Molybdenite Polytypism and Its Implications for Processing and Recovery: A Geometallurgical-Based Case Study from the Bingham Canyon Mine, Utah

Abstract

Contrary to other sulfide minerals , where recovery is principally liberation controlled, the recovery of molybdenite is more complex. It is this complexity that initiated a geometallurgical investigation of molybdenites from Bingham Canyon. The aim of this investigation was to determine the effect mineralogy and/or mineralogical attributes have on recovery . All samples were analyzed using normal mineralogical techniques. The results reveal the presence of two distinctly different types of molybdenite . These have been identified as the two polytypes of molybdenite : i.e. hexagonal (2H); and rhombohedral (3R). The 2H-polytype occurs as textbook-shaped particles in quartz -molybdenite veins that are located in the current pit bottom. The 3R-polytype occurs as disseminated, “ball”-shaped particles with a dull or frosted appearance and are concentrated along the margins of the current pit. Concerning their metallurgical behavior, each type exhibits unique metallurgical properties that are consistent with those reported in the published literature. The 2H-polytype is easily ground and kinetically “faster” floating with surface attributes that are amenable to higher rates of recovery . This results in the production of a high quality concentrate under normal operating conditions. The 3R-polytype, by comparison, is difficult to grind and kinetically “slower” floating with surface attributes that are less amenable to recovery . Therefore, in deposits with higher concentrations of the 3R-polytype, modifications to the normal operating parameters may be necessary to improve recovery . This investigation highlights the necessity for understanding the mineralogical characteristics of any economic mineral(s), as these will have a direct impact on recovery .

Introduction

Molybdenite , or molybdenum disulfide (MoS2), the primary ore mineral of molybdenum , is primarily recovered as a byproduct of the copper mining and concentration process. As documented in the published literature (see discussion below), the metallurgical behavior of molybdenite is more complex than that of copper sulfides and does not appear to be consistent. In some deposits high rates of molybdenum recovery are easily achieved, while other operations regularly obtain lower than expected recoveries. In some instances these losses may be related to liberation , however, in most instances fully liberated molybdenite grains are lost. In most cases these losses are attributed to—(1) variations in ore mineralogy ; (2) slimes coatings; (3) optimizing copper recovery at the expense of molybdenite ; (4) grinding and liberation ; (5) particle attributes; (6) flotation reagents; and (7) ore blending—despite the documented presence of two naturally occurring types of molybdenite . In spite of this, a review of the published literature reveals that very few publications have evaluated the metallurgical performance of these different types of molybdenite and/or their impact to concentration, as well as froth flotation . Clearly a better understanding of the type of molybdenite present is critical for concentration and recovery as this is one of the key controls on the physical and chemical properties exploited in the processing circuit. In accordance, a geometllurgical investigation of molybdenites from the Bingham Canyon Cu-(Au-Mo ) mine was undertaken to determine what effect, if any, mineralogy and/or mineralogical attributes have on the recovery of molybdenite .

The crystalline structure of molybdenite comprises a series of closely-packed, tabular S-Mo-S layers that are weakly held together by van der Waal’s forces. The bonding between molybdenum and sulfur in the individual layers is covalent. As already stated, molybdenite naturally consists of two distinctly different types or polytypes. A polytype is defined as: “…any of a number of forms of a crystalline substance that differ in only one of the dimensions of the unit cell” [1]. Dickinson and Pauling [2] were the first to establish the hexagonal crystal structure of molybdenite . Later, Bell and Herfert [3] synthesized molybdenite with a rhombohedral crystal structure , which Traill [4] later found naturally occurring in the Yellowknife area of Canada. Although both polytypes consist of tabular sheets (Fig. 1), the most significant difference between them is the elongation and degree of rotation about the c-axis (long axis). In the 2H-polytype, Mo ions lay directly above or below S ions in immediately adjacent layers [1], resulting in stronger and energetically more stable Mo-S bonds. By comparison, in the 3R-polytype Mo and S ions are outset by a 60° rotation about the c-axis (long axis) [3]. The significance of this slight difference results in a slight attenuation of the Mo-S bonds and a higher interval of repetition resulting in weaker Mo-S bonds [1, 5]. This rotation also results in an elongation of the c-axis [1, 3, 6]. It is generally accepted that the cause for such rotations is due to internal strains generated by high impurity contents during crystal growth [1]. Although no detailed studies of impurity contents and molybdenite polytypism are known in the literature. Higher impurity contents (i.e. Re, but also Ti, Sn, Bi, W, Fe) are suggested to correlate with higher 3R contents [1, 5, 7, 8].

../images/468727_1_En_242_Chapter/468727_1_En_242_Fig1_HTML.gif — Fig. 1
Crystal structure of the 2H- and 3R-molybdenite polytypes emphasizing the different stacking patterns, see boxes.
Modified from [1]

Methodology

For this investigation, roughly 280 samples were collected from various plant feed and products produced by the Rio Tinto Kennecott’s (RTKC) Copperton Concentrator. For all samples, plant stream composite or grab samples were collected and prepared following RTKC internal standard operating procedure for sample collection and preparation. For mineralogical investigations, all samples were mounted in epoxy and polished in-house following the RTKC standard operating procedure for mineralogical sample preparation. Mineralogical investigations were carried out using normal polarized reflective light microscopy (optical), powder X-ray diffractometery (XRD ) and automated mineralogy (Mineral Liberation Analyzer or MLA) consisting of two Bruker energy dispersive X-ray spectroscopy (EDX) detectors attached to a JKTech/FEI Quanta 600 Mark I scanning electron microscope on polished blocks. Operating conditions were set to 25 kV acceleration voltage and roughly 40 µA emission current using a 6.9 μm spot size. A copper metal standard was used to calibrate the backscatter electron grey-scale. The MLA was used to estimate the modal sulfide mineralogy , sulfide mineral associations, textural and morphology attributes, liberation and general sizing characteristics of molybdenite grains. The powder XRD analysis was conducted using a PANalytical X’Pert Pro Empyrean X-ray diffractometer equipped with a Cu kα radiation source and PIXCel3D detector. Operating conditions were set to 45 kV/40 mA with scan ranges from 4.5 to 70° 2θ, in 0.007° 2θ increments with a total scan time of 34 min. Phase identification and analysis, as well as phase quantification were performed using the Rietveld refinement software HighScore Plus version 3.0. The mineralogical assemblage for each sample has been normalized to 100% and used to calculate the S/F ratio (see discussion), calculated as:

$S/F \, Ratio \, = \frac{Abundance \, \left( \% \right) \, of \, Slow - floating\,molybdenite\,(\it{3}R)}{Abundance \, \left( \% \right) \, of \, Fast - floating\,molybdenite\,(\it{2}H)}$

(1)

During the course of this investigation, only two physical attributes were measured optically on approximately 3,000 particles, they are: (1) particle size; and (2) aspect ratio. Both of these measurements were made optically with the particle size recorded as the maximum length in microns. It should be noted that while every attempt was made to avoid any sampling bias, the nature of this techniques does introduce some bias. The aspect ratio is defined as the ratio between the length of the major axis relative to the length of the minor axis of the best fit ellipse and is calculated using the formula:

$Aspect \, Ratio \, = \frac{Major \, Axis \, Length}{Minor \, Axis \, Length}$

(2)

Case Study from the Bingham Canyon Mine: Mineralogy and Geology

Located approximately 25 km southwest of Salt Lake City, Utah, the Bingham Canyon Cu-(Au-Mo) mine is classified as a classical copper porphyry deposit [9, 10]. Sulfide mineralization is characterized by the typical concentric pattern consisting of a barren core surrounded by successive shells of molybdenum , copper -gold , iron and lead -zinc-silver in the form of stacked, inverted bowls. In the molybdenum zone and throughout the deposit, S. Nelson in [11], describes the occurrence of molybdenite as: (1) cross-cutting to stockworked veins of quartz -molybdenite ; and (2) disseminated crystals within the groundmass of the host rock. Although veined molybdenite accounts for the majority of the molybdenite in the deposit, disseminated molybdenite locally encompasses a significant portion of the ore. The quartz -molybdenite veins measure up to 60 mm in width and vary in composition depending on the size of the vein. For example, large veins (>20 mm wide) comprise an outer rim of large (>1 mm), shiny molybdenite flakes overgrown by quartz , chalcopyrite and calcite. Smaller (<1 mm wide) veins, on the other hand, are generally composed of smaller molybdenite with a sooty-appearance. By comparison, disseminated molybdenite occurs as small amorphous grains dispersed or “peppered” throughout the host rock.

Mineralogical investigations of the mill’s feed and concentrates reveale the presence of two distinctly different molybdenite crystal forms. The two forms of molybdenite are: (1) more abundant textbook-shaped flakes informally termed fast-floating; and (2) less common spherical to elliptical or ball-shaped particles informally termed slow-floating. Identified in all samples analyzed, these morphologies have also been noted by previous investigators [11–13]. Macroscopically, the fast-floating form has a bright, glassy appearance, shatters easily and displays an aspect ratio of around 6, while the slow-floating form exhibits a dull to frosted appearance, easily deforms or distorts with an aspect ratio of around 2. Microscopically, the fast-floating form displays the typical tabular-shape commonly attributed to molybdenite crystals (Fig. 2a) with a high level of color change (i.e. bireflectance) from white to a dull blue grey. It also displays very strong, wavy change in color under polarized light (i.e. undulatory anisotropism). The slow-floating form, by comparison, display rounded to subangular, spherical to elliptical shapes with highly irregular or porous to pitted faces with a very bright white reflectance with little to no change in color (i.e. bireflectance; Fig. 2b). They also display a weak, dark sooty color under polarized light (i.e. isotropism). Lastly, it should be noted that the major element chemistry of these two forms of molybdenite are nearly identical.

../images/468727_1_En_242_Chapter/468727_1_En_242_Fig2_HTML.gif — Fig. 2
Plane polarized reflected light photomicrographs of: (a) tabular fast-floating molybdenite crystals in concentrate and (b) spherical, highly reflective slow-floating molybdenite crystal in concentrate. Abbreviations: Cpy = Chalcopyrite , Bn = Bornite, Ff Moly = fast-floating molybdenite , Sf Moly = slow-floating molybdenite

Discussion and Implications to Mineral Processing

In their analysis of molybdenite ’s metallurgical behavior from various deposits, Hernlund [14], Sutulov [15], and Shirley [16] concluded that molybdenite occurs as either: (1) quartz vein-hosted, clean or bright, coarse-crystalline, textbook-shaped molybdenite that does not require fuel oil for flotation and collection; or (2) sooty, smeared or dull-looking, amorphous molybdenite that does require fuel oil for recovery . Hernlund [14] and Sutulov [15] also found that this latter type of molybdenite is virtually unfloatable as it has a tendency to be emitted from the grinding circuit as coarse, saucer-shaped particles that are difficult to float.

Newberry [17] was the first to note the correlation between molybdenite polytype, geographic location and host rock alteration. In his analysis of porphyry copper deposits, he found that the 2H-polytype exhibits a close spatial and temporal association with regions of intense host rock alteration located with the core of most porphyry deposits (Fig. 3). In contrast, the 3R-polytype is characterized by a general lack of temporally associated host rock alteration and is concentrated along the outer margins of the mineralized intrusive body and host rocks (grey regions in Fig. 3). With regards to the Bingham Canyon deposit, XRD analyses of concentrates derived from ores mined throughout the pit show a consistent pattern of increasing slow/fast (S/F) ratios relative to the center of the mine. For instance, as illustrated in Table 1, the distribution of molybdenite polytypes follows that described by Newberry [17] (Fig. 3). In accordance, the vein-hosted (ore) or fast-floating molybdenite (concentrate) account for the majority of the molybdenite observed, exhibit the classical textbook-shape and display a close association to regions of intense potassic alteration in the central portion of the deposit. It stands to reason that the vein-hosted and fast-floating molybdenites are equivalent. The rarity of the disseminated (ore) and slow-floating molybdenite (concentrate), combined with their amorphous or elliptical morphology and association with distal, meta-sedimentary dominated ore types implies they are equivalent. A morphological description of the disseminated or slow-floating molybdenites as either the 3R-polytype or “slower”-floating molybdenite from other deposits [4, 18, 19] supports this correlation. In accordance, it is proposed that the majority of Bingham’s vein-hosted or fast-floating molybdenites belong to the 2H-polytype, while the disseminated or slow-floating molybdenites conforms to the 3R-polytype.

../images/468727_1_En_242_Chapter/468727_1_En_242_Fig3_HTML.gif — Fig. 3
Schematic cross section through a hypothetical porphyry copper deposit illustrating the spatial distributions of the 3R-polytype and Re contents. For instance, note the higher concentrations of the 3R-polytype, highlighted dark grey, along the top and margins of the porphyry intrusion, which holds true for the Bingham Canyon deposit (see [26]; Table 1)
(Modified from [17])

Table 1

Comparison of molybdenite polytype and associated attributes relative to geographical distribution within the Bingham Canyon deposit

Host rock	Location	Molybdenum grade	Alteration type	S/F ratio
Proximal porphyritic ores	Central/pit bottom	High	Intense potassic	Low (<0.5)
Distal host rock ores	Distal/marginal	Low	Generally lacking	High (>0.5)

Polytype Abundance

Frondel and Wickman [5] were the first to study the worldwide abundance and polytype of naturally-occurring molybdenite . They found that the majority of Cu-(Mo) porphyry deposits contain a mixture of the 2H- and 3R-polytypes in varying proportions. Newberry [1, 17], subsequently, documented the first occurrences of the 3R-polytype at Bingham Canyon, which he estimated contains some of the highest 3R-molybdenite contents of any porphyry, i.e. up to 95% with a mean of around 50% [17]. While the presence of the 3R-polytype has been observed in the concentrates, see previous paragraph, abundances far less than those documented by Newberry [1, 17] have been observed in the feed and concentrates.

Implication for Mineral Processing

While it has been well documented that molybdenite naturally exhibits multiple shapes or forms [1, 4, 6, 14, 15, 16, 17, 20, this study], documentation of their impact on the mineral processing circuit appears to be lacking in the published literature. This is in spite of data emphasizing the importance of understanding the effect these forms have upon their metallurgical behavior [11, 13, 14, 15, 16, this study]. The layered crystalline structure, combined with the presence of weak van der Waals forces enables the 2H-polytype to be easily liberated and separated from the other mineral phases present. In contrast, in the 3R-polytype, rotation and elongation of the crystal structure causes attenuation of the Mo-S bonds and this has a profound impact on the metallurgical behavior and subsequent performance. For instance, in the grinding circuit, the strong and energetically stable layered crystalline structure of the 2H-polytype enables easy separation of the layers during crushing and grinding. Although this characteristic is regularly exploited for liberation and grain size reduction , if not carefully managed it can result in molybdenite grains being easily slimmed. In contrast, attenuation and whereby weakening of the Mo-S bonds in the 3R-polytype, results in the creation of elliptical, coarse, malleable particles. However, these differences do not stop with the grinding circuit. As noted by Forbes and Bradshaw [12] and Triffett and Bradshaw [13] in their analysis of mill products, long plate-like molybdenite particles, i.e. 2H, tend to report to the concentrate, while compact or rounded molybdenite particles, i.e. 3R, tend to report to the tails stream. More recently, it has been observed that the 2H-polytype particles are best concentrated in a lower energy environment , while the 3R-polytype particles require a more turbulent environment to induce flotation . This conclusion has been confirmed by sized-based MLA samples of bulk concentrate where particles of the 3R-polytype are restricted to the coarser size fractions, while particles of the 2H-polytype occur in all size fractions.

Moreover, the layered crystalline structure of molybdenite also allows for the creation of the two opposing electrochemical surfaces. They are: (1) faces, which are produced by the rupture of van der Waals forces; and (2) edges that are produced by the breaking of the covalent bonds. As illustrated above for the grinding circuit, the crystal structure of the 2H-polytype allows for easy rupturing of van der Waals bonds resulting in the formation of non-polar surfaces that are responsible for molybdenite ’s natural hydrophobicity [15, 21, 22]. In contrast, rotation of the crystal structure of the 3R-polytype results in the formation of more edges. This, in turn, results in the formation of chemically active, polar surfaces that have a high affinity for water through the creation of various hydrophilic species (i.e. molybdates [22]). This suggestion has been confirmed by Triffett et al. [11], Zanin et al. [23] and Gerson et al. [24] who performed Time of Flight Laser Ionization Mass Spectrometery (TOFLIMS) analyses on molybdenite from the bulk flotation circuit. All of these studies found that the majority of the molybdenites reporting to the concentrate are of the “faster” floating variety (2H-polytype), which exhibits higher sulfide sulfur contents. The preferential recovery of “faster” floating molybdenites indicates a higher face to edge ratio, which is suggestive of a higher hydrophobicity and enhanced easy of recovery . Conversely, the “slower” floating molybdenites (3R-polytype), as well as the molybdenite of the tails, contain lower sulfide sulfur content with higher concentrations of oxidized products, e.g. MoO3, as well as Ca, Fe, Mg and K ions. Elevation of these ions and oxidation products is suggestive of a lower face to edge ratio or a higher percentage of polar surfaces. These polar surfaces prevent the spreading of hydrocarbon droplets reducing its efficiency and making these grains more difficult to recovery during froth flotation [25].

Table 2 lists the various attributes of the two polytypes, as well as their apparent metallurgical impact. From this table, it is apparent that in most industrial Cu-Mo porphyry concentrating operations, which are optimized for the recovery of copper sulfides, the crystal structure of the 2H-polytype generates high aspect ratio particles with high face/edge ratios that are easily recovered and concentrated under normal operating conditions [22]. On the contrary, the 3R-polytype crystal structure results in the formation of particles with low aspect ratios and low face/edge ratios that require a different set of operating parameters for optimal recovery . These might include running at a lower pH to increase the hydrophobicity and lowering of the zeta potential [22], using a smaller bubble size in the flotation tanks to decrease the mineral-bubble contact angle [R. Sorensen; Pers. Com.], using other collectors (e.g. none hydrocarbon-based) to name a few.

Table 2

Comparison of the attributes for the two molybdenite polytype and their apparent metallurgical impact to the processing circuit

Attributes	Fast-floating	Metallurgical impact	Slow-floating	Metallurgical impact
Polytype	2H	1. “Easily” recovered in Cu bulk flotation 2. Favorable for recovery 3. High probability of reporting to the concentrate	3R	“Difficult” to recover in Cu bulk flotation High probability of reporting to the tails
Natural abundance	Common		Rare
Crystal habit	Tabular sheets		Balls
Aspect ratio	High		Low
Face/edge ratio^a	High		Low
Surface properties	Sulfur -rich		Oxide-rich
Affinity for water	Low (hydrophobic)		High (hydrophilic)
Burner oil addition	Good—increases contact angle		Little to no impact
Ca ion/lime addition	Good—decreases zeta potential		Bad—creates an oxide coating
Floatability	Easy/fast		Difficult/slow
Cell energy	Low		High
Grindability	Brittle	Easily slimmed	Malleable	Difficult to grind
Re-content	Low		High	Economic boost

^aEquivalent to non-polar/polar surface ratio

Conclusion

The results of this investigation highlight the importance of understanding the mineralogical variations within a given ore body, but more importantly the mineralogical variations, i.e. chemical or structural, within given mineral species as these can have a direct impact on plant performance. For instance, a geometallurgical investigation of molybdenite from the Bingham Canyon mine confirms the findings of pervious investigations that recognized no less than two geologically, mineralogically and metallurgically distinct forms. A review of the published literature has assisted in identifying these as the 2H- and 3R-polytypes. The 2H-polytype is primarily located within quartz -molybdenite veins to veinlets in the current pit bottom, where they exhibit classical textbook-shape morphology . The 3R-polytype, in contrast, occurs as disseminated, amorphous grains located along the margins of the current pit. Macroscopically, they occur as elliptical or ball-shaped particles that exhibit a dull or frosted appearance. Metallurgically, the 2H-polytype is easily ground and kinetically “faster” floating with surface attributes amenable to higher rates of recovery resulting in the production of a high quality product under normal operating conditions. However, the crystal structure of the 3R-polytype results in particles that are difficult to grind and kinetically “slower” floating with surface attributes that are not easily recovered under normal operating conditions. In those operations with elevated concentrations of the 3R-polytype, modifications to the normal operating parameters may be necessary to improved molybdenum rates of recovery .

Acknowledgements

The author would like to acknowledge the Management of Rio Tinto Kennecott for granting time and resource to compile and write the work in this report. Many thanks also to the many individuals throughout RTKC for the useful discussions and assistance with various matters. In particular I would like to thank Amy Lamb, Michael MacDonald, Sarah Schwarz, Tracy Smith, Ron Sorensen and Steve Whites for their valuable feedback. I would also like to thank Cameron MacArthur for preparing all of the polished blocks and Nicolas Brady for his assistance with the running and processing some of the samples on the XRD .

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