Reaction behavior and mechanism of anatase in digestion process of diasporic bauxite

LI Xiao-bin(李小斌), FU Wei-an(付伟岸), ZHOU Qiu-sheng(周秋生),

LIU Gui-hua(刘桂华), PENG Zhi-hong(彭志宏)

 School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 15 December 2008; accepted 29 March 2009

Abstract: Based on the study of influence of temperature, digestion time, amount of CaO added and composition of aluminate solution on reaction behavior of pure anatase in high-pressure digestion process of bauxite, reaction mechanism of anatase was preliminarily determined. Anatase first reacts with caustic soda to produce Na₂TiO₃, then the resultant Na₂TiO₃ reacts with 3CaO·Al₂O₃·6H₂O resulting from the reaction of CaO with sodium aluminate solution to produce CaO·2TiO₂·H₂O which eventually converts into CaTiO₃. Higher temperature, concentration of free Na₂O_k (the caustic soda uncombined with aluminate anions in the form of Na₂O) and molar ratio of CaO to TiO₂ are favorable to the conversion of CaO·2TiO₂·H₂O to CaTiO₃. And Al(OH)₄^- shows the function of a catalyzer in the reaction of anatase with caustic soda with or without CaO added during the digestion process of diasporic bauxite.

Key words: anatase; hydroxy calcium titanate; calcium titanate; high-pressure digestion; reaction mechanism

1 Introduction

There is 2%-4% of TiO₂ mainly in the form of anatase, rutile or brookite in almost all kinds of bauxite[1-3]. In the digestion process at relatively low temperature for gibbsitic bauxite, chemical reactions between these titanium-containing minerals and components of aluminate solution will not take place. But as for the digestion of diasporic bauxite in China, higher digestion temperature ranging from 210 ℃ to 270 ℃ is required. In this case, titanium-containing minerals will react with components of aluminate solution. According to the results reported in Refs.[4-5], sodium titanates with different compositions produced by the reaction of titanium-containing mineral with caustic soda appear in a dense thin film covering on the surface of bauxite particles. The film can prevent diaspore from contacting with caustic solution, which significantly lowers the extraction rate of alumina from diasporic bauxite. In order to eliminate the inhibition effect of titanium-containing mineral, lime is always added to the original slurry in the high pressure digestion process for diasporic bauxite in the alumina industry. With the addition of lime in the digestion process, titanium-containing minerals react with calcium-bearing compounds to produce calcium-titanium compounds, such as CaO·TiO₂ and CaO·2TiO₂·H₂O[6]. These compounds usually form substantial scale which is rather difficult to be cleaned up on the surface of heat exchanger during the bauxite digestion process so as to increase energy consumption and cause wastage of equipments productivity[7].

Up to now, although some research work was carried out on the composition, crystal morphology and lattice parameters of calcium-titanium compounds in the red mud[5], it is still unclear about the formation conditions of calcium-titanium compounds and reaction mechanism of titanium-containing minerals with the additive of lime in the alkaline aluminate solution. Therefore, it is of great importance to theoretically make clear the reaction behavior and mechanism of anatase in Bayer high-pressure digestion process of diasporic bauxite for scale control and removal to reduce manufacture cost and improve equipment efficiency.

However, as there are so many other minerals besides diaspore and anatase in bauxite ore, such as limestone, kaolinite, illite, hematite, goethite, etc, the chemical reactions and influencing factors are complex during high-pressure digestion process of diasporic bauxite, which is one of the reasons that the reaction mechanism of anatase in high-pressure digestion process of bauxite has not yet been revealed clearly. So, it is necessary to employ pure titanium-containing mineral to explore the reaction mechanism of titanium-containing mineral in high-pressure digestion process of diasporic bauxite. With consideration of the presence state and the sequence of reaction ability of titanium-containing minerals with caustic soda[8], anatase, the most representative titanium-containing mineral present in diasporic bauxite in China, was chosen as the starting material in this research work.

2 Experimental

2.1 Materials

TiO₂ powder was a chemical reagent in the phase of anatase; NaOH and Na₂CO₃ were reagents with analytically pure grade; Sodium aluminate solution was synthesized in laboratory by heating the mixture of NaOH and Al(OH)₃ with analytically pure grade; and CaO was prepared by calcination of Ca(OH)₂ with analytically pure grade at 800 ℃ for 2 h.

2.2 Methods

Digestion operations of anatase were carried out in high-pressure autoclaves (made in Machine Factory of Central South University, China) with mixed nitrate molten salts as heating medium. The resultant slurry from digestion process was abruptly cooled down by cold water, then filtered by vacuum pump, and washed with boiling water three times. The filter cake was dried at 95-105 ℃, and finally weighed by electrical balance with a accuracy of 0.1 g. Then 0.50 g of filter cake was put into 100 mL hydrochloric acid solution with a volume ratio of hydrogen chlorine to water of 1, then heated at 100 ℃ for 15 min in thermostat and filtered while it was hot. The concentration of titanium dissolved in hydrochloric acid was measured by H₂O₂  colorimeter with 7230G spectrometer (Shanghai Analytical Apparatus Company, China).

Symbol ω(Ti) represents mass ratio of TiO₂ dissolved in hydrochloric acid to the total amount of TiO₂ present in the anatase added. Na₂O_k represents caustic soda (NaOH) in the form of Na₂O in alkali aluminate solution, and free Na₂O_k represents the caustic soda uncombined with aluminate anions in the form of Na₂O. Na₂O_c represents sodium carbonate (Na₂CO₃) in the form of Na₂O.

3 Results and discussion

3.1 Influence of temperature and time on reaction behavior of anatase

It was reported that the apparent activation energy of de-titanization in diasporic bauxite digestion slurry is 84.4 kJ/mol in the temperature range of 180-260 ℃, and surface chemical reaction is the controlling step of the de-titanization reaction in this temperature range[9-11]. Temperature and time have great influence on reaction ability and solubility of all minerals present in bauxite, and on the composition of scale and scaling speed  during the bauxite digestion process. Composition of scale formed by Ti-containing minerals varies with temperature and time, such as CaO·2TiO₂·H₂O, titanium-containing hydrogarnet, and perovskite.

Fig.1 shows the influence of temperature on ω(Ti). It is obviously that ω(Ti) first increases slowly to reach its maximum and then decreases sharply with the increase of temperature. When temperature is lower than 180 ℃, ω(Ti) increases with the increase of time; when temperature is higher than 180 ℃, ω(Ti) first increases and then decreases with the prolongation of time.

Fig.1 Influence of temperature on ω(Ti) of residue resulting from reaction of anatase with alkaline aluminate solution (ρ(Na₂O_k)=233.11 g/L, ρ(Al₂O₃)=183.01 g/L, n(CaO)/n(TiO₂)= 1.0)

Fig.2 represents XRD patterns of residue obtained at different digestion temperatures for various time. When digestion temperature is lower than 200 ℃, the main phases of residue are CaO·2TiO₂·H₂O and 3CaO·Al₂O₃·6H₂O with a small amount of CaTiO₃ as shown in Fig.2(a), and CaO·2TiO₂·H₂O converts into more stable CaTiO₃ at higher digestion temperature and longer time, as shown in Fig.2(b).

Fig.2 XRD patterns of residue prepared under different conditions (ρ(Na₂O_k)=233.11 g/L, ρ(Al₂O₃)=183.01 g/L, n(CaO)/ n(TiO₂)=1.0): (a) 170 ℃, 40 min; (b) 250 ℃, 80 min

CaTiO₃ in the phase of perovskite is the most stable phase of titanium-containing compounds in residue in the digestion process of diasporic bauxite, which is also the main component of scale formed on the surface of heat exchanger in alumina refinery. Well crystallized CaTiO₃ dissolves completely only in hydrofluoric acid, and with a small solubility in boiling hydrochloric acid[12-14], while poor crystallized CaTiO₃, Na₂TiO₃ and CaO·2TiO₂·H₂O can completely dissolve in the boiling hydrochloric acid solution. With increasing the digestion temperature and time, more CaTiO₃ with better crystallization degree is produced, which leads to the decreases of ω(Ti) rapidly. So, it should be noted that ω(Ti) is not always a reliable parameter to characterize reaction degree of titanium-containing mineral, which is different from the common knowledge.

3.2 Influence of composition of sodium aluminate solution on reaction behavior of anatase

When concentration of Na₂O_k or free Na₂O_k is fixed, influence of concentration of Al₂O₃ on reaction behavior of anatase is shown in Fig.3(a). ω(Ti) is almost invariable with the increase of concentration of Al₂O₃ while concentration of free Na₂O_k is fixed. However, when concentration of Na₂O_k is fixed, ω(Ti) decreases with the increase of concentration of Al₂O₃. It may be explained that the concentration of free Na₂O_k decreases with the increase of concentration of Al₂O₃ in the alkaline aluminate solution. And it is the same case in Fig.3(b). The reason will be discussed later in detail.

Fig.3 Influence of composition of sodium aluminate solution on ω(Ti): (a) 180 ℃, 30 min, n(CaO)/n(TiO₂)=1.0; (b) ρ(Al₂O₃)=183.01 g/L, 180 ℃, 30 min, n(CaO)/n(TiO₂)=1.0

3.3 Influence of molar ratio of CaO to TiO₂ on reaction behavior of anatase in sodium aluminate solution

During the high-pressure digestion process, Na₂TiO₃ isolates diaspore from caustic soda so as to inhibit digestion of alumina in the diasporic bauxite. Calcium aluminate hydrate (3CaO·Al₂O₃·6H₂O) is rapidly produced once CaO is added to the aluminate solution, then other reactions may take place. As for the familiar compounds, CaO·2TiO₂·H₂O and CaTiO₃, theoretical molar ratios of CaO to TiO₂ are 0.5 and 1.0. Research on the phase transformation of silicon-containing and titanium-containing minerals in high-pressure digestion proves that TiO₂ mainly converts into CaO·2TiO₂·H₂O when molar ratio of CaO to TiO₂ is less than 0.76, and that CaO·2TiO₂·H₂O becomes unstable and changes into more stable calcium-titanium compound gradually if more CaO is added[15-19].

Fig.4 indicates the relationship between ω(Ti) and molar ratio of CaO to TiO₂. ω(Ti) increases with the increase of molar ratio of CaO to TiO₂ with relatively low concentration of free Na₂O_k, whereas ω(Ti) first increases and then decreases with the increase of molar ratio of CaO to TiO₂ under the condition of high concentration of free Na₂O_k. For example, ω(Ti) decreases from 84.70% to 35.63% with molar ratio correspondingly ranging from 0.5 to 3.0 when concentration of free Na₂O_k is fixed at 220 g/L. Fig.5 shows the XRD pattern of residue obtained under the following conditions: the molar ratio of CaO to TiO₂ is 3, and the concentration of free Na₂O_k is 220 g/L. It can be revealed that CaO·2TiO₂·H₂O changes into more stable CaTiO₃ which is hard to dissolve in hydrochloric acid.

Fig.4 Influence of molar ratio of CaO to TiO₂ on ω(Ti) (ρ(Al₂O₃)=180.94 g/L, 180 ℃ for 30 min)

Fig.5 XRD pattern of residue prepared with high molar ratio of CaO to TiO₂ and high concentration of free Na₂O_k (ρ(Al₂O₃)=180.94 g/L, n(CaO)/n(TiO₂)=3.0, ρ(free Na₂O_k)=220 g/L, 180 ℃, 30 min)

3.4 Reaction mechanism of anatase in high-pressure digestion process

In order to explore the reaction laws and its mechanism of titanium-containing mineral during high-pressure digestion process of diasporic bauxite, digestion experiments of anatase in different solutions were carried out at 180 ℃ for 60 min. Experimental results are listed in Table 1.

Table 1 Reaction behavior of anatase in different solutions

From results listed in Table 1 and the reaction behaviors of anatase discussed above, reaction mechanism of anatase in sodium aluminate solution during the high-pressure digestion process can be presumably determined as follows.

1) Neither soda (Na₂CO₃) nor Ca(OH)₂ can react with anatase under the experimental conditions. However, anatase can react with NaOH to form Na₂TiO₃ which may further react with Ca(OH)₂ to form CaTiO₃ if CaO is added to the system, as shown in reactions (1) and (2):

2NaOH+TiO₂=Na₂TiO₃+H₂O                    (1)

Ca(OH)₂+Na₂TiO₃=CaTiO₃+2NaOH              (2)

2) Aluminate anion seems to remarkably catalyze reactions of anatase with caustic soda solution. Results of experiment 3 and experiment 4 show that, under the almost same concentrations of free Na₂O_k (Na₂O_k 116.00 g/L), ω(Ti) increases from 3.05% to 37.88% in the presence of aluminate anion without CaO, and ω(Ti) also increases from 7.16% to 84.87% in the presence of aluminate anion when CaO is added with molar ratio of CaO to TiO₂ of 1. So, the conclusion can be drawn that aluminate anions plays catalysis role in the reaction process. The catalysis mechanism of aluminate anions could be explained as follows:

2NaAl(OH)₄+TiO₂=Na₂TiO₃+2Al(OH)₃+H₂O       (3)

NaOH+Al(OH)₃=NaAl(OH)₄                    (4)

The total reaction can be expressed as

2NaOH+ TiO₂=Na₂TiO₃+H₂O                   (5)

3) Hydroxy calcium titanate (CaO·2TiO₂·H₂O) is the product of reaction of Na₂TiO₃ with calcium aluminate hydrate (3CaO·Al₂O₃·6H₂O) resulting from the reaction of CaO added with sodium aluminate solution, as shown in chemical reactions (6) and (7):

3Ca(OH)₂+2NaAl(OH)₄=3CaO·Al₂O₃·6H₂O+2NaOH (6)

3CaO·Al₂O₃·6H₂O+6Na₂TiO₃+6H₂O=

3[CaO·2TiO₂·H₂O]+2NaAl(OH)₄+10NaOH    (7)

Calcium titanate (CaTiO₃) may be formed from two ways. One is from reaction of Na₂TiO₃ and Ca(OH)₂, and the other is from reaction of CaO·2TiO₂·H₂O with 3CaO·Al₂O₃·6H₂O and NaOH, as shown in chemical reaction (8):

3[CaO·2TiO₂·H₂O]+3CaO·Al₂O₃·6H₂O+2NaOH=

6CaTiO₃+2NaAl(OH)₄+6H₂O               (8)

According to the theoretical calculation, reaction (8) may take place only if molar ratio of CaO to TiO₂ is more than 0.5. Result from experiment 4 in Table 1 also indicates that more CaO·2TiO₂·H₂O converts into CaTiO₃, which results in the decrease of ω(Ti), if molar ratio of CaO to TiO₂ is 3. From the discussion mentioned above, conclusion can be made that elevating the digestion temperature, prolonging the duration time, increasing concentration of free Na₂O_k and molar ratio of CaO to TiO₂ favor the conversion of CaO·2TiO₂·H₂O to CaTiO₃.

4 Conclusions

1) In sodium aluminate solution, aluminate anion has a catalysis function for reaction of anatase with caustic soda.

2) During high-pressure digestion process of diasporic bauxite, the reaction degree of anatase depends on the concentration of free Na₂O_k rather than the concentration of Al₂O₃, so long as there is aluminate anions in alkali solution.

3) Anatase first reacts with free Na₂O_k to form Na₂TiO₃ which further reacts with 3CaO·Al₂O₃·6H₂O to produce CaO·2TiO₂·H₂O, and CaO·2TiO₂·H₂O eventually changes into CaTiO₃. Moreover, higher temperature, longer time, higher concentration of free Na₂O_k and molar ratio of CaO to TiO₂ all favor the conversion of CaO·2TiO₂·H₂O to CaTiO₃.

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Foundation item: Project(2005CB6237-02) supported by the National Basic Research Program of China

Corresponding author: ZHOU Qiu-sheng; Tel/Fax: +86-731-88830453; E-mail: csqszhou@163.com

DOI: 10.1016/S1003-6326(09)60111-4

(Edited by YANG Hua)