US2737298A - Waste disposal process - Google Patents

Description

United States wastes containing cyanides. More particularly, the invention relatesto an electrolytic process for treating solutione containing cyanides to convert the cyanides ,either to less toxic cyanates, or to completely break down the cyanides and cyanates to liberate the nitrogen component thereofias a gas. 7 v

Current methods for the destruction of cyanides in liquid industrial wastes generally involve chlorination under alkaline conditions according to reactions of the following type:

() I NaCN+Clz+2NaOH=NaCNO+2NaCl+HzO and (2-) 2NaCN+5Cl2+10NaOH 7 V =2NaHCOa+l0NaCl+Nai-l4HzO The choice between these two reactions de n pen w the'r' the conversion to cyanate is sufiicienttoper; niit safe disposal, or whether completefbreakldown of the eyanideto nitrogen is-required before the wastes can be disposed of. m The. cost of the chemicals required i thesereaction's, particularly when full destruction of cyanides tonitrogen isfrequi red, and the special equipment needed for chlorination a'nd, for the handling of causticused infreactions, present: serious problems and add excessively tothe overheadin electroplating, and other processes which yield substantial amounts of cyanide wastes. v I-have now discovered a new and practical procedure for treating cyanide wastes to convert the cyanidesthere-v in, .either to cyanates or to free nitrogen, without ,any need for handling free chlorine and caustic, while, at the sarnetime eifecting the conversion at only a fraction of the cost involved in the chemical process above menned- "l he newv process, in accordance iiivenjti comprises adding to a solution of cyanide's: an; alkali nieta lihalide, and subjecting the resulting solution-to elee} trol'y's is for a time, which may be varied from a minimum required to decompose all cyanide present, toa maximum required to decompose all cyanide originally present to free nitrogen. I 5:, t

Inthis process, any alkali metal halide canlbe employed; but, since economy is a primary.tactor,, it is.

preferable to use a cheap form of sodium chloride; such as rock salt, brine, or even sea water, and thesalt can-be' added in solution, or as a solid which is permitted-to dissolve as electrolysis proceeds. It is to be noted in this conneetion, however, that the alkali metal halt al-. though taking. part in the process andbeing essential to the process, is not consumed. In other words, the alkali metal halide behaves somewhat in the ,manner of a catalyst. This will be more readily apparent from the following breakdown of reactions which may he considered to take place in the electrolysis and involving the" alkali metal halide:

(4) On cathode: lNa++10=l0Na (5) On cathode: 1ONa+10HzO=lONa0H+5Hi (6) On anode: 10Cl-+10+=5Cl2 igt r theelectrolysis, no separ'ation of atiolyte from catholyte and" hence the chlorine andsodium hydroxide which are' formed at the anode and, cathode may reac with cyanides and/or with themselves, according to the reaction: 7 y

(7) 2NaOH+Cl2=NaC IO-I NaCl-t-HQO .I d' 'm. slllw e arme Lm t e n. o ever, is as powerfulan o ridiaing agent as treechlorine, d wi l, onv t yan d s't z nat sio ev n nit o Furthermore, the nascent chlorine as ;it i sformed in: the electrolysis reacts: faster-with eyatiides, and with cyanates than does thechlorinedntroduced as a; gas in accordance W th P s 'du e he t f re; m Qi d .T us; t e possibility of interreaction betweerrthe chlorine and sodium hydroxide as formeddoes-not; interfere with the cyanide destruction in; the electrolysis.

.when reactions 2 and 310 o-aretcombined, thefol lowing reaction is .obtained as' representing what takes place the electrolysis -F -F-| .n ='2NaHCOs+N2-] 5H2 Iii-this reaction, hydrogen evolves at the cathode and nitrogenevolves at the anode, and it will be noted that in the overall "reaction', the initiallsodiu m chloride has been cancelled or ofi'set by the sodium chloride formed in the reaction. a

-For .complete breakdowneof cyanide to nitrogen as indicatedin Equation 8', I havexfound that 5 to 6 kw. of electric. energy are'required for each pound of NaCN or its equivalent. On-.tlie other hand,:-if tlie presence' of cyanates :in the-resulting solutioni is; permitted, the energy can be reduced tocabout 2-2.5 kw. h. per pound of NaCNzor its equivalent. In. this-connection,- it will be noted that incthe initial'stages'of the electrolysis, i. e.,- while the primary change involved is conversion of cyanide to cyanate, no nitrogen is evolved at the anode, and, as a practical matter, the time at-whichtheevolution of nitrogen commences can' be'talten as indicating s'ubstan: tial: completion of the breakdown from-cyanide to cyanate.

In supplying eIe'ctricenergy-for electrolysis ,'I preferably employ-a relativelylow'voltage, i-. e.,-a'bout'5' to 7 volts. The a'mperag'e" may' be varied to a considerable extent depending upon the nature of thewa'stesolution being =treatedj and the time required for the electrolysis will accordingly vary' inversely with respect to the amper-age, V v 7 I Th'e amountof sodium-chloride or other alkali" metal halide to be employed depends up'oiithe dilution of the cyanides and upon whether thewas'tes' are treated batchwise; or in .aIfiow-th'rough oricoritinuous process. The lower the quantityfof electrolytes in the wastes, the more sodium chloride should be added, and in batch operation less sodium: chloride ma be used as the chlorine and sodiumj hyd'roxide which are formed as intermediates react. within thesameenvironmentto again form sodium. chloride. ;In flow-through operation, the sodium chloride formedjis constantly.-.being carried-away, and-hence a somewhat greater amount of sodium chloride should be;

andthe cost of 6i-kw. h. of electricity'mayfbe about 5- to. .,l%, making) a total of 16 to 27 -.per pound of.

NaCN. This represents but a small fraction of the cost 3 of materials required in the currently available methods originally mentioned.

Further in the interest of economy, it should be noted that the equipment required is relatively simple and inexpensive. Suitable electrolytic cells for use in the process can be made out of mild steel, or if desired steel lined with concrete. The capacity should be such that at least four hours retention time is insured. This can be achieved by employing a plurality of cells or a single cell of suitable size. Each cell, whether small, or large, should have a number of alternately arranged anodes and cathodes, and in a flow-through system these should be arranged to form a series of baflies so that the waste being treated will fiow in zigzag fashion between them.

The cathodes may be made of carbon steel, although the anodes should be formed of lead, graphite, or other material suitably resistant to the action of chlorine. The cell should of course be set up either outdoors or in a space suitably ventilated for the elimination of hydrogen as it is evolved at the cathodes.

Wastes to be subjected to electrolysis should have a pH of about 8.5 or more, and this pH should be adjusted by the addition of alkali if it is too low. It may also be desirable to clarify wastes by settling, filtering, or the like, before adding the same to the electrolytic cells. On the other hand, if a waste liquor is strongly alkaline, this alkalinity may be partly reduced by the sodium bicarbonate which is formed in the electrolysis. Further adjustment of the pH of efiluent from the cells can be made either by addition of sulfuric acidor the like, or by blending this effiuent with other wastes of an acidic nature.

It will also be noted that valuable metals which may remain in the Wastes may be deposited in the cathodes and thus recovered. To the extent that metals are deposited in this way at the cathode, the amount of hydrogen liberated may be reduced.

The following example will serve to indicate how the process, in accordance with my invention, can be carried out, but it is to be understood that this example is given by way of illustration, and not of limitation:

Example 0.676 oz. of sodium cyanide was dissolved in one gallon of water and placed in a glass vessel. Two fiat electrodes facing each other were submerged. The submerged part was 5" x 3". The distance between the electrodes was 4". The temperature was 70 F. pH-14. The cathode was made of stainless steel, while the anode was made of lead.

Four ounces of sodium chloride in commercial crystalline form was then dumped to the bottom of the vessel. The contents of the vessel were not agitated and hence sodium chloride was dissolving during the electrolysis very slowly. A- current of six volts and amps. was applied for three hours. Hydrogen was evolving in great quantities on the cathode. Nitrogen on the anode started to evolve only after one hour of electrolysis.

NaCN, Time oz./gal

Start, 9:00 a. m 0.676 14 10:00 0.273 14 11:00 11.111 0.013 13 End, 12:00 a. m 0.000 11 are embraced by the appended claims, it is to be understood that they constitute part of my invention.

I claim:

1. The process for treating aqueous cyanide waste solution that comprises adding to the said solution the amount of alkali metal halide at least approximately equivalent to the cyanides in the solution, adjusting said solution to an alkaline pH in excess of about 8.5, and subjecting the resulting alkaline solution to electrolysis at a potential of about 5-7 volts until 2 to 6 kw. h. of energy has been consumed for each quantity of cyanide equivalent to one pound of NaCN to thereby decompose all of the cyanides originally present.

2. The process as defined in claim 1 wherein the alkali metal halide is a form of sodium chloride selected from the group consisting of rock salt, brines, and sea water.

3. The process as defined in claim 2 wherein 5 to 15 pounds of sodium chloride is employed for each cyanide equivalent of one pound of NaCN.

4. The process as defined in claim 3 wherein 5 to 10 pounds of sodium chloride is employed and the electrolysis is conducted batchwise.

5. The process as defined in claim 3 wherein 7 to 15 pounds of sodium chloride is employed and the electrolysis is conducted in continuous flow-through operation.

6. The process for treating aqueous cyanide waste solution that comprises adding to the said solution the amount of alkali metal halide at least approximately equivalent to the cyanides in the solution, adjusting said solution to an alkaline pH in excess of about 8.5, and subjecting the resulting alkaline solution to electrolysis at a potential of 5-7 volts until about 2-2.5 kw. h. of energy has been consumed for each quantity of cyanide equivalent to one pound of NaCN, to thereby convert the cyanides present in said solution substantially to cyanates.

7. The process for treating aqueous cyanide waste solution that comprises adding to the said solution the amount of alkali metal halide at least approximately equivalent to the cyanides in the solution, adjusting said solution to an alkaline pH in excess of about 8.5, and subjecting the resulting alkaline solution to electrolysis at a potential of 5-7 volts until about 5-7 kw. h. of energy has been consumed for each quantity of cyanide equivalent to one pound of NaCN, to thereby decompose the cyanides present in said solution substantially to free nitrogen.

8. The process for treating aqueous cyanide waste solution that comprises adding to the said solution 5 to 15 pounds of sodium chloride for each quantity of waste solution containing cyanides equivalent to one pound of NaCN, adjusting said solution to an alkaline pH in excess of about 8.5, and subjecting the resulting solution to electrolysis at a potential of 5-7 volts until at least 2 kw. h. of energy has been consumed for each such quantity of waste solution to thereby decompose all cyanides present in said waste solution.

9. The process as defined in claim 8 wherein the electrolysis is continued until 2-2.5 kw. h. of energy has been consumed to thereby convert cyanides in said waste solution substantially to cyanates.

10. The process as defined in claim 8 wherein the electrolysis is continued until 5-6 kw. h. of energy has been consumed to thereby decompose cyanides in said waste solution substantially to free nitrogen.

References Cited in the file of this patent 0 UNITED STATES PATENTS 641,571 Witter Jan. 16, 1900 775,597 Cassel Nov. 22, 1904 2,520,703 Wagner Aug. 29, 1950 OTHER REFERENCES Plating, Apr. 1949, pp. 341-347.

Claims (1)

1. THE PROCESS FOR TREATING AQUEOUS CYANIDE WASTE SOLUTION THAT COMPRISES ADDING TO THE SAID SOLUTION THE AMOUNT OF ALKALI METAL HALIDE AT LEAST APPROXIMATELY EQUIVALENT TO THE CYANIDES IN THE SOLUTION, ADJUSTING SAID SOLUTION TO AN ALKALINE PH IN EXCESS OF ABOUT 8.5, AND SUBJECTING THE RESULTING ALKALINE SOLUTION TO ELECTROLYSIS AT A POTENTIAL OF ABOUT 5-7 VOLTS UNTIL 2 TO 6 KW. H. OF ENERGY HAS BEEN CONSUMED FOR EACH QUANTITY OF CYANIDE EQUIVALENT TO ONE POUND OF NACN TO THEREBY DECOMPOSE ALL OF THE CYANIDES ORIGINALLY PRESENT.