With the impact of a recession taking its toll, processing plant operators have become very conscious about containing costs and increasing efficiency in mineral recovery. But are their air-only sparging systems helping their cause in column floatation cells? African Mining Brief explores.
For industrial applications that need efficient recovery of fine particles and high grade concentrate from material, air-only column floatation technology is preferred over conventional mechanical floatation, mainly in greenfield projects. In particular, in column flotation mineral separation, air-only spargers are selected over air-water spargers, mainly due to the following benefits: Low maintenance and Lower capex (no recycling pump required).
Using air-sparging devices, bubbles are produced in the cell to carry the minerals to the surface for recovery of hydrophobic particles or processing. The bubble size and distribution determines the level of the column’s performance – bigger and evenly distributed bubbles are preferred.
Expectation vs. reality
In theory, air-only sparging sytems are supposed to work like a charm, flawlessly enabling mines to extract more value from mineral separation processes. However, in reality, as most mining managers have recounted, air-spargers can fail to deliver when needed the most.
Air-only sparger failure results in poor recovery of minerals and production of low mineral concentrate. To a mine, this can translate into hundreds of millions of dollars in loss of potential revenue, which could impact on its bottom-line.
Failure not uncommon
However, air-only sparger system failure is not unusual, emphasises Thierry Monredon, Product Manager of Flotation Engineering at Metso Canada/France. He has noticed that, more often than not, poor recovery and low concentrate production are down to poor bubble production – bubble size and distribution.
Corroborating this view, commentator Vivian Ramirez Coterio says, poor bubble production is a result of the limitation of air-only sparging device that are used, says. In his research, Optimisation of air-injection spargers for column floatation applications, he notes that, a malfunctioning air-sparger system is symptomatic of two factors – improper design of the sparging system and poor operation practices plant operators employ.
Poor or lack of optimisation
With reference to the design of the air-spargers, Coterio comments: “Few aerators (air-only sparger) systems offer the possibility to monitor, control, and even less, predict the bubble size generated within the column. This unfavorable situation can create a number of issues within the column operation, including poorer concentrate grade and lower recovery, among others.” The inefficiency, he adds, could be due to poor optimisation, specifically design, of the respective sparging system.
Evidently, despite their popularity, especially comparatively lower maintenance levels over air-water spargers), the shortfalls of air-only spargers, in terms of correct bubble size and bubble size distribution, have been adequately documented.
The game is changing. Thanks to milestones in Research & Development (R&D), innovations in air-spargers have improved the performance of column flotation products.
There are notable developments in air-sparger technology, amongst others – the air-slurry and air-spargers with high rate of bubble area.
Air-slurry spargers have emerged as alternative to “conventional” air-only spargers because they can produce the right bubble size and bubble size distribution. There is a minimum of 15% increase in column stage recovery when an air-only sparger system is replaced by an air-slurry sparger such as Metso’s Microcel. In a Microcel sparger, the use of a VFD for the sparger recycling pump (Microcel sparger) allows for the manipulation of the bubble size and changes it as process parameters change.
High rate of bubble surface area
Through the generation of a fine buble that is evenly distributed, Eriez’s EFD SlamJet™ sparging systems promote the attachment and recovery of hydrophobic particles. FD sparging systems are designed to generate high rates of bubble surface area which guarantees a high probability of attachment and improved recoveries. Further, designed for and used with EFD flotation systems, they are also easily retro-fitted to improve the performance of other flotation columns.
Client preferences are changing, says Monredon. He has noticed that customers are now more aware about the long-term benefits of products beyond initial capex (capital expenditure). “From our experience, production people are realising the limitation of conventional air-only spargers after commissioning and switching to air-slurry spargers. The game is changing. Despite their higher Capex, air-slurry spargers are more and more specified during the engineering phase of greenfield projects due to their ability to improve metallurgical performance of the column cells.”
Retrofitting a sparger
Retrofitting a sparger to improve performance is regarded as a viable alternative to costly replacement. But what if it does not always work?
It is always advisable that a sparger retrofit must always be done with a different sparger technology, says Monredon. “Many clients have been disappointed when charging from one air-only sparger supplier to another air-only sparger supplier expecting a metallurgical improvement. They must understand doing such a change will lead to marginal improvement of the column metallurgical performance since the principles of bubble generation remain the same and average bubble size produced will not change.”
In a nutshell, three things are important and must be taken into account prior to any air-slurry sparger retrofit:
- ensure there is enough space to install the sparger recycling pump and there is no interference such as piping near the column wall to install the slurry manifold
- check what is the actual air flow used in the flotation columns. Many installed airflow meters are lacking calibration and/or maintenance and indicate wrong airflow. This is not an issue for airflow control since only the relative value is considered. However, airflow will affect the sparger sizing and therefore, the real value must be considered for the sparger design.
- check what are the expectations from production personnel in terms of recovery or grade improvement and assess if these are achievable.