SYSTEM AND METHOD FOR THE EXTRACTION OF CYANIDE FROM TAILS LIQUOR

Abstract: The invention relates to a method and system for separating by aeration a component, such as cyanide, from a liquid containing the component. The method comprises passing the liquid through an array of aeration columns arranged in stages so that the liquid flowing from one aeration column in a first stage is divided into two or more streams which are introduced into separate aeration columns in a successive second stage. The method also comprises passing air through the aeration columns counter-current to the liquid to entrain at least a portion of the component in the air thereby to separate at least the portion from the liquid.

Description: SYSTEM AND METHOD FOR THE EXTRACTION OF CYANIDE FROM TAILS LIQUOR The present invention relates to a method and a system for treating by aeration a liquid containing a component to separate the component from the liquid. In particular, the present invention relates to a method and a system for treating by aeration a liquid containing cyanide to separate cyanide from the liquid. It is known to aerate a liquid to separate from the liquid any dissolved gases in the liquid. For ' example, it is known to aerate liquid containing dissolved cyanide to separate the cyanide from the liquid. One problem associated with known aeration techniques is that a considerable volume of air is required in proportion to the volume of liquid. An object of the present invention is to alleviate the problem described in the preceding paragraph and to provide a more efficient and cost effective method and system for treating by aeration a liquid containing a component to separate the component from the liquid. According to the present invention there is provided a method of separating by aeration a component from a liquid containing the component comprising, in a first stage, passing the liquid counter-current to a stream of air to entrain a portion the component in the air thereby to separate the portion. from the liquid, subsequently dividing the liquid into a plurality of divided portions, and, in a second stage, passing each divided portion counter-current to a stream of air so as to entrain a further portion of the component in the air thereby to separate the further portion from the liquid. Preferably, the component comprises cyanide. Preferably, according to the present invention there is provided a method of separating by aeration cyanide from liquid containing cyanide, comprising: (a) adjusting the pH of the liquid to be in the acidic range to cause the formation of free hydrogen cyanide gas in the liquid; (b) passing the liquid through an array of aeration columns arranged in stages so that the liquid flowing from one aeration column in a first stage is divided into two or more streams which are introduced into separate aeration columns in a successive second stage; and (c) passing air through the aeration columns counter-current to the liquid to entrain the free hydrogen cyanide gas, thereby to separate cyanide from the liquid. According to the present invention there is also provided a system for separating a component from a liquid containing the component, comprising: (a) a plurality of aeration columns to receive the liquid therethrough, the aeration columns arranged in a series of stages so'that, in use, liquid which flows through one said aeration column in a first stage is subsequently divided into at least two separate streams and the streams are directed into separate said aeration columns in a second stage downstream from the first stage; and (b) means to direct air through the aeration columns counter-current to the liquid to entrain the component in the air, thereby to separate the component from the liquid. Preferably, when the system is used to separate cyanide from a liquid, in accordance with the invention the system comprises a means to adjust the pH of the liquid to be in the acidic range to cause the formation of free hydrogen cyanide gas in the liquid prior to the introduction of the liquid into the aeration columns. The term "column" is understood herein to mean any container or the like, such as an open ended tube which is adapted to receive liquid therein. The term "aeration" is understood herein to mean a process in which a liquid is exposed to the action of air or any other aeration gas. The invention is now described in more detail with reference to a specific embodiment thereof. The description is made with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a tailings treatment process in its entirety; Figure 2 is a detailed schematic drawing showing the particular arrangement of the aeration columns and caustic circulation tanks used in the process; Figure 3 is a block diagram showing the principal of passing air through the aeration columns in a serial manner without changing the volume of air relative to each aeration column; and Figure 4 Is a detailed schematic diagram showing the manner in which air passes through a single pair of separation and absorption aeration columns counter-current with the liquor and caustic solution. The specific embodiment concerns a system and process for -treating tailings from an extraction plant for the recovery of gold or other minerals. In one known process for extracting gold from its ore, cyanide is dissolved in water and the resultant solution is used to leach gold from deposits of ore. In the leaching process, some of the cyanide remains in the tailings or the residue from the plant. The purpose of the system and process described herein is to treat the tailings so as to remove sufficient of the cyanide to enable the tailings to be disposed of in an environmentally satisfactory condition. The system and process initially comprises forming a tails liquor from the tailings dump which is decanted by known means 1 and pumped to a mixing chamber 2. Acid stored in a bulk acid storage tank 3 is pumped by an acidification dosing pump to the chamber for admixture with the tails liquor so as to adjust to the pH of the liquor into the acid range thereby to cause the formation of free hydrogen cyanide gas which is retained in the liquor. From the mixing chamber 2, the liquor is pumped into a sand-floc contact tank 4. Coagulant, flocculant and make-up sand from respective storage tanks 5, 6 and 7 are added to the sand-floc contact tank 4. The mixture is agitated and flows by gravity to a constant density tank 8. The underflow from the constant density tank 8 is pumped to a cyclone 9. The cyclone underflow is returned to the sand-floc contact tank 4 for re-use. The cyclone overflow is directed to a settling pond 10 from which liquor is drawn off and returned to the sand-floc contact tank 4 and the liquid is transferred to pondage. The liquor from the constant density tank 8 flows by gravity through sand filters 12 and then is pumped to a column 13a. The column 13a is a preliminary aeration column for the separation of cyanide from the liquor. In this regard, the liquor is sprayed into the column 13a and flows downwardly therethrough counter-current to a stream of air directed upwardly through the column 13a. The stream of air is drawn from the constant density tank 8 and has the effect of entraining a portion of the free hydrogen cyanide gas in the liquor. The air is drawn from the top of column 13a and is directe'd through an absorption column 13b for absorption of the hydrogen cyanide gas into a caustic solution. The liquor, with a reduced cyanide concentration, which collects at the bottom of the column 13a is pumped to an array of aeration columns, generally identified by the numeral 14, to remove a further• portion of the cyanide in the liquor. The aeration columns in the array 14 are arranged in stages so that the liquid ' flowing from one aeration column in a first stage is divided into two streams which are introduced into two separate aeration columns in a successive second stage. The aeration columns in the array 14 are also arranged to receive therethrough air in counter-current flow to the flow of liquor, thereby to entrain free hydrogen cyanide gas in the air. The array 14 of aeration columns is described in more detail hereinafter. The liquor, with a substantially reduced cyanide concentration, which passes through the final stage of the array 14 of aeration columns, is conveyed to a final aeration column 15. The liquor is passed downwardly through the aeration column 15 counter-current to an upward flow of air drawn from the atmosphere. The liquor from the final aeration column 15, being clean and substantially free of cyanide, is passed through columns 16 which contain carbon to collect any residual gold in the liquor- The liquor from the columns 16 is suitable for disposal. Such disposal may be by direct transfer to a tailings impoundment system or via a pH correction stage 17 where a suitable reagent such as lime or caustic soda is added to raise the pH of the liquor to a level suitable for discharge into the environment. The array 14 of aeration columns is now described in more detail with reference to Figures 2 and 4. The array comprises a plurality of pairs of aeration columns 14a, 14b, ... c' 1 ' which are arranged in three stages. The first stage comprises the pair of-aeration columns 14a, the second stage comprises the pairs of aeration columns 14b, 14b' and the third stage comprises the pairs of aeration columns 14c, 14c', 14c' ' and 14c* * ' . Each pair of aeration columns 14a, 14b ... 14c' ' ' comprises, an aeration column 25 for use in connection with the separation of free hydrogen cyanide gas frorα liquor by means of a counter-current flow of air, and an aeration column 26 for use in connection with the absorption of hydrogen cyanide gas from the air into a stream of caustic solution. - The separation column 25 and absorption column 26 in each pair are separated by an intermediate air duct 29 which maintains liquor in separation column 25 and caustic solution in absorption column 26 whilst allowing air flow 27 to pass serially through both columns from an inlet air port 28 disposed in the separation column 25 to an outlet air port 30 disposed in the absorption column 26. In this manner air is passed serially through the columns 25, 26 in each pair. The liquor is introduced in a spray through an inlet liquor port 31 disposed near the top of each separation column 25 and flows downwardly and is discharged through an outlet liquor port 33 near the base of each separation column 25. Each separation column 25 contains a bed of packing material 32 suspended therein to promote dispersion of the downward flow of liquor and upward flow of air thereby to enhance aeration of the liquor and, as a consequence, to enhance entrainment of the free hydrogen cyanide gas in the air. ' Similarly, the caustic solution is introduced in a spray through a caustic inlet port 34 disposed near the top of each absorption column 26 and flows downwardly and is discharged through a caustic outlet port 36 disposed near the base of each absorption column 26. In addition, each absorption column 26 contains a bed of packing material 35 suspended therein to promote dispersion of the downward flow of caustic solution and upward flow of air thereby to enhance aeration of the caustic solution and, as a consequence, to enhance absorption of the hydrogen cyanide gas in the caustic solution. The pairs of aeration columns 14a, 14b, ... 14c 1 * ' are interconnected to allow selective circulation of the liquor and the caustic solution. Figure 2 shows the physical arrangement of the pairs of aeration columns and the circulation system for liquor and caustic solution, and Figure 3 shows the conceptual disposition of the separation columns 25 in each pair of aeration columns at the various stages. The liquor is introduced into the first stage of the array 14 which comprises the pair of aeration columns 14a. The liquor is introduced into the separation column 25 thereof at the inlet liquor port 31 and flows downwardly to the base of the separation column 25. The liquor discharged from the outlet port 33 is circulated by a liquor pump 37 to the second stage of the array 14 which comprises the pairs of aeration columns 14b and 14b'. The stream of liquor is split equally so that half of the original volume is introduced through inlet port 31 into the separation column 25 of the pair of aeration columns 14b and the other half is introduced through the inlet port 31 into the separation column 25 of the pair of aeration columns 14b* . After flowing through the second stage the liquor is discharged from the outlet ports 33 of respective separation columns 25 and is collectively circulated by a second stage pump 38 to the third stage of the array 14 which comprises the pairs of aeration columns 14c, 14c', 14c' 1 and 14c' ''. The recollected liquor is split equally Into four streams such that a quarter of the original volume is introduced into the separation column.25 of the pair of aeration columns 14c and so on up to the pair of aeration columns 14c'' 1 . After discharge from the third stage the liquor is combined In a single pipeline 38 and is transferred to the final aeration stage 15 and thence to the columns 16 prior to eventual discharge as shown in Figure 1 of the drawings; The caustic solution is pumped from a bulk caustic storage tank 19 via a pipe line 39 and initially is discharged into a caustic circulation tank 20a intended to supply the third stage of the array 14 of aeration columns, i.e. converse to the supply of liquor which is fed initially to the first stage of the array 14 as described in the preceding paragraph. Water from a water storage tank 23 is also pumped into the third stage caustic circulation tank 20a via a pipe line 22. In a closed circuit arrangement, the diluted caustic solution accumulated within the third stage circulation tank 20a is circulated by means of a third stage caustic recirculation pump 40 from the circulation tank 20a to each of the absorption columns -26 in the third stage of the array 14, i.e. the absorption columns 26 in the pairs of aeration columns 14c, 14c', 14c' ' and 14c 1 - ' . In each of the absorption columns 26 of the third stage, the caustic solution is sprayed from the top through the inlet liquor ports 34 and flows through the absorption columns 26 and subsequently is discharged through the outlet liquor ports 36. Each of the outlet ports 36 of the absorption columns 26 of the third stage are connected to a common return pipe line 41 which discharges back into the third stage recirculation tank 20a, wherein excess caustic solution subsequently is fed • via pipe line 42 to a second stage circulation tank 20b. The accumulated caustic solution in the second stage circulation tank 20b in turn is pumped via a second stage caustic recirculation pump 43 to each of the absorption columns 26 of the second stage, i.e. the absorption columns 26 in the pairs of aeration columns 14b and 14b'. As in the third stage, the caustic solution is sprayed into the absorption columns 26, through inlet ports 34 and flows downwardly and is discharged through the outlet ports 26 thereof. Subsequently, the discharged caustic solution is fed to a common return pipe line 44 for return to the second stage circulation tank 20b, thus providing a closed loop circulation. The excess caustic solution contained within the second stage caustic circulation tank 20b in turn is supplied to a first stage caustic circulation tank 20c via pipe line 45. Caustic solution supplied to the first stage circulation tank 20c, in turn is pumped via the first stage recirculation pump 46 to the absorption column 26 of the first stage; i.e. the absorption column 26 of the pair of aeration columns 14a. The caustic solution, as in the previous stages, flows downwardly and subsequently is discharged through the outlet port 36 for return via pipe line 47 to the first stage circulation tank 20c. To complete the circuit, excess caustic solution in the first stage circulation tank 20c in turn is fed to a final caustic circulation tank 20d to be subsequently returned for use in the mineral recovery plant for leaching. It should be evident that the concentration of absorbed cyanide within the caustic solution increases as the caustic solution is circulated from the third stage to the first stage for collection in the final circulation tank 20d. It should also be evident that each stage of the array 14 of aeration columns receives therethrough substantially the same volume of liquor and that each pair of aeration columns 14a, 14b, ... 14c 1 * ' in each stage receives therethrough substantially the same volume of liquor and caustic solution. Air is drawn serially through the pairs of aerator columns 14a, 14b, ... 14c' ' ' by fans 48a and 48b disposed respectively at the outlet air ports 30 of the pair of aeration columns 14c' and the pair of aeration columns 14c' ' ' . The inlet air ports 28 of the pairs of aeration columns 14a, 14b are vented to the atmosphere so that air may be drawn from the inlet ports 28 to the outlet ports 30 in a serial fashion through the aeration columns. As has been previously described, the intermediate air ducts 29 between adjacent separation and absorption columns 25, 26 in each pair of aeration columns are arranged so as to create rising air streams counter-current to the downward flow of liquor and caustic solution in the aeration columns. An important aspect of the embodiment of the invention as described herein is the circulation of liquor and air through the stages of the array 14 of aeration columns. This is best illustrated in Figure 3 where it can be seen that an initial volume of liquor is passed wholly through the first stage comprising the pair of aeration columns 14a and is subsequently divided into two half volumes which respectively pass through the second stage comprising the pairs of aeration columns 14b, 14b' . The half volumes of cyanide liquor are subsequently divided again at the third stage comprising the pairs of aeration columns 14c, 14c', 14c' ', 14c- 1 ' so that quarter volumes of the original liquor volume are passed through each pair of aeration columns of the third stage. These quarter volumes are then recombined into a single supply pipe for eventual delivery to the final aeration stages as previously described. Simultaneously with the reduction in volume of the liquor, a single volume of air is passed through the various columns serially so that each separation and absorption column 25, 26 is treated with the same volume of air. Therefore, the air to liquor ratio increases as the liquor passes through the pairs of aeration columns in successive stages without an increase in the amount of air required in the system. As a consequence, there is an improvement in the contact time for the liquor and air. and a consequential improvement in the separation of cyanide from the liquor. It should be appreciated that the scope of the present invention is not limited to the particular embodiment herein described. In particular, the invention is not limited to application to gold extraction processes and can be used elsewhere for the separation of cyanide from a liquid. Further, the invention is not restricted to the separation of cyanide from a liquid and relates to the separation of any dissolved gas from a liquid. There exists scope to vary both the application of the present invention and the form of the invention from the embodiment herein described. In this regard, other applications of the invention include all cyanide containing waste solutions and liquors such as : (a) mineral processing liquors; (b) electroplating solutions; (c) electrowinning solutions; and (d) tannery liquors. In addition, variations to the embodiment herein described include: (a) the use of all and varied flocculating and coagulating agents ; (b) the use of any inert, particulate material herein before referred to as "sand" ; (c) the use of a multiplicity of any or all of steps 1-8 shown in Figure 1; (d) the use of a multiplicity of any or all of the separation columns 25; (e) the use of a multiplicity of any or all of the absorption columns 26; (f) . the use of any and all combinations of separation and absorption columns 25, 26 so as to effect variation in the air:liquor ratios employed; (g) the use of a multiplicity of final aeration stages 15 shown In Figure 1; (h) the use of any subsequent water conditioning process specifically to regulate discharge pH and composition including repeating the application of any or all of the stages herein before described; and (i) the use of agents to materially change and/or control the temperature at which the process takes place. In particular, whilst in the preferred embodiment a stream of liquor flowing from a separation column in one stage is divided into two equal-volume portions for introduction into separation columns in the successive stage, it can readily be appreciated that the invention is not so limited and the stream of liquor could be divided into more than two portion ' s which may or may not be of equal volume. Further, whilst in the preferred embodiment the array 14 of aeration columns is arranged in three stages, it can readily be appreciated that any suitable number of stages could be used. Further, whilst in the preferred embodiment the air is passed through the pairs of aeration columns serially, it can readily be appreciated that any suitable air flow could be used.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of separating by aeration a component from a liquid containing the component, comprising: (a) in a first stage, passing the liquid and air in counter-current relationship to entrain a portion of the component in the air thereby to separate the portion from the liquid; (b) subsequently dividing said liquid into a plurality of divided portions; and (c) in a second stage, passing each divided portion and air in counter-current relationship to entrain a further portion of the component in the air thereby to separate the further portion from the liquid.. 2. The method defined in Claim 1, further comprising dividing each of said divided portions into a plurality of further divided portions and, in a third stage, passing each further divided portion and air in counter-current relationship to entrain an additional further portion of the component in the air thereby to separate the additional further portion from the liquid. 3. The method defined in Claim 1 or Claim 2, wherein the divided portions in the or each stage are of substantially equal volume within the or each stage. 4. The method defined in any one of the preceding claims, comprising controlling the flow of air and/or liquid so that the ratio of air to liquid increases as the liquid flows through the successive stages. 5. The method of any one of the preceding claims, wherein air is passed serially through said liquid in each stage and between said stages. 6. The method defined in any one of the preceding claims, wherein the component is cyanide. 7. A system for separating a component from a liquid containing the component comprising: (a) a plurality of aeration columns to receive the liquid therethrough, the aeration columns arranged in a series of stages so that in use liquid which flows through one said aeration column in a first stage is subsequently divided into at least two separate streams and the streams are directed into separate said aeration columns in a second stage downstream from the first stage; and (b) means to direct air through the aeration columns in counter-current relationship to the liquid to entrain free hydrogen cyanide gas in the air, thereby to separate cyanide from the liquid. 8. The ' system defined in Claim 7, further comprises a plurality of pipes or conduits for transferring the liquid from the first stage to the second stage. 9. A method of separating cyanide from a liquid, comprising: (a) adjusting the pH of the liquid to be. in the acidic range to cause the formation of free hydrogen cyanide gas in the liquid; (b) passing the liquid through an array of aeration columns arranged in stages so that the liquid flowing from one aeration column in a first stage is divided into two or more streams which are introduced into separate aeration columns in a successive second stage; and (c) passing air through the aeration columns counter-current to the liquid to entrain at least a portion of the free hydrogen cyanide gas, thereby to separate at least the portion of the cyanide gas from the liquid. 10. The method defined in Claim 9, wherein the liquid is divided into two equal-volume streams. 11. The method defined in Claim 9 or Claim 10, comprising controlling the flow of air and/or liquid through the aeration columns so that the ratio of air to liquid increases as the liquid flows through the aeration columns in successive stages. 12. The method defined in any one of Claims 9 to 11, wherein a given column of air flows serially through aeration columns in each stage. 13. The method defined in Claim 12, comprising extracting free hydrogen cyanide gas from the air by contacting air after it flows from each " aeration column with a solution capable of absorbing free hydrogen cyanide gas. 14. The method defined in Claim 13, comprising pass air flowing from each aeration column through an absorption column and passing the absorption solution counter-current to the air. 15. A system for separating cyanide from a liquid, comprising: (a) a means to adjust the pH of the liquid to be in the acidic range to cause the formation of free hydrogen cyanide gas in the liquid; (b) a plurality of aeration columns to receive the liquid therethrough, the aeration columns arranged in a series of stages so that in use liquid which flows through one said aeration column in a first stage is subsequently divided into at least two separate streams and the streams are directed into separate said aeration columns in a second stage downstream from the first stage; and (c) means to direct air through the aeration columns counter-current to the liquid to entrain free hydrogen cyanide gas in the air, thereby to separate cyanide from the liquid. 16. The system defined in Claim 15, further comprises a plurality of absorption columns for receiving therethrough air from the aeration columns and for receiving therethrough a counter-current flow of solution capable of absorbing the hydrogen cyanide gas entrained in the air. 17. The system defined in Claim 16, wherein the aeration and absorption columns are arranged in pairs with each pair interconnected so that in use air flowing from the aeration column in one pair flows into the absorption column in the same pair.

Inventors: LOBB KEVIN JOHN, LOBB, KEVIN, JOHN
Applicants: GOLCONDA ENG MINING SERV
Application year: 1988
Main classification: C02F1
IPC codes: B01D21, C02F1, C02F9