Process Control Considerations for Online Slurry Analysis in Metallurgical Plants FAQs

A: The following process control elements should be considered in selecting the most suitable configuration:

• Elements to be analyzed. For example, analyzers utilizing the Prompt Gamma Neutron Activation Analysis (PGNAA) technique have a distinct advantage over analyzers using X-ray Fluorescence by being able to measure light elements — those lower than calcium on the periodic table.
• Frequency of analysis (assay update time) as dictated by the process control strategy
• Residence time of upstream process
• Importance of stream in overall control strategy
• Confidence required in assay-based control decisions.

A:  There are three physical plant layout considerations when selecting an online slurry analysis system:

1. Length of small-bore sample transport piping from primary samplers to a centralized analyzer unit
2. Number of pumps required for sample transport, both supply and return
3. Whether or not gravity can be used for transportation of stream in order to eliminate the needs for sample pumps, thereby improving sample availability and reducing OPEX

When using the multi-stream analyzers one tries to minimize the length of the small-bore sample transport piping and hoses from the primary samplers to the analyzer. If one stream is a long way from others in the plant, from an on-going maintenance point of view, it would be prudent to use a dedicated analyzer for that stream and transport critical process data digitally rather than transport small volume slurry samples over long distances as this is inherently a low availability, high OPEX solution

To further minimize on-going maintenance, multi-stream analyzer unit(s) should be situated to minimize the number of pumps required to transport sampled streams. Ideally, the analyzer(s) should be located so that the sub-streams flow under gravity, or the pressure of the main line from which they were taken, to the analyzer and then gravity feedback to a convenient point in the process.

If there are large distances in the plant between the streams which are to be measured, it may be prudent to locate centralized analyzers near a cluster of sample points and use several analyzers, instead of a single centralized highly-mulitplex analyzer requiring double pumped sampling systems. While the CAPEX may be higher, a study of OPEX, availability, and utilization will show that this is more than offset by a substantial reduction in on-going maintenance costs. As the downtime is minimized, greater availability is realized resulting in greater utilization and realizing full ROI of the investment.

A:  When selecting the right online analysis system, there are several capital constraints that should be considered, including

• Capital investment,
• Economic benefits expected
• Cost of on-going maintenance
• Flexibility of system modules
• Incremental benefits of less essential assays.

The economic benefits of having an on-line analysis system coupled to a control system, be it manual or fully automatic, comes from one or more of the following: increase in metal recovery, improvement in concentrate grade and control of impurities, reduction in reagent consumption, decrease in operating costs, and improvement in stability of the operation.

These benefits have to be weighed against the capital investment of the analysis system and the cost of on-going maintenance including mechanical repairs, sample pump servicing or replacement, sample transport system troubleshooting, electronic repairs, analyzer downtime and calibration. The flexibility of hybrid systems of dedicated analyzers for the critical streams and multi-stream analyzers for the less critical streams enables the most cost effective analysis system to be selected for each particular plant. In working out what streams are to be measured a cost benefit analysis should be carried out to guide the decision making process.

A:  Before selecting an online analysis system, consider if light elements will need to be measured and if the measurement technique is amenable, given the expected variation in mineralogical and particle size for the process. In addition, look at the streams to be measured and ask these questions:

• What is critical for control of process (usually includes Feed, Final Tail and Concentrate)?
• Is there a need for understanding trends within the process (usually includes Rougher Concentrate and Cleaner Tails)?
• What are the elements to be measured in each stream?

Based on this information, the various trade-offs taking into account all factors between centralized and dedicated analyzers, Prompt Gamma Neutron Activation Analysis (PGNAA) and X-ray Fluorescence (XRF), capital and maintenance cost etc., can be worked out and a recommendation made for the optimum  system configuration for the particular plant

For example, in a nickel concentrator, it is essential to control the concentration of talc (or MgO) in the concentrate stream. To be able to control the concentration of talc in the concentrate requires measurement of Ni and talc in each of the feed, rougher concentrate and final concentrate streams so that the appropriate concentration gradients between these can be optimized and the ratio of Ni/talc can be maximized at each stage for minimum reagent usage. It may also be useful to measure Fe and S in the feed stream because this may give an indication of the nickel mineralogy entering the plant. In all other streams, it is only necessary to measure Ni because the information from these streams is used only for monitoring the recovery of Ni. Thus, PGNAAA would be required with multiplexing for the three main streams, and possibly a multi-stream analyzer (using XRF technology) for the other streams).

A:  The elements to be analyzed are determined by the objectives of the process control strategy and the particular metallurgical problems which are anticipated from prior metallurgical test work. The frequency of analysis required, often referred to as the assay update time, depends on the following criteria:

• The fluctuation in assays in a given process stream considering the residence times of the processes immediately upstream
• The stability of the circuit
• As a minimum the assay update times of the analyzers for the critical streams should be less than half of the retention time of the preceding process stage.

Therefore, in the tailings stream from a scavenger bank of cells with a retention time of 5 minutes, the grade can be expected to vary considerably in 2 minutes during upset conditions or reactions to process control actions so on-line analysis should be made at an interval less than this to provide the best visibility of real-time plant performance. To obtain these sorts of assay update times, requires dedicated analyzers or a centralized analyzer with just a few streams located nearby the process sample points.

A:  If the critical streams are monitored frequently according to the above criteria, the operators should be able to control the plant to give overall stability and best metallurgical results at minimum cost. The less critical intermediate streams can then be monitored at a lower frequency for the fine tuning of the circuit.

The degree of confidence required in the assay-based control decisions must be known. Streams that are more critical for control of the plant need to be monitored more frequently. Trends in plant performance will then be shown in more detail, showing effect of control actions on grade in real-time and giving greater confidence in control decisions. For example, in a base metal concentrator, the main objective might be to minimize metal losses in primary floatation while producing a particular concentrate grade in the cleaners. In addition, test work may show that recirculating loads tend to build up in the cleaning stages which is a result of recovery of excessive gangue in the rougher concentrate. Continuous analysis of tailings grades provides a critical tool in the operation of rougher flotation.  On-line analysis of concentrates provides a tool to manage grade-recovery in the cleaners and better control impurities to meet the smelter requirements. 

A:  To fully monitor the recirculating load, it is also necessary to monitor the rougher and scavenger concentrates and cleaner tailing streams. As recirculating loads tend to build up slowly with time, these analyses are not required on a minute-by-minute basis so these streams can be monitored with a lower cost-per-stream centralized analyzer.