JPS6035285B2 - Treatment method for fluorine-containing sulfuric acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating a sulfuric acid solution containing fluorine, and particularly to a method for recovering fluorine as calcium fluoride from a sulfuric acid waste solution containing fluorine.

There are situations in industry where waste sulfuric acid solutions containing fluorine must be treated.

An example of a waste sulfuric acid solution containing fluorine is a dilute sulfuric acid waste solution discharged from a sulfur dioxide gas cleaning process. Sulfur dioxide gas generated from the wire manufacturing process of non-ferrous metals such as steel contains impurities such as phosphorus and fluorine contained in ore and dust. When such sulfur dioxide gas is used in the production of sulfuric acid, it is necessary to perform purification treatments such as dust removal and washing in advance. Among these, in the cleaning process, a method of self-circulating water to clean the gas is used. As a result, the cleaning water becomes diluted sulfuric acid, and during cleaning, impurities such as phosphorus and fluorine are dissolved as diluted sulfuric acid and hydrofluoric acid, and gradually accumulate there, while moisture in the gas is absorbed and diluted. Because this will lead to an increase in the amount of sulfuric acid, a portion of the dilute sulfuric acid is periodically extracted from the system. This extracted dilute sulfuric acid is referred to as the dilute sulfuric acid waste solution. In addition, examples of sulfuric acid waste liquid containing fluorine include pickling waste liquid for iron materials, etc. Since hydrogen fluoride is usually added to the sulfuric acid solution to improve the pickling effect, the sulfuric acid waste solution contains fluorine. Furthermore, fluorine may also be contained in sulfuric acid waste liquid from various plating factories and electrolysis factories. In the present invention, these sulfuric acid waste liquids are collectively referred to as waste sulfuric acid. If waste sulfuric acid contains a large amount of fluorine, it is not allowed to neutralize and dump it as is.
- Conventionally, the following methods have been used to treat fluorine in waste sulfuric acid: {1}
After precipitating and removing phosphorus as phosphorus sulfide with a sulfurizing agent, free sulfuric acid in the waste sulfuric acid is reduced by using calcium hydroxide etc.
After precipitating and collecting the gypsum with a small amount of fluorine content by neutralizing it to leave it at night, the pH is raised to 10 or more with calcium hydroxide to separate the fluorine as a mixture of gypsum and calcium fluoride. Method. (2) A method of heating and concentrating waste sulfuric acid to lower the solubility of borosulfite and hydrofluoric acid, precipitating and removing phosphorous acid, and releasing hydrofluoric acid as vapor out of the system.

However, in method [1], the product is a mixture of gypsum and calcium fluoride, and the two cannot be separated, so it cannot be said to be an effective method for recovering fluorine.

Even when trying to use this by-product stone pipe as a raw material for cement, the fluorine content is low. However, it cannot be used because it slows down the cement solidification rate. Therefore, the gypsum with a high fluorine content that is produced as a by-product in method (m) has no choice but to be stored or returned to the steel smelter, but since the amount is relatively large, if you want to store it, you need to find a place and how to store it. On the other hand, returning the steel to the steel smelting process only results in an increase in the amount of fluorine in the system. In method (2), there is no effective method for recovering the hydrofluoric acid vapor produced by heating and concentrating pseudosulfuric acid, which increases the cause of air pollution.

Furthermore, the heating concentration method uses heavy oil, which has the disadvantage of increasing running costs. Therefore, the method {2} cannot be said to be an effective method since the fluorine in the waste sulfuric acid cannot be treated after all. Thus, conventional methods for recovering fluorine from fluorine-containing waste sulfuric acid are far from being useful because fluorine cannot be recovered alone in a useful form. In view of the current state of the art, the present inventor has conducted repeated research on a method for recovering fluorine independently from waste sulfuric acid containing fluorine, and has developed a method that allows fluorine to be precipitated and collected independently as calcium fluoride. completed. The calcium fluoride collected is of good quality and can be reused. To summarize, the present invention involves the step of neutralizing only the free sulfuric acid content of a fluorine-containing sulfuric acid solution with caustic soda to the extent that the fluorine does not become sodium fluoride, and then adjusting the pH to 4 by adding calcium carbonate or calcium hydroxide. The present invention provides a method for treating a fluorine-containing sulfuric acid solution, which comprises a step of adjusting the sulfuric acid solution to .0 to 10.0 to precipitate fluorine as calcium fluoride.

It is also possible to pre-treat the raw sulfuric acid solution and precipitate and remove the stone pipe therefrom. The details of the present invention will be explained below. The sulfuric acid concentration in the waste sulfuric acid solution varies depending on its source, but for example, the sulfuric acid concentration in the waste sulfuric acid solution from the sulfur dioxide gas cleaning process depends on the amount of dilute sulfuric acid used and the S03 in the waste gas.
Depending on the concentration, it is approximately 50 to 200 pm/k.

Depending on the source of the waste sulfuric acid, the sulfuric acid concentration may be very low, for example, 20 pm/pm. When arsenic is contained in the waste sulfuric acid solution, arsenic can be easily precipitated and removed as boronic sulfide by a sulfurizing agent, so boronate is removed first as has been conventionally done. After that, the free sulfuric acid concentration increased by 50%/
If the temperature is higher than that, high-purity gypsum can be precipitated and recovered by neutralization treatment with calcium carbonate, as was done in the conventional method {1-1-- mentioned above. It is a good idea to do so.
When the waste sulfuric acid solution is neutralized with calcium carbonate, etc. as a pretreatment to precipitate gypsum, in order to use this gypsum as a raw material for cement, it is necessary to keep the fluorine content in the gypsum to a very small amount from the viewpoint of cement solidification speed. There is.

Theoretically, even if neutralized to a pH of about 2, no calcium fluoride precipitation reaction will occur, and therefore fluorine should not be mixed into the precipitated gypsum. However, in reality, when calcium carbonate is added, the pH locally increases. Therefore, in order to obtain high quality gypsum from the waste sulfuric acid solution, the sulfuric acid content in the waste sulfuric acid solution should be neutralized while leaving the sulfuric acid content for 30 to 50 minutes. It is preferable. By leaving a sufficient amount of sulfuric acid to prevent a local pH increase, only stone tubes are precipitated without causing a calcium fluoride precipitation reaction. Precipitated gypsum is separated and removed by a furnace filtration operation.
If the concentration of free sulfuric acid in the waste sulfuric acid solution is less than 30 μm/kg, good quality gypsum cannot be recovered from the waste sulfuric acid solution, so the above-mentioned pretreatment is omitted.

After pretreatment as desired, the mixture of gypsum and calcium fluoride has conventionally been precipitated and collected using calcium hydroxide to increase the prl to 10 or more.

As mentioned above, this mixture is relatively large and cannot be used thereafter. If calcium fluoride can be collected alone with high purity, the collected calcium fluoride can be used as a raw material for fluorine compounds such as fluorine or silicofluoric acid, or as a slag additive, making it an effective method for recovering fluorine from waste sulfuric acid solution. This means that the process has been completed. According to the present invention, a sufficient amount of caustic soda is added to the waste sulfuric acid solution that has undergone the above-mentioned pretreatment or is left as it is without undergoing any pretreatment to neutralize only the free sulfuric acid content to the extent that fluorine is not converted to sodium fluoride. added.

At this time, the pH of the waste sulfuric acid solution is approximately 3. In this state, add calcium carbonate or calcium hydroxide to adjust the pH.
If the value is adjusted to 4.0 to 10.0, highly pure CaF2 can be precipitated and collected. It is important to neutralize with caustic soda so that only the sulfuric acid content is neutralized.

By doing this, sulfuric acid changes to Na2S04, but hydrofluoric acid remains as it is. When the neutralization pH with caustic soda is increased to 3 or more, for example 5 to 6, most of the hydrofluoric acid becomes sodium fluoride. Even if calcium hydroxide or calcium carbonate is added in this state, CaF2 will not precipitate unless the pH of the solution is brought to the alkaline side. For example, Ca
Even if F2 is precipitated, CaF obtained in this state
2 contains a lot of gypsum produced from the following reaction: N
a2S04 10CaC03 → CaS04 10Na2C03
...{3' Therefore, the purity of the precipitated CaF2 becomes very poor, and it cannot be used later.

If the neutralization pH with caustic soda is 3 or less, free sulfuric acid remains, and even if calcium carbonate is added thereto, precipitation of only CaF2 cannot be obtained.

In this way, in the present invention, when only free sulfuric acid is neutralized with caustic soda and the pH is adjusted to 4.0 to 10 by adding calcium carbonate or calcium hydroxide, a high purity of 85 to 90% can be obtained. of calcium fluoride is obtained. Most of the fluorine in the waste sulfuric acid solution is precipitated as calcium fluoride, and the amount of fluorine remaining in the waste sulfuric acid is less than 50%. Due to the fact that free sulfuric acid is converted into the form of sodium chloride, and the pH is adjusted to 3.2 to 11, almost no stone tube formation reaction occurs. After all, PHIO. This is because when the temperature is below 0, no gypsum is generated due to the reaction between ramie and coal. In cases where arsenic remains in the waste sulfuric acid solution due to insufficient removal of sulfuric acid, raising the pH above 4 will cause arsenic to co-accumulate and reduce the purity of CaF2. When it is desired to increase the purity, it is preferable to suppress the pH to about 4. When regenerated calcium fluoride is to be used effectively, for example, when it is used as a raw material for producing silicofluoric acid, it is naturally required to have high purity.

Calcium fluoride obtained by the present invention has a purity of 85-9.
0%, and the remainder is gypsum. This level of purity is sufficient for most uses, but if you want to further increase the purity of calcium fluoride, you can wash the precipitate with water to elute and remove the stone tube. As mentioned above, the waste liquid after recovering calcium fluoride contains up to 50m9/fluorine.
Some amount of water remains.

In order to lower the amount of fluorine in the waste liquid, a mixture of gypsum and calcium fluoride is precipitated by raising the pH to 10 or more with calcium hydroxide, and the fluorine in the waste liquid is reduced to 10 m
9/or less. The resulting mixture of gypsum and calcium fluoride is very small in quantity and is either stored or returned to the copper melting furnace. If the desaturation step is not sufficient, sulfur will co-precipitate and be mixed in at this time. As mentioned above, in the treatment of 9E solution that also contains boron, by adjusting the pH to 4.0 to 8, even if boron remains, it is prevented from being mixed into the calcium fluoride. Obtaining highly pure calcium fluoride is also a side effect of pH adjustment. As explained in detail above, the present invention can recover fluorine from a waste sulfuric acid solution containing fluorine as highly pure calcium fluoride, and makes it possible to effectively use the recovered calcium fluoride using conventional methods. It is much more effective than

Examples of the present invention are presented below. Example day 2S〇4 200 evening/evening, F 8 evening/so, AS O-
In treating the waste sulfuric acid solution containing 3 ml of sulfuric acid according to the present invention, first, as a pretreatment, the free sulfuric acid was neutralized with CaC03 slurry for 200 ml of sulfuric acid until the free sulfuric acid reached 40 ml of sulfuric acid.

The generated gypsum was removed by filtration. After that, 200 evenings/
The pH was adjusted to 3 with NaOH solution. Thereafter, 200 ml of CaC03 slurry or 200 ml of Ca(OH)2 slurry was added to 500 cc of this adjustment solution to adjust the pH to 4 to 8. The composition of the furnace liquid and precipitate after the tailing operation was analyzed. The results are shown in the table below. The quality of calcium fluoride in the produced precipitate was around 87%, and the residual fluorine in the furnace solution was 0.03 to 0.05 m/s. table

Claims (1)

[Claims] 1. Neutralize only free sulfuric acid in a sulfuric acid solution containing fluorine with caustic soda to an extent that fluorine does not become sodium fluoride, and adjust the pH to 4.0 to 10 by adding calcium carbonate or calcium hydroxide. A method for treating a fluorine-containing sulfuric acid solution, comprising the step of adjusting the sulfuric acid solution to 0.0 and precipitating fluorine as calcium fluoride.