UG2 Sparger Cell Technology
Extensive investment in research and development (R&D) has made a meaningful contribution in increasing efficiencies in Northam's metallurgical plants. The introduction of an external sparger column cell as a final concentrate cleaner in the company's UG2 concentrator plant has been a successful innovation resulting in improved concentrate upgrade ratios, higher PGM recoveries, and ultimately reducing the total chromite (Cr2O3) content in the feed to the smelter plant and avoiding the associated build-up of chrome in the smelter.
The relatively high incidence of chromite in UG2 ore has for a long time presented a number of metallurgical challenges in the PGM industry, as it has an impact on PGM recovery in the concentrator circuit, and chrome content higher than 1.6% in concentrate will build up in the furnace at a temperature of 1 485°C. It could, and often does have an economic impact on the producers of concentrate as most mineral processing plants will impose penalties for chrome content above 1%.
Spearheading this innovation at Northam were metallurgical manager Danie Minnaar, and senior metallurgist Danie Smit, by running a series of rigorous tests and pilot plant trials at the Northam UG2 plant, where the traditional processing circuit had been unable to achieve the required Cr2O3 grades as indicated in laboratory tests in the plant design phase.
The trials involved the installation of a pilot plant flotation column as a re-cleaner on the final concentrate stream of the UG2 plant. The pilot flotation column, 0.6 metres in diameter and 6.1 metres high, was tested over a four-day period to compare the effectiveness of the internal and external sparger in these key performance areas:
- Overall PGM recovery
- Overall PGM upgrade ratio (head grade to final concentrate grade)
- PGM/Cr2O3 ratio in concentrate
To ensure consistency, all comparative tests were done three times. Results from the trials showed that the external sparger column (a static mixer situated on a slurry circulation stream at the base of the column) was more effective than the internal sparger:
- PGM recovery improved by 23%
- PGM upgrade ratio improved by 19%
- Cr2O3 reduction improved by 29%
Based on these results it was decided to install an industrial-sized flotation column measuring 1.42 metres in diameter and 9.5 metres high as a re-cleaner. During this next phase of optimisation trials lasting 31 days, normal concentrate was fed through the circuit under different conditions.
Three operating conditions were tested, namely air addition rate, pulp level and slurry feed rate, and 315 composite samples were taken at each setting within the pilot plant circuit to determine PGM and Cr2O3 content. The standard settings for the trials were a plant feed rate of 115 tons per hour (t/hr) at a constant feed grade (this represented an increase in current throughput from 105 t/hr); an air flow rate of 65 Normal metres³ per hour (Nm³/hr); pulp level of 700 millimetres (mm); and sparger pressure of 2.0 bar.
Equally pleasing results were recorded:
- PGM recovery improved by more than 2%
- PGM upgrade ratio improved by more than 9% (from 78 to 85 and 105 achieved on a number of occasions)
- Cr2O3 content in final concentrate reduced by 38%
After the installation of the flotation column PGM recoveries exceeded 81%. Before the installation of the external sparger column PGM recovery in excess of 81% could only be achieved with an upgrade ratio of 60. After the installation, the PGM upgrade ratio, at a PGM recovery of 81%, increased to 85.
In commercial metallurgical applications this innovation does not only provide the opportunity for additional revenue, but, by avoiding the penalties associated with Cr2O3 in concentrate, will result in additional savings.
Against the background of potential expansion into the Bushveld Complex's eastern limb, where UG2 reef is more prevalent than Merensky, this development is likely to hold significant future metallurgical benefits. Northam has since filed a patent on the external sparger column cell technology.
