Abandoned potential

Clay Technology magazine
16 Aug 2018

Abandoned mine waste has been successfully transformed into glass, which can be used to create glass-ceramics, as Ellis Davies reports. 

Mine waste materials, which are commonly left in open-air tanks with no use, have been transformed into glass-ceramic materials by researchers at the Miguel Hernández University of Elche, Spain. The resulting ceramics can be used to cover façades, or as flooring for high-use buildings such as supermarkets. Researchers say they are cheaper to produce than traditional composite materials. This will also save on the use of raw materials.

The waste in question is a combination of a clay residue – vermiculite – and a phosphate – amblygonite. The former can be found in several deposits in Spain, and has been mined in Huelva for use as an isolation material. However, these open deposits are now abandoned, researchers report, along with deposits of amblygonite in the region of Extremadura, which were formerly exploited for decoration stone. 

Finding new uses

Vermiculite is a natural occurring mineral that is lightweight, non-combustible, highly absorbent, pH neutral, inert, non-reactive to all but very strong acids, and compressible. It is a type of clay consisting of two tetrahedral silicate layers coupled with one octahedral magnesium hydroxide-like layer. While mined for these properties, and for use as insulation, it has received some bad press relating to vermiculite insulation produced up until the 1990s, which was made from vermiculite mined in a specific USA mine that was known to contain asbestos deposits. Vermiculite mines are now tested for contamination.

Researchers were able to obtain a green, transparent glass from a mixture of the two waste materials, replacing the traditional raw materials of kaolin and commercial sodium borate, which can be processed into a glass-ceramic with a metallic effect on its outer layer, using a waste vitrification process. Samples of vermiculite were taken from the Huelva site, and amblygonite from Valdeflores. Researchers formulated several batches of vermiculite-amblygonite mix ranging from 90/10 to 50/50. They compared the mixture with another made up of vermiculite and sodium borate.

The mixtures were melted at 1,400oC in a furnace and moulded. They were then cooled for annealing by thermal heating at 500oC for one-hour and allowed to return to room temperature. The result was a green glass, which was then subjected to thermal treatments in a camera Khantal furnace to form transparent, opal, and opaque glass-ceramics. 

To assess the glass-ceramics’ capabilities, the team used thermal analysis. This showed that the materials have the appearance of a metallic surface, which was not present on the compared sample of vermiculite and sodium borate. The surface of the ceramic is enriched with high amounts of iron, which could be due to the easy diffusion of iron at high treatment temperatures. Overall, the ceramics’ mechanical properties were improved. Researchers were also able to show that the glass made using vermiculite and amblygonite has an increased tendency to crystallise, which will give the material advantageous properties such as high purification and low energy requirements.