One of the objectives of DFID / BGS Technology Development Research (TDR) project R6226 \"Mitigation of mining-related mercury pollution hazards\", was to provide improved methods for gold recovery for small scale miners to replace mercury amalgamation. Previous work carried out for this project indicated that the most appropriate method of recovering fine-grained gold is through the use of a shaking table type of gravity separator (Mitchell et al, 1997). Shaking tables are one of the most environmentally friendly methods of mineral processing as the only reagent used is water. Shaking tables are widely used in commercial mines but have found little use by small-scale miners due to their relatively high cost. The purpose of this phase of the project was to design, construct and test a cheap, simple shaking table that could be produced for use by small-scale miners in developing countries.
A set of test samples of gold ores and tailings from processing had been collected from small-scale mines on a previous visit to the Philippines. Laboratory characterisation of these samples established the gold content of the ores and tailings and grain size distribution of gold within the samples. The assays showed that ores varied from 12 to 96 grams/ton (for one very rich ore) and the tailings from 9 to 25 grams/ton, which is higher grade than ore material in many other areas. The grain size measurements showed that they contained dominantly fine-grained gold with one of the main samples studied having 60%
A series of mineral separation and gold recovery tests were carried out in the laboratory using a commercially produced, laboratory scale Wilfley table (a type of shaking table). The high-grade ore from Acupan contained mainly fine-grained gold and a recovery of only 20% of the gold was achieved. In contrast the lower grade ore from Kias contained slightly coarser gold and a recovery of nearly 80% was achieved.
A simple, shaking table was designed and constructed of cheap materials that are available in developing countries and has a drive mechanism using bicycle gears and chains and rubber bands made from car tyre inner tubes. A hand crank powers the drive mechanism of this prototype but it can readily be modified to be powered by a bicycle, motor cycle or a motor, either electric or diesel. Its design was particularly aimed at the recovery of fine-grained gold. Laboratory trials showed that for the separation of fine-grained gold, this simple table was as good as and probably slightly more effective than the commercial Wilfley table.
The laboratory trials had been carried out in almost perfect conditions; the samples were washed and deslimed prior to tests, the table was set up on large flat benches and a well controlled, even pressure water supply was available. It was important to ascertain whether similar good results could be obtained in the far from ideal setting of a small scale mine. The BGS shaking table was taken to the Philippines and field trials were carried out in collaboration with the Mines and Geosciences Bureau. The trials were held at Kias Creek and Acupan Benquet mine in the Baguio mining district, the area from which the test samples had been collected. Trials at Kias Creek were very successful; the table was easily set up and adjusted to a stable configuration and heavy mineral concentrates were readily recovered. Field examination with a hand lens showed that considerable amounts of fine-grained gold had been recovered from the tailings that had previously been processed by the miners' normal methods. Subsequent laboratory examination revealed that the effectiveness of the simple table was quite remarkable as most of the gold was only around 40 µm in size and grains as small as 10 µm had been recovered.
Trials at Acupan were problematical and it was far more difficult to set up the table in a stable configuration. This revealed shortcomings in the design of the prototype apparatus and gave valuable information for simple modifications that should make it more adaptable. Heavy concentrates were collected but with more difficulty than at Kias. Laboratory examination of the concentrates showed that despite the difficulties significant amounts of gold had been recovered from ores and tailings and again substantial amounts were very fine-grained around 30 µm in size.
The field trials demonstrated that the simple shaking table is an effective apparatus for the recovery of fine-grained gold. Its use is more difficult in adverse conditions where material to be processed is muddy, stability is a problem and water pressure is variable. The staff of MGB who took part in the trials and the small-scale miners were impressed by the table's simple construction and effective performance. The miners were very interested in such an apparatus as an alternative to cyanidation of their tailings if it were available at an appropriate price. The table has been donated to MGB for further trials, demonstrations, development and possible local manufacture. This is particularly in areas where mercury use is widespread and environmental benefits would be greatest.
It has been demonstrated that a simple shaking table is an environmentally friendly method of recovering fine-grained gold. It could have a significant role in the reduction of the use of mercury by small-scale miners by providing them with an alternative to mercury amalgamation for gold recovery.
This report is available to download in full colour (5466 kb) and in black and white (2711 kb).