Settling of titanium liquid

Titanium was soluble iron ore to titanium as raw materials by sulfuric acid decomposition obtained turbid complex composition having both properties of ionic reaction solution, colloids have characteristics in which the main component can be divided into two categories: a soluble sulfate Salt, in addition to soluble sulfates mainly composed of titanium and iron, sulfates such as manganese , chromium , vanadium , tin , copper , antimony and rare earth elements; the other is solid suspended solids insoluble in sulfuric acid. solid impurities are more l0μm mechanical impurities, mainly non-acid solution of ilmenite, rutile insoluble in sulfuric acid, gangue, zircon, horn stone, mud and sand, and calcium, lead, carbon compound. There is a class of 0.1 ~ l0μm fine colloidal solid impurities and their total number of suspended solids such as 20% to 30%: silica, hydrolyzed early metatitanic acid, aluminum salts and the like. If these impurities are not cleaned, they will not only seriously affect the quality of the final product, but also make the filtration difficult, and block the pores of the filter cloth, so that the filtration loss is high and the ferrous sulfate after crystallization is dirty. Therefore, the titanium solution after acid hydrolysis must be purified before it can be used for the production of titanium dioxide. The first step of purification for the sulfuric acid process is sedimentation (precipitation).

Generally, the most effective way to remove solid suspended solids is to filter, but the titanium liquid after leaching and reduction has high viscosity, strong acidity and many colloidal substances. It is difficult to complete the purification operation by filtration method, and the sedimentation is first used in industrial production. The method firstly purifies the titanium liquid and then filters it.

The sedimentation is the separation of insoluble impurities and partial colloidal particles in the titanium liquid from the suspension of the crude dispersion system by means of gravity.

(1) Method of settlement

The method of sedimentation generally divides into gap settlement and continuous sedimentation.

a. Gap settlement is in an acid-resistant lining sedimentation tank with large cross-sectional area and relatively small diameter. Natural sedimentation is carried out with the help of sedimentation agent. Generally, 2/3 of the solid suspension can be settled after 6 hours. Continue to prolong the settling time. Because the ultrafine particles remaining in the solution are affected by the Brownian motion, it is difficult to settle completely under the action of gravity. The other part of the charged colloidal particles is very stable. It is not easy to settle by gravity and continue to settle. The time effect is not obvious. It can be seen from Table 1 that the residue settled in 0~8h is the main solid suspended matter in the solution, and the sedimentation effect is not large within 8~24h, mainly the colloid which is difficult to settle.

Table 1 Titanium liquid residue gravity settlement table

b. Continuous Settlement: Continuous settling is also a type of gravity settling, but it is only a large capacity for continuous operation. Continuous sedimentation is to continuously add the titanium liquid to be settled together with the sedimenting agent to the thickener (or Dow type settler), and the supernatant liquid after the upper layer is discharged continuously from the overflow port, and the thick slurry collected after the sedimentation is increased. At the bottom of the thickener, the slurry is continuously discharged from the discharge port by means of a low-speed mixer. This method is restricted by the sedimentation effect. Another kind of gap type continuous sedimentation is to remove the upper part of the clear liquid through the sedimentation in the above-mentioned gap sedimentation tank for several hours, continue to put the second tank material, and then settle for several hours to remove the supernatant liquid for 3~4 batches. After thoroughly removing the bottom residue.

(2) Settlement agent

Without the help of a settling agent, it is difficult to achieve the purpose of clarifying the titanium liquid by gravity-based natural sedimentation. Add some polymer flocculants or some negatively charged substances and positively charged colloidal suspensions to electrically neutralize it. The zeta potential is reduced to zero and coagulation occurs. Commonly used settlers are:

a. Organic flocculant is not suitable for the deposition of titanium liquid. There are not many organic flocculants, which is mainly due to its harsh flocculation environment. It requires flocculant to resist strong acid and high temperature (60~75°C) and can be kept longer. Does not break down during the time. At present, polyacrylamide (PAM) modified aminomethylated polyacrylamide (AMPAM) is an organic polymer flocculant widely used in the titanium dioxide industry.

Polyacrylamide itself is a water-soluble polymer compound obtained by polymerizing acrylamide. . The non-ionic type itself has no charge and exhibits the greatest flocculation at pH 6.5. In order to make it usable in a strong acid titanium solution, it must be modified by introducing a methyl group and an amine group into its molecular chain to stretch the original curly polyacrylamide molecular chain. The original polar group is fully exposed and new polar groups are added. The amine methylated polyacrylamide modified by formaldehyde and dimethylamine exhibits negative electronegativity, and can fully exert flocculation effect without degradation in a strong acid environment at 60-70 °C.

The modification method of polyacrylamide is as follows: the conventional modified ratio is a molar ratio of polyacrylamide to formaldehyde and dimethylamine of 1:0.75:0.75. First, the commercially available polyacrylamide with a molecular weight of ≥ 4 million (about 8%) is diluted with water to 1%. Since the colloidal polyacrylamide is very viscous, it can be added in several portions. This dissolution and dilution process is very long, and then formaldehyde is added. was heated to 40 ~ 50 ℃, pH was adjusted to 10.5 with 10 to 1% trisodium phosphate and incubated with stirring for 1.5 ~ 2h formaldehyde reaction, the following reaction formula.

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Because the dialo group in formaldehyde is positively charged, and the nitrogen atom in PAM is negatively charged. If the solution is acidic, H+ will combine with NH2-, which will affect the negative charge of the nitrogen atom. Therefore, the reaction is alkaline. The next operation is better. Then, dimethylamine was further added, and the temperature was further raised to 65 to 75 ° C for amination reaction.

After adding dimethylamine, it can be kept for 0.5 h.

To prepare amine methylated polyacrylamide, first choose polyacrylamide with high molecular weight, the molecular weight requirement is not less than 4 million, otherwise the flocculation effect is not good. Recently, solid polyacrylamide with molecular weight of more than 10 million is used. The product has a good flocculation effect and a small amount. In the modification process, formaldehyde is not suitable for long-term storage, especially in the presence of water. Therefore, the selection of high-quality formaldehyde is also one of the key factors for the success or failure of modification. Secondly, dimethylamine is volatile at high temperature. The time of methylamine addition should not be too early.

Recently, it has been found that amine methylation is a reversible reaction. The ratio of dimethylamine in PAM, HCHO, (CH ₃ ) ₂ NH3 raw materials is slightly better than that of formaldehyde, so as to avoid excess formaldehyde and The methyl group formed by the polyacrylamide reaction removes the meridine to form a methylimine, and the imine reacts with the amide to form an insoluble matter, so the ratio of the three is changed to: 1:0.9:1 (mol) It is also possible to obtain an amine methylated polyacrylamide having a good flocculation effect.

A large number of polar groups are distributed in the modified polyacrylamide, because of the large electron cloud density on the nitrogen atom in the molecular structure, and a strong affinity for the suspended particles, so that the polymer chain is in the suspended particles. The adsorption bridge is carried out, and the zeta potential of the colloidal particles can be lowered, and the polymer chains adsorbing the suspended particles are entangled with each other by stirring, and flocculated into a cluster to settle.

Figure 1 is a schematic view showing the structure of polyacrylamide. According to Michaels' research, the degree of hydrolysis of polyacrylamide is about 33%, which indicates that the partially hydrolyzed polyacrylamide carbon chain is relatively straight and long, and the volume of flocculation entangled after adsorption and bridging is also large. Therefore it is easier to settle.

The modified polyacrylamide was diluted to about 1/1000 before use and added from the precipitation tank. The diluted modified polyacrylamide should not be stored for a long time, and is generally diluted and used now. However, when the pH of the modified polyacrylamide aqueous solution adjusted by the hydrolysis waste acid is 2.5~4.5, it can maintain a good flocculation effect after being stored at 20~24 °C for two weeks. [next]

. b inorganic flocculant antimony trioxide (Sb ₂ O ₃₎ - iron sulfide (of FeS) is the classic sulfate process titanium dioxide production used in the first flocculating agent which is a high valency, co-sedimentation can be dissolved in a strong acid The agent is still in use until now.

Antimony trioxide is generally supplied with the ore powder. When pyrolysis is carried out at high temperature, antimony trioxide reacts with sulfuric acid to form barium sulfate into the titanium liquid.

Sb ₂ O ₃ +3H ₂ SO ₄ →Sb ₂ (SO ₄ ) ₃ +3H ₂ O

After the titanium liquid is placed in the precipitation tank, the ground iron sulfide is uniformly dispersed into the precipitation tank, at which time the iron sulfide reacts with sulfuric acid to form hydrogen sulfide, and the hydrogen sulfide reacts with barium sulfate in the solution to form barium sulfide.

FeS+H ₂ SO ₄ →FeSO ₄ +H ₂ S↑

Sb ₂ (SO ₄ ) ₃ +3H ₂ S→Sb ₂ S ₃ ↓+3H ₂ SO ₄

The micelle structure of barium sulfide is {[Sb ₂ S ₃ ]·nHS ^- (nx)H ⁺ } ^{x -} x H, which is a negatively charged sol with a large volume, which is positive with titanium liquid. The charged suspended particles and the colloidal particles generate electrical neutralization, which reduces the zeta potential of the surface of the colloidal particles, and causes the colloidal particles in the dispersed state to agglomerate. Because of the high density of barium sulfide, these impurity particles agglomerate rapidly around Sb ₂ S ₃ and settle down quickly. When Sb ₂ O ₃ -FeS inorganic coagulant is used, the acidity (F value) of the solution is controlled to be higher because not only the reaction consumes a little sulfuric acid, but it is important that Sb ₂ O _{3 is} not only generated in the case of low acidity. Barium sulfide is also likely to form soluble thioguanate and cannot be removed. If Sb ₂ O ₃ is not used, copper sulfate can also be used, and iron sulfide can also be replaced by sodium sulfide.

Inorganic coagulant (Sb ₂ O ₃ -FeS) is a classic coagulant. It does not increase the viscosity of titanium solution, does not cause filtration difficulties, and can coagulate smaller colloidal particles, but the sedimentation time is longer and slightly larger. Solid agglomerated particles are not as good as AMCAM. In addition, H ₂ S gas is toxic, polluting the environment, corrosive equipment, especially copper (copper coil) and stainless steel equipment will corrode the product after corrosion, but residual Sb2O3 can be prevented in the product. The occurrence of light color interconversion.

Organic flocculant (AMPAM) flocculates and settles on slightly larger solid particles, generally reaching 0.25~0.37m/h. Although the liquid after it settles looks very clear, there is still a small amount of very fine colloidal particles which are difficult to settle together. When the polyacrylamide flocculant is added in a large amount, foam is generated to increase the viscosity of the titanium liquid.

In order to allow the suspended solid impurities and the colloidal particles in the titanium liquid to settle, it is preferable to use an inorganic coagulant and an organic flocculant in combination, so that a better sedimentation effect can be obtained. Method General Procedure: antimony oxide with the addition of a slag, until complete extraction of the iron sulfide was added before discharge from the acid solution tank, so that the hydrogen sulfide gas may be discharged to reduce the hazard to the operator of the chimney, and then added in the precipitation tank Modified polyacrylamide.

c. Mixed Settling Agent: The two components of this type of settling agent have both coagulation and flocculation effects. Since the organic flocculant polyacrylamide has been widely used, such mixed settling agents have been rarely used. A more typical mixed settling agent is tannic acid-calf, and the tannic acid has the formula C ₆ H ₆ O ₆ [C ₆ H ₂ (OH) ₃ COOC ₆ H ₂ (OH) ₂ CO]5 It is a An organic acid with a molecular weight of 1700 and a content of 85% or more. The tetravalent titanium ions in the tannic acid and titanium solution form an orange-red tannin titanium complex with negative electronegativity, which can neutralize the positively charged colloidal particles in the titanium solution to cause coagulation. It is an organic acid that can co-precipitate with metal elements such as Al, Be, Ca, Ti, V, Nb, Zn, Ta, etc., which used to be used in analytical chemistry. Bovine gum is a natural polypeptide bond polymer compound, which contains many -COOH, -OH, -NH ₂ polar groups in the molecule. These polar groups can act as a colloidal particle flocculation and precipitation. .

Other settling agents are: polyalcohol and polypropylene ester compounds, hydroxy ripolyacrylamide, pull-opening powder, sodium alkyl sulfonate, polyamide propylene oxide, methanol esterified pectin, etc., but actually used There are very few factories.

See Table 2 for a comparison of the sedimentation effects of various settlers.

Table 2 Comparison of sedimentation and fruit contrast of various flocculants

(3) Main factors affecting the sedimentation effect of titanium liquid

a. The effect of titanium temperature on the sedimentation effect, generally the high temperature of the titanium liquid viscosity is conducive to sedimentation, but the temperature too high polymer movement will form convection is not conducive to sedimentation, and the stability of the iron liquid is also unfavorable, usually should be maintained at 50 Let it settle naturally between ~65 °C. Winter insulation is very important. In the summer, it is also necessary to pay attention to heat dissipation. Otherwise, the material that is liberated by acid is not easy to dissipate heat and will affect the sedimentation effect.

b. The influence of the concentration of the titanium liquid on the sedimentation effect, the concentration of the titanium liquid is the relative density of the titanium liquid or the TiO ₂ content in the titanium liquid. Generally, the concentration is high, the viscosity is large, and the sedimentation speed is slow; the concentration is low, and the sedimentation speed is fast, but the burden of the subsequent concentration process is increased. Generally, the requirement is not less than 120 g/L, and the field operation is usually measured by a densitometer.

c. The effect of the quality of the titanium liquid on the sedimentation effect, the high F value, the high trivalent titanium content, the good stability of the titanium liquid, the good settlement and the opposite stability, the trivalent titanium or the early hydrolysis of the titanium liquid and the mineral The titanium liquid obtained by the high content of SiO ₂ and Al ₂ O ₃ has difficulty in sedimentation and dry separation because of the presence of a large amount of colloidal particles.

d. The effect of stirring strength on the sedimentation effect. In the case of using inorganic coagulant, because the ion chemical reaction requires sufficient agitation, but it is not too strong, it will escape a large amount of H ₂ S gas, which not only pollutes the environment but also condenses. The effect is good. In the case of using a modified polyacrylamide flocculant, it is necessary to mix well rather than strongly stir, and strong agitation will split the large floc cluster into small flocculation clusters, and even break the chain of the long polymer chain, not only It is also possible to disperse without flocculation (see Figure 2). Therefore, some plants put the modified polyacrylamide and the acid-decomposing material into the sedimentation tank simultaneously in a mixer without stirring with compressed air.

e. The effect of the amount of settler on the sedimentation effect. When Sb ₂ O ₃ -FeS coagulant is used, the addition amount of Sb ₂ O ₃ is 0.1%~0.2% of the mass of the ore powder, and the theoretical amount of FeS should be Sb ₂ 3 times the amount of O ₃ , of course, the use of ilmenite with higher sulfur content (such as Panzhihua ilmenite) can be relatively reduced. When using AMPAM at the same time, the total amount of Sb ₂ O ₃ -FeS can be relatively reduced. When using AMPAM, the effect of increasing the sedimentation is better, but there is no obvious effect, but there is a tendency to decrease. The general dosage is 30~50g per cubic meter of titanium liquid (based on AMPAM dry powder), too much will make titanium liquid Sticky and foamy.

f. The influence of the concentration of the settling agent on the sedimentation effect. The concentration of the settling agent has a great influence on the sedimentation effect. Generally, the concentration is high, the viscosity is too large to be dispersed in the titanium liquid, and the concentration is too high, and the polymer chain is curled. The ball loses its flocculation effect. When the test proves that the AMPAM concentration exceeds 0.7%, the sedimentation effect deteriorates drastically, and it is usually diluted to 0.1% to 0.2% at the angle of the stool. The concentration is low, although the viscosity of the titanium liquid is easy to disperse evenly, but the concentration is too low, the increase of the amount will dilute the concentration of the titanium liquid, and the AMPAM whose concentration is too low is not easy to exist for a long time.

The sedimentation effect of titanium liquid is good or bad. Qualitative inspection generally takes a certain amount of titanium liquid and filters it in a Buchner funnel. After cleaning the filter paper with a small amount of water, check the depth of the trace on the filter paper (usually looking at the second filter paper) to judge the sedimentation effect. Good or bad. Quantitative inspection is to take a certain amount of titanium liquid in a Buchner funnel, and then determine the sludge content on the filter paper, generally controlled at about 300mg / L. It is also possible to determine the effect of sedimentation by means of light transmittance by first oxidizing the trivalent titanium in the titanium solution to tetravalent titanium with 30% hydrogen peroxide to change the titanium liquid from purple black. The transparent liquid is then visually or measured by a spectrophotometer. The higher the light transmittance, the better the sedimentation effect. [next]

The residue at the bottom of the sediment still contains a small amount of titanium liquid, which should be further recovered. It is usually diluted with water (or diluted acid) and filtered. The filtrate is used as a small amount of water. The residue is washed again to dryness and then thrown away as inorganic waste.

Although the sedimentation of titanium liquid is a simple physical process, it is very strict, and it has a great influence on the quality, yield and production schedule of the future. It is best to succeed once, because the titanium solution cannot be lowered due to poor sedimentation. Heating, heating is very likely to cause stability degradation, and even early hydrolysis. If the sedimentation effect is not good and the temperature is not too low, Sb ₂ O ₃ can be dissolved in hydrochloric acid and added to the titanium solution, and then a small amount of flocculant is added for secondary sedimentation. If it is not enough, the upper layer can be slightly better. The titanium liquid is taken in small portions and transferred to the next batch of titanium liquid to continue to settle. The supernatant liquid may also be diluted with light waste acid or a small amount of water, and then added as a flocculating agent to be used as a small amount of water, or brought to the next batch of titanium liquid to continue to settle. Severe early hydrolysis, or a large amount of colloidal material that cannot settle, cannot be used to produce pigment-grade titanium dioxide. As for the reason that the settlement is not easy to find out the quality of the flocculant, the reassortment should be reversed. It is forbidden to attempt to improve the effect by increasing the dosage, and the sedimentation effect will be worse.

Organic flocculant can not be stored for too long, animal glue such as gelatin is highly perishable and deteriorated. The modified polyacrylamide can not be stored for too long in summer. It should be used immediately after dilution to 0.1%, usually diluted now. use.

Regardless of whether organic flocculants or inorganic coagulants are used, they are naturally settled by gravity. It is inevitable that many particles will not settle due to various reasons, especially when the titanium solution is sent to the crystallization process from the precipitation tank, whether it is the lower part. If the material is discharged or the upper siphon method, it is possible to take the sediment away. In order to improve the quality of the product, many factories filter the plate filter press once before crystallization (also called hot filtration), so the temperature of the titanium liquid Still high (about 45~55 °C), low viscosity and good filtration, which not only can reduce the burden of controlling filtration later, but also the quality of ferrous sulfate crystallized.

The chemical composition of the titanium liquid after sedimentation is generally as follows:

Titanium dioxide content (TiO ₂ ) 120~150g/L

Acidity coefficient (F value) 1.7~2.0

Trivalent titanium (calculated as TiO ₂ ) 1.0~5.Og/L

Stability ≥350mL

The residue in the sedimentation tank is mainly gangue, rutile, unacidified or unacidified ilmenite. This part of the residue is produced in a non-pigment grade titanium dioxide factory. It is incorporated into the next batch as a part of the mine. Re-acidification in the mineral powder. The residue is usually treated with water, small water or light waste acid, and then sent to a thickener by pump or vacuum. The supernatant containing soluble titanium is used as a small amount of water, and the lower layer is used for the mud. The frame filter press filters, washes, and blows it, and then treats it as an inorganic solid waste.

The treatment of the precipitate residue is not important in the entire production process, but is an indispensable step for increasing the yield of the titanium liquid.

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