Treatment of waste gas in titanium dioxide production

Titanium white powder sulfuric acid production of environmentally harmful exhaust gases are: the exhaust gas when the exhaust gas of the rotary kiln and calcined at acidolysis which treatment method is as follows.

1. Treatment of acid hydrolysis reaction waste gas

The total amount of exhaust gas from the acid hydrolysis reaction is not large, but the exhaust gas per reaction is concentrated in a few minutes, and the amount discharged per unit time is very large, very violent, the temperature is as high as 160~180 °C, containing acid mist, Dust and a large amount of water vapor and non-condensable gas, the main components are H ₂ SO ₄ 2~3g/m ³ , SO ₃ 7~8g/m ³ , SO ₂ 1~2g/m ³ , H ₂ O500g/m ³ Sometimes, due to improper operation or very intense reaction, the acid mist is even as high as 20g/m ³ , which is the most serious problem of disturbing people in the domestic factories and the biggest reflection of the masses.

When dealing with the acid-decomposed reaction gas, it is first necessary to find out how much the emissions are. The theoretically calculated emissions are quite different from the actual emissions. Some people in the test measured the amount of condensate finally produced by the acid-decomposed waste gas to calculate, the exhaust gas per ton of ore powder was 56m ³ /min, and was replaced by 3360m ³ /h; the exhaust gas emission of the 17m ³ acid hydrolysis reaction tank of a factory in Shanghai was 25500Nm ³ /h, equivalent to 7300~8200Nm ³ /h per ton of ore powder; a method of measuring flow velocity by dynamic pressure type on the chimney of a 30m ³ acid decomposing tank (6t each time) in a design factory in Shanghai in terms of flow, the maximum flow rate measured 26m / s, the exhaust gas pressure 2.8 ~ 7.999kPa, flow 61000 ~ 64000m ³ / h, equivalent emissions per ton of ore 10000m ³ / h, approximately. The reasons for such a big difference are mainly the degree of leakage of reaction equipment (inlet holes, pipes, chimneys, valves, etc.) of the acid solution tank, the amount of compressed air that is introduced during the reaction, and the type of minerals and the concentration of acid. And the length of the main reaction time, etc., the most influential is the degree of leakage of the reaction equipment and the length of the main reaction time.

According to the nature of the above-mentioned acid-decomposed exhaust gas composition, the main pollutants and treatment focus are sulfuric acid mist and sulfur oxides. Under normal circumstances, when the acid mist concentration is as high as 10g/m ³ or more, the condensation method is mainly used, when the concentration is <4000mg/ Nm ³ is dominated by absorption method. The efficiency of absorption is related to the absorption temperature. The absorption coefficient of SO ₂ gas in water is 56.65 at 10 ° C, and the absorption coefficient of SO ₂ in water is 18.77 at 40 ° C. It is known that the temperature can effectively absorb acid mist and sulfur oxidation. Therefore, the most effective and simple method for the treatment of acid-decomposed waste gas is to use water spray.

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Figure 1 is a flow diagram of a conventional water spray treatment of acid-decomposed waste gas. The treatment process of the process is to wash the exhaust gas in the main reaction of the acid hydrolysis into the washing tower, wash, cool and remove dust with a large amount of water. The tower can be sprayed by using a sieve tray tower or an empty tower, and the exhaust gas after washing and cooling is re-entered. The absorption tower is washed and absorbed by the dilute alkali liquid, and the alkali liquid can neutralize the acidic substance in the exhaust gas, so that the treatment is relatively complete. Its reaction formula is as follows:

2NaOH+SO ₃ →Na ₂ SO ₄ +H ₂ O

2NaOH+SO ₂ →Na ₂ SO ₃ +H ₂ O

Na ₂ SO ₃ +SO ₂ +H ₂ O→2NaHSO ₃

NaHSO ₃ + NaOH → Na ₂ SO ₃ + H ₂ O

2NaOH+H ₂ SO ₄ →Na ₂ SO ₄ +2H ₂ O

The absorption tower generally uses a packed tower, and the upper part is provided with a defoaming layer (or a glass fiber mist eliminator), and the exhaust gas absorbed by the alkali liquid (5%) is sent to the chimney by the fan. The process is more thorough, but the material requirements for the fan are extremely strict. Many factories in China have processing devices with similar processes, which are often damaged due to the high temperature erosion of the fan and the high temperature erosion of water droplets.

The method shown in Fig. 2 has simple structure, convenient operation, no need of a fan, and relies only on the natural wind extraction of the chimney, avoiding the defects of using the fan in the above method, and some titanium in Europe, Japan, Poland, Czech, etc. There are applications in the white powder factory. There are also factories in China that use this method to treat acid-decomposed waste gas. The principle of the method is to introduce the acid-decomposed waste gas into a spray chimney with a large diameter and a large volume. The lower half of the chimney is an enlarged section, which is sprayed with water in the middle or upper part of the chimney due to the diameter and volume of the spray chimney. Larger, thus reducing the flow rate of the acid-decomposed exhaust gas, increasing the contact area and contact time of the water and the exhaust gas. Because the chimney is relatively high and the diameter is relatively large, the natural wind extraction of the chimney can be utilized without having to provide a fan, and the material of the chimney can be used. Steel lined rubber, temperature resistant glass steel and treated wood. [next]

In recent years, there has also been a method of water jet absorption which has been applied to the treatment of acid-decomposed waste gas. Water jet absorption has long been used in other chemical industries. A water jet absorber for acid-decomposed waste gas treatment is a three-stage. The porous water ejector is made of stainless steel and can be used in parallel in several stages. The negative pressure generated by water injection is used to draw the exhaust gas together with water to cool and absorb. Therefore, this method does not require a fan, but the water pressure is high. A high lift pump is required. The above three methods can all meet the national emission standards (acid mist content 260mg/m ³ ) under normal conditions. But to completely eradicate the sulfur oxide absorbent is preferably an alkali to increase the flow, Germany Sachtleben company acidolysis using water spray and then exhaust through the electrostatic precipitator, SO ₂ with H ₂ O ₂ oxidation to SO ₃ and then absorbed in water.

The key points of the acid hydrolysis waste gas treatment operation are as follows:

a. The whole system (equipment, pipeline, chimney, inlet hole, valve, etc.) should not leak during operation, otherwise the treatment effect is poor;

b. As far as possible, the acid hydrolysis reaction is smoothed from the process operation, prolonging the time of the main reaction of acid hydrolysis, and reducing the flow rate when the exhaust gas is discharged to increase the time of vapor-liquid contact;

c. Ensure that there is sufficient water and water pressure. In the case of circulating water, the water temperature should not exceed 40 °C, because the rate of gas dissolution and absorption decreases with increasing temperature, otherwise only Can play a simple cooling effect without the effect of absorption;

d. During the design, the maximum exhaust gas emissions and the highest flow rate during the discharge should be measured or correctly estimated. The volume of the processing equipment should have a certain margin, minimize the pipeline distance and reduce the system resistance.

2. Treatment of calcined waste gas

When the metatitanic acid is calcined, the exhaust gas is characterized by a certain temperature, a large moisture content, acid mist and sulfur oxides, titanium dioxide dust, water vapor, non-condensable gas, etc., but the discharge speed and flow rate are relatively uniform, unlike The acid-decomposed exhaust gas is concentrated and discharged in a matter of minutes. About 15,000~20000m ^{3 of} exhaust gas should be discharged for each 1t pigment grade titanium dioxide. The temperature of exhaust gas is 200~400°C, acid mist is 1000~2000mg/m ³ , SO _{3 is} about 10g/m ³ , SO ₂ 100~500mg/m ³ , TiO _{2 is} about 0.15g / m ³ , according to the material balance calculation, the exhaust gas also contains N ₂ 54%, H ₂ O35%, O ₂ 7%, CO ₂ 4%.

A domestic unit had a rotary kiln Ñ„1800X38000mm station measured, while the yield 420kg / h, coal coke oven gas as fuel, exhaust emissions 6019.91Nm ³ / h, containing mist (sulfuric acid basis) 1645.4mg / m ³ , equivalent to 9.905kg / h; a factory in Shanghai measured the amount of exhaust gas 6000m ³ / h, acid mist (sulfur oxide) 5000 ~ 8000mg / m ³ in the same size of the kiln. Since the above-mentioned exhaust gas has a large moisture content and the dust (titanium dioxide) in the exhaust gas is hydrophilic, it is generally first treated by a wet method.

In some early-stage small and medium-sized titanium dioxide factories in China, several sets of water sprinklers were installed at different heights in the exhaust gas exhaust stack. The spray water flows from the lower part of the chimney into the sedimentation tank, and the clean water after the precipitation of titanium dioxide is discharged from the upper overflow. The treatment effect of this method is not good, the chimney is easily damaged, and a sedimentation tank with a large volume is needed. The acidic wastewater is still to be treated, and is no longer adopted. Others are first cooled by venturi spray cooling, and then sent to the chimney through the fan after washing into the washing tower or water bath dust collector; also using the ammonia water spray tri-state tower used in the past small sulfuric acid plant, the effect is not satisfactory. and it will affect the atmosphere of the calcination kiln (pressure), the impact of iron oxide calcined. It is recognized that the ideal method at home and abroad is to use electric dust removal or electrostatic defogging, and the treatment efficiency can reach more than 95%. When the operation effect is good, almost no white smoke can be observed with the naked eye.

In addition to high processing efficiency, electric demisting has a large range of defogging particle size, and can minimize “capture” to 0.01μm particles (aerosol), and the pressure loss during processing is small, only 98~196Pa, although the voltage is very high. However, the current is small, so the power consumption is small and the processing capacity is large. After special design, the high temperature gas of 350~500 °C can be directly processed. For example, the electrostatic mist eliminator used in a 1MW thermal power station can treat millions of exhaust gases. Cubic meters / h. Table 1 is a comparison of the effects of various dust removal and defogging methods.

Table 1 Comparison of various dust removal and demisting devices

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1824 Hohfeld first proposed the electrostatic deposition method, the 1885 Walker and Harkin Sri Lanka in lead smelter built the first electrostatic precipitator in the world, in 1907, Professor Kurt Demirel University of California for the electrostatic demisting The treatment of sulfuric acid mist has been rapidly extended to cement plants, power plants, smelters, paper mills, etc. for more than 90 years. See Figure 3 for the electric defogging process of the calcined waste gas from the sulfuric acid method titanium dioxide factory.

The exhaust gas discharged from the calcining kiln is generally first removed in the flue (or settling chamber) by inertia and gravity sedimentation, and then enters the venturi scrubber, and the gas is accelerated in the venturi throat. It is quickly crushed by high-speed airflow to collide effectively. When the gas and liquid enter the venturi, the deceleration is caused, and the water droplets collide with the dust particles and gas to cool again. Some of the SO ₃ becomes acid mist, and the dust is captured by the water droplets and wetted. Condensed into large particles with water discharged into the recovery tank, while the temperature of the exhaust gas can be reduced to 60~70. The gas then enters the scrubber (packed column or foam tower) to lower the temperature of the gas to below 50 and enters the electric mist eliminator. Under the action of the high voltage electrostatic field, the acid mist and a very small amount of dust are removed, and the tail gas passing the discharge standard is The fan is sent to the chimney for discharge, and the SO ₂ and acid mist in the calcined exhaust gas treated by the electric defogger can reach 50mg/m ³ or less, which meets the national emission standard (SO ₂ 34kg/h or 145mg/m ³ , acid mist 260mg/m ³ ).

The corona discharge process is the main principle of electric defogging. High-voltage electrons close to the cathode are strongly accelerated by a strong electric field, and electron avalanches are formed by collision. These electrons form negative ions with charged gases and particles (SO ₂ , O ₂ , dust particles, etc.), and then these negative ions are transferred to the positive electrode in the electric field. During the transfer to the positive electrode, some of the particles in the gas are again charged to charge the particles, and some of them directly move to the positive electrode, and the neutral molecules in the positive electrode are trapped.

Corona can be divided into two zones, one called the plasma zone, which is surrounded by a corona wire, which includes anions, cations, free electrons, and gas molecules in a normal state and an excited state. This region is small and does not actually display a charge. Therefore, it is called the plasma zone, and the occurrence of ionization is mainly here. After ionization, a number of positive charges move toward the negative electrode, and free electrons get a large amount of energy to be accelerated, and collide with the molecules to form a sharp electron avalanche. At night, the corona discharge is observed, and there is a clear blue light around the cathode (glow effect) ). The other area is the anion space charge zone, also known as the corona outer zone. The corona outer zone actually occupies the entire volume of the electrical zone, and has more neutral gas molecules, a small amount of anions and free electrons, particle charging and recycling. It happened in this area.

The plasma region is the source of free electrons. The strength and number of free electrons determine the efficiency of electric defogging. The formation of the plasma region mainly depends on the electric field, and the electric field strength comes from the corona voltage. The flow rate and flow rate of the treated exhaust gas in the electric demisting design determine the outer dimensions of the electric mist eliminator. The electric field strength determines the effect of electric defogging and is the only place that can be controlled during operation.

After the above-mentioned process fan is installed in the electric defogger, the electric defogger belongs to the negative pressure operation; there is also the positive pressure operation of the fan before the electric demisting, and the venturi arrangement can be either horizontal or vertical, and the vertical can be reduced. The dust is clogged, and some factories consider the venturi's resistance to be large and do not use venturi. Instead, they use two-stage spray cooling to enter the electric defogger. The electric mist eliminator can be used for the treatment of titanium dioxide calcination waste gas. It is a rotary kiln with the specification of Ñ„1800mm×38000mm. It uses 92 tube (Ñ„250mm×4000mm) tubular electrostatic mist eliminator to generate electricity. Halo voltage 4kV, secondary spark voltage 60kV, corona current 50mA, the electrode is made of steel wire lead-out hexagonal star strip (or thorn line), the precipitate electrode tube is made of hard PVC pipe (or hard lead pipe), the processing capacity is 8000~10000m ³ / h.