Fluorine elution amount reduction method of fluorine-containing precipitates

Abstract

To provide a technique for suppressing elution of fluorine again from precipitates generated by pH adjustment of waste acid containing fluorine.SOLUTION: In a reduction method of a fluorine elution amount of a fluorine-containing precipitate obtained by solid-liquid separation of precipitates generated by pH adjustment by addition of an alkali agent to waste acid discharged from a wet washing apparatus for treating an exhaust gas containing sulfurous acid generated from a smelting furnace, such as waste acid containing fluorine, the fluorine-containing precipitate is subjected to heating dehydration in an atmospheric air for a predetermined time.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for reducing the amount of fluorine eluted from a fluorine-containing precipitate. [Background technology]

[0002] In dry copper smelting plants, copper concentrate with a copper content of about 20-30%, obtained by classifying and dressing sulfide ores, mainly consisting of chalcopyrite, is used as the raw material, and by processing this in a smelting furnace such as a flash furnace or converter, the impurities iron and sulfur contained in the copper concentrate are oxidized and separated and removed from the copper as slag and sulfurous acid (SO2) gas, respectively. This produces refined crude copper with a copper content of about 99.8%, which is then used for the subsequent electrolytic refining process.

[0003] The hot exhaust gas containing sulfurous acid (SO2) generated by the oxidation of sulfur is sent in its entirety to a sulfuric acid manufacturing plant as a raw material for sulfuric acid. In the sulfuric acid manufacturing plant, the hot exhaust gas is first introduced into a waste heat boiler to recover waste heat by heat exchange with boiler feed water, and the exhaust gas cooled by this heat exchange is then introduced into a gas purification means to remove dust. The exhaust gas from which the dust has been removed is introduced into a converter having a vanadium pentoxide catalyst to oxidize the SO2 in the exhaust gas to SO3, and the SO3 is further absorbed by water in a downstream absorption tower to produce concentrated sulfuric acid.

[0004] The gas purification means is generally composed of an electric dust collector and a wet scrubber. The electric dust collector in the front stage removes most of the dust made of heavy metals such as copper, arsenic, cadmium, and zinc contained in the exhaust gas, and the wet scrubber in the rear stage removes some of the dust that could not be removed by the electric dust collector with cleaning water. When removing the dust with the cleaning water, some of the SO3 gas contained in the exhaust gas is also absorbed in the cleaning water, so the cleaning water containing the removed heavy metals becomes dilute sulfuric acid. Since the cleaning water is generally circulated in the wet scrubber, it is necessary to prevent the concentration of sulfuric acid and heavy metals such as copper and arsenic in the cleaning water in the circulation system of the wet scrubber. Therefore, a part of the cleaning water in the circulation system is extracted as waste acid and sent to a waste acid treatment facility. In the waste acid treatment facility, the extracted waste acid is subjected to various purification treatments such as neutralization treatment to separate and remove the heavy metals in the form of precipitates, and the removed precipitates are recovered and recycled to the copper smelting plant.

[0005] For example, Patent Document 1 discloses a wastewater treatment method in which wastewater containing a large amount of dissolved heavy metals such as iron, manganese, copper, tin, lead, nickel, mercury, zinc, and cadmium and having a pH of about 2.5 to 5 is treated with a neutralizing agent to reduce the amount of heavy metals. Specifically, Patent Document 1 discloses a technique in which carrier particles such as silica and a neutralizing agent such as sodium hydroxide are mixed, a slurry prepared by this is mixed with wastewater containing heavy metals and the pH is adjusted to about 8.4 to 10.6 to precipitate most of the heavy metals, and the resulting slurry containing the precipitates is subjected to solid-liquid separation to remove the precipitates in the form of sludge concentrated to a solid content concentration of at least 10% by weight, thereby carrying out purification treatment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 61-000156 Summary of the Invention [Problem to be solved by the invention]

[0007] However, waste acid generated by wet treatment of exhaust gas discharged from a copper smelting plant may contain fluorine in addition to heavy metals. In this case, it is difficult to remove fluorine by neutralization treatment using the above-mentioned neutralizing agent, so it is necessary to perform fluorine removal treatment after precipitating and recovering heavy metals. In fluorine removal treatment, fluorine is generally separated and removed in the form of precipitate (also simply called sediment) by adding an alkaline agent to the waste acid to adjust the pH, but depending on the alkaline agent used, there may be a problem that fluorine contained in the sediment is re-eluted. The present invention has been made in consideration of the above circumstances, and aims to provide a technology for suppressing the re-elution of fluorine from the sediment by adjusting the pH of fluorine-containing waste acid. [Means for solving the problem]

[0008] In order to achieve the above object, the method of the present invention for reducing the amount of fluorine eluted from a fluorine-containing precipitate is characterized in that the fluorine-containing precipitate is dried by heating in an air atmosphere for a predetermined period of time. Effect of the Invention

[0009] According to the present invention, it is possible to prevent fluorine from being easily eluted again from the fluorine-containing precipitate, and therefore the industrial value of the present invention is extremely great. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block flow diagram of an embodiment of a method for reducing the amount of fluorine eluted from a fluorine-containing precipitate according to the present invention. [Diagram 2] FIG. 1 is a process flow diagram of equipment that can suitably carry out the method for reducing the amount of fluorine eluted from a fluorine-containing precipitate according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the method for reducing the amount of fluorine eluted from a fluorine-containing precipitate according to the present invention will be described with reference to the drawings. As shown in the block flow diagram of Fig. 1, the method for reducing the amount of fluorine eluted from the fluorine-containing precipitate according to the embodiment of the present invention comprises a fluorine-containing precipitate production step S1 in which a neutralizing agent is added to a waste acid containing fluorine to produce a fluorine-containing precipitate, a settling separation step S2 in which a slurry containing the fluorine-containing precipitate produced in the fluorine-containing precipitate production step S1 is separated by gravity settling into a concentrated slurry and a clear liquid, a dehydration step S3 in which the concentrated slurry obtained in the settling separation step S2 is dehydrated to obtain a fluorine-containing precipitate wet cake, and a heat drying step S4 in which the fluorine-containing precipitate wet cake obtained in the dehydration step S3 is heated and dried to convert it into a dry product from which fluorine is hardly eluted.

[0012] To explain in detail with reference to the process flow diagram of FIG. 2, the method for reducing the amount of fluorine elution according to the embodiment of the present invention is directed to waste acid generated during purification treatment of sulfurous acid-containing exhaust gas discharged from a copper smelting furnace such as a flash furnace. Specifically, high-temperature sulfurous acid-containing exhaust gas discharged from a copper smelting furnace is first introduced into a waste heat recovery boiler 1, where it is heat exchanged with boiler feed water (BFW) flowing through a group of water tubes to generate steam. The exhaust gas cooled by heat exchange with the boiler feed water is then introduced into an electric dust collector 2, where most of the dust contained in the exhaust gas is removed, and further, in a wet scrubber 3 at the rear stage, it is brought into countercurrent gas-liquid contact with the washing water circulating in a packed tower to remove dust that was not removed by the electric dust collector 2 at the front stage. The exhaust gas cooled and purified in this way is used as a raw material for sulfuric acid.

[0013] In the above-mentioned wet scrubbing apparatus 3, part of the sulfurous acid contained in the exhaust gas is absorbed by the wash water during countercurrent gas-liquid contact with the wash water, so that the wash water circulating within the system of the wet scrubbing apparatus 3 becomes dilute sulfuric acid. Furthermore, ores that are raw materials for dry copper smelting may contain a trace amount of fluorine. In this case, fluorine is distributed in the above-mentioned exhaust gas containing sulfurous acid, so that fluorine is also absorbed by the wash water during countercurrent gas-liquid contact with the wash water in the wet scrubbing apparatus 3.

[0014] When fluorine absorbed in the cleaning water in the circulation system of the wet cleaning device 3 is concentrated, it generates highly corrosive hydrofluoric acid, etc., which may cause a problem of wear on the liquid-contacting parts of the equipment. In order to prevent this problem, for example, industrial water (work water) is continuously or periodically supplied to the circulation system of the wet cleaning device 3, and the same amount of cleaning water is continuously or periodically extracted from the circulation system as waste acid for treatment. Specifically, the waste acid extracted from the wet cleaning device 3 is introduced into a neutralization reaction tank 4 equipped with a stirrer, where a neutralizing agent such as caustic soda is added while stirring to adjust the pH to 5.5 to 6.5. As a result, a precipitate containing fluorine (referred to as a fluorine-containing precipitate) is generated. The slurry containing the fluorine-containing precipitate generated in the neutralization reaction tank 4 is supplied to a gravity settling device, for example, a thickener 6, via a first slurry pump 5. Here, gravity settling separation is performed to cause the clear liquid to overflow, and a concentrated slurry containing the fluorine-containing precipitate is extracted from the bottom of the thickener.

[0015] The concentrated slurry concentrated in the thickener is pressurized by a second slurry pump 7, and then introduced into a dehydrator 8, such as a filter press, for dehydration. The wet cake of the fluorine-containing precipitate thus obtained is charged into a heat drying apparatus 9, and is heated and dried by maintaining it in an air atmosphere at an atmospheric temperature of preferably 80 to 120°C, more preferably about 100°C, for preferably 10 to 15 hours. This allows the fluorine-containing precipitate to be modified into one having little fluorine elution, from which fluorine is less likely to be eluted.

[0016] If the atmospheric temperature during the above heat drying is less than 80°C, the temperature is too low and it is difficult to convert the fluorine-containing precipitate into one that is difficult to dissolve in fluorine, and conversely, if it exceeds 120°C, the degree of conversion to one that is difficult to dissolve in fluorine will not improve and energy consumption will increase, which is not preferable. Furthermore, if the holding time during the above heat drying is less than 10 hours, the conversion to one that is difficult to dissolve in fluorine may not progress sufficiently, and conversely, if it exceeds 15 hours, the progress of the conversion to one that is difficult to dissolve in fluorine will not improve any further and energy consumption will increase, which is not preferable. The reason why fluorine is less likely to dissolve from the fluorine-containing precipitate by heat drying under the specified conditions as described above is thought to be due to a decrease in moisture content. EXAMPLES

[0017] High-temperature exhaust gas discharged from the smelting furnace of a dry copper smelting plant was subjected to waste heat recovery in a waste heat boiler and dust collection in an electric dust collector, and then wet washing was performed by countercurrent gas-liquid contact with circulating washing water in a packed tower. The waste acid continuously discharged from this circulating washing water was treated according to the process shown in Figure 1 to produce a dried product consisting of fluorine-containing precipitate. Specifically, as shown in Figure 2, first, caustic soda was added as a neutralizing agent in the neutralization reaction tank 4 to adjust the pH to 6.0, and the slurry containing the precipitate thus produced was concentrated in a thickener 6 and then dehydrated in a filter press as a dehydration device 8 to obtain a wet cake.

[0018] The wet cake obtained above was divided into two, and one of them was heated and dried in air at an atmospheric temperature of 100°C for 12 hours. The obtained dried precipitate and the other of the two wet cakes were subjected to an elution test (based on Environment Agency Notification No. 13 (H)) without the use of any elution inhibitor. When the fluorine elution value of the wet cake that was not subjected to heat drying was taken as 100%, the fluorine elution value of the dried product that was heated and dried was 60-80%, confirming that the amount of fluorine elution was significantly reduced by heat treatment. [Explanation of symbols]

[0019] 1. Waste heat recovery boiler 2. Electrostatic precipitator 3. Wet cleaning equipment 4. Neutralization Reactor 5. No. 1 slurry pump 6. Thickener 7. No. 2 Slurry Pump 8 Dehydration equipment 9 Heating drying device

Claims

1. A method for reducing the amount of fluorine eluted from a fluorine-containing precipitate, comprising heating and drying the fluorine-containing precipitate in an air atmosphere for a predetermined period of time.

2. 2. The method for reducing the amount of fluorine eluted from a fluorine-containing precipitate according to claim 1, wherein the fluorine-containing precipitate is obtained by subjecting a precipitate produced by adjusting the pH of a fluorine-containing waste acid by adding an alkaline agent to the precipitate and then subjecting the precipitate to solid-liquid separation.

3. 3. The method for reducing the amount of fluorine eluted from a fluorine-containing precipitate according to claim 2, wherein the waste acid is discharged from a wet scrubbing device for treating an exhaust gas containing sulfurous acid generated from a smelting furnace.

Images