Global Benchmarking of Lithium Grade in Geothermal Brine and the Feasibility of Recovery from the Menengai Field in Nakuru County Kenya

The limits used for assessing lithium recovery from geothermal brine are those widely cited as 50 mg/L, but not always tested against the systems that have been developed. In this study, the Menengai geothermal field was compared against thirty two geothermal and brine fields around the world and the following questions were asked: What are the physical properties of the brines that make them developed or undeveloped? Twenty-nine samples of brine from the Menengai area were analysed by flame atomic absorption spectrophotometry and comparison systems were taken from published geochemical surveys. The capacity of each property to discriminate developed systems was quantified by the area under the ROC curve with stratified bootstrap confidence intervals, and position was expressed in terms of percentile rank, after regressing the data for each property on total dissolved solids to remove the confounding effect of bulk salinity. Principal component analysis and Ward clustering were used to analyze the behavior of multivariate structure. Menengai was the 28th of 33 systems with a grade of Lithium, and ranked at the 18th percentile. There is a significant global correlation between salinity and lithium (slope = 1.019, 95% CI = 0.734 to 1.303; R² = 0.632), and Menengai had 0.32 times the lithium expected for its salinity, ranking it one of the most depleted in this regard. When discriminated developed systems were considered, grade discriminated almost perfectly (AUC 0.951, 95% CI 0.852 to 1.000) with a Youden-optimal threshold set at 150mg/L (95% CI 150 to 195), a threefold increase from the nominal and eighty-eightfold increase from the Menengai mean. The ratio of magnesium and lithium did not discriminate between developed and non-developed systems, but five out of six developed systems had higher ratios than Menengai. The purity of geothermal brines is not the factor that determines what is developed, it is the grade, and at the present time, the brines at Menengai aren't sufficiently pure to allow recovery of lithium.

benchmarking; bootstrap; economic feasibility

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