The coarse particle recovery process
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The coarse particle recovery (CPR) process captures coarse particles that are not recoverable by regular flotation. Several porphyry copper operations in the world operate analogous circuits that recover non-floated copper from tailings; however, the case presented here for Minera Escondida Ltda. is unique in that extensive underlying test work has allowed the authors to elucidate some fundamental principles behind recovery enhancement.
The concept is to refloat the coarse fraction of the rougher tailings, a fraction that contains about 2/3 of the copper in about 1/3 of the tailings mass. The concentrate from this stage will be reground and cleaned in a conventional copper cleaning circuit as shown in the figure. The process will treat an average 0.18% copper over the life of the project, producing a total of 3,470 kt (dry basis) of concentrate with an average grade of 3% copper. With this process about 35% of the copper in the rougher tailings stream is expected to be recovered.
When the tailings are cut at the optimum size of 106 μm, the overall enrichment ratio in the coarse fraction is typically in the order of 1.6 to 1.8. At a rougher tailings grade of 0.2% copper, the resulting CPR feed will therefore range from 0.36% to 0.38% copper. Coarse separation of tailings upgrades copper relative to iron. The phenomenon is important, since the coarse copper in tailings could not be recovered in the conventional rougher flotation circuit without simultaneously recovering unacceptable quantities of fine pyrite.
Virtually no liberated copper is present in the coarse cyclone underflow (p80 of about 300 microns). The role of scavenger flotation in the CPR circuit is to establish the most favourable conditions for the recovery of these coarse (heavy) particles that have relatively low percentages of exposed hydrophobic surfaces. Such particles are extremely sensitive both to shear (detachment of particles from bubbles in the pulp) and to drop-back from the flotation froth. It should be noticed that the flotation conditions outlined here are different from the common concept of “floating harder.” A typical operator strategy for a scavenger circuit is to increase airflow in an attempt to achieve more aggressive flotation. In this particular operation, increased air raises shear, lowers particle suspension, and requires deeper froth at any given mass transfer rate in order to avoid cell flooding. All of these actions would dramatically lower the recovery of coarse, locked particles. Thus, some operator re-education is required regarding what constitutes “floating smarter” for this particular system. The successful transfer of these concepts to the operations level is a fundamental prerequisite for success of the overall project.
Flotation concentrate and tailings grades both show a predictable linear relationship with the feed grade, scaling upward as a function of the underflow grade. The quantitative relationships are:
c = 3.92 f + 0.682t = 0.191 f + 0.079
It is important to apply strong frothers that are effective in recovering coarse particles. One such reagent is Cytec’s F507, a strong polyglycol. Besides frother, sufficient collector (Cytec 3477 was selected) must also be metered into the pulp to ensure maximum recovery.
The approximate liberation size of the sulphides in CPR scavenger concentrate is equivalent to a p80 of ~35 to 45 μm (70% to 80%, -400#), which is finer than for most regrind applications in copper flotation. This requires a regrind energy of approximately 19.8 kWh/mt (at a BMWI of ~22), corresponding to the equivalent of about 0.8 kWh/t of original ore.
The cleaning circuit is conventional, consisting of a mechanical first cleaner followed by column upgrading. In terms of grade, the first cleaner of the CPR is analogous to the conventional rougher of the main plant, taking a feed of ~1% to 2% Cu and upgrading it to ~15% Cu. This upgrade ratio is clearly the result of liberation realized during regrinding of the scavenger concentrate. As expected, a rising cleaner feed grade will produce an improved first cleaner concentrate grade. The first cleaner tailings are a discard stream. Because the products recovered in CPR tend to originate as particles locked with silica rather than pyrite, the contaminants in the CPR concentrate include more silica and less pyrite at any given copper grade than one would normally encounter in concentrate from the main circuit.