Impact of air distribution profile on banks in a Zn cleaning circuit

Abstract Introduction

A campaign during the greater part of 2001 at Noranda’s Brunswick mine concentrator investigated the role of gas (air) distribution to the cells in the final Zn cleaning stage. Air-flow was measured as the superficial gas rate (Jg, i.e. the volumetric flow rate per unit cell crosssection) using the Jg sensor designed by the McGill mineral processing group. The distribution is referred to as the “Jg profile.” The final stage comprises two parallel banks of seven Denver DR 100 cells, one bank being the “manipulated” the other acting as the “control.”

Test Program

The first step was to establish the operating range of each cell. This was accomplished by measuring gas holdup (using another sensor designed by the group) as a function of gas rate. This range was respected when designing the profiles. On the manipulated bank, a three-level experimental design was implemented on three variables: froth depth, total gas rate, and Jg profile. The three profiles were: “balanced” (each cell with the same Jg), “increasing” (from cell 1 to 7 proportionally), and “decreasing” (from cell 1 to 7 proportionally). The results were compared with the “as found” profile in the control bank (i.e.
Jg as set by the operators according to established practice).
Sixteen surveys were completed spanning five months. The experimental design allowed for two confounding factors inevitable over this extended period, changes in ore type, and circulating load.

Results and Discussion

The Jg profile proved to be the statistically most significant variable. The impact of the profile was on sphalerite selectivity against non-sulphide gangue (NSG), with no effect against pyrite. The increasing profile consistently gave the best metallurgy (down-the-bank grade/recovery). The improved performance was in the response of the first cells in the bank where the increasing profile gave the highest selectivity against NSG. The other profiles gave lower selectivity and higher zinc recovery in the first cells, which combined to reduce overall bank performance. The higher selectivity with the Increasing profile is attributed to the low gas rate in the first cells reducing water recovery and hence limiting entrainment of NSG. Analysis of froth density response to gas rate supported this interpretation.

Implementation

The results attracted the attention of operations. A trial was conducted with the final two cleaner banks (i.e. third and fourth) configured to the increasing Jg profile. An increase in plant concentrate grade with no loss in recovery prompted configuration of all cleaner stages to this strategy, which remains the current practice.

Conclusions

The availability of a gas rate (Jg) sensor has permitted investigation of the Jg profile to a bank of cells. In the example here of a cleaner bank, an increasing profile (low gas rate in the first cell and increasing down the bank) was found the best through control of entrainment in the first cells. All four cleaner stages at the plant were subsequently configured to this strategy.

The increasing profile is not suggested for all duties but the tools are available to find the appropriate one. This introduces the concept of “gas distribution management” as part of optimizing circuit performance.
Keywords: Air distribution, Profiles, Flotation circuit, Sensors, Brunswick mine
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