Aluminium and the untapped secondary resource

By Pritii Tam Wai Yin

Let’s talk about aluminium

Right at this moment in time you're probably reading this blog article from a screen contained in a device. And right at this moment in time, that particular device is most likely encased within an aluminium-derived covering. Now let’s talk about what we know about this delightful metal!

Aluminium is the third most abundant element found in the Earth’s crust accounting for up to 8.1% (by weight) of total elements (The highest two elements being oxygen at 46.7% and silicon at 27.7%)[1]. It is a lightweight non-toxic metal discovered by Hans Christian Oersted in 1825[2], and the pervasiveness of aluminium products in our modern times, allowing us to create vehicles, cans, electronic casing, household equipment etc. has resulted in aluminium production becoming the second largest of the world’s metal markets[3], second only to steel.

Hydrometallurgical processes

The prefix “hydro-“ often relates to water in Greek. In the case of extractive metallurgy, hydrometallurgical processes comprise numerous processing techniques involving aqueous solutions. This spans from the use of acid or caustic leaching liquors, ionometallurgical and solvometallurgical-related solvents and biohydrometallurgical pairings of aqueous system with microbes to recover metals from mined ores and concentrates.

In the extraction of aluminium, a famous economically viable process was developed by Carl Josef Bayer. The Bayer process describes alumina extraction from bauxite ores with caustics, producing waste known as bauxite residue. Unfortunately, the Bayer process also incurs aluminium losses from the system due to the formation of desilication products (DSP). DSP is the result of silicate precipitation in the digestion unit after alumina and silicate materials are dissolved. It contains caustic, alumina and silica approximately in the ratio of: 1:1.25:1.5[4]. DSP is often discarded together with bauxite residue.

Ionic Liquids: A look into the future?

One of the breakthrough technologies in hydrometallurgy is the discovery of ionic liquids that introduce a non-conventional processing approach. Ionic liquids are a group of salts in liquid form with melting points below 100oC and in its pure state, composed entirely with cations and anions [5]. The solvation of ionic liquids is unique, in which the existent ion-ion interactions of dissolved salts with the ionic liquid creates a highly-polar environment, as opposed to polarity dependent on molecule-molecule interactions in conventional aqueous solutions. Other important properties of ionic liquids include negligible vapour pressure, non-flammability, electricital conductivity and high thermostability. These contribute to speed and selectivity of reaction processes. The interest in understanding ionic liquids improves the likelihood of increased commercial and large-scale applications in the hydrometallurgical field.

Potential of secondary resource

In the last five years, there have been a shift of global aluminium supply market with China dominating slightly more than half of the market as indicated from June 2015 reports[6].

Image Credit: The International Aluminium Institute (Extracted from[6]

Increased demand and manufacture of aluminium leads to more ore processing, and thus to further increasing waste production (i.e. Bauxite residue). This presents an interesting and compelling scenario to address in the future years as the amount of bauxite residue continues to grow. Depending on the ore quality, bauxite residue consists of predominantly iron and aluminium compounds, followed by titanium and rare earth elements. The high market values of high-demand elements such as rare-earths, coupled with resource that is readily available to be tapped into, further stresses the fact that bauxite residue is actually an incredibly valuable secondary resource that can no longer be sidelined in the global economy.