METHOD FOR DESULFURIZING SMOKE

Abstract: The present invention is connected with a method for desulfurizing smoke which comprises an absorbing process of cleaning a combustion exhaust gas with a limestone slurry; an oxidizing process of blowing air into the slurry coming from the absorbing process in order to oxidize calcium sulfite in the slurry to gypsum; a circulating process of circulating, through the absorbing process, the slurry coming from the oxidizing process; and a separating process of drawing a part of the slurry from the circulating process, separating and recovering gypsum therefrom, and returning the remaining slurry to the circulating system; the aforesaid method being characterized in that a separator for separating the fed slurry into an over flow and an under flow is used, and the fed slurry is irradiated with ultrasonic wave to accelerate the separation of gypsum particles from limestone particles, whereby the limestone particles and the gypsum particles are transferred to the over flow and the under flow, respectively.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method for desulfurizing smoke in which a combustion exhaust gas is cleaned by the use of an absorbing slurry containing limestone particles suspended therein, and in particular, it relates to a method for desulfurizing smoke by first separating an absorbing slurry, which has absorbed SO₂, into a gypsum slurry and a limestone slurry, returning the limestone slurry to an absorbing process, and recovering gypsum from the gypsum slurry. Fig. 3 attached hereto shows a schematic view of an apparatus for carrying out a conventional method for desulfurizing somke. In Fig. 3, a combustion exhaust gas 202 is cleaned in an absorbing tower 201 of a smoke-desulfurizing apparatus. In an absorbing tower tank 203, an absorbing slurry is stored in which calcium sulfite particles, gypsum particles and limestone particles are suspended. A sulfurous acid gas SO₂ present in the exhaust gas is absorbed in the absorbing slurry by spraying the absorbing slurry from the absorbing tower tank 203 through a spray pipe 205 with the aid of an absorbing tower circulating pump 204. The absorbing slurry sprayed in the exhaust gas becomes a mist state and is then entrained by the exhaust gas. The mist is afterward removed from the exhaust gas by means of a mist eliminator 206 and is then discharged as a cleaned gas 207 from the system. In this mist eliminator 206, the gypsum particles are liable to adhere thereto and to clog it eventually, and thus the mist eliminator 206 is washed by spraying water through a washing nozzle 208. On the other hand, a limestone slurry which is an absorbing agent is fed to the absorbing tower tank 203 through a limestone feed pipe 209 and is then reacted with SO₂ absorbed in the absorbing slurry to produce calcium sulfite. In this case, the thus produced calcium sulfite is partially oxidized by oxygen contained in the exhaust gas, thereby converting calcium sulfite into gypsum. Further, in order to heighten an SO₂ absorbing power of the absorbing slurry, limestone may be fed thereto through the feed pipe 209 in an amount in excess of a level necessary to absorb SO₂. In consequence, at this point of time, the absorbing slurry stored in the absorbing tower tank 203 is composed of a mixture of the calcium sulfite particles, the gypsum particles and the limestone particles. In order to recover gypsum as a by-product, as described in Japanese Utility Model Provisional Publication No. 10833/1984, the absorbing slurry is drawn out through a valve 211 and is led to a separator 212, in which the absorbing slurry is separated into a gypsum slurry 213 and a mixed slurry 214 containing calcium sulfite and limestone, and the mixed slurry 214 containing calcium sulfite and limestone is then returned to the absorbing tower tank 203 and the gypsum slurry 213 is delivered to a by-product gypsum separator (not shown) via a gypsum slurry tank 215. However, even when the absorbing slurry is separated into the gypsum slurry 213 and the mixed slurry 214 containing calcium sulfite and limestone by means of a centrifugal separator such as a hydrocyclone and a weight sedimentation separator such as a thickener, the result of this separation is insufficient. Accordingly, in the conventional technique, the by-product gypsum is contaminated with calcium sulfite particles and limestone particles, and for this reason, high-purity gypsum cannot be recovered. Inventors of the present application have investigated the cause of such a contamination, and finally it has been found that the ground limestone particles having an average diameter of 10 µm and the calcium sulfite particles having an average diameter of 10 µm which have been produced due to the SO₂ absorbing reaction tend to coagulate in the absorbing slurry, and the coagulated calcium sulfite particles and limestone particles embrace the gypsum particles therein having an average diameter of 40 µm, which fact makes their separation difficult. In addition, another process for desulfurizing smoke is suggested in Japanese Patent Provisional Publication No. 61070/1979, and in this process, an absorbing slurry is irradiated with ultrasonic wave in order to precipitate CaSO₄ which has been dissolved in a supersaturating state in the absorbing slurry, and the precipitated CaSO₄ is then separated and recovered by a thickener. The aqueous alkali solution from which the precipitate has been removed is supplied with Ca(OH)₂ and is then circulated through a desulfurizing tower. 3. OBJECT AND SUMMARY OF THE INVENTION The present invention intends to provide a method for desulfurizing smoke by which the drawback of conventional smoke-desulfurizing methods is overcome, that is, by which coagulated particles in an absorbing slurry are broken up in order to accelerate the separation of gypsum particles and to thereby recover the high-quality gypsum. The present invention is directed to a method for desulfurizing smoke which comprises an absorbing process of cleaning a combustion exhaust gas with a limestone slurry; an oxidizing process of blowing air into the slurry coming from the absorbing process in order to oxidize calcium sulfite in the slurry to gypsum; a circulating process of circulating, through the absorbing process, the slurry coming from the oxidizing process; and a separating process of drawing a part of the slurry from the circulating process, separating and recovering gypsum therefrom, and returning the remaining slurry to the circulating system; the aforesaid method being characterized in that a separator for separating the fed slurry into an over flow and an under flow is used, and the fed slurry is irradiated with ultrasonic wave to accelerate the separation of gypsum particles from limestone particles, whereby the limestone particles and the gypsum particles are transferred to the over flow and the under flow, respectively. 4. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a smoke-desulfurizing apparatus for carrying out a method of the present invention; Fig. 2 is a graph illustrating an effect of ultrasonic wave irradiation in the present invention; and Fig. 3 is a schematic view of a conventional smoke-desulfurizing apparatus. 5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The function of the present invention will be described in detail with reference to Fig. 1. An absorbing slurry in which gypsum particles and limestone particles are suspended is stored in an absorbing tower tank 103. This absorbing slurry is sprayed into an absorbing tower 101 through a spray pipe 105 by means of an absorbing tower circulating pump 104. A combustion exhaust gas 102 in the absorbing tower 101 is cleaned by the sprayed absorbing slurry, and at this time, SO₂ present in the exhaust gas is absorbed in the absorbing slurry. The absorbing slurry which has been sprayed into the exhaust gas becomes a mist state and is then entrained by the exhaust gas, and the mist is afterward removed from the exhaust gas in a mist eliminator 106 and is then discharged as a cleaned gas 107 from the system. In this mist eliminator 106, the gypsum particles are liable to adhere thereto and to clog it eventually, and thus the mist eliminator 106 is washed by spraying water through a washing nozzle 108. On the other hand, a limestone slurry which is an absorbing agent is fed to the absorbing tower tank 103 through a limestone feed pipe 109 and is then reacted with SO₂ absorbed in the absorbing slurry to produce calcium sulfite. This calcium sulfite is oxidized by oxyge n blown from an air sparger 110 disposed in the absorbing tank 103, thereby converting all of the calcium sulfite into gypsum. In consequence, the absorbing slurry stored in the absorbing tower tank 103 can be treated so as to be free from the calcium sulfite particles. In order to enhance an SO₂-absorbing power of the absorbing slurry, limestone may be fed thereto through the feed pipe 109 in an amount in excess of a level necessary to absorb SO₂. Therefore, the absorbing slurry stored in the absorbing tower tank 103 is composed of a mixed slurry of the gypsum particles and the limestone particles. A part of the absorbing slurry is led to a separator 112 through a valve 111. The separator 112 is provided with, for example as shown in Fig. 1, a slurry feed inlet 120, an outlet 121 for an under flow and an outlet 122 for an over flow, and is constituted so that a feed rate of the slurry coming through the feed inlet 120 and a discharge rate going out through the outlet 121 may be adjusted so as to form the over flow streaming upward from the feed inlet 120 and the under flow streaming downward therefrom. An ultrasonic wave oscillator 113 is preferably disposed in the vicinity of a diverging point of the over flow and the under flow. Reference numeral 114 is an ultrasonic wave transmitter. The ultrasonic wave oscillator 113 gives ultrasonic wave vibrations to the absorbing slurry fed to the separator 112 in order to break up and disperse the coagulated limestone particles. The absorbing slurry contains the limestone particles having a diameter of about 10 µm and the gypsum particles having a diameter of about 40 µm and the limestone particles are coagulated embracing the gypsum particles, but since the oscillator 113 breaks up and disperses the coagulated limestone particles, some difference arises between sedimentation rates of the gypsum particles and the limestone particles, so that both the particles are easily separable. The thus separated limestone particles are transferred to the side of the over flow and the gypsum particles are transferred to the side of the under flow. In consequence, the limestone slurry 115 and the gypsum slurry 116 are obtained separately. An amount of the gypsum slurry 116 which should be drawn from the separator 112 can be adjusted by opening and closing a valve 118 in response to a signal from a slurry concentration detector 117 (a pycnometer) for the absorbing slurry which is equipped in the absorbing tower tank 103. The concentration of the gypsum particles in the absorbing slurry is preferably controlled so that the gypsum particles may serve as an advocate for the prevention of scale. The gypsum slurry 116 is delivered to a by-product gypsum separator (not shown) via a gypsum slurry tank 119. Further, the limestone slurry 115 which overflows the separator 112 is returned to the absorbing tower tank 103 and is then utilized again for the absorption of SO₂. Next, reference will be made to a separation effect of the gypsum particles from the limestone particles by means of the ultrasonic wave vibration. The absorbing slurry containing about 100 grams of the gypsum particles per kilogram of H₂O and about 15 grams of the limestone particles per kilogram of H₂O was fed to the separator, an average diameter of each gypsum particle being 40 µm, an average diameter of each limestone particle being 10 µm. In this case, the feed flow rate of the absorbing slurry to the separator was adjusted all the time so that liquid flow rates of the over and under flows might be substantially equal to each other. Changing the feed flow rate of the slurry to the separator causes the flow rate of the over flow to vary (the flow rate of the under flow was also changed so as to become substantially equal to the flow rate of the over flow). Therefore, the change in the feed flow rate of the slurry to the separator can be grasped as the change in an average rise velocity of the over flow liquid in the separator. In the case that the slurry was not irradiated with any ultrasonic wave, the limestone particles were coagulated, and they behaved like the gypsum particles, as illustrated by dotted lines having black symbols in Fig. 2. In addition, when a rise velocity of the over flow liquid was 4 m/h or less, this liquid was in a clarifying state. As be apparent from Fig. 2, the limestone particles and the gypsum particles had substantially equal sedimentation rates, and therefore the respective particles were hard to separate from each other. However, when the slurry was irradiated with ultrasonic wave, the limestone particles became easily separable from the gypsum particles, as illustrated by solid lines having white symbols in Fig. 2. The data in Fig. 2 were obtained by irradiating the slurry in the 10-liter separator with the ultrasonic wave having an output of 400 W and a frequency of 28 KHz. According to the present invention in which the above-mentioned constitution is employed, limestone-free and calcium sulfite-free gypsum can be recovered, and the absorbing slurry from which gypsum has been removed can be returned to the exhaust gas-cleaning system to thereby heighten the absorbing power of the absorbing agent.

Claims: A method for desulfurizing smoke which comprises an absorbing process of cleaning a combustion exhaust gas with a limestone slurry; an oxidizing process of blowing air into said slurry coming from said absorbing process in order to oxidize calcium sulfite in said slurry to gypsum; a circulating process of circulating, through said absorbing process, said slurry coming from said oxidizing process; and a separating process of drawing a part of said slurry from said circulating process, separating and recovering gypsum therefrom, and returning the remaining slurry to said circulating system; said method being characterized in that a separator for separating said fed slurry into an over flow and an under flow is used, and said fed slurry is irradiated with ultrasonic wave to accelerate the separation of gypsum particles from limestone particles, whereby said limestone particles and said gypsum particles are transferred to said over flow and said under flow, respectively. A method for desulfurizing smoke according to Claim 1 wherein the vicinity of a diverging point of said over flow and said under flow is irradiated with said ultrasonic wave.

Inventors: ONIZUKA, MASAKAZU HIROSHIMA TECHNICAL INSTITUTE, SHINODA, NAOHARU HIROSHIMA TECHNICAL INSTITUTE, TAKASHINA, TORU HIROSHIMA TECHNICAL INSTITUTE, TATANI, ATSUSHI HIROSHIMA TECHNICAL INSTITUTE
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: B01D53
IPC codes: C02F1, B01D53, C01F11