Treatment of wastewater in titanium dioxide production

Titanium white powder production process waste sulfuric acid is discharged mainly acidic wastewater, mainly containing free sulfuric acid, ferrous sulfate, metatitanic acid, and other metal ions and sulfate. The concentration of sulfuric acid in the wastewater is generally 17%~22% of the concentrated waste acid according to the process. It is also called concentrated acid (mainly the mother liquor of metatitanic acid after filtration and hydrolysis). The first type is 4%. ~10% bleaching waste acid (filtering the mother liquor of metatitanic acid after bleaching or the front filtrate recovered when the above-mentioned sorbic acid is washed with water); the other type is acidic wastewater of 2% or less (washing the filtrate of metatitanic acid), this The discharge of the class is the largest, and the treatment should be treated differently according to the concentration of the acid.

1. Treatment of concentrated acid (hydrolysis waste acid)

In the sulfuric acid method titanium dioxide factory, some of the concentrated acid can be directly returned to the acid hydrolysis process and used to adjust the acidity coefficient of the titanium liquid during leaching, but its dosage only accounts for 15%-20% of the total discharge. A large amount of concentrated acid cannot be treated by neutralization together with acidic wastewater, so that the workload during treatment is too large, and it is preferable to treat concentrated waste acid, dilute waste acid and acid waste water separately. In the past, some factories used this part of the concentrated acid to supply the nearby steel mills for pickling steel or to supply paper mills, printing and dyeing plants, etc., but the factories far from these users were transported and stored. Can't learn from. Some plants also use it to produce manganese sulfate, rust P, not solve the problem but the amount is too small, the production is preferably used after partially returns it was concentrated, mostly for the production of phosphate fertilizer, the fertilizer is a large sulfuric acid The large and small phosphate fertilizer plants are distributed throughout the country, and many phosphate plants have phosphate fertilizer production facilities. The supply of phosphate fertilizer plants can basically solve the problem of treatment of concentrated and waste acid in small and medium-sized titanium dioxide factories. A higher content will lower the available phosphorus content in the phosphate fertilizer (calcium).

The waste acid concentration can be carried out by immersion combustion and vacuum concentration. The immersion combustion is to directly spray the high-temperature gas (about 1200 ° C) generated in the combustion chamber into the waste acid, so that the water in the waste acid evaporates to concentrate the waste acid, and dissolves in the waste due to the increase in the concentration of sulfuric acid. The ferrous sulfate in the acid is precipitated in the form of ferrous sulfate monohydrate. The concentrated sulfuric acid concentration in this method is not high, and the equipment is very corroded. The former Soviet Union used this method to concentrate the spent sulfuric acid to 55% and then sell or supply the phosphate fertilizer. use.

According to its evaporation intensity and concentration series, about 20% of the waste acid can be concentrated to 40%, 50%, 70% or even more than 90%. Figure 1 is the Bayer sulfuric acid titanium dioxide factory in Germany. The process of recycling waste acid recycling. In recent years, the company has jointly developed a concentration process for waste sulfuric acid with the Swiss company Bertrams. The waste acid is first used in a multi-effect falling film evaporator and a forced circulation concentrator, using steam as a heat source to remove 20% of waste acid. Concentrated to 78%, and then concentrated from 78% to 96% with a new concentrator, a 43t (100% H2SO4) waste acid concentration system was established at the Bayer Lefkhausen plant.

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Luqi Company of Germany has also built a waste acid concentrating device similar to Bayer Company for Saatchi, Germany, which can process 80x10 ⁴ t23% waste sulfuric acid annually. The concentrated acid concentration after concentration can reach 70%~80%, waste Ferrous sulfate in acid is used in pyrite roasting to produce sulfur cut; Canadian International Chemical Company built a waste acid concentration unit with a daily treatment capacity of 110t in Tioxide's titanium dioxide factory in Canada. Concentration can reach 93%~96%; Switzerland Sulzer Escher Wyss has built 3 sets of 11~24t/h waste acid concentrating devices in Poland Polis Titanium Dioxide Factory, Finland Kemira Company, Germany, Kronos Company, after concentration The concentration of spent acid can reach 70%.

The Third Design Institute of the Ministry of Chemical Industry and the Coatings Research Institute of the former Ministry of Chemical Industry established a pilot plant for the waste acid concentration in Nanjing and Zhenjiang in the early 1980s. The process was to firstly remove about 20% of the waste acid from the waste. Then, the waste acid is concentrated to more than 30% by vacuum concentration, and then frozen at 0 to 5 ° C, so that the iron salt in the spent acid is precipitated as ferrous sulfate heptahydrate, and the concentration of the waste acid can be increased to 40. %about. Later on this basis, the second stage of concentration was added to increase the concentration of waste acid to 65%. However, the tube of the evaporator blocked by the ferrous sulfate monohydrate formed by the dehydration of ferrous sulfate heptahydrate failed to be put into industrial production.

The method of foreign waste acid concentration is more commonly used to concentrate to 50% in two stages, or to concentrate to 70% in the third stage. In order to prevent the ferrous sulfate monohydrate from clogging the tube of the evaporator in the tube wall, a special type is usually adopted. The pump is forced to circulate, and then the ferrous sulfate monohydrate is separated by filtration. The ferrous sulfate monohydrate contains a certain amount of free acid and cannot be used directly. Most of the plants are dissolved in water and then recrystallized to form ferrous sulfate heptahydrate and then sold. 2 is a commonly used three-stage concentration process of waste acid. Because a large amount of water is evaporated during the concentration process, the steam condensation heat is recycled. The first stage heat source comes from the second stage steam, the second stage and the third stage. With fresh steam, the second stage operates at atmospheric pressure and the third stage operates under vacuum. In this way, it is necessary to cool the spent acid in the second stage to precipitate ferrous sulfate or to force circulation with a pump.

Another method is to concentrate 20% of the waste acid to 50%~60% under normal pressure, because the solubility of ferrous sulfate in sulfuric acid is the lowest at 50%~60%, then 70~80°C. After aging for 3 hours, the ferrous sulfate crystals are grown, and then the ferrous sulfate is separated by filtration (the curing temperature can not be lower than 50 ° C, otherwise the filtration is difficult), the waste acid is then continuously concentrated to 60% to 80% under vacuum, and then matured and crystallized. Separating the residue and crystallization again, and finally obtaining 80% to 85% of the recovered concentrated sulfuric acid. [next]

After concentrating waste acid to 50%~80%, the scope of transportation and application is much larger. The concentrated waste acid used in the treatment of titanium dioxide is more popular in Europe and Japan, but the waste acid concentration equipment is very Expensive, energy consumption and operating costs are also very high, the cost of concentrating 1t of waste acid is much more expensive than buying 1t of fresh sulphuric acid. Therefore, most of the small and medium-sized sulphuric acid titanium dioxide factories in China are not afraid. US sulfuric acid plant because of cost and the use of waste acid after concentration, concentration process is generally not used, but with the spent acid and gypsum production, process waste acid by American Cyanamid and gypsum produced as follows.

First, different types and concentrations of acidic wastewater in production should be treated separately to reduce the burden on the neutralization unit. The company divides the wastewater in the plant into four categories.

a. The wastewater discharged from the surface treatment process is “clean” with slightly acidic or slightly alkaline, and the water is separately separated to reduce the load of the neutralization process;

b. The slightly acidic cooling water discharged from the atmospheric condenser and the like is also stored separately, and is mainly used for neutralization process of lime water;

c. The acidic wastewater discharged from the water washing process, including the water cooled by the calcining exhaust gas spray, is first settled and recovered, and then sent to the neutralization treatment;

d. Hydrolyzed spent acid (concentrated spent acid) is neutralized separately.

Figure 3 shows the waste acid treatment process of the American Cyanamide Company with a de-ironing device. The process is to first neutralize about 20% of waste sulfuric acid with lime milk to pH2.5, and filter to obtain low-iron gypsum. The quality of this gypsum is similar to that of natural gypsum. It can be used as building materials, and the acid filtrate continues to be around 2%. The acidic waste water in the water washing process is further neutralized with lime milk to pH 4.5, and the iron sulphate dissolved in the waste acid is precipitated by using air and slaked lime in the iron removal reactor. This will ensure that the iron content in the wastewater does not exceed the standard. After the iron-removed material is thickened by the thickener, the high-iron gypsum is filtered. If the local environmental protection department does not control the dissolved iron standard in the wastewater, this process can be omitted, and the final gypsum is still low-iron gypsum. The overflow in the thickener is then neutralized with slaked lime to pH 6-9, and CO ₂ can be liquefied and sold. The operation cost of the process is also very high. The key technology is to correctly master the crystallization method of gypsum. Otherwise, filtration will be very difficult. In addition, 5t of gypsum per ton of titanium dioxide should be used as a by-product, and the use of gypsum should be sought. Otherwise, a large amount of gypsum storage is also a problem.

The quality index of gypsum produced by the US cyanamide company with neutral acid is: gypsum (CaSO ₄ · 2H ₂ O) content 94%~97%; solid content 75%~80%; CaCO ₃ 2%~4%; Fe ₂ O ₃ 0.1%~0.3%; TiO ₂ 0.5%~1.0%; Al ₂ O ₃ 0.6%~0.8%; Cr ₂ O ₃ <0.5%. Table 1 and Table 2 are the composition of the waste acid and neutralized wastewater from the US cyanamide company before neutralization (discharge 76,650 m ³ /d).

The composition of the sewage discharged in and after Table 1

Table 2 Composition of spent acid before concentration

The quality of neutralized gypsum obtained by the process of cyanamide in the United States is good, but the cost is as high as $90 per ton of waste acid treatment (1980 price). Each ton of cement can be sold for 13 US dollars for gypsum and 4 US dollars per ton of agricultural gypsum. The iron-containing gypsum (red gypsum) after neutralization in other countries is treated as inorganic waste. [next]

2. Treatment of acidic wastewater

The acidic wastewater in the production of titanium dioxide by sulfuric acid method (about 2%) mainly comes from the washing water in the washing process of metatitanic acid, the treatment of acid hydrolysis, the cooling and washing waste water of the calcined waste gas, the slightly acidic waste water discharged from the concentrated steam condenser, and Cleaning equipment, acid-containing wastewater at the operation site, etc. The average concentration of this type of wastewater is not high, but the acidity range fluctuates greatly, from a few milligrams per liter to tens of grams per liter, and the large amount of emissions is the largest workload in wastewater treatment.

The treatment of acidic wastewater mainly relies on alkali neutralization. The basic substances used are mainly dolomite, limestone , lime, calcium carbide slag, waste alkali, etc. The treatment methods mainly include the following two.

(1) Dolomite upflow expansion and neutralization of acidic wastewater In the expansion filter of Fig. 4, CaSO ₄ and MgSO _{4 are} formed with dolomite, and the chemical reaction formula is as follows:

CaCO ₃ ·MgCO ₃ +2H ₂ SO ₄ →CaSO ₄ ↓+MgSO ₄ +2H ₂ O+CO ₂ ↑

Since MgSO _{4 has a} large solubility and is not easy to precipitate, the neutralization reaction can continue, and the precipitation of CaSO ₄ deposits on the surface of the unreacted dolomite, and the generated CO ₂ gas entrapped on the dolomite surface hinders the reaction from proceeding. The upflow expansion neutralizer is used, and the air blown in the bottom makes the bed material loose. During the reaction, the dolomite particles rub against each other, and the deposits and gases covering the material are easily accompanied by the rising water flow and the air flow. Go, let the reaction effect be more complete. The rising flow rate of this method is the key, generally required to reach 20~40m/h. In addition, since the dolomite is weak base, the acid concentration in the wastewater can not be too high, generally controlled at about 4g/L, and the pH after neutralization can only be It can reach 4~5, and can reach pH5~6 after aeration. In order to reach the discharge standard of pH6~9, it is necessary to add alkali or lime milk (or calcium carbide slag) to further neutralize the acidic substances in the wastewater, and then pass After settling, the supernatant liquid is discharged and the lower layer suspension is filtered, and the filter residue is treated as inorganic waste. The chemical reaction formula is as follows:

Ca(OH) ₂ +H ₂ SO ₄ →CaSO ₄ ↓+2H ₂ O

Ca(OH) ₂ +FeSO ₄ →Fe(OH) ₂ +CaSO ₄

4Fe(OH) ₂ +O ₂ +2H ₂ O→4Fe(OH) ₃ ↓

Ca(OH) ₂ + MgSO ₄ → CaSO ₄ ↓ + Mg(OH) ₂ ↓

The expansion neutralizer of this method is more expensive. Once the neutralization pH reaches the emission standard, it needs to be re-neutralized. Currently, it is rarely adopted by large factories. The schematic flow is shown in Figure 5.

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(2) Lime neutralization. Most foreign large-scale sulfuric acid titanium dioxide factories use lime (lime milk) to neutralize acidic wastewater. Figure 6 shows the lime neutralization process of a Japanese company. The method is to neutralize acidic wastewater with calcium carbonate (or limestone) and calcium oxide (slaked lime) because calcium carbonate (limestone) is cheaper than lime and can reduce the cost of treatment. The first stage is first neutralized to pH 4 with calcium carbonate, then neutralized to pH 7 with Ca(OH) ₂ (lime milk) in the second stage, while introducing compressed air to oxidize part of the ferrous iron in the wastewater to ferric iron. And can reduce the COD value of the wastewater, and finally add the flocculant to settle in the thickener, the clear liquid is discharged by the overflow, the lower thick slurry is filtered by the filter press, the filter residue (gypsum) is sold or used as the inorganic waste landfill. The pH of the acid waste water treatment by this method up to 6 ~ 9, COD100X10 ^-6 or less, and several suspension 70mg or less, in line with the national emission standards in China.

It is difficult to settle and filter the gypsum (calcium sulfate) formed by using sulfuric acid to neutralize the sulfuric acid in the wastewater. Generally, the sedimentation equipment uses a thickener with a large area. Recently, there is a membrane treatment equipment at home and abroad such as Gore membrane (organic membrane) of Gore, and ceramic membrane (inorganic membrane) of Nanjing University of Chemical Technology, which can replace the bulk thickener for the concentration of suspension after neutralization. The effect is very good, the turbidity of the discharged water is also very low, but the price of the membrane is very expensive (the domestic ceramic membrane is cheap to import the membrane), but from the cost of the fixed assets investment equipment (thickener and membrane processor) The difference is small, the operating cost of the membrane is higher than the thickener.

The wastewater that has been neutralized to neutral (pH 6~9) has no significant impact on the ecological environment. According to the report of the Ohio Health Committee and the Aquatic Advisory Committee, there is no direct harm to fish in the range of pH 5~9.5. However, fish produced in the water range of pH 6.5~8.2, and the development of fish eggs at pH 6.0~7.2 is most suitable. However, although the above-mentioned neutralized and filtered wastewater can reach the discharge standard and the surface looks clear and transparent, the water has been saturated with calcium sulfate, and it is not suitable for returning to the factory as a process water. Recently, Finland Kay Mila has invented a patent (GB2249086) that uses water vapor compression evaporation to evaporate acidic wastewater containing 1.5% sulfuric acid to 15% sulfuric acid for use in a spent acid concentration plant. The condensate from the evaporation process is available. The surface treatment process is used as ion exchange water.

So far, there is no other new method for the treatment of acidic wastewater in the production of titanium dioxide by sulfuric acid. In fact, in addition to changing the raw material route and production process, the world's sulfuric acid titanium dioxide factory uses this kind of lime-like neutralization. The key issue is that the factory must do a good job of “cleaning and diverting the flow”. It is necessary not only to separately store and discharge the acidic wastewater with different concentrations, but also to recycle the water. Other water, such as the cooling of machinery and equipment. Water, domestic sewage, bathroom drainage, and even room rainwater cannot be mixed into the discharge pipe of acidic wastewater to reduce the burden on the wastewater neutralization device.