Feasibility study on physical beneficiation of low-grade PGM flotation ...

2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong

Feasibility Study on Physical Beneficiation of Low-Grade PGM Flotation Tailings using Spiral Classifiers and Enhanced Gravity Separators J. Siame, and H. Kasaini

and chemical processes are used to extract the desired product from the run of the mine ore and produce a waste stream known as tailings. This process of product extraction is never 100% efficient, nor is it possible to reclaim all reusable and expended processing reagents and chemicals. The unrecoverable and uneconomic metals, minerals, chemicals, organics and process water are discharged, normally as slurry, to a final storage area commonly known as a Tailings Management Facility (TMF). Tailings are generally stored on the surface either within retaining structures or in the form of piles (dry stacks) but can also be stored underground in mined out voids by a process commonly referred to as backfill [1]. The challenges associated with tailings storage are ever increasing. Advances in technology allow lower grade ores to be exploited, generating higher volumes of waste that require safe storage. Environmental regulations are also advancing, placing more stringent requirements on the mining industry, particularly with regard to tailings storage practices. This ultimately places added pressure on the operators of a tailings facility who carry out the day to day roles of tailings discharge and water management [2]. The economic benefits of physical beneficiation of ores include the reduction of the impact on the environment, capital and operational expenditures which are often associated with extractive metallurgical operations (concentrators, roasting, smelting, leaching and purification of solutions). Furthermore, physical beneficiation can be applied to reduce the carbon print of a mining firm by scaling down its dependence on chemicals, energy and water for extracting minerals. In general, physical pre-treatment of low grade ores is necessary for reducing the total mass of reagent-consuming minerals and other gangue minerals in order to improve metallurgical plant efficiency and productivity. There are over 450 million tonnes of flotation tailings and smelter slags containing appreciable amounts of PGMs in South Africa. This is a huge resource for secondary PGMs. Several researchers have carried out feasibility studies of recovering PGMs (often associated with gold) by recycling the tailings back to the flotation banks or by lixiviation with HCl/H 2 SO 4 acid [3]. The average grade of total PGMs in tailings vary from one mining firm to another. The average PGM recoveries on concentrator feed solids vary between 80 and 88% across the industry and depending on the feed grade

Abstract— A study on physical beneficiation of platinum group metals (PGMs) flotation tailings was carried out by means of spiral classifiers and a gravity separator (Knelson Concentrator – KC-CVD 6 unit). The economic mineral reserves for PGMs are found in the Busheveld Complex geological formation of South Africa particularly in the narrow strata of Merensky Reef, (Platreef) and the UG2 chromitite layer. In addition, there are about 450 million tonnes of mine waste material (tailings and smelter slags) across the platinum industry which contains sufficient PGMs for economic exploitation. The major minerals in UG2 flotation tailings are chromite {FeCr 2 O 4 , S.G = 4.5 – 4.8}, orthopyroxene {(Mg, Fe) 2 Si 2 O 6 , S.G = 3.2 – 3.9} and plagioclase {Na 0.5 Ca 0.5 Si 3 AlO 8 , S.G = 2.6 – 2.8} with small amounts of talc {Mg 3 Si 4 O 10 (OH) 2 , S.G = 2.7 – 2.8} and clay minerals (chlorites/phlogopite, S.G = 2.2 – 2.75). PGMs in flotation tailings are either associated with silicates (60%) or are free PGMs particles (10%) while 30% of them are associated with sulphide minerals (pentlandite, chalcopyrite, pyrrhotite pyrite and millerite). On the basis of mineralogical data, about 70% of total PGMs associated with silicates and native alloys can be separated from oxide and sulphide minerals using gravity separators. This was confirmed by results from the spiral classifier and Knelson Concentrator where 71% of total PGMs were recovered to the tailings (lighter fraction) and the overall grade of PGMs increased from 0.75 g/t to 1.07 g/t. The highest amount of palladium to be transferred from the feed solids (10 kg) to tailings (5 kg) was about 84%. The optimum conditions for operating a Knelson Concentrator (KC-CVD 6) and spiral with regards to PGMs beneficiation are discussed further in this study. Even though the feed solids to the KC-CVD 6 gravity separator were deslimed, its capacity to upgrade PGMs from 0.7 g/t to 1.11 g/t at an average recovery of 71% compared well with the results from a spiral classifier where the fines remained part of the feed. The recovery of PGMs by gravity separation was about 75% at an upgrade ratio of 1.8.

Keywords— Beneficiation, Classifier, Gravity Separation, Knelson Concentrator, Platinum, Spiral Classifier, UG2 Tailings. I. INTRODUCTION

T

AILINGS consist of ground rock and process effluents that are generated in a mine processing plant. Mechanical

J. Siame is with the Copperbelt University, School of Mines and Mineral Sciences, Department of Chemical Engineering, P.O. Box 21692, Kitwe, Zambia. (Phone: +260 969 206022; Fax: +260 212 228212; E-mail: [email protected]). H. Kasaini is with Rare Element Resources Corporation, Denver, USA. (E-mail: [email protected]). 46


(4–8 g/t); the final flotation tailings may contain significant amounts of PGMs (0.7–1.6 g/t). In the past three decades, PGM losses to flotation tailings were very high mainly because there were no reliable analytical tools to verify the PGM assays in concentrator tailings until the mini laparoscopy-assisted (MLA) technique was introduced to the platinum industry. Therefore, stockpiles of flotation tailings constitute an important secondary resource of PGMs for the global market. But, beneficiation of PGM tailings by further flotation has limited benefits since only about 30% of PGMs are associated with base-metal sulphides and the rest are either inclusions or attached to clay minerals, with a small fraction being free or native minerals of PGMs. Fine grinding is required to liberate the PGMs which are locked in either chromite matrix or silicates [4]. Owing to the mineral hardness of silicates and chromite (5–6 Mohs scale) energy consumption in the milling process is expected to be high. The mineralogical profile of PGM feed solids to the concentrators is illustrated in Table I along with PGMs which are associated with or attached / enclosed in 25% oxide, 12% silicate, 36% sulphide or 27% native minerals. Mineralogical data on UG2 samples was generated by means of MLA technique. The UG2 samples are flotation tailings from the Northam Platinum Limited plant operations in South Africa. Complete recovery of PGMs which are locked in silicates, native minerals and chromite matrix is difficult. Therefore, most of the PGMs in flotation tailings are attached or enclosed in silicates [5], [6].

Table II illustrates the mineralogical profile of UG2 flotation tailings which contain about 0.7 g/t, total PGMs (Northam Platinum Ltd.). The most floated PGMs are associated with metal oxides and sulphides while PGMs enclosed or attached to silicates are lost to the tailings together with significant amount of free PGM particles [7]. On the basis of mineralogical data shown in Table II, about 57% of total PGMs in flotation tailings are attached or enclosed in silicates while 13% of total PGMs are associated with native minerals. Both silicates and native minerals are lighter minerals compared to metal oxides and sulphides and therefore physical separation of these minerals is possible under optimized granulometric conditions. Therefore, about 70% of total PGMs can be transferred to lighter minerals resulting in PGMs upgrade. The extent of the upgrade ratio depends on the partition coefficient of solids. Further milling of the heavier minerals from the gravity separators may be necessary to liberate 30% of PGMs which are associated or attached to metal sulphides or the heavy minerals (almost silica or clay free minerals) may be subjected to froth flotation in order to recover PGMs [8], [9]. TABLE II MINERALOGICAL DATA ON UG2 TAILINGS. Minerals PGM Distribution Oxides Enclosed Silicates Association Attached Enclosed Sulphides Association Attached Enclosed Free or Native Minerals Free

TABLE I MINERALOGICAL DATA ON UG2 FEED SOLIDS TO THE CONCENTRATOR. Chemical Formula S.G PGMs Oxide Minerals

Chromite

FeO.Cr 2 O 3

4.7

Silicate Minerals

Orthopyroxene

(Mg, Fe) 2 Si 2 O 6

3.5

Plagioclase

Na 0.5 Ca 0.5 Si 3 AlO 8

2.7

Talc

Mg 3 Si 4 O 10 (OH) 2

2.7

Chlorites

X 4-6 Y 4 O 10 (OH,O) 8

2.4

phlogopite

KMg 3 AlSi 3 O 10 F(OH)

2.8

Pentlandite

(Fe, Ni) 9 S 8

4.8

Chalcopyrite

CuFeS 2

4.2

Pyrrhotite

Fe(1 - x)S; x = 0 - 0.17

4.5

Pyrite

FeS 2

5.0

Millerite

NiS

5.5

Sulphide Minerals

Free or PGMs Native Minerals N.B.: X = Mn, Mg, Li, Fe, Ni, Zn Y = Al, Si

Attached/Enclosed (25%)

2% 25% 21% 11% 26% 2% 2% 13%

In this study, the KC-CVD was used to treat rougher flotation tailings that were pretreated by desliming (removal of Attached/Enclosed –45 µm fractions) using a spiral classifier. The final tailings (12%) from the spiral classifier (less heavy minerals) were transferred to the pilot enhanced gravity separator at a flow rate of 1.69 m3/hr. The rationale of the concept of upgrading PGMs in flotation tailings lies in the fact that a decrease in chromite minerals (heavier and abundant) will almost invariably lead to an upgrade of PGMs. Chromites are almost not physically or chemically associated with PGMs in tailings. Association In industry, gravity separation units are classified (3%) according to the average particle size of the feed; fine solids Attached/Enclosed (40 - 75 µm) require units that permit settling of particles in a fluid media under a centrifugal force (Knelson Concentrator, (33%) Kelsey, Falcon). These units have capacities up to 23 000 ton/day (Knelson CVD) and can take a closely sized feed. Shaking tables such as Mozley Multi Gravity Separators (MGS), with high throughput capacities (45 ton/day) could handle fine particles down to 1 µm from a closely sized feed. However, mineral jigs and spirals are heavy duty equipment Free (27%) which can handle feed solids with wide particle size distribution but are generally better for large particle sizes (>100 µm for spirals and > 1 mm for mineral jigs) [10], [11], [12]. The concentration criterion (C c ) is a mathematical expression which predicts the separation of minerals by 47


gravity taking into account the density (Á) of heavy and light minerals which are settling at different rates in a fluid of a known density. The value of C c should be equal or greater than 2.5 for easier separation irrespective of the sign [13]. 𝐶𝑐 =

𝜌ℎ𝑒𝑎𝑣𝑦−𝜌𝑓𝑙𝑢𝑖𝑑 𝜌𝑙𝑖𝑔ℎ𝑡 −𝜌𝑓𝑙𝑢𝑖𝑑

≥ ±2.5

were subjected to treatment by spirals in order to recover chromium. Chromium assay of the rougher flotation tailings varied between 25-29%. After gravity separation, a concentrate with about 40% chromium was produced. The tailings from the spiral classifier were discarded.

(1)

where, Á is the density of the fluid or minerals (kg/m3). The specific gravities of minerals are shown in Table I. In this study, 30% of the total feed solids were above 106 µm. The solids in the fine fraction (-45 µm) accounted for 43%. The middle size ranges (-106 +75µm and -75 +45µm) contained about 14% and 13% of total solids, respectively. The PGMs distribution and particle size distribution (PDS) of feed solids are shown in Table II and III, respectively. The objective of the present study was to investigate and establish the extent to which PGM assays in UG2 flotation tailings can be upgraded using gravity separation methods (spiral classifier) and enhanced gravity separation process (Knelson concentrator continuous variable-discharge – KCCVD). The grade of upgraded PGM tailings should make the PGM tailings amenable to effective leaching processes. II. EXPERIMENTAL PROCEDURE Fig. 1 Schematic diagram of heavy metal spiral classifier test rig.

A. Materials Solid samples of UG2 tailings were drawn from the rougher flotation circuit. The feed was used without further grinding. Table III shows the results of assay to particle size distribution (PSD) in the feed sample.

The operating variables such fluidization water flow rate, and G-force were selected using Knelson proprietary “Independent Control System (ICS)”. A series of on-site test runs were conducted over a period of one and half months in batch mode for an 8-hour shift. It is worth noting here that the four (4) test runs presented in this study were done at constant operating conditions (Table IV). Furthermore, the test runs were not performed consecutively due to the shutdown of the plant to allow for the maintenance of equipment to be carried out. The percent recoveries were calculated using the twoproduct formula and the average percent recoveries are presented in this study for the period of one and half months.

TABLE III PSD OF FEED SOLIDS TO LABORATORY SPIRAL AND PGMS DISTRIBUTION. Size fraction (µm) Wt. %, solids PGMs Distribution (%) +106 33 30 -106 +75 15 14 -75 +45 17 13 -45* 35 43 *The finer solids were not deslimed.

TABLE IV OPERATING PARAMETERS FOR THE FOUR (4) KC - CVD TESTS. Feed capacity (tons/hr.) 1.0 Fluidization water flow rate (m3/hr.) 2.7 Feed density (% solids/weight) 60 G - Force (G) 30 Sampling time (s) 5.0

B. Laboratory Apparatus: Spiral Classifier Test Rig Figure 1 shows the experimental set up of a spiral classifier in the laboratory. The flow rate (1.69 m3/hr.) and feed densities (10–45% solids) were optimized. The feed size (Table III) remained constant. In this set up, there was a provision to recycle the tailings and middlings back to the spiral classifier.

In a KC-CVD process, the heavier particles settle out of the fluid and report to the cone ring(s) of the device from where they are discharged continuously into a concentrate launder. The finer particles (tailings) flow out to the top of the cone into the tailings launder. The KC-CVD specifications are shown in Table V. The particle size distribution (PSD) of the spiral classifier tailings which were the feed solids are given in Table VI.

C. Pilot Scale: Spiral Classifier and Knelson Concentrator (KC – CVD) The purpose of the Knelson pilot tests was to recover PGMs from spiral’s cleaner tailings and upgrade them. The spiral tailings were treated as received from Northam Chrome Producers (NCP) plant at Northam Platinum Ltd., South Africa. Figure 2 shows the connection between a Knelson concentrator and spiral classifier. The spiral’s cleaner tailings were tapped off using a 4 inch pipe and routed to the Knelson device at a flow rate of 1 ton/hr. Initially, the rougher tailings (Table III) from the rougher banks were deslimed and the slimes were discarded. Subsequently, the deslimed fine solids

D. Sample analysis Samples of different masses for the concentrate and tailing solids were collected. The samples were filtered and then dried at 50 ºC for 3 hours. All test runs were repeated in triplicates. Solid samples (Pt, Pd, Rh, Cr and Fe) were 48


analyzed by Northam Platinum Ltd. and SGS Lakefield Research Africa (Pty) Ltd., Johannesburg, South Africa using a combination of analytical procedures: Fire Assay (FA), inductively coupled plasma (ICP-AES and ICP-OES,) and Xray fluorescence (XRF).

averaged 0.75 g/t, with a platinum fraction of 62%. Chromium and iron assays averaged 15.18 and 19.2% in the feed. About 10 kg of the PGM tailings were then subjected to spiral separation. After a few test runs, the optimum feed density was found to be 30% solids at a flow rate of 1.69 m3/hr. The assays of PGMs in the spiral tailings (lighter fraction) and concentrates (heavier fraction) are shown in Tables VIII and IX. But, the final partition of solids and PGMs are shown in Table X. According to Table VIII, the PGMs were upgraded from 0.75 to 1.07 g/t which represent an upgrade ratio of 1.4. The recovery of PGMs to tailings was about 71% according to the minerals distribution data (Table X). This result was achieved by lowering the grade of chromium in the feed by about 5% which implied that there was great potential to beneficiate the tailings further. About 29% of PGMs reported to the concentrate or heavier mineral fraction due to inclusions in metal sulphides which implies that PGMs should be liberated further by milling.

TABLE V KC-CVD specifications. Feed capacity (tons/hr. solids) 0.5 - 2.0 Fluidization water flow rate (m3/hr.) 1.1 2.7 Feed density (% solids/weight) 0 - 75 Maximum total volumetric throughput (m3/hr.) 4.0 Feed size (mm) (Recommended) 1.7 Concentrate volume (% of feed rate) 1 - 50 1 - 50 (Variable) Concentrate weight (% of feed rate) 1 - 50 1 - 50 (Variable) Concentrator net weight (kg) 230 Motor Horse Power (HP) 1.5 Independent Control System (ICS) Standard with machine Variable Gravity Variable G-Force

Pt (g/t) Pd (g/t) Rh (g/t) Total PGMs (g/t) % Fe % Cr Solids (g)

Fig. 2 Schematic diagram showing the connection of the Knelson concentrator- KC-CVD (1 ton/hr.) to spiral classifiers at Northam Platinum Ltd [14].


TABLE VI PSD OF SPIRAL TAILINGS WHICH WERE THE FEED SOLIDS TO KC-CVD. Size Fraction % Wt. % Cum Wt. +212 2.2 2.2 -212 +150 13.3 15.5 -150 +75 57.1 72.6 -75 +53 20.3 92.9 -53 +45 3.7 96.6 -45* 3.4 100 *The finer solids were deslimed.

A. Laboratory Spiral Classifier Test Rig The feed to the spiral classifier was sampled and PGMs in each size fraction was determined (Table VII).


-45 0.70 0.31 0.09 1.10 16.05 17.94 1304

TABLE IX UG2 SPIRAL CONCENTRATE (HEAVIER FRACTION). +106 -106 +75 -75 +45 -45 Composite 0.47 0.21 0.19 0.27 0.41 0.13 0.12 0.11 0.15 0.16 0.09 0.11 0.08 0.09 0.13 0.69 0.44 0.38 0.51 0.70 15.73 16.43 17.36 17.21 17.32 21.21 21.39 20.81 21.78 21.52 658 1235 2318 789 5000

B. Knelson concentrator continuous variable-discharge (KC-CVD The average feed rate was 1 t/hr. (0.15 kg/s) and the sampling frequency was every 5 seconds. Since the G-Force, fluidization fluid and feed rate were constant, composite samples of concentrates and tailings were created for each test run. Table XI shows the assays of solids in the feed, concentrates and tailings of the KC-CVD. The result obtained in the first run of KC-CVD tests showed that 71% of PGMs can be recovered to 45% of solids (Table XI). In this case, the PGMs were upgraded from 0.70g/t in the feed to 1.11g/t in the tailings. This represents an upgrade ratio of 1.6 which is slightly higher than the value obtained in spiral classifier (1.4). Even though the feed solids to the KC-CVD device were deslimed (removal of the -45 µm fraction), the upgrade and recoveries achieved compared very well with similar results from a laboratory spiral classifier. The G-force (gravitational force) was not varied beyond 30. Therefore, further tests are required to study the effect of the G-Force (30 – 120) and fluidization rate of water in the KC-CVD.

III. RESULTS AND DISCUSSION

TABLE VII UG2 FEED TO THE SPIRAL. +106 -106+75 -75+45 0.93 0.38 0.31 0.23 0.18 0.21 0.09 0.06 0.08 1.25 0.62 0.60 15.00 16.57 16.65 18.42 18.61 18.10 1298 2535 4863

TABLE VIII UG2 SPIRAL TAILINGS (LIGHTER FRACTION). +106 -106 +75 -75 +45 -45 Composite 0.95 0.38 0.57 0.42 0.60 0.40 0.27 0.40 0.33 0.36 0.18 0.12 0.18 0.16 0.11 1.53 0.77 1.15 0.91 1.07 16.75 17.40 17.85 17.22 18.27 13.82 13.96 13.56 13.46 14.40 650 1290 2415 645 5000

Composite 0.47 0.19 0.09 0.75 15.18 19.20 10000

The total PGMs in rougher tailings over a two-week period of sampling from the Northam Platinum Ltd., South Africa 49


successfully to beneficiate the flotation tailings which were first pre-treated by spiral classifiers. About 71% of PGMs were recovered from flotation tailings by using spiral classifiers and a further 70% of PGMs was recovered from the spiral classifier tailings using KC-CVD. Total recovery of PGMs to the lighter minerals or silicates was 75% at an upgrade ratio of 1.8.

IV. CONCLUSIONS Even though the feed solids to the KC-CVD gravity separator were deslimed, its capacity to upgrade PGMs from 0.7g/t to 1.11 g/t at an average recovery of 71% compared well with the results from a spiral classifier where the fines remained part of the feed. The KC-CVD was applied

APPENDIX

Element(s) Pt (g/t) Pd (g/t) Rh (g/t) Total PGMs (g/t) % Fe % Cr Solids (g)

Feed (g) 0.00460 0.00215 0.00080 0.00755 1819.5 1930.0 10000.0

TABLE X UG2 SPIRAL CONCENTRATE (HEAVIER FRACTION). MASS DISTRIBUTION Tailings (g) Conc. (g) 0.00300 0.00160 0.00180 0.00035 0.00055 0.00025 0.00535 0.00220 913.5 906.0 720.0 1210.0 5000.0 5000.0

PERCENT DISTRIBUTION Feed (%) Tailings (%) Conc. (%) 100.0 65.2 34.8 100.0 83.7 16.3 100.0 68.8 31.3 100.0 70.9 29.1 100.0 50.2 49.8 100.0 37.3 62.7 100.0 50.0 50.0

TABLE XI KC-CVD TEST RESULTS: PGM ASSAYS OF FEED, CONCENTRATES AND TAILINGS. SOLIDS AND PGMS DISTRIBUTION.

[5]

ACKNOWLEDGMENT

The authors are grateful to Tshwane University of Technology, Department of Science & Technology (Innovation and Technology Fund), South Africa and The Copperbelt University, Kitwe, Zambia for financial support.

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[2]

[3]

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