtaining the ability to resist rust.
Once for some reason, the film is continually destroyed, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will be continuously separated, forming loose iron oxide, and the metal surface will be continuously rusted. Because the Fe2+-containing salt in the backwater reacts electrochemically with the steel wool material, the steel wool breaks due to corrosion and loses its magnetic permeability. Therefore, it is necessary to remove the Fe2+-containing salt in the return water as much as possible before it can be used for production.
(3) If the water containing Fe2+ is directly used for reproduction, Fe2+ will be converted into Fe3+ in an aerobic and acid environment, which inevitably increases the content of ferric ions in the slurry, which is in the subsequent bleaching process. It will consume more sulfuric acid and sodium dithionite to increase production costs and affect product quality.
2. Treatment process of kaolin bleaching wastewater
(1) Neutralization + precipitation treatment
1 Kaolin bleaching wastewater is collected into the initial sedimentation tank, and the precipitated kaolin in the sedimentation tank is filtered by a filter press to be a kaolin product;
2 The pressure filtration wastewater is precipitated through the primary sedimentation tank and sent to the neutralization tank of the anti-corrosion treatment of the sewage treatment station. In the pool and adding lime solution, the PAC polyaluminum chloride, will also sulphate the wastewater to form calcium sulfate precipitated in the wastewater in the form of Fe2 + to precipitate Fe (OH) 3 precipitated.
3 After the wastewater is neutralized in the neutralization tank, it enters the sedimentation tank of the anti-corrosion treatment to flocculate and precipitate; the sediment in the sedimentation tank is filtered by a filter press, and the wastewater discharged by the pressure filtration is sent to the tertiary sedimentation tank.
The tail water in the 4 three-stage sedimentation tank can be recycled.
The kaolin bleaching wastewater is treated by the neutralization + precipitation treatment method, and the removal efficiency of the solid suspension SS can reach more than 95%, and the removal rate of the iron ions can reach 99.9% or more.
(2) Neutralization callback method
1 Using lime as a neutralizing agent and adding it by wet method, accelerating the neutralization reaction by stirring, increasing the contact area of ​​the reaction, and achieving the best effect at a pH of 10-11;
2 After stirring for about 15 minutes, the neutralization reaction has been basically completed, and the mixture is precipitated for 10 minutes, and then subjected to solid-liquid separation;
3 callback separation of the supernatant pH to 6-8, the resulting mixed solution is oxidatively aerated with air, aeration for 15min;
4 Add PAM solution as flocculant, stir and mix for 1 min, and then let the reaction stand for 10 min. The pH of the effluent after the reaction is 6-8, and the Fe content is about 2 mg/L.
The kaolin bleaching wastewater is treated by the “neutralization-precipitation-pH back-regulation-oxidation-flocculation†process, and the removal rate of iron ions can reach over 98%.
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