Mineral Replacement Reactions Enabled through Electrokinetic Fluid Flow within Pores

Open Access
Author:
Mays, Benjamin E
Area of Honors:
Chemical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Darrell Velegol, Thesis Supervisor
  • Themis Matsoukas, Honors Advisor
Keywords:
  • mineral
  • replacement
  • electrokinetics
  • diffusiophoresis
  • pseudomorphic
  • potassium
  • bromide
  • chloride
  • ion exchange
  • quantum dots
Abstract:
Pseudomorphic mineral replacements involve a change in the composition or structure of a mineral without a change in the shape and size, as well as pore formation in the product phase. Here we explore pseudomorphic mineral replacements to determine the reaction process, focusing on how fluid flows affect the kinetics of the process. Though the thermodynamics of the phenomenon is well understood, the kinetics of this process have yet to be adequately represented. This work challenges the widely accepted diffusion-limited, coupled dissolution-precipitation reaction process, and it explores the question of whether convective flows emerging from concentration gradients are driving the pseudomorphic mineral replacement process. The question was approached experimentally using confocal and transmission microscopy. Quantum dots were intentionally trapped within a potassium bromide (KBr) crystal and extracted using saturated potassium chloride (KCl) via mineral replacement. The results showed that the 20 nm blue quantum dots were extracted within time scales significantly shorter than predicted by diffusion. We attempted to extract 4 μm particles, however no extraction occurred at first. SEM imaging showed that over time, the pores sealed. A flow-through system was then created, and it was found that replacement rates and pore cross-sectional area both increased as external flow rates increased. The data thus indicated that dissolution and precipitation occur at separate fronts, in order to set up a concentration gradient which drives electrokinetic flows in and out of the mineral pores, through diffusioosmosis. Understanding that these concentration-driven flows play a large role in the replacement process, we were able to determine flow rates at which optimum mass transfer occurs and extract 4 micron particles trapped within a KBr mineral. Understanding this reaction process further will enable a more holistic understanding of more geologically relevant systems and allow for more efficient extraction of oil and gas from rocks and minerals. This system served as an effective nexus between chemical engineering fundamental principles and geological settings.