JPS5934644B2 - Fluorine removal method from waste sulfuric acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing fluorine from waste sulfuric acid, and particularly relates to a method for removing fluorine from fluorine-containing waste sulfuric acid, such as waste liquid discharged from a sulfur dioxide gas cleaning process in a sulfuric acid manufacturing process. related to methods for removing gypsum and recovering gypsum and other by-products.

In nonferrous smelting, most of the processed concentrate is sulfide, so exhaust gas containing sulfur dioxide gas is generated during various smelting processes including roasting.

Normally, sulfur dioxide gas is recovered from this exhaust gas and used as a raw material to produce sulfuric acid.

In the sulfuric acid manufacturing process, dust removal, cleaning, cooling, purification, and
A series of gas purification processes such as drying are performed.

Among these, dilute sulfuric acid is used as a cleaning liquid in the gas cleaning process.

This dilute sulfuric acid for cleaning absorbs impurities in the gas while being recycled and the impurities are accumulated there, so it is necessary to discharge a portion of it to the outside of the system.

Hereinafter, this discharged dilute sulfuric acid will be referred to as waste sulfuric acid.

By the way, the raw material concentrate used in nonferrous smelting contains trace amounts of fluorine, arsenic, etc. as impurities, and some of the arsenic and most of the fluorine are gasified during the smelting process.
It is carried into the sulfuric acid manufacturing process as part of the exhaust gas.

Therefore, these arsenic and fluorine dissolve in the dilute sulfuric acid used in the gas cleaning process, and accumulate up to a level of 3 to 10<9>/l in the waste sulfuric acid that is periodically extracted.

The sulfuric acid content of waste sulfuric acid is generally recovered as gypsum (CaSO, .2H20) using a neutralization reaction by adding an alkaline agent such as quicklime, calcium hydroxide, or calcium carbonate.

However, the gypsum obtained in this way is contaminated with impurities such as arsenic and fluorine.

Of these, there is no problem with arsenic, as there is an established removal technology using the sulfurization method, in which arsenic is made into arsenic sulfide by adding a sulfiding agent as a pre-process of the neutralization process, and then separated from house to house.However, with regard to fluorine, there is no problem. Appropriate removal methods have not been established.

Most of the gypsum produced is used as a raw material for cement, but when fluorine is mixed in, the hardening of cement is delayed.
Therefore, the fluorine content of gypsum should be 0.2% or less.

Therefore, when producing gypsum as a by-product from waste sulfuric acid, measures are needed to ensure that almost no fluorine is mixed into the gypsum.

To date, the aluminum addition method (Japanese Patent Publication No. 53-34119) has been proposed as a method for removing fluorine.
and iron (III) addition method.

The former method is impractical due to the cost of producing gypsum as the aluminum source is generally expensive; the latter method is also impractical.
There is a problem in that iron (1) is reduced by sulfur dioxide gas absorbed in waste sulfuric acid during use, becomes iron (II), and loses its effectiveness.

Therefore, it is difficult to adopt any of the conventional methods, and the development of a new fluorine separation method is awaited.

In view of this situation in the industry, it is an object of the present invention to provide a fluorine separation method that does not use expensive additives such as aluminum and can effectively prevent fluorine migration into gypsum.

The present invention involves converting fluorine into fluorosilicic acid (H2SiF6) by adding silicon dioxide to the waste sulfuric acid in advance before adding an alkaline agent to the waste sulfuric acid for gypsum production. It is characterized in that fluorosilicic acid is further converted into calcium fluorosilicate upon addition of Nodame's alkaline agent.

The calcium fluorosilicate produced has high solubility, so
Does not migrate into precipitated gypsum.

In this way, gypsum containing less than acceptable fluorine is recovered.

Thus, in one aspect, the present invention includes (a) adding a silicon dioxide-containing substance to waste sulfuric acid containing fluorine to convert fluorine into fluorosilicic acid; and (b) waste sulfuric acid containing fluorosilicic acid. The concentration of sulfuric acid in the solution is at least 220 g/l.
a step of converting fluorosilicic acid into calcium fluorosilicate at the same time as precipitating gypsum by adding C until the gypsum is precipitated; and a step of separating the precipitated gypsum from a liquid containing the remaining calcium fluorosilicate. Provided is a method for separating fluorine from waste sulfuric acid.

If necessary, in order to recover fluorine, following the above steps, step 1) add a substance containing sodium to the separated solution containing calcium fluorosilicate until the pH becomes 4 or higher to convert the calcium fluorosilicate into sodium fluorosilicate. It is also possible to recover fluorine as sodium fluorosilicate and commercialize it by additionally performing the step of converting into sodium fluorosilicate and (e) separating the produced sodium fluorosilicate from the remaining liquid. .

Hereinafter, the present invention will be specifically explained with reference to the flow sheet shown in FIG.

As mentioned above, the object of the present invention is waste sulfuric acid from the sulfur dioxide gas cleaning process of a sulfuric acid manufacturing factory, which usually has a
Contains 300 g/l sulfuric acid and 3-10 g/1
Contains 1 fluorine.

If arsenic is contained in the waste sulfuric acid, it is preferable to remove it in advance by a known method such as a sulfurization method at an appropriate point in the series of steps described below after straining.

The fluorine-containing waste sulfuric acid is first sent to a silica fluoridation step by addition of a material containing a source of silicon dioxide, such as silica stone or diatoms.

In this silicon fluoridation step, fluorine contained in waste sulfuric acid is converted to fluorosilicic acid (H2SiF6).

By the way, fluorine in waste sulfuric acid is generally thought to exist in the form of hydrogen fluoride, and some of it reacts with the acid-resistant bricks used in cleaning equipment and the glass fibers of glass fiber-reinforced plastic materials to form fluorine. It is estimated that it is made of silicic acid, fluoroaluminic acid, etc.

In any case, in this silicon fluoridation process, most of the fluorine in the waste sulfuric acid is in the form of fluorosilicic acid, including the fluorine that has already been converted to fluorosilicic acid before being brought into the process. It becomes like this.

The purpose of the fluorosilicate step is to produce fluorosilicic acid as a precursor for producing highly soluble calcium fluorosilicate in the next gypsum production step.

The waste sulfuric acid that has undergone fluoridation of the fluorine content is then sent to the gypsum manufacturing process.

In the gypsum manufacturing process, calcium-containing alkaline agents such as calcium carbonate, calcium oxide, and calcium hydroxide are added, and gypsum (Ca5O,.2H20) is produced by a neutralization reaction between these additives and sulfuric acid.

This reaction process itself is well known.

In the gypsum manufacturing process, at the same time as gypsum is produced,
Calcium fluorosilicate (CaSi
F6).

The solubility of calcium fluorosilicate is 1 at 22°C.
Since it is as high as 0.6 g/l, it does not precipitate together with gypsum and remains dissolved in the liquid.

Therefore, fluorine will not be mixed into the produced gypsum.

In this gypsum manufacturing process, the amount of alkaline agent added is very important.

That is, the amount of fluorine mixed into gypsum is affected by the concentration of residual sulfuric acid, and when the sulfuric acid concentration becomes 20 g/l or less, the amount of fluorine mixed into gypsum increases rapidly.

Therefore, neutralization with a calcium-containing alkaline agent reduces the free sulfuric acid concentration to a minimum of 20j! /l.

This fact will be explained with reference to the graph in FIG.

In the graph of FIG. 2, which is related to the examples presented later, curve A shows the relationship between sulfuric acid concentration and gypsum fluorine content.

The fluorine content in gypsum remains at an extremely low level with a sulfuric acid concentration of 207i711 or higher, but the sulfuric acid concentration is 209/l! It increases rapidly below.

The lower limit of the sulfuric acid concentration is slightly influenced by the reaction tank temperature, reaction tank throughput, type of alkali agent, etc., but generally 20 g/l is sufficient.

The upper limit of sulfuric acid concentration is not particularly restricted from the perspective of gypsum fluorine content, as shown in the graph, but a high sulfuric acid concentration means that the amount of gypsum produced will be smaller (and the use of neutralizing agent in the next process). This means that the amount added is large, and it is practically desirable that it be as low as possible.

Thus, the amount of alkaline agent added is practically controlled so that the free sulfuric acid concentration is close to 20 g/l.

For this purpose as well, it is preferable to add the alkaline agent in small batches.

After adding the required alkaline agent, the reaction solution is classified using an elutriation tank or a wet cyclone, and the fine crystals are recycled into the reaction tank.

The underflow from the cyclone is subjected to sedimentation and separation treatment using a thickener.

The settled gypsum spigot is sent to a centrifuge where it is recovered as product gypsum.

In this gypsum recovery process, the reaction solution is sedimented and separated using a thickener, the precipitated gypsum spigot is classified using an elutriation tank or a wet cyclone, the fine crystals are returned to the reaction tank, and the underflow is sent to a centrifuge and recovered as product gypsum. It is also possible to do so.

The recovered gypsum is of high quality and meets specifications for use as a raw material for cement, containing less than permissible levels of fluorine.

Through the above steps, the intended purpose of separating fluorine in waste sulfuric acid is achieved, and the waste sulfuric acid is separated into gypsum that does not contain fluorine and a liquid solution that contains fluorine.

In order to recover fluorine from this gypsum separation solution, this solution is subsequently sent to a defluorination step.

In the defluoridation process, the gypsum drained liquid is neutralized by adding a sodium-containing neutralizing agent, typically sodium hydroxide.

Calcium fluorosilicate is converted to sodium fluorosilicate (Na2SiF6) by addition of a sodium source.

It is important that the amount of sodium source added is such that the pH is 4 or higher.

At pH 4 or lower, the reaction for producing sodium fluorosilicate is not completed, and when the pH increases to 4 or higher, the reaction is completed, and the solubility of sodium fluorosilicate is quite large (although it is quite large by itself), and the coexisting salt of sodium ( For example, due to the Glauber's Salt effect, the amount becomes 0.029 wt% at 25°C, and sodium fluorosilicate can be precipitated.

However, if the pH becomes too high, other hydroxides such as iron will precipitate, so the preferred pH value is 4 to 5.

After neutralization, the liquid is treated with a wet cyclone or the like to separate sodium fluorosilicate, and recovered as crystals with a pit or the like.

If desired, the recovered sodium fluorosilicate can be decomposed with sulfuric acid to produce a product as hydrofluorosilicic acid (H2SiF6).

A portion of the liquid after crystal separation is returned to the neutralization tank if necessary, and the remainder is sent to another processing step and subjected to appropriate treatment.

Example 1 100 g of silica powder was added to 51 waste sulfuric acid containing 219 g/l of sulfuric acid and 8.59/IJ of fluorine while stirring,
Fluorine was oxidized to silicon fluoride.

Stirring was carried out by a magnetic stirrer and the reaction temperature was at room temperature.

After standing for 68 hours, this waste sulfuric acid 21 was taken up in a beaker, and while stirring, calcium carbonate was added at a rate of 19/l·min, and neutralized until the sulfuric acid concentration became 25.69/l.

After the addition was completed, the mixture was stirred for 10 minutes and then filtered with suction through 5°CF paper.

The fluorine content in the obtained gypsum was 0.05% on a dry weight basis, which was sufficiently acceptable as a product.

At the same time, the liquid sampled at each point during the addition of calcium carbonate was suction-filtered at 5° C. to collect gypsum.

These analysis results are shown in Table 1 together with the above results.

The above-mentioned curve A in FIG. 2 is a graph showing these results.

A case in which silica fluoride was not carried out in the reference cutting is also shown in Table 1 as a comparative example, and in FIG. 2 as curve B.

From a comparison of curves A and B, it can be seen that the silicate-hydrofluoric acid treatment that characterizes the present invention is effective in preventing the incorporation of fluorine into gypsum.

Example 2 Sodium hydroxide was gradually added to 1.51 g of the gypsum drained solution obtained in Example 1 with stirring until the pH reached 4.

After stirring the neutralized solution for 10 minutes after the addition, the solution was stirred at 5° (
Filter with suction using J' paper.

As a result of analyzing the obtained door paper after washing with water and drying,
It was confirmed that it was sodium fluorosilicate (Na2SiF6).

The purity was over 95%. As explained above, the present invention has developed a technology to easily and effectively separate fluorine from waste sulfuric acid by using inexpensive reagents, and will greatly contribute to the treatment of waste sulfuric acid in place of conventional methods. .

The gypsum produced as a by-product is of high purity and does not contain fluorine, and passes specifications as a raw material for cement.

[Brief explanation of drawings]

FIG. 1 shows a flow sheet of the method of the invention, and FIG.
The figure is a graph showing the relationship between sulfuric acid concentration and gypsum fluorine content in the gypsum manufacturing process.

Claims (1)

[Claims] 1. (a) A step of converting fluorine into fluorosilicic acid by adding a silicon dioxide-containing substance to waste sulfuric acid containing fluorine, and (0) an alkaline agent containing calcium to waste sulfuric acid containing fluorosilicic acid. The sulfuric acid concentration in the solution is at least 209/l.
a step of converting fluorosilicic acid into calcium fluorosilicate at the same time as precipitating gypsum by adding fluorine until A method for separating fluorine from waste sulfuric acid containing 2 (a) A step of converting fluorine into fluorosilicic acid by adding a silicon dioxide-containing substance to waste sulfuric acid containing fluorine, and (→ adding calcium to waste sulfuric acid containing fluorosilicic acid. The concentration of sulfuric acid in the solution is at least 20 g/IJ.
A step of converting the fluorosilicic acid into calcium fluorosilicate by adding the precipitated gypsum to precipitate the gypsum at the same time; a step of converting the calcium fluorosilicate into sodium fluorosilicate by adding a substance containing sodium to the solution containing sodium fluorosilicate until the pH becomes 4 or higher; (e) separating the produced sodium fluorosilicate from the remaining solution; A method for separating fluorine from fluorine-containing waste sulfuric acid comprising the steps of: