Chemical behaviour of geothermal silica after precipitation from geothermal fluids with inorganic flocculating agents at the Hawaii Geothermal Project Well-A (HGP-A) Page: 5 of 85
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CHEMICAL BEHAVIOUR OF GEOTHERMAL SILICA AFTER PRECIPITATION
FROM GEOTHERMAL FLUIDS WITH INORGANIC FLOCCULATING AGENTS
AT THE HAWAII GEOTHERMAL PROJECT WELL-A (HGP-A)
INTRODUCTION
The report presented herein summarizes the results of experiments carried
out during the period July 1985-June 1986 at the Hawaii Geothermal Project
Well-A (HGP-A). The experiments performed are a continuation of previous work
dealing with the problem of removal of waste-silica from spent fluids at the
experimental power generating facility'in the Puna District of the island of
Hawaii.
Geothermal discharges from HGP-A represent a mixture of meteoric and sea-
waters which has reacted at depth with basalts from the Kilauea East Rift Zone
under high pressure and temperature (Thomas, 1982). The well which was com-
pleted in 1976 was tested intermittently between 1976 and early 1981, and has
been under commercial production since March 1982. The geothermal fluid from
the well is composed of approximately 43% steam and 57% liquid. The unflashed
brine (at the well-head) is nearly 0.3 M in NaCl, and is highly enriched over
its seawater componemt in Ca, K, Li, and SiO2(aq) (De Carlo and Thomas 1985).
The bottom hole temperature of 350 degrees Celsius leads to basalt/water
interactions resulting in a dissolved silica concentration between 800 and
900 mg/L governed not by the extent of seawater intrusion into the system but
by the amorphous solubility of silica at the equilibration temperature
(Fournier and Rowe, 1966; Iler, 1979). Analysis of the brine has shown that
although the concentration of metal ions contributed by the seawater component
of the geothermal fluid has increased (indicating a higher seawater influx to
the geothermal system, the silica content has remained nearly constant since
the beginning of production at the well thus confirming both the origin and
controlling mechanism mentioned above.
After separation of the steam phase of the geothermal fluid from the
liquid phase and a final flashing stage to 100 degrees Celsius and atmospheric
pressure, the concentration of the silica increases to approximately 1100
mg/L. This concentration represents five to six times the solubility of
amorphous silica in this temperature range, and the silica remains in a meta-
stable state (colloidal suspension ?) which precipitates at a very slow rate.
The high silica content results in a slow deposition of significant amounts of
scale both in pipes and conduits, as well as in the final settling ponds thus
contributing to a long-term clogging problem. Although alternative brine
disposal methods such as reinjection of the fluids at depth under pressure and
secondary use of the fluids in heat exchangers have been considered at HGP-A
and will most likely be necessary should the power producing facilities be
expanded, these techniques are presently not feasible due to the problems
associated with silica scale.
We have evaluated and successfully developed bench scale techniques
utilising adsorptive bubble flotation for the removal of colloidal silica from
the spent brine discharge in the temperature range of 60-90 degrees C. The
methods employed resulted in recovery of up to 90% of the silica present above
its amorphous solubility in the experimental temperature range studied. The
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De Carlo, E.H. Chemical behaviour of geothermal silica after precipitation from geothermal fluids with inorganic flocculating agents at the Hawaii Geothermal Project Well-A (HGP-A), report, January 1, 1987; United States. (https://digital.library.unt.edu/ark:/67531/metadc1196794/m1/5/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.