WO2000035816A1

WO2000035816A1 - Biodegradation of thiocyanate and cyanide

Info

Publication number: WO2000035816A1
Application number: PCT/ZA1999/000137
Authority: WO
Inventors: John D. Batty; Gary V. Rorke; Ritva M. MÜHLBAUER; Trevor H. Tunley
Original assignee: Billiton SA Ltd
Current assignee: BHP Billiton SA Ltd
Priority date: 1998-12-14
Filing date: 1999-12-14
Publication date: 2000-06-22
Anticipated expiration: 2001-06-14
Also published as: AU2226000A; PE20001531A1

Abstract

A method of treating a solution which contains at least one of thiocyanate and cyanide which includes the step of contacting the solution in a first reactor with at least the active bacteria contained in a microbial culture of the type deposited at the Australian Government Analytical Laboratories under the Accession number NM 98/11331.

Description

BIQDEGRADATION OF THIOCYANATE AND CYANIDE

BACKGROUND OF THE INVENTION

This invention relates to the removal of thiocyanate and cyanide and, optionally, heavy metals from effluent.

Thiocyanate and cyanide ions are common constituents of effluents which arise in metallurgical plants, particularly in gold recovery operations which make use of the cyanidation process.

Where an ore contains sulfur in any form, thiocyanate invariably occurs in the effluent due to the fusion of alkaline cyanides with sulfur.

Many processes have been used to recover cyanide from effluents or to destroy cyanide in effluents. Thiocyanate is however generally resistant to recovery or destruction and remains in the effluent.

In recent years biological sulphide oxidation techniques have been used with gold ores to obtain improved gold recovery rates. The sulfide minerals are oxidised by a biological process and the gold is released for recovery by the cyanidation process.

There is however a disadvantage to having the cyanide process and the biological sulphide oxidation process on the same site as the organisms, which are responsible for the biological oxidation of mineral sulphides, can be poisoned by thiocyanate and cyanide. The water circuits of the two processes must therefore be kept strictly separate. This can be very difficult as the effluents are sometimes re-used. Even a few parts per million of thiocyanate or cyanide are toxic to the bacteria used in the biological sulphide oxidation process.

US patent specification No 4440644 describes a method for the biological removal of free and complex cyanides and thiocyanate from waste water wherein cyanides, thiocyanate and metal cyanide complexes in the waste water are degraded by contacting the waste with an acclimated strain of bacteria known as Pseudomonas paucimobilis mudlock. This strain has been deposited at the ATCC under the Accession number 39204. This bacterial strain has subsequently been reclassified as Sphingomonas paucimobilis, (see Appl Environ Microbiol 1998 Mar. 64(3):836-42 Nishikawa S. et al; Microbiol Immunol 34:99-119 (1990) Yabuuchi E. et al).

The aforementioned method is implemented either by using rotating disks which carry the bacteria and which alternately expose the bacteria to the aqueous waste and to air, or by making use of an activated sludge process, which is also known as a suspended growth process, wherein the aqueous waste and sludge which contains the bacteria and which is used as seed for continued biological treatment are alternately agitated and allowed to settle with continuous aeration.

SUMMARY OF THE INVENTION

The invention is concerned with an alternative method of treating a thiocyanate and cyanide solution.

The invention provides a method of treating a solution which contains at least one of thiocyanate and cyanide, which includes the step of contacting the solution in a first reactor with at least active bacteria contained in a microbial culture of the type deposited at the Australian Government Analytical Laboratories under the Accession number NM98/11331.

The phrase "active bacteria" is further described hereinafter.

Details of Deposit

Australian Government Analytical Laboratories, 1 Suakin Street, PO Box 385 Pymble NSW, 2073, Australia. Date of deposit: 9 December 1998; Accession number: NM98/1 331. The viability of the deposited microorganism was tested on 8 February 1999, and the microorganism on that date was viable. The microbial culture can be freeze dried for storage and reactivated when required.

Although the method may be implemented by making use of the rotating disk system, described for example in the specification of the US patent No 4440644, it is preferred to make use of the activated sludge system referred to hereinbefore.

The method may include the step of introducing air into the solution in the first reactor.

In one form of the invention the method includes the steps of subjecting material, discharged from the first reactor, to a liquid/solid separation stage and returning at least a portion of the separated solids to the first reactor to maintain an active biological population in the first reactor.

Preferably the method includes the step of varying the absolute retention time of the solution, in the first reactor, from 4 to 24 hours depending on the influent concentration of thiocyanate and cyanide.

In a variation of the invention, prior to contacting the solution with at least the said active bacteria, the solution is subjected to a heavy metal treatment stage.

The heavy metal treatment stage may be carried out by feeding the thiocyanate and cyanide solution into a counter current heavy metal adsorption reactor containing at least the said active bacteria.

Preferably the method includes the steps of subjecting material in the counter current reactor to a liquid/solid separation stage and removing at least a portion of the separated solids for heavy metal recovery.

It is preferred to make use of the said active bacteria in combination with a fungal culture selected from the species Fusarium Oxysporum. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 is a block diagram representation of a first form of the method of the invention, and

Figure 2 illustrates a modification of the process of Figure 1 , adapted for the removal of heavy metals from a solution which is being treated.

DESCRIPTION OF PREFERRED EMBODIMENTS

The applicant has isolated a microbial culture from a mine in South Africa where a natural destruction of effluent containing thiocyanate and cyanide takes place. The applicant has determined the conditions under which a fast rate of activity is obtained. The microbial culture has been deposited at the Australian Government Analytical Laboratories under the Accession number NM98/11331.

The applicant has found that the microbial culture requires no organic nutrient for its growth, nor is an organic nutrient necessary for the formation of a stable sludge. The culture can draw its carbon and nitrogen requirements directly from the breakdown of thiocyanate or cyanide ions, or both, although, on the other hand, organic matter may be present and may also be used by the microbial culture as a carbon source.

The active bacterial species in the culture were isolated using standard techniques. Solution was plated out onto a selected solid nutrient medium containing thiocyanate. At least two bacterial species were identified which were capable of utilising the thiocyanate for growth, and these are referred to herein as the "active bacteria".

In waste processing equipment the biological population responsible for destroying the effluent component, i.e. the thiocyanate and cyanide ions, is always present in the so- called media. As described in the specification of US patent No 4440644 the media may be supported on a solid support such as a rotating disk. In an alternative approach, which is referred to as the suspended growth process and which is the preferred process for implementing the method of the present invention, the media are supported on a sludge biomass, which is the product of the growth of biological components. In this case in a first form of the invention the biomass is separated from the treated water so that the treated water is a clean, clear liquid. The biomass, or at least a portion thereof, is returned to an aerated reactor so that an active biological population is maintained. Feed to the reactor, which contains the thiocyanate and cyanide ions, is fed to the aerated mixture of biomass sludge and water. Excess biomass is purged.

In a variation of the invention which is useful when the feed to the reactor contains large amounts of heavy metals, the feed is directed to a counter current heavy metals removal stage before being fed to the aerated mixture of biomass sludge and water. This process differs from the process specified in US patent No. 4440644 in that a counter current heavy metals stage precedes the thiocyanate and cyanide removal stage. Biomass for heavy metal removal is fed to the counter current reactor from the aerated reactor when excess biomass sludge is purged.

Figure 1 illustrates the first form of the invention wherein an aqueous thiocyanate and cyanide solution 10 is fed to a reactor 12, which is aerated. Air 14 is blown into the liquid in the reactor by any appropriate means, e.g. self-induced agitation, a compressed air blower, or surface agitation.

The reactor 12 contains sludge 16 into which at least the active bacteria contained in the aforementioned microbial culture have been introduced.

The bacteria rapidly degrade the thiocyanate and cyanide ions and reduce the thiocyanate and cyanide content to a very low level, below an amount of 2mg/litre (This value is the detection limit of the measuring equipment which was used). The thiocyanate and cyanide are degraded to relatively harmless inorganic compounds of carbonate or carbon dioxide, sulfate and ammonium, and carbonate or carbon dioxide and ammonium respectively.

Material which is discharged from the reactor 12 is subjected to a liquid/solid separation step 18. At least a portion of the separated sludge 20 is returned to the reactor 12 to maintain the active biological population. A minimum amount of sludge 16, of the order of 15%, is maintained in the reactor 12. Excess sludge 22 is withdrawn from the reactor 12 and purged to prevent heavy metal poisoning of the biomass.

Liquid 24 produced in the separation stage 18 is clean and clear and, as indicated, has a very low thiocyanate and cyanide concentration and may be discharged to waste or further treated , e.g. in a reed bed, for the removal of excess phosphate and ammonium.

Liquid produced in the separation stage 20 can be re-used in the biological sulphide oxidation process.

For reasons which are not fully understood the thiocyanate and cyanide degradation may be enhanced by using the aforementioned active bacteria together with a fungal culture selected from the species Fusarium Oxysporum, which is present in the microbial culture which forms the subject of the said deposit.

If the solution 10 contains heavy metals such as copper, gold, arsenic and nickel which are to be removed, e.g. for commercial or environmental reasons, then a modified process of the invention, shown in Figure 2, may be employed. In Figure 2 reference numerals which are the same as those used in Figure 1 are used to indicate similar steps and materials.

Referring to Figure 2 an aqueous thiocyanate and cyanide solution 10 is fed to a counter current metal adsorption reactor 26. Heavy metals and cyano-metal complexes are removed from the solution by adsorption onto the biomass. The biomass sludge from this reactor is not agitated. This allows the biomass sludge 28 to settle in the reactor 26, thus automatically giving rise to a liquid/solid separation step 30 although, for the sake of convenience, this step is separately designated. The separated biomass sludge 28, containing heavy metals, is purged, and thereafter treated in any appropriate way to recover the heavy metals.

Solution 32, produced by the step 30, containing thiocyanate and cyanide ions is fed to a reactor 12, which is aerated. Air 14 is blown into the liquid in the reactor by any appropriate means, e.g. self-induced agitation, a compressed air blower, or surface agitation.

The reactor 12 contains sludge 16 into which at least the aforementioned active bacteria have been introduced. Preferably the microbial culture i.e. the active bacteria and the fungal culture are introduced to the reactor.

As is the case in the Figure 1 process, the microbial culture rapidly degrades the thiocyanate and cyanide ions and reduces the thiocyanate and cyanide content to a very low level, below an amount of 2mg/litre. Again it is to be noted that this is the detection limit of the measuring equipment which was used. The thiocyanate and cyanide are degraded to relatively harmless inorganic compounds of carbonate or carbon dioxide, sulfate and ammonium, and carbonate or carbon dioxide and ammonium respectively.

The purged biomass can be fed to the counter current metal adsorption reactor 26 for the heavy metal adsorption stage. The liquid which is produced in the separation stage 18 can be treated or used in the manner described in connection with Figure 1.

Under demonstration scale test conditions a reactor 12 of volume 25m³ was fed with a solution at absolute retention times with a variation of between 4 and 24 hours depending on the influent concentration of thiocyanate and cyanide, which was as high as 200mg/litre. In all cases, thiocyanate and cyanide degradation was such that less than 2mg/litre remained in the effluent. Heavy metal adsorption tests showed that copper and gold were removed from the solution, in the sludge 28, almost completely. In excess of 80% arsenic and about 50% nickel were removed from the solution by adsorption onto the biomass.

Claims

1. A method of treating a solution which contains at least one of thiocyanate and cyanide which includes the step of contacting the solution in a first reactor with at least the active bacteria contained in a microbial culture of the type deposited at the

Australian Government Analytical Laboratories under the Accession number NM 98/11331.

2. A method according to claim 1 which includes the step of introducing air into the solution in the first reactor.

3. A method according to claim 1 or 2 which includes the step of varying the absolute retention time of the solution, in the first reactor, from 4 to 24 hours depending on the influent concentration of thiocyanate and cyanide.

4. A method according to claim 1 , 2 or 3 which includes the steps of subjecting material, discharged from the first reactor, to a liquid/solid separation stage and returning at least a portion of the separated solids to the first reactor to maintain an active biological population in the first reactor.

5. A method according to any one of claims 1 to 4 which includes the step of purging excess sludge, withdrawn from the first reactor, to prevent heavy metal poisoning of biomass in the first reactor.

6. A method according to any one of claims 1 to 4 wherein, prior to contacting the solution in the first reactor with at least the said active bacteria, the solution is subjected to a heavy metal treatment stage.

7. A method according to claim 6 wherein the heavy metal treatment stage is carried out by feeding the thiocyanate and cyanide solution into a counter current heavy metal adsorption reactor containing at least the said active bacteria.

8. A method according to claim 7 wherein solution, separated in the counter current reactor, is fed to the first reactor.

9. A method according to claim 7 or 8 which includes the step of removing solids from the counter current reactor for heavy metal recovery.

10. A method according to any one of claims 6 to 9 which includes the steps of purging excess sludge, withdrawn from the first reactor, and feeding at least a portion of the purged sludge to the counter current reactor.

11. A method according to claim 10 wherein a portion of the purged sludge is returned to the first reactor to maintain an active biological population in the first reactor.

12. A method according to any one of claims 1 to 11 wherein the said active bacteria are used in combination with a fungal culture selected from the species Fusarium

Oxysporum.