New England antimony mining company, recently had a small gold mine and beneficiation plant put into operation, the mine is located Hilger mining area about 30 km east of Armidale in northern New South Wales.
The mineralization of stibnite is produced in low-temperature hydrothermal ore urea with quartz and calcite . Symbiosis minerals: pyrite, pyrrhotite, arsenopyrite, scheelite, chlorite and natural gold.
The Freehold Mine currently has a monthly production of 1,750 tons and an average grade of approximately 4.5% and 9g/t. The mine began production in 1969. Initially the ore can only be recovered by flotation. As the price and grade of gold increased, the gold recycling process was studied and a re-election system was installed in 1976.
The ore discharged from a 1.2 m × 3 m rubber-lined ball mill was sieved with a drum sieve having a hole diameter of 3 mm. The screened product is returned to the ball mill for regrind; the undersize product is loaded into a conical concentrator to produce a crude gold concentrate. The concentrate is selected on a spiral concentrator, and the concentrate on the spiral concentrator is further enriched with a shaker. The final grade of Ag is about 2000 g/t.
The tailings of the conical concentrator are graded in a 375 mm cyclone, and the grit is returned to the ball mill. The overflow size of the cyclone is about 60% -200 mesh, and the ore is loaded into the flotation system. Flotation is carried out in the Denver Type 23 Small A flotation cell. The stibnite was activated with lead nitrate, sodium ethyl citrate as a collector, and MIBC as a foaming agent. The flotation concentrate was treated with two series shakers to reduce the arsenic content and recover the fine free gold. The shaker concentrate is re-selected with the concentrate to produce the final free gold concentrate.
Before the expansion of the mine in 1981, the tantalum concentrate with a grade of 68% was recovered from materials containing about 4% niobium and 9 g/t gold, and the recovery rate of niobium was 95%. The recovery of gold from the shaker concentrate is approximately 35%.
Usually the gold content of the gold ore is 30-40g/t, and this gold content is not paid when it is sold to the smelter. In order to recover gold from antimony concentrates, a new process was initiated in 1981.
Other leaching agents were investigated because of the ineffectiveness of the cyanidation process. It has been demonstrated that acidic thiourea can effectively leach exposed gold under controlled conditions. Based on the results of laboratory tests, a new immersion gold plant was commissioned in March 1982. Intermittent operations are 8h per shift, and each batch is about 8t concentrate. Its flow chart is shown in the figure.
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Important parameters for thiourea leaching operations are pH, redox potential and thiourea concentration. Since the redox potential drops rapidly, a relatively high concentration of thiourea and iron ions is required, which requires that the leachate should be mixed as quickly as possible with the concentrate, and the leaching time is less than 15 min.
The gold is recovered by an activated carbon adsorption method and sold as a carbon concentrate having a gold content of 6000-8000 g/t. The thiourea solution can be recycled after adjusting the redox potential with hydrogen peroxide.
Several problems have been encountered in the first few months of production, and it has now been determined that these problems are due to the adsorption of gold onto flotation concentrates. At the time of flotation, the Aero 633 inhibitor was used and a small amount of diesel was added to the leachate to overcome this problem. The recovery rate of gold is between 50% and 80%, depending on the exposure of gold in the concentrate.
Based on the experience of industrial applications of thiourea leaching by New England. It has been decided to carry out thiourea leaching tests on various ores, concentrates and calcines. The results of this test are very encouraging. When carefully controlling the leachate conditions, a leaching time of up to 15 min is sufficient, and the general thiourea consumption is less than 2 kg/t. The maximum recovery of gold has been obtained from a thiourea solution, i.e., when using a noble liquid of less than 0.5 x 10 -6 .
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