JPS5835930B2 - Method for purifying mirabilite solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying mirabilite solution, and more particularly, the present invention relates to a method for purifying mirabilite solution produced in a flue gas desulfurization equipment using the mirabilite method.

Dry methods and wet methods are known as methods for removing sulfur dioxide gas contained in combustion exhaust gas, but methods that use caustic soda or aqueous sodium sulfite solutions as absorbents achieve a high desulfurization rate and eliminate by-products. Due to the usefulness of Glauber's salt, its excellence as an elimination plant has been recognized and it has been widely put into practical use.

These methods are mainly limited to the reactions that produce sodium sulfite according to the following reaction formulas (1) to (3), and the reaction of formula (4) that oxidizes the generated sodium sulfite to produce mirabilite: However, the above methods The glauber's salt solution produced by this method contains heavy metals such as vanadium, which are impurities in the fuel.For example, in the case of vanadium, the concentration of vanadium mixed in varies depending on the type of fuel, but when C heavy oil is exclusively burned, mirabilite solution is usually used. The concentration is several W to several + ppm relative to the sulfate solution recovered at a concentration of about 3.0%.

Furthermore, since some amounts of other heavy metals such as iron and nickel are also mixed in, its use as an industrial raw material is limited.

Therefore, various methods have been proposed to remove heavy metals such as vanadium from the sodium sulfate solution, and among them, a method using co-precipitation with iron hydroxide precipitation is common.

For example, in JP-A-52-146799, a certain amount of ferrous salt is added to an aqueous solution containing vanadium, and then a caustic alkali is added to form an iron hydroxide precipitate as p) 128, which is adsorbed and removed. A method is disclosed in which the composition is acidified with a mineral acid and then treated with a caustic alkali.

However, in all of these methods, the precipitate produced is bulky, has poor settling properties, and extremely poor filtration properties, making it impossible to separate efficiently, and the filtration area of the filter is limited. The drawback was that it had to be very large.

The present inventors have investigated various methods of obtaining high-purity Glauber's salt that can overcome the above-mentioned drawbacks and can be industrially advantageously used as a variety of raw materials. By adding a small amount of soluble calcium salt while stirring, the sedimentation and filterability of the precipitate are greatly improved, and the adsorption capacity of the produced iron hydroxide is
It was discovered that the value is maximum in the H5 to H7 range, and the present invention was achieved.

That is, an object of the present invention is to provide an industrially advantageous method for purifying Glauber's salt solution.

Another object of the present invention is to efficiently remove heavy metals and economically recover high-purity Glauber's salt, which has high industrial utility value.

The above purpose is to add ferrous salt to an aqueous solution containing vanadium, and use a mineral acid to lower the pH of the solution to 3.5 or less.
Next, an alkaline substance is added to bring the pH of the liquid to 5 to 7, and the pH of the liquid is temporarily maintained, and then the pH of the liquid is adjusted to 10 or more with an alkaline substance, and a soluble calcium salt is further added to cause a reaction, and the generated precipitate is separated and removed. This can be achieved by a method for purifying mirabilite solution characterized by the following.

The significance of each step in the above method is summarized as follows: (a) After adding ferrous salt to the mirabilite solution, p) 13.5
Aim for sufficient ionization of ferrous salts and heavy metal ions such as vanadium as follows: (b) Maintain the pH at 5 to 7 to generate a sufficient amount of iron hydroxide to fully function the adsorption function; (c) Further, the pH is raised to 10 or more, and excess ferrous ions are completely insolubilized as hydroxide, while other heavy metals such as nickel and zinc are also completely insolubilized. (a) Further, soluble calcium salts are added. , coexisting so, "- to produce calcium sulfate and improve the physical properties of the produced insoluble matter.

In the present invention, the ferrous salt added to co-precipitate heavy metal ions such as vanadium in the sodium sulfate solution with ferrous hydroxide is, for example, ferrous chloride misulfate used in the conventional method. - Iron is suitable, and the amount of iron to be added is about 2 to 8 times the weight of the heavy metals present.

In addition, alkaline substances added for pH adjustment include:
Usually, milk of lime, caustic soda, etc. are used, and the time to maintain the pH at 5 to 7 is sufficient for the heavy metal ions to be adsorbed by the produced iron hydroxide, and 5 to 15 minutes is generally used.

Further, as the mineral acid, sulfuric acid, hydrochloric acid, nitric acid, etc. are used.

When the pH is raised to 10 or more, the soluble calcium salt added is, for example, calcium chloride, calcium nitrate, etc., and the amount added is in the range of 6 to 12 mol per 1 mol of the ferrous salt added. preferable.

In the present invention, vanadium can be removed to the extent that it can be used as various industrial raw materials by maintaining the iron hydroxide in a pH range of 5 to 7, where its vanadium adsorption capacity is at its maximum, for a certain period of time.

The precipitate that adsorbs heavy metals produced by the method of the present invention is mainly composed of iron hydroxide, but if soluble calcium salts are not added, the precipitate becomes bulky, has poor sedimentation properties, and has extremely poor filterability. By adding a small amount of soluble calcium salt while stirring, the sedimentation and filterability of the precipitate can be greatly improved.

This is because fine gypsum dihydrate is generated in the iron hydroxide crystals, and the iron hydroxide aggregates using these as nuclei, reducing the bulk, increasing the apparent specific gravity of the precipitate, and causing the gypsum particles themselves to become hydroxylated. The synergistic effect of weighing down the sediment improves the sedimentation properties and reduces the apparent volume of the sediment.

Furthermore, during filtration, the gypsum particles act as a filter aid, which improves the filtration compressibility of the precipitate and improves the filtration operation.

In general, the depleted sodium sulfate solution is recovered at a concentration of about 30% and recovered as anhydrous sodium sulfate by evaporation and concentration, but depending on the application, it may be used as it is as a low concentration solution (for example, about 15%).

In such a case, the viscosity of the solution is naturally low, so the sedimentation property is better than at a concentration of about 30%, but it is inferior to the case where soluble calcium salts are added, and it also has a lower settling property than when a soluble calcium salt is added. Since the solution volume is large and the overflow properties are poor, it is necessary to add soluble calcium salts for the overflow operation.

As described above, the present invention not only makes it possible to effectively remove vanadium and other heavy metals through relatively simple processing to produce highly pure Glauber's salt, but also
The present invention provides an industrially advantageous method that improves the filterability and settling properties of precipitates and achieves their effective separation.

The present invention will be explained in more detail below using examples.

Example 1 21 g of ferrous sulfate (heptahydrate) was added to a Na2SO4 concentration of 27.
8%, ■ Concentration: 38.6 ppfl.
After that, milk of lime (Ca09511/l) was added to adjust the pH of the solution to 6.1, which was maintained for 10 minutes.

Milk of lime was added to this liquid to make the liquid level 10.7, and Ca
290 ml of calcium chloride solution with a Cl2 concentration of 15% were added.

Take this slurry into a measuring cylinder and slurry liquid temperature 5
As a result of measuring the sedimentation rate at 3°C, the sedimentation rate was 1
．． The speed was 28m/hour.

The volume percentage of the precipitate after 30 minutes was 26.3%.

Further, p-filtration was carried out in a conventional manner, and the concentration of the p-solution was measured and found to be 0.24 pINIl.

Example 2 13g of ferrous sulfate (heptahydrate) was dissolved in Na2SO4 degree 15.2
%,■ Added to Glauber's salt solution 91 with a concentration of 17.3 ppIII while stirring, and then added sulfuric acid to adjust the pH of the solution to 2.8. Milk of lime (Ca095.!?/l) was added to the solution. The pH was set to 6.3 and held for 10 minutes.

Milk of lime was added to this solution to adjust the pH of the solution to 10.4.
6OTrLl of 5% calcium chloride solution was added.

When this slurry was rubbed on a cylinder IC and the sedimentation speed was measured, it was found to be 4.7 m/hour.

The slurry liquid temperature was 56°C.

Also, the volume percent of the precipitate is 8.5%, and the concentration of the p solution is 0.28.
It became p4.

Example 3 21 g of ferrous sulfate (heptahydrate) was added to a Na2SO4 concentration of 27
%, ■ Added with stirring to Glauber's salt solution 91 with a concentration of 35 ppIn, and then added hydrochloric acid to bring the pH of the solution to 3, and then
An 8% caustic soda solution was added to bring the pH of the solution to 6, which was maintained for 10 minutes.

Add caustic soda solution to this solution to adjust the pH of the solution to 10.5.
and 560 ml of 15% calcium chloride solution was added.

This slurry was placed in a cylinder and the sedimentation speed was measured and found to be 1.15 m/hour.

Also, the precipitation is 24% by volume, and the concentration of p liquid is 0.2F.
Met.

Example 4 Slurry treated in the same manner as in Example 3 was mixed with 11crI
Vacuumed with a Nutsche (degree of vacuum -400 Hg,
5CP paper used).

The amount of slurry completed in 3 minutes was 500 TLl.

On the other hand, when a slurry with no soluble calcium salt added was similarly passed through, only 10,011 Ll could be passed through in 3 minutes.

Comparative Example 1 (a) When no soluble calcium salt is added, (o
) When the pH is not maintained at 5 to 71, the conditions are the same as in Example 1 except for the case where no mineral acid is added. The sedimentation rate, sedimentation volume %, and concentration of the p solution were measured, and the results are shown in the table below. The following results were obtained.

Comparative Example 2 The same operation as in Example 2 was carried out except that soluble calcium salt was not added.
As a result of measuring the concentration of the offshore liquid (2), the sedimentation speed was 1.25 m/hour, the sedimentation volume % was 13.8%, and the concentration (2) was 0.25 folds.

Claims (1)

[Claims] 1. Add a ferrous salt to a vanadium-containing aqueous solution of Glauber's salt, lower the pH of the solution to 3.5 or less using a mineral acid, then add an alkaline substance to bring the pH of the solution to 5 to 7 and maintain it once.
A method for purifying a sodium sulfate solution, which further comprises adjusting the pH of the solution to 10 or more with an alkaline substance, further adding a soluble calcium salt to cause a reaction, and separating and removing a precipitate formed.