JP7839656B2 - Method for producing sulfur-containing compositions - Google Patents

Description

This invention relates to a method for producing a sulfur-containing composition.

A method for obtaining a sulfur-containing composition is known, which involves heating raw materials containing a sulfur source and a carbon source in a heating furnace such as a rotary kiln (for example, Patent Document 1). As the carbon source, powder containing plastics, such as waste plastics, may be used. The plastic powder burns in the rotary kiln, generating reducing gases. The reaction proceeds when these reducing gases come into contact with the sulfur source.

Japanese Patent Publication No. 2005-306911 Japanese Patent Publication No. 2004-337677

In the conventional method, crushed plastic, along with the sulfur source, was dropped into the kiln's main body at the rear using a chute or similar device. In this case, the introduced plastic burns at the rear of the kiln. This generates reducing gases at the rear, resulting in a short contact time between the newly introduced sulfur source and the reducing gases, leaving room for improvement in reaction efficiency.

This invention has been made in view of the above circumstances, and aims to provide a method for producing sulfur-containing compositions more efficiently.

The present invention provides a method for producing a sulfur-containing composition, comprising a heating step in which a sulfur source and plastic are heated in a rotary kiln, wherein the plastic is introduced in the heating step on the kiln side relative to the sulfur source.

In the heating process described above, the plastic may be introduced while being guided by an airflow.

In the heating process described above, the material may be introduced to a position further forward than 80% of the total length of the rotary kiln body, as viewed from the front end of the kiln body.

The temperature at the front of the rotary kiln may be between 800 and 1300°C.

The above-mentioned plastic may be fed into the rotary kiln from the rear towards the front.

The above-mentioned plastic may be introduced from the front of the rotary kiln.

According to the present invention, a method for producing sulfur-containing compositions more efficiently can be provided.

Figure 1 is a schematic diagram showing the overall structure of a firing apparatus according to one embodiment. Figure 2 is a schematic diagram showing the overall structure of a firing apparatus according to one embodiment. Figure 3 is a diagram illustrating a method for producing a sulfur-containing composition according to one embodiment. Figure 4 is a diagram illustrating a method for producing a sulfur-containing composition according to one embodiment. Figure 5 is a diagram illustrating a method for producing a sulfur-containing composition according to the prior art.

The method for producing the sulfur-containing composition of this embodiment includes a heating step in which a sulfur source and plastic are heated in a rotary kiln, and in the heating step, the plastic is introduced to the front of the kiln, ahead of the sulfur source.

Furthermore, a method for producing a sulfur-containing composition according to one embodiment of the present invention includes a heating step of heating a sulfur source and plastic in a rotary kiln, wherein the plastic is introduced while being guided by airflow to the front of the kiln from the sulfur source. Alternatively, the method for producing a sulfur-containing composition according to this embodiment may include a heating step of heating a sulfur source and plastic in a rotary kiln, wherein the plastic is introduced to the front of the kiln from the sulfur source and to a position beyond 80% of the total length of the rotary kiln body from the front end of the main body.

In the manufacturing method of this embodiment, a sulfur source is used as a raw material. Any raw material containing sulfur is acceptable as the sulfur source. The sulfur source may contain a compound with a positive oxidation state of sulfur, and may be a compound containing sulfur oxoacids (such as sulfur oxoates). Examples of such compounds include sulfates, sulfites, and bisulfates (bisulfates). These salts may also be hydrates. The sulfur source may contain one or more sulfur-containing compounds.

Furthermore, the sulfur source may contain metallic elements. The metallic elements are not particularly limited and may include alkali metals, alkaline earth metals, etc. Examples of alkali metals include lithium, sodium, potassium, rubidium, and cesium. Examples of alkaline earth metals include magnesium, calcium, barium, and strontium, and may be magnesium or calcium alone. The sulfur source may contain one or more metallic elements.

The sulfur source may specifically include gypsum. The sulfur source containing gypsum is not particularly limited and includes anhydrous gypsum, hemihydrate gypsum, dihydrate gypsum, waste gypsum board, gypsum molds for casting, gypsum molds for industrial models, and other gypsum waste materials. Waste gypsum board may also contain paper. When using waste gypsum board as the sulfur source, a crushing step may be included before the mixing step. The crushing step can be carried out using a conventional crusher. The particle size of the sulfur source may be 20 mm or less, 10 mm or less, or 1 mm or less. The particle size of the sulfur source can be measured using a metal sieve in accordance with JIS Z 8801-1:2019.

In the manufacturing method of this embodiment, plastic is used as the carbon source. The plastic used as the raw material may consist solely of plastic, or it may be plastic with other materials such as paper or wood attached. The plastic may be waste such as plastic waste, or it may be crushed plastic powder (plastic fragments). The plastic may be an organic polymer. An example of plastic waste is RPF (Refuse Paper & Plastic Fuel). RPF is a high-quality solid fuel mainly made from waste paper and waste plastics, which are industrial waste materials that are difficult to recycle. The particle size of the plastic may be 1 to 50 mm, preferably 3 to 40 mm, more preferably 5 to 30 mm, and particularly preferably 5 to 20 mm. When the particle size of the plastic is within the above range, it does not burn out immediately after being blown in, so sufficient contact time is obtained, and the mixing of unburned plastic into the product can be suppressed. The particle size of plastics can be measured using a metal sieve conforming to JIS Z 8801-1:2019. Alternatively, the particle size may be measured using a caliper or similar device.

The firing apparatus used in this embodiment will now be described with reference to Figure 1. Figure 1 is a schematic diagram showing an example of a firing apparatus according to this embodiment. The firing apparatus 200 comprises a rotary kiln 20, a sulfur source introduction passage 18 connected to the kiln end 22 of the rotary kiln 20, and a plastic introduction passage 50. The sulfur source is supplied to the rotary kiln 20 through the sulfur source introduction passage 18. The sulfur source may be supplied by sliding it down by gravity, transporting it by a screw, conveyor, etc. The plastic is supplied to the rotary kiln 20 from the kiln end 22 toward the kiln front 28 through the plastic introduction passage 50. The plastic may be supplied by being blown out with air by a blower 52 in Figure 1, or by being carried by the airflow. This allows the plastic to be supplied to the kiln front 28 side of the sulfur source. In the rotary kiln 20, the raw material, the material to be processed 58, supplied into the main body 26 is heated and reacted by the combustion of the burner 24 located at the rear end of the main body 26 to form a sulfur-containing composition, which is then discharged from the rear end of the main body 26.

Figure 2 is a schematic diagram showing the overall structure of a calcination apparatus according to another embodiment. As shown in Figure 2, the calcination apparatus 100 may include a preheating and calcination section 10 for preheating and calcining a sulfur source, a rotary kiln 20 for calcining the introduced raw materials to obtain a sulfur-containing composition, and a cooler 30 for cooling the sulfur-containing composition obtained in the rotary kiln 20. The preheating and calcination section 10 has four cyclones C1, C2, C3, and C4 (preheaters) and a calcination furnace 12.

The kiln end 22 of the rotary kiln 20 and the calcination furnace 12 of the preheating calcination section 10 are connected by a rising duct 14. The rising duct 14 may also be connected to a bypass section that extracts exhaust gas from within the rising duct 14. The exhaust gas extracted into the bypass section may be converted, for example, by contacting it with a calcium compound and reacting it with _SO₂ contained in the exhaust gas to produce calcium sulfite.

The manufacturing method of this embodiment may further include a step of contacting kiln exhaust gas with a slurry containing a calcium compound to obtain sulfites. This allows for the capture and effective utilization of _SO2 contained in the kiln exhaust gas.

The sulfur source introduced from the connection point between cyclone C1 and cyclone C2 flows through cyclone C1, cyclone C2, cyclone C3, rising duct 14, calcination furnace 12, and cyclone C4, and is introduced into the kiln end 22 of the rotary kiln 20 via the raw material input chute 16. Plastic is introduced through the plastic introduction passage 50 connected to the kiln end 22. In the rotary kiln 20, the introduced raw materials are heated by combustion in the burner 24 located at the rear end of the main body 26 to form a sulfur-containing composition. The resulting sulfur-containing composition is cooled in the cooler 30. After cooling in the cooler 30, the sulfur-containing composition is obtained.

Examples of sulfur-containing compounds in the sulfur-containing composition obtained by the heating process include compounds containing sulfur with a lower oxidation state than the sulfur contained in the sulfur source of the raw material, such as metal sulfides and metal sulfites (i.e., compounds in which the sulfur has been reduced). Such sulfur-containing compounds containing sulfur with a lower oxidation state can be used as reducing agents for hexavalent chromium. Examples of metal sulfides include calcium sulfide, calcium polysulfide, and magnesium sulfide. The metal sulfide may be calcium sulfide. Examples of metal sulfites include calcium sulfite and magnesium sulfite.

In the heating process, for example, if gypsum ( _CaSO₄ ) is used as the sulfur source, the reaction represented by the following reaction equation (1) proceeds. Simultaneously, the reaction represented by the following reaction equation (2) may also proceed, generating _SO₂ .
CaSO ₄ +2C→ CaS+2CO ₂ (1)
CaSO ₄ +CaS→ 2CaO+2SO ₂ (2)

In the heating process, it is preferable that the number of moles of gypsum ( _CaSO₄ ) supplied to the heating section and the number of moles of plastic (C) satisfy the following formula (3), more preferably the following formula (4), and even more preferably the following formula (5). This makes the reaction between gypsum and plastic proceed more easily.
C/CaSO ₄ ≧3 (3)
C/CaSO ₄ > 4 (4)
C/CaSO ₄ >5 (5)

The molar ratio of carbon in the plastic introduced during the heating process to sulfur in the sulfur source may be 1.0 or higher, 1.5 or higher, 2.0 or higher, or 2.2 or higher, from the viewpoint of increasing the content of sulfur-containing compounds in the resulting sulfur-containing composition. Furthermore, the molar ratio of carbon in the plastic introduced during the heating process to sulfur in the sulfur source may be 10 or lower, 5 or lower, 3 or lower, or 2.5 or lower, from the viewpoint of reducing unreacted carbon and suppressing the generation of oxides due to excessive reaction.

For example, if the sulfur source contains gypsum ( _CaSO₄ ), the ratio of the plastic to the sulfur source can be expressed as [C]/[ _CaSO₄ ], which is the molar ratio of carbon (C) to _CaSO₄ contained in the plastic.

The mass ratio of the sulfur source supplied to the rotary kiln as raw material to the plastic may be 0.5:1 to 5:1, or 1:1 to 4:1.

The sulfur-containing composition may contain components other than sulfur-containing compounds. Such components include calcium oxide, unreacted gypsum, and impurities derived from raw materials such as gypsum. By adding the sulfur-containing composition containing sulfides to the grinding process, which is one of the cement manufacturing processes for producing cement compositions, and grinding it together with cement clinker and gypsum, a cement composition capable of reducing hexavalent chromium leaching can be produced. The sulfur-containing composition may also be added in the grinding process as a dry powder or alkaline slurry.

The content of metal sulfides in a sulfur-containing composition can be determined, for example, by analyzing the diffraction pattern obtained by powder X-ray diffraction measurement using the Rietveld method. The metal sulfide content of the sulfur-containing composition is preferably 5% by mass or more, more preferably 30% by mass or more, even more preferably 50% by mass or more, and particularly preferably 70% by mass or more. From the viewpoint of ease of manufacture, the metal sulfide content of the sulfur-containing composition may be 95% by mass or less. The metal sulfide contained in the sulfur-containing composition may be calcium sulfide.

The sulfur-containing composition may optionally contain metal oxides produced by further combustion of sulfur contained in the metal sulfide. Furthermore, the sulfur-containing composition may contain unreacted sulfur sources. The total amount of metal sulfides and metal oxides converted to the amount (e.g., by mass) of the raw material sulfur source, plus the amount of unreacted sulfur sources, may be 20% or more, 30% or more, 50% or more, or 60% or more. The content of unreacted sulfur sources and/or metal oxides in the sulfur-containing composition may be substantially 0%.

Furthermore, the manufacturing method of this embodiment will be explained using Figures 3 to 5. First, Figure 5 is a diagram for schematically explaining a conventional manufacturing method. Conventionally, a sulfur source such as gypsum and plastic were mixed in advance before being introduced into the rotary kiln 20, and the mixed raw materials 62 were introduced together through the raw material introduction passage 60. As a result, the plastic was burned at the kiln end 22, and as indicated by the arrow 56 in Figure 5 which shows the flow of reducing gas, the generated reducing gas only passed near the kiln end and could not be effectively utilized in the reaction. Examples of reducing gases include carbon monoxide (CO), hydrocarbon gases ( _CH4 , _C3H8 , _{C4H10 , etc.} ), and hydrogen ( _H2) .

Figure 3 is a diagram illustrating a manufacturing method according to one embodiment. As shown in Figure 3, in the manufacturing method of this embodiment, the plastic 54 is introduced while being blown away by the airflow ejected from the blower 52, and is introduced on the kiln front 28 side of the sulfur source. Therefore, as indicated by the arrow 56, the plastic burns on the kiln front 28 side than in the conventional method, generating reducing gas. This increases the contact time between the material to be processed 58 and the reducing gas, allowing for more effective utilization of the reducing gas. In Figure 3, the sulfur source introduction passage 18 is connected below the plastic introduction passage 50, making it easier to introduce the plastic 54 on the kiln front 28 side of the sulfur source. The introduction position of the plastic 54 may be on the kiln front 28 side of the position 80% of the total length in the longitudinal direction of the main body 26 of the rotary kiln 20 when viewed from the end of the main body 26 on the kiln front 28 side of the main body 26, and may be within the range of 25-75%. In other words, the total length of the main body 26 in the longitudinal direction is the length from the end of the main body 26 on the kiln front 28 side to the end on the kiln rear 22 side. The position where the plastic 54 is inserted may be closer to the kiln front 28 than 80% of the total length of the main body from the end of the main body 26 on the kiln front 28 side, and may be within the range of 25-75%.

One method for introducing plastic is to guide it in using airflow. One example is introducing the plastic into the main body 26 of the rotary kiln by blowing it out with airflow using a blower 52. Alternatively, the plastic may be introduced by extending the tip of the plastic introduction channel 50 into the interior of the main body 26 of the rotary kiln, and then introducing it on the kiln front 28 side of the sulfur source.

Figure 4 is a diagram illustrating a manufacturing method according to another embodiment of the present invention. In Figure 4, unlike Figure 3, the plastic introduction passage 50 is connected to the front of the kiln 28, and the plastic is introduced from the front of the kiln 28 side. In this case, it is preferable because it is easier to introduce the plastic to the front of the kiln than the sulfur source. The position in which the plastic is introduced may be on the front of the kiln 28 side of the position 80% of the total length in the longitudinal direction of the main body 26 when viewed from the end of the main body of the rotary kiln on the front of the kiln 28 side, and may be in the range of 25 to 75%. As indicated by the arrow 56 in Figure 4, the contact time between the introduced sulfur source and the reducing gas is long, and the reducing gas can be utilized more effectively. Also, in Figure 4, the plastic introduction passage 50 is connected below the burner 24. In this embodiment, when the plastic is introduced by guiding it into the airflow with the blower 52, it may be introduced by blowing it onto the workpiece 58 inside the rotary kiln 20. In this method of introduction, the plastic reacts with the heat of the material being processed while in direct contact with it, allowing the reaction to proceed more efficiently. Furthermore, because it is less likely to come into direct contact with the flame of the burner 24, it prevents the plastic from burning out before it comes into contact with the material being processed.

In the heating process, the temperature of the front part of the rotary kiln may be 800 to 1300°C, 1000 to 1200°C, or 1100 to 1150°C. The oxygen concentration inside the rotary kiln may be 20% by volume or less, 3 to 20% by volume, or 5 to 15% by volume. The rotation speed of the rotary kiln may be 2 ^min⁻¹ or less, 1.5 ^min⁻¹ or less, 1.0 ^min⁻¹ or less, or 0.01 to 0.8 ^min⁻¹ .

The following were prepared as raw materials.
Sulfur source: Crushed waste gypsum board (containing 94% dihydrate gypsum by mass, including paper)
Carbon source: RPF
A sulfur-containing composition was obtained by introducing a sulfur source and a carbon source into a rotary kiln from the kiln's tail end and firing them. In Comparative Examples 1 and 2, the RPF was pre-mixed with the sulfur source and introduced together via a screw conveyor. In Examples 1 to 6, only the sulfur source was introduced via a screw conveyor, and the RPF was introduced from the kiln's tail end by blowing in airflow with a blower. The carbon source was introduced by blowing in airflow from the front of the kiln, further forward than the sulfur source, and from the front end of the rotary kiln's main body towards a point approximately 75% of the kiln's total length. The product temperature (kiln temperature) near the carbon source's drop point was measured using an infrared radiation thermometer. X-ray diffraction experiments were performed on the obtained sulfur-containing composition, and its chemical composition was analyzed by XRD Rietveld analysis. The obtained sulfur-containing composition contained unreacted gypsum ( _CaSO₄ ) and the products calcium sulfide (CaS) and calcium oxide (CaO). The amounts of CaS and CaO produced are converted to the amount of _CaSO₄ used as raw material. Table 1 shows the content of these (in gypsum equivalent %) relative to the total amount of unreacted _CaSO₄ and the converted CaS and CaO. In Table 1, the reaction rate is the percentage (%) of the total amount of converted CaS and CaO relative to the total amount in the obtained sulfur-containing composition.

In the example where RPF was added by blowing, gypsum reacted to produce calcium sulfide and calcium oxide. Furthermore, the reaction rate increased with higher kiln temperatures and a larger amount of RPF added. The reaction rate also tended to increase when the kiln rotation speed was reduced, increasing the residence time of the raw materials.

10…Preheating and calcination section, 12…Cacination furnace, 14…Rising duct, 18…Sulfur source introduction path, 20…Rotary kiln, 22…Kiln end, 24…Burner, 26…Main body, 28…Kiln front, 30…Cooler, 50…Plastic introduction path, 52…Blower, 100, 200…Firing equipment, C1, C2, C3, C4…Cyclone.

Claims (6)

The process includes a heating step in which a sulfur source and plastic are heated in a rotary kiln.
A method for producing a sulfur-containing composition, wherein in the heating step, the plastic is introduced on the side of the kiln in front of the sulfur source. The method for producing a sulfur-containing composition according to claim 1, wherein the plastic is introduced while being guided by an airflow during the heating step. A method for producing a sulfur-containing composition according to claim 1 or 2, wherein, in the heating step, the material is introduced to a position closer to the front of the rotary kiln than 80% of the total length in the longitudinal direction of the main body, as viewed from the front end of the main body of the rotary kiln. A method for producing a sulfur-containing composition according to any one of claims 1 to 3, wherein the temperature of the front portion of the rotary kiln is 800 to 1300°C. A method for producing a sulfur-containing composition according to any one of claims 1 to 4, wherein the plastic is fed into the rotary kiln from the kiln end toward the kiln front. A method for producing a sulfur-containing composition according to any one of claims 1 to 4 , wherein the plastic is introduced from the front of the rotary kiln.