GB1560038A - Process for removing chloride ions from the wet-gas processing system - Google Patents

Description

(54) PROCESS FOR REMOVING CHLORIDE IONS FROM THE WET FLUE-GAS PROCESSING SYSTEM (71) We, IDEMITSU KOSAN COM PANY LIMITED, No. 1-1, 3-Chome, Marunouchi, Chiyoda-Ku, Tokyo, Japan, a company organised and existing under the laws of Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is concerned with improvements in or relating to a process for removing chloride ions from flue-gas.

For the wet flue-gas processing system, various procedures have hithereto been employed to completely avoid the formation of waste water so as to prevent pollution. These procedures, however, suffer from the disadvantage in that small amounts of chlorine contained in industrial water, heavy fuel oil and the reagents employed are concentrated and accumulated in the circulating absorbent solution during processing. When the chlorine concentration in the circulating absorbent solution reaches 4,000 to 6,000 ppm, stress corrosion cracking and pitching corrosion for example of stainless steel, of which an absorbing tower is principally made, results.

Therefore, it has long been desired to effectively remove chloride ions.

In the past, in order to prevent or reduce the accumulation of chloride ions, a part or the whole of the circulating absorbing solution has been withdrawn from the processing system when the chloride ion concentration reaches a certain level.

This procedure, however, possesses the disadvantages that it is difficult to render harmless the circulating absorbent solution which is withdrawn and that when the circulating absorbent solution is abandoned, secondary pollution is caused. In addition there is the economical disadvantage that a fresh absorbent solution should be supplemented in the amount corresponding to that of the absorb bent solution withdrawn, and that large amounts of dissolved salts are contained in the absorbent solution withdrawn.

The present invention provides a process for removing chloride ions from flue-gas comprising the steps of bringing a flue-gas containing sulfur dioxide into contact with a circulating slurry containing a magnesium oxide (absorption step), separating solids from a part of the circulating absorbent slurry (solidliquid separation step), drying and calcining the solids to regenerate the magnesium oxide (dry-calcination step) which is to be re-used in the absorption step, and washing the gas formed at the dry-calcination step with water in a washing tower (washing step), at least part of the solution withdrawn from the washing tower being removed and any remainder being returned to the absorption step, and the water content of the solids to be fed to the dry-calcination step and the amount of solution withdrawn from the washing tower to be removed being controlled so that the accumulation of the chloride ions is substantially prevented, the amount of solution withdrawn from the washing tower to be removed being inversely proportional to the water content of the solids to be fed to the dry-calcination step.

In carrying out a process according to the invention preferably the chloride ion concentration is maintained below 3,000 ppm.

Processing of a flue-gas containing sulfur dioxide by the wet processing system is shown in the drawing. Referring to the flow diagram shown in the drawing, an absorbent slurry containing magnesium compounds such as magnesium oxide, magnesium sulfite and magnesium sulfate is brought into contact with the flue-gas in an absorption tower 1 to thereby absorb the sulfur dioxide therein. For example, the chemical reactions between the magnesium compounds and the sulfur dioxide are as follows:

The reaction products and the absorbing agents are separated in a centrifugal separator 2, and the mother liquor is returned to the absorption tower for re-use.

The solids (wet cake) separated are dried in a dryer 3 and then calcined in a calcining unit 4 to recover sulfur dioxide and at the same time, to regenerate the magnesium compounds. These magnesium com- pounds are re-used as absorbing agents. Dust is removed from the recovered gas which contains sulfur dioxide in washing tower 6, and it then used as a feed for production of a by-product such as sulfur or sulfuric acid. The solution withdrawn from the washing tower is returned to the circulating absorption system.

In the above flue-gas processing system, the chloride ions not only circulate through the circulating absorption system together with the other components, but some of the chloride ions circulate in the following manner: absorption system o solid-liquid separation step 9 drying step o calcining step < washing step o absorption system.

We have recognized that the solids obtained in the centrifugal separator by the solidliquid separator by the solid-liquid seperation (wet cake) usually contain 5 to 30% by weight of water and that the chloride ions contained in the water are liberated as hydrogen chloride to the gas formed at the dry-calcination step and at least about 90% thereofisfinallytrap ped in the solution at the washing tower. The amount of chlorine trapped exceeds the amount of chloride ions formed in all the other steps.

Thus it has been found that the accumulation of the chloride ions in the absorption system can be effectively prevented by discharging a part of the solution withdrawn from the washing tower out of the processing system.

Since the solution withdrawn from the washing tower 6, as described above, contains a large amount of chloride ions, it is possible to prevent the accumulation of chloride ions in the processing system and to maintain its concentration below a certain level by taking a part or the whole of the solution withdrawn from the washing tower out of the processing system.

The ratio of the solution withdrawn to be taken out of the processing system is controlled taking into account the relation between the desired chloride ion concentration of the absorbing slurry and the water content of the wet cake For example, where the water content of the wet cake is 15% by weight, if the whole of the solution withdrawn from the washing tower is taken out of the processing system, the chloride ion concentra- tion in the absorbing system is maintained at about 340 ppm. If the ratio of the solution withdrawn to be taken out of the processing system is controlled at about 11%, the chloride ion concentration in the absorption system is as high as 3,000 ppm.

If the chloride ion absorption system is maintained below about 3,000 ppm, it is possible to remove problems such as corrosion of the absorption tower, and thus stable operation is possible for a long period of time.

In accordance with the present invention, if the water content of the wet cake to be fed to the dryer is above 2% by weight, it is possible to maintain the chloride ion concentration in the absorption system below 3,000 ppm by taking a certain amount of the solution withdrawn from the washing tower out of the processing system. The ratio of the quantity of solution withdrawn to that taken out of the processing system is made inversely proportional to the water content of the solids to be fed to the regeneration processing step.

That is to say, the ratio of the solution withdrawn to be taken out of the processing system can be lowered by increasing the water content of the wet cake. Therefore, it is possible to transfer a part of the circulating absorbing slurry directly to the dryer without passing through the centrifugal separator.

The removal of the solution withdrawn from the washing tower out of the processing system can be carried out either continuously or discontinuously. Also, by increasing the chloride ion concentration of the solution withdrawn from the washing tower by temporarily increasing the water content of the wet cake, the amount of the solution to be taken out of the processing system can be decreased.

The solution withdrawn from the washing tower which has been taken out of the processing system generally has a pH of from 1.5 to 3.5, a slurry concentration of from 0 to 3 percent by weight, total S03 concentration including the dissolved SO2 gas of from 5 to 8 percent by weight and COD concentration in supernatant liquid of from 700 to 1500 ppm. By neutralizing this solution with alkali reagent in a vessel 7 and, if necessary, adding a flocculating agent, followed by filtration or settling in a tank 8 to separate the solid and liquid, the solution thus separated reduces its solid content from 20 to 10 ppm, its total SO, concentration from 3000 to 2000 ppm and its COD concentration from 30 to 25 ppm.

Further, total S03 concentration of the solution can be reduced from 100 to 20 ppm by subjecting the solution to air exposure processing in a vessel 9, and thus it is possible to abandon the solution without causing any secondary pollution. As alkali reagents employed for neutralizing, it is possible to use magnesium oxide slurry of the flue-gas processing as shown in the drawing, and the solids separated are again returned to the flue-gas processing step. In carrying out the present invention, temperatures at which the drying and calcination steps are carried out, and operation conditions under which all the steps are effected, can be set as in usual operations.

For example, the drying step is usually care ried out at 200 to 2500C and the calcination step is carried out usually at 500 to 1,0000C.

WHAT WE CLAIM IS: 1. A process for removing chloride ions from flue-gas comprising the steps of bringing a flue-gas containing sulfur dioxide into contact with a circulating slurry containing a magnesium oxide (absorption step), separating solids from a part of the circulating ah- sorbent slurry (solid-liquid separation step), drying and calcining the solids to regenerate the magnesium oxide (dry-calcination step) which is to be re-used in the absorption step, and washing the gas formed at the dry-calcination step with water in a washing tower (washing step), at least a part of the solution withdrawn from the washing tower being removed and any remainder being returned to the absorption step, and the water content of the solids to be fed to the dry-calcination step and the amount of solution withdrawn from the washing tower to be removed being controlled so that the accumulation of the chloride ions is substantially prevented, the amount of solution withdrawn from the washing tower to be removed being inversely proportional to the water content of the solids to be fed to the dry-calcination step.

2. A process according to claim 1 wherein the chloride ion concentration is maintained below 3,000 ppm.

3. A process according to clarm 1 substantially is hereinbefore described with reference tially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. and calcination steps are carried out, and operation conditions under which all the steps are effected, can be set as in usual operations. For example, the drying step is usually care ried out at 200 to 2500C and the calcination step is carried out usually at 500 to 1,0000C. WHAT WE CLAIM IS:

1. A process for removing chloride ions from flue-gas comprising the steps of bringing a flue-gas containing sulfur dioxide into contact with a circulating slurry containing a magnesium oxide (absorption step), separating solids from a part of the circulating ah- sorbent slurry (solid-liquid separation step), drying and calcining the solids to regenerate the magnesium oxide (dry-calcination step) which is to be re-used in the absorption step, and washing the gas formed at the dry-calcination step with water in a washing tower (washing step), at least a part of the solution withdrawn from the washing tower being removed and any remainder being returned to the absorption step, and the water content of the solids to be fed to the dry-calcination step and the amount of solution withdrawn from the washing tower to be removed being controlled so that the accumulation of the chloride ions is substantially prevented, the amount of solution withdrawn from the washing tower to be removed being inversely proportional to the water content of the solids to be fed to the dry-calcination step.

2. A process according to claim 1 wherein the chloride ion concentration is maintained below 3,000 ppm.

3. A process according to clarm 1 substantially is hereinbefore described with reference tially as hereinbefore described with reference to the accompanying drawing.