Hello folks,
It wasn't long ago we heard that the Naxos folks were working on a process that started with an attrition grind of the sample. The end result was a comminuted (ground) sample that attained a particle size that met the standards of 1000 Mesh Tyler. It was a thread discussion that was kind of amusing as no one that added to the post, even those knowledgeable folks, knew how to prove if Dr. Johnson was right or wrong. Besides, what difference did it make if his process worked? Well, the fun is over and now I feel I should correct or further explain grinds.
In early mineralogy a person learns that there are minerals that are metals, crystals or amorphous (without structure). Shapes identify the crystals. The shapes are caused by lattice structures. Almost all, if not all, minerals can be positively identified by the lattice structures. Some are so close to another minerals structure, an x-ray spectrographic analysis is necessary to do the job if color and hardness tests are not enough for final classification. The bulk of the earth is probably composed of crystalline minerals with definite structure. Clays are generally amorphous. These also are quite common. The igneous rocks and volcanic ash falls are largely crystalline. Granite is almost entirely composed of crystals of feldspar and quartz. If you melt these minerals and cool them relatively slow, but not as slow as an igneous rock might cool, the results are glasses. Glasses are liquids by some definitions. Eventually glass will re-crystallize. When a magma cools it separates into individual crystalline species along the lines iron rich crystals first, potassium and other feldspars second and quartz last. The crystals thus formed are relatively pure. Other contaminants are generally along side the individual mineral crystals acting as divider zones. As crystals, these minerals generally weather into different materials. The quartz becomes sand. If the sand contains some fresh feldspar we call it arkosic. In weathering the crystals are even more pure as individual species. The feldspar that becomes weathered or altered turns to clay. Clays are frequently trashy mineral mixtures. The crystals that have impurities in their lattices as inclusions decompose along those lattice lines first.
So, here is the key to breaking up particles. First grind to a minus-50 Mesh Tyler sized particles. That is a screen size where there are 50 separations per inch using a wire mesh. Now if you could grind the sample so only the crystal weaknesses can be the fractured planes, the results would be particles that are defined by cleavages rather than size. By breaking particles into fractions by those parameters, the contained inclusions of precious and trash metals will be open to exposure for processing. The larger particles that are very pure crystals will be left alone. If a particle is shot at a hard surface, the impact compression followed by decompression does just exactly that to some degree. Another method I am aware of allows a miller to re-grind using the low cost standard ball mill to attain a grind by particle quality much more efficiently for such minerals as quartz, feldspar, zeolite cages, mafic minerals and zircon. A few crystals such as diamonds are not effected by this grind technique. Also, most of the minerals mentioned that are very pure (barren) and without lattice inclusions are also unaffected. The results are that only the impure lattices are destroyed. I believe that when this occurs the metals and clusters are liberated. Certainly some of the particles that result are particles of 1000 Mesh Tyler and below and going down to who knows what "ity bity" particle size. The remainder is mostly left unaffected and at particle sizes up to the original 50 Mesh Tyler. The cost is more expensive than traditional grinding, but inexpensive when you consider the job that is being done. I believe this low cost comminution method can be done at 10,000 and even 50,000 tons per day. In defense of Johnson & Lett, let me say that the grind that they are doing just might be getting the job done that has to be done. Johnson has been working DDs a long time. He is a pioneer. I have read some of his reports in the past. I have not known him to produce seriously flawed conclusions.
mike |
