Ore enters the jaw crusher and is reduced to ~½” minus to be fed into the hammer mill. Primary crushing to -½” material substantially reduces wear on the hammer mill.
The material then enters the hammer mill with a small amount of water to keep dust down and to mix the material into a wet slurry. Dry powder does not “wet out” thoroughly for sluicing and tabling. Retention time in the mill is minimized in order to avoid over-grinding the free milling gold and sulfides. When limited to a gravity recovery system, over-grinding is the enemy of recovery, since abraded “micron” gold is lost in the tailings unless chemical leaching is used.
The discharge from the mill has a bell curve distribution of sizes: a few larger pieces, some extremely fine material, and a high percentage somewhere in the middle (~20-100 mesh). The oversized gangue material needs to be recirculated back to the inlet of the mill for finer grinding, but the larger gold needs to be removed from the ore stream to avoid over grinding. The discharge of the mill enters a “nugget trap” sluice where the larger free gold (~50 mesh and larger) is removed. The discharge of the sluice is then screened to a nominal +20 mesh, and now barren of the oversized gold, is sent back to the mill. The -20 mesh slurry from the sluice, containing fine gold, sulfides and gangue, goes to the shaker table.
Many people ask about ball mills for fine grinding. Ball mills are the industry standard for fine grinding, but they are expensive, permanent, and take a lot of power. They are best suited for long term installations where all the slurry is ground to a fine texture and recovery is via flotation and/or cyanide leaching. A serious challenge with ball mills for gravity circuits is to avoid over-grinding the values. Hammer mills have a high rate of wear, but are cost effective, easy to maintain, and have a high through-put for their small footprint.
Once the material exits the sluice and screen, it feeds directly onto the shaker table as a -20 mesh wet slurry. The MBMM shaker table has been designed and tuned to capture very fine free gold (smaller than 325 mesh) and has the unique feature of a ramp and plateau to specifically separate the concentrates, middlings and gangue into three separate streams. The sulfide middling product will have very little free gold (less than 5%), but there may be fine gold contained within in the sulfides that can later be processed to release gold (The sulfide middling’s processed from our hard rock property assay 3-5 oz/ton with no free gold).
With ore that fractures along crystalline boundaries, grinding to a -20 mesh slurry often provides a satisfactory recovery of values on the table, with a minimum of treatment. This provides immediate cash flow for the operator. If significant values are still present in the table tailings, the operator can stockpile the tailings for later re-processing, or classify the tailings with a simple gravity spiral classifier. When a spiral classifier is used, the oversized material (based on the operator’s specific ore) is sent back to the mill for further grinding and re-run again on the shaker table. Testing for the practical liberation size will determine the cut-off for the maximum size of ore particles sent to the tailings.