JPH0312305A - Method for recovering spent hydrochloric acid - Google Patents

Abstract

PURPOSE:To inexpensively recover high-quality hydrochloric acid from spent hydrochloric acid with no need for a dust collector or a dust collecting equipment such as a strainer at the time of recovering hydrochloric acid from a spent pickling soln. for a steel sheet, etc., by the roasting-type hydrochloric acid recovery method by allowing hydrochloric acid recovered in an absorption tower to stay in a solution tank for a fixed time and then cooling the hydrochloric acid. CONSTITUTION:Spent hydrochloric acid is decomposed into hydrogen chloride and iron oxide in a roasting furnace 1, and the hydrogen chloride is introduced into an absorption tower 3 to recover hydrochloric acid which is then cooled in a cooler 4. The following constitution is used in this roasting-type hydrochloric acid recovery method. Namely, the hydrochloric acid recovered in the absorption tower 3 is introduced into the solution tank 10, the solid iron oxide in the recovered hydrochloric acid is dissolved and removed, and then the hydrochloric acid is cooled in the cooler 4. The hydrochloric acid leaving the absorption tower 3 is allowed to stay in the solution tank 10 under the conditions to completely dissolve the solid iron oxide by the self-retaining heat (e.g. at 80 deg.C and >=10-15% hydrochloric acid concn. for >=5min retention time) and the solid iron oxide is almost completely dissolved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates primarily to a method for recovering hydrochloric acid from pickling waste liquid used in the pickling treatment of steel materials such as wire rods. The present invention relates to a method for recovering high-quality hydrochloric acid by removing solid iron oxide mixed in the recovered hydrochloric acid.

BACKGROUND OF THE INVENTION In the conventional cold rolling process of steel sheets and the like, pickling treatment with hydrochloric acid is performed as a pretreatment to remove surface scale. One of the methods to recover hydrochloric acid from this pickling waste liquid is
There is a roasting method for recovering hydrochloric acid.

This method utilizes hydrothermal decomposition, and its specific equipment includes a roasting furnace (1), a cyclone (2), an absorption tower (3), and the treatment process shown in Figure 2. It consists of a cooler (4) and a recovered hydrochloric acid tank (5).

In other words, waste hydrochloric acid is decomposed into hydrogen chloride and iron oxide in the roasting furnace (1), and iron oxide is taken out from the lower part of the furnace, while hydrogen chloride is mixed in the exhaust gas and flows out from the upper part of the furnace and is sent to the cyclone. It is introduced into the absorption tower (3) via (2).

In the absorption tower (3), industrial water and circulating water are sprayed onto hydrogen chloride to recover hydrochloric acid from the bottom.

The recovered hydrochloric acid is introduced into a cooler and cooled to a predetermined temperature, and a portion is recycled and used for spraying in the absorption tower.
The rest is sent to factories.

However, the recovered @acid may contain solid iron oxide that was not removed by the cyclone (2). This solid iron oxide gradually dissolves in hydrochloric acid, but the dissolution rate is slow, and if it remains undissolved in the recovered hydrochloric acid, there are concerns that it may accumulate in transportation piping or tanks, or wear out valves. In addition, it adheres to the surface of the pickled steel sheet, causing quality problems.

Therefore, it is necessary to remove or reduce the solid iron oxide in the recovered hydrochloric acid as much as possible.

Therefore, measures to reduce solid iron oxide in the recovered hydrochloric acid are to install a high-efficiency dust collector (for example, an electric precipitator) in the exhaust gas line at the entrance of the absorption tower (3), or to install a strainer in the recovered acid line. It is said that the method of installing iron oxide is effective for removing solid iron oxide.

However, it cannot be said to be a preferable method because the equipment such as an electrostatic precipitator or a strainer is expensive and requires a great deal of effort to maintain.

Problems to be Solved by the Invention In view of the above-mentioned circumstances, the present invention has been developed to effectively collect solid iron oxide using relatively simple equipment without requiring dust collection equipment such as a dust collector or strainer. The purpose of this paper is to propose a method for recovering waste hydrochloric acid that can be removed and high-quality hydrochloric acid can be recovered from the waste hydrochloric acid at low cost.

Means for Solving the Problems The present inventor has observed that solid iron oxide in hydrochloric acid dissolves in the hydrochloric acid due to self-retained heat, but the rate of dissolution depends on the residence time. It was found that securing this residence time before cooling is effective in reducing solid iron oxide.

Based on this knowledge, we adopted a method of temporarily introducing hydrochloric acid recovered from the absorption tower into the dissolution tank as a method to ensure the residence time until the solid iron oxide melts with its own heat.

That is, in the roasting-type hydrochloric acid recovery method, the present invention first introduces hydrochloric acid recovered in an absorption tower into a dissolution tank,
The gist of this method is to melt and remove solid iron oxide in recovered hydrochloric acid using its own heat, and then cool it.

Function In the torrefaction type hydrochloric acid recovery equipment, the hydrochloric acid recovered in the absorption tower usually has a temperature of 90°C and a hydrochloric acid concentration of 17 to 19%. The conditions for the solid iron oxide mixed in this recovered hydrochloric acid to completely dissolve due to its own heat are as follows: temperature is 80'C or higher, hydrochloric acid concentration is 10
The test results revealed that the retention time was 15% or more and the residence time was 5 minutes or more.

Therefore, the hydrochloric acid leaving the absorption tower is
Since the salt content M'a degree) is satisfied, the solid iron oxide can be almost completely dissolved by ensuring the residence time of the recovered hydrochloric acid in the dissolution tank.

Note that the dissolution tank is not particularly limited;
Any tank that can maintain the temperature of the recovered hydrochloric acid leaving the absorption tower may be used.

Embodiment FIG. 1 shows the flow of waste hydrochloric acid processing equipment showing an embodiment of the present invention, in which (10) is a dissolution tank, (11) is a hydrochloric acid concentration meter, and V+ and V2 are on-off valves.

That is, as mentioned above, the waste hydrochloric acid is first transferred to the roasting furnace (1).
The hydrogen chloride is decomposed into hydrogen chloride and iron oxide, and the hydrogen chloride is introduced into the absorption tower (3) via the cyclone (2) together with the exhaust gas.

The hydrochloric acid recovered in the absorption tower is put into a dissolution tank (10),
Remain for a predetermined time. During retention in this tank, solid iron oxide mixed in the recovered hydrochloric acid is dissolved by its own heat.

The recovered hydrochloric acid retained in the dissolution tank (10) for a predetermined period of time is recovered to the recovered hydrochloric acid tank (5) via the cooler (4).

Note that when the circulating acid concentration is low, solid iron oxide is deposited at the bottom of the dissolution tank (10), and it takes time for the hydrochloric acid recovery equipment to start up. Therefore, in order to shorten the rise time as much as possible, a bypass pipe (8) was installed in the dissolution tank (l○), and a hydrochloric acid concentration meter (9) was installed at the outlet of the cooler.
Open/close valve installed in the bypass pipe when the specified hydrochloric acid concentration is reached v
3 and open the on-off valves V+ and V2, stable start-up is possible in a short time.

Next, pickling waste liquid (Fe (1232%, HCj 0.
5%) was treated using the equipment flow shown in FIG. 1, the results are shown below.

In this example, 4000 kCJ+ of the waste hydrochloric acid was decomposed into hydrogen chloride and iron oxide in a roasting furnace, and the obtained hydrogen chloride was sprayed in an absorption tower to recover 3 aooh,'h of about 18% hydrochloric acid. The recovered hydrochloric acid had a temperature of 92°C and a hydrochloric acid concentration of 18%.

Met.

As a result of investigating the content of solid iron oxide in the hydrochloric acid obtained by allowing the recovered hydrochloric acid to stay in the dissolution tank for 15 minutes, the content of solid iron oxide was found to be 0.2 to 0.4 wt% before the implementation. However, after implementation, the amount was reduced to almost nothing.

As described in detail, according to the method of this invention, the content of solid iron oxide in recovered hydrochloric acid can be significantly reduced, so high quality hydrochloric acid can be recovered from waste hydrochloric acid, and transportation piping and Valve wear can be significantly reduced, and quality problems with pickled steel sheets are also eliminated.

Further, the present invention can be easily applied to existing equipment, and the equipment cost is not high, which is a great effect on the treatment of pickling waste liquid.

[Brief explanation of the drawing]

FIG. 1 is an equipment flow diagram showing one embodiment of the present invention. FIG. 2 is a diagram showing a conventional equipment flow diagram.

Claims (1)

[Claims] In a roasting-type hydrochloric acid recovery method in which waste hydrochloric acid is decomposed into hydrogen chloride and iron oxide in a roasting furnace, hydrogen chloride is introduced into an absorption tower to recover hydrochloric acid, and the recovered hydrochloric acid is cooled in a cooler. ,
A method for recovering waste hydrochloric acid, comprising introducing the hydrochloric acid recovered in the absorption tower into a dissolution tank, dissolving and removing solid iron oxide in the recovered hydrochloric acid, and cooling the recovered hydrochloric acid in a cooler.