AU617332B2 - Chemical/biological process to oxidize multimetallic sulphide ores - Google Patents

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I- C OH HO N W 9 A L T H 0 F A U S T R A LX A ITwtENT AcT 195i COIAPLETE SPBC1 FICATION 617332 (ORIGXNAL) POR OFFICE USE CLASS TNT. CL\SS Application Number: Lodged: ConplCt Specificationt Lodged: Accepted: Published: Priority: Related Art-:

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0 0D 0 00C 00 0 o o 0 0 NAME OF APPLICANT: ADDRESS OF APPLICANT: NAME(S) OF INVENTOR(S) GIANT BAY BIOTECH INC.

Suite 43C Discovery Park, 3700 Gilore way, Burnaby, B.C.

Canada V5G 4M1 Ralph Petcr HACKL Albert BR.UYNESTEYN Frank Ra'ph WRIGHT o 9 .r A DDRRSS FOR SPAMCE: DAVIES COLUSU~s Patent Attorneys I Littiel Collins Street, Melbourne, 3000.

MtAPLWE SVWXFXCA',1'XO0 FOR THE XU QMXN W9)LTth10: "CHEMICAL/BIOLOGICAL PROCESS TO OXIDIZE MULTIMETALLIC sULPH:DE ORES" The follwaing sta~eteot is a fUl deiaiptoa of this jvention, including te best method of performing, it knou% to us i 2 This application relates to an improved method for oxidizing multimetallic sulphide ores and concentrates, using a combination chemical/biological leaching process and at least three different types of bacteria.

The extraction of metals from sulphide minerals through the mediation of the bacterium Thiobacillus Sferrooxidans has been known for many years. It appears that as much as one quarter of the copper produced in Arizona is through the biological leaching of low grade 15 copper sulphide wastes produced from open pit copper mining operations.

At present the only other known commercial application of biological leaching is at the Denison Mine in the Elliot Lake area of Ontario, Canada, where the o 20 bacterium T. ferrooxidans is used to extract uranium from pyrit.i uranium ores.

S. Although biological leaching methods have been developed for the oxidation of sulphide minerals in low grade waste ores, a process taking place naturally, the kinetics of such processes are so slow that they are o" applied only to low value wa.ste materials. It is not uncommon that in 10 to 15 years of leaching, only extraction is obtained in such cases. However, in the laboratory, when working under optimized conditions, the kinetics of the biological leaching process can be improved several hundred thousand times. As a result of such laboratory work, Duncan et al., in U.S. Patent 3,607,235, describe a biological leaching process for sulphide minerals. The metal sulphide, in finely ground form, is suspended in an acidic, air-sparged solution, I 3 maintained at a pH of 2.0, together with a culture of sulphide oxidizing bacteria, identified as T. ferrooxidans. Extraction in the process is a function of particle size and regrinding of the leached residue is necessary to obtain extractions in excess of 90%. McElroy et al., U.S. Patent 3,856,913, describe the use of silver as a catalyst in the oxidation of the mineral chalcopyrite, again using T. ferrooxidans, and Bruynesteyn et al. in U.S. Patent 4,571,387 describe and claim a further modification to the biological process which also uses T. fetrooxidans for the production of elemental sulphur from the sulphi e portion of the chalcopyrite o° mineral.

Extensive cowmi'rcial application of the biological leaching process is hampered by the fact that T. ferrooxidans is highly sensitive to high concentrations of hydrogen ions (low pH), and to the presence in relatively low concentrations of certain elements, such as arsenic and antimony, which have an inhibitory or toxic effect.

The effect of acidity on the activity of S'I T. ferrooxidans has been researched extensively. Buchanan Gibbon, reported that T. ferrooxidans can grow at pH values between 1.4 and 6.0. Tuovinen et al, repoct that T. ferrooxidans does not grow at pH values below but can be adapted to oxidize ferrous iron at pH 1.3 by successive culturing in media of progressively greater acidity. Tuovinen et al point out however that during the.

subculturing steps, the pH of the media rises to 1.7 as a result of the acid consumption of the ferrous iron oxidation, so it is questionable whether their strain was active for any length of time at pH 1.3. Lui observed the same pH rise during his experiments.

Tomizuka et al, reported that, when oxidizing ferrous iron in a continuous fermentor, the i I I 4optimum pH was in the range 2.3-2.7. He also showed that the specific growth rate decreased to zero at pH 0.8 and was 40% and 56% of the maximum rate at pH 1.3 and respectively. Golomzik and Ivanov used serial transfers into successively more acidic media in an attempt to adapt T. ferrooxidans to a low pH environment.

Although they quote success in obtaining growth at pH values as low as 1.0, the rate of ferrous iron oxidation at this low pH was only 17mg/l/h. At pH 2.3, rates of 500-1,000 mg/l/h have been reported Thus, the adaptations obtained were of limited significance.

o. Bruynesteyn et al reported that T. ferrooxidans can be adapted, by continuous culturing techniques, to pH values as low as 1.25 while retaining S 15 its rapid ferrous iron oxidation capabilities.

SBruynesteyn et al also report that hydrogen ions react in a syiergistic manner with uranium in causing inhibition to T. ferrooxidans, particularly at pH 1.6.

Thus, T. ferrooxidans does not oxidize ferrous iron or sulphides at pH values below 1.0. Since ferric iron is an oxidant which assists in the oxidation of mineral sulphides and since the solubility of ferric iron increases with decreasing pH, the organism's pH sensitivity prevents its use in the highly oxidative solutions formed by strong ferric sulphate in solution.

Additicnally, many sulphide ores and concentrates are sufficiently high in ,.ulphide content that the biologically produced sulphuric acid lowers the pH below 1.2, effectively stopping the biological leaching process.

U.S. Patent 4,497,778 to P. Pooley describes a process which overcomes some of the difficulties in biological leaching of pyritic and arsenopyritic ores and concentrates, by first subjecting the mineral to a partial roast to remove part of the contained sulphur by converting the contained pyrite to pyrrhotite. The patent claims improved extraction results from this process.

5 Little information is available on the pH sensitivity of T. thiooxidans, other than that this organism, in contrast to T. ferrooxidans, can oxidize elemental sulphur at pH values below 1.0. Its sensitivity to inhibitors is assumed to be similar as that of T. ferrooxidans. Groudev has shown that T. thiooxidans, when growing at a pH of 2.3, is capable of oxidizing certain sulphide minerals such as zinc sulphide, nickel sulphide, and cobalt sulphide, but could not oxidize pyrite and arsenopyrite. It was not determined whether this oxidation is direct or via a chemical oxidation step. In the latter case, the bacterium is thought to oxidize the elemental sulphur resulting from the chemical oxidation of the sulphide.

Norris also reports that T. thiooxidans does not oxidize pyrite.

Leptospirillum ferrooxidans is still a relatively d unknown organism which is reported to be similar to T. ferrooxidans. Norris reports that Leptospirillum oxidizes ferr is iron at pH values as low as 1.4 but o cannot oxidize elemental sulphur. Norris also reports that there are some indications that LepjtospiJillum can oxidize pyrite as well as does Thiobacillus ferrooxidans.

At present, no one has been able to develop an economically viable biological treatment process for rfractory ores and concentrates containing arsenic, because dissolved arsenic concentrations as low as 1,000 mg/l are toxic to the leaching bacterium, while, as stated above, at the low pH values resulting from the acid produced from the pyrite when leaching pyritic arsenopyritic ores, the activity of the bacterium is severely inhibited, Little information on the inhibitory effect of arsenic on any of T. ferrooxidans, Leptospirillum ferrooxidans and T. thiooxidans is known, although it is 6 believed that arsenic is inhibitory to microorganisms because it tends to replace phosphorus in the microbip".

enzyme systems.

Brown et al (10) report that they have found T. ferrooxidans in Alaskan streams in the presence of up to 0.347 mg/i dissolved arsenic.

Livesey-Goldblatt, (11) reports that he adapted a strain of T. ferrooxidans, in a solution of pH 1.7, to arsenic roncentrations as high as 4,000 mg/1.

During the Sixth International Symposium on o Biohydrometallurgy (1985), Karavaiko (12) reported that, o while leaching arsenopyrite with Thiobacillus ferrooxidans °o in a solution of pH 2.0, he encountered bacterial inhibition due to arsenic concentrations in the range oV 15 10-20 g/1 and iron concentrations in the range 20-40 g/l.

The potential commercial significance of the inhibition by arsenic is demonstrated by attempts to use genetic engineering techniques to construct arsenic resistant strains of T. ferrooxidans. A recent patent application ly Gencor relates to work carried out at the University of Capetown by D.E. Rawlings, I. Pretorius o

B

and D.R. Woods These authors studied the arsenic resistance in a strain of Thiobacillus ferrooxidans found to be resistant to as much as 2,048 mg/l pentavalent S. 25 arsenic per liter, and were able to isolate and replicate the relevant plasmids. However, there is no information on how much arsenic resistance can be engineered or how such resistance can be replicated in the organisms. The patent is concerned with the genetic manipulations only.

At the optimum pH value for T. ferrooxidans of 2.4, and at values in excess thereof, ferric iron produced during the biological leachi:.j process from the pyrites and arsenopyrites present, has a limited solubility and tends to precipitate partly as an hydroxide. This is a waste material which is very difficult to separate from ;r ~n^ -7the suspension. It tends to coat mineral surfaces, thus interfering with the leaching process. Also, when leaching arsenopyritic ores, the dissolved arsenic may, at these high pH values, partly precipitate as calcium arsenate, a slightly water soluble compound which is not acceptable for disposal in tailings ponds. Therefore, it is of advantage to carrying out the leach at a pH in the range 0.3 to 1.5 and preferably of 1.0, at which value the solubility of ferric iron is increased to more than 100 g/l, as compared to less than 1 g/l at pH 2.3. Such high concentrations facilitate the chemical oxidation of metal sulphides.

We have found that in the present invention a combined chemical-biological treatment process for multimetallic ores such as arsenopyrite can be made to work rapidly and to as much as 98% sulphide oxidation, particularly when the finely ground ore or concentrate is leached in agitated, air sparged tanks, with the strains of three different bacteria, T. thiooxidans, T. ferrooxidans, and Leptospirillum ferrooxidans.

In accordance with a first aspect of the invention there is provided a process for oxidizing oo multi-metallic arsenic sulphide containing ores, concentrates or other material, the process comprising: 25 contacting the ore, concentrate or material with a leach solution comprising a mixed culture containing as the effective ore leaching components o Thiobacillus thiooxidans, Leptospirillum ferrooxidans and Thiobacillus ferrooxjdans in a first stage, at an Eh of less than 750 mV; and contacting the products from the first stage with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans in a second stage, operated at an Eh of 750 mV or more; wherein the pH of said leach solutions is 910715,himdaL104,a:L892 Lgiars,7 7a maintained at 0.3 to 2.8.

A second aspect of the invention provides a process for oxidizing multi-metallic arsenic and sulphide containing ores and concentrates, the process comprising: contacting the arsenic ore with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus thiooxidans, Leptospirillum ferrooxidans in the first stage, at an Eh of less than 750 mV, to oxidize elemental sulphur; and contacting the products from the first stage with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans in a second stage, operated at an Eh of 750 mV or more, to oxidize ferrous iron and sulphides; f said process being conducted under conditions o of low shear agitation and accompanied by sparging with bubbles of a gas containing oxygen; said conditions of low shear agitation being insufficient to prevent effective attachment of bacteria in said bacterial culture to sulphide mineral surfaces in said ore and insufficient to cause rupturing of the cell o 25 walls of said bacteria; and wherein the pH of said leach solution is maintained at 0.3 to 2.8.

A third aspect provides a process in which said conditions of low shear agitation are those produced by an axial flow impeller.

A fourth aspect of the invention provides a mixed culture of T. thiooxidans, T. ferrooxidans and L. ferrooxidans uniquely adapted to the second stage of the above described processes. A deposit of the mixed culture was made at ATCC under deposit No. 53618.

Leptospirillum ferrooxidans is quite similar to T. ferrooxidans and obtains its energy for growth from J 910909,nndat104,a:\ 18921giLares,8 7b the oxidation of ferrous iron.

L. ferrooxidans has not been extensively researched yet and one of the most recent articles, published in 1983 by Dr. P.R. Norris, shows that this organism can operate in the same pH range as T.

ferrooxidans and is not able to work at pH values below 1.3. The organism is not known to be able to oxidize sulphides or elemental sulphur, although Norris, in his paper, found that Leptospirillum-like bacteria did oxidize a pyrite substrate.

We have now found that the culture of bacteria, identified as Leptospirillum ferrooxidans-like bacteria, can oxidize both ferrous iron and pyrite at pH values as low as 0.3.

AhIM- )q/'~JJ 910909,inundaL104,aA 1892lgilares,9 8 It has also been observed that the low pH culture is accompanied by a fungus, something which has not been observed withi higher pHi cultures. The fungus has not been identified. We have not ruled out the possibility that a beneficial interaction exists between LepikosL1 jrilm ferrooxidans and the fungus, =otributinq to the ability of the bacteria to function in the extremely acid environment.

It is therefore felt that 1j. ferrooxidans can play an active role in the first 6tage of our process by oxidizing th, ferrouis iron lissolved ftom the mineral, as well as that formed during the ferric iron oxidation of the mineral sulphides. In addition, it is likely that the organism also oxidiz~es sumie of the pyrite present in the process, producing the ferric iron necessary for the chemical oxidation of the multimetallic sulphidos such as arsenopyrite.

We must also consider that L, ferrooxidans can play a role in the sooond st~igje as it is known to oxidlize fe- rous i :on as well as T. ferrouxid.I-ns.

T, thiooxidans useleliJ&nt-11 sulp~hur as a substrate and is active at highly acidic condlitions suoh as pHi 0.3-1.5. T. ferrooxidans uses both sulphides and dissolved ferrous iron as substrates, but cannot oxidlize elemental sulphur at low pH- values. In the present invention both of these st rainm have boon adapted by continuous culturing clniques to low 1)11 values and high dlissolved arsenic oonoontrations, In Fome cases we adapted these to operate at p11 values as l.ow as 0.3, and arsenic as high as 29.tg/L and dissolved iront up to 90 g/l.

Our process differs from the prior art in that one stage of the vInuti-stage biologial. leach uses T. thiooxjdtins as the main leaching organism. During this leach, operated at an Eh of 600-750, preferably 650-720 typically arsonopyrite and pyrite are partially 9oxidized to elemental sulphur as an intermediary product.

This elemental sulphur would normally coat the active mineral surfaces and inhibit further rapid oxidation of the sulphides. However, in our process, the strain of the elemental sulphur oxidizing bacterium, ZT. Lhiooxidans, rapidly converts the elemental sulphur to sulphate, thereby allowing the oxidation of sulphides to go to completion rapidly. In addition, partial chemical oxidation of iron sulphides such as pyrite and arsenopyrite, by -)xycler, and ferric sulph,-te is possible, I'lhich also producus elemkontal sulphur. The bacterium Tj. thipoxidans will also oxidize this chemically produced elemental sulphur rapidly, thus allowing the more rapid conversion of the mineral sualphide into metal sulphate and sulphuric acid.

The oxidizing capability of fertic iron is crihanced by the aotions T, ferrooxidans, and L.

ferrooxi l'ns which rapidlly regenerate ferric iron from the feruW iron produced. When L. ferrooxidans is the predominant, organism, this oxidation can be carried out at pHl values as low as 0, 3. The pro(css appears to be applicable to any multi ietallic ;nlphidos that can be oxilize-l by oxygen or fetric iron and produce elemental sulphur. P' e eA /I Tho chemical biological leach process ls/~carried out as described below.

The sulpide "raterial is first crushied aknd ballmilled, if recjUired, to typically 100% ininus 200 mao;h (Tyler sjtandlardj screen-scale dlesignation). The finely ground feed is then slurried with water and fed, to the fir~st of a series of bioleaoh reactors which contain a high population of oxidizing microorganisms. rOepending on the nature of the material, leaching is carried out at a pulp density typically between 1% and 65% and a pUl range of 0.3-2,8l.

The tovqperature of lcehin ohoul1 be in the ran~je 10C to, 4511C and px-rera-hty- 30 0 0 to 40 0

C.

Most sulphide materials will have a high enough sulphide content to enable the bacteria to produce sufficient acid to neutralize acid consuming constituents contained in the material; however for those feeds with low sulphide content it may be niecessary to provide for the adldition of extra acid. Any souroe of sulphuric acid, not no,'.ossarily pure, will suffice.

The leach reactors are agitated by conventional meohanical or air-lift moans. Air is blown into the reoctors; o pruvide oxyge--n for the ';ulpbide oxidation reactioii and for bauterial ro:wth, Sli,irry passes from one tank to the next by rioans of jjravity overflow. The number of loach stagi~s requoirec-. dopkinds on the nature tf the foed but will valry f rum uric stage to five stages. A distinct advantage of the prooe.,s over the prior art is that the cbemioal/biolocgic'al le~ioh is so efficient that the retention time is roeduood to 1-5 LIays and as nuch as or more of the oxygen in the air blown into the reoctor is vitilizeO, Slurry exiting the final bioleach reactor undI !rjoes a solid-Iiquid separation step, and the u. tals of economio interest are recovered by moethods well known in the in~ustry.

During large scale pilot plant testing of the bioleach process* a surprising discovery was made.

Excessive agitation shear stress was found to have a detrimental effect on bioleaching. This phenomc ion has, to the best of the invenitors' knowledge, never before been reported in the literature for sulphidle oxidizing microorganisms, Shear stress is dependent on the type of impeller used and also on impeller tip speed. The conventional impeller used for proceoses Which require high oxygjen uptake, such as our bioleach process, is the radial flow (Iushton) Lurbine. TPhis type of impeller was fritnd to cause a drastic reduction in bioleach rates 3$ during largoe-scale tests when an impeller tip speed of 4 metres per second was exceeded. The phenomenon was riot evident during small-scale lab tests because impeller tip speeds were substantialty lower, never exceeding 2 mnetres per second. Excessive shear is popstulated to affect preventinig effective attachment of the bacteria to the oulphide mineral surfaces, or by rupturing the bacteria's ceill wall and killing the bacteria.

We have discovered that the problem of excessive agitaion shoar c~an be solved by replacinj the Rushton turbjines with 459 pitched-blade axial flow impellers. Th e latter produce much 1'-ss shear stress even at tip speeds a s high as R meitres per secind. Th1is type of impeller would niot normally be considered for processes requiring high oxygen tiptake, but, we have found that it work~s well for the bioloachi procc~is. Other low-shear impellers which would work well are hydrofoils and ma-:ine-type propellers. Further dEltails of the *hear stress phenic nn are given in F)on -mples 4 and 9.

what is iviportant is that the conditions of agitation be low s7 oar aglttition insufficient to prevent effective attaohme0. uf the bacteria in said bacterial culture to sulphida minerali surfaces in said ore and insufficient to cause ruptures of the cell walls of the bacteria.

In a variation of the process, the inatrial can be treated by convk-ntioaal heap, leach methods, in which caise the oxidation step would require 1-12 months to complete but would be much less expensiv.e than mechanical or air agitation methods.

The bacteria require certain nutrients) the most important of which are sources of nitrogen, phosphorus and carbon dioxide. Often, the sulphide material itself will contain enough nutrients to sustain activity* but for feeds high in sulphide content it may be advantageous to augment the available nutrient supply with a source of -12 ammonium sulphate and potassium phosphate, which are ommonly available as agricultural fertilizers. Amounts sufficient for optimum biological activity are from 0.5-10 kg (NH 4 2

SO

4 and 0.1-2 kg KH 2

PO

4 per tonne feed. Similarly, it is sometimes advantageous to provide minute amounts of carbon dioxide to supplement the carbon dioxide content of the air. Sources of carbon dioxide include the gas which can be injected into the air supply to a concentration of about or any carbonate source such as limestone w>ich will react with acid in the tanks to form

CO

2 The progress of the leach can be readily followed by measurement of the Eh of the leach solutions, which gives an indication of the nature and degree of sulphide oxidati(n taking place.

The Eh is a measure of the reduction-oxidation potential of a solution (redox). A higher Eh indicates stronger .xidizing conditions.

Theoreti !ally, the Eh is a function of the pH, the gas constant, the temperature and the quotient of the uxidized species ani reduced species. Fur a pure acidic iron sulphate solution tunder conditions of standard temperature and pressure, the Eh will be about 680 mV when the amount of ferric iron in solution equals the amount of ferrous iron.

Thus, according to one emboditment of the present invention, one stage of the leach is operated under aerobic conditions with a mixture of T. thiooxidans, T. forrooxidans, and L. ferrooxidans at a pH of 2.5 or lower and at an Eh of less than 750 mV. Under these conditions T. thiooxidans grows rapidly, probably by oxidizing elemental sulphur. This causes the T. ferrooxidans and L. ferrooxidans which cannot oxidize elemental sulphur rapidly, but are capable of oxidizing 3$ ferrous iron, to become secondary organisms. Because the 13 ferric iron is reduced faster than it can be regenerated by T. ferrooxidans and L. ferrooxidans, the leach progresses at the relatively low Eh of 600-750 mV.

Because most of the multimetallic sulphides are removed in the first stage of the leach, one or more later stages of the leach operate at an Eh of typically 750 mV or more, because T. ferrooxidans is the predominant microorganism, oxidizing pyritic sulphide and dissolved ferrous iron.

The resultant dissolved ferric iron can then, through pH control, be precipitated as bal-ic ferric sulphate. This compound does not interfere with the leaching process. It can be readily separated from the leach suspension by gravitational settling, and can be disposed of safely in a tailings pond.

Since ferric iron is an effective oxidant for many sulphide minerals, it is an advantage of the present invention that the leach can be operated at pH values as low as 0.3, conditions at which ferric iron is 10-100 times more soluble than it is at pH 2.3.

An important advantage of the invention is that in the case of arsenopyrite, under the highly oxidative conditions of the later stage of the leach, the dissolved j3-valent arsenic, which is a potential pollutant, will be oxidized to its 5-valent form and precipitated as ferric arsenate, an environmentally safe waste product that can be easily separated from the leach solution and disposed of in a tailings pond without environmental impact.

Similar oxidative conditions apply to antimony.

The process of the invention is preferably a continuously operating process utilizing more than one stage. Most of the chemical/biological action using the bacterium T. thiooxidans preferably takes place in the first stage, while in the subsequent stages the activity of T. thiooxidans is decreased in favour of the activity of the bacteria T. ferrooxidans and L. ferrooxidans, which 14 oxidize the by then more readily available sulphide portion of the pyrite minerals present in most arsenopyrite ores and concentrates.

In a variation of the process, the leach is carried out in a single stage with a sufficient residence time to allow for the initial development of predominantly T. thiooxidans, followed by the later development of T.

ferrooxidans and L. ferrooxidans.

In our invention, in a preferred aspect the bacteria T. ferrooxidans and L. ferrooxidans will also rapidly oxidize most of the dissolved ferrous iron to ferric iron which in turn assists in the oxidation of the metal sulphides. In addition, the ferric iron reacts with the arsenate ions to produce the environmentally acceptable ferric arsenate waste product, as well as the equally acceptable basic ferric sulphate or jarosite.

When the leach is conducted at a pH of less than 1.3, L.

ferrooxidans is the active iron oxidizer. Each stage may te' conducted in a separate reactor. Since the precious )0 metals are frequently not finely disseminated in the sulphides, it is an advantage of the process of the on invention that in many cases it is not necessary to oxidize all the sulphide material present.

Our I cterial cultures were originally derived from a naturally occurring strain isolated from aoid mine drainage occurring at the now-closed Britannia Copper Mine near Squamish, B.C. The original strain was adapted to progressively higher concentrations of dissolved arsenic by the following method. A culture of the bacteria is innoculated into a 50 g/L suspension of an arsenopyrite concentrate containing, for example, 14% arsenic. Once the bacteria have developed through their lag phase, rapid bioleaching occurs and the dissolved arsenic concentration increases to as high as 7 g/L. Subsequently, the pulp density is increased in increments of 10 g/L by adding 15 more arsenopyrite, allowing for a three-day adaptation time for the bacteria between increases. In this way the dissolved arsenic concentration increases steadily and would theoretically reach, for example, 35 g/l at 250 g/l pulp density.

The adaptation can also be carried out while Leaching continuously in a plurality of tanks, by incrementally increasing the pulp density of arsenopyrite in the feed tank. Both adaptation methods allow for the optimim development of a mixed culture of T. ferrooxidans, T. thiooxidans and L. ferrooxidans. More details of the adaptation methods are given in Examples 1 and 2.

A process according to the preferred embodiment of the invention has been demonstrated by operating on a continuous bench scale leach for over 6 months, using these special cultures in a suspension containing 12 g/l j dissolved arsenic and 30 g/l iron at a pH of 1.1, to oxidize an arsenopyrite/pyrite mixture.

*A distinct difference between the present invention anil the prior art is that the prior art refers to the bacterium T. ferrouxidans as the sulphide oxidizing 0. orga'nism. This invention employs at least two and o.oo possibly three distinctly different bacteria. In the multimetallic sulphide oxidizing stage, the predominant bacterium is a sulphur oxidizer, T. thiooxidans, different from T. ferrooxidans in that it operates at low Eh and pH values and oxidizes elemental sulphur only and not dissolved ferrous iron. Only in those stages where there is little arsenopyrite substrate left, does T. ferrooxidans play a dominant role, or, if the pH is low, does L. ferrooxidans play a dominant role. Thus the invention employs or emphasizes three different organisms, one an elemental sulphur oxidizer and the other two sulphide/ferrous iron oxidizers. When the amount of acid produced from the oxidation of elemental sulphur causes 16 the pH of the leach solution to fall below 1.0, L.

ferrooxidans tends to become selective over T. ferrooxidans and may also oxidize some of the sulphide present.

Therefore, T. ferrooxidans and L. ferrooxidans differ mainly from T. thiooxidans in that the former are capable of oxidizing ferrous iron and the latter is not.

In addition, only T. thiooxidans can oxidize elemental sulphur at pH values below 1.0. T. f i errooxians does oxidize elemental sulphur at pH values above 1.0, but relatively slowly compared with T. thiooxidans. The elemental sulphur oxidizing capabilities of L. ferrooxidans is supposed to be zero, but we have not carried out any tests to confirm this.

The rrocess is applicable to those multimetallic sulphide materials which form elemental sulphur during oxid-tive leaching. These include, but are not limited to, sulphide materials containing the minerals pyrite, arsenopyrite, pyrrhotite, ttrahedrite, chalcopyrite, sphalerite, millerite and cobaltite. The process can also be used to desulphurize coal by oxidation of the contained pyrite.

The mixed cultures have been identified by taxonomy studies, and are described below.

The mixed cultures, arsenic resistant and low pH resistant cultures have been deposited in the following culture collection: American Type Culture Collection 12301 Parklawn Drive Rockville, Maryland

U.S.A.

Mixed Culture, Coded "GBB mixed"-ATCC No. 53,618.

This culture contains T. thiooxidans, T. ferrooxidans, and Leptospirilum ferrooxidans. The culture is effective at normal pH ranges of about pH 1.3 to 2.8, but is also effective at lower pH, such as bel<w 1.

L

4' 17 Low pH Resistant Culture coded "GBB IRON"- ATCC No. 53,625 This culture contains mainly Leptospirillum ferrooxidans, as well as some T. ferrooxidans. It is capable of oxidizing ferrous iron at low pH, below 1.

This culture is particularly useful when treating strong acid producing materials amenable to ferric iron oxidation.

High Arsenic Resistant Culture Coded "GBB "ooD Sulfur"-A'CC 53,619.

°o 10 This culture is unique T. thiooxidans. It is for o" 0 its arsenic resistance and can be used for the leaching of elemental sulphur produced during the leaching of o multimetal3ic sulphides such as arsenopyrite.

These cultures have also been deposited with: Olli H. Tuovinen, Ph.D.

o° Professor of Microbiology .o The Ohio State University Department of Microbiology 484 W. 12th Avenue C lumbus, Ohio U.S.A.

4'210-1292 o.S Thus, the above cultures can be used, at normal pH with a lower limit of 1.3, to oxidize multimetallic sulphides utilizing T. thiooxidans and T. ferrooxidans.

By utilizing L. ferrooxidins, the mixed culture can be used to oxidize multimetallic sulphides at pH values below 1, and as low as 0.3. When the mixed cultures are made up from arsenic resistant strains, arsenic containing multimetallic sulphides can be leached. The culture of L.

ferrooxidans can be used to oxidize pyrite and similar sulphides under highly acidic conditions. In addition, the arsenic resistant strains can be used, individually or collectively, to oxidize arsenic containing sulphides.

The bacterial cultures are further characterized as follows: 18 Mixed Mineral Leaching Bacteria GBB Mixed This culture is a mixture of acidophilic bacteria capable of growing with arsenopyrite as the sole source of energy. The culture was derived as explained in Example I. The culture is aerobic and utilizes carbon dioxide as the source of carbon. The major important properties of the mixed culture are the ability to oxidize arsenopyrite for energy ar, growth; the ability to oxidize ferrous iron for erergy and growth, due to the presence of Thiobacillus ferrooxidans and Lepltos irillum ferrooxidans types of bacteria; and the ability to oxidize elemental sulfur at and below pH 1.0 due to the prescice of Thiobacillus thiooxidans type of bacteria.

Two cultures derived from this mixed culture by utilizing ferrous sulfate and elemental sulfur as the respective substrates have been submitted to the ATCC as separate depositions. However, the optimum composition of the mixed culture is best realized by maintaining the mixed r culture in arsenopyrite-containing liquid media.

The mixed culture is maintained in shake flasks at room temperature in a mineral salts medium. The following medium is used for routine cultivation.

(NH4 2 4 3.0 g/liter KCI 0.1 g/liter K2HPO 4 0.5 g/liter MgSO 4 .7H 2 0 0.5 g/liter Ca(NO 3 2 0.01 g/liter Use sulfuric acid to adjust to pH 2.0. The mineral salts solution can be autoclaved or filter-sterilized. After sterilization, add aseptically finely ground arsenopyrite as the substrate (20 g/100 ml). Because arsenopyrite is not commercially available, a sample of about 200 g is enclosed with this culture deposition. Other arsenopyrite as well as pyrite concentrate materials may also be suitable if they are *:ty 19 finely ground to provide a large surface area as suspended solids in the final medium.

An active culture releases iron and arsenic into solution during arsenopyrite oxidation and produces sulfuric acid because of the usual presence of pyrite. An I actively growing culture lowers the pH to below 1.0 during i the incubation; normally this will occur in eight to twelve days with a 7.5% (vol/vol) intculum in batch culture. Microscopic examination of the culture initially and during the incubation may also be used to verify increases in cell numbers.

It should be noted that many commercially available pH probes display poor linearity in the pH 0.5-2,0 range. It is necessary to calibrate the pH meter with both pH 1.0 and pH 2.0 buffer solutions for accurate measurements. In the event of poor linearity, intermediate pH values of sami le solutions should be checked against a calibration buffer that has the closest pH value pH 1.0 or pH STORAGE: Store under liquid nitrogen.

Mixed Mineral Lea<'ingBacteria GB1 Iron non This culture contains ferrous ion oxidizing acidophilic bacteria, specifically characterized by their ability to grow at pH 0.9 with ferrous sulfate as the sole source of energy. The culture was derived by subculturing GSB Mixed on ferrous iron at pH 0.9. The culture is aerobic and utilizes carbon dioxide as the source of carbon. The culture resembles Leptospirillum ferrooxidans in its morphological characteristics and in its ability to grow with ferrous ion as the energy source, but is differentiated from previously described L. ferrooxidans cultures by its ability to grow at pH 0.9. The ferrooxidans type is the predominant morphological form in the culture. Additionally, the culture contains as a minor component short straight rods which resemble the 20 morphological description of Thiobacillus ferrooxidans.

Again, the straight rod shaped cells are uniquely different from previous descriptions of T. ferrooxidans due to the pH 0.9 growth conditions. Filamentous fungi of unknown taxonomic status are also present in this culture. These fungi tend to form small suspended pellicles of white-greyish color.

The culture is maintained in shake flasks at room temperature in a mineral salts medium. The following medium is used for ututine cultivation:

(NH

4 2 S0 4 3 g KC1 0.1 g K 2 PO4 0.5 g MgSO .7H 2 0 0.5 g Ca(NO3) 2 0.01 g FeSO 4 .7H 2 0 44.22 g Distilled w er 1000 ml 12 N H 2

SO

4 28 ml Final pH 0.9 The me" un can be filter-sterilized or it can be prepared in separate portions as follows: ferrous ilfate Cissolved in distilled water acidified with sulfuric acid (filter-sterilize), and (ii) minerals salts dissolved in distilled water (autoclave). The size of the inoculum is usually 7.5% (vol/vol). Growth of the culture can be monitored by any of the following methods: (i) microscopic examination of the cell density; (ii) determination by chemical methods of the residual ferrous ion; and (iii) measurement of the redox potential of the culture.

NOTE 1. Fully grown cultures should be transferred to fresh media within 3 to 4 days to avoid cell death and prolonged lag periods. Note 2. Many commercially available pH probes display poor linearity in the low pH range required for calibration of the pH meter. The pH meter should be calibrated with pH 1.0 buffer solution and 21 the final pH of each batch of media should be determined to ensure the desired pH.

STORAGE: Store under liquid nitrogen.

Mixed Mineral ILeachinq Bacteria GBB Sulphur This culture contailis sul fur-oxidi zing, arsenic resistant, acidophilic bacteria, specifically characterized by their ability to grow at pH 1.0 with elemental sulfur ar, the sole source energy. Thel clire 'ierob i d utilizes carbon dioxide as the source of carbon. The culture was dlerived by suli-oltiiring GBB mixed on elemental sulphur at pH 1.0, The culture resembl~es Thiobacillus thinoxidans in its cjenoral chiaractLori stics and lihas been specifically derived from a mixed culture of bacteria p~reviously maintained uniler selective conditions in arsenopyrite-containing mineral leaching s~ispensions. The parent mixed culture has been deposited at the same time with the ATCC ("Mixed Mineral Leaching Bacteria, GI3B Mixed, ATCC #53618").

The culture is maintained in shs'ike flasks at room t empera tutre in a mineral salts medium. rhe following medlium is used for routine cultivation.

(NH

4 2 S0 4 3.0 g/liter KCI 0.1 g/liter K HPO 4 0.5 g/liter M9SO 4 '7 12 0 0.5 g/liter Ca(N0 3 2 0.01 g/liter Use sulfuric acid to adjust to pH 1.0. The mineral salts solution can be aUtoclavod or filter-sterilized. After sterilization, add aseptically precipitated sulfur (flowers of sulfur) as the substrate (1-4g/100 ml). The sulfur is c.teamsterilized three times f~or 30 min each during three conoecutive days. Growth of the culture can be monitored by microscopic examination of the cell density and by measurement of the pHi durinig incubation. The size of the inoculum is 7.5% (vol/vol).

22 NOTE: Many commercially available pH probes display poor linearity in the pHl 0.5-1.0 range. It is necessary to calibrate the pli meter with pH 1.0 buffer solution and check the calibration against pHi 2.0 buffer solution for accurate measurements.

STORAGE: Store under liquid nitrogen.

Another advantage of this invention is that when the invention is used for the solubilization of refractory sulphides containing precious metals, liberation of the precious metials can often be zcoompl ished by only partial oxidation of the SuLpjhide(s.

The following oxaples illustrate the invention: x ample One A irixed culture (..ntaining T. ferrooxidans, T. thiooxid ans and ferrooxidans, and resistant to high levels of dissolved areiwas dleveloped by our method of step-wise adaiptaltion to projressively h~gher levels of arsenic.

The foed used wnls a mixed pyrite--arren(opyrite concetrate recive-1 from the Giant Yellowknife Mine, similar to the cofl( tntrate sample de~iribed in Example #1.

but varyiii4 slightly in iron and arsenic content. The coacentrate used i this example assayed 20.08% iron and 5.70% arsenic.

A 5 litre 1tch fermentation test was slarted at 1.00 q/IL solids using the eq~uipment and procedures described in Example After initial acidification of The polp to pH 2, the Coest was innoculated with 200 mE. of ,a culturo routinely maintained on arsenopyrite at 10% pulp density.

Within 8 days on actively growing culture had developed which had oxidized virtually all of the arsenic (5.7 g/L present in solution) and had achieved an Eh greater that 750 reV. At this time another 10 q/b dry concentrate was added to bring the pulp density up to 110 23 g/L, after which leaching continoed unabated. Thereafter g/L of conceatrate was idded at 2-3 day intervals and dissolved iron, arsenic, pHi and Flh was monitored.

Iron and ar 3enic levels increased steadily and the Eh remaiv ed above 750 mV.

After 7 weeks of step,-wise increases in solids d ens itLy, the pulp density had reached 450 g/L, dissolved iron 80.1 g/L, dissolved arsenic 29.1 gI4, Eh 8130 mV and pH 0.66. At this time the arsenic tesistant culL~ire w, s used to innoculate several sbakeflasks: ferrous iron at pHi 0,9 and 2.0, and elciintal su lpliur at pli 0.9 and All flasks were indergoing rapid oxidation within 7 r1 .y, proving that the bacterial culture bazl beer, active iav healthy when eXIposed to 29.1 g/b itrstnalc and that the culture contained T. ferrooxidztns T. thiooxida-ns and L, ferronxicdhns.

E~x ample Two A mixed pyriLe-ai sonopyrlte cunceiitrate recei v-.

from the Giant Yellowknife Mine at Yellowknife, Northiwo Territories, Canada# was processed tbr(-_.jh a benc h soiJe co.ntinuous biol eachi circuit over 10 mo:nthjs to devel<;, din arsenic-resistant strain oE bacteria and evaluate the process of the invention for this concentrate.

o The as-received concenrtrate was reo'milled to 90%-400 meosh and assayed 18,6% Fe, 5.9% As, 15.0% 8 2 74 g/t Au and 28 g/t Ag. Concentrate in a feed ta-nk was slurried to the desired pulp dentiity with water and bacterial nutrient sailts whi0 c Civisted of l0kg

(MH

4 2

SO

4 /t conc. and I %q KH 2

PO

4 /t conc.

The bioleach circuit consisted of three 5 liter capacity turbine agitated tanks connected in series, with pulp pumped from one tank to the next with peristaltic, pumps. Air enriched with 1% Co 2 was sparged in directly underneath the tanks to provide oxygen for sulphide oxIclation and bacterial growth. Carbon dioxide erce 00" 24 air was not essential, but +the extra 002 did appear to decrease the bacterial adaptation times required, and improve leach rates by 20-33%. Tank temperature was controlled at 350 C.

To start the bacterial adaptation process, each of the three leach tanks was inoculated with an active mixed culture containing T. ferrooxidans, T. thiooxidans, and L. ferroo(.xiclans. The circuit wlas left in batob mode for 3 days to allow -the :wActeria to grow and multiply, at which time slurried feed containing 100 g/l conceitrate and bacterial nutrients were pumped slowly at a rate of ml/h to the first reactor in the bioleach circliit.

Simultaneously, tank 1 contents w(.re pumpod ait the samne rate to tank 2, tank 2 contents to tank 3, and tarik 3 contents to a pr Th,1ct tank.

Over the course of the next nonth, the feed rate was gradually increased inr-.eii,.ntally until a rate of 11.0 ml/h, corresponding to a retention time of 45 hours por tank, was reachedl. S$.,c--eesful adaptation was evident when bioleach rates were ol;served to Increase In direct Iroportlon to the increase in feed rate.

After a feed rate of 110 nd/h had been sucocessfully achieved 1 the feed pulp dJensity was inorea- sed cradually, in increments of 2 g/l, uid-il a pkip density of 200 g/1 had been reached.. Agailn successful adaptation was indicated by ui~cchrates increasing in direct proportion to the increilse in pulp detnsity. Soluble arsenic concentrations of 12 (il Were attaine-d withjout any adverse effects on the adapted bacteria.

Three distinctly different 8ttains of bacteria were identified in eaoh tank, Thiiobacillus thiooxidlans Which oxidiZes Only OleMeltC11 Sulphur, and Thiobacilltis ferrooxidans and Leptognirillum fo 2 roo9idanls whlich oxidize primarily pyrite and ferrou5 iron, and to a lesser extent, elemental sulphur.

00 once steady state leach conditions had been achieved at 200 g/1 soiids and 110 mi/h flow rate, slurry was removed from each tank, filtered and the solids retained for analysis and cyanidaition testing. Sulphide analyses revealed that sulphide extractions (cumulative) achieved in the bioleach circuit were: tank I 57. 9%; tank 2 89. tanl< 3 91. The solids also contained some elemental sulphur; tank I 1.1% S 0; tank 2 9% so tank 3 1.5% S The bioleachate exiting the last leach tank contained 30.0 9/1 iron (as Fe 3+)0 11.3 g/l arsenic (as As 5+ and registered a p11 of 1.1 and an Eh of 813 mV, wb:_ ceas the Eh of the solution in the first tank was only 720 mV. This low pH showed, that L. fetrooxidans played a dominant role in the oxid,-tion processes taking place in the last tank.

Solids wein'.t loss was 40,2%. The bioleachate was nelitralized to pH. 4.0 with limestone and ti rn further neutralizedl to phl 6.5 vsitth lime. This procedure ensured that all ar~iinic preoipitate"C 4s ferric arsenate, with exces:1 iron precipitated as jarosite and excess sulphate as gypsaim. After filtration to remove the tailings for disposal, the treited bioleachate and make-up wAter was recycled back to the feed tank.

Solids exiting the last leach tank assayed 6.9% re, A~i, 2. 1% S 9.65% 504 2 g1 /t Au anid 38 g/t Ag. Aased on rolids arid solution assays, iron extraction wa% and arsenic extraction was 95%. Some oxidized iron h~ad re-precipitated as jarosilte during the leach and remained with the solids.

Tbe untreated head concentrate and solids from aach tank underwent standard 24h bottle-roll cyanidation testing. The results are -,uimarized below.

elemental sulphur rapidly, but are capable of oxidizing ferrous iron, to become secondary organisms. Because the -26- Head Tank Tank Tank Conc. #21# CN Tail Assays: Au, g/t 41.56 6.31 2.47 2.77 Ag, g/t 8.91 9.26 16.80 17.49 Extractions M:) KAu 36.8 92.8 98.1 97.5 Ag 55.9 64.1 61.5 50.2 For a control test, a sample of thle same f inely ground concentrate was bioleached in batch mode at a pulp density of 200 g/1. Thle test was inoculated with a culture of T. ferrou.xidzjns previously grown batch-wise on Giant Yellowknife food. Becaut;e the culture had not originated from a prolonged continuous run, which illows development of both T. ferrooxidans and T. thiooxida-ns, only T ferrooxidlans was present.

After 14 days, leaching of iron and ar ;enopyrite had stopped, with only 26.7% iron and 75.8% arsenic oxidation achieved. Thle bioleachate registered a pH of 1.58 and an Bh of only 650 mV. Wie believe leaching stopped prematurely because of the inability of T. ferrooxidans to oxidize elemental sulphur at a rate *fast eaoujh to prevent S 0 from coating the sulphide minerals. In c'o.ntrasti when leaching in continuous mode, both T. ferro( xidans and T. thiooxidans develop with the latter oxidizing S 0 and allowing oxidation of the vilphides to go to comp .etion.

ExapleThree 'TwQ tonnes of a mixed pyrite-arsenopyrite concentrate, received from t'he Campbell Red Lake Mine in falmerton, Ontario, Canada, were processed through a pilot plant. Conventional cyanide treatment of the concentrate typically achieved only 60-70% gold extraction and 50-70% silver extractionj therefore Campbell Red Lake pretreats I ~1~1 27 the concentrate by roasting to enhance gold recovery to 97%.

The concentrate was re-milled to 90% -400 mesh 2and assayed 22.5% Fe, 6.9% As, 15.7% S 122 g/t Au and 33 g/t Ag. Concentrate was slurried to 17.5% solids (200 g/1) with treated recycled bioleachate and make up water in a feed tank. About 10 kg (NH4) 2

SO

4 /t cone. and 1 kg KH 2

PO

4 /t cone. were added as nutrients for the bacteria.

Slurried feed was pumped continously to the first reactor in the bioleach circuit at a rate of 3.73 1/h (0.75 kg solids/h). The leach circuit consisted of three 167 liter capacity, turbine agitated tanks connected in series with pulp passing from one tank to the rext by gravity overflow. Retention time was 45 hours per tank for a total leach residence time of 135 hours. Fach tank S. contained a mixture of three distinctly different strains of bacteria, Thiobacillus ferrooxidans and Thiobacillus o thiooxidans, and L. ferrooxidans. Air was sparged in directly underneath the turbines to provide oxygen for sulphide oxidation and bacterial growth. Tank temperature was controlled at 35° C, although temperatures as high as S42' C presented no problems for the bacteria.

Measurement of the sulphide contents of solids extracted from each leach tank indicated that cumulative sulphide oxidations achieved were 48.3% in tank 1, 75.3% in tank 2 and 93.5% in tank 3. In addition, 1.0-1.5% So was present in each case. Minimum oxygen utilizations achieved were 55% in tank 1, 35% in tank 2 and 30% in tank 3.

Product exiting the last tank contained 147 g/l solids, representing a solids weight loss of 26.5%. The solids assayed 8.3% Fe, 0.9% As, 1.4% 2

SO

4 170 g/t Au and 39 g/t Ag. The solution registered a pH of 1,45 and Eh of 796 mV, whereas the Eh

-I

I

0 0 ou

C

28 of the solution in the first tank was 700 mV. The solution in the last tank contained 32.8 g/l Fe, 12.6 g/l As, 79.0 g/l SO 4 1.8 g/l Mg, 0.7 g/l Ca and trace amounts of Cu, Co, Ni and Zn. Iron was present totally as Fe 3 and arsenic solely as As 5 Based on solid and solution assays, 72% iron, 89% arsenic and 71% sulphur dissolution was achieved. A substantial portion of the oxidized iron and sulphur had re-precipitated as jarosite.

The head sample and unwashed solids from each tank underwent standard 24 hour bottle-roll cyanidation testing. Results are summarized below.

Head Tank Tank Tank Cone #i #2 #3 CN Tail Assays: Au, g/t 10.1 8.88 4.66 3.50 Ag, g/t 15.6 18,17 17.14 13.37 Extractions Au 65.0 92.0 96.2 98.0 Ag 60.0 47.7 54.8 65.8 Product bioleachate exiting the last reactor wAs neutralized to pH 4.0 with slurried limestone, and further neutralized to pH 6.5 with slurried lime. After thickening, the solution was recycled back to the feed tank, and the solids, which contained gypsum jarosite and ferric arsenate, were disposed of as tailings.

Example Four The detrimental effect of excessive agitation shear stress on bioleaching was discovered While conducting laboratory pilot plant bioleach tests on Giant Yellowknife Mines' Red 24 ore, during which two different types of impellers were tried. The equipment used has been described previously in Example The pilot plant treated a refractory gold ore assaying 2.92% iron, 0.75% arsenic, 0.87% sulphide sulphur, and 21.04 g/t gold.

A

29 Giant Yellowknife personnel obtained only 65-75% gold recovery when processing the ore through their Salmita gold mill in the Northwest Territories. Sulphide minerals present in the ore consisted of pyrrhotite, arsenopyrite and pyrite.

The pilot plant was operated with three stages of leaching. Slurried ore at 23% pulp density was pumped to the first stage, which consisted of two parallel tanks (tanks lA and 1B). Slurry overflowed from tanks 1A 1 0 and 1B to tle second stage (tank which overflowed into the third stage (tank About 20% of the slurry in the o o third stage was recycled to tanks 1A and IB to provide acid and extra bacteria for the first stage bioleaching.

a Slurry exiting the final stage was pumped directly to neutralization and cyanidation.

The bioleach tanks were air-sparged at a rate of L/min to tanks IA and IB, 4.5 L/min to tank 2 and o L/min to tank 3. Temperature in the tanks was controlled ao at 35°C. First stage leaching required continuous addition of sulphuric acid to maintain the pH at about Sas the biologically produced acid was insufficient to neutralize acid consuming calcite in the ore. The amount of extra acid required was about 9 kg H 2 S0 4 per tonne 0o of ore. The PH in the< final step of 1,-laching in the last a< ao .ioleoah tank should be at least Tank IA was fitted with a 127 mm diameter, disk-type radial flow impeller (Rushton turbine). Tank IB was fitted with a 133 mm diameter, 45° pitched-blade impeller (axial flow). Tank 2 was fitted with a 127 mm dieeter axial flow impeller, and tank 3 was fitted with a 102 mm diameter Rushton turbine. Rushton turbines are traditionally used when a high degree of air bubble shear is desired in order to maximize uptake of oxygen into solution. Axial flow impellers are used when high oxygen uptake rates are not required. They provide less shear, better solids suspension and consume less power than the 30 Rushton turbine. One objective of the pilot plant test was to determine whether the more energy efficient axial flow impeller would provide a high enough oxygen uptake to sustain maximum possible bioleach rates.

During continuous operation, tank 1A stirring speed was set at 800 rpm, tank IB was set at 750 rpm, and tanks 2 and 3 were set at 850 rpm. The feed pulp density was set at 250 g/L. Nutilents, which were added to the feed tank, consisted of 0.3 g/L (NH 4 2 S0 4 and 0.05 g/L KH 2

PO

4 The feed rate was 7 L/h to tank IA ?nd 7 L/h to tank IB, for an overall retention time in the bioleach circuit of 2 days.

Throughout co tinuous operation, bioleach performance wvas surpr singly much superior in tank IB, equipped with the axial flow impeller, than in tank lA, fitted with the radial flow impeller. The Eh in tank lA remained at about 600 mV even when the flow rate ''as decreased to 5 L/h, indicative of poor bioleaching for this ore. The Eh in tank IB remained above 700 mV throughout the campaign, indicative of good leaching.

o This result was surprising because it had previously been assumed that the radial flow impeller would give the best possible results because of its superior oxygen uptake ability. Because the only difference between the two tanks was impeller type, bacterial shear stress was suspected.

The shear imparted by an impeller is proportional to the impeller's tip speed, so the stirring speed for tank 1A was decreased from 800 rpm to 500 rpm, to determine whether bioleach performance would be improved by a reduction in shear stress to the bacteria. This reduced the impeller tip speed from 5.3 metres per second to 3.3 m/s. Within two days, the Eh in tank 1A increased to over 700 mV, indicating that bioleach performance had improved.

31 These tests demonstrated conclusively that Rushton turbines can produce shear stress high enough to impair bioleach performance, a phenomenon which to the best of the inventors knowledge, has never before been reported in the literature for sulphide oxidizing microorganisms. Furthermore, axial flow impellers are superior to radial flow impellers for bioleaching of low-sulphur ores because of the greatly reduced shear stress produced by the former.

Example Five A mixed pyrite-arsenopyrite ore from the Lander County area of Nevada was treated by the process using conventional heap leach methods. The ore assayed 3.97% iron, 1.33% arsenic, sulphide sulphur and 7.10 grams per tonne gold. Standa i cyanide bottle-roll leach tests Son finely pulverized oie (minus 200 mesh) demonstrated that only 18.7% of the gold could be extracted. The remainder of the gold was presumably encapsulated within the sulphide minerals, and therefore not amendable to cyanide extraction.

A 3.0 \g sample of the ore was crushed to minus 0.64 cm and packed into a 7 cm diameter by 84 cm long plastic column. The solution application system consisted of a reservoir bucket holding 3-4 liters solution, a peristaltic pump, and a discharge bucket. Solution was pumped from the reservoir through the column at a rate of liters per hour per square meter cross-sectional area, and allowed to collect in the discharge bucket. After each 3-4 day leach cycle, discharged solution was returned to the reservoir bucket and water added to compensate for evaporation. The leach cycle was then repeated.

The column was initially saturated with water and the water uptake volume was recorded. The test was then acidified by adding sulphuric acid to the reservoir bucket and pumping acidified water through the column until the 32pH had stabilized at 2. The column was then inoculated with a mixed culture which had previously been adapted to the same, finely ground, ore. Leach progress was monitored by sampling leachate on a weekly basis and measuring soluble iron, arsenic, pH and Eh.

The ore was leached for 130 days. The Eh remained below 740 mV for the first 90 days, after which it increased ultimately to 890 mV by the end of the leach. The pH decreased from 2.0 to 1.41 during the course of the leach. Ir and arsenic extractions achieved were 65.5% and 86.4% respectively. Sulphide oxidation achieved was 45.9%.

After the bioleach treatment, the ore was washed and brought to a pH of 10.9 by pumping a weak lime solution through the column. The ore was then cyanide leached for 27 days. Gold recovery based on a calculated head gold content of 7.38 g/t was 82.0%. Thus the process, using heap leach methods, improved gold recovery from 18.7% to 82.0%.

Exa mnple Six Cultures of bacteria from the first and last reactors of a 3-reactor leaching process as described in Ex. 1 were grown on pyrite, And elemental sulphur. The latter substrate was at pH 0.9.

On pyrite substrate, a rapid and consistent drop in ulution pH and increase in Eh was noted in the test containing the culture from the last reactor, but no acid was produced with the culture from the first reactor, indicating that it was not capable of oxidizing sulphides, and therefore contained predominantly T. thiooxidans.

On elemental sulphur substrate, both cultures were able to oxidize elemental sulphur at pH 0.9, indicating that T. thiooxidans was present and active in both reactors.

These results indicate that the bacterium T. thiooxidans is present in all reactors, but 33predominates in the first reactor where it exists on the oxidation of elemental sulphur which is produced from the chemical reaction of arsenopyrite with ferric iron and oxygen. Elemental sulphur removal in the first reactor by T. thiooxidans is rapid enough to ensure substantially complete chemical oxidation of the arsenopyrite. A portion of the resultant elemental sulphur passes into the subsequent reactors, providing a continuing source of substrate for T. thiooxidans.

Although T. ferrooxidans is present in all teactors, it predominates in the last reactor where it oxidizes pyrite and ferrous iron.

ExamiAe Seven A culture grown- on an arsenopyritic concentrate at a pH of 0.28 was grown successfully on ferrous iron at pH 0.9 and 2.3 as well as on ele! intal sulphur at pH 0.9.

It must therefore be concluded that the culture contained both T. thiooxidaris and a special bacterial strain capable of oxi lizing ferrolus iron at extremely low pH values.

This special strain was identified by visual observation to be Ieptospirillum ferrooxidans.

Exanl Eaht A culture of bacteria was grown on an arsenopyritic concentrate containing 37.48% iron and 3.90% arsenic. Using a suspension containing 40% w/w solids, numerous serial transfers were carried out, ultimately producing a culture active in a solution of pH containirg 26.94 g/l dissolved arsenic and 89.76 g/l dissolved iron.

When samples of this culture were grown on ferrous iron at pH 0.9, it oxidized the ferrous iron rapidly. It also oxidized elemental sulphur at pH 0.9 and oxidized ferrous iron at pH 2.3, proving that it contained Leptosirillum ferrooxidans, Thiobacillus thiooxidans and Thiobacillus ferrooxidans.

hiC- 34 Example Nine The process has been tested on a semi-commercial scale at Giant Yellowknife's Salmita gold mill in the Northwest Territories. The plant was operated for a 2 month test period during which the Red 24 gold ore, described previously in Example 4, was treated.

The bioleach section of the plant consisted of fo'ur 3.05 m diameter by 3.43 m high stainless steel tanks, air sparged and agitated by overhead stirrers.

T'emperature in each tank was controlled at 35 0 °C by the flow of hot or cold water through cooling coils inserted o in each tank. Leaching was carried out in three stages in the same manner as described in Example 4. Acid consumption in stage 1 leaching was about 9 kg H 2

SO

4 per tonne of ore. Nutrient onsumptions were 1.1 kg (H4)S04 and 0,2 kg KH 2

PO

4 per tonne of ore.

Total retention time was 2.5 dlays at a pulp density of 23% for a processing capaicity of 9.1 tonnes of ore per day. The ore was ballmilled to 80% minus 200 iesh before bioleaching.

Oxilized product exiting the third leach stage underwent three stages of neutralization with slurried lime. The pH was increased step-wise from about 1.9 to 3.5 in stage 1, from 3.5 to 7,0 in stage 2, and from to 11,0 in stage 3. Lime consumption amounted to 17 kg Ca(OH1) 2 per tonne of ore.

The neutralized product was then cyanide leached with the gold precipitated on to zinc dust and smelted to produce a dore bar. Overall gold recovery from the ore was 95.6%.

During the initial stages of the run, impaired bioleaching due to shear stress was again noted.

Substitution of Rushton turbines for 450 pitched blade impellers in all tanks improved bacterial activity dramatically, giving the expected bioleach rates.

35 Example Ten A mixed culture containing T. ferrooxidans, T. thiooxidans and L. ferrooxidans was used to innoculate a 10% suspension of copper concentrate. The concentrate assayed 21.44% copper, 28.59% iron and 26.95% sulphur. The primary copper mineral in the concentrate was chalcopyrite, CuFeS 2 After a 2-3 day lag period, rapid bacterial growth and leaching of copper commenced.

Mircoscopic examination showed a large population of bacteria. The culture was maintained on the copper concentrate for 7 months by serial transfer techniques.

The culture was then used to innoculate several flasks: ferrous iron at pH 0,9 and 2.0, and elemental sulphur at pH 0.9 and 2.0. Within 3 days all flasks tested positive for bacterial growth.

This test proved the mixed culture is effective for leaching copper concentrate. Furthermore, after 7 months maintenance on the concentrate, the culture still contained an unchanged active mixture of T. ferrooxidans, O0 T. thio.,uxidans and L. ferrooxidans.

While the present invention has been particularly described with reference to certain specific embodiments thereof it will be understood that various modifications may be made to the pro'ess by persons skilled in the art without departing from the spirit and scope of the invention. It is intended therefore that this invention be limited only by the claims which follow.

-L

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6. Bruynesteyn, A. Vizsolyi, A. and R. Vos. 1930. The effect of low pHi on the rate of ferrous iron oxidation by ThioIacillus ferrooxid1Anq. Presented at tbe con reince:- U_.e fr coo~nisms in lyilrometallurgy, Pecs, Hungary.

7. IBruynesteyn, A. anti A. Vizstolyi. 1981. The effect of pHi and Eh on the chemnical and biological leching of a pyritic uranium ore. SME-SPR Interntion,1l 8. Groudev, S.N. 1983. Participition of Tiobac ill -us thiooxidans in the leaching of initals ~rmsihd minral. Pesetedat: Fifth Intiternatiojal lspaposium on BiohydromoLallurqX, C a -iai, tay 9. Norris, P.R. 1983. iron and Mineral oxidation with leptospiril.Ivt-like bacteria. Presented att Fifth lnltetnational Symrosium anB.oy9jtalr,, Cagliari, Italy.

Brown, Luong, R.V. and J.M. Forshaug. 1982.

The occuirrence of Firobacillus ferrooxidans and arsenic in subarctic st~~safctdby 9.old-mine drainage. Arctic 35 No.3:417-421, Livesey-Goldblatt, Philippe Norman, P. and DR.

bivesey-Goldblatt.. "Gold recovery from Arsenopyrite/-Pyrite ore by Bacterial Leaching and Cyanidation". Presented at: Fifth International on Bi oy~do mta~u~ 9 raiiii Italy.

12. Karavaiko, Chuchalin, L.K. and T.A. Pivovarova.

1985. Microbiological leaching of metals from arsenopyrite containing concentrates. Presented at: Sixth International S ibdomtlu2y Vanc o uvje -Ca na da.

13. General Mining Corporation. "Plasmidi vectors resistant to arsenic-capable of replication in Thiobacillus ferrooxidans". RSA No. 8406735.

14. D.A. Rawlings, I. Pretorius and D.R. Woods. 1984.

"Expression of a Thiobacillus ferrooxidans origin of replication in Esch iF. chia1 Y6ofl" J.A ofBacteriola Vol. 158, No. 2.i77~7

Claims (17)

1. A process for oxidizing multi-metallic arsenic sulphide containing ores, concentrates or other material, the process comprising: contacting the ore, concentrate or material with a ]each solution compri' ig a mixed cultvce containing as effective ore leaching components Thiobacillus thiooxidans, Leptospirillum ferrooxidans and Thiobacillus ferrooxidans in a first stage, at an Eh of less than 750 mV; and contacting the products from the first stage with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans in a second stage, operated at an Eh of 750 mV or more; wherein the pH of said leach solutions is maintained at 0.3 to 2.8. 2, A process as claimed in claim 1 in which the ore contains ars.iopyrite.

3. A process as claimed in claim 1 in which the ore contains pyrite and other multi-metallic sulphides which produce elemental sulphur during chemical leaching.

4. A process as claimed in claim 1 including a preliminary step of milling the ore to facilitate rapid bacterial action. A process as claimed in claim 1 including supplying the oxygen required for the chemical and biological oxidation for each stage by sparging air or oxygen.

6. A pj <as as claimed in claim 5 in which the air is enriched with carbon dioxide. 9107l5,immdatl04,al18921gia.rs,38 -39-

7. A process as claimed in claim 6 in which the carbon dioxide is supplied in the form of carbonate in solution.

8. A process as claimed in claim 1 in which the temperatures of the leach solutions are controlled in the range of 1 0

9. A process as claimed in claim 1 in which the temperatures of the leach solutions are controlled in the range of 30"-40"C. A process as claimed in claim 1 in which the leach solutions are treated to maintain their dissolved arsenic concentration at less than 5 g/l.

11. A process as claimed in claim 1 in which the mixed culture of bacteria used is resistant to pH values as low as 0.3, to dissolved arsenic concentrations as high as 29 g/l and to dissolved iron concentrations as high as g/l.

12. A process as claimed in claim 1 in which the pH of the leach solutions is maintained in the range of 0.3-1.5. S13. A process as claimed in claim 1 including a preliminary step of bna rial adaption.

14. A process as claimed in claim 13 in which the bacterial adaption comprises: placing the slurry of the ore to be treated in a plurality of bioleaching tanks; introducing into each tank active mixed cultures comprising as the effective ore leaching components Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans; allowing the bacteria to develop under stable 9107 15,immdaL 104,a:\ 18921gi.res,39 1 conditions; then feeding a slurry of ore and nutrients to the first tank; pumping the contents of each tank to a subsequent tank, the contents of the last bioleaching tank being pumped to a product tank; and gradually increasing the pulp density of the feed until adaption of the bacteria is complete. A process as claimed in claim 14 in which the two stages each consist of one to five bioleaching tanks.

16. A process as claimed in claim 14 in which the pH of the product from the last bioleach tank is increased to a pH above

17. A process as claimed in claim 14, in which the leach solutions are treated with a neutralizing agent to maintain their pH above 1.3.

18. A process as claimed in claim 1 in which said contacting is by conventional heap leach methods.

19. A process as claimed in claim 1 in which the imaterial contains a mineral selected from pyrite, arsenopyrite, pyrrhotite, tetrahedrite, chalcopyrite, millerite, sphalerite and cobaltite. A process as claimed in claim 1 in which said contacting is by conventional mechanical or air-lift agitation methods.

21. A process as claimed in claim 1 wherein the material is comminuted coal, and whe~ain the process is for desulfurization of the coal.

22. A process for oxidizing multi-metallic arsenic and S 9i 715,1nmndat 104,at 18921giares,40

41- sulphide containing ores and concentrates, the process comprising: contacting the arsenic ore with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus thiooxidans, Leptospirillum ferrooxidans in the first stage, at an Eh of less than 750 mV, to oxidize elemental sulphur; and contacting the products from the first stage with a leach solution containing a mixed culture comprising as the effective ore leaching components Thiobacillus ferrooxidans, Leptospirillum ferrooxidans and Thiobacillus thiooxidans in a second stage, operated at an Eh of 750 mV or more, to oxidize ferrous iron and sulphides; said process being conducted under conditions of low shear agitation and accompanied by sparging with bubbles of a gas containing oxygen; said conditions of low shear agitation being insufficient to prevent effective attachment of bacteria in said bacterial culture to sulphide mineral surfaces in said ore and insufficient to cause rupturing of the cell walls of said bacteria; and wherein the pH of said leach solution is maintained g at 0.3 to 2.8. 23. A process as claimed in claim 22 in which said conditions of low shear agitation are those produced by an axial flow impeller. 24. A process as clait.<d in claim 22 in which said conditions of low shear agitation are those produced by a radial flow impeller having a tip speed of less than about four meters per second. A process as in claim 24, which the ore is contacted with said mixed bacterial culture at a pH of 0.3 to 910715,!mmdLI04,a:l8921g'iairs,4 -42- 26. A mixed culture of acidophilic bacteria useful in the process of any preceding claim; coded GBB-MIXED, identified as ATCC deposit No. 53618; and containing T. ferrooxidans, thiooxidans and L. ferrooxidans. 27. A process according to claim 1 or 23 substantially as hereinbefore described with reference to the Examples. DATED this 15th day of July, 1991. GIANT BAY BIOTECH INC. By Its Patent Attorneys DAVIES COLLISON i, i 1; I:a s 9107 15,mmdat. 104,a,\ 18921 giares,2