copper ore crushing, grinding & flotation
You will note that the Oracle Ridge project has utilized a two-stage crushing circuit with a double acting jaw crusher and cone crusher. In order to utilize this system, the jaw crusher is oversized to produce all -5 material for the cone mill with a screen in closed circuit. Normally a three-stage crushing circuit would have been more conventional, but with the type of rock processed, its crushing characteristics and its . high bulk density, this two-stage system should work out well.
Conventional crushing and grinding plants are safe and conservative. The ore can be tested by proven techniques and crushers and mills selected with assurance that they will meet performance requirements. Operation of crushing plants, rod mills and ball mills is understood by many, and most operators are comfortable with the use of this type of equipment which has been around for over 70 years.
Autogenous and semi-autogenous grinding is still quite new and mysterious to many, although there have been over 276,000 connected horsepower sold into the copper industry. There are few who understand completely the application of these mills or their operation. Indications are that these type mills are not often selected due to this lack of understanding and concern about misapplication. Yet when properly applied, these mills can offer economics which might mean the difference between profitable or unprofitable operation.
The attraction of semi-autogenous grinding in copper operations is that they can accommodate ores that are hard or soft, wet or dry, sticky or otherwise. The SAG mill can handle everything that is presented to it, regardless of coarseness or fineness, or hardness or moisture content, and from it prepare satisfactory feed for subsequent secondary grinding by conventional means to the required flotation product sizing. A parallel point not to be forgotten is that the SAG mill circuit completely eliminates fine crushing, screening and binning of sometimes wet and sticky ores, generally regarded as the most disruptive and awkward operations in a conventional concentrator.
The only published data giving valid economic comparisons between conventional and SAG milling is Pimas 1973 paper. They had the unique opportunity of processing the same ore both conventionally and in a semi-autogenous circuit and were able to compare capital and operating costs. The capital cost analysis at the time of the expansion showed 53,000.00 per ton for the conventional plant and $2,000.00 per ton for the SAG plant. Their operating cost comparison shows a ratio of 100 versus 88.9 for conventional versus SAG. We understand that prior to the plant shutdown in 1977, that the comparison was 100 versus 80.
Hundreds of pilot tests have been run on a wide variety of ores. Although new applications on new ores ideally should be studied by a pilot plant test of a 50-ton sample, if the ore resembles ore previously tested, it is possible to make confident mill selections on very small ore samples. No one has developed an autogenous mill selection method comparable to the Work Index Method for selection of rod mills and ball mills. We do not anticipate a simple index type system applying to these type mills because it is not that simple to describe ore characteristics.
Fine ore at minus 19mm () sizing is fed at a controlled rate into the open-circuit 2600mm x 3960mm (8-6 x 13-0) Hardinge rod mill at an average feed rate of 2,106 STPD or 87.75 STRH. Rod mill discharge at a nominal size of minus 16 mesh and a pulp density of 75 percent solids combines with ball mill discharge and is pumped to a bank of three Wemco hydrocyclones, two operating and one standby. Cyclone underflow at 74% solids gravitates to the 3500mm x 4600mm (11-6 x 15-0) Hardinge ball mill and overflow goes into the flotation circuit. A Denver two-stage sampler is installed in cyclone overflow launder.
Process water is added under ratio control to the rod mill feed and additional water is added to the primary cyclone feed, while provisions also exist for water addition to the feed end of the ball mill. A second source of water to the grinding section is derived from fresh water to the crushing and ore storage dust collection systems when operating.
Flotation consists of one stage of rougher flotation of three cells, three stages of rougher scavenger flotation and one stage each of cleaner (3 cells), recleaner (2 cells) and cleaner scavenger flotation (3 cells).
The regrind section comprises a 2130mm x 3660mm (7 -0 x 12-0) Hardinge regrind ball mill in closed circuit with a pump and cyclone. Feed to the regrind section has a nominal sizing of 55 percent minus 325 mesh while the regrind cyclone overflow product, which is returned to the cleaner flotation section, has been reduced in size to 90 percent minus 325 mesh. The 80 percent passing sizes for feed and product are 74 microns and 32 microns respectively.
semi-autogenous grinding of copper ores
The spectrum of comminution mechanisms in a mill range from shattering of the rock by imposition of a load to abrasion by surface contact with other rocks. Since SAG represents a combination of autogenous and ball milling, a better understanding of the mechanism of the comminution process in a SAG mill can be obtained by examining first the information available on autogenous grinding.
Where D0 represents a size modulus representing the initial size of the particles, and n corresponds to the inclination (n = tan) of the cumulative particle size distribution in the mill after comminution (plotted on a log-log scale).
The pilot tests in the 10-ft. mill showed that when the autogenous mill was fed with a narrow fraction of particle sizes, ranging from 150-mm. to 40-mm, after comminution the inclination factor n ranged from 1.0 for the larger to 1.7 for the finer size particles.
By comparing these values of n with the value of k, which according to the juxtaposition of equations (1) and (2) would be associated with pure surface abrasion, Bergstedt and Fagremo concluded that the mechanism of comminution in an autogenous grinding mill depends principally on impact type of breakage. When a stone enters the mill, corners and sharp edges are quickly broken off and from there on the stone is reduced in size by abrasion, until impact with the liner or other stone breaks it up to smaller pieces and the polishing off is repeated.
As in the case of autogenous grinding, the comminution process occurring in a SAG mill is also a combination of impact and abrasion. Operating conditions, which favor abrasion, for instance low loading of small size balls, may lead to high wear of balls and liners and result in operational problems due to the buildup of coarse material in the mill and the grinding of the feed to extra fine particles, at a relatively high energy consumption. Impact breakage, on the other hand, yields a wide distribution in product sizes and minimizes energy consumption, but unless it is properly controlled it may result in cracking of the mill liners. A proper balance of the two mechanisms is, therefore, of the utmost importance. This depends on the proper design of the factors discussed below.
The length and diameter of a SAG mill are determined principally by the desired throughput capacity and the grindability of the ore. For conventional ball mill grinding, grindability is usually expressed in terms of the Bond Work Index, which is determined in laboratory tests and expresses numerically the power required to reduce rock from a large size (e.g. >10-inch diameter) to 80% passing 100 microns.
For conventional ball milling, the Operational Work Index can be related to the Bond Work Index by the introduction of efficiency factors as described by Rowland. To date, the authors have been unable to draw a corresponding relationship between Bond Work Index and Operational Bond Work Index for semi-autogenous grinding plants. The inability to do so appears to be related to rock competency, i.e. the fact that the work index is not constant over the range of size reduction under consideration. Hence, it is believed that the sizing of SAG production mills can only be done by extrapolation of performance data from carefully conducted pilot test work.
The critical speed of the mill is termed to be that at which the centrifugal force created by the rotation of the mill becomes equal to the force of gravity and, therefore, the material within the mill ceases to tumble.
Through operating experience, it has been established empirically that autogenous and SAG mills operate best at a speed of rotation equal to 72-76% of the critical speed. The mill speed is fixed during design and there are no SAG plants which attempt to use speed of rotation as an operating parameter. However, some Swedish fully autogenous mills are so equipped and recently Afton Mines in British Columbia commissioned an autogenous mill with variable drive.
Most SAG operations use volumetric ball charges, i.e., % of mill volume occupied by the balls including interstitial volume, of 6-10% depending on the work index of the ore to be milled and the desired throughput capacity. In addition to varying the volumetric ball charge, operators may also alter the mix of balls , depending on the work load to the mill.
For instance, at Lornex Mining, one of the most successful SAG installations, the ball load varies normally from 6-7% by volume of the mill; the ball make-up consists usually of 80% 4-inch diameter balls and 20% 5-inch diameter balls. The ratio of the larger balls is increased when the mill is processing hard ore and decreased with soft ore. The ball load can also be used to shift work from the primary to the secondary mill; a greater load of large balls results in increased impact breakage, and may produce a coarser discharge from the SAG mill.
The power input required to rotate the empty mill is termed tare power and consists of the energy for rotation plus the mechanical and electrical losses in the drive system. The cross power to the mill is the sum of the tare power plus the energy required to move the pulp within the mill.
One of the conclusions reached very early in Kennecotts investigation of autogenous and semi-autogenous grinding was that, though conventional rod and ball mill grinding circuits can be designed from bench scale test data, the design of SAG circuits required pilot testing to develop design parameters. The principal reason was that the Kennecott ores varied widely in competency, and, therefore, in their capability to act as grinding media; even within a given mine, wide variations were found between rock from different locations. This variability was sufficient to rule out the application of fully autogenous grinding and to require detailed pilot testing and evaluation of projected SAG application.
Test the various rock types to be encountered in an actual mining operating, determine the Operational Work Indices for the primary and secondary mills, and correlate Operational and Bond Work Indices as an assessment of grinding efficiency.From the data for the individual ore type samples, calculate the operating work index for grinding average mine ore in a SAG circuit and the variation in work index to be expected because of the differences in competency and work index of ore as delivered to the concentrator by the mining operation.
The throughput capacity of the primary SAG mill exceeds the capacity of the secondary mill and also, as expected, varies with the characteristics of the particular ore type ground and the opening of the recycle screen. Therefore, in order to match the SAG mill throughput to the secondary mill capacity, the screen undersize is split into two streams, the first used to fully load the secondary ball mill and the remainder discarded.
grasberg open pit copper mine, tembagapura, irian jaya, indonesia - mining technology | mining news and views updated daily
Grasberg mine has the single largest known gold reserve and the second largest copper reserves in the world. It is located 96km north of Timika, at Tembagapura in Irian Jaya the most easterly of Indonesias provinces on the western half of the island of New Guinea.
Grasberg minerals district includes open-pit and underground mines. It has produced 528 billion ounces of copper and 53 million ounces (Moz) of gold, including more than 432 billion ounces of copper and 46Moz of gold from the Grasberg open pit during 1990 to 2019.
Extraction of ore from the Grasberg Block Cave underground mine commenced in the second quarter of 2019, which is the same ore body mined from the surface in the Grasberg open pit. The mining of the final phase of the Grasberg open pit was completed in Q4 2019 and the mine transitioned from open-pit mining to large-scale underground mining.
Freeport McMoran and Rio Tinto sold a majority interest in Grasberg to Indonesias state-owned aluminium mining company, Indonesia Asahan Aluminium (Inalum). Prior to the $3.85bn transaction, Freeport held a 90.64% stake in the mine operation while Rio Tinto had a 40% participating interest. Under the deal, the government agreed to provide a special licence to Freeport to enable the company to continue the extraction of ore until 2041. To support the mine and its workforce, PT Freeport has built an airport, a port at Amamapare, 119km of access road, a tramway, hospital, housing, schools and other facilities.
The mine stands at the collision of the Indo-Australian and the Pacific tectonic plates. Two distinct phases of intrusion have led to the production of nested coaxial porphyry ore bodies and sulphide-rich skarn at the margins, while sedimentary strata include Eocene clastic carbonate limestone with siltstones and sandstones near the base.
The second intrusive stage, the Main Grasberg Stock (MG), is composed of non-fragmental, porphyritic monzodiorites, forming a quartz-magnetite dilational stockwork with veinlet-controlled copper-gold mineralisation. This is a high-grade resource, with averages of 1.5% copper and 2g/t gold.
The workings comprise an open-pit mine, an underground mine and four concentrators. The open-pit mine, which forms a mile-wide crater at the surface, is a high-volume low-cost operation, producing more than 67 million tonnes of ore and providing more than 75% of the mill feed in 2006.
Designed to be fully mechanised, using 6.2m Caterpillar R1700 load-haul-dump vehicles (LHDs) at the extraction level with a truck haulage level to the gyratory crusher, the Deep Ore Zone (DOZ) block cave mine is one of the largest underground operations in the world.
After 2004, when the DOZ mine averaged 43,600tpd, a second underground crusher and additional ventilation were installed to increase daily capacity to 50,000 tonnes. Ramp-up production has begun at the Deep Mill Level Zone (DMLZ) underground mine, which lies below the DOZ underground mine and to the east of the Grasberg ore body.
Production equipment includes 30m42m buckets, a 170-strong fleet of 70t330t haul trucks, together with 65 dozers and graders, with radar, GPS and robotics used in the mines state-of-the-art slope-monitoring system.
The ore undergoes primary crushing at the mine, before being delivered by ore passes to the mill complex for further crushing, grinding and flotation. Grasbergs milling and concentrating complex is the largest in the world, with four crushers and two giant semi-autogenous grinding (SAG) units processing a daily average of 240,000t of ore.
A flotation reagent is used to separate concentrate from the ore. Slurry containing 60-40 copper concentrate is drawn along three pipelines to the seaport of Amamapare, more than 70 miles away, where it is dewatered. Once filtered and dried, the concentrate containing copper, gold and silver is shipped to smelters around the world.