As gold continues to shine in the modern world —whether in jewelry, finance or the components of everyday electronic devices —the reclamation of this valuable metal remains a vital activity. But how does gold recovery work, and how has science improved it?

Prof. Zhichang Liu, who heads the Laboratory of Supramolecular Organic Functional Assemblies (SOFA Lab) at Westlake and served as a postdoctoral fellow at Northwestern University, has pioneered a new and more environmentally friendly method for gold recovery. We asked him to explain the science behind gold recovery and share with us his novel approach.

What exactly is gold recovery?

This is an important topic because gold is very valuable. We use gold in many areas, from finance to making electronics. Some 80% of the gold now produced is used in the electronics industry. Gold can also be used in medicine, so gold is very significant.

Gold is also unique because you can find it everywhere, but in very small amounts. So normally 1 part per million (ppm) is already valuable to recovery. But how do you take that gold out?

In nature, gold always takes the form of gold metal. But you must extract material from thousands of mines to end up with something like 1 ppm of gold. How do you do that?

One process for gold recovery is known as cyanide leaching. We use a cyanide solution to dissolve the gold out from the gold mine, collect the solution, and then use a reducing agent to reduce the gold out and retrieve it. Cyanide leaching produces almost 80% of the gold in the world. But cyanide is very toxic, so it is very hazardous to the environment.

You might also see people washing gold from the river. That’s another way to get pure gold, but it yields a very tiny amount.

There’s also a process for gold recovery called gold amalgamation. You just use mercury to dissolve gold and make a gold-mercury solution. Then you isolate the gold by heating the mixture and evaporating the mercury. But it's also not good for the environment.

Additionally, people can use aqua regia to dissolve and recover gold. Aqua regia is made from two strong acids -- hydrochloric acid and nitric acid. It’s a powerful acid, so the mixture can oxidize gold, but it's also environmentally unfriendly.

Could you tell us about your research into gold recovery, and the more environmentally friendly approach you uncovered?

My discovery was made by accident.

One day I was trying to mix cyclodextrin, a type of cyclic sugar, with gold salt, to make something known as a metal organic framework.

I had these two water-based solutions, one with cyclodextrin and the other with gold salt. But when I combined the two together, I just got this solid precipitate.

At the time, I was upset because I expected the two to create a mixed solution that I would have used to accomplish something else.

However, I noticed the solution was a bit shiny, so I used scanning electron microscopy to check the precipitate, and I discovered these single-crystal needles.

We found the cyclodextrin and the gold formed a one-dimensional supramolecular complex.

Cyclodextrin can stack up to form a one-dimensional tube. Then, inside the tube the gold salt forms a chain.

So we thought about it, and hypothesized that we could use this to purify gold.

Normally, when you are purifying gold, you need to make a gold solution first. However, the gold is normally mixed with other metals such as copper, zinc or silver. So if we could highly selectively extract the gold, then we could just use this sugar – cyclodextrin -- to recover gold and avoid the use of cyanide.

When we did our experiment, we found that alpha-cyclodextrin was able to extract the gold with high selectivity and high yield.

If people want to recover gold without impacting the environment, what should they do?

First, you should avoid the use of cyanide or mercury in gold recovery because these are very environmentally hazardous.

You can choose our method using alpha-cyclodextrin. Alpha-cyclodextrin is a food additive, so it’s eco-friendly. We’ve already proved alpha-cyclodextrin is cost-efficient, and we’ve also seen some successful experiments with it which others have done.

In fact, our method is especially useful for recovering gold from waste electronics, such as your old cell phone or your old computer. This is known as urban mining.

The gold content in such waste electronics is very high -- much higher than the content in gold ore. In gold ore it might be 1 ppm but in your cell phone it might be something like 100 ppm. And in a computer, such as your RAM or memory, you see all this shiny gold, which is highly pure.

In an electronic board, you always have other metals like copper or silver, so how do you end up with pure gold? Here, you need high selectivity and high yield. Our method using alpha-cyclodextrin can selectively recover the gold, so we can get pure gold.

In our first experiment using alpha-cyclodextrin we recovered gold from gold alloy, which is a mixture of gold with zinc, copper and silver.