Porosity and Permeability of Round Top Mountain Rhyolite (Texas, USA) Favor Coarse Crush Size for Rare Earth Element Heap Leach

Abstract

Water-saturation porosity and dye-penetration permeability measurements of Round Top Mountain rhyolite confirm that a ½-inch (13-mm) crush size would permit efficient acid heap leaching of yttrium and heavy rare earth elements (YHREEs) hosted in yttrofluorite, a YHREE-substituted variety of fluorite. Laboratory acid leaching has extracted up to 90% of the YHREEs. The bulk insoluble gangue mineralogy of the rhyolite, 90% to 95% quartz and feldspars, assures low acid consumption. Different crush sizes were weighed, soaked in water, and reweighed over time to determine water-penetration estimated porosity. Typical porosities were 1% to 2% for gray and 3% to 8% for pink varieties of Round Top rhyolite. The same samples were re-tested after soaking in dilute sulfuric to simulate heap leaching effects. Post-leach porosity favorably increased 15% in pink and 50% in gray varieties, due to internal mineral dissolution. Next, drops of water-based writing ink were placed on rhyolite slabs up to~10 mm thick, and monitored over time for visual dye breakthrough to the lower side. Ink penetration through 0.5 to 2.5-mm-thick slabs was rapid, with breakthrough in minutes to a few hours. Pink rhyolite breakthrough was faster than gray. Thicker slabs, 4 to 10 mm, took hours to three days for breakthrough. Porosity and permeability of the Round Top rhyolite and acid solubility of the yttrofluorite host should permit liberation of YHREEs from the bulk rock by inexpensive heap leaching at a coarse and inexpensive nominal ½-inch (13-mm) crush size. The rate-limiting step in heap leach extraction would be diffusion of acid into, and back-diffusion of dissolution products out of, the crushed particles. The exceptional porosity and permeability that we document at Round Top suggest that there may be other crystalline rock deposits that economically can be exploited by a coarse-crush bulk heap leach approach.

ArcGIS<sup>TM</sup> and Principal Component Analysis of Probe Data to Micro-Map Minerals in Round Top Rare Earth Deposit

“…This porosity is evident in the mineral maps. Using ArcGIS™, the percentage of pore spaces was calculated to range from 1.4% to 4.0% in the 7 samples, consistent with earlier measurements of porosity by water saturation [5].…”

“…Previous studies have shown that Round Top Mountain rhyolites have 1% to 2% porosity in the gray varieties and 3% -8% porosity in the pink varieties [5].…”

Remarkably Consistent Rare Earth Element Grades at Round Top Yttrofluorite Deposit

“…Because the deposit is exposed at the surface there is little overburden to be excavated before surface mining could begin. Furthermore, the Round Top rhyolite has proved both porous and permeable, making high leach YHREE recoveries, up to 90%, possible without fine grinding [26]. It also is important to note that the YHREEs at Round Top present a compact, 3-dimensional mining opportunity.…”

Microprobe Mapping of Rare Earth Element Distribution in Round Top Yttrofluorite Deposit

“…After performing extensive laboratory tests, Texas Mineral Resources Corporation has proposed to extract the YHREEs via a heap leach with dilute H 2 SO 4 [27] [28] [29]. The YHREE host yttrofluorite was found to be soluble in dilute sulfuric acid, a relatively inexpensive and widely available industrial chemical.…”