A new process for nickel ammonium disulfate production from ash of the hyperaccumulating plant Alyssum murale

Abstract

The extraction of nickel (Ni) from ultramafic soils by phytomining can be achieved using Alyssum murale cultures. This study presents a new process for the valorization of Ni accumulated by this plant through the production of a Ni ammonium disulfate salt (Ni(NH₄)₂(SO₄)₂.6H₂O). The process comprises an initial leaching of the ashes of A. murale with a sulphuric acid solution (1.9 M H₂SO₄, T = 95 °C, t = 240 min, TS = 150 g ash L^− 1), producing a leachate rich in Ni (10.2 g Ni L^− 1; 96% Ni solubilisation), Mg, P, K, Fe, Ca and Al. The pH of the acid leachate is increased to 5.0 with NaOH (5 M), followed by an evaporation step which produced a purified solution rich in Ni (21.3 g Ni L^− 1) and an iron hydroxide precipitate. The cold crystallization (T = 2 °C, t = 6 h) of this solution by the stoichiometric addition (× 1.2) of ammonium sulfate generates a Ni ammonium disulfate salt, containing 13.2% Ni, that is potentially valuable to industry.

Introduction

Phytomining uses metal accumulating plants as miners to extract commercially valuable metals, these metals being initially present in mineralized soils that cannot be exploited by traditional mining methods (Chaney et al., 2007, Barbaroux et al., 2009). The application of phytoextraction principles for phytoremediation or phytomining proposes the use of plants that can annually accumulate sufficient quantities of metals like Ni, Zn, Cd, Se, Co, As or Ti (Baker and Brooks, 1989).

More than 400 species of plants, representing about 45 families, have been identified as hyperaccumulators and two thirds of these accumulate Ni (Brooks, 1998, Chaney et al., 2007).

Nickel is present at trace concentrations in all compartments of terrestrial environments, except in serpentine minerals and soils, where concentrations are higher. Serpentine soils stem from the alteration of ultramafic rocks and are widely distributed around the world (Brooks, 1987, Ghaderian et al., 2007). These soils are characterized by a pH between 6 and 8 and abnormally high concentrations of metals like Ni, Cr and Co, as well as high concentrations of Fe and Mg and low concentrations of Ca. Other elements K, P and S are present at trace concentrations (Shallari et al., 1998). The concentration of Ni in these soils varies from 0.5 to 8 g Ni kg^− 1 of soil (Bani et al., 2007).

Nickel based chemicals are used for electrolytic plating, fabrication of batteries (nickel hydroxide is used in Ni-Cd batteries), paint pigment fabrication and as catalysts for a number of chemical reactions (Kerfoot et al., 1995). Ni being a transition metal, Ni compounds are numerous and not all of them are interesting. However, chemicals like Ni ammonium disulfate salts used for electrolytic plating are more valuable than, for example, nickel destined for the production of alloys (Kerfoot et al., 1995). The precipitation of Ni ammonium disulfate salts is possible through the addition of ammonium sulfate to a concentrated nickel sulfate solution. The physical properties of nickel sulfate and ammonium in solution have been studied and quantified (Linke, 1965, Mullin and Osman, 1967, Mullin and Osman, 1976).

The hyperaccumulating plant Alyssum murale can accumulate up to 20,000 mg Ni kg^− 1 dry weight and a biomass of 10,000 kg ha^− 1 can be harvested per year (Chaney et al., 2007). Serpentine soils contain Ni at concentrations between 1000 and 7000 mg kg^− 1. These concentrations are well below the exploitation threshold required by traditional mining (30,000 mg kg^− 1), but sufficient to allow the extraction and accumulation of Ni by hyperaccumulating plants (Baker and Brooks, 1989, Shallari et al., 1998, Broadhurst et al., 2004).

Once harvested and dried, A. murale can be treated by pyrometallurgy in a smelter (Chaney et al., 2007) to produce metallic nickel or by leaching to produce a concentrated nickel leachate (Wood et al., 2006). It would therefore be plausible to produce a pure and commercializable Ni from harvested and chemically treated A. murale grown on serpentine soils.

It has been demonstrated that the leaching of A. murale seeds in sulphuric acid (0.5 M at 90 °C) for 120 min with a 15% solids fraction, followed by two washing steps, allowed the extraction of 97% of the available Ni (Barbaroux et al., 2009). However, a previous study demonstrated that the recovery of Ni in this type of leachate using selective precipitation or electrodeposition was not economically feasible (Barbaroux et al., 2011). In effect, Ghorbani et al. (2002) demonstrated the influence of organic complexes during metal electrolysis. Only a process using the solvent Cyanex 272, a phosphonic acid known for its ability to separate nickel from other metals (e.g. Co), allowed the selective extraction of Ni to an aqueous solution (Barbaroux et al., 2011). Despite the selectivity and high yield of the selective extraction using solvents, a difficult optimization procedure would be required given the prohibitive cost of Cyanex 272 (Barbaroux et al., 2011).

Koppolu et al. (2004) demonstrated that the incineration of hyperaccumulating plants concentrated in Ni, Cu and Zn at 600 °C allowed the recovery of 99% of the total metals as an ash concentrate. The concentrations increased by a ratio of 3.2 to 6 compared to concentrations found in the dry plant matter. This approach appears to be the most feasible from both an economic and environmental standpoint (Harris et al., 2009).

Metal extraction technologies using hyperaccumulating plants and their valorization present difficulties and uncertainties that put into question the cost effectiveness of phytomining. Therefore, there exists a need to remove some of the disadvantages associated with these metal production technologies that would lead to a reliable and profitable production of metals, and their derivatives, extracted by plants. This study describes the steps required to produce a Ni ammonium disulfate salt from the ashes of the plant A. murale.