FLSmidth recently installed two turnkey SuperCell flotation machines—the world’s largest flotation cells—at Rio Tinto’s Kennecott Utah Copper concentrator near Salt Lake City, Utah, USA. In the past, extensive testing has been required to fine-tune and validate the performance of flotation cells after installation. In this application, FLSmidth said it substantially reduced the amount of testing required by using statistical Design of Experiments (DoE) software to simultaneously test the effects of all factors, including multiple factor interactions. DoE was also used to determine the optimum values of cell parameters under a wide range of operating conditions.
“The first of two SuperCells was built, tested, and commissioned in 110 days from start to finish, much less time than has been required in the past,” said Dariusz Lelinski, product development manager for FLSmidth.
Before the cells were installed, negotiations between the mine owner and vendor stipulated that the mine owner only pays for the installation after it has been proven to meet a performance guarantee. Statistical analysis of the results showed that the first cell exceeded the agreed-upon performance guarantee: copper recovery of 85% with a copper concentration of 20% and molybdenum recovery of 85% over the full range of operation.
Both of the SuperCells provide significantly more than a 1% increase in recovery of both copper and molybdenum, which provides a substantial revenue increase to the mine owner.
The SuperCells were designed to accept any of three types of flotation mechanisms manufactured by FLSmidth. The first cell, with a volume of 300 m3 (10,600 ft3), was commissioned with a WEMCO self-aspirated mechanism with a top-mounted rotor and internal launders.
The second cell has the same internal size but the use of external launders provides a volume of 330 m3 (11,650 ft3) when fitted with a bevel tank bottom and 350 m3 (12,360 ft3) without the bevel bottom.
The second cell was installed and tested using two forced-air designs; the Dorr-Oliver cell with a conventional bottom rotor, and the newer XCELL mechanism which has a mid-rotor design. With the first cell in service, testing of the second cell progressed on a less-urgent timetable, taking roughly five months for water and slurry testing with the Dorr-Oliver setup and slightly longer for slurry testing with the XCELL design.
Three 1.5-m3 (53-ft3) pilot cells also were installed with each of the three types of mechanisms.
According to FLSmidth, an extensive two-step validation process is required with this type of equipment. First, the basic assumptions used to design the cell are validated by running the cells with water only and comparing the results with the computational fluid dynamics (CFD) simulations. Next, the cells are run with actual slurry and their performance—especially recovery and grade—is compared with vendor performance guarantees. This validation process has traditionally been long and expensive because each run ties up expensive equipment and requires a lengthy setup process. Many runs are needed to validate a wide range of potential operating conditions.
“We heard that DOE might have the potential to reduce the time and cost involved in cell validation while also providing the potential to improve cell performance,” Lelinski said. “We looked at several different statistical software packages with DoE capabilities but they seemed to be targeted more at statisticians than engineers. Design-Expert software from Stat-Ease, on the other hand, is designed for people like us—engineers that want to solve a problem but don’t want to get neck-deep in statistical methods.”
Prior to installation, operational information from more than 100 existing 257-m3 (9,000-ft3) SmartCell flotation units was used to develop CFD models of the newly designed cells. The CFD models in turn were used to optimize the design of the new SmartCells. After installation of the first cell, the cell hydrodynamics were extensively tested with water in order to validate the accuracy of the simulations. Hydrodynamic experiments typically require a 10-hour process to set up the cell for each run. The FLSmidth team used DoE to design an experiment that would provide the desired test coverage with the least possible number of runs. They explained to Stat-Ease technical support that the time required to perform the experiment could be substantially reduced by “blocking” the experiments to reduce the amount of changeover required between runs.
Stat-Ease’s technical support team helped design an experiment that reduced the validation time from three months on similar previous projects to only three days on this project. Three cell factors—submergence, engagement and rotor speed—were evaluated over a very wide range to ensure a statistically significant influence on the responses of cell power, aeration rate and pulp circulation. Pulp recirculation was calculated by measuring linear velocity in the draft tube as shown in Figure 2.
The measured results matched the CFD predictions within +/-15%, providing confidence in achieving the anticipated metallurgical performance. Besides validating the CFD results, DoE also revealed interactions between variables that had been hidden in previous validation campaigns. “We were made aware of an interaction between the rpm and the flow rate of the cell that enabled us to improve the performance of the cell,” Lelinski said.
After the hydrodynamic evaluation was completed, the first cell was commissioned on slurry and went into production. The FLSmidth team designed a two-factor central composite experiment with seven responses to evaluate the performance of the cell as measured by copper and molybdenum recovery and grade over a wide range of operating parameters; they were also looking for guidance in setting the two controllable cell parameters to maximize cell performance. Testing was performed on weekends over a compressed 24-hour-per-day schedule to reduce the likelihood of “noise” from ore-blend or process changes upstream from the cell. Statistical analysis with Design-Expert showed that all campaign results were statistically significant.
The factors were froth height and rotor speed. The measured responses included feed assay, concentrate assay, tailings assay, aeration rate, absorbed power, feed rate and solids content.
The FLSmidth team performed the experiments on both the pilot cells and the full-size cells, as shown in Figure 3, so the scale-up factors can easily be determined for any set of operating conditions. In the future, this will make it possible for the mine owners to optimize performance under any operating conditions by using inexpensive and quick experiments in the pilot cells.
An important benefit of DoE is calibrating the effect of operating conditions in the pilot cell vs. the SuperCell, as shown in Figure 4. When it is necessary to produce a certain grade, the mine owner can now easily determine the operating conditions that will produce that grade with a high degree of accuracy in order to maximize the recovery, which in turn generates the highest revenues.
As shown in Figure 5, results indicate that copper and molybdenum recovery is influenced by absorbed power. Recovery of copper increased from 86% to 88.7% and molybdenum from 82.9% to 86% with a power input increase from 0.74 to 0.84 kW/m3.
“DoE played a major role in the success of this project,” Lelinski said. “It reduced the time required to prove the cells meet the performance guarantee. This substantially reduced validation costs. DoE also contributed to the improved performance of these cells.”
FLSmidth—e-mail: [email protected], Web: www.flsmidth.com/;
Stat-Ease—e-mail: [email protected], Web: www.statease.com.