Study on preparation and properties of supported compound desulfurizer for Bayer dissolution

In view of the serious harm caused by sulfur ion to alumina production and the high cost of desulfurization in Bayer dissolution process of high sulfur bauxite leaching at high temperature and high pressure, the supported compound desulfurizer was prepared by melt infiltration method to realize simultaneous desulfurization in Bayer dissolution process. In this paper, Zn(NO3)2·6H2O and Cu(NO3)2·3H2O as the main components, kaolin, sepiolite, activated alumina as the active carrier, the supported compound desulfurizer was prepared. The active carrier was optimized and the optimum preparation parameters were determined by means of DSC, IR, TG, SEM and XRD. The results show that kaolin is more suitable as the active carrier of compound desulfurizer. The best melting temperature of the precursor is 100 °C, the best calcination temperature is 400 °C, and the best calcination time is 2 h. The supported compound desulfurizer mainly consists of ZnO, CuO, AlOOH, SiO2 and Mullite. The results provide a basis for the active preparation of the supported compound desulfurizer.


Introduction
China's bauxite resources are relatively rich, mainly the monohydrate duralumin type, accounting for 98.46% of the reserves, the trihydrate type is less, accounting for only 1.54% of the total [1].Among them, the monohydrate stibnite type has higher sulfur content (generally the sulfur content can reach more than 0.7%), and the sulfur mainly exists in the form of pyrite FeS 2 [2].China's rich reserves of high-sulfur bauxite as the existing resources [3].However, the high sulfur bauxite usually contains higher content of sulfur and silicon, resulting in its difficult to be effectively utilized on a large scale.In Bayer method of high-temperature and high-pressure dissolution process, sulfur enters the solution in the form of ions and accumulate continuously during the production process, which caused great harm to the production of alumina, such as corrosion of steel equipment [4].Therefore, the biggest problem in making full use of the existing high quality high sulfur bauxite resources is how to desulfurize.Aiming at the difficult problem of desulfurization in the process of alumina production, scholars at home and abroad have carried out a large number of in-depth studies [5,6].At present, the commonly used desulfurization technology includes the flotation desulphurization, wet process desulfurization, roasting pretreatment desulfurization and adding reducing agent sintering desulphurization [7].
This paper focuses on the preparation and desulfurization of in wet process desulfurization, and also puts forward related requirements for desulfurizer in view of the problems in dissolution process, such as high activity, high sulfur capacity, fast reaction speed, stability and easy to control the side reaction [7].The results of Westmoreland and Harrison's thermodynamic study of a series of sulfidation reactions of 28 metal oxides at medium and high temperatures showed that oxides of 11 metals can be used for desulfurization [8], namely Ba, Ca [9], Co, Cu [10], Fe [11], Mn, Mo, Sr, W, V and Zn.The oxides of these metals have the ability to desulfurize at medium and high temperatures and are easily regenerated.Among them Zn, Fe, Cu, Mn, Ca, Ba and other metal oxides are used more as desulfurizers.
Metal desulfurizers currently in common use can generally be divided into three types, namely, the single metal oxide desulfurizers, the composite metal oxide desulfurizers, and the loaded metal oxide desulfurizers [12].At present, although there are many types of single-metal desulfurizers in China, there are fewer desulfurizers that meet the requirements of large working sulfur capacity, high precision, both inorganic and organic sulfur removal, and low price, and all kinds of desulfurizers are accompanied by insurmountable shortcomings [13].For example, although zinc oxide desulfurizer is widely used as a desulfurizer in the fields of Fischer-Tropsch synthesis, ammonia synthesis, petroleum refining, etc, it also has the disadvantages of small specific surface area, small pore volume and easy to be sintered, which leads to the smaller sulfur penetration capacity at room temperature.The desulfurization performance of single metal oxides is low, and the researchers have optimized the performance of desulfurizers by adding additives to increase the reactivity.The Cu-Zn-Al system desulfurizer prepared by Dahao [14] et al can effectively improve the catalytic activity of the desulfurizer by adjusting the atomic ratio of Co/Zn.Baird [15] et al uniformly dispersed Fe 2 O 3 and CuO on the surface of ZnO through nitrate decomposition, effectively improving the absorption and conversion of the desulfurizer.Therefore, increasing the active point of the reaction can effectively increase the desulfurization performance of the desulfurizer.Xue [16] et al prepared a series of metal oxides and synthesized them with ZnO, all of which had high sulfur capacity.Compared with the single metal oxide desulfurizer, the compound metal oxide desulfurizer can effectively increase the reactivity, so that the reaction of desulfurizer with hydrogen sulfide is more efficient, thus showing better desulfurization performance.As monometallic desulfurizers began to no longer meet the needs of desulfurization, the researchers have carried out research on the preparation of compound desulfurizers [17,18].Whether it is structural characteristics or reaction performance, they have greater advantages over the single metal oxides [19].It is mainly manifested in several aspects [20,21], improving the desulfurization efficiency, increasing the sulfur capacity, effectively preventing or reducing the volatilization or loss of metal oxides, improving the dispersion of active components, and enhancing the mechanical strength and the stability.Zinc ferrate is a relatively common, which combines the advantages of high desulfurization accuracy of zinc oxide desulfurizer and high sulfur capacity of iron oxide desulfurizer with fast reaction speed, and the regeneration is relatively easy, and the stability and the mechanical strength of desulfurizer can be improved by adding binder.Akyuritlu [22] added vanadium oxide to zinc ferrate, which effectively inhibited the volatilization of zinc, and the more vanadium oxide was added, the greater the cyclic stability of zinc ferrate, and the desulfurization accuracy of the desulfurizer is also improved.Zinc titanate is developed on the basis of zinc ferrate research.Compared with zinc ferrite, zinc titanate can effectively inhibit the loss of active components caused by the reduction of zinc oxide to elemental zinc.Yi, Chang-Keun [23] et al conducted an earlier study on zinc titanate desulfurizer and found that the addition of titanium dioxide could not only effectively increase the stability of zinc oxide, but also effectively improve the conversion rate of zinc titanate.In addition, the supported compound desulfurizer is prepared on the basis of the compound desulfurizer by using a loose and porous substance as an active carrier, which can improve the dispersion of the active component on its surface, provide more reactive active points, and effectively alleviate the situation that the desulfurizer surface has closed holes or become dense, and improve the desulfurization performance of the desulfurizer [24].In view of the problems existing in the desulfurization of high sulfur bauxite wet process, the supported compound desulfurizer was prepared by melt infiltration method in this paper.The pore structure of the compound desulfurizer was changed by adding active carrier with ZnO as the main desulfurizer and CuO as the desulfurizer.At the same time, based on the analysis of surface properties, the best preparation parameters of the compound desulfurizer were determined, which provided a theoretical reference for the desulfurization of high-sulfur bauxite by Bayer dissolution process.

Experiment
Appropriate amounts of kaolin, activated alumina, sepiolite, Zn(NO 3 ) 2 •6H 2 O, Cu(NO 3 ) 2 •3H 2 O, all analytically pure, were weighed.The nitrate of different proportions was mixed with the active carrier and then ground, and the proportions of nitrate were converted according to the molar ratio of ZnO and CuO.The ground powder was placed in the hydrothermal reactor in the oven at a certain temperature for melting.After the melting infiltration, the supported compound desulfurizer was obtained by calcining the precursor of the impregnated compound desulfurizer in Muffle furnace at a certain temperature.
The schematic diagram of the preparation of supported compound desulfurizer is shown in figure 1.

Influence of active carriers on the surface morphology of supported compound desulfurizers
In order to discuss in depth the effect of different active carriers on the surface morphology of supported compound desulfurizers, SEM morphology of the supported compound desulfurizers was examined, as shown in figure 3, where a, b and c are SEM morphologies of the composite desulfurizer precursors prepared by different active carriers after melting infiltration, respectively, while d, e and f are SEM morphologies of their corresponding composite desulfurizers after calcination.
After melting infiltration, the compound desulfurizer precursor prepared with the sepiolite as the active carrier has a larger particle size before calcination (figure a) and has agglomerated, resulting in a smaller specific surface area of the sample.The sample prepared with alumina as the active carrier has also most of the blocks before calcination (figure b), with a few particulate sample on the surface.While the precursor prepared with kaolin as the active carrier has a more homogeneous distribution of particles before calcination (figure c), and there is no large agglomeration, except for a small number of particles, most of the particles has a diameter of 3-4 μm, compared with the other two, the compound desulfurizer precursor prepared with kaolin has more uniform particle distribution and larger specific surface area, and kaolin is cheaper and less easy to absorb water, which can effectively reduce experimental errors.Therefore, the kaolin is a good active carrier for the preparation of compound desulfurizers.
After calcination, the precursor particles with kaolin as the active carrier are snowflake shaped, and the diameter of the sample is in the range of 3-4 μm.Compared with sepiolite and alumina, the specific surface area is also the largest and the distribution is relatively uniform.Therefore, the kaolin was selected as the active carrier for the supported compound desulfurizer.

Effect of melting infiltration temperature on supported compound desulfurizers
Differential scanning thermal analysis (DSC) was performed on the compound desulfurizer precursors that was infiltrated for 24 h at different infiltration temperatures (85 °C, 100 °C, 105 °C), and the results are shown in figure 4.
As can be seen from figure 4, at the infiltrating temperature of 85 °C, the cold crystallization exothermic peak appeared between 45.5 °C and 60 °C, followed by the melting heat absorption peak, and the precursor had begun to decompose at 75 °C.It is seen that only a small portion of the precursor enters the carrier pores at the infiltrating temperature of 85 °C.When the infiltrating temperature is 100 °C, the cold crystallization curve still exists in DSC spectra, but the heat absorption peak of melting has disappeared, which shows that the precursor has melted and infiltrated into the carrier pores at 100 °C.In order to determine the precise infiltration temperature, it is changed to a small extent.At the infiltration temperature of 105 °C, the melting heat absorption peak has disappeared, indicating that the sample is completely permeated.It can be judged that the infiltration temperature of the compound desulfurizer precursor is between 100 °C and 105 °C.Therefore, 100 °C is chosen as the optimum infiltration temperature.

Effect of calcination temperature on supported compound desulfurizer
The decomposition of Zn(NO 3 ) 2 •6H 2 O and Cu(NO 3 ) 2 •3H 2 O in the sample can be well judged by TG analysis, so as to determine the optimal decomposition temperature for calcination.
Figure 5 shows TG analysis after 24 h of melt infiltration at infiltration temperature.It can be seen that there are two significant weight changes in TG curves.The first change occurs at 50 °C to 150 °C, corresponding to the removal of adsorbed water and crystal water.The second change occurs in the range of 250 to 350 °C and is accompanied by the release of NO 2 , which is caused by the decomposition of Zn(NO 3 ) 2 .The analysis shows that Zn(NO 3 ) 2 and Cu(NO 3 ) 2 loaded on the active carrier have been completely decomposed below 400 °C.Therefore, 400 °C is selected as the calcination temperature of the precursor.As can be seen in figure 6, the samples after fusion were calcined at 1 h, 2 h, 4 h and 6 h, and there was no characteristic peak of nitrate at 1380 cm −1 , indicating that the calcination of 2 h is sufficient to decomposition of Zn(NO 3 ) 2 •6H 2 O completely.For comparison, the desulfurizer of the precursor calcined for 1 h was characterized, which showed that the characteristic peak of nitrate appeared at 1380 cm −1 when calcined for 1 h, indicating that Zn(NO 3 ) 2 •6H 2 O calcined for 1 h was not completely decomposed.Therefore, to completely decompose the nitrate at 400 °C, the calcination time is determined to be 2 h.

Scanning electron microscope analysis (SEM)
SEM surface morphology analysis can clearly observe the surface morphology and particle state of the samples after calcination.SEM surface morphologies of the compound desulfurizer with kaolin as the carrier under different calcination times are shown in figure 7.
As can be seen in figure 7, the larger particles appeared on the surface of the samples after calcination for 1 h.The surface of the sample after calcination for 2 h is basically granular and snowflake, and the particles are relatively smaller, while the surface of the sample after calcination for 4 h basically become snowflake.According   to SEM surface morphology analysis, after calcination for 1 h there are still a few particles on the surface, indicating that the calcination time is slightly shorter.The surface particles of the samples calcined for 2 h and 4 h show a better snowflake shape with the best morphology.However, considering the possibility of overburning and economic costs, the optimal calcination time is determined to be 2 h combined with IR analysis.

Physical composition analysis (XRD)
In order to study the effect of different calcination times on the physical phase composition of the compound desulfurizers, XRD analysis was conducted, and the results are shown in figure 8.As can be seen that the positions and intensities of x-ray diffraction peaks under different calcination times are the same, indicating that the physical phase compositions of the compound desulfurizers under calcination for 1 h, 2 h and 4 h are the same.It was analyzed using X'Pert HighScore Plus software, which shows that the supported compound desulfurizer is mainly composed of ZnO, CuO, AlOOH, SiO 2 and Mullite.

Conclusions
The supported compound desulfurizer was prepared by the melt infiltration method, and the main conclusions obtained by testing and analyzing the physical and structural properties of the precursor and the supported compound desulfurizer.
(1) The surface morphology of the desulfurizers prepared from sepiolite, kaolin, and activated alumina was analyzed and kaolin was determined to be the best active carrier.
(2) The optimum melt infiltration temperature of the precursor was obtained as 100 °C and the optimum calcination temperature was 400 °C by DSC and TG analysis of the precursor after melting infiltration.
(3) With the help of SEM surface morphology and IR spectral analysis, the optimum calcination time of the precursor was determined to be 2 h.

3. 1 .
Carrier selection 3.1.1.Nitrate loading in active carriers Figure 2 shows IR spectrum of activated Al 2 O 3 , activated Al 2 O 3 -Zn(NO 3 ) 2 •6H 2 O and activated Al 2 O 3 -Zn(NO 3) 2 •6H 2 O-Cu(NO 3 ) 2 •3H 2 Oafter 24 h of melt infiltration.As can be seen from figure2, in IR spectra of alumina, there is no characteristic peak of nitrate appearing at 1380 cm −1 , while after the addition of Zn(NO 3 ) 2 •6H 2 O and Cu(NO 3 ) 2 •3H 2 O, an obvious characteristic peak appeared at 1380 cm −1 of the infrared spectrum.This peak position is caused by the symmetric stretching vibration of the N-O bond in nitric acid, and is one of the characteristic characteristics of nitric acid, so the presence of nitrate can be confirmed.It also indicates that the nitrate has been fused well to Al 2 O 3 carrier.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of supported compound desulfurizer.

Figure 2 .
Figure 2. Analysis of nitrate loading in active carriers.

3. 4 .
Effect of calcination time on supported compound desulfurizer 3.4.1.Infrared spectral analysis (IR) In order to study the effect of calcination time on the decomposition of Zn(NO 3 ) 2 •6H 2 O and Cu(NO 3 ) 2 •3H 2 O, the calcined samples at 400 °C were characterized by infrared (IR), and IR spectra of the compound desulfurizers after different calcination times are given in figure 6.

Figure 4 .
Figure 4. Effect of infiltrating temperature on compound desulfurizer precursor.

Figure 5 .
Figure 5. TG analysis curves of the compound desulfurizer precursors.

Figure 6 .
Figure 6.IR spectra of precursors at different calcination times.

Figure 8 .
Figure 8.Effect of calcination time on the physical phase composition of compound desulfurizers.