Published: September 24, 2025
Author: Stephen D. Wagy, M.S. Chemistry, Texas Tech University
Abstract
A novel method has been developed to clean native aluminum and zinc nuggets using a chemical process called phosphating.
Mineral Deposit Removal from Native Aluminum and Zinc Nuggets
The native aluminum and zinc nuggets that I prospected were coated with a layer of mineral deposits. Most nuggets contained pockets of carbonaceous material and inorganic salts. Attempts were made to remove mineral deposits and embedded rocks with 5% acetic acid. This proved ineffective suggesting granite mineral deposits, which is consistant with the geology of the area prospected.
Most embedded rocks were removed physically due to the high reactivity of aluminum and zinc. Mineral deposits, inorganic salts and carbonaceous material were effectively removed using stainless steel wire brushes - bowl and pen type for rotary tools (e.g. Dremel). The lowest speed of the rotary tool was used. For safety and to improve visual appearance, burnishing was performed with a worn pen brush to smooth any sharp, decomposed edges and pockets and to remove any loose pieces. In most cases the results were aluminum and zinc nuggets with a hard, dark outer layer.
Phosphating Native Aluminum and Zinc Nuggets
Internet sources revealed that dark silver nuggets are only dark on the surface. Cross sections of dark silver nuggets showed the interior to be bright silver in color. This suggests that native aluminum and zinc nuggets have a similar outer layer that could be removed. A "chemical peel" to remove the dark layer on the surface of native aluminum and zinc nuggets was proposed and two chemical reactions were considered:
2Al + 2H₃PO₄ → 3H₂ + 2AlPO₄
3Zn(s) + 2H₃PO₄(aq) → Zn₃(PO₄)₂(aq) + 3H₂(g)
After removing as much of the dark surface as possible by burnishing with a worn pen brush, I treated the native aluminum and zinc nuggets I prospected with 85% Food Grade Phosphoric Acid for up to one hour at 25°C, stirring the solution and flipping the nuggets often to ensure even phosphating. Specimens were then rinsed well with local tap water (pH 7.6) to neutralize the phosphoric acid. Specimens were air dried one day, then brushed with stainless steel wire brushes - bowl and pen type. The result was a much cleaner looking and brighter surface on all specimens.
It is best to phosphate specimens individually. Some specimens that proved to be native zinc nuggets required only a few minutes to effectively phosphate due to their high reactivity to phosphoric acid. Some specimens that proved to be native zinc nuggets were less reactive to phosphoric acid than other native zinc nuggets and required an additional brushing and phosphating treatment. This was probably due to the thickness of the dark layer on the surface of the nugget and the chemical composition of the nugget.
Conclusion
Phosphating and wire brushing proved to be an effective way to clean native aluminum and zinc nuggets yielding specimens that have a bright metallic color and a high luster.
References
Phosphating.net
Aalberts Surface Technologies
M&M International
BOP Products