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Characteristics of Products from the Acid Ammonium Oxalate Treatment of Manganese Minerals

Published online by Cambridge University Press:  02 April 2024

Efraím Mendelovici  and
Amaya Sagarzazu
Efraím Mendelovici
Affiliation:
Materials Physico-Chemistry Laboratory, Department of Materials Science, IVIC, Apartado 21827, Caracas 1020A, Venezuela
Amaya Sagarzazu
Affiliation:
Materials Physico-Chemistry Laboratory, Department of Materials Science, IVIC, Apartado 21827, Caracas 1020A, Venezuela
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Abstract

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To determine the parameters that control the attack of Mn minerals by acid ammonium oxalate in darkness (AAOD), rhodochrosite, pyrolusite, manganosite, hausmannite, and bixbyite were shaken with AAOD for 2 hr. These treatments were followed systematically by X-ray powder diffraction (XRD) and AAOD-extractable Mn analyses. About 5% of original hausmannite (surface area = 6 m2/g) remained in the solid residue of the AAOD treatment; however, if the hausmannite surface area was increased to 8 m2/g, by grinding, it completely dissolved in oxalate. Synthetic hausmannite of high surface area (37 m2/g) and rhodochrosite were completely dissolved by oxalate. Manganosite (1.5 m2/g) and especially pyrolusite (~ 1 m2/g) were more resistant to AAOD attack. Ground manganosite (4.2 m2/g) dissolved completely, but ground pyrolusite (7.2 m2/g) was only partially attacked by AAOD, inasmuch as about 25% of pyrolusite was found in the residue. An increase of the extraction time to 4 hr did not completely dissolve the ground pyrolusite.

As a result of the AAOD treatment, MnC2O4 · 3H2O and MnC2O4 · 2H2O precipitated from the oxalate solutions with all starting minerals, except pyrolusite (~ 1 m2/g), which only slightly dissolved. The seldom reported MnC2O4 · 3H2O phase was identified in residues of freshly extracted samples by its strong characteristic peak at 6.5.-6.6 Å, the intensity of which gradually decreased and disappeared over several days when the sample was exposed to ambient conditions (22°C and 70% relative humidity). The trihydrate phase also collapsed after heating AAOD-treated rhodochrosite at 50°C; α-MnC2O4 · 2H2O was identified as the main crystalline product. Heating the α-MnC2O4 · 2H2O product at 115°C overnight transformed most of it to MnC2O4. The color of the oxalate-treated samples ranged from pinkish-gray to black (7.5 YR); their surface area ranged from about 20 to 30 m2/g. The degree of transformation of Mn minerals by oxalate depended on the surface area and structural characteristics of the starting materials.

Keywords

Acid ammonium oxalateDissolutionGrindingManganese oxidesSurface areaX-ray powder diffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 39 , Issue 3 , June 1991 , pp. 264 - 269
Copyright
Copyright © 1991, The Clay Minerals Society

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