WO2022225469A1

WO2022225469A1 - High-purity nanosilica and nanosilicon manufacturing process

Info

Publication number: WO2022225469A1
Application number: PCT/TH2022/000017
Authority: WO
Inventors: Nonglak Meethong; Sarawut PONGHA; Pilan NGIEWLAY; Thanitsorn WORACHOTPHAISAN; Sutthida HA-KHAM; Thanthika WORATIYA; Tassama MONGKOLDEE
Original assignee: Khon Kaen University; Rpcg Public Company Limited.; Petro-Instruments Corp., Ltd.
Priority date: 2021-04-19
Filing date: 2022-04-18
Publication date: 2022-10-27
Also published as: CN117120370A

Abstract

Manufacturing processes of high purity nanosilica and nanosllicon from rice husks. Heating the rice husks in water and acid followed by calcining two times to produce nanosilica．The mixture of nanosilica, magnesium and sodium chloride is then calcined in an Ar atmosphere．The resulting product is heated to remove unwanted substances twice in acid solutions. Next, the sediment is washed and dried to obtain nanosilicon. The nanosilica produced from rice husks is 99.99 percent pure and its particle size is between 10-50nm. The produced nanosilicon is 99.63 percent pure and its particle size is 10-100nm. Both of processes can greatly reduce production costs and environmental impacts for industries that need to produce nanometer-sized silica and silicon particles.

Description

TITLE OF INVENTION

HIGH-PURITY NANOSILICA AND NANOSILICON MANUFACTURING PROCESS

TECHNICAL FIELD

This invention relates to manufacturing process of high-purity nanosilica and nanosilicon from agricultural waste resource, belongs to the material chemistry and technology fields.

BACKGROUND ART

At present, some industries are extremely important to people. This is especially so for those industries engaged in the production of items that are necessary for people such as consumer products, medicines, batteries, etc. Each industry uses different raw materials to produce good quality products and to reduce production costs that will affect the profit. For example, in Thailand, the important raw material in the glass industry is sand, which is abundant in the country. Therefore, it is not necessary to import this raw material from abroad, which results in savings in transportation and production costs. However, there are still many industries that import raw materials from abroad. This may be due to the fact that the imported raw materials have the required properties or the required raw materials cannot be obtained within the country.

Silica, or silicon dioxide (S1O₂₎ by its chemical name, is one of the raw materials used in many industries such as glass, tire, concrete, pharmaceutical and cosmetic, food production, etc. These industries’ production costs are quite high because they import large amounts of silica from abroad. Therefore, research and inventions are needed to be able to produce the silica that these industries need using natural agricultural residues such as bagasse, com husks and rice husks, etc.

Additionally, there is a need for silicon (silicon, Si) which can be produced from silica when it undergoes further chemical processing. Silicon is essential in some industries. For example, the battery industry has a demand for silicon as a component of batteries. This is especially true for high energy density and fast charging lithium-ion batteries, which are very important at present.

Rice is a staple food of Asian people and is widely cultivated. Additionally, it is an important export product of Thailand and other countries in the region. However, rice husks are not efficiently used and become agricultural wastes. Therefore, this invention has proposed the production of silica and silicon from rice husks to add value to the rice husks without having to burn them after cultivation causing great environmental air pollutions. Using them to produce silica also reduces costs for industries that need to import high-priced silica and silicon from abroad. If silica and silicon can be inexpensively produced in high quantities, they could become export products, which is another way to generate income for the countries.

General manufacturing process of nano silica and nano silicon from rice husks use the following steps.

1. Clean the rice husks

2. Heat the rice husks in an acid solution. After heating, wash the rice husks several times, bring the husks to a neutral acidity (pH of about 7) and dry.

3. Calcine the rice husks at a temperature ranging from 400-1,600 C to obtain silica. 4. Mix the resulting silica with potassium (K), calcium (Ca), aluminum (Al) or magnesium

(Mg), etc., These substances are used to bind oxygen which is released from the dissociation of silica into silicon during calcination.

5. The mixture produced from step 4 above is calcined to obtain silicon.

The production processes for nanosilica and nanosilicon from rice husks discussed above are based on research and patents that have been used to produce silica from rice husks and silicon from silica, which are very similar processes as shown in Figure 1. However, this invention has improved and devised the method for producing silica and silicon providing several advantages as follows.

US Patent No. US 7998448 is a method for producing silica by cleaning rice husks and heating them with 5-10_% carboxylic acid solution at 80 °C for 6 hours. The solution will have a dark color due to the release of lignin contained within the rice husks during the heating process. As a result, the heated acid solution must be discarded after every use. Therefore, this invention added an initial step of heating the rice husks in water to remove lignin, before heating them in an acidic solution, so that the acid solution can be recovered. The rice husks can be heated 30 more times before discarding the acid solution. This will greatly reduce production costs and environmental impacts.

Chinese Patent No. CN 108069429A is a method of producing silica by calcining rice husks in a closed system at 600 °C for 2 hours. As it is not treated by gas or air, it takes a long time to completely remove the carbon. Therefore, this invention conducted incineration in open conditions and flowed air through the system. This will help to more quickly bum and remove carbon. Incineration of rice husks to produce silica takes only a few minutes. This again will greatly reduce production costs and environmental impacts.

US Patent No. US 6696036 is a method for producing nanosilica by burning rice husks inside a rotating tube in two cycles. In the first cycle, rice husks are burned at 350-400 °C for 30-60 minutes. The second cycle uses the rice husk ash from the first process and calcines it at 700-1000 °C for 1 hour with oxygen gas flowing through the system. The shortcomings of this method are the following: The first incineration was at a low temperature, thus causing a resin to come out from husks in the smoke. As a result, the resin will deposit inside the tubes used for this step. Therefore, they have to be frequently cleaned and it takes a long time to bum. Therefore, this invention has switched from incineration in tubes to using conveyor belts to bring the rice husks into the furnace and out on the other side, as well as passing air into the furnace. This results in better combustion, thereby reducing resin problems and it take only 5-10 minutes to bum the rusks. Additionally, according to US Patent No. US 6696036, in the second firing cycle, the rice husks are calcined in closed tubes with oxygen gas flowed through the system. Oxygen is an expensive gas, so this invention switched the combustion method to an open system that does not seal the ends of the pipes and flows atmospheric air through the system. As a result, production costs are greatly reduced.

In US Patent No. US 2012/0230904A1, silicon is made by mixing rice husk ash and magnesium together. Then, the mixture is calcined at 800-1600 °C under argon gas. After that, it was heated twice in an acid solution. The resulting silicon is 98% pure. The shortcoming of this method is that the purity of the resulting silicon was not high enough and due to the high calcination temperature, and large silicon particle sizes result. Therefore, this invention mixed silica with magnesium in ball milling method. It is then calcined in a closed system inside a rotating tube with argon gas flowing through the system. After this calcination, the resulting substance was heated in acid solution two more times, resulting in silicon with a purity of up to 99.63% and nanometer scale particle sizes.

From the aforementioned invention and research, the inventors have developed processes of making higher purity nanosilica and nanosilicon to solve various problems that arose from previous research. After addressing these problems, the process can produce nanosilica with particle sizes between 10 50 nm and purities of up to 99.99_%. Nanosilicon can be produced with particle sizes of 10 100 nm and purities of up to 99.63_%.

CHARACTERISTICS AND INTENTION OF THE INVENTION Here in, the invention is a method for producing nanosized silica and silicon from rice husks by bringing the rice husks through a process of heating in water and acid and firing two times to produce nanosilica. The resulting nanosilica is calcined and twice heated in acid, then washed and dried to obtain nanosilicon. Nanosilica produced from rice husks is a white powder. Its particle sizes are between 10 - 50 nm with a purity of 99.99_%. Nanosilicon produced by this process has particle sizes of 10 - 100 nm and a purity of 99.63_%. Both are suitable for industrial purposes. Nanosilicon can be used as raw material for producing electrodes in the Li-ion battery industry.

The purpose of this invention is to develop processes for producing nanosilica and nanosilicon from rice husks, which are considered a waste material in Thailand. The time required for this production process is shorter. The nanosilica and nanosilicon produced by the process have high purity and can be used for making electrodes in the Li-ion battery industry.

DISCLOSURE OF INVENTION

1. The invention has following steps to manufacture high purity nanosilica:- a. Heat rice husks in deionized water at 80-90 °C for 6 -8 hours. The weight ratio of rice husks to deionized water is 1-3 : 5-10, while the optimal ratio is L 7. Lignin from the chaff was removed by this step. b. Heat the rice husks from Step la in a 2 10_% hydrochloric acid (HC1) solution at 80-90 °C for 6-8 hours. The optimum acid solution concentration is 5_%. The weight ratio of rice husks to acid solution is 1-3 : 5-10, while the optimal ratio is L8. This step is to remove metal substances from the husks. Then, the rice husks are washed with water until their acidity is neutral. Subsequently, the rice husks were dried. c. Bum the dried rice husks from Step 1(b) at 400-600 °C, with the optimum temperature between 540-560 °C, for 5-10 minutes in an open system to allow air to pass into the combustion chamber. This step is to remove carbon and substances such as cellulose from the rice husks. The rice husk that has been burned in this step will become rice husk ash. d. Rice husk ash from Step 1(c) is calcined at a temperature of 700-800 °C, with an optimum temperature between 770-780 °C, for 10-15 minutes in a closed system with a continuous air flow into the system. The tubular system is used that has a rotation speed of 5-10 rpm. Rotation causes the husk ash to mix and maintain good contact with the air that passes through it. It thus enables rapid combustion to remove carbon and cellulose. The rice husk ash that has been calcined in this way will be a white powder.

Analyze the white powder obtained from the calcination in Step Id above using an X-ray diffraction (XRD) technique to determine its crystal structure. The substance has a broad peak around the angle of 22° 20, which is the amorphous silica peak position shown in Figure 2. Additionally, the white powder analyzed by Fourier Transform Infrared Spectroscopy (FTIR) shows oscillation peaks at 1062.41 cm ¹ and 802.42 cm.¹, which are the positions of silicon-oxygen- silicon (Si-O-Si) and silicon-oxygen (Si-O), respectively, as shown in Figure 3. From the above data, it can be concluded that this substance is silica or silicon dioxide (S1O2).

When the morphology of silica were analyzed using transmission electron microscopy (TEM), the particles were found to be rather spherical with sizes in the range of 10-50 nm, as shown in Figure 4. Therefore, these nanoparticles of Silica can be referred to as nanosilica.

When nanosilica was analyzed using an X-ray Fluorescence (XRF) technique, 46.742%, silicon (Si), 53.251% oxygen (O) and 0.007% iron (Fe) were present in the form of 99.99% silica (S1O2) and 0.01% hematite (Fe₂0₃₎ as shown in Figure 5. From the above data, it can be concluded that the nanosilica produced has an amorphous structure with particle sizes of 10-50 nm and a purity of up to 99.99%.

2. The invention has following steps to manufacture high purity nanosilicon :- a. Mix nanosilica (obtained according to the nanosilica manufacturing process described above), magnesium (Mg) and sodium chloride (NaCl) at a weight ratio of 1-3 : 2-4 : 1-3, respectively. The optimal weight ratio is T 2 : 1. Then, the material is subjected to a ball milling technique for 10-30 hours. Then, the resulting mixture was calcined at 500-700 °C, with the optimum temperature being between 590-610 °C for 10-15 minutes in an oxygen-free closed system with continuous argon gas injection into the system at a rate of 1-5 liters per minute. The tubular system is rotated at a speed of 10-30 rpm. In this calcination process, silica (S1O2) and magnesium (Mg) react as follows:

Si0_{2 +} Mg ® Si +2Mg0

This reaction is exothermic. It causes very high heat inside the sintering tubes that may result in an explosion. Sodium chloride (NaCl)has a high capability of absorbing heat. Therefore, sodium chloride is added to the mixture to absorb the heat generated by this reaction and reduce the risk of explosion in the furnace. After this sintering step, a small amount of unreacted nanosilica remains. The material obtained after calcination consists of at least silicon (Si), silica (S1O2) and magnesium oxide (MgO). b. Mix the substance obtained from the calcination from Step 2a in 2-8% hydrochloric acid (HC1) solution. The optimum acid solution concentration is 6-7% at a ratio of 1-3: 8-12 by weight. Heat the mixture at 40-80 °C along with stirring for 3-7 hours, then filter out the sediment and wash it with water until the sediment becomes neutral (pH = 7). This step will remove magnesium oxide (MgO) according to the reaction:

MgO + 2HC1 ® MgCli + H₂0

When magnesium oxide (MgO) reacts with hydrochloric acid (HC1), the resulting chemical reaction forms water (H₂0) and magnesium chloride (MgCl₂₎. MgCl₂ is water soluble, therefore, when washing and filtering the precipitate, it is leached out, leaving only silica (Si0₂₎ and silicon (Si). The material can now be used as anode materials for Li-ion batteries. c. If desired, the sediment can be furthered purified by mixing with a 2-8% hydrofluoric acid (HF) solution at a weight ratio of 1-3:30-34 and heated at 40-80 °C with continuous stirring for 3-7 hours. Then, the sediment is filtered out and washed with water until the acidity of the sediment becomes neutral. In this step, silica is removed as shown in the reaction below:

Si0_{2 +} 4HF — » SiF_{4 +} 2H₂0

When silica (Si0₂₎ reacts with hydrofluoric acid (HF), then water (H₂0) and silicon tetrafluoride (S1F4) are formed and evaporated. Then, they flow through a toxic material trap and are captured to prevent contamination of the environment. Finally, when the sediment is washed, filtered and dried at 60 80 ⁰ C, nanosilicon powder is obtained. Figure 6 shows the high purity and crystalline nature of silicon nanoparticles. It can be clearly seen from this X-ray diffraction (XRD) data that the sample consisted of a pure-phase silicon. All peaks at 28.442°, 47.326°, 56.121°, 69.121° and 76.364° were indexed according to the Fd3m space group of a cubic structure. The results illustrate the successful synthesis of a pure silicon material using the process of this invention.

Silicon nanoparticles were analyzed to examine their morphology by transmission electron microscopy (TEM). It was found that they are rather spherical with particle sizes ranging from

10-100 nm, as shown in Figure 7. When analyzed using an X-ray Fluorescence (XRF) technique, it was found that the produced substance consisted of up to 99.63_% of silicon, as shown in Figure 8. Therefore, it can be concluded that the produced nanosilicon is characterized by a crystalline structure with particle sizes of 10-100 nm and a purity of 99.63_%.

Nanosilica produced by this invention is suitable for use in applications such as forensic science work using nanosilica to detect fake signatures on important documents. In addition, it is suitable for the production of plastic bags that use nano-silica to produce nano porous films to extend shelf-life of high value fruits and vegetables, etc. Most importantly, the small particle sizes of nanosilicon make it suitable for use as an ingredient in the production of battery electrodes allowing high energy density and fast charging Li-ion batteries with long cycle life and low cost.

From the above experimental data, it can be seen that adding and modifying the steps of the new processes resulted in better nanosilica and nanosilicon, compared to the previous silica and silicon production methods. Tables 1 and 2 compare, in detail, the nanosilica and high-purity nanosilicon produced by previous processes with nanosilica and high-purity nanosilicon produced by this invention.

Table 1 shows a comparison of silica production methods.

Table 2 shows a comparison of silicon production methods.

Brief description of the drawing

Figure 1. The process of manufacturing nanosilica and nanosilicon from rice husks. Figure 2. Amorphous structure of nanosilica from rice husks obtained from X-ray diffraction (XRD) technique. Figure 3. Transmittance vs. wavenumber derived from Fourier transform infrared spectrometry (FTIR) technique of nanosilica from rice husks.

Figure 4. Morphology of nanosilica from rice husks obtained from transmission electron microscopy (TEM) technique.. Figure 5. Chemical compositions of nanosilica from rice husks obtained from X-ray

Fluorescence (XRF) technique..

Figure 6. Crystal structure of nanosilicon obtained from X-ray diffraction (XRD) technique.

Figure 7. Morphology of nanosilicon obtained from transmission electron microscopy (TEM) technique.

Figure 8. Chemical compositions of nanosilicon obtained from X-ray Fluorescence (XRF) technique.

Best mode for Carrying out the invention

As mentioned in the Disclosure of Invention

Claims

1. The process to manufacture high purity nanosilica has the following steps: a. Heat the rice husks in deionized water at 80-90 °C for 6-8 hours. The weight ratio of rice husks to deionized water is 1-3 : 5-10. b. Next, heat rice husks in a 2-10 weight percent solution of hydrochloric acid (HC1) at

80-90 °C for 6-8 hours. The ratio of rice husks to the acid solution is 1-3 : 5-10. The resulting rice husks are then washed with water until the rice husks have pH ₌ 7. After that, the rice husks are dried. c. After that, calcine the dried rice husks at 400-600 °C for 5-10 minutes in an open system to allow air to pass into the combustion system. The calcined rice husks are thus converted into rice husk ash. d. Next, the system containing rice husk ash was rotated in a rotary kiln at 5-10 rpm and calcined at a temperature of 700-800 °C for 10-15 minutes in a closed system with continuous air flow into the system. The product of this step is nanosilica powder.

2. The process to manufacturing high purity nanosilicon is as follows. a. The nanosilica described in Claim 1, magnesium (Mg) and sodium chloride (NaCl) in weight ratios of 1-3 : 2-4 : 1-3, respectively, were mixed using a ball milling method for 10 30 h. The resulting mixture was rotated at 10-30 rpm and calcined at 500 700 °C for 10-15 min in an oxygen-free closed system with a continuous supply of argon gas flowing into the system at a rate of 1-5 liters per minute. b. Then, mix the substance obtained from the previous calcination with a solution of 2-8_% hydrochloric acid (HC1) at a ratio of 1-3 : 8-12 by weight. Heat it at 40-80 °C and stir for 3-7 hours. After that, filter out the sediment and washed it with water until the sediment has a pH ₌ 7. c. The washed sediment is mixed with a solution of 2-8_% hydrofluoric acid (HF) in a ratio of 1-3 : 30-34 by weight. It is heated at 40-80 °C with stirring for 3-7 hours. Then, filter out the sediment and wash it with water until its acidity is neutral (pH ₌7). Next, dry the sediment at 60 80 °C to obtain nano silicon powder.

3. The ideal weight ratio of rice husks to deionized water described in Claim 1 is L 7.

4. The optimum weight ratio of rice husks to the hydrochloric acid (HC1) solution described in Claim 1 is 1: 8.

5. The suitable concentration of hydrochloric acid (HC1) solution described in Claims 1 and

4 is 5_%. 6. The optimum temperature for calcinating rice husks described in Claim 1 is between 540 and 560 °C.

7. The optimum temperature for calcinating rice husk ash described in Claim 1 is between 770 and 780 °C.

8. The optimal weight ratio of nano-silica, magnesium (Mg) and sodium chloride (NaCl) described in Claim 2 is 1:2:1, respectively.

9. The optimum temperature for firing nanosilica, magnesium (Mg) and sodium chloride (NaCl) mixtures in described in Claim 2 is between 590 and 610 °C.

10. The appropriate concentration of the hydrochloric acid (HC1) solution described in Claim 2 is 6-7 percent.

11. The silica particle size in described Claim 1 is 10 - 50 nm.

12. The silicon nanoparticle particle size described in Claim 2 is 10 - 100 nm.