Production of Titanium Dioxide by Sulfuric Acid Method--Removal of Titanium Residue (I)

First, the production of titanium residue
After decomposition of the acid solution in the pot with sulfuric acid and iron titanium ore by leaching the resulting solution, a complex system of turbid. This solution has both the properties of a true solution and the nature of a colloidal solution. It contains both soluble sulfates based on titanium and iron; it also contains insoluble, larger particles of suspended mechanical impurities and smaller particles with higher stability and colloidal impurities. The latter two insoluble solid impurities are referred to as titanium residue. Larger particles of mechanical impurities, mainly insoluble or decomposition of ilmenite, rutile, monazite, wrong cristobalite, gangue, mud, and carbon, lead, calcium and the like compounds. Smaller particles of colloidal impurities, mainly silicic acid, aluminum salts, and metatitanic acid. Silicic acid and aluminate colloids are mainly formed by the action of sediment and sulphuric acid in ilmenite. The metatitanic acid colloid is mainly produced by acid hydrolysis and leaching operation conditions, and some titanium liquid is produced by early hydrolysis.
Second, the harm of titanium residue
1. The sedimentation time of the titanium liquid is prolonged, and the slag liquid is difficult to stratify. When the titanium liquid is extracted, the colloidal impurities in the upper layer of the residue are often extracted.
2. Separation and filtration of ferrous sulfate crystals and molten iron and plate-frame pressure filtration for further purification of titanium liquid will cause the filter layer to clog, and filtration is difficult to carry out, so as to greatly affect the production of titanium dioxide.
3. Contaminated ferrous sulfate crystals.
4. Colloidal impurities will become irregular crystal centers during hydrolysis, so that the shape of the metatitanic acid particles is irregular and angular, which makes the particles of the product after calcination hard, poor in color and low in purity.
5. Colloidal impurities are positively charged particles. When the colloidal particles are hydrolyzed into a crystalline center, some of the adsorbed metal ions will eventually be mixed with the finished product, reducing the purity of the finished product, while some colored iron, vanadium , chromium , and manganese The mixing of harmful metal ions such as lead and drill will also affect the whiteness of the product.
In order to accelerate the sedimentation of the residue, accelerate the filtration rate of ferrous sulfate crystals, accelerate the speed of plate and frame filter press, and produce regular metatitanic acid and easy to wash, in order to improve the yield and quality, and produce high quality ferrous sulfate, it is necessary. The residue of the titanium liquid is removed.
Third, the principle of removing titanium residue
For insoluble, large particles of mechanically suspended impurities, they can be separated by natural sedimentation under gravity (see Figure 1). For colloidal impurities with a content of about 20% to 36% of the total insoluble impurities, which are small and dispersed, due to the adsorption of H ⁺ , with the same positive charge, these colloids cannot be electrostatically repulsed. The mutual condensation depends on gravity to settle. However, if a negatively charged sol is added, it can be electrically neutralized, and the fine particles form larger particles in the collision and settle under gravity. It is also possible to utilize the polar adsorption and bridging effects generated by the affinity of certain high molecular polar groups, so that the suspended particles are connected to each other by means of the action of the flocculant molecules, and the network becomes a large aggregate and rapidly settles.

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Fourth, the method of removing titanium residue
First of all, we must fundamentally eliminate the source of the residue, so as to produce as little residue as possible, which requires the selection of mines to minimize the composition of sediment; the second is to strictly control the production operations of acid hydrolysis and leaching so that they do not occur as much as possible. Or early hydrolysis is less likely to occur, resulting in no or less production of metatitanic acid or orthotitanic acid colloid.
As for the value of whether the residue is recycled and used for acid hydrolysis, the Ti0 ₂ content is still more than 40% from the verification of the residue dry product. If it can be recycled, it really benefits a lot. In order to use these residues, a residue from the washing and drying of the suspended gelatin was returned to a small amount for re-acidification. As a result, the residue accumulated more and more, and there were several hundred piles on the floor. Tons, but the recovery rate of titanium is still very low, the unit consumption of acid and mine is still high, indicating that the residue is not necessary for recovery and acidolysis.
The author believes that this part of the undecomposed titanium ore may have a different structure, which may be a poorly soluble rutile component, which is extremely difficult to decompose with normal acid hydrolysis conditions. These residues did not dissolve under the severe reaction conditions of the previous acid hydrolysis, and the same conditions were obtained in the next acid hydrolysis, and certainly did not dissolve. If you use more acid or smash the residue to a finer, or at higher temperatures, I am afraid it will dissolve. It may be used for the production of electric welding rods or for the treatment of titanium dioxide by chlorination.
There are various methods for removing titanium residue. According to different sedimentation methods, it can be divided into batch method and continuous method. According to the sedimentation mechanism, there are coagulation method, flocculation method and mixed sedimentation method.
1. Batch method The batch method is carried out in a settling tank. The slag-containing titanium liquid is stirred under compressed air, and a certain amount of the settling agent is added. After the dispersion is uniform, the titanium liquid is allowed to stand for 8 to 12 hours. After the inspection meets the process requirements, the supernatant liquid can be taken out for use in the next process. After the lower residue is added with water and stirred well, the mixture is pumped and filtered by a plate and a filter press, and the filtrate is a light titanium solution for immersing the solid phase material in the next acid solution pot. Residues for paving or draining sewers.
Although this batch method has a large area, a long production cycle and a low production capacity, the process adaptability is strong. The titanium dioxide factory with a small production scale in China currently uses a batch method.
At present, China's 10,000-ton manufacturers have increased. This small-scale batch method has not adapted to the needs of production development. Some large factories have adopted the following large-scale batch method. The operation process is as follows: the titanium liquid leached after acid hydrolysis is added in proportion to the flocculant, and the mixer is used to pump 2-3 sedimentation tanks with a slight slope at the bottom. The volume of each sedimentation tank is equivalent to the total volume of titanium produced per day (the volume of the settling tank introduced by foreign technology is above 300 m ³ ). After 2.5-3 h of sedimentation, the clear titanium solution with a depth of about 100 cm at the top reaches the clarification requirement, and the supernatant (about 35-45 m ³ ) is taken up by a siphon that can move up and down to be introduced into the centrifugal pump and sent to the next step. After each settling tank is treated with 10-13 cans of titanium liquid, the bottom sludge is about 50 cm thick. At this time, another settling tank is used to receive the titanium liquid. Before cleaning the sludge, the upper turbid suspended titanium liquid is introduced into another sedimentation tank through a centrifugal pump, and a certain settling agent is added at the same time. Then washed with water cannons mud bottom slag, and the resulting slurry washed suspension into the filter press through the bottom plate and frame filter press slurry pump valve directly. The filtrate is stored and used for acid leaching and leaching of the solid phase. The slag is flushed with a high-pressure water gun to sluice the sewer or used for paving (the mud suspension is also passed through the bottom of the pool, and the diameter of the pump is 3 m, the length is 5. 5 m. , a single filter area 52m ² vacuum drum filter filtered). If the sedimentation effect is poor and needs to be treated with an inorganic aiding agent, it can be treated in the acid solution pot before the acid liberation material.
2. Continuous process The continuous process is carried out in a thickener. The prepared settling agent solution and the slag-containing titanium liquid are continuously flowed into the thickener at a certain ratio of the flow rate, and the sedimentation floc is under gravity and mechanical stirring under 0. 1-0. 2r/min. Concentrated by the centripetal force, concentrated on the bottom of the thickener, timed or continuously discharged. The supernatant liquid continuously flows out from the overflow port. A Φ12m*1.5m thickener with a monthly output of 750 tons.
This continuous method has large production capacity and can be operated continuously, but it is restricted by the sedimentation effect. Many titanium dioxide factories in China are unwilling to use it. Larger manufacturers in foreign countries (such as the former Soviet Union) are using this continuous method. [next]
3. Coagulation method The commonly used coacervation method is cerium oxide -ferrous ferrous sulfide method. The cerium oxide is injected into the acid hydrolysis pot together with the ilmenite powder, and the cerium oxide reacts with the sulfuric acid during acid hydrolysis to form strontium sulfate [Sb ₂ (SO ₄ ) ₃ ]. Before the titanium liquid settles, ferrous sulfide or sodium sulfide is added, and the sulfide reacts with the free acid in the titanium liquid to form hydrogen sulfide, and the generated hydrogen sulfide encounters barium sulfate in the iron liquid, that is, an insoluble sulfide sulfide precipitate is formed. Its relevant reaction formula is as follows:

Since the last formed ruthenium sulfide is a negatively charged micelle, it can be electrically neutralized with the positively charged silicic acid and aluminate colloidal impurities in the titanium solution, so that the cohesive force of the colloidal particles is increased, thereby causing cohesion. Accelerate the settlement.
The addition of ferrous sulfide does not increase the viscosity of the titanium solution, nor does it cause filtration difficulties. At the same time, heavy metal ions such as Cu ²⁺ and Pb ²⁺ in the titanium solution act on it to form insoluble heavy metal sulfide precipitates (CuS, PbS), which are removed together with the residue.
However, this material is consumed in large sedimentation method, high cost, not agglomerated larger particles, poor cleaning effect, the sedimentation rate is slow, resulting sulfide precipitation will affect the value of F, it tends to decrease the stability of the hydrogen sulfide gas produced It is toxic to the human body, harmful to equipment, capable of corroding copper equipment, and its corrosion product CuS will contaminate the product. Therefore, although this method was very popular in the 1970s, it was replaced by the flocculation method in the 1980s.
4. Flocculation method The flocculation method is a flocculant (also called AMPAM) using a polymethylamine polymer compound modified with aminomethyl group. The polymer compound has two functions: 1 the modified AMPAM is negatively charged, and many negatively charged polar groups carried on the molecular chain are electrically connected with the positively charged colloidal particles in the titanium liquid. And, thereby adsorbing into larger particles and sedimenting; 2 polar groups on the polymer chain have a great affinity for the suspended particles, so that the polymer chain can create bridging ability between the particles, so that the suspended particles and The colloidal particles are firmly adsorbed on the surface of the flocculant, and then the dispersed suspended particles and the colloidal particles are networked by cross-linking of the polymer chains, and become large aggregated flocs which are easy to settle and rapidly settle.
The amount of AMPAM added depends on the source of the mine. Generally, 1m ^{3 of} titanium liquid is added to the concentration of 1% of the concentrated liquid by about 3 to 7L, and then diluted with water to 0.1%. When adding people, the temperature should not be lower than 55 °C, the mixing time should not exceed 7mino. AMPAM flocculant has low cost, no toxicity, no pollution, weak corrosion to equipment, fast settling speed, high clarification speed, dense slag layer, at 60- The long non-degradation time in a strong acidic solution at 70 ° C is beneficial to improving the quality and yield of titanium dioxide and labor productivity. Therefore, it is used as an ideal settling agent by many titanium dioxide factories. However, it is still necessary to further improve its influence on the filtration of titanium liquid.
V. Method for aminomethylation modified polypropylene decylamine
1. Purpose of Modification Polyacrylamide (PAM) is a polymer compound which is polymerized from acrylamide monomer and is soluble in water. Most manufacturers use non-ionic (and a few manufacturers use anionic or cationic). It has no charge and has good flocculation effect under neutral, weakly acidic and weakly alkaline conditions. It exhibits maximum flocculation at pH 6.5. However, under strong acidic conditions, its flocculation effect is poor. In order to adapt it to use in strong acidic iron liquid, it still has good flocculation effect, and it must be modified by aminomethylation. The modification is to introduce a methyl group and an amino group in the molecular chain to extend the originally crimped polyacrylamide molecular chain, not only to fully expose its original polar group, but also to add a new polar group. The group has a large electron cloud density on the nitrogen atom in the molecular structure, and exhibits a negative electric property, thereby having a strong affinity for the positively charged suspended particles and causing the polymer chain to be carried out between the suspended particles. Adsorption bridge. At the same time, the equipotential of the colloidal particles can be reduced, and then the polymer chains adsorbing the suspended particles are entangled by stirring, flocculated into a cluster and rapidly settled, and in the highly acidic titanium liquid, the flocculation can still be fully exerted, and the suspension is suspended. The particles are removed by sedimentation. [next]
2. Proportion of raw materials used for modification According to the conventional modification method, the molar ratio of the three main raw materials is: acrylamide monomer (C ₃ H ₅ ON), formaldehyde (CH ₂ 0), dimethylamine (C) ₂ H ₇ N) =1:0. 75:0. 75. However, due to the volatilization of formaldehyde and dimethylamine, in order to avoid the reaction of excess formaldehyde with polyacrylamide to form a methylol group, the methylamine is removed to form a methylimine, and the imine reacts with the amide to form an insoluble matter. Therefore, in the ratio, the proportion of dimethylamine is slightly higher than that of formaldehyde, so that the molar ratio of the three raw materials used in the modified polyacrylamide in recent years reaches 1:0. The ratio is: C ₃ H ₅ ON: CH ₂ O: C ₂ H ₇ N=71:30:45, according to the molar ratio of the ratio: 1:0. 9:1, then the mass ratio is: 71:27 :45. Can be reduced to: 10:3.8:6.34. Since the contents of formaldehyde and dimethylamine are about 37% and 40%, respectively, the required amounts of the formaldehyde solution and the dimethylamine solution are: 3.8÷37%=10.3 and 6.34÷40%=15.85, respectively.
3. Modified method Add about 1 ton of water to the enamel pot, and then add 10 kg of polyacrylamide with a relative molecular mass of 4 million to 5 million, so that the content is 1% (if the relative molecular mass is ≥900) For a time, the concentration should be appropriately reduced to 0.8%, otherwise it is too thick). Heat to 40-50 ° C, stir until completely dissolved, and then add 10.3 kg of formaldehyde solution to make a formaldehyde reaction. Its reaction formula is as follows:

Since the carbonyl group in formaldehyde is positively charged, the nitrogen atom in the polypropylene decylamine is negatively charged. If the solution is acidic, H+ will combine with NH2- to affect the negative polarity of the nitrogen atom. the reaction should be operated under alkaline conditions, the use of 1% trisodium phosphate solution was adjusted between 11 pH at a value of 10.5. 5至小时。 After stirring at a temperature of 40-45 ° C 1. 5-2h. Subsequently, 15.85 kg of a dimethylamine solution was added, and heating was continued to 70-75 ° C to carry out an amination reaction. Its reaction formula is as follows:

The whole reaction process is completed after half an hour of heat preservation. When it is cooled to room temperature, it can be packed in a plastic bucket for use. The production cost per ton of aminomethylated modified polyacrylamide (abbreviated as AMPAM) is about 300 yuan. When using, add 10 times water to dilute to a concentration of one thousandth.
4. Precautions 1 The formaldehyde will become polyoxymethylene below 10 °C and can no longer be used. Keep stocks in winter or store above 10°C.
2 dimethylamine is flammable and explosive, should be kept away from fire, cover it and avoid sun exposure.
3 The quality of raw materials must be guaranteed. The dry powder of polyacrylamide is best purchased from regular manufacturers. The relative density and content of formaldehyde and dimethylamine should be measured and calculated according to the measured content.

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