AU619998B2 - Process for reducing the cyanide content of a solution - Google Patents

Description

OPI DATE 17/01/91 APPLN- I D 59361 PCT AOJP DATE 07/03/91 PCT NUMBER PCT/AU90/00289 IINTERNATIONAL iI.-l iul--- (PCT) (51) International Patent Classification 5 C02F9/00, 1/20, 1/66 C02F 1/76, C22B 11/08 (11) International Publication Number: WO 91/00248 Al 'e p (43) Internationa PublicationDale: l uar 91 (10.01.91) 0/00289 (74)Agent: OBERIN, Colin, James; Oberihs,66 Croydon Road, Croydon, VIC 3136 (AU).

15.07.90) (21) International Application Number: (22) International Filing Date: PCT/AU9i 5 July 1990 (0 Priority data: PJ 5097 5 July 1989 (05.07.89) (81) Designated States: AT (European patent), AU, BE (European patent), CA, CH (European patent), DE (European patent)*, DK (European patent), ES (European patent), FR (European patent), GB (European patent), IT (European patent), LU (European patent), NL (European patent), SE (European patent), US.

Published With international search report.

(71) Applicant (for all designated States except US): MINPROC TECHNOLOGY PTY. LIMITED [AU/AU]; 364 Albert Street, East Melbourne, VIC 3002 (AU).

(72) Inventors; and Inventors/Applicants (for US only) COSTELLO, Michael, Charles [AU/AU]; 15 Elmshurst Way, Greenwood, W.A.

6024 ROGERS, Douglas, Gordon [AU/AU]; 17 Ebro Way, Willetton, W.A. 6155 GUERNEY, Philip, John [AU/AU]; 29 Moorfields Street, Fig Tree Pocket, QLD 4069 (AU).

(54) Title: PROCESS FOR REDUCING THE CYANIDE CONTENT OF A SOLUTION (57) Abstract A process for treatment of waste aqueous process streams from industrial processes containing both cyanide ions and dissolved ammonia to reduce the cyanide content, comprising the consecutive steps: adding a halogenating agent to the solution while maintaining the pH of the solution at greater thar. 7; removing ammonia from the solution by aeration; acidifying the solution to a pH of less than 7; substantially neutralising the solution; and removing the ammonia from the solution by aeration.

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WO 91/00248 PCT/AU90/00289 1 TITLE: PROCESS FOR REDUCING THE CYANIDE CONTENT OF A

SOLUTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for reducing the cyanide content of a solution.

The use of an alkaline aqueous solution of a cyanide salt for recovery of gold and/or silver values from ores, tailings and wastes has been well known and documented for a long period. While this process is well known and widely used for such precious metal recovery it is also to be understood that the waste water streams from such processes contain cyanide in solution which could be dangerous if allowed to enter the environment and particularly the water supply.

For this reason, waste aqueous process streams from gold treatment plants using alkaline cyanide solutions as the leachant must meet specific composition limits before they are discharged into the environment. In particular, the total cyanide and free cyanide concentrations must be below the maximum limits set by local, state and federal licensing authorities.

Similar requirements are imposed on the effluent discharged from cyanide production plants and from other industrial processes some of which produce waste water streams containing other species notably ammonia as well as cyanide ions.

2. Discussion of the Prior Art A number of processes have been proposed to bring about the chemical breakdown of the total and free cyanide components of typical effluent streams. Some of the processes have been patented and several are presently used on the commercial scale.

High temperature hydrolysis brings about the total destruction of the cyanide ion, but the high temperatures and extended retention times required makes this process inefficient in cost terms. Some chemical reagents convert 1 I_

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WO 91/00248 PCAU90/00289 2 the cyanide ion only to cyanate. In many localities, discharge of cyanate is not permitted. Further destruction of cyanate may be inefficient or costly in reagent consumption terms, or in fact require a different chemical reagent.

The complexity of cyanide destruction is compounded by the presence of other species in solution that interfere with the destruction mechanism and/or substantially increase the capital and operating costs. One such species is dissolved ammonia.

Treatment of an effluent or processing liquor containing, for example, an equivalent concentration of cyanide ions and dissolved ammonia, by the standard cyanide destruction methods is unsatisfactory in terms of additional reagent consumption, increased capital and operating costs and/or incomplete cyanide destruction. These methods include the use of hydrogen peroxide, chlorine, chlorine/calcium hydroxide, hypochlorite, or sulphur dioxide.

The applicants are aware, for example, of a proposal described in Australian Patent Application 73261/81 (PCT International Publication Number W082/00288) for removing cyanide ions from solutions. This process provides for adjusting the solution to a pH of at least about 11 and to a halide content at least equivalent to the cyanide concentration. Electrolysis is performed using intense agitation of the electrolyte to result in deposition of solid carbon on the anode. Electrolytic methods generally require a readily available source of cheap electricity for economic operation and this is not always available in remote areas.

Further, this proposal does not deal with any dissolved ammonia also present in the waste water stream.

Australian Patent Application 17053/88 (PCT International Publication Number WO 88/08408) relates to a method 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 column 1 WO 91/00248 PCT/AU90/00289 in a first stage is divided into two or more streams which are introduced intoseparate aeration columns in a successive second stage. However this process is likewise unsuitable I for use in remote areas and does not effectively deal with any dissolved ammonia present in the waste water system.

Australian Patent Application 76929/81 relates to the chlorination of waste water containing ammonia, cyanide and phenol. This method involves adding a chlorinating compound to the waste water in a first vessel at pH 8.5 to 10, passing the waste water to a second vessel at pH 6 to 8 and controlling the addition of the chlorinating compound in response to the redox potential in the second vessel so as to maintain the redox potential at +625 millivolts to +750 millivolts. This method requires close monitoring of redox potentials and is specifically directed at waste water containing phenol as well as ammonia and cyanide contaminants.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide in one embodiment an improved process for reducing the cyanide content of a solution containing both cyanide and dissolved ammonia.

We have now found that the cyanide concentration of an effluent or process stream containing both cyanide and dissolved ammonia can be lowered to a level acceptable for discharge by a combination of steps. Accordingly, the present invention provides in one embodiment a process for reducing the cyanide content of a solution containing both cyanide ions and dissolved ammonia, comprising the following consecutive steps: adding a halogenating agent to the solution while maintaining the pH of the solution at greater than 7; removing ammonia from the solution by aeration; acidifying the solution to a pH of less than 7; substantially neutralising the solution; and removing the ammonia from the solution by aeration.

WO 91/00248 PCT/AU90/00289 4 We have found that this combination of steps is effective in terms of both capital and operating costs and suitable for use in remote locations.

In step the pH may be between 11 and 12 and most preferably about 12. The preferred reagent for raising the pH in step and/or step is sodium hydroxide.

iIn step the preferred pH is less than 5 and most preferably from 3 to 4.

In step the pH of the substantially neutralised solution may be about 7 and most preferably in the range 6 to 7.

The ammonia may be removed from the solution by aeration in the form of air stripping. Removed or stripped ammonia is preferably collected in a suitable gas scrubbing system.

The halogenation agent is preferably a chlorination agent.

The chlorination agent may be any agent capable of generating and/or liberating chlorine in aqueous solution. Chlorine gas and agents capable of generating and/or liberating chlorine gas in the solution are particularly preferred for use as halogenation agents in accordance with the present invention.

Hypochlorites such as sodium hypochlorite have been found suitable for use as chlorination agents in accordance with the present invention.

Where the chlorination agent is added in the form of gaseous chlorine the addition to the liquor may be achieved by means of a suitable gas sparging or bubbling system to oxidise the cyanide to cyanate.

In the process provided by the present invention, cyanide is converted by oxidation in step to cyanate, which in step is hydrolysed to ammonia and carbon dioxide. The carbon dioxide is readily removed from the reaction system.

Step may be considered as an acidification and hydrolysis step for converting the cyanate ion from the substantially ammonia free solution produced in step to produce ammonia and carbon dioxide.

Steps and have the effect of re-neutralising the liquor derived from step followed by removing the WO 91/00248 PCT/AU90/00289 dissolved ammonia produced by the acid hydrolysis of the cyanate ion. The final tail solution should be low in ammonia and contain little if any cyanide.

Each step may be carried out at temperatures close to ambient temperature (typically 15-30 0 C) and does not require the use of exotic materials of construction, nor the use of complex chemical engineering principles.

The present invention uses the effective combination of various known processes, none of which on its own, or in any other combination, would achieve the results achieved by the present invention by carrying out the present process, sequentially involving steps and The process thus provides a unique method having substantial and unexpected advantages for treatment of waste waters.

More importantly, because the oxidation of cyanide to cyanate and the removal of dissolved ammonia are carried out separately, reagent consumption is minimised. This particularly applies to step where tests show that complete oxidation of cyanide to cyanate is achieved by addition of typically 1.05 times the stoichiometric requirement of the halogenation agent.

Another significant advantage of the present process is that decomposition of cyanate to ammonia and carbon dioxide does not require the use of costly reagents.

EXAMPLES

The present invention is further described in the following non-limiting examples of illustrative embodiments.

Example 1 Initial laboratory scale testing of individual steps in the process of the invention were carried out to demonstrate under controlled conditions the operation of individual steps.

A. Examples of the Oxidation Step As the source of chlorine, the laboratory testwork used as a chlorinating agent a solution of sodium hypochlorite with an equivalent free chlorine content of 13.1 g/L as determined

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WO 91/00248 PCT/AU90/00289 6 by titration against standard thiosulphate solution.

A solution (200 mL) containing 1000 ppm of cyanide ion (as sodium cyanide) and 1000 ppm of ammonia was treated with the above solution of sodium hypochlorite and the cyanide concentration was measured by ion selective electrode as a function of the added equivalent chlorine. In Test 1, shown in Table 1, the weight of chlorine sufficient to oxidise the amount of cyanide present in the solution to cyanate ion, was 0.54 g. The pH was 11.9-12.2 during the test.

Table 1 Test 1: Oxidation of Cyanide (1000 ppm) to Cyanate with Chlorine in the presence of Ammonia (1000 ppm) Chlorine Added Cyanide Concentration (gms) (gms) 0.26 311 0.39 136 0.50 37 0.52 17 0.55 6 0.59 0 Test 2 was carried out in the same way as Test 1 except that the initial solution was 100 mL in volume and contained 2000 ppm of cyanide ion and 2000 ppm of ammonia.

Table 2 Test 2: Oxidation of Cyanide (2000 ppm) to Cyanate with Chlorine in the presence of Ammonia (2000 ppm) Chlorine Added Cyanide Concentration (gms) (gms) 0.26 547 0.39 241 0.50 64 0.52 29 0.55 6 0.59 2 B. Examples of the Hydrolysis Step WO 91/00248 PCT/AU90/00289 7 As a pure source of cyanate ion, a solution of 3.1 g/L potassium cyanate was used.

In Test 3, 200 mL of this cyanate solution was treated with sufficient sulphuric acid solution to maintain pH 3.0 at room temperature. The increase in ammonia concentration as hydrolysis of cyanate proceeded was measured by specific ion electrode. The results are shown in Table 3.

Table 3 Test 3: Hydrolysis of Cyanate ion to Ammonia at pH Time Ammonia Conversion to (min) Concentration to Ammonia (ppm) 1 152 340 34 689 69 1,034 103 Test 4 was carried out in the same way as Test 3 except that the pH was maintained at pH Table 4 Test 4: Hydrolysis of Cyanate ion to Ammonia at pH Time Ammonia Conversion to (min) Concentration to Ammonia (ppm) 1 124 195 483 48 759 76 105 1,027 103 Example 2 A test run was carried out on a prepared aqueous solution containing 2,000 mgm NH 3 per litre and 1200 mgm CN- per litre to simulate a waste process stream from an industrial process WO 91/00248 PCT/AU90/00289 8 containing both cyanide ions and dissolved ammonia.

Chlorination: The solution was subjected to chlorination to-oxidise all CN- to CNO~ using the minimum C1 2 /OCl- possible.

It was believed that a sharp rise in redox would occur at the 100% stoichiometric Cl 2 addition point i.e.

where CN- concentration 1 ppm.

The pH during the chlorination was held at 11.9- 12.0.

Chlorination was conducted by the addition of sodium hypochlorite. The results are summarised in Table 1 which shows that the stoichiometry at the mV rise was 100-105%.

Table 1 TIME HYPO ADDED REDOX pH CN- (mins) Stoich) (mV) (ppm) 0 0 116 11.4 1,210 2 11 22 10 11.9 21 33 24 716 29 44 37 46 11.9 38 56 47 384 53 67 78 92 11.9 79 78 105 100 89 129 78 11.9 18 134 100 148 15 11.9 157 4 161 103 174 5 12.0 176 105 189 0.3 191 8 12.0 192 108 206 20 12.0 First Aeration WO 91/00248 PCT/AU90/00289 9 The CNO-/NH 3 mixture formed at around pH 12 in the chlorination step was passed through a packed column to remove the bulk of the NH 3 to atmosphere leaving a weak

SNH

3 /CNO- solution.

The column used for all aerations was a 7 m length of 300 Smm Class 6 PVC pipe packed with 5 m of 1" tellerettes. Air flow was produced by fan suction with flow measured at the fan outlet using a digital turbine anenometer. Liquor flow was visually monitored via flow meter but physically measured by level change in the head tank.

Liquor flow was produced by a centrifugal pump running at 2800 rpm.

Ammonia removal was 89% at an Air:Liquor ratio of 4900:1.

The area specific flow rates were: Liquor 0.55 1m-2sec- 1 Air 162 m 3 min-1m 2 Exit gas assays via drager tubes indicated 450 ppm v/v

NH

3 The drager tube reacts to NH 3 and other basic species (amines) in the sample gas, e.g. monochloramine, etc.

Acid Hydrolysis The solution formed in the first aeration was made acidic to hydrolyse all CNO- to NH 4 followed by caustic addition to raise the pH, converting NH 4 to NH 3 CNO- destruction was very rapid, probably less than minutes.

The acid and caustic requirements for this stage were about 6 kg H 2 S0 4 m 3 for pH 12 to pH 3 and 5 kg NaOH m 3 to raise pH from 3 to 12. It was noted that some foaming occurs when pH falls below 7.

Second Aeration The NH 3 in the neutralised solution formed in the acid hydrolysis stage was removed to the atmosphere in a second aeration.

Ammonia removal during this second aeration was 92% at an Air:Liquor Ratio of 4750:1.

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WO 91/00248 PC/AU90/00289 The area specific flow rates for this second aeration were: Liquor 0.57 Im- 2 sec

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Air 164 m 3 min-lm 2 The drager tube sampling method was again used on the exit gas giving an indicated 200 ppm v/v NH 3 in the exit gas.

Calculations based on air and liquor flows and the reduction in the solution NH 3 levels predict 170 ppm v/v NH 3 in the gas phase. Hence, the drager tubes appear to give realistic figures in this stage, at least.

The conclusion which can be drawn from this example is that chlorination of the CN- can be achieved by near stoichiometric addition of chlorine. A small excess seems necessary, possibly due to reaction with ammonia at the lower CN- levels.

Stripping of NH 3 from the alkaline solutions is very efficient with good tail solutions being obtained.

Chlorinated amines in the exit gas would not appear to be a problem especially with accurate chlorination dosing control.

Cyanate conversion is very rapid indeed and thus is not a problem.

While it has been convenient to describe the invention herein in relation to particularly preferred embodiments, it is to be appreciated that other embodiments and arrangements are also considered as falling within the scope of the invention. Various modifications, alterations, variations and/or additions to the embodiments and arrangements described herein are also considered as falling within the scope and ambit of the present invention as defined by the claims.

Claims (10)

1. A process .f or reducing the cyanide content of a solution containing both cyanide ions and dissolved ammonia, comprising the following consecutive steps: adding a halogenating agent to the solution while maintaining the pH of the solution at greater than 7; removing ammonia from the solution by aeration; acidifying the solution to a pH of less than 7; substantially neutralising the solution; and removing the ammonia from the solution by aeration.

2. A process according to Claim 1 wherein the aeration steps comprise air stripping for removal of ammonia from the solution.

3. A process according to Claim 1 or Claim 2 wherein the halogenation agent is a chlorination agent.

4. A process according to Claim 3 wherein the chlorination agent is chlorine gas.

A process according to Claim 4 wherein the chlorine gas is gener.e-d in solution.

6. A process according to any preceding claim wherein the pH is maintained at a level of between 11 and 12 during step

7. A process according to any preceding claim wherein sodium hydroxide is used to raise pH in step and/or step

8. A process according to any preceding claim wherein ammonia removed from the solution is collected in a gas scrubbing system. I iI See back of page 1 INTERNATIONAL SEARCH REPORT International Application No. PC!/AJ 90/00289 I. CASSInICA.TION OF SBJ MATTER (if several classification symbols apply, indicate all) 6 I According to International Patent CLassification (IPC) or to both National CLassification and IPC Int. Cl. C02F 9/00, 1/20, 1/66, 1/76, C22B 11/08 4 II. FIaDS SEAHDI minimum Documentation Searched 7 I Classification system I Classification Symbols TFC C02F 9/00, 1/20, 1/66, 1/76 Us Cl. j 210/904 Documentation Searched other than Minimum Documentation to the Extent that such Documents are Included in the Fields searched 8 AU IPC as above III. DCUMETS C NSIDM OI BE REEVANT

9 Category- Citation of Document, with indication, where appropriate, I Relevant to of the relevant passages 12 j Claim No 13 A I GB,A, 895741 (DEUTSCIE GOLD-UND-SILBER-ScIEDANSTALT VORMALS I (1) ROESSLER) 9 May 1962 (09.05.62) A GBA, 1418743 EL.1M-SPIGEVER1K1 A/S) 24 December 1975 (24.12.75) (1) See example, figure. A US,A, 4029557 (aIpiSTENSEq et al) 14 June 1977 (14.06.77) I (1) See claim 1, example. A I US,A, 4070281 (TAGASHIRA et al) 24 January 1978 (24.01.78) (1) See claims, P.xaples. A US,A, 4105545 (MULER et al) 8 August 1978 (08.08.78) See claims. I (1) (continued) Special categories of cited documents:

10 T' later document published after the international filing date or priority date document defining the general state of the and not in conflict with the application but art which is not considered to be of cited to understand the principle or theory particular relevance underlying the invention earlier document but published on or document of particular relevance; the after the international filing date claimed invention cannot be considered novel "L document which may throw doubts on priority or cannot be considered to involve an claim(s) or which is cited to establish the inventive step publication date of another citation or document of particular relevance; the other special reason (as specified) claimed invention cannot be considered to document referring to an oral disclosure, involve an inventive step when the document use, exhibition or other means is combined with one or more other such document published prior to the documents, such combination being obvious to international filing date but later than a person skilled in the art. the priority date claimed document member of the same patent family IV. CERTIFICATION Date of the Actual Completion of the Date of Mailing of this International International Search I search Report 4 October 1990 (04.10.90) jICoc.tobe.r- 1't International Searching Authority Signaruae of Authorized Officer AUIstralimn atmet Office BROYM Form PCT/ISA/210 (second sheet) (January 1985) A A IA A I V A This in I 7(2)(a) 1 I 2.~ 3 .E VI his mt I as follow I1-[C A s J A w I J N in it I 4. i j As Ith I Remark an IC] The a I I: J No pr Form PCT/ISA, International A ication No. PCT/AJ 90/00289 FUR'IER INFORMATION CONTINUED FROM THE SECOND SHEET A Derwent Abstract Accession no. 85591B/47, Class 015, (1) SU,A, 650983 (NONFERR METAL RES.) 5 March 1979 (05.03.79) A Patents Abstracts of Japan, C-15, page 64, (1) JP,A, 55-49191 (HITACHI PLANT KENSETSU KK) 10 March 1978 (10.03.78) A EP,A, 051967 (USS ENGINEERS CONSULTANTS INC) 19 May 1982 (1) (19.05.82) Claim 1, examples. A Patents Abstracts of Japan, C-226, page 164, (1) JP,A, 32978 (KAWASAKI SEITETSU IKK) 16 August 1982 (16.08.82) V. OBSERVATIONS WHERE CERTAIN CLAIMS WERE FOUND UNSEARCHABLE 1 This international search report has not been established in respect of certain claims under Article 17(2)(a) for the following reasons: Claim numbers because they relate to subject matter not required to be searched by this Authority, namely: 1 Claim numbers because they relate to parts of the international application that do not comply with the prescribed requirements to such an extent that no meaningful international search can be carried out, specifically: Claim numbers because they are dependent claims and are not drafted in accordance with the second and third sentences of PCT Rule 6.4 VI. OBSERVATIONS WHERE UNITY OF INVENTION IS LACKIN 2 This International Searching Authority found multiple inventions in this international application as follows: As all required additional search fees were timely paid by the applicant, this international search report covers all searchable claims of the international application. As or y some of the required additional search fees were timely paid by the applicant, this inte',national search report covers only those claims of the international application for which fees were paid, specifically claims: 2 No required additional search fees were timely paid by the applicant. Consequently, this international search report is restricted to the invention first mentioned in the claims; it is covered by claim numbers: 4. As all searchable claims could be searched without effort justifying an additional fee, the International Searching Authority did not invite payment of any additional fee. Remark on Protest The additional search fees were accompanied by applicant's protest. C No protest accompanied the payment of additional search fees. Form PCT/ISA/210 (supplemental sheet (January 1985) ANNEX o TOME IN RTIONAL SEARCH REPORT ON iNIERNATIONAL APPLICATION m. =A 70C29 This Annex lists the known publication level patent family mnbers relating to the patent documents cited in the above-mentioned international search report. The Australian Patent Office is in no way liable for these particulars which are merely given for the purpose of information. Patent Document CiLei in Search Patent Family Members Report GB 1418743 AU 51631/73 BE 794877 BR 7300735 CA 1005933 DE 2305457 ES 411034 FR 2169928 GB 1418744 IT 974761 JP 48084459 US 3950250 ZA 7209010 US 4059514 US 4070281 DE 2534458 IT 1040320 JP 51017178 JP 51055774 JP 51121957 JP 51025474 EP 51967 AU 76929/81 BR 8106874 ES 506690 ES 8206389 JP 57132590 ZA 8107073 US 4366064 END OF ANNEX