Increasing recoveries with flotation upgrades

Recently, a large copper mine in Australia approached FLSmidth asking for a new flotation mechanism design that would improve copper recovery, reduce power consumption, improve the life of wear parts and reduce maintenance costs. FLSmidth was able to accomplish all of the requirements by converting the old Agitair™ flotation technology to new Dorr-Oliver® flotation mechanisms. The customer is now planning to upgrade the rest of the flotation circuit to benefit from this increase in overall copper recovery across all of the flotation machines.

The problem
The customer's metallurgical team was investigating options to improve the metallurgical performance of their flotation circuit. At the time, the majority of the flotation duty was being performed by Agitair machines—shallow (914 mm), low volume (1.7 m3) cells; 304 cells in total. The existing flotation machines had been installed in 1971 and were reaching the end of their serviceable life, so the mechanical stability of each bank was unknown, but certainly not in original condition.

Batch testing results had indicated that copper losses in the flotation circuit were mostly due to flotation kinetics and mechanical constraints rather than surface chemistry, and that recovery gains might be achieved with improved circuit and flotation machine design.

The motors, cabling and control hardware were operating at full capacity, so power could not be increased without cost-prohibitive expenditures on upgrades in addition to a scarcity of available power. So, any solution would require the limited power draw to be maintained or reduced.

A secondary objective for the customer was to reduce the cost of flotation consumable wear parts as part of ongoing initiatives to reduce operating costs. In addition to the outright purchase price of the consumables, the wear life would be critical. The frequency of having to replace the stator and rotor not only impacts the overall consumable expenditure, but the cost to remove and replace wear parts contributes heavily to that price.

Possible solutions
The client considered several options to overcome their present challenges in order to meet their goals. The option to build a new concentrator building did not meet internal CAPEX approval requirements nor did it complement the current life-of-mine or future expansion planning. The client considered putting entirely new flotation machines into the current concentrator building as well. While costing less than a new concentrator building, this option required too much capital expenditure for the time being. Refurbishing the existing flotation machines and their technology was considered, but this would possibly cost more than replacing the banks with new machines.

The most feasible option for the current plant conditions and budget was to upgrade only the mechanisms within the tanks. This would be the lowest cost option that would be quick and simple to test. It would also be relatively easy to convert the entire circuit after the initial tests, provided that the performance targets had been met with the conversion to the new mechanism.

Final solution
The Dorr-Oliver® flotation mechanism for forced-air flotation machines provides:

  • High-efficiency rotor
  • Energy-intensive zone
  • Effective solids suspension
  • Multiple passes of unattached particles
  • Contact with the air bubbles
  • Maximum air dispersion
  • Minimum froth disturbance

Dorr-Oliver® flotation technology
The Dorr-Oliver flotation mechanism forced-air flotation machine has been specifically designed to maximize fine particle recovery. The Dorr-Oliver streamlined, high-efficiency rotor is a very powerful pump that works together with the stator to generate and define an energy-intensive zone in the bottom of the cell. This ensures effective solids suspension and the ability to restart in even the most difficult of applications. The well-defined turbulence zone at the bottom of the cell results in unattached particles through the highest energy dissipation area of the cell where fine particles are driven into contact with the air bubbles. The stator design, in addition to providing good separation of the zones in the cell, also serves to redirect the rotor jet uniformly across the tank to allow the maximum amount of air to be dispersed into the cell without disturbing the surface. Maximizing the air dispersion capability is an important consideration for fine particle recovery.

The plant trial
It was decided to proceed with a staged plant trial. The trial consisted of the installation of two standard Dorr-Oliver 14RS mechanisms to conduct reliability testing. This was followed by the conversion of a full bank (16 mechanisms) to the Dorr-Oliver mechanisms. Experimental design and block surveys were done before and after the mechanisms conversions. Conditions were monitored closely and controlled carefully to detect and minimize any variables. The metallurgical performance was then tested to determine the impact of the mechanism design on a single bank and on the overall plant recovery rates.

The Results
Grade and recovery
The grade and recovery increased significantly in the Dorr-Oliver bank of cells. On the basis of size-by-recovery, recovery rates increased across all size fractions, particularly the coarse range.

Maintenance – rotor/stator life
The old Agitair parts were lasting, on average, 12 months. Dorr-Oliver rotors that were installed for testing were still in use after exceeding 18 months of service life. Although this satisfied the testing criteria, FLSmidth is continuing development work with alternate engineering materials to further extend wear life to reduce maintenance costs.

Power draw
When the bank was started up, the air feed rates were reduced to lower mass pull in order to prevent the concentrate launders from overflowing. This in turn increased the pulp density and therefore increased motor power draw. The resultant power draw observed across the eight motors was either maintained or marginally increased. It was possible to further optimize the bank to achieve reduced power draw, but the customer was satisfied with the overall increase in recovery instead of pursuing power draw reductions at the cost of recovery.

The customer was pleased with the results on the test bank of cells and now plans to implement the upgrade to Dorr-Oliver mechanisms throughout the rest of their flotation circuit.

FLSmidth's in-house flotation research teams include:

  • Seven dedicated flotation research scientists
  • Dawson Metallurgical Laboratories
  • Ore Characterization and Process Mineralogy Laboratory
  • Pilot plant facility
  • Field testing team
  • Research alliance with Virginia Tech, USA

Flotation R&D
Although the Dorr-Oliver mechanism design met and exceeded this customer's performance expectations, FLSmidth continues to evaluate the performance
of "new generation" FLSmidth rotor/stator designs.

FLSmidth has invested over USD $3M in the last three years in the area of forced-air mechanical cell design improvements.

The aim of the research into flotation machine design improvements was to extend the range of coarse and fine particle recovery. Approximately 30 stator and rotor designs, including our competitors' designs, were tested hydraulically in FLSmidth's Salt Lake City, USA laboratory. The best designs were further tested for metallurgical performance at bench and pilot scale.

The result of these R&D efforts is a new generation of flotation mechanism designs with the following benefits:

  • Increased fine and coarse particle recovery
  • Reduced power draw
  • Longer wear life of stators and rotors resulting in reduced maintenance time and cost
  • Ability to upgrade any mechanical flotation machine

CONTACT: Richard Jenner

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