Evaluation of blended collectors for improved recovery of PGEs from western bushveld UG2 deposit.
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Date
2018
Authors
Moja, Malebogo Gloria
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Abstract
Lonmin mining company located in the Bushveld Complex of South Africa is one of the main platinum group elements (PGEs) producers in the world. Its core operations are made up of 11 shafts and inclines. There are resources of 181 million troy ounces of 3PGE + Au, and there are reserves of 32 million ounces of 3PGE + Au.
One of the ore type produced at Lonmin is UG2 ore which is dominated by the high presence of chromite. The UG2 ore is also associated with PGE assemblages divided into sulphides and non-sulphides, and it is beneficiated through the froth flotation technique. Froth flotation is a physico-chemical process that is used for separation of desired valuable minerals from the gangue minerals by utilising the difference in surface properties.
The process has been achieving lower recoveries with P4 (shaft name) UG2 ore compared to Eastern Platinum Limited (EPL) UG2 ore when using similar reagents suite, this leads to loss of valuable minerals to the tailings, both ores were from Lonmin. The first step was to conduct the mineralogical analysis conducted using Scanning electron microscopy- energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and optical microscopy to study the mineral composition of the two ores, and to identify any differences between them (two ores) considering that EPL UG2 ore is a blend of P1, P2, P3 (shaft names) and P4 UG2 ores while P4 UG2 ore is not blended with any other ores. The mineralogical results showed the presence of chromite, plagioclase, enstatite and sulphide minerals. The PGEs could not be detectable by any of the techniques used due to their small size and rarity. However, X-ray diffraction detected differences in concentrations of minor gangue constituents such as talc, muscovite, chlorite and actinolite and these results suggest that reagent consuming gangue mineralogy may have contributed to the differences in PGE recoveries by flotation.
Batch flotation tests were also conducted. The existing reagent suite consisted of CuSO4 as an activator, Sodium n-propyl xanthate (SPNX) as a collector, carboxymethyl cellulose CMC as a depressant, and Senfroth 200 as a frother, and this was a single collector system. Therefore it was imperative to conduct flotation n investigation on alternative collector blends in order to improve the recovery of P4 UG2 ore. SNPX was used as the primary collector and it was blended with the following co-collectors: alkyl dithiocarbamates (DTC), two formulations of S-alkyl-N-butyl thionocarbamates (ABTC C1 & ABTC C2), and two formulations O-isopropyl-N-ethyl thionocarbamate (IPETC 30 & IPETC 31), one co-collector at a time. The first test incorporated the SNPX at dosage of 150 g/t without a blend and this dosage was selected based on the current optimum practice used at Lonmin and to use as a benchmark for the project. Trying to maintain the same dosage of 150 g/t of collectors, SNPX + co-collector were blended at two different dosages of 100 g/t + 50 g/t, and was also due to the fact that the co-collectors were highly concentrated and small dosages were expected to perform very well with SNPX. Lastly, the SNPX + co-collector at dosages 100 g/t + 125 g/t, here the dosage of co-collector was very high compared to 50 g/t and this was to check the effect of high dosages of highly concentrated collectors on the performance of the ore.
The flotation results showed that the use of 50 g/t of co-collectors yielded optimum PGEs + Au recoveries and grades, while the dosage of 125 g/t decreased recoveries and grades. The high dosage quantities of collectors do not necessarily mean they will yield improved recoveries and grades. Different chain structures can be used to alter the behaviour of a collector, and these may increase or decrease their capabilities to cause higher recoveries. By using a collector with a longer hydrocarbon chain the flotation limit may be extended without loss of selectivity, consequently bringing about greater water repulsion, instead of increasing the concentration of a shorter chain collector.
At 100 g/t of SNPX and 50 g/t of co-collector i.e. SNPX + IPETC 30 yielded improved 3PGE + Au recovery of 85.7 % at 3PGE + Au grades of 60.14 g/t, compared to the unblended SNPX (150 g/t) which yielded 3PGE + Au recovery of 81.1 % but insignificantly higher grade of 60.53 g/t. On the other hand, SNPX + IPETC C1 blend yielded low 3PGE + Au recoveries compared to SNPX + IPETC 30 and SNPX + IPETC 31 blends, but it achieved the highest grade of 76.1 g/t. Evidently, this proves that the relationship between recovery and grade is a trade-off.
The results have also shown the synergic effects, especially for SNPX blended with IPETC 30, and SNPX blended with IPETC 31 at dosage of 100 g/t (SNPX) and 50 g/t (IPETCs). It can be concluded that the different interaction obtainable from the thionocarbamate (ROCSNHR), effectively complement that from the xanthate ion (ROCS2–) to achieve more collector interaction at surface sites otherwise interactable for xanthate only. Therefore the collector blends rendered the mineral of interest hydrophobic and as a result the minerals were recovered to the concentrate.
On the other hand, too much of collectors may not be beneficial. At the dosage of 100 g/t of SNPX and 125 g/t of collectors, SNPX + DTC attained lower recoveries compared to SNPX, SNPX + IPETC 30, SNPX + IPETC 31, however the grade was higher than achieved SNPX + IPETC 30, SNPX + IPETC 31 and SNPX + ABTC C1. Nevertheless, comparing these results to the dosage of 50 g/t of the co-collectors, the 125 g/t did not perform well at all. The dosage of 125 g/t of co-collectors lead to loss of collecting power and selectivity, especially for SNPX + IPETC 30, SNPX + IPETC 31, and SNPX + ABTC C1 blends. It is therefore wise to conduct an optimisation test to determine the correct dosing rate.
In addition, the chromite entrainment was below the smelter limit and is very beneficial since chromite is detrimental to the furnace.
Therefore, it is concluded that the blends of SNPX with IPETC 30 and IPETC 31 at a dosage 50 g/t have shown satisfying recoveries and the FeCr2O4 recovery is less than 1 % meaning there will not be any smelter penalties for FeCr2O4 content. Therefore, these are the recommended collector blends.
It is recommended that further mineralogical study of the ores be conducted so that it may provide deeper insight into the causes of low recoveries under SNPX only. The system of blended collectors and its optimisation would be beneficial and can be practiced. The Chemisorption studies between the minerals and co-collectors used will provide more specific insight and details into the actual interaction synergy that gave the improved recoveries.
Description
M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology.
Keywords
Bushveld Complex, Platinum group elements, Flotation, Scanning electron microscopy