Copper country - metal extraction techniques
Michael Forrest talks to Stephen Twyerould, CEO of Excelsior Mining Corp about metal extraction techniques.
Mining is an energy-intensive business. One of the most significant costs is the breaking and crushing of ore to release the metals it contains – and these costs are escalating, due to increased energy prices and the increasingly fine grain of many mineral deposits. A more elegant solution is to extract the valuable metals by hydrogeochemistry, otherwise known as in-situ leaching. This is a primary technique in uranium production, especially in Kazakhstan, which is the world’s largest producer.
The theory behind in-situ leaching is simple. A weak solution of acid – usually around one per cent concentration – is pumped through a porous ore body, dissolving the desired mineral that is captured at the surface and recovered by precipitation or electrolysis. The solution is then refreshed and recirculated through the ore body.
More difficult is ensuring the circulating fluid, the lixiviant and its dissolved metals, are contained within the orebody and do not leak into the surrounding ground water. This usually requires a geological seal in the form of an impervious cap rock below the ore body, or is achieved by differential pumping that creates a low pressure zone in the water column above and around the deposit. The second consideration is the suitability of the ore to leaching, and that usually means metal in oxides.
Most base metals occur as sulphides in the Earth’s crust. However, there are a number of deposits that have been altered by weathering or metasomatism to oxide minerals. Most prevalent are copper oxide caps that overlay the sulphide porphyry copper deposits of the Pacific coast of the Americas, for example Tintaya, Peru, and Collahuasi, Chile. Arizona has similar copper deposits, most notably the Morenci and Bagdad mines. In the southeast of the state lies the Gunnison copper project of the Excelsior Mining Corp. It is within the basin and range geology of the western States and is typified by fault bounded troughs and underlying intrusions.
Gunnison is part of the Mexican Highlands, an upstanding mass covered by 140m of alluvium overburden. Comprised of a number of calcareous and sedimentary horizons, it is underlain by an impervious basal shale. These rocks are highly fractured and permeable with the copper mineralisation related to the Texas Canyon quartz monzonite located on the western margin of the deposit. The intrusion has formed wide zones of calc-silicate and hornfels alteration as well as extensive low-grade copper sulphide mineralisation within the Palaeozoic rocks above the basal Pioneer and Pinal horizons. The intrusion has metamorphosed the surrounding host rock with the highest grades next to the intrusion, fading to dolomite to the east. Copper oxide mineralisation extends over a strike length of 9,800ft, has an aerial extent across strike of up to 2,500ft and in places is more than 900ft thick.
Excelsior’s CEO Stephen Twyerould has been examining this deposit and its suitability for in-situ leaching for the past five years. ‘Arizona has a track record for permitting in-situ leaching of copper oxide deposits over the past 30 years. Magma Copper (later acquired by BHP Billiton, Melbourne, Australia) began with the Pinto Valley deposits of its Miami operation where some 1.47 billion pounds (700,000t) were recovered in an in-situ leach solvent extraction electro-winning plant over a period of more than 20 years. At San Miguel, the same company produced copper at US$0.40/lb.’ At Gunnison there is a indicated resource of 3.21 billion pounds (1.46Mt) contained in 511Mt of oxide copper grading 0.31% Cu that is amenable to in-situ leaching. The area is well served by roads, power and railway, yet the property is not occupied or farmed. Hydrological tests have revealed that its North Star deposit on the property is suitable for in-situ leaching and that a circulatory regime can be established.
The copper is mainly in the form of chrysocolla, a hydrous copper silicate, with subsidiary malachite and azurite. Oxidation continues to a depth of 1,600ft, forming a flat-lying tabular mass that reflects a former water table. The mineralisation is fairly uniform, with occasional pockets of higher grades up to 1.3% copper, and occurs as a coating between the broken rock with little value within fragments.
Excelsior has carried out preliminary hydrological research in this complex terrain. The mineralised units most suitable for in-situ leaching are the Abrigo and Martin formations, a series of consolidated shales and dolomites that were intruded by the Texas Canyon quartz monzonite and metamorphosed to a skarn (calc-silicate rocks). The contained chrysocolla is leachable as long as a pH of the lixiviant is below 2.0, similar to that in copper heap-leach operations. In test work on large core samples, 89% of the contained copper was leachable, but acid consumption was high, reflecting the carbonate content. As dissolution of the copper begins, other gangue minerals associated with the fractures are also mobilised, especially calcite when sulphuric acid is used. This could result in the deposition of gypsum within the fracture zones.
Over the past 40 years a number of companies have carried out metallurgical tests on ore from the North Star deposit. Most results indicated acid consumption of 10-12 pounds per pound of copper produced. However, tests by Magma Copper indicated that sulphurous acid (H2SO3), made by mixing SO2 gas and water, reduced acid consumption to 8-9 pounds per pound of recovered copper. In 2007, these tests were completed by Hazen Inc, a US company specialising in leaching. Its results indicated that, although sulphurous acid did not attack chrysocolla as aggressively as sulphuric, it eliminated the secondary precipitation of gypsum by not having the sulphate ion in the lixiviant.
The regional hydrological regime is complex, reflecting the faulting and fracturing with potential vertical pathways. Overlying the mineralised rock is semi-consolidated alluvium where the regional water occurs, rendering the underlying formations saturated. Groundwater recharge occurs to the west of the mineralisation along the contact of the monzonite outcrop and alluvium, and moves west to east above and through the mineralised zone.
In trials by the company, a number of boreholes have established a method for containing the circulating lixiviant. It is based on differential water pressure stimulated by the drilling of five holes in a square pattern with a central injection point. The remaining bores are pumped at a rate greater than that of injection, thus causing negative pressure, sucking in surrounding ground water and isolating the lixiviant from the aquifer. It will result in extraction exceeding injection rates and require neutralisation of excess water. The lack of habitations or stock on the property and surrounding area is one of the factors in granting the necessary permits for in-situ leaching.
Logistically the roads and railway in the area reduce the infrastructure cost of the project. Acid generation will be the major consideration, and in past operations have amounted to as much as 40% of overall costs. The railway links the project to the west coast oilfields of California, and their by-product (sulphur) can be purchased to make cheap sulphuric acid on site. Twyerould explains, ‘The project has the potential to recover copper at significantly lower capital and operating costs compared to conventional mine and leach operations. We are encouraged by the metallurgical recovery rates and look forward to progressing the leaching metallurgy undertaken by the company. This project has the potential to transform copper mining in Arizona and beyond.’
Stephen Twyerould, 1240-1140 West Pender Street, Vancouver, BC, V6E 4G1, Canada. Tel: +1 (604) 681 8030.