JPS61222918A - Production of reducing agent for producing solar cell-grade silicon - Google Patents

Abstract

PURPOSE:To produce a reducing agent for the production of an inexpensive solar cell-grade silicon and resistant to cracking even by the rapid heating to a high temperature, by aggregating carbonaceous powder with an aqueous solution of a sugar, drying the aggregate, and heating in an inert atmosphere. CONSTITUTION:Carbonaceous powder (e.g. carbon black) is aggregated by using a binder consisting of an aqueous solution of a sugar such as sucrose, glucose, lactose, etc. The aggregated carbon particle is dried to a water-content of <=6% and heat-treated in an inert atmosphere at 280-320 deg.C to obtain the objective reducing agent for producing a solar cell-grade silicon. The obtained carbon particle is resistant to cracking even by the rapid heating to about >=1,500 deg.C in the production of the solar cell-grade silicon. Accordingly the operational troubles can be prevented, and the agent can be produced at a low cost because the use of expensive binder is unnecessary.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for producing a carbonaceous reducing agent used in the production of solar cell grade silicon, and in particular to a method for producing a carbonaceous reducing agent for producing solar cell-grade silicon, in particular a method for agglomerating high-purity carbon powder with an aqueous solution of sugars. Relating to a method of manufacturing.

<Prior art and its problems> 5 Solar cell grade silicon (5olar Grade-9i
Currently, 5OG-Si (abbreviated as 5OG-Si) is produced by chlorinating metallic silicon obtained by reducing silica stone to form a gas, and then purifying and reducing this gas, but the energy required for production is extremely large and low. It is said that there are limits to cost reduction.

On the other hand, a technology has recently been established for refining domestically produced low-grade silica sand to a purity that can be used as a raw material for 5OG-Si. As described above, highly purified carbon powder such as activated carbon or carbon black is mixed with a binder such as sugar or carbohydrate and agglomerated to be used as a reducing agent when producing silicon from 5i02.

DE 30 13 319 A1 describes tablets in which carbon activated in a high temperature melt process is combined with quartz sand with a binder such as sugar or cellulose for the reduction step of 5i02 to produce 5OG-9i. A method for using it in the form of
The carbon powder is preferably granulated with phenolic resin and pre-granulated high purity 5i02
In addition, a method for reduction at high temperature is disclosed.

In addition, JP-A No. 52-66895 describes a method of granulating carbon black by mixing a sugar-tight aqueous solution, and JP-A No. 55-1
No. 36116 contains activated carbon and carbon black with sugar,
A method of pelletizing with a binder such as starch or cellulose has been disclosed.

However, carbon grains, which are commonly found in these types of carbon powders, are simply granulated with a binder such as sugar and then dried.
When rapidly exposed to a high temperature atmosphere of soo~2000℃, thermal cracking occurs significantly.

During operation, there is a risk of operational difficulties due to the generation of a large amount of fine powder and hanging shelves. In addition, in the case of phenolic resin,
Although the degree of thermal cracking mentioned above can be reduced, it still has drawbacks such as the fact that it is not completely eliminated and the price is high.

<Objective of the Invention> Therefore, the object of the present invention is to produce solar grade silicon that does not require the use of expensive binders, does not generate cracks when rapidly heated to high temperatures, and can be operated stably and inexpensively. The purpose of the present invention is to provide a method for producing a reducing agent for use in reducing agents.

<Configuration of the Invention> The present invention provides a method for producing a reducing agent for producing solar cell-grade silicon by agglomerating carbonaceous powder. Provided is a method for producing a reducing agent for producing solar cell-grade silicon, which comprises drying the converted carbon grains to a moisture content of 6% or less, and then subjecting them to heat treatment at 280 to 320°C in an inert atmosphere. It is.

The present invention will be explained in more detail below.

In the 5OG-Si production process, carbon grains are charged into a shaft furnace and 5i02 is introduced from the bottom to reduce the grain size.
~20mm, porosity 45-55%, crushing strength 3 Kgf
Therefore, carbon particles with even higher reactivity are required.

As a raw material for such carbon grains, creosote oil obtained by tar distillation or ethylene bottom can be used.
It can be said that carbon black produced by spraying into a high-temperature flame of 200 to 1,600°C is superior in terms of reactivity, productivity, and cost (see Figure 1).

Carbon black is difficult to agglomerate due to its fine particle size of 20 to 60 mm, narrow particle size distribution, and low surface activity, and even if it does agglomerate, its strength is low. However, an aqueous solution of sugars Good blending, e.g. sucrose 10-3
In the case of a 0% aqueous solution, use a dish-type granulator etc. to produce 10 to 15 tons.
Although it is possible to agglomerate carbon grains into
40%, and the strength is low. This problem can generally be solved by providing a drying process.

According to the results of experiments conducted by the present inventors, the method of blow drying at 120 to 130°C has the least cracking of agglomerated carbon grains during drying, has a good crushing strength of 3 to 5 kgf, and is economical. Normally, it is used as a reducing agent at this stage, but in the case of the 5OG-9i process, it is charged into the shaft furnace and instantly reduced to 1,500 to 20
Since it is exposed to a high temperature of 00°C, cracks will occur due to the rapid thermal decomposition reaction of sucrose and stress caused by the temperature difference with the inside, and the crushing strength will drop to less than 1 kgf, making operation difficult.

Therefore, the present inventors studied the thermal decomposition characteristics of carbonaceous powder and saccharide as a binder, and based on the results of numerous heat treatment experiments, we decided to use carbon powder in an aqueous solution of saccharide as a method for producing a reducing agent suitable for the 5OG-Si process. After drying the carbon particles obtained by agglomerating the carbon particles to a moisture content of 6 at% or less, the carbon particles are dried at 280 to 320°C, preferably 300°C in an inert atmosphere such as N2.
It has been found that a method of heat treatment at a temperature around °C is effective. If the moisture content before heat treatment exceeds 6%, the strength of the carbon grains will be low and cracks will occur.

Next, regarding the heat treatment temperature, as typified by the example of thermal decomposition characteristics of carbon black and sucrose shown in Figure 1, sucrose as a binder melts and expands at around 180°C.
It exhibits rapid thermal decomposition centered on dehydration reaction from around ℃,
This can be explained by the behavior that almost ends at around 400°C.

In other words, if the temperature is below 280°C, the amount of decomposition is too small and cracks will form during the process of using the reducing agent (1,500-1700°C), and if it exceeds 320°C, the amount of decomposition will be too large and the rate of degassing will increase, causing heating. Although cracks are generated during the treatment process, cracks do not occur because the amount of decomposition at the heat treatment temperature range of 280 to 320° C. according to the present invention is strong enough to cope with stress generation due to degassing. Therefore,
When applying heat treatment in the temperature range of the present invention, the amount of sugars such as sucrose that determines the strength of carbon grains is also important, and according to experiments conducted by the present inventors, the amount is required to be 10 to 30 wt%. It has become. This is because if the amount of saccharide is less than 10 wt%, cracking will become noticeable during the heat treatment process, and if it exceeds 30 wt%, it is not economical.

As for sugars, it can be applied to other sugars that exhibit similar thermal decomposition properties to sucrose, such as monosaccharides such as glucose, disaccharides such as maltose and lactose, and polysaccharides such as cellulose and lignin, and can be applied to similar heat treatments. Carbon particles suitable as a reducing agent for the 5OG-Si process can be produced by this method. These saccharides may be used alone or in combination of two or more.

Example: Carbon plastron with an average particle size of 25 mm was tumble granulated using a 30% sucrose aqueous solution in a 1000φ single-layer dish-shaped granulator to obtain lO~15 particles containing 17% sucrose. Peng carbon grains were produced. The water content of the carbon particles at this time was 37 wt%.

This was air-dried at 125°C for 70 minutes to reduce the moisture content to 6%, and then heated in a Matsufuru furnace under an N2 atmosphere to a 250-3
Five types of carbon grains were obtained by holding at five different temperatures in the range of 50°C for 60 minutes and then cooling. Then, weight loss due to decomposition, crack formation, crushing strength, etc. were measured. Furthermore, these heat-treated products were placed in a furnace at 1500° C. to rapidly heat them, held for 10 minutes, and then cooled to examine the occurrence of cracks and the crushing strength.

These results are shown in Table 1.

In the case of 150°C treatment, the heating loss is mainly due to water loss.8.
4%, no crack formation occurred, but 1% in the next process.
It was completely destroyed by rapid heating at 500℃, and the crushing strength was 0.
．． It decreased to 3KgF.

In the case of treatment at 350°C, the loss on heating reached IB, 2%, which means that in addition to water, about 80% of the sucrose was decomposed.

In this case, the occurrence of cracks in carbon grains reaches approximately 70%,
The crushing strength has decreased to 1.2Kgf.

In all cases conducted at 280 to 320°C, which is the range of the present invention, crack formation was slight, and in the case of 300°C, there was no cracking.Furthermore, even when rapidly heated to 1500°C, no cracking was observed, and the crushing strength was 4Kgf. It was excellent.

A similar experiment was conducted with the water content of the carbon grains lower than 6 wt% and the sucrose content changed.

Similar results were obtained.

Table 1 Total number of carbon grains <Effects of the invention> The carbon grains produced according to the present invention do not generate cracks when subjected to rapid heating of 1500°C or more in the 5OG-9i process, and troubles with operation can be prevented. Moreover, since no expensive binder is used, the 5OG-3i process has the advantage of being able to operate stably and at low cost.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the thermal decomposition characteristics of raw materials (carbon black and sucrose) and dried carbon particles. In this figure, sucrose starts to decompose at 200°C, and 40°C
At 0°C, 80% of the total amount of decomposition occurs. Carbon black, which has a carbonization yield of 22% at 800°C, hardly decomposes,
The carbon yield at 800°C is shown to be 98.5%. FIG, 1 Temperature (°C)

Claims (1)

[Claims] In a method for producing a reducing agent for producing solar cell-grade silicon by agglomerating carbonaceous powder, the carbonaceous powder is agglomerated in an aqueous solution of at least one type of sugar, and the water content of the agglomerated carbon grains is reduced to 6 After drying to less than 28% under an inert atmosphere
A method for producing a reducing agent for producing solar cell-grade silicon, which comprises performing a heat treatment at 0 to 320°C.