GB2042493A - Method of removing cyanide from aqueous liquid - Google Patents

Abstract

Cyanide is removed from an aqueous liquid containing uncomplexed cyanide ions or an inorganic cyanide complex or both by adjusting the pH of the liquid to not more than 0.8 using a mineral acid, and adding thereto a chlorine-containing oxidizing agent thereby causing the production of free chlorine and causing said uncomplexed cyanide ions or inorganic cyanide complex or both to decompose into a carbon-containing gas and a nitrogen- containing gas.

Description

SPECIFICATION Method of removing cyanide The present invention relates to a method of treating a cyan-containing waste liquor, for example, an industrial waste liquor containing free cyan or a heavy metal cyanide complex such as red or yellow prussiate of potash etc., or a blood examination waste liquor containing cyanmethemoglobin (cyanide methemoglobin) discharged from a laboratory diagnosis chamber of a hospital, a medical center and the like.

Heretofore, as a method of treating a cyanide-containing waste liquor, there has been used an oxidation decomposition method such as alkali chlorine method, ozone oxidation method or electrolytic oxidation method, a precipitate-separation method such as prussian blue method, a biochemical treatment method, a gasification method, a heat distillation method or an ultra-violet ray decomposition method etc.

Among these treating methods, an ozone oxidation method, an ultra-violet ray decomposition method and a heat distillation method have been said effective in treating a stable heavy metal cyanide complex such as red or yellow prussiate of potash. However, they have many problems in practice in that they necessitate complicated operation, special apparatus and severe reaction conditions etc. In addition, all the aforementioned treating methods contain still 0.1-1 mg/e of cyan in treated liquors and can scarcely decrease cyan content in the treated liquors below the National Environmental Standard Value of not over than 0.02 mg/t (calculated as total cyan) concerning human health.

Meanwhile, a treatment method has not been known to treat a macromolecular substance such as cyanmethemoglobin wherein cyan is coordinated to iron. As a quantitative method of analyzing total hemoglobin in blood, cyanmethemoglobin method is an internationalized clinical diagnostic examination method which, however, discharges a detrimental organic cyanide cyanmethemoglobin as an examination waste liquor. Therefore, an effective method of treating the blood examination waste liquor has become necessary. However, cyanmethemoglobin existing in the waste liquor is a macromolecular substance wherein CN groups are combined in a stable form with haem irons. Hence, it has been considered that cyanmethemoglobin can never be decomposed and separated from cyan by a usual oxidation decomposition treatment.Thus, cyanmethemoglobin has been a cause of environmental pollution, because a method of decomposing cyanmethemoglobin for removing cyan has not yet been known and blood examination waste liquors have been discharged after being diluted.

An object of the present invention is to provide a method of preventing environmental pollution due to cyan-containing waste liquors.

Another object of the present invention is to provide a method of treating a waste liquor containing free cyan.

A further object of the present invention is to provide a method of treating a waste liquor containing an organic cyanide compound, for example, an organic macromolecular cyanide such as cyanmethemoglobin or the like.

Another object of the present invention is to provide a method of treating a waste liquor containing an inorganic cyanide compound and an organic cyanide compound.

We have made various studies and experiments to separate and remove cyan from waste liquors containing free cyan and inorganic cyanide compound such as red or yellow prussiate of potash, as well as from waste liquors containing organic cyanide compounds, for example, organic macromolecular cyanides such as cyanmethemoglobin, leading to the present invention wherein waste liquors containing free cyan and/or inorganic cyanide compound and/or organic cyanide compound can be treated to remove cyan therefrom at an efficiency of substantially 100%.

The present invention provides a method of treating a waste liquor containing free cyan and an inorganic cyanide compound, for example, an inorganic heavy metal cyanide complex such as red or yellow prussiate of potash, wherein the waste liquor is added with a mineral acid such as sulfuric acid or hydrochloric acid to obtain a strong acidity of a pH value of not more than 0.8, and then added with a chlorine-containing oxidizing agent such as hypochlorite or bleaching powder, and if necessary agitated, thereby to effect a reaction at an ambient temperature to produce free chlorine, and the free cyan and the inorganic cyanide compound are decomposed and converted to generate a gaseous carbonaceous compound and a nitrogeneous compound by the produced free chlorine at an efficiency of substantially 100% and removed from the waste liquor.

The present invention provides also a method of treating a waste liquor containing an organic cyanide compound, for example, an organic macromolecular cyanide such as cyanmethemoglobin, in addition to free cyan and an inorganic cyanide compound such as red or yellow prussiate of potash, which comprises a first process wherein the waste liquor is added with a mineral acid such as sulfuric acid or hydrochloric acid to obtain a pH value of 1-3, and then added with a flocculant comprising strong electrolytic macromolecular substance anions (strongly polyacidic anions) or strong polyanions to flocculate and precipitate the organic cyanide compound, and the precipitated organic cyanide compound is separated and removed from filtrate, and a second process wherein the filtrate is further added with a mineral acid such as sulfuric acid or hydrochloric acid to obtain a strong acidity of a pH value of not more than 0.8, and then added with a chlorine-containing oxidizing agent such as hypochlorite or bleaching powder, and if necessary agitated, thereby to effect a reaction at an ambient temperature to produce free chlorine, and the remaining inorganic cyanide compound and free cyan are decomposed and converted to gegenrate a gaseous carbonaceous compound and a gaseous nitrogenous compound by the produced free chlorine at an efficiency of substantially 100% and removed from the waste liquor.

Figure 1 is a characteristic curve illustrating a relation between used sulfuric acid concentrations and removed cyan percentages after the treatment of a waste liquor containing a heavy metal cyanide complex, and Figure 2 is a characteristic curve illustrating a relation between pH values of a waste liquor containing an organic macromolecular cyanide and per cent transmittance of filtrate after flocculating and removing the organic macromolecular cyanide.

We noticed a behaviour of cyanmethemoglobin that it acts as an amphoteric polyelectrolyte or polyampholite having dissociated functional groups of acidic property and basic property and found out that a waste liquor containing cyanmethemoglobin can exceedingly easily be treated by reacting cyanmethemoglobin with a flocculant comprising macromolecular electrolyte ions or strong polyanions to form polyion complex, flocculating and precipitating the polyion complex, and separating the precipitate from the reaction system.

We have found out also that the flocculant comprising strong polyanions is preferably potassium polyvinyl sulfate, potassium (polyvinyl alcohol) sulfate, polystyrene sulfonic acid, chondroitin sulfuric acid or ligninsulfonic acid.

We proceeded further studies and found out that usual cyan-containing waste liquors contain inorganic cyanide compounds in addition to such organic macromolecular cyanide as cyanmethemoglobin, and that these compounds cannot be removed from the usual waste liquors even by means of the above method of flocculating, precipitating and filtering. Because it has been said that among such inorganic cyanide compounds heavy metal cyanide complexes such as red or yellow prussiate of potash are most difficult to separate or decompose, we made studies mainly of heavy metal cyanide complexes.

Sometimes, red prussiate of potash is called as potassium ferrocyanide and yellow prussiate of potash is called as potassium ferricyanide, and they are said to have most high stability constants among heavy metal cyanide complexes. Therefore, they do not decompose substantially in an acidic aqueous solution of a pH value of about 1.

According to our experiments, even when an aqueous solution of potassium ferrocyanide having a cyan concentration of 15.8 mgle and a pH value of 0.3 was agitated for 19 hours, the cyan concentration slightly decreased to 14.8 mg/f. However, if the aqueous solution having the same cyan concentration was made to pH 2.5 and added with a small amount of an aqueous solution of sodium hypochloride and agitated for an hour, the cyan concentration decreased to 10.7 mgleo Moreover, when added 6% (WN) of sulfuric acid for further increasing its acidity and 40 me of 50% aqueous solution of sodium hypochlorite and agitated for an hour, the cyan concentration decreased indeed to 0.009 mgleX These facts have hitherto never been known at all and indicate that even a stable heavy metal cyanide complex can be decomposed and the decomposition reaction proceeds exceedingly under a strong acidic condition and in the presence of free chlorine due to addition of a chlorine-containing oxidizing agent.

For instance, Figure 1 shows an example of removed cyan percentage when an aqueous solution of potassium ferrocyanide was added with sulfuric acid to yield several batches of the solution having various concentrations of sulfuric acid and added with sodium hypochlorite and agitated sufficiently and left at 260C for 1 hour. As apparent from Figure 1, the decomposition reaction becomes remarkable when the sulfuric acid concentration is 3.5% (W/V) (pH of the batch reaction solution is 0.7-0.8), and cyan is decomposed and removed at an efficiency of 100% when the batch reaction solution has a strong acidity of a sulfuric acid concentration of 4.5% (W/V) and a pH value of 0.4-0.5. Herein, removal percentage of 100% or the like expression means a state wherein remaining cyan cannot be detected by means of an Official Analysis Method JIS K 0102.

Detailed mechanism of the oxidation decomposition reaction under the above strong acidic condition is not clear at present. However, it is considered that sodium hypochlorite is converted to hypochlorous acid under an acidic condition, and if the hypochlorous acid is combined with CN coordinated to iron to result

the complex becomes unstable to liberate cyanic acid HOCN which is rapidly decomposed under an acidic condition to form ammonia and carbon dioxide. Additionally, cyan ion directly liberated by the decomposition from the complex is presumed to behave in the following two manners, one being an immediate change to HCN to volatize and the other one being a change to cyanic acid HOCN to decompose as described above.

Meanwhile, cyanmethemoglobin produced from an examination of hemoglobin in blood is an iron cyanide complex wherein cyan is coordinated to iron in a macromolecular compound. The above method of examining total hemoglobin in blood by means of cyanmethemoglobin has been widely used as an international standard examination method and has been called as cyanmethemoglobin method.A solution of potassium cyanide or a mixed solution of potassium cyanide and potassium ferricyanide is used in order to change hemoglobin in blood to cyanmethemoglobin, and such stock solutions have high total cyan concentration of 100 m9lez Though blood examination waste liquors containing cyanmethemoglobin resulting from the examinations are rather minor in quantity as compared with waste liquors from factories, they have been considered as most difficult cyan-containing waste liquors to treat, since they contain free cyan, cyan combined with macromolecular substance and sometimes potassium ferricyanide. Direct application of the above oxidation decomposition treatment under a strong acidic condition to such blood examination waste liquor is possible only in principle.However, in practice, a large amount of oxidizing agent is consumed in oxidizing the organic macromolecular substance existing in the waste liquor and a lot of organic matter remain in the treated waste liquor. Accordingly, in order to treat such blood examination waste liquor, the organic macromolecular substance must be removed from the waste liquor and thereafter subjected to the oxidization and decomposition under a strong acidic condition in the presence of free chlorine resulting from a chlorine-containing oxidizing agent used for the treatment of he#avy metal cyanide complexes.The removal of the organic macromolecular substance in the waste liquor can be performed by the formation reaction of polyion complex between strong polyanions and cyanmethemoglobin under a pH value of the waste liquor adjusting to not over than 3 wherein the basic groups in cyanmethemoglobin, i.e., amino, imidazol and guanidyl groups, are dissociated completely.

Formation of polyion complexes between cyanmethemoglobin and strong polyanions is largely influenced by pH value of the system. Figure 2 illustrates a relation between pH values of a blood examination waste liquor containing cyanmethemoglobin which is subjected to the flocculation and precipitation of cyanmethemoglobin with potassium polyvinyl sulfate and transmittance in % of a filtrate after separation of the precipitates. Figure 2 shows that cyanmethemoglobin is substantially completely flocculated and removed from a blood examination waste in the pH range of 1-3.

The mechanism of forming the ion complex is based on a stoichiometric salt bonding reaction between basic groups in cyanmethemoglobin and acidic groups in a strong electrolytic macromolecular substance, whereby a portion of cyan combined with haem iron is dissociated and liberated from the haem iron by the reaction of forming a polyion complex between cyanmethemoglobin and the strong electrolytic macromolecular substance.

Cyan waste liquors discharged from cyan-plating factories and the like factories contain free cyan in forms of potassium cyanide and sodium cyanide in addition to various heavy metal cyanide complexes. If such cyan waste liquor is adjusted to a strong acidic condition, prussic acid is formed which volatizes to the atmosphere. According to our experiments, if an aqueous solution of potassium cyanide having a cyan concentration of 10.1 mg/ & was made to pH 4 and agitated for 5 hours, the cyan concentration decreased to 0.21 mg/ & . Therefore, if such waste liquor containing free cyan is treated under a strong acidic condition, prussic acid is vaporized from the waste liquor. However, the prussic acid can be collected in an alkaline trap.

From sodium hypochlorite or bleaching powder added as an oxidizing agent is formed hypochlorous acid which volatizes and diffuses and is absorbed together with the prussic acid in the alkaline trap wherein a usual alkali chlorination decomposition reaction takes place whereby cyan is treated or decomposed.

As described above, characteristic properties of the present invention are firstly that cyanide compounds having cyan coordinated to iron in a macromolecular substance and heavy metal cyanide complexes which heretofore have been considered difficult to treat can be treated satisfactorily, secondly that waste liquors containing free cyan in addition to an inorganic cyanide compound such as a heavy metal cyanide complex or an organic cyanide compound such as an organic macromolecular cyanide can be treated; thirdly that the treatment can be effected at a mild condition (reaction at an ambient temperature), fourthly that a complicated apparatus is not necessary and the treatment can be performed in a sealed vessel equipped with a gas trap, and fifthly that cyan can be removed at an efficiency of substantially 100% (95-99.99%) by using a cheap apparatus such as plastics vessel because the treatment reaction can be effected at an ambient temperature.

Hereinafter, the present invention will be explained in more detail with reference to preferred examples.

In the examples, all percentages are shown by weight basis, unless otherwise specified.

Example 1 A 500 mt gas-absorbing flask containing 200 mb of 3N-sodium hydroxide is connected to a 3,000 mt sealed glass vessel through a rubber tube. The sealed container is charged with 100 me of aqueous potassium ferricyanide solution having a cyan concentration of 50 m9le and 50 mt of sulfuric acid and a sufficient amount of distilled water to give a total volume of 2,000 m~. The resulting solution has a total cyan concentration of 2.5 milt. The solution is added with 20 g of bleaching powder to start reaction with stirring.

The reaction is continued at an ambient temperature for 10 hours, while agitating and adding each 2 g of bleaching powder every 1 hour until 8th hour. After completion of the reaction, the solution is filtered through a No. 2 filter paper and chlorine remaining in the filtrate is decomposed by sodium sulfite and total cyan in the filtrate is analyzed. As a result, cyan concentration if 0.018 mgle and cyan cannot be detected from the solution in the gas absorbing flask.

Example 2 50 mb of aqueous K2Ni(CN)4 having a cyan concentration of 250 mgK is charged in the same apparatus as described in example 1 and added with 70 mt of concentrated sulfuric acid and a sufficient amount of distilled water to give a total volume of 1,800 mt and further added with an aqueous sodium hypochlorite solution having an effective chlorine concentration of 10% to make total volume 2,000 mf. Thus obtained solution has a cyan concentration of 6.25 mg/i'. The solution is reacted at an ambient temperature for4 hours while agitating by a magnetic stirrer. After completion of the reaction, excess chlorine is decomposed in the same manner as described in example 1 and total cyan in the treated solution is analyzed.No cyan is detected in the treated liquor, nor in the gas absorbinb flask.

Example 3 1,000 mle of a cyanmethemoglobin-containing blood examination waste liquor discharged from a laboratory diagnosis chamber of a hospital having a total cyan concentration of 1.1 mg/i' is made to pH 2.5 by adding sulfuric acid and added with 30 m4 of an aqueous solution of potassium polyvinyl sulfate having a concentration of 1.79 9/100 mt to form a polyion complex by reacting cyanmethemoglobin with potassium polyvinyl sulfate.Thereafter, each 10 g of activated clay and active carbon are added as filter aids and the resulting precipitates are filtered through a No. 2 filter paper. 800 mt of the filtrate is added with 35 mt of concentrated sulfuric acid and 100 mb of an aqueous sodium hypochlorite solution and decomposition treatment is effected similarly as in example 2. Treated solution has a cyan concentration of 0.021 mule. No cyan is detected in the gas absorbing flask.

Example 4 200 mt of a cyan-containing waste liquor having a total cyan concentration of 52 mgle discharged from a cyan-plating plant is added with 70 mt of concentrated sulfuric acid and distilled water to make total volume 2,000 me. Thus obtained solution is treated in the similar manner as in example 2. Treated solution has a cyan concentration of 0.016 mg/i'. No cyan is detected in the gas absorbing flask.

Example 5 The treatment of example 3 is repeated, except that ligninsulfonic acid is used as a flocculant instead of potassium polyvinylsulfate. Treated solution has a total cyan concentration of 0.01 mg/i'.

As described above, according to the present invention, waste liquors containing free cyan in a form of potassium cyanide, sodium cyanide or the like, an inorganic cyanide compound, for example, an inorganic heavy metal cyanide complex such as red or yellow prussiate of potash and an organic cyanide compound such as cyanmethemoglobin discharged from hospitals, factories and the like, which have heretofore been considered most difficult to separate and remove cyan therefrom, can be separated or removed from cyan in an economical way in an extremely simple and cheap apparatus using a plastic reaction vessel and an alkaline trap in a short period of time by a reaction at an ambient temperature, and total cyan in the waste liquor can be decomposed and removed at an efficiency of substantially 100%, i.e., 95-99.99% and a total cyan concentration in the treated waste liquor can be decreased to an undetectable extent of not over than 0.02 mg/i. Accordingly, the present invention is an epoch-macking industrially useful invention having a distinguished remarkable effect of preventing environmental pollution occurred by cyan-containing waste liquors.

Although the present invention has been explained with reference to specific values and embodiments, it will of course be apparent to those skilled in the art that the present invention is not limited thereto and many variations and modifications are possible without departing from the broad aspect and scope of the present invention as defined in the appended claims.

Claims (16)

1. A method of removing cyanide from an aqueous liquid containing uncomplexed cyanide ions or an inorganic cyanide complex or both, which process comprises adjusting the pH of the liquid to a value of not more than 0.8 using a mineral acid, adding thereto a chlorine-containing oxidizing agent, and if necessary agitating the oxidizing agent-containing liquid, thereby causing the production of free chlorine and causing said uncomplexed cyanide ions or inorganic cyanide complex or both to decompose into a carboncontaining gas and a nitrogen-containing gas.

2. A method according to claim 1, wherein the inorganic cyanide complex is an inorganic heavy metal cyanide complex.

3. A method according to claim 2, wherein the inorganic heavy metal cyanide complex is potassium ferrocyanide or potassium ferricyanide.

4. A method according to any one of the preceding claims wherein the mineral acid is sulfuric acid or hydrochloric acid.

5. A method according to any one of preceding claims wherein the chlorine-containing oxidizing agent is sodium hypochlorite or bleaching powder.

6. A method according to any one of the preceding claims wherein the production of free chlorine is effected at ambient temperature.

7. A method according to any one of the preceding claims wherein the decomposition reaction is effected at ambient temperature.

8. A method according to any one of the preceding claims wherein the carbon-containing and nitrogen-containing gases are passed into a trap containing an aqueous alkaline solution to trap HCN gas.

9. A method according to any one of the preceding claims wherein the aqueous liquid containing uncomplexed cyanide ions or an inorganic cyanide complex or both has been obtained from a liquid which also contains an organic cyanide by a process which comprises adjusting the pH of the liquid also containing the organic cyanide to a value of from 1 to 3 using a mineral acid, adding a flocculant comprising strong polyanions to flocculate and precipitate the organic cyanide and removing the precipitated organic cyanide from the liquid.

10. A method according to claim 9, wherein the organic cyanide is an organic macro-molecular cyanide.

11. A method according to claim 10, wherein the organic macro-molecular cyanide is cyanmethemoglobin.

12. A method according to any one of claims 9 to 11 wherein the mineral acid employed to adjust the pH to a value of from 1 to 3 is sulfuric acid or hydrochloric acid.

13. A method according to any one of claims 9 to 12 wherein the flocculant is potassium polyvinyl sulfate, potassium (polyvinyl alcohol) sulfate, polystyrene sulfonic acid, chondroitin sulfuric acid or ligninsulfonic acid.

14. A method according to any one of claims 1 to 8 wherein the aqueous liquid containing uncomplexed cyanide ions or an inorganic cyanide complex or both is a waste liquor.

15. A method according to any one of claims 9 to 13 wherein the liquid containing the organic cyanide is waste liquor.

16. A method of removing cyanide from an aqueous liquid substantially as hereinbefore described with reference to any one of the Examples.