WO2012165841A2

WO2012165841A2 - Method of manufacturing silicon carbide-containing heat storage material from waste silicon sludge

Info

Publication number: WO2012165841A2
Application number: PCT/KR2012/004230
Authority: WO
Inventors: Hyun Jae Lee; Woo Teck Kwon; Young Hee Kim; Soo Ryong Kim
Original assignee: Enbion Inc.; Korea Institute Of Ceramic Engineering And Technology
Priority date: 2011-05-31
Filing date: 2012-05-30
Publication date: 2012-12-06
Also published as: KR101257458B1; WO2012165841A3; CN103732561A; KR20120133439A

Abstract

Provided a method of method of manufacturing a silicon carbide-containing heat storage material, including the steps of: providing a silicon sludge produced from a silicon wafer cutting process; heat-treating the silicon sludge in a non-oxidative atmosphere to remove a part of oil; mixing the silicon sludge with a binder to prepare a slurry; extruding the slurry to form a honeycombed compact; and reaction-sintering the honeycombed compact at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere. The method is advantageous in that a silicon carbide-containing heat storage material having high thermal conductivity, heat accumulation characteristics and chemical resistance can be manufactured at a low cost.

Description

METHOD OF MANUFACTURING SILICON CARBIDE-CONTAINING HEAT STORAGE MATERIAL FROM WASTE SILICON SLUDGE

The present invention relates to a method of recycling silicon sludge that is a byproduct occurring when a silicon wafer is crushed, and, more particularly, to a method of manufacturing a silicon carbide-containing heat storage material from waste silicon sludge.

Silicon is generally used to manufacture a solar cell and a semiconductor wafer, and a silicon ingot is sliced in the form of a wafer using a wire saw. When the silicon ingot is sliced, an abrasive slurry containing silicon carbide having an average particle size of 10 μm is used, and thus an abrasive sludge containing silicon (main component), silicon carbide and other oxides is produced.

As such, waste sludge is produced in an amount of 10,000 tons or more a year in the process of slicing a silicon ingot. It is predicted that silicon sludge will be produced in an amount of about 25,000 tons a year in 2012, depending on a program for increasing solar cell wafer output.

In the past, silicon sludge was buried in the ground by a waste disposal company. However, recently, attempts to recover a large amount of silicon, silicon carbide or the like from silicon sludge have been conducted.

Typical examples of conventional silicon sludge recycling technologies may include a technology of regenerating abrasive slurry, a technology of separating and recovering solids and a technology of synthesizing silicon carbide.

For example, Korean Unexamined Patent Publication No. 2003-84528 discloses a method of recycling waste slurry, wherein the waste slurry is reacted with 1 ~ 20% by weight of a nonionic surfactant and 2 ~ 50% by weight of a solvent (alcohol) for a predetermined amount of time (5 minutes ~ 10 hours), the reaction product is separated into layers due to differences of specific gravity using a centrifugal separator, and then each of the layers is put into a container using an oil pump, dried and classified according to size.

Further, Korean Unexamined Patent Publication No. 2004-55218 discloses a method of preparing high-purity silicon carbide, comprising the steps of: filtering waste slurry to separate solids such as silicon, silicon carbide, copper powder, iron powder and the like; removing copper powder and iron powder using specific gravity selection and magnetic force selection; washing the resultant with hydrochloric acid having a high concentration of 30% at room temperature to obtain a mixed powder of silicon powder and silicon carbide powder; mixing the mixed powder with graphite powder at 1600℃ or more to obtain a silicon carbide composite; and crushing and pulverizing the silicon carbide composite and removing impurities therefrom.

However, this method is problematic in that an excessive number of processes are required to separate and recover sludge solids, thus reducing the efficacy of obtaining silicon carbide from cheap waste sludge, and in that the obtained silicon carbide powder must be further formed and sintered in order to form a sintered silicon carbide body.

Meanwhile, a heat storage type of combustion apparatus includes a combustion chamber for burning and oxidizing a process gas, a heat storage layer, and a rotor for supplying and discharging the process gas into/out of the combustion chamber. The process gas is passed through the heat storage layer by the rotor, is burned in the combustion chamber, is again passed through the heat storage layer, and is then discharged to the outside by the rotor. In this process, thermal energy is stored in the heat storage layer disposed at the discharge side of the combustion gas, and this thermal energy is used to preheat the process gas introduced by the rotor. When such a heat storage type of combustion apparatus is used, harmful materials can be converted into harmless gases and then discharged, and energy consumption required to discharge the harmless gas can be minimized.

In a conventional heat storage type of combustion apparatus, a heat storage layer is generally made of cordierite or alumina porcelain. However, this heat storage layer does not have sufficient thermal conductivity, heat accumulation characteristics and chemical resistance. Therefore, it has been required to use a silicon carbide-containing heat storage material, but it is difficult to practically use the silicon carbide-containing heat storage material because the production cost thereof is high.

Accordingly, the present invention has been devised to solve the above-mentioned problems, and an object of the present invention is to provide a method of manufacturing a silicon carbide-containing heat storage material at a low cost by recycling waste silicon sludge produced in the process of manufacturing a semiconductor wafer or solar cell wafer.

In order to accomplish the above object, an aspect of the present invention provides a method of manufacturing a silicon carbide-containing heat storage material, including the steps of: providing a silicon sludge produced from a silicon wafer cutting process; heat-treating the silicon sludge in a non-oxidative atmosphere to remove a part of oil; mixing the silicon sludge with a binder to prepare a slurry; extruding the slurry to form a honeycombed compact; and reaction-sintering the honeycombed compact at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere.

Another aspect of the present invention provides a method of manufacturing a silicon carbide-containing heat storage material, including the steps of: providing a silicon sludge produced from a silicon wafer cutting process; heat-treating the silicon sludge in a non-oxidative atmosphere to remove a part of oil; reaction-sintering the silicon sludge at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere to obtain silicon carbide (SiC) powder; mixing the silicon carbide (SiC) powder with a binder to prepare a slurry; extruding the slurry to form a honeycombed compact; and sintering the honeycombed compact at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere.

Here, the silicon sludge may include silicon carbide (SiC) powder.

Further, the step of heat-treating the silicon sludge for removing oil may be performed at a temperature of 100 ~ 600 ℃.

Further, the silicon sludge, from which a part of oil was removed, may include 0.1 ~ 10 wt% of oil.

Further, the binder may include an inorganic binder.

Further, the slurry may include carbon powder.

The method of manufacturing a silicon carbide-containing heat storage material according to the present invention is advantageous in that a silicon carbide-containing heat storage material can be manufactured from waste silicon sludge at a low cost. The silicon carbide-containing heat storage material manufactured by this method can be used to form a heat storage layer of a heat storage type of combustion apparatus because it can realize thermal conductivity, heat accumulation characteristics and heat resistance peculiar to silicon carbide.

Particularly, the method can be used to treat all kinds of waste silicon sludge because it can be applied to silicon sludge discarded after the separation and recover of solids as well as typical waste silicon sludge.

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing the external appearance of a honeycombed heat storage material of the present invention;

FIG. 2 shows photographs of the outer appearances of a compact and a sintered body before and after sintering according to an embodiment of the present invention;

FIG. 3 is a graph showing the results of XRD analysis of a sintered body obtained by changing the sintering temperature according to an embodiment of the present invention;

FIG. 4 shows electron microscope photographs of a powder that was heat treated by changing the sintering temperature according to an embodiment of the present invention; and

FIG. 5 is a graph showing the results of XRD analysis of a powder that was prepared at 1450℃.

Hereinafter, preferred embodiments of the present invention will be described in detail.

The method of manufacturing a heat storage material according to the present invention includes a process of separating solids from silicon sludge, a process of removing oil from the solids, a process of preparing a slurry, a process of forming a compact, and a process of sintering the compact.

First, solids and oils are separated from the silicon sludge obtained from a wafer manufacturing factory using centrifugation or the like. The solids include silicon, silicon carbide (SiC), and a small amount of impurities.

An analysis of the composition of impurities included in the solids of the silicon sludge after centrifugation is given in Table 1 below.

Table 1

Na (mg/kg)	K(mg/kg)	Ca(mg/kg)	Fe(mg/kg)	Al(mg/kg)	Cu(mg/kg)
30	35	48	72	39	13

It can be seen from Table 1 that the silicon sludge includes a small amount of alkali metals or metals. Among these metals, iron (Fe) and copper (Cu) are derived from a cutting machine.

Excluding the impurities, the centrifuged silicon sludge includes oil, silicon, and silicon carbide (SiC) powder. The oil and silicon carbide (SiC) are derived from cutting oil and cutting materials from a cutting process. The oil is generally composed of ethylene glycol (EG), polyethylene glycol (PEG) or diethylene glycol (DEG). In the present invention, the content of silicon in the silicon sludge solid including silicon and silicon carbide (SiC) may be variously changed.

For example, the silicon sludge obtained from a wafer manufacturing process has a high silicon content, whereas the silicon sludge obtained from a waste water disposal plant, in which a silicon solid is separated and then discarded, has a high silicon carbide (SiC) content. Regardless of the change of silicon content, the silicon sludge can be easily applied to the method of the present invention. As described later, the method of the present invention is advantageous in that it is usefully used in the manufacture of a silicon carbide sintered body from a silicon sludge containing no silicon carbide (SiC).

The process of removing oil from the centrifuged silicon sludge is performed as follows. First, in the present invention, oil is removed at a temperature of 100 ~ 600℃. The process of removing oil is performed in a non-oxidative atmosphere, and preferably in a reduction atmosphere. The reduction atmosphere prevents silicon powder from being oxidized in the oil removal process. When silicon powder is oxidized, the carbonization reaction of silicon is caused by the so-called Acheson reaction, thus incurring the problem of increasing the carbonization reaction temperature. As described later, in the present invention, the oil removal process is conducted in a reduction atmosphere, and thus sintering can be performed at a low temperature of 1350℃. In the present invention, in order to rapidly remove the oil, a pressing process may be performed simultaneously with the heat treatment.

In the present invention, the oil removal process is conducted such that the amount of residual oil in the silicon sludge is 0.1 ~ 10 wt%, and preferably 1 ~ 10 wt%. The residual oil is uniformly distributed on the surface of silicon powder. The residual oil functions as a carbon supply source in the following sintering process. The uniform distribution of residual oil enables silicon to be carbonized at a relatively low temperature.

Subsequently, a slurry is prepared in order to form the silicon sludge in a desired shape. The slurry is prepared by mixing the silicon sludge with water and an organic binder. The organic binder may be at least one selected from the group consisting of polyvinyl alcohol, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, polyvinyl acetate, and polyethylene glycol. The organic binder functions as a binding agent in a forming process. In the present invention, it is preferred that the organic binder be included in an amount of 2 ~ 15 parts by weight based on 100 parts by weight of the silicon sludge which passed through the oil removal process.

The slurry of the present invention may further include an inorganic binder. The inorganic binder may be at least one selected from the group consisting of clay, feldspar, alumina, silica-alumina, aluminum silicate, aluminum titanate, and silica. The inorganic bind functions as a sintering agent in a sintering process. In the present invention, it is preferred that the inorganic binder be included in an amount of 2 ~ 30 parts by weight based on 100 parts by weight of the silicon sludge.

Meanwhile, a carbon source may be additionally provided in the forming process. For example, the carbon source may be a carbon powder such as carbon black.

The silicon sludge is formed into a honeycombed compact. FIG. 1 shows a honeycombed compact. As shown in FIG. 1, the honeycombed compact 100 includes a plurality of cells 110 forming channels in a length direction, and the cells 110 are closed by partition walls 120.

In the present invention, the honeycombed compact may be formed by a general extruding process. For example, the honeycombed compact of the present invention may be configured to have a predetermined number (20 x 20, 43 x 43, etc.) of cells in an area of 150 mm x 150 mm. The honeycombed compact may be formed in a desired size by cutting it to a suitable size.

The honeycombed compact obtained in this way is dried by a general drying method such as microwave drying, hot air drying, wet drying, or the like.

Subsequently, the honeycombed compact is sintered at a temperature of 1300 ~ 1800℃. In the sintering process, silicon (Si) is converted into silicon carbide (SiC). The present invention is characterized in that silicon (Si) can be converted into silicon carbide (SiC) even at a low temperature of 1300℃. It is inferred that this is caused by the residual oil included in the silicon sludge.

Conventionally, when silicon carbide (SiC) is prepared by reaction-sintering silicon, the reaction sintering is generally performed at a temperature of 1450℃ or more. However, since the melting point of pure silicon is 1412℃, the frame of silicon cannot be maintained at the sintering temperature. Therefore, a preform is first formed using a compact composed of SiC and/or a carbon source, and then the preform is dipped into a silicon melt to obtain a sintered silicon carbide body. However, this conventional process of forming a sintered silicon carbide body is very complicated and expensive.

In the present invention, a silicon carbide-containing heat storage material, the shape of which is maintained although a compact is formed by mixing silicon sludge with a binder without a preform, can be manufactured. The reason for this is that residual oil included in silicon sludge is uniformly distributed on the surface of silicon powder and thus the carbonization reaction proceeds at a lower temperature than the melting point of silicon.

As described above, the method of directly manufacturing a heat storage material from silicon sludge according to the present invention was explained. However, the heat storage material of the present invention may be manufactured by preparing silicon carbide powder from silicon sludge, mixing the silicon carbide powder with the above-mentioned binder and then forming the mixture.

In this case, the silicon sludge, which has passed through the oil removal process, is sintered. If necessary, organic and inorganic binders and a carbon source may be added to the silicon sludge. Subsequently, as described above, a slurry is prepared by adding a binder and a solvent to the silicon carbide (SiC) obtained by sintering the silicon sludge. Subsequently, the slurry is formed into a honeycombed compact by an extruding process, and then the honeycombed compact is sintered to manufacture a silicon carbide-containing heat storage material. In this case, since a relatively high sintering temperature is required, the sintering temperature must be lowered by adjusting the amount of the inorganic binder.

Example 1: Preparation of sintered silicon carbide body

Silicon sludge obtained from a domestic semiconductor wafer manufacturing factory was centrifuged to recover silicon. The composition of impurities included in the solid of the obtained silicon sludge is given in Table 1 above, and the silicon sludge includes silicon (Si) in addition to the impurities. Subsequently, the obtained silicon sludge was heat-treated at a temperature of 300℃ for 120 minutes in a reduction atmosphere to remove oil therefrom. As a result, the content of oil remaining in the silicon sludge was 5 wt%.

Subsequently, the silicon sludge was mixed with carbon black having a particle size of 1 μm, manufactured by Korea Carbon Black Co., Ltd., to form a pellet-type compact. In this case, the molar ratio of carbon black to silicon was 1:1.

The compact was sintered for 1 hour at 1350℃, 1650℃, 1750℃ and 1850℃ in a vacuum atmosphere. In this case, the sintering temperature was increased at a rate of 10 ℃/min. The external appearance of the obtained sintered body was observed, and the sintered boy was photographed by XRD. In order to compare the XRD pattern of the sintered body of Example 1 with that of a conventional sintered body, the XRD pattern of Marketch powder is shown in FIG. 2 together with the XRD pattern of the sintered body of Example 1.

FIG. 2A is a photograph showing the shape of the sintered body before sintering, and FIG. 2B is a photograph showing the shape of the sintered body after sintering at 1550 ℃.

Referring to FIG. 2, it can be ascertained that the shape of the pellet is accurately maintained before and after sintering. That is, the silicon sludge, which is used as a raw material for reaction sintering in the present invention, maintains its shape even at a temperature equal to or higher than the melting point of silicon. The result is based on the fact that the silicon carbide (SiC) produced by the carbonization reaction of silicon at a temperature equal to or lower than the melting point of silicon maintains the frame of the sintered body.

FIG. 3 is a graph showing the results of XRD analysis of the sintered body obtained depending on the change of sintering temperature.

In the graph of FIG. 3, "WJ-2" indicates the result of XRD analysis of raw powder.

Referring to FIG. 3, it can be seen that the SiC phase exists in each of the sintered body samples obtained depending on each sintering temperature. Further, it can be seen that silicon peaks were scarcely observed even in the sample sintered at 1350 ℃. Consequently, in the case of the sintered body of the present invention, it can be seen that the reaction of Si into SiC is easily conducted even at very low temperature. However, it can be seen that a small amount of carbon exists in the sample sintered at 1350 ℃. Since the existence of carbon is not confirmed at other sintering temperatures, it can be seen that the conversion of Si into SiC is more easily conducted depending on the increase of sintering temperature. Further, the existence of (Fe, Si)C compounds can be confirmed in each of the sintered body samples obtained depending on each sintering temperature.

Example 2: Preparation of silicon carbide powder

The solid of the silicon sludge obtained in the same manner as in Example 1 was mixed with a carbon source, and was then heat-treated at a temperature of 1450 ~ 1850℃ for 1 hour. In this case, the heat treatment temperature was increased at a rate of 10 ℃/min. The silicon carbide (SiC) powder was prepared in the same manner as in Example 1, except that the silicon carbide powder was not formed in the form of a pellet.

The external appearance of the silicon carbide (SiC) powder prepared in this way was photographed with an electron microscope.

FIGS. 4A to 4D show electron microscope photographs of the SiC powder samples heat-treated at 1450℃, 1650℃, 1750℃ and 1850℃, and FIG. 5 is a graph showing the results of XRD analysis of the silicon carbide (SiC) powder prepared at 1450℃.

From FIG. 5, it can be seen that β-SiC powder was already formed (KICET 5:5 in the graph). The "Marktech" and "SIKA" in the graph indicate the XRD patterns of the SiC powder commercially available as brand names of Marktech and SIKA.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. These embodiments are set forth to illustrate the technical ideas of the present invention, not to restrict them, and the scope of the present invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements should be considered to be within the scope of the invention, and the detailed scope of the invention will be disclosed by the accompanying claims.

Claims

A method of manufacturing a silicon carbide-containing heat storage material, comprising the steps of:

providing a silicon sludge produced from a silicon wafer cutting process;

heat-treating the silicon sludge in a non-oxidative atmosphere to remove a part of oil;

mixing the silicon sludge with a binder to prepare a slurry;

extruding the slurry to form a honeycombed compact; and

reaction-sintering the honeycombed compact at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere.
A method of manufacturing a silicon carbide-containing heat storage material, comprising the steps of:

providing a silicon sludge produced from a silicon wafer cutting process;

heat-treating the silicon sludge in a non-oxidative atmosphere to remove a part of oil;

reaction-sintering the silicon sludge at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere to obtain silicon carbide (SiC) powder;

mixing the silicon carbide (SiC) powder with a binder to prepare a slurry;

extruding the slurry to form a honeycombed compact; and

sintering the honeycombed compact at a temperature of 1300 ~ 1900℃ in a non-oxidative atmosphere.
The method according to claim 1 or 2, wherein the silicon sludge includes silicon carbide (SiC) powder.
The method according to claim 1 or 2, wherein the step of heat-treating the silicon sludge for removing oil is performed at a temperature of 100 ~ 600℃.
The method according to claim 1 or 2, wherein the silicon sludge, from which a part of oil was removed, includes 0.1 ~ 10 wt% of oil.
The method according to claim 1 or 2, wherein the binder includes an inorganic binder.
The method according to claim 1, wherein the slurry further includes carbon powder.