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IN-SITU LEACHING of GOLD DEPOSITS * D Connelly1 1 Mineral

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IN-SITU LEACHING of GOLD DEPOSITS * D Connelly1 1 Mineral IN-SITU LEACHING OF GOLD DEPOSITS * D Connelly1 1 Mineral Engineering Technical Services Pty Ltd (METS) Level 6, 524 Hay Street Perth, Australia, 6832 (*Corresponding author: [email protected]) ABSTRACT In-Situ Leaching (ISL) process plants are low cost producers for uranium deposits where the geology is suitable. In the 1970s, a three-year pilot ISL program occurred on gold ore from the Ajax Mine, Cripple Creek District, Colorado, using a chloride and iodide solution. [1] The test was halted due to poor results. With gold and silver the international ISL progress has been slow; however the potential for low CAPEX and OPEX with large low grade deposits is real. Unless the metallurgy has been demonstrated there are significant risks which could be fatal flaws to an ISL gold or silver project. The metallurgy of gold and silver ISL projects is far from simple and it is early days with project development. INTRODUCTION ISL is also known as in-situ recovery (ISR) or solution mining. Developed independently in both the USSR and USA in the early 1960s for extracting uranium from the sandstone type uranium deposits. Copper ISL was practiced by Chinese as early as 977 AD. Gold ISL was trialled in US in 1970’s using halide solutions – abandoned due to low recoveries (Cripple Creek, Ajax Mine).[1] ISL is a well-established technology that accounted for more than 27% of the world’s uranium production in 2007, used for copper and potash as well as tar sands and commercial salt production [2]. This paper explores the work that has been done in considering ISL for a unique gold deposit where it is believed the technology could be applied. THE ISL CONCEPT In-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining, is a mining process used to recover minerals such as copper, uranium, potash, commercial salt and tar sands through boreholes drilled into a deposit. In-situ leach mining involves pumping of a leachate solution into the ore body via a borehole (injection), which circulates through the porous rock dissolving the ore and is extracted via a peripheral borehole (recovery). The leachate solution varies according to the ore deposit: for salt deposits the leachate can be fresh water into which salts can readily dissolve. For copper, sulphuric acid is generally needed to enhance solubility of the ore minerals within the solution including an oxidant if found necessary. For uranium ores, the leachate may be sulphuric acid or sodium bicarbonate depending on the carbonate content of the rock. The process initially involves drilling holes into the ore deposit. Explosive or hydraulic fracturing may be used to create open pathways in the deposit for solution to penetrate. Leaching solution is pumped into the deposit where it makes contact with the ore. The solution bearing the dissolved ore content is then pumped to the surface and processed. This process allows the extraction of metals and salts from an ore body without the need for conventional mining involving drill-and-blast, open-cut or underground mining. HISTORY ISL was first developed for uranium processing in the 1960s in USSR and USA [3]. The process was used to dissolve uranium ore are either acid (sulfuric acid or less commonly nitric acid) or carbonate (sodium bicarbonate, ammonium carbonate, or dissolved carbon dioxide). Dissolved oxygen is sometimes added to the water to mobilize the uranium The first uranium ISL in the US was in the Shirley Basin in the state of Wyoming, which operated from 1961-1970 using sulfuric acid. Since 1970, all commercial-scale ISL mines in the US have used carbonate solutions. Australia has two key ISL uranium mines [3]. These are Beverley, which was the first mine to use ISL in Australia and Honeymoon Mine [3]. Kazakhstan has 19 ISL mines in 2010, with the highest use of ISL globally [3]. ISL is also used for copper processing for ores like copper carbonates malachite and azurite, the oxide tenorite, and the silicate chrysocolla. It is important to note that ISL leaching has not been used on a commercial scale for gold mining based on extensive searches of a number of data bases. In fact the searches revealed very little written on the subject. The only reference found was a three-year pilot program was undertaken in the 1970s to in-situ leach gold ore at the Ajax mine in the Cripple Creek district in the US, using a chloride and iodide solution. After obtaining poor results, perhaps because of the complex telluride ore, the test was halted [1]. WHY NOT GOLD? ISL leaching is used for uranium, copper, salt, potash, tar sands yet it has been very slow to gain traction for gold. At first this appears to be odd however a deeper investigation and discussion with colleagues suggested that environmental aspects may be the real reason. One of the biggest perceived problems has been the environmental issues related to pumping cyanide into the gold ore systems underground. The significant environmental issues of ground water contamination, remediation and public perception have appeared to be insurmountable hurdles preventing resource companies investigating the option. Even when initially raised with a client as a possible solution they ruled it out immediately as a first reaction. It is interesting to note the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia has a research project for ISL on gold ores but this does not include cyanide at this stage as the lixiviant [4]. Instead this research project has tested the use of sodium thiosulfate and ferric EDTA as the lixiviant [4]. CSIRO’s project is also aiming to overcome one of the key issues of ISL on gold ores, which is the permeability of oxide gold ores [4]. CSIRO is currently considering the use of permeability enhancement methods like mechanical rock breakage by strategies like hydraulic fracturing methods [4]. Table 1- Aspects Of Different Mining Processes Aspects Of Conventional Mining Aspects Of In-situ Leaching Massive Operation Uses leach solution to recover minerals Massive rock removal Ore in place (no rock movement) Costly processing (crushing, milling) Little or no surface disturbance Tailings and waste rock Minimal tailings or waste Substantial environmental impact Major reduction in water use Environmental and closure advantages Cost effective GEOLOGICAL CONSIDERATIONS ISL can only be used for ore situated in permeable rocks (sandstone) confined by impermeable layers. The ore deposit geometry must be amenable and there needs to be sufficient size and continuity to enable economic metal extraction. The ore zone needs to be a permeable host rock confined by impermeable country rock (confining rocks) to prevent the migration of leach solutions into the surrounding environment. Highly permeable ore hosted systems allow the mining solution to access and interact with the mineralisation. The hydrogeology geometry must prevent metal bearing fluids from migrating in unwanted directions. The low permeability layers such as shales or clays, isolates the metal-producing horizon from overlying and underlying aquifers. The mineralisation should be located in a hydrological saturated zone. ENVIRONMENTAL ISSUES The leaching liquid used for ISL contains the leaching agent sodium cyanide. The risks are migration of cyanide liquors into the surrounding rocks where the cyanide does not break down. There is a risk of spreading of the leaching liquid outside of the gold deposit, involving subsequent groundwater contamination, including the unpredictable impact of the leaching liquid on the rock of the deposit. The Feasibility of restoring natural groundwater conditions after completion of the leaching operations is a prime consideration. Without remediation after ISL mining and even Detox (cyanide destruction) of the site it is unlikely authorities would permit the process. These two aspects are critical in getting ISL gold projects permitted. HYDROLOGY ISSUES Groundwater is a primary concern in mineral processing and ISL, as it is generally difficult to access and measure. It is predicted that many plants will start to use groundwater more readily in the future, due to projected lower and less reliable surface runoff in southern Australia [5]. The Groundwater Hydrology Program works in the Australian context where subdued topography and high rainfall deficit is manifested as low and episodic recharge, very low hydraulic gradients and large variability in salinity [5]. In hypersaline environments the use of cyanide for ISL is likely to be less of an issue than systems where the groundwater is potable [5]. For an ISL project it is essential to understand and quantify the biophysical processes at a range of scales that support the capacity for long-term use of groundwater without unacceptable or inadvertent impacts on surface water, the environment, groundwater levels, or water quality [5]. It is necessary to understand the extent to which terrestrial and aquatic ecosystems depend on groundwater and to evaluate management options at the surface water-vegetation-groundwater nexus, particularly with respect to salinity and cyanide impacts (e.g. stygofauna) [5]. PUBLIC ACCEPTANCE ISL techniques are often controversial, sometimes because of acid leachate solution. The concerns of environmental groups and landholders would centre on possible migration of cyanide contaminated groundwaters. Also the mobilisation of potentially hazardous heavy metals and, in the case of gold ISL, weak acid dissociable (WAD) complexes into the surrounding water systems would be significant issues they would need re assurance on. Other possible concerns could be a disturbance of the groundwater table, mixing of groundwater aquifers and general disturbance of the land atop of the ore body. More recently the destruction of habitat for stygofauna and other rock-inhabiting organisms, bacteria, et cetera has been raised as a concern for other projects where cyanide has not been involved. Potential spills of cyanide and cyanide metal-bearing or salt- bearing leachates upon the surface would also be a concern.
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