«2012 Americas School of Mines Basics of Mining and Mineral Processing W Scott Dunbar University of British Columbia Agenda Geological Concepts ...»
2012 Americas School of
Basics of Mining and Mineral Processing
W Scott Dunbar
University of British Columbia
Mineral Processing Methods
Mine Waste Management
Mining and Money
A Future of Mining
Mining Methods: The Topics
Open pit mining
PwC Mining Methods 3
Open pit mining
Generally low grade, shallow orebodies
Mining rate 20,000 tonnes per day Tailings pond Concentrator or Processing plant Waste dump Marketable product Waste Ore Possible expansion PwC Mining Methods 4 Open pit mining Open pit mines are used to exploit low grade, shallow ore bodies. However, some pits are quite deep – about 1 km.
The mining rate is greater than 20,000 tonnes per day (tpd) but is usually much greater.
Some pits operate at a rate of more than 100,000 tpd Open pit mining results in two waste streams: waste rock which contains no economic quantity of minerals but which must be removed to gain access to the orebody, and tailings which are the result of a mineral separation process in the concentrator or processing plant. The mining rate includes the mining of both waste and ore.
Open pit mining is non‐selective – all high and low grade zones of the orebody are mined The significant design issues of an open pit mine are: location of haul roads, equipment – size of trucks and fleet, pit slope angle and stability, control of water PwC Mining Methods 5 Bagdad Pit, Arizona – looking west Porphyry copper 170,000 tpd 9,200 ft 5,800 ft pit 828 Mt sulfide and oxide reserves: 0.28% Cu, 0.022% Mo (2007 10K filing) Copper and Molybdenum concentrates Pressure leach facility for concentrate, SX/EW leach plant for oxides PwC Mining Methods 6 Notes: Bagdad Pit Arizona – looking west First claims staked in 1882. Property changed ownership numerous times through first half of 20th century. First mill began operation in 1928 to process ore from the underground mine. Transition to open‐pit mining began in 1945. A $240 million expansion in 1973 included new haul trucks, shovels, nearly 400 housing units and concentrator. Bagdad became part of the Phelps Dodge mining portfolio in 1999 with the acquisition of Cyprus Amax Minerals Co. In 2007, Freeport McMoran merged with Phelps Dodge.
The ore is a porphyry with disseminated primary sulfides (chalcopyrite and molybdenite) with gold and silver. Low grade secondary sulfide and oxide ores are present which are soluble by acid.
Copper and molybdenum concentrates with gold and silver credits are produced and smelted at the Miami smelter. Copper is produced at a SX/EW plant for oxide ore (operating since 1970) and at a new (2003) concentrate leach facility, the world’s first.
Porphyry copper, 170,000 tons mined/day (tpd) 451 Mt reserves: 0.38% Cu, 0.007% Mo (2007 Teck Cominco report) 90% of production from Valley pit Copper concentrate with gold and silver, Molybdenum concentrate PwC Mining Methods 8 Notes: Highland Valley Pit British Columbia The Highland Valley Copper mine, located in British Columbia, 60 km southwest of Kamloops is 97.5% owned by Teck Cominco.
Operations at Highland Valley began over 20 years ago by predecessor companies. The present operation is a combination of the Lornex mine and mill, the Valley Copper orebody and the Highmont mine and mill. The present mill is the old Lornex mill into which Highmont mill equipment was integrated.
The Valley pit contains two in‐pit crushers feeding a 12,000‐tonne per hour conveying system that delivers ore to stockpiles at the mill. Ore from the Lornex mine is hauled to two permanent crushers with discharge to common stockpiles.
Concentrates are transported by rail eastward to domestic markets and to Vancouver for shipment to overseas markets.
The life of the mine has been extended from 2013 to 2019. The new mine plan will require a push‐back of the west wall of the Valley pit. Total capital costs of the expansion project are estimated at $300 million, including $130 million for capital equipment and the balance in pre‐production stripping over the period of 2009 through 2013. Approximately $50 million of mobile mining equipment will be ordered in 2007 to permit waste stripping to commence in 2009.
Source: Teck Cominco 2006 Annual Report
Coal and tar sands operations typically involve moving large amounts of waste (often called overburden) to gain access to the economic mineral.
Eagle Mountain is part of the Fording River operations in southeastern British Columbia.
Fording River produces both metallurgical (coking) coal for the steel industry and thermal coal for power plants in Alberta. Fording River's measured and indicated reserves total over 200Mt of clean coal plus a further 286Mt in resources, over 65% of which is contained in the Eagle Mountain deposit. The coal has a low sulfur content and its volatile content ranges from medium to high. Three distinct coking coal types are available at Fording River. Fording River can mine at a rate of 10Mt/year or about 28,000 tpd.
Source: http://www.mining‐technology.com/projects/fording Oil sand is a mixture of bitumen (a thick sticky form of crude oil), sand, water and clay. The Suncor Mines extract oil sand from mines north of Fort McMurray in Northern Alberta.
Using shovels with 100 ton buckets and 240 and 360 ton trucks the mine extracts about 450,000 tonnes of oil sands per day. The material is crushed and sized and made into a slurry which is delivered to the processing plant via 86 km (54 mi) of pipelines In the plant, the sands are mixed with hot water to separate the oil from the sand. In 2005 the Suncor Mines produced 171,300 barrels of bitumen per day, which, after upgrading, is ready for refining. Suncor’s leases contain a resource of more than 13 billion barrels of bitumen.
Source: www.suncor.com PwC Mining Methods 14 March 2, 2012 Pre-stripping at Oyu Tolgoi, Mongolia
An open pit mine is developed as a series of nested pits, each larger in area than the previous pit. A pushback is the removal of material required to proceed from one pit to the next. The revenue from the ore must pay for the cost of excavating the waste from the pushback and for excavating the ore. But the slope cannot exceed 45˚ and remain stable so at some point it becomes impossible and/or uneconomic to continue mining.
The slopes of a deep pit are cut into a series of locally steep slopes ( 450) each about 5‐15 meters high depending on the stability of the rock and the equipment in use. Such steep slopes can become unstable and therefore benches are formed at the bottom of each slope to contain any slope failures. Although parts of the slope are steep, the overall slope angle is low, say 30‐400.
In some pits the rock may be strong enough to allow “double‐benching” where slopes about 20‐30 meters high are built. The available equipment must be able to excavate such heights. The objective is to minimize waste excavation, but design and monitoring of such slopes can be difficult.
The need for locally steep slopes is illustrated by the following example. For a 500 m deep pit, the difference in volume for a pit slope of 45 slopes and a pit slope of 40 is about 25 million m3. If the rock density is 2.7 t/m3, that is equivalent to 67.5 Mt. Since it costs $2‐3 to move a tonne of rock, the extra volume amounts to quite a large amount of money.
Notes: Straight faults and circular holes As the magma containing the minerals for an orebody rises up, it generates stresses in the host rock, rupturing it and causing faults. Thus most orebodies are related to faulting in the earth’s crust. Faults are long linear features and so if an orebody is mined with a circular pit, it is likely to intersect a fault. This can lead to instability in at least two parts of the pit slope.
In the case of the Homestake Pitch uranium mine, pit excavation near the fault on the northeast slope led to a series of slope failures soon after mining started in 1977 and continuing through to 1980. In 1983 extreme climatic conditions led to an excess accumulation of water which weakened the northeast slope and led to the failure shown in the picture. The mine was placed under reclamation soon after the 1984 failure.
Cremeens, J., 2003. Geologic controls on complex slope displacement at the Pitch reclamation project. Engineering Geology in Colorado, Contributions, Trends, and Case Histories. AEG Journal
There are 40 of these around the Cortez pit pumping water out of the ground at a total rate of 30,000 gallons per minute in order to keep the pit dry. Dewatering also helps to keep the slopes dry and more stable.
Applies to an open pit mine Waste Strip ratio = Ore SR is the mass of waste to be mined to obtain one unit mass of ore. (ore that goes to concentrator)
Oil sands deposits in Alberta: SR = 1.0‐1.5 Highland Valley: SR ~0.45 (2007 Teck annual report) Bagdad mine: SR =1.4 (2007 FCX 10K filing, p.5, but oxide ore also mined – see notes) Red Dog mine: SR = 0.8 Cortez mine: SR = 2.2 (2007 Barrick annual report) PwC Mining Methods 26 Notes: Strip Ratio Mistakes are often made when computing the strip ratio. It’s all in the words and you have to watch the flow of material and how it is classified. For example, according to Freeport McMoran’s 2007 10K filing, the concentrator is capable of processing 75,000 metric tons per day of primary sulfide ore and the mining fleet is capable of moving 180,000 metric tons per day. Thus If everything is working to capacity (and it usually works close to capacity) and the material moved is assumed to be waste plus primary sulfide ore, the strip ratio is (180‐75)/75 = 1.4.
However, the material moved includes waste plus oxide/secondary sulfide ore and primary sulfide ore. The 10K filing and other information provided does not classify the amounts mined. However, the 2005 Phelps Dodge report states that Bagdad mined 64,093 thousand tons (kt) of material and processed 26,592 kt in the concentrator. The report also states that 23,857 kt of ore (oxides/secondary sulfides) was placed on the leach stockpiles. This means the waste is 64093‐26,592‐ 23,857 = 13,644 kt, about 21% of the material mined. So does this mean their strip ratio, waste/ore = 0.21/0.79 ~ 0.27 Not so fast. According to footnote h on page 11 of the 2005 report the leach ore includes “previously considered waste material that is now being leached.” This means that some leach ore was mined prior to 2005 and was re‐classified from waste to ore. The report does not state how much was mined in 2005 and placed on the leach stockpile but, according to a contact at Bagdad, the amount mined was minimal – the 23,857 kt is mostly a re‐classification.
Hard to define a strip ratio when what was waste becomes ore and vice versa.
During production, stripping costs may be either capitalized or expensed. If capitalized, the amortization of the costs is based on the estimated value of the underlying ore and the amortization schedule will change over the life of the mine as the value of the underling ore changes. If expensed, production costs per ton of ore will vary over the life of the mine as the amount of ore produced changes.
The pushback of the west wall of the Valley pit at Highland Valley, BC will provide access to additional ore. The waste stripping associated with the pushback will be capitalized and amortized based on the estimated value of the additional ore. However, the amounts of waste and ore are subject to a number of uncertainties and could change over time. If the amount and/or value of ore changes, the amortization schedule will have to be changed.
The cost of the north wall pushback at Bagdad is treated as a cost of current production and is not related to the ore underlying the pushback. The cost per ton of ore will therefore change as the current strip ratio changes. In effect, Bagdad is buying an option on the underlying ore, the value of which is uncertain.
Underground mines are used to exploit high grade, deep ore bodies. However, there is no limiting grade above which underground mining is always done, nor is there a limiting depth below which underground mining is always done. It depends on the mining method used.
For underground mines the mining rate is typically less than 20,000 tonnes per day (tpd);
10,000 tpd is a large capacity (and highly mechanized) underground mine. However, the block caving method can achieve mining rates much greater than 20,000 tpd.
Underground mining results in one waste stream: tailings which are the result of a mineral separation process in the concentrator or processing plant. There is very little waste rock generated as a result of sinking the shaft or driving the tunnels to gain access to the ore.
Underground mining is generally more selective than open pit mining, but the degree of selectivity depends on the underground mining method.
The significant design issues of an underground mine are: geometry of underground mining, ground support, and logistics of materials handling.
In general for underground mines:
Small output mines (4,000 tpd) ‐ hauling is done on several levels, tonnage handled on each level is small, and light equipment is used. High output mines (4,000 tpd) ‐ a main haulage level is used and all the ore is dropped to that haulage level via ore passes.
A level includes all the horizontal workings tributary to a shaft station. Ore excavated in a level is transported to the shaft to be hoisted to the surface.