A. C. Johnson, Jr., Ph.D.

4/25/05

Not all naturally occurring precious “metals” are mature to the extent that they can be assayed using so-called conventional methods, but then, are these “hidden elements” really in a physiochemical state where this is possible? It has been clearly demonstrated that there is a progressive growth in particle size from a single existing metal atom possessing few, if any, metallic properties, to a bulk metal size at which point the various elemental properties may be identified, and the metal assayed, and the metal and subsequently recovered.

Does this progressive growth of metal atoms to like-composition bulk metal particles occur in nature? The preponderance of evidence from laboratory studies, industrial applications of catalysis, and geologic investigations indicate a most definite –YES! This sequence of particle size increase is considered to progress from a single metal atom to clusters of atoms; from clusters to nanoparticles (to approx. 100 nm); then to colloids (approx. 1000 nm); and finally larger particles with bulk metal properties such that the resulting metal can be identified using conventional methods. Actually, the appearance of bulk metal properties does not appear to occur at any single growth stage, but progressively from the cluster stage to the bulk metal stage; sort of an aging or maturation process.

Obviously, without being pre-treated in some fashion, geologic samples containing small, non-bulk metal size particles of precious metal are not going to be indicated as present in a typical fire assay, chemical leach, or spectrographic analysis. Of course, if you could convert these particles to bulk metal size and produce them, this would be yet another matter!

Interesting References:

Raithby, Paul R., 1998, The build-up of bimetallic transition metal clusters: Platinum Metals Rev., v. 42, p. 142-157.

Lewis, L. N., 1996, Catalysis by colloids: Tech Info. Ser., 96CRD135, Class I, GE Research and Development Center, 23 p.

A.C.J.

7/26/04

Mixing or a Mess?

The proper mixing of various fluid suspensions on a laboratory scale or in a production mode is a procedure that can easily be taken for granted. Actually, there are a number of important variables that must be considered with fluid mixing or the results of a test or production run can be misleading or even disastrous! Phenomena such as macromixing and micromixing, the importance of baffle plates and how they should be placed, the type and shape of a container or tank to use, what type and size of agitator and impeller give the best results, and should the impeller push upwards or downwards are merely a few of the functions of proper fluid mixing that need to be considered.

5/10/05

One of the major problems in dealing with the recovery of solubilized precious elements using various specialized ion exchange resins is separating the precious metal product from the resin. Due to the high unit value of precious metals, virtually all of the adsorbed precious metal must be removed and recovered from the subject resin. As it turns out, this is not an easy task to accomplish.

In most precious metal recovery systems that utilize ion exchange resin, the final act of total recovery is ashing. Anyone trying this, even on a laboratory scale, realizes the pervasive and acrid odors that are involved. Scrubbers are available, however, they are expensive and frequently not totally successful. Equally important are the serious precious metal losses (up to 50% or more) involved when using various types of laboratory to commercial-scale ashing systems.  

In our laboratory we have long sought after a non-odorous, inexpensive, and quantitative method for the recovery of precious metals from commercial ion exchange resins. Les Price, who manages our lab, has finally devised such a method. It is simple, inexpensive, and it works; at least on small amounts and, most likely, it could be scaled to a commercial size. The ion exchange resin used in the following was Dow Chemical MSA-1.

1. Immerse the “loaded” resin in a suitable amount of water. While agitating, reduce the pH to approximately 2-3 using HCl.  Incrementally add VenMet or VenPure (product of Rohm and Haas) to a final pH of 9-10. Continue agitation for 30 to 60 minutes.
   
2. Filter and at least partially dry resin.
   
3. Immerse the resin in a 25% solution of NaOH for 24 to 48 hours.
   
4. Filter the resin and let it “dry”.
   
5. Place resin in a clay crucible; approximately 50 to 75 % full.
   
6. Furnace at 2000 F until the resin is carbonized and the reduced metal has had an opportunity to form a liquidus.
   
7. Pour into a mold. If necessary, clean the metal button.
   

A.C.J.