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DETOXIFICATION OF BOMBING SOLUTION WASTES CONTAINING CYANIDE ION
Norman C. Peterson
Department of Chemistry
Polytechnic University
Brooklyn, NY 11201
INTRODUCTION
In the jewelry manufacturing industry, soldered products are
brightened and cleaned of tarnish by a technique known as "bombing." The bombing
technique has been described as follows. First cover the gold object with
hydrogen peroxide solution in a small bowl. Second, add a solution of sodium
cyanide heated almost to boiling. Finally, stand back! (Behind a safety shield.)
The remaining solution contains sodium cyanide and a small
amount of hydrogen peroxide after being used to clean the oxide residues from
the soldered jewelry. When this technique is used to clean jewelry, the vigorous
evolution of oxygen from the decomposition of hydrogen peroxide induces the
removal of the oxide layer. The metal oxides are dissolved through the formation
of metal cyanide complex ions. After use the solutions may contain gold, copper
and other metal ions. The metals can be recovered by electrolysis.
A variety of compositions have been used for jewelry bombing,
and no standard of preparation is known to US. The residues from the bombing
process, after recovering the metals by electrolysis, are solutions requiring
detoxification.
Cyanide waste solutions are harmful to aquatic life, bacteria
and other microorganisms in sewage treatment plants. Detoxification of these
toxic waste solutions is required before they can be released into the
environment. It is known that some of the cyanide ion in these solutions can be
removed by electrolytic oxidation. Complete removal requires chemical oxidation,
but electrolysis can reduce the concentration of cyanide ion in the solution by
oxidizing it to cyanate ion. The hazard from the toxic cyanogen chloride
intermediate in the oxidation process is thereby reduced. The chemical oxidation
of cyanide ion can be accomplished by allowing it to react with excess
hypochlorite ion, in alkaline solution. When the cyanide ion reacts to
completion, it is oxidized to nitrogen gas and carbonate ion. The chemistry of
the oxidation is described in the following section. Experimental tests were
performed to demonstrate the time required for the reaction to occur and to
measure the extent of- conversion to nitrogen.
Cut to the chase. Go
directly to the cyanide destruction procedure.
CHEMICAL BASIS OF CYANIDE
DETOXIFICATION BY HYPOCHLORITE ION
The chemical reactions taking place in the reaction of cyanide
ion with hypochlorite ion have been discussed by Arnold, based on data from the
chemical literature. These reactions are described briefly in this section.
Direct oxidation of cyanide ion
(CN-) by hypochlorite ion
(ClO ) will produce cyanate ion
(CNO) in alkaline solution by means of an atom transfer
process as in equation (1).
CN + CIO CNO + Cl
(1)
In addition these reactants can form cyanogen chloride
(ClCN), according to the chemical equation:
CNO + ClO + H2O ClCN
+ 2OH (2)
Cyanogen chloride produced in reaction
(2) undergoes hydrolysis in an alkaline solution to
produce cyanate ion according to reaction
(3).
ClCN + 2OH CNO + H2O
+ Cl (3)
When cyanogen chloride is oxidized by hypochlorite ion,
nitrogen is produced by reaction (4).
CICN + 3ClO + 6OH- N2
+ 2CO3
2 +5Cl + 3HO
(4)
The cyanate ion produced in steps (1)
and (3) is in turn
oxidized by hypochlorite ion to produce nitrogen according to reaction (5).
2CNO + 3ClO + 2OH N2
+ 2CO3
2 + 3Cl + HO (5)
It is evident that the oxidation of cyanide ion to nitrogen
can proceed through several pathways. This can take place via reactions (1)
and (5), or through
reactions (2), (3)
and (5),
or via reactions (2)
and (4). The overall
process can be described by equation (6).
2CN + 5ClO + 2OH N2
+ 2CO3 2 +5Cl +HO (6)
In alkaline solution in the presence of excess sodium
hypochlorite, this reaction will go to completion. Eden et al. found that at pH
8 part of the cyanide was oxidized to nitrate. Some of these reactions are
measurably slow and therefore sufficient time must elapse before all of the
cyanide nitrogen atoms are converted to molecular nitrogen.
Mapstone and Thorn measured the rate of reaction of
hypochlorite with cyanide ion, i.e. the combined reactions (1)
and (2). They found
that these combined reactions take place on a time scale of seconds, and
suggested that the two processes have similar rates.
The rate of reaction of cyanogen chloride in alkaline
solutions containing hypochlorite ion was measured by Eden et al. The rate of
disappearance of cyanogen chloride depends on the concentration of hypochlorite
ion. At pH 11, the reaction takes place on a time scale of tens of minutes under
conditions where excess hypochlorite is present. Their experiment did not
distinguish between reactions (3) and (4), but they showed that at pH 9 the
reaction takes place in about 15 minutes when equivalent amounts of cyanogen
chloride and sodium hypochlorite are present initially. They also demonstrated
that the overall reaction of potassium cyanide and hypochlorite ion (6) has a
rate which depends strongly on the pH and which is much faster at pH 8 than pH
11. The time scale of the reaction is hours at pH II. Much faster rates were
reported by Mapstone and Thorn by timing the appearance of nitrogen bubbles at
pH values between 9 and 10 at 18 C. It is likely that their method is only
useful for measuring the initial rates.
EXPERIMENTAL
A solution of sodium cyanide prepared for jewelry bombing was
titrated with silver nitrate to determine the concentration. The cyanide ion
concentration of the room temperature solution was found to be 0.103 M. To
provide a standard of comparison for the toxic waste solution, 10 ml of the
sodium cyanide solution was diluted to 100 ml of solution with aqueous potassium
hydroxide so that the final pH was 11. The diluted solution had a cyanide ion
concentration of 0.0103 M.
A solution of waste residues from jewelry bombing was obtained
for testing. This solution had been electrolyzed to remove the metal ions. The
cyanide concentration of a sample of this electrolyzed waste bombing solution
was found to be 0.016 M. This suggests the interpretation that electrolysis
reduced the initially present cyanide ion concentration by as much as 80%. The
electrolytic oxidation of cyanide ion produces cyanate ion. The cyanate content
of the bombing solution sample was demonstrated by the amount of nitrogen
produced from the hypochlorite oxidation which was in considerable excess of
that expected from the cyanide ion concentration.
Exactly 4 ml of sodium hypochlorite solution (available
chlorine 15%) was diluted to 100 ml of solution in a volumetric flask by adding
potassium hydroxide of pH 11.
These solutions were separately introduced into a Warburg
flask, in the ratio of 2 ml of cyanide test solution to 1 ml of sodium
hypochlorite solution. The flask was attached to a Warburg manometer and allowed
to come to thermal equilibrium in a constant temperature bath regulated at 25 C.
After thermal equilibrium was established, as indicated by a constant pressure
the solutions were mixed. The increase in pressure due to the formation of
nitrogen gas was measured. Typical results for that measurement are shown
graphically in Figure 1. A yield of 100% of the theoretical nitrogen was found
after a reaction time of eight hours. Under the conditions of these experiments
nitrate ion is not formed in detectable amounts.
DETOXIFICATION PROCEDURE
This procedure is specifically tailored to removing the
cyanide ion from "bombing" solutions after the heavy metal ions have been
recovered by electrolysis. Follow the instructions exactly and carefully.
Cyanide ion is a toxic material and should be treated with
caution. Use plastic or rubber gloves to avoid skin contact with the solution
and the other chemicals. Wear safety glasses. When solutions of cyanide ion are
made acidic, the toxic gas hydrogen cyanide can be released from the solutions.
This procedure uses alkaline conditions which will not generate hydrogen
cyanide. Cyanogen chloride, a poisonous gas, is an intermediate in the oxidation
of cyanide ion with hypochlorite ion. Fortunately cyanogen chloride and hydrogen
cyanide are soluble in water and little of these gases escape from the solution.
Good ventilation is strongly recommended, and a gas mask can be worn. Some
people do not have the ability to smell cyanide compounds in the air. Should a
chemical be spilled on the skin, immediately wash the affected area with large
quantities of water.
The following instructions assume that one has not more than
45 gallons of the electrolyzed cyanate and cyanide containing waste bombing
solution in a 55 gallon drum ready for processing. Do not attempt to detoxify
more than 7.5 pounds of sodium cyanide by this procedure. Extra head space is
needed because of the volume of the solutions which will be added. Before
beginning the detoxification process be sure that the chemical reagents are
available in sufficient quantities. These materials are described in the
chemicals section below. Refer to the section on amounts to find the necessary
quantities of chemical reagents needed for the process.
1. Test the pH of the solution using phydrion test paper. The
pH of the solution must be adjusted to 11 by the addition of caustic soda
solution. Add portions of the caustic soda solution slowly. After stirring, test
the pH with the test paper. continue the addition of portions of caustic soda
solution with stirring until pH 11 is indicated using the test paper.
2. Refer to the section on amounts and select the volume of
sodium hypochlorite solution needed. Add the required amount of sodium
hypochlorite solution to the solution of step 1. Stir the solution and wait for
fifteen minutes for the chemical reaction to occur. Bubbles of nitrogen gas can
be observed. After the reaction stops generating gas bubbles, test for excess
chlorine using the chlorine test paper. If chlorine is not indicated, add
another one gallon portion of sodium hypochlorite solution, stir, wait and test.
When excess chlorine is present allow the solution to stand overnight.
3. It is essential that excess chlorine is indicated using the
test paper before preceding with the next step. If the test
paper does not indicate excess chlorine, this indicates that
the amount of sodium cyanide was incorrectly estimated. In that event, add an
additional gallon of sodium hypochlorite solution with stirring, and allow the
solution to stand overnight again. If excess chlorine is indicated proceed with
the next step.
4. After the reaction of part 3. has been completed, adjust
the pH of the solution to 8 to 8.5 by the addition of dilute sulfuric acid with
stirring. Now test with chlorine test paper again to be sure that there is
excess chlorine. The solution is now detoxified.
CHEMICALS
Sodium cyanide:
A solution is prepared by dissolving dry pellets (eggs) of sodium cyanide in hot
water. A typical preparation is to dissolve four troy ounces of sodium cyanide
in one gallon of hot water.
Hydrogen peroxide:
Commercially available as a water solution, 30% hydrogen peroxide, specific
gravity 1.16
Bombing solution:
A "bombing solution" is made by mixing the hot potassium cyanide solution with
the 30% solution of hydrogen peroxide in the volume ratio of 4 to 1.
Sodium hypochlorite:
Commercially available as an aqueous solution containing 15% available chlorine.
(Specific gravity 1.10) The commercial sodium hypochlorite with 10% available
chlorine also can be used, and proportionally more solution will be required.
Sulfuric acid:
Concentrated sulfuric acid (H SO 4) is available commercially as 96% by weight
(66 Be, specific gravity 1.84)
Dilute Sulfuric acid:
Prepare a dilute solution by adding one quart of concentrated sulfuric acid
slowly and with stirring to one quart of cold water. The solution will become
hot, and can spatter if the mixing is done too rapidly.
Sodium hydroxide (caustic soda):
Dissolve 1 pound of sodium hydroxide pellets in one gallon of cold water with
vigorous stirring. The temperature will increase rapidly during this step.
pH test paper:
An absorbing paper impregnated with several indicator compounds is available
under the name phydrion. The pH of the solution describes the acidity or
alkalinity on a scale of 0 to 14. A solution of pH 7 is neutral, i.e. neither
acid nor alkaline. A solution of pH less than 7 is acidic. A solution of pH
greater than 7 is alkaline. The pH of a solution is determined by immersing the
paper in the solution and comparing the color with the standards on the package.
Chlorine test paper:
Absorbent paper impregnated with starch and potassium iodide is available
commercially. When the test paper is immersed in a solution of sodium
hypochlorite, a chemical reaction takes place, generating iodine. Iodine causes
the starch to turn blue, indicating the presence of chlorine.
AMOUNTS
It is essential that the detoxification reaction is carried
out at the specified pH and in the presence excess sodium hypochlorite solution.
The rate of the reaction depends on obtaining both of these conditions. In
addition excess sodium hypochlorite is needed to be sure that all of the cyanide
and cyanate in the solution is destroyed.
The quantity of sodium hypochlorite to oxidize a known weight
of sodium cyanide can be calculated exactly based on equation (6). It is evident
that 2.5 gram moles of sodium hypochlorite is the minimum amount needed to
oxidize one gram mole of sodium cyanide. The total amount of sodium cyanide used
for the bombing process and collected in the waste solution is a good basis for
determining the volume of sodium hypochlorite solution needed. Table I can be
used to find the volume of sodium hypochlorite needed to detoxify a known weight
of sodium cyanide present in the waste as a total of sodium cyanide and sodium
cyanate.
Table 1
Calculated volume of sodium hypochlorite solution required for
detoxification of sodium cyanide wastes, based on the total amount of sodium
cyanide originally used for the bombing process. For intermediate weights of
sodium cyanide, use the next higher entry in the table.
Sodium Cyanide
(in pounds) |
Sodium Hypochlorite
(in gallons, 15% concentration) |
Sodium Hypochlorite
(in gallons, 10% concentration) |
| 1 |
2 |
3 |
| 1.5 |
2.5 |
4 |
| 2 |
3 |
4.5 |
| 2.5 |
3.5 |
5.5 |
| 3 |
4 |
6 |
| 3.5 |
4.5 |
7 |
| 4 |
5 |
7.5 |
| 4.5 |
5.5 |
|
| 5 |
6 |
|
| 5.5 |
6.5 |
|
| 6 |
7 |
|
| 6.5 |
7.5 |
|
| 7 |
8 |
|
| 7.5 |
8.5 |
|
REFERENCES
K. Arnold, Galvanotechnik, 57, 760 (1966).
G. E. Eden, B. L. Hampson and A. B. Wheatland, J. Soc. Chem.
Ind. Lond.,
69, 244 (1950).
A. I. Gladysheva, A. I. Gutman and I. V. Saramukova, Uch. Zap.
Tsent. Nauch.-Issled. Inst. Olovyanoi Prom. No. 2, 3 (1969). (CA76(6):28118k).
S.N. Joshi, Chem. Techn., 35, 465 (1983).
G. E. Mapstone and B. R. Thorn, J. appl. Chem. Biotechnol.,
28, 135 (1978).
C. C. Price, T. E. Larson, K. M. Beck, F. C. Harrington, L. C.
Smith and I. Stephanoff, J. Am. Chem. Soc., 69, 1640 (1947).
R. Weiner and Ch. Leiss, Metalloberflaeche, 26, 169
(1972).
W. Zabban. and R. Helwick, Pl ating and Surface Finishin 56
August (1980).