Whiteness is one of the most important quality indexes of titanium dioxide. The factors affecting the whiteness of titanium dioxide are complex. In the actual production of titanium dioxide, the impurity content, particle size and particle size distribution in titanium dioxide are of practical significance. Therefore, in order to improve the whiteness of titanium dioxide, the impurity content should be reduced as much as possible. The insoluble impurities in titanium liquid are removed during sedimentation and fine filtration. Soluble impurities are removed in the water washing process of metatitanic acid.
At present and for a long time in the future, the production of titanium dioxide in China is still dominated by sulfuric acid method. In this production process, the discharge of "three wastes" is large, and the environmental governance problem of titanium dioxide industry becomes more prominent, which seriously restricts the development of the industry. The cost of pollution control is increasing. The adoption of advanced production equipment and processes to minimize waste emissions without reducing product quality has become the goal of enterprises and the only way for China's titanium dioxide industry to grow. The filtration equipment is used to filter and wash titanium dioxide, so as to realize the separation of titanium dioxide from other impurities. The development and progress of the filtration equipment provide a guarantee for energy conservation and emission reduction in the titanium dioxide industry.
1 Classification of filtering equipment
There are many kinds of equipment used for filtration operation in industry, which can be divided into intermittent and continuous according to different operation modes; According to the different driving forces of filtration, it can be divided into gravity filtration, pressure filtration, vacuum filtration and centrifugal filtration; According to the different filter media, it can be divided into granular media filtration, fabric media filtration and micro hollow tube filtration.
The filtration equipment commonly used in titanium dioxide industry is as follows:
Horizontal screw centrifuge ；
Disc filter;
Vacuum drum filter;
Vacuum leaf filter;
Plate and frame filter press;
Tubular filter.
Solid-liquid separation
2 Basic principle of filtration
Under the effect of pressure difference, the liquid in the suspension passes through the filter medium, and the solid particles are trapped by the medium, so as to realize solid-liquid separation. The complete filtration process includes four stages: filtration, washing, drying and unloading.
The production capacity of filtration equipment is usually expressed by the amount of filter cake accumulated per unit area per unit time. When the filtering area is fixed, the production capacity depends on the filtering speed. The filtering equation proposed by Darcy is:
u=dV／Adt=K△P／(?L)
U -- average linear velocity of liquid passing through the filter layer, m ／ s
V - cumulative filtrate volume within time t, m3
A -- Filtering area, m2
T - filtering time, s
K - permeability coefficient of filter layer, m2
△ P - pressure drop across the filter layer, Pa
?-- Viscosity of filtrate, Pa · s
L - thickness of filter layer, m
It can be seen from the above formula that under a certain material, the filtering speed is proportional to the filtering pressure, that is, the filtering pressure is large and the filtering speed is fast.
Although the basic principle of filtering is the same, different filtering devices have different characteristics, and are applied to different filtering processes according to their different characteristics. The selection of filter is mainly based on the filtration characteristics of filter slurry, the characteristics of filter slurry, production scale and other factors.
The application and progress of several filtration equipment in titanium dioxide production are briefly introduced below.
3 Working characteristics of different types of filtration equipment and their application and progress in titanium dioxide production
The filtration process in titanium dioxide production is divided into three parts: A) separation and purification of titanium liquid; B) Filtration and washing of hydrolyzed metatitanic acid; C) Filtration and washing of post-treatment materials.
3.1 Filtration equipment for separation and purification of titanium liquid
3.11 Centrifuge
It is mainly used for the separation of ferrous sulfate in titanium liquid. It is also divided into tripod centrifuge, cone and blue centrifuge and horizontal piston centrifuge. The tripod centrifuge is operated intermittently with high labor intensity. Conical and blue centrifuges and horizontal piston centrifuges are operated continuously with high degree of automation, low water content of ferrous sulfate, less washing water, high purity of ferrous sulfate produced, and less loss of titanium dioxide. The disadvantage is that the concentration of suspension is required to be high, and the corrosion resistance of the filter screen is poor, especially the titanium ore with high sulfur content has greater impact, so the filter screen needs to be replaced frequently. Its production capacity is small and suitable for small-scale production. Some domestic enterprises use imported horizontal screw scraper centrifuges with large separation capacity and high degree of automation, which are suitable for large-scale production.
3.1.2 Horizontal disc filter
It is mainly used to separate ferrous sulfate from titanium liquid. It is a continuous vacuum filtration equipment. Continuous operation of filtration, washing and drying can obtain better washing effect. Large production capacity, stable operation and few vulnerable parts. The disadvantage is the high water content of ferrous sulfate.
3.1.3 Plate and frame filter press
Plate and frame filter press is a kind of intermittent operation equipment, which is arranged by multiple filter plates. It is also divided into box type plate and frame filter press and diaphragm plate and frame filter press. The efflux mode is open flow and underground flow. The sludge separation and control filtration of black titanium liquid mainly use box type plate and frame filter, which is generally open flow, so as to check whether the filter cloth is damaged and the filtration quality.
3.2 Filtration and washing equipment applied to hydrolysis of metatitanic acid
Filtration and washing of hydrolyzed metatitanic acid plays an important role in the production of titanium dioxide. Hydrolyzed metatitanic acid is a gel with granular structure and high viscosity. The particle size distribution and impurity content of hydrolyzed metatitanic acid are internal factors affecting the water washing quality. The correct application of filtration equipment is an external factor affecting the quality of water washing.
3.2.1 Vacuum leaf filter
This is a long-term water washing equipment used by many titanium dioxide enterprises in China. It is mainly used for filtration and washing of hydrolyzed metatitanic acid and dehydration of salt treatment slurry. The leaves are rectangular. Each group has 10 to 30 leaves, each of which is covered with acid resistant filter cloth. Under the effect of negative pressure, the filtrate passes through the filter cloth and is pumped into the pouring tank through the vacuum tube in the blade.
There are two ways to use the leaf filter for water washing, including high-level water washing and low-level water washing. High level water washing means that the level difference between the water washing tank and the reverse tank is more than 10 m, and the filtrate enters the reverse tank under the action of gravity. The vacuum is stable, the air flow is smooth, and the water washing speed is fast. Foreign enterprises that use leaf filters for water washing mostly use high-level water washing. Low level water washing means that the water washing tank and the liquid pouring system are on the same level. This arrangement affects the continuous removal of filtrate, causes vacuum loss and instability, prolongs the water washing time, and increases the water consumption. This is one of the reasons why many domestic enterprises have a long water washing time and high water consumption.
3.2.2 Vacuum drum filter
The vacuum drum filter completes filtering, washing, drying, unloading and other operations in the rotating drum, with high production efficiency and large production capacity. The disadvantage is that the water content of filter cake is large and the washing is not complete. At present, titanium dioxide industry is mainly used for dehydration of salt treatment slurry. It is reported abroad that metatitanic acid is used for water washing.
3.2.3 Diaphragm plate and frame filter press
Diaphragm plate and frame filter press is a filtering and washing equipment applied in titanium dioxide industry in recent years. The filter system is composed of diaphragm plate and box plate arranged in cross intervals. The filter chamber is composed of diaphragm plate, chamber plate and filter cloth sandwiched between filter plates. Filtration and washing operations are carried out in the filter chamber, reducing environmental pollution. The operation process generally includes: pressing the filter plate, feeding filtration, pre pressing, central washing and angle washing, final pressing, reverse blowing and angle blowing, and return pulling the plate. The diaphragm plate and frame filter press is a positive pressure filtering and washing equipment, so the filtering and washing time is short, saving time and water, and the washing efficiency is high. One production cycle is 2-3 hours, and the unit consumption of metatitanic acid for washing is within 25 m3. The disadvantage is that the regeneration of the filter cloth is troublesome and the disassembly is tedious. The obtained filter cake is hard and takes time to beat. The high cantilever diaphragm filter press imported by Shandong Jinhong Taibai Chemical Co., Ltd. is equipped with high-pressure water washing equipment, which can wash the filter cloth, extending the service life of the filter cloth and improving the working efficiency.
Because the diaphragm plate and frame filter press can press materials under high pressure, the water content of the filter cake obtained is low, and the free water content is only 45%. Therefore, it can be applied to dehydration of salt treatment materials before calcination, and will obtain better energy saving effect. The water content of filter cake obtained by vacuum drum filter is 60%, which is 15% more than the material of diaphragm plate and frame filter press. Calculated according to the production of lt titanium dioxide, materials before calcination:
Water content of filter cake of diaphragm plate and frame filter press is l ／ 55% - 1=0.82 (t)
Water content of filter cake of vacuum drum filter is l ／ 40% - 1=1.5 (t)
Heat demand for evaporation of 1t water Q:
(1) Heat demand Q1 of 1t water rising from 20 ℃ to 100 ℃
Ql = cm△t =4．2×10''J／(Kg·℃)×1 000 Kg·(100℃-20℃) = 3.36×108 J
(2) Heat demand Q2 (evaporation heat of water is 2.26 × 103 J/g) for 1t water changing from 100 ℃ liquid to gas
Q2 = 2．26×103 J／g ×106g = 2.26×109 J
Q = Ql + Q2 = 2.6×109 J
The water content of the filter cake of the evaporation vacuum drum filter is higher than that of the filter cake of the evaporation diaphragm plate and frame filter press
2．6×109 J×(1．5-0．82) = 1．77×109 J = 4．21×105 Kcal
If natural gas is used as fuel for calcination in rotary kiln, and the calorific value of natural gas is 8500 Kcal/m3, the production of 1 t titanium dioxide by vacuum drum filter consumes more natural gas than by diaphragm plate and frame filter press:
4．2l×105 Kcal／8 500 Kcal／m3 = 49．5 m3
If the annual output of titanium dioxide is 50 000 t and the current natural gas price is 2 yuan/m3, the production cost will be increased:
2 × 49.5 × 50 000=4.95 × 106 yuan
The disadvantage of diaphragm plate and frame filter press used for dehydration of salt treatment slurry is that with the extension of filter cloth service time, the composition of salt treatment agent fluctuates greatly, affecting the calcination effect.
3.2.4 Tubular filter
The production and application of titanium dioxide in China has not been reported yet. Nanjing titanium dioxide has been introduced from abroad and has not yet been put into production (reported by Tibai. com). It is said that each ton of titanium dioxide can save 20 tons of water. It is washed with positive pressure water, and there is no dead corner. Feeding, washing, drying and unloading are carried out dynamically and continuously, the filter cloth is easy to clean, closed production and occupies a small area. Therefore, it has advantages that other filtering equipment cannot match.
3.3 Filtration and washing of post-treatment materials
At present, the filtering and washing equipment used for post-treatment materials are mainly vacuum leaf filter and diaphragm plate and frame filter press. Because the coated post-treatment material does not have the characteristics of colloid, it is easy to filter and wash. Also, the materials pressed by the diaphragm plate and frame filter press are dry, which is conducive to the dehydration and drying of the next process and can achieve the effect of energy saving.
4 Conclusion
The selection of suitable filtration equipment for different materials in the production process of titanium dioxide has a positive significance for energy conservation and emission reduction. There are many factors affecting the filtration washing. The quality of materials is the internal factor, and the filtration equipment is the external factor. The good quality of materials is the prerequisite for smooth filtration and washing. Under the condition of stable material quality, some filtration processes are suitable for positive pressure filtration equipment and some are suitable for negative pressure filtration equipment, which cannot be generalized. The titanium dioxide industry is a major polluter. Choosing good filtration equipment and energy conservation and emission reduction is one of the correct development directions of the industry.
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