GB2121776A - Method for recovering gypsum at waste gas desulphurization - Google Patents

Abstract

A wet-lime gypsum-process waste gas desulphurization apparatus in which CaSO4 dihydrate is produced secondarily comprises an absorption column 2 for bringing a waste gas containing SO2 into contact with a suspension containing calcium carbonate and/or calcium hydroxide and an oxidation column 6, for oxidizing, with oxygen gas, calcium sulphite produced by desulphurization reaction in the absorption column. To recover calcium sulphate, the suspension in the oxidation column 6 is adjusted to a temperature of 60 DEG C or more and a pH of 6.5 or more to oxidize the calcium sulphite, the resultant CaSO4 dihydrate crystals, contained in the suspension drawn from the oxidation column are collected and the suspension without the CaSO4 is returned to the absorption column 2 for recycling. <IMAGE>

Description

SPECIFICATION Method for recovering gypsum at waste gas desulphurization This invention relates to an improved method for obtaining high-purity gypsum by oxidizing calcium sulfite, more particularly to a method which can be utilized practically and widely in a wet-lime gypsum-process waste gas desulfurization apparatus.

A method which has heretofore been employed on an industrial scale comprises treating a waste gas containing SO2 with a suspension containing, as an absorbent, a calcium compound such as calcium carbonate or calcium hydroxide; feeding the suspension containing the resultant calcium sulfite to an oxidation column; and dispersing a gas containing oxygen gas into the suspension in the oxidation column to oxidize the calcium sulfite and to thereby obtain calcium sulfate (gypsum dihydrate).

Such a method is described in "Mitsubishi Juko Giho", Vol. 10, No.5, p.44-51(1973 Sep.).

Conventionally, in the oxidizing step of such a lime sypsum process, there has been employed a way of rendering the suspension acidic in order to facilitate the solubility, to the suspension, of calcium sulfite which has an extremely poor solubility to neutral water, and to thereby accelerate the oxidation reaction.

Accordingly, said conventional method requires to maintain the pH of the suspension at an acidic side by adding sulfuric acid for the purpose of obtaining a practical oxidation rate.

As be definite from the foregoing, it is an inevitable requirement to provide an acidic atmosphere of pH 6.5 or less during the oxidation of calcium sulfite. Such a technique is described in, for example, Japanese Patent Publication Nos. 43839/1976, 43039/1976,43837/1976 and 43840/1976.

However, the conventional methods have the following drawbacks: (1) An additive such as sulfuric acid to be added to the oxidation column and facilities for feeding it are needful. Further, the residue of an alkaline calcium compound such as calcium carbonate or calcium hydroxide used as an absorbent for SO2 is present in the suspension to be fed to the oxidation column together with calcium sulfite, and thus sulfuric acid is consumed by the residue, which requires a noticeably increased amount of sulfuric acid.

(2) Since the suspension in the oxidation column is made acidic, the oxidation column and devices downstream therefrom must be made of an acid resistant material, which raises costs of the facilities.

This invention intends to overcome the aforementioned drawbacks and has been established as a result of researches on a method for achieving the oxidation reaction of calcium sulfite under high-pH conditions, i.e.

in a neutral or alkaline atmosphere which has heretofore been considered to be impractical in the art.

This is to say, this invention is directed to a method for recovering gypsum at waste gas desulfurization comprising the steps of adjusting a suspension containing calcium sulfite and an alkaline calcium compound such as calcium carbonate or calcium hydroxide to pH 6.5 or more and a temperature of 600C or more and blowing a gas containing oxygen gas into the suspension in order to oxidize a portion or all of the calcium sulfite to gypsum dihydrate; collecting selectively the resultant gypsum dihydrate crystal from the oxidized suspension; returning the suspension without the gypsum to the absorption column for recycling.

It has heretofore been known that the oxidation reaction of calcium sulfite under neutral or alkaline conditions does not progress at a practical rate. This fact has also been confirmed from experiment by the inventors of the present case, and in the course of the experiment the knowledge for this invention has been acquired.

The inventors have found that when a suspension containing neutral or alkaline calcium sulfite and calcium carbonate is heated up to a temperature of 600C at outside while dispersing air thereinto, the oxidation reaction rate of the calcium sulfite will be heightened remarkably, as shown in Figure 1 attached here.

In other words, indeed at a temperature of 60 C or less, calcium sulfite cannot be oxidized unless in an acidic atmosphere, as usually considered. However, the inventors has found that the oxidation reaction of calcium sulfite becomes appreciable, even under neutral to alkaline conditions, by elevating the temperature of the suspension to 60 C or more, even if not acidic. This invention has been established on the basis of such a finding.

Heretofore, there are known a manner of facilitating the oxidation to some extent by the use of carbonic acid and a magnesium compound (Japanese Patent Publication Nos. 40396/1975 and 11680/1977), and another manner of accelerating the oxidation by heating, while maintaining a low pH by the use of SO2 gas (Japanese Patent Publication No. 10793/1977). However, it is manifest that no prior art suggests the industrially very beneficial present invention in which the oxidation is accomplished at a practically enough rate under neutral or alkaline conditions without any additive.

The method according to this invention can be applied to a wet-lime gypsum-process waste gas desulfurization apparatus by which gypsum dihydrate is produced secondarily and which is composed of an absorption column for bringing a waste gas containing SO2 into contact with a suspension containing, as an absorbent, at least one calcium compound of calcium carbonate and calcium hydroxide, and an oxidation column for oxidizing, with air, calcium sulfite produced by the desulfurization reaction in the absorption column.

In the waste gas desulfurization apparatus, the suspension in the oxidation column is adjusted to a temperature of 60 C or more and pH of 6.5 or more in order to oxidize a portion or all of calcium sulfite therein, the resultant gypsum dihydrate is collected selectively from the suspension drawn from the oxidation column, and calcium carbonate or calcium hydroxide and unoxidized calcium sulfite are recycled to the absorption column.

As understood from the above, it is unnecessary to add sulfuric acid to the oxidation column as in the conventional case, therefore calcium carbonate or calcium hydroxide which is contained in the suspension drawn from the absorption column is not decomposed with sulfuric acid and remains in the suspension in the oxidation column as it is. Conveniently, calcium carbonate and calcium hydroxide as well as unoxidized calcium sulfite are different from the gypsum dihydrate crystal in crystalline morphology, and the former compounds are of a smaller grain size than the latter. Therefore, these compounds can be separated therefrom by utilizing the difference in sedimentation rate or using a suitable sieve.The separated calcium carbonate or calcium hydroxide and unoxidized calcium sulfite are returned to the absorption column as a portion of the absorbent, whereby the loss of the absorbent can be inhibited.

As some manners of warming the suspension in the oxidation column to a temperature of 60 C or more, there are, for example, a manner of causing a heat transfer medium to pass through a jacket disposed outside the oxidation column and a manner of warming previously the suspension to be fed to the oxidation column up to 600C or more.

Needless to say, since the oxidation reaction is exothermic, the heat generated here can be utilized, and it is also possible to feed directly steam to the suspension. These warming manners may be used in combination of two or more.

However, when the temperature in the oxidation column is heated to 1000C or more, a-type gypsum hemihydratewill be produced. Therefore, in orderto ensure the preparation of gypsum dihydrate,the temperature therein is preferably set within the range of 60 to 900C.

Adjusting the pH of the suspension in the oxidation columm to 6.5 or more may be accomplished by causing calcium sulfite to exist in the suspension carrying calcium hydroxide or calcium carbonate as an absorbent for desulfurization from the absorption column to the oxidation column.

This pH adjusting operation can be carried out involving no or little loss in the industrial economy and operation. The adjustment of the pH to 6.5 or more may be achieved only by stopping feed of an additive such as sulfuric acid as in the conventional case.

For the improvement in the efficiency of SO2 absorption in the absorption column, it is advantageous that calcium carbonate or calcium hydroxide is abundantly present in the suspension in the absorption column.

Therefore, this invention in which the oxidation column not requiring sulfuric acid is utilized can provide especially excellent effects.

This invention will be described in detail in accordance with a comparative example showing the conventional oxidation process and examples of this invention and with reference to the accompanying drawings, in which: Figure 1 is a graph showing the temperature effect of an oxidation according to this invention; and Figure 2 is a flow chart illustrating an embodiment of a method according to this invention.

Comparative Example So as to maintain the pH of the suspension in an oxidation column at a level less than 6.5, sulfuric acid was added to the suspension, and oxidation was then carried out therein. In this case, it was confirmed that the oxidation reaction progressed, without heating especially the suspension therein at outside to adjust its temperature to 600C or more, but in order to maintain the practically stable oxidation reaction, it was required to adjust the pH of the suspension in the oxidation column to 5 or less.

Example 1 A boiler waste gas containing 1200 ppm of SO2, 12% of CO2, 3% of O2 and 10% of H2O was subjected to a desulfurization treatment in an absorption column, with slaked lime as an absorbent. Properties of the suspension drawn from the absorption column at this time are set forth in Table 1 below.

TABLE 1 Properties of the Suspension Drawn from the Absorption Column Calcium Sulfite 0.50 mol/1 (as CaSO3,1/2H2O) Gypsum Dihydrate 0.45 mol/1 (as CaSO4-2H20) Calcium Carbonate 0.04 mol/1 (as CaCO3) pH of Suspension 6.5 The following Table 2 sets forth specifications of the oxidation column which was used to oxidize the suspension drawn from the absorption column having the properties of Table 1 above.

TABLE 2 Specifications of the Oxidation Column Used in the Experiment Type: Vertical cylindrical oxidation column with jacket Size: Inner diameter 200 m and height 4,000 mm Volume: Standard volume 200 f Aerator: Foam dispersing device based on a rotary atomizer (hollow cylindrical rotary type) is fixed to the bottom of the oxidation column.

Blowing gas: Compressed air 20 m3N/H Supplementary Item: In the jacket, an automatic temperature-controllable liquid passing system is em ployed.

The suspension having a pH 6.5 drawn from the absorption column was fed to said oxidation column, and when oxidation was carried out under aeration, the pH of the suspension in the oxidation column became 6.5 or more. In such a way, the oxidized suspension always transferred to a higher pH than of the suspension in the absorption column, i.e. to an alkaline side. The gap between the pH values of the respective suspensions in the absorption column and oxidation column would result from differences in properties of gas therein.

Measurements of the oxidation reaction rate of calcium sulfite which were obtained by varying the temperature of the suspension in the oxidation column are shown in Figure 1.

Figure 1 shows that at 600C or more the oxidation rate is high, but at a temperature of less than 600C a practically usable oxidation rate can scarcely be obtained.

It has heretofore been considered that the oxidation reaction progresses at a practically usable rate when the suspension of calcium sulfite has a pH of 6.5 or more, but, as be definite from the above, such a fact is attributable to a low temperature of the suspension. It has just been manifested that the oxidation reaction markedly proceeds even under neutral or alkaline conditions, if warmed to 600C or more.

Example 2 A boiler waste gas containing 1000 ppm of SOP was treated by a wet-lime gypsum-process waste gas desulfurization plant in which the flow chart shown in Figure 2 is designed.

A shown in Figure 2, a waste gas 1 containing 1000 ppm of SO2 was blown and introduced into an absorption column 2 at 2000 m3N/H (dry) in order to prepare a desulfurized waste gas 3. While the suspension recycling through the absorption column was pumped up to the top of the absorption column by means of a recycle pump 4 for the absorption column, a portion of the suspension was forwarded to an oxidation column 6 through a line 5, with equilibrium between incoming and outgoing materials balance. In the oxidation column 6, the temperature of the suspension was controlled by adjusting the amount of a heat transfer medium in a jacket 6' so as to be within the range of 60 to 900C, and the column was supplied with air from the bottom thereof.The suspension was fed from the oxidation column 6 to a 200-mesh sifting device 8 through a line 7, and the collected gypsum dihydrate crystal was taken out through 9 and the suspension 10 which was allowed to pass through the sifting device was introduced into a tank 11 from an absorbing material. A calcium carbonate powder 12 which passed through a 325-mesh sieve was placed in the tank 11 above to prepare an absorption liquid, and thus prepared absorption liquid was added to the absorption column-through recycling suspension via a pump 13 and a line 14 in proportion to an absorption amount of 802. In such a way, a continuous operation was carried out.

Table 3 below sets forth analytical data of the respective compositions of the suspension (in line 5) drawn from the absorption column, the suspension (in line 7) drawn from the oxidation column, the gypsum dihydrate (in line 9) and the suspension (in line 10) allowed to pass through the sifting device.

TABLE 3 Analytical Data of Suspension Compositions Line No.

in Figure 2 5 7 9 10 Suspension Suspension Gypsum Suspension Drawn from Drawn from Dihyd- Allowed to Suspension Absorption Oxidation rate Pass through Column Column Sifting Device Flow Rate (1/H) 65 65 9 56 Composition CaSO4-2H2O 0.970 1.466 9.07 0.250 (mol/1) CaSO3.1/2H2O 0.546 0.050 0.050 0.050 (mol/1) CaCO3(mol/1) 0.160 0.160 0.090 0.160 pH 5.5-6.0 6.5-8.3 6.5-8.3 6.5-8.3 As be definite from the analytical date in Table 3 above, the suspension in the line 7 drawn from the oxidation column included calcium carbonate (CaCO3) which was advantageous as the absorbent for desulfurization, and some unoxidized calcium sulfite (CaSO3.1/2H2O). Further, the coarse crystal (in line 9) collected by the sifting device having a 200-mesh sieve was a high-purity gypsum dihydrate (CaSO4-2H2O) including trace CaSO3.1/2H2O and Cacao3 Furthermore, the suspension which was allowed to pass through the sifting device and which was flowing through the line 10 contained calcium carbonate (CaCO3), the absorbent effective for desulfurization, and calcium sulfite (CaSO3#1/2H2O), therefore this suspension was returned to the absorbing material tank 11 to be reused in the absorption column.

It was found on the basis of the observation by a microscope that CaSO4.2H2O was of a plate prism crystal, a longitudinal axis of which was about 100 > in length and CaCO3 was of a spherical crystal an average diameter of which was about 10ii.

Functional effects of this invention are as follows: Since the oxidation may be carried out under neutral to alkaline conditions (at a high pH level), facilities for feeding an additive such as sulfuric acid, which has been formerly required, are unnecessary.

(2) Expensive acid proof materials are not required for equipments in which oxidation and treatments subsequent to separation are carried out, therefore costs thereof can be reduced.

(3) The temperature in the oxidation process is 600C or more, preferably within the range of 60 to 90 C, therefore a heat source can be easily available and it is also possible to utilize the heat of the oxidation reaction.

(4) Since not decomposed with sulfuric acid or the like in the oxidation column, the suspension can be separated from gypsum dihydrate by the use of differences in shape of crystal or other properties, and the said suspension can be reused as the absorbent, which can inhibit the loss of the absorbent.

Claims (2)

1. A method for recovering gypsum at waste gas desulfurization in a wet-lime gypsum-process waste gas desulfurization apparatus by which gypsum dihydrate is produced secondarily and which is composed of an absorption column for bringing a waste gas containing 802 into contact with a suspension containing at least one of calcium carbonate and calcium hydroxide and an oxidation column for oxidizing, with oxygen gas, calcium sulfite produced by desulfurization reaction in said absorption column, characterized by the steps of adjusting the suspension in said oxidation column to a temperature of 60 C or more and a pH of 6.5 or more to oxidize the calcium sulfite, collecting the resultant gypsum dihydrate crystal which is contained in the suspension drawn from said oxidation column, and returning the suspension without the gypsum to said absorption column for recycling.

2. A method for recovering gypsum at waste gas desulfurization according to claim 1 wherein the suspension in said oxidation column is adjusted to a temperature within the range of 60 to 90 C.