Exhaust gas treating method

Abstract: A method of treating an exhaust gas containing SO₂ and HF which is characterized by using an absorbing liquid containing CaCO₃ and Ca(OH)₂ as absorbents to be fed to an absorption tower for the exhaust gas, circulating said absorbing liquid from a second slurry tank to the absorption tower, from the absorption tower to a first slurry tank, and from the first slurry tank to the second slurry tank, introducing Ca(OH)₂ into the second slurry tank to keep the pH value of the absorbing liquid in the second slurry tank in the range of 5.5-7.0, and blowing air into the absorbing liquid in the first slurry tank.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method for treating an exhaust gas essentially loaded only with SO₂, HF and Al wherein SO₂ and HF are reacted in a liquid environment containing CaCO₃ in order to form non-hazardous CaCO₃ and CaF₂ respectively. When known exhaust gas desulfurization according to a wet type lime process is carried out, the resulting exhaust gas may contain not only SO x but also HF as harmful components. For example, the exhaust gas from the combustion of coal includes about 1,000 ppm of SO x and about 40 ppm of HF. Such exhaust gas is treated in a wet type exhaust gas treating tower using CaCO₃ as an SO₂-absorbing agent to bring about the following reactions: CaCO₃ + SO₂ → CaSO₃ + CO₂ (1) CaCo₃ + 2HF → CaF₂ + CO₂ + H₂O (2) When the exhaust gas includes a large amount of dust, the dust contains Al as a component which dissolves and reacts with HF to produce a fluoride of Al (which is referred to as AlF x hereinafter). It is known that this AlF x prevents linestone (CaCO₃) from dissolving, as discussed in Japanese Patent Provisional Publication JP-A-551 67023/1980. However, the addition of a basic sodium salt can prevent the above disadvantage, as described also in the above-mentioned publication. Accordingly, by adding a basic sodium salt in an amount which depends on that of HF to treat an exhaust gas containing SO₂ and HF, a method having no disadvantage due to AlF x should be achieved. In this regard, however, the thus added sodium remains in an absorbent liquid, and a decrease in the amount of waste water therefore causes an increase in the concentration of sodium in the absorbent liquid. Accordingly, when SO₂ is recovered in the form of gypsum, the purity of the recovered gypsum tends to be lowered by the coexisting sodium. Also, Japanese Patent Provisional Publication JP-A-601 22029/1985 discloses another method having no disadvantage due to AlF x by adding in a pulse-like manner an alkali compound to the absorbing liquid in order to temporarily increase the pH value thereof. It is to be noted that this method may result in lower desulfurization efficiency which lasts, though only for a short period of time, between the lowering and recovery of the desulfurization efficiency. GB-A-2 171 400 describes the use of CaCO₃ and Ca(OH)₂ as neutralizing agents in an exhaust gas desulfurizing plant, which comprises an absorption tower, a neutralizing zone and an oxidation zone, wherein the gas is contacted by an absorption liquid in the absorption zone, the liquid is hereafter moved to the oxidation zone where air is blown into the liquid and thereafter the liquid is moved into the neutralization zone where CaCO₃ and/or Ca(OH)₂ are replenished. This process is proposed to scrub exhaust gases including i.a. SO₂ and HCl. The process according to this GB document includes further the use of other alkali compounds. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a method for treating an exhaust gas which overcomes the disadvantage concerning the obstacle of insolubility of CaCO₃ due to AlF x and in which gypsum can be recovered in a highly pure form. The gist of the present invention resides in a method as mentioned above which is further characterized by the following steps : providing a first slurry tank, a second slurry tank and an absorption tower, said first and second slurry tanks containing absorption liquid ; moving said absorption liquid from said first slurry tank to said second slurry tank; adding to said absorption liquid essentially no other than CaCO₃ and Ca(OH)₂, whereby said Ca(OH)₂ is added into said second slurry tank in controlled amounts such as to maintain the pH of the absorbing liquid in said second slurry tank between 5,5 and 7,0, whereby said CaCO₃ is added in an amount sufficient to control the rest concentration of SO₂ in the treated exhaust gas to a desired value; introducing said exhaust gas into said absorption tower ; moving absorption liquid from said second slurry tank to said absorption tower, in order to contact said exhaust gas ; blowing air into said absorption liquid in the first slurry tank; moving said absorption liquid from said absorption tower to said first slurry tank; extracting treated exhaust gas from said absorption tower. The present invention can eliminate the harmful influence of HF and dust included in the exhaust gas, maintain the activity of CaCO₃, decrease the amount of the expensive Ca(OH)₂ to be used, and recover gypsum in a highly pure form. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing an Example of the present invention. FIG. 2 is a flow chart showing Comparative Example 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As described above, in the present invention an absorbing liquid including CaCO₃ and Ca(OH)₂ as absorbents in used, Ca(OH)₂ is added to the absorbing liquid in a second slurry tank to be introduced to an absorption tower for exhaust gases to control the absorbing liquid at a pH value of 5.5-7.0, the air for oxidation is blown into the absorbing liquid in a first slurry tank into which the absorbing liquid from the absorption tower is introduced, and the absorbing liquid is circulated from the second slurry tank to the first slurry tank through the absorption tower. In addition to the above basic sodium salts, a variety of alkali compounds have been proposed to decompose AlF x . This is because Al, which is a component of AlF x , is transferred to the solid phase by alkali. The inventors of the present invention have further investigated the effect of these alkali additives to find that these compounds are equal as to the effect of decomposing AlF x , but a considerable amount of F is left in the absorbing liquid when Na and Mg are used as a source of alkali, and even when AlF x is decomposed once by alkali AlF x easily reappears in the dissolved form if the pH value becomes low. It is presumed that this is because the use of Na and Mg salts brings about the production of relatively soluble compounds such as NaF and MgF₂. On the other hand, the use of Ca(OH)₂ lowers the concentration of F in the absorbing liquid because F also deposits in the solid phase as a relatively insoluble CaF₂ together with Al. As a result, the amount of F which appears again in the absorbing liquid is greatly reduced. This is the reason that the absorbing liquid containing CaCO₃ and Ca(OH)₂ as absorbents is used in the present invention. Moreover, the inventors have studied the way of supplying CaCO₃ and Ca(OH)₂, and found that the effect of said Ca(OH)₂ is remarkable when two slurry tanks are used for holding the absorbing liquid to be fed into the absorption tower for the exhaust gas; the absorbing liquid is circulated from the second slurry tank to the absorption tower, from said absorbing tower to the first slurry tank, and from said first slurry tank to said second slurry tank; the air is blown into the absorbing liquid in said first slurry tank for oxidation; and Ca(OH)₂ is added to the absorbing liquid in said second slurry tank to adjust its pH value in the range from 5.5 to 7.0. That is to say, as the inventors found, even when Ca(OH)₂ is added to the second slurry tank, AlF x is not sufficiently decomposed by Ca(OH)₂ at a pH value of less than 5.5, and the reactivity of CaCO₃ cannot therefore be fully recovered. On the other hand, it was found that even when the amounts of the sources of AlF x (e.g., fly ash and HF) included in an exhaust gas are changed, the reactivity of stable CaCO₃ can be kept by controlling the pH value of the absorbing liquid in the second slurry tank in the range of 5.5-7.0. When the pH value of the absorbing liquid in the second slurry tank is over 7, the oxidation rate of sulfite becomes low to produce calcium sulfite, and gypsum cannot be recovered with high purity. When Ca(OH)₂ is fed to the first slurry tank, the amount of Ca(OH)₂ is more than that fed to the second slurry tank. It was found that Ca(OH)₂ is consumed in neutralization reactions because the reaction rate of Ca(OH)₂ is higher than that of CaCO₃. Namely, it was found that when fed to the second slurry tank the amount of Ca(OH)₂ required to recover the reactivity of CaCO₃ after the decomposition of AlF x is minimized. Thus, in the present invention Ca(OH)₂ is introduced into the second slurry tank and the absorbing liquid in said second slurry tank is kept at a pH value of 5.5-7.0. Also, in said first slurry tank, calcium sulfite is oxidized to gypsum by means of blowing the air into the absorbing liquid. It is to be noted that the effect of Ca(OH)₂ is not different when CaCO₃ is added to either the first slurry tank or the second slurry tank. As described above, the present invention can exclude the harmful influence of HF and dust in exhaust gases, maintain the activity of CaCO₃, decrease the amount of expensive Ca(OH)₂ to be used and recover highly pure gypsum. EXAMPLE An example of the present invention will be explained with reference to FIG. 1. First, 200 m³(s.t.p.)/h of exhaust gas taken out of a small pulverized coal burner (which is not shown in FIG. 1) was treated according to the method for treating an exhaust gas shown in the flow chart in FIG. 1. The inlet temperature of the treated exhaust gas was kept at 110°C by a heat-exchanger (which is not shown in FIG. 1), and the dust concentration was adjusted at about 200 mg/m³(s.t.p.) by a bag filter (which is not shown in FIG. 1). The inlet concentrations of SO₂ and HF were 1,200 ppm and 53 ppm, respectively. The exhaust gas to be treated was introduced into an absorption tower 5 by line 3 and discharged from a line 4 after the removal of SO₂ and HF therefrom. Under the absorption tower 5 were placed a slurry tank 1 (which is called the first slurry tank) and a slurry tank 2 (which is called the second slurry tank) both holding an absorbing liquid. The absorbing liquid was sent from the slurry tank 2 to the absorbing tower 5 by means of an absorbing liquid pump 6, sent from the absorbing tower 5 to the slurry tank 1 and then recycled from the slurry tank 1 to the slurry tank 2. Slurry of CaCO₃ was fed to the slurry tank 1 by line 7. In this case, the amount of CaCO₃ slurry fed was automatically adjusted by a control valve 8 through a controller 9 to obtain a SO₂ concentration of 50 ppm in the gas in line 4 discharged from the absorption tower. On the other hand, slurry of Ca(OH)₂ was fed into the slurry tank 2 by line 10. The flow rate of Ca(OH)₂ slurry was automatically adjusted by a control valve 11 through a pH controller 12 to obtain a pH value of 5.7 in the absorbing liquid sent from the slurry tank 2 to the absorbing tower 5. Also, a given amount of the air for oxidation was introduced into the slurry tank 1 by line 13. A part of the absorbing liquid to be fed to the absorbing tower 5 by means of the absorbing liquid pump 6 was taken out by line 14. It is to be noted that 15 is a stirrer, 16 is a barrier placed between the slurry tank 1 and the slurry tank 2, and 17 is a passage placed between the barrier and the tank bottom. Under the above-mentioned conditions, a stationary run was carried out in which SO₂ in the outlet gas was stable at 50 ppm and the pH value of the absorbing liquid in the slurry tank 1 was 5.2. Also, the CaCO₃ concentration in the absorbing liquid was 0.04 mol/liter, the reactivity of CaCO₃ was good and no calcium sulfite was recognized. The concentration of Al and F in the filtrate of the absorbing liquid were determined to find trace amounts of 1.3 mg/liter and 17 mg/liter, respectively. The molar ratio of CaCO₃ to Ca(OH)₂ introduced in the Example of the present invention was Ca(OH)₂/[CaCO₃ + Ca(OH)₂] = 0.025. COMPARATIVE EXAMPLE 1 A run was carried out with the same equipment under the same conditions as given in the above Example of the present invention except for using only CaCO₃ as the absorbent. That is, in FIG. 1, the Ca(OH)₂ slurry was not fed from line 10. In Comparative Example 1, the pH values of the absorbing liquids in the slurry tanks 1 and 2 became gradually lower, after starting the run, to 4.5 and 4.8, respectively, toward the final stage. The concentration of CaCO₃ in the absorbing liquid was 0.11 mol/liter and the reactivity of CaCO₃ was exceedingly lower in comparison with the above Example of the present invention. The concentrations of Al and F in the filtrate of the absorbing liquid were 75 mg/liter and 120 mg/liter, respectively. COMPARATIVE EXAMPLE 2 In comparative Example 2, a run was carried out with the equipment shown in FIG. 2 under the same gas conditions as given in the above Example of the present invention. It is to be noted that the numerical figures and signs given in FIG. 2 indicate the same parts as given in FIG. 1, except that FIG. 2 has only one slurry tank holding the absorbing liquid to be fed to the absorbing tower 5. In Comparative Example 2, the CaCO₃ slurry and the Ca(OH)₂ slurry were introduced into the slurry tank 1 at the same time. Under the above-mentioned conditions, a run was carried out so that the SO₂ concentration in the outlet could be kept at a constant value of 50 ppm. The concentration of CaCO₃ in the absorbing liquid was 0.04 mol/liter, and the reactivity of CaCO₃ was good. In Comparative Example 2, the mol ratio of Ca(OH)₂ to CaCO₃ fed was Ca(OH)₂/[CaCO₃ + Ca(OH)₂] = 0.12. This revealed that the amount of Ca(OH)₂ used in Comparative Example 2 was much more than that in the above Example of the present invention. The concentrations of Al and F in the absorbing liquid in Comparative Example 2 were 1.5 mg/liter and 20 mg/liter, respectively. As explained above, the present invention can provide a superior method for treating an exhaust gas which eliminates the harmful influence of HF and dust in exhaust gases, maintains the activity of CaCO₃, decreases the amount of the expensive Ca(OH)₂ to be used and recovers gypsum in a highly pure form.

Claims: A method for treating an exhaust gas essentially loaded only with SO₂, HF and Al, wherein SO₂, and HF are reacted in a liquid environment containing CaCO₃ in order to form non-hazardous CaSO₃ and CaF₂ respectively, characterized by the following steps : providing a first slurry tank (1), a second slurry tank (2) and an absorption tower (5), said first and second slurry tanks containing absorption liquid ; moving said absorption liquid from said first slurry tank (1) to said second slurry tank (2) ; adding to said absorption liquid essentially no other than CaCO₃ and Ca(OH)₂, whereby said Ca(OH)₂ is added into said second slurry tank (2) in controlled amounts such as to maintain the pH of the absorbing liquid in said second slurry tank (2) between 5,5 and 7,0, whereby said CaCO₃ is added in an amount sufficient to control the rest concentration of SO₂ in the treated exhaust gas to a desired value; introducing said exhaust gas into said absorption tower (5); moving absorption liquid from said second slurry tank (2) to said absorption tower (5), in order to contact said exhaust gas; blowing air into said absorption liquid in the first slurry tank (1); moving said absorption liquid from said absorption tower (5) to said first slurry tank (1); extracting treated exhaust gas from said absorption tower (5). The method of claim 1, characterized in that CaCO₃ is added into said first slurry tank (1).

Inventors: IWASHITA KOUICHIRO, OKINO SUSUMU C O HIROSHIMA TEC, SHIMIZU TAKU, UKAWA NAOHIKO C O HIROSHIMA TE, IWASHITA, KOUICHIRO, OKINO, SUSUMU C/O HIROSHIMA TECHNICAL INSTITUTE, SHIMIZU, TAKU, UKAWA, NAOHIKO C/O HIROSHIMA TECHNICAL INSTITUTE
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: B01D53
IPC codes: B01D53