The i-botmprocess and related treatments mine waste remediation

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1 The i-botmprocess and related treatments mine waste remediation for K. McEwanl & D. Ralph2 Micron Research Pty. Ltd, Australia 2Division of Science, Murdoch University Australia Abstract Acid mine drainage is an ongoing problem associated primarily with sulphidic mine wastes. Remediation of the mine sites is expensive. Micron Research Pty Ltd has developed the Z-BOTMProcess for treatment of a particularly difficult to remediate tailings dumps located near Tennant Creek in the Northern Territory of Australia. Application of the i-botm Process to the tailings dams results in an increase of the rate of natural weathering and hence acid production. Years of weathering are condensed into months allowing the acid drainage to be controlled and collected. The contained gold, cobalt and copper are then available to downstream metallurgical recovery processes making the remediation process profitable. 1 Introduction Centuries of mining and metallurgical treatment of ore bodies have left a legacy of billions of tonnes of tailings and low-grade waste. The sulphur contained in tailings is thermodynamically unstable and exposure to oxygen and water results in the oxidation of the sulphur containing minerals to sulphuric acid, generating acidic solutions that contain a cocktail of base metals and impurities. Hence, the remediation liability is exacerbated by seasonal parameters and natural oxidation of the exposed mineral, creating additional environmental problems in the form of acid and base metal drainage into the surrounding land, eventually polluting the waterways. The high cost of the remediation of mine sites frequently results in the liability being passed onto successive operators, or being ignored.

2 526 Browtfield Sites: Assessment, Rehabilitation and De~elopntent Micron Research Pty, Ltd. focuses on developing processes that profitably recover precious and base metals from these low-grade resources, making the rehabilitation of mine wastes economically attractive. Our proprietary process for the in situ bio-oxidation of low-grade mineral resources, the i-botmprocess, increases the rate of the natural weathering, using naturally occurring lithotrophic bacteria to oxidise the mineralsl. Contained precious metals are then available to conventional recovery processes. 2 The Peko Tailings The i-botm Process was developed to rehabilitate a refractory and impermeable tailings dump, located at Tennant Creek, in the Northern Territory of Australia. The 4 Mt of mine tailings at this site contain mostly magnetite (- 80 %), clay minerals and small amounts of mineral sulphides with the most common being pyrite (FeS2), chalcopyrite (FeCuSJ and cobaltiferous pyrite (FeCoSJ2>3. Gold is also present (1.2 ppm average)4 and the combined value of the base metals and gold is approximately $A 80 per tonne. Despite the contained metal value, the refi-actory, impermeable nature of the tailings makes conventional recovery options uneconomic and remediation expensive. An aerial photograph of the dump is presented in Figure 1. Fine black acidic dust blows from the dump into the town of Tennant Creek and local indigenous communities, Figure 1: The Peko Tailings dump The dump contains the tailings from two processing plants, which operated from 1954 to 1976 and treated the Peko ore body and that of four nearby mines. The rehabilitation obligation was passed between subsequent owners of the mining tenements.

3 Browwfield Sites: Assessntent, Rehabilitation and Del elopwent 527 The area is subject to inundation from monsoonal rainfall, The rainwater is contaminated by acid and base metal drainage fiorn the dump and subsequently pollutes the local waterways, 3 The remediation strategy The tailings dump was comprehensively sampled and assays indicated that natural weathering had oxidized the outer surface layers to a depth of ca. two meters (ea. 380 ktonne). The gold recovery from the oxidized surface layers was 65% to 95%, with cyanide consumption of 2 kg/t and lime consumption of 5 kg/t; whereas the gold recovery from the lower layers was 17 /0 to 50 /0with cyanide

4 528 Brow} field Sites: Assessment, Rehabilitation and De~elopntent and lime consumption of 10 kglt and 27 kglt, respectively. The surface layer was acidic, with a ph of 3.5, the ph increases with depth, the bottom being ph 8.2. The method used for depositing the tailings has resulted in very low vertical percolation rates for water and air resulting in high runoff volumes and a limit to the extent of natural oxidation. These observations provided some insights into the behavior of the tailings dump. It was clear, that the metal sulphides contained in the surface layers, which are exposed to water and air, enabling the oxidative activity of naturally occurring bacteria had become oxidized by natural weathering. The gold bearing minerals become non-refractory during this process, the base metals and acid are released into solution. The chemical reaction is described below: MSX + y02 + H20 + IV(S04 ), +H, SO, (1) (M= Cu, Co, Fe) The treatment strategy that appeared most promising was to maximize the rate of natural weathering that has already oxidized the outer layer of the tailings heap over the last 25 years. It was postulated that this could be done by providing large numbers of Iithotrophic bacteria and optimizing the conditions of ph, water, oxygen and carbon dioxide availability. Scarifying the tailings surface to a depth of 0.5 m (assuming a treatment area of 2.5 x 105 m2) provides a broken layer containing 0.3 Mt, or ca. 150 days supply for a conventional CIP plant. Oxidizing enough of the sulphur in this layer to give a high recovery of precious and base metals imposes only a modest oxygen and carbon dioxide demand to be supplied by diffhsion over 150 to 200 days. Water could be added by an irrigation system to maintain adequate moisture levels in the broken layer while collecting the drainage would enable base metals to be extracted. The strategy also included a means to increase the number of bacterial cells present in the tailings without relying on the natural increase occurring within the broken layer. 3.1 Inoculum Generator (IG) To provide large numbers of lithotrophic bacteria, cells were grown in an external vessel and transferred to the tailings oxidation test pads (also called farms). Draining the IG vessel gave a flow of fluid that was acidic (ph 2), contained dissolved iron (mainly as the ferric form) and a large number of cells. This fluid was used to inundate the pad of tailings. Collecting the run-off after a few hours showed that: 1. Cells contained in the IG fluid tended to attach to the tailings and not report to the run-off sump. 2. The ferric ions in the IG fluid were reduced to ferrous form by contact with the tailings in the farm

5 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent 529 3, The acid in the IG fluid created a tailings slurry in the Farm of ph favorable to bacterial growth. 3.2 Tailings oxidation farms The tailings oxidation Farm allowed a thin layer of tailings to be exposed to favorable oxidation conditions so that the extent of the oxidation and the recovery of base and precious metals could be quantified. The laboratory scale farms were inundated (with IG fluid) and ploughed on a regular basis to maximize their exposure to air. Tailing from all levels within the heap were tested in the oxidation farms. 4 Materials and methods 4,1 The Inoculum Generator (IG) The waste rock used in the IG was obtained horn the Geko mine waste pile near Tennant Creek. This sulphide mineral is stockpiled and exposed to air and water, generating continuous acid drainage. The Geko mine waste rock is a rehabilitation liability to the current owners of the mining tenements on which it is situated. This mineral (averaging 3 /0S) was crushed and sieved. The -12 mm fraction was loaded directly into the IG column (1600 mm length by 300 mm diameter) while the large fraction was returned. Sulphuric acid was added to 50 L of water until the ph was 1.7 and small amounts of ammonium sulphate and potassium hydrogen phosphate added and the fluid used to fill the column. The air sparge was set to add ca. 100 ml per minute to the base of the column. This basic design was scaled up by a factor of 10 (in 4 separate units) to provide inoculum for the full scale of farm test-work. Conventional cyanidation test work was conducted on milled and unmilled material both before exposure to the bacterial cultures and after 450 days of inoculum generation. 4.2 Tailing Farms The tailings were sampled in a comprehensive program of drilling. Samples collected at the various depths were stored in airtight bags until used. The tailings collected at different levels in the dump (e.g. O 2 m) were weighed, sampled and spread in a plastic tray (1200 x 500 x 150 mm) to give a layer of ca. 10 cm. The tailings (usually kg) were inundated with fluid from the IG. After 24 hours, the fluid was drained and the cycle of flooding and scarifying described below was begun. 1. Tailings were irrigated with fluid from the IG. 2. The excess fluid drained naturally to a sump and was collected. 3. The tailings bed was allowed to dry to a particular moisture content. 4. The tailings bed was then ploughed to allow access of air.

6 530 Brow} field Sites: Assessment, Rehabilitation and De~elopntent This cycle of farm treatment was continued for 150 to 250 days and solid samples were withdrawn at intervals to test for copper and cobalt removal and gold recovery after conventional cyanidation. Lights arranged above the surface of the tailings (between mm) gave a radiant energy of 1000 W m-2and were operated for 12 of every 24 hour cycle. These lights were setup to simulate the effect of the sun in evaporating moisture ftom the tailings farm surface 5 Results and discussion 5.1 Inoculum Generator Fluid from the inoculum generator used to inundate the tailings farms was replaced with tap water. After a period, the acid, ferric ions and cell numbers in the diluted vessel settled to constant values that were a function of the dilution rate. The Geko mine waste rock contained 2ppm Au, 3.3 /0S2-,2.5 /0Cu and 17.5% Fe. An IG column containing 35 kg of Geko rock and 50 L of fluid, yielded ca. 1 L of fluid containing 1-2 g of sulphuric acid (ph ), 2 g of ferric ions and ca. 1 x 108 cells each day over a period of ca. 450 days. Over this period there was a mass loss of 5.3% and 88% sulphide mineral oxidation, there was no longer an acid drainage problem from the spent mineral from the IG. The results of conventional cyanidation treatment of milled and unmilled rock, both before and after exposure to the bacterial inoculum are presented in Table 1. Table 1: Gold dissolution from the Geko mine waste rock before and after residence in the Inoculum Generator Gold Cyanide Lime Treatment Dissolution Consumption Consumption (%) (kg/t) (kg/t) Untreated Unmilled Milled day IG Unmilled residence Milled Gold dissolution from the unmilled and milled waste rock increased by 250% and respectively, following the 450 day treatment period. There was no substantial change in cyanide consumption. Lime consumption for the cyanidation of the milled rock increased from 5.9 kg/t in the untreated samples to 8.5 kglt in the oxidized samples. Recovery of gold from this waste rock used in the IG makes its rehabilitation process profitable.

7 Browtzfield Sites: Assessment, Rehabilitation and De~ elopntent Tailings Farms The Tailings Farms were used to test tailings extracted from all levels of the heap and each was treated with the same methodology. A four day cycle of inundation of the tailings with solution containing bacteria from the inoculum generators and aeration by scarifying was carried out and representative solid samples were extracted regularly, Conventional cyanidation test work was conducted on the samples. Since the tailings originate from several different mines, their composition varies with their location in the dumps. The high grade tailings contain 7g/t Au, 0.9% Cu and 18!Z0Fe. The lower grade material contains 1.2g/t Au, 0.4 to 0.8% Cu and 40 % Fe. The S2-concentration ranges from 1 to 7Y0. The tailings from various locations within the dumps were treated in farms until the drainage of acid and base metals ceased, Between 50 to 600/0 of the cobalt and copper was solubilized as a result of this process. The recovery by proprietary techniques of valuable base metals from these solutions is currently being developed at Micron Research. The effect of the treatment on gold dissolution is presented in Table 2 for both the high grade and low grade materials, Table 2: The effect of the i-botm Process on gold dissolution Treatment Time (days) Gold Dissolution (%0) Low Grade High Grade o Not Available Gold dissolution from the low grade material improved from 40~o to 60% over a four month period. Gold dissolution from the high grade material improved from 17% to 87% over a 7 month period. After application of the i-botm Process to the tailings and subsequent gold recovery by CIP, the tailings residue will no longer leach acid and base metals and will be disposed of in a nearby open cut mine, covered with top soil and revegetated, The gold and base metal recovery from the tailings makes their rehabilitation profitable and rehabilitation of the area economically attractive. 6 Conclusions The i-botm Process was developed as a low cost, viable treatment option for the Peko tailings dumps. By using the Geko mine waste, this material is also

8 532 Brow} field Sites: Assessment, Rehabilitation and De~elopntent remediated. Furthermore, the tailings can then be used as an inert fill to rehabilitate a nearby open cut mine. The oxidation process solubilizes 50% to 60% of the contained cobalt and copper and renders 65% to 87% of the contained gold recoverable by conventional cyanidation techniques. The low input cost for the recovery of the cobalt, copper and gold makes the application of this process and hence the rehabilitation of the Peko tailings dumps, Geko mine waste rock and a nearby open cut mine, attractive to the owners of the mining tenements on which they are situated. References [1] Crundwell, F., Holmes, P. R., & Fowler, T.A,, The Mechanism of Bacterial Leaching of Pyrite by Thiobacillus Ferrooxidans Applied and Environmental Microbiology, 65 pp , 1999 [2] Henley, K., Amdel Report Number G6404/86. [3] Radke, F., Amdel Report Number G259P098. [4] Mujdrica, S., & Hatcher, M., Peko Tailings Resource Report, Peko Tailings Project Tennant Creek Tennant Creek Library No , Kent Town Library No , 1997 Acknowledgements The authors wish to thank Peko Rehabilitation Project Pty Ltd for permission to publish this article.