Study on the Preparation of Silica Using Residues of Oil Shale

Usage of oil shale produces abundant oil shale residue, which may not only directly cause serious pollution to the environment, but also result in a waste of resource. In this paper, acid leaching method and alkali soluble method were compared to obtain the optimum processing conditions for the preparation of white carbon black, which was determined as follows: the mass fraction of NaOH was 8 %, the alkali dissolving time was 5 h, and the reaction temperature was 100 °C. Then the performance characteristics of the obtained white carbon black were analysed by XRD, SEM and IR spectroscopy. The results showed that the product of silica has a particle size between 20 – 30 nm and a purity of 98.4 %. Preparation of the high quality white carbon black might be a good way to make sufficient and reasonable use of oil shale residues, which will bring many environmental and social benefits.


Introduction
With the increasing demands for energy and reduction of available petroleum resources, the long-term oil prices tend to increase.Oil shale has drawn much attention in recent years as an important alternative energy resource.However, the waste residue from each oil shale extraction method is fairly high.Even concerning the maximum oil content of oil shale (around 30 %), there will be 70 % waste residue of the spent oil shale after retorting. 1The main components of oil shale waste are SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO and SO 3 etc.Besides, there are many kinds of trace elements, including poisonous and harmful elements. 2China and the United States have the largest oil shale ash heap in the world, but now there are only Estonia, Russia, Germany and another six countries utilizing the ash and slag.The analysis of the ash shows that it contains heavy metal elements and trace amounts of radioactive elements, which are very likely to cause pollution in water, soil and surrounding flora and fauna.How to effectively improve the comprehensive utilization of oil shale ash and increase the added value of oil shale resources is a technical problem to be solved.
White carbon black is a porous, white, amorphous substance with a large internal surface area, which has good resistance to acids, and is insoluble in water and acids (except for hydrofluoric acid).[6] Reports on the use of oil shale residue to make silica are prevalent in recent years.8][9] Therefore, in order to improve the application performance of silica generated from oil shale residue, further improvement of the preparation method is necessary.

Oil shale samples
The oil shale used in this study was obtained from the open pit mine located in Fu Shun (FS), China.It was received as hard, dark grey blocks with no particular smell.After burning, breaking, and lapping, the samples were prepared.The chemical composition of the oil shale residue was obtained by X-ray fluorescence spectrometer (Table 1), in which SiO 2 accounted for more than 60 % of the oil shale residue.The reaction equations are: [12][13][14][15][16] In order to study the influence of the concentration of H 2 SO 4 on the leaching rate of SiO 2 , adopted were four different concentrations of H 2 SO 4 analysis.The outcomes of different concentrations of H 2 SO 4 are shown in Table 2.The results showed that the leaching rate of SiO 2 increased along with the concentration of H 2 SO 4 .When the concentration of H 2 SO 4 was low, not only was the leaching rate of silica low, but also the silica colour was not white, suggesting the quality of the product was low.When 1.0 ml H 2 SO 4 was used, the colour of the obtained silica was earthy red, indicating Fe 3+ existed in the white carbon black.The acid dissociation method can exclude Fe 3+ from the oil shale, thus it can influence the quality of obtained silica effectively.

Preparation of silica by alkaline method
Alkali method mainly uses the reaction of oil shale ash, SiO 2 , Al 2 O 3 and NaOH to generate water soluble Na 2 SiO 3 , and separate the Na 2 SiO 3 from the other components of the oil shale ash by filtration.To carry out the reaction successfully, the solution pH has to be controlled in a certain range.The reaction equations are: In the reaction, 10 g oil shale ash, 30 ml water, and 15 g NaOH was added into a 250 ml three-neck flask, and heated at 95 °C in a water bath to react for about 2 h.The pH was adjusted to 8 -9 to produce white floc, then filtered.
[19][20][21][22][23] In order to study the influence of the concentration of NaOH on the leaching rate of SiO 2 , reactions were carried out in different concentrations of NaOH, and the leaching rates of SiO 2 are shown in Table 3. Results showed that the alkali concentration had a significant effect on the leaching rate of SiO 2 (Table 3).By comparing the results listed in Table 2 and Table 3, it can be seen that the prepared silica by alkaline method was much higher than that with the acid extraction method.
The alkaline dissolution method is more suitable for the industrial production of silica, so this study adopted the alkali dissolving methods to analyse the influence factors of the silicon extraction rate.

Influence of the dissociation temperature
To investigate the influence of dissociation temperature on the leaching rate of SiO 2 , the reaction temperature was changed to perform the preparation.The results indicated that by increasing the reaction temperature, the extraction rate of SiO 2 had also increased (Fig. 1), which might be attributed to the improved diffusion and chemical reaction rates at higher temperature.The extraction rate of SiO 2 increased rapidly with temperature up to 90 °C, while it became stable in the range of 90-100 °C.Considering the industrial steam heating, 100 °C might be the optimum extraction temperature.

Influence of the dissociation time
The effect of dissociation time was investigated at various times.As the reaction time proceeded, the extraction rate of SiO 2 increased in the first 5 h, after which it declined.A plausible explanation for this may be due to the separation of Si from the lattice, as the reaction lasted over 5 h, and silicic acid precipitated on the remaining mineral surface, thus slowing down the reaction.Therefore, the appropriate dissociation time for the alkali might be 5 h.The results showed that the leaching rate increased as the mass fraction of NaOH increasing up to 8 %.When the mass fraction of NaOH was over 8 %, the extraction rate noticeably declined (Fig. 3).Extracting silica with sodium hydroxide is a solid-liquid multiphase chemical reaction, which may be controlled by diffusion or chemical reaction.In either case, the leaching rate is proportional to the concentration of sodium hydroxide.Because a portion of silica may be attached on the surface of the aluminum dross, the concentration of NaOH should be low; otherwise, the solution will exhibit high viscosity, resulting in difficulties in solid-liquid separation, and thus influencing the extraction rate.

Results and discussion
Silica as an important inorganic chemical raw material in industry that has many product indicators, including the contents of impurities, silicon dioxide, and so on.

Purity analysis
The purity of the product was analysed by X-ray fluorescence spectrometry, as shown in the following Table 4. Results showed that there were very low amounts of impurities in the product.

XRD analysis
The crystal structure of the product recognition with XRD is shown in Fig. 4. The XRD spectrum of the SiO 2 product did not show a sharp crystal diffraction peak, while only amorphous diffraction peaks appeared at 20 -30°, indicating that the obtained products contained no other crystalline phases, and the product structure was amorphous.

IR analysis
In the infrared spectrum, the 1100 cm -1 and 466 cm -1 peaks represented the Si-O-Si bond of antisymetric stretching vibration, while the peak at 950 cm -1 represented the bending vibration.The 1640 cm -1 absorption peak indicated the bending vibration of surface water H-O-H.The peak at 3400 cm -1 indicated the product internal structure of water (Fig. 5).

SEM analysis
The results of the scanning electron microscope analysis of the product is shown in Fig. 6.The SEM image showed that the product was in aggregation and the outline was not clear, which might be due to the internal temperature gradient of silica in the process of heating.Dehydration between the hydroxyl functional groups (OH-Si-OH) caused the aggregation of the particles, whose size was between 20 -30 nm.

Conclusions
The alkali dissociation method is more suitable for the preparation of silica from oil shale residues than the acid dissociation method, because it results in a higher yield of SiO 2 .
The optimum conditions for the preparation of silica from FS oil shale residue are as follows: temperature 100 °C,mass fraction of NaOH 8 %, and alkali dissolving time 5 h.
Through XRD, IR and SEM analysis, it was found that the produced silica had a purity of 98.4 % and a particle size between 20 -30 nm, indicating the product is high-quality white carbon black.

Table 4 -
Composition of the silica obtained by the alkali dis-