Abstract
Experiments were conducted in sand-pack, flow-through reactors to study labradorite solubility and dissolution kinetics at 50' and IOO'C and 13.8 MPa. Labradorite grains were reacted with distilled deionized water, HCI solutions (pHi=3.5, 4.7), pH-buffered solutions of 0.07m acetate(pHi=3.5, 4.7, 5.4), 0.07m acetate0.005m oxalate (pHi=4.4), and 0.07m acetate-0.005m citrate (pHi=4.4). Pore fluid flow was essentially constant (25-30 ml/hr) for rate studies and semi-static and slow (0.03-0.3 ml/hr) for solubility determinations. Under mildly acidic condition at I OO'C, all the organic acids used in this study significantly increased both solubility and dissolution rate of labradorite compared to distilled deionized water and HCI in the order citrate > oxalate > acetate. However, at 50'C, the order of the dissolution rate changes to oxalate > citrate > acetate. Al was below detection limit in the studies with 0.07m acetate solution. On the other hand, both oxalate and citrate in pH-buffered acetate solution can result in significant Al (up to 4 mmolal) in solution. Calculated activation energies of dissolution was different for different fluids and ranged between 18 KJ/mol for the acetate-oxalate solution to 42 KJ/mol for the acetate-citrate solution. Although, these values are lower than that predicted by surface reaction models, considering the experimental conditions, reacting solid, and reaction mechanisms, they are quite reasonable. Labradorite dissolution is weakly dependent on pH in acetate buffer solution over the pH range of 3.5 to 5.5. In the neutral region, the rates in the acetate buffer, DDW, and HCI are essentially identical. Scanning electron microscope observations revealed secondary precipitates, which is the probable cause of removal of oxalate from solution and also a possible reason for incongruent dissolution observed in this investigation. SEM images also show extensive leach-pits on reacted labradorite surfaces indicating surface controlled processes. The reaction history has little effect on measured dissolution rates. The reproducibility between studies are very good.
Ahmed, MD. Raquib Uddin (1995). Labradorite dissolution in aqueous organic acid solutions: an experimental study. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1995 -THESIS -A367.