[Image above] Picture of a tailings pond, which contains the waste left over from mining. In the future, rather than being left in the environment, tailings may be processed further to extract valuable minerals like feldspar and quartz. Credit: James St. John, Flickr (CC BY 2.0)
Lithium, copper, nickel, cobalt—these and other critical minerals have been a huge focus for governments in recent years with the transition to clean energy technologies and infrastructure.
What is often overlooked, however, is that identifying and mining ore deposits is only the first step in securing the supply chain. The ore still needs to undergo processing to extract the valuable minerals, and this process can be far from simple.
Take feldspar and quartz, for example. These minerals are some of the most abundant silicate minerals in the Earth’s crust. Quartz is widely used in various high-technology fields, such as aerospace, semiconductors, and solar energy, while feldspar is indispensable to the ceramics, glass, potash fertilizer, and insulation industries.
Currently, feldspar and quartz are sourced largely from open-pit mines that focus specifically on extracting these minerals. However, a lot of feldspar and quartz goes to waste in other mining operations that focus on extracting other materials.
For example, feldspar and quartz are major components of the granite-based ores from which nonferrous metals are extracted. But after the metals are extracted, these silicates are simply thrown away as mine waste (tailings).
Researchers suspect these tailings may become the primary source of feldspar and quartz in the future. In addition to being more cost effective than open-pit mining, processing the tailings provides a way to dispose of the waste without depositing it in the environment.
Extracting feldspar and quartz from the tailings is complicated, however, by the minerals’ physicochemical similarities. This similarity prevents the use of physical separation methods, such as filtration and distillation, because the minerals cannot be selectively separated out.
Recent studies, such as here, have demonstrated the potential of flotation to selectively separate out feldspar and quartz. Unfortunately, the most effective flotation methods to date rely on strongly acidic conditions, which comes with the risks of environmental pollution and equipment corrosion.
Depressants may provide a way to separate out feldspar and quartz under more favorable conditions. These substances increase the efficiency of the flotation process by selectively inhibiting the interaction of certain minerals with the collector, i.e., the reagent used to form a thin hydrophobic layer on the mineral surface. By preventing the collector from attaching to the undesired minerals, the desired minerals can be separated out more easily.
Very few studies have explored the use of depressants to aid in flotation separation of quartz and feldspar. One recent study, though, achieved promising results that hopefully will inspire more research.
The authors of the study come from Central South University and Southwest University of Science and Technology in China. They investigated using hydroxypropyl starch (HPS) as a depressant to help separate feldspar and quartz.
HPS is a modified starch that had its hydroxide groups replaced with propylene epoxide via a nucleophilic substitution reaction in a sodium hydroxide solution. It is widely used as a binder, suspending agent, and thickening agent in paper, textile, oil drilling, and daily chemical industries due to its nontoxicity and stable chemical properties.
For this study, the researchers used HPS in combination with magnesium chloride as the activator* and sodium oleate as the collector. They conducted micro-flotation tests on pure quartz and feldspar samples as well as bench-scale flotation tests on granite tailings.
*Activators are reagents that attach to a mineral’s surface and allow it to react with the collector.
The micro-flotation tests showed that without depressants, quartz and feldspar could be only partially separated at a pH of 10.50. However, the addition of HPS increased the selectivity index from 4.57 to 7.52. The bench-scale flotation tests confirmed the effectiveness of HPS as a depressant.
Raman and X-ray photoelectron spectroscopy analysis revealed the mechanisms behind HPS’ effectiveness. The modified starch reacted with aluminum sites on the feldspar surface, thereby reducing the adsorption of the sodium oleate collector. By preventing the attachment of the collector, HPS inhibited the flotation of feldspar but not the flotation of quartz.
“The research findings are expected to provide a new strategy for efficient flotation separation of quartz from feldspar and the comprehensive utilization of tailings,” the researchers conclude.
The paper, published in Minerals Engineering, is “Selective flotation of quartz from feldspar using hydroxypropyl starch as depressant” (DOI: 10.1016/j.mineng.2023.108022).