Method for producing modified calcium fluoride-containing solids

Abstract

This invention provides a method for separating the lime component from a calcium fluoride-containing solid while suppressing the loss of calcium fluoride, without using acids or alkalis, thereby producing a modified calcium fluoride-containing solid with reduced lime content. [Solution] A method for producing a modified calcium fluoride-containing solid with reduced lime content by separating the lime component from a calcium fluoride-containing solid containing calcium fluoride and at least one lime component selected from calcium carbonate, calcium hydroxide, and calcium oxide, wherein a carbon dioxide-containing gas is supplied to a slurry containing the calcium fluoride-containing solid and water, and the pH of the slurry is maintained at 6.0 or below to elute the lime component.

Description

Technical Field

[0001] The present invention relates to a method for separating a lime component from a calcium fluoride-containing solid containing calcium fluoride and a lime component, and reducing the lime component in the calcium fluoride-containing solid.

Background Art

[0002] Conventionally, hydrofluoric acid has been produced by reacting fluorite (calcium fluoride) with concentrated sulfuric acid, and much of it is used in semiconductor manufacturing, glass processing, etc. Used hydrofluoric acid used in semiconductor manufacturing, etc. is precipitated as calcium fluoride with a lime-based additive together with waste liquid generated in other manufacturing lines, and then, after solid-liquid separation and dehydration treatment, it becomes calcium fluoride sludge. At this time, calcium carbonate and calcium hydroxide contained in the lime-based additive also precipitate together with calcium fluoride, and thus these are contained in the calcium fluoride sludge (calcium fluoride-containing solid) as impurities.

[0003] On the other hand, fluorite is a mineral resource, and there are concerns about resource depletion and price increases. Therefore, using calcium fluoride sludge as a raw material in the production of hydrofluoric acid has been considered. However, when using calcium fluoride sludge as a substitute for fluorite, for example, if calcium carbonate is contained in the calcium fluoride sludge, carbon dioxide gas is generated during the production of hydrofluoric acid, and if calcium hydroxide is contained in the calcium fluoride sludge, water is generated, leading to a deterioration in the quality of hydrofluoric acid.

[0004] For this reason, in order to recover high-purity calcium fluoride from calcium fluoride sludge, methods using acids or alkalis such as hydrochloric acid and hydrofluoric acid have been proposed (see, for example, Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] However, strong acids such as hydrochloric acid and nitric acid react with calcium fluoride in calcium fluoride sludge to produce hydrofluoric acid, resulting in the loss of calcium fluoride. Furthermore, acids and alkalis are highly dangerous to handle, and particular care must be taken when handling hydrofluoric acid.

[0007] The object of the present invention is to provide a method for safely separating the lime component from a calcium fluoride-containing solid while suppressing the loss of calcium fluoride, without using acids or alkalis, and for producing a modified calcium fluoride-containing solid with reduced lime content. [Means for solving the problem]

[0008] The inventors of the present invention have discovered that by supplying a carbon dioxide-containing gas to a slurry containing calcium fluoride-containing solids and water, and maintaining the pH of the slurry at 6.0 or lower, the lime component in the calcium fluoride-containing solids can be bicarbonated and eluted, thereby reducing the lime component in the calcium fluoride-containing solids, and have completed the present invention.

[0009] In other words, the present invention is as follows: [1] A method for producing a modified calcium fluoride-containing solid in which the amount of calcium fluoride is reduced, by separating the lime component from a calcium fluoride-containing solid containing calcium fluoride and at least one lime component selected from calcium carbonate, calcium hydroxide and calcium oxide, A method for producing a modified calcium fluoride-containing solid, characterized by supplying a carbon dioxide-containing gas to a slurry containing the calcium fluoride-containing solid and water, maintaining the pH of the slurry at 6.0 or lower, and eluting the lime component.

[0010] [2] A method for producing the modified calcium fluoride-containing solid according to [1], characterized in that the content of the lime component in the modified calcium fluoride-containing solid is 0.5% by mass or less.

[0011] [3] A method for producing a modified calcium fluoride-containing solid according to [1] or [2] above, characterized in that the pH of the slurry is maintained at 5.0 or higher and 6.0 or lower. [4] A method for producing a modified calcium fluoride-containing solid according to any one of [1] to [3] above, characterized in that the pH of the slurry is maintained at 6.0 or below for 15 minutes or more.

[0012] [5] A method for producing a modified calcium fluoride-containing solid according to any one of [1] to [4] above, characterized in that the calcium fluoride-containing solid contains 60% by mass or more of calcium fluoride on a dry matter basis. [6] A method for producing a modified calcium fluoride-containing solid according to any one of [1] to [5] above, characterized in that the carbon dioxide-containing gas is exhaust gas generated from a thermal power plant or exhaust gas generated from a cement manufacturing facility. [Effects of the Invention]

[0013] According to the present invention, it is possible to safely separate the lime component from a calcium fluoride-containing solid while suppressing the loss of calcium fluoride, without using acids or alkalis, and to produce a modified calcium fluoride-containing solid with reduced lime content. [Modes for carrying out the invention]

[0014] The present invention provides a method for producing a modified calcium fluoride-containing solid, comprising separating the lime component from a calcium fluoride-containing solid containing calcium fluoride and at least one lime component selected from calcium carbonate, calcium hydroxide, and calcium oxide, to produce a modified calcium fluoride-containing solid with reduced lime component, characterized by supplying a carbon dioxide-containing gas to a slurry containing the calcium fluoride-containing solid and water, maintaining the pH of the slurry at 6.0 or lower, and eluting the lime component.

[0015] Here, calcium fluoride-containing solids refer to solids containing calcium fluoride (CaF2) and at least one lime component selected from calcium carbonate (CaCO3), calcium hydroxide (Ca(OH)2), and calcium oxide (CaO). Examples include calcium fluoride sludge obtained by treating fluorine-based wastewater generated in semiconductor manufacturing, liquid crystal manufacturing, glass processing, etc. Specifically, examples include calcium fluoride sludge produced by adding lime-based additives containing calcium carbonate or calcium hydroxide to fluorine-based wastewater. This calcium fluoride sludge may be sludge that has undergone dewatering treatment after solid-liquid separation, or sludge that contains moisture.

[0016] The calcium fluoride-containing solid preferably contains 60% by mass or more of calcium fluoride on a dry matter basis (the total value of components excluding water is taken as 100% by mass), more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more. On the other hand, there is no particular upper limit, but for example it is about 90% by mass.

[0017] This calcium fluoride-containing solid may contain other components in addition to calcium fluoride and lime components. These other components vary depending on the production route of the calcium fluoride-containing solid. For example, in the case of calcium fluoride sludge, examples include aluminum compounds (including metallic aluminum) and silicon compounds (including silicon).

[0018] In the present invention, by supplying a carbon dioxide-containing gas to a slurry containing a calcium fluoride-containing solid and water and maintaining the pH of the slurry at 6.0 or less, the lime component in the calcium fluoride-containing solid can be bicarbonated and eluted. Therefore, the lime component can be separated from the calcium fluoride-containing solid without using an acid or an alkali, and a calcium fluoride-containing solid with a reduced lime component can be obtained.

[0019] That is, in the present invention, as shown in the following formula, calcium carbonate contained in the calcium fluoride-containing solid is bicarbonated and eluted into the slurry.

[0020] CaCO3(s)+H2O(l)+CO2(g) → Ca(HCO3)2(aq)

[0021] In addition, calcium hydroxide and calcium oxide, which are lime components in the calcium fluoride-containing solid, also change to calcium carbonate and are further bicarbonated with the supply of the carbon dioxide-containing gas.

[0022] Further, in the present invention, by not using an acid or an alkali, the loss of calcium fluoride can be suppressed or prevented, and the lime component in the calcium fluoride-containing solid can be safely separated and reduced. In addition, since the amount of waste of the calcium fluoride-containing solid can be reduced, environmental pollution can be suppressed, and by reusing the calcium fluoride-containing solid, effective utilization of resources can be achieved and the use of fluorite, which is a mineral resource, can be reduced.

[0023] Hereinafter, the method for producing the modified calcium fluoride-containing solid of the present invention will be specifically described.

[0024] First, a calcium fluoride-containing solid and water are added into a container to prepare a slurry (slurry preparation step).

[0025] The proportion of calcium fluoride-containing solids (dried material) in the slurry can be appropriately adjusted depending on the type of calcium fluoride-containing solids, for example, preferably 0.1 to 20% by mass, more preferably 0.2 to 15% by mass, even more preferably 0.5 to 10% by mass, and particularly preferably 0.5 to 5% by mass. This allows the calcium fluoride-containing solids to be sufficiently dispersed in the liquid and to be effectively brought into contact with carbon dioxide-containing gas, thereby increasing the production efficiency of modified calcium fluoride-containing solids.

[0026] Next, carbon dioxide-containing gas is supplied to the slurry containing calcium fluoride-containing solids and water to maintain the slurry's pH at 6.0 or below (carbon dioxide supply step). This allows the lime components in the calcium fluoride-containing solids to be bicarbonated and dissolved into the liquid.

[0027] This carbon dioxide supply process can be carried out in a continuous or batch manner. As an example of a continuous method, one can describe a configuration in which a predetermined amount of calcium fluoride-containing solid material and water are continuously supplied to and discharged from a stirring vessel, while carbon dioxide gas is supplied to the stirring vessel to maintain the pH of the slurry at 6.0 or below. The rate at which calcium fluoride-containing solids and water are supplied into the stirring vessel, i.e., the processing time (residence time) within the stirring vessel, is preferably adjusted appropriately according to the lime content in the calcium fluoride-containing solids, the amount of carbon dioxide supplied, and other factors. In actual operation, the lime content in the modified calcium fluoride-containing solids discharged from the stirring vessel can be measured, and various conditions can be adjusted accordingly.

[0028] One example of a batch-type method is to supply carbon dioxide gas to maintain the pH of the slurry at 6.0 or lower while stirring the slurry in a stirring container, and to maintain this pH for a predetermined time. The processing time in the stirring container should preferably be adjusted as appropriate depending on the amount of lime component in the calcium fluoride-containing solid, the amount of carbon dioxide supplied, and other factors. After processing, the modified calcium fluoride-containing solid and the water from which the lime components have been dissolved are removed from the stirring container, and the above procedure is repeated.

[0029] The carbon dioxide-containing gas supplied to the slurry is not particularly limited as long as it contains carbon dioxide (CO2), but examples of preferred examples include exhaust gas generated from a thermal power plant or exhaust gas generated from a cement manufacturing facility. This allows carbon dioxide generated from thermal power plants and cement manufacturing facilities to be used as a resource without being released into the atmosphere, thus promoting the efficient use of resources.

[0030] This carbon dioxide-containing gas is not particularly limited as long as it contains carbon dioxide (CO2), but it is preferably 10% by volume or more of carbon dioxide, more preferably 30% by volume or more, even more preferably 50% by volume or more, particularly preferably 80% by volume or more, and most preferably carbon dioxide alone (100% by volume). This makes it possible to improve the production efficiency of the modified calcium fluoride-containing solid.

[0031] The pH of the slurry is usually around 7.0 (neutral) before the supply of carbon dioxide-containing gas, and can decrease to 6.0 or below with the supply of carbon dioxide-containing gas. However, from the viewpoint of efficiently dissolving the lime components, it is preferable to maintain the pH between 4.0 and 6.0, more preferably between 4.5 and 6.0, and even more preferably between 5.0 and 6.0.

[0032] Here, the retention time of the slurry at pH 6.0 or below is preferably 15 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more, from the viewpoint of ensuring the reaction time necessary for the bicarbonation treatment (elution) of the lime components. On the other hand, there is no particular upper limit, but considering the production efficiency of the modified calcium fluoride-containing solid, it is about 120 minutes.

[0033] After dissolving the lime components in water, the slurry can be subjected to solid-liquid separation to obtain a modified calcium fluoride-containing solid, after removing other components (impurities) as needed. It is preferable to wash the solid-liquid separated modified calcium fluoride-containing solid with water to remove any attached substances. Examples of equipment used for solid-liquid separation include atmospheric pressure filtration, vacuum filtration, filter press, screw press, centrifuge (screw decanter), and thickener.

[0034] The lime content in the manufactured modified calcium fluoride-containing solid is preferably 0.5% by mass or less, more preferably 0.4% by mass or less, even more preferably 0.3% by mass or less, and particularly preferably 0.25% by mass or less, in order to maintain high quality hydrofluoric acid produced using this modified calcium fluoride-containing solid. On the other hand, while a lower limit is preferable, considering manufacturing efficiency, it is approximately 0.1% by mass.

[0035] Here, the amount of lime component in the modified calcium fluoride-containing solid is the value (amount of calcium carbonate) obtained by heating the material from room temperature to 1000°C at a rate of 10°C / min using a TG-DTA (thermogravimetric-differential thermal analysis) instrument under a nitrogen atmosphere, and measuring the weight loss due to thermal decomposition at 680-755°C. [Examples]

[0036] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0037] (sample) As a sample, we used dried calcium fluoride sludge (calcium fluoride-containing solid) with the component composition shown in Table 1.

[0038] [Table 1]

[0039] (Test method) 1 g of the above sample was mixed with 100 mL of water to disperse the sample in the water and form a slurry, to which a carbon dioxide-containing gas was supplied. As the carbon dioxide-containing gas, industrial exhaust gas with a carbon dioxide content of 100 vol% (Examples 1 and 2) and simulated exhaust gas composed of nitrogen and carbon dioxide with a carbon dioxide content of 15 vol% (Example 3) were used. The supply rate of industrial exhaust gas (carbon dioxide) to the slurry was set to 0.5 L / min, and the supply rate of simulated exhaust gas was set to 1.18 L / min (carbon dioxide supply rate of 0.18 L / min).

[0040] The test conditions and test results are shown in Table 2. Note that the reaction times in Table 2 represent the processing time after carbon dioxide is supplied and the pH drops below 6.0.

[0041] Furthermore, the calcium content in the samples before and after treatment was calculated using a TG-DTA (thermogravimetric-differential thermal analysis) instrument, where the temperature was raised from room temperature to 1000°C at a rate of 10°C / min under a nitrogen atmosphere, and the weight loss due to thermal decomposition at 680-755°C was measured. Furthermore, the bicarbonation rates in Table 2 were calculated using the following formula.

[0042] Bicarbonation rate (%) =[{(CaCO3 concentration before reaction) × (total weight of sample before reaction) - (CaCO3 concentration after reaction) × (total weight of sample after reaction)} / {(CaCO3 concentration before reaction) × (total weight of sample before reaction)}]×100

[0043] [Table 2]

[0044] As shown in Examples 1-3 of Table 2, by supplying carbon dioxide and maintaining the pH of the slurry at 6.0 or below, the lime component (calcium carbonate) in the modified calcium fluoride-containing solid was reduced from before the treatment. In particular, as in Examples 2 and 3, by extending the time for maintaining the slurry's pH at 6.0 or below to 15 minutes, the bicarbonation rate was increased compared to Example 1, and the lime component in the modified calcium fluoride-containing solid was reduced to 0.5% by mass or less.

[0045] In summary, it has been confirmed that the present invention can effectively reduce the lime component in calcium fluoride-containing solids without the use of acids or alkalis. [Industrial applicability]

[0046] The present invention's method for producing modified calcium fluoride-containing solids is industrially useful because it allows for the production of high-quality calcium fluoride-containing solids that can be used as raw materials for hydrofluoric acid.

Claims

1. A method for producing a modified calcium fluoride-containing solid in which the amount of calcium fluoride is reduced, by separating the lime component from a calcium fluoride-containing solid containing calcium fluoride and at least one lime component selected from calcium carbonate, calcium hydroxide, and calcium oxide, A method for producing a modified calcium fluoride-containing solid, characterized by supplying a carbon dioxide-containing gas to a slurry containing the calcium fluoride-containing solid and water, maintaining the pH of the slurry at 6.0 or below, and eluting the lime component.

2. A method for producing a modified calcium fluoride-containing solid according to claim 1, characterized in that the content ratio of the lime component in the modified calcium fluoride-containing solid is 0.5% by mass or less.

3. A method for producing a modified calcium fluoride-containing solid according to claim 1, characterized in that the pH of the slurry is maintained at 5.0 or higher and 6.0 or lower.

4. A method for producing a modified calcium fluoride-containing solid according to claim 1, characterized in that the pH of the slurry is maintained at 6.0 or lower for 15 minutes or more.

5. A method for producing a modified calcium fluoride-containing solid according to claim 1, characterized in that the calcium fluoride-containing solid contains 60% by mass or more of calcium fluoride on a dry matter basis.

6. The method for producing a modified calcium fluoride-containing solid according to claim 1, characterized in that the carbon dioxide-containing gas is exhaust gas generated from a thermal power plant or exhaust gas generated from a cement manufacturing facility.

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