A synthetic pore water solution, based on HD-PERM-1 pore water composition,8 was evaporated about 3400 times at 95°C in a series of experiments to iteratively concentrate the solution in manageable quantities of approximately 2 l and to monitor precipitation. The chemical composition of each successive leg was based on the brine composition toward the end of the previous leg (Table I). Experiments FEC 9, 12, 13, and 14 are referred to as legs 1, 2, 3, and 4 throughout this paper. The solutions were prepared at room temperature using analytical grade salts. Differences between the ending and starting compositions for each leg reflect the difficulty in exactly synthesizing the solutions and changes that occur when the solution is prepared at 25°C and then heated to the experimental temperature of 95°C. At the beginning of each new leg, the prepared solution did not include the undissolved solids that were present at the end of the previous leg. During the preparation of the starting solution for legs 3 and 4, an amorphous magnesium silicate precipitate formed in the 25°C solutions. This precipitate was not removed from the starting solutions and only partially dissolved when the solution was heated up to the experimental run temperature (95°C).

Evaporation was conducted in a vented, halar-lined vessel heated to 95°C in a fluidized sand bath furnace which provided optimal heat transfer for this method. The solution was stirred constantly and HEPA (high efficiency particulate air) filtered air was streamed over the solution to help control the evaporation rate. The solution vapor was refluxed to prevent evaporative water loss as it was heated to run temperature. Once the solution was at 95°C, the evolving water vapor was condensed into a separate container to monitor the extent of evaporation. Samples of the evaporating solution were periodically extracted and filtered at 95°C and analyzed to determine the water chemistry. The 95°C filtered samples withdrawn for cation and anion analysis were immediately diluted by directly injecting the sample into a known quantity of room temperature de-ionized water to prevent precipitation on cooling. Undiluted samples withdrawn for total dissolved inorganic carbon analysis, ∑CO₂(aq), were immediately stored by filling gas tight vials to prevent equilibration with air at room temperature. Separate undiluted samples for solution pH were stored in a closed container and pH was measured as soon as they cooled to room temperature. In the last two samples of leg 4, precipitates formed in the pH and carbon samples as they cooled. Samples of the condensed water vapor were also periodically extracted and analyzed to monitor gas volatility. After the last sample was taken for each leg, the evaporation was continued to dryness. The solid precipitate was collected at the end of each leg of the experiment, dried in an oven at 40°C to facilitate sample preparation, and analyzed by powder x-ray diffraction (XRD).

Sample pH was measured at room temperature with a combination electrode, which is reliable in solutions with an ionic strength less than 0.1 molal.11 The sample cooled to room temperature and pH was measured within a half hour of sampling. The pH measurements for legs 3 and 4, where the ionic strength of the solution was greater than 0.1 molal, represent uncorrected values and have not been corrected for ionic strength. Total dissolved carbon, ∑CO₂(aq), was measured with an infrared carbon analyzer and had a detection limit of 1 ppm. Dissolved calcium, magnesium, silica, and sodium were measured with an inductively coupled plasma-atomic emission spectrometer, dissolved potassium was measured using an atomic absorption spectrophotometer, and fluoride, chloride, nitrate, and sulfate anions were determined using ion chromatography. Reproducibility of these techniques is typically better than ±2%. The mineralogical composition was determined by powdered XRD using a Cu K_α source from 10° to 90° 2θ at 0.02° per step. The XRD instrument was calibrated using NIST traceable silicon (# 640 c) and mica (# 675) standards for high angle and low angle peaks, respectively. XRD cannot detect amorphous solids or minerals that are present at <2 wt %. Mineral identification was based on the presence of the three most intense peaks in the XRD pattern for a given mineral. In some cases where the most intense peaks overlapped with other mineral peaks, identification was based on the presence of lower intensity diagnostic peaks.

Solution compositions were modeled using the EQ3/6 geochemical code, and a high temperature Pitzer ion-interaction model that is further described in Table II.1121314 The high temperature Pitzer ion-interaction model approximates nonideal behavior of solutions at elevated ionic strength and temperature. The predictive models were generated to mirror the experimental design and analysis, in which synthetic pore water was evaporated over a discrete range for each leg, with a cumulative evaporation up to 3500× for the overall experiment. The concentration factor, CF, can then be defined as