Decomposition seed production is an important step of the Bayer process alumina, aluminum hydroxide quality of their products directly affects the quality of the final alumina product, also affect the cycle efficiency of the Bayer process and other manufacturing processes. At present, the problem of crystallized sodium aluminate solution crystallized aluminum hydroxide has long decomposition time and low decomposition rate, which has been the bottleneck restricting the production of alumina by Bayer process. Studies on seed crystal decomposition around sodium aluminate solution have focused on enhanced decomposition, such as seed activation, external field strengthening, and organic additives. Industrial sodium aluminate solution in the presence of various impurities, these impurities include silicon oxide, sodium carbonate, sodium sulfate, sodium chloride, and other organic trace impurities, mainly from bauxite, lime, coal burning, and so on up base. The presence of impurities not only affects the decomposition rate of the solution, but also affects the quality of the decomposed product. The presence of sodium sulfate and sodium carbonate not only increases the viscosity of the solution, but also increases the stability of the sodium aluminate solution, and dissolves and grows in the alumina production process. The processes of fractionation and evaporation have different degrees of influence; the accumulation of Cl - in sodium aluminate solution easily causes corrosion of the equipment and adversely affects the decomposition and evaporation processes. Based on the actual production situation, this paper mainly studies the influence of inorganic salt ions Cl - , S0 4 2- and C0 3 2- on the seed decomposition rate of sodium aluminate solution and its influence mechanism, which provides guidance for optimizing alumina production process.
First, the experimental part
(1) Instruments and raw materials
Homemade stainless steel vertical seed tank (2L) (Central South University Machinery Factory), JS94H micro-electrophoresis (Shanghai Zhongchen Digital Technology Equipment Co., Ltd.), DT-102 automatic interface tension meter (Zibo Huakun Electronic Instrument Co., Ltd.) .
The sodium aluminate solution is prepared by using industrial sodium hydroxide and industrial aluminum hydroxide, and has a molecular ratio of 1.45 to 1.50 and an alumina concentration of 165 to 185 g/L. The seed crystal is an industrial aluminum hydroxide dried at 80 to 100 ° C.
Inorganic salt impurities added in the experiment, sodium chloride, sodium carbonate and sodium sulfate are analytically pure reagents. The amounts of sodium carbonate and sodium sulfate are represented by Na 2 0 C and Na 2 O S , respectively.
(2) Experimental methods
The temperature of the water bath in the decomposition tank was raised to 65 ° C, and 1 L of the prepared sodium aluminate solution was added to each of the four tanks, and different amounts of inorganic salts were added to the respective tanks according to the experimental requirements, and the mixture was stirred with a glass rod. Dissolve, add 500g each seed crystal, seal, stir at 120r/min and start timing. When the decomposition time reaches 9, 21, 33h, the temperature is lowered by 10, 5, 5 °C. Samples were taken from the sampling holes at regular intervals (analyze caustic, alumina content and calculate the decomposition rate of the solution). The relationship between decomposition temperature and time is shown in Figure 1.
The caustic analysis was carried out by acid-base neutralization titration; the alumina analysis was carried out by EDTA complexometric titration. The decomposition rate is calculated as:
In the middle After the reaction time t hours, the decomposition rate of the sodium aluminate solution, %; The caustic ratio of the sodium aluminate solution at the beginning of the reaction; The caustic ratio of the sodium aluminate solution after t hours of reaction.
Second, the results and discussion
(1) Effect of NaCl concentration on the decomposition rate of sodium aluminate solution
The experiment first studied the effect of sodium chloride concentration on the seed decomposition rate of sodium aluminate solution, as shown in Figure 2.
It can be seen from Fig. 2 that at the same decomposition time, the decomposition rate of sodium aluminate solution decreases with the increase of NaCl concentration in the solution, and the higher the NaCl concentration, the greater the influence on the decomposition rate. When the NaCl concentration in the solution is less than 10g/L, the inhibition on decomposition is not significant; when the NaCl concentration is greater than 10g/L, the decomposition is obviously inhibited. For every 10g/L increase in NaCl concentration in the solution, the decomposition rate is reduced by about 2%. When the NaCl concentration is 30g/L, the decomposition rate is reduced by 6% to 7% compared with the blank sample. This is because NaCl is present in the form of simple sodium ions and chloride ions in the solution, and the seed crystal preferentially adsorbs simple ions, thereby reducing the ability of the aluminate ions to interact with the seed crystals, thereby reducing the activity of the seed crystals. When the concentration of NaCl is small, the coverage of chloride ions on the surface of the seed crystal is small, which is not enough to inhibit the decomposition of sodium aluminate solution. However, when the concentration of NaCl is large (such as concentration greater than 10g/L), due to chloride ion As the coverage of the seed crystal surface increases, the decomposition process of the sodium aluminate solution is significantly inhibited.
(2) Effect of Na 2 S0 4 concentration on seed decomposition rate of sodium aluminate solution
The effect of sodium sulfate (as Na 2 O S ) concentration on the seed decomposition rate of sodium aluminate solution was investigated experimentally, as shown in Figure 3.
It can be seen from Fig. 3 that at the same decomposition time, the decomposition rate of sodium aluminate solution decreases with the increase of Na 2 O S concentration in the solution. When the concentration of Na 2 O S is less than 5 g/L, the inhibition effect on decomposition is not Significantly; when the Na 2 O S concentration is greater than 5g / L, the decomposition has a significant inhibitory effect, and as the Na 2 O S concentration increases, the inhibition of the decomposition rate gradually increases. The reason why sodium sulfate inhibits the decomposition of sodium aluminate solution can be explained by its effect on the equilibrium solubility of Al 2 0 3 in the solution. According to the Misra-White equilibrium solubility equation:
C ∞ =Ï(Na 2 0 k )exp(a+b/T+cÏ(Na 2 0 k )/T+dÏ(Na 2 0 c )/T+...+gÏ(Na 2 0 s )/T)
When the temperature is constant, the increase of sodium sulfate (Na 2 s ) increases the Ï(Na 2 s )/T term, which increases the equilibrium solubility of alumina in sodium aluminate solution, resulting in solution The supersaturation (C-C ∞ ) is reduced, which reduces the driving force of the decomposition process. It can be seen from Fig. 3 that when the concentration of Na 2 0 s in the solution is constant, the effect on the decomposition rate of the seed in the early stage of decomposition is greater than that in the later stage, because the supersaturation of the solution is large in the early stage of decomposition, and the seed crystal has a higher Good activity and more high energy points, the decomposition rate is fast, Na 2 SO 4 will be preferentially adsorbed on the surface of the seed crystal in a simple ion form, hindering the interaction between the aluminate ions and the seed crystal, thereby affecting the sodium aluminate solution. break down. In the late stage of decomposition, as the depth of decomposition increases, the solution supersaturation decreases, the decomposition driving force decreases, the decomposition rate decreases, and the effect of sodium sulfate on the decomposition of sodium aluminate decreases. Therefore, the degree of inhibition of the decomposition rate of sodium aluminate solution by sodium sulfate in the late stage of decomposition is less than that in the early stage of decomposition.
(III) Effect of Na 2 C0 3 Concentration on Seed Decomposition Rate of Sodium Aluminate Solution
The effect of sodium carbonate (as Na 2 O C ) concentration on the seed decomposition rate of sodium aluminate solution was investigated experimentally, as shown in Figure 4.
As can be seen from Fig. 4, at the same decomposition time, the decomposition rate of the sodium aluminate solution decreases as the Na 2 O C concentration in the solution increases. When the concentration of Na 2 O C is 10g/L, the decomposition rate of solution at 0.2, 33, 45h is 0.23%, 0.21%, and 0.33% higher than that of the blank. It can be seen that when the concentration of Na 2 O C is less than 10g/L, Seed crystal decomposition does not produce inhibition; when the concentration of Na 2 O C is more than 10g / L, it will significantly inhibit the decomposition of sodium aluminate solution, such as when the concentration of Na 2 O C is 30g / L, The decomposition rate is reduced by 5% to 6% compared to the blank. Na 2 O C has a similar effect to Na 2 O S , that is, when Na 2 SO 4 and Na 2 C0 3 are added, it will exist in a simple ionic form in the solution, and the seed crystal preferentially adsorbs simple ions to make the seed crystal active. Reduce, which affects the decomposition of the solution. When the concentration is small, the coverage of ions on the surface of the seed crystal is small, which is not enough to inhibit the decomposition of sodium aluminate solution. However, when the concentration is large, the coverage of ions on the surface of the seed crystal increases. The sodium solution decomposition process is significantly inhibited.
In summary, the effects of these three inorganic salt ions on the decomposition rate of sodium aluminate solution have the same law. When the concentration is small, the degree of decomposition is small, and as the concentration increases, the decomposition is generated. Significant inhibition. The reason is analyzed. On the one hand, after the three inorganic salts are added, they are present in the solution as simple ions. The seed crystals will preferentially adsorb simple ions, and the adsorbed ions will cover the surface of the seed crystal, thereby hindering the seed crystal and aluminum. The role of acid ions, as the concentration of inorganic salts increases, this inhibition is further enhanced; on the other hand, the presence of inorganic salts increases the equilibrium solubility of alumina in sodium aluminate solution, thereby reducing the supersaturation of the solution and reducing decomposition. The driving force of the process.
Third, the action mechanism of inorganic salt impurities on the decomposition of sodium aluminate solution
(I) Effect of inorganic salts on the zeta potential of Al(OH) 3 surface
Dilute sodium aluminate solution with α k = 3.00 and deionized water to pH=12, and add 100 mL to 7 beakers respectively, and order them as 1 # ～7 # , 1 # as blank samples without inorganic salts, and the rest. 6 different inorganic salts were added separately, stirred and dissolved, and then 20 g of seed crystal Al(OH) 3 solid was added to each beaker, stirred for 2 min, then allowed to stand for 15 min, and Al was measured by JS94H microelectrophoresis apparatus. (OH) 3 particle zeta potential. Fig. 5 and Fig. 6 show the changes of zeta potential on the surface of Al(OH) 3 particles in sodium aluminate solution under different NaCl, Na 2 O C and Na 2 O S concentrations.
The Zeta potential is a physical quantity that describes the surface charge properties of a particle and is a potential at a distance from the surface of the particle. It can be seen from Fig. 5 and Fig. 6 that the addition of NaCl has no significant effect on the zeta potential of the aluminum hydroxide particles in the sodium aluminate solution, and the zeta potential change is small, while the addition of Na 2 C0 3 and Na 2 S0 4 is correct. The effect of the Zeta potential is very significant, making the zeta potential more negative. When an inorganic salt is added to the solution, the anion will be ion-exchanged with A1(OH) 4 - on the surface of the aluminum hydroxide crystal particles. Since the charge of the carbonate ion and the sulfate ion are both valence, the concentration increases. Large, the ion exchange capacity also increases, the potential of the surface of the seed crystal will change significantly, making the zeta potential more negative; while the charge of the chloride ion is monovalent, the zeta potential changes with the NaCl concentration due to ion exchange and Not obvious. It can be seen from Fig. 5 and Fig. 6 that the change of the zeta potential value of the aluminum hydroxide particles in the solution is more obvious than the change of the zeta potential value of the sodium carbonate to the aluminum hydroxide particles, for example, the concentration of Na 2 O 5 is 20 g. At /L, the zeta potential value is -35.6324 mV, and when the Na 2 0 C concentration is 20 g/L, the zeta potential value is -24.4273 mV. This may be caused by the different structures of the two ions, sulfate and carbonate. The sulfate ion is a regular tetrahedral structure, and the carbonate has an equilateral triangle structure. In the sodium aluminate solution, the aluminate is mainly tetrahedral Al ( The OH) 4 - form exists. It is known from the similar principle that the sulfate ion is more easily ion-exchanged with the aluminate ion. Therefore, the zeta potential value is more remarkable with the change of sodium sulfate.
The Zeta potential is an important parameter affecting the properties of the solid-liquid interface, and the decomposition of the sodium aluminate solution is carried out at the solid-liquid interface. Therefore, the zeta potential will directly affect the decomposition of the sodium aluminate solution. The presence of inorganic salts makes the zeta potential more negative, which is detrimental to the adsorption of aluminate ions by the aluminum hydroxide crystal particles, thereby inhibiting the decomposition of the sodium aluminate solution.
(B) the effect of inorganic salts on the surface tension of sodium aluminate solution
Take 100mL of prepared sodium aluminate solution (Ï(Na 2 O)=153.48g/L; Ï(A1 2 0 3 )=170.63g/L; α k =1.47) respectively, add 7 beakers, and then edit 1 # ～7 # ,1 # is a blank sample without inorganic salt, the other 6 are added with different amounts of the specified inorganic salt, stirred to dissolve, and the surface tension of the solution is tested with a DT-102 automatic interfacial tension meter. Fig. 7 and Fig. 8 show changes in the surface tension of the sodium aluminate solution at different concentrations of NaCl, Na 2 0 C and Na 2 0 S.
It can be seen from Fig. 7 and Fig. 8 that the inorganic salt has a certain influence on the surface tension of the sodium aluminate solution, and the surface tension of the solution increases as the concentration of the inorganic salt increases. From a molecular point of view, surface tension is a force that is parallel to the surface and attempts to shrink the surface. The decrease of surface tension will make the wettability of the solution on the surface of the crystal better, which is more conducive to the spreading of ions on the solid surface, and will also reduce the critical nucleation radius and accelerate the secondary nucleation, which will adversely affect the decomposition. influences.
When the seed crystal is decomposed, the aluminate ions in the sodium aluminate solution nucleate on the surface of the seed aluminum hydroxide with the aluminum hydroxide particles as a nucleation center, and the process includes: 1 diffusion of the aluminate ions onto the surface of the aluminum hydroxide particles; 2 adsorption on the surface of the particles; growth and detachment of the 3 cores. Therefore, adsorption is a necessary condition for nucleation. The adsorption in the electrolyte solution has the following rules: 1 selecting a solute similar to the adsorbent; 2 selecting an ion having a lattice size similar to that of the adsorbent; 3 selecting an insoluble or insoluble ion on the surface of the adsorbent. In the sodium aluminate solution, the aluminate ions have a complex morphology and a large ion cluster. Therefore, the aluminum hydroxide crystal particles preferentially adsorb simple inorganic salt ions. At the same time, the presence of the inorganic salt increases the surface tension of the sodium aluminate solution, so that the wettability of the solution on the surface of the crystal is deteriorated, hindering the adsorption of the aluminate ions, thereby inhibiting the decomposition of the seed crystal.
Fourth, the conclusion
(1) The presence of impurities such as sodium chloride, sodium carbonate, and sodium sulfate in the sodium aluminate solution adversely affects the decomposition of the seed crystal. When the NaCl concentration is greater than 10g / L, Na 2 0 S concentration greater than 5g / L, Na 2 0 C concentration greater than 10g / L, sodium aluminate solution will have seed precipitation significantly inhibited.
(B) Zeta potential directly affects the decomposition of sodium aluminate solution. The presence of inorganic salt impurities affects the zeta potential of the surface of the aluminum hydroxide particles in the sodium aluminate solution, making the zeta potential more negative, which is not conducive to the adsorption of aluminate ions by the aluminum hydroxide crystal particles, thereby dissolving the sodium aluminate. The decomposition of the liquid crystal species produces an inhibitory effect.
(3) Inorganic salt impurities increase the surface tension of the sodium aluminate solution, and also hinder the adsorption of the aluminate ions on the crystal surface, which adversely affects the decomposition of the crystal.
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