Background
Solvent Extraction (SX) Mixer Organic/Aqueous (O/A) ratio estimator provides real time estimates of the O/A volume ratio in the mixing units. It uses values of various readings, such as mixer motor current drawn, loaded organic pump current drawn, pregnant leach solution flowrate - (PLS) flowrate -inline sensor, spent electrolyte flow rate - inline sensor from which it estimates the O/A ratios in the associated SX mixer stage.
The O/A ratio in a mixer has a significant effect on entrainment. This is particularly true for SX circuits where crud (entrained impurities in the form of solids, settling aids (flocculant) and air) are prevalent. Most SX systems deal with one or more of the above crud forms and therefore, for the purpose of this training document, it can be considered that maintaining the correct mixer O/A ratios is an important control for all SX plants. Maintaining the optimum O/A ratio reduces impurity entrainments being transferred in the advancing organic phase from any individual mixer-settler. Operating any individual SX mixer in the correct mixer O/A ratio has a pronounced impact in the physical separation of the two mixed phases once they discharge from the mixer into the associated SX settler.
Intended users
The O/A ratio estimator was designed for the needs of the SX control room operator and shift metallurgist in mind.
Principles
Data used
The model takes the history of the following quantities
input aqueous flow rates (PLS and Spent Electrolyte Flows)
loaded organic pump performance variables
SX individual mixer stages motor performance variables
Data quality requirements
There must not be any data gap longer than two minutes in the history used
all pumps and motors considered must have been running continuously throughout the period used as a history
Should either of these conditions not hold, the estimator does not calculate the ratios. This is indicated by gaps in the plot of the estimated O/A ratios.
Outputs
Minutely point estimate of the O/A ratio in a mixing unit.
Confidence bands of the estimate. For instance, the ninety percent confidence band informs the user that ninety percent of the time the real O/A ratio is inside of the band. See Figure 1. for graphical explanation.
Figure 1. Measured O/A ratio (yellow line), O/A ratio estimate (blue line) upper and lower limits of the 90% confidence band (green and red lines, respectively. The measured O/A ratio is expected to be inside of confidence band 90% of the times.
Where to Find the Outputs?
The outputs of the detector is placed in the widget and dashboard to best suit the needs of the client. As such, the exact location may change site-to-site. As a default, the "SX Plant Dashboard" dashboard would contain the widget displaying the detector outputs.
Click on the "SX Plant Dashboard" and scroll to the bottom of the screen.
Limitations
model refusal: the estimator refuses to estimate the O/A ratio in a mixing unit if any of its linked motors or pumps was switched off in the preceding twenty minutes
model validity range: If the operating regime of any linked motors or pumps change significantly with respect to its normal operating regime, the chance of the estimates being inaccurate increase. The widening of the confidence bands of the estimates indicate that the operating regime may be shifting.
Figure 2. Measured O/A ratio (yellow line), O/A ratio estimate (blue line) upper and lower limits of the 90% confidence band (green and red lines, respectively. Compare the width of the confidence band on the left to that on the right. As the band widens, the estimated O/A ratio drifts below the measured values.