CN1962031A - Method for removing sulfur dioxide and dust from coal and oil burning flue gas - Google Patents

Abstract

The invention relates to a method for removing low-density SO2 and powder from the smoke of coal with sodium citrate method, and converting the SO2 into sulfur flour, wherein the pH value of sodium citrate buffer solution can be controlled at 4.0-4.5, the density of Ci3- is controlled at 0.1-0.5mol.1-1, while the 0.225mol.l-1 is best. The invention can be used to treat the tail gas from burning coal. And the reacted solution can be repeated used, and the content of sulfate radical ion is reduced more than traditional method.

Description

From the coal and oil flue gas, remove the method for sulfur dioxide and dust

Technical field

The present invention relates to a kind of sodium citrate buffer of utilizing and remove low concentration SO in the flue gas ₂With dust and with SO ₂Change into the method for Cosan, the present invention is particularly useful for handling coal and oil institute discharge tail gas, wherein SO ₂Concentration is usually less than 2000ppm, preferably in the 30-1500ppm scope, more preferably in the 40-1000ppm scope.

Background technology

The natrium citricum method is to be put forward by Norway and Sweden scientist the seventies, referring to JamesFarrington, and Sune Bengtsson.Citrate solution absorbs SO ₂[J] .chemicalengineering, June 16,1980:88～89; Olav Erga.SO ₂Recovery by a sodiumcitrate solution scrubbing [J] .Chemical EngineeringScience, 1980,35:162～169; Binay K.Dutta, Ranjan K.Basu, Amit Pandit, and Parthasarathi Ray.Absorption of SO2 in Citric Acid-Sodium CitrateBuffer Solutions [J] .Industrial Engineering chemical research, 1987,26:1291～1296.It is reported that it can remove the SO more than 90% in the flue gas ₂, saturated absorption liquid is regenerated by steam pyrolysis, can produce the SO that concentration is 25%-90% ₂, be used to produce sulfuric acid or liquid SO ₂But on the traditional sense, this sulfur method can only be used for handling greater than 3000ppm, is preferably greater than 3% high concentration SO ₂, and have the problems such as a large amount of generations of side reaction product sodium sulphate.The domestic first cover natrium citricum method is produced the liquid SO of 500t/a ₂Being installed on nineteen eighty-two builds up in second chemical plant in Changzhou, and the furnace gas that is used to absorb after sulfuric acid plant purifies (contains SO ₂About 7%), absorptivity reaches more than 99%, and this factory is also to SO ₂The waste gas of content 3%-12% has carried out commerical test, has all reached satisfied absorptivity, referring to: Zhang Jinsheng, " the natrium citricum method is produced liquid SO ₂Theoretic discussion and domestic manufacturer's production case study ", " sulfuric acid industry ", 1998 the 3rd phases: 3-6 page or leaf; Xu Guohong, " handling the production practices of tail gas with the natrium citricum method ", " sulfuric acid industry ", 1998 the 6th phases: 35-38 page or leaf; Zhang Songbai, " domestic natrium citricum legal system liquid SO ₂Defective ", " sulfuric acid industry ", the 3rd phase of nineteen ninety-five: 40-42 page or leaf.Thereafter, also used this method in a plurality of industrial and mining enterprises such as smeltery, Fuchunjiang River, Hangzhou, Wuhan City sulfuric acid plant and Suzhou fine chemistry industry Group Co.,Ltd, referring to: hole good and honest virtue, " the natrium citricum method is produced sulfur dioxide liquid ", " sulfuric acid industry ", the 3rd phase of calendar year 2001: 50-52 page or leaf.

The nontoxic free from extraneous odour of natrium citricum, nonflammable, production operation safety, three-waste free discharge.Have the advantage that desulfurization degree height, desulfurization product can be realized resource, meet very much the scientific idea of the recycling economy of being advocated at present.But this method generally only is used for SO ₂On the sulfuric acid that content is higher and the treating tail gas of metallurgy industry, for the low SO that coal and oil discharged ₂The concentration industrial smoke, the present case history that there is no application success both at home and abroad.If the natrium citricum method can be on the coal and oil boiler smoke be administered application success, will widen this The Application of Technology scope, make its technical merit bring up to a new step.Coal and oil institute discharged flue gas is different from sulfuric acid purification tail gas and smelts waste gas, the not only SO in the flue gas ₂Concentration is lower, is usually less than in the 2000ppm scope, have only tens ppm when minimum, and flue-gas temperature is higher, and has the dust that measures catalysed oxidn greatly, and this has not only increased the difficulty of desulfurization, also makes the SO that can absorb ₂The easier sulfate that is oxidized to, thus SO caused ₂The regeneration difficulty and the consumption of natrium citricum excessive.How overcoming these problems, avoid the generation of side reaction in the sweetening process, is a difficult problem of realizing this method industrial applications on the coal and oil boiler.

Summary of the invention

The objective of the invention is according to component specific in the coal and oil discharged flue gas and actual conditions, provide a kind of and from the coal and oil discharged flue gas, remove SO ₂With dust and with SO ₂Change into the method for Cosan, thereby realize the industrial applications of natrium citricum method aspect coal and oil flue gas desulfurization.The present inventor has finished the present invention through unremitting research.

Particularly, the present invention relates to a kind ofly from the coal and oil discharged flue gas, remove SO ₂With dust and with SO ₂Change into the method for Cosan, SO in the wherein said flue gas ₂Concentration be lower than 2000ppm, preferably in the 30-1500ppm scope, more preferably in the 40-1000ppm scope.Described method comprises utilizes sodium citrate buffer solution to absorb SO ₂Step, utilize ultrafilter to remove the step of dust and utilize H ₂S is with the SO in the rich solution ₂Change into the step of Cosan.Absorb SO at the described sodium citrate buffer solution that utilizes ₂Step in, the pH of described sodium citrate buffer solution in the 4.0-4.5 scope, [Ci ^3-] concentration at 0.1-0.5mol.l ^-1In the scope, preferred [Ci ^3-] concentration be 0.225mol.l ^-1The aperture that is preferred for the ultrafilter of dedusting is 0.1～0.3 μ m, more preferably 0.2 μ m.In a preferred embodiment of the invention, utilizing H ₂S is with SO ₂Change in the step of sulphur powder H ₂S and SO ₂Before the reaction, the pH value of rich solution is 4.0.

The specific embodiment

Below from the control of absorption liquid pH value and citrate ion concentration, the selection of dust arrester and the SO the rich solution ₂This tripartite the present invention of facing of reaction condition who generates Cosan by liquid Claus method discusses in detail.

The control of absorption liquid pH value and citrate ion concentration

Utilize the natrium citricum method to absorb SO ₂Be the buffering effect of performance absorption liquid, make at SO ₂In the absorption process pH value be unlikely to descend too fast, thereby absorb more SO ₂This absorption process can be undertaken by following reaction:

SO ₂(g)SO ₂(l) (1)

SO ₂(l)+H ₂OH ⁺+HSO ₃ ^- (2)

HSO ₃ ^-H ⁺+SO ₃ ^2- (3)

H ⁺+ Ci ^3- HCi ^2-(Ci represents citrate ion) (4)

H ⁺+HCi ^2-H ₂Ci ^- (5)

H ⁺+H ₂Ci ^-H ₃Ci (6)

Wherein reaction (1), (2) and (3) expression SO ₂Dissolving and ionization reaction, the ionic dissociation equilibrium of citrate is represented in reaction (4), (5) and (6).

At flue gas SO ₂In the absorption system, there is Na ₂SO ₃, NaHSO ₃And NaH ₂Ci, Na ₂HCi and Na ₃Ci two big sodium salt buffer solution systems.Because 0.1mol.l ^-1SO in the natrium citricum saturated absorption liquid ₂Concentration generally have only 4～10 grams per liters (about 0.1mol.l ^-1), still 0.1mol.l can calculate 25 ℃ the time ^-1The pH value of following sodium salt solution.

(1), Na ₂SO ₃PH value of aqueous solution (K ⁰ ₂=6.16 * 10 ^-8)

$\[{\mathrm{OH}}^{-}\]=\sqrt{\frac{{K_w}^0}{{K_2}^0}\×0.1}=1.27\×{10}^{-4}$ pH＝14-pOH＝10.11

(2), NaHSO ₃PH value of aqueous solution (K ⁰ ₁=1.29 * 10 ^-2K ⁰ ₂=6.16 * 10 ^-8)

$\left[H^{+}\right]=\sqrt{{K^0}_1\×{K^0}_2}=2.82\×{10}^{-5},$ pH＝4.55

(3), Na ₃Ci pH value of aqueous solution (K ⁰ ₃=4 * 10 ^-7)

$\left[{\mathrm{OH}}^{-}\right]=\sqrt{\frac{{K_w}^0}{{K_3}^0}\×0.1}=5\×{10}^{-5}$ pH＝14-pOH＝9.7

(4), Na ₂HCi pH value of aqueous solution (K ⁰ ₁=7.4 * 10 ^-4K ⁰ ₂=1.73 * 10 ^-5K ⁰ ₃=4 * 10 ^-7)

$\left[H^{+}\right]=\sqrt[3]{{K^0}_1\×{K^0}_2\×{K^0}_3}=1.72\×{10}^{-5},$ pH＝4.76

(5), NaH ₂Ci pH value of aqueous solution (K ⁰ ₁=7.4 * 10 ^-4K ⁰ ₂=1.73 * 10 ^-5)

$\left[H^{+}\right]=\sqrt{{K^0}_1\×{K^0}_2}=1.13\×{10}^{-4},$ pH＝3.95

From above calculating as can be seen: if select Na for use ₃Ci solution (0.1mol.l ^-1, pH=9.7) absorb SO ₂Although can obtain assimilation effect preferably, because NaHSO ₃The pH of solution has only 4.55, therefore, has a large amount of with SO ₃ ^2-The SO that form exists ₂Can not be decomposed.And select NaH for use ₂Ci solution (0.1mol.l ^-1, pH=3.95), although NaHSO ₃In SO ₂Can decompose fully, but because the pH value is too low, absorptivity is limited.Na ₂The pH=4.76 of HCi solution is a bit larger tham 4.55, still has the undecomposed NaHSO of small part when using separately in the solution ₃Exist, the pH value that only makes stoste could guarantee that solution had both had higher SO about 4.5 ₂Uptake can make the SO that resolves in the solution of back again ₂Few as much as possible, to avoid HSO ₃ ^-Circulate repeatedly and round oxidation.

Proportioning test by 25 ℃ of following sodium citrate buffer solutions is found (table 1): as citric acid (C (H ₃Ci.H ₂O, molecular weight 210.14) be 0.1mol.l ^-1The time, natrium citricum (Na ₃Ci.2H ₂O, molecular weight 294.12) addition and correspondingly the pH value following Changing Pattern is arranged:

Table 1 sodium citrate buffer pH value is along with the situation of change of different quality proportioning

W (H3Ci.H2O) *Gram	C (H3Ci.H2O)mol.l -1	W (Na3Ci.2H2O) *Gram	C (Na3Ci.2H2O)mol.l -1	The pH value
					21 21 21 21	0.1 0.1 0.1 0.1	30 35 38 40	0.101 0.110 0.120 0.130	4.33 4.40 4.48 4.52

^*Weight number (gram) in every liter of solution

As seen sodium citrate buffer will reach pH=4.5, when citric acid concentration is 0.1mol.l ^-1The time, sodium citrate concentration is 0.125mol.l ^-1, the molecular formula of forming the natrium citricum mixture is Na _1.7H _1.3Ci, [Ci ^3-]=0.225mol.l ^-1, citric acid content 21 grams per liters wherein, natrium citricum 39 grams per liters.In a preferred embodiment of the invention, [Ci ^3-] concentration at 0.1-0.5mol.l ^-1In the scope, preferred [Ci ^3-] concentration be 0.225mol.l ^-1If absorbed SO ₂Sodium citrate buffer solution circular flow overlong time, then be unfavorable for SO ₂Stable (can change into sulfate), and feed H ₂S is with SO ₂The best pH of reaction that is converted into Cosan is 4.0 (seeing description hereinafter for details), therefore adopt the sodium citrate buffer of low concentration to help to shorten the circular flow time, quickly pH is reduced to 4.0 from 4.5, utilizes ultrafilter to remove behind the grain dust then by adding H ₂S is with SO ₂Be converted into Cosan.Adopt the sodium citrate buffer of higher concentration then can cause pH to descend slowly, the circular flow overlong time makes more SO ₂Change into sulfate.

The inventor adopts the low concentration SO of natrium citricum method to 1 35 tons of boiler emission of Ci County, Hebei province coal gangue power generation branch company of six directions Industrial Co., Ltd ₂Carry out flue gas desulfurization and dedusting, the citrate ion concentration in the experimentation is 0.225mol.l ^-1(be equivalent to W (H ₃Ci.H ₂O)=21Kg/M ³, W (Na ₃Ci.2H ₂O)=39Kg/M ³), actual pH value of solution=4.48 that record, 10:10 start in morning, afternoon, 15:55 finished, and moved 5 hours 45 minutes altogether, initial cycle liquid measure 9M ³, circulating fluid volume 5.8M during end ³SO during terminal point ₂Solubility in circulation fluid is 4.35g/L.Circulating absorption solution density 1.05～1.08g/cm during terminal point ³, average 1.07g/cm ³, solution viscosity η=0.758 * 10 ^-3Pa.S.

Operating condition record sheet in table 2 absorption process

Sample number into spectrum	Sample time	The absorption liquid pH value	SO in the absorption liquid 2Content (g/l)	Flue gas desulphuization rate (%) *
					1	10:25	4.45	0.576	91.2% (Measuring Time: 10:25)
2	10:40	4.32	1.286	86.51％
					3	10:55	4.09	2.43	76.7% (Measuring Time: 11:00)
4	11:10	4.07	2.18	70.5% (Measuring Time: 11:15)
					5	11:25	4.05	2.38	62.26％
6	11:40	4.03	2.98	58.28％
					7	11:55	4.03	2.11	58.28％
8	12:10	3.99	4.35	54.22％
					9	12:25	3.99	3.20	54.22％
10	12:40	3.99	3.59	54.22％
					11	12:55	4.00	3.84	55.23％
12	13:10	3.94	4.22	50.33％
					13	13:25	3.95	4.16	51.18％
14	13:45	3.92	3.712	48.64％
					15	14:15	3.90	3.648	46.97％
16	14:30	3.89	3.904	45.83％
					17	15:05	3.91	3.584	47.55％
18	15:20	3.82	3.392	30.70% (Measuring Time: 15:20)
					19	15:35	3.79	4.224	30.03％
20	15:55	3.75	4.352	29.67％

^*That Measuring Time is arranged is the result of actual measurement, and other is for passing through the simulation equation result calculated.

By above experimental result as can be seen: flue gas SO ₂The 91.2% (time: 10:25) slowly drop to final 29.67% (time: 15:55) of absorptivity when just beginning.Move after 3 hours 15 minutes (13:25) SO ₂Solubility reach capacity basically (4.16g/L), this moment the pH=3.95 in the solution, SO ₂Absorptivity be 51.18%.Thereafter, SO ₂Solubility in circulation fluid slowly descends, (during 15:55) SO during terminal point ₂Solubility also has only 4.224g/L, and desulfurization degree is about about 30%.As seen, sodium citrate buffer absorbs SO ₂Substantially reach capacity near pH=4.00, make total flue gas desulphuization rate greater than 70%, the pH of absorption liquid should be not less than 4.00 (flue gas desulphuization rate of pH at 4.00 places is 55.23%).Therefore, utilizing sodium citrate buffer solution to absorb SO ₂Step in, the pH of sodium citrate buffer solution should be in the 4.0-4.5 scope.

Utilize sulfate ion content in the ion chromatography solution, the results are shown in Table 3.As seen, the content of sulfate ion is starkly lower than traditional in order to produce liquid SO ₂The natrium citricum method, the content of sulfate ion only is about 1/3rd of conventional method.

Sulfate ion content list in the table 3 ion chromatography solution

(proportion d=1.07 gram per centimeter ³)

Sample number	Sample time	Original solution concentration	Be converted into volumetric concentration
				12 20	15:20 15:55	577.35ppm 641.62ppm	0.617g/L 0.686g/L

In whole operational process of craft, if the sulfate ion concentration in the circulating absorption solution is too high, then separate out and occluding device with the form of sodium sulphate easily, therefore, the present invention is because the sulfate ion that generates is less and especially favourable.

The selection of dust arrester

According to the preferred embodiments of the invention, the aperture of ultrafilter that is used to remove the fine particle thing is preferably 0.2 μ m in the scope of 0.1-0.3 μ m.

Coal and oil factory only adopts the method for electrostatic precipitation that dust in flue gas is implemented effectively control usually, though design efficiency of dust collection＞99.7%, catch particle diameter 〉=0.1 μ m, but in the actual mechanical process, electrostatic precipitator can only be removed the bigger dust of particle diameter, and can not remove the fine particle thing (micron order) in the flue gas.And method of the present invention also comprises and utilizes ultrafilter to remove dust tiny in the flue gas, and the aperture of ultrafilter is preferably 0.2 μ m in the scope of 0.1-0.3 μ m.Utilize TH-880VI type intelligence parallel sampling instrument of flue dust and the telescopical detection of DW10-II type Lin Geman to show: behind electrostatic precipitation, air-introduced machine exit dust content is 120～123mg/Nm ³, have only 47～120～123mg/Nm by the content of dust in the flue gas of washing back, absorption tower ³, have only 47～51mg/Nm by the content of dust in the flue gas of washing back, absorption tower ³, average dust removal efficiency 64.66%, lingemann blackness is less than 1.

By with the JL-1155 particle size analyzer to the mensuration (table 2) of moving dust size in the sodium citrate buffer after 5.5 hours as can be known: accumulating 50% particle diameter is 0.95 μ m, and accumulating 90% particle diameter is 1.29 μ m; Reach filtration to dust 100%, the aperture of filter should be smaller or equal to 0.2 μ m, but the aperture of filter is too little, then can air pressure excessive, therefore according to the preferred embodiments of the invention, the aperture of ultrafilter that is used to remove the fine particle thing is preferably 0.2 μ m in the scope of 0.1-0.3 μ m.

The measurement result of dust granularity in table 4 flue gas

Unit (μ m)	Cumulative percentage rate (%)	Unit (μ m)	Cumulative percentage rate (%)
				0.2	0	0.8	2.67
0.3	1.76	0.9	29.52
				0.4	2.67	1.0	67.75
0.5	2.67	1.2	100
				0.6	2.67	1.4	100

In a preferred embodiment of the invention, the ceramic core filter of having selected Zhongcai Advanced Materials Co., Ltd's research and development for use is as ultrafilter, and its aperture is 0.2 μ m, by trial run, has reached good filter effect.

SO in the rich solution ₂Generate the reaction condition of Cosan by liquid Claus method

The method according to this invention, in the citric acid solution system, the SO in the rich solution ₂, select direct feeding H for use ₂S produces the technology path of Cosan.Former study is found: utilize H ₂S is very complicated with the reaction that sulphite is reduced into Cosan, has a large amount of rudimentary polythionate and SO in the reaction product ₂Reaction forms senior polythionate, when thiosulfate and polythionic acid reaction, can generate sulphur, this system height complexity, and do not understand fully as yet, referring to: " exhuast gas desulfurization ", [U.S.] A.V. Si Rec, Shanghai City Light Industry Designing Institute technical information group, Shanghai Tongji University heating ventilation teaching and research room, China Construction Industry Press, liquid pH value was at 3.3～3.7 o'clock, and the productive rate of sulphur can reach more than 90%, and side reaction can be controlled, and can under closed environment, make Cosan, and be unlikely to cause the accumulation of unfavorable byproduct, but the pH value is low more, SO ₂Removal efficiency low more, SO ₂Overflow from solution system easily, it is multifactor to take all factors into consideration above storage, rich solution and H ₂The S reaction pH value is best when left back 4.0, and this moment, the productive rate of sulphur was left back 80%.Therefore absorbing SO ₂Process in, when the pH of natrium citricum rich solution value reduces to 4.0, just rich solution is separated from the circulation absorption system, utilize ultrafilter to remove particle dust after, feed H ₂The S reaction is with the SO that absorbs ₂Change into sulphur, the lean solution after the filtration enters and absorbs SO ₂System, recycling:

Na ₃Ci+3SO ₂+3H ₂O＝＝＝3NaHSO ₃+H ₃Ci

3NaHSO ₃+6H ₂S+H ₃Ci＝＝＝9S+Na ₃Ci+9H ₂O

By control rich solution and H ₂The S reaction pH value can draw satisfactory sulfur product.And because SO ₂Feed H in the saturated absorption liquid ₂S can make the natrium citricum absorption system be in reducing condition, thereby also reduce SO ₂Suffer oxidation to become the probability of sulfate.

In whole technical process, if the excessive concentration of sulfate ion, then separate out and occluding device with the form of sodium sulphate easily, therefore need in time to detect the concentration of the sulfate ion in the circulation fluid, if sulfate ion concentration is too high, can be removed by adding active carbonic acid barium, and the pH value of natrium citricum buffering absorption system is returned in the scope of 4.0-4.5.

Embodiment

Following examples are used to the present invention that explains, but do not limit the scope of the invention.

Ci County, Hebei province coal gangue power generation branch company of six directions Industrial Co., Ltd is positioned at Guan Taizhen east, Ci County, need implement the desulfurization and dedusting transformation to 1 35 tons boiler wherein at present, and the boiler model is SHF20-25 type (a river pot product), and the outlet air quantity is 61928～75197M ³/ h, outlet blast 6285～5204Pa, flue-gas temperature is less than 140 ℃, and fuel adopts gangue+coal slime, and its ratio is 7: 3, and as fired fuel industry low heat value is 7500～9200kJ/Kg, sulphur content about 0.3% in the coal.Utilize TH-990 type intelligent flue gas analyzer, the TH-880VI type intelligence parallel sampling instrument of flue dust and the telescopical detection of DW10-II type Lin Geman to show: after flue gas is handled with electrostatic precipitator, SO in the air-introduced machine exit actual measurement flue gas ₂Content is 101～123mg/Nm ³(about 35-43ppm), flue-gas temperature is 126～130 ℃, dust content is 120～123mg/Nm ³, blackness of exhaustion is less than 1.

Then above-mentioned flue gas being fed citrate ion concentration is 0.225mol.l ^-1Sodium citrate buffer solution in (be equivalent to W (H ₃Ci.H ₂O)=21Kg/M ³, W (Na ₃Ci.2H ₂O)=39Kg/M ³), the actual initial soln pH=4.48 that records when rich solution pH drops to 4.0, removes particle dust with this rich solution with the ultrafilter of Zhongcai Advanced Materials Co., Ltd's research and development, and wherein the aperture of ultrafilter is 0.2 μ m, records SO this moment ₂Concentration is 3.84g/l, presses the proportioning (H of liquid Claus reactant then ₂S: SO ₂=2: 1) in rich solution, feed H ₂S gas has led to the back and has continued to keep after 30 minutes, is the filter filtration of 0.1 μ m with the sulphur sediment that generates with the aperture, and its productive rate is more than 80%.Lean solution after the filtration can be used for absorbing the SO of flue gas once more ₂

After flue gas utilization the inventive method of coal and oil is handled, removed wherein most SO ₂And dust, remove SO in the flue gas of back ₂Content is at 8.9-55.4mg/Nm ³In the scope, mean value 36.3mg/Nm ³, reached environmental requirement fully, and with the SO that absorbs ₂Be converted into Cosan, produced tangible economic benefit.

Claims (8)

1. one kind removes SO from the coal and oil discharged flue gas ₂With dust and with SO ₂Change into the method for Cosan, SO in the wherein said flue gas ₂Concentration be lower than 2000ppm, described method comprises utilizes sodium citrate buffer solution to absorb SO ₂Step, utilize ultrafilter to remove the step of particle dust and utilize H ₂S is with the SO in the rich solution ₂Change into the step of Cosan.

2. according to the method for claim 1, it is characterized in that SO in the described flue gas ₂Concentration in the 30-1500ppm scope.

3. according to the method for claim 1, it is characterized in that SO in the described flue gas ₂Concentration in the 40-1000ppm scope.

4. according to each method among the claim 1-3, it is characterized in that, absorb SO at the described sodium citrate buffer solution that utilizes ₂Step in, the pH of described sodium citrate buffer solution in the 4.0-4.5 scope, [Ci ^3-] concentration at 0.1-0.5mol.l ^-1In the scope.

5. according to the method for claim 4, [Ci wherein ^3-] concentration be 0.225mol.l ^-1

According among the claim 1-3 each method, it is characterized in that utilize in the step that ultrafilter removes particle dust described, the aperture of described ultrafilter is 0.1～0.3 μ m.

7. according to the method for claim 6, it is characterized in that the aperture of described ultrafilter is 0.2 μ m.

8. according to each method among the claim 1-3, it is characterized in that, at the described H that utilizes ₂S is with the SO in the rich solution ₂Change in the step of Cosan H ₂S and SO ₂Before the reaction, the pH value of rich solution is 4.0.