JPS5833309B2 - Corrosion prevention method for wastewater treatment equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a corrosion prevention method for wastewater treatment equipment, and more particularly to a method for effectively preventing corrosion of wastewater treatment equipment by adding iron ions in advance to wastewater containing cyanide and hydrogen sulfide.

Wastewater discharged from heavy oil cracking equipment such as fluid catalytic cracking equipment, hydrocracking equipment, and hydrodesulfurization equipment contains cyanide, and equipment that processes that wastewater is subject to severe corrosion due to cyanide. .

Conventionally, known methods for preventing corrosion in wastewater treatment equipment include neutralizing cyanide with sodium polysulfide, or forming a film on surfaces subject to corrosion using amine-based corrosion inhibitors. There is.

However, the former method has little effect unless a large amount of sodium polysulfide is used; however, if used in a lucky amount, the corrosive nature of sodium polysulfide cannot be ignored, and at the same time, it becomes easy to block the sodium polysulfide pipes. Moreover, it has disadvantages such as precipitating sulfur when exposed to air and becoming cloudy when discharged into a rainwater system.

The latter method requires the use of a large amount of amine-based corrosion inhibitor, and has the disadvantage that the formed film is easily peeled off.

The present inventors have conducted extensive research in order to eliminate the drawbacks of such conventional methods and to develop a corrosion prevention method for wastewater treatment equipment that can be carried out simply and effectively.

In the course of this research, we investigated the mechanism of corrosion of equipment due to cyanide in wastewater, and found that corrosion of wastewater treatment equipment can be effectively prevented by adding iron ions to wastewater in advance, which led to the invention of the present invention. I was able to complete it.

That is, the present invention provides wastewater characterized in that, before introducing wastewater containing cyanide and hydrogen sulfide into a wastewater treatment device, iron ions are added in advance to the wastewater in an amount equal to at least the molar amount of cyanide contained in the wastewater. A method for preventing corrosion of processing equipment is provided.

In the present invention, cyanide refers to a compound that can dissociate in water to generate cyanide ions.

According to the research conducted by the present inventors, the mechanism of corrosion of treatment equipment by cyanide in wastewater is thought to be as follows.

First, wastewater contains hydrogen sulfide, which reacts with iron in the treatment equipment as shown in the following equation.

20Fe+H2S-+FeS+2H+(I) Originally, ferrous sulfide (Fed) forms a stable film in an alkaline region and the progress of corrosion is attenuated.

However, if cyanide is present in wastewater, the reaction of the following formula will proceed, and corrosion will continue to progress.

FeS+6CN−−+ Fe(CN)7 +S2−
(II) Iron ion (Fe2+) involved in these reactions
is eluted from wastewater treatment equipment, and the more intense the reaction is, the more severe the corrosion of the equipment is.

Therefore, in the method of the present invention, iron ions are added in advance to the wastewater before it is introduced into the treatment equipment.

This iron ion and hydrogen sulfide and cyanide in the wastewater react as shown in formulas (I) and (n) above to form a stable compound such as iron ion and cyanide such as Fe'(Fe(CN)6)3. A complex salt is formed and the compound no longer has corrosive potential.

By stabilizing the cyanide in the wastewater before introducing it into the treatment equipment, the equipment will not be corroded by the wastewater and can be operated continuously for a long time.

The amount of iron ions to be added to the wastewater must be the amount necessary to turn cyanide in the wastewater into a stable compound, and specifically, the amount of iron ions to be added to the wastewater must be at least equivalent to the amount of cyanide contained in the wastewater. mol, preferably 2 to 40 mol per mol of cyanide.

If the amount of iron ions added is less than the equivalent mole of cyanide, unreacted cyanide remains, which causes corrosion of the processing equipment.

On the other hand, if the amount of iron ions added is too large, all of the cyanide in the wastewater can be stabilized, which is effective in preventing corrosion of the equipment, but the surplus iron ions reduce the efficiency of wastewater treatment within the equipment. It doesn't drop and it doesn't rain.

Therefore, it is preferable to suppress the amount of iron ions added to at most about 40 moles per mole of cyanide in the wastewater.

Further, the type of iron ion to be added in the method of the present invention may be either ferrous ion (Fe'') or ferric ion (Fe''), or may be a mixture of both.

Furthermore, the iron ion source is not particularly limited and may be a solid such as scrap iron as long as it can supply ferrous or ferric ions, or an aqueous solution containing iron ions such as ferrous sulfate. Even liquids such as aqueous solutions can be used.

As described above, according to the method of the present invention, cyanide in wastewater is stabilized as ferrocyanate (for example, Fe (Fe (CN) 6) 3 ) or ferricyanate before being introduced into the treatment equipment. Therefore, corrosion hardly progresses even if the wastewater treatment equipment is operated continuously for a long period of time.

Furthermore, there is no need to worry about troubles such as clouding of the processing liquid during discharge, which may occur during operation of the apparatus.

Moreover, since the method of the present invention requires only an extremely simple operation of adding a predetermined amount of iron ions to wastewater in advance, it can be applied almost as is without changing existing equipment.

Therefore, if the method of the present invention is applied to the operation of various industrial wastewater treatment equipment containing cyanide, the equipment can be extremely effectively protected against corrosion.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 (see Figure 1) Wastewater discharged from a heavy oil fluid catalytic cracking unit (not shown) (hydrogen sulfide 2000 ppm, ammonia 1000 ppm)
Ferrous sulfate (FeS) with a concentration of 20φ was added from iron ion tank 1 to iron ion tank 1 (containing 100 ppm of cyanide (CN-)).
O4) By adding aqueous solution, iron ions in wastewater are reduced to 4000
It was set to ppm.

After the ferrous sulfide generated in the wastewater was removed by the filter 2, the wastewater was introduced into the wastewater stripper 3 under operating conditions of a temperature of 116° C., a pressure of 0.8, and 9/iG.

Next, the wastewater is passed from the top of the stripper 3 to a cooler 4.
There, after cooling, it was circulated to the waste water stripper 3 via the receiving tank 5.

Ammonia and other acidic gases were recovered and collected from the receiving tank 5.

The treated wastewater treated by the wastewater stripper 3 had a hydrogen sulfide content of 30 ppm or less, ammonia content of 50 ppm or less, and cyanide content of 1 ppm or less.

The wastewater was continuously treated under the above conditions for 6 months.
The desired results were achieved, with almost no corrosion observed across the entire plant (including pipes, etc.).

The material of the wastewater treatment equipment used here is that the cooler 4 and the tray inside the wastewater stripper 3 are made of stainless steel (SUS3).
04), and the others are carbon steel.

Example 2 (see Fig. 2) Wastewater having the same components as in Example 1 was first led to a scrap iron packing tower 6 and was treated with scrap iron (the amount of scrap iron was approximately 3 tons of lathe scraps, or 1 m3, per 1 ton/hour of waste water).

) was brought into contact with.

Next, this wastewater was introduced into the wastewater stripper 3, and the following operation was performed under the same operating conditions as in Example 1.

As a result, the treated wastewater contained less than 30 ppm of hydrogen sulfide, less than 50 ppm of ammonia, and less than 11) pm of cyanide.

When wastewater was continuously treated for three months under the above conditions, some thinning was observed in the cooler downstream piping (carbon steel) of wastewater stripper 3, but this was compared with the corrosion situation when no iron ions were added to the wastewater. The anticorrosion effect was clearly demonstrated.

Example 3 (See Figure 3) Iron ions prepared by electrolysis from the iron ion tank 1 were added to wastewater having the same components as in Example 1, so that the iron ions in the wastewater were 2000 ppm.

This wastewater was processed into wastewater sl-IJ under the same operating conditions as in Example 1.
It was introduced into Tuba-3.

Next, among wastewater treatment equipment, wastewater sl-IJ tube 3
Considering that the overhead system (cooler, piping) is severely corroded, 200% of iron ions are added to the top steam extracted from the top of the stripper 3 from the iron ion tank 1.
0 ppm was newly added, and after being led to the cooler 4 and cooled, ferrous sulfate and the like in the waste water were removed by the filter 2, and the waste water was circulated through the receiving tank 5 to the waste water bath I-IJ tube 3.

Ammonia and other acidic gases were recovered and collected from the receiving tank 5.

Hydrogen sulfide in the treated wastewater was 30 ppm or less, ammonia was 50 ppm or less, and cyanide was 11) pm or less.

Wastewater was continuously treated for three months under the above conditions, but almost no corrosion was observed throughout the wastewater treatment equipment.

[Brief explanation of the drawing]

1 to 3 are schematic diagrams of equipment for treating wastewater by applying the method of the present invention, respectively. In the figure, 1 is an iron ion tank, 2 is a filter, 3 is a wastewater stripper, 4 is a cooler, 5 is a receiving tank, and 6 is a scrap iron packed tower.

Claims (1)

[Claims] 1. When introducing wastewater containing cyanide and hydrogen sulfide into a wastewater treatment device, iron ions are added in advance to the wastewater in an amount equal to at least the mole of cyanide contained in the wastewater. Corrosion prevention method for wastewater treatment equipment. 2. The method according to claim 1, wherein the amount of iron ions added is 2 to 40 (molar ratio) to cyanide contained in the wastewater. 3. The method according to claim 1, wherein the iron ion is a ferrous ion and/or a ferric ion.