Refractory gold-bearing ores: a review of treatment methods and recent advances in biotechnological techniques

Hydrometallurgy, 34 (1994) 383-395 383 Elsevier Science B.V., Amsterdam

Refractory gold-bearing ores: a review of treatment methods and recent advances in

biotechnological techniques

Nieves Iglesias and Francisco Carranza Chemical Engineering Department, University of Sevilla, 41012 Sevilla, Spain

(Received January 10, 1993; revised version accepted March 16, 1993)

ABSTRACT

The problem of the refractory nature of gold-bearing sulphide ores is described and possible pre- treatments are reviewed. Among them, bioleaching is an interesting one from an environmental point of view, but up to the present the process has been too slow. The IBES (Indirect Bioleaching with Effect Separation) process improves the kinetics of the bioleaching by means of a physical separation of the chemical and biological effects involved in what we call the indirect contact mechanism. This permits the enhancement of each stage. This process, which has been developed for the treatment of high-grade sulphide flotation concentrates, is proposed for the pretreatment of gold-bearing sulphide ores.

1. I N T R O D U C T I O N

1.1 Refractoriness

Gold ores are considered refractory if gold extractions from a conventional cyanidation process are less than 80% even after fine grinding [ 1 ]. These low extractions do not normally allow economic recovery of the metal [2-6] .

These resources can consist of ores, flotation concentrates, mill railings and other reserves. Frequently, precious metals are locked and finely dissemi- nated in iron sulphide minerals (principally pyrite and/or arsenopyrite ).

1.2 Nature of the refractoriness

A significant percentage of gold in an ore may occur in submicroscopic form, either very finely disseminated or in solid solution with sulphide. This mode of occurrence may not be detected by optical microscopy or electron probe

0304-386X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 4 - 3 8 6 X ( 9 3 ) E 0 0 2 0 - O

384 N. IGLESIAS AND F. CARRANZA

microanalysis. Even fine grinding may fail to liberate the gold for recovery by cyanide leaching, in which case we define it as "physical" refractoriness. This kind of refractoriness is due to the inability of cyanide ions to gain access to the submicron size gold particles inside the dense sulphide grains [ 5-7 ].

The refractoriness is called "chemical" in nature if the gangue matrix can react with, for example, sulphide minerals, which decompose to species which combine with and deplete the cyanide available to dissolve gold.

The presence of some intermediate species, such as ferrous iron, sulphide ion, thiosulphates and arsenites, can consume vital oxygen for gold dissolu- tion in cyanide. Furthermore, these species tend to reprecipitate the gold al- ready oxidized [ 8 ].

In all cases, an oxidative pretreatment that breaks up the sulphide is re- quired before cyanidation, in order to render gold amenable to the subse- quent cyanide leaching.

1.3 Feasible pretreatments

Pretreatments may be classified into two main groups: the pyrometallurgi- cal ones, which include the traditional sulphide roasting method, and the hy- drometallurgical ones, such as chemical oxidation or bioleaching.

1.3.1 Roasting In the roasting process (Fig. 1 ) the sulphides are convened into highly po-

rous oxides. This is often carried out in rotary or circulating fluid bed roaster at 800-900°C [9-15 ]. It is an expensive process and the gaseous effluents have to be treated. In ores with a high sulphide content this treatment in- volves the production of sulplmric acid. This is an undesirable feature, due to the already saturated market for sulphuric acid. In ores with a low sulphide content it requires gas purification equipment. In some cases, this method of pre-oxidation can result in a poor metal recovery, particularly of silver, due

CONCENTRATE

Air T~WO:~{ Gas cleaning As~ Sulphuric ~-~H2SO , ~-~Roasting h Acid Plant

water v TM I I H21~r~ ~ ~ v

L_o Nootralizatioo ] Noot oliz t o i i v Lime v v

) ' ,

[ v

NaCN

v GOld Recovery

Fig. 1. Block diagram of the two-stage roasting process.

BIOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES 3 8 5

CONCENTRATE i v Reagent

~ CHEMICALLEACHING ~4~Sol!tion preparation 1

t S/L I separation 1

t Lime

~eutralizationl 4 r [

"ANIO 'ION I NaCN

v Gold Recovery

Fig. 2. Block diagram of the chemical leaching process.

IWaste disposal I

to fusion compounds which consume cyanide in the subsequent cyanidation stage. Therefore, considerable research has been devoted to the hydrometal- lurgical pretreatment.

1.3.2 Chemical oxidation at atmospheric pressure In this process (Fig. 2 ) the sulphides are transformed into sulphates or el-

emental sulphur. Several oxidants, such as ozone, permanganate, chlorine, Caro's acid (peroxymonosulphuric acid), hypochlorite, oxygen, hydrogen peroxide, nitric acid and ferric iron have been used for the oxidation precyan- idation of gold bearing refractory ores [ 1,16-21 ]. Reagent availability and cost, materials of construction, environmental concerns and development progress all contribute to limiting the application of these techniques [22 ].

1.3.3 Pressure oxidation In this process (Fig. 3 ) the sulphides are completely oxidized to sulphates

by pressure leaching with oxygen at high temperature. The following pro- cesses, among others, have been developed:

( 1 ) Sherrit-Gordon's process, which needs temperatures of 170-190 °C and pressures of 1800-2200 kPa [23 ].

(2) The ARSENO process, this differs from the first one in the use of ni- trate as a catalyst; for this reason this process needs less extreme conditions ( 100°C and 700 kPa) [24].

(3) Pressure leaching under alkaline conditions, this is carried out between 100 and 200°C [25].

These pretreatment processes have been successfully used for refractory precious metal ores with residence times in the range of 45-200 min. In many instances, however, their use is precluded by capital and operating cost rela- tive to the value of the ore.

386 N. IGLESIAS AND F. CARRANZA

CONCENTRATE

~ P r e e o n d i t i o n i n g I

v

~ox PRESSURE I ~ o~ IDATIVE LEACHING[ ~ steam

I F v l

S/L I s e p a r a t i o n ~

i v

Lime Neutralization

I v L

E CY IOATION f NaCN I

v Gold Recovery

o [Waste dieposal]

Fig. 3. Block diagram of the acid pressure oxidation process.

CONCENTRATE

Air~ i ~OLEACHINGI v Nutrients

STAGE ~ I Sulphurie Acid

1 I Conditi°ning l

s/L I ' ' - - t Waste disp°ssll s e p a r a t i o n ]

i v I iLim e

INeutralization~--~ r v I

[ CYANIDATION ~---492 NaCN

v G o l d R e c o v e r y

Fig. 4. Block diagram of the bacterial leaching process.

1.3.4 Bioleaching

An alternative to the aforementioned methods can be found in bacterial leaching (Fig. 4), a biological process whereby the leaching of sulphide min- erals is catalyzed by microorganisms.

2. BIOLEACHING AS A PRETREATMENT

This pretreatment is based on the action of bacteria in oxidizing reduced sulphur species and ferrous iron to sulphate and ferric iron, respectively.

BIOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES 387

2.1 Characteristics of bacteria

The most important bacteria for hydrometallurgical applications are Thiobacillus, Sulfolobus and Acidianus. Thiobacilli ferrooxidans are small, rod- shaped cells, gram-negative, mobile by means of a single polar flagellum, non- spore-forming and strictly aerobic. These chemolithoautotrophic bacteria de- rive carbon from carbon dioxide and energy from ferrous iron and reduced sulphur species. They grow at pH values between 1 and 3.5 and at tempera- tures of 20-40 ° C.

Genus Sulfolobus and genus Acidianus are very similar in morphology: spherical cells with lobes, gram-negative, non-mobile and without flagella. In chemolithotrophic conditions the sources of energy for their growth are re- duced sulphur species and, by chance, ferrous iron. They can obtain carbon from both organic and inorganic compounds. They are acidophilic and ther- mophilic, with a growth range at pH values between 1 and 6 and temperatures of 50-90 ° C for the genus Sulfolobus and between 45-70 ° C for the genus Aci- dianus. Sulfolobus (strict aerobe ) needs oxygen while Acidianus is a faculta- tive anaerobe [ 26 ].

2.2 Mechanism of bacterial oxidation

The biochemical reactions taking place during the biological leaching of sulphide minerals may proceed either by a direct or an indirect mechanism.

The direct mechanism requires a close physical contact between bacteria and the mineral surface in order to obtain the bacterial attachment to the mineral, while the indirect mechanism involves the action of bacterially gen- erated ferric sulphate.

The bacterial attack on pyrite takes place mainly by a direct contact mech- anism [27]:

4FeS2 + 1502 + 2H2 obacteriaFe2 ( S O 4 )3 "~- 2H2 SO 4 ( 1 )

the ferric sulphate produced in this reaction has not got oxidative power because it is chelated by the organic metabolites that these bacteria excrete to the medium [ 27 ].

The direct contact mechanism of arsenopyrite follows reaction (2), al- though several authors propound that arsenious acid may be formed as an intermediate product of biological oxidation, followed by ferric oxidation of the trivalent arsenic to the pentavalent form (reactions 3 and 4):

4FeAsS + 1302 + 16H2 obacteria4H3 AsO4 + 4FeSO4 (2)

4FeAsS + 1102 + 2H2 obacteria4H3 AsO3 + 4FeSO4 ( 3 )

388 N. IGLESIAS AND F. CARRANZA

H3 AsO3 -1- 2Fe2 ( 804 ) 3 "JI- H2 O ~ H 3 AsO4 -t- 2FeSO4 -t- Ha 804 (4)

According to the indirect contact mechanism of arsenopyrite, reactions (5) and (6) are possible. The first one, leading to elemental sulphur formation, is the most probable. The cycle is closed by the bacterial oxidation of the produced ferrous iron and elemental sulphur (7 and 8 ):

2FeAsS+ 2Fe2 (SO4) 3 + 3H2 O+ 5/202 --.2H3AsO4 + 6FeSO4 + 2S (5)

2FeAsS+ 2Fe2 (SO4)3 + 4 H 2 0 + 602 ~2H3AsO4 + 4FeSO4 -J- H2504 (6)

4FeSO4 + 02 + 2H2 SO4 bacteria2Fe2 (SO4) 3 + 2H20 ( 7 )

2S+ 2 H 2 0 + 302bacteria2H2SO4 (8)

2.3 Results of the bioleaching tests

Table 1 shows the results of some recent laboratory bioleaching tests [ 28- 35 ]. In all of the cases the contact between ores, nutrient media and bacteria was carried out simultaneously. It can be seen that Thiobacillusferrooxidans is the most commonly used. Generally, the use of the thermophilic ones is not justified from an economic point of view.

In most cases the gold recovery following bioleaching pretreatment has been spectacularly enhanced. Nevertheless, the results are poor from a kinetic point of view because reaction time ranges between 1 and several weeks.

The extent of the oxidation required to obtain high extraction yields of gold depends on the nature of the sulphide and the type of gold-sulphide associa- tion. The arsenopyrite is oxidized at a greater rate than pyrite because it has a lower electrode potential [ 36 ], so that the gold is released faster if it is as- sociated to arsenopyrite. Moreover, galvanic contact of pyrite-arsenopyrite leads to a preferential oxidation of the latter, acting as the anode, and passi- vation of pyrite (the more noble) acting as the cathode. In this way, Table 1 shows bioleaching times between 2-10 d, if gold is mainly distributed in an arsenopyrite matrix, and times between 1 and 5 weeks if it is distributed in a pyrite matrix.

In general, only a partial sulphide oxidation is required for complete gold liberation. This is because although gold is distributed throughout the sul- phide matrix, fine gold particles are associated with structural dislocations in the sulphide lattice, which are preferred sites of corrosion. This means that even refractory ores and concentrates where gold is evenly distributed among pyrite may respond to selective bacterial oxidation [ 37 ]. Besides, bacterial attack of the sulphide appears to be preferential in gold-rich regions because gold acts like the cathode in the gold-sulphide association [ 34,38 ].

Another conclusion from these experiments is that continuous operations

BIOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES

TABLE 1

Results of some recent laboratory bioleaching tests

389

Main Bacteria Temp. Operation Pd Time Fe Au recovery matrix ( ° C ) mode (w/w%) ( d ) oxide

(%) pre post (%) (%)

Pyrite T.f. 35 batch 20 35 87 24 81 T.f. 30-35 batch - 30 60 25 90 T.f. 30-35 continuous - 12 60 25 90 T.f. 33 batch 20 15 60 T.f. 33 batch 10 7 28 Sulfo. 64 batch 20 16 8 Sulfo. 64 batch 10 16 4 F.ther. ~ 50 batch 5 17 60 0 74 T.f. 35 batch 15 14 45 7 70

15 33 85 7 94 15 70 100 7 100

T.f. 30-35 batch 15 30 90 10 95 T.f. 30 continuous 18 5 90 21 95 T.f. 30 batch 10 30 65 90

continuous 10 16 50 90

Arsenopyrite T.f. 30 batch 5 8 29 6 56 S161 50 5 8 47 6 56 Sulfo. 60 5 8 84 6 91 T.f. 35 continuous 13 2 70(As) 15 73 Moderate continuous 2 7 15 63 Thermoph. 43 3 7 27 88 T.f. 35 continuous 15 6 31 55 90

15 10 80 55 99 T.f. + T.t. 2 35 continuous 12 7 71 92 T.f. batch 20 54 85

~Facultive Thermopile. 2Thiobacillus ferrooxidans+ Thiobacillus thioxidans.

substantially reduce the bioleaching time. This fact is due to the absence of the lag phase of bacterial growth in these processes.

A number of pilot plants and even semi-commercial operations for the bio- leaching of refractory ores and concentrates are known (Table 2 ). However, these operations require residence times of the order of days to get an extrac- tion of gold of 90% in the following cyanidation stage. These long residence times cause excessive operational costs [ 39-43 ] and so it is desirable to im- prove the bioleaching kinetics.

3. I M P R O V E M E N T O F B I O L E A C H I N G K I N E T I C S

First of all, in order to improve the bioleaching kinetics, it is necessary to consider the nature of the bioleaching mechanism. If bioleaching takes place by a direct contact mechanism the improvements are based on the develop- ment of more active cultures, either by the discovery of new natural species or by the modification of the known ones, through techniques of adaptation or genetic manipulation.

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BIOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES 391

Short-term chances of improving the kinetics of the indirect bioleaching are more promising. They lie in the physical separation of the two simulta- neous processes which take place in the bioreactor: the chemical oxidation of the sulphide by ferric iron and the bacterial regeneration of the ferrous iron produced (reaction 7 ), in order to enhance both.

The ferric attack on the sulphide minerals takes place by an electrochemical mechanism based on their semiconductor properties. The kinetics of these reactions are greatly influenced by temperature. At room temperature they are very slow but at moderately high temperatures (60-90 °C) they are much faster. This is due to the fact that the conductivity of semi-conductors rises as the temperature increases, enhancing the electronic diffusion. However, op- eration at high temperature is impeded by the mesophyllic nature of the ma- jority of bacteria; this difficulty can be avoided by a physical separation of the chemical and biological processes.

The chemical stage can also be improved by control of the pH and the use of catalysts to create artificial galvanic couples in order to achieve selective leaching [ 44 ].

The main role of the bacteria is to regenerate the ferric iron as a primary leaching agent (reaction 7). The rate of this stage is limited by several phe- nomena which take place in a single reactor. One of these phenomena is the abrasive effect that the mineral particles exert on bacteria. Bacterial abrasion has two negative consequences: firstly the active bacterial populations de- crease and, secondly, organic substances from the broken cells inhibit the

CONCENTRATE i Fe(IIl) [

v i

CHEMICAL LEACHING

STAGE i v I

S/L I Fe(II) s e p a r a t i o n ~

Washing

n e u t r a l i z a t i o n ~ L ime Water

1CYANIDATION 1 402 NaCN

v G o l d r e c o v e r y

Fig. 5. Block diagram of the IBES process.

] H~SO4 pH

conditioning

Fe(III) [

BIO-OXIDATION STAGE

392 N. IGLESIAS AND F. CARRANZA

growth of the autotrophic bacteria. The result is that, after few days, the activ- ity of the culture decreases. In addition, bacteria also need a suitable supply of CO2 and 02, the former as the only source of carbon and the latter as the last acceptor of electrons.

The biological process (ferrous iron bio-oxidation) can take place using one of two different models: suspended bacterial populations or supported bacterial films. It has been proved that the oxidative efficiency of Thiobacil- lus ferrooxidans attached to solid surfaces is higher than for suspended pop- ulations, probably because microorganism adsorption to a surface in close proximity to the nutrients saves the bacteria energy otherwise used to move against the nutrient concentration gradient [45 ].

Two types of reactor design, where bacteria are attached to supports, have been tested: the packed-bed bioreactor and the rotating biological contactor (RBC).

The packed-bed bioreactor consists of columns or heaps randomly packed with particles (generally an inert solid) and its design tries to achieve an as large as possible contact surface between the solution and the bacterial film. Sometimes forced aeration is installed [46 ].

The RBC unit consists of a group of disks mounted on a shaft above a semi- cylindrical trough. The disks rotate around the shaft in such a way that half of the surface of each disk is submerged in the solution and the other half is in contact with the atmosphere where CO2 and 02 are more easily diffused.

The IBES (indirect bioleaching with effect separation) process (Fig. 5), that has been developed for the treatment of bulk flotation concentrates of non-ferrous sulphides, applies the physical separation of the biological and chemical stages [47,48 ]. The process has shown an excellent flexibility with regard to the mineral feed composition and the level of the bacterial activity has remained high and invariable during a period of continuous operation. This is in spite of the high metal concentrations of the liquors [49].

A semi-industrial pilot plant of this process is being tested in order to carry out an economic evaluation. This pilot plant, which has been set up in Rio- tinto Mines, Spain, has a capacity of 1.2 t /d.

The chemical stage is carried out in stirred reactors at 60 ° C. After the first chemical reaction stage, a solid-liquid separation of the pulp takes place in settlers or in filters. The liquor is then passed through a heat exchanger and fed to a bioleaching heap, where the ferric iron is regenerated by the Thioba- cillusferrooxidans bacterium at room temperature. After a conditioning stage, involving pH adjustment and repulping, five chemical leaching stages take place in a cascade of reactors.

The selective dissolution of Zn from a Cu/Zn concentrate ( 15:9 ) has been tested in this demonstration plant. A total of 85% of Zn and 16% of Cu ex- tractions were obtained in 12 h of residence time (chemical+biological stages). An oxidation rate of 27 g Fe ( I I ) / h / m 2 of heap was achieved. The

BIOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES 393

energy cost was only 100 KWh/t concentrate and the contribution of reagents to the operating cost was almost nil.

At this moment the application of the IBES process to refractory gold-bear- ing ores is being studied. Batch and semi-continuous tests have already shown the viability of this process for the pretreatment of arsenopyrite-pyrite gold- bearing concentrates [ 50 ]. A refractory gold concentrate, assaying 20 ppm Au, 25.1 ppm Ag, 12% As, 15.8% Fe and 10.3% S has been tested. The main mineralogical species have been identified as arsenopyrite, pyrite and silica by XRD, optical microscopy and SEM. After 1 h of leaching with ferric sul- phate (6 g/ l ) at 80°C, pH= 1.25 and 5.5 mg of silver (Ag2SO4) as a catalyst, 35% of the arsenic was extracted. Bacterial cultures adapted to silver and to high arsenic concentrations are able to regenerate the ferric ion required for the chemical stage. After ferric sulphate pretreatment, 85% of gold recovery by cyanidation was obtained; in contrast to 54% achieved by direct cyanidation.

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394 N. IGLESIAS AND F. CARRANZA

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B IOTECHNOLOGICAL TECHNIQUES FOR REFACTORY GOLD ORES 395

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