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Jun 14, 2017

Flotation process audit and bottleneck identification

In flotation operations, the metallurgical team follows daily production and is prepared to act on any changes to the process in order to maintain the concentrate quality at the specified level and to maximize recovery. Common questions during daily operations are: “What are today’s recovery and grade?”, “Are the mills run with maximal throughput?”, and “Are we within budget?”. What is often forgotten is: “What could be the recovery?”, “Where are we losing valuable minerals?” and “Is the equipment operating at its best?”. To answer these questions, a process audit coupled with laboratory scale tests is required. When combined with process modeling, this procedure can be used to evaluate the possible benefits of plant modernizations.
Flotation cells

CARRYING OUT PROCESS AUDITS

In order to assess the metallurgical performance of a process in detail and to identify its bottlenecks, an in-depth process audit is required. The objective of the audit is to collect data from the process, generate mass balance, and analyze the performance of the process from the established mass balance. The data required for the mass balance includes throughputs, solids contents, as well as chemical compositions of the process streams. Part of the data can be collected from the automation system or the daily production reports, but most of the data needs to be based on a sampling campaign. 

The objective of sampling campaigns is to collect samples from the selected process streams giving solids and water balance for the assessed process or part thereof. Prior to commencement of a sampling campaign, it is necessary to define the scope and identify the limitations. The most important stage during planning is the identification of sampled streams, of the most suitable sampling locations and of the sampling methodology. 

The process audit gives a detailed analysis of the process performance at the time of auditing. When plant sampling is coupled with laboratory tests carried out using a fresh process sample collected from a dedicated location, the plant performance can be compared to the performance at the laboratory scale. The maximum possible recoveries achievable with the ore can be estimated from the kinetic laboratory scale tests. A kinetic flotation model is then fitted to the test data. The model solves mass portions of floatability classes – typically fast, slow and non-floating – and gives the associated rate constants for each mineral. 

The depth of the process audit can be increased through down-the-bank surveys. Instead of considering a bank of flotation cells as one black box, the feed, concentrate and tailings of each flotation cell can be sampled individually giving down-the-bank performance. This can be further used, for example, for froth surface area optimization based on the calculated froth carry rates or for air and level profiling down the bank of cells. 

Another layer of depth for the process audit can be developed by carrying out a gas dispersion characterization campaign together with sampling. This gives in-depth information on whether the flotation cells are generating an adequate bubble size range, which relates to adequate mixing. 

SAMPLING CAMPAIGN

The purpose of sampling campaigns is to gather in-depth information from the process that is not available from the automation system or daily sampling. Before the campaign, it is important to review the circuit flowsheet, assess data available from the control system and previous surveys, and review chemical addition points and dosage.

In the sampling campaign, the first step is to determine the sampled streams. The streams are to be selected based on the scope of the sampling campaign. The rule of thumb is that all streams required to establish the mass balance for the process or stages thereof must be sampled. This means collecting all inputs and outputs from each individual equipment or process stage. An example of a sampling program to establish the down-the-bank mass balance for a rougher flotation line is illustrated in Figure 1. Remes (2016a) presented the mass balancing procedure in Minerva 2/2016.

Flotation cells - sampling program
Figure 1: Down-the-bank sampling program for rougher flotation line.

Then, it is necessary to determine which elements are assayed from the collected samples. The elemental analyses should cover the main elements present in the ore, enabling satisfactory element-to-mineral conversions. In addition to the elemental assays, wet and dry weights from the sampled streams also need to be measured.

When the solids percentage measurements are combined with the specific gravities of the samples, which can be determined from their mineralogical compositions, the solid flow rates (t/h) can be transferred to slurry flow rates (m3/h). The residence times of the process stages and individual equipment can be determined based on the slurry flow rates. 

Finally, the best practices to sample the selected streams need to be determined. The best practice is always the safest. It depends largely on the location and access to the sampling point. The sampling equipment must be selected based on these. 

The following section presents some of the basic sampling equipment required for sampling campaigns. Note that the sampler type also depends on the capacity of the sampled circuit.

PULP SAMPLE FROM THE CELL

A dip sampler is required to sample tailings of an individual flotation cell. The sampler is used to collect slurry near the cell bottom (near the tails port). After the sampler is lowered to the desired position, the sampler lid is quickly opened by pulling a rope. The length of the sampler can be adjusted according to the desired sampling depth. Figure 2 shows an Outotec dip sampler designed to sample slurry inside a flotation cell.

A pump can also be used to sample pulp from the cell. As moving between different sampling points requires more time and personnel resources, the pump sampler is more suitable when sampling only one location or when generating mixing profiles for flotation cells. Figure 3 shows an example of a pump sampler.

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CONCENTRATE SAMPLE FROM THE CELL

Flotation cell concentrate can be sampled directly from the cell launder with a lip sampler. The lip sample can be timed and, based on the time, it is possible to estimate the sample dry weight, the lip length, the length of the sampler and the mass pull of the cell. 

When collecting lip samples, it is important to clean the launder prior to sampling, move the sampler in steady motion along the lip and not overfill the sampler. Figure 4 shows an example of a lip sampler.

SLURRY SAMPLE FROM AN OPEN PIPE

Cutters are suitable for sampling downward-flowing slurries from open pipelines. When cutting a slurry stream, it is important to collect the sample from the whole stream, keep the cutter in steady motion while cutting the stream and not overfill the sampler. Figure 5 shows an example of a slurry cutter. 

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FIXED PRIMARY SAMPLES WITH SECONDARY SAMPLER

Fixed samplers, such as Outotec Metallurgical Sampler Assembly (MSA), Launder Sampler Assembly (LSA) and Pressurized Sampler Assembly (PSA), are designed to collect representative samples from the process. When the collected samples are further divided with a secondary sampler, such as Courier® Multiplexer or Linear Moving Cutter (LMC) sampler, the sample is collected from the process as representatively as possible. Therefore, fixed samplers should be used in a sampling campaign whenever possible. 

SAMPLING METHODOLOGY

Sampling is a form of art with a heavily theoretical background (Gy, P. 1979; Johnson, B. 2010; and Napier-Munn, T. 2010). In order to collect representative samples, a very large sample size and many individual samples are often required. This is often too time-consuming and unpractical for process audits where the sampling campaign should be completed relatively quickly and with minimal changes to the process. Therefore, more practical methods developed from the theory are often used. 

Below are a few practical guidelines on how to carry out sampling campaigns:

  • At least four sampling rounds
  • Individual samples collected from each round
  • Each individual sample (collected each round) should have as many cuts as practically possible – the recommendation is a minimum of 20 cuts with cutters and 3 with lip and dip samplers
  • Take three repeatability samples from the same stream with the same sampling equipment to determine the sampling error of each device
  • The same person should  take the same samples each round to minimize the effect of human error on the results between sampling rounds
  • Record the commencement and end of the sampling campaign

LABORATORY FLOTATION TESTING

“Hot laboratory flotation” tests are a common method to assess the floatability of the slurry collected from an operating flotation circuit. The objective of hot flotation tests is to transfer process slurry properties to a laboratory scale flotation cell by collecting a test feed sample directly from a process stream. 

The main outcomes of hot flotation tests are the recoveries, grades and kinetics of the main minerals from a certain flotation stage. On the basis of the kinetics of the main minerals at different flotation stages, it is possible to simulate the feed mineralogy, flowsheet, flotation cell sizes and feed flowrates at each stage of the continuously operating industrial-scale circuit. The built simulator can be used as a tool to indicate how the performance of the process can be enhanced through modernization. Remes (2016b) described how a simulator can be built from test data in Minerva Issue 3/2016, and Mattsson et al. (2015) presented how simulators can be used to evaluate the benefits of plant modernization.

PLANT PERFORMANCE VS LABORATORY PERFORMANCE

The mass balance calculation gives the basis to assess the performance of the surveyed circuit. The laboratory test gives the recovery limits for the ore processed during the survey when the samples for the laboratory test are collected during the sampling campaign. By comparing these results, it is possible to evaluate whether the process is operating at full performance. The evaluation is carried out by means of process simulations. A more detailed approach on the simulation as a tool for flotation plant surveys was presented by Mattsson et al. (2015). The flotation plant simulator’s built up was briefly presented by Remes (2016-2). For mass balance calculation, floatation kinetics fitting and flotation process simulation, the Outotec HSC Chemistry® software package is used.

OUTCOMES OF PROCESS SURVEY

The process survey data is handled to form: 1) mass balance of the circuit, 2) flotation kinetic models of the laboratory tests and 3) process simulation model. The mass balance is the basis for performance assessment. also It is also used to obtain the volumetric flow rates and flotation cell residence times, and it may be accompanied by gas dispersion measurement results. The flotation kinetics tests are compared to the plant results to obtain the scale-up factor for the process. The simulation model is used for evaluating the benefits of best-performing, modernized plants and possible alternative processing configurations. 

Both the mass balance and the simulation model are applied for bottleneck identification. The following are examples of  typical flotation circuit performance bottlenecks:

  • Recirculation loads from the cleaner: The low performance flotation stages are to be identified.
  • Enrichment ratios not in the recommended ranges: These stages or individual cells are discovered.
  • Residence times: Are they sufficient to achieve the target recoveries with the current plant capacity and feed composition? Is there indication of cell short-circuiting?
  • Is the rougher distribution box delivering material evenly (flow rates and compositions)? An uneven rougher line by line operation can result in significant recovery losses.
  • Mass pull of the cells: Are they operating consistently in the down-the-bank profile?
  • Lip loads of the cells: They should not exceed the predefined limits.
  • Froth carry rates (t/h/m2): They should be within the recommended stage-specific ranges. If they are too high or too low, the rates will reduce the froth recovery. It must be at the optimal operating point.
  • Plant performance differs abnormally from the laboratory flotation: Scale-up factors and froth recovery characteristics are estimated and compared to the best predicted performance.
  • Poor recovery of fine or coarse fractions: The survey can be carried out by mineral basis to reveal particle size-based performance problems
  • Flotation selectivity: If problems are detected, a comparison to laboratory results and  a review of process operating conditions are necessary.
  • Mineral liberation-based grade-recovery problems: Separate mineralogical samples are collected and sent for mineral liberation analysis (MLA). This will give an in-depth particle composition study, addressing the needs for grinding and regrinding optimization during flotation.
  • To deepen the flotation assessment, cell froth recovery measurements, gas dispersion measurements (pulp phase bubble size distributions and superficial gas velocities) and/or reagent dosage analyses can also be included.

In addition to process sampling, the metallurgical assessment includes visual inspection of cell operation, especially froth stability and transportation characteristics. Furthermore, the process audit can be extended to include both process automation assessment and equipment mechanical assessment. Also, process audits can be expanded to cover assessments of grinding and/or dewatering sections.

SUMMARY

Process audits consist of process stream sampling, sample treatment, chemical analysis, data reconciliation and mass balancing. Based on the established solids and water balances, it is possible to evaluate the performance of the process and to identify its bottlenecks. 

When the established mass balance is compared to the measured flotation performance on the laboratory scale, it is possible to evaluate what the maximal recovery could be. The residence times on the plant and laboratory scale give valuable information on the required flotation volumes. Similarly, by comparing the results, it is possible to detect recovery issues related to the froth area or froth transportation.

REFERENCES

Gy, P. 1979. Sampling of particulate minerals – theory and practice. Developments in Geomathematics 4. Elseview Scientific Publishing Company.
Johnson, B. 2010. Flotation Plant Optimisation. A Metallurgical Guide to Identifying and Solving Problems in Flotation Plants. Chapter 2: “Existing Methods for Process Analysis”. Ed: Greet, C. The Australasian Institute of Mining and Metallurgy. Spectrum Series 16. < br /> Napier-Munn, T., 2010.Flotation Plant Optimisation. A Metallurgical Guide to Identifying and Solving Problems in Flotation Plants. Chapter 10: “Designing and Analysing Plant Trials”. Ed: Greet, C. The Australasian Institute of Mining and Metallurgy. Spectrum Series 16.
Mattsson, T., Remes A., and Tirkkonen M. 2015. Flotation Circuit Simulation as a Tool to Evaluate Benefits of Flotation Cell Modernization. SME conference.
Remes, A. 2016a. Minerva 2016-2. Mass Balancing of Concentrator Data.
Remes, A. 2016b. Minerva 2016-3.
Utilizing Plant Simulation to Design and Optimize Flotation Process.
 Outotec HSC Chemistry

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