Titanium dioxide is an important pigment widely applied in different industries, such as ink, coatings,masterbacth, PVC, paper, and cosmetics. knowing the titanium dioxide production process is key to understand the complex and accuracy in the production of titanium dioxide. We will get idea of different steps of process needed in the production. By understanding this process, we can better value the importance of titanium dioxide in various applications.
Two Production processes of Titanium Dioxide
There are two different production process of titanium dioxide, which are the production process of chloride method and the production process of sulfuric acid method.By sulfuric acid process, titanium powder will be reacted with concentrated sulfuric acid to get titanium sulfate. With hydrolyzation, metatitanic acid can be generated. Then with calcination and crushing, titanium dioxide can be obtained. Both Rutile and Anatase titanium dioxide can be produced in this way.
There are several advantages by sulfuric acid process. Its raw material is ilmenite and sulfuric acid, and they are easy to get. Its production technology is relatively mature which requires simple equipment.Also its anti-corrosion materials are easy to handle. But sulfate process also has some disadvantages. Its production process is long and can only be operated intermittently. It is by wet method operation which costs high consumption of water and sulfuric acid. There are various wastes by-products which may lead to pollution to the enviroment.
By chlorination process, rutile or high-titanium slag powder are mixed with coke. And titanium tetrachloride can be produced by high-temperature chlorination. With oxidization at high temperature, titanium dioxide can be obtained after filtering, washing, drying and crushing. And only rutile type titanium dioxide can be produced by chloride process.
Chlorination process also has advantages. Its production process is short, continuous automation is high, the production capacity can beexpand easily, and its energy consumption is much lower. Three wastes are lower, but higher quality products can be produced. But it also has some disadvantages. It requires high investment, complex equipment, higher quality raw materials. Its material also needs good resistant to high temperature and corrosion. The maintenance of the device and R&D is difficult.
Titanium Dioxide by Sulfate Process
The sulfate process is mainly grouped into five steps: preparation of titnaium ore or titanium slag; preparation of titanium sulfate; preparation of hydrated titanium dioxide; calcination of hydrated titanium dioxide; after-treatment of titanium dioxide.
The above five steps includes following segments: drying, magnetic separation and grinding, acid hydrolysis, purification, concentration, crystallization and hydrolysis, water washing, bleaching and washing after bleaching, salt treatment, calcination, dispersion and classification of titanium dioxide, inorganic surface treatment, water washing, drying, airflow crushing and organic treatment, packaging, recycling, by-products treatment and utilization. Various types of titanium dixoide such as anatase, rutile, and enamel type can be obtained through the sulfuric process.
Main segments in sulfate process production process
Acid hydrolysis. The titanium slag or dryilmenite is acidolyzed with concentrated sulfuric acid at a temperature of 150-180°C.The higher dilution of sulfuric acid is used when the ilmenite has lower titanium content. 85% of sulfuric acid concentration is suitable for treatment of rock ore; while 91-92% of sulfuric acid concentration is suitable for treatment of titanium slag. In the acidolysis process, ferric iron needs to be turned into ferrous iron.
Ferric iron is easy to be adsorbed on the surface of titanium dioxide particles, which will lead to the lower whiteness titanium dioxide.So it is quite important to maintain iron in a divalent form in the production process. Also the acid hydrolysis will produce the most of the air pollution. During the reaction, a big amount of acid mist, sulfur oxides, and unreacted raw material particles are produced in a short time.
Sedimentation and filtration. The cooled acidolysis solution, solid inert substances and unreacted raw material residue solution will be transfered from thebottom of acidolysis tank into the settling tank. Itspurpose is to remove soluble residues caused by titanium ore impurities.
These residues may include zircon,sulphate, silica, rutile or leucolite. By adding starch, casein, or other organic flocculants, the liquid settles in the settling tank with simple gravity decomposition. First the solid matter is removed from the tank. Then raw materials will be recovered with washing of waste acid. Also the residual liquid will be washed by water. With fine filtering, the residual particles will be removed from settled titanium solution. These residues and other solids collected in the settling tank will be delivered to the storage yard.
Hydrolysis.Water-insoluble hydrated titanium dioxide precipitate or metatitanic acidcan be obtained with the hydrolyzation of soluble titanyl sulfate at 90°C. To getqualified hydrolyzed products at required particle size,the heating rate, ferrous ions content, tetravalent titanium ion content and other factors of titanium liquid need to be strictly controlled. The control of ferric ions is the key in this process. During rehydrolysis,crystal seeds is necessary to be added to control the hydrolysis rate, the filtration and washing performance. This can make the final product with better fineness and other quality indicators.
Seeds can be added in two ways: self-seeding and external seeding. The self-seeded crystal added during the carry out of the hydrolysis process and no additional preparation of the crystal seed is required. External crystal seed is to add rutile or anatase seed crystals to the titanium solution. Adsorbed iron and other metals can be removed under acid-leached with sulfuric acid after the hydrolyzed sediment slurry is filtered and washed. It is also referred as bleaching. This is the main step for causing waste acid in the sulfuric process.
Calcination.At 900-1250°C,the hydrated titanium dioxide is calcined. During the calcination process, water and sulfur trioxide can be deducted. At the same time, anatase titanium dioxide can be transformed into the rutile type. Calcination can determine the particle size of the final titanium dioxide and enhance the chemical inertness. Titanium dioxide is grind into particles and then will be after-treated. In the calcination stage, some fine titanium dioxide particles are entrained with the discharge of sulfur trioxide and acid mist. This is the main source of waste gas in the sulfuric process.
After-treatment. After treatment process has following aspects:wet grinding, inorganic coating, drying, jet milling, and organic coating.
Different Stages In After Treatment Process
During wet grinding, titanium dioxide particles will be dispersed together with organic or inorganic dispersants. The aggregated titanium dioxide particles have to be depolymerized as much as possible, which can facilitate the inorganic coating. Wet milling equipment mainly includes ball milling machine, sand mill, dense medium mill, organic ball mill, etc.
After wet grinding and depolymerization, titanium dioxide needs to be treated with inorganic coating. This can shield ultraviolet light and strengthen its weather resistance. Inorganic coating agents are aluminum oxides, silicon oxides, water and hydroxides. In recent years, zirconium has been adopted for mixed coating to achieve better gloss and weather resistance. In drying process, a belt dryer or a spray dryer is used.
After inorganic coating, jet crushing and organic coating is made. Flat airflow pulverizers are commonly used currently. Different organic coating agents are added in this process. On the one hand, organic coating can help in neutralizing the surface energy of titanium dioxide to avoid its re-agglomeration. On the other hand, it can change the physical and chemical properties of titanium dioxide, which help improving its compatibility in the paint and plastics formulation. In paint application, amines, alcoholamines, and silanes are used for organic coating. In plastics application, dimethylsilane, polyethanol, and triethanolamine are used.
Chlorination production process of titanium dioxide
Chlorination production process of titanium dioxide are as follows: preparation of raw ore, preparation of titanium chloride, titanium chloride, titanium dioxide surface treatment. It includes following segments: ore coke drying, ore coke crushing, chlorination, titanium chloride refining,oxidation of titanium chloride , dispersion and classification of titanium dioxide, inorganic surface treatment, water washing, drying, air crushing and organic treatment.
Differnt Segments in the Chlorination production process
Titanium ore will be added into the boiling furnace from the boiling layer with certain proportion of petroleum coke. The circulating chlorine gas back from the oxidation process will be added in from the bottom of the furnace. This reaction is carried out at 925~1010℃ in a continual process. TiO2 and oxides of iron, silicon, alum, calcium, magnesium and other metals are transformed into corresponding chlorides. They will get out of the chlorination furnace with gas. The reaction gas needs to go through the cyclone separator.
And unreacted solid dust and non-volatile chloride in the reaction gas will be separated. After cooling and condensing, the ferrous chloride is separated first, and then ferric chlorine is separated. After this, low-boiling titanium tetrachloride (containing silicon tetrachloride and alum trichloride, etc.) will be collected by using cold titanium tetrachloride spray. After mud removing, crude tetrachloride is obtained. Crude titanium tetrachloride will be made into high-purity refined titanium tetrachloride by removing low-boiling impurities such as silicon tetrachloride.
the oxidation reactor needs refined titanium tetrachloride preheate, oxygen preheated, nucleating agent and crystal transformation accelerator to react in it. The initiation temperature of the oxidation reaction is above 800 °C. The main reaction is generally processed at 1300-1800 °C. The airflow from the reactor (entraining the titanium dioxide particles generated by the reaction) is quenched with low-temperature circulating chlorine gas to below 700 ° C, and then cooled to 200 ° C through the pipeline along the way, and the titanium dioxide is gradually separated from the cyclone separator and bag collector The chlorine gas is separated, and the titanium dioxide is sent to the post-treatment process after dechlorination treatment.
The post-treatment section includes following process: beating, grinding, chemical preparation, surface treatment, filtration and washing, spray drying, micro-grinding and packaging. Following are the details.
After oxidation process , the TiO2 powder will get into the beating tank. Dispersant and desalted water are measured and put into the beating tank. The qualified slurry will go to the grinding process.
After beating, the slurry enters into sand mill’s feedtank and get redispersion. By the feed pump, the slurry gets into sand mill. It will be ground to slurry with particle size less than 1 μm. After grinding, the slurry will go to the storage thank of sand mill. It will be pumped in the classifier as per different grades. The coarse material gets back to sand mill again. The fine material gets to the surface treatment tank.
After measuring, the slurry comes into the surface treatment tank. Different surface treatment agents are added in to the slurry. After the reaction, it goes into the filtration process.
The treated slurry gets pumped into the suction tank. In the suction tank, the leaf filter is vacuum-filtered . After the film loads up, the bridge crane get up the leaf filter. It is put into desalted water for washing. After it meets the requirement, the bridge crane gets up the leaf filter and unloads the slurry in the discharge tank.
The powder will go into airflow conveying system. At the entrance of the airflow mill, a pump delivers the powder. After adding crushing aids, the powder will get crushed into an average particle size of around 0.3μm. The bag filter separates the entrained TiO2 powder. The powder cools down by the air and gets seperated by cooling cyclone. The air flows out of the bag filter and goes into the atmosphere through the big fan. After the cooling cyclone and bag filter, the Tio2 powder gets into packaging process.
The TiO2 finished product comes into the screw conveyor. And it will go into the finished product warehouse. The package of TiO2 is 25 kg/bag or 1000 kg/bag by packaging machine as per customer needs
The Current Capacity Situtaion For Two Processes
Chloride process of titanium dioxide is the development trend. The chlorination method and the sulfuric method will exist together for a long time. Instead of a substitute relationship, they are complementary. In china, the total capacity of Chloride titanium dioxide from China is around 40MT. And the total capacity of sulfate titanium dioxide in China is 3.3million MT. The production capacity of chloride titanium dioxide increases in recent 10 years. The producers of chloride titanium dioxide in China has increased from 1 producer to 5 producers.
We have stated about the two different production process and each section of the titanium dioxide production process. It provides the chemical transformations and equipment involved in different process. And you will get a clear idea about the quality control of titanium dioxide.