Mining Integration - Zinc Processing

Once zinc ore has been extracted from the deposit, it must be treated to remove the gangue (waste rock). This process is called beneficiation. Several beneficiation methods may be used, but at industrial scale for sulfide zinc ores, froth flotation is the most common method [1].

In the first step of froth flotation, the zinc ore is crushed and milled, then ground to an optimal particle size. Typical flotation feed sizes range from 75-125 micrometers [2]. However, over-grinding should be avoided: if too many particles are reduced below ~10 microns, they form slimes that interfere with flotation and are often lost in the tailings (left-over materials) [3]. Hydrocyclones or desliming screens help separate usable material from ultrafines or gangue. Dispersion agents improve the manageability of slimes [8].

The ground ore is then mixed with water to form a slurry. This slurry is treated with a specific sequence of flotation reagents [4]:

• Collectors (such as xanthates) selectively bind to zinc minerals like sphalerite, making them hydrophobic so they attach to air bubbles [1].
• Frother (e.g., methyl isobutyl carbinol or pine oil) help create and stabilize froth, allowing zinc particles to float to the surface more easily [5].
• Depressants are added to inhibit the flotation of unwanted minerals like pyrite or silicates by making them hydrophilic [5].
• Activators particularly copper sulfate (CuSO₄), are often used in zinc flotation to make sphalerite more responsive to collectors [6].

After reagents are added, the slurry is conditioned — mixed thoroughly so that reagents interact effectively with the mineral surfaces. Air is then introduced into the flotation cell. Hydrophobic zinc particles attach to the rising air bubbles and are carried to the surface to form a froth, while hydrophilic gangue particles remain in the pulp [1]. The froth, which contains the zinc concentrate, is skimmed or allowed to overflow and then collected. The remaining slurry at the bottom is called the tailings [2]. Because the initial concentrate still contained impurities, multiple flotation stages are employed:

• The rougher stage collects as much zinc as possible in a single pass, though the grade is relatively low.
• The scavenger stage treats rougher tailings to recover residual zinc; its output is recycled into the rougher feed.
• Biosorption columns made of dead biomass treated with microbes can be used to passively separate zinc from water, reducing the load on the bioreactor [9].
• Engineered bacteria or microalgae can be designed to uptake zinc, repel toxicity, and increase harvest efficiency [9].
• The cleaner stages (sometimes called middlings for intermediate steps) progressively purify the concentrate until a final, smeltable product is obtained [3].
• Using acid wash to separate zinc and feeding it back into the circuit turns the bioreactor into a secondary recovery unit, instead of just a waste treatment step. E. coli expressing metal-binding peptides is an active field [3].
• Alternatively, converting non-floatable zinc oxides or silicates into soluble forms through bioleaching can make zinc recoverable with electrowinning or precipitation methods [8].

Tailings from the middlings are often recycled into earlier stages to maximize recovery. This circuit of recycling and staged cleaning ensures both high recovery and acceptable concentrate grade [4].

The industrial application for the Metlock system is taking the zinc up following the removal of the foam from the remaining water, exiting it through a purge stream that goes to a bioreactor that sequesters the remaining zinc from the environment. Using feed sources such as treated wastewater and fine sediment underflow may increase total zinc output.

[1] Fuerstenau, M. C., Jameson, G. J., & Yoon, R. H. (2007). Froth flotation: A century of innovation. Society for Mining, Metallurgy, and Exploration.

[2] JXSC Mining. (2021). Four-step process of lead-zinc ore beneficiation. Retrieved from https://www.jxscmining.com

[3] Chen, W., & Zhang, L. (2020). Key factors influencing the flotation efficiency of lead-zinc ores. Minerals Engineering, 145, 106111. https://doi.org/10.1016/j.mineng.2019.106111

[4] Xinhai Mining. (2021). Zinc ore beneficiation - Sphalerite flotation methods. Retrieved from https://www.xinhaimining.com

U.S. Environmental Protection Agency. (1994). Technical resource document: Extraction and beneficiation of ores and minerals - Lead-zinc (EPA 530-R-94-011). Retrieved from https://nepis.epa.gov

[6] Wills, B. A., & Finch, J. (2016). Wills' mineral processing technology (8th ed.). Butterworth-Heinemann.

[7] Rawlings, D. E., & Johnson, D. B. (2007). Biomining. Springer.

[8] Volesky, B. (2003). Sorption and biosorption. BV Sorbex.