Titanium dioxide, commonly known as titanium dioxide, is an important chemical raw material and excellent performance of the white pigment widely used in paint, plastics, paper and other industries. At present, the industrial production methods of titanium dioxide mainly include sulfuric acid method and chlorination method. Sulfuric acid process titanium iron ore as a raw material, the production process a large amount of waste acid, and sulfur-containing acidic waste water of flue gas, serious environmental pollution; large and the high temperature chlorination chloride oxidation process difficult, and the production process of toxic chlorine The compound is difficult to handle and requires deep well landfill. Therefore, there is an urgent need to develop an environmentally friendly new process for the clean production of titanium dioxide. Institute of Process Engineering successfully developed a titanium dioxide alkaline cleaner production of new technology, the use of this process can produce rutile and anatase titanium dioxide.
First, the test part
(1) Test principle
The main component of the high titanium slag is Ti 3 O 5 , and Ti 3 O 5 reacts as follows in the NaOH molten salt:
The reaction product is washed with water, Na + in Na 2 TiO 3 is ion-exchanged with H + in water, and part of Na + is leached into the liquid phase to form a certain concentration of alkali solution. The main reaction can be expressed as:
The washed solid phase (xNa 2 O·TiO 2 ·yH 2 O) reacts with sulfuric acid to form a titanium sulfate solution, and the reaction formula is as follows:
Hydrolysis of titanium sulfate solution to form metatitanic acid precipitation, the reaction formula is:
(2) Test materials and instruments
The NaOH and sulfuric acid used in the test were all analytically pure, and the water was deionized water; the molten salt reaction product was prepared according to the literature, and the water washing material obtained after washing was the test raw material, and the composition is shown in Table 1.
Table 1 Chemical composition of washed materials %
The product phase was determined by X-Pert PRO MPD X-ray diffractometer from PANalytical, the Netherlands, and the elemental composition in the raw material was analyzed by Optima 5300 DV inductively coupled plasma atomic emission spectroscopy (ICP-OES) from Perkin-Elmer, USA. The particle size distribution of metatitanic acid after hydrolysis was measured by a laser particle size analyzer, and the crystal form was analyzed by XRD.
(3) Test methods
1. Preparation of titanium sulfate solution
Different concentrations of sulfuric acid solution were prepared and added to the three-necked flask in a certain ratio with the water-washed material. The three-necked flask was placed in a constant temperature oil bath, and after reacting for a certain period of time at a set temperature, it was taken out and filtered, and the filtrate was a titanium sulfate solution. The mass concentration (calculated as TiO 2 ) and the F value of the titanium sulfate solution were determined according to the literature.
2. Hydrolysis of titanium sulfate solution
A titanium sulfate solution was added to the three-necked flask, and placed in a constant temperature oil bath which had been heated to a certain temperature, and the reaction was timed. During the reaction, the samples were periodically sampled, filtered, and the mass concentration of titanium in the filtrate was determined. After completion of the reaction, the mixture was filtered, and the mass concentration of titanium in the filtrate was measured to calculate the hydrolysis rate. The metatitanic acid filter cake is sufficiently washed and calcined by salt treatment to obtain titanium dioxide. The hydrolysis rate of the titanium sulfate solution is calculated as follows:
Where: ρ(Ti) 0 is the total titanium mass concentration in the titanium sulfate solution at the beginning of the reaction; ρ(Ti) t is the total titanium mass concentration in the solution at the reaction t.
(4) Test process
Test principle The process flow chart is shown in Figure 1.
Figure 1 Titanium dioxide cleaning production process
The process mainly includes five operating units: sodium hydroxide molten salt reaction unit, multi-stage countercurrent washing and medium circulation unit, titanium sulfate solution preparation unit, titanium sulfate solution hydrolysis unit, salt treatment and calcination unit. The new process uses high titanium slag as raw material, sodium hydroxide molten salt as reaction medium; titanium is converted into sodium titanate; sodium titanate is washed by multistage countercurrent washing, sodium ion is leached to liquid phase to form a certain concentration of alkali liquid, alkali After the liquid is removed by impurities, concentrated by evaporation, and returned to the molten salt reaction unit for recycling, the titanium is left in the solid phase after washing, and then dissolved in dilute sulfuric acid to form a titanium sulfate solution; the titanium sulfate solution is heated and hydrolyzed to prepare metatitanic acid, and the mother liquor is hydrolyzed. The membrane is separated and removed, and recycled, and the titanic acid is further subjected to salt treatment and calcination to obtain titanium dioxide. The preparation and hydrolysis process of the titanium sulfate solution is the core part of the process, which determines the properties of the product titanium dioxide, including purity, particle size distribution and pigment properties. The preparation and hydrolysis of titanium sulfate solution is also the core operation unit of the traditional sulfuric acid method, but the composition (ion type, concentration, etc.), preparation process and hydrolysis mechanism of the titanium sulfate solution in the new process are different from the traditional sulfuric acid method.
Second, the results and discussion
(1) Preparation of titanium sulfate solution
1. Test design and results
Take different mass fractions of sulfuric acid and a certain amount of water-washed materials, react at 50 ° C for 4 h, and filter. The effects of the amount of water washing material and the mass fraction of sulfuric acid on the mass concentration of the product titanium sulfate solution (calculated as TiO 2 ) and the F value are shown in Table 2.
Table 2 Effect of Sulfuric Acid Mass Fraction and Washing Material Amount on the Properties of Titanium Sulfate Solution
2. Relationship between the amount of water washing materials and the mass concentration of titanium sulfate
The amount of water washed in Table 2 is plotted against the mass concentration of titanium sulfate, as shown in Figure 2. The mass concentration of titanium sulphate increases with the increase of the amount of washing material, and the relationship between them is about 2 times:
Where: a, b, c are constants. Fitting with MATLAB, the results a = 0.00676, b = 0.773, c = 76.1. Therefore, the amount of water washing material required to prepare titanium sulfate solutions of different mass concentrations can be calculated by formula (7):
The correlation coefficient of equation (7) is r2=0.992, and the fitting result of the quadratic curve agrees with the experimental results.
Figure 2 Relationship between mass concentration of titanium sulphate and the amount of washed materials
3. Relationship between mass fraction of sulfuric acid and properties of titanium sulfate solution
It can be seen from Table 2 that the mass fraction of sulfuric acid has an effect on the mass concentration and F value of the titanium sulfate solution. Suppose that the mass fraction of sulfuric acid and the mass concentration of titanium sulphate solution and the acidity of titanium sulphate solution meet the quadratic curve relationship, that is,
Where: w represents the mass fraction of sulfuric acid; ρ represents the mass concentration of the titanium sulfate solution; F represents the acidity of the titanium sulfate solution; h, i, j, k, l, m are constants. The data of Table 2 was fitted with MATLAB to obtain h = 0.000378, i = -27.7, j = 0.195, k = -0.247, l = 83.9, m = -54.5. Therefore, the relationship between the mass fraction of sulfuric acid and the mass concentration and F value of the titanium sulfate solution can be expressed as:
The w in the above formula is plotted against ρ and F, and the result is shown in FIG.
Fig. 3 Relationship between mass concentration and F value of titanium sulfate and sulfuric acid mass fraction
It can be seen that when the mass concentration of the titanium sulfate solution is constant, the sulfuric acid mass fraction increases as the F value increases; when the F value is constant, as the mass concentration of the titanium sulfate solution increases, the required sulfuric acid mass concentration increases. This is because the mass concentration of the titanium sulphate solution is increased, the consumption of the water-washed material is increased, and the amount of sulfuric acid required to dissolve the water-washed material is increased.
(2) Hydrolysis of titanium sulfate solution
1. Effect of hydrolysis temperature on hydrolysis rate of titanium sulfate
The effect of temperature on hydrolysis was examined under the conditions of F = 1.75 and titanium mass concentration (calculated as TiO 2 ), and the results are shown in Fig. 4. It can be seen that increasing the temperature can increase the hydrolysis rate, because the hydrolysis reaction is an endothermic reaction, and raising the temperature can promote the positive movement of the reaction, which is favorable for the hydrolysis. It is also seen from Fig. 4 that the hydrolysis process is roughly divided into three stages: the first stage is 0 to 30 min, in which no solid phase precipitates, the hydrolysis rate is low, and the liquid phase composition does not change significantly; the second stage is 30 to 90 min. At this stage, a large amount of metatitanic acid solid precipitated, and the precipitation rate was faster; after the third stage was 90 minutes, the precipitation rate of titanium sulfate was slowed down at this stage.
Figure 4 Effect of reaction temperature on hydrolysis rate of titanium sulfate
2. Effect of F value on hydrolysis rate of titanium sulfate
The effect of F value on hydrolysis of titanium sulfate was investigated under the conditions of titanium concentration (calculated by TiO 2 ) 200 g/L and hydrolysis temperature 110 ° C. The results are shown in Fig. 5. It can be seen that as the value of F increases, the reaction speed increases. According to the reaction (4), reducing the mass fraction of sulfuric acid in the solution facilitates the forward movement of the hydrolysis reaction. Therefore, the preferred hydrolysis F value is determined to be 1.9 to 2.0, and the hydrolysis time is 4 hours. At this time, the hydrolysis rate of titanium sulfate is 90% or more.
Fig. 5 Effect of F value on hydrolysis rate of titanium sulfate
3. Effect of mass concentration of titanium sulfate on hydrolysis rate
The effect of the mass concentration of titanium sulfate on the hydrolysis was investigated at F = 2.0 and a hydrolysis temperature of 110 ° C. The results are shown in Fig. 6. It can be seen that as the mass concentration of titanium sulfate increases, both the hydrolysis rate and the final hydrolysis rate decrease. This is because after the mass concentration of titanium sulfate is increased, the ionic strength in the solution is increased, and the salary phase is more difficult to precipitate. When the mass concentration of titanium sulfate is 204 g/L, the hydrolysis rate is greater than 90%. Therefore, it is determined that the preferred titanium sulfate mass concentration is 200 g/L.
Fig. 6 Effect of mass concentration of titanium sulfate on hydrolysis rate of titanium sulfate
4. Particle size distribution of metatitanic acid during hydrolysis
The titanium sulfate solution having a concentration of F=1.75 and a titanium concentration (calculated as TiO 2 ) of 140 g/L was heated to 102° C. for hydrolysis. During the reaction, the particle size distribution of the hydrolyzate metatitanic acid was analyzed periodically. The particle size distribution of metatitanic acid at different reaction times is shown in Fig. 7. It can be seen that the particle size of metatitanic acid increases with time during the initial stage of the reaction; the particle size remains basically unchanged at the end of the reaction.
Figure 7 Particle size distribution of metatitanic acid at different reaction times
5. Determination of the preparation and hydrolysis process of barium sulfate solution
According to the above test results, the optimal hydrolysis conditions of the titanium sulfate solution were determined as follows: the mass concentration of titanium sulfate was 200 g/L, F=1.9, the reaction temperature was 110 ° C, and the hydrolysis time was 4 h. Substituting the above parameters into the formulas (7) and (9), the optimum conditions for preparing the titanium sulfate solution are: sulfuric acid mass fraction 43.4%, acid-solid mass ratio 1:1.
(III) SEM characterization of metatitanic acid
The reaction product of metatitanic sufficiently dried, dispersed in a clean metal surface of the aluminum foil, which is analyzed by SEM morphology, the results shown in Fig. It can be seen that the metatitanic acid is substantially a very small particle of about 10 nm, and after agglomeration, it is a large particle of about 2 μm, and the agglomerate is approximately spherical. This structure illustrates the two processes of single particle formation and particle polymerization occurring during the hydrolysis process.
Figure 8 SEM spectrum of metatitanic acid product
(4) Analysis of hydrolysis mechanism
By analyzing the process of hydrolysis of titanium sulphate solution to form metatitanic acid and the morphology of the reaction product metatitanic acid, it is concluded that the hydrolysis mechanism is: in the first stage, the titanium ions in the titanium sulphate solution are first combined with each other to form a linear polymer. Conducive to the formation of anatase type titanium dioxide, therefore, all of the anatase type metatitanic acid is formed after hydrolysis of the titanium sulfate solution;
In the second stage, the particle size of the solid phase is 1 to 3 μm, and the particle size of the primary particles of metatitanic acid is only 10 nm, that is, the titanium sulfate solution is formed into primary particles of about 10 nm, and then the primary particles are mutually polymerized to form a particle size of 1 to 3 μm. Aggregates. As the reaction time increases, the particle size of the agglomerates gradually becomes larger;
In the third stage, after 90 minutes of reaction, at this stage, the hydrolysis rate increased slowly, and the particle size of metatitanic acid did not change significantly.
The reaction time of the above three stages varies slightly with the reaction temperature: the higher the temperature, the shorter the first stage time; and the higher the reaction rate and the hydrolysis rate.
(5) Preparation of titanium dioxide by calcination of metatitanic acid
Titanium dioxide can be obtained by heating and calcining the metatitanic acid obtained by hydrolysis. The effect of calcination temperature on the titanium dioxide crystal form is shown in Figure 9.
Figure 9 XRD pattern of titanium dioxide at different calcination temperatures
It can be seen that the crystalline form of metatitanic acid is anatase at 100 ° C; the crystal form is unchanged when the calcination temperature is low, and the product is anatase titanium dioxide; when the calcination temperature is as high as 800 ° C, the product gradually It is converted into rutile type titanium dioxide; when the temperature is 1000 ° C, the product is completely rutile type titanium dioxide. This indicates that after the titanic acid prepared by the method is calcined at different temperatures, both rutile-type titanium dioxide and anatase-type titanium dioxide can be prepared.
Third, the conclusion
(1) The titanium sulfate solution can be obtained by dissolving the water-washed material after the high-titanium slag molten salt reaction, and the sulfuric acid mass fraction and the acid-solid mass ratio can be controlled to obtain the titanium sulfate solution with different F values and mass concentrations. The optimum conditions for the preparation of titanium sulphate solution were determined by MATLAB fitting: sulfuric acid mass fraction 43.4% and acid-solid mass ratio 1:1. The titanium sulfate solution prepared under this condition had a mass concentration of 190 g/L and F = 1.9.
(2) During the hydrolysis of titanium sulfate, reducing the F value of the solution and the mass concentration of titanium sulfate, increasing the hydrolysis temperature, and prolonging the hydrolysis time can increase the hydrolysis rate. The optimal hydrolysis conditions were: temperature 110 ° C, time 4 h, titanium sulfate mass concentration 190 g / L, F = 1.9. Under this condition, the hydrolysis rate of titanium sulfate is greater than 90%.
(III) XRD, SEM and particle size distribution characterization results show that the hydrolysis of titanium sulfate is carried out in three different stages.
