Supergene alteration of sulphides, V. Laboratory studies on the dispersion of Ni, Cu, Co and Fe

Chemical Geology, 26 (1979) 135--149 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

135

S U P E R G E N E A L T E R A T I O N O F S U L P H I D E S , V. L A B O R A T O R Y S T U D I E S ON T H E D I S P E R S I O N O F Ni, Cu, Co A N D F e

M.R. THORNBER Division of Mineralogy, CSIRO, Wembley, W.A. 6014 (Australia)

(Received December 28, 1977; revised and accepted November 24, 1978)

ABSTRACT

Thornber, M.R., 1979. Supergene alteration of sulphides, V. Laboratory studies on the dispersion of Ni, Cu, Co and Fe. Chem. Geol., 26: 135--149. The results of experiments are presented in which Ni, Co, Cu and Fe(II) ions in agar gel are allowed to diffuse under the influence of oxygen and metal concentration gradients and to interact with the surfaces of goethite, serpentine, magnesite and talc. Ni, Cu, Co and Fe are fixed by serpentine and to a lesser extent by magnesite, both minerals releasing Mg into solution. Ni, Cu, Co and probably Fe(II), and also Mg, released from serpentine and magnesite, are all strongly fixed by existing goethite. Talc was shown to be comparatively inert.

Ni and Co were not coprecipitated with ferric hydroxide during its formation but there was evidence that Cu and Mg could be coprecipitated. These results are used to give a possible explanation of why most of the base metals released from a weathering nickel sulphide ore are dispersed in solution rather than fixed in the gossan.

INTRODUC~ON

Oxidative w e a t h e r i n g o f sulphide ores is a c o r r o s i o n process w i t h t h e c o n d u c tive ore c o n n e c t i n g t h e e l e c t r o d e s and t h e circuit c o m p l e t e d t h r o u g h the ionically c o n d u c t i v e g r o u n d w a t e r s . As t h e sulphides oxidize, metals are released t o t h e g r o u n d w a t e r , s o m e being q u i c k l y r e p r e c i p i t a t e d as p s e u d o m o r p h s in t h e gossan, s o m e spreading b e y o n d t h e c o n f i n e s o f t h e ore t o p r e c i p i t a t e in mineral f o r m or t o be a d s o r b e d o n pre-existing minerals and t h e r e m a i n d e r dispersing so widely as t o be lost f r o m t h e r e c o g n i z a b l e e n v i r o n m e n t o f t h e o r e (Nickel et al., 1 9 7 4 ; T h o r n b e r , 1975a, b; T h o r n b e r and Nickel, 1976). T h e m a j o r base metals released f r o m nickel sulphide ores are Fe(II), Ni(II), Cu(II) and Co(II), and o f these Fe(II) u n d e r g o e s o x i d a t i o n t o Fe(III) which is readily h y d r o l y z e d t o p r o d u c e o n e o f a n u m b e r o f iron o x i d e o r o x y h y d r o x i d e minerals. T h e e x t e n t and m a n n e r b y which Cu, Co and Ni are f i x e d b y Fe(III) as it precipitates d u r i n g t h e actual sulphide t o gossan processes, is d e p e n d e n t n o t o n l y o n t h e c o m p o s i t i o n o f sulphide ore b u t also o n t h e c h e m i c a l environm e n t and t h e m e c h a n i s m s o f t h e r e a c t i o n s t h a t t a k e place. As well as this, subs e q u e n t t o t h e f o r m a t i o n o f t h e gossan, base metals can be l e a c h e d f r o m or a d d e d t o it b y processes which d e p e n d o n t h e minerals present, h o w t h e base m e t a l o c c u r s in t h e m and t h e chemical p r o p e r t i e s o f t h e solutions involved.

136

Thus the ultimate base metal content of a gossan can vary greatly and quite often it may bear little relationship to that of the original sulphide. Adsorption, replacement and coprecipitation processes can also fix base metals as they are dispersed beyond the immediate weathering environment into surrounding rocks, including preformed gossan materials in another part of the profile. Although much useful information is available on the adsorption of base metals onto existing soil materials (Grimme, 1968; O'Connor and Kester, 1975; Kinniburgh et al., 1976; Forbes et al., 1976; Korte et al., 1976) no study has been made where base metals in solution are interacting with the surfaces of minerals in an oxidizing environment, as expected in a supergene weathering environment. The experiments presented here show how Cu, Ni, Co and Fe(II) interact with surfaces of goethite, serpentine, magnesite and talc in an environment where Fe(II) is also oxidizing and hydrolyzing. EXPERIMENTAL

A technique (Thornber, 1975a) using agar gel to immobilize the electrolyte to convection currents was extended so that Fe, Ni, Cu and Co ions could diffuse and interact with crushed minerals suspended in the gel while being acted on by an oxidation gradient. Agar is a cross-linked polysaccharide polymer of interlinking galacto pyranose groups and was chosen for its excellent gelling properties and good gel strength over the pH range to be studied. The gel does not bind metal ions, and studies on similar polymers have shown that there is no measurable interference with the surface properties of minerals (J. Lyklema, pers. commun., 1976). The gel can be broken down by strong acid for chemical analysis and the only interfering property is that reducing sugars produced in the acid slowly reduce Fe(III) to Fe(II) and make it difficult to establish the ratio between these species. By comparison, silica gels are sensitive to pH changes, interact with mineral surfaces and are likely to form compounds with base metal ions. In the three-dimensional experiments described below the base metals Fe and Ni were supplied to the gel as sulphates in the same proportions as these m e ~ occur in the supergene nickel ore of the Lunnon Shoot, Kambalda, Western Australia. Somewhat higher proportions of Cu and Co were used to facilitate analysis.

Three-dimensional experiment A disc of gel, ~ 5 c m diam. and ~ 1 c m thick, was placed in the bottom centre of a rectangular dish, 27 c m X 38 cm, and then the dish was filled to a depth of 6 c m with agar gel containing 0.1 M NaCl as an electrolyte. In its approximate 15 ml volume the disc contained 2.10 g Fe(II), 0.69 g Ni, 0.57 g Cu and 0.19 g Co, all dissolved as sulphates. The tank was allowed to stand with a glass cover to limit the evaporation, but with air access to the surface. A n y evaporation loss was made up by a fine spray of deionized water from time to time. After some 20 days a brown precipitate of Fe(III) formed above the centre and gradually spread over the whole of the tank. After 150 days the gel

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was tipped out and one quadrant was sectioned into blocks 1 cm × 1 cm in horizontal dimensions and 2 cm deep. Measurements of pH and Eh were made on each of these blocks by standard electrode techniques. They were then analyzed for F e t o ~ , Ni, Cu and Co by atomic adsorption techniques, and Fe (tI) colorimetrically using 2,2'-dipyridyl. This gave analyses for three layers -bottom, middle and top; the results are summarized in Fig.1.

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142

(b) A series of nine tubes was prepared, each with a 1 cm thick layer of gel containing 8800 ppm Ni, 5240 ppm Cu, 25600 ppm Fe and 2370 ppm Co as sulphates, sandwiched between layers of gel solution, about 10 cm from the b o t t o m of the tube. Powders of goethite, magnesite, talc and serpentine were mixed with the gel solution before it set, and these mixtures were arranged as layers at different levels and in various combinations in the tubes. The natural goethite used was crushed to less than 300 mesh and had a base metal content of 0.014 wt.% Ni, 0.21 wt.% Cu and 0.002 wt.% Co. The serpentine was a mixture of antigorite, chrysotile and lizardite with 3.25 wt.% Fe, 0.23 wt.% Ni, 0.004 wt.% Cu and 0.003 wt.% Co. The magnesite was prepared from a wet slurry of laboratory reagent grade MgCO3" Mg(OH)2- 3H20 held in a Pyrex® glass tube open at the t o p and standing in a vertical stainless-steel bomb. The cold b o m b was pressurized to 90 bar with CO2, then heated to 200 ° C causing the pressure to rise to 100 bar and held under these conditions for 5 days. The crystalline product was identified by XRD as magnesite and its only significant impurity was 0.04 wt.% Fe. The talc used was of commercial grade with no significant base metal content. The gels were extruded from the tubes after 150 days, sectioned into 1 cm lengths and broken down with 5 M HC1. The solutions so obtained were analysed by atomic absorption techniques as for the other experiments; however, any crushed mineral present was filtered off after the extraction with 5 M HC1. Only small portions of the added goethite, serpentine and talc dissolved but all of the magnesite went into solution, hence the need for especially pure magnesite. All of the analyses were plotted as ppm b y weight in Figs. 3--5 so that the results for bands to which minerals have been added are affected by the higher density of the mineral--gel mixture as compared with the gel on its own. That the measured concentrations of some base metals in bands of talc and magnesite (Fig.3c) dips below their general level in the gels is due to this dilution of gel by the mineral. However, the comparative effects are still valid and reference to these apparent negative effects will be made in quotation marks to indicate that they do not necessarily imply a real decrease in concentration. The results of the analyses and vertical layering of each tube are displayed in Figs. 3--5. The vertical length of each gel was 20 + 2 cm and this is represented by the total vertical length for each graph. RESULTS AND DISCUSSION

The movements of metal ions in these experiments and in the sulphide weathering zones are dependent on the mobilities o f the metal ions and the degree to which they are complexed in solution. Table I gives a comparison between the absolute ion mobilities, hydrolysis constants, stability constants for sulphate and chloride, and the solubility products for the hydroxides and carbonates that are pertinent to both the experimental and the weathering conditions. No data on the mobilities of the various metal complexes are readily available although it could be reasonably expected that they would be similar

143 TABLE I C o m p a r i s o n o f t h e mobilities and c o m p l e x i n g c o n s t a n t s o f t h e base m e t a l ions ~c (× (1)

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10 - ° ' 2 s no complex 10 -°'3 101"4s

(1) Uc = a b s o l u t e ionic m o b i l i t y in m 2 S-1 V - 1 (2) K ( h y d r . ) = [M(OH) +] . [H + ] / [ M 2÷ J (3) K s o ( M ( O H ) n ) = [M n+ ] [ O H - ] n (4) g I (SO4) = [M(SO4) ° ] / [ M s÷ ] • [ S O ~ - ] ( 2 ) - - ( 6 ) a c c o r d i n g to Sill~n and Martell (1964,

(Dobas, 1975). (5) K, (Cl) = [M(CI) + ] / [M s+ ] . [ C l - ] (6) Kso(MCO3) = [M 2+ ] I C O n - - ] 1971).

for each metal complex and a lesser mobility could be expected where the complex is large. All the bivalent metal ions show similar mobilities, all have the same property of being more strongly complexed b y sulphate than by chloride and they are all hydrolyzed to some degree forming significant M(OH) ÷ complexes above pH 7. The mobility of Fe(III) b y contrast is dominated b y its hydrolysis reactions. The formation of Fe(OH)3 precipitates is not an instantaneous reaction and has been shown to be preceded b y the formation of polycations that are related to the complexing anion (Murphy et al., 1976). The experiments are designed so that the base metals are introduced to the gels as sulphates at low pH in conditions similar to the release from weathering sulphides (Thornber, 1975b) and they are free to move by diffusion processes as M 2+ and MSO4° ions to where the pH is higher, C1- replaces SO~- and Fe(II) can be oxidized to Fe(III). This same t y p e of movement is envisaged as taking place in the sulphide weathering zones and the experiments demonstrate in an empirical way the types of interaction that can be expected. Fig.1 shows the movement, after 150 days, of base metals that were originally positioned in the b o t t o m layer as indicated by the arrows. Both Fe(III) and F e t o ~ show a marked concentration in the t o p section and in t w o small areas on the surface a heavy precipitation of Fe has nucleated. However, Ni, Cu and Co are n o t preferentially concentrated either in these areas or more generally in the t o p section. Eh, shown as volts vs. standard hydrogen scale, and pH did not appear to vary significantly throughout the tank. However, the Eh measurement was probably affected b y air access during the slow process of dissecting the gel during examination. The Eh--pH relationship to the movement of Fe is better demonstrated b y the one-dimensional experiment (Fig.2). Air access to the t o p of the tube

144

caused an increase in Eh, favouring the formation of Fe{III) nearer the upper surface. Thus Fe(II} reaching this zone was consumed to form Fe(III) which in turn hydrolyzed and precipitated, causing the decrease in pH. The consumption of Fe(II) maintained the concentration gradient, and Fe{II) continued to diffuse at a high rate. It has been shown (Thornber and Wildman, 1979} that coprecipitation and/or adsorption of Ni and Co with iron oxide hydroxides is enhanced by higher pH, therefore the lowering of pH at the sites where Fe(III) is hydrolyzing and precipitating would militate against Ni mad Co actually coprecipitating with Fe(III) while it is hydrolyzing. The overall final pH of the three-dimensional experiment was low, ~ 3 - - 3 . 6 (Fig.l), which could account for there apparently being no Ni or Co associated with the Fe(III} precipitate. However, there are additional complicating factors in the case of Cu and these are better demonstrated by the one-dimensional experiments. Where Fe 2÷, Cu 2+, Ni 2÷ and Co 2÷ are diffusing in one-dimensional tubes, the influence of different crushed minerals is quite profound and is best explained by surface interactions. In Figs. 3--5 the packing sequences of the vertical tubes, each about 20 cm in length, are represented in the left-hand columns and the corresponding compositional variations are shown graphically. The presence of Fe(OH)3 precipitated during the run is also indicated in the left-hand column. Fig.3a is a blank run with no added minerals and should be used to compare with the other results. The pH and Eh scales cover slightly different ranges for each of the three figures. In Fig.3a the Eh increases towards the air interface at the t o p b u t the whole system was influenced by the low pH, of a b o u t 3.1, which did not allow the degree of Fe 3+ hydrolysis to take place that occurred in the experiment represented by Fig. 2. In fact, precipitation of Fe had just started to take place on the t o p of the tube at the time of sampling. This can be seen in the slight Fe increase and pH decrease at the very top. The partial pressure of oxygen is thermodynamically linked to both the pH and Eh by the relationship: log Po2 =.67.6 Eh + 4 pH -- 83.2

(Garrels and Christ, 1965)

so that where the oxygen activity is increasing, both the Eh and pH can increase. In Fig.3a, while the Eh increases towards the top, the pH begins to increase part of the way up the uppermost "clear" length of gel b u t this increase is arrested and then a pH decrease occurs near the surface. At the same position, the Fe curve flattens o u t and shows a s ~ t increase near the surface. All of this can be interpreted as incipient creation of Fe(OH)3 by the formation of soluble hydrolysis complexes of Fe 3÷ such as Fe(OH) 2÷, Fe(OH)~, Fe2(OH)~ ÷ and Fe3(OH)~ ÷ as well as the large polycations recognized by Murphy et al. (1976). The surprising result shown in Fig.3a is the increase of Cu concentration towards the top surface, This effect is demonstrated in all the columns except those represented by Fig.Sa and c where no Cu has been able to diffuse past the serpentine layers. Hydrolysis of the Cu 2÷ and/or the formation of carbonate complexes due to higher carbonate activity attributable to atmospheric CO2 could be the explanation. On the other hand, if carbonate complexing is the

145

answer then an interaction of Cu and magnesite might be expected (Figs.3b, c; 4b, c), but here the Cu prefers to move towards the surface rather than to stay with the magnesite. There is no obvious theoretical connection between the increase in Cu concentration near the surface and the increase in oxidation potential; the Eh isalways in the range of Cu 2÷ stability. All of the cases where Cu increases towards the surface (Figs. 3a, b, c; 4a, b, 4c), except for the talc column (Fig.5b), show precipitation of Fe(OH)3 at the air--gel interface. Possibly the best suggestion is that in these cases the Cu 2÷ is involved in the Fe(II) oxidation and hydrolysis, causing it to coprecipitate by the same mechanism as described for the coprecipitation of Cu when goethite forms (Thornber and Wildman, 1979). If this explanation is correct, however, then the lack of Cu concentration at the top of the tank in the three-dimensional experiment (Fig.l) has to be explained. Possibly the vertical sectioning in that experiment may not have been sufficiently closely spaced to reveal this effect, if present. Explanation of the result at the top of the talc column (Fig.5b), where an increase in Cu is associated with a decrease in Fe, remains a problem. Mg is also concentrated near the top surface (Figs.3a; c; 4b, 4c; 5a) and in each case an Fe(OH)3 precipitate has formed. No explanation can be offered on the grounds of solubility data (Table I) so that it can only be assumed that Mg is coprecipitated with the Fe(OH)3. The strong adsorption of Ni, Cu, Co and Mg by goethite relative to the other minerals tested is amply demonstrated in Figs.3b, and 4a, b and c. Goethite alone has little effect on pH and Eh, as shown in Fig.3b, but the adsorption takes place nonetheless. The slight drop in pH over the goethite is probably due to adsorption of the base metals taking place by a hydrolysis process, i.e. surface H ÷ ions are released from the goethite as the metals adsorb (Forbes et al., 1976). Although it cannot be seen from the analysis of the goethite segment, adsorption of Fe(II) can also be implied from examination of the background Fe profile which shows the same shape as for Ni, Cu and Co, i.e. a down gradient towards the goethite, indicating diffusion towards the goethite. By contrast the Mg background profiles round the magnesite, talc and serpentine show down gradients away from the bands consistent with Mg being released and diffusing away. Serpentine layers cause an increase in pH, lowering of Eh, adsorption of Cu, Ni, Co and Fe, and release of Mg; this is probably due to acid dissolution of the serpentine and hydrolysis of the base metals. This effect is demonstrated most clearly in Fig. 5a and c where the serpentine has increased the pH ot more than 7. Cu is completely adsorbed into the serpentine layer when it stands alone as in Fig. 5a and c, but some Cu passes through rather thinner serpentine layers where they are combined with layers of goethite, magnesite and talc, as in Fig. 4a, b and c. Any effect from the magnesite is slight compared with the serpentine (see Figs.5a, 3b, and compare Fig.5b and 5c) and the response from talc is "negative" for Ni, Cu and Co and only slightly positive for Fe (see Fig.3c); any response in the talc/carbonate is probably due to the magnesite component. Comparing Fig.3b with Fig.4a, b or c, it can be seen that the goethite is

146

generally more adsorptive when it is combined with serpentine or magnesite that can act to produce and maintain a high pH. This is consistent with the resuits of the adsorption experiments of Forbes et al. (1976) and supports the mechanisms proposed that involve a surface hydrolysis process for the adsorption of base metals onto goethite. The well recognized parallel behaviour of Co and Ni in solution is again demonstrated; Cu by comparison appears to be more strongly b o u n d to serpentine as shown in Figs: 4a, and 5a and c. Where talc is involved, Cu appears to behave similarly to Ni and Co, as in Fig.3c, b u t in Fig.5b Cu forms a broad band that shows it to be adsorbing slightly more strongly than the Ni and Co. Fe also adsorbs to the talc and this can be seen as a brown stain of ferric h y d r o x ide; the band of Cu could be associated with this Fe precipitate.

Sulphide weathering One can apply these data to the supergene zone of a nickel sulphide ore b o d y that is weathering by a galvanic corrosion mechanism. Anodic weathering of violarite takes place as in the equation: (NiFe)3S4 + 16H20 -* 3 (NiFe) 2÷ + 4SO~- + 32H ÷ + 30e-

(1)

violarite

The weathering anode is in electrical contact with more noble pyrite and at the pyrite surface electrons from the half-cell reaction (1) are consumed in reducing oxygen to produce hydroxide (Thornber, 1975a) according to the reaction: 02 + 2H20 + 4e- -~ 4OH-

(2)

If the oxidation potential is high enough, then a significant amount of the Fe 2÷ produced at the anode will be oxidized to Fe3+: Fe 2÷ -> Fe 3÷ + e-

(3)

and some of this can hydrolyze in situ thus producing a pseudomorph of the violarite texture and increasing the local acidity still further. Experimental results described here and previously ( T h o m b e r and Wildman, 1979) indicate that these conditions, high acidity and sulphate, are not conducive to the coprecipitation of Ni or Co released from the violarite and thus less of these base metals can be expected to be contained in these pseudomorphs than would be expected for adsorption processes more remote from sulphide weathering processes. Some of the Fe 3÷ would be likely to diffuse towards the hydroxide producing cathode where it would precipitate to produce a coating on the surface of the pyrite grain and this may build a b o x w o r k texture. The experimental evidence presented here suggests that only Cu would be likely to coprecipitate with the Fe in this way and could explain why Cu is often preferentially enriched in gossans over Ni ores (MazzuccheHi, 1972).

147

If the sulphide weathering environment apart from the surrounding rocks is considered, then the anodic and cathodic reactions (1) and (2) taking place within its confines can be combined to give: 2(NiFe)sS4 + 2H20 + 1502 -~ 4H ÷ + 6 (FeNi) 2÷ + 8SO~-

(4)

and the similar equation for the ultimate weathering of the more inert pyrite can also be included: 2FeS2 + 2H20 + 7 0 2 -~ 2 Fe2÷ + 4SO42- + 4H ÷ Thus the overall reactions within this environment not only produce base metals in solution but consume oxygen and produce both sulphate and acid. These conditions are conducive to Fe remaining in the ferrous state and to all metals moving out of the immediate environment of the ore. Such movement away would be governed mainly by the direction of groundwater flow rather than by diffusion due to a concentration gradient or the flow of electrical currents (Webber, 1975}. However, these experiments demonstrate several mechanisms by which the base metals can be fixed when they move away from the immediate ore environment: (1) Cu may coprecipitate with Fe as it penetrates to more oxidizing environments and is oxidized and hydrolyzed. (2) Ni, Cu and Co will be fixed on existing goethite surfaces. (3) Serpentine and to a lesser extent magnesite will produce more alkaline conditions, releasing Mg and taking up Ni, Co and Cu. This could explain the Ni, Cu, Fe rich ultramafic bedrock near to the ore in the weathered zones at Kambalda reported by MazzuccheUi (1972). This also gives a possible explanation why much of the Ni released from an ultramafic rock during laterite formation can be associated with the smectite remnants of the serpentine and why the Cu is mainly associated with the goethite (Smith, 1977).

Storage o f mine and mill waste It has been suggested (W.E. Ewers, pers. commun., 1978) that the gel technique could be used to test the efficacy of rocks that could be built into the base of embankments, or the final covering of mine and mill waste dumps, to limit the migration of base metals and other elements which might prejudice the establishment of plants on the dump surface. The experiments reported here indicate that rocks containing a combination of goethite and serpentine minerals would be worth consideration. CONCLUSIONS

The experiments described here are semiquantitative as many of the variables can only be partially controlled. However, they have provided some insight into the weathering processes and their products both in the immediate ore environment and in the nearby country rocks.

148

(1) Fe(II) in solution will move towards higher oxygen activities where it will oxidize to Fe(III) which can undergo hydrolysis and precipitate, producing a more acid environment. (2) Cu(II) in solution can move in the same way and coprecipitate with the Fe(III) probably due to a catalytic involvement in the Fe(II)--Fe(III) oxidation process. Mg also shows a tendency to coprecipitate with the Fe(III). (3) The surface of an existing goethite has been shown to be ideal for adsorption of all the metals studied, Fe, Ni, Co, Cu and Mg. The adsorption produced a localized lowering of pH in accord with the hydrolysis mechanisms proposed by Forbes et al, (1976). Where goethite is associated with minerals such as serpentine and magnesite that raise the pH locally, the adsorption of all metals on the goethite will be increased. (4) In the experiments serpentine produced an environment up to 2.5 pH units higher than the background. Fe, Ni, Co and especially Cu are fixed by the serpentine, due mainly to the pH effect. Mg is released. Magnesite appears to behave in the same way but to a much lesser extent and the effect from talc is slight by comparison; some Fe is fixed and Cu may be associated with it but there is no apparent adsorption of Ni and Co. Thus as base metals disperse from an acid sulphide weathering environment t h e y will tend to be fixed in the more alkaline rocks such as those containing serpentine minerals. (5) If these results are applied to within the sulphide weathering environment then it is postulated that the pseudomorphic replacement of sulphides by iron oxide at the initial stages would create a localized low pH (~3) and high sulphate conditions where Ni and Co are not favoured to coprecipitate. These goethitic pseudomorphs could adsorb base metals at a later stage when the sulphides have been leached away completely and the pH is more nearly neutral. ACKNOWLEDGEMENTS

Thanks go to C.E.S. Davis and M.J. Willing for their careful analytical work, to M. Bussell for preparing pure magnesite and to E.H. Nickel, W.E. Ewers and A.W. Mann for reviewing the manuscript.

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