THE ROLE OF THE DISTURBED LAYER IN SILICA DISSOLUTION AND GROWTH
The economic recovery of precursor-type, precious element-bearing a.c.s. (amorphous colloidal silica) depends on dissolving the subject silica, subsequently nucleating the contained precious element clusters to a bulk metal size, and finally recovering the newly formed precious metal as a product. The ever present existence of what is referred to as the external “disturbed layer,” largely controls both the rate and amount of silica that dissolves from, and resorbs back to, potentially any silica particles remaining in an aqueous system; ex. ground silica/silicates inany leach solution. According to Rieck, et al (1), and Faimon (2) the “disturbed layer” in ambient conditions is in the approximate order of 13.3% of the silica mass for finely ground silica of approximately 3 microns in size. The control of silica dissolution and growth by the disturbed layer is a poorly recognized phenomenon which, to date, has apparently not been previously investigated outside of the laboratory and, most certainly, not in the mineral industry. The good news is; even though the “disturbed layer” is always present in a production situation, the effect of the disturbed layer can besubstantially controlled using appropriate system modifications!
In acidic, neutral, or basic aqueous solutions that are under static (ambient) conditions, the highly soluble silica disturbed layer dissolves relatively quickly and saturates the leach solution with silica. As silica saturation/supersaturation is achieved, the dissolved silica phase begins to redeposit back on the partially depleted disturbed layer. In this sense, the disturbed layer may be considered as a rather exotic exchange layer. At some point, the concentration of dissolved silica from the disturbed layer becomes increasingly stable, with a certain amount being continually redeposited on the remaining undissolved, silica particle disturbed layers. This dissolution/redeposition process continues, seeking some semblance of equilibrium. The net result, however, is that under same-system conditions, only the formed disturbed layer is affected; the majority of the silica particle remains essentially buffered from the leach solution. A portion of the disturbed layer remains in solution, and the remaining portion is resorbed onto the disturbed layer. In effect, the disturbed layer controls the overall dissolution dynamics of the silica-leach system.
Agitation, rather than static conditions as described in most of the research publications, is commonly used in silica leaching systems to effectively increase the rate of dissolution. However, the dynamics of the silica-disturbed layer are still present and effective as they are under static conditions, but take place over a shorter period of time and consequently often become more evident, troublesome, and contribute to a poor product recovery. Consider the potentially unexpected problems that might be encountered when operating a heap leaching system where the ore mineral is a silicate or perhaps, even worse, encapsulated in silica!