Factors Affecting Alkali Activation of Laterite Acid Leaching Residues

Konstantinos Komnitsas,Vasiliki Karmali,Georgios Bartzas,Evangelos Petrakis

doi:10.3390/environments8010004

Konstantinos Komnitsas, Vasiliki Karmali + Show 2 more

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https://doi.org/10.3390/environments8010004

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Abstract

In this experimental study, the alkali activation of acid leaching residues using a mixture of sodium hydroxide (NaOH) and alkaline sodium silicate solution (Na2SiO3) as activators is investigated. The residues were also calcined at 800 and 1000 °C for 2 h or mixed with metakaolin (MK) in order to increase their reactivity. The effect of several parameters, namely the H2O/Na2O and SiO2/Na2O ratios present in the activating solution, the pre–curing time (4–24 h), the curing temperature (40–80 °C), the curing time (24 or 48 h), and the ageing period (7–28 days) on the properties of the produced alkali activated materials (AAMs), including compressive strength, porosity, water absorption, and density, was explored. Analytical techniques, namely X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and elemental mapping analysis were used for the identification of the morphology and structure of the final products. The experimental results show that the laterite acid leaching residues cannot be alkali activated in an unaltered state, and the compressive strength of the produced AAMs barely reaches 1.4 MPa, while the mixing of the residues with 10 wt% metakaolin results in noticeably higher compressive strength (41 MPa). Moreover, the calcination of residues at 800 and 1000 °C has practically no beneficial effect on alkali activation. Alkali activated materials produced under the optimum synthesis conditions were subjected to high temperature firing for 2 h and immersed in distilled water or acidic solution (1 mol L−1 HCl) for 7 and 30 days in order to assess their structural integrity under different environmental conditions. This study explores the potential of alkali activation of laterite leaching residues amended with the addition of metakaolin for the production of AAMS that can be used as binders or in several construction applications in order to enable their valorization and also improve the environmental sustainability of the metallurgical sector.

Highlights

Heap leaching is one of the oldest low-cost hydrometallurgical processes that can be used for the extraction of base and precious metals, mainly from low-grade ores and secondary resources [1,2,3,4]
The objective of the present study was to explore the alkali activation potential of laterite acid leaching residues in the presence of metakaolin (MK) and develop an integrated approach that will result in their valorization and the production of high added value products that can be used as binders or construction elements, in line with the zero-waste approach and circular economy principles
Most studies carried out to assess the reactivity and subsequently the potential of various raw materials for alkali activation focused on the calculation of SiO2 /Al2 O3 molar ratios in the raw materials [54,55,56,57,58]

Summary

Introduction

Heap leaching is one of the oldest low-cost hydrometallurgical processes that can be used for the extraction of base and precious metals, mainly from low-grade ores and secondary resources [1,2,3,4]. The recovery of the metals from the pregnant leach solution (PLS) generated after leaching is mainly accomplished with the use of solvent extraction and electrowinning (SX/EW). One of the major environmental concerns associated with heap leaching is the management of the final leaching residues, the so-called spent ore, which may contain residual metal values and chemicals. The current industrial practice indicates that these residues are mainly disposed/stockpiled on site or leached at later periods for additional metal extraction. If residues are not properly managed, e.g., are left

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