TAILINGS 101

So let’s talk about tailings. This conversation is overdue.  In particular, let’s talk about hard-rock, milled tailings. There are other materials at other types of mining properties that are sometimes referred to as tailings. For example, at a diamond mine there is the ground waste product, but the material isn’t milled. The mineral sands industry likewise has a material that is similar to tailings. The fly ash at power plants can be quite similar to milled tailings.

Okay, back to the basics. By “hard-rock”, I am leaving out things like coal and salts. When I say “milled”, I am referring a mechanical pulverizing process where crushed ore is placed in a spinning drum, normally with water. The drum (mill) spins, and the rock gets further crushed. Normally, metal balls or rods are used in the mills to aid in the pulverizing the ore.

To recover the metals and other minerals, the necessary quantities of rock are mined, crushed, pulverized, and processed to recover metal and other mineral values. A fine grind is often needed to release metals and minerals, so the mining industry produces enormous quantities of fine rock particles, in sizes ranging from sand-size grains down to as low as just a few microns. These fine-grained wastes are known as “tailings.” Tailings are a waste product of the mining industry, and they need to be managed as such. Sometimes the tailings are benign with respect to the environment, but even so, they are typically transported via pipeline or open channel, and deposited into a tailings facility.

The purpose of a tailings facility is to contain the fine-grained tailings, often with a secondary purpose of conserving water for use in the mine and milling operation. This has to be accomplished in a cost-effective manner that provides for long-term stability of the tailings facility and the long-term protection of the environment. When designing any tailings facility, cost, stability, and environmental performance must be balanced, with site-specific conditions.

In a nutshell, milled tailings are the waste byproduct of the mining industry, but they must be handled and accommodated in a very well-thought-out manner. The management and stewardship of tailings is becoming increasingly important to the industry, to society and to the environment.

Tailings may also be deposited as a dilute slurry; it may be thickened or sent through a high-density thickener. It may be thickened in a paste-like consistency, or it may be “de-watered” to a filter cake, which has approximate the consistency of used coffee grounds from a drip coffee maker. Each type of tailings technology has its own specific uses, limitations and advantages. I’ll make a point to discuss some of that in a future blog.

I have made much of my career as a tailings facility designer. Tailings facility design encompasses so many aspects of geotechnical (and civil) engineering. It includes soil mechanics, site investigation, laboratory testing, slope stability and seepage analyses, earthquake engineering, hydrology and hydraulics, civil site layout and material handling and so on.

Who the heck wouldn’t want to be a tailings dam designer with all those fascinating aspects involved?