RESOLVING THE ‘RED NUCLEATION SEED’ PROBLEM Experimenting with aqueous nucleation systems for the recovery of precursor-type precious elements presents an enigma that, at times, seems almost impossible to resolve. A sequential batch series of bench or pilot tests utilizing conventional chemistry and production mechanics typically, at first, will respond by recovering significant amounts of precious metals over and above any added like- precious metal that was used as seed. However, after a week or two of continuous batch operation, the net return of additional precious metal decreases until little or no precious metal gain can be measured. Consistently, however, when seeding and recovering for precursor-type Au and/or Pt the precious element seed fraction initially turns from a medium gray to a faint pinkish violet color. As a portion of the same seed is reused as seed for additional batch runs, it turns to a progressively darker violet to purple color. Even though the total weight of the seed/recovery fraction is progressively increasing, a batch-by-batch assaying of this seed material indicates that the total ore recovery of precious metal is progressively decreasing. With additional batch runs, the seed/product color of each successive run becomes less intense, light pink to violet, until the color of the seed fraction is merely a light stain. At this stage, nucleation of the precursor precious elements, as determined by conventional assaying, is generally near a standstill. In many instances a significant apparent loss or “disappearance” of the precious metal seed and/or gain is observed. In summary, at the beginning of a series of batch runs, a significant increase occurs in precious metal recovery from the precursor ore over that of the added seed. After additional batch runs, there is a progressive “loss” of Au and/or Pt values until little or no conventionally assayable Au and/or Pt seed is present.
After researching this situation, it became apparent that the problem resides mainly in the category of cluster chemistry rather than in the mechanics and chemistry of nucleation, both of which appeared to be working as predicted. Atomic clusters often occur as like-element-composition atoms arranged in specific “loose-net” geometric form; much of their surface area is exposed. In most instances, especially with naturally formed clusters, they are apparently bonded to a molecular substrate of some sort; silica, Si(OH)4, being a natural and common geochemical substrate. The cluster model being used here for the recovery of naturally occurring precursor-type precious elements essentially consists of small, less than bulk-metal size clusters bonded to molecules of amorphous colloidal silica (a.c.s.). If these less than bulk-metal size precious element clusters, which because of their small size exhibit the properties of a non-metal, are to be converted to metal, one approach is enlargement by nucleation using like-precious metal ions, clusters, etc. as seed for growth. For Au, and also at least Pt, this transition by growth-through-nucleation to bulk metal can possibly result in a complexing with certain types of ligands (anion complexes) that prevent or greatly inhibit the actual conversion of these enlarged cluster complexes to particles that exhibit the metallic properties of the subject precious metal. As certain of these precious metal cluster complexes are formed, they have been observed to take on physiochemical properties that are essentially nonmetallic. They are largely insoluble in mineral acids or combinations of such. They tend not alloy with other metals and have peculiar melting characteristics. Often, when combined with specific ligands, they volatize at low temperatures. If the subject clusters are Au and/or Pt they have distinct color variations from pink to violet/purple which they give to macro-samples. By comparison, if these clusters are not ligated and formed as described above, they would likely be metallic in character, not colored as noted, and identifiable as Au and/or Pt using standard analytical procedures.
In view of this information, a comprehensive study and re-interpretation of our production model led to a simplified, continuous batch production system that is more efficient, gives a continuous and improved precious metal recovery; all with the general absence of a pink/red/violet/purple coloration. As per usual, the initial testing of this model was performed on various coal combustion products because of the comparatively simple chemistry of this type of material. After a “proof of process,” small samples of several other geologic materials were tested with even more favorable precious metal recoveries. Some of the best results were derived from finely ground basaltic scoria. A.C.J.