AU656899B2

AU656899B2 - Determining cyanide concentration

Info

Publication number: AU656899B2
Application number: AU82492/91A
Authority: AU
Inventors: Frank Kenneth Crundwell
Original assignee: DEBEX Pty Ltd
Current assignee: University of the Witwatersrand, Johannesburg
Priority date: 1990-08-16
Filing date: 1991-08-16
Publication date: 1995-02-23
Anticipated expiration: 2011-08-16
Also published as: AU8249291A

Description

65689,9

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): ADDRESS FOR SERVICE: 0 00 00 0 0 0 0 00 0 C 00 0 0 00 to0 DA-VIES COLLISON -Patent Attorneys 1 Little Cclins Street, Mall1 oaMo~b~ boume.300% r('A ExL.aoo t INVENTION TITLE: Determining cyanide concentration 00 00 .60 0 0 ato The following statement is a full description of this invention, including the best method of performing it known to me/us:a 0 0 -OU 0 a 0 0 0.

BACKGROUND OF THE INVENTION This invention relates to a method of and apparatus for determining the concentration of cyanide ions in a solution. The invention is applicable, but not limited, to the determination of the concentration of cyanide ions in a leaching solution for treatment of gold-bearing ore.

The cyanide ion is an essential reagent in the solubilization of gold. To assure high recoveries in the metallurgical treatment of gold-bearing ores, an excess of this reagent is added, either as sodium cyanide or calcium cyanide. The method of adding cyanide varies from plant to plant. Most plants dose leaching solutions continuously, in proportion to the bulk flow of solids or slurry S 15 being fed to the leach tank. The cyanide is usually °o dispensed in dissolved form by metering pumps or proo portionating valves.

oo 0 0900 0 00 4 a Control over the amount of reagent addition is commonly 0 0o o o 20 effected by manual titration of filtered leach slurries or pulps. The end point of the titration is often difficult :o000 0to detect, and is subject to considerable operator error.

o0O0 In eldition, and perhaps more importantly, the process of oo filtering the sample of leach solution is slow, so that control over the cyanide concentration is not accurate.

0 Because of this, it is common practice to maintain the 00° cyanide concentration in the leaching solution at a higher level than is strictly necessary, to allow for a margin or error. However, since cyanide is expensive, this causes a significant increase in the cost of the gold recovery process, and may result in it being non-viable to treat raw material, such as old mine dump deposits, having low gold concentrations.

Methods of monitoring the concentration of cyanide in a leaching solution have been proposed. One process which has been proposed involves the measurement of the potential of a silver button in contact with the solution d to be measured. However, this method proved not to 7/ 4W- 2- -3provide a satisfactory means of controlling reagent addition to the leach tank, due to the necessity for cleaning and recalibrating the response of the silver button electrode at frequent intervals.

It has also been proposed to use an integrated filtration/potentiometric titration system in which a measured volume of clear filtrate is automatically titrated with silver nitrate. However, this system also suffers form a time delay associated with the inline filtration process and the use of a titration system, which reduces the effectivity of closed-loop control. In addition, this system is prone to blockages and mechanical failure.

It is an object of the invention to provide an alternative method and apparatus.

o SUMMARY OF THE INVENTION o According to the invention there is provided a So 0• 0 method of determining the concentration of cyanide ions in S 20 a solution including the steps of establibhing an electrolytic cell having at least an anode and a cathode including respective selected metals with a potential difference between them, so that a current exists in the solution between the anode and the cathode; maintaining 25 the potential of the anode substantially constant at a .selected value that lies within a predetermined range so that the magnitude of the current is substantially u unaffected by ions other than cyanide ions in the solution; measuring the current; and calculating the concentration of cyanide ions in the solution from the magnitude of the measured current.

Preferably, the anode and cathode are metallic and are selected to ensure the existence of a "window" within which the potential of the anode can be maintained to avoid electrolytic activity involving ions other than cyanide ions in the cell.

The cell may be a three-terminal cell including a /clu third, reference electrode, wherein a constant potential difference is maintained between the anode and the

I

-4reference electrode.

Further according to the invention there is provided apparatus for determining the concentration of cyanide ions in a solution including an electrolytic cell having at least an anode and a cathode including respective selected metals; means for agitating the solution in controlled manner relative to the anode and cathode; circuit means for establishing a potential difference between the anode and the cathode and for maintaining the potential of the anode substantially constant at a selected value that lies within a predetermined range so that the magnitiude of a current between the anode and cathode is substantially unaffected by ions other than cyanide ions in the solution; and measurement means for measuring the magnitude of the current and for providing an output related to the i magnitude of the current which corresponds to the concentration of cyanide ions in the solution.

0.o The anode may comprise copper or silver, and the cathode may comprise platinum.

"00The anode potential is preferably maintained 00 between 400 mV (vs SCE) and 200 mV (vs SCE).

Preferably the apparatus includes circulation means such as a pump for directing the controlled flow of ofat the solution past the anode and cathode.

i The electrolytic cell may include a third, reference electrode, the circuit means being adapted to maintain a constant potential difference between the anode and the reference electrode.

The apparatus may further include control means responsive to the output of the measurement means to vary the amount of cyanide added to the solution, thereby to €s~ -j L I -IN i I- -~C maintain the concentration of cyanide in the solution at a predetermined or desired level.

BRIEF DESCRIPTION OF THE DRAWINGS 09 os o 0eo 0 9 *0o0 00 0 0 00 0 09 0 0 Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figures 6 and 7 Figure 8 is a schematic illustration of a conventional electrolytic cell; is a graph illustrating the dissolution of copper at the electrolytic cell anode showing the effect of varying cyanide concentration; is a graph showing the dissolution of copper at the anode of a similar cell, showing the effect of agitation of the cyanide solution; is a schematic circuit diagram of a three electrode electrolytic cell forming part of apparatus according to the invention; is a graph showing current-voltage curves for the dissolution of copper and silver and other ions in a cyanide leach solution; are graphs illustrating, respectively, the effective of cyanide and the effect of solution agitation on the limiting current of the electrolytic cell; is a schematic illustration of apparatus according to the invention for determining the concentration of cyanide ions in a leach solution; Mal.-i Figure 9(a) is a detail view illustrating an electrode assembly of the apparatus of Figure 8; and Figure 9(b) illustrates an alternative electrode assembly.

DESCRIPTION OF EMBODIMENTS The invention will now be described with reference to preferred embodiments thereof in which the concentration of cyanide ions in a leaching solution for treatment of gold-bearing ore is monitored.

The leaching of gold by oxygen in the presence of cyanide proceeds by the reaction: 4Au 0 2 8CN~ 2H20 4Au(CN-)2 o o 0 00 SO*'o Cyanides are present as cyanide ions and as metal o complexes of widely varying stability. Cyanide compounds "00~ are classified as free cyanides (that is, ionic cyanide, 0 20 CN or as complexed metal cyanide (for example, 0 0 4- 00.. Fe(CN)4- or Zn (CN) 3 The term total cyanide is used for all cyanides, including free as well as j o oo Scomplexed cyanides. The concentration of free cyanide in S a solution depends on its pH and on its content of metals 25 capable of forming cyanide complexes. It is the concentration of free cyanide that we are interested in, 0o since it is this species of cyanide that is available for reaction with gold. The composition of a typical gold Sleach solution is shown in Table 1 below.

Table 1 Composition of a typical gold leach solutidn (ppm) Au 5.4 Fe 1 Ca 590 CNS Ag 0.69 Zn 1.5 Ba 3 S20 4 Cu 7 Pb <1 Na 310 S0 2 1680 4 Ni 2 Hg 0.01 K 45 Cl1 240 Co <1 Al <1 Si 20 Free 92 6

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7 A typical electrochemical cell is shown schematically in Figure 1. A reduction reaction occurs at the cathode (negative electrode) and an oxidation reaction occurs at the anode (positive electrode). The equilibrium potential of an electrode reaction is a thermodynamic measurement and is given by the Nernst equation Fe 3 e" Fe 2 E E° RT In Fe 3 oo nF [Fe I 0 a where E* is the standard reduction potential.

0O 1°3 The current of a cell is a kinetic measurement, that is, o 0 0 °it relates to the rate at which an electrode reaction oo occurs. The rate is given by: rate i ,,rrent density nF number of electrons transferred x Faraday's constant o o° The current density (that is, the current per unit area) is a function of the voltage and other normal kinetic O0 oa o parameters, such as concentrations of the reacting species, conditions of agitation, temperature etc.

In carrying out the method of the invention, it is desired to measure the electrolytic current of the reaction: Cu 2CN" e" Cu(CN)2 This reaction occurs at the anode of the electrolytic cell and typical plots of the cell current against the anode potential for various cyanide concentrations and agitation or stirring conditions are shown in Figures 2 and 3.

It is clear from Figure 2 above that a determination of is possible by controlling or fixing the potential rC ICL of the cell and measuring the cell current. This can be achieved by a cell of the type shown in Figure 1.

However, the potential is not only dependent on the potential of the anode, which should preferably be fixed, but also on the potential of the cathode and on the IR drop across the cell. Therefore, a more sophisticated technique would be to measure the anode potential against a third electrode whose potential in the solution is known and constant, that is, a reference electrode. This technique is known as controlled electrolysis, and an o a electrolytic cell of this type is illustrated schematically in Figure 4.

0 o0 o 0 0 0o- In a typical leach solution, the ions that may interact 00 0 "o with any determination that employs an a"0 electrochemical cell or technique are the following: Cu present as Cu(CN) -or Cu(CN); Fe present as Fe(CN)4- Ni present as Ni(CN)2" *Zn present as Zn(CN)2" 4 I" In addition to the above, there is the reduction of oxygen, given by the reaction: 02 4H 2 0 4e" H 2 0 2 The interaction of such ions in the electrolytic process would negatively affect the accuracy of the cyanide determination. However, as illustrated in Figure 5, a j suitable choice of anode operating potential will allow the determination of without interference by the above ions. From Figure 5 it can be seen that if one chooses a potential at an anode of silver or copper of between 400 mV (vs SCE) and 200 mV (vs SCE), the only current that will be measured is that due to the dissolution of the anode according to the reaction: Cu 2CN' Cu(CN); e" r 9 It is apparent from Figure 5 that both the copper and silver electrodes reach a limiting current density due to the diffusion of to the electrode surface. This means that the current is limited by the supply of cyanide to the electrode, and that the rate will simply be proportional to the cyanide concentration in solution, and on the conditions of agitation in the cell. This is confirmed by the results illustrated in Figures 6 and 7.

From Figures 6 and 7 it can be seen that if the degree of agitation can be controlled, the concentration of cyanide o°o 8 in the solution can be measured directly. Experiments o 00 oo0 were conducted to confirm the above results, and the results of the experiments are given in Table 2 below.

0000 0 0900 So Table 2 0 0o co o00 a 0 o 0 O t0 00 07 Ion Concentration Current Current without with addition addition (AA) (MA) Fe (CN)6 100 ppm Fe 110 106 Ni (CN) 4 100 ppm Ni 110 114 Zn 37 ppm Zn 48 42 Cl" 100 ppm Cl" 131 124 SO 2- 2 000 ppm SO2 131 124 4 SCN 100 ppm SCN" 131 130 S203" 100 ppm S 2 O" 131 132 It can be seen that the presence of the listed salts has very little effect on the electrolytic cell current.

In order to implement the method of the invention, the apparatus illustrated in Figures 8 and 9 is used. Gold is normally leached in mechanically agitated tanks or air agitated pachuca tanks. Because the measurement of cyanide concentration according to the above described method depends on the conditions of agitation as well as on the concentration of cyanide, it is desirable to place I M the measurement cell in an environment where the flow conditions can be controlled. Thus, in the proposed apparatus, a small stream of slurry is pumped from the leaching tank through an electrolytic measurement cell and back into the tar k. The measurement apparatus is conveniently mounted on the top of the leaching tank, as shown in Figure 8.

The circulating apparatus comprises a small pump 10, an inlet 12 which extends into the leaching tank below the surface of the slurry, a conduit 14 including a special conduit section 16 which defines the measurement cell, and an outlet 18 which returns the slurry to the tank.

The special section of the conduit 16 is shown in more S0 detail in Figure 9(a) and is seen to comprise a section 0° of flanged pipe formed of an electrically non-conductive o0 material such as PVC, with a pair of electrodes 20 and 22 disposed on the inner walls of the pipe. The electrode is a copper anode, while the electrode 22 is a 0,,0 platinum cathode. The electrodes are curved to match the 0444 curvature of the inner wall of the pipe, and offer minimal resistance to the passage of the slurry through o the pipe. Conductive leads 24 and 26 are connected to the respective electrodes 20 and 22 and pass through the pipe walls, for connection to an electrical measurement 0 circuit.

It will be appreciated that by controlling the flow of slurry or leach solution through the pipe 16, the variable of solution agitation is effectively held constant so that the electrolytic cell current between the electrodes 20 and 22 will be related directly to the concentration of cyanide in the solution.

The measurements of the apparatus can be used in a process control function, to control the addition of cyanide to the leach solution on a continuous basis. In Figure 8, si.h a process control system is illustrated schematically. Slurry is fed to a leaching tank 30 via 1 7'i 11 a pipe 32. An agitator 34 stirs the slurry in the tank.

The leads from the electrodes 20 and 22 are fed to a measuring circuit 38 and thence to a current/voltage convertor 40 and a potentiostat 42. The potentiostat is of a conventional design, which can be, for example, an analog design using op-amps, or a digital design. The current/voltage convertor can be a simple resistor through which the cell current flows, so that the voltage drop across the resistor is proportional to the cell current. The output of the current/voltage convertor is o O applied to a control circuit 46 which controls a proportionating valve 48 in a cyanide feed line 50. The O circuit is arranged in a feedback mode so that a o cf reduction in the measured cyanide concentration in the -q C °o leach solution relative to a reference concentration will 0 cause the valve 48 to be opened, adding more cyanide to

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the solution. Conversely, if an increase in cyanide concentration is measured, the valve 48 will be closed.

The controller 46 is a conventional process controller.

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I

Figure 9(b) shows an alternative electrode assembly, which can be lowered into a tank of slurry, for example.

The assembly comprises a pneumatic motor 52 with an cI insulated shaft 54.

First and se, ond electrodes 56 and 58 in the form of spaced apart circumferential rings serve as an anode and a cathode, while a third electrode 60 at the end of the shaft serves as a reference electrode. Rotation of the shaft in the slurry or solution being measured ensures appropriate conditions of agitation.

The described method and apparatus has numerous advantages compared to the prior art. Firstly, the system provides an accurate linear response to the concentration of free cyanide in a leach solution. The response is rapid, enabling effective process control to take place. The system is substantially immune to interference from other ions in the leach solution.

2358L 12 There is no need for filtration of the leach solution, which reduces the possibility of blockages and mechanical failure of the measurement cell. Finally, the measurement cell operates by constant dissolution of the anode, which means that the surface of the anode is continually being renewed. In fact, the potential region in which the cell operates is one that is known as electropolishing, so that the surface of the anode is continually being polished in use.

S0 o 0 0 0 0 0i 'ft

A

Claims

1. A method of determining the concentration of cyanide ions in a solution including the steps of establishing an electrolytic cell having at least an anode and a cathode including respective selected metals with a potential difference between them, so that a current exists in the solution between the anode and the cathode; maintaining the potential of the anode substantially constant at a selected value that lies within a predetermined range so that the magnitude of the current is substantially unaffected by ions other than cyanide ions in the solution; measuring the current; and calculating the concentration of cyanide ions in the S solution from the magnitude of the measured current.

2. A method according to claim 1 wherein the anode and the cathode are metallic, the anode being maintained 20 at a potential within a range determined by the metals of O the anode and cathode so that electrolytic activity involving ions other than cyanide ions in the cell does not occur.

3. A method according to claim 1 or claim 2 wherein tee ~25 the cell is a three-terminal cell including a third, reference electrode, a constant potential difference being maintained between the anode and the reference electrode. S4. A method according to any one of claims 1 to 3 wherein the anode potential is maintained between 400 mV (vs SCE) and 200 mV (vs SCE). A method of controlling the concentration o' cyanide ions in a solution including determining the concentration of cyanide ions in the solution according to the method of any one of claims 1 to 4, and further including the steps of comparing the calculated cyanide ion concentration with a reference value, and automatically increasing or decreasing the rate of addition of cyanide ions to the solution according to whether the calculated cyanide ion concentration is lower r -14- or higher than the reference value, respectively.

6. Apparatus for determining the concentration of cyanide ions in a solution including an electrolytic cell having at least an anode and a cathode including respective selected metals; means for agitating the solution in controlled manner relative to the anode and cathode; circuit means for establishing a potential difference between the anode and the cathode and for maintaining the potential of the anode substantially constant at a selected value that lies within a predetermined range so that the magnitiude of a current between the anode and cathode is substantially unaffected by ions other than cyanide ions in the solution; and measurement means for measuring the magnitude of the current and for providing an output related to the magnitude of the current which corresponds to the •0 concentration of cyanide ions in the solution.

7. Apparatus according to claim 6 wherein the anode 0oo comprises copper or silver.

8. Apparatus according to claim 6 or claim 7 wherein *o the cathode comprises platinum. 0000 9. Apparatus according to any one of claims 6 to 8 wherein the apparatus includes circulation means for directing a controlled flow of the solution past the anode 0oo 25 and cathode.

10. Apparatus according to claim 9 wherein the 0 40 circulation means includes a pump. o 11. Apparatus according to claim 9 or claim :oo wherein the circulation means includes a conduit through 0 0150C r lil rcli c which the controlled flow of the solution is directed, the anode and the cathode being located within the conduit.

12. Apparatus according to claim 11 wherein the conduit is a length of pipe of insulating material, the anode and cathode being located on the inner wall of the pipe in a spaced apart relationship.

13. Apparatus according to any one of claims 6 to 8 o wherein the circulation means comprise a support o member on which the anode and cathode are formed, cop, a and means for moving the support member relative to a D the solution. ao ae 4 V 14. Apparatus according to claim 13 wherein the support member is a shaft, and the means for moving the support member is a motor arranged to rotate the shaft about its axis. 4 o E 15. Apparatus according to claim 14 wherein the anode and cathode are formed as spaced apart 0 0 circumferential rings on the shaft.

16. Apparatus according to any one of claims 6 to including a third, reference electrode, the circuit means being adapted to maintain a constant potential difference between the anode and the reference electrode.

17. Process control apparatus for controlling the concentration of cyanide ions in a solution '~.a'\apparatus according to any one of claims 6 to 16, and further including control means responsive to the output of the measurement means to vary the rate of addition of cyanide ions to the solution, thereby to maintain the concentration of 4, cyanide ions in the solution at a predete.mined or desired level. r ,i 16

18. Process control apparatus according to claim 17 wherein the control means includes a conduit for directing a flow of cyanide into the solution, a feedback control circuit responsive to the output of the measurement means, and a valve responsive to the feedback control means to increase or decrease automatically the rate of addition of cyanide to the solution according to whether the measured cyanide ion concentration is lower or higher than a predetermined reference value.

19. A method of determining the concentration of cyanide ions in a solution substantially as herein described with reference to the accompanying drawings. o 00 o 00 o°°S°o 20. A method of controlling the concentration of cyanide o 00 .o0o ions in a solution substantially as herein described o o with reference to the accompanying drawings. 0 0 0 00 0

21. Apparatus for determining the concentration of cyanide ions in a solution substantially as herein described with reference to the accompanying o So o drawings

40...0 22. Process control apparatus for controlling the °So concentration. of cyanide ions in a solution substantially as herein described with reference to o the accompanying drawings. DATED: 15 November 1993 PHILLIPS ORMONDE FITZPATRICK Attorneys for: DEREX (PROPRIETARY) LIMITED ^O^A49 L i i i II-- ABSTRACT A method of determining the concentration of cyanide ions in a solution comprises establishing an electrolytic cell having at least an anode (20; 56) and a cathode (22; 58), and preferably a third, reference electrode The potential of the anode is maintained within a voltage window so that the magnitude of the current between the anode and the cathode is unaffected by ions other than cyanide ions in the solution. The current is measured, oo and the concentration of cyanide ions is calculated from o o a o the magnitude of the current. The invention extends to 00 a process control method which compares the calculated .cyanide ion concentration with a reference value, and o a 0 automatically increases or decreases the rate of addition 0a 0of cyanide ions to the solution according to whether the calculated cyanide ion concentration is lower or higher than the reference value. Apparatus for carrying out the methods s also disclosed. 09806 o0 or0 0 0 0 6 1h,- 1 -1