Acid hydrolysis of ilmenite (solid phase method)

Decomposition titanium iron ore with sulfuric acid to chemical reaction:
FeTiO 3 +3H 2 SO 4 →Ti(SO 4 ) 2 +FeSO 4 +3H 2 O (1)
FeTiO 3 +2H 2 SO 4 →TiOSO 4 +FeSO 4 +2H 2 O (2)
The equation for this chemical reaction can also be expressed as titanium and iron in ilmenite as a separate component.
TiO 2 +2H 2 SO 4 →Ti(SO 4 ) 2 +2H 2 O+Q (3)
TiO 2 +H 2 SO 4 →TiOSO 4 +H 2 O+24.45kJ (4)
FeO+H 2 SO 4 →FeSO 4 +H 2 O+121.45kJ (5)
Fe 2 O 3 +3H 2 SO 4 →Fe 2 (SO 4 ) 3 +3H 2 O+141.5k1 (6)
It can be known from the above chemical reaction equation that when the ilmenite is decomposed with sulfuric acid, the reaction product is mainly: Ti(SO 4 ) 2 (titanium sulfate or titanium orthosulfate), TiOSO 4 (titanyl sulfate or titanyl sulfate), FeSO 4 (ferrous sulfate or low-iron sulfate), Fe 2 (SO 4 ) 3 (high-iron sulfate) and water.
In actual production, whether the reaction of formula (1) produces titanium sulfate Ti(SO 4 ) 2 or (2) reacts to form titanyl sulfate TiOSO 4 , or it may be hydrolyzed to form sulfuric acid by the following reaction (7) Titanium oxide, free part of the sulfuric acid, and then participate in the reaction with this part of the acid:
Ti(SO 4 ) 2 +H 2 O→TiOSO 4 +H 2 SO 4 (7)
It is generally believed that the main component of the reactants, Ti(SO 4 ) 2 or TiOSO 4 , is determined by the specific conditions of the reaction, that is, according to the amount of sulfuric acid used in the reactions (3) and (4).
According to the reaction (3) TiO 2 in the ilmenite reacts with H 2 SO 4 to form Ti(SO 4 ) 2 , at which time 1 mol of TiO 2 consumes 2 mol of H 2 SO 4 , and if all the sulfuric acid participating in the reaction is involved at this time In combination with titanium dioxide in the ore, regardless of the presence of free sulfuric acid that does not participate in the reaction, then at this time When the reaction, the F value>2.453 should be regarded as “acidic”, <2.453 should be regarded as “alkaline”, and F=2.453 proves that all the sulfuric acid is combined with titanium to form Ti(SO 4 ) 2 . In theory, the solution should be “medium”. Sex."
For example, according to the reaction (4), lmol of TiO 2 consumes only 1 mol of H 2 SO 4 , and the F value at this time should be 1.226 according to the same assumption as above, then, in the case of the reaction product TiOSO 4 of the formula (4), F= 1.226 The solution should also be in a "neutral" state. In actual production, whether there is a large amount of free acid in the reactants of F=2.453 or F=1.226, it is reasonable to say that these free acids should continue to participate in the reaction, but actually Did not continue to participate in the reaction. Therefore, based on the above reaction theory and actual production analysis, it is concluded that Ti(SO 4 ) 2 and TiOSO 4 are present in the solution of ilmenite decomposed by sulfuric acid. When F>2.45, Ti(SO 4 ) 2 accounts for Most of them, when F<2.45, the TiOSO 4 in the solution accounts for the majority, and the F-value process control range of industrial titanium dioxide production is generally 1.7~2.1. It should be understood that the reactants are mainly TiOSO 4 (titanium sulfate). Some people also refer to the amount of acid required for the calculation of the acid content in the titanium solution to be greater than that of Ti(SO 4 ) 2 as the "acid titanium solution"; the effective acid content in the titanium solution is lower than that of Ti(SO 4 ) 2 The amount of acid required is called "basic titanium liquid". Currently, sulfuric acid titanium dioxide factories all over the world use alkaline titanium liquid to produce various brands of titanium dioxide.
Titanium sulphate [Ti(SO 4 ) 2 ] is a white deliquescent powder which cannot be precipitated from aqueous solution and can only be crystallized and isolated in fuming sulfuric acid. It is considered that tetrabasic titanium anhydrous titanium sulfate Ti(SO 4 ) 2 is hardly present, and there are described hydrates of nine-water, four-water and three crystal water of titanium sulfate. [next]
Titanium sulphate (TiOSO 4 ) is a white deliquescent crystalline powder that is soluble in water and completely soluble in 0.5% to 1.0% dilute sulfuric acid. It has the lowest solubility in 40% to 60% concentrated sulfuric acid. , only 0.04% ~ 0.09%, the acid concentration increased, and the solubility rose again. Among the various sulfates of titanium, the hydrated titanyl sulfate TiOSO 4 ·2H 2 O crystal is the most stable, and it is also the most studied one. It is white coarse needle crystal (Fig. 1), in the air. Direct heating removes the crystal water and disulfide to form titanium dioxide, and the dihydrated titanyl sulfate dissolves slowly in cold water, and is easily hydrolyzed upon heating.
Figure 2 is a triangulation diagram of the concentration of titanium dioxide-sulfur trioxide-water. The composition of the system is expressed by mo1%, with the point as the symbol of a certain compound, the TiO 2 -H 2 O side in the triangle diagram is the hydroxide of titanium, and the SO 3 -H 2 O side is sulfuric acid, pyrosulfuric acid and its hydrate. The TiO 2 -SO 3 side is a titanium sulfate, and is most stable in titanium sulfate dihydrate sulfate titanate (TiO 2 ·SO 3 ·2H 2 O) among numerous titanium sulfates.
The viscosity of the titanium sulfate solution is very high, and it is generally considered to be both a true solution and a colloidal solution, and the two states sometimes exist in balance with each other. All of the titanium sulphate solutions showed a damaging effect, that is, the solution was carefully concentrated to form a crystalline precipitate of a fixed composition, but a semi-liquid semi-solid colloidal substance was formed. It is generally believed that most of the concentrated solution in a strong acid medium is in an ionic state, while in a dilute solution of a weak acid medium, it is mainly in a colloidal state.
The acid decomposition product of ilmenite (titanium sulfate solution), in addition to the above TiOSO 4 and Ti(SO 4 ) 2 , many researchers believe that there are other tetravalent titanium sulfates present, such as TiO 2 and SO 3 Sulfates with ratios ranging from 1:2 to 7:1 are likely to exist. Their composition is not fixed and their properties are different. Some are insoluble in water, some are insoluble in dilute acid and insoluble in acid, or even precipitate. . In order to prevent these substances from being produced as much as possible, it is necessary to strictly control the acid hydrolysis reaction according to different conditions. [next]
In the solution after decomposition of ilmenite and sulphuric acid, the ratio of TiO 2 to H 2 SO 4 has a great influence on the production process and the quality of the final product.
There are different forms of a sulfuric acid solution of titanium sulphate in the free acid is reacted with sulfuric acid --- not participate ilmenite;.. B bind to an iron sulfate;. C in combination with other metals and iron sulfate. The sum of the free acid and the sulfuric acid combined with titanium is called "effective acid", and the sulfuric acid combined with titanium can be calculated from the difference between the effective acid and the free acid, and only the effective acid is meaningful in actual production.
For example, a titanium solution containing 150 g/L TiO 2 and 270 g/L effective H 2 SO 4 has an F value of 1.8. F = 270 / 150 = 1.8.
The F value can not only show the ratio of TiO 2 to H 2 SO 4 in the titanium solution, but also evaluate the quality of the titanium solution. The titanium liquid with high F value generally has good stability, but the hydrolysis rate is slowed down, and the hydrolysis yield is affected, so that the particle size of the metatitanic acid after hydrolysis is fine. The titanium liquid having a low F value generally has poor stability, and the hydrolysis rate is fast, but the obtained metatitanic acid has a large particle diameter and poor pigment properties.
However, the F value does not fully explain the change of the material in some processes during production, because it reflects only the ratio of acid to titanium in the titanium liquid. For example, when the titanium liquid is frozen and crystallized, a large amount of ferrous sulfate is precipitated from the solution, and the F value of the titanium liquid does not change at this time; when the titanium liquid is concentrated or diluted, the total titanium and the effective acid concentration occur. The change does not change the F value; even if the ratio of TiOSO 4 and Ti(SO 4 ): generated in the acid hydrolysis reaction changes, the effective acid content in the titanium solution is affected, and the F value does not change. . In addition, the F value is mainly determined by the ratio of mineral acid in the acid hydrolysis of ilmenite, followed by the waste sulfuric acid added during leaching. The amount of these two acids changes when the total acid amount is constant, and the F value No change will happen. It is generally believed that the high F value can improve the stability of the titanium liquid, and mainly relies on the free acid in the effective acid to inhibit the early hydrolysis of the titanium liquid, and the F value cannot visually indicate the content and effect of the free acid.
The control range of F value in industrial titanium dioxide production is 1.7~2.1, and the end point color development when adding effective acid is not obvious, the analysis error is large, and in some cases, FOA or BAS value is better.
Where 1.2265 is the ratio of sulfuric acid to titanium dioxide when the reaction of 1 mol of sulfuric acid and 1 mol of titanium dioxide is formed by the following reaction (98/79.9).
TiO 2 +H 2 SO 4 →TiOSO 4 +H 2 O
The above calculation formula can also be expressed by the following formula:
FOA=F-1.2265/1.2265
For example, with the same example as above, a titanium solution containing 150 g/L of TiO 2 and 270 g/L of an effective acid (F value = 270/150 = 1.8) is represented by a FOA value of 46.76%.
Since the FOA value and the BAS value indicate a wide range of acidity coefficients, they are more intuitive in life, and it is easier to adjust the control during the operation of the worker.
Titanium liquid tends to age when placed (that is, the solution becomes cloudy, and localized water decomposes white colloidal particles). The degree of this tendency is called the stability of the titanium liquid. Titanium liquid with poor stability is prone to early hydrolysis, and precipitated TiO 2 colloidal particles make precipitation and filtration operations difficult. When washed, the titanate particles are more severely filtered. Since the particle size of the meta-titanic acid which is hydrolyzed in the early stage is irregular, the pigment grade titanium white powder produced by such a product has poor pigment properties such as whiteness and achromatic power.
The main factors affecting the stability of titanium liquid from the actual production experience are the acid ratio of titanium solution, the concentration (total TiO 2 content in titanium solution) and the temperature of 3 elements. The 3 elements can also be derived from the hydrolysis equation of titanium liquid. Theoretically proved.
It can be seen from the above three titanium sulfate hydrolysis chemical reaction equations that all of the three hydrolysis reactions produce free sulfuric acid, which indicates that the acidity in the solution is high, the reaction can be inhibited to the right, and the rate of hydrolysis reaction can be reduced; In the hydrolysis reaction equation, water is one of the reactants. When the water is too much, the concentration of the titanium liquid must be lowered, so that the hydrolysis reaction is accelerated to the right; the other three hydrolysis reactions are endothermic reactions, and the temperature can accelerate the hydrolysis reaction. s speed. [next]
The acid hydrolysis process of ilmenite is usually carried out in industrial production by solid phase process. Generally, there are three operation modes as follows.
a. High temperature method
First put concentrated sulfuric acid into the acid hydrolysis tank, put the metered ilmenite into the acid hydrolysis tank and stir evenly under the stirring of compressed air, then add the calculated dilution water, dilute the heat released by sulfuric acid, and then use steam. The acid hydrolysis reaction starts after heating to a certain temperature.
b. Low temperature method
First, after diluting the metered concentrated sulfuric acid into the concentration and temperature specified in the process in the acid solution tank, the metered ilmenite is put into the acid solution tank under the stirring of compressed air, and after stirring, the direct steam is induced to initiate the acid hydrolysis reaction. .
c. Premixing method
Mix the concentrated sulfuric acid and ilmenite in a premixing tank first, then put the viscous slurry into the acid hydrolysis tank, add a certain amount of dilution water, and use the dilution heat to initiate the acid hydrolysis reaction (if diluted) The heat is not enough to initiate the reaction and can be heated by a small amount of direct steam).
The high temperature method is mainly suitable for winters with low ambient temperature, or low content of ferric oxide (low heat of reaction) in ilmenite, relatively smooth minerals and titanium ore with high titanium dioxide content and low total iron content (including acid). Soluble titanium slag).
The premixing method is mainly used for large-scale acid decomposing tanks, because the reaction speed of acid hydrolysis is fast, and the amount of mineral powder invested by equipment is large. It is not easy to stir evenly by the above two methods, and the ilmenite can be thoroughly mixed with sulfuric acid by premixing. Evenly, in order to improve the acid hydrolysis rate, reduce the solid phase material that is difficult to be leached, and prevent the fine ore powder from running away with the chimney when the ore powder is put into the acid hydrolysis tank.
(1) Determination of the ratio of mineral to acid
The first step of the acid hydrolysis reaction is to determine the ratio of mineral to acid. The correct ratio of mineral acid can not only save the amount of sulfuric acid, but also improve the acid hydrolysis rate to make the reaction more complete. Generally, the TiOSO 4 solution formed by the too low reaction of mineral acid is unstable and easy to be hydrolyzed early, while the ratio of mineral acid is too high to only waste sulfuric acid and inhibit the hydrolysis reaction. The Ti(SO 4 ) 2 formed by the reaction is insoluble during leaching. In water, it also causes the hydrolysis product particles to be difficult to wash.
According to the following reaction equation of ilmenite and sulfuric acid, the theoretical ratio of mineral to acid can be calculated:
FeTiO 3 +3H 2 SO 4 →Ti(SO 4 ) 2 +3H 2 O+FeSO 4 (8)
FeTiO 3 +2H 2 SO 4 →TiOSO 4 +2H 2 O+FeSO 4 (9)
According to formula (8), the ratio of mineral to acid is 1:1.93, and the formula (9) is only 1:1.29. However, the composition of ilmenite is very complicated. In addition to FeO and TiO 2 , there is a large amount of Fe 2 O. 3 , Al 2 O 3 , SiO 2 , MgO, CaO, MnO, etc. participate in the reaction with sulfuric acid at the same time, so the ratio of mineral acid is difficult to calculate, generally based on TiO 2 content in ilmenite, total iron content ( The ratio of Fe 2 O 3 and FeO, and other impurity levels, are determined by experimentation.
According to the quality and acid hydrolysis operation method of ilmenite used in China, the ratio of mineral acid is generally controlled at 1:1.55~1.65 (100% sulfuric acid), which is based on the quality and actuality of each batch of mineral powder. Flexible production conditions such as: the amount of waste acid used in leaching, the F value required by the process, etc., should not be forced to move hard.
According to the law of chemical reaction, increasing the amount of sulfuric acid in the main reaction can increase the rate of acid hydrolysis and the rate of acid hydrolysis. When the ratio of mineral acid to 1:2 is adjusted, the acid hydrolysis rate can be increased by 10%, but continues to increase to 1:15. The acid hydrolysis rate is only increased by 6% to 7%, which means that the amount of sulfuric acid is not as high as possible. [next]
(2) Determination of the concentration of sulfuric acid in the reaction
The concentration of the raw material sulfuric acid and the concentration of the diluted diluent during the reaction have a significant influence on the quality of the acid hydrolysis reaction. As mentioned above, theoretically, the concentration of raw material sulfuric acid can be used from 85% to 98%, but it is better to use 92.5%~95% sulfuric acid because the raw material sulfuric acid concentration is too high (>96%) not only when diluted. More heat is released, and the concentration of H + and SO 4 2- in the high concentration of sulfuric acid is high, the activity is large, and the reaction speed is increased, while the concentration of the raw material sulfuric acid is too low (<92%) because the heat of dilution is less, the acid hydrolysis reaction is gentle, The solid phase is not easy to be immersed and has poor stability. However, the concentration of sulfuric acid can be higher by premixing.
The concentration of the dilution in the sulfuric acid reaction is also important, which directly affects the rate of reaction and the degree of gradual and gentle reaction. Although the reaction of ilmenite with sulfuric acid is an exothermic reaction, ilmenite is a complex oxide composed of many different covalent bonds and ionic bonds, and its specific heat capacity is only 0.743 kJ/(kg·°C). The energy can be about 56.9kJ/mol. The effect of ilmenite and sulfuric acid is very slow at normal temperature. It is often carried out after heating to a certain temperature, and the reaction will proceed at a relatively fast speed. Until the main reaction is completed. Therefore, in order to complete the reaction, it must be heated and activated at the initial stage of the reaction. From the data in Figure 3-4 (mineral acid ratio 1:2), it can be seen that the temperature increase can shorten the reaction time and increase the acid hydrolysis rate. This test is based on the acid hydrolysis effect of 50% sulfuric acid at about 122 °C (requires 20h reaction to end), but when the sulfuric acid concentration is increased to 90%, the reaction temperature can be increased to 190~210 °C, and the reaction rate is obvious. Accelerate, the reaction can be completed quickly, and the acid hydrolysis rate can be further improved. In actual production, the reaction temperature is usually increased by 10 ° C, and the reaction rate can be increased by 2 to 4 times.
In China's titanium dioxide factory, the dilution concentration of sulfuric acid is generally (88 ± 2)%, because the dilution concentration is too low (less than 85%), the dilution heat needs less steam, the reactants are not thickened, and it is not easy to form solid. Phase, even paste, incomplete reaction, difficult to leach, low acid hydrolysis rate, stability of titanium liquid is not good, if the dilution concentration is too high (>90%), the reaction heat is large, the reaction speed is fast, the main The short reaction time and high temperature of the reactants can also cause early hydrolysis, even a pan accident or an unreacted solid phase to reduce the acid hydrolysis rate.
Generally, the principle of selecting the concentration of the raw material sulfuric acid and determining the dilution concentration during the reaction is to select a sulfuric acid having a higher concentration in the winter and a sulfuric acid having a lower concentration in the summer. When using a higher concentration of sulfuric acid, the concentration at the time of the reaction dilution is lower. When the sulfuric acid having a lower concentration is used, the concentration at the time of the reaction dilution may be higher. Large acid solution tank (90m 3 or more), due to the input of more sulfuric acid and ilmenite, in order to avoid the reaction is difficult to control, the dilution concentration of sulfuric acid can be lower, the fineness of the mineral powder can be larger, which can be extended The acid hydrolysis reaction time is convenient for operation control.
Since the concentration of sulfuric acid is important for the acidolysis reaction and safe operation of ilmenite, the sulfuric acid used in each batch should be carefully and carefully tested, and the concentration of sulfuric acid is generally determined by chemical analysis. [next]
(3) Operation of acid hydrolysis reaction
The operation of the acid hydrolysis reaction generally firstly puts the metered sulfuric acid into the acid hydrolysis tank first, and puts the mineral powder under the stirring of the compressed air, that is, first adding the acid and then adding the ore. The dilution of sulfuric acid usually adopts the low temperature method, that is, the sulfuric acid is first diluted to a certain process concentration and cooled to a certain temperature to re-mine the powder; in winter, the high temperature method is usually adopted, that is, the acid is added first, then the sand powder is added, and then the metering is added. Dilute the water and use a dilute heat of sulfuric acid to initiate the reaction.
In addition to the above-mentioned order of addition of sulfuric acid, mineral powder and dilution water, the preheating temperature of sulfuric acid before the input of the ore powder is also important. The preheating temperature of sulfuric acid is too high, and the quenching reaction will occur soon after the mineral powder is put in, which may cause a pan accident. Moreover, because the reaction speed is too fast, the ore powder and sulfuric acid have not been stirred and the reaction has ended, and the acid hydrolysis rate is low. The solid phase is not completely prone to occur; while the sulfuric acid temperature is too low, the reaction is slow, the main reaction is not obvious, the solid phase is soft, and the reaction is incomplete, and the unreacted solid phase is also present, and the acid hydrolysis rate is low. Generally, the preheating temperature of sulfuric acid is controlled at 80~120°C (the upper limit of winter temperature can be upper limit and the lower limit of summer temperature can be lower limit). In the cold season of winter, even if the heat is diluted by high temperature method, it is not enough to reach the temperature specified by the process, sometimes it is preheated by steam. In the summer, the low temperature method is adopted. The temperature of the diluted sulfuric acid is still very high. It is not advisable to immediately input the mineral powder reaction. The compressed air should be stirred and cooled, but sometimes the temperature of the compressed air is also very high and it is difficult to reduce the temperature to a specified temperature in a short time. At this time, the recovered dilute acid can be used instead of water to dilute the concentrated sulfuric acid to reduce the heat of dilution. Of course, it must be accurately calculated to prevent the acidity coefficient from being too high or the ratio of mineral acid to too high.
According to the above method, the reaction generally reaches 120~130°C after 10~20min after the input of the ore powder. At the beginning of the reaction, white smoke in the tank overflows from the inlet cover and has a slight vibration. When the temperature reaches 160°C, the temperature is fierce. The acid hydrolysis reaction begins. At this time, the steam heating should be stopped immediately. Since the exothermic temperature of the reaction rises sharply, the white smoke sometimes overflows and sucks up. The temperature rises by a hundred degrees in a few minutes, and the vibration is obviously up to about 200 °C. Sense, the reaction volume expands instantaneously and generates a large amount of foam, which discharges a large amount of water vapor, acid mist, sulfur dioxide, sulfur trioxide and entrained mineral dust from the chimney. Because the reaction temperature is very high, the water in the sulfuric acid evaporates quickly, and the blowing force of the compressed air is increased to make the reactants become a porous solid phase after the thickening stage, so as to facilitate the subsequent dissolution (leaching). The main reaction usually ends in about 5 minutes, but the large acid solution tank is thicker, the sulfuric acid concentration is lower, and the reaction time is longer. The main reaction is generally about 10 minutes, and the temperature at the main reaction is also low (160~190). °C). Thermodynamic studies indicate that there are two thickening processes in the acid hydrolysis reaction, which is an objective reaction in which the temperature of titanium and different valent iron oxides reacts with sulfuric acid.
After the acid hydrolysis reaction is solidified, the compressed air should be stopped (the temperature is about 180~200 °C), and the mature (cooked) heat is allowed to cool for a while, so that the unreacted ore powder can continue to use this temperature. The free acid reacts to increase the acid hydrolysis rate. Generally, the acid hydrolysis rate of titanium concentrate can reach 95%~97%, and the actual acid solution rate of 85%~90% is completed in the main reaction, and can be increased by 5%~10% during the ripening period. The maturity time varies with the size of the reaction equipment and the temperature. The ripening time of the 10m 3 acid decanting tank is 40~80min, and the 132m 3 large acid decomposing tank is 1.5~2.5h. Generally, the summer is long and the winter is short. The reasonable maturity time is also related to the temperature of the solid phase after the reaction in the tank. The solid phase temperature is high, the heat dissipation time can be longer, and the solid phase temperature can be relatively shorter. The mature temperature is 200 ° C. The ripening temperature of 180 ° C can increase the acid hydrolysis rate by up to 7%.
After the maturity time is reached, the compressed air is introduced. At this time, the compressed air is mainly used to help the solid phase material to be further cooled. When the solid phase material is cooled to 90-120 ° C, it can be leached by water to dissolve the solid formed by the reaction. Phase.
The titanium salt formed by the reaction between ilmenite and sulfur enemy is very complicated, mainly Ti(SO 4 ) 2 , TiOSO 4 ·2H 2 O, TiOSO 4 , H 2 SO 4 ·H 2 O, TiOSO 4 ·H 2 SO 4 · 2H 2 O, TiOSO 4 ·H 2 O, TiOSO 4, etc., these sulfates have different sulfate stability and solubility, and their solubility in water and dilute sulfuric acid decreases in the following order: Ti(SO 4 ) 2 , TiOSO 4 · H 2 SO 4 ·H 2 O, TiOSO 4 ·H 2 SO 4 ·2H 2 O, TiOSO 4 ·2H 2 O, TiOSO 4 ·H 2 O, TiOSO 4 . Therefore, it is necessary to strictly control the mineral acid during the acid hydrolysis operation. The process conditions such as ratio and reaction temperature are as high as possible to produce TiOSO 4 ·2H 2 O required by the process, and avoid formation of anhydrous iron oxysulfate (TiOSO 4 ) which is poorly soluble in water.
(4) Leaching of solid phase materials
The so-called leaching is to dissolve the reaction solid phase with water or a part of light waste acid, a small amount of water (reduced titanium dioxide solution with a low concentration of TiO 2 ) under the condition of tight control of temperature and concentration under compressed air. The leaching has a great relationship with the temperature of the solid phase, the method of adding water, the order of adding water, the rate of water addition and the temperature and concentration of the solution. Otherwise, the leaching is incomplete, the acid hydrolysis rate is low, the stability is lowered, and even early hydrolysis may occur.
When adding water, it is generally preferred to fasten and then slow down. After a large amount of water is quickly immersed in the solid phase under the agitation of compressed air, water is added to the acid hydrolysis tank at a certain rate. Because the solid phase temperature is still high at the beginning of the leaching, if the water content is too small, the surface leaching temperature is too high (including the heat released by the dilution of water and free acid), which is likely to cause early hydrolysis, but it cannot be added too fast because There is a process for the dissolution of the solid phase. Otherwise, the local solution concentration and acidity are too low in a short time, and the early hydrolysis is also likely to occur when the temperature is high.
In industrial production, a portion of the dilute waste acid (the mother liquor after hydrolysis filtration) and a small amount of water are often added during the leaching. In this case, water or a small amount of water is usually added after the diluted acid is first added. The addition of dilute waste acid during acid leaching can not only reduce the temperature during leaching, but also increase the acidity of the solution to prevent early hydrolysis, and at the same time reduce the mineral acid ratio during the reaction to save sulfuric acid consumption. The small water is a low-concentration titanium solution containing TiO2 for washing sludge, ferrous sulfate and titanium storage tank. Substituting a small amount of water can increase the yield of titanium and reduce waste. Therefore, many factories mainly rely on waste acid during leaching. And small water, tap water is used very little.
Generally, small acid decomposing tanks, water, waste acid, and small amount of water are directly added from the upper part of the tank. Large acid decomposing tanks are simultaneously fed from the bottom or the upper and lower sides to facilitate rapid submersion of solid phase materials to avoid early hydrolysis due to excessive local temperature. [next]
Although the leaching process is only the dissolution process of the acid-dissolved solid phase, the quality of the leaching directly affects the quality index (stability and acid hydrolysis rate) of the titanium liquid. The main factors affecting the leaching operation are:
a. Whether the solid phase is porous, the porous solid phase is easily dissolved due to the solid-liquid contact area, and the solid phase of the solid plate is difficult to be leached;
b. The temperature of the solid phase, if the temperature of the solid phase is too high, it has the advantage of fast dissolution, but it will lead to early hydrolysis, which will reduce the stability; if the temperature of the solid phase is too low, it will make The leaching speed is slower;
c. Solid-liquid ratio at the time of leaching, that is, the ratio of solid to liquid at the time of leaching and the concentration of TiO 2 in the final solution after leaching. Generally, the high concentration of titanium liquid has good stability, but the leaching speed is slow; the low concentration not only increases the burden of the subsequent concentration process, but also induces early hydrolysis, which is more difficult once the early hydrolysis leaching occurs.
d. The quality of the stirring depends mainly on the temperature and pressure of the compressed air, whether the opening of the air distribution plate at the bottom of the tank is uniform, whether there is any foreign matter clogging, etc., and the compressed air is unevenly stirred, which is the main cause of residual unreacted solid phase. one of the reasons;
e. The acid ratio (F value) of the solution is prone to early hydrolysis when the acid ratio (F value) of the reactant is low.
Early hydrolysis means that the titanium liquid begins to precipitate metatitanic acid and orthotitanic acid without thermal hydrolysis. The amorphous milky white colloidal particles have fine and irregular particle size, which not only affects the leaching but also partially precipitates with the sludge. One settling makes the yield of titanium low, and these colloidal particles block the filter cloth during filtration, making the filtration operation difficult, and at the same time, these colloidal particles form a poor crystallization center during hydrolysis, and the pigment properties of the hydrolyzed product become Bad, only low-grade non-pigment grade titanium dioxide can be produced.
To identify the quality of titanium liquid, whether it is prone to early hydrolysis can be expressed by "stability". The stability of titanium liquid refers to the number of milliliters of distilled water consumed by lml titanium solution at a certain temperature (25±1 °C), gradually diluted with distilled water until the titanium solution is hydrolyzed (when it becomes turbid)
The stability of the acid solution titanium solution (after precipitation) should not be lower than 350ml. Otherwise, the titanium solution is unstable, and there are many factors affecting the stability of the titanium liquid. From the viewpoint of acid hydrolysis process operation, it is mainly temperature and acidity. And concentration.
Among the three factors, the temperature is the most important. Some people have done experiments. The stability of the same titanium solution at 400 ° C is 400 mL, the stability is improved to 500 mL at 12 ° C, and the stability is reduced to 200 mL at 65 ° C. When the temperature is raised to 100 ° C, the stability is almost equal to zero, and it is hydrolyzed upon contact with water.
In general, the stability of titanium liquid at a lower temperature (below 80 ° C) increases with increasing acid ratio and concentration (TiO 2 content). However, if the temperature is too low, the leaching speed will be slow, and the low viscosity will affect the quality of the precipitate. If the temperature is too high (more than 80 ° C), the titanium solution with high acidity is also unstable. The concentration of the titanium liquid is high, and although the stability is good, the early hydrolysis is not easy to occur, but the excessively high concentration of the titanium liquid increases the precipitation and filtration. Similarly, F should not be too high. Although it is too high, it has good stability, but it inhibits hydrolysis to make the particles during hydrolysis fine, the hydrolysis rate is low, and the water washing and filtration loss is large.
Usually, the leaching temperature of high-grade pigment-grade titanium dioxide is controlled at 55~75°C, and it is controlled at about 65°C during reduction. Because it is exothermic during reduction, the temperature will rise by 5~6°C, and the F value is controlled at 1.8~1.95. The concentration of titanium solution is TiO 2 120~150g/L. In addition to temperature, the F value and TiO 2 concentration sometimes fluctuate depending on the hydrolysis process.
Temperature is very important in the acid hydrolysis reaction, and it is one of the important factors for the success or failure of acid hydrolysis reaction. The temperature needs to be controlled from the preheating of sulfuric acid, the main reaction to the maturity, leaching and reduction. It is worth noting that there is no temperature measuring device or air and steam flow meter on the acid hydrolysis reactor of most titanium dioxide factories in China. One of the main reasons is that the high temperature and high temperature corrosion resistant thermoelectric protection cannot be found. casing. The fistula is generally used abroad, but the price is too expensive. In recent years, China has developed a special alloy tube of 3YC-24 to solve the problem of high strength and high temperature corrosion resistance. The acid hydrolysis reaction of ilmenite is fast and fierce. It is difficult to control the acid hydrolysis reaction process without temperature display and air and steam flowmeters. There is no guarantee for product quality, yield and safety. At present, this method of operation only by experience It is not acceptable. This is also one of the major gaps in the production of titanium dioxide in China compared with foreign countries.
In addition, the acidity and concentration of the titanium liquid are also important, especially during the leaching process, controlling a certain concentration and acidity is one of the keys to the success of the leaching operation. Experienced operators do not use water or use less water in the black process (acidolysis to concentration) in the production of titanium dioxide, whether it is acid hydrolysis, leaching, washing mud, washing sulfuric acid Iron should not use tap water as much as possible, but use light waste acid or small water, which not only saves water, recovers waste acid and small amount of water, but also avoids the decrease of acidity and concentration caused by dilution of water. . [next]
(5) Reduction of titanium liquid
The composition of the acid hydrolysis reaction of ilmenite is very complicated. The most important components of ilmenite are titanium and iron. The composition of titanium is mainly ternary system of TiO 2 -SO 3 -H 2 O, mainly TiOSO. 4 · 2H 2 O exists. In the iron component, the divalent iron oxide and the trivalent iron oxide in the QDY iron ore are reacted with sulfuric acid to form ferrous sulfate FeSO 4 and high-iron Fe 2 (SO 4 ) 3 , and the ferrous sulfate is acidic. The solution is relatively stable. At pH 5, hydrolysis begins to form iron hydroxide precipitate. The reaction formula is:
FeSO 4 +2H 2 O→Fe(OH) 2 ↓+H 2 SO 4
High-sulfur sulfate is unstable in an acidic solution, and begins to hydrolyze at pH 2.5 to form a basic sulfate or hydroxide precipitate. The reaction formula is:
Fe 2 (SO 4 ) 3 +2H 2 O→2Fe(OH)SO 4 ↓+H 2 SO 4
Fe 2 (SO 4 ) 3 +6H 2 O→2Fe(OH) 3 ↓+3H 2 SO 4
These iron hydroxides are harmful. When the titanium liquid is hydrolyzed, they are precipitated into the metatitanic acid and cannot be removed by water washing. When calcined, they become iron oxide, which causes the titanium dioxide to change color and the whiteness is seriously affected. In order to avoid this phenomenon, it is necessary to reduce the ferric ions in the solution to divalent iron ions, and then separate the ferrous sulfate from the solution by crystallization.
In addition to the majority of titanium and iron sulfates, titanium is also a solid suspension of sulfuric acid (mainly undecomposed ilmenite, rutile, gangue, mud sand, etc.), which can be removed by sedimentation. The other part is small but harmful. It is a kind of trace heavy metal sulfate dissolved in sulfuric acid. This part of the metal salt also has high price and low price. It also needs to be reduced to a low price for later. Remove as much as possible during the operation.
The reducing agent for titanium liquid in industrial production mainly uses metal iron powder, iron filings and iron sheet. Because iron powder, iron filings and thin iron sheet have large specific surface area, they can increase their reaction area to obtain better reduction effect. Although metal zinc , aluminum , sodium sulfite, sodium thiosulfate can also play a reducing role, and even the reduction efficiency is higher, but no iron powder, iron filings, iron sheets are cheap and easy to obtain. The reduction reaction formula of high-sulfur sulfate and metal iron powder is as follows:
Fe+H 2 SO 4 →FeSO 4 +2[H]
Fe 2 (SO 4 ) 3 +2[H] →2FeSO 4 +H 2 SO 4
Or 2Fe 3+ +Fe→3Fe 2+
When the reducing agent iron powder reduces the high-iron sulfate to the ferrous sulfate, the iron sulfate in the titanium liquid is also reduced from the tetravalent titanium to the trivalent titanium, and the reaction formula is as follows:
2Ti(SO 4 ) 2 +2[H]→Ti 2 (SO 4 ) 3 +H 2 SO 4
The reduction is generally carried out in an acid hydrolysis tank, and a reducing agent is added in the later stage of the leaching. If the reduction is not complete at this time, the iron is reduced in the precipitation step until the process requirements are met. When tetravalent titanium is reduced to trivalent titanium in the solution, the solution changes from khaki to purple-black, because the trivalent titanium in the solution is actually a violet of Ti 2 (SO 4 ) 3 ·6H 2 O. Black complex. In foreign countries, it is also reduced in a reducer. The reducer is a tower-type rubber-lined equipment. The titanium-containing liquid in the tower is circulated in the tower until the time is reached.
Reduction is an exothermic reaction. Premature addition of reducing agent not only makes the strong reducing agent generated by the reaction of iron and sulfuric acid - hydrogen loss is more, but also the stability of titanium liquid decreases due to the increase of temperature, if Fe 2 in the ore powder The content of O 3 is very high. In order to avoid more exothermic reaction, the reducing agent can be added in batches. Generally, the temperature rise is not more than 6 ° C when the reduction is controlled.
How much to reduce the reducing agent depends on the level of the reduced material, mainly Fe 2 O 3 in ilmenite. The iron filings used should be oil-free, free of silicon (such as silicon steel sheets) or other alloys, metal-free coatings or iron filings of paint, because organic impurities such as oil can foam during reduction, and other impurities such as silicon are harmful to product quality. .
The degree of reduction of the titanium solution depends on how much trivalent titanium appears in the solution. According to the electrode potential of the redox reaction, the reducing agent in the solution first reacts with the ferric iron which has a strong oxidizing effect. After the ferric iron in the solution is all reduced to divalent iron, the tetravalent titanium participates in the reaction. It is reduced to trivalent titanium. Therefore, once trivalent titanium is present in the solution, it means that the trivalent iron in the solution has been all reduced to divalent iron. In industrial production, the reduction is generally artificial and the trivalent titanium content in the solution is maintained (calculated as TiO 2 ). It is around 1~3g/L.
Maintaining a certain amount of trivalent titanium in the solution can prevent the metal ions in the solution from being reduced to a low-priced state, and reoxidize to a high-valent state in the subsequent production, but the trivalent titanium does not participate in the hydrolysis reaction of the titanium salt, only at the pH. When it is more than 3, it can be hydrolyzed, so the trivalent titanium content is too high, although it is good for the quality, it will lower the hydrolysis rate and increase the loss rate of titanium, which is uneconomical. [next]
In addition to the use of reducing agent reduction, some people have done electrochemical industrial tests, in the case of current 2 ~ 4A, voltage 2.75 ~ 3.1V, temperature 50 ~ 60 ° C, in an asbestos diaphragm electrolytic cell, restore lkg three The price of iron consumes 1.7kW·h, because this method has high power consumption and complicated operation and cannot be industrialized.
The time of leaching and reduction varies depending on the size of the acid hydrolysis reactor, generally 2~8h. The reaction cycle of the acid solution tank of 95m 3 in a foreign factory is as follows (170 mesh of mineral powder, 20t of primary ore powder): acid release 30 min, 40 min of ore casting, l0 min of blowing, 20 min of steam, mature for 60 min, immersed for 420 min, and discharged for 110 min for 11.5 h.
The material of the acid hydrolysis reactor (Fig. 4) is very high. Generally, a layer of lead (especially the bottom part of the cone) is first placed on the steel plate, then two layers of acid-resistant ceramic tiles are built, and the acid-resistant reinforced concrete is used as the tank. Acid-resistant bricks, the more advanced foreign countries are first lead, followed by a layer of hard rubber, and then build two layers of acid-resistant bricks, said to have a service life of up to 50 years.
Since the reaction of ilmenite with sulfuric acid is extremely intense, the utilization rate of the acid hydrolysis reactor is not high, and generally the volume of 1 m 3 can only react 200 kg of ilmenite. China's titanium dioxide factory is relatively small, so the volume of the acid hydrolysis reactor is relatively small, usually only 10~30m 3 , the largest is only 50m 3 (Nanjing oil chemical plant), foreign countries are generally above 90m 3 , recently introduced three in China The acid hydrolysis reactor of the sulfuric acid titanium dioxide production unit has a volume of 132 m 3 , which is basically similar to a large foreign factory.
In addition to giving priority to the corrosion resistance, temperature resistance and shock resistance of the material, the design of the chimney is much larger than that of the general chemical reactor. This can make a large amount of acid gas such as water vapor generated during the main reaction. Discharge in time.其次底部的分布板设计也很重要,分布板的开孔与角度要保证搅拌用的压缩空气能均匀地吹到反应器的四周,否则会在罐壁产生固相物,多孔板的材质有用铅- 锑合金,也有用陶瓷材料的,大型酸解反应器的多孔板常设计成泡罩形,这样空气分布更均匀,也可防止杂物堵塞分布板的孔眼。酸解反应器底部的放料阀门设计也很独特,它既要耐温、耐腐蚀,又要能满足放料、通压缩空气、通蒸汽、通水的功能,还要保证放料时无死角(防止泥渣堵塞阀门)和压缩空气分布均匀。

"Gravity Die Casting. A permanent mould casting process, where the molten metal is poured from a vessle of ladle into the mould, and cavity fills with no force other than gravity, in a similar manner to the production of sand castings, although filling cn be controlled by tilting the die."

Gravity Die Casting

Sometimes referred to as Permanent Mould, GDC is a repeatable casting process used for non-ferrous alloy parts, typically aluminium, Zinc and Copper Base alloys.

The process differs from HPDC in that Gravity- rather than high pressure- is used to fill the mould with the liquid alloy.

GDC is suited to medium to high volumes products and typically parts are of a heavier sections than HPDC, but thinner sections than sand casting.

There are three key stages in the process.

  1. The heated mould [Die or Tool] is coated with a die release agent. The release agent spray also has a secondary function in that it aids cooling of the mould face after the previous part has been removed from the die.
  2. Molten metal is poured into channels in the tool to allow the material to fill all the extremities of the mould cavity. The metal is either hand poured using steel ladles or dosed using mechanical methods. Typically, there is a mould [down sprue" that allows the alloy to enter the mould cavity from the lower part of the die, reducing the formation of turbulence and subsequent porosity and inclusions in the finished part.
  3. Once the part has cooled sufficiently, the die is opened, either manually or utilising mechanical methods.

Advantages

  • Good dimensional accuracy
  • Smoother cast surface finish than sand casting
  • Improved mechanical properties compared to sand casting
  • Thinner walls can be cast compared to sand casting
  • Reverse draft internal pockets and forms can be cast in using preformed sand core inserts
  • Steel pins and inserts can be cast in to the part
  • Faster production times compared to other processes.
  • Once the tolling is proven, the product quality is very repeatable.
  • Outsourced Tooling setup costs can be lower than sand casting.

Gravity Casting Parts

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