Method for the recovery of cyanide from solutions

Cyanide-containing solution is subjected to two-stage membrane electrolysis in units each comprising a cathode, an anode and one or more bipolar electrodes interspaced with cationic membranes defining alternating anode and cathode compartments containing anolyte and catholyte, respectively. Electrolysis causes generation of hydrogen cyanide in the recirculating first stage anolyte, and alkali metal hydroxide is formed in the recirculating first-stage catholyte. The hydrogen cyanide is stripped from the first stage anolyte and is subsequently absorbed in catholyte in an absorption unit for the recovery of alkali metal cyanide-containing solution. Partially cyanide-depleted first-stage anolyte is mixed with an amount of an alkali metal chloride and passed as second-stage anolyte through the second-stage unit for the generation of chlorine with destruction of residual cyanide in the anolyte and the formation of alkali metal hydroxide in the recirculating second-stage catholyte from which a portion is directed to the absorption. A substantially cyanide-free effluent is withdrawn from the circulating second-stage anolyte. To allow for the formation of solid precipitates in the anolyte of the membrane electrolysis stages, the width of the anode compartments may be increased to at least 5 mm. Electrolysis is carried out at current densities in the range of 20 A/m.sup. 2 to 350 A/m.sup.2 and at ambient temperatures.

This invention relates to a method for the recovery of cyanide from 
cyanide-containing solutions, and, more particularly, to a method for the 
recovery of cyanide and the subsequent destruction of residual cyanide. 
BACKGROUND OF THE INVENTION 
Cyanide-containing solutions are being used in a number of industrial 
processes which include metal plating processes and metallurgical 
processes for the recovery of precious metals, especially gold. These and 
other processes are the source of cyanide-containing waste solutions which 
must be treated for the removal of cyanide prior to discharge into the 
environment. 
Many methods have been developed for the removal of cyanide from waste 
waters and solutions. The removal processes include such operations as 
dialysis, electrodialysis, membrane electrolysis and electrochemical 
oxidation and reduction with and without the addition of alkali metal 
chloride, alkali metal oxychloride or chlorine. The methods disclosed in 
the prior art are generally directed either to the concentration and 
recovery of cyanide or to the destruction of cyanide in solution. Either 
type of method has a serious disadvantage. The concentration and recovery 
methods leave a residual solution that still contains cyanide, and such 
residual solutions can not be discharged to the environment. The 
destruction methods remove substantially all cyanide without recovery 
thereby losing cyanide that must be replaced in the process wherein 
cyanide is used. 
SUMMARY OF THE INVENTION 
I have now found that cyanide-containing solutions may be successfully 
treated in a two-stage process, wherein a major portion of the cyanide 
content is recovered in the first stage, and residual cyanide in solution 
from the first stage is destructed in the second stage with the formation 
of a substantially cyanide-free effluent. 
More specifically, alkali metal cyanide-containing feed solution obtained 
from a cyanide-using operation is subjected to a first-stage membrane 
electrolysis (ME) in a ME unit comprising at least two cells. Each cell of 
the ME unit consists of an anode (anodic) compartment and a cathode 
(cathodic) compartment separated by a monovalent cation permselective 
membrane. In a two-cell unit, the compartments contain a monopolar anode 
and a monopolar cathode as terminal electrodes with an intermediate 
bipolar electrode forming two anode and two cathode compartments. In a 
more-than-two-cell unit, bipolar electrodes are positioned between a 
monopolar anode and a monopolar cathode forming a multiplicity of 
alternating anode and cathode compartments. The units may either have a 
rectangular shape or a cylindrical shape or a combination of a rectangular 
and a cylindrical unit may be used. 
The feed solution is fed to the anode compartments of the first-stage ME 
unit. The electrical current applied to the electrodes causes the 
generation of hydrogen cyanide in the first-stage anolyte, which is being 
circulated through the anode compartments, forming a partially 
cyanide-depleted anolyte, and the formation of alkali metal hydroxide in 
the first-stage catholyte which is being recirculated through the cathode 
compartments. The generated hydrogen cyanide is stripped from the 
first-stage anolyte in a stripping unit, and is fed to an absorption. A 
portion of the circulating first-stage catholyte is withdrawn and is also 
fed to the absorption, where the hydrogen cyanide is absorbed with the 
formation of alkali metal cyanide-containing solution. This alkali metal 
cyanide-containing solution, which contains a major portion of the cyanide 
in the feed solution, is recovered and returned to the cyanide-using 
operation. 
The first-stage anolyte is circulated through the anode compartments and 
the stripping unit. A portion of the circulating, partially 
cyanide-depleted first-stage anolyte is withdrawn from circulation, is 
mixed with a small amount of an alkali metal chloride, and the mixture is 
passed as second-stage anolyte to and is circulated through the anode 
compartments of the second-stage ME unit. The second-stage ME unit is 
similar to the first-stage ME unit. In the second-stage unit, electrolysis 
causes the formation of chlorine in the second-stage anolyte circulating 
through the anode compartments, and causes the formation of alkali metal 
hydroxide in the second-stage catholyte circulating through the cathode 
compartments. The chlorine destructs the residual cyanide in the anolyte, 
and a substantially cyanide-free anolyte is withdrawn as effluent. The 
effluent may contain free chlorine dissolved therein. A portion of the 
circulating second-stage catholyte is fed to the absorption for hydrogen 
cyanide. 
The first-stage and second-stage catholytes may be passed to a catholyte 
circulation vessel wherefrom catholyte is withdrawn for circulation to the 
first-stage and second-stage ME cathode compartments, and to the 
absorption of hydrogen cyanide. The second-stage anolyte may be circulated 
through an anolyte circulation vessel. Partially cyanide-depleted 
first-stage anolyte and the alkali metal chloride are added and the 
cyanide-free anolyte is withdrawn as effluent from the anolyte circulation 
vessel. 
The process is operated at ambient temperatures, at a current density for 
both ME stages in the range of about 20 A/m.sup.2 to 250 A/m.sup.2, and 
with a feed solution to the ME that may contain as little as 10 mg/L but, 
preferably, should contain at least about 100 mg/L total cyanide. About 
90% of the free cyanide in the feed solution can be recovered and the 
substantially cyanide-free effluent usually contains &lt;1 mg CN/L. 
In order to allow for the formation of solid precipitates in the anolytes 
of the ME, the ME units may be designed with an appropriately chosen 
spacing between a membrane and an anode such that any solids may be 
flushed out by the anolyte flowing through the anode compartments. In case 
of solids formation, the spacing between a membrane and an anode should be 
at least about 5 mm. 
Accordingly, there is provided a method for the recovery of cyanide from an 
alkali metal cyanide-containing feed solution and the formation of an 
essentially cyanide-free effluent by two-stage membrane electrolysis in a 
first-stage electrolysis unit and a second-stage electrolysis unit, each 
unit comprising electrodes consisting of an anode, a cathode and at least 
one bipolar electrode intermediate said anode and said cathode and 
monovalent cation permselective membranes between said anode, cathode and 
bipolar electrode forming alternating anode and cathode compartments, said 
method comprising the steps of circulating anolyte through the anode 
compartments and circulating catholyte through the cathode compartments of 
each of said first- and second-stage electrolysis units; feeding said 
alkali metal cyanide-containing feed solution into circulating first-stage 
anolyte; applying an electrical current between the anode and the cathode 
of said first-stage unit, said current causing the generation of hydrogen 
cyanide in said first-stage anolyte in the anode compartments of said 
first-stage unit with the formation of partially cyanide-depleted solution 
containing generated hydrogen cyanide; forming alkali metal hydroxide in 
the circulating first-stage catholyte in the cathode compartments of said 
first-stage unit; stripping hydrogen cyanide from said partially 
cyanide-depleted solution containing generated hydrogen cyanide; mixing 
partially cyanide-depleted solution substantially free of hydrogen cyanide 
with an amount of an alkali metal chloride to form a mixture; circulating 
said mixture through the anode compartments of the second-stage 
electrolysis unit as second-stage anolyte; applying an electrical current 
between the anode and the cathode of said second-stage unit, said current 
causing the generation of chlorine in said second-stage anolyte in the 
anode compartments of said second-stage unit, said chlorine causing 
destruction of cyanide in said mixture with the formation of said 
substantially cyanide-free effluent; forming alkali metal hydroxide in the 
circulating second-stage catholyte in the cathode compartments of said 
second-stage unit; absorbing stripped hydrogen cyanide in at least a 
portion of said circulating first-stage catholyte and a portion of said 
circulating second-stage catholyte for the formation of alkali metal 
cyanide-containing solution; recovering formed alkali metal 
cyanide-containing solution for the recovery of a major portion of the 
alkali metal cyanide contained in said feed solution; and removing said 
substantially cyanide-free effluent. 
According to a preferred embodiment, at least one of the first- and the 
second-stage electrolysis units has anode compartments with a distance 
between the electrode and the membrane of at least about 5 mm. Preferably, 
anode compartments have a distance between the electrode and the membrane 
in the range of about 5 to 100 mm, and cathode compartments have a 
distance between the electrode and the membrane of not greater than about 
1 mm. 
It is an aspect of the present invention to provide a method for the 
recovery of cyanide from cyanide-containing solutions. 
It is another aspect to provide a method for the substantial destruction of 
cyanide in cyanide-containing solutions. 
It is yet another aspect to provide a method for the recovery of cyanide 
from cyanide-containing solutions and for the destruction of any residual 
cyanide. 
It is further aspect to recover cyanide from and destruct residual cyanide 
in cyanide-containing solutions using an integrated two-stage membrane 
electrolysis process.

DETAILED DESCRIPTION 
Cyanide-containing solutions that can be treated according to the method of 
the present inventions are solutions that contain alkali metal cyanide. 
Solutions are obtained from processes for the preparation of chemicals, 
metal treatments such as plating, coating, etching and the like processes, 
and metallurgical processes, particularly precious metal recovery 
processes, wherein cyanide solutions are used. The alkali metal cyanide 
may be either sodium cyanide or potassium cyanide. After being used in 
these processes, the residual solutions contain the alkali metal cyanide, 
and contain cyanide as cyanide ions, and may contain metal ions, such as, 
for example, iron, nickel, cadmium, zinc or copper ions, or complex metal 
cyanide ions originating from the metal treatment, the chemical processes 
or the metallurgical processes. The complex cyanides may include those 
containing the metals iron, nickel, cadmium, zinc, copper, silver and 
gold. When the cyanide-containing solution contains silver or gold cyanide 
complexes, the solution is preferably treated to remove these complexes 
prior to submitting the solution to the process of the invention. The 
relative cyanide and metal ion concentrations determine whether the metal 
is present as metal ion or as complex metal cyanide ion or, in some cases, 
as both. The cyanide solutions to be treated according to the instant 
invention should be essentially free of solids but may contain 
constituents, such as metals and/or complex metal cyanides, that may form 
solid precipitates such as cyanides and hydroxides during treatment. The 
cyanide-containing solutions that can be treated may contain as little as 
10 mg total cyanide/L but, to make recovery worthwhile, should contain at 
least about 100 mg total cyanide/L. The solutions may contain as much as 
several thousand mg total cyanide/L. If desired, dilute solutions may be 
concentrated prior to treatment. 
The following description is made with reference to a sodium 
cyanide-containing feed solution, the use of sodium chloride and the 
generation of sodium hydroxide. It is understood that the process can be 
carried out equally well with the corresponding potassium compounds. 
With reference now to FIG. 1, cyanide-containing solution that contains 
cyanide ions and may, additionally, contain dissolved complex cyanides or 
dissolved metals or both, is fed as feed solution 1 to a first-stage 
membrane electrolysis (ME) unit, generally indicated with 2. 
The first-stage ME unit 2 comprises a number of two-compartment, bipolar 
electrode cells. A four-cell unit is illustrated, but a unit may contain 
from two to a multiplicity of cells. The unit 2 comprises a housing 3, a 
terminal cathode 4 and a terminal anode 5 placed at opposite ends of 
housing 3, and at least one electrode/membrane group. Each 
electrode/membrane group consists of a cationic membrane 7 and an 
intermediate electrode, which is a bipolar electrode 8 having a cathodic 
side 8a and an anodic side 8b. A terminal (additional) cationic membrane 6 
is positioned between terminal anode 5 and the cathodic side 8a of the 
adjacent bipolar electrode 8. The number of electrode membrane groups, and 
hence the number of electrode cells, depends on the desired capacity of 
the unit. A cathode compartment 9 is defined between a cationic membrane 7 
and cathodic side 8a of a bipolar electrode 8, between a cationic membrane 
7 and terminal cathode 4, and between terminal cationic membrane 6 and the 
cathodic side 8a of adjacent bipolar electrode 8. An anode compartment 10 
is formed between each cationic membrane 7 and the anodic side 8b of a 
bipolar electrode 8 and between terminal cationic membrane 6 and terminal 
anode 5. Thus, the unit has alternating anode and cathode compartments. 
The terminal electrodes 4 and 5 are connected to a source (not shown) of 
direct electrical current. The terminal anode 5 is made of an 
acid-resistant material such as, for example, lead, graphite, platinum or 
iridium; lead alloys of silver, antimony or calcium; or platinum-coated or 
iridium oxide-coated valve metals. The terminal cathode 4 is made of an 
alkali-resistant material such as, for example, copper, lead, nickel, 
iron, steel, tin, silver, graphite, gold, platinum, palladium or 
platinum-plated titanium, iridium or iridium oxide, zirconium or niobium, 
or alloys of lead or nickel. 
Each bipolar electrode 8 has a cathodic side 8a and an anodic side 8b, and 
is made from a suitable, electrically conductive material or composite 
that, when the direct current is applied between the terminal electrodes 4 
and 5, causes formation of oxygen at the anodic side 8b and formation of 
hydrogen at the cathodic side 8a. Suitable materials for the bipolar 
electrodes comprise, for example, graphite, metals such as lead, alloys 
such as antimony-lead, silver-lead or calcium-lead; and composites such as 
titanium coated with a noble metal, or a metal with a cathodic side of, 
for example, nickel and an anodic side of platinum, or platinum-plated 
niobium, tantalum, titanium or zirconium, iridium or iridium oxide-coated 
titanium or a bimetallic electrode with a cathodic side of steel and an 
anodic side of any of the suitable materials listed above for the terminal 
anode. 
The cationic membranes 6 and 7 are suitable monovalent cation permselective 
membranes such as those that have, for example, strongly acidic active 
groups and a membrane matrix of a styrene di-vinyl benzene co-polymer on a 
polyvinyl chloride base, the active groups being sulfonic acid radicals 
(R--SO.sub.3 H). Suitable cationic membranes include sulfonated or 
carboxylated per fluorocarbon membranes. Suitable membranes 6 and 7 are 
treated Selemion.TM. CMR, Selemion.TM. CMD, Selemion.TM. CSR, Selemion.TM. 
CMT and, especially, treated Selemion.TM. CMF membranes, manufactured by 
the Asahi Glass Company of Japan, and equivalent membranes manufactured by 
other companies. 
With reference now to FIGS. 1 and 2, a sodium cyanide-containing feed 
solution 1 is indirectly fed to each of the anode compartments 10 of 
first-stage electrolysis unit 2. (In FIG. 2, the anode and cathode 
compartments, respectively, are schematically indicated as single 
compartments). A first-stage anolyte 11 is supplied to and is circulated 
through anode compartments 10. The circulating anolyte 11 is also passed 
through a stripping unit 12, and feed solution 1 is, preferably, added to 
the circulating first-stage anolyte 11 in stripping unit 12. A first-stage 
catholyte 13 is supplied from a catholyte circulation vessel 14 to and 
circulated through cathode compartments 9. Although the flows of catholyte 
and anolyte through the ME units are shown in FIGS. 1 and 2 as being 
co-current, the flows may also be counter-current. First-stage anolyte 11 
is a cyanide-containing solution that is becoming partially depleted in 
cyanide, as will be explained. First-stage catholyte 13 is a solution that 
is becoming enriched in sodium or other monovalent ions, as will be 
explained. A direct electrical current is applied from a source of direct 
current (not shown) between terminal cathode 4 and terminal anode 5. The 
current should be sufficient to cause the generation of hydrogen cyanide 
in anolyte 11 in anode compartments 10 forming partially cyanide-depleted 
solution containing generated hydrogen cyanide. Sodium and any other 
monovalent cations present in the feed solution pass through the 
monovalent cation permselective membranes 6 and 7 from the anode 
compartments 10 into first-stage catholyte 13 circulating through cathode 
compartments 9. First-stage catholyte 13, thereby, becomes enriched in 
sodium hydroxide and other monovalent cations. Multivalent ions will 
substantially remain in the anolyte, while metal cyanide complexes may 
either dissociate with the generation of hydrogen cyanide or precipitate 
depending on the concentration of a cyanide complex in the anolyte. 
In stripping unit 12, the hydrogen cyanide generated in circulating anolyte 
11 is stripped with a flow of air passed into the unit forming a flow of 
hydrogen cyanide 15 and partially cyanide-depleted solution 18 
substantially free of hydrogen cyanide. Flow 15 and solution 18 are 
separately removed from unit 12. The stripping unit is preferably a 
cylindrical column. 
The flow of hydrogen cyanide 15 is conducted to an absorption 16 wherein it 
is absorbed into a portion of catholyte passed from catholyte circulation 
vessel 14 with the formation of a sodium cyanide-containing solution 17 as 
product, which is returned for use in the metal treatment or in the 
chemical or metallurgical process. 
The cyanide-containing feed solution 1 is usually strongly alkaline, and 
may have a value of the pH of about 10 to 13. As a result of the reactions 
taking place in the first stage ME, the acidity strongly increases so that 
the anolyte 11 usually has a pH of about 1 to 4. 
Partially cyanide-depleted solution 18 is removed from stripping unit 12, 
usually at a rate about equal to the rate of addition of feed solution 1 
to unit 12. Using well-known means, no "short circuiting" in stripping 
unit 12 occurs between feed 1 and solution 18. The solution 18 is passed 
to an anolyte circulation vessel 19. Anolyte from vessel 19 is passed to a 
second-stage ME unit, generally indicated with 20. Second-stage ME unit 20 
is similar to unit 2 as described with reference to FIG. 1. A second-stage 
anolyte 21 is supplied to and circulated through anode compartments 10. 
Anolyte 21 is also passed through anolyte circulation vessel 19. A 
second-stage catholyte 22 is passed to and is circulated through cathode 
compartments 9 from catholyte circulation vessel 14. Vessel 19 is 
preferably a cylindrical vessel. In anolyte circulation vessel 19, 
solution 18 and second-stage anolyte 21 are mixed with an amount of sodium 
chloride. The amount of sodium chloride should be sufficient to cause the 
generation of a quantity of chlorine in second-stage ME unit 20. An amount 
of sodium chloride added in the range of about 2 to 15 g/L of solution 18 
has generally been found adequate for the substantially complete 
destruction of residual cyanide in solution 18 (and anolyte 21). 
A direct electrical current is applied between terminal cathode 4 and 
terminal anode 5 from a source (not shown) of electrical current. The 
current passing between the terminal electrodes causes the electrolysis of 
the sodium chloride in second-stage anolyte 21 with the generation of 
chlorine in the anode compartments 10 and the formation of sodium 
hydroxide in the cathode compartments 9. The amount of sodium chloride in 
anolyte 21 must be sufficient to generate a quantity of chlorine that is 
at least sufficient to destruct substantially all the cyanide present in 
the partially cyanide-depleted first-stage anolyte, i.e. solution 18. The 
second-stage anolyte 21 would normally have a pH similar to that of the 
first-stage anolyte 11, but the pH of anolyte 21 may be controlled by 
adding an amount of catholyte 24 from catholyte circulation vessel 14 to 
anolyte circulation vessel 19. Sodium ions and any other monovalent 
cations present in the feed (anolyte 21) to second-stage ME unit 20 pass 
through monovalent cation permselective membranes 6 and 7 from anode 
compartments 10 into cathode compartments 9 with the formation of sodium 
hydroxide. 
The chlorine generated in anode compartments 10 of second-stage ME unit 20 
reacts with the cyanide in the anolyte 21 passing through compartments 10 
for the substantial destruction of the cyanide, including any metal 
cyanide complexes, to carbon dioxide and nitrogen. The substantial 
destruction of cyanide in second-stage anolyte 21 causes the formation of 
a substantially cyanide-free anolyte, which, if desired, may contain an 
excess of dissolved chlorine. An excess of chlorine in the effluent may be 
desired, for example, in processes for the recovery of precious metals. 
Depending on the ambient temperatures an excess of about 1 to 4 g/L may be 
desirable. As sodium chloride is regenerated in the second stage, only a 
small amount must be added to make up for losses and to provide any excess 
chlorine. The substantially cyanide-free anolyte is an effluent 23 which 
is removed from anolyte circulation vessel 19. Using well-known means, no 
short-circuiting occurs between portion 18 and effluent 23. If feed 
solution 1 contains dissolved metals or metal cyanide complexes, 
multivalent metals will be present in effluent 23. Effluent 23 must then 
be treated such as, for example, by adjusting the pH to about 9.5 for the 
further removal of such metals prior to removal of the effluent to a 
tailings pond, other containment or the environment. 
The circulating first-stage catholyte 13 and the circulating second-stage 
catholyte 22 are both passed through catholyte circulation vessel 14 and 
the catholyte streams for circulation to units 2 and 20 are common 
solution from vessel 14. A portion of the catholyte is withdrawn from 
vessel 14 and is passed to absorption 16, where it absorbs the flow of 
hydrogen cyanide 15 from stripping unit 12 for the formation of sodium 
cyanide-containing product solution 17. Fresh solution, which may be water 
or dilute sodium hydroxide solution, may be added to the catholyte in 
vessel 14 as necessary to substantially replenish any portion withdrawn 
from circulation. 
The feed solution 1 to the ME units 2 and 20 should be substantially free 
of solids, but the solution may contain complex cyanides and metal cations 
such as iron, nickel, cadmium, copper, zinc and other metals. In 
particular, the ions of metal cyanide complexes of the aforesaid metal 
cations may form precipitates in the anode compartments of either one of 
or both the first-stage unit 2 and second-stage unit 20. The distances 
between membranes and electrodes in a pack of cells of ME units are 
normally in the order of about 1 mm. In case of solids formation, it has 
been found that these solids accumulate in the ME units and cause clogging 
of the units. In order to accommodate such precipitates and avoid clogging 
of the units, it has been found that when the width of the anode 
compartments 10 is increased solids are readily removed and no clogging 
occurs. The width may be increased in either one or both the ME units. The 
distance between a membrane and an electrode of an anode compartment in 
case of solids formation is increased to at least about 5 mm. The width of 
the anode compartments 10 is, preferably, increased to a distance in the 
range of about 5 mm to 100 mm, most preferably about 5 mm to 50 mm. The 
width of the cathode compartments 9 may also be increased but may also be 
retained at the usual value of about 1 mm or less. 
The ME units 2 and 20 may be constructed in a rectangular shape having 
parallel electrodes and membranes. Alternatively the units may have a 
cylindrical shape having concentric electrodes and at least one membrane. 
In its simplest form, a cylindrical unit may comprise a cylindrical 
housing with a central rod-like anode, a cylindrical cathode on the inside 
wall of the housing and a cation exchange membrane mounted concentrically 
in the space between the two electrodes forming a cathode compartment and 
an anode compartment. A combination of a rectangular cell and a 
cylindrical cell may be used. 
With reference to FIGS. 3 and 4, a cylindrical ME unit, according to the 
invention, comprises a cylindrical housing 30, a cylindrical terminal 
cathode 31 on the inside wall of housing 30 or in spaced proximity 
thereto, and a cylindrical or rod-like terminal anode 32 mounted in the 
centre of housing 30. Between housing 30 and terminal anode 32 are 
positioned at least one electrode/membrane group, each consisting of a 
cationic membrane 34 and a bipolar electrode 33 having a cathodic and an 
anodic side, and mounted concentrically around the anode 32 in spaced 
relationships. Cathode compartments 35 are defined between a membrane and 
the cathode or the cathodic side of a bipolar electrode, and anode 
compartments 36 are defined between a membrane and the anode or the anodic 
side of a bipolar electrode. An additional membrane is to be used to 
ensure that a membrane is present between each of the cathodic and anodic 
electrode surfaces. As with rectangularly shaped units, the membranes 34 
may be the same as those described with reference to FIG. 1. The anode 32 
the cathode 31 and the bipolar electrode 33 may be made of the same 
materials as described with reference to FIG. 1. The cathode may also be 
made of steel mesh. Catholyte 37 is circulated through cathode 
compartments 35, and anolyte 38 is circulated through anode compartments 
36. The directions of flow may be either co-current or counter-current. 
The flows of streams to, from and through a cylindrically shaped unit are 
similar to those of a rectangularly shaped unit. 
The method of the invention is carried out at ambient temperatures, such as 
in the range of about 5.degree. C. to 50.degree. C. The first- and 
second-stage ME units are operated with currents equivalent to current 
densities, expressed as A/m.sup.2 of membrane surface area, in the range 
of about 20 A/m.sup.2 to 350 A/m.sup.2, preferably 50 A/m.sup.2 to 350 
A/m.sup.2. The current applied to the units may be the same or may have 
different values for each unit. Below about 20 A/m.sup.2, the current is 
too low to be effective, while operating above about 350 A/m.sup.2 would 
require a high final ionic concentration in the anolyte. Feed solution 1 
is supplied to the process at a rate in the range of about 3 to 50 
L/h.m.sup.2, based on the first-stage unit membrane area. The rate chosen 
depends on the cyanide concentration in the feed. As destruction of 
residual cyanide in the second-stage is very efficient and fast, the 
second-stage can be fed at a much higher rate, that is, at least 2 times, 
and up to about 10 times higher, than the first-stage. Circulation rates 
of anolytes 11, 21 and 38, and catholytes 12, 22 and 37 are generally in 
the range of about 1.5 m.sup.3 /h.m.sup.2 to 5 m.sup.3 /h.m.sup.2 membrane 
surface area. These circulation rates may be the same or have different 
values for each solution and for each unit. 
Using the method of the invention, 90% or more of the free cyanide in the 
feed solution may be recovered, and the substantially cyanide-free 
effluent usually contains less than about 1 mg total cyanide per liter. 
The invention will now be illustrated by means of the following 
non-limitative examples. 
EXAMPLE 1 
A sodium cyanide-containing solution from a gold ore leaching operation and 
containing 335 mg/L total cyanide and 135 mg/L copper was recirculated 
through the anode compartments of the (first-stage) ME unit as described 
with reference to FIG. 1. The unit comprised a Pb-Ag alloy anode, a 
stainless steel cathode, three bipolar electrodes made of Pb-Ag alloy and 
four Selemion.TM. CMD cation permselective membranes. 
A total of 5 L of solution was treated at a feed flow rate of 19.5 
L/h.m.sup.2 of membrane surface area. The current density was 95 A/m.sup.2 
and the temperature was ambient (22.degree. C. to 26.degree. C.). The pH 
of the feed solution was 11.5. Water was initially circulated through the 
cathode compartments. Hydrogen cyanide was absorbed in sodium hydroxide 
solution. The final solutions were analyzed. The now partially 
cyanide-free solution had a pH of 1.5 and contained 19 mg/L total cyanide 
and 16 mg/L copper. The catholyte contained 13 g/L sodium hydroxide. It 
follows that a major portion (94%) of the cyanide was converted into 
hydrogen cyanide, which was recovered as 66% sodium cyanide-containing 
solution, and copper cyanide precipitate, equivalent to 28% of total 
cyanide in the feed. The precipitate was removed as solids in the anolyte. 
EXAMPLE 2 
Using the same electrolysis unit as in Example 1, 5 L of the solution of 
Example 1 were treated as in the second-stage electrolysis described with 
reference to FIG. 1. The unit was operated with a current density of 200 
A/m.sup.2 and with a rate of feed of 65 L/h.m.sup.2. The feed solution 
having a pH of 11.5, was mixed with 25 g sodium chloride prior to 
electrolysis. No attempt was made to control the pH in the anolyte. Some 
solids were formed in the anolyte which were found to be predominantly 
copper cyanide. 
The final solutions were analyzed. The substantially cyanide-free effluent 
had a pH of 1.6 and was found to contain 0.4 mg/L total cyanide and 59 
mg/L copper, and contained 1.2 g/L chlorine. The catholyte contained 16 
g/L sodium hydroxide. The results show that the cyanide content was 
reduced from 335 to 0.4 mg/L, a level normally suitable for discharge to 
the environment. 
EXAMPLE 3 
This example illustrates the concentration of a relatively dilute 
cyanide-containing solution prior to feeding the solution to the ME 
process of the invention. The concentration was carried out by 
electrodialysis. The electrodialysis unit used for the concentration had 
alternating Selemion.TM. CMR and ASR cationic and anionic membranes, 
respectively, with a total effective membrane pair area of 1720 cm.sup.2. 
Cyanide-containing solution obtained from the cyanide leach of a gold ore 
was fed into the circulating diluate stream. A concentrate stream was 
withdrawn from the circulating concentrate. The withdrawn concentrate 
stream was suitable as feed to the ME process of the invention. The 
concentration was carried out with two different dilute solutions. The 
feed rate of both solutions was 8.75 L/h.m.sup.2 and electrodialysis was 
carried out at 70 A/m.sup.2. The cyanide concentrations in the feed, 
diluate and concentrate solutions are given in Table I. 
TABLE I 
______________________________________ 
mg/L CN 
Feed Diluate Concentrate 
______________________________________ 
1 245 39 3700 
2 485 83 7390 
______________________________________ 
EXAMPLE 4 
This example demonstrates the use of rectangular cell configuration in ME 
with bipolar electrodes in two stages, as illustrated in and described 
with reference to FIG. 2, to recover cyanide and to destroy residual 
cyanide from a cyanide-containing waste solution. The ME units were 
assembled according to the schematic shown in FIG. 1. The terminal 
cathodes and anodes were made of stainless steel and Pb-Ag alloy, 
respectively. The bipolar electrodes were bi-metal plates with a stainless 
steel cathodic surface and a Pb-Ag anodic surface. The unit had a total 
effective membrane area of 480 cm.sup.2 and employed Selemion.TM. CMT 
membranes. The electrodes and membranes were spaced 3 cm apart at the end 
compartments and 5 cm apart for the other compartments. The anolyte and 
catholyte solutions were circulated at a linear velocity of 1.22 m/h which 
ensured that the fine precipitate formed in the anolyte did not settle to 
the bottom of the units. 
In the first-stage, the feed solution containing 820 mg/L cyanide was 
introduced at a rate of 15.6 L/h.m.sup.2 by feeding it into circulating 
anolyte at the stripping column. Air was supplied to the stripping column 
to strip HCN, which was absorbed in a caustic solution in an absorption 
column. A portion of the circulating anolyte approximately equal to the 
raw feed solution was withdrawn and fed to the second-stage ME. Metal 
compound precipitates formed in the anolyte and were found to be 
predominantly copper cyanide. The feed to the second-stage ME unit 
contained 15 mg/L cyanide and was introduced at a rate of 62.4 L/h.m.sup.2 
by blending it with the second-stage circulating anolyte in the anolyte 
circulation vessel, together with sodium chloride added to give 5 g/L NaCl 
in solution. The pH of the second-stage anolyte was controlled at a value 
between 8.0 and 9.5 with caustic. Except at the initial stages of 
operation, a portion of the catholyte product was used for achieving the 
pH control. The catholytes from the two stages were combined and 
circulated through all cathode compartments, while a portion was withdrawn 
as catholyte (caustic) by-product. Both ME units were operated at a 
current density of 200 A/m.sup.2. The effluent withdrawn from the anolyte 
circulation vessel contained less than 1 mg/L cyanide. 
Caustic was generated in the cathode compartments at a current efficiency 
of 73.3% or a generation rate of 219 g/h.m.sup.2 of membrane area. The 
overall cyanide removal and destruction amounted to 99.96% with 
approximately 85% recovered from the absorption of HCN to give a 3200 mg/L 
cyanide solution. 
EXAMPLE 5 
This example illustrates the use of a cylindrical cell configuration for 
treating cyanide-containing solutions in a single stage. 
The cell was constructed as a simple cell with a single cation exchange 
membrane with an effective membrane area of 1216 cm.sup.2 between a 
rod-like graphite anode and a cathode screen. The raw feed (pH 12.5) 
solution was introduced to a circulating anolyte at a rate which resulted 
in the anolyte pH controlled at a value of from 2.0 to 3.6. The anolyte 
was circulated at a linear velocity of 2.25 m/h through the anode 
compartment. The catholyte was circulated through the cathode compartment 
at a linear velocity of 4.3 m/h (although the linear velocity could have 
been selected in the range of 1 to 5 m/h). The anolyte was circulated 
through a stripping column in which air was used to strip the HCN 
generated, as in example 4. The formation of metal cyanide precipitates 
(predominantly CuCN.sub.2) was observed in the anolyte. The feed solution 
which contained 1100 mg/L cyanide was fed at a measured rate of 9.6 
L/h.m.sup.2 to the ME unit by introducing it into the stripping column. 
The ME was carried out at a current density of 72 A/m.sup.2. 
A portion of the circulating anolyte approximately equal in volume to the 
raw feed was removed as treated solution. A portion of circulating 
catholyte was also withdrawn as a caustic by-product. 
Thus, the treatment yielded a sodium cyanide solution containing 3950 mg/L 
cyanide, a caustic by-product and a treated solution containing less than 
1 mg/L cyanide, suitable after pH adjustment for discharging to a tailing 
pond such as found in a gold recovery plant. 
EXAMPLE 6 
This example illustrates the use of the cylindrical cell configuration in a 
two-stage treatment of a cyanide-containing waste stream. The unit as 
described in Example 6 was employed in a two stage treatment according to 
which the first-stage ME comprised two units in parallel and the second 
stage ME comprised a single such unit. Auxiliary process steps, HCN 
stripping and absorption, second stage sodium chloride addition, feed and 
circulating anolyte handling, were similar to the scheme described in 
Example 5. The catholytes from both the first-stage and second-stage 
cathode compartments were circulated using a common recirculating tank out 
of which a caustic product was withdrawn. 
The anolyte and catholyte were circulated at linear velocities of 2.25 m/h 
and 4.3 m/h, respectively. The waste stream containing 900 mg/L cyanide 
was fed at a rate of 14.4 L/h.m.sup.2 into the stripping column and the 
feed rate to the second ME unit (second stage anolyte) was 57.6 
L/h.m.sup.2. The units were operated at a current density of 72 A/m.sup.2. 
The first stage anolyte contained 45 mg/L cyanide. 
The treated anolyte discharged from the second stage contained less than 1 
mg/L cyanide and contained dissolved chlorine gas, which was beneficial 
when discharged to a tailing pond containing other cyanide-bearing 
effluents. 87% of the cyanide in the initial feed was recovered as a 
re-useable cyanide solution. 
EXAMPLE 7 
This example illustrates the removal of dissolved metal from waste streams 
fed to the process. 
The results from the operation of the rectangular cell configuration under 
operational conditions as in Example 5, and those for cylindrical cell 
configuration under conditions described in Example 6 are shown in Table 
II. Table II shows dissolved metal removals achieved during the first 
stage membrane electrolysis. 
TABLE II 
__________________________________________________________________________ 
mg/L IN SOLUTION 
TEST ANOLYTE 
FEED TREATED ANOLYTE 
# CELL TYPE 
pH Cu Zn 
Fe 
Ni 
Cu Zn Fe Ni 
__________________________________________________________________________ 
1 Rectangular 
1.8 463 
44 
-- 
12 
7 2 -- 1.3 
2 Rectangular 
2.5 470 
49 
8 
10 
9 2 0.4 
6 
3 Cylindrical 
1.6 500 
29 
58 
-- 
12 1.4 
0.6 
-- 
__________________________________________________________________________ 
The results show that the process of the present invention gives 
significant metal removal from solution. 
It is understood that changes and modifications may be made in the method 
according to the invention without departing from the scope of the 
appended claims.