Slag Recycling in Modern Production

ABSTRACT

The here studied slags derived from the ferronickel production, Doniambo, New Caledonia. The microstructural characterization shows that the ferronickel slags (FNS) are microcrystalline heterogeneous material whose main components are vitreous-like silicate, forsterite, quartz and enstatite with traces of chromite, calcite and akaganeite. Our results show that we can use up to 20% of FNS slag as a partial replacement of cement in mortar without compromising the mechanical properties. Slag recycling will lead to shorter storage time, reduce the waste management and environmental impact. This leads to significant cost reduction. Furthermore, the C footprint will be reduced.

 Introduction

The French company Société Le Nickel (ERAMET-SLN) has been producing nickel for 145 years in New Caledonia [1]. The plant at Doniambo processes New Caledonian nickel laterite ore with a nickel content of 2 % and a silica to magnesia ratio of 1.6 [2]. After beneficiation of the ore, the preconcentrate is dried in a single rotary orde dryer and pre-reduced in two 130-meter-long x 5.5 m diameter coal-fired rotary kilns. Coal scoops are used to feed reductant coal at two locations along the kilns. The resulting calcine and residual coke is fed into a single 22.2-m diameter, 3-electrode 94 MW / 120 MVA electric furnace which operates in the shielded-arc mode [3]. The slags are the waste of nickel manufacturing. SLN is currently producing around 2 million tons of FNS per year with an existing stockpile of 25 million tons immediately available [3]. At present, only 8% of the annual production is used in cement production and road construction [3]. The rest is stockpiled on site and may present environmental risks for surface and groundwater for example. Thus, waste reduction and/or elimination is desirable.

The FNS from SLN is chemically stable and free of harmful substances [3]. It provides excellent properties such as high density, hardness and toughness, good compaction with high water permeability, and high fire resistance with low thermal expansion [3]. It is widely used for many purposes such as asphalt aggregate, construction materials, molding sand and as a raw material for fertilizers. FNS presents an excellent potential for construction applications in the Pacific region and in particularly for Australia, taking advantage of its strategic positioning and the increasing risk of the decreasing supply of natural aggregates and other supplementary cementitious materials. This product is largely employed in road building, waterway stabilization, agriculture and in many other sectors [3]. Further industrial application of these alternative materials would contribute to minimize the extractive quarrying of primary aggregates thereby protecting more of natural resources and landscape.

The cement and concrete industries contribute considerably to CO2 emissions. Cement is one of the world’s most widely used construction materials. About 1.6 billion tons of cement is used each year, releasing 1.5 billion tons of CO2 and about 5-7% of the total anthropogenic footprint [4]. The enhanced use of slag as additives in the cement manufacturing will contribute to a reduction of the CO2 emissions.

The use of slags in concrete opens a whole new range of possibilities in the recycling of materials in the building industry, while reducing wastes [5]. Furthermore, these materials provide an added value to the cement properties. The ferronickel slag could also be used as building materials if they present appropriate physicochemical characteristics [6,7]. In the past 50 years, study showed that FNS are unsuitable for structural use [8,9] and a very limited number of studies are available on the use of FNS from New Caledonia in concrete. However, some problems have been encountered related to durability aspects [3]. Therefore, the FNS are actually employed only as base filler for road construction [3]. As the secondary raw material, slag, is classified as waste, it has to cost-competitive at the same or better quality compared to equivalent products on the market.

In this study, we focus on the use of FNS as a structural component with a partial replacement in concrete. For that purpose, an experimental study of the soundness and compressive strength of specimens made with slags is carried out. Five mortar mixtures using different percentages of slags (0%, 5%, 10%, 15%, and 20%) are cast and tested up to failure.

 Materials and Methods

Characterization techniques

Imaging and elemental analyses were performed using a SUPRA™ 55 (SAPPHIRE; Carl Zeiss, Jena, Germany) scanning electron microscope equipped with an energy dispersive X-Ray analysis (EDX). The samples were carbon coated and images were recorded using an acceleration voltage of 20 kV. For EDX, 25 spectra were recorded on different zones and were averaged.

X-ray powder diffraction data was performed on a D8 Advance Vario 1 Bruker (two-circle diffractometer, θ-2θ Bragg-Brentano mode) at room temperature and using a pure Cu Kα radiation (λ = 1.54059 Å) selected by an incident beam Ge (111) monochromator. Data are collected for 2θ varying from 15° to 100° for 2sec per 0.01° step (2 h/scan). NIST SRM-660b LaB6 standard powder was used to calibrate the instrumental [10]. Crystalline phase identification and quantification were performed using the Full-Pattern Search-Match procedure and the Crystallography Open Database [11]. The Rietveld quantification was performed using MAUD software [12].

Raman spectroscopy were performed at room temperature with a Thermo Scientific DXR raman microscope (Thermo Fisher Scientific Inc., Waltham, MA, USA) with a 532 nm laser as an excitation source. The Raman spectrometer is equipped with a 900 lines/mm diffraction grating. A 50X magnification long working distance objective was used to focus the laser beam onto the surface and collect the scattered light in a backscattering geometry. Data was collected equipped with a 900 lines/mm diffraction grating. A 50X magnification long working distance objective was used to focus the laser beam onto the surface and collect the scattered light in a backscattering geometry. Data was collected over a range of 50-2200 cm-1. The spot size of the laser was estimated to 0.8 μm and the spectral resolution to 3 cm-1. Raman spectra were systematically recorded twice at mow laser power (1 Mw) and with an integration time of 400 s. We used the Origin software and Lorentzien curves as elementary fitting functions for silica glass. The compositions in terms of mineral phases were determined by comparing the collected Raman spectra to those reported in the Raman Open Database ROD [13].