The main process steps of the sulfuric acid process are: The main waste generated by the process is waste acid (including washing water) and FeS0 4 ·7H 2 0 produced by using ilmenite as raw material; the waste acid is generally very rare, and the H 2 S0 4 content is less than 25%, some Titanium dioxide producers try to separate the strong acid waste generated by the first filtration during acid hydrolysis and the weak acid waste generated by subsequent filtration and washing. The by-products produced by the sulfuric acid process per ton of titanium dioxide produced and the yields are as follows. Towel ring is used to hang towels at home. Generally, its shape is ring or semicircle. It is used to place towels and wipe hands after washing hands. Depending on the designer, there will be some other shapes. The shape of the towel ring is also changeable and distinctive, which can meet the needs of different bathrooms.
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1TiO 2 raw material is acidified with sulfuric acid;
2 Settling, separating soluble titanium oxysulfate from solid impurities;
3 hydrolyzing titanium oxysulfate to form an insoluble hydrolysis product or called metatitanic acid;
4 calcination removes moisture to form dry pure Ti0 2 .
If the initial raw material ingredients used have a high iron content or a low titanium content, a process step of removing and recovering FeS0 4 ·7H 2 0 and concentrated titanium liquid is added between purification and hydrolysis.
1. Acidolysis Grinded, dried ilmenite (containing 42%-60% TiO 2 ) and / or acid-soluble titanium slag (TiO 2 content 72% -78%) is generally used in lead- lined reactor with concentrated sulfuric acid at 150 Acidolysis at a temperature of -180 °C. In order to facilitate acid hydrolysis, the raw materials are usually ground to about 200 mesh. It is noted that, leucoxene, rutile and synthetic rutile is insoluble in sulfuric acid, with sulfuric acid can not be titanium dioxide production process method.
The acid mixture is generally agitated with air and heated by blowing steam. Most plants use 85%-92% sulfuric acid, and the intense exothermic reaction begins around 160 °C. In some plants, acid premixed materials are first used to help mitigate violent reactions.
The higher the iron content in the ilmenite (the lower the TiO 2 content), the higher the degree of dilution of the sulfuric acid used. A suitable acid concentration for processing rock ore is 85%. In the case of treating titanium slag, the H 2 S0 4 in the acid is generally 91% to 92%. In order to obtain a gentle reaction, it is not necessary to use sulfuric acid which is more concentrated than this.
After that, the titanium solution is gradually diluted, first with acid and then with water. Regardless of the concentration of the acid, the morphology of the reaction solid phase is a loose porous cake whose main components are Fe 2 (S0 4 ) 3 and TiSO 04 (titanium sulfate). Since the vanadium , chromium and other metals present in the raw materials are decomposed in the sulfuric acid, such porous metal salts are also contained in the porous cake.
The acidolysis reaction is usually carried out batchwise using an acid hydrolysis tank capable of containing 30-40 t of reactant. The violent exothermic reaction generally lasts for about 30 minutes, and then the porous cake solid phase is cooled for about 3 hours.
Most of the air pollution produced by the sulfuric acid process comes from acid hydrolysis. In the reaction, a large amount of sulfur oxides, acid mist and entrained unreacted raw material particles are released in a short time. These leaps are temporarily collected into a gas scrubber and a solids dust removal system.
Some factories, such as the Merrill Lynch in Salvador (Brazil), Huntsman's plant in Tirukalong (Malaysia) and Titanium Industries in Ube (Japan) and Hankou (Korea) to facilitate better control of reactions And to reduce the release of sulfur oxides, a continuous acid hydrolysis process is employed.
Next, the sulfate porous cake is leached with water and/or dilute sulfuric acid. The decomposition of this cake and the reduction of ferric/divalent iron usually take 11-12h, so that the total reaction time in the acid hydrolysis tank is 14-15h.
If ilmenite is used as the raw material, the titanium liquid is treated with iron filings to reduce ferric iron (Fe 3+ ) to divalent iron (Fe 2+ ). If titanium slag is used as the basic material, this step is omitted because only Fe2+ is used. The reduction continues until some trivalent titanium (Ti 3+ ) is produced in order to maintain all of the iron in its divalent form in subsequent steps. Usually the indicator is 2% Ti 3+ , and most of the titanium is in the form of tetravalent (Ti 4+ ). As with continuous acid hydrolysis, reduction can also be carried out in a continuous manner.
If Fe 3+ is allowed to enter the hydrolysis stage, they will adsorb on the surface of the TiO 2 particles, resulting in a low whiteness index of the final titanium dioxide product. Therefore, it is important to maintain iron in a bivalent form throughout the process. [next]
2. Clarification/Settling The cooled acid hydrolyzate, solid inert material and unreacted raw material residue solution are all discharged from the bottom of the acid hydrolysis tank to the wide bottom low sedimentation tank/settling tank.
Here, the soluble residue formed by the titanium ore impurities is removed. These residues may include silica , zircon /zirconium sulfate, leucoxene and/or rutile. Add casein, starch or other organic flocculant, and the liquid is precipitated in the settling tank by simple gravity separation. The sedimentation of the soluble residue can be carried out at this stage in the form of precipitation of strontium sulfide (SbS 3 ). To this end, it is necessary to add cerium oxide to the original raw material in the acid hydrolysis stage, and add sodium sulfide to precipitate SbS 3 when it settles.
The solid matter is removed from the settling tank by a rotating crucible. Typically, there is a concentrated discharge point at the bottom of the settling tank. After the solid matter is removed, it is washed with spent acid to recover unreacted raw materials, and then the residual acid is washed away with water. The settled titanium solution removes fine residual particles by fine filtration. These fine filtration slags are combined with other solids collected from the settling tank and sent to a licensed yard.
The entire settling process is approximately 8 h.
3. Green sputum recovery Titanium solution is cooled to about 10 °C to precipitate most of the iron in the form of "green sputum". The green mites are mainly FeS0 4 ·7H 2 0, mixed with chromium, vanadium, manganese and other metal sulfates. These metals are entrained in the original material. The remaining Fe 2+ remains in the titanium solution. Green mites can be filtered out in the form of red mud. Disposal of green mites is one of the major problems in the sulphuric acid process. In modern sulphuric acid plants, green mites are removed using a special vacuum freeze crystallization system designed to produce large FeS0 4 ·7H 2 0 crystals with minimal chromium and vanadium impurities. It is important that large crystals are easy to handle and store, as most titanium white producers process recycled green oysters into by-products, such as soil conditioners or water treatments.
If only titanium slag is used as the raw material, there is not a large amount of iron precipitated at this stage. In this way, the titanium dioxide producer avoids the problem of green enamel treatment that comes with it. The critical ratio of FeSO 4 :Ti0 2 is 7:10. If the FeSO 4 content of the titanium liquid is high, the green mites must be removed at this stage. However, if the ratio is less than 0.7, it is not necessary to remove the green cockroach.
After the green mites are removed, the remaining titanium liquid is typically concentrated by vacuum evaporation to a relative density of 1.67 at 25 degrees Celsius. After this step, if the raw material is ilmenite, the Ti0 2 content of the titanium liquid is 230 g/L. If the raw material is titanium slag, the TiO 2 content is 250 g/L. Despite the precipitation of green mites and iron, the iron content in the titanium liquid obtained by ilmenite is still higher than that of titanium slag.
4. Hydrolysis The hydrolysis process is a very critical step in the production of titanium dioxide by sulfuric acid. In this step, the soluble titanyl sulfate is hydrolyzed at 90 ° C into a water-insoluble hydrated Ti 2 2 precipitate, or metatitanic acid. To obtain a high quality hydrolyzate of the desired particle size, conditions such as heating rate, Fe 2+ and Ti 4+ content of the titanium liquid, and other factors must be strictly controlled. As mentioned earlier, avoiding Fe 3+ is the key to this link.
In order to control the hydrolysis rate, the filtration washing performance of the hydrolyzate, and the fineness and quality index of the final product, it is necessary to add a seed crystal during the hydrolysis. There are two ways to add seed crystals: self-seeding (Blumenfeld method, 1928) and additional seed crystals (Mecklenburg method, 1930). Both methods produce the same quality. [next]
The self seed crystal is subjected to a hydrolysis process by using a seed crystal produced by pre-adding a hydrolyzed titanium liquid and water at the time of hydrolysis, without separately preparing a seed crystal.
Additional seed crystals, as the name suggests, are rutile or anatase seed crystals prepared separately from titanium liquid to control the rate of hydrolysis and the final crystal type of the titanium dioxide product. The rutile seed crystal for this purpose is prepared using ferric acid-hydrochloric acid or pure TiCl 4 , and the anatase seed crystal is produced by adding metatitanic acid, sodium hydroxide or adding titanium or human acid or acid to the titanium liquid.
The seed crystal added is cleaved, and the TiO 2 precipitated from the titanyl sulfate solution is anatase. However, at this stage the addition of rutile seeds are easy to make into a metatitanic acid precipitate is converted to rutile Ti0 2 at the time of firing. If anatase or self-crystal seed is used, it is advantageous to add rutile-type calcined seed crystal during calcination to facilitate the formation of rutile Ti0 2 .
The precipitation of metatitanic acid is achieved by a few hours of boiling of the titanium liquid. At the end of the precipitation, sometimes a certain amount of water is added to increase the rate of hydrolysis. However, the addition of excessive water will destroy the quality of the Ti0 2 precipitate, and the entire hydrolysis and precipitation process takes 3-5 hours.
The hydrolyzed precipitate slurry is filtered, washed, and leached with sulfuric acid under reducing conditions to remove the last trace of adsorbed iron and other metals, so-called bleaching, about 7%-8% of S0 3 is tightly adsorbed in the slurry. In the material, it cannot be washed off. In fact, it is necessary to undergo filtration and washing to separate the metatitanic acid precipitate. Most of the spent acid produced by the sulfuric acid titanium dioxide is produced therefrom.
The filtrate was first filtration (called "concentrated spent acid") generally contains H 2 SO 4 22% -24% . Usually, every 10 lb of "concentrated waste acid" is produced for each of the finished titanium white powder. If the slag as the raw material, this acid decomposition still contains ferrous sulfate, also contains a large amount of aluminum sulfate and magnesium sulfate.
5%。 The H2SO4 is less than 0.5%. According to the quantity of washing and filtering, the dilute acid produced by each finished It titanium dioxide can reach 60t.
To control the particle size growth modifiers to be added to the metatitanic acid, such as potassium, phosphorus, potassium and zinc. It is sometimes necessary to further add rutile seed crystals in this step to promote the formation of rutile-type Ti0 2 during calcination. The material ultimately used for calcination is a hydrated TiO 2 slurry having a solids content of from 35% to 50%.
5. Calcination Calcination is carried out in a micro-dip internal combustion rotary kiln. Under the action of gravity, the hydrated Ti0 2 slurry is slowly advanced in the rotary kiln. The calcination temperature is 900 - 1250 degrees Celsius. In order to achieve the desired type of titanium dioxide, the actual temperature needs to be strictly controlled in several levels. Generally, the temperature required to produce rutile titanium dioxide is higher. The calcination step removes moisture and removes residual traces of S0 3 , while also converting the anatase form into a rutile form. Calcination also helps to enhance the chemical inertness of the final titanium dioxide product and to determine its particle size, although the determination of particle size is primarily in the hydrolysis stage. Sampling by process and physical detection of color, achromatic force and particle size can assist in the tight control of the calcination process.
In the calcination stage, with the elimination of S0 3 and acid mist, it is inevitable to entrain some fine TiO 2 particles, and the flue gas is clearly visible. Therefore, the plant must be equipped with means for recovering Ti0 2 particles, dust removal and acid mist removal so that the calcined tail gas does not pose a hazard to the environment.
After calcination, Ti0 2 is ground to crush the sintered particles. Thereafter, some titanium dioxide is sold in the form of uncoated Ti0 2 for use in applications requiring primary products such as enamels and welding rods. It is primarily and is the surface treatment that is required for almost all titanium dioxide plants, ie post-treatment. However, most manufacturers perform surface treatment of Ti0 2 on the same site. Only a small number of manufacturers carry out post-processing in different places. The surface treatment steps are described in the next three subsections. [next] 
As ilmenite raw material: 3-4t FeS0 4 · 7H 2 0; 7-8t (23% H 2 S0 4 ) waste acid;
Raw material of titanium slag: 4-6t (25% H 2 S0 4 ) waste acid.
In addition, in the calcining stage, there are 7-8 kg of S0 3 per ton of titanium dioxide or must be recycled to reduce air pollution.
