EFFECT OF FIBRE DISINTEGRATION ON FLOTATION OF AN ULTRAMAFIC NI-ORE. S. Uddin*, M. Mirnezami, S. R. Rao and J. A. Finch

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1 EFFECT OF FIBRE DISINTEGRATION ON FLOTATION OF AN ULTRAMAFIC NI-ORE S. Uddin*, M. Mirnezami, S. R. Rao and J. A. Finch Department of Mining and Materials Engineering McGill University 3610 University street Montreal, Canada, H3A 2B2 (*Corresponding author: ABSTRACT Ultramafic ores are a potential major resource of nickel. Recovery by flotation is challenged by the fibrous nature of these ores owing to the presence of serpentine minerals. The fibres create physical entanglement that reduces selectivity and hinders bubble motion. As an approach, technology developed to improve CO 2 sequestration of serpentines is considered as an ore pre-treatment step. This involves strong acid attack to dissolve magnesium from the serpentine lattice and weaken the structure. Coupled with mechanical attrition this leads to fibre disintegration. An ultramafic ore was subjected to acid attack (up to 15 wt% HCl) in a ceramic ball mill. Subsequent conventional flotation (amyl xanthate, soda ash and MIBC) gave significantly improved results over untreated ore. 1

2 INTRODUCTION There are three major sources of nickel, namely sulphide ores, laterites and ultramafic ores. Sulphide ores are characterized by high pyrrhotite gangue content that represents the major separation challenge in recovery of the main Ni-mineral, pentlandite. Canadian examples are the ores in the Sudbury district, Raglan in northern Quebec and Voisey s Bay in Labrador. The ores typically respond to treatment by combinations of xanthate collectors, soda ash ph modifier, carboxy-methyl-cellulose (CMC) dispersant and MIBC frother, for discussion purposes referred to as conventional sulphide flotation. Laterites are the result of extensive weathering and the nickel is present in a variety of nickel silicate-type minerals in a largely iron oxy-hydroxide matrix. The main current processing option is pressure acid leaching, technically and economically more challenging than processing sulphide ores. Because of decreasing sulphide ore reserves and the cost of treating laterites increasing attention is directed to the third source, ultramafic ores. The name reflects the high content of magnesium-iron silicate minerals derived from weathering of primary minerals such as olivine. The sulphide content is low, typically < 1%Ni mainly as pentlandite, with some pyrrhotite. The silicates include fibrous serpentines which present the processing challenge. These minerals produce viscous pulps that hinder grinding at normal operating densities and separation of the nickel minerals by conventional sulphide flotation. The separation challenges include entanglement with the fibres in the roughing stage and slime coating in the cleaning stages resulting from the opposite surface charging of pentlandite and serpentines at typical flotation ph 10 [1-]. In this communication an ore pre-treatment approach based on fibre disintegration is explored. The notion comes from the research conducted into the use of serpentine minerals for carbon dioxide sequestration. FIBRE DISINTEGRATION: BACKGROUND To illustrate the process, one of the most common polymorphs of serpentine is considered, chrysotile, with idealized chemical composition Mg3SiO 2 5( OH ). The mismatch in spacing between Mg and Si atoms makes chrysotile curl into thin rolled sheets which wrap into a spiral to form hollow tubelike fibres [5]. The structure is broken down to unit fibrils during comminution. The fibril structure is essentially composed of brucite-like (magnesium hydroxide) sheets bonded to tridymite-like sheets (polymorph of quartz) [6]. It is known that chrysotile can react with CO 2 to form environmentally stable MgCO 3 : Mg3Si2O5( OH ) + CO2 MgCO3+ SiO2 + H2O + 6 KJ / mole (1) Experimental work found that structural disruption of serpentine by chemical and mechanical treatments enhances this reaction [7, 8]. In the present work we used HCl attack in a ceramic grinding mill. The reaction can be written: Mg SiO( OH) + 6H 3Mg + 2 SiOH ( ) + HO (2) where grinding helps maintain exposure to the Mg layers. A factor that impeded reaction was precipitation of the leached metal ions [9]. A strong chelating agent would thus retard formation of precipitated layers and promote dissolution: we used EDTA (Ethylenediaminetetraacetic acid) for this purpose. 2

3 The paper discusses the impact of this fibre disintegration ore pre-treatment step on processing an ultramafic ore by otherwise conventional sulphide flotation. An integrated site with acid derived from smelter off-gas and tailings used for CO 2 sequestration is considered as a possibility. Ore mineralogy EXPERIMENTAL Vale Inco provided the sample. It comprised mostly serpentine (63%) and olivine (12%) with minor dolomite and Mg chlorite as the major sources of Mg. From X-ray diffraction the major serpentine mineral was identified as clinochrysotile as is shown in Figure 1. Naturally hydrophobic talc was low (<1%). The main iron minerals were pyrrhotite (5%) and magnetite (6%). The principal Ni sulphide mineral was pentlandite with minor violarite, mackinawite and millerite. There were essentially no Cubearing minerals. One hundred gram sub-samples were used in grinding and flotation. Figure 1 The X-ray powder diffraction pattern of the ore together with the match for clinochrysotile from a search of the Powder Diffraction File # [9], (vertical lines) Grinding Grinding was performed in a 15 cm diameter and 15 cm tall ceramic ball mill with a 15% media charge of 3 and 1.8 cm diameter zirconium oxide balls. The 100g-sample was slurried with 800 ml deionized water. Acid (HCl 10N) added to 5wt%, 10wt% and 15wt% along with 1wt% EDTA. Grinding time was set to hrs. To the ground sample CMC was added (0.05g) and the product was aged in the mill for 12 hrs during which the ph rose to near neutral. A sample of supernatant was taken for assaying. The slurry was then transferred to the flotation cell. 3

4 Flotation A Denver flotation cell was employed. The total slurry volume was adjusted to 1L (i.e., slurry density is 10wt% solids). The ph was adjusted and stabilized at ph ca. 10 using soda ash. As collector, 0.00g potassium amyl xanthate (PAX), purified by acetone dissolution and precipitation into petroleum ether, was used with a conditioning time of 5 minutes. Frother MIBC (0.002g) was then added and the system conditioned for a further for 5 minutes prior to introducing air. Three concentrates were taken at 1, 2 and minutes. They, along with the tails, were filtered, oven dried (at 100 o C) weighed and assayed (AAS and ICP-OES). A schematic of the procedure is shown in Figure 2. Two sets of test were performed at each acid concentration and the standard deviations are shown as error bars. General Features Figure 2 Flow sheet for fibre disintegration / flotation tests RESULTS A micrograph of the ore sample using field emission SEM is shown in Figure 3. The fibrous nature is evident with particles entangled among long, thin fibres.

5 Figure 3 SEM micrograph of the ore sample; (A), (B) and (C) Chrysotile fibres in ore; (D) Microanalysis Figure Settling slurry in different ph conditions; (A) Dispersed, (B) Partially agglomerated and (C) Agglomerated 5

6 The nature of the pulp was explored using settling experiments. Figure 3 reveals the transformation from dispersed at low ph to agglomerated at higher ph (ca. 10) where the pulp became viscous and difficult to mix. Fibre Disintegration: Mg dissolution As a guide to fibre disintegration Figure 5 shows the concentration of Mg in solution after grinding as a function of acid addition. Higher dissolution of Mg corresponds to higher dimensional instability and disintegration of fibres. There is a notable increase in dissolution of Mg from 0wt% to 5wt% HCl with further increases on acid addition up to 15wt% at which point approximately 10wt% Mg had been extracted into solution. At the same time approximately 3wt% Ni and 2wt% Fe were lost to solution as shown in Figure Distribution (%) HCl (wt%) Figure 5 Distribution of wt% Mg in the solution as function of HCl concentration Distribution (%) Ni Fe HCl (wt%) Figure 6 Distribution of wt% Ni and Fe in the solution as function of HCl concentration 6

7 Mineralogical examination (not shown) indicated the continued presence of fibres even at 15wt% HCl. Thus the present treatment only disintegrates a portion of the fibres. More aggressive treatments are capable of giving clear morphological changes (e.g. Figure 3 of [7]) Flotation Figure 7 shows the froth appearance as a function of HCl addition. At zero to 5wt% HCl the froth appeared barren but at 10wt% froth took on the shiny metallic lustre typical of a froth loaded with sulphide minerals. Figure 7 Froth appearance at (A) 0wt% HCl, (B) 5wt% HCl, (C) 10wt% HCl and (D) 15wt% HCl The corresponding Ni upgrade ratio vs. recovery is shown in Figure 8 where the upgrade ratio (or enrichment), R = concentrate grade/feed grade. At zero HCl the result confirms the limited upgrading suggested by Figure 7A although recoveries exceed 60%. By 15wt% HCl the upgrade ratio-recovery relationship shows significant improvement, again in line with Figure 7, with upgrade ratio reaching 10 in concentrate 1 and remaining above 1 in cumulative concentrate 3 with recovery ca. 85%. 7

8 12 8 0% HCl 5% HCl 10% HCl 15% HCl R Ni Recovery (%) 100 Figure 8 Ni upgrade ratio vs. recovery as a function of HCl concentration The other major sulphide mineral recovered is pyrrhotite, evidenced in Figure 9 which shows Fe recovery reaches 0-50% after acid treatment. 3 5% HCl 10% HCl 15% HCl R Fe Recovery (%) Figure 9 Fe upgrade ratio vs. recovery as a function of HCl concentration What has decreased is recovery of Mg, as shown in Figure 10. At zero acid recovery is 0% (by cumulative concentrate 3) but at 15wt% acid recovery is less than 15%. Compared to Ni recovery (Figure 8), which increases from ca. 60% to 85%, this represents a significant advance in selectivity between Ni and Mg minerals. 8

9 1 0.8 R Mg Recovery (%) 0% HCl 5% HCl 10% HCl 15% HCl Figure 10 Mg upgrade ratio vs. recovery as a function of HCl concentration As another indication of enhanced selectivity Figure 11 shows insol content in concentrate 1 and in the tail is similar at zero HCl but by 15wt% HCl there is significant difference, concentrate 1 showing < 10% insol while the tail approaches 0%. Figure 11 Insol content in first concentrate and tail as a function of acid concentration DISCUSSION The results have shown a notable improvement in Ni and Mg mineral selectivity using conventional sulphide flotation after applying the fibre disintegration treatment. The visual evidence is striking (Figure 7) where the froth, initially appearing barren, at 15wt% HCl treatment showed evidence of high loading with sulphide minerals. As a measure of increased separation efficiency we can use recovery difference (recovery of Ni minus recovery of Mg). Comparing Figures 8 and 10 shows, commensurate with 9

10 the visual clue, that treatment at 15wt% HCl took the initial recovery difference from ca. 20% (60 0) to ca. 70% (85 15). Indeed, at zero acid performance is close to a mass split with similar Ni and Mg recoveries (60% vs. 0%) and upgrading ratios (both ~ 1). This improvement in performance can be reasonably attributed to disintegration of the fibres through combined chemical (HCl and EDTA) and physical (grinding) attack that reduces entanglement. Disintegration was not apparent visually (SEM micrographs showed fibres remained) but the extensive loss of Mg to solution (up to 10%) does indicate a significant level of disintegration of the major fibrous mineral present, clinochrysotile. This is seen as significant as chrysotile is one of the strongest asbestos type fibres [11, 12]. Selectivity against Mg-minerals may be further promoted by the chemical attack lowering the Mg/Si ratio on the mineral surface, which reduces the iso-electric-point [13] giving serpentines a negative charge at flotation ph similar to pentlandite and thus countering the tendency for heterocoagulation (slime coating). The results do show the treatment promotes pyrrhotite recovery but technologies to enhance selectivity against this mineral are available (part of conventional sulphide flotation ). This ore pre-treatment based on technology for enhancing carbon dioxide sequestration of serpentine minerals appears technically attractive. The economics depend on several factors. An integrated site with SO 2 capture at the smelter may supply the acid for the ore treatment, which not only enhances flotation selectivity but also serves to neutralize the acid. Future work will focus on the use of sulphuric acid. If there is nearby CO 2 emission source the tails may represent a sequestration opportunity. As the demand to fix CO 2 grows the definition of nearby might be quite flexible. Future investigations should include the CO 2 sequestration capacity of the tails. CONCLUSION Using a fibre disintegration ore pre-treatment based on emerging technology for enhancing carbon dioxide sequestration, the flotation selectivity of a serpentine fibre-rich ultramafic Ni ore was significantly improved. With otherwise conventional sulphide flotation conditions, the difference in Ni and Mg recovery increased from ca. 20% with no treatment to 70% after treatment with up to 15wt% hydrochloric acid. This corresponded to a marked visual change in the froth suggestive of high sulphide content after treatment. ACKNOWLEDGEMENT The funding of this work and the permission to publish by Vale Inco is greatly appreciated. We, also, like to acknowledge Vale Inco for organizing the progress review meetings on Ultramafic Ore Research. This is an exceptional opportunity to share knowledge between researchers from McGill University, McMaster University, Columbia University, Cytec Industries Inc. and Vale Inco Technical Services Ltd. REFERENCES 1. G.R. Edwards, W.B. Kipkie and G.E. Agar, The Effect of Slime Coating of the Serpentine Minerals, Chrysotile and Lizardite on Pentlandite Flotation, International Journal Mineral Processing, 7, 33 2, Z. Dai, J-A. Bos, P. Quinn, A. Lee and M. Xu, Flowsheet Development for Thompson Ultramafic Low-grade Nickel Ores, In Advances in Mineral Processing Science and Technology (Ed. C.O. Gomez, J.E. Nesset and S.R. Rao), CIM, , H. Mani, M. Xu, P. Quinn and R. Stratton-Crawley, The Effect of Ultramafic Mineralogy on Pentlandite Flotation, In Processing of Complex Ores (Ed. J.A. Finch, S.R. Rao and I. Holubec), CIM, 63-76,

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