Crushing and Grinding
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.
Autogenous, semi-autogenous, and conventional rod mill-ball mill grinding were considered:
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 Pima’s 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.
The leading suppliers of SAG type mills are building up experience in the testing of ores, sizing of machines and in proper operation.
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.
Because of lack of time and an adequate supply of sample material to do the necessary testing required to select a SAG or AG circuit, O.R.M.P. decided to select the conventional grinding circuit.
Grinding and Flotation
Fine ore at minus 19mm (¾”) 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.
The nominated circulating load in the grinding circuit is 300 percent and the mesh of grind 70 percent minus 200 mesh.
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).
Retention times in the individual flotation stages are as shown in Figure 5, Flotation Stage Retention Times.
All three rougher scavenger concentrates and cleaner scavenger concentrates gravitate to the regrind section while cleaner scavenger tail is pumped to the No. 1 rougher scavenger bank.
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.