KR20020051011A - Apparatus and method for purifying Coke oven gas - Google Patents

Abstract

PURPOSE: A purifying equipment of cokes gas and method thereof is provided, which can remove hydrogen sulfide, ammonia and hydrogen cyanide efficiently by removing carbon dioxide prior to absorption of hydrogen sulfide. CONSTITUTION: The system comprises a hydrogen sulfide absorber(2), an ammonia absorber(3), a high purity ammonia solution tower(6), an ammonia distillation tower(7) and a condenser(17a). The purifying method of cokes gas(1) comprises the following steps; (i) absorb carbon dioxide by feeding condensate to the cooling water supplied to the cooling stage(11) at the bottom of hydrogen sulfide absorber(2) and by recirculating ammonia solution; (ii) feed the absorbent of carbon dioxide to recover of ammonium thiocyanate, followed by contact with acidic gas to be supplied with hydrogen sulfide and ammonium thiocyanate required to synthesize thiocyanic acid; and (iii) produce ammonium thiocyanate by oxidizing hydrogen sulfide, the absorbent of carbon dioxide being supplied with oxygen.

Description

Apparatus and method for purifying Coke oven gas}

The present invention relates to a coke gas purification apparatus and method, and more particularly to a coke gas purification apparatus and method capable of removing carbon dioxide and recovering ammonium thiocyanate.

Carbon dioxide is an interfering substance in the absorption of hydrogen sulfide in coke gas. It is advantageous in the gas purification process if it is absorbed and removed in advance. Also, if the ammonium thiocyanate has a certain degree of purity and can be obtained relatively easily, the gas purification efficiency is increased. At the same time, there is an advantage that other useful chemicals can be obtained.

Coke gas is a gas generated when coking coal to make coke, and it contains various components. Among them, ammonia, hydrogen sulfide, and hydrogen cyanide cause corrosion and closure of gas pipes or pollution when used as fuel gas. Therefore, it is lowered below a certain concentration and used as fuel.

Conventional methods used for refining coke gas have been used to first remove ammonia and absorb it in sulfuric acid solution or to decompose it in a decomposition furnace and dissipate it in the air.

At this time, when ammonia is decomposed in a cracking furnace, when ammonia and hydrogen sulfide are mixed, there is a problem that sulfur oxide is included in the cracking furnace gas and is discharged as a pollutant.

Another method of removing ammonia is the ginseng method, which is absorbed in a phosphate solution and recovered as pure ammonia.

However, hydrogen sulfide and hydrogen cyanide are more difficult to remove and cause pollution than ammonia, and numerous processes have been developed to remove them.

Hydrogen sulfide was started by absorbing and removing limestone, and the dry oxidation method of oxidizing iron and removing it with sulfur by using iron oxide as a catalyst, and the wet process were developed to overcome the disadvantages of the dry oxidation process. It has been developed to reduce the required labor and the purity of the sulfur produced.

In this method, hydrogen sulfide is absorbed in an alkaline solution and then removed by oxidizing hydrogen sulfide using an oxygen carrier. At this time, while absorbing hydrogen sulfide by the catalyst in the absorption tower to increase the hydrogen sulfide absorption rate, the catalyst is regenerated in the regeneration tower.

Representative of the wet hydrogen sulfide removal method is the Fummaks Process (Aromatics 27, 174, 1975) developed in Japan, which uses picric acid as an oxygen carrier catalyst and similar Takahax Process (Takahax Process, Chemical Economy & Engineering Review 2, 27, 1970) uses sodium naphthoquinone-2-sulfonic acid as a catalyst.

Other wet processes are those using similar oxygen carriers.

Since such a wet process uses an expensive catalyst, an oxidation and regeneration tower for regenerating the catalyst is required, and accordingly, side effects due to the accumulation of various compounds in the circulation process occur. Representative accumulations are thiosulfate and thiocyanate. In order to remove this, a part of the circulating fluid is removed and discarded, or a method of oxidizing thiosulfate to sulfate with hydrogen peroxide.

Another method to remove ammonia and hydrogen sulfide is the Feld Process (Feld Process, Gas Purification, 4th ed., Gulf Publishing Company, 484, Houston, 1985), which absorbs and removes ammonia and hydrogen sulfide simultaneously. It is necessary to use a relatively expensive polythionate to regenerate it, and because it oxidizes hydrogen sulfate to several steps through hydrogen sulfide, it does not solve the problem of controlling the complicated chemical reaction included in this process. I couldn't.

3 is a block diagram showing the configuration of a hydrogen sulfide and ammonia purification apparatus in coke gas using an ammonia solution.

A conventional method of removing hydrogen sulfide and ammonia is a process of removing hydrogen sulfide using an ammonia solution.

Such a method is currently widely adopted in Germany as a coke gas refining process. Hereinafter, a method of removing hydrogen sulfide using an ammonia solution will be described.

The coke gas 1 is cooled below 30 ° C. in the cooling stage 11 before entering the hydrogen sulfide absorption tower 2. The cooled gas is absorbed and removed in the hydrogen sulfide absorption tower 2 by a solution having a high ammonia concentration in which hydrogen gas, which is an acid gas, and hydrogen cyanide, are alkaline.

As the ammonia solution for hydrogen sulfide absorption, the high concentration ammonia solution obtained from the lower part of the high concentration ammonia solution production tower 6 plays a primary role. In addition, a solution containing a large amount of ammonia discharged from the lower part of the ammonia absorption tower 3 is used for hydrogen sulfide absorption.

The liquid generated under the hydrogen sulfide absorption tower (2) is charged to the high concentration ammonia solution production tower (6) so that the hydrogen sulfide is distilled and made into a high concentration ammonia solution by ammonia vapor supplied from the ammonia distillation column (7), and part of the solution. Is sent to the hydrogen sulfide absorption tower (2), and part of it is charged into the ammonia distillation tower (7) and all the distillable material at 100 ° C is distilled and then discharged to the bottom.

A part of the liquid discharged to the lower part of the ammonia distillation column 7 is used as the recycle liquid 8 for absorbing ammonia.

As the solution for absorbing ammonia, soft water 4 is used in addition to the recycle liquid 8. The coke gas condensate (5) used as the ammonia absorption liquid is condensate condensed in the coke gas (1) generated when coal is carbonized to produce coke. This is moisture contained in raw coal, and hot gas generated in the coke oven When cooled, it is condensed with tar and separated by tar and specific gravity, and then supplemented with ammonia absorption tower 3 through a filter to be used as ammonia absorption liquid.

However, phenol and many other organic substances and various pollutants are saturated, which plays a major role in increasing the concentration of various pollutants in the absorbent liquid.

The problem of this process is that cyan is absorbed together with hydrogen sulfide, which causes difficulties in wastewater treatment due to cyan remaining in the wastewater generated at the bottom of the ammonia distillation column (7).

In addition, even if the cyan and thiocyanate compounds contained therein are to be recovered, they are severely contaminated by various pollutants contained in the coke gas condensate (5) and contain a large amount of liquid at a very low concentration. There is a problem that needs to be addressed.

The conventional method for recovering thiocyanate compounds is to recover ammonium thiocyanate by supplying sulfur in the absorbent liquid of the cyan absorption tower. In this process, the solid sulfur must be dissolved in the solution and supplied. It is not currently used.

In addition, a process for absorption into sodium polysulfide solution and recovery to sodium thiocyanate (NaCNS) has recently been developed (Coke and Chemistry, No. 5, pp.20-22, 1984). Sodium polysulfide must be supplied, resulting in an increase in chemical costs and difficulty in operation.

In the case of carbon dioxide, not only does it interfere with the absorption of hydrogen sulfide, but also combines with ammonia to form a very stable compound in water, which acts as a burden in the distillation column during distillation and sometimes causes distillation failure.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a coke gas purification apparatus and method for improving the hydrogen sulfide absorption rate by lowering the concentration of the carbon dioxide acting in advance to interfere with the absorption of hydrogen sulfide. It is.

In addition, by lowering the carbon dioxide content in the absorbent liquid, energy saving and distillation efficiency during distillation are increased and at the same time, the thiocyanate compound is recovered in the form of ammonium thiocyanate salt without supply of solid sulfur or sodium polysulfide. The present invention provides a coke gas purification apparatus and method for facilitating recovery of ammonium thiocyanate by allowing ammonium thiocyanate to be recovered from a solution in a high concentration and relatively pure form.

1 is a block diagram illustrating a method for purifying hydrogen sulfide and ammonia in coke gas to which a pretreatment of carbon dioxide and a method for recovering ammonium thiocyanate according to the present invention are added.

Figure 2 is a block diagram showing an ammonium thiocyanate recovery group according to the present invention.

Figure 3 is a block diagram showing the configuration of a hydrogen sulfide and ammonia purification apparatus in coke gas using a conventional ammonia solution.

<< Explanation of symbols for main part of drawing >>

1.coke gas 2. hydrogen sulfide absorption tower

Ammonia absorption tower 4

5.Coke gas condensate 6 ... High concentration ammonia solution

7.ammonia distillation column 8 ... recycle absorbent

9 ... concentrated ammonia solution 10 ... heat exchanger

11 Cooling stage 12 Acid gas

13 ... CO2 absorber 14 ... Ammonium thiocyanate recovery machine

15 ... ammonium thiocyanate 16 ... oxygen

17 Distillate Condensate 17a Condenser

18 Carbon Dioxide 19 Acid Gas Absorption Tower

20 ... oxidation tower 21 ... evaporator

22 ... solid-liquid separator

In order to achieve the above object, the coke gas purification apparatus of the present invention comprises a condenser connected between a high concentration ammonia solution production tower and the heat exchanger; And an ammonium thiocyanate recoverer connected to the high concentration ammonia solution production tower and a condenser, connected to a purified coke gas supplied from the hydrogen sulfide absorption tower to the ammonia absorption tower, and supplied with a carbon dioxide absorption liquid.

In the above configuration, the ammonium thiocyanate recovery unit, the acid gas absorption tower to which the carbon dioxide absorption liquid and the acid gas is supplied; Oxidation tower is supplied with the liquid formed from the acid gas absorption tower and oxygen, and recycles the liquid generated by the reaction of the liquid and oxygen to the acid gas absorption tower; A portion of the remaining liquid that is recycled and supplied is connected to an evaporator for crystallizing ammonium thiocyanate and to be added to the evaporator to recover ammonium thiocyanate and to be added into a circulating fluid circulating between the acid gas absorption tower and the oxidation tower. It is preferred to include a solid-liquid separator.

The purification method using such a coke gas purification device is as follows.

That is, the coke purification method includes the step of absorbing carbon dioxide using the ammonia concentration by circulating the steam condensate discharged from the upper portion of the high concentration ammonia solution to the cooling water supplied to the cooling stage located at the bottom of the ammonia absorption tower; The carbon dioxide absorbing liquid discharged from the ammonia absorption tower is fed to the ammonium thiocyanate recoverer, and then contacted with the acid gas from which the condensate is removed to receive hydrogen sulfide and hydrogen cyanide necessary for the synthesis of thiocyanate; And the carbon dioxide absorbing liquid supplied to the ammonium thiocyanate is supplied with oxygen again to produce ammonium thiocyanate as the hydrogen sulfide is oxidized.

In the above method, it is preferable to keep the temperature of the cooling stage at 30 ° C to 35 ° C.

In the above-described method, the carbon dioxide absorbing liquid is supplied to the acid gas absorption tower of the ammonium thiocyanate recovering group, and then acidic gas is added to combine with the ammonium group in the carbon dioxide absorbing liquid; The liquid discharged after being combined with the ammonium group is sent to the oxidation tower so that hydrogen sulfide is oxidized in contact with oxygen, sulfur generated in the oxidation reaction is combined with hydrogen cyanide and ammonia to form ammonium thiocyanate; The supplied oxygen is supplied so that the amount of sulfur generated by reacting with hydrogen sulfide is supplied in an amount greater than the equivalent to react with hydrogen cyanide to form thiocyanic acid; The liquid generated by the oxidation reaction with the oxygen is recycled to the acid gas absorption tower and sent to the evaporator a part of the circulated amount to crystallize the ammonium thiocyanate; And the crystallized ammonium thiocyanate is further recovered ammonium thiocyanate through a solid-liquid separator, wherein the generated liquid is preferably added to the circulating fluid circulating between the acid gas absorption tower and the oxidation tower.

In the above method, the amount of liquid discharged to the evaporator is preferably less than 1/10 of the amount of circulating liquid circulating between the acid gas absorption tower and the oxidation tower.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. For reference, some of the reference numerals described in the present invention and the same reference numerals are used for the sake of a smooth description of the present invention.

1 is a block diagram illustrating a method for refining hydrogen sulfide and ammonia in coke gas to which a pretreatment of carbon dioxide and a method for recovering ammonium thiocyanate according to the present invention are added, and FIG. 2 is a thiou according to the present invention. A block diagram showing an ammonium cyanate recovery group.

1, a condenser 17a connected between a high concentration ammonia solution production tower 6 and a heat exchanger 10 in a conventional coke gas purification apparatus according to the present invention; And a high concentration of ammonia solution manufacturing tower 6 and a condenser 17a, connected to a purified coke gas furnace supplied from the hydrogen sulfide absorption tower 2 to the ammonia absorption tower 3, and a carbon dioxide absorbing liquid 13 Ammonium thiocyanate recovery 14 is supplied.

In particular, the ammonium thiocyanate recovery unit 14 includes: an acid gas absorption tower 19 to which the carbon dioxide absorbing liquid 13 and the acid gas 12 are supplied; Oxidation tower 20 for supplying the liquid and oxygen formed in the acid gas absorption tower 19, and recycling the liquid generated by the reaction of the liquid and oxygen to the acid gas absorption tower 19; Part of the recycled and the remaining liquid is supplied to the evaporator 21 and the evaporator 21 to crystallize the ammonium thiocyanate to recover the ammonium thiocyanate (15) and then the acid gas absorption tower 19 and It is preferable to include a solid-liquid separator 22 connected to be added into the circulating fluid circulating between the oxidation tower 20.

The present invention will be described in more detail as follows.

The steam condensate discharged from the upper portion of the high concentration ammonia solution manufacturing tower 6 is supplied to the cooling water supplied to the cooling stage 11.

When the condensate is discharged from the upper part, the steam close to 100 ° C condenses to about 80 ° C, and the condensate is recycled back to the top of the high concentration ammonia solution manufacturing tower 6, and the condenser 17a cools the vapor of 80 ° C to about 50 ° C The condensate 17 saturated with ammonia can be obtained here.

In this distillate condensate (17), since the ammonia is highly water soluble, ammonia is contained at a very high concentration in almost saturated state, and since hydrogen sulfide has a solubility as low as 1/100 of that of ammonia, the solution contains almost ammonia. Can be.

In addition, hydrogen cyanide is contained in a saturated state. When this condensate is introduced into the cooling stage 11 and circulated, carbon dioxide is absorbed due to the high ammonia concentration, and the temperature of the condensate is higher than the temperature in the other absorption tower, and the amount supplied is dependent on the cooling water circulation amount or the size of the cooling stage 11. Because of its small size, the residence time is very long, accounting for most of the absorption of ammonia and carbon dioxide, which makes it a more stable compound than hydrogen sulfide.

The carbon dioxide absorbing liquid 13 discharged here is supplied to the ammonium thiocyanate recoverer 14, where the condensate 17 is contacted with the acidic gas 12 from which the condensate 17 is removed, and hydrogen sulfide and hydrogen cyanide required for the synthesis of thiocyanic acid are received. When oxygen is supplied again, hydrogen sulfide is oxidized to produce ammonium thiocyanate (15).

Ammonium thiocyanate (15) manufacturing process is prepared through a process as shown in FIG.

That is, the carbon dioxide absorbing liquid 13 is supplied to the acid gas absorbing tower 19 where the acidic gas 12 is added to secure an amount capable of as much as possible bonding to the ammonium groups in the carbon dioxide absorbing liquid.

The discharged liquid is sent to the oxidation tower 20 so that hydrogen sulfide is oxidized in contact with oxygen 16, and sulfur generated in the oxidation reaction is combined with hydrogen cyanide and ammonia to produce ammonium thiocyanate.

Oxygen (16) supplied here minimizes the amount of excess sulfur by supplying an amount of sulfur generated by reacting with hydrogen sulfide to be slightly larger than the equivalent of reacting with hydrogen cyanide to form thiocyanic acid. The liquid is recycled to the acid gas absorption tower 19 and 1/10 of the circulated amount is sent to the evaporator 21 to crystallize the ammonium thiocyanate 15.

And, the reason for recycling about 10 times of the amount sent to the evaporator 21 is that the oxygen is spread evenly in the system so that the acid gas 12 is absorbed and oxidized at the same time, so that hydrogen cyanide and hydrogen sulfide in the acid gas absorption tower 19 To maximize absorption and to convert hydrogen cyanide and hydrogen sulfide as much as possible to ammonium thiocyanate (15).

The liquid exiting to the evaporator 21 is taken from the bottom of the acid gas absorption tower 19 so that the oxygen content in the liquid can be minimized.

Since hydrogen sulfide and hydrogen cyanide in the liquid react rapidly and have a large amount of circulating fluid, the concentration is low and only 1/10 of the circulating liquid is taken and sent to the evaporator, so the gas generated in the evaporator 21 is mainly carbon dioxide and unreacted ammonia, and evaporator is generated. Traces of impurities are removed in the ammonia absorption tower 3 because the vapor is mixed with the coke gas in front of the ammonia absorption tower 3.

In the evaporator 21, heat is applied to evaporate the water, and since ammonium thiocyanate (15) is much more stable than a compound of ammonia and carbon dioxide, ammonium thiocyanate (15) is almost free of impurities, where most carbon dioxide and thio Most of the ammonia, which cannot be bound in the form of cyanate, evaporates and flies away, and only the carbon dioxide separated takes up the majority because the ammonia, which was initially combined with carbon dioxide to form a compound, is fixed as ammonium thiocyanate (15). .

Carbon dioxide (18) generated here is included in the coke gas in front of the ammonia absorption tower (3) to enter the ammonia absorption tower (3). Thus, most of the carbon dioxide is absorbed by the cooling stage 11 before the hydrogen sulfide absorption tower, bypasses the hydrogen sulfide absorption tower 2, and is combined with coke gas to enter the ammonia absorption tower 3.

In the hydrogen sulfide absorption tower (2), the competition between carbon dioxide and hydrogen sulfide is alleviated, and the collection rate of hydrogen sulfide can be greatly improved, and the content of carbon dioxide which makes ammonia and a stable compound is small in the collection liquid containing hydrogen sulfide. It can prevent defects and save energy.

In addition, in the conventional method, when the temperature of the hydrogen sulfide absorption tower 2 rises, the absorption of carbon dioxide proceeds very rapidly, so that the hydrogen sulfide absorption efficiency deteriorates rapidly, but in the case of the present invention, after the carbon dioxide in the coke gas is removed in advance, the hydrogen sulfide absorption tower ( As it enters 2), even when the temperature rises, it is possible to maintain the hydrogen sulfide absorption rate by increasing the ammonia concentration in the absorbent liquid or increasing the flow rate of the absorbent liquid.

The liquid discharged from the evaporator 21 is treated in the solid-liquid separator 22 to recover ammonium thiocyanate 15, and the generated liquid is circulated between the acid gas absorption tower 19 and the oxidation tower 20. It is added to the liquid.

The amount of liquid supplied to the ammonium thiocyanate recovery unit 14 is equal to the amount of the distillate condensate 17 supplied to the cooling stage 11 and this amount is the amount supplied to the evaporator 21. Water is evaporated in the evaporator 21 to produce crystals of ammonium thiocyanate (15). The resulting crystals are produced as a product of ammonium thiocyanate (15).

In the cooling stage 11, mainly carbon dioxide is absorbed, and hydrogen sulfide and hydrogen cyanide are supplied from the acid gas 12, so that it is possible to obtain a high concentration of ammonium thiocyanate solution with relatively little contaminants.

EXAMPLE

Nitrogen gas was used as the base gas and the contents of carbon dioxide, ammonia, hydrogen sulfide, and hydrogen cyanide were 10 g / Nm3, 5g / N㎥, 5g / N㎥, and 1.5g / N㎥, respectively. The gas flow rate was 70 Nm 3 / h. The absorption towers were all packed towers. The high concentration ammonia solution manufacturing tower 6 used a 12 stage sieve tray and the ammonia distillation tower 7 used a 16 stage bubble cap tray. The flow rate circulated to the cooling stage 11 was 100 L / h, and the distillate condensate 17 introduced therein was supplied at an ammonia concentration of 150 g / L, flow rate 1 L / h.

The high concentration ammonia solution supplied to the hydrogen sulfide absorption tower 2 was supplied at 70 L / h at an ammonia concentration of 15 g / L and the recycle absorbing solution was supplied at 30 L / h. The soft water supplied thereto was 25 L / h and the coke gas condensate was 25 L / h. Steam used for distillation of the ammonia distillation column was saturated steam having an absolute pressure of 6 bar and a temperature of about 160 ° C.

After being absorbed by the cooling stage 11, the carbon dioxide content was lowered to 5 g / Nm 3 and entered the hydrogen sulfide absorption tower 2. The amount of crystal recovered with ammonium thiocyanate was about 400 g / h.

When the absorption tower temperature was maintained at 25 ° C., the concentration of hydrogen sulfide in the coke gas dropped to 0.05 g / Nm 3 after the final purification. The concentration of ammonia was 0.01 g / Nm 3. The concentration of hydrogen cyanide was 0.05 g / Nm 3. This is much lower than 0.2 g / Nm3 of hydrogen sulfide, 0.03 g / Nm3, and 0.1 g / Nm3 of hydrogen cyanide, which are values that can be obtained in the conventional manner without using a method of recovering ammonium thiocyanate.

As described above, the present invention can significantly improve the purification rate of hydrogen sulfide, ammonia, and hydrogen cyanide in coke gas as compared to the conventional method, and can recover a thiocyanate compound, which is a kind of contaminant, as a useful compound. As a result, waste water quality can be improved.

Claims (6)

The coke gas cooled by the cooling water of the heat exchanger is absorbed and removed by hydrogen sulfide and hydrogen cyanide from the hydrogen sulfide absorption tower, and the absorption liquid is made of high concentration ammonia solution from the high concentration ammonia solution manufacturing tower, and part of the hydrogen sulfide absorption tower is partially ammonia distillation tower. In the coke gas purification apparatus which is charged to, and a portion of the liquid discharged to the lower portion of the ammonia distillation column is used for absorption of ammonia in the ammonia distillation column as a recycle liquid, A condenser connected between the high concentration ammonia solution production tower and the heat exchanger; And An ammonium thiocyanate recoverer connected to the high concentration ammonia solution manufacturing tower and a condenser, connected to a purified coke gas supplied from the hydrogen sulfide absorption tower to the ammonia absorption tower, and supplied with a carbon dioxide absorption liquid. Coke gas purification apparatus comprising a. The method of claim 1, The ammonium thiocyanate recovery group, An acid gas absorption tower to supply the carbon dioxide absorption liquid and the acid gas; Oxidation tower is supplied with the liquid formed from the acid gas absorption tower and oxygen, and recycles the liquid generated by the reaction of the liquid and oxygen to the acid gas absorption tower; An evaporator in which a portion of the remaining liquid that is recycled is supplied to crystallize ammonium thiocyanate, and A solid-liquid separator connected to the evaporator to recover ammonium thiocyanate and then added to the circulating fluid circulating between the acid gas absorption tower and the oxidation tower Coke gas purification apparatus comprising a. Injecting steam condensate discharged from the upper portion of the high concentration ammonia solution production tower into the cooling water supplied to the cooling stage located at the bottom of the ammonia absorption tower and circulating to absorb carbon dioxide using the ammonia concentration; The carbon dioxide absorbing liquid discharged from the ammonia absorption tower is fed to the ammonium thiocyanate recoverer, and then contacted with the acid gas from which the condensate is removed to receive hydrogen sulfide and hydrogen cyanide necessary for the synthesis of thiocyanate; And The carbon dioxide absorption liquid supplied with hydrogen sulfide and hydrogen cyanide is supplied with oxygen again to produce ammonium thiocyanate as the hydrogen sulfide is oxidized. The method of claim 3, wherein Coke gas purification method characterized in that the temperature of the cooling stage is maintained at 30 ℃ to 35 ℃. The method of claim 3, wherein The carbon dioxide absorbing liquid is supplied to the acid gas absorption tower of the ammonium thiocyanate recovery unit, the acid gas is added to combine with the ammonium group in the carbon dioxide absorbing liquid; The liquid discharged after being combined with the ammonium group is sent to the oxidation tower so that hydrogen sulfide is oxidized in contact with oxygen, sulfur generated in the oxidation reaction is combined with hydrogen cyanide and ammonia to form ammonium thiocyanate; The supplied oxygen is supplied so that the amount of sulfur generated by reacting with hydrogen sulfide is supplied in an amount greater than the equivalent to react with hydrogen cyanide to form thiocyanic acid; The liquid generated by the oxidation reaction with the oxygen is recycled to the acid gas absorption tower and sent to the evaporator a part of the circulated amount to crystallize the ammonium thiocyanate; And The crystallized ammonium thiocyanate is recovered through ammonium thiocyanate again through a solid-liquid separator, and the generated liquid is added to the circulating fluid circulating between the acid gas absorption tower and the oxidation tower. Purification method. The method of claim 5, The amount of liquid discharged to the evaporator is a coke gas purification method characterized in that less than 1/10 of the amount of the circulating liquid circulating between the acid gas absorption tower and the oxidation tower.