HydroFloat® Reduces Energy and Water Usage
Cozamin, a polymetallic sulfide mine with pay-metals of copper, zinc, lead and silver, is located in Zacatecas, Mexico and operated by Capstone Mining Corporation. The mill processes 3800 tonnes/per day, which is crushed in two stages and split between two parallel 12 foot diameter x 14 foot length ball mills, each receiving about 80 tonnes of fresh solids feed per hour and consuming about 800 kW of power. Both mill lines recombine and feed two parallel flash flotation cells, followed by conventional rougher flotation and then a flowsheet with re-grinding of rougher concentrate and selective flotation and production of separate concentrates of copper, lead and zinc. The rougher recoveries for the main pay-metals of copper, zinc, lead and silver are 95 percent, 70 percent, 50 percent and 80 percent, respectively. The final tail of this plant has a particle size of mostly less than 230 microns and is thickened and pumped to a conventional tailing impoundment at 55 percent solids by weight.
To evaluate the Tail Scavenging (TS) and Coarse Gangue Rejection (CGR) applications using the same ore body and mine, two 50 kg samples were taken from the Cozamin flow-sheet in 2016; one from the final tail and the other from the ball mill cyclone underflow.
Samples from each location were prepared and floated using an Eriez® 150 mm diameter lab HydroFloat® unit. The Eriez HydroFloat uses a fully developed steady state liquid fluidized bed which requires some limit to the polydispersity or spread of the feed size distribution. For this reason, any CPF plant based on the HydroFloat technology will involve some size classification of the feed. In this test work, the fines below 160 micron and the coarse component above 700 microns are removed using two stages of screens. The pass-band material (+160 micron/-700 micron) is fed to the HydroFloat. In commercial applications, HydroFloat feed preparation can utilize cyclones, screens, the Eriez CrossFlow hydro-sizer or any combination of this conventional classification technology.
Applying the two stages of screening, each sample is split into three size classes: below 160 microns, between 160 and 700 microns (HydroFloat feed) and greater than 700 microns. The corresponding mass splits for each application are shown in Table 1. The third column indicates the mass fraction of the total feed that is suitable for flotation in the HydroFloat.
Table 1. HydroFloat feed preparation and mass splits
|Flotation sample||Percent of -160 micron removed [% of unscreened feed]||Percent of +700 microns removed [% of unscreened feed]||Percent of feed remaining after 2-stage screening -this is the HydroFloat feed
[% of unscreened feed]
|CGR (Mill CU)||30%||20%||50%|
|TS (Final Tail)||70%||0%||30%|
The overall metal recoveries in the HydroFloat are shown in Table 2. In Table 3, the 80th percentile of the size distribution (d80) is used to characterize the size distribution for each sample.
Table 2. Overall HydroFloat pay-metal recoveries
|Flotation sample||Copper (%)||Lead (%)||Zinc (%)||Silver (%)|
|CGR (Mill CU)||95.0-95.3||80.9-90.0||85.0-86.8||87.0-90.0|
|TS (Final Tail)||86.4||63.9||77.4||88.3|
Table 3. HydroFloat feed and tail size and mass splits
|Flotation sample||d80 HydroFloat feed (m)||d80 HydroFloat tails
|HydroFloat Mass pull
|CGR (Mill CU)||500||560||30.9|
|TS (Final Tail)||330||375||9.4|
The TS results show a significant incremental recovery of metal units that are lost to final tails. The CGR results show recoveries of all pay-metals, which are comparable to conventional flotation.
There are a number of opportunities for studying how the CGR results can be used to optimize the conventional mill circuit shown in Figure 1. As an example to illustrate the concept, two-stage screening of the ball mill cyclone underflow stream could be introduced as a way to scale up the experimental CGR results presented in the previous discussion.
In this configuration, the -160 micron fine fraction, which is normally misplaced in the cyclone underflow, can be sent to conventional flotation while the coarse +700 micron fraction can be returned to the mill, thereby reducing the mass but increasing the size of returning ore. The pass-band ore, (consisting of +160 micron/-700 micron) could be fed to the HydroFloat. The results shown in Table 2 and Figure 2 indicate that the HydroFloat would produce a bulk float concentrate with comparable recoveries as the conventional circuit in this scenario. With a CGR circuit installed, a significant portion of the final plant tail can be produced as a coarse final tail with a size of about 560 microns, with the remaining portion being produced as a fine final tail with a size of about 230 microns.
Figure 1: Conventional closed ball mill circuit with circulating load
To quantitatively evaluate the effect of the CGR flowsheet configuration on the power and production of coarse and fine tails for the same mill capacity, a JKSimMet model was built and calibrated against the plant data from the mill circuit shown in Figure 1. Then two models were run; one as shown in Figure 1 and one as shown in Figure 2. In both cases they received the same amount of fresh feed. JKSimMet is a steady state process modelling software which has models for standard mineral processing unit operations. This package allows each solid stream to be modeled as a family of size classes and the equations of mass continuity are solved for total mass as well as size using a population balance approach. The conventional circuit shown in Figure 1 uses a ball mill model and cyclone model as the main unit operations, and the CGR circuit adds two stages of screens and a HydroFloat.
Figure 2: Hypothetical mill circuit reconfiguration to allow the CGR concept to be practiced
SUMMARY AND CONCLUSIONS
The results of this study have been combined in Table 3. Comparing with the base-case, the main quantifiable benefits of the Tail Scavenging option are in the capture of additional metal units. Optionally, there may be the possibility to increase throughput, but this is difficult to quantify properly since it requires knowledge of the interaction between conventional flotation and HydroFloat flotation. The size classification required for the HydroFloat process will allow of degree of sizing, which would be comparable to conventional sand cycloning.
Table 3: Estimates of the benefits of TS and CGR CPF configurations at Capstone’s Cozamin
|Circuit||Increase in plant recovery Cu recovery [%}||Reduction in ball mill power [%]||Reduction in float capacity [%}]||Reduction in impoundable fine tails/ water [%]|
|Base-case with TS||See Table 3||—||—||?|
In the case of the Coarse Gangue Rejection application, there is the possibility of an increase in overall recovery, but this cannot be quantified at this level of study. A combination of experimental and simulation results indicate that placing the HydroFloat in the mill circuit would result in a significant decrease in the mill load, resulting in a power decrease of 50 percent. It also shows that approximately 30 percent of the total run of mine mill feed could be removed from the circuit prior to conventional flotation, resulting in a decrease of conventional flotation capacity by 40 percent and reduction in tailing management costs of 30 percent. Some typical North American numbers were provided to show the financial benefits of these improvements. At sites that are more remote, or where water is scarcer, these results could be even more significant.
The HydroFloat CPF technology creates new possibilities for extraction companies and engineers to renew their social contract with society by improving the efficiency and minimizing the detrimental impact of mining. This study shows the possibility for one new unit operation to dramatically reduce energy, impoundable tailings, water and plant requirements