Our customers want to know how fine to grind their gold ore (liberation size) when using a shaker table for gravity recovery. Below is a simple test to determine liberation size for a commercially-viable recovery of values.
In processing our local ore with the MBMM jaw crusher, hammer mill and shaker table, we get a p80 of ~30-50 mesh (80% passing 30 or 50 mesh sieve screen), depending upon the slot size in the hammer mill screen. Results show nearly 50% of the gold recovered was at <100 mesh, and a surprising amount of gold was at <325 mesh. So, contrary to some beliefs, gold ore does not need to be ground to 200 mesh to liberate 200 mesh gold. Here’s the video:
https://www.youtube.com/watch?v=NlAXVFFFiEI
Overgrinding past the desired liberation size slows production and increases wear. Worse yet, overgrinding can result in values so fine that they will be lost in a gravity recovery circuit. For instance, the best gravity shaker tables begin to lose effectiveness at recovering gold smaller than about 50-35 microns (300-400 mesh). Smaller gold and finely-ground gold-bearing sulfides are lost to gravity tailings and commonly recovered only with flotation or leaching circuits.
Here is a simple procedure to determine the practical liberation size for the small scale miner with a high performance shaker table. It is a modified sizing-sorting-assay test. The strategy is to process the ore thru the system and test the various cuts off the shaker table to see where additional values might be found. The MBMM shaker table has 4 discharge ports. The ore samples-of-interest are from port 3 (middlings, mostly sulfides) and port 4 (quartz tailings).
Simplified testing for liberation size with gravity recovery systems:
- Weigh an amount of head ore
- Run the ore through the system
- Methodically sample the discharge from ports 3 and 4. Typically, catch the discharge in a container for 5-10 seconds every minute or two to get a cross-section of the ore makeup from each port.
- When all the head ore is processed, drain and weight the total amounts from ports 1, 2 and 3 (not just the samples). Compensate for the water by multiplying by 0.85 to approximate the dry weights. Or dry them and then weigh them. There’s no need to weight the tailings, since the total ore weight was found before processing.
Processing the sample from port 3, middlings
- Port 3 will capture a high percentage of sulfides (if any) and some quartz gangue. Carefully pan the port 3 sample to detect any free-milling gold. We usually find less that 3% of the pannable, free milling gold reports to port 3. The port 3 middlings can be re-run on the table to extract detectable gold, if desired.
- Now, the sample should be barren of any pannable, free-milling gold (less than about 400 mesh).
- Screen the sample into these fractions: +200 and -200 mesh. Send these 2 fractions off for assay to a professional assayer. The results are too important for owner-testing.
- If gold is found, it is likely from gold-bearing sulfides. It’s not too important to know if the gold is in coarse or fine sulfide particles, as the circuit will not be tuned based on the size of the sulfides. Most small scale miners will save gold-bearing sulfides and send them to a refiner for processing.
Processing the sample from port 4, tailings
- Screen the sample into these fractions: +30 mesh, 30-100, 100-200, 200-325, <325. Send these off for assay to a professional assayer.
- If gold is found in the +30, 30-100, or 100-200 fractions, additional grinding may release it for capture via a gravity shaker table.
- Retest with smaller classification ranges to zero-in on the liberation size. Tune the grinding circuit to only grind fine enough to attain the commercially-viable liberation size.
- If gold is found in the 200-300 or <325 mesh fractions, it is likely gold-bearing sulfides or free milling gold <~400 mesh. This gold is not practical to recover via gravity concentration, so flotation or leaching could be used.
Additional notes
-Total gold-per-ton is computed using the 1) port 1 and 2 gold recovery, 2) assayer’s results and discharge weights from ports 3 and 4 to compute gold weight at these ports, divided by 3) weight of all material discharged from all ports.
–Percentage recovery of total gold is computing by taking total gold recovered in ports 1 and 2 (free milling, >~400 mesh), divided by total gold in ports 1, 2, 3, and 4 (includes very fine free gold and gold-bearing sulfides from assayer).
A hammer mill or impactor produces a fairly coarse grind. Usually, about 30-50 mesh is the practical limit. They are also high-wear machines that need frequent wear-part replacement and maintenance. But, they are comparatively inexpensive and are very useful for proof-of-concept testing for new prospects, or for limited production runs of smaller volumes. If testing shows that a finer grind liberates additional gold, the tailings from the hammer mill circuit can be stockpiled and rerun when a ball mill is installed in a more permanent installation.
Ball mills routinely produce P80 of 200 mesh, in preparation for flotation or leaching, but can be tuned to produce sizes from 50 mesh and smaller. Often, the discharge from the ball mill is processed on a shaker table to remove coarser gold. Then a classifier (hydrocyclone or spiral) is used to isolate the oversized table tailings material and send it back to the ball mill for re-grinding. The undersized slurry from the classifier is sent for further processing or to the settling pond.
Sincerely,
Steve Gaber (Co-Owner)