Method for producing cement clinker having reduced hexavalent chromium content

Abstract

To provide a method for producing a cement clinker, capable of reducing hexavalent chromium content even when waste is contained as a cement clinker raw material.SOLUTION: This invention relates to a method for producing a cement clinker having reduced hexavalent chromium content wherein, when a cement clinker raw material containing a waste is supplied to a rotary kiln and calcined, a sulfate is charged near a burning point of a main burner of the rotary kiln, and a combustible material is charged in a range from a vicinity of the burning point to an outlet of a cement kiln.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a hexavalent chromium-reduced cement clinker. [Background technology]

[0002] In recent years, the cement industry has been developing cement clinker that uses industrial waste, general waste, and other waste materials as raw materials. However, some of these waste materials contain hexavalent chromium, and when the hexavalent chromium concentration is high, the amount of these waste materials used must be reduced, which hinders the effective use of waste materials.

[0003] Conventionally, in the cement clinker burning process, it has been reported that by feeding a consolidated or granulated product of waste plastics and gypsum as auxiliary fuel into at least one location in a rotary kiln, the waste plastics coated with gypsum can be fed into the rotary kiln without melting near the tip outlet of the auxiliary fuel injection pipe, thereby preventing the waste plastics from welding or clogging near the tip outlet of the auxiliary fuel injection pipe and also suppressing the generation of hexavalent chromium (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5605274 specification Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method for producing cement clinker that can reduce the amount of hexavalent chromium even when waste materials are used as raw materials for the cement clinker. [Means for solving the problem]

[0006] The present inventors have found that, when cement clinker raw materials containing waste materials are supplied to a rotary kiln and burned, by adding a sulfate near the burning point of the main burner of the rotary kiln and adding a combustible substance in the range from the burning point to the outlet of the cement kiln, cement clinker containing reduced hexavalent chromium can be produced even when the cement clinker raw materials contain waste materials.

[0007] That is, the present invention provides the following [1] to [3]. [1] When cement clinker raw materials containing waste are supplied to a rotary kiln and burned, sulfates are charged near the burning point of the main burner of the rotary kiln, and combustible materials are charged in the range from the burning point to the rotary kiln outlet. A method for producing hexavalent chromium-reduced cement clinker. [2] The manufacturing method according to [1] above, wherein the sulfates include gypsum. [3] The manufacturing method according to [1] or [2] above, wherein the combustible material comprises one or more selected from bituminous coal, coke, waste plastic, combustible waste, heavy oil, heavy oil sludge, and crude oil sludge. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for producing cement clinker that can reduce the amount of hexavalent chromium even when waste materials are used as raw materials for the cement clinker. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view showing an example of a cement clinker calcining apparatus applicable to a manufacturing method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the following embodiments are merely examples for explaining the present invention, and are not intended to limit the present invention to the following contents. Furthermore, for convenience of illustration, the dimensional proportions of the drawings do not necessarily correspond to those in the description.

[0011] FIG. 1 is a schematic cross-sectional view showing an example of a cement clinker burning apparatus applicable to the manufacturing method according to this embodiment. The cement clinker calciner 10 shown in FIG. 1 includes a rotary kiln 1 for calcining cement clinker raw materials (hereinafter simply referred to as "raw materials") including waste, and a main burner 3 for injecting a main fuel such as pulverized coal from the front end of the rotary kiln on the outlet 2 side. Above the main burner 3, a combustible material feed pipe 4 for feeding combustible materials into the rotary kiln 1 from the front end of the rotary kiln on the outlet 2 side, and a sulfate feed pipe 5 for feeding sulfates into the rotary kiln 1 from the front end of the rotary kiln on the outlet 2 side, are provided. From a cost perspective, the combustible material feed pipe 4 and the sulfate feed pipe 5 may be integrated with the main burner 3; however, it is preferable to provide separate feed pipes for the combustible materials and sulfates, as shown in FIG. 1, so that they can be fed to desired positions within the rotary kiln 1.

[0012] The raw materials are fed into the rotary kiln 1 and are fired by the main burner flame 6 while being rolled, and the material to be fired 7 is moved toward the rotary kiln outlet 2. The temperature of the material to be fired 7 gradually increases inside the rotary kiln 1, and after reaching about 1450°C at the firing point of the main burner 3, the temperature of the material to be fired 7 drops to about 1300°C, turning into cement clinker 8, which is then discharged from the rotary kiln outlet 2.

[0013] In this specification, the "burning point" refers to the point at which the main burner of a rotary kiln reaches its maximum temperature. Furthermore, assuming that the inner diameter of the rotary kiln is D and the longitudinal distance from the end of the kiln to the outlet of the rotary kiln is L, the burning point of the main burner is typically located at a position where L / D = 3. Furthermore, in this specification, the "near the burning point" typically refers to a position where L / D = 3 ± 1, and preferably a position where L / D = 3 ± 0.5. While the size of a rotary kiln is not uniform, for example, if it is cylindrical, the diameter is typically 4 to 6 m and the longitudinal length is typically 60 to 100 m.

[0014] (Cement clinker raw materials including waste) Examples of cement clinker raw materials include general Portland cement clinker raw materials, such as CaO raw materials such as limestone, quicklime, and slaked lime; SiO2 raw materials such as silica stone and clay; Al2O3 raw materials such as clay; and Fe2O3 raw materials such as iron slag and iron cake. The waste material may be, for example, one or more selected from industrial waste, general waste, and construction waste soil. The bottom ash collected at the bottom of an incinerator when these materials are incinerated may be used. The bottom ash may also include incineration fly ash, which is soot and dust in the exhaust gas from incineration. Examples of industrial waste include raw concrete sludge; various sludges such as sewage sludge, water purification sludge, construction sludge, and steelmaking sludge; construction waste, concrete waste, waste plastics, scrap metal, foundry sand, rock wool, waste glass, ceramic waste, slag, and rubble; as well as shredder dust generated by crushing scrapped automobiles and discarded home appliances. Examples of general waste include dried sewage sludge powder and municipal solid waste incineration ash. Examples of construction waste soil include soil and waste soil generated at construction sites and other work sites, as well as waste soil. Among the above-mentioned raw materials, some of them, such as clay, iron slag, industrial waste, general waste, and construction waste soil, contain chromium.

[0015] The mixing ratio of the waste and the cement clinker raw materials can be selected as appropriate, but for example, they can be mixed so that the hydraulic modulus (HM) is preferably within the range of 1.8 to 2.3, more preferably 2.0 to 2.25.

[0016] (sulfates) In the present invention, sulfates are added near the burn point of the main burner of the rotary kiln. Hexavalent chromium is believed to exist in the cement clinker in the form of potassium chromate (KCrO). Adding sulfates produces KSO, etc., and it is presumed that the hexavalent chromium is reduced to trivalent chromium. Adding sulfates to the same location as combustible materials reduces the sulfates themselves, resulting in insufficient reduction of the hexavalent chromium.

[0017] Examples of sulfates include alkali metal sulfates, alkaline earth metal sulfates, ammonium sulfate, copper sulfate, iron sulfate, zinc sulfate, and aluminum sulfate. Examples of alkali metal sulfates include lithium sulfate, potassium sulfate, and sodium sulfate, and examples of alkaline earth metal sulfates include magnesium sulfate, calcium sulfate, and barium sulfate. One or more types of sulfates can be used, and they may be hydrated or anhydrous.

[0018] Among these, sulfates are preferably sulfates of alkaline earth metals, and more preferably calcium sulfate. Gypsum is suitably used as calcium sulfate. Therefore, it is preferable that the sulfates contain gypsum. Examples of gypsum include gypsum dihydrate, gypsum hemihydrate, and anhydrous gypsum, and these can be used alone or in combination of two or more. Furthermore, for example, natural gypsum, flue gas desulfurization gypsum, and waste gypsum boats may be used as gypsum, and may be pulverized and / or granulated as necessary to facilitate feeding into the rotary kiln.

[0019] Sulfates are charged into the rotary kiln through a sulfate charging pipe provided in front of the rotary kiln, and the charging speed of the sulfates can be set appropriately depending on their particle size, density, shape, etc. For example, when the sulfate is gypsum, it can be set to 5 m / s or more. Note that a blower or the like can be provided to adjust the charging speed and charging position of the sulfates.

[0020] The amount of sulfates added can be set appropriately depending on the hexavalent chromium content in the raw materials. From the viewpoint of preventing oxidation of trivalent chromium and promoting reduction of hexavalent chromium, the amount of sulfates added is preferably 0.15% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, converted into SO3 relative to the cement clinker, and is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less.

[0021] (Flammable material) In the present invention, a combustible substance is supplied from a combustible substance supply pipe, separate from the main fuel (pulverized coal, heavy oil, etc.) of the main burner, for the purpose of preventing the oxidation of trivalent chromium or promoting the reduction of hexavalent chromium. That is, since the generation of hexavalent chromium is an oxidation reaction that occurs in the oxidation region of the rotary kiln, a combustible substance is added to the rotary kiln as a substance that oxidizes itself and consumes oxygen so as to prevent the trivalent chromium in the material to be burned from being oxidized to hexavalent chromium or to reduce the hexavalent chromium that has already been oxidized to trivalent chromium.

[0022] The combustible material may be either a liquid fuel or a solid fuel, provided that it has a burning rate equal to or slower than that of the main fuel. Examples of liquid fuels include heavy oils such as heavy oil A, heavy oil B, and heavy oil C, waste lubricating oil, vegetable oil, heavy oil sludge, and crude oil sludge. Examples of solid fuels include bituminous coal, coke, waste plastics, and also combustible wastes obtained by compressing and solidifying wastes such as waste plastics, waste wood, and municipal waste. When using solid fuels, the particle size is preferably about 0.1 to 5 mm. Among these, the combustible material preferably includes one or more selected from bituminous coal, coke, waste plastic, combustible waste, heavy oil, heavy oil sludge, and crude oil sludge.

[0023] The generation of hexavalent chromium requires not only a high temperature but also an oxidizing atmosphere (a state with a relatively high oxygen concentration). Because the oxygen in the combustion air supplied from the main burner is consumed in the combustion of the main fuel, the oxygen concentration decreases near the firing point. However, as the temperature approaches the rotary kiln exit, the oxygen concentration increases due to the influence of air sent from the cooler side. Therefore, the inventors have confirmed that the generation of hexavalent chromium occurs in the range from near the firing point to the rotary kiln exit. Therefore, in order to effectively suppress the generation of hexavalent chromium, it is preferable to introduce the combustible substance into the rotary kiln in the range from near the firing point to the rotary kiln exit. More specifically, the combustible substance is introduced when the temperature of the material to be fired is preferably in the range of 1450°C to 1250°C, more preferably in the range of 1425°C to 1275°C, and even more preferably in the range of 1400°C to 1300°C.

[0024] Combustible materials are fed into the rotary kiln through a combustible material feed pipe installed in front of the kiln. The feed speed of the combustible material can be set appropriately depending on the particle size, density, shape, etc. For example, for general combustible materials, it can be set to 5 m / s or more. A blower or other device can be provided to adjust the feed speed and feed position of the combustible material.

[0025] The amount of combustible material added is preferably 0.5 to 15%, more preferably 1 to 12%, and even more preferably 2 to 10%, in terms of heat quantity relative to the heat quantity of the main burner, from the viewpoint of preventing oxidation of trivalent chromium and promoting reduction of hexavalent chromium. [Example]

[0026] The following examples will explain the present invention in more detail, but the present invention is not limited to the examples below.

[0027] 1. Raw materials used in this example (1) Cement clinker raw material containing municipal waste incineration ash (2) Reagent gypsum dihydrate (sulfates) (3) Heavy oil (flammable material)

[0028] 2.Water-soluble Cr 6+ Analysis of The analysis was carried out according to the Cement Association Standard Test, JCAS 1-53:2018 (method for determining trace elements in cement). 6+ of water-soluble Cr in clinker after reburning 6+ The percentage of water-soluble Cr was calculated. 6+ The ratio was calculated.

[0029] Examples 1 to 3 and Comparative Examples 1 to 4 Using the cement clinker calcination apparatus shown in Figure 1, the calcination point temperature was set to 1450°C, and sulfates and combustible materials were charged at the positions shown in Table 1 to produce cement clinker. Note that "near the calcination point" here refers to a position where L / D = 3 ± 0.5, where D is the inner diameter of the rotary kiln and L is the longitudinal distance from the end of the rotary kiln to the rotary kiln outlet. The "kiln cooling zone" typically refers to a position where L / D = 0 to 3, but the position where combustible materials are charged here refers to a position where the temperature of the material to be calcined is in the range of 1400°C to 1300°C. Reagent gypsum dihydrate was injected into the cement clinker as sulfates in the amounts shown in Table 1 in terms of SO3 equivalents. Heavy oil was also injected as a combustible material at a heat rate of 5% of the heat rate of the main burner.

[0030] [Table 1]

[0031] From Table 1, it can be seen that when cement clinker raw materials including waste materials are burned in a rotary kiln, by adding sulfates near the burning point of the main burner of the rotary kiln and adding combustible substances in the range from the burning point to the outlet of the cement kiln, cement clinker with reduced hexavalent chromium can be produced. [Explanation of symbols]

[0032] 1. Rotary kiln 2 Rotary kiln outlet 3 Main Burner 4 Flammable substance input pipe 5. Sulfate inlet pipe 6 Main burner flame 7 Object to be fired 8. Cement clinker 10. Cement clinker calciner

Claims

1. When cement clinker raw materials containing waste materials are supplied to a rotary kiln and burned, sulfates are charged near the burning point of the main burner of the rotary kiln, and combustible materials are charged in the range from the burning point to the cement kiln outlet. A method for producing hexavalent chromium-reduced cement clinker.

2. The method of claim 1 , wherein the sulfate salts include gypsum.

3. 3. The method according to claim 1, wherein the combustible material comprises one or more selected from the group consisting of bituminous coal, coke, waste plastic, combustible waste, heavy oil, heavy oil sludge, and crude oil sludge.

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